JP2000049363A - Schottky diode and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Schottky diode in which a Schottky barrier in a reverse directional bias is high and the Schottky barrier in a forward directional bias is low. SOLUTION: An n- type layer 3 composed of 3C-SiC having a band gap smaller than that of an n- type epitaxial layer 2 is provided on an upper face of the n- type epitaxial layer 2 composed of 4H-SiC or 6H-SiC, also a trench part 4 passing the n- type layer 3 and reaching the n- type epitaxial layer 2 is provided, and an Al film 5 is brought in Schottky contact with the n- type layer 3 and the n- type epitaxial layer 2. With such a structure, at reverse bias, the contact part of the n- type layer 2 with the Al film 5 in a mesa part is pinched off by a depletion layer which extends to the n- type epitaxial layer 2, and at reverse bias, the potential barrier in the mesa part is made higher. Thus, in a reverse directed bias, the potential barrier can be made high in the n- type epitaxial layer 2, and in a forward directed bias, the potential barrier can be lowered in the n- type layer 3. Then, it is possible to realize reduction in the consumption power of a Schottky diode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Schottky diode (Schottky barrier diode) utilizing a Schottky barrier between a metal and a semiconductor, and a method of manufacturing the same.
It is suitable for application to ET.

[0002]

2. Description of the Related Art Conventionally, a Schottky diode using a Schottky barrier between a metal and a semiconductor has been known. This Schottky diode is a majority carrier device in which majority carriers dominate operation, and has no minority carrier accumulation effect unlike a PN junction diode.
This is effective because the switching speed is fast.

In order to improve the breakdown voltage of the Schottky diode, research on a Schottky diode using silicon carbide (SiC) has been advanced. FIG. 6 shows a Schottky diode using this silicon carbide. High on the surface concentration of the n ⁺ -type silicon carbide substrate 50 low concentration n ^- -type and epitaxial layer 51 is formed, the n ^- -type Al film 52 on the epitaxial layer 51 is n ^- -type epitaxial layer 51 and shot It is formed so as to make key contact. On the back surface of n ⁺ -type silicon carbide substrate 50, a metal film 53 made of Ti or Ni is formed so as to make ohmic contact with n ⁺ -type silicon carbide substrate 50.

Further, the n ⁻ -type epitaxial layer 51 includes:
4H or 6H silicon carbide, which has a large Schottky barrier, is used, thereby improving the breakdown voltage.

[0005]

The power consumption of the Schottky diode depends on the contact resistance caused by the Schottky barrier in the forward bias and the leakage current in the reverse bias. Therefore, in order to reduce power consumption,
Ideally, the Schottky barrier is low in forward bias and high in reverse bias.

However, in the above-described conventional Schottky diode using silicon carbide, not only the breakdown voltage in the reverse bias is increased, but also the contact resistance due to the Schottky barrier in the forward bias is increased, and the power consumption is increased. There is a problem that the amount increases. An object of the present invention is to provide a Schottky diode having a high Schottky barrier in a reverse bias and a small Schottky barrier in a forward bias, and a method of manufacturing the same.

[0007]

In order to achieve the above object, the following technical means are employed. In the inventions described in claims 1 to 7, a second semiconductor layer (3) having a smaller band gap than the first semiconductor layer is provided on the upper surface of the first semiconductor layer, and the second semiconductor layer is formed on the upper surface of the first semiconductor layer. Penetrate first
And a second metal layer (7) is in Schottky contact with the second semiconductor layer and the first semiconductor layer.

[0008] As described above, the second semiconductor layer having a smaller band gap than the first semiconductor layer is provided, and the second semiconductor layer is also brought into Schottky contact with the second metal layer, thereby forming the first semiconductor layer. The Schottky barrier between the layer and the second semiconductor layer increases, and the Schottky barrier between the second semiconductor layer and the second metal layer decreases. Therefore, the withstand voltage can be increased in the first semiconductor layer in the reverse bias, and the contact resistance due to the Schottky barrier can be reduced in the second semiconductor layer in the forward bias. Thus, the power consumption of the Schottky diode can be reduced.

