JP2015213100A - Semiconductor device and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a semiconductor device arranged so that a leak current in an opposite direction can be suppressed; and a method for manufacturing such a semiconductor device.SOLUTION: A semiconductor device according to the present invention comprises: an AlGaN barrier layer 4; and a SiN surface protection film 8 formed on the AlGaN barrier layer 4. The surface protection film 8 is higher in the rate of nitrogen in terms of Si/N ratio in comparison to a stoichiometric SiN. In the surface protection film 8, the content of hydrogen is less than 10%.

Description

  The present invention relates to a semiconductor device having a SiN surface protective film and a method for manufacturing the same.

  A nitride semiconductor device (GaN-based transistor), which is typically known as an AlGaN / GaN heterojunction structure, is expected to operate at high voltage and high power because GaN has a higher breakdown voltage than Si or GaAs. Device. Therefore, from the viewpoint of ensuring reliability, the nitride semiconductor device desirably has as low a reverse leakage current as possible to the gate when a high bias is applied.

  The reverse leakage current is considered to be related to the impurity level and the interface level in the semiconductor surface, or the density of these levels. In addition, in order to suppress reverse leakage current, gate electrode materials that directly contact the semiconductor surface and film types of protective films have been studied, or field plate structures that reduce electric field concentration at the gate electrode end have been applied. Yes.

  Conventionally, as a semiconductor surface protective film, SiN which has high insulation and can be formed relatively easily is generally used as a practical protective film (SiN film) (for example, Patent Documents 1 and 2 and Non-Patent Documents). Reference 1).

  However, since the reverse leakage current may not be sufficiently suppressed by simply using the SiN film as a surface protection film, the desired content can be obtained by defining the content of hydrogen (H) in the SiN film. Technology for obtaining characteristics (for example, see Patent Document 3), or technology for suppressing reverse leakage current by containing fluorine (F) or carbon (C) at a specific ratio (for example, see Patent Document 4) ) Is disclosed.

JP 2007-234986 A (pages 8-9) JP 2012-175089 A (6th and 9th pages) JP-A-2005-286135 (page 5) Japanese Patent No. 5347228 (pages 3 to 4) JP2013-115323A (page 3)

Fumio Hasegawa and Akihiko Yoshikawa, “Wide Gap Semiconductor Optical / Electronic Devices”, Morikita Publishing Co., Ltd., 2006, p. 224-245

  As described above, although the SiN film is preferably used as a surface protective film in terms of characteristics, the reverse leakage current may not be sufficiently suppressed by simply applying the SiN film to the surface protective film. .

  In Patent Document 3, an attempt is made to suppress the reverse leakage current by defining the hydrogen content in the SiN film. However, in the case where the SiN film is formed using a gas not containing hydrogen, the reverse leakage current is not always the same. It is known that it is not suppressed.

  In Patent Document 4, reverse leakage current is suppressed by containing fluorine and carbon in a specific ratio in the SiN film. However, variation in fluorine content in the SiN film when exposed to a high temperature environment. In addition, it is difficult to say that the controllability and reproducibility are sufficient technologies in consideration of the fact that unintentional carbon content can occur regardless of the film formation conditions.

  Moreover, although the technique which prescribed | regulated the conditions of SiN film | membrane more simply was disclosed (for example, patent document 5), patent document 5 is not a technique for suppressing a reverse direction leakage current, but is a structure for a high frequency use. Not suitable.

  On the other hand, when attention is focused on the semiconductor surface side, there is an idea that the generation of nitrogen vacancies on the semiconductor surface is one of the causes for increasing the reverse gate leakage current. Therefore, in order to stably obtain an SiN film effective for suppressing reverse leakage current, the effect of the SiN film itself is considered while considering the influence on the semiconductor surface rather than applying an additional technique to the SiN film. It is more important to define basic specifications.

  The present invention has been made to solve such a problem, and an object thereof is to provide a semiconductor device capable of suppressing reverse leakage current and a method for manufacturing the same.

In order to solve the above-described problems, a semiconductor device according to the present invention includes a nitride semiconductor layer and a protective film made of SiN formed on the nitride semiconductor layer, and the protective film is stoichiometric Si _3. The Si / N ratio is higher than that of N ₄ , and the hydrogen content in the protective film is less than 10%.

