JP2000101101A - Sic schottky diode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an SiC Schottky diode which has reduced leakage current, a reverse voltage stop characteristic is improved without increasing voltage drop at conducting current in the forward direction and withstand voltage and has current heavy. SOLUTION: An SiC semiconductor substrate 1, which has a pair of main surfaces and in which the two semiconductor layers of a heavily-high doped concentration 3 and a lightly-doped 2 of a first conductivity-type are stacked, Schottky metal 5 which is formed on the first surface of the substrate 1 and forms a Schottky barrier 51 with the semiconductor layer of the lightly-doped first conductivity-type and a cathode electrode 6, which is formed on the second main surface of the substrate 1 and brought into contact with the semiconductor layer of the heavily-doped first conductivity-type in terms of ohmic resistance are installed. A plurality of buried layers 7 of a second conductivity-type, which forms a p-n junction with the semiconductor layer of the lightly-doped first conductivity-type in a part close to the first main surface in the substrate 1, and the intervals are enlarged.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a SiC Schottky diode, and more particularly, to a SiC Schottky diode for controlling a high breakdown voltage and a large current.

[0002]

2. Description of the Related Art As the operating frequency of power conversion devices such as inverters becomes higher, the speed of semiconductor switching elements and the speed of freewheeling diodes and freewheel diodes connected in parallel to switching elements are strongly required. I have. Since these diodes are required to have a function of rectifying a high voltage and a large current with low loss, a pn junction diode is generally widely used. But p
Since the n-junction diode has a property that a large reverse current flows during a turn-off transition due to minority carriers accumulated inside the semiconductor when a current flows, not only an excessive loss occurs when the switching element is turned on, but also an excessive noise occurs. It is a major factor that hinders the operation of the power converter from having a higher frequency. For this reason, various pn junction diodes with improved recovery characteristics have been developed, but this type of diode with minority carrier injection has an inherent limitation in reducing the reverse current during recovery. There is a Schottky diode as a rectifier diode meeting such a demand. In the Schottky diode, the carrier that carries the current inside the semiconductor is only the majority carrier, and the minority carrier is not injected or accumulated even when the current is supplied, so that the reverse current at the time of turn-off can be extremely reduced. However, the conventional Schottky diode using silicon as a base material has a problem that the on-resistance at the time of current conduction is high, the generation loss is extremely large, and it cannot be applied to a high-voltage, large-current power converter. This problem can be solved by using SiC as a base material. However, even a SiC Schottky diode has the property that the leakage current when a reverse voltage is applied increases. In particular, when a high voltage near the withstand voltage is applied at a high temperature, the leakage current increases, so that the generation loss in the reverse voltage blocking state increases. This has the disadvantage that it occurs locally in the diode element and the element is easily destroyed by partial thermal runaway.

As a conventional technique for improving the leakage current when a voltage is applied in the reverse direction to such a Schottky diode,
For example, Solid-State Electronic
s, Vol. 28. No. 11, pp. 1089-10
93 (1985). J. Junction-Barrier-Cont applied to a Si-based Schottky diode by Baliga
rolled Shottky (JBS) Rect
A technique called "ifier" is disclosed. FIG. 2 is a sectional view showing a schematic configuration of the Schottky diode. In FIG. 2, reference numeral 1 denotes a semiconductor substrate comprising an n + type layer 2 having a high impurity concentration and an n− type layer 3 having a relatively low impurity concentration;
Reference numeral 6 denotes a cathode electrode which is in ohmic contact with the n-type layer 2 having a low resistance, and 5 denotes a Schottky metal which forms a Schottky barrier 51 on the surface of the n-type layer 3 having a relatively low impurity concentration.
A p + -type layer 4 for card ring for relaxing a local electric field is provided at the surface terminal portion, and a plurality of p + -type layers 8 are provided at a Schottky barrier portion. The p + -type layers 8 are dispersedly arranged at intervals such that the depletion layers extending from the pn junction of the adjacent p + -type layers toward the n − -type layer 3 when a reverse voltage is applied overlap each other.
It is described that this has the function of reducing the electric field strength of the reverse voltage applied to 1 and the function of reducing the leakage current in the Schottky barrier portion. If this conventional technique is applied to a Schottky diode using SiC as a base material, it can be expected that the reverse voltage blocking characteristic can be significantly improved.