Specifically, when a reverse bias is applied, a depletion layer extending into the first semiconductor layer below the second semiconductor layer pinches off when a reverse bias is applied. By setting the width of the second semiconductor layer, the breakdown voltage can be provided at the bottom of the groove having a high Schottky breakdown voltage. Thus, the Schottky barrier is low at the time of reverse bias, and the second semiconductor layer having a low withstand voltage can be prevented from having a withstand voltage.

According to a third aspect of the present invention, the depth of the groove is larger than the thickness of the second semiconductor layer. Accordingly, it is possible to prevent the interface between the first semiconductor layer and the second semiconductor layer from being broken down due to electric field concentration when a voltage is applied. It is preferable that the second semiconductor layer is made of 3C—SiC having a low Schottky barrier, and the first semiconductor layer is formed of a Schottky barrier. It is preferable to use high 4H-SiC or 6H-SiC.

According to a seventh aspect of the present invention, the main surface (1a) of the semiconductor substrate (1) is a (0001) Si plane. Thus, as the main surface, (0
By using the (001) Si plane, crystal defects can be reduced and the Schottky barrier can be increased. In the invention according to claims 8 to 10, the main surface (1
a) a step of preparing a semiconductor substrate (1) of the first conductivity type having a high concentration and having a back surface (1b) which is the opposite surface to the semiconductor substrate; Forming a first conductive type first semiconductor layer (2) having a lower concentration than the first semiconductor layer; and forming a lower conductive layer on the upper surface of the first semiconductor layer with a semiconductor having a smaller potential barrier than the first semiconductor layer. First of concentration
Forming a conductive type second semiconductor layer (3); forming a groove (4) penetrating through the second semiconductor layer to reach the first semiconductor layer; Forming a first metal layer (6) in ohmic contact with the semiconductor substrate; and forming a Schottky contact with the second semiconductor layer and the first semiconductor layer on the second semiconductor layer including the inside of the groove. Forming a second metal layer (5).

By manufacturing a Schottky diode by using such a process, the Schottky diode according to the first aspect can be manufactured. According to a tenth aspect of the present invention, in the step of forming the second semiconductor layer, the second semiconductor layer is formed on the first semiconductor layer by epitaxial growth.

When the second semiconductor layer is formed by epitaxial growth, the second semiconductor layer can be formed with good controllability. Note that the reference numerals in parentheses described above indicate the correspondence with specific means described in the embodiment described later.

[0014]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a first embodiment of the present invention. FIG. 1 shows a cross-sectional configuration of a Schottky diode to which one embodiment of the present invention is applied. Hereinafter, the structure of the Schottky diode will be described with reference to FIG. Note that, in the present embodiment, one embodiment of the present invention is applied to an n-type semiconductor whose mobility can be increased.

The Schottky diode has a main surface 1a
And a high-concentration n ⁺ -type silicon carbide substrate 1 having back surface 1b opposite to the surface. N ⁺ -type silicon carbide substrate 1 is made of silicon carbide such as 6H-SiC or 4H-SiC, and main surface 1a of n ⁺ -type silicon carbide substrate 1
The (0001) Si plane is adopted as the method. By using this (0001) Si plane, crystal defects can be reduced and the Schottky barrier can be increased.

[0016] On the main surface 1a of the n ⁺ -type silicon carbide substrate 1, n as a first semiconductor layer of lower concentration than the n ⁺ -type silicon carbide substrate 1 ^- -type epitaxial layer (hereinafter, n ^- -type epitaxial layer 2) are formed. This n ^- type epi layer 2
Is made of silicon carbide such as 6H-SiC or 4H-SiC. Then, this on the n ^- -type epitaxial layer 2, n ^- n as a second semiconductor layer having a type epi layer 2 and the equivalent concentration ^- -type layer 3 is formed. This n ^- type layer 3 is 3C-
It is made of SiC. 3C-SiC is 6H-Si
The Schottky barrier is lower than that of C or 4H-SiC.

