JP3595098B2 - Thin film capacitors - Google Patents

Description

【００７７】
以上のようにして成膜した膜厚７７ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜の組成分析の結果を、下記表２に示す。組成分析は、ＩＣＰ発光分光法により行った。下部電極としてＳｒＲｕＯ_３ を用いている関係で、膜の溶解には、アンモニア水、過酸化水素およびＥＤＴＡの混合物からなるエッチング液を使用した。組成は、Ｂａ／（Ｂａ＋Ｓｒ）比が約０．１２であり、（Ｂａ＋Ｓｒ）／Ｔｉ比が約１．０６であった。
【００７８】
【表２】

【００７９】
図２に、膜厚３９ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜に関するＸ線回折（θ−２θ法）の結果を示す。基板として使用したＳｒＴｉＯ_３ からの回折ピーク以外に、下部電極として使用したＳｒＲｕＯ_３ および誘電体膜であるＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ からの回折ピークが、分離せずに観察されているものと考えられる。この回折ピークは、いずれも（００ｌ）面からのものに限られており、良好な結晶配向性を示している。
【００８０】
Ｘ線回折から求めた格子定数は、ＳｒＲｕＯ_３ 膜、Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜ともに等しく、膜面方向の方向の格子定数ａが０．３９０５ｎｍ、膜厚方向の格子定数ｃが０．３９８８ｎｍであった。即ち、Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜が成長する導電性基板表面すなわちＳｒＲｕＯ_３ 膜表面の格子定数は、ａ_ｓ ＝０．３９０５ｎｍである。
【００８１】
従って、誘電体本来の格子定数ａ_０ ＝０．３９１５ｎｍと、基板表面の格子定数ａ_ｓ とは、ａ_０ ／ａ_ｓ ＝１．００２５なる関係にあり、格子不整合が存在する。一方、成膜後の誘電体膜の膜面方向の格子定数ａ＝０．３９０５ｎｍと、基板表面の格子定数ａ_ｓ とは、ａ／ａ_ｓ ＝１．００の関係にあり、膜面方向の格子定数は緩和せずに誘電体膜が成長していることがわかる。
【００８２】
誘電体膜の単位格子体積の変化を比較すると、Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ の本来の対称性（立方晶）格子定数（ａ_０ ＝０．３９１５ｎｍ）から算出した単位格子体積Ｖ_０ が０．０６００１ｎｍ^３ であるのに対して、エピタキシャル成長後のＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜の単位格子体積Ｖは０．０６０８１ｎｍ^３ であり、単位格子体積の変化Ｖ／Ｖ_０ は、１．０１３であった。
【００８３】
図３に、ＳｒＲｕＯ_３ （００２）およびＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ （００２）の両者が含まれると考えられる回折ピークに関するロッキングカーブを示す。半値全幅（ＦＷＨＭ）の値は、約０．１２５°であり、ＳｒＲｕＯ_３ 膜およびＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜ともに、比較的結晶性が良好であることを示している。
【００８４】
図４に、膜厚７７ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜における比誘電率のバイアス電界依存性、図５に誘電損失のバイアス電界依存性を示す。測定にはＬＣＲメータを用い、１０ｋＨｚ，０．１Ｖにて行った。図４および図５から、誘電率および誘電損失は、バイアス電界ゼロ付近で最大値を取り、バイアス電界が強くなるにつれて減少することがわかる。比誘電率の低下は、分極飽和に伴うものと考えられる。比誘電率の最大値は約７４０であり、ＳｉＯ_２ の比誘電率を４と仮定して算出したＳｉＯ_２ 換算膜厚は、約０．４２ｎｍに相当する。
【００８５】
図６および図７に、膜厚３９ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜における、比誘電率、誘電損失係数のバイアス電界依存性を示す。測定は４０ｋＨｚ，０．１Ｖにて行った。比誘電率の最大値は約５９０であり、膜厚を薄くすることにより比誘電率が低下していることがわかる。しかしながら、ＳｉＯ_２ 換算膜厚は約０．２６ｎｍに相当し、膜厚を薄くすることによる蓄積電荷量の増大は表れている。
【００８６】
図８に、膜厚３９ｎｍおよび７７ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜における、比誘電率および誘電損失係数の温度依存性を示す。測定は室温から２００℃までの温度範囲について、周波数１ｋＨｚにて行った。図８から、膜厚７７ｎｍ，３９ｎｍともに温度上昇とともに比誘電率が低下していることがわかる。従って、強誘電相転移温度（キュリー温度）が、室温より低温側に存在するものと考えられる。比誘電率の温度変化は、厚い７７ｎｍについては急激であるのに対して、薄い３９ｎｍについては比較的緩やかである。室温においては膜厚による比誘電率の差が大きいが、温度上昇とともに減少する傾向にある。
【００８７】
このような比誘電率および誘電損失係数の温度依存性の振る舞いは、電界をかけたときに見られる誘電率の温度依存性に似ている。すなわち、キュリー温度付近では電界の有無による誘電率の差は大きいが、キュリー温度から遠ざかるにつれて誘電率の大きさ自体が減少し、電界の有無による差も小さくなる。したがって、膜厚が薄くなることによる比誘電率の低下は、薄い膜では膜の内部に内部電界が存在する（あるいは電極近傍に電界が存在する）と仮定することにより説明することができるように思われる。
【００８８】
誘電損失は、１００℃以下の温度範囲では約２％であり、温度によらずほぼ一定の値を示すが、１５０℃以上では温度とともに徐々に増大する傾向が見られる。薄い膜の方が高温での損失の増大が大きい。温度上昇に伴い、何らかの伝導キャリアが励起され、誘電損失を増大させている可能性があり、薄い膜の方がキャリア密度が高いと予想される。
【００８９】
図９に、膜厚３９ｎｍおよび７７ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜における、比誘電率および誘電損失係数の周波数依存性を示す。図９から、１００ｋＨｚ以下の周波数領域では、膜厚７７ｎｍ，３９ｎｍともに周波数が高くなるにつれて比誘電率が徐々に低下しており、誘電分散現象が観察されている。この範囲では誘電損失はほぼ２％で一定である。
【００９０】
周波数１００ｋＨｚ以上では、誘電損失が急激に増大するとともに、比誘電率も減少している。この原因としては、キャパシターに直列に入っているＳｒＲｕＯ_３ 膜の抵抗成分が、高周波ではキャパシターの充放電を律速するためと考えられる。これを裏付けるように、周波数をリニアスケールによってプロットすると、この領域では、誘電損失が周波数にほぼ比例している。
【００９１】
図１０に、膜厚３９ｎｍおよび７７ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜を誘電体膜とするキャパシタに、交流電圧（５００Ｈｚ，三角波）を印加したときの、キャパシターに蓄積される電荷密度の変化（Ｄ−Ｖ曲線）を示す。測定には演算増幅器を使用して自作した改良型のソーヤタワー回路を使用した。
【００９２】
電圧０Ｖ付近では曲線傾きが大きく比誘電率が大きいことに対応している。膜厚によらず、電圧が高くなるにつれて電荷密度は飽和する傾向がある。電圧の上昇時と下降時でわずかにヒステリシスが観測されているが、これが強誘電性によるものか、リーク電流などによるものかは、この測定からだけでははっきりしない。
【００９３】
図１１に、膜厚３９ｎｍおよび７７ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜を誘電体膜とするキャパシタにおける、交流電圧の振幅（ｐｅａｋ ｔｏ ｐｅａｋ）の値と、蓄積電荷密度の振幅（ｐｅａｋ ｔｏ ｐｅａｋ）の関係を示した。Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜の膜厚が３９ｎｍと７７ｎｍの間で比較すると、薄い方が低電圧領域において電荷量の立ち上がりが急峻であり、薄膜化による効果が現れている。膜厚３９ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜についてみると、電圧振幅１．５Ｖ_ｐ−ｐ のときに電荷密度の振幅が約０．２Ｃ／ｍ^２ である。これよりＳｉＯ_２ の比誘電率を４として換算膜厚を計算すると、約０．２６５ｎｍが得られる。この値は、比誘電率のバイアス電界依存性における比誘電率の最大値から見積もった換算膜厚にほぼ等しい。
【００９４】
高誘電率膜では一般に、比誘電率のバイアス電界依存性が大きい。したがってこの誘電体膜のＤＲＡＭへの応用を考える場合には、蓄積電荷量の測定値からＳｉＯ_２ 換算膜厚を算出した値を用いる方が妥当であると考えられる。
【００９５】
この誘電体膜のＤＲＡＭへの応用を考えた場合、パルスに対する蓄積電荷量の変化を調べる必要がある。図１２に交流パルス波形（ａ）と、キャパシターへの充放電電流の波形（ｂ）を示す。パルス電圧は−１Ｖから＋１Ｖまでの振幅とし、周期は１０μｓとした。電流波形は、キャパシターに直列に１００Ωの抵抗を接続し、この両端の電圧をモニターした。電圧、電流波形はデジタルオシロスコープによりモニターした。
【００９６】
充放電電流波形は１０００ｎｓ程度で十分減衰しているので、電荷量を求めるための積分時間は約２０００ｎｓとした。なお試料の容量は約１ｎＦである。
【００９７】
図１３に、このようにして測定した蓄積電荷量のパルス印加回数による変化を示す。蓄積電荷量は約０．２４Ｃ／ｍ^２ であり、これは図１１に示した電圧振幅２Ｖ_ｐ−ｐ のときの電荷密度にほぼ一致している（若干小さい値であるのは、測定周波数の影響であると考えられる）。パルス印加回数１０^９ 回まで電荷量の変化を調べたが、この範囲では顕著な電荷量の劣化は認められなかった。
【００９８】
図１４および図１５に、それぞれ膜厚７７ｎｍ，３９ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜に関する電流密度−電圧特性（Ｊ−Ｖ特性）を示す。図１４および図１５から、低電圧領域では電圧とともにむしろ電流が低下する現象が観測されることがわかる。これは吸収電流によるものであると考えられる。低電圧領域で吸収電流が大きいのは、誘電率が大きいために膜中の誘電率分布が大きいためと考えられる。ＤＲＡＭ適用の目安とされている、電流密度１×１０^−８Ａ／ｃｍ^２ 以下の電圧範囲は、膜厚７７ｎｍの場合、−４Ｖから＋７．５Ｖまでの計約１１．５Ｖであり、膜厚３９ｎｍの場合、−２．３Ｖから＋３．２Ｖまでの計約５．５Ｖであった。
【００９９】
誘電体膜に直流電圧を長時間印加すると、次第に絶縁抵抗が劣化する現象が知られている。図１６に、厚さ３９ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜に、直流の正電圧（ａ）および負電圧（ｂ）を長時間印加し続けたときのリーク電流の時間的な変化を示す。＋５Ｖおよび−５Ｖはそれぞれ電界強度約１．３ＭＶ／ｃｍに相当するが、図１６から、３０分間かけ続けてもリーク電流の増加はほとんど見られないことがわかる。
【０１００】
図１７に、厚さ３９ｎｍのＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜において、温度を室温から１５０℃まで変えて測定した場合の、リーク電流密度と電圧の関係を示す。図１７から、高電圧領域では、温度が高いほどリーク電流は増える傾向があり、リーク電流の伝導が熱的に励起されたキャリアの数に支配されていることがわかる。しかしながら、絶対温度の逆数（１／Ｔ）と電流密度の関係は、必ずしも直線的にはならなかった。これは、伝導キャリアの数が、熱的に励起される確率だけで増減しているわけではないことを示している。
【０１０１】
電圧上昇時と下降時でリーク電流密度の値に差が見られる。低温では、電圧下降時のリーク電流の値は上昇時と比較して小さい。電圧上昇時にはいわゆる吸収電流がリーク電流に重畳されているために大き目の測定値が得られ、下降時に吸収されて電荷が放出されるために、リーク電流とは逆方向の電流として重畳されるためと考えられる。
【０１０２】
一方高温側では、電圧上昇時よりも電圧下降時の方がリーク電流密度が増加している。この原因は必ずしも明らかではないが、高温では測定中の電圧により抵抗劣化が促進されている可能性がある。
【０１０３】
また、図１８は、横軸に電圧の平方根を取り、これと電流密度との関係を示したものである。図１８から、高い高圧領域では、電流密度と電圧の平方根が直線的な関係にあり、伝導キャリアがクーロン・ポテンシャルに打ち勝って励起されるような、ショットキー放出モデルあるいはプールフレンケル型の伝導で説明できることがわかる。
【０１０４】
この直線の傾きは温度によらずほぼ保存されている。温度によって励起されるキャリアの数の変化は単純な熱的励起で説明することはできないものの、キャリアが拘束されているポテンシャル形状は常に等しい、すなわちキャリアの種類は同じであることを示している。
【０１０５】
ここで、実施例１において、上部電極１４としてＳｒＲｕＯ_３ 膜を使用した場合について述べる。図１９は、上述と同様な方法で作製したＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚２３ｎｍ）上に、上部電極としてＳｒＲｕＯ_３ 膜を形成した場合の、Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜の誘電率および誘電損失の電界依存性を示すものである。このＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜は、膜厚が２３ｎｍと薄いにも関わらず、最大の比誘電率として、７００以上が得られている。このように、本発明において下部電極と誘電体の格子定数の関係を制御することによって誘電率が向上するが、更にこれに加えて、上部電極として導電性ペロブスカイト型酸化物を用いることにより、さらなる誘電率の向上に効果があることがわかる。
【０１０６】
比較例１
比較例として、ＭｇＯ（１００）基板上に下部電極としてＰｔ膜をエピタキシャル成長させ、その上にＢａ_０．２４Ｓｒ_０．７６ＴｉＯ_３ 膜をエピタキシャル成長させた場合について述べる。Ｐｔの本来の格子定数は０．３９２３ｎｍであり、Ｂａ_０．２４Ｓｒ_０．７６ＴｉＯ_３ は本来立方晶に属し、その格子定数はａ_０ ＝０．３９３ｎｍである。
【０１０７】
エピタキシャル成長後の膜の格子定数は、Ｐｔ膜の場合、膜面内の方向の格子定数ａ_ｓ は０．３９１５ｎｍであり、膜厚方向の格子定数ｃ_ｓ は０．３９３５ｎｍであった。これに対して、Ｂａ_０．２４Ｓｒ_０．７６ＴｉＯ_３ 膜の格子定数は膜の面内方向の格子定数ａが０．３９２５ｎｍ、膜厚方向の格子定数ｃが０．４００１ｎｍであった。このＢａ_０．２４Ｓｒ_０．７６ＴｉＯ_３ 膜の場合、本来の格子定数ａ_０ ＝０．３９３ｎｍは基板の格子定数ａ_ｓ ＝０．３９１５ｎｍに対する比ａ_０ ／ａ_ｓ が１．００４である。
【０１０８】
また、単位格子体積の変化を見積もると、本来の立方晶格子（ａ＝０．３９３ｎｍ）の単位体積Ｖ_０ は０．０６０６９ｎｍ^３ であるのに対して、エピタキシャル成長後の単位格子体積Ｖ＝ａ^２ ｃの値は０．０６１６３となり、体積変化の比率Ｖ／Ｖ_０ は１．０１５である。このような下部電極との格子不整合および単位格子体積の膨張により、本来立方晶に属する結晶構造が正方晶に歪み、膜厚方向の格子定数が伸ばされていると考えられる。
【０１０９】
しかしながら、下部電極との膜面内の格子定数を比較すると、下部電極Ｐｔの面内の格子定数ａ_ｓ が０．３９１５ｎｍであるのに対して、誘電体膜Ｂａ_０．２４Ｓｒ_０．７６ＴｉＯ_３ 膜のエピタキシャル成長後の面内の格子定数ａは０．３９２５ｎｍとなり、緩和の度合いを表わす比率ａ／ａ_ｓ は１．００２５となる。このように、歪みに緩和が生じている点が、比較例１が実施例１と異なる点である。
【０１１０】
図２０に、比較例１により作製したＢａ_０．２４Ｓｒ_０．７６ＴｉＯ_３ 膜に関する比誘電率と誘電損失の周波数依存性を示す。比較例１に係るＢａ_０．２４Ｓｒ_０．７６ＴｉＯ_３ 膜においては、誘電率の周波数依存性が非常に大きく、また誘電損失が広い周波数範囲に渡って、６−８％と大きい値を示している。このような誘電特性の周波数分散の原因は、歪みの緩和による、すなわち誘電体膜の内部に比誘電率あるいは伝導率、あるいは両者の分布が存在することが原因である。このような格子定数の歪みの緩和は、もともと格子定数の不整合により歪みが導入された誘電体膜において顕著に観測されるものである。
【０１１１】
図２１においては、比較例１により作製したＢａ_０．２４Ｓｒ_０．７６ＴｉＯ_３ 膜に関するリーク電流特性と、実施例１によるＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜に関するリーク電流特性の比較をおこなった。格子歪みの緩和の度合いが大きい比較例１の誘電体膜においては、緩和の度合いの少ない実施例１の誘電体膜よりも、大きなリーク電流が流れている。このように、歪みの緩和が大きい膜でリーク電流が大きい理由は、歪みの緩和に伴い、結晶転位が導入されるため、この結晶転位が伝導キャリアの供給源になるためと考えられる。
【０１１２】
実施例２
本発明の第２の実施例は、本発明を下部電極表面の格子定数よりも本来同じ程度か小さな格子定数を持つ誘電体の薄膜に適用した場合について述べたものである。
【０１１３】
図１に示すように、ＭｇＯ（１００）単結晶からなる基板１上に、立方晶Ｐｔ膜からなる下部電極２、ＳｒＴｉＯ_３ 膜からなるペロブスカイト誘電体膜３および所定形状にパターニングされたＰｔ膜からなる上部電極４を順次形成し、薄膜キャパシタを以下のようにして作製した。
【０１１４】
ＲＦスパッタ法により４インチ径のＰｔターゲットをスパッタし、ＭｇＯ（１００）単結晶基板１上に、膜厚５０ｎｍのＰｔからなる下部電極２を形成した。このとき、Ａｒ流量５０ｓｃｃｍの雰囲気中で、基板温度を６００℃に設定し、ターゲットへの投入ＲＦ電力を３００Ｗとした。
【０１１５】
形成されたＰｔ膜２をＸ線回折および反射電子線回折を用いて評価した。ＭｇＯ基板１とＰｔ膜２との結晶方位関係は、（１００）ＭｇＯに対し（１００）Ｐｔ、［００ｌ］ＭｇＯに対し［００ｌ］Ｐｔであり、Ｐｔ膜２がエピタキシャル成長していることがわかった。
【０１１６】
次に、ＲＦスパッタ法により、４インチ径のＳｒＴｉＯ_３ 焼結体ターゲットをスパッタし、下部電極２上に膜厚８０ｎｍのＳｒＴｉＯ_３ （ＳＴＯ）からなる誘電体薄膜３を堆積した。このとき、まずＡｒ／Ｏ_２ ＝４０ｓｃｃｍ／１０ｓｃｃｍの雰囲気で基板温度６００℃に設定し、ターゲットへの投入ＲＦ電力を４００Ｗとして厚さ１０ｎｍ以下の薄膜を堆積したのち、Ｏ_２ ＝１００ｓｃｃｍの雰囲気に切り替えて基板温度および投入電力は上記と同一条件で残りの厚さ７０ｎｍの薄膜を堆積した。
【０１１７】
このような２段階の堆積を行った理由は、最初からＯ_２ ＝１００ｓｃｃｍの雰囲気で堆積すると、Ｐｔ表面のわずかな酸化のために、ＳＴＯのエピタキシャル膜を得ることができないためである。
【０１１８】
形成されたＳＴＯ膜３をＸ線回折および反射電子線回折を用いて評価した。Ｐｔ膜とＳＴＯ膜との結晶方位関係は、（１００）Ｐｔに対し（１００）ＳＴＯ、［００ｌ］Ｐｔに対し［００ｌ］ＳＴＯであった。Ｘ線回折に基づいてＳｒＴｉＯ_３ 膜３の格子定数を測定したところ、膜厚方向（［００ｌ］方向）の格子定数ｃは０．３９８１ｎｍ、膜面に平行な方向（［１００］方向）の格子定数ａは０．３８８８ｎｍであった。ｃ／ａ比は１．０２であり、正方晶構造であった。
【０１１９】
単位格子体積の変化を調べると、ＳｒＴｉＯ_３ の本来の対称性（立方晶）格子定数（ａ＝０．３９０５ｎｍ）から算出した単位格子体積Ｖ_０ が０．０５９５５ｎｍ^３ であるのに対して、エピタキシャル成長後のＳｒＴｉＯ_３ 膜３の単位格子体積Ｖは０．０６０１８ｎｍ^３ であり、単位格子体積の変化Ｖ／Ｖ_０ は、１．０１１であった。
【０１２０】
その後、所定パターンのレジスト膜を形成した後、ＲＦスパッタ法により室温にて膜厚１００ｎｍのＰｔ膜を堆積し、リフトオフ法によりパターニングして上部電極４を形成した。上部電極４のサイズは１００μｍ×１００μｍとした。
【０１２１】
比較例２
実施例２と同様の方法を用いたが、下部電極２上に膜厚８０ｎｍのＳｒＴｉＯ_３ （ＳＴＯ）誘電体薄膜３を堆積する際に、雰囲気の切り替えを行わずに、Ａｒ／Ｏ_２ ＝４０ｓｃｃｍ／１０ｓｃｃｍの雰囲気だけで厚さ８０ｎｍの薄膜を堆積した。形成されたＳＴＯ膜３をＸ線回折および反射Ｘ線回折を用いて評価した。Ｐｔ膜２とＳＴＯ膜３の結晶方位関係は、（１００）Ｐｔに対し（１００）ＳＴＯ、［００ｌ］Ｐｔに対し［００ｌ］ＳＴＯであった。
【０１２２】
Ｘ線回折に基づいてＳｒＴｉＯ_３ 膜３の格子定数を測定したところ、膜厚方向の格子定数ｃは０．３９１０ｎｍ、膜面に平行な方向の格子定数ａは０．３９１５ｎｍであった。ｃ／ａ比は約１．００であり、ほぼ立方晶のままであった。このように、比較例２では実施例２と異なり、ＳＴＯ膜３の結晶格子の歪みは認められなかった。
【０１２３】
一方、成膜後の単位格子体積Ｖは、０．０５９９３ｎｍ^３ であり、単位格子体積の変化Ｖ／Ｖ_０ は、１．００６であった。以下、実施例２と同様にして、図１に示す構造を有する薄膜キャパシタを作製した。
【０１２４】
次に、上記のようにして作製した実施例２および比較例２の薄膜キャパシタの電気的特性を調べた。
【０１２５】
薄膜キャパシタの３００Ｋでの比誘電率−バイアス電界特性を図２２に示す。比誘電率は、比較例２に係る薄膜キャパシタでは３００であるのに対して、実施例２に係る薄膜キャパシタでは４５０と、バルク結晶（比誘電率約３５０）よりも大きな値を示した。実施例２のＳｒＴｉＯ_３ 膜が大きな比誘電率を示す理由は、酸素含有率の高い雰囲気で堆積したため、酸素のピーニング効果により、単位格子体積が膨張し、また膜面の格子定数が下部電極の基板に拘束されるために、結晶格子が膜厚方向に伸びて、結晶構造が正方晶に変化したことによると考えられる。
【０１２６】
比誘電率の温度依存性を測定した結果を図２３に示す。比誘電率の最大値は、実施例２のＳｒＴｉＯ_３ 膜では−５０℃近傍、比較例２のＳｒＴｉＯ_３ 膜では−１５０℃近傍である。この図からわかるように、実施例２のＳｒＴｉＯ_３ 膜では室温付近の広い温度範囲で大きな比誘電率を示す。
【０１２７】
なお、ＳｒＴｉＯ_３ 誘電体薄膜の組成（ＳｒとＴｉのモル比）を分析したところ、Ｓｒ／Ｔｉ比の値は、実施例２のＳｒＴｉＯ_３ 膜では１．０１／０．９９、比較例２のＳｒＴｉＯ_３ 膜では１．０４／０．９６であり、実施例２のＳｒＴｉＯ_３ 膜の方が化学量論組成に近いことがわかった。
【０１２８】
実施例３
本発明の第３の実施例は、本発明を歪みによって強誘電性が誘起された強誘電体薄膜に適用した場合である。この場合には、強誘電体薄膜において、残留分極の周波数特性が改善される。
【０１２９】
まず、図１に示すように、表面が平滑なＳｒＴｉＯ_３ （１００）単結晶基板１の上に、下部電極２として（００ｌ）配向のＢａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 薄膜を基板温度６００℃で、Ａｒ：Ｏ_２ ＝４０：１０（ｓｃｃｍ）雰囲気中（全体の圧力約０．６Ｐａ）にて、ＲＦマグネトロンスパッタリング法により形成し、これを本実施例における導電性の基板とした。このとき、Ｂａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜３の膜厚は約５０ｎｍとした。
【０１３０】
Ｘ線回折により、Ｂａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜２がペロブスカイト型の結晶構造を持つ（００ｌ）配向膜であることを確認すると共に、このような方法でスパッタ法により形成した場合には、膜厚方向の格子定数が約０．３９９ｎｍ、面方向の格子定数が約０．３９３５ｎｍであることを確認した。従って、このようにして作成したＢａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜は、正方晶の結晶対称性を持ち、本発明が規定するところの下部電極に該当する。
【０１３１】
また、本発明において規定する下部電極の面内の格子定数ａ_ｓ は、面内の格子定数０．３９３５ｎｍが該当する。
【０１３２】
このＢａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜の上に、強誘電体膜３として膜厚約２００ｎｍのＢａＴｉＯ_３ 膜を、ＲＦマグネトロンスパッタリング法により形成した。スパッタターゲットとしては、ＢａＴｉＯ_３ 焼結体（４インチ径，５ｍｍ厚）を用いた。成膜中の基板温度を６００℃とし、スパッタの雰囲気はアルゴン（Ａｒ）と酸素（Ｏ_２ ）の混合ガス（Ａｒ：Ｏ_２ ＝４０：１０ｓｃｃｍ）とし、基板・ターゲット間距離を１４０ｍｍとした。また、ターゲットに投入するＲＦ電力を６００Ｗとした。作製した膜の組成をＩＣＰ法で分析し、いずれもほぼ化学量論組成であることを確認した。
【０１３３】
この場合、Ｂａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜の面内の格子定数ａ_ｓ は０．３９３５ｎｍ、ＢａＴｉＯ_３ （正方晶系）のａ軸の材料本来の格子定数ａ_０ は約０．３９９２ｎｍであることが知られており（ＢａＴｉＯ_３ の本来のｃ軸は、０．４０３６ｎｍ）、本実施例についてはａ_０ ／ａ_ｓ ＝１．０１４であることになる。従って、本発明においてａ_０ ／ａ_ｓ が満たすべき、１．００２以上１．０１５以下という条件の範囲内である。
【０１３４】
次に、比較例３として本実施例と同様な方法で、表面が平滑なＳｒＴｉＯ_３ （１００）単結晶基板１の上に、下部電極として同様の（００ｌ）配向のＢａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 薄膜２を、基板温度６００℃で上述した同じ成膜条件にてＲＦマグネトロンスパッタリング法により形成し、比較例における導電性の基板とした。このとき、Ｂａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜２の膜厚を約５０ｎｍとした。Ｘ線回折により、Ｂａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜が（００ｌ）配向膜であることを確認すると共に、この膜の面内の格子定数が実施例３と同じ約０．３９３５ｎｍであることを確認した。
【０１３５】
このＢａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜２の上に、強誘電体膜３として膜厚約２００ｎｍのＢａＴｉＯ_３ 膜を実施例３とは異なる条件にて、ＲＦマグネトロンスパッタリング法により形成した。スパッタターゲットとしてはＢａＴｉＯ_３ 焼結体 （４インチ径，５ｍｍ厚）を用いた。成膜中の基板温度を６００℃とし、スパッタの雰囲気はアルゴン（Ａｒ）と酸素（Ｏ_２ ）の混合ガス（Ａｒ：Ｏ_２ ＝４０：１０ｓｃｃｍ）とし、基板・ターゲット間距離を１７０ｍｍとした。また、ターゲットに投入するＲＦ電力を４００Ｗとした。
【０１３６】
即ち、比較例３のスパッタ条件としては、基板・ターゲット間距離と投入電力が実施例３とは異なり、これ以外は実施例３と同じ条件にてＢａＴｉＯ_３ 膜を作製した。
【０１３７】
このようにしてスパッタリング条件を変えて作製した２種類のＢａＴｉＯ_３ 膜（実施例３と比較例３）について、Ｘ線回折法及びＲＨＥＥＤ法によって、詳細に結晶方位の調査及び格子定数の測定を行ったところ、次のような結果が得られた。即ち両者とも、同じようにＢａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ （００ｌ）膜の上に、（００ｌ）配向したＢａＴｉＯ_３ 膜が得られており、しかも面内の回転方向についても、下部電極２であるＢａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜と同じ方位関係、即ちＢＢＳＲ（（Ｂａ，Ｓｒ）ＲｕＯ_３ ）（１００）に対しＢＴ（ＢａＴｉＯ_３ ）（１００）、かつＢＳＲ（００ｌ）に対しＢＴ（００ｌ）なる関係が成立していることが確認された。
【０１３８】
しかしながら、ＢａＴｉＯ_３ 膜の格子定数については、実施例３と比較例３では下記表３に示すような差異が見られた。
【０１３９】
【表３】

