JP7528681B2 - Method for polishing SiC polycrystalline substrate - Google Patents

Description

The present invention relates to a method for polishing a SiC polycrystalline substrate, which polishes the main surface of a SiC polycrystalline substrate that has been subjected to grinding, and in particular to an improved polishing method that can flatten the main surface of a SiC polycrystalline substrate to near atomic size.

Compared to silicon (Si), silicon carbide (SiC) has a band gap three times larger (4H-SiC: 3.8 eV, 6H-SiC: 3.1 eV, silicon: 1.1 eV) and a high thermal conductivity (5 W/cm K, silicon: 1.5 W/cm K). In recent years, SiC single crystals have begun to be used as a substrate material for power devices. For example, compared to conventional Si power devices, SiC power devices have a withstand voltage 5 to 10 times larger and an operating temperature several hundred degrees higher. Furthermore, they can reduce the power loss of the element to about one-tenth, so they have been put to practical use in inverters for railway vehicles, etc.

SiC single crystals used as substrate materials are usually produced by a gas phase method called sublimation recrystallization (modified Lely process) (see, for example, Non-Patent Document 1), and are processed to the desired diameter and thickness.

The modified Lely process is a method for growing bulk SiC single crystals by heating and sublimating solid SiC raw material (usually powder) at high temperatures (2,400°C or higher), and transporting the sublimated Si and carbon atoms as vapor at 2,400°C through an inert gas atmosphere by diffusion, where they become supersaturated and recrystallize on a seed crystal placed at a lower temperature than the raw material.

However, because the process temperature in the modified Lely process is extremely high at over 2,400°C, it is extremely difficult to control the temperature and convection during crystal growth, and to control crystal defects. SiC single crystal substrates produced by this method contain numerous crystal defects called micropipes and other crystal defects (stacking faults, etc.), making it extremely difficult to manufacture high-quality SiC single crystal substrates that can withstand electronic device use with a high yield. As a result, high-quality SiC single crystal substrates with few crystal defects that can be used for electronic devices are very expensive, and devices using such SiC single crystal substrates are also expensive, which has hindered the widespread use of SiC single crystal substrates.

In response to this, in recent years, a method has been proposed in which a SiC substrate is manufactured by preparing a SiC single crystal substrate and a SiC polycrystalline substrate, bonding the SiC single crystal substrate and the SiC polycrystalline substrate together, and then thinning the SiC single crystal substrate to produce a SiC substrate in which a thin SiC single crystal layer is formed on the SiC polycrystalline substrate (see, for example, Non-Patent Document 2).

This method for manufacturing SiC substrates allows the thickness of SiC single crystal substrates to be reduced by several times to several hundred times compared to conventional methods. This allows the cost of SiC substrates to be significantly reduced compared to conventional methods in which the entire substrate is made of expensive, high-quality SiC single crystals. In addition, because elements such as power devices can be formed on a high-quality SiC single crystal layer with few crystal defects, device performance and manufacturing yields can be significantly improved.

In the process of bonding such a SiC single crystal substrate to a SiC polycrystalline substrate, the SiC polycrystalline substrate is required to be dense, highly pure, and highly flat.

In order to obtain a semiconductor substrate having a highly flat mirror surface, a single crystal is generally grown by a growth method such as the Czochralski (Cz) method or the Vertical Gradient Freezing (VGF) method, and then the crystal is processed into a cylindrical shape and cut into a substrate of a specified thickness using a multi-wire saw. However, the cut surface of the substrate has a process-induced layer containing microcracks and dislocations. For this reason, after cutting out the substrate, it is necessary to remove the process-induced layer and waviness on the substrate surface by further performing processes such as beveling, double-sided grinding (lapping), polishing, and cleaning. In particular, a method of polishing compound semiconductor crystal materials such as silicon (Si) and gallium arsenide is known in which an alkaline aqueous polishing slurry with a pH of about 9 to 12 in which colloidal silica is dispersed is supplied between the polishing pad and the workpiece while performing chemical mechanical polishing (CMP) (Patent Documents 1 to 3).