[0017] The Schottky diode, n ^- through the type layer 3 n ^- groove portion reaching -type epitaxial layer 2 (trench) 4 is formed, n by the groove portion 4 ^- -type layer 3 and the n ^- The surface portion of the mold epi layer 2 has a mesa shape. The groove 4 has a depth n ^- is larger than the thickness of the mold layer 3, n ^- -type layer 3 and the n ^- boundary between type epi layer 2 is positioned on the side surface of the groove 4.

The width Wm of the n ⁻ -type layer 2 is formed smaller than the width Wt of the bottom surface of the groove 4 and smaller than the depth of the groove 4. Further, an Al film 5 as a second metal layer is formed so as to cover the inside of the groove 4 and the n ⁻ -type layer 3.
Is provided. This Al film 5 constitutes an anode electrode. The Al film 5 is in Schottky contact with the n ⁻ -type layer 3 and the n ⁻ -type epi layer 2.

On the back surface 1b of the n ⁺ type semiconductor substrate 1, a NiAl film 6 is formed as a first metal layer.
This NiAl film 6 constitutes a cathode electrode. The NiAl film 6 and the n ⁺ type semiconductor substrate 1 are in ohmic contact. In the Schottky diode configured as described above, when a forward bias is applied so that the potential of the NiAl film 6 is higher than that of the Al film 5, a forward current using electrons as carriers flows.

At this time, since the n ⁻ -type layer 3 made of 3C—SiC having a low potential barrier is in contact with the Al film 5, the forward current flows through the n ⁻ -type layer 3 having a low potential barrier. Can flow. A voltage-forward current characteristic during forward bias was simulated. The result is shown in FIG. This figure shows a voltage-forward current characteristic when a forward bias is applied when 3C-SiC and Al are brought into Schottky contact. In FIG. 2, the forward current is represented by a magnitude flowing per 1 μm ² .

As shown in this figure, when 3C-SiC and Al are brought into Schottky contact, the voltage value when the forward current rises becomes low. Therefore, 3C
By providing the Schottky contact between the n ⁻ -type layer 3 made of —SiC and the Al film 5, the potential barrier can be reduced. Thereby, in the forward bias, the height of the potential barrier can be set to the height of 3C-SiC, and the potential barrier can be reduced.

When a reverse bias is applied so that the NiAl film 6 side has a lower potential than the Al film 5 side,
The Schottky barrier between the n ⁻ -type epi layer 2 and the n ⁻ -type layer 3 and the Al film 5 makes it difficult for reverse current to flow. The extension of the depletion layer in the reverse bias is shown by a dotted line in FIG. As shown in this figure, the depletion layer has an n ⁻ -type epitaxial layer 2 according to the difference in work function between silicon carbide and metal.
And n ^- type layer 3. Then, n ^- -type depletion Correspondingly work function difference 6H-SiC and Al extends in the epitaxial layer 2, the n ^- in the lower type epi layer 2 ^- n by a depletion layer extending -type epitaxial layer 2 Pinch off.
Since the width Wm of the n ⁻ -type layer 3 is set to the above width, pinch-off can be easily performed by the depletion layers extending from both sides of the groove 4.

For this reason, the Schottky breakdown voltage in the reverse bias is 4H-SiC constituting the n ⁻ -type epi layer 2.
Alternatively, it is determined by the Schottky barrier between 6H—SiC and the Al film 5. As described above, in the reverse bias, the height of the potential barrier can be set to 4H-SiC or 6H-SiC, and the potential barrier can be increased. As described above, the n ⁻ -type epi layer 2 made of 4H-SiC or 6H-SiC having a high Schottky barrier and the n ⁻ -type layer 3 made of 3C-SiC having a low Schottky barrier are provided at the contact portion between the metal and the semiconductor. By combining the above, a Schottky diode with a high reverse bias and a small potential barrier with a forward bias can be obtained. Thus, the power consumption of the Schottky diode can be reduced.