【０１４０】
上記表３から、実施例３であるところの基板・ターゲット間距離１４０ｍｍかつ投入ＲＦ電力６００Ｗにて作製したＢａＴｉＯ_３ 膜については、膜厚方向の格子定数が０．４３２ｎｍ、面内の格子定数ａが０．３９４ｎｍであることがわかる。即ち、膜形成後のＢａＴｉＯ_３ 膜の面方向の格子定数ａと基板であるＢａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜の面方向の格子定数ａ_ｓ の比率、ａ／ａ_ｓ が１．００２未満という本発明が規定するところの範囲に含まれている。
【０１４１】
一方、比較例３である、基板・ターゲット間距離１７０ｍｍかつ投入ＲＦ電力４００Ｗにて作製したＢａＴｉＯ_３ 膜については、膜厚方向の格子定数が０．４２３ｎｍ、面方向の格子定数ａが０．３９９ｎｍであった。即ち、膜形成後のＢａＴｉＯ_３ 膜の面方向の格子定数ａと基板であるＢａ_０．１ Ｓｒ_０．９ ＲｕＯ_３ 膜の面方向の格子定数ａ_ｓ の比率、ａ／ａ_ｓ の比率が１．０１以上となり、１．００２未満という本発明が規定するところの範囲から逸脱している。
【０１４２】
これらのＢａＴｉＯ_３ 膜の上に、上部電極４としてＰｔ膜をＲＦスパッタリングにより作製した。Ｐｔ膜はリフトオフ法によって、１００μｍ×１００μｍの形状に加工した。
【０１４３】
このようにスパッタリング条件とその結果得られた格子定数の異なる２種類のＢａＴｉＯ_３ 膜（実施例３と比較例３）について、様々な周波数にてＤ−Ｅヒステリシス測定を実施した。測定にはソーヤタワー回路を用い、電圧の最大振幅は１０Ｖとし、室温（２２℃）にて行った。
【０１４４】
図２４に、実施例３と比較例３のＢａＴｉＯ_３ 膜の残留分極の２倍、２Ｐｒ （Ｃ／ｍ^２ ）の周波数依存性を示す。図２４から、６０Ｈｚ以下の低い周波数においては、実施例３のＢａＴｉＯ_３ 膜と比較例３のＢａＴｉＯ_３ 膜はほぼ同等の残留分極が得られているが、周波数が高くなるにつれて、比較例３のＢａＴｉＯ_３ 膜の残留分極が低下し、１００ｋＨｚでは実施例３のＢａＴｉＯ_３ 膜の約半分の残留分極しか得られないことがわかる。
【０１４５】
これに対し、実施例３のＢａＴｉＯ_３ 膜においては、５Ｈｚから１００ｋＨｚまでの範囲で残留分極の低下が僅かに５％程度である。このように、本発明により規定した範囲のエピタキシャル強誘電体薄膜は、高い周波数まで良好な強誘電特性を維持し得ることが分る。
【０１４６】
実施例４
図１に示すように、まず、表面が平滑なＳｒＴｉＯ_３ （１００）単結晶基板１の上に、下部電極２として（００ｌ）配向のＳｒＲｕＯ_３ 薄膜を、基板温度６００℃で、Ａｒ：Ｏ_２ ＝４０：１０（ｓｃｃｍ）雰囲気中（全体の圧力約０．６Ｐａ）にてＲＦマグネトロンスパッタリング法により形成し、これを本実施例における導電性の基板とした。このとき、ＳｒＲｕＯ_３ 膜２の膜厚は約５０ｎｍとした。
【０１４７】
Ｘ線回折により、ＳｒＲｕＯ_３ 膜がペロブスカイト型の結晶構造を持つ（００ｌ）配向膜であることを確認すると共に、このような方法でスパッタ法により形成した場合には、膜厚方向の格子定数が約０．３９８ｎｍ、面方向の格子定数が約０．３９２５ｎｍであることを確認した。従って、このようにして作成したＳｒＲｕＯ_３ 膜は、正方晶の結晶対称性を持ち、本発明に規定する下部電極に該当する。また、本発明に規定する下部電極の面方向の格子定数ａ_ｓ は、面方向の格子定数０．３９２５ｎｍが該当する。
【０１４８】
このＳｒＲｕＯ_３ 膜の上に、強誘電体膜３として膜厚約１３０ｎｍのＢａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 膜を、ＲＦマグネトロンスパッタリング法により形成した。スパッタターゲットとしては、Ｂａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 焼結体（４インチ径，５ｍｍ厚）を用いた。成膜中の基板温度を６００℃とし、スパッタの雰囲気はアルゴン（Ａｒ）と酸素（Ｏ_２ ）の混合ガス（Ａｒ：Ｏ_２ ＝４０：１０ｓｃｃｍ）とし、基板・ターゲット間距離を１４０ｍｍとした。また、ターゲットに投入するＲＦ電力を６００Ｗとした。作製した膜の組成をＩＣＰ法で分析し、いずれもほぼ化学量論組成であることを確認した。
【０１４９】
この場合、ＳｒＲｕＯ_３ 膜の面方向の格子定数ａ_ｓ は０．３９２５ｎｍ、Ｂａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ （正方晶系）のａ軸の材料本来の格子定数ａ_０ は約０．３９５ｎｍであることが知られており、本実施例については、ａ_０ ／ａ_ｓ ＝１．００６であることになる。従って、本発明においてａ_０ ／ａ_ｓ が満たすべき、１．００２以上１．０１５以下という条件の範囲内である。
【０１５０】
次に、比較例４として本実施例と同様な方法で、表面が平滑なＳｒＴｉＯ_３ （１００）単結晶基板１の上に、下部電極２として同様の（００ｌ）配向のＳｒＲｕＯ_３ 薄膜を、基板温度６００℃で上述した同じ成膜条件にてＲＦマグネトロンスパッタリング法により形成し、比較例４における導電性の基板とした。このとき、ＳｒＲｕＯ_３ 膜２の膜厚を約５０ｎｍとした。Ｘ線回折により、ＳｒＲｕＯ_３ 膜２が（００ｌ）配向膜であることを確認すると共に、この膜の面方向の格子定数が実施例４と同じ約０．３９２５ｎｍであることを確認した。
【０１５１】
このＳｒＲｕＯ_３ 膜の上に、強誘電体膜３として膜厚約１３０ｎｍのＢａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 膜を実施例４とは異なる条件にて、ＲＦマグネトロンスパッタリング法により形成した。スパッタターゲットとしてはＢａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 焼結体（４インチ径，５ｍｍ厚）を用いた。成膜中の基板温度を６００℃とし、スパッタの雰囲気はアルゴン（Ａｒ）と酸素（Ｏ_２ ）の混合ガス（Ａｒ：Ｏ_２ ＝４０：１０ｓｃｃｍ）とし、基板・ターゲット間距離を１７０ｍｍとした。また、ターゲットに投入するＲＦ電力を４００Ｗとした。即ち、比較例４のスパッタ条件としては、基板・ターゲット間距離と、投入電力が実施例４とは異なり、これ以外は実施例４と同じ条件にてＢａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 膜を作製した。
【０１５２】
このようにしてスパッタリング条件を変えて作製した２種類の（Ｂａ，Ｓｒ）ＴｉＯ_３ 膜（実施例４と比較例４）について、Ｘ線回折法及びＲＨＥＥＤ法によって、詳細に結晶方位の調査及び格子定数の測定を行ったところ、次のような結果が得られた。即ち、両者とも、同じようにＳｒＲｕＯ_３ （００ｌ）膜の上に、（００ｌ）配向したＢａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 膜が得られており、しかも面方向の回転方向についても、下部電極であるＳｒＲｕＯ_３ 膜と同じ方位関係、即ちＳＲ（１００）に対しＢＳＴ（１００）、かつＳＲ（００ｌ）に対しＢＳＴ（００ｌ）なる関係が成立していることが確認された。
【０１５３】
しかしながら、Ｂａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 膜の格子定数については、実施例４と比較例４では下記表４に示すような差異が見られた。
【０１５４】
【表４】