However, silicon carbide (SiC) is a much more chemically stable material than silicon (Si), and is also the third hardest material after diamond and boron carbide. As such, just like the silicon substrates mentioned above, polishing using colloidal silica after grinding (lapping) alone requires a considerable amount of time to obtain a highly flat mirror surface.

In order to shorten the polishing time, methods have been proposed, such as mechanical polishing of silicon carbide (SiC) using a polishing solution containing diamond abrasive grains with an average particle size of 1 to 3 μm (Patent Document 4), a method of shortening the polishing time by changing the surface of SiC into a mechanically brittle oxide or compound (Patent Document 5), specifically, a method of chemical mechanical polishing in the presence of an oxidizing agent (hydrogen peroxide solution) on the polishing surface, and a method of chemical mechanical polishing by adding catalytic chromium (III) oxide.

Special Publication No. 7-12590 Japanese Patent Application Publication No. 10-308379 Japanese Patent Application Publication No. 11-31675 Japanese Patent Application Publication No. 8-323604 JP 2001-205555 A

Yu. M. Tairov and V. F. Tsvetkov: J. of Cryst. Growth 43 (1978) 209 Journal of the Japan Society for Precision Engineering, 2017, Vol. 83, No. 9, pp. 833-836

In the mechanical polishing of SiC single crystals using diamond abrasive grains as described in Patent Document 4, so-called linear scratches usually occur. These linear scratches are caused by the surface of the SiC single crystal being scratched by coarse particles of diamond abrasive grains, SiC polishing debris, the convex parts of the table used during processing, etc., and are accompanied by the above-mentioned processing-affected layer. However, these linear scratches, except for particularly deep ones, can be removed by the chemical mechanical polishing (CMP) as described in Patent Documents 1 to 3 above.

On the other hand, in the case of SiC polycrystals, mechanical polishing with diamond abrasive grains produces the linear scratches described above, just as in the case of SiC single crystals. However, in addition to the linear scratches, mechanical polishing with SiC polycrystals can also produce pit-like defects. Because SiC polycrystals are an aggregate of tiny single crystal grains, when an overload is applied during mechanical polishing, the single crystal grains are further destroyed or parts of them fall off, resulting in pit-like defects. Even if chemical mechanical polishing (CMP) is performed with pit-like defects, once pit-like defects are produced, the chemical elements act strongly from the starting point, making it impossible to polish the surface flat.

Furthermore, unlike single crystal SiC, polycrystalline SiC has a different growth orientation for each single crystal grain. Silicon carbide (SiC) is a compound composed of two elements, silicon atoms and carbon atoms, which have significantly different atomic radii, but carbon is known to be more susceptible to oxidation than silicon. For this reason, speed differences due to chemical mechanical polishing (CMP) are likely to occur depending on the crystal orientation and crystal system, the amount of polishing differs for each single crystal grain, and unevenness (grain boundary steps) occurs along the grain boundaries.

For these reasons, it has been difficult to flatten the surface of polycrystalline SiC to near atomic size using the mechanical polishing method described in Patent Document 4, which uses a polishing solution containing diamond abrasive grains with an average particle size of 1 to 3 μm, and the chemical mechanical polishing (CMP) method described in Patent Document 5, which simply uses an oxidizing agent or catalyst.

The present invention was made with a focus on the above problems, and its objective is to provide a polishing method that can flatten the main surface of a SiC polycrystalline substrate to near atomic size.

The inventors conducted the following technical studies to solve the above problems.

First, silicon carbide (SiC) is a material that is said to be the third hardest after diamond and boron carbide. The bond between silicon atoms and carbon atoms is extremely strong, but some of the bonds are broken on the crystal surface. For this reason, when an oxidizing agent, a catalyst, etc. are added to colloidal silica and chemical mechanical polishing (CMP) described in Patent Document 5 is performed on a SiC polycrystalline substrate, the broken carbon atoms are preferentially chemically bonded with oxygen atoms and vaporized and desorbed. It is believed that the reaction in which the remaining silicon atoms are then dissolved in colloidal silica and desorbed occurs repeatedly on the crystal surface. Therefore, if silicon atoms and carbon atoms, which have different processes for desorption from SiC polycrystalline, can both desorb at similar speeds, it is believed that the above-mentioned unevenness (grain boundary steps) along the grain boundaries will be less likely to occur even if the crystal orientation and crystal system are different.