As described above, the depth of the groove 4 is n
^- is set to be larger than the thickness of the mold layer 3, n ^-
The interface between the mold layer 3 and the n ⁻ -type epi layer 2 is located on the side surface of the mesa shape. This is because electric field concentration is likely to occur at the boundary between the bottom surface and the side surface of the groove portion 4, and when the interface between the n ⁻ -type layer 3 and the n ⁻ -type epi layer 2 where the crystal form changes is located at this portion, insulation is caused by the electric field concentration. This is because the interface between the n ⁻ -type layer 3 and the n ⁻ -type epi layer 2 may be located on the side surface of the mesa shape to prevent dielectric breakdown. it can.

Next, a method of manufacturing the Schottky diode shown in FIG. 1 will be described. 4 to 5 show a manufacturing process of the Schottky diode. [Step shown in FIG. 4A] First, 6H of about 100 μm
A high concentration n ⁺ -type silicon carbide substrate 1 made of -SiC (or 4H-SiC) is prepared. Then, on main surface 1a of n ⁺ -type silicon carbide substrate 1, a film having a thickness of about 10.0 μm
Low concentration n consisting of H-SiC (or 4H-SiC) ^-
The epitaxial layer 2 is epitaxially grown.

[Step shown in FIG. 4B] n ^- type epi layer 2
Of Si ions and C ions are implanted from the upper surface of the substrate. Thus, a damage layer is formed on the surface of the n ⁻ -type epi layer 2. This damaged layer is in an amorphous state, for example. Note that the ratio of Si ions and C ions to be implanted at this time is set to 1: 1 and that the Si ions and C ions are reacted by an annealing process performed later without excess or shortage so that all become SiC.

[Step shown in FIG. 4C] An annealing process is performed to recrystallize the damaged layer. Thereby, 6H
The crystallinity of the surface layer of n ⁻ -type epi layer 2 made of —SiC changes, and n ⁻ -type layer 3 made of 3C—SiC is formed. [Step shown in FIG. 5A] Next, a trench 4 penetrating through the n ⁻ -type layer 3 and reaching the n ⁻ -type epi layer 2 is formed by photoetching. Thereby, a part of the n ⁻ -type epi layer 2 and n
^-The mold layer 3 has a mesa shape partially projecting.

[0028] [5 step shown in (b)] The NiAl film 6 is formed on the back side of the n ⁺ -type silicon carbide substrate 1, by, for example heat treatment and the n ⁺ -type silicon carbide substrate 1 and the NiAl film 6 To make ohmic contact. Thereby, a cathode electrode of the Schottky diode is formed. [Step shown in FIG. 5C] Subsequently, n including the inside of the groove 4
^An Al film 5 is formed on the upper surface of the n ^- type layer 3, and the n-type layer 3 and n
^The Schottky contact is made between the -type epi layer 2 and the Al film 5.
Thereby, the cathode electrode of the Schottky diode is thereby formed, and the Schottky diode is completed.

The Schottky diode thus formed can be used, for example, for producing a power switching element. (Other Embodiments) In the above embodiment, the n ⁻ -type layer 3 made of 3C—SiC is formed by ion implantation, but the n ⁻ -type layer 3 made of 3C—SiC may be formed by epitaxial growth. In this case, 3C-Si with good controllability
The effect that C can be formed is also obtained.

In the above-described embodiment, Si ions and C ions are implanted to form the n ⁻ -type layer 3. However, even if argon, hydrogen, helium, or the like that does not act as an impurity is used, the same applies. The effect of is obtained. Further, in the above embodiment, the n ⁻ -type layer 3 is made of 3C—SiC in order to reduce the Schottky barrier, but the potential barrier is lower than the crystal form of silicon carbide forming the n ⁻ -type epi layer 2. , The effect of reducing power consumption can be obtained.

In the above embodiment, the Al film 5 is used as an electrode (anode-side electrode) to be brought into Schottky contact, but this has a high potential barrier when brought into Schottky contact with 4H-SiC or 6H-SiC. This is because a material having a low potential barrier when it is brought into Schottky contact with 3C-SiC is selected, and the above-described effect can be obtained even if another electrode material is used.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a Schottky diode according to an embodiment of the present invention.