The method for manufacturing a semiconductor device according to the present invention includes (a) a step of forming a nitride semiconductor layer and (b) a step of forming a protective film made of SiN on the nitride semiconductor layer. ), The protective film is formed so that the ratio of nitrogen is higher in the Si / N ratio than stoichiometric Si ₃ N ₄ and the hydrogen content in the protective film is less than 10%. Features.

According to the present invention, a semiconductor device includes a nitride semiconductor layer and a protective film made of SiN formed on the nitride semiconductor layer, and the protective film is more Si / N than stoichiometric Si ₃ N _4. Since the ratio of nitrogen is high and the hydrogen content in the protective film is less than 10%, the reverse leakage current can be suppressed.

The method for manufacturing a semiconductor device includes (a) a step of forming a nitride semiconductor layer, and (b) a step of forming a protective film made of SiN on the nitride semiconductor layer. In the step (b), Since the protective film is formed so that the ratio of nitrogen is higher in the Si / N ratio than stoichiometric Si ₃ N ₄ and the hydrogen content in the protective film is less than 10%, reverse leakage is caused. The current can be suppressed.

It is sectional drawing which shows an example of a structure of the semiconductor device by Embodiment 1 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 1 of this invention. It is a figure for demonstrating the effect by Embodiment 1 of this invention. It is a figure which shows an example of a structure of the semiconductor device by Embodiment 2 of this invention. It is a figure which shows an example of a structure of the semiconductor device by Embodiment 3 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 3 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 3 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 3 of this invention. It is a figure which shows an example of a structure of the semiconductor device by Embodiment 4 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 4 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 4 of this invention. It is a figure which shows an example of a structure of the semiconductor device by Embodiment 5 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 5 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 5 of this invention. It is a figure which shows an example of a structure of the semiconductor device by Embodiment 6 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 6 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 6 of this invention. It is a figure which shows an example of the manufacturing process of the semiconductor device by Embodiment 6 of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

<Embodiment 1>
First, the configuration of the semiconductor device according to the first embodiment of the present invention will be described.

  FIG. 1 is a cross-sectional view showing an example of the configuration of the semiconductor device according to the first embodiment.

  As shown in FIG. 1, a buffer layer 2, a GaN channel layer 3, and an AlGaN barrier layer 4 (nitride semiconductor layer) are sequentially stacked on a substrate 1. On the AlGaN barrier layer 4, a source electrode 5 and a drain electrode 6 are formed apart from each other with a gate electrode 7 interposed therebetween. A surface protective film 8 made of a SiN film is formed so as to cover the AlGaN barrier layer 4, the source electrode 5, the drain electrode 6, and the gate electrode 7. The source electrode 5 and the drain electrode 6 may be formed at opposite positions.

  The substrate 1 may be any substrate such as a SiC substrate, a sapphire substrate, a silicon substrate or the like as long as the GaN channel layer 3 can be formed later.

  The buffer layer 2 may be any material as long as it can be formed using GaN, AlN, or the like, and the GaN channel layer 3 can be formed later.

  The source electrode 5 and the drain electrode 6 are formed so as to be in ohmic contact with the surface of the AlGaN barrier layer 4, and typically a Ti / Al system is used. For example, an ohmic contact can be obtained by forming Ti (20 nm) / Al (100 nm) / Ti (40 nm) / Au (30 nm) and performing an annealing process at 600 ° C. or higher. In addition, any electrode structure and ohmic formation process can be applied as necessary.

  The gate electrode 7 is formed so as to be in Schottky contact with the surface of the AlGaN barrier layer 4 and typically has a Ni / Au structure. For example, Ni (50 nm) / Au (300 nm) is formed by vapor deposition or sputtering lift-off process. In addition, a Pt or Pd-based electrode material having a high Schottky barrier can be applied as necessary.