However, due to the limitations on the manufacturing process peculiar to SiC described below, this structure is directly used for SiC.
There is a great difficulty in applying this to Schottky diodes. That is, in the above conventional example, the p + type layer 8
In order to reduce the electric field strength of the reverse voltage applied to the intervening Schottky barrier 51, it is necessary to narrow the interval so that the depletion layers overlap each other when the reverse voltage is applied, and to increase the reverse voltage of several hundreds to several thousand volts. In order to maintain the voltage blocking ability with respect to the voltage, it must be formed as a relatively deep high concentration layer. However, in the case of SiC, the junction design takes advantage of the physical property that the junction breakdown electric field is about 10 times that of Si. Therefore, the impurity concentration of the n − -type layer 3 is generally 70 to 100 times that of Si. Set to high concentration,
Therefore, the expansion of the depletion layer is significantly reduced. In order to exhibit the depletion layer pinch-off effect, the p +
Is required to be a very narrow value of about 1 μm.
As a result, there arises a problem that the voltage drop significantly increases when the current is conducted. For forming a deep p-type layer, acceptor impurities such as boron and aluminum are diffused by thermal diffusion in Si. However, in SiC, since the diffusion coefficients of these impurities are extremely small, this diffusion method cannot be applied. Ion implantation must be applied. But,
It is extremely difficult to selectively form a deep implanted layer of 1 μm or more by ion implantation. This is because it is difficult to form an implantation mask having a large thickness, and crystal defects that cannot be erased are induced. Therefore, it is almost impossible to directly apply the JBS structure made of Si, which can reduce the leakage current, to a Schottky diode made of SiC.

[0005]

As described above, the power Schottky diode with a high breakdown voltage and a large current has the conventional problem of the Schottky diode of reducing the leakage current at the time of reverse voltage at high temperature and high voltage. In addition, it is necessary to solve the problem that the voltage drop significantly increases during forward current conduction. SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide a high-breakdown-voltage, large-current SiC which has a reduced leakage current and a remarkably improved reverse voltage blocking characteristic without increasing a voltage drop during forward current conduction. It is to provide a Schottky diode.

[0006]

In order to solve the above problems, a Schottky barrier is formed, and a plurality of buried layers of the opposite conductivity type to the substrate are dispersedly arranged in a substrate near the surface of the SiC semiconductor substrate. Further, the impurity concentration of the semiconductor substrate interposed between the formation of the buried layer and the Schottky barrier is reduced.