【０１５５】
即ち、実施例４であるところの基板・ターゲット間距離１４０ｍｍ、かつ投入電力６００Ｗにて作製したＢａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 膜については、膜厚方向の格子定数が０．４１３ｎｍ、面方向の格子定数ａが０．３９３ｎｍであった。即ち、膜形成後のＢａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 膜の面方向格子定数ａと基板であるＳｒＲｕＯ_３ 膜の面方向の格子定数ａ_ｓ の比率、ａ／ａ_ｓ が１．００２未満という本発明が規定する範囲に含まれている。
【０１５６】
一方、比較例４における基板・ターゲット間距離１７０ｍｍかつ投入電力４００Ｗにて作製したＢａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 膜については、膜厚方向の格子定数が０．４１１ｎｍ、面方向の格子定数ａが０．３９４ｎｍであった。即ち、膜形成後のＢａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 膜の面方向の格子定数ａと基板であるＳｒＲｕＯ_３ 膜の面方向の格子定数ａ_ｓ の比率、ａ／ａ_ｓ の比率が１．００３以上となり、１．００２未満という本発明が規定するところの範囲から逸脱している。
【０１５７】
これらのＢａＳｒＴｉＯ_３ 膜の上に、上部電極４としてＰｔの膜をＲＦスパッタリングにより作製した。Ｐｔ膜はリフトオフ法によって、１００μｍ×１００μｍの形状に加工した。
【０１５８】
このようにして作製した実施例４及び比較例４の２種類のＢａ_０．５ Ｓｒ_０．５ ＴｉＯ_３ 膜について分極の保持特性を比較した。保持特性の測定は、上部電極４に＋５Ｖの矩形のパルス（パルス幅２００ｎｓ）をかけた場合と、−５Ｖの矩形パルス（パルス幅２００ｎｓ）をかけた場合について、１秒後及び１０分後に再び＋５Ｖの読み出しパルス（２００ｎｓ）をかけた時に、電極に流れ込む電荷量を比較した。このとき、最初に＋５Ｖのパルスをかけた場合に読み出し時に流れ込む電荷量をＱｎｓとし、−５Ｖをかけたときに読み出し時に流れ込む電荷の量をＱｓｗとし、その差ｄＱ＝Ｑｓｗ−Ｑｎｓを比較した。
【０１５９】
その結果、実施例４においては、１秒後及び１０分後の電荷量差ｄＱは共にｄＱ＝０．１２Ｃ／ｍ^２ であった。これに対して比較例４においては、１秒後の電荷量差がｄＱ＝０．１０Ｃ／ｍ^２ であったにも拘らず、１０分後にはｄＱ＝０．０１Ｃ／ｍ^２ 以下の値にまで低下してしまった。即ち、実施例４においては、強誘電性に起因する十分な保持特性が得られているのに対して、比較例４においては十分な保持特性が得られておらず、比較例４は不揮発性メモリへの応用には適さないのが分る。
【０１６０】
実施例５
図１に示すように、まず、表面が平滑なＳｒＴｉＯ_３ （１００）単結晶基板１の上に、下部電極２として（００１）配向のＳｒＲｕＯ_３ 薄膜を、基板温度６００℃で、Ａｒ：Ｏ_２ ＝４０：１０（ｓｃｃｍ）雰囲気中（全体の圧力約０．６Ｐａ）にてＲＦマグネトロンスパッタリング法により形成し、これを本実施例における導電性の基板とした。このとき、ＳｒＲｕＯ_３ 膜２の膜厚は約５０ｎｍとした。
【０１６１】
Ｘ線回折により、ＳｒＲｕＯ_３ 膜がペロブスカイト型の結晶構造を持つ（００１）配向膜であることを確認すると共に、このような方法でスパッタ法により形成した場合には、膜厚方向の格子定数が約０．３７９ｎｍ〜０．３９９１ｎｍの間にあり、面方向の格子定数が約０．３９０６ｎｍ〜０．３９１３ｎｍの間にあることを確認した（表６）。従って、このようにして作成したＳｒＲｕＯ_３ 膜は、正方晶の結晶対称性を持ち、本発明に規定する下部電極に該当する。また、本発明に規定する下部電極の面方向の格子定数ａ_ｓ は、面方向の格子定数０．３９１ｎｍが該当する。
【０１６２】
このＳｒＲｕＯ_３ 膜の上に、強誘電体膜３として膜厚約５０ｎｍの（Ｂａ_ｘ Ｓｒ_{１−ｘ−ｙ} Ｃａ_ｙ ）ＴｉＯ_３ 膜を、３つのターゲットを使用した３元のＲＦマグネトロンスパッタリング法により形成した。スパッタターゲットとしては、ＢａＴｉＯ_３ 焼結体、ＳｒＴｉＯ_３ 焼結体、およびＣａＴｉＯ_３ 焼結体（それぞれ４インチ径，５ｍｍ厚）を用いた。成膜中の基板温度を６００℃とし、スパッタの雰囲気はアルゴン（Ａｒ）と酸素（Ｏ_２ ）の混合ガス（Ａｒ：Ｏ_２ ＝４０：１０ｓｃｃｍ）とし、基板・ターゲット間距離を１４０ｍｍとした。また、ターゲットに投入するＲＦ電力を、それぞれＢａＴｉＯ_３ 焼結体には６００Ｗ、ＳｒＴｉＯ_３ 焼結体には４５０Ｗの一定の電力を供給する一方で、ＣａＴｉＯ_３ 焼結体には０Ｗから５００Ｗまで可変とした。
【０１６３】
作製した膜の組成をＩＣＰ発光分析法で分析した結果を下記表５に示す。また、ＣａＴｉＯ_３ 焼結体ターゲットに投入したＲＦ電力と誘電体膜中のＣａ量の関係を求めたところ、図２５に示す結果を得た。図２５から、誘電体膜中のＣａ量は、ＣａＴｉＯ_３ 焼結体ターゲットに投入したＲＦ電力にほぼ比例することがわかる。また、段差計を使用して測定したＢａ_ｘ Ｓｒ_{１−ｘ−ｙ} Ｃａ_ｙ ＴｉＯ_３ 膜の厚さは、下記表５に示すように、４６〜５２ｎｍの間に分布することがわかった。
【０１６４】
【表５】

【０１６５】
このようにして形成した、Ｃａ含有量が異なる５種類の（Ｂａ_ｘ Ｓｒ_{１−ｘ−ｙ} Ｃａ_ｙ ）ＴｉＯ_３ 膜の結晶構造をＸ線回折により解析したところ、いずれの膜も （００１）配向しており、いわゆるｃｕｂｅ ｏｎ ｃｕｂｅの関係をもってＳｒＲｕＯ_３ 膜上にヘテロエピタキシャル成長していることが確認された。その格子定数を測定したところ、下記表６に示すように、Ｃａの量によって若干の相違が見られたものの、いずれの場合も膜厚方向の格子定数ｃは０．４２２ｎｍから０．４２８の間で本来の格子定数と比較して５％以上伸びているのに対して、面方向の格子定数ａは、下部電極として用いたＳｒＲｕＯ_３ 膜のａ軸と完全に一致しており、緩和していないことが確認された。
【０１６６】
【表６】