FIG. 2 is a diagram showing the extension of a depletion layer when no bias is applied to the Schottky diode of FIG. 1;

FIG. 3 is a diagram showing a voltage-forward current characteristic when a forward bias is applied to a device in which 3C-SiC and Al are brought into Schottky contact.

FIG. 4 is a view for explaining a manufacturing process of the Schottky diode shown in FIG. 1;

FIG. 5 is a view illustrating a manufacturing step of the Schottky diode following FIG. 4;

FIG. 6 is a cross-sectional view of a conventional Schottky diode.

[Explanation of symbols]

1 ... n ⁺ -type silicon carbide substrate, 2 ... n ^- -type epitaxial layer, 3 ... n ^-
Mold layer, 4 groove portion, 5 Al film as second metal layer, 6
... NiAl film as first metal layer.

Claims (10)

[Claims] 1. A semiconductor substrate (1) of a first conductivity type having a main surface (1a) and a back surface (1b) opposite to the main surface and comprising a high concentration, and on a main surface of the semiconductor substrate. A first semiconductor layer of a first conductivity type having a lower concentration than the semiconductor substrate.
A low-concentration second semiconductor layer of a first conductivity type provided on an upper surface of the first semiconductor layer and having a band gap smaller than that of the first semiconductor layer; and a second semiconductor A groove (4) penetrating through the layer to reach the first semiconductor layer; a first metal layer (6) in ohmic contact with the back surface of the semiconductor substrate; and a Schottky contact with the second semiconductor layer. And a second metal layer (7) that is also in Schottky contact with the first semiconductor layer via the groove. 2. The width of the second semiconductor layer is such that a depletion layer extending into the first semiconductor layer below the second semiconductor layer pinches off when a reverse bias is applied. 2. The setting is set.
The Schottky diode according to the above. 3. The Schottky diode according to claim 1, wherein a depth of the groove is larger than a thickness of the second semiconductor layer. 4. The semiconductor device according to claim 1, wherein the second semiconductor layer has a mesa shape formed by the groove, and a width of the groove is larger than a width of the second semiconductor layer having the mesa shape. The Schottky diode according to any one of claims 1 to 3, wherein 5. The Schottky diode according to claim 1, wherein the second semiconductor layer is made of 3C—SiC. 6. The Schottky diode according to claim 1, wherein the first semiconductor layer is made of 4H—SiC or 6H—SiC. 7. The Schottky diode according to claim 1, wherein the main surface is a (0001) Si plane. 8. A step of preparing a first conductivity type semiconductor substrate (1) having a main surface (1a) and a back surface (1b) opposite to the main surface and having a high concentration, and Forming a first conductivity-type first semiconductor layer (2) having a lower concentration than the semiconductor substrate on the main surface of the first semiconductor layer; and forming an upper surface of the first semiconductor layer on the upper surface of the first semiconductor layer. Forming a low-concentration second semiconductor layer (3) of the first conductivity type composed of a semiconductor having a small band gap; and penetrating the second semiconductor layer and forming the second semiconductor layer (3) on the first semiconductor layer. A step of forming a groove (4) that reaches; a step of forming a first metal layer (6) in ohmic contact with the semiconductor substrate on the back surface of the semiconductor substrate; and a step of forming the second semiconductor including the inside of the groove. A Schottky contact with the second semiconductor layer and the first semiconductor layer on the layer Method of manufacturing a Schottky diode which comprises forming a composed second metal layer (5), a. 9. The method of manufacturing a Schottky diode according to claim 8, wherein in the step of forming the groove, the depth of the groove is larger than the thickness of the second semiconductor layer. Method. 10. The shot according to claim 8, wherein, in the step of forming the second semiconductor layer, the second semiconductor layer is formed on the first semiconductor layer by epitaxial growth. Manufacturing method of key diode.