JP2009231438A - Silicon carbide wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a silicon carbide wafer, can significantly enhance the reverse breakdown voltage, when it is applied to a Schottky barrier diode (SBD). <P>SOLUTION: In the silicon carbide wafer, obtained by allowing the epitaxial film of a cubic silicon carbide on the surface of a cubic silicon carbide substrate, the concentration of the dopant contained in the epitaxial film is made to be equal to or lower than 3×10<SP>15</SP>atoms/cm<SP>3</SP>, the thickness of the epitaxial film is set to 20 μm or larger, and the surface roughness of the epitaxial film is set to 3 nm or smaller by arithmetic mean roughness (Ra), in a region which is 10 square μm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a silicon carbide wafer, particularly a cubic silicon carbide epitaxial wafer, which is intended to advantageously improve Schottky characteristics.

In recent years, as a semiconductor material used in fields such as electric power and communication, there is an increasing demand for low loss, high speed control, high temperature operation, and the like.
Looking at recent progress, it is approaching the material limit that cannot be exceeded by improvement by microfabrication, and silicon carbide (SiC) having a wide band gap is being developed with the aim of further improving the characteristics.
SiC is capable of controlling both p and n conductivity types, has a characteristic that the breakdown electric field is an order of magnitude higher than that of silicon (Si), and has a high saturation drift velocity and thermal conductivity. Therefore, it is expected as a material that can produce a large-capacity, low-loss power device that cannot be realized.

  SiC has a so-called polymorph that has the same composition and a different crystal structure depending on how the atoms are stacked. The type of SiC polymorph is represented by 3C, 4H, and the like. The capital letters “C” and “H” represent a crystal system, which means cubic and hexagonal, respectively. The numbers before “C” and “H” indicate the repetition period. For example, the stacking period of the unit stacking surface of 3C-SiC is 3 (ABCABBC...), And 4H-SiC is 4 (ABCBABCB...).

Cubic SiC is not a stable phase at a high temperature of 2000 ° C. or higher, that is, not a high-temperature polymorph, and is usually produced by epitaxial growth on a Si substrate at around 1000 ° C. Cubic SiC is advantageous in that it does not have the problem of a micropipe generated in hexagonal SiC, and the substrate can be easily increased in diameter.
However, when a Schottky barrier diode (SBD) is manufactured using cubic SiC, there is a problem that it is difficult to obtain good characteristics due to undulations on the surface of the epitaxial layer caused by crystal defects.
In the conventional report, when the dopant concentration of the epitaxial layer is set to 5 × 10 ¹⁵ atoms / cm ³ and the epitaxial film thickness is set to about 10 μm, the reverse breakdown voltage is 200 to 240 V in the case of the Ni electrode, and in the case of the Au electrode. The value of 140-180V is shown (refer nonpatent literature 1).
`` Y.Ishida et al. Mater.Sci.Forum, 338-342 (2000), 1235. ''

However, in other polymorphs, for example, 4H-SiC, SBDs having a higher reverse breakdown voltage have been developed (see Non-Patent Document 2).
Therefore, in order to use 3C-SiC as an SBD for practical use, a large reverse breakdown voltage comparable to or higher than 4H-SiC is required.
“Hiroyuki Matsunami, Semiconductor SiC Technology and Applications P.177-185”

  The present invention has been developed in view of the above situation, and provides a silicon carbide wafer that can obtain a large reverse breakdown voltage compared to the prior art when applied to SBD, that is, has excellent Schottky characteristics. With the goal.

As a result of intensive studies to achieve the above problems, the inventors have obtained the following knowledge.
That is, in order to increase the reverse breakdown voltage of the 3C-SiC epitaxial wafer,
(1) Control the dopant concentration and film thickness of the epitaxial film within an appropriate range.
(2) reduce the surface roughness of the epitaxial film,
I found it important.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. A silicon carbide wafer in which an epitaxial film of cubic silicon carbide is grown on the surface of a cubic silicon carbide substrate, the dopant concentration contained in the epitaxial film is 3 × 10 ¹⁵ atoms / cm ³ or less, and the thickness of the epitaxial film is A silicon carbide wafer characterized by having an arithmetic average roughness (Ra) of 3 nm or less in an area of 20 μm or more and an epitaxial film having a surface roughness of 10 μm square.