According to the present invention, when a reverse voltage is applied to the element,
Due to the reverse voltage applied to the pn junction between the semiconductor substrate and the buried layer, the depletion layers extending there overlap, and the electric field intensity applied to the Schottky barrier can be reduced. At this time, the depletion layer between the buried layers is easier to spread than the conventional surface layer, so that the distance between the layers can be set wider. Also,
The buried layer becomes a substantially deep pn junction, allowing blocking capability to withstand high voltages. By the above operation, a SiC Schottky diode with a high withstand voltage in which the leakage current in the reverse direction is reduced can be realized without increasing the voltage drop when the current flows in the forward direction. Further, by interposing a layer having a low impurity concentration between the Schottky barrier and the buried layer, the reverse bias electric field applied to the Schottky barrier can be further reduced, so that the leakage current can be further reduced and the blocking voltage can be improved.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view of a high-breakdown-voltage SiC Schottky diode according to a first embodiment of the present invention, which has improved reverse characteristics. In FIG. 1, a parallel-plate-shaped SiC semiconductor substrate 1 having upper and lower main surfaces is a low-resistance n having an impurity concentration of about 3 × 10 19 and a thickness of about 200 μm.
+ Type layer 2, impurity concentration of about 1 × 10 16, thickness of about 1
A n-type layer 3 having a high resistance of 2 .mu.m, and a low resistance ohmic contact Ni contacting one main surface where the n + type layer 2 is exposed.
A Schottky metal 5 such as Ti / A1 or Pt serving as an anode electrode is provided on the other main surface where the cathode electrode 6 of metal or the like and the n− type layer 3 are exposed, respectively. A Schottky barrier 51 is formed in a portion in contact with. In the portion where the Schottky metal 5 terminates, an implantation amount of about 1 × 10 15 / cm ² formed by boron ion implantation into the n − -type layer 3 from the other main surface,
A relatively high concentration p + type layer 4 having a depth of about 0.5 μm is provided,
On the surface thereof, the Schottky electrode 5 makes ohmic contact with low resistance. Then, in a portion to be a main functional area at the center of the substrate surface, n at a position of about 1 μm from the other main surface.
A plurality of relatively high-concentration p + type buried layers 7 formed by boron implantation having a height of about 0.5 μm and a width of about 1 μm are provided in the − type layer 3 at intervals of about 3 μm, that is, at a wide interval. A pn junction 71 is formed with the mold layer 3. FIG. 1 shows three buried layers 7, but in an actual device, many buried layers 7 are arranged according to the size of the semiconductor substrate.

The operation of each part in this embodiment will be described below. The rectifying action of the current as a diode operates in a portion of the Schottky barrier 51 formed between the n − -type layer 3 and the Schottky metal 5. That is, when a voltage in a direction in which the Schottky metal 5 has a positive potential with respect to the cathode electrode 6 is applied, the voltage of the Schottky barrier 51 becomes approximately 0.1.
Electrons flow from the Schottky metal 5 to the n − -type layer 3 beyond the relatively low barrier of 1 to 0.5 V, and further flow toward the cathode electrode 6 to conduct. When a voltage in the opposite direction is applied, the flow of electrons is reduced by the Schottky barrier 5.
1 to block the flow of current. The p + -type layer 4 provided at the end of the Schottky metal 5 prevents the breakdown voltage from decreasing due to the local concentrated electric field applied to the Schottky barrier 51 in the state where a reverse voltage is applied, and utilizes the high reverse voltage blocking characteristic of the p + n junction. are doing. In this example, a so-called guard ring structure that is commonly used is shown, but other structures such as a field limiting ring (FLR), a field plate (FP), or a junction termination extension (J) are used.
TE) can also be applied.

In this embodiment, a plurality of p + type buried layers 7 is a novel point. When a reverse voltage is applied, the depletion layer first expands into the n − -type layer 3 from the Schottky barrier 51, but at a reverse voltage of about 100 V, the depletion layer becomes p +
The mold buried layer 7 is reached. Due to the higher reverse voltage, the depletion layer spreads from the p + -type buried layer 7, and the depletion layer spreading from the adjacent p + -type buried layer 7 at a voltage of about 400 V overlaps. At a reverse voltage higher than that, the depletion layer becomes n + in the n− type layer 3.
It spreads uniformly toward the mold layer 2, and at about 1200 V, its tip reaches the n + type layer 2, and the element breaks down by punch-through. In the operation when the reverse voltage is applied, the electric field in the reverse direction applied to the Schottky barrier 51 initially increases as the reverse voltage increases. However, when the reverse voltage is higher than the voltage at which the depletion layer spreading from the p + -type buried layer 7 overlaps, the reverse electric field increases. Due to the pinch-off effect of the parts, no higher strength electric field is created. By this function, it is possible to prevent a remarkable increase in leakage current in Schottky barrier portion 51 when a high voltage is applied. Further, the interval between the p + type layers 8 of the conventional example shown in FIG.
In contrast to the extremely narrow value of about m, in the present embodiment,
Since the p + type buried layers 7 are dispersedly arranged at intervals of about 3 μm, that is, at wide intervals, the voltage can be lowered without increasing the voltage drop during forward current conduction.