However, even if various measures are implemented, the rate at which silicon atoms are desorbed cannot keep up with the rate at which carbon atoms are desorbed. For this reason, it is necessary to minimize the processing time of chemical mechanical polishing (CMP) and to suppress the occurrence of the above-mentioned grain boundary steps due to differences in polishing speed.

It has been found that a good method for reducing the processing time of chemical mechanical polishing (CMP) is to smooth the surface roughness of the main surface of the SiC polycrystalline substrate measured by the fringe scan method to 0.7 nm or less in terms of arithmetic mean height Sa specified in ISO 25178 by mechanical polishing before chemical mechanical polishing (CMP), and then perform chemical mechanical polishing (CMP) under appropriate polishing conditions. That is, in the polishing process of SiC polycrystalline substrates, which has been considered very difficult to polish ultra-smoothly, the inventor first performed mechanical polishing using the synergistic action of diamond abrasive grains and a urethane resin pad, and then performed chemical mechanical polishing (CMP) under conditions in which the pH of the polishing liquid, the main component of which is colloidal silica, is optimized, thereby discovering that the main surface of the SiC polycrystalline substrate can be flattened to near atomic size. The present invention was completed based on such technical discoveries.

That is, the first invention according to the present invention is,
1. A method for polishing a SiC polycrystalline substrate, comprising polishing a main surface of a SiC polycrystalline substrate that has been subjected to grinding, comprising:
A first step of mechanically polishing a SiC polycrystalline substrate by dropping a diamond slurry containing diamond abrasive grains having an average grain size of 0.5 μm to 1.1 μm onto a urethane resin pad until the surface roughness of the main surface of the SiC polycrystalline substrate measured by a fringe scan method is 0.7 nm or less in terms of arithmetic mean height Sa defined in ISO 25178;
A second step of chemically and mechanically polishing the surface with a urethane resin pad using an aqueous colloidal silica polishing solution adjusted to a pH of 6.7 to 9.2;
The present invention is characterized by having the following.

Next, the second invention according to the present invention is
In the method for polishing a SiC polycrystalline substrate according to the first aspect of the present invention,
In the second step, the pH of the aqueous colloidal silica polishing liquid is adjusted to 7.7;
The third invention is
In the method for polishing a SiC polycrystalline substrate according to the first or second invention,
In the second step, the SiC polycrystalline substrate is chemically and mechanically polished until the surface roughness of the main surface measured by a fringe scan method is 0.5 nm or less in terms of arithmetic mean height Sa defined in ISO 25178;
The fourth invention is
In the method for polishing a SiC polycrystalline substrate according to the first, second or third invention,
The surface roughness of the main surface of the ground SiC polycrystalline substrate, measured by a fringe scan method, is 3 nm or less in terms of arithmetic mean height Sa defined in ISO 25178.

According to the method for polishing a SiC polycrystalline substrate of the present invention,
In the first step before chemical mechanical polishing, the SiC polycrystalline substrate is mechanically polished until the surface roughness of the main surface, measured by the fringe scan method, is 0.7 nm or less in terms of arithmetic mean height Sa defined in ISO 25178. This reduces the processing time of the chemical mechanical polishing in the second step, and makes it possible to suppress the occurrence of grain boundary steps due to differences in polishing speed resulting from differences in crystal orientation or crystal system.

This has the effect of flattening the main surface of the SiC polycrystalline substrate to nearly atomic size.

FIG. 2 is a graph showing the relationship between the average particle size (μm) of diamond abrasive grains used in the mechanical polishing in the first step and the surface roughness (Sa) of the SiC polycrystalline substrate after mechanical polishing. FIG. 2 is a graph showing the relationship between the pH of the colloidal silica aqueous polishing liquid used in the chemical mechanical polishing in the second step and the surface roughness (Sa) of the SiC polycrystalline substrate after chemical mechanical polishing.