When the surface protective film 8 is based on a stoichiometric Si ₃ N ₄ film so as to cover the surfaces of the AlGaN barrier layer 4, the source electrode 5, the drain electrode 6, and the gate electrode 7, the Si / N ratio The SiN film is excessive in nitrogen and the content of hydrogen contained in the surface protective film 8 is less than 10%. That is, the surface protective film 8 has a higher nitrogen ratio in the Si / N ratio than stoichiometric Si ₃ N ₄ , and the hydrogen content in the surface protective film 8 is less than 10%.

  Next, a method for manufacturing a semiconductor device will be described with reference to FIG. FIG. 2 is a diagram illustrating an example of a manufacturing process of a semiconductor device.

  First, as shown in FIG. 2, a buffer layer 2 made of GaN or AlN is formed on a semi-insulating SiC substrate 1. Next, a GaN channel layer 3 and an AlGaN barrier layer 4 are sequentially stacked on the buffer layer 2 (AlGaN / GaN structure). Next, on the AlGaN barrier layer 4, a source electrode 5 and a drain electrode 6 made of Ti (20 nm) / Al (100 nm) / Ti (40 nm) / Au (30 nm) and Ni (50 nm) / Au (300 nm) are made. A gate electrode 7 is formed.

  Next, a SiN film is formed as a surface protective film 8 so as to cover the surfaces of the AlGaN barrier layer 4, the source electrode 5, the drain electrode 6, and the gate electrode 7. The film thickness of the surface protective film 8 is, for example, 80 nm. As a result, a semiconductor device as shown in FIG. 1 is obtained.

As a method for forming the surface protective film 8, for example, it is possible to intentionally form a film using only hydrogen-free gas (Ar gas and N ₂ gas) and to freely adjust the gas flow rate during film formation. A high-frequency ECR (Electron Cyclotron Resonance) sputtering method is used. As for the gas flow rate, for example, when the gas flow rate for forming a stoichiometric Si ₃ N ₄ film is used as a reference, only the N ₂ gas flow rate is set to 150%. Accordingly, when the stoichiometric Si ₃ N ₄ film is used as a reference, the Si / N ratio is excessive in nitrogen and the content of hydrogen contained in the surface protective film 8 is less than 10%. A film can be formed as the surface protective film 8.

  From the above, according to the first embodiment, the SiN film having an excess of nitrogen in the Si / N ratio is used as the surface protective film 8, thereby forming a nitride semiconductor layer (in the example of the first embodiment, an AlGaN barrier layer). It plays a role of supplementing the nitrogen depletion occurring on the surface of 4), and the reverse leakage current due to the nitrogen depletion can be suppressed. Further, by defining the content of hydrogen in the surface protective film 8, it is possible to reduce the impurity level caused by the hydrogen contained in the surface protective film 8, and the reverse leakage due to the reduction of the impurity level. Current or electron capture can be suppressed. For example, as shown in FIG. 3, the semiconductor device using the surface protective film 8 according to the first embodiment has a reverse leakage than the semiconductor device using a nitrogen-deficient SiN film or stoichiometric SiN film. It can be seen that the current is suppressed. Therefore, unintended reverse leakage current due to the formation of the SiN film as the surface protective film 8 can be suppressed, and the reliability of the semiconductor device can be improved.

  In the first embodiment, it is intended to suppress the increase of the unintended reverse leakage current when the SiN film is formed as the surface protective film 8 and to surely obtain the effect of suppressing the reverse leakage current. The film thickness is not specified in detail. However, by applying the surface protective film 8 according to the first embodiment, an effect corresponding to the thickness of each design can be obtained.

<Embodiment 2>
First, the configuration of the semiconductor device according to the second embodiment of the present invention will be described.

  FIG. 4 is a diagram showing an example of the configuration of the semiconductor device according to the second embodiment of the present invention.

  As shown in FIG. 4, the second embodiment is characterized in that the surface protective film is formed by laminating a plurality of layers (in the example of FIG. 4, the lower surface protective film 9 and the upper surface protective film 10). It is said. Other configurations are the same as those in the first embodiment, and thus description thereof is omitted here.

The lower surface protective film 9 is the lowest layer formed on the most AlGaN barrier layer 4 (nitride semiconductor layer) side among the plurality of layers forming the surface protective film, and is lower than stoichiometric Si ₃ N _4. The SiN film has a high nitrogen ratio in the / N ratio and the hydrogen content in the lower surface protective film 9 is less than 10%. In addition, the film thickness of the lower surface protective film 9 shall be 10 nm, for example.