【０１６７】
このように形成した、Ｃａ含有量が異なる５種類の（Ｂａ_ｘ Ｓｒ_{１−ｘ−ｙ} Ｃａ_ｙ ）ＴｉＯ_３ 膜上に、上部電極４としてＰｔ膜をスパッタリング法により形成し、リフトオフ法により、１００μｍ×１００μｍの形状に加工した。
【０１６８】
このようにして作製した（Ｂａ_ｘ Ｓｒ_{１−ｘ−ｙ} Ｃａ_ｙ ）ＴｉＯ_３ 膜の電流−電圧特性を測定したところ、図２６に示すように、Ｃａを含まない膜（ｙ＝０）と比較して、Ｃａを含む膜は一様に高電圧領域におけるリーク電流の低減が認められた。
【０１６９】
このようなリーク電流の低下の度合いは、Ｃａの量ｙに依存せず、ｙの値が２％以上では、ｙ＝０の膜と比較して、ｙの大きさに依らず、ほぼ同じ程度の効果が認められた。
【０１７０】
一方、これらの膜の分極Ｐと電圧Ｖの関係をソーヤタワー回路によって５００Ｈｚで測定したところ、図２７に示すように、Ｃａ含有量が異なる５種類のいずれの（Ｂａ_ｘ Ｓｒ_{１−ｘ−ｙ} Ｃａ_ｙ ）ＴｉＯ_３ 膜においても、強誘電性を示すヒステリシス曲線が観測された。これらの誘電体膜は、いずれもバルクならば、本来は室温で常誘電性を示すはずの組成を有するので、これらの膜においても、強誘電性は格子不整合により導入されたものである。しかしながら、そのヒステリシス曲線の形状には、Ｃａ量による相違が観測された。
【０１７１】
図２８は、ヒステリシス曲線を測定するのに用いる交流電圧の振幅と、その電圧振幅で得られる残留分極の関係を示したものものである。この図から、Ｃａ量ｙが増加すると、Ｃａ量に応じて、強誘電的なヒステリシスを示し始める電圧が低下することがわかる。しかも、分極が飽和したときの残留分極の絶対値は、Ｃａを置換しない場合と比べても、それほど遜色ないことがわかる。
【０１７２】
なお、本発明は上述した各実施形態に限定されるものではない。前記誘電体膜としては、ＢａＴｉＯ_３ や（Ｂａ，Ｓｒ）ＴｉＯ_３ に限られるものではなく、エピタキシャル成長したペロブスカイト型結晶構造の誘電体材料を用いることができる。例えば、先に（課題を解決するための手段）の項で述べたような、チタン酸ストロンチウム（ＳｒＴｉＯ_３ ），チタン酸カルシウム（ＣａＴｉＯ_３ ），スズ酸バリウム（ＢａＳｎＯ_３ ），ジルコニウム酸バリウム（ＢａＺｒＯ_３ ）などに代表される単純ペロブスカイト型酸化物、マグネシウム酸タンタル酸バリウム（Ｂａ（Ｍｇ_１／３ Ｔａ_２／３ ）Ｏ_３ ），マグネシウムニオブ酸バリウム（Ｂａ（Ｍｇ_１／３ Ｎｂ_２／３ ）Ｏ_３ ）などの複合ペロブスカイト型酸化物、さらにこれらの中から複数の酸化物を同時に固溶させたものなどを用いることができる。
【０１７３】
また本発明で、組成式がＡＢＯ_３ で表されるペロブスカイト型結晶構造を有する誘電体の組成式において、Ａとしては主としてＢａからなるものであるが、Ｂａの一部Ｓｒ，Ｃａのうち少なくとも一種類の元素で置換しても構わない。Ｂとしては主としてＴｉであるが、同様にＴｉの一部をＺｒ，Ｈｆ，Ｓｎ，（Ｍｇ_１／３ Ｎｂ_２／３ ），（Ｍｇ_１／３ Ｔａ_２／３ ），（Ｚｎ_１／３ Ｎｂ_２／３ ），（Ｚｎ_１／３ Ｔａ_２／３ ）のうち少なくとも一種類からなる元素で置換しても構わない。さらに、誘電体膜の成膜方法としては、ＲＦスパッタリング以外に、反応性蒸着，ＭＯＣＶＤ，レーザーアブレーション，ゾルゲル法などの方法を用いることもできる。
【０１７４】
また、実施形態では正方晶系の導電性基板を用いたが、立方晶系の導電性基板を用いることも可能である。その他、本発明の要旨を逸脱しない範囲で、種々変形して実施することができる。
【０１７５】
【発明の効果】
以上詳述したように、本発明によれば、成膜条件による単位格子体積の膨張や下部電極との格子定数の不整合を利用して誘電率を大きくしたり、あるいは強誘電性を誘起した場合に問題となっていた、誘電率の周波数依存性や高い誘電損失、残留分極の周波数依存性を抑制することができる。
【０１７６】
従って、鉛やビスマスなど低融点金属を成分として含まず、誘電率が大きく、あるいは残留分極が大きな、信頼性の高い高誘電率膜あるいは強誘電体薄膜を作製することができ、半導体メモリの大容量化及び高集積化に適した薄膜キャパシタを実現することが可能となり、ＤＲＡＭ技術あるいは不揮発性メモリ技術に貢献するところ大である。
【図面の簡単な説明】
【図１】本発明の一実施例に係る薄膜キャパシタを示す断面図。
【図２】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ）のＸ線回折パターンを示す図。
【図３】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ）の（００２）に関するロッキングカーブを示す図。
【図４】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚７７ｎｍ）の比誘電率のバイアス電界依存性を示す図。
【図５】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚７７ｎｍ）の誘電損失のバイアス電界依存性を示す図。
【図６】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ）の比誘電率のバイアス電界依存性を示す図。
【図７】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ）の誘電損失のバイアス電界依存性を示す図。
【図８】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ，７７ｎｍ）の比誘電率および誘電損失係数の温度依存性を示す図。
【図９】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ，７７ｎｍ）の比誘電率の周波数依存性を示す図。
【図１０】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ，７７ｎｍ）の蓄積電荷密度と電圧との関係を示す図。
【図１１】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ，７７ｎｍ）の測定電圧振幅と電荷振幅との関係を示す図。
【図１２】交流電圧負荷試験に用いた電圧波形と電流波形とを示す図。
【図１３】交流電圧印加回数による蓄積電荷量の変化を示す図。
【図１４】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚７７ｎｍ）の電流・電圧特性を示す図。
【図１５】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ）の電流・電圧特性を示す図。
【図１６】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ）に直流の正電圧（ａ）および負電圧（ｂ）を印加したときのリーク電流の時間的な変化を示す図。
【図１７】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚３９ｎｍ）の電流・電圧特性の温度依存性を示す図。
【図１８】電圧の平方根と電流密度との関係を示す図。
【図１９】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜（膜厚２３ｎｍ）上に上部電極としてＳｒＲｕＯ_３ 膜を形成した場合のＢａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜の誘電率および誘電損失の電界依存性を示す図。
【図２０】比較例１におけるＢａ_０．２４Ｓｒ_０．７６ＴｉＯ_３ 膜の比誘電率と誘電損失の周波数依存性を示す図。
【図２１】Ｂａ_０．１２Ｓｒ_０．８８ＴｉＯ_３ 膜のリーク電流特性を比較例１と実施例１とで比較して示す図。
【図２２】実施例２および比較例２の薄膜キャパシタにおける、比誘電率−電界特性を示す図。
【図２３】実施例２および比較例２の薄膜キャパシタにおける、比誘電率の温度依存性を示す図。
【図２４】実施例３および比較例３のＢａＴｉＯ_３ 膜の残留分極の周波数依存性を示す図。
【図２５】ＣａＴｉＯ_３ 焼結体ターゲットに投入したＲＦ電力と誘電体膜中のＣａ量の関係を示す図。
【図２６】Ｃａの比率によるリーク電流の変化を示す図。
【図２７】Ｃａの比率によるヒステリシス曲線の変化を示す図。
【図２８】Ｃａの比率とヒステリシス曲線の立上がりとの関係を示す図。
【符号の説明】
１…単結晶基板
２…下部電極
３…誘電体膜
４…上部電極[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a thin film capacitor used for a thin film capacitor used for a semiconductor integrated circuit and its peripheral circuits.
[0002]
[Prior art]
LSI memories, particularly dynamic random access memories (DRAMs), have been highly integrated at a speed of four times in three years in the past, and are expected to continue to be highly integrated at the same speed in the future. DRAM requires a certain capacitor capacity from the viewpoint of securing the sensitivity of the sense circuit and preventing soft errors due to radiation. Therefore, the effective area of the capacitor in the memory cell cannot be reduced in proportion to high integration. . As a result, as long as a silicon oxide film or the like is used as a dielectric material, the proportion of a capacitor occupied in a memory cell is gradually increasing as the degree of integration increases.
[0003]
The use of a high dielectric constant thin film as a dielectric film of a capacitor of a memory cell of a DRAM has been actively studied. This is intended to reduce the area of the capacitor and reduce the size of the highly integrated memory cell by using a high dielectric constant thin film as a dielectric film of the capacitor. When a high dielectric constant thin film is used as a dielectric film of a capacitor of a memory cell of a DRAM, it is necessary that a large amount of electric charge can be stored per unit area and a small leak current. Further, in order to operate the memory at high speed, it is required that charging and discharging be performed at sufficiently high speed.
[0004]
As an application to a memory cell of a DRAM, a silicon oxide film (SiO₂: Relative dielectric constant of about 4) and silicon nitride film (Si₃N₄: Instead of the relative dielectric constant of about 7), Ta₂O₅(Relative permittivity of about 28) is being studied. However, Ta₂O₅However, the dielectric constant is insufficient for reducing the capacitor area and achieving high integration.
[0005]
Therefore, as a material having a higher dielectric constant, strontium titanate (SrTiO) is used.₃) Or barium strontium titanate (Ba)_xSr_1-xTiO₃) Are being actively studied. (Ba, Sr) TiO₃Is BaTiO₃And SrTiO₃And has a perovskite-type crystal structure. SrTiO₃Is a paraelectric, BaTiO₃Is known to be a ferroelectric material having a Curie temperature of about 120 ° C.
[0006]
Since Sr and Ba are less likely to evaporate than Pb and Bi, (Ba, Sr) TiO₃It is relatively easy to control the composition in the preparation of the thin film. Also, SrTiO₃And BaTiO₃When is crystallized, it hardly takes a crystal structure other than the perovskite type (for example, pyrochlore type).
[0007]
However, Ba_xSr_1-xTiO₃However, there is a problem that when the film thickness is reduced to 100 nm or less, the dielectric constant decreases, and sufficient charges cannot be accumulated.
[0008]
On the other hand, storage devices (ferroelectric memories) using a ferroelectric thin film for a capacitor as a storage medium have been developed, and some of them have already been put to practical use. The ferroelectric memory is non-volatile, so that its memory contents are not lost even after the power is turned off. In addition, when the film thickness is sufficiently small, the spontaneous polarization is reversed quickly, and writing and reading can be performed at the same speed as DRAM. There are features such as. Further, since a 1-bit memory cell can be manufactured using one transistor and one ferroelectric capacitor, it is suitable for increasing the capacity.
[0009]
It is also under consideration to operate a ferroelectric memory as a DRAM. This is used as a memory cell capacitor of a DRAM (volatile memory) without inverting the spontaneous polarization of the ferroelectric during normal operation. However, the device is used as a nonvolatile memory by utilizing the remanent polarization of the ferroelectric thin film only before the power of the device is turned off. This method is effective in reducing the influence of a phenomenon (fatigue) in which ferroelectricity is degraded as polarization inversion is repeated, which is considered to be the biggest problem of ferroelectric memories.
[0010]
A ferroelectric thin film suitable for a ferroelectric memory has a large remanent polarization, a small temperature dependence of the remanent polarization, a small deterioration (fatigue) of the remanent polarization due to repeated polarization inversion, and a long remanent polarization. It is necessary to be able to hold time (retention). In addition, in order to operate the memory at high speed, it is required that the reversal of spontaneous polarization be sufficiently fast.
[0011]
At present, lead zirconate titanate (PZT) is mainly used as a ferroelectric material. PZT is lead zirconate (PbZrO)₃) And lead titanate (PbTiO)₃The solid solutions of (1), which are dissolved in a molar ratio of about 1: 1, are considered to be excellent as storage media because they have a large spontaneous polarization and can be inverted even in a low electric field. PZT has a relatively high transition temperature (Curie point) between the ferroelectric phase and the paraelectric phase of about 300 ° C. or higher. Therefore, in a temperature range where ordinary electronic circuits are used (120 ° C. or lower), stored contents are not stored. There is little worry about heat loss.
[0012]
However, it is known that it is difficult to produce a high quality thin film of PZT. First, lead (Pb), which is the main component of PZT, tends to evaporate at 500 ° C. or higher, so that it is difficult to control the composition accurately. Second, ferroelectricity appears only when PZT forms a perovskite crystal structure, but it is difficult to obtain PZT having this perovskite crystal structure. Instead, a crystal structure called pyrochlore can be easily obtained, but when this crystal structure is adopted, it does not show ferroelectricity. In order to form the PZT film into a perovskite structure, a temperature of a certain degree or more (about 500 ° C.) is required. However, when the temperature is increased, Pb evaporates or diffuses as described above, and the composition deviates from a desired composition. .
[0013]
Recently, SrBi, a kind of Bi layered perovskite compound, has been recently developed.₂Ta₂O₉Researches on such devices have been actively conducted for applications to ferroelectric memories and the like. However, although Bi is a low melting point element like Pb, sufficient crystallization is necessary to obtain hysteresis. Therefore, Bi is evaporated by heat treatment at a high temperature (700 ° C. or higher). Or the diffusion of Bi into the electrodes and the like is inevitable. In addition, when it is necessary to use a non-oriented polycrystalline film in spite of the fact that it is a material having a strong crystal anisotropy, there is a concern about variation in ferroelectric characteristics when miniaturized.
[0014]
Although it is difficult to produce high-quality PZT thin films and Bi layered compound thin films with good reproducibility, PZT and Bi-based compounds are now widely studied as storage media (capacitors) for memories. Besides, no ferroelectric material suitable for the memory capacitor has been found.
[0015]
Other than PZT, barium titanate (BaTiO₃) Are known as materials exhibiting ferroelectricity at room temperature. BaTiO₃Has the same perovskite structure as PZT, and is known to have a Curie temperature of about 120 ° C. Since Ba is less likely to evaporate than Pb, BaTiO₃It is relatively easy to control the composition in the preparation of the thin film. In addition, BaTiO₃When is crystallized, it hardly takes a crystal structure other than the perovskite type (for example, pyrochlore type).
[0016]
Despite these advantages, BaTiO₃The reason why the thin film capacitor described above has not been studied so much as a storage medium for a ferroelectric memory is that remnant polarization is smaller and temperature dependence of remnant polarization is larger than that of PZT. This is because BaTiO₃The characteristic Curie temperature Tc is relatively low (about 120 ° C.), and the coercive electric field is low. The Curie temperature Tc is a temperature specific to a ferroelectric material that undergoes a phase transition from a ferroelectric phase to a paraelectric phase. Even if a material exhibits ferroelectricity at room temperature, it does not exhibit ferroelectricity at temperatures higher than the Curie temperature. For this reason, BaTiO₃When a ferroelectric memory is manufactured by using a thin film capacitor using GaN, when exposed to a high temperature (about 120 ° C.) for some reason, not only may the stored contents be lost, but also the electronic circuit is usually Even in the used temperature range (85 ° C. or less), the temperature dependence of remanent polarization is large and the operation is unstable.
[0017]
Therefore, conventionally, BaTiO₃It has been considered that a thin film capacitor using a ferroelectric thin film made of is not suitable for use as a storage medium of a ferroelectric memory.
[0018]
On the other hand, recently, BaTiO having a thickness of 60 nm was epitaxially grown on a Pt / MgO single crystal substrate.₃A phenomenon in which the Curie temperature of a film rises to 200 ° C. or higher has been reported (Kenji Iijima et al., Applied Physics, Vol. 62, No. 12, 1993, pp. 1250-1251). According to this document, such a phenomenon occurs because BaTiO epitaxially grown to match the lattice constant of Pt.₃It is thought that in the perovskite lattice, the a-axis contracts and the c-axis length increases. However, in this document, such a phenomenon is observed only in BaTiO having a film thickness of 60 nm or less.₃It is stated that a thicker film will return to the original lattice constant due to misfit transition (because it becomes larger than the critical film thickness).
[0019]
On the other hand, it is said that the ferroelectric thin film generally tends to have a smaller remanent polarization as the thickness becomes smaller in a region of 1 μm or less. In fact, the BaTiO prepared in₃In the case of an epitaxial film, the residual polarization is 2-3 μC / cm for a film of 100 nm or less.²It has been reported that: Accordingly, in the region where the film thickness is 60 nm or less, practical remanent polarization cannot be obtained as a ferroelectric thin film even if the Curie temperature can be increased.
[0020]
For the reasons described above, the conventional BaTiO₃Even if the thin film capacitor described above could raise the Curie temperature by the epitaxial effect, it has been considered that it is difficult to use it as a storage medium of a ferroelectric memory.
[0021]
In view of the above-mentioned problems in the conventional ferroelectric thin film, the present inventors have proposed a dielectric material (for example, Ba) having a lattice constant relatively close to the lattice constant of the lower electrode (for example, Pt)._xSr_1-xTiO₃) And adopting the RF magnetron sputtering method, which is a film formation method in which misfit dislocations are relatively unlikely to occur in the film formation process, so that the film thickness is relatively large, about 200 nm. It has been found that the lattice constant (c-axis) extends in the film thickness direction and the lattice constant in the in-plane direction (a-axis) can be kept smaller than the original lattice constant obtained by the epitaxial effect. As a result, it is possible to realize a ferroelectric thin film that shifts the ferroelectric Curie temperature Tc to a high temperature side, shows a large remanent polarization in a room temperature region, and can maintain a sufficiently large remanent polarization even when the temperature is increased to about 85 ° C. I confirmed that there is.
[0022]
For example, Pt (lattice constant a = 0.329231 nm) which is hardly oxidized is used as the lower electrode, and barium strontium titanate (Ba) is used as the dielectric._xSr_1-xTiO₃: BST), ferroelectricity is exhibited even in a compositional region (x ≦ 0.7) which should not exhibit ferroelectricity at room temperature, and originally, It has been experimentally confirmed that, in a composition region (x> 0.7) that exhibits ferroelectricity at room temperature, a practically preferable ferroelectric characteristic in which the Curie temperature which is higher than room temperature is further increased can be realized.
[0023]
In addition, the method of introducing strain into the dielectric by utilizing the mismatch between the lattice constant of the lower electrode and the dielectric has a great effect on improving the dielectric constant of the paraelectric thin film. Have been found by the present inventors. That is, Ba_xSr_1-xTiO₃It has been found that when a thin film having a composition with a Ba content x of about 0.24 is epitaxially grown on Pt, ferroelectricity is not exhibited, but the dielectric constant is greatly increased. By utilizing such an effect, the amount of charge stored in the dielectric film used for the memory cell of the DRAM can be greatly increased.
[0024]
[Problems to be solved by the invention]
However, from subsequent experiments conducted by the present inventors, these systems, that is, Ba as a dielectric on the lower electrode, were used._xSr_1-xTiO₃The epitaxial effect obtained by utilizing the effect of lattice mismatch, that is, the ferroelectricity is given to the paraelectric material, or the ferroelectricity is given in the case of the originally ferroelectric material. It has been clarified that, when the dielectric constant of the reinforced or paraelectric material is improved, a problem may occur in high-speed driving.
[0025]
That is, when the obtained film is a ferroelectric, when the DE hysteresis is drawn at a high frequency, the remanent polarization may decrease compared to the case where the DE hysteresis is drawn at a low frequency. I understand. Further, when the obtained film is a paraelectric material, the frequency dependence of the dielectric constant increases, and a phenomenon that the dielectric constant decreases as the frequency increases may be observed.
[0026]
SUMMARY OF THE INVENTION An object of the present invention is to provide a thin film capacitor capable of storing a large amount of charges in a small area.
[0027]
Another object of the present invention is to provide a thin film capacitor suitable for increasing the capacity and integration of a semiconductor memory.
[0028]
[Means for Solving the Problems]
In order to solve the above problem, the present invention (claim 1) provides a first electrode having a cubic (100) plane or a tetragonal (001) plane on its surface, A dielectric thin film epitaxially grown thereon and having a cubic perovskite structure, and a second electrode formed on the dielectric thin film, wherein the dielectric thin film originally belongs to a cubic system. Perovskite crystal structure (lattice constant a₀) Is the unit cell volume₀= A₀ ³, Unit cell volume distorted to tetragon after epitaxial growth
(Lattice constant a = b ≠ c) is calculated as V = a²When expressed as c,
V / V₀≧ 1.01
And the ratio c / a between the lattice constant c in the film thickness direction and the lattice constant a in the direction parallel to the film surface is
c / a ≧ 1.01
A thin film capacitor characterized by satisfying the following relationship:
[0029]
The present invention (Claim 2) provides a first electrode having a cubic (100) plane or a tetragonal (001) plane on its surface, and a first electrode epitaxially grown on the first electrode. A dielectric thin film having a cubic perovskite structure; and a second electrode formed on the dielectric thin film, wherein the dielectric thin film has a lattice constant in a plane direction of a surface of the first electrode. a_s, The lattice constant of the dielectric material represented by the a-axis length of the perovskite type crystal structure belonging to the cubic system is represented by a₀When
a₀/ A_s≤1.002
And the ratio c / a between the lattice constant c in the film thickness direction and the lattice constant a in the direction parallel to the film surface is satisfied.
c / a ≧ 1.01
A thin film capacitor characterized by satisfying the following relationship:
[0030]