The following describes an embodiment of the present invention in detail.

The method for polishing a SiC polycrystalline substrate according to the present invention comprises the steps of:
1. A method for polishing a SiC polycrystalline substrate, comprising polishing a main surface of a SiC polycrystalline substrate that has been subjected to grinding, comprising:
A first step is to drop a diamond slurry containing diamond abrasive grains having an average particle size of 0.5 μm to 1.1 μm onto a urethane resin pad, and mechanically polish the SiC polycrystalline substrate until the surface roughness of the main surface measured by a fringe scan method (a method using an optical surface roughness measuring device) is 0.7 nm or less in terms of arithmetic mean height Sa defined in ISO 25178;
A second step of chemically and mechanically polishing the surface with a urethane resin pad using an aqueous colloidal silica polishing solution adjusted to a pH of 6.7 to 9.2;
The present invention is characterized in that it has the following features.

(1) Average particle size of diamond abrasive grains used in the first mechanical polishing step FIG. 1 is a graph showing the relationship between the average particle size (μm) of diamond abrasive grains used in the first mechanical polishing step and the surface roughness (Sa) of the SiC polycrystalline substrate after mechanical polishing.

When the average grain size of the diamond abrasive grains exceeds 1 μm, as shown in FIG. 1, the smaller the average grain size of the diamond abrasive grains, the smaller the surface roughness (Sa) of the SiC polycrystalline substrate after mechanical polishing.

However, when the average grain size of the diamond abrasive grains is 1 μm or less, it was found that the smaller the average grain size of the diamond abrasive grains, the greater the surface roughness (Sa) of the SiC polycrystalline substrate after mechanical polishing, as shown in Figure 1. The reason for this is presumably that as the average grain size of the diamond abrasive grains becomes smaller, the cutting ability decreases and the substrate becomes more susceptible to brittle fracture, destroying the single crystal grains of the SiC polycrystalline substrate, or causing pit-like defects by partly dropping off the single crystal grains, thereby increasing the surface roughness (Sa).

The graph in Figure 1 confirms that the average particle size of diamond abrasive grains that can set the surface roughness (Sa) of the SiC polycrystalline substrate after the first mechanical polishing step to 0.7 nm or less is 0.5 μm to 1.1 μm.

(2) pH of the colloidal silica aqueous polishing solution used in the second step of chemical mechanical polishing
FIG. 2 is a graph showing the relationship between the pH of the colloidal silica aqueous polishing liquid used in the chemical mechanical polishing in the second step and the surface roughness (Sa) of the SiC polycrystalline substrate after chemical mechanical polishing.

The second step, chemical mechanical polishing, is a process for removing the altered layer and linear scratches that occurred during the first step of mechanical polishing, and it is desirable to make the surface roughness (Sa) of the SiC polycrystalline substrate 0.5 nm or less by making the desorption rates (polishing rates) of the silicon and carbon that make up the SiC polycrystalline substrate as close as possible.

The pH of the colloidal silica aqueous polishing solution that can set the surface roughness (Sa) of the SiC polycrystalline substrate to 0.5 nm or less by the chemical mechanical polishing in the second step is confirmed to be pH 6.7 to 9.2, from the graph in Figure 2, and it is confirmed that a pH of around 7.5, that is, pH 7.7, is particularly preferable.

It has been confirmed that if the pH falls outside the range of 6.7 to 9.2, grain boundary steps will appear on the surface of the SiC polycrystalline substrate, and the surface roughness (Sa) will increase.

(3) Surface roughness (Sa) of the ground SiC polycrystalline substrate
The surface roughness of the main surface of the ground SiC polycrystalline substrate, measured by a fringe scan method, is desirably 3 nm or less in terms of arithmetic mean height Sa defined in ISO 25178.