The upper surface protective film 10 is an Al ₂ O ₃ film having a larger band gap than the lower surface protective film 9 that is a SiN film. The film thickness of the upper surface protective film 10 is, for example, 70 nm.

  Next, a method for manufacturing a semiconductor device will be described.

  First, the buffer layer 2, the GaN channel layer 3, and the AlGaN barrier layer 4 are stacked on the substrate 1, and the source electrode 5, the drain electrode 6, and the gate electrode 7 are formed on the AlGaN barrier layer 4. Since these manufacturing methods are the same as those in the first embodiment (see FIG. 2), description thereof is omitted here.

  Next, a SiN film is formed as a lower surface protective film 9 (for example, 10 nm) so as to cover the surfaces of the AlGaN barrier layer 4, the source electrode 5, the drain electrode 6, and the gate electrode 7. Next, an upper surface protective film 10 (for example, 70 nm) is formed on the lower surface protective film 9. As a result, a semiconductor device as shown in FIG. 4 is obtained.

As a method for forming the lower surface protective film 9 and the upper surface protective film 10, for example, an ECR-sputtering method or an ALD (Atomic Layer Deposition) method excellent in film thickness controllability is used. Thereby, the SiN film having a higher nitrogen ratio in the Si / N ratio than the stoichiometric Si ₃ N ₄ and the hydrogen content in the lower surface protective film 9 is less than 10% is changed to the lower surface protective film 9. A film having a larger band gap than the lower surface protective film 9 can be formed as the upper surface protective film 10.

  From the above, according to the second embodiment, by forming the surface protective film with a plurality of layers (lower surface protective film 9 and upper surface protective film 10), an unintended reverse caused by the formation of the SiN film. A surface protective film having a higher breakdown voltage than that of the SiN film can be formed while suppressing the direction leakage current. Therefore, the reliability of the semiconductor device can be improved as compared with the first embodiment.

In the second embodiment, the case where the surface protective film is formed by laminating two layers (the lower surface protective film 9 and the upper surface protective film 10) has been described. It may be the above. In this case, the lowermost layer may be a SiN film in which the ratio of nitrogen is higher in the Si / N ratio than the stoichiometric Si ₃ N ₄ and the hydrogen content in the lowermost layer is less than 10%, It is only necessary that layers other than the lowermost layer have a larger band gap than the lowermost layer.

<Embodiment 3>
First, the configuration of the semiconductor device according to the third embodiment of the present invention will be described.

  FIG. 5 is a diagram showing an example of the configuration of the semiconductor device according to the third embodiment of the present invention.

  As shown in FIG. 5, the third embodiment is characterized in that a field plate structure is formed by the gate electrode 12 and the surface protective film 11. Other configurations are the same as those in the first embodiment, and thus description thereof is omitted here.

The surface protective film 11 is formed so as to cover the surfaces of the AlGaN barrier layer 4, the source electrode 5, and the drain electrode 6, and has an opening at a position where the gate electrode 12 is to be formed. Note that the surface protective film 11 is similar to the surface protective film 8 of the first embodiment, when the stoichiometric Si ₃ N ₄ film is used as a reference, the Si / N ratio is excessive in nitrogen, and the surface protective film 11 The film 11 is formed by a SiN film having a hydrogen content of less than 10%.

  The gate electrode 12 is formed so as to fill the opening of the surface protective film 11 and partially cover the surface protective film 11. The configuration (layer structure) of the gate electrode 12 is made of Ni (50 nm) / Au (300 nm) as in the gate electrode 7 of the first embodiment.

  Next, a method for manufacturing a semiconductor device will be described with reference to FIGS. 6-8 is a figure which shows an example of the manufacturing process of a semiconductor device.

  First, as shown in FIG. 6, the buffer layer 2, the GaN channel layer 3, and the AlGaN barrier layer 4 are stacked on the substrate 1, and the source electrode 5 and the drain electrode 6 are formed on the AlGaN barrier layer 4. . Since these manufacturing methods are the same as those in the first embodiment (see FIG. 2), description thereof is omitted here. At this time, the gate electrode 12 is not formed.