The present invention (Claim 3) provides a first electrode having a cubic (100) plane or a tetragonal (001) plane on its surface, and a first electrode epitaxially grown on the first electrode. It has a cubic perovskite structure, a dielectric thin film made of a dielectric material having a Curie temperature of 200 ° C. or lower, and a second electrode formed on the dielectric thin film, wherein the dielectric thin film is Let the lattice constant of the first electrode surface in the plane direction be a_sThe original lattice constant of the dielectric material represented by the a-axis length of the perovskite type crystal structure belonging to the cubic or tetragonal system is a₀When
1.002 ≦ a₀/ A_s≤1.015
When satisfying the following relationship, and the lattice constant in the plane direction after the dielectric is epitaxially grown is a,
a / a_s<1.002
A thin film capacitor characterized by satisfying the following relationship:
[0031]
The present invention (Claim 4) is characterized in that, in the above-mentioned thin film capacitor (Claims 1 to 3), the dielectric thin film is made of a compound represented by the following formula.
[0032]
ABO₃
Wherein A is at least one selected from the group consisting of Ba, Sr, and Ca, and B is Ti, Zr, Hf, Sn, Mg_1/3Nb_2/3, Mg_1/3Ta_2/3, Zn_1/3Nb_2/3, Zn_1/3Ta_2/3, Ni_1/3Nb_2/3, Ni_1/3Ta_2/3, Co_1/3Nb_2/3, Co_1/3Ta_2/3, Sc_1/3Nb_2/3, And Sc_1/3Ta_2/3At least one selected from the group consisting of: )
According to a fifth aspect of the present invention, in the above-described thin film capacitor (the first to third aspects), the first electrode is formed of a general formula ABO₃Wherein A is at least one selected from the group consisting of Ba, Sr, Ca, and La, and B is selected from the group consisting of Ru, Ir, Mo, W, Co, Ni, and Cr. (At least one kind) of a conductive compound having a perovskite crystal structure.
[0033]
According to the present invention (claim 6), in the above-mentioned thin film capacitor (claim 1 or 2), the dielectric thin film may be (Ba)_xSr_1-x) TiO₃(0 <x <0.9), wherein the first electrode is made of SrRuO₃It is characterized by comprising a conductive compound having a perovskite crystal structure represented by (0.9 <Sr / Ru <1.1).
[0034]
The present invention (Claim 7) provides the above-mentioned thin film capacitor (Claim 1) wherein 1.03 ≧ c / a ≧ 1.01 when the dielectric thin film is a paraelectric, and 1. 08 ≧ c / a ≧ 1.01.
[0035]
The present invention (Claim 8) is characterized in that, in the above-mentioned thin film capacitor (Claim 2), the dielectric thin film satisfies 1.1 ≧ c / a ≧ 1.01.
[0036]
According to the present invention (claim 9), in the above-mentioned thin film capacitor (claim 3), the dielectric thin film may have a ratio of 1.004 ≦ a₀/ A_s≦ 1.011 is satisfied.
[0037]
The present invention (claim 10) provides a conductive substrate having a cubic (100) plane or a tetragonal (001) plane on its surface in an atmosphere having an oxygen content of 70% or more. Forming a dielectric thin film having a perovskite structure in which a ratio c / a of a lattice constant c in a film thickness direction to a lattice constant a parallel to a film surface is 1.01 or more by epitaxially growing a perovskite dielectric material. The present invention provides a method for producing a perovskite-type dielectric thin film comprising:
[0038]
The thin film capacitor of the present invention is based on the following principle.
[0039]
In other words, by growing a dielectric film epitaxially on the lower electrode, the dielectric constant can be increased without changing the paraelectric material, or the ferroelectric characteristics can be imparted to the paraelectric material. And enhance the sex. The reason for such an effect (epitaxial effect) is that the dielectric is grown epitaxially so as to match the lattice constant of the lower electrode. Is to grow.
[0040]
Generally, a perovskite-type dielectric or ferroelectric material has the chemical formula ABO₃It is considered that the origin of the large polarization is due to the relative displacement of cations and anions. In particular, it is considered that the displacement of the B site ion governs the polarization characteristics. When a two-dimensional compressive stress is applied to a dielectric having such a perovskite structure, the lattice expands in a direction perpendicular to the stress, and the displacement of ions, particularly the displacement of B-site ions, is facilitated and polarization is facilitated. Become. Thereby, in the case of a paraelectric, the dielectric constant increases, and in some cases, the dipole interaction in the film thickness direction increases, thereby inducing ferroelectricity. In addition, when a ferroelectric substance is used, the magnitude of spontaneous polarization increases.
[0041]
Such an effect is likely to appear when a dielectric material having a larger lattice constant than the lower electrode is epitaxially grown, but also appears when the lattice constant of the dielectric is equal to or smaller than that of the lower electrode. In particular, the unit cell volume V of the dielectric film is reduced to the original unit cell volume V by sputtering.₀It becomes remarkable when it is larger than 1%. However, when a dielectric material having a lattice constant larger than that of the lower electrode is originally epitaxially grown, even if a dielectric film having the same orientation as the crystal orientation of the lower electrode is obtained, the lattice constant in the plane direction is not necessarily equal to that of the lower electrode. It doesn't always grow in perfect agreement. That is, the lattice mismatch is relaxed at or near the electrode interface, and an attempt is made to return to the original lattice constant of the dielectric. It is considered that such a relaxation of the lattice constant is mainly caused by a mismatch transition in the dielectric film. It goes without saying that the extent to which such relaxation occurs depends on the method of forming the dielectric, the film forming conditions, the crystallinity, surface roughness, and surface state of the lower electrode.
[0042]
However, the degree of such lattice relaxation greatly affects the characteristics of the obtained ferroelectric film or dielectric film. That is, as will be described in detail in the following embodiments, when the dielectric constant of the dielectric thin film is remarkably relaxed, the ferroelectric characteristic or the dielectric characteristic does not follow at a high frequency. On the other hand, in the case of a material with little relaxation of the distortion, the dielectric characteristic follows up to a high frequency.
[0043]
The dielectric constant can be followed up to a sufficiently high frequency (1 MHz) with little relaxation of the distortion because the lattice constant a in the plane direction of the lower electrode obtained by X-ray diffraction_sRatio of the lattice constant a in the plane direction of the dielectric after epitaxial growth to the ratio a / a_sBut a / a_s<1.002. Ratio a / a_sIs 1.002 or more, the frequency dispersion of the dielectric characteristics and ferroelectric characteristics is large, and sufficiently high dielectric characteristics cannot be obtained at a high frequency.
[0044]
Based on the above principle, in the first embodiment of the present invention, the original cubic system (lattice constant a₀)), The unit cell volume of the perovskite crystal structure belonging to₀= A₀ ³The volume of a unit lattice (lattice constant a = b ≠ c) of a dielectric film deformed into a tetragonal crystal after epitaxial growth is represented by V = a²When expressed as c,
V / V₀≧ 1.01
And a ratio c / a of a lattice constant c in a film thickness direction and a lattice constant a in a direction parallel to the film surface is 1.01 or more. Is provided.
[0045]
Preferred V / V₀Is 1.01. Note that V / V₀Is not particularly limited as long as the crystal is not destroyed, but is usually 1.05.
[0046]
The upper limit of c / a is preferably 1.03 when the dielectric thin film is paraelectric, and 1.08 when it is ferroelectric.
[0047]
In the second aspect of the present invention, when epitaxially growing a cubic perovskite-type dielectric material on an electrode, the lattice constant of the bulk original dielectric material is set to a₀, The lattice constant a of the electrode material surface_sThen, a₀Is a_sIs approximately equal to or less than
a₀/ A_s≤1.002
Wherein the ratio c / a of the lattice constant c in the film thickness direction and the lattice constant a in the direction parallel to the film surface is 1.01 or more. Is provided.
[0048]
As an electrode material and a perovskite dielectric material satisfying such a relationship, for example, Pt and SrTiO₃Combinations. Note that the lattice constant a of bulk cubic Pt_sIs 0.3923 nm, bulk cubic SrTiO₃Has a lattice constant of 0.3905 nm.
[0049]
Thus, when a perovskite-type dielectric is epitaxially grown on the cubic electrode, the thin film becomes close to a single crystal. In general, it is known that a perovskite-type dielectric material has a larger dielectric constant as a structure closer to a single crystal without a grain boundary. In addition, since the grain boundaries of a thin film closer to a single crystal are very small, the leakage current is smaller than that of a polycrystalline film.
[0050]
In the perovskite dielectric thin film according to the present invention, the ratio c / a of the lattice constant c in the thickness direction, ie, the [001] direction, to the lattice constant a in the direction parallel to the film surface, ie, the [100] direction, is 1.01 or more. Means that the lattice constant in the thickness direction increases, the lattice constant parallel to the film surface shrinks, the cubic crystal structure is broken, and the thin film has a tetragonal crystal structure. .
[0051]
The present inventors have found that, as described above, a large dielectric constant can be obtained in a perovskite-type dielectric thin film having a crystal structure close to a tetragonal structure due to a distorted crystal lattice. As described above, the reason why a large dielectric constant is obtained when the crystal lattice is distorted is, for example, SrTiO.₃This is considered to be because Ti atoms are easily displaced in the crystal lattice and polarization is facilitated.
[0052]
The value of the ratio c / a of the lattice constant c in the thickness direction of the perovskite thin film of the present invention to the lattice constant a parallel to the film surface is preferably 1.1 or less. This is because if the value of c / a exceeds 1.1, a desired crystal structure cannot be maintained.
[0053]
On the other hand, when ferroelectricity is induced by utilizing lattice mismatch, as in the third aspect of the present invention, a₀/ A_sIs desirably 1.002 or more. At a ratio smaller than this, a rise in the Curie temperature due to the epitaxial effect is not seen, or even if it is seen, it is difficult to obtain a ferroelectric substance having a large spontaneous polarization. On the other hand, a₀/ A_sThe reason for limiting the ratio to 1.015 or less is that if the ratio is larger than this, misfit transition occurs during the epitaxial growth of the dielectric film on the substrate. This is because a sufficient increase in Curie temperature cannot be obtained.
[0054]
a₀/ A_sIs preferably 1.004 or more in order to obtain more remarkable ferroelectricity. If the ratio is 1.011 or less, misfit of the lattice constant is small. Regardless, a dielectric film of epitaxial growth having relatively uniform lattice constant and good crystallinity is easily obtained.
[0055]
In the present invention, as described above, a₀/ A_sWhen the ratio is 1.002 or more, the lattice constant a in the in-plane direction after film formation is a / a_sIs 1.002 or less. The reason is that, when the lattice constant in the plane direction of the substrate and that of the dielectric after film formation match, the strain inside the dielectric film introduced by the epitaxial effect becomes constant, and as a result, This is because the ferroelectric characteristics or the distribution of the dielectric characteristics is reduced, good frequency characteristics of the ferroelectric characteristics and the dielectric characteristics are obtained, and high-speed driving becomes possible.
[0056]
As described above, in order to produce a dielectric film having a lattice constant (a-axis) having a size equal to the lattice constant (a-axis) of the lower electrode, the surface treatment of the lower electrode before forming the dielectric film Alternatively, it is necessary to optimize the initial formation conditions for producing the dielectric film. For example, by spraying oxygen activated by plasma onto the surface of the lower electrode before manufacturing the dielectric, a dielectric film with less lattice constant mismatch between the dielectric and the lower electrode can be formed. In the initial stage of dielectric film formation, forming a dielectric film at a slow film formation rate of 1 nm / min or less for the first few atomic layers has a lattice constant more consistent with that of the lower electrode. This is advantageous in forming a dielectric film.
[0057]
When the ferroelectricity is induced by strain in the present invention, the Curie temperature specific to the dielectric material is desirably 200 ° C. or less. The reason is that a material having a Curie temperature exceeding 200 ° C. usually contains lead or bismuth as a main component, so that it is difficult to suppress a change in composition due to evaporation of lead or bismuth at the time of forming a thin film, and, consequently, a good dielectric material. This is because it is difficult to obtain a body membrane. In addition, when lead or bismuth is integrated, it is difficult to control the composition because it easily diffuses from the dielectric film to other electrodes, insulating films, and the like. Furthermore, since the Curie temperature of a dielectric material having a Curie temperature exceeding 200 ° C. is originally sufficiently high, there is no problem even if the Curie temperature is used as it is, and the effect obtained by applying the present invention is small.
[0058]
The perovskite-type dielectric materials and electrode materials that can be used in the thin-film capacitor of the present invention described above include the following.
[0059]
That is, as an example of a dielectric material having a perovskite crystal structure, the formula ABO is used.₃The compound represented by these is mentioned.
[0060]
In the formula, A is at least one selected from the group consisting of Ba, Sr, and Ca, and B is Ti, Zr, Hf, Sn, Mg_1/3Nb_2/3, Mg_1/3Ta_2/3, Zn_1/3Nb_2/3, Zn_1/3Ta_2/3, Ni_1/3Nb_2/3, Ni_1/3Ta_2/3, Co_1/3Nb_2/3, Co_1/3Ta_2/3, Sc_1/3Nb_2/3, And Sc_1/3Ta_2/3At least one selected from the group consisting of:
[0061]
Among these, in particular, (Ba_xSr_1-x) TiO₃A compound represented by (0 <x <0.9, preferably 0 <x <0.7, more preferably 0.05 <x <0.24), (Ba_xSr_1-xyCa_y) TiO₃Compounds represented by (0 <x <0.9, preferably 0 <x <0.7, more preferably 0.05 <x <0.24, 0.01 <y <0.12) are preferred.
[0062]
Specifically, barium titanate (BaTiO₃), Strontium titanate (SrTiO₃), Calcium titanate (CaTiO₃), Barium stannate (BaSnO)₃), Barium zirconate (BaZrO)₃), Barium magnesium tantalate (Ba (Mg_1/3Ta_2/3) O₃), Magnesium barium niobate (Ba (Mg_1/3Nb_2/3) O₃), And oxides obtained by simultaneously dissolving a plurality of oxides among these oxides.
[0063]
As described above, the composition formula is ABO₃In the dielectric having a perovskite crystal structure represented by the formula, A is mainly composed of Ba, but a part of Ba may be replaced with at least one element of Sr and Ca. B is mainly Ti, but a part of Ti is similarly converted to Zr, Hf, Sn, (Mg_1/3Nb_2/3), (Mg_1/3Ta_2/3), (Zn_1/3Nb_2/3), (Zn_1/3Ta_2/3) May be replaced with at least one element. The reason why these compositions are desirable is that the oxides of these constituent elements (BaO, SrO, CaO, etc.) all have a sufficiently high melting point of 1000 ° C. or more, so that even if the film is formed at about 600 ° C. Is less likely to evaporate and compositional deviation is less likely to occur.
[0064]
Therefore, although it is not desirable to replace the A site with a low-melting element such as Pb, it is preferable that the content be 20% or less even if the replacement is required. The Curie temperature of a perovskite oxide composed of a combination of these elements is BaTiO, which has the highest Curie temperature when not replaced by Pb at all.₃Is 120 ° C., and is not practical as a ferroelectric thin film as described above if the material is at its original Curie temperature.
[0065]
The first electrode (lower electrode) in the present invention is made of a cubic or tetragonal electrode material.₃Wherein A is at least one selected from the group consisting of Ba, Sr, Ca, and La, and B is selected from the group consisting of Ru, Ir, Mo, W, Co, Ni, and Cr. (At least one kind) of a conductive compound having a perovskite crystal structure.
[0066]
Among these materials, SrRuO₃Is preferred. In this case, the ratio of Sr to Ru (Sr / Ru) preferably satisfies 0.9 ≦ Sr / Ru ≦ 1.1. Also, SrRuO₃Regarding the crystallinity, the full width at half maximum of the rocking curve is preferably 0.2 ° or less.
[0067]
Pt, Ir, Rh, Au can be used as an electrode material other than the above-mentioned conductive compound. In particular, Pt is SrRuO.₃The same effect as is expected.
[0068]
Note that the lattice constant a of the lower electrode_sIs selected so that a dielectric having a larger lattice constant can be selected when a dielectric to be formed thereon is selected._sIs preferably a material of 0.3935 nm or more.
[0069]
In the present invention, the growth direction of these perovskite-type dielectrics when epitaxially growing on a substrate is such that the (100) plane of the perovskite-type dielectric is grown parallel to the (100) plane of the conductive substrate. desirable. In the case where the conductor substrate is further epitaxially grown on the substrate therebelow and strain is introduced due to lattice constant mismatch with the substrate, the above-mentioned range is defined._sInstead of using the original lattice constant of the conductor material, it is natural to apply the lattice constant in the plane direction after strain is introduced by epitaxial growth.
[0070]
Further, in addition to the above-described conditions regarding the relationship between the lower electrode and the lattice constant of the dielectric, the use of a conductive perovskite oxide as the upper electrode further increases the dielectric constant or, in the case of ferroelectricity, a large residual amount. It is possible to obtain polarization.
[0071]
Examples of the method for forming the dielectric film include reactive evaporation, RF sputtering, MOCVD, laser ablation, and sol-gel method. Among them, a sputtering method in which a large strain is easily introduced is preferable.
[0072]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, various embodiments of the present invention will be described with reference to the drawings.
[0073]
Example 1
FIG. 1 is a sectional view showing a schematic configuration of a thin film capacitor according to a first embodiment of the present invention. In this embodiment, the lower electrode material is SrRuO₃, Ba as a dielectric_0.12Sr_0.88TiO₃A membrane was used. Note that SrRuO₃Has an intrinsic lattice constant of about 0.393 nm,_0.12Sr_0.88TiO₃Has a perovskite-type structure originally belonging to a cubic structure at room temperature, and the original lattice constant of its bulk is a₀= 0.3915 nm.
[0074]
SrTiO₃The lower electrode SrRuO was formed on a (100) single crystal substrate (10 mm × 10 mm) 1 by RF magnetron sputtering.₃Film 2 and dielectric film Ba_0.12Sr_0.88TiO₃The film 3 was heteroepitaxially grown. The respective film forming conditions including the Pt film 4 as the upper electrode are shown in Table 1 below.
[0075]
Ba_0.12Sr_0.88TiO₃The film 3 is formed of BaTiO₃And SrTiO₃Was performed by binary sputtering using the sintered target of the above. Two films having a sputtering time of 30 minutes and 60 minutes were formed, and the thicknesses of the respective dielectrics measured by a step gauge were 39 nm and 77 nm.
[0076]
[Table 1]