The reason for this is that if the surface roughness (Sa) of the ground SiC polycrystalline substrate exceeds 3 nm, when a diamond slurry containing diamond abrasive grains is dropped onto a urethane resin pad to perform mechanical polishing, the polishing time required to reduce the surface roughness (Sa) of the SiC polycrystalline substrate to 0.7 nm or less becomes extremely long.

The following is a detailed explanation of the present invention, including comparative examples.

[Example 1]
(Grinding)
A 6-inch diameter SiC polycrystalline substrate was ground by a precision grinder using a diamond grinding wheel with a grit size of 7000 (ISO 8486 designation). The surface roughness of the ground SiC polycrystalline substrate (arithmetic mean height Sa defined in ISO 25178, the same applies below) was measured by the fringe scan method, and the average of five points on the substrate (four points on the periphery of the substrate and one point at the center of the substrate) was 2.3 nm.

(First step: mechanical polishing)
Next, a diamond slurry in which diamond abrasive grains having an average grain size of 1.0 μm were mixed at a concentration of 2 g/L in an aqueous solution having a viscosity of 3×10 ⁻³ Pa·S was added at a constant rate of 10 ml/min. The solution was dropped onto a platen, and mechanical polishing (diamond lapping) was carried out at a platen rotation speed of 80 rpm, a head rotation speed of 64 rpm, and a pressure of 30 kPa.

The platen used was made of stainless steel and had a urethane resin pad (Shore D hardness: 75) attached to it.

As a result, the large linear scratches that had been generated by the grinding process using the precision grinder disappeared after 30 minutes of mechanical polishing, and when the surface roughness (Sa) of the mechanically polished SiC polycrystalline substrate was measured using the fringe scan method, the average of five points on the substrate (four points on the periphery of the substrate and one point at the center of the substrate) was 0.63 nm (0.7 nm or less).

(Second process: chemical mechanical polishing)
Next, an aqueous polishing solution of colloidal silica (silica weight ratio: 5 wt%) with an average particle size of 25 nm, adjusted to pH 7.7 by adding sodium hydroxide and hydrogen peroxide, was applied to the surface of the platen at a rate of 100 ml/min. Then, chemical mechanical polishing (CMP) was carried out for 20 minutes at a platen rotation speed of 40 rpm, a head rotation speed of 32 rpm, and a pressure of 40 kPa.

The platen used was made of stainless steel and had a urethane resin pad (Shore D hardness: 63) attached to it.

Next, the surface roughness (Sa) of the chemically mechanically polished (CMP) SiC polycrystalline substrate was measured using the fringe scan method, and it was found to be 0.44 nm at the center of the substrate, 0.38 nm at the periphery of the substrate, and 0.42 nm at the midpoint between the center and periphery of the substrate, confirming that the surface roughness (Sa) of the entire substrate was 0.5 nm or less.

These results are shown in Table 1.

[Example 2]
Grinding using a precision grinder and mechanical polishing (diamond lapping) were performed under the same conditions as in Example 1, and chemical mechanical polishing (CMP) was performed for 20 min under the same conditions as in Example 1, except that the pH of the polishing solution during CMP was 9.2.

The surface roughness (Sa) of the chemically mechanically polished (CMP) SiC polycrystalline substrate was measured using the fringe scan method, and was found to be 0.50 nm at the center of the substrate, 0.45 nm at the periphery of the substrate, and 0.48 nm midway between the center and periphery of the substrate, confirming that the surface roughness (Sa) of the entire substrate was 0.5 nm or less.

These results are also shown in Table 1.

[Example 3]
Grinding using a precision grinder and chemical mechanical polishing (CMP) in the second step were carried out for 20 min under the same conditions as in Example 1, and mechanical polishing (diamond lapping) was carried out under the same conditions as in Example 1, except that the average particle size of the diamond abrasive grains in the first step of mechanical polishing (diamond lapping) was set to 0.5 μm.

The surface roughness (Sa) of the mechanically polished (diamond lapped) SiC polycrystalline substrate was measured using the fringe scan method, and the average of five points on the substrate (four points on the periphery and one point at the center of the substrate) was 0.68 nm (less than 0.7 nm).