  Next, as shown in FIG. 7, a surface protective film 11 (for example, 80 nm) is formed so as to cover the surfaces of the AlGaN barrier layer 4, the source electrode 5, and the drain electrode 6, and the gate electrode 12 is to be formed. An opening (gate opening pattern) is formed using lithography and dry etching.

  Next, as shown in FIG. 8, a pattern 13 for forming the gate electrode 12 is formed by lithography in accordance with the opening of the surface protective film 11.

  Next, the gate electrode 12 is formed by applying the evaporation lift-off method using Ni (50 nm) / Au (300 nm) similar to the gate electrode 7 of the first embodiment. As a result, a semiconductor device having a field plate structure as shown in FIG. 5 is obtained.

  From the above, according to the third embodiment, the electric field concentration is alleviated by the field plate structure while suppressing the unintended reverse leakage current resulting from the formation of the SiN film, and thus the finite interface exists. It becomes possible to further suppress the reverse leakage current via the level and the impurity level. That is, by adopting the configuration of the third embodiment, the effect of the field plate structure can be further enhanced and the reverse leakage current can be suppressed. Therefore, the field plate structure using the SiN film is adopted as a hand. Reliability can be improved as compared with a semiconductor device.

In the third embodiment, the case where the surface protective film 11 has one layer has been described. However, the present invention is not limited to this. For example, as in the second embodiment, the surface protective film 11 may be formed by stacking a plurality of layers. In this case, the lowermost layer (the layer in contact with the AlGaN barrier layer 4 in the example of FIG. 5) has a higher nitrogen ratio in the Si / N ratio than the stoichiometric Si ₃ N ₄ , and the hydrogen content in the lowermost layer Any SiN film whose amount is less than 10% may be used, and layers other than the lowermost layer may have a larger band gap than the lowermost layer.

<Embodiment 4>
First, the configuration of the semiconductor device according to the fourth embodiment of the present invention will be described.

  FIG. 9 is a diagram showing an example of the configuration of the semiconductor device according to the fourth embodiment of the present invention.

  As shown in FIG. 9, the fourth embodiment is characterized in that a MIS (Metal-Insulator-Semiconductor) structure is formed by the gate electrode 15, the surface protective film 14, and the AlGaN barrier layer 4 (nitride semiconductor layer). It is said. That is, the surface protective film 14 has a function of protecting the surface of the semiconductor device and a function as an insulating film in the MIS structure. Other configurations are the same as those in the first embodiment, and thus description thereof is omitted here.

The surface protective film 14 is formed so as to cover the surfaces of the AlGaN barrier layer 4, the source electrode 5, and the drain electrode 6, and is formed between the AlGaN barrier layer 4 and the gate electrode 15. The surface protective film 14 is similar to the surface protective film 8 of the first embodiment, and has a nitrogen excess in the Si / N ratio when the stoichiometric Si ₃ N ₄ film is used as a reference. The film 14 is formed by a SiN film having a hydrogen content of less than 10%. Further, the film thickness of the surface protective film 14 is, for example, 5 nm.

  The gate electrode 15 is formed on the surface protective film 14. The configuration (layer structure) of the gate electrode 15 is assumed to be Ni (50 nm) / Au (300 nm), as in the gate electrode 7 of the first embodiment.

  Next, a method for manufacturing a semiconductor device will be described with reference to FIGS. 10 and 11 are diagrams illustrating an example of a manufacturing process of a semiconductor device.

  First, similarly to FIG. 6 of the third embodiment, the buffer layer 2, the GaN channel layer 3, and the AlGaN barrier layer 4 are formed on the substrate 1 by laminating, and the source electrode 5 and the drain electrode 6 are formed on the AlGaN barrier layer 4. Form. Since these manufacturing methods are the same as those in the third embodiment, description thereof is omitted here. At this time, the gate electrode 15 is not formed.

  Next, as shown in FIG. 10, a surface protective film 14 (for example, 5 nm) is formed so as to cover the surfaces of the AlGaN barrier layer 4, the source electrode 5, and the drain electrode 6.