FIG. 3 shows a second embodiment of the present invention.
FIG. 2 shows a cross-sectional view of a Schottky diode of the first embodiment of FIG. 1 with further improved reverse voltage characteristics. The same reference numerals as those in the first embodiment shown in FIG. 1 indicate the parts having the same structure, conductivity type and function. In FIG. 3, the point that a plurality of p + -type buried layers 7 having a relatively high concentration are provided in the n − -type layer 3 is the same as in the first embodiment, but is further provided above the p + -type buried layers 7. High resistance n−− with impurity concentration of 1 to 3 × 10 15
The difference is that the mold layer 31 is laminated in contact with the interface 313 with the n − -type layer 3. With this configuration, when a reverse voltage is applied to the element, the voltage at which the depletion layer extending from the Schottky barrier 51 reaches the p + -type buried layer 7 is reduced to 50 V or less, and the pinch-off between the p + -type buried layers 7 is reduced. This voltage is maintained even after the state. As a result, the electric field applied to the Schottky barrier 51 is reduced, and the leakage current when a reverse voltage is applied can be significantly reduced.

In this embodiment, at a junction temperature of 150 ° C., the leakage current density when a voltage of 1000 V is applied in the reverse direction is extremely reduced to about 1 × 10 −4 A / cm ² , and the breakdown voltage order is reduced. A Schottky diode as high as 1200 V and having an extremely low voltage drop of 1.1 to 1.2 V at a current density of 100 A / cm ² at the time of forward current conduction is obtained. Therefore, in this embodiment, a high breakdown voltage and a large current
An iC Schottky diode can be realized.

FIG. 4 shows the main steps of the manufacturing method of the second embodiment shown in FIG. The same reference numerals as in the second embodiment shown in FIG. 3 denote the same parts in the structure, conductivity type and function as those in the second embodiment shown in FIG. FIG.
(A) is one main surface portion of the n − -type layer 3 and a portion of the semiconductor substrate such as a lower n + -type layer is omitted.
In FIG. 4B, the p + -type buried layer 7 is partially formed by selective ion implantation of boron from the surface of the n − -type layer 3. The implantation amount is about 1 × 10 15 / cm ² and the implantation energy is 50 keV, 30 keV and 10 keV.
V is implanted into three stages to form a box-shaped impurity distribution, and then annealing is performed at about 1500 ° C. to perform an activation process.
Subsequently, in FIG. 4C, an n − -type layer 31 having a concentration of 1 to 3 × 10 and a thickness of about 1 μm is stacked using nitrogen as a dopant by an epitaxial growth method. Then, FIG.
In (d), a functional region of the device is manufactured by depositing a Schottky metal of Ti / A1 on the surface.

In the embodiment of the present invention, the case where the conductivity type of the semiconductor substrate 1 is n-type has been described. However, if all the described conductivity types are of the opposite conductivity type, the present invention can be applied to the case of p-type.

[0015]

As described above, according to the present invention,
The leakage current characteristics in the reverse direction of a SiC Schottky diode with a high breakdown voltage and a large current can be significantly improved. In addition, by interposing a low impurity concentration layer between the Schottky barrier and the buried layer, the reverse bias electric field applied to the Schottky barrier can be further reduced, thereby further reducing the leakage current and improving the blocking voltage. Become. Also,
Without increasing the voltage drop during forward current conduction,
It is possible to realize a SiC Schottky diode with a high breakdown voltage and a large current, which is excellent in both the conduction characteristics and the reverse voltage characteristics in which the leakage current in the reverse direction is reduced.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a high-breakdown-voltage SiC Schottky diode with improved reverse characteristics according to a first embodiment of the present invention.