2. 2. The silicon carbide wafer according to claim 1, wherein the surface of the silicon carbide wafer is subjected to a treatment for removing a surface oxide film with hydrofluoric acid after the oxidation treatment.

3. 3. The silicon carbide wafer according to 1 or 2 above, wherein the dopant contained in the epitaxial film is nitrogen.

  According to the present invention, when a Schottky barrier diode (SBD) is manufactured using cubic SiC, the reverse breakdown voltage can be significantly improved as compared with the conventional case.

The present invention will be specifically described below.
In the present invention, the concentration of the dopant introduced into the epitaxial film is 3 × 10 ¹⁵ atoms / cm ³ or less because the depletion layer width is not sufficiently increased when the dopant concentration exceeds 3 × 10 ¹⁵ atoms / cm ^3. For this reason, it is difficult to obtain a reverse breakdown voltage of 600 V or more which is expected in a device using cubic SiC of the present invention. However, if the dopant concentration is too low, there is a disadvantage that it is difficult to prevent, for example, nitrogen contamination, which becomes a dopant in the epitaxial process, so the lower limit of the dopant concentration is about 1 × 10 ¹⁴ atoms / cm ³ . It is preferable. As a dopant, nitrogen or the like is advantageously adapted.

In addition, the thickness of the epitaxial film is set to 20 μm or more because if the film thickness is less than 20 μm, the depletion layer intended to expand with the dopant concentration of 3 × 10 ¹⁵ atoms / cm ³ or less This is because, since the expansion of the depletion layer is limited by the small epitaxial film thickness, it is difficult to obtain a reverse breakdown voltage of 600 V or more in a device using cubic SiC. However, if the epitaxial film thickness becomes too large, the epitaxial process takes a long time, which increases the cost. Therefore, the epitaxial film thickness is preferably about 150 μm or less.

Furthermore, the reason why the surface roughness is 3 nm or less in terms of arithmetic average roughness (Ra) in a 10 μm square area is as follows.
If the surface irregularity of the epitaxial film is large, when applied to SBD, electric field concentration occurs due to the surface irregularity, leading to a decrease in reverse breakdown voltage.
Therefore, in the present invention, asperities on the surface of the epitaxial film that are manifested due to stacking faults, twins, and facets such as {111} planes are removed as much as possible to obtain a smooth surface, thereby avoiding electric field concentration and in the reverse direction. It is necessary to realize a Schottky junction with a high breakdown voltage.
Therefore, in the present invention, the surface roughness of the epitaxial film is limited to 3 nm or less by the arithmetic average roughness (Ra) in a 10 μm square region.
Incidentally, mechanical polishing or chemical mechanical polishing is particularly advantageously adapted as a process for smoothing the surface of the epitaxial film.

  In addition, an epitaxial film whose surface is smoothed by mechanical polishing or chemical mechanical polishing is further oxidized to form an oxide film on the surface, and then the oxide film on the surface is removed by hydrofluoric acid. Alternatively, it is useful for removing processing strain that is introduced by chemical mechanical polishing and may remain. Such processing strain causes a decrease in reverse breakdown voltage.

Next, the suitable manufacturing method of the cubic silicon carbide epitaxial wafer of this invention is demonstrated.
The substrate to be used for growing the cubic silicon carbide epitaxial film of the present invention is not particularly limited. For example, a {100} single crystal provided with an off angle of about 2 to 4 ° in the <110> direction. A material obtained by heteroepitaxially growing cubic silicon carbide on a silicon substrate or a {100} single crystal silicon substrate provided with a relief pattern can be used. Examples of the undulation pattern include a plurality of undulation patterns extending parallel to one position. Thus, by providing an off-angle on the surface of the substrate to be grown, it is possible to easily reduce the defect density called APB (Anti-Phase Boundary) generated in the substrate to be grown.