  Next, as shown in FIG. 11, a pattern 16 for forming the gate electrode 15 is formed on the surface protective film 14 by lithography.

  Next, the gate electrode 15 is formed by applying the evaporation lift-off method to the pattern 16 using Ni (50 nm) / Au (300 nm) similar to the gate electrode 7 of the first embodiment. As a result, a semiconductor device having a MIS structure as shown in FIG. 9 is obtained.

  In a semiconductor device having an MIS structure, an interface level and an impurity level between an insulating film and a semiconductor layer can contribute to a leak path of a reverse leakage current. Therefore, by adopting the surface protective film 14 according to the fourth embodiment in the semiconductor device having the MIS structure, the effect of the MIS structure can be further enhanced and the reverse leakage current can be suppressed. The reliability can be improved as compared with the semiconductor device adopting the used MIS structure.

<Embodiment 5>
First, the configuration of the semiconductor device according to the fifth embodiment of the present invention will be described.

  FIG. 12 is a diagram showing an example of the configuration of the semiconductor device according to the fifth embodiment of the present invention.

  As shown in FIG. 12, the fifth embodiment is characterized in that the surface protective film is formed by laminating a plurality of layers (in the example of FIG. 12, the lower surface protective film 17 and the upper surface protective film 18). It is said. Other configurations are the same as those in the fourth embodiment, and thus description thereof is omitted here.

The lower surface protective film 17 is the lowest layer formed on the most AlGaN barrier layer 4 (nitride semiconductor layer) side among the plurality of layers forming the surface protective film, and is lower than stoichiometric Si ₃ N _4. The SiN film has a high nitrogen ratio in the / N ratio and a hydrogen content in the lower surface protective film 17 of less than 10%. In addition, the film thickness of the lower surface protective film 17 shall be 5 nm, for example.

The upper surface protective film 18 is an Al ₂ O ₃ film having a larger band gap than the lower surface protective film 17 that is a SiN film. In addition, the film thickness of the upper surface protective film 18 shall be 5 nm, for example.

  The gate electrode 19 is formed on the upper surface protective film 18. The configuration (layer structure) of the gate electrode 19 is assumed to be Ni (50 nm) / Au (300 nm) as in the gate electrode 15 of the fourth embodiment.

  Next, a method for manufacturing a semiconductor device will be described with reference to FIGS. 13 and 14 are diagrams illustrating an example of a manufacturing process of a semiconductor device.

  First, as in the fourth embodiment, the buffer layer 2, the GaN channel layer 3, and the AlGaN barrier layer 4 are stacked on the substrate 1, and the source electrode 5 and the drain electrode 6 are formed on the AlGaN barrier layer 4. . Since these manufacturing methods are the same as those in the fourth embodiment, description thereof is omitted here. At this time, the gate electrode 19 is not formed.

  Next, as shown in FIG. 13, a SiN film is formed as a lower surface protective film 17 (for example, 5 nm) so as to cover the surfaces of the AlGaN barrier layer 4, the source electrode 5, and the drain electrode 6. Next, an upper surface protective film 18 (for example, 5 nm) is formed on the lower surface protective film 17. As a method for forming the lower surface protective film 17 and the upper surface protective film 18, for example, an ECR-sputtering method or an ALD method excellent in film thickness controllability is used.

  Next, as shown in FIG. 14, a pattern 20 for forming the gate electrode 19 is formed on the upper surface protective film 18 by lithography.

  Next, the gate electrode 19 is formed by applying the evaporation lift-off method to the pattern 20 using Ni (50 nm) / Au (300 nm) similar to the gate electrode 15 of the fourth embodiment. As a result, a semiconductor device having a MIS structure as shown in FIG. 12 is obtained.

  From the above, according to the fifth embodiment, by forming the surface protective film with a plurality of layers (lower surface protective film 17 and upper surface protective film 18), unintended reverse caused by the formation of the SiN film. A surface protective film having a higher breakdown voltage than that of the SiN film can be formed while suppressing the direction leakage current. Therefore, the reliability of the semiconductor device can be improved as compared with the fourth embodiment.