[0077]
Ba having a thickness of 77 nm formed as described above_0.12Sr_0.88TiO₃The results of the composition analysis of the film are shown in Table 2 below. The composition analysis was performed by ICP emission spectroscopy. SrRuO as lower electrode₃In order to dissolve the film, an etching solution composed of a mixture of ammonia water, hydrogen peroxide and EDTA was used. The composition had a Ba / (Ba + Sr) ratio of about 0.12 and a (Ba + Sr) / Ti ratio of about 1.06.
[0078]
[Table 2]

[0079]
FIG. 2 shows a Ba having a thickness of 39 nm._0.12Sr_0.88TiO₃4 shows the results of X-ray diffraction (θ-2θ method) on the film. SrTiO used as substrate₃SrRuO used as the lower electrode other than the diffraction peak from₃And Ba as a dielectric film_0.12Sr_0.88TiO₃It is considered that the diffraction peak from was observed without separation. These diffraction peaks are all limited to those from the (00l) plane, and show good crystal orientation.
[0080]
The lattice constant obtained from X-ray diffraction is SrRuO₃Membrane, Ba_0.12Sr_0.88TiO₃The films were the same, and the lattice constant a in the film surface direction was 0.3905 nm, and the lattice constant c in the film thickness direction was 0.3988 nm. That is, Ba_0.12Sr_0.88TiO₃The conductive substrate surface on which the film grows, ie, SrRuO₃The lattice constant of the film surface is a_s= 0.3905 nm.
[0081]
Therefore, the original lattice constant a of the dielectric₀= 0.3915 nm and the lattice constant a of the substrate surface_sMeans a₀/ A_s= 1.0025, and there is a lattice mismatch. On the other hand, the lattice constant a = 0.3905 nm in the film surface direction of the dielectric film after film formation, and the lattice constant a of the substrate surface_sMeans a / a_s= 1.00, indicating that the dielectric film grows without relaxing the lattice constant in the film surface direction.
[0082]
Comparing the change in the unit cell volume of the dielectric film, Ba_0.12Sr_0.88TiO₃The original symmetry (cubic) lattice constant (a₀= 0.3915 nm)₀Is 0.06001 nm³In contrast, Ba after epitaxial growth_0.12Sr_0.88TiO₃The unit cell volume V of the film is 0.06081 nm³And the change in unit cell volume V / V₀Was 1.013.
[0083]
FIG. 3 shows SrRuO₃(002) and Ba_0.12Sr_0.88TiO₃7 shows a rocking curve related to a diffraction peak which is considered to include both (002). The value of the full width at half maximum (FWHM) is about 0.125 °, and SrRuO₃Membrane and Ba_0.12Sr_0.88TiO₃Both films show that the crystallinity is relatively good.
[0084]
FIG. 4 shows a Ba film having a thickness of 77 nm._0.12Sr_0.88TiO₃FIG. 5 shows the bias electric field dependence of the relative permittivity of the film, and FIG. 5 shows the bias electric field dependence of the dielectric loss. The measurement was performed at 10 kHz and 0.1 V using an LCR meter. FIGS. 4 and 5 show that the dielectric constant and the dielectric loss reach their maximum values near zero bias electric field and decrease as the bias electric field increases. It is considered that the decrease in the relative dielectric constant is accompanied by polarization saturation. The maximum value of the relative dielectric constant is about 740, and SiO 2₂Calculated assuming that the relative permittivity of is 4₂The reduced film thickness corresponds to about 0.42 nm.
[0085]
FIG. 6 and FIG._0.12Sr_0.88TiO₃4 shows the bias electric field dependence of the relative permittivity and the dielectric loss coefficient of the film. The measurement was performed at 40 kHz and 0.1 V. The maximum value of the relative dielectric constant is about 590, and it can be seen that the relative dielectric constant decreases as the film thickness decreases. However, SiO₂The converted film thickness corresponds to about 0.26 nm, and the increase in the accumulated charge amount by reducing the film thickness is apparent.
[0086]
FIG. 8 shows the thicknesses of 39 nm and 77 nm Ba._0.12Sr_0.88TiO₃3 shows the temperature dependence of the relative dielectric constant and the dielectric loss coefficient of a film. The measurement was performed at a frequency of 1 kHz in a temperature range from room temperature to 200 ° C. From FIG. 8, it can be seen that the relative permittivity decreases with increasing temperature for both the film thicknesses of 77 nm and 39 nm. Therefore, it is considered that the ferroelectric phase transition temperature (Curie temperature) exists on a lower temperature side than room temperature. The temperature change of the relative dielectric constant is abrupt for a thick 77 nm, while it is relatively slow for a thin 39 nm. At room temperature, the difference in relative dielectric constant depending on the film thickness is large, but tends to decrease as the temperature rises.
[0087]
Such behavior of the temperature dependence of the relative dielectric constant and the dielectric loss coefficient is similar to the temperature dependence of the dielectric constant observed when an electric field is applied. That is, near the Curie temperature, the difference in the dielectric constant due to the presence or absence of an electric field is large, but as the distance from the Curie temperature increases, the magnitude of the dielectric constant itself decreases, and the difference due to the presence or absence of the electric field also decreases. Therefore, the decrease in the relative dielectric constant due to the decrease in the film thickness can be explained by assuming that an internal electric field exists inside the film (or an electric field exists near the electrode) in a thin film. Seem.
[0088]
The dielectric loss is about 2% in a temperature range of 100 ° C. or less, and shows a substantially constant value regardless of the temperature. Thin films have a greater loss at high temperatures. As the temperature rises, some conduction carriers may be excited to increase the dielectric loss, and it is expected that a thin film has a higher carrier density.
[0089]
FIG. 9 shows the thickness of Ba of 39 nm and 77 nm._0.12Sr_0.88TiO₃4 shows the frequency dependence of the relative dielectric constant and the dielectric loss coefficient of a film. From FIG. 9, in the frequency region of 100 kHz or less, the relative dielectric constant gradually decreases as the frequency increases for both the film thicknesses of 77 nm and 39 nm, and a dielectric dispersion phenomenon is observed. In this range, the dielectric loss is constant at approximately 2%.
[0090]
At a frequency of 100 kHz or more, the dielectric loss sharply increases and the relative dielectric constant also decreases. This is due to the SrRuO in series with the capacitor.₃It is considered that the resistance component of the film controls the charge and discharge of the capacitor at high frequencies. To support this, if the frequency is plotted on a linear scale, the dielectric loss is almost proportional to the frequency in this region.
[0091]
FIG. 10 shows the thicknesses of 39 nm and 77 nm Ba._0.12Sr_0.88TiO₃5 shows a change in charge density (D-V curve) stored in the capacitor when an AC voltage (500 Hz, triangular wave) is applied to a capacitor having a film as a dielectric film. For the measurement, an improved Sawyer tower circuit that was self-made using an operational amplifier was used.
[0092]
In the vicinity of the voltage of 0 V, the slope of the curve is large, which corresponds to a large relative permittivity. Regardless of the film thickness, the charge density tends to be saturated as the voltage increases. Although a small amount of hysteresis is observed when the voltage rises and falls, it is not clear from this measurement alone whether this is due to ferroelectricity or leakage current.
[0093]
FIG. 11 shows the thicknesses of 39 nm and 77 nm Ba._0.12Sr_0.88TiO₃The relationship between the amplitude of the AC voltage (peak to peak) and the amplitude of the accumulated charge density (peak to peak) in a capacitor using the film as a dielectric film is shown. Ba_0.12Sr_0.88TiO₃When the thickness of the film is compared between 39 nm and 77 nm, the thinner the film, the sharper the rise of the charge amount in the low voltage region, and the effect of the thinner film appears. Ba with a thickness of 39 nm_0.12Sr_0.88TiO₃Looking at the film, the voltage amplitude is 1.5V_ppWhen the amplitude of the charge density is about 0.2 C / m²It is. From this SiO₂When the converted film thickness is calculated with the relative dielectric constant of 4 as a value, about 0.265 nm is obtained. This value is substantially equal to the reduced film thickness estimated from the maximum value of the relative permittivity in the bias electric field dependence of the relative permittivity.
[0094]
In general, a high dielectric constant film has a large bias electric field dependence of the relative dielectric constant. Therefore, when the application of this dielectric film to a DRAM is to be considered, the measured value of the amount of stored charge indicates that the SiO 2₂It is considered more appropriate to use the value calculated for the converted film thickness.
[0095]
When the application of this dielectric film to a DRAM is considered, it is necessary to examine a change in the amount of accumulated charge with respect to a pulse. FIG. 12 shows an AC pulse waveform (a) and a charge / discharge current waveform (b) for the capacitor. The pulse voltage had an amplitude from -1 V to +1 V, and the period was 10 μs. As for the current waveform, a resistor of 100Ω was connected in series with the capacitor, and the voltage between both ends was monitored. The voltage and current waveforms were monitored using a digital oscilloscope.
[0096]
Since the charge / discharge current waveform is sufficiently attenuated at about 1000 ns, the integration time for obtaining the charge amount was set to about 2000 ns. The capacity of the sample is about 1 nF.
[0097]
FIG. 13 shows the change in the amount of accumulated charge measured in this manner depending on the number of times of pulse application. The accumulated charge amount is about 0.24 C / m²Which is the voltage amplitude 2V shown in FIG._pp(The slightly smaller value is considered to be due to the effect of the measurement frequency.) Number of pulse application 10⁹A change in the charge amount was examined up to the second time, but no significant deterioration in the charge amount was observed in this range.
[0098]
FIGS. 14 and 15 show the thicknesses of Ba and 77 nm, respectively._0.12Sr_0.88TiO₃5 shows current density-voltage characteristics (JV characteristics) of the film. From FIGS. 14 and 15, it can be seen that in the low voltage region, a phenomenon in which the current rather decreases with the voltage is observed. This is considered to be due to the absorption current. It is considered that the reason why the absorption current is large in the low voltage region is that the dielectric constant in the film is large because the dielectric constant is large. Current density 1 × 10, which is a guideline for DRAM application^-8A / cm²The following voltage range is a total of about 11.5 V from -4 V to +7.5 V for a film thickness of 77 nm, and a total of about 5.5 V from -2.3 V to +3.2 V for a film thickness of 39 nm. there were.
[0099]
It is known that when a DC voltage is applied to a dielectric film for a long time, the insulation resistance gradually deteriorates. FIG. 16 shows a Ba having a thickness of 39 nm._0.12Sr_0.88TiO₃FIG. 6 shows a temporal change in a leak current when a DC positive voltage (a) and a negative voltage (b) are continuously applied to the film for a long time. Although +5 V and -5 V correspond to an electric field strength of about 1.3 MV / cm, it can be seen from FIG. 16 that almost no increase in the leakage current is observed even after 30 minutes.
[0100]
FIG. 17 shows a Ba having a thickness of 39 nm._0.12Sr_0.88TiO₃The relationship between the leak current density and the voltage when the temperature is changed from room temperature to 150 ° C. in the film is shown. From FIG. 17, it can be seen that, in the high voltage region, the leak current tends to increase as the temperature increases, and the conduction of the leak current is governed by the number of thermally excited carriers. However, the relationship between the reciprocal (1 / T) of the absolute temperature and the current density was not always linear. This indicates that the number of conduction carriers does not increase or decrease only by the probability of being thermally excited.
[0101]
There is a difference in the value of the leak current density between when the voltage rises and when the voltage falls. At a low temperature, the value of the leak current at the time of voltage drop is smaller than that at the time of voltage rise. When the voltage rises, a so-called absorption current is superimposed on the leak current, so that a large measurement value is obtained. When the voltage falls, the absorption current is absorbed and the charge is released, so the current is superimposed as a current in the opposite direction to the leakage current. it is conceivable that.
[0102]
On the other hand, on the high temperature side, the leak current density is higher at the time of voltage drop than at the time of voltage rise. Although the cause is not necessarily clear, at a high temperature, there is a possibility that resistance deterioration is promoted by a voltage during measurement.
[0103]
FIG. 18 shows the relationship between the square root of the voltage on the horizontal axis and the current density. From FIG. 18, in the high-pressure region, the current density and the square root of the voltage are in a linear relationship, and the explanation is based on the Schottky emission model or the Pool-Frenkel conduction in which the conduction carriers are excited by overcoming the Coulomb potential. We can see that we can do it.
[0104]
The slope of this straight line is almost conserved regardless of the temperature. Although the change in the number of carriers excited by temperature cannot be explained by simple thermal excitation, it indicates that the potential shapes in which the carriers are constrained are always equal, that is, the types of carriers are the same.
[0105]
Here, in the first embodiment, SrRuO₃The case where a membrane is used will be described. FIG. 19 is a view showing Ba manufactured by the same method as described above._0.12Sr_0.88TiO₃SrRuO as an upper electrode on the film (thickness: 23 nm)₃Ba when a film is formed_0.12Sr_0.88TiO₃4 shows the electric field dependence of the dielectric constant and dielectric loss of the film. This Ba_0.12Sr_0.88TiO₃Although the film is as thin as 23 nm, a maximum relative dielectric constant of 700 or more is obtained. As described above, the dielectric constant is improved by controlling the relationship between the lattice constant of the lower electrode and the dielectric in the present invention. In addition to this, the use of a conductive perovskite oxide as the upper electrode further increases the dielectric constant. It can be seen that there is an effect in improving the dielectric constant.
[0106]
Comparative Example 1
As a comparative example, a Pt film was epitaxially grown as a lower electrode on an MgO (100) substrate, and Ba was formed thereon._0.24Sr_0.76TiO₃The case where the film is epitaxially grown will be described. The original lattice constant of Pt is 0.3923 nm, and Ba_0.24Sr_0.76TiO₃Originally belongs to the cubic system, and its lattice constant is a₀= 0.393 nm.
[0107]
In the case of a Pt film, the lattice constant of the film after epitaxial growth is the lattice constant a in the in-plane direction of the film._sIs 0.3915 nm, and the lattice constant c in the film thickness direction is_sWas 0.3935 nm. On the other hand, Ba_0.24Sr_0.76TiO₃Regarding the lattice constant of the film, the lattice constant a in the in-plane direction of the film was 0.3925 nm, and the lattice constant c in the film thickness direction was 0.4001 nm. This Ba_0.24Sr_0.76TiO₃In the case of a film, the original lattice constant a₀= 0.393 nm is the lattice constant a of the substrate._s= Ratio a for 0.3915 nm₀/ A_sIs 1.004.
[0108]
Further, when the change in the unit cell volume is estimated, the unit volume V of the original cubic lattice (a = 0.393 nm) is obtained.₀Is 0.06069 nm³Whereas the unit cell volume after epitaxial growth V = a²The value of c becomes 0.06163, and the volume change ratio V / V₀Is 1.015. It is considered that due to such lattice mismatch with the lower electrode and expansion of the unit lattice volume, the crystal structure originally belonging to the cubic system is distorted into the tetragonal system, and the lattice constant in the film thickness direction is extended.
[0109]
However, comparing the in-plane lattice constant with the lower electrode, the in-plane lattice constant a_sIs 0.3915 nm, whereas the dielectric film Ba_0.24Sr_0.76TiO₃The in-plane lattice constant a after the epitaxial growth of the film is 0.3925 nm, and the ratio a / a representing the degree of relaxation._sIs 1.0025. Thus, Comparative Example 1 is different from Example 1 in that the strain is relaxed.
[0110]
FIG. 20 shows the Ba produced in Comparative Example 1._0.24Sr_0.76TiO₃3 shows the frequency dependence of relative permittivity and dielectric loss for a film. Ba according to Comparative Example 1_0.24Sr_0.76TiO₃In the film, the frequency dependence of the dielectric constant is very large, and the dielectric loss shows a large value of 6-8% over a wide frequency range. The frequency dispersion of the dielectric characteristics is caused by the relaxation of the strain, that is, the distribution of the relative permittivity or the conductivity or the distribution of the both inside the dielectric film. Such relaxation of the lattice constant distortion is remarkably observed in a dielectric film to which distortion is originally introduced due to lattice constant mismatch.
[0111]
In FIG. 21, Ba manufactured according to Comparative Example 1 was used._0.24Sr_0.76TiO₃Leakage current characteristics of the film and Ba according to Example 1_0.12Sr_0.88TiO₃The leak current characteristics of the films were compared. In the dielectric film of Comparative Example 1 in which the degree of relaxation of the lattice strain is large, a larger leak current flows than in the dielectric film of Example 1 in which the degree of relaxation is small. As described above, it is considered that the reason why the leakage current is large in the film in which the strain is relaxed is large is that the crystal dislocation is introduced as the strain is relaxed, and the crystal dislocation becomes a supply source of the conduction carrier.
[0112]
Example 2
The second embodiment of the present invention describes a case where the present invention is applied to a dielectric thin film having a lattice constant which is essentially the same as or smaller than the lattice constant of the lower electrode surface.
[0113]
As shown in FIG. 1, a lower electrode 2 made of a cubic Pt film and a SrTiO₃A perovskite dielectric film 3 made of a film and an upper electrode 4 made of a Pt film patterned in a predetermined shape were sequentially formed, and a thin film capacitor was manufactured as follows.
[0114]
A 4-inch diameter Pt target was sputtered by RF sputtering to form a lower electrode 2 made of Pt with a thickness of 50 nm on the MgO (100) single crystal substrate 1. At this time, the substrate temperature was set to 600 ° C. and the RF power applied to the target was set to 300 W in an atmosphere with an Ar flow rate of 50 sccm.
[0115]
The formed Pt film 2 was evaluated using X-ray diffraction and reflection electron beam diffraction. The crystal orientation relationship between the MgO substrate 1 and the Pt film 2 was (100) Pt for (100) MgO and [00l] Pt for [00l] MgO, indicating that the Pt film 2 was epitaxially grown. .
[0116]
Next, a 4-inch diameter SrTiO 3 was formed by RF sputtering.₃A sintered target is sputtered and SrTiO having a thickness of 80 nm is formed on the lower electrode 2.₃A dielectric thin film 3 made of (STO) was deposited. At this time, first, Ar / O₂= 40 sccm / 10 sccm, the substrate temperature was set to 600 ° C., the RF power applied to the target was set to 400 W, and a thin film having a thickness of 10 nm or less was deposited.₂= 100 sccm and the remaining thin film having a thickness of 70 nm was deposited under the same conditions of the substrate temperature and the input power as described above.
[0117]
The reason for performing such a two-stage deposition is that O₂This is because an STO epitaxial film cannot be obtained due to slight oxidation of the Pt surface when deposited in an atmosphere of = 100 sccm.
[0118]
The formed STO film 3 was evaluated using X-ray diffraction and reflected electron beam diffraction. The crystal orientation relationship between the Pt film and the STO film was (100) STO for (100) Pt and [00l] STO for [00l] Pt. SrTiO based on X-ray diffraction₃When the lattice constant of the film 3 was measured, the lattice constant c in the film thickness direction ([00l] direction) was 0.3981 nm, and the lattice constant a in the direction parallel to the film surface ([100] direction) was 0.3888 nm. Was. The c / a ratio was 1.02 and had a tetragonal structure.
[0119]
Examining the change of the unit cell volume, SrTiO₃Unit cell volume V calculated from the original symmetry (cubic) lattice constant (a = 0.3905 nm) of₀Is 0.05955 nm³Whereas SrTiO after epitaxial growth₃The unit cell volume V of the film 3 is 0.06018 nm³And the change in unit cell volume V / V₀Was 1.011.
[0120]
Thereafter, after forming a resist film having a predetermined pattern, a 100 nm-thick Pt film was deposited at room temperature by an RF sputtering method, and patterned by a lift-off method to form an upper electrode 4. The size of the upper electrode 4 was 100 μm × 100 μm.
[0121]
Comparative Example 2
A method similar to that of Example 2 was used, except that an SrTiO film having a thickness of 80 nm was formed on the lower electrode 2.₃(STO) When depositing the dielectric thin film 3, Ar / O₂A thin film having a thickness of 80 nm was deposited only in an atmosphere of 40 sccm / 10 sccm. The formed STO film 3 was evaluated using X-ray diffraction and reflection X-ray diffraction. The crystal orientation relationship between the Pt film 2 and the STO film 3 was (100) STO for (100) Pt and [00l] STO for [00l] Pt.
[0122]
SrTiO based on X-ray diffraction₃When the lattice constant of the film 3 was measured, the lattice constant c in the film thickness direction was 0.3910 nm, and the lattice constant a in the direction parallel to the film surface was 0.3915 nm. The c / a ratio was about 1.00 and remained almost cubic. Thus, in Comparative Example 2, unlike in Example 2, no distortion of the crystal lattice of the STO film 3 was observed.
[0123]
On the other hand, the unit cell volume V after film formation is 0.05993 nm³And the change in unit cell volume V / V₀Was 1.006. Hereinafter, in the same manner as in Example 2, a thin film capacitor having the structure shown in FIG.
[0124]
Next, the electrical characteristics of the thin film capacitors of Example 2 and Comparative Example 2 manufactured as described above were examined.
[0125]
FIG. 22 shows the relative dielectric constant-bias electric field characteristic of the thin film capacitor at 300K. The relative dielectric constant of the thin film capacitor according to Comparative Example 2 was 300, whereas the relative dielectric constant of the thin film capacitor according to Example 2 was 450, which was larger than that of the bulk crystal (relative dielectric constant of about 350). SrTiO of Example 2₃The reason why the film shows a large relative dielectric constant is that the unit cell volume is expanded by the peening effect of oxygen because the film is deposited in an atmosphere having a high oxygen content, and the lattice constant of the film surface is restricted by the substrate of the lower electrode. In addition, it is considered that the crystal lattice extended in the film thickness direction and the crystal structure changed to tetragonal.
[0126]
FIG. 23 shows the result of measuring the temperature dependence of the relative dielectric constant. The maximum value of the relative dielectric constant is the value of SrTiO of Example 2.₃In the film, the temperature was around −50 ° C., and SrTiO of Comparative Example 2 was used.₃The temperature is around -150 ° C for the film. As can be seen from this figure, the SrTiO of Example 2₃The film shows a large relative dielectric constant over a wide temperature range around room temperature.
[0127]
In addition, SrTiO₃When the composition (molar ratio of Sr and Ti) of the dielectric thin film was analyzed, the value of the Sr / Ti ratio was found to be SrTiO 2 of Example 2.₃The film was 1.01 / 0.99, and SrTiO of Comparative Example 2 was used.₃In the film, it is 1.04 / 0.96, and the SrTiO of Example 2 is used.₃It was found that the film was closer to the stoichiometric composition.
[0128]
Example 3
The third embodiment of the present invention is a case where the present invention is applied to a ferroelectric thin film in which ferroelectricity is induced by strain. In this case, the frequency characteristics of remanent polarization in the ferroelectric thin film are improved.
[0129]
First, as shown in FIG.₃On a (100) single crystal substrate 1, (00l) oriented Ba is used as the lower electrode 2._0.1Sr_0.9RuO₃A thin film is formed at a substrate temperature of 600 ° C. by Ar: O₂= 40:10 (sccm) in an atmosphere (total pressure of about 0.6 Pa) by RF magnetron sputtering, which was used as a conductive substrate in this example. At this time, Ba_0.1Sr_0.9RuO₃The thickness of the film 3 was about 50 nm.
[0130]
By X-ray diffraction, Ba_0.1Sr_0.9RuO₃The film 2 was confirmed to be a (001) oriented film having a perovskite-type crystal structure. When the film 2 was formed by the sputtering method, the lattice constant in the film thickness direction was about 0.399 nm, and the It was confirmed that the lattice constant in the direction was about 0.3935 nm. Therefore, the Ba thus created_0.1Sr_0.9RuO₃The film has a tetragonal crystal symmetry and corresponds to a lower electrode as defined by the present invention.
[0131]
Further, the lattice constant a in the plane of the lower electrode defined in the present invention a_sCorresponds to an in-plane lattice constant of 0.3935 nm.
[0132]
This Ba_0.1Sr_0.9RuO₃On the film, BaTiO having a thickness of about 200 nm is formed as a ferroelectric film 3.₃The film was formed by an RF magnetron sputtering method. BaTiO is used as a sputtering target.₃A sintered body (4 inch diameter, 5 mm thickness) was used. The substrate temperature during the film formation was set at 600 ° C., and the sputtering atmosphere was argon (Ar) and oxygen (O 2).₂) Mixed gas (Ar: O₂= 40:10 sccm), and the distance between the substrate and the target was 140 mm. The RF power applied to the target was set to 600 W. The composition of the prepared film was analyzed by the ICP method, and it was confirmed that all of the films had almost stoichiometric compositions.
[0133]
In this case, Ba_0.1Sr_0.9RuO₃In-plane lattice constant a of the film_sIs 0.3935 nm, BaTiO₃(Tetragonal) a-axis material lattice constant a₀Is known to be about 0.3992 nm (BaTiO₃The original c-axis is 0.4036 nm).₀/ A_s= 1.014. Therefore, in the present invention, a₀/ A_sSatisfies the condition of 1.002 or more and 1.015 or less.
[0134]
Next, as Comparative Example 3, SrTiO having a smooth surface was produced in the same manner as in this example.₃  On a (100) single crystal substrate 1, Ba of the same (001) orientation is used as a lower electrode._0.1Sr_0.9RuO₃The thin film 2 was formed at a substrate temperature of 600 ° C. by the RF magnetron sputtering method under the same film forming conditions as described above, and was used as a conductive substrate in a comparative example. At this time, Ba_0.1Sr_0.9RuO₃The thickness of the film 2 was about 50 nm. By X-ray diffraction, Ba_0.1Sr_0.9RuO₃It was confirmed that the film was a (001) oriented film, and that the in-plane lattice constant of this film was about 0.3935 nm, which is the same as in Example 3.
[0135]
This Ba_0.1Sr_0.9RuO₃BaTiO having a thickness of about 200 nm as a ferroelectric film 3 on the film 2₃The film was formed by RF magnetron sputtering under conditions different from those in Example 3. BaTiO is used as a sputtering target.₃A sintered body (4 inch diameter, 5 mm thickness) was used. The substrate temperature during the film formation was set at 600 ° C., and the sputtering atmosphere was argon (Ar) and oxygen (O 2).₂) Mixed gas (Ar: O₂= 40:10 sccm), and the distance between the substrate and the target was 170 mm. The RF power applied to the target was 400 W.
[0136]
That is, as the sputtering conditions of Comparative Example 3, the distance between the substrate and the target and the applied power are different from those of Example 3, and the BaTiO 3 is the same as that of Example 3 except for this.₃A film was prepared.
[0137]
Thus, two types of BaTiO manufactured under different sputtering conditions.₃For the film (Example 3 and Comparative Example 3), when the crystal orientation and the lattice constant were measured in detail by the X-ray diffraction method and the RHEED method, the following results were obtained. That is, in both cases,_0.1Sr_0.9RuO₃On the (001) film, (001) oriented BaTiO₃A film is obtained, and also in the in-plane rotation direction, Ba as the lower electrode 2_0.1Sr_0.9RuO₃The same orientation relationship as the film, ie, BBSR ((Ba, Sr) RuO₃) (100) to BT (BaTiO₃) (100), and the relationship of BT (001) with BSR (001) was confirmed.
[0138]
However, BaTiO₃Regarding the lattice constant of the film, there was a difference between Example 3 and Comparative Example 3 as shown in Table 3 below.
[0139]
[Table 3]