Here, in order to form an undulation pattern on a substrate made of single crystal silicon, for example, polishing can be used, and by changing the size of the abrasive grain size and the processing pressure in the polishing, It is possible to control the depth.
Next, the above silicon substrate is carbonized by raising the temperature to 1050-1300 ° C. in an atmosphere in which propane, ethylene or acetylene is added to hydrogen. A silicon carbide film (3C—SiC) is manufactured by a growth process in which epitaxial growth is continuously performed on the carbonized substrate thus obtained. In this manufacturing process, hydrogen is used as a carrier gas, silane (SiH ₄ ), dichlorosilane (SiH ₂ Cl ₂ ), or trichlorosilane (SiHCl ₃ ) is used as a Si source gas, and propane (C ₃ H ₈ ) or ethylene (C ₂ ). It is preferable to heteroepitaxially grow between 1050 and 1415 ° C. using H ₄ ) or acetylene (C ₂ H ₂ ) as a C source gas.
By removing the Si substrate by etching or the like from the cubic silicon carbide heteroepitaxial substrate thus obtained, a growth substrate used in the present invention can be obtained.

In the present invention, when cubic silicon carbide is epitaxially grown on a cubic silicon carbide substrate which is a growth substrate produced as described above, the dopant concentration of the epitaxial film is 3 × 10 ¹⁵ atoms / cm ³ or less, It is also important to grow the epitaxial film so that its thickness is 20 μm or more.
The epitaxial growth step in the present invention is performed by supplying a source gas and a carrier gas at a predetermined flow rate into a chamber of an epitaxial growth apparatus and performing epitaxial growth. For example, silane (SiH ₄ ), dichlorosilane (SiH ₂ Cl ₂ ), or trichlorosilane (SiHCl ₃ ) is used as a Si source gas, and propane (C ₃ H ₈ ), ethylene (C ₂ H ₄ ), or acetylene (C ₂ H). ₂ ) can be used as the C source gas and hydrogen as the carrier gas.

  The substrate temperature during epitaxial growth is preferably about 1500 to 1800 ° C. This is because when the substrate temperature is lower than 1500 ° C., the growth rate is reduced and it takes a long time to grow a film thickness of 20 μm or more, whereas when it is higher than 1800 ° C., the hexagonal crystal is a high-temperature polymorph of SiC. This is because it is not preferable because of the mixing of the system SiC.

Furthermore, unevenness is evident on the surface after epitaxial growth due to facets such as stacking faults, twins, and {111} planes. This unevenness causes electric field concentration in the Schottky barrier diode, and causes a decrease in reverse breakdown voltage. Therefore, in the present invention, the surface roughness of the epitaxial film is reduced to 3 nm or less in terms of arithmetic average roughness (Ra) in a 10 μm square region.
The surface polishing method is not particularly limited, but a method of performing mechanical polishing with diamond abrasive grains or chemical chemical polishing using colloidal silica after that is particularly advantageous.

Note that some processing strain may remain after mechanical polishing or chemical mechanical polishing as described above. Such processing strain may cause a decrease in reverse breakdown voltage, so it is desirable to remove it.
As a method for removing such processing strain, a method of removing the surface oxide film with hydrofluoric acid after oxidizing the surface of the epitaxial film is preferable.

Thus, on the surface of the cubic silicon carbide substrate, the arithmetic average roughness (Ra) in the region where the dopant concentration is 3 × 10 ¹⁵ atoms / cm ³ or less, the film thickness is 20 μm or more, and the film surface roughness is 10 μm square. An epitaxial film of 3 nm or less can be formed.