In the fifth embodiment, the case where the surface protective film is formed by laminating two layers (the lower surface protective film 17 and the upper surface protective film 18) has been described. However, the present invention is not limited to this. It may be the above. In this case, the lowermost layer may be a SiN film in which the ratio of nitrogen is higher in the Si / N ratio than the stoichiometric Si ₃ N ₄ and the hydrogen content in the lowermost layer is less than 10%, Further, it is only necessary that layers other than the lowermost layer have a larger band gap than the lowermost layer.

<Embodiment 6>
First, the configuration of the semiconductor device according to the sixth embodiment of the present invention will be described.

  FIG. 15 is a diagram showing an example of the configuration of the semiconductor device according to the sixth embodiment of the present invention.

  As shown in FIG. 15, in the sixth embodiment, the gate electrode 23, the lower surface protection film 21, and the AlGaN barrier layer 4 (nitride semiconductor layer) form an MIS structure, and the gate electrode 23 and the upper surface protection. A field plate structure is formed by the film 22. Other configurations are the same as those of the fifth embodiment, and thus description thereof is omitted here.

The lower surface protective film 21 is formed so as to cover the surfaces of the AlGaN barrier layer 4, the source electrode 5, and the drain electrode 6. The lower surface protective film 21 is the lowest layer formed on the most AlGaN barrier layer 4 (nitride semiconductor layer) side among the plurality of layers forming the surface protective film, and is formed by stoichiometric Si ₃ N ₄ . This is a SiN film in which the nitrogen ratio is high in the Si / N ratio and the hydrogen content in the lower surface protective film 21 is less than 10%. In addition, the film thickness of the lower surface protective film 21 shall be 5 nm, for example.

The upper surface protective film 22 is formed so as to cover the surface of the lower surface protective film 21 and has an opening at a position where the gate electrode 23 is to be formed. The upper surface protective film 22 is an Al ₂ O ₃ film having a larger band gap than the lower surface protective film 21 that is a SiN film. The film thickness of the upper surface protective film 22 is, for example, 5 nm.

  The gate electrode 23 is formed so as to fill the opening of the upper surface protective film 22 and partially cover the upper surface protective film 22. The configuration (layer structure) of the gate electrode 23 is assumed to be Ni (50 nm) / Au (300 nm), as in the gate electrode 19 of the fifth embodiment.

  Next, a method for manufacturing a semiconductor device will be described with reference to FIGS. 16 to 18 are diagrams illustrating an example of the manufacturing process of the semiconductor device.

  First, as in the fifth embodiment, the buffer layer 2, the GaN channel layer 3, and the AlGaN barrier layer 4 are stacked on the substrate 1, and the source electrode 5 and the drain electrode 6 are formed on the AlGaN barrier layer 4. . Since these manufacturing methods are the same as those in the fifth embodiment, description thereof is omitted here. At this time, the gate electrode 23 is not formed.

  Next, as shown in FIG. 16, a SiN film is formed as a lower surface protective film 21 (for example, 5 nm) so as to cover the surfaces of the AlGaN barrier layer 4, the source electrode 5, and the drain electrode 6. Next, an upper surface protective film 22 (for example, 5 nm) is formed on the lower surface protective film 21. As a method for forming the lower surface protective film 21 and the upper surface protective film 22, for example, an ECR-sputtering method or an ALD method excellent in film thickness controllability is used.

  Next, as shown in FIG. 17, openings (gate opening patterns) are formed in the upper surface protective film 22 at locations where the gate electrodes 23 are to be formed using lithography (not shown) and dry etching. . As a method of forming the opening in the upper surface protective film 22, there is a method by dry etching using chlorine gas, methane gas, and argon gas, or wet etching using a strong alkaline developer.

  Next, as shown in FIG. 18, a pattern 24 for forming the gate electrode 23 is formed by lithography in accordance with the opening of the upper surface protective film 22.

  Next, the gate electrode 23 is formed by applying the evaporation lift-off method using Ni (50 nm) / Au (300 nm) similar to the gate electrode 19 of the fifth embodiment. As a result, a semiconductor device having a MIS structure and a field plate structure (MIS type semiconductor device having a field plate structure) as shown in FIG. 15 is obtained.