[0140]
From the above Table 3, it is found that BaTiO manufactured in Example 3 with a distance between the substrate and the target of 140 mm and an applied RF power of 600 W.₃As for the film, the lattice constant in the film thickness direction is 0.432 nm, and the lattice constant a in the plane is 0.394 nm. That is, BaTiO after film formation₃The lattice constant a in the plane direction of the film and the substrate Ba_0.1Sr_0.9RuO₃Lattice constant a in the plane direction of the film_sRatio, a / a_sIs less than 1.002 within the range defined by the present invention.
[0141]
On the other hand, the comparative example 3 was made of BaTiO manufactured at a substrate-target distance of 170 mm and an input RF power of 400 W.₃Regarding the film, the lattice constant in the thickness direction was 0.423 nm, and the lattice constant a in the plane direction was 0.399 nm. That is, BaTiO after film formation₃The lattice constant a in the plane direction of the film and the substrate Ba_0.1Sr_0.9RuO₃Lattice constant a in the plane direction of the film_sRatio, a / a_sIs not less than 1.01 and less than 1.002, which is outside the range defined by the present invention.
[0142]
These BaTiO₃A Pt film was formed on the film as the upper electrode 4 by RF sputtering. The Pt film was processed into a shape of 100 μm × 100 μm by a lift-off method.
[0143]
Thus, two types of BaTiO having different sputtering conditions and resulting lattice constants are used.₃For the films (Example 3 and Comparative Example 3), DE hysteresis measurements were performed at various frequencies. The measurement was performed at room temperature (22 ° C.) using a Sawyer tower circuit with a maximum voltage amplitude of 10 V.
[0144]
FIG. 24 shows BaTiO of Example 3 and Comparative Example 3.₃Twice the remanent polarization of the film, 2Pr (C / m²) Shows the frequency dependence of the above. From FIG. 24, at a low frequency of 60 Hz or less, the BaTiO of Example 3 was used.₃Film and BaTiO of Comparative Example 3₃Although the film has almost the same remanent polarization, the BaTiO of Comparative Example 3 becomes higher as the frequency increases.₃The remanent polarization of the film is reduced, and at 100 kHz, the BaTiO of Example 3 is used.₃It can be seen that only about half the remanent polarization of the film can be obtained.
[0145]
In contrast, the BaTiO of Example 3₃In the film, the decrease in remanent polarization is only about 5% in the range from 5 Hz to 100 kHz. Thus, it can be seen that the epitaxial ferroelectric thin film within the range specified by the present invention can maintain good ferroelectric properties up to high frequencies.
[0146]
Example 4
As shown in FIG. 1, first, SrTiO₃A (00l) oriented SrRuO as a lower electrode 2 on a (100) single crystal substrate 1₃A thin film is formed at a substrate temperature of 600 ° C. by using Ar: O.₂= 40: 10 (sccm) in an atmosphere (total pressure of about 0.6 Pa) by an RF magnetron sputtering method, which was used as a conductive substrate in this example. At this time, SrRuO₃The thickness of the film 2 was about 50 nm.
[0147]
By X-ray diffraction, SrRuO₃The film was confirmed to be a (001) oriented film having a perovskite-type crystal structure. When the film was formed by sputtering in such a method, the lattice constant in the film thickness direction was about 0.398 nm, and the Was confirmed to have a lattice constant of about 0.3925 nm. Therefore, the SrRuO created in this way is₃The film has tetragonal crystal symmetry and corresponds to the lower electrode defined in the present invention. Further, the lattice constant a in the plane direction of the lower electrode defined in the present invention is_sCorresponds to a lattice constant of 0.3925 nm in the plane direction.
[0148]
This SrRuO₃On the film, Ba having a thickness of about 130 nm is formed as the ferroelectric film 3._0.5Sr_0.5TiO₃The film was formed by an RF magnetron sputtering method. Ba is used as a sputtering target._0.5Sr_0.5TiO₃A sintered body (4 inch diameter, 5 mm thickness) was used. The substrate temperature during the film formation was set at 600 ° C., and the sputtering atmosphere was argon (Ar) and oxygen (O 2).₂) Mixed gas (Ar: O₂= 40:10 sccm), and the distance between the substrate and the target was 140 mm. The RF power applied to the target was set to 600 W. The composition of the prepared film was analyzed by the ICP method, and it was confirmed that all of the films had almost stoichiometric compositions.
[0149]
In this case, SrRuO₃Lattice constant a in the plane direction of the film_sIs 0.3925 nm, Ba_0.5Sr_0.5TiO₃(Tetragonal) a-axis material lattice constant a₀Is known to be about 0.395 nm, and for this example, a₀/ A_s= 1.006. Therefore, in the present invention, a₀/ A_sSatisfies the condition of 1.002 or more and 1.015 or less.
[0150]
Next, as Comparative Example 4, SrTiO having a smooth surface was produced in the same manner as in this example.₃  On a (100) single crystal substrate 1, a similar (00l) oriented SrRuO₃The thin film was formed by the RF magnetron sputtering method at the substrate temperature of 600 ° C. under the same film forming conditions as described above, and was used as the conductive substrate in Comparative Example 4. At this time, SrRuO₃The thickness of the film 2 was about 50 nm. By X-ray diffraction, SrRuO₃It was confirmed that the film 2 was a (001) oriented film, and that the lattice constant in the plane direction of this film was about 0.3925 nm, which is the same as that of Example 4.
[0151]
This SrRuO₃On the film, Ba having a thickness of about 130 nm is formed as the ferroelectric film 3._0.5Sr_0.5TiO₃The film was formed by RF magnetron sputtering under conditions different from those in Example 4. Ba as a sputtering target_0.5Sr_0.5TiO₃A sintered body (4 inch diameter, 5 mm thickness) was used. The substrate temperature during the film formation was set at 600 ° C., and the sputtering atmosphere was argon (Ar) and oxygen (O 2).₂) Mixed gas (Ar: O₂= 40:10 sccm), and the distance between the substrate and the target was 170 mm. The RF power applied to the target was 400 W. That is, as the sputtering conditions of Comparative Example 4, the distance between the substrate and the target and the input power were different from those of Example 4, and the conditions were the same as in Example 4._0.5Sr_0.5TiO₃A film was prepared.
[0152]
The two types of (Ba, Sr) TiO 2 thus manufactured under different sputtering conditions.₃For the film (Example 4 and Comparative Example 4), when the crystal orientation and the lattice constant were measured in detail by the X-ray diffraction method and the RHEED method, the following results were obtained. That is, both have the same SrRuO₃On the (001) film, (001) oriented Ba_0.5Sr_0.5TiO₃A film was obtained, and the lower electrode SrRuO₃It was confirmed that the same azimuthal relationship as the film, that is, the relationship of BST (100) for SR (100) and the relationship of BST (001) for SR (001) was established.
[0153]
However, Ba_0.5Sr_0.5TiO₃Regarding the lattice constant of the film, there was a difference between Example 4 and Comparative Example 4 as shown in Table 4 below.
[0154]
[Table 4]