A 2-inch n-type (100) cubic silicon carbide substrate was used as the growth substrate. Nitrogen is used as a dopant for the substrate, and its concentration is 3 × 10 ¹⁸ atoms / cm ³ . N-type cubic silicon carbide is epitaxially grown on this substrate by using 8 sccm of silane (SiH ₄ ) as the Si source gas, ₄ sccm of propane (C ₃ H ₈ ) as the C source gas, and hydrogen gas as the carrier gas. It was. Nitrogen gas was used as the dopant gas, and epitaxial films having various dopant concentrations and thicknesses were formed. Next, the surface of the epitaxial film was mechanically polished using a diamond slurry of 6 μm to 0.5 μm, and then chemical mechanical polishing was performed using a slurry containing colloidal silica.
Thereafter, heat treatment was performed at 1150 ° C. for 90 minutes in an oxygen atmosphere to oxidize the surface of the epitaxial film, and then the oxide film was removed using hydrofluoric acid.
In addition, an Au electrode with a diameter of 200 μm is deposited on the front surface as a Schottky electrode, and an ohmic electrode is formed by depositing Al on the back surface. did.
Table 1 shows the results of investigating the film thickness, dopant concentration, and reverse breakdown voltage of the epitaxial film. In the table, “surface roughness” is an arithmetic average roughness (Ra) evaluated in an area of 10 μm square using an AFM (atomic force microscope). In Table 1, No. 19 is an example in which the surface oxide film is not removed by oxidation treatment-hydrofluoric acid, and No. 20 is chemical mechanical polishing treatment (oxidation treatment-hydrofluoric acid). This is an example in which both of the surface oxide film removal treatment by (2) were not performed.

As shown in the table, the dopant concentration was reduced from 1 × 10 ¹⁷ atoms / cm ³ to 5 × 10 ¹⁵ atoms / cm ³ and the epitaxial film thickness was 30 μm, as in Nos. 1 to 7 (comparative example). It can be seen that the reverse breakdown voltage reaches a peak at about 500V even when the voltage is increased to about 500V. In addition, as shown in No. 8 (comparative example), even if the dopant concentration is reduced to 3 × 10 ¹⁵ atoms / cm ^3, if the epitaxial film thickness is less than 20 μm, good reverse breakdown voltage can still be obtained. I can't.
In contrast, as shown in Nos. 9 to 11 and 13 to 18 (invention examples), the dopant concentration was reduced to 3 × 10 ¹⁵ atoms / cm ³ or less, and the epitaxial film thickness was increased to 20 μm or more. In some cases, a reverse breakdown voltage exceeding 600V could be obtained. In particular, as shown in Nos. ¹⁵ to 18, when the dopant concentration is reduced to 1 × 10 ¹⁵ atoms / cm ³ and the epitaxial film thickness is increased to 40 μm or more, a reverse breakdown voltage exceeding 1200 V is obtained. I was able to. Furthermore, as shown in Nos. 16 to 18, when the epitaxial film thickness was 50 μm or more, a very high reverse breakdown voltage exceeding 1500 V could be obtained.
As shown in No. 12 (comparative example), even if the dopant concentration is as low as 1 × 10 ¹⁵ atoms / cm ³ , if the epitaxial film thickness is less than 20 μm, only a reverse breakdown voltage of about 300 V is obtained. I couldn't.

In addition, as shown in No. 19 (invention example), even if chemical mechanical polishing is performed, if the surface oxide film is not removed by oxidation treatment-hydrofluoric acid, the reverse direction is applied. Although the withstand voltage decreases somewhat, satisfactory reverse withstand voltages of 540 V are obtained in each case.
Furthermore, as shown in No. 20 (comparative example), not only (oxidation treatment-removal treatment of surface oxide film with hydrofluoric acid) but also chemical mechanical polishing treatment, the reverse of about 150V Only directional pressure resistance was obtained.

Claims (3)

A silicon carbide wafer in which an epitaxial film of cubic silicon carbide is grown on the surface of a cubic silicon carbide substrate, the dopant concentration contained in the epitaxial film is 3 × 10 ¹⁵ atoms / cm ³ or less, and the thickness of the epitaxial film is A silicon carbide wafer characterized by having an arithmetic average roughness (Ra) of 3 nm or less in an area of 20 μm or more and an epitaxial film having a surface roughness of 10 μm square.   2. The silicon carbide wafer according to claim 1, wherein the surface of the silicon carbide wafer is subjected to a removal treatment of a surface oxide film with hydrofluoric acid after the oxidation treatment. 3.   The silicon carbide wafer according to claim 1 or 2, wherein the dopant contained in the epitaxial film is nitrogen.