Furthermore, the surface roughness (Sa) of the chemically mechanically polished (CMP) SiC polycrystalline substrate was measured using the fringe scan method, and was found to be 0.49 nm at the center of the substrate, 0.47 nm at the periphery of the substrate, and 0.48 nm at the midpoint between the center and periphery of the substrate, confirming that the surface roughness (Sa) of the entire substrate was 0.5 nm or less.

These results are also shown in Table 1.

[Comparative Example 1]
Grinding using a precision grinder and mechanical polishing (diamond lapping) were performed under the same conditions as in Example 1, and chemical mechanical polishing (CMP) was performed for 20 min under the same conditions as in Example 1, except that the pH of the polishing solution during CMP was 10.2.

The surface roughness (Sa) of the chemically mechanically polished (CMP) SiC polycrystalline substrate was measured using the fringe scan method, and was found to be 0.68 nm at the center of the substrate, 0.61 nm at the periphery of the substrate, and 0.63 nm at the midpoint between the center and periphery of the substrate, and the surface roughness (Sa) was not less than 0.5 nm at any point on the substrate.

These results are also shown in Table 1.

[Comparative Example 2]
Grinding using a precision grinder and mechanical polishing (diamond lapping) were performed under the same conditions as in Example 1, and chemical mechanical polishing (CMP) was performed for 20 min under the same conditions as in Example 1, except that the pH of the polishing solution during CMP was set to 5.4.

The surface roughness (Sa) of the chemically mechanically polished (CMP) SiC polycrystalline substrate was measured using the fringe scan method, and the in-plane average surface roughness (Sa) was 1.10 nm, and the surface roughness (Sa) was not less than 0.5 nm at any point on the substrate.

These results are shown in Table 2.

[Comparative Example 3]
Grinding using a precision grinder and mechanical polishing (diamond lapping) were performed under the same conditions as in Example 1, and chemical mechanical polishing (CMP) was performed for 20 minutes under the same conditions as in Example 1, except that the pH of the polishing solution during CMP was 6.3.

The surface roughness (Sa) of the chemically mechanically polished (CMP) SiC polycrystalline substrate was measured using the fringe scan method, and the in-plane average surface roughness (Sa) was 0.60 nm, and the surface roughness (Sa) was not less than 0.5 nm at any point on the substrate.

These results are also shown in Table 2.

[Comparative Example 4]
Grinding using a precision grinder and mechanical polishing (diamond lapping) were performed under the same conditions as in Example 1, and chemical mechanical polishing (CMP) was performed for 20 minutes under the same conditions as in Example 1, except that the pH of the polishing solution during CMP was 12.3.

The surface roughness (Sa) of the chemically mechanically polished (CMP) SiC polycrystalline substrate was measured using the fringe scan method, and the in-plane average surface roughness (Sa) was 0.94 nm, and the surface roughness (Sa) was not less than 0.5 nm at any point on the substrate.

These results are also shown in Table 2.

[Comparative Example 5]
After grinding was performed using a precision grinder under the same conditions as in Example 1, mechanical polishing (diamond lapping) was performed under the same conditions as in Example 1, except that the average particle size of the diamond abrasive grains in the first mechanical polishing (diamond lapping) step was set to 0.2 μm.

The surface roughness (Sa) of the mechanically polished (diamond lapped) SiC polycrystalline substrate was measured using the fringe scan method, and the average of five points on the substrate (four points on the periphery and one point at the center of the substrate) was 0.81 nm, and did not fall below 0.7 nm.

Therefore, the mechanical polishing (diamond lapping) was performed for an additional 30 min, but the surface roughness (Sa) remained almost unchanged at 0.84 nm, averaged over five points on the substrate (four points on the periphery and one point at the center).

Then, the SiC polycrystalline substrate that had been mechanically polished (diamond lapping) for an additional 30 minutes was subjected to the second process of chemical mechanical polishing (CMP) for 20 minutes, and the surface roughness (Sa) of the chemically mechanically polished (CMP) SiC polycrystalline substrate was measured using the fringe scan method. The in-plane average surface roughness (Sa) was 0.78 nm, and the surface roughness (Sa) was not less than 0.5 nm at any point on the substrate.

These results are also shown in Table 2.

[Comparative Example 6]
After grinding was performed using a precision grinder under the same conditions as in Example 1, mechanical polishing (diamond lapping) was performed under the same conditions as in Example 1, except that the average particle size of the diamond abrasive grains in the first mechanical polishing (diamond lapping) step was set to 2.0 μm.

The surface roughness (Sa) of the mechanically polished (diamond lapped) SiC polycrystalline substrate was measured using the fringe scan method, and the average of five points on the substrate (four points on the periphery and one point at the center of the substrate) was 2.4 nm, which was not less than 0.7 nm.

Then, the SiC polycrystalline substrate that had been mechanically polished (diamond wrapping) was subjected to the second step of chemical mechanical polishing (CMP) for 20 minutes, and the surface roughness (Sa) of the chemically mechanically polished (CMP) SiC polycrystalline substrate was measured using the fringe scan method. The in-plane average surface roughness (Sa) was 1.20 nm, and the surface roughness (Sa) was not less than 0.5 nm at any point on the substrate.

These results are also shown in Table 2.

[Comparative Example 7]
After grinding was performed using a precision grinder under the same conditions as in Example 1, mechanical polishing (diamond lapping) was performed under the same conditions as in Example 1, except that the average particle size of the diamond abrasive grains in the first mechanical polishing (diamond lapping) step was 4.0 μm.

The surface roughness (Sa) of the mechanically polished (diamond lapped) SiC polycrystalline substrate was measured by a fringe scan method, and the average of five points on the substrate (four points on the periphery of the substrate and one point at the center of the substrate) was 5.3 nm, which did not become 0.7 nm or less. The SiC polycrystalline substrate that had been mechanically polished (diamond lapped) was subjected to the second step of chemical mechanical polishing (CMP) for 20 minutes, and the surface roughness (Sa) of the chemically mechanically polished (CMP) SiC polycrystalline substrate was measured by a fringe scan method, and the in-plane average of the surface roughness (Sa) was 3.90 nm, and the surface roughness (Sa) at no point on the substrate was 0.5 nm or less.

These results are also shown in Table 2.

The method of the present invention can flatten the main surface of a SiC polycrystalline substrate to near atomic size, and therefore has industrial applicability as a method for polishing SiC polycrystalline substrates used in the manufacture of SiC substrates.

Claims (4)

1. A method for polishing a SiC polycrystalline substrate, comprising polishing a main surface of a SiC polycrystalline substrate that has been subjected to grinding, comprising:
A first step of mechanically polishing a SiC polycrystalline substrate by dropping a diamond slurry containing diamond abrasive grains having an average grain size of 0.5 μm to 1.1 μm onto a urethane resin pad until the surface roughness of the main surface of the SiC polycrystalline substrate measured by a fringe scan method is 0.7 nm or less in terms of arithmetic mean height Sa defined in ISO 25178;
A second step of chemically and mechanically polishing the surface with a urethane resin pad using an aqueous colloidal silica polishing solution adjusted to a pH of 6.7 to 9.2;
A method for polishing a SiC polycrystalline substrate, comprising the steps of: The method for polishing a SiC polycrystalline substrate according to claim 1, characterized in that in the second step, the pH of the aqueous colloidal silica polishing liquid is adjusted to 7.7. The method for polishing a SiC polycrystalline substrate according to claim 1 or 2, characterized in that in the second step, the SiC polycrystalline substrate is chemically and mechanically polished until the surface roughness of the main surface measured by the fringe scan method is 0.5 nm or less in terms of the arithmetic mean height Sa specified in ISO 25178. The method for polishing a SiC polycrystalline substrate according to claim 1, 2 or 3, characterized in that the surface roughness of the main surface of the ground SiC polycrystalline substrate measured by the fringe scan method is 3 nm or less in terms of the arithmetic mean height Sa defined in ISO 25178.