  From the above, according to the sixth embodiment, in addition to the effect of the fifth embodiment, the effect of electric field relaxation by the field plate structure can be obtained. Therefore, the reliability of the semiconductor device can be improved as compared with the fifth embodiment.

In the sixth embodiment, the case where the surface protective film is formed by stacking two layers (the lower surface protective film 21 and the upper surface protective film 22) has been described. It may be the above. In this case, the lowermost layer may be a SiN film in which the ratio of nitrogen is higher in the Si / N ratio than the stoichiometric Si ₃ N ₄ and the hydrogen content in the lowermost layer is less than 10%, Further, it is only necessary that layers other than the lowermost layer have a larger band gap than the lowermost layer. The gate electrode may be formed so that a part thereof is in contact with the lowermost layer.

  It should be noted that the present invention can be freely combined with each other within the scope of the invention, and each embodiment can be appropriately modified or omitted.

  1 substrate, 2 buffer layer, 3 GaN channel layer, 4 AlGaN barrier layer, 5 source electrode, 6 drain electrode, 7 gate electrode, 8 surface protective film, 9 lower surface protective film, 10 upper surface protective film, 11 surface protective film , 12 Gate electrode, 13 patterns, 14 Surface protective film, 15 Gate electrode, 16 patterns, 17 Lower surface protective film, 18 Upper surface protective film, 19 Gate electrode, 20 patterns, 21 Lower surface protective film, 22 Upper surface protective film , 23 Gate electrode, 24 patterns.

Claims (10)

A nitride semiconductor layer;
A protective film made of SiN formed on the nitride semiconductor layer;
With
The protective film is characterized in that the ratio of nitrogen is higher in the Si / N ratio than stoichiometric Si ₃ N ₄ and the hydrogen content in the protective film is less than 10%. . The protective film is formed by laminating a plurality of layers,
The lowermost layer formed on the nitride semiconductor layer side of the plurality of layers has a higher nitrogen ratio in the Si / N ratio than stoichiometric Si ₃ N ₄ , and hydrogen in the lowermost layer. The content is less than 10%,
2. The semiconductor device according to claim 1, wherein a layer other than the lowest layer of the plurality of layers has a band gap larger than that of the lowest layer. A gate electrode formed on the nitride semiconductor layer;
The semiconductor device according to claim 1, wherein a field plate structure is formed by the gate electrode and the protective film. A gate electrode formed on the protective film;
The semiconductor device according to claim 1, wherein a MIS (Metal-Insulator-Semiconductor) structure is formed by the gate electrode, the protective film, and the nitride semiconductor layer.   The semiconductor device according to claim 4, wherein a field plate structure is formed by the gate electrode and the protective film. (A) forming a nitride semiconductor layer;
(B) forming a protective film made of SiN on the nitride semiconductor layer;
With
In the step (b), the protective film has a higher nitrogen ratio in the Si / N ratio than stoichiometric Si ₃ N ₄ , and the hydrogen content in the protective film is less than 10%. A method for manufacturing a semiconductor device, comprising: In the step (b),
The protective film is formed by laminating a plurality of layers,
The lowermost layer formed on the nitride semiconductor layer side of the plurality of layers has a higher nitrogen ratio in the Si / N ratio than stoichiometric Si ₃ N ₄ , and hydrogen in the lowermost layer. The content is less than 10%,
The method for manufacturing a semiconductor device according to claim 6, wherein a layer other than the lowermost layer of the plurality of layers has a band gap larger than that of the lowermost layer. (C) further comprising a step of forming a gate electrode on the nitride semiconductor layer;
8. The semiconductor device according to claim 6, wherein a field plate structure is formed by the protective film formed in the step (b) and the gate electrode formed in the step (c). Production method. (D) further comprising a step of forming a gate electrode on the protective film;
MIS (Metal-Insulator-Semiconductor) is formed by the gate electrode formed in the step (d), the protective film formed in the step (b), and the nitride semiconductor layer formed in the step (a). 8. The method of manufacturing a semiconductor device according to claim 6, wherein a structure is formed.   The method of manufacturing a semiconductor device according to claim 9, wherein a field plate structure is formed by the gate electrode and the protective film.

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