[0155]
That is, the Ba that was produced in Example 4 with the substrate-target distance of 140 mm and the input power of 600 W._0.5Sr_0.5TiO₃Regarding the film, the lattice constant in the film thickness direction was 0.413 nm, and the lattice constant a in the plane direction was 0.393 nm. That is, Ba after film formation._0.5Sr_0.5TiO₃The lattice constant a in the plane direction of the film and the substrate SrRuO₃Lattice constant a in the plane direction of the film_sRatio, a / a_sIs less than 1.002 within the range defined by the present invention.
[0156]
On the other hand, the Ba prepared in Comparative Example 4 with a substrate-target distance of 170 mm and an input power of 400 W_0.5Sr_0.5TiO₃As for the film, the lattice constant in the film thickness direction was 0.411 nm, and the lattice constant a in the plane direction was 0.394 nm. That is, Ba after film formation._0.5Sr_0.5TiO₃The lattice constant a in the plane direction of the film and the substrate SrRuO₃Lattice constant a in the plane direction of the film_sRatio, a / a_sIs not less than 1.003 and less than 1.002, which is outside the range defined by the present invention.
[0157]
These BaSrTiO₃A Pt film was formed on the film as the upper electrode 4 by RF sputtering. The Pt film was processed into a shape of 100 μm × 100 μm by a lift-off method.
[0158]
The two types of Ba of Example 4 and Comparative Example 4 thus manufactured._0.5Sr_0.5TiO₃The polarization retention characteristics of the films were compared. The retention characteristics were measured after 1 second and 10 minutes after applying a +5 V rectangular pulse (pulse width 200 ns) to the upper electrode 4 and a −5 V rectangular pulse (pulse width 200 ns). The amount of charge flowing into the electrodes when a +5 V read pulse (200 ns) was applied was compared. At this time, when the pulse of +5 V is applied first, the amount of charge flowing in the read operation is Qns, and the amount of charge flowing in the read operation when -5 V is applied is Qsw, and the difference dQ = Qsw-Qns is compared.
[0159]
As a result, in Example 4, the charge amount differences dQ after 1 second and 10 minutes were both dQ = 0.12 C / m.²Met. On the other hand, in Comparative Example 4, the charge amount difference after one second was dQ = 0.10 C / m.²Despite being 10 minutes later, dQ = 0.01 C / m²It has dropped to the following value. That is, in Example 4, sufficient holding characteristics due to ferroelectricity were obtained, whereas in Comparative Example 4, sufficient holding characteristics were not obtained, and Comparative Example 4 was non-volatile. You can see that it is not suitable for application to memory.
[0160]
Example 5
As shown in FIG. 1, first, SrTiO₃A (001) oriented SrRuO as a lower electrode 2 on a (100) single crystal substrate 1₃A thin film is formed at a substrate temperature of 600 ° C. by using Ar: O.₂= 40: 10 (sccm) in an atmosphere (total pressure of about 0.6 Pa) by an RF magnetron sputtering method, which was used as a conductive substrate in this example. At this time, SrRuO₃The thickness of the film 2 was about 50 nm.
[0161]
By X-ray diffraction, SrRuO₃The film was confirmed to be a (001) oriented film having a perovskite-type crystal structure, and when formed by a sputtering method using such a method, the lattice constant in the film thickness direction was about 0.379 nm to 0.1 nm. It was confirmed that it was between 3991 nm and the lattice constant in the plane direction was between about 0.3906 nm and 0.3913 nm (Table 6). Therefore, the SrRuO created in this way is₃The film has tetragonal crystal symmetry and corresponds to the lower electrode defined in the present invention. Further, the lattice constant a in the plane direction of the lower electrode defined in the present invention is_sCorresponds to a lattice constant in the plane direction of 0.391 nm.
[0162]
This SrRuO₃On the film, a (Ba) film having a thickness of about 50 nm_xSr_1-xyCa_y) TiO₃The film was formed by a ternary RF magnetron sputtering method using three targets. BaTiO is used as a sputtering target.₃Sintered body, SrTiO₃Sintered body and CaTiO₃A sintered body (each having a diameter of 4 inches and a thickness of 5 mm) was used. The substrate temperature during the film formation was set at 600 ° C., and the sputtering atmosphere was argon (Ar) and oxygen (O 2).₂) Mixed gas (Ar: O₂= 40:10 sccm), and the distance between the substrate and the target was 140 mm. In addition, the RF power applied to the target is₃600 W, SrTiO for sintered body₃While supplying a constant power of 450 W to the sintered body, CaTiO₃The sintered body was variable from 0 W to 500 W.
[0163]
Table 5 below shows the result of analyzing the composition of the formed film by ICP emission spectrometry. In addition, CaTiO₃The relationship between the RF power applied to the sintered target and the amount of Ca in the dielectric film was determined, and the results shown in FIG. 25 were obtained. From FIG. 25, the amount of Ca in the dielectric film was CaTiO₃It can be seen that it is almost proportional to the RF power applied to the sintered target. Ba measured using a level difference meter_xSr_1-xyCa_yTiO₃It was found that the thickness of the film was distributed between 46 and 52 nm as shown in Table 5 below.
[0164]
[Table 5]

[0165]
Five kinds of (Ba) having different Ca contents formed in this manner_xSr_1-xyCa_y) TiO₃When the crystal structures of the films were analyzed by X-ray diffraction, all the films were (001) -oriented, and SrRuOO was in a so-called cube-on-cube relationship.₃It was confirmed that the film was heteroepitaxially grown on the film. When the lattice constant was measured, as shown in Table 6 below, although a slight difference was observed depending on the amount of Ca, the lattice constant c in the film thickness direction was 0.422 nm to 0.428 in each case. In comparison with the original lattice constant, the lattice constant a in the plane direction is 5% or more compared with the original lattice constant.₃It was completely coincident with the a-axis of the film, and it was confirmed that the film was not relaxed.
[0166]
[Table 6]

[0167]
Five kinds of (Ba) having different Ca contents formed in this way_xSr_1-xyCa_y) TiO₃On the film, a Pt film was formed as the upper electrode 4 by a sputtering method, and processed into a shape of 100 μm × 100 μm by a lift-off method.
[0168]
The thus manufactured (Ba)_xSr_1-xyCa_y) TiO₃When the current-voltage characteristics of the film were measured, as shown in FIG. 26, the film containing Ca exhibited a uniform reduction in the leak current in the high voltage region as compared with the film not containing Ca (y = 0). Admitted.
[0169]
The degree of such a decrease in the leak current does not depend on the amount y of Ca, and when the value of y is 2% or more, it is almost the same as that of the film of y = 0, regardless of the magnitude of y. The effect was recognized.
[0170]
On the other hand, when the relationship between the polarization P and the voltage V of these films was measured at 500 Hz using a Sawyer tower circuit, as shown in FIG._xSr_1-xyCa_y) TiO₃A hysteresis curve indicating ferroelectricity was also observed in the film. If these dielectric films are all bulk, they originally have a composition that should exhibit paraelectricity at room temperature. Therefore, also in these films, ferroelectricity is introduced by lattice mismatch. However, a difference in the shape of the hysteresis curve due to the amount of Ca was observed.
[0171]
FIG. 28 shows the relationship between the amplitude of the AC voltage used to measure the hysteresis curve and the remanent polarization obtained at that voltage amplitude. From this figure, it can be seen that as the Ca amount y increases, the voltage at which ferroelectric hysteresis starts to appear decreases according to the Ca amount. Moreover, it can be seen that the absolute value of the remanent polarization when the polarization is saturated is not inferior to the case where Ca is not replaced.
[0172]
Note that the present invention is not limited to the above embodiments. BaTiO 3 is used as the dielectric film.₃And (Ba, Sr) TiO₃However, the present invention is not limited to this, and a dielectric material having a perovskite crystal structure epitaxially grown can be used. For example, strontium titanate (SrTiO 2) as described in the section (Means for solving the problem) above.₃), Calcium titanate (CaTiO₃), Barium stannate (BaSnO)₃), Barium zirconate (BaZrO)₃), Barium magnesium tantalate (Ba (Mg_1/3Ta_2/3) O₃), Magnesium barium niobate (Ba (Mg_1/3Nb_2/3) O₃And the like, and those obtained by solid-solution of a plurality of oxides at the same time among them.
[0173]
In the present invention, the composition formula is ABO₃In the composition formula of the dielectric having a perovskite crystal structure represented by the formula, A is mainly composed of Ba, but Ba may be partially substituted with at least one element of Sr and Ca. . B is mainly Ti, but a part of Ti is similarly converted to Zr, Hf, Sn, (Mg_1/3Nb_2/3), (Mg_1/3Ta_2/3), (Zn_1/3Nb_2/3), (Zn_1/3Ta_2/3) May be replaced with at least one element. Further, as a method of forming the dielectric film, other than RF sputtering, a method such as reactive evaporation, MOCVD, laser ablation, or sol-gel method can be used.
[0174]
In the embodiment, a tetragonal conductive substrate is used, but a cubic conductive substrate may be used. In addition, various modifications can be made without departing from the scope of the present invention.
[0175]
【The invention's effect】
As described in detail above, according to the present invention, the dielectric constant is increased or ferroelectricity is induced by utilizing the expansion of the unit cell volume due to the film formation conditions or the mismatch of the lattice constant with the lower electrode. It is possible to suppress the frequency dependence of the dielectric constant, the high dielectric loss, and the frequency dependence of the remanent polarization, which have been problems in such a case.
[0176]
Therefore, it is possible to manufacture a highly reliable high dielectric constant film or ferroelectric thin film which does not contain a low melting point metal such as lead or bismuth as a component and has a large dielectric constant or a large remanent polarization. This makes it possible to realize a thin film capacitor suitable for increasing the capacity and increasing the integration density, and greatly contributes to DRAM technology or nonvolatile memory technology.
[Brief description of the drawings]
FIG. 1 is a sectional view showing a thin film capacitor according to one embodiment of the present invention.
FIG. 2 Ba_0.12Sr_0.88TiO₃The figure which shows the X-ray diffraction pattern of a film (film thickness 39nm).
FIG. 3 is Ba_0.12Sr_0.88TiO₃The figure which shows the rocking curve regarding (002) of a film (film thickness 39nm).
FIG. 4 Ba_0.12Sr_0.88TiO₃The figure which shows the bias electric field dependence of the relative dielectric constant of a film (film thickness 77nm).
FIG. 5: Ba_0.12Sr_0.88TiO₃The figure which shows the bias electric field dependence of the dielectric loss of a film (film thickness 77nm).
FIG. 6: Ba_0.12Sr_0.88TiO₃The figure which shows the bias electric field dependence of the relative dielectric constant of a film (film thickness 39nm).
FIG. 7: Ba_0.12Sr_0.88TiO₃The figure which shows the bias electric field dependence of the dielectric loss of a film (film thickness 39nm).
FIG. 8: Ba_0.12Sr_0.88TiO₃The figure which shows the temperature dependence of the relative dielectric constant and dielectric loss coefficient of a film (film thickness 39nm, 77nm).
FIG. 9: Ba_0.12Sr_0.88TiO₃The figure which shows the frequency dependence of the relative dielectric constant of a film (film thickness 39nm, 77nm).
FIG. 10: Ba_0.12Sr_0.88TiO₃FIG. 9 is a graph showing a relationship between a stored charge density of a film (thickness: 39 nm and 77 nm) and voltage.
FIG. 11: Ba_0.12Sr_0.88TiO₃The figure which shows the relationship between the measured voltage amplitude of a film (film thickness 39nm, 77nm), and a charge amplitude.
FIG. 12 is a diagram showing a voltage waveform and a current waveform used in an AC voltage load test.
FIG. 13 is a diagram showing a change in the amount of stored charges depending on the number of times of application of an AC voltage.
FIG. 14: Ba_0.12Sr_0.88TiO₃FIG. 9 is a graph showing current-voltage characteristics of a film (thickness: 77 nm).
FIG. 15: Ba_0.12Sr_0.88TiO₃FIG. 9 is a diagram showing current-voltage characteristics of a film (thickness: 39 nm).
FIG. 16: Ba_0.12Sr_0.88TiO₃The figure which shows the time change of the leak current when a DC positive voltage (a) and a negative voltage (b) are applied to a film (film thickness 39nm).
FIG. 17: Ba_0.12Sr_0.88TiO₃FIG. 9 is a graph showing temperature dependence of current / voltage characteristics of a film (thickness: 39 nm).
FIG. 18 is a diagram showing a relationship between a square root of a voltage and a current density.
FIG. 19: Ba_0.12Sr_0.88TiO₃SrRuO as an upper electrode on the film (thickness: 23 nm)₃Ba when a film is formed_0.12Sr_0.88TiO₃FIG. 4 is a view showing electric field dependence of dielectric constant and dielectric loss of a film.
FIG. 20 shows Ba in Comparative Example 1._0.24Sr_0.76TiO₃FIG. 4 is a diagram illustrating the frequency dependence of the relative dielectric constant and dielectric loss of a film.
FIG. 21: Ba_0.12Sr_0.88TiO₃FIG. 7 is a diagram showing a comparison between the leakage current characteristics of a film in Comparative Example 1 and Example 1.
FIG. 22 is a graph showing relative dielectric constant-electric field characteristics in the thin film capacitors of Example 2 and Comparative Example 2.
FIG. 23 is a diagram showing the temperature dependence of the relative dielectric constant of the thin film capacitors of Example 2 and Comparative Example 2.
FIG. 24 shows BaTiO of Example 3 and Comparative Example 3.₃FIG. 4 is a diagram illustrating frequency dependence of remanent polarization of a film.
FIG. 25: CaTiO₃The figure which shows the relationship between the RF electric power supplied to the sintered compact target, and the Ca amount in a dielectric film.
FIG. 26 is a diagram showing a change in leakage current depending on the ratio of Ca.
FIG. 27 is a diagram showing a change in a hysteresis curve depending on the ratio of Ca.
FIG. 28 is a view showing the relationship between the ratio of Ca and the rise of a hysteresis curve.
[Explanation of symbols]
1: Single crystal substrate
2 ... Lower electrode
3. Dielectric film
4: Upper electrode

Claims (9)

A first electrode having a cubic (100) plane or a tetragonal (001) plane on its surface, and a dielectric having an originally cubic perovskite structure epitaxially grown on the first electrode. A body thin film and a second electrode formed on the dielectric thin film, wherein the dielectric thin film has a unit cell volume of an original perovskite crystal structure (lattice constant a ₀ ) belonging to a cubic system. When v ₀ = a ₀ ³ and the unit cell volume (lattice constant a = b ≠ c) distorted into a tetragon after epitaxial growth is expressed as V = a ² c,
V / V ₀ ≧ 1.01
And the ratio c / a between the lattice constant c in the film thickness direction and the lattice constant a in the direction parallel to the film surface is c / a ≧ 1.01.
A thin film capacitor characterized by satisfying the following relationship: A first electrode having a cubic (100) plane or a tetragonal (001) plane on its surface; and a dielectric having an originally cubic perovskite structure epitaxially grown on the first electrode. A body thin film, and a second electrode formed on the dielectric thin film, wherein the dielectric thin film has a lattice constant in the plane direction of the surface of the first electrode as _s and a perovskite belonging to a cubic system. When the original lattice constant of the dielectric material represented by the a-axis length of the type crystal structure is a ₀ ,
a ₀ / a _s ≦ 1.002
And the ratio c / a between the lattice constant c in the film thickness direction and the lattice constant a in the direction parallel to the film surface is c / a ≧ 1.01.
A thin film capacitor characterized by satisfying the following relationship: A first electrode having a cubic (100) plane or a tetragonal (001) plane on its surface; and a first cubic perovskite structure epitaxially grown on the first electrode. A dielectric thin film made of a dielectric material having a Curie temperature of 200 ° C. or lower, and a second electrode formed on the dielectric thin film, wherein the dielectric thin film has a surface direction of the first electrode surface. When the lattice constant is a _s , and the original lattice constant of the dielectric material represented by the a-axis length of the perovskite type crystal structure belonging to the cubic or tetragonal system is a ₀ ,
_{1.002 ≦ a 0 / a s ≦} 1.015
When satisfying the following relationship, and the lattice constant in the plane direction after the dielectric is epitaxially grown is a,
a / a _s <1.002
A thin film capacitor characterized by satisfying the following relationship: 4. The thin film capacitor according to claim 1, wherein the dielectric thin film is made of a compound represented by the following formula.
ABO ₃
Wherein A is at least one selected from the group consisting of Ba, Sr, and Ca, and B is Ti, Zr, Hf, Sn, Mg _1/3 Nb _2/3 , Mg _1/3 Ta _{2 / 3} , Zn _1/3 Nb _2/3 , Zn _1/3 Ta _2/3 , Ni _1/3 Nb _2/3 , Ni _1/3 Ta _2/3 , Co _1/3 Nb _2/3 , Co _{1 /} It is at least one selected from the group consisting of ₃ Ta _2/3 , Sc _1/3 Nb _2/3 , and Sc _1/3 Ta _2/3 .) The first electrode has a general formula of ABO ₃ (where A is at least one member selected from the group consisting of Ba, Sr, Ca, and La, and B is Ru, Ir, Mo, W, Co, The thin film capacitor according to any one of claims 1 to 3, comprising a conductive compound having a perovskite crystal structure represented by at least one selected from the group consisting of Ni and Cr). Said dielectric thin film _{is, (Ba x Sr 1-x} ) consists _{TiO 3 (0 <x <0.9} ) a compound represented by the first _{electrode, SrRuO 3 (0.9 <Sr /} Ru 3. The thin-film capacitor according to claim 1, comprising a conductive compound having a perovskite crystal structure represented by <1.1). 2. The dielectric thin film satisfies 1.03 ≧ c / a ≧ 1.01 when it is a paraelectric, and 1.08 ≧ c / a ≧ 1.01 when it is a ferroelectric. 3. The thin film capacitor as described in the above. 3. The thin film capacitor according to claim 2, wherein the dielectric thin film satisfies 1.1 ≧ c / a ≧ 1.01. It said dielectric thin film, a thin film capacitor according to claim 3, characterized in that satisfying _{1.004 ≦ a 0 / a s ≦} 1.011.

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