FIG. 2 is a sectional view showing a conventional example.

FIG. 3 is a sectional view showing a second embodiment of the present invention.

FIG. 4 is a diagram showing a manufacturing process of the Schottky diode of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... SiC semiconductor base, 2 ... n + type layer, 3 ... n- type layer,
31 ... low-concentration n- type layer, 313 ... interface joining n-type layer and n- type layer, 4 ... p + type layer serving as guard ring,
5: Schottky metal serving as anode electrode, 51: Schottky barrier, 6: cathode electrode, 7: relatively high concentration p
+ Type layer buried layer, 8 ... p + type layer

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Toshiyuki Ohno 7-1-1, Omikacho, Hitachi City, Ibaraki Prefecture Inside Hitachi, Ltd. Hitachi Research Laboratory, Ltd. (72) Inventor Hidekatsu Onose 7-1, Omikamachi, Hitachi City, Ibaraki Prefecture No. 1 Inside Hitachi, Ltd.Hitachi Research Laboratories (72) Katsunori Asano 3--22 Nakanoshima, Kita-ku, Osaka-shi, Osaka Prefecture Kansai Electric Power Co., Inc. (72) Tomoki Hayashi Nakanoshima, Kita-ku, Osaka, Osaka 3-2-2, Kansai Electric Power Co., Inc. (72) Inventor Yoshitaka Sugawara 3-2-2, Nakanoshima, Kita-ku, Osaka-shi, Osaka F-term in Kansai Electric Power Co., Inc. 4M104 AA03 CC03 FF31 FF35 GG18 HH20

Claims (4)

[Claims] 1. An SiC semiconductor substrate having a pair of main surfaces, on which two semiconductor layers of a first conductivity type having a high impurity concentration and a low impurity concentration are stacked, and formed on a first main surface of the substrate. A Schottky metal forming a Schottky barrier between the first conductive type low impurity concentration semiconductor layer and the first conductive type high impurity concentration semiconductor layer formed on the second main surface of the base; In a SiC Schottky diode comprising a cathode electrode in ohmic resistance contact, a pn junction is formed between the first conductive type low impurity concentration semiconductor layer and a portion near the first main surface in the base. An SiC Schottky diode, wherein a plurality of buried layers of two conductivity types are dispersedly arranged. 2. An SiC semiconductor substrate having a pair of main surfaces, on which two semiconductor layers of a high conductivity concentration and a low impurity concentration of a first conductivity type are stacked, and formed on a first main surface of the substrate. A Schottky metal forming a Schottky barrier between the first conductive type low impurity concentration semiconductor layer and the first conductive type high impurity concentration semiconductor layer formed on the second main surface of the base; In a SiC Schottky diode comprising a cathode electrode in ohmic resistance contact, a pn junction is formed between the first conductive type low impurity concentration semiconductor layer and a portion near the first main surface in the base. A plurality of two-conductivity-type buried layers are dispersed and arranged, and the first-conductivity-type low-impurity-concentration semiconductor layer has a further reduced low-impurity-concentration region between the buried layer and the Schottky barrier. An SiC Schottky diode, comprising: 3. The buried layer of the second conductivity type according to claim 1, wherein a plurality of the buried layers of the second conductivity type are dispersed, so that the distance between the buried layers is widened to reduce a voltage drop during forward current conduction. A SiC Schottky diode. 4. The semiconductor device according to claim 1, wherein when a voltage having a direction in which the cathode electrode has a positive potential with respect to the Schottky metal is applied between the main electrodes, the voltage is sufficiently higher than the breakdown voltage of the semiconductor element. At a low voltage, a depletion layer extending from the Schottky barrier reaches the buried layer of the second conductivity type, and the depletion layers extending between the buried layers are set so as to overlap each other by applying a voltage higher than that. Characteristic SiC Schottky diode.