JP7433170B2 - Wafer polishing method and wafer polishing device - Google Patents

Description

The present invention relates to a wafer polishing method and a wafer polishing apparatus.

Patent Documents 1 and 2 disclose methods for simultaneously polishing both sides of a wafer of Si, SiC, etc. In these wafer polishing methods, the first surface of the wafer is polished by relatively moving the first surface of the wafer and the first polishing pad under a predetermined surface pressure, and the second surface, which is the back side of the first surface of the wafer, is polished. The second surface is polished by moving the second polishing pad relative to the second polishing pad. At this time, a polishing liquid containing polishing particles is interposed between the first surface of the wafer and the first polishing pad and between the second surface of the wafer and the second polishing pad.

Specifically, in the wafer polishing method disclosed in Patent Document 1, the frictional resistance of the first polishing pad against the first surface of the wafer and the frictional resistance of the second polishing pad against the second surface of the wafer are made different. There is. More specifically, a hard urethane foam is used as the first polishing pad, and a soft nonwoven fabric pad made by impregnating and hardening a urethane resin in a nonwoven fabric is used as the second polishing pad. The frictional resistance of the first polishing pad is smaller than the frictional resistance of the second polishing pad. The polishing liquid used is an alkaline etching liquid in which colloidal silica as polishing particles is dispersed. According to this wafer polishing method, both sides of the wafer can be polished simultaneously, improving wafer manufacturing efficiency. Further, according to this wafer polishing method, it is possible to stabilize the rotation direction and speed of the wafer, and it is said that the wafer can be highly flattened.

Further, the wafer polishing method disclosed in Patent Document 2 polishes a SiC wafer having an epitaxial layer stacked on the Si surface side. It is known that in a CMP (Chemical Mechanical Polishing) method, SiC wafers have a lower polishability on the Si surface than on the C surface. In this wafer polishing method, in the CMP method, a polishing liquid containing alumina as polishing particles is employed, and under a predetermined surface pressure and relative movement speed, the polishing efficiency for the Si surface of the SiC wafer is set to 0.5 to 3. 0 (μm/hr), and the polishing efficiency for the C-plane of the SiC wafer is 3 to 15 (μm/hr). More specifically, as a preliminary investigation, the Si surface and C surface of the wafer were polished separately by CMP using different polishing pads and polishing liquids, and the relationship between the polishing efficiency of each surface was obtained. . Then, based on the relationship, the polishing efficiency when simultaneously polishing the Si surface and the C surface is set. This wafer polishing method also allows both sides of the wafer to be polished simultaneously, improving wafer manufacturing efficiency. Furthermore, according to this wafer polishing method, it is possible to remove roughness on the surface of the epitaxial layer without removing too much of the Si-plane epitaxial layer, and it is also said to be possible to effectively remove roughness on the C-plane. .

Patent No. 3791302 Patent No. 6106535

However, the wafer polishing methods of Patent Documents 1 and 2 above employ a polishing liquid containing abrasive particles during polishing. Therefore, in these wafer polishing methods, polishing particles may behave unexpectedly during polishing, and scratches may occur on the surface of the wafer.

In addition, in the wafer polishing method of Patent Document 2, when polishing a wafer such as SiC whose first surface and second surface have different polishability, different polishing pads and polishing liquids are used to polish the first surface and the second surface of the wafer. A preliminary investigation must be conducted to polish the second surface separately, and the polishing efficiency for simultaneously polishing the first and second surfaces must be determined based on the relationship obtained. Therefore, in this wafer polishing method, in addition to selecting the first polishing pad and the second polishing pad, it is also necessary to select the polishing liquid, more specifically, the polishing particles in the polishing liquid, and repeat a preliminary test. For this reason, in this wafer polishing method, errors may occur in the selection or the errors may become large, resulting in poor workability and accuracy.

The present invention has been made in view of the above-mentioned conventional situation, and is capable of suppressing the occurrence of scratches on the surface of the wafer when polishing a wafer whose first surface and second surface have different polishability. The problem to be solved is to provide a wafer polishing method and a wafer polishing apparatus that can achieve high workability and precision.

The wafer polishing method of the present invention includes a first surface plate having a first polishing pad extending in a direction perpendicular to the first axis and rotating around the first axis;
a second surface plate that extends in a direction perpendicular to the first axis, has a second polishing pad facing the first polishing pad, and is rotated around the first axis;
a fixing portion extending in a direction perpendicular to a second axis parallel to the first axis, facing the first polishing pad and facing the second polishing pad, and rotating around the second axis; Using a wafer polishing device equipped with a carrier,
A wafer is fixed to the fixing part so that a first surface faces the first polishing pad and a second surface, which is the back side of the first surface, faces the second polishing pad,
Under a predetermined surface pressure, the first surface and the first polishing pad are relatively moved to polish the first surface, and the second surface and the second polishing pad are relatively moved to polish the second surface. A wafer polishing method for polishing a surface,
The wafer is made of SiC in which the first surface is a Si surface and the second surface is a C surface,
The second surface of the wafer has a higher polishability than the first surface,
The first polishing pad and the second polishing pad are made of resin and have a base material in which a plurality of pores are formed, and abrasive particles held in the base material and housed in the pores,
The porosity of the base material in the first polishing pad is lower than the porosity of the base material in the second polishing pad,
The content rate of the abrasive particles in the first polishing pad is equal to or higher than the content rate of the abrasive particles in the second polishing pad,
The polishing efficiency of the second surface by the second polishing pad is 1.2 to 2.0 times the polishing efficiency of the first surface by the first polishing pad .

Further, the wafer polishing apparatus of the present invention includes a first polishing pad that extends in a direction perpendicular to the first axis, and a first surface plate that is rotated around the first axis;
a second surface plate that extends in a direction perpendicular to the first axis, has a second polishing pad facing the first polishing pad, and is rotated around the first axis;
a fixing portion extending in a direction perpendicular to a second axis parallel to the first axis, facing the first polishing pad and facing the second polishing pad, and rotating around the second axis; and a carrier to be
A wafer is fixed to the fixing part so that a first surface faces the first polishing pad and a second surface, which is the back side of the first surface, faces the second polishing pad,
The wafer is made of SiC in which the first surface is a Si surface and the second surface is a C surface,
The second surface of the wafer has a higher polishability than the first surface,
The first polishing pad and the second polishing pad are made of resin and have a base material in which a plurality of pores are formed, and abrasive particles held in the base material and housed in the pores,
The porosity of the base material in the first polishing pad is lower than the porosity of the base material in the second polishing pad,
The content rate of the abrasive particles in the first polishing pad is equal to or higher than the content rate of the abrasive particles in the second polishing pad,
The polishing efficiency of the second surface by the second polishing pad is 1.2 to 2.0 times the polishing efficiency of the first surface by the first polishing pad .

The wafer polishing method and wafer polishing apparatus of the present invention employs a first polishing pad and a second polishing pad having polishing particles held in a base material. In these first polishing pads and second polishing pads, the polishing particles are elastically held on the base material and are difficult to fall off from the base material. For this reason, if the first and second surfaces of the wafer are polished using the first polishing pad and the second polishing pad, the polishing particles are less likely to behave unexpectedly, and polishing in which the polishing particles move freely together with the polishing liquid Compared to the case where a liquid is used, scratches are less likely to occur on the wafer surface.

In addition, the first polishing pad and the second polishing pad have a base material and polishing particles integrated in advance. Therefore, when polishing a wafer whose first surface and second surface have different polishability, if the first polishing pad and the second polishing pad are selected, there is no need to make any other selections. Therefore, in this wafer polishing method and wafer polishing apparatus, errors are unlikely to occur in their selection, and their errors will not become large.

In the wafer polishing method and wafer polishing apparatus of the present invention, when polishing a wafer whose first surface and second surface have different polishability, it is possible to suppress the occurrence of scratches on the surface of the wafer, and to improve workability. and accuracy can be achieved.

FIG. 1 is a side view of essential parts of a wafer polishing apparatus according to an embodiment.

In the wafer polished by the wafer polishing method and wafer polishing apparatus of the present invention, the polishability of the second surface is greater than the polishability of the first surface. Examples of such wafers include SiC, SiC with an epitaxial layer stacked on the Si surface side, and GaN. Here, the polishability means the polishing depth per hour by a polishing body with a prescribed polishing efficiency under a prescribed surface pressure and relative movement speed. That is, polishability means the ease with which the first or second surface of the wafer is polished.

The first polishing pad and the second polishing pad are abrasive-containing polishing pads also called LHA (Loosely Held Abrasive) pads. That is, the first polishing pad and the second polishing pad are made of resin and have a base material in which a plurality of pores are formed, and polishing particles held by the base material and accommodated in the pores .

The resins that make up the base material include polyether, rigid polyurethane foam, epoxy resin, polyethersulfone (PES) resin, as well as polyvinyl fluoride, vinyl fluoride/hexafluoropropylene copolymer, polyvinylidene fluoride, and fluoride. It is possible to employ fluorine-based synthetic resins such as vinylidene chloride/hexafluoropropylene copolymer, polyethylene resin, polymethyl methacrylate, and the like. The porosity, pore diameter, etc. of the base material can be set in various ways.

Abrasive particles held in the matrix and contained within the pores include silica, diamond, cubic boron nitride, boron carbide, silicon carbide, alumina, zirconia, titania, ceria, manganese oxide, barium carbonate, chromium oxide, It is possible to employ iron oxide or the like. The diameter of the abrasive particles, the content ratio of the abrasive particles to the base material, etc. can be set variously. The average particle diameter of the abrasive particles may range from 0.005 μm to 10 μm. The diameter of the abrasive particles can be measured by photon correlation method or laser diffraction scattering method. The average particle diameter means the value of D50 in a particle size distribution determined by a photon correlation method or a laser diffraction scattering method.

The polishing efficiency of the first polishing pad and the second polishing pad can be set by selecting and combining the type of base material, porosity, pore diameter, diameter of the abrasive particles, content ratio of the abrasive particles to the base material, and the like. Here, the polishing efficiency (μm/hr) refers to the efficiency with which a predetermined object to be polished can be polished per hour under a predetermined surface pressure and relative movement speed. That is, polishing efficiency means the ease of polishing in the first polishing pad and the second polishing pad.

When the wafer is SiC and the first surface is the Si surface and the second surface is the C surface, the polishing efficiency of the second surface by the second polishing pad is the same as the polishing efficiency of the first surface by the first polishing pad. It is preferably 1.2 to 2.0 times the polishing efficiency. The polishing efficiency of the second surface by the second polishing pad is more preferably 1.2 to 1.4 times the polishing efficiency of the first surface by the first polishing pad. The Si surface has a low polishability and is difficult to polish, while the C surface has a high polishability and is easy to polish. Therefore, the polishing efficiency of the first surface by the first polishing pad is higher than that of the second surface by the second polishing pad. If the polishing efficiency is low, scratches and damage are less likely to occur.

Although it is possible to interpose a polishing liquid between the wafer and the first polishing pad and between the wafer and the second polishing pad, in the wafer polishing method and wafer polishing apparatus of the present invention, the polishing liquid does not contain polishing particles. It is preferable not to include it. If the polishing liquid does not contain abrasive particles, management, recovery, and circulation of the polishing liquid can be simplified, and polishing costs can be reduced. As the polishing liquid, an alkaline aqueous solution, a potassium permanganate aqueous solution, a hydrogen peroxide aqueous solution, and the like having oxidizing ability can be used.

In the first polishing pad and the second polishing pad, if the porosity of the base material is high, the content of polishing particles stored in the pores will be high, and the polishing efficiency will be high. On the other hand, if the porosity of the base material is low, the content of abrasive particles stored in the pores will be low, and the polishing efficiency will be low. Examples of the porosity of the base material of the first polishing pad include 40 to 70% by volume and 45 to 60% by volume. Similarly, the porosity of the base material of the second polishing pad can be 40 to 70% by volume or 45 to 60% by volume. For example, when the wafer is SiC, and the first surface is the Si surface and the second surface is the C surface, the porosity of the base material of the first polishing pad is preferably 40 to 60% by volume. For example, the porosity of the base material of the second polishing pad is 50 to 70 volume%, preferably 55 to 60 volume%.

By making the base materials of the first polishing pad and the second polishing pad different in porosity, it is possible to provide a difference in polishing efficiency. For example, if the wafer is SiC and the first surface is the Si surface and the second surface is the C surface, the difference in porosity of the base material between the first polishing pad and the second polishing pad is as follows. I can give an example of -8 to -13 points.

In the first polishing pad and the second polishing pad, if the content of abrasive particles is high, the polishing efficiency will be high, and if the content of abrasive particles is low, the polishing efficiency will be low. Examples of the content of the polishing particles in the first polishing pad include 5 to 30% by volume and 10 to 25% by volume. Similarly, the content of abrasive particles in the second polishing pad may be 5 to 30% by volume or 10 to 25% by volume. For example, when the wafer is made of SiC and the first surface is the Si surface and the second surface is the C surface, the content of abrasive particles in the first polishing pad is preferably 15 to 25% by volume. For example, the content of the abrasive particles in the second polishing pad is 8 to 25 volume %, preferably 10 to 18 volume %.

By making the first polishing pad and the second polishing pad have different content rates of polishing particles, it is possible to provide a difference in polishing efficiency. For example, if the wafer is SiC, and the first surface is the Si surface and the second surface is the C surface, the difference in the content of abrasive particles between the first polishing pad and the second polishing pad is as follows: An example is 0 to 12 points.

The first polishing pad and the second polishing pad may have different polishing efficiencies by using different types of polishing particles and resins. If harder abrasive particles are used, the polishing efficiency will be higher, and if softer abrasive particles are used, the polishing efficiency will be lower. Further, by selecting the resin, the elastic force that holds the abrasive particles can be changed, and the polishing efficiency can be changed.

Examples 1 to 5 embodying the present invention, Reference Examples 1 and 2 , and Comparative Example 1 will be described below. The wafer polishing apparatus 10 used in Examples 1 to 5, Reference Examples 1 and 2, and Comparative Example 1 includes a first surface plate 1, a second surface plate 3, and a carrier 5, as shown in FIG. ing. The wafer polishing device 10 is a device that polishes both surfaces W1 and W2 of a wafer W at the same time.

A first polishing pad 7 is provided on the lower surface of the first surface plate 1, and a second polishing pad 9 is provided on the upper surface of the second surface plate 3. The first surface plate 1 is provided with a first rotating shaft 1a, and the second surface plate 3 is provided with a second rotating shaft 3a. The first rotation shaft 1a and the second rotation shaft 3a extend in the direction of the first axis O1, and are rotated by the first drive device 11 around the first axis O1 at a predetermined speed. Further, the first surface plate 1 and the second surface plate 3 can be moved closer to each other and separated from each other. The first polishing pad 7 and the second polishing pad 9 extend horizontally in a direction perpendicular to the first axis O1 and face each other.

The carrier 5 is provided between the first polishing pad 7 and the second polishing pad 9. The carrier 5 is provided with a driving force transmission mechanism (not shown), and is rotated by a second drive device 13 around a second axis O2 at a predetermined speed. The second axis O2 is parallel to the first axis O1.

The fixed portion 5a of the carrier 5 extends horizontally in a direction orthogonal to the second axis O2, and faces the first polishing pad 7 and the second polishing pad 9. A wafer W is fixed to the fixing portion 5a. The carrier 5 has a plurality of fixing parts 5a, and a plurality of wafers W can be mounted thereon. Further, the wafer polishing apparatus 10 can be provided with a plurality of carriers 5.

Each wafer W is made of SiC. The Si surface of SiC is the first surface W1, and the C surface of SiC is the second surface W2. The first surface W1 of the wafer W faces the first polishing pad 7, and the second surface W2 faces the second polishing pad 9.

A nozzle 15 is arranged on the side of the first surface plate 1 and the second surface plate 3, and the nozzle 15 is connected to a tank (not shown) that stores polishing liquid. The polishing liquid stored in the tank can be supplied between the first polishing pad 7 and the second polishing pad 9 by the nozzle 15 .

When the wafer polishing apparatus 10 is operated with the wafer W fixed to the fixed part 5a, the first surface plate 1 and the second surface plate 3 approach each other, and each wafer W fixed to the fixed part 5a of the carrier 5 and The first and second polishing pads 7 and 9 are pressurized to a predetermined surface pressure, and the first drive device 11 and the second drive device 13 are driven to The board 3 and carrier 5 rotate. As a result, the first surface W1 of the wafer W and the first polishing pad 7 move relative to each other at a predetermined speed to polish the first surface W1, and the second surface W2 of the wafer W and the second polishing pad 9 move at a predetermined speed. to polish the second surface W2. Then, after a predetermined period of time has elapsed, polishing of the first surface W1 and polishing of the second surface W2 can be completed at the same time.

In Examples 1 to 5 and Reference Examples 1 and 2 , LHA pads were used as the first polishing pad 7 and the second polishing pad 9. The LHA pad consists of a polyether matrix and abrasive particles held within the pores of the matrix. As the polishing particles, silica having a particle size of about 200 nm is used. The content rate (volume %) of polishing particles and the porosity (volume %) of the base material in the first polishing pad 7 and the second polishing pad 9 are as shown in Table 1. Further, in Examples 1 to 5 and Reference Examples 1 and 2 , a potassium permanganate aqueous solution was used as the polishing liquid. Polishing efficiency (μm/hr) when only the Si side of the wafer W is polished with the same surface pressure and speed using the first polishing pad 7, and when only the C side of the wafer W is polished with the same surface pressure using the second polishing pad 9. Table 1 also shows the polishing efficiency (μm/hr) when polishing was performed in advance at the same speed.

On the other hand, in Comparative Example 1, commercially available nonwoven fabric pads were used in place of the first polishing pad 7 and the second polishing pad 9. In Comparative Example 1, a slurry in which abrasive particles made of silica having a particle size of about 200 nm were blended into an aqueous potassium permanganate solution was used as the polishing liquid. Polishing efficiency (μm/hr) when only the Si side of the wafer W is polished in advance with the same surface pressure and the same speed using a nonwoven fabric pad, and the polishing efficiency (μm/hr) when only the C side of the wafer W is previously polished with the same surface pressure and the same speed using the nonwoven fabric pad. Table 1 also shows the polishing efficiency (μm/hr) when the polishing was performed.

The value of the ratio (times) of the polishing efficiency of the C surface by the second polishing pad 9 to the polishing efficiency of the Si surface by the first polishing pad 7, the porosity of the base material of the first polishing pad 7 and the polishing efficiency of the second polishing pad 9. Table 1 also shows the difference (points) between the porosity of the base material and the difference (points) between the abrasive particle content of the first polishing pad 7 and the abrasive particle content of the second polishing pad 9.

The wafer polishing apparatus 10 was operated under the following conditions to simultaneously polish the Si surface and C surface of SiC.

Number of revolutions of first surface plate 1 and second surface plate 3: 60 (rpm)
Size of first and second polishing pads 7 and 9 and non-woven pad: diameter 300 (mm)
Rotation speed of carrier 5: 60 (rpm)
Processing surface pressure: 20.5 (kPa)
Polishing liquid supply rate: 10ml/min Polishing time: 10 minutes x 3 times

The above results are shown in Table 1. In addition, in the double-sided polishing results shown in Table 1, ◎ means that polishing progressed well even when 10 or more wafers W were placed in the wafer polishing apparatus 10, and ○ means that the polishing proceeded well even when 10 or more wafers W were placed in the wafer polishing apparatus 10. This means that polishing progressed well even when 5 to 10 wafers W were arranged.

As shown in Table 1, in Comparative Example 1 in which a nonwoven fabric was used and a slurry containing abrasive particles was used as the polishing liquid, the wafer W was seriously damaged. On the other hand, in Examples 1 to 5 and Reference Examples 1 and 2 , in which LHA pads were used as the first and second polishing pads 7 and 9, and an aqueous solution containing no polishing particles was used as the polishing liquid, damage to the wafer W was prevented. It has been reduced. For this reason, in Examples 1 to 5 and Reference Examples 1 and 2 , in the first polishing pad 7 and the second polishing pad 9, the abrasive particles are held elastically by the base material and are difficult to fall off from the base material. It can be seen that the abrasive particles are less likely to behave unexpectedly and scratches are less likely to occur on the surface of the wafer W.

Further, in Examples 1 to 5 and Reference Examples 1 and 2 , the first polishing pad 7 and the second polishing pad 9 have a base material and polishing particles integrated in advance. Therefore, even when polishing a wafer W whose first surface W1 and second surface W2 have different polishability, if the first polishing pad 7 and the second polishing pad 9 are selected, other selections can be made. There's no need to do it. Therefore, in Examples 1 to 5 and Reference Examples 1 and 2 , errors are unlikely to occur in their selection, and their errors do not become large.

Furthermore, it can be seen that if the polishing efficiency of the C surface by the second polishing pad 9 is 1.2 to 2.0 times the polishing efficiency of the Si surface by the first polishing pad 7, scratches and damage are unlikely to occur. In particular, if the polishing efficiency of the C surface by the second polishing pad 9 is 1.2 to 1.4 times the polishing efficiency of the Si surface by the first polishing pad 7, scratches and damage will not occur.

In addition, according to Examples 1 to 5, it can be seen that scratches and damage do not occur because the first polishing pad 7 and the second polishing pad 9 have different porosity of the base materials. In particular, in Examples 1, 2, 4, and 5, the first polishing pad 7 and the second polishing pad 9 have different content rates of polishing particles, so that scratches and damage are less likely to occur.

Therefore, in the wafer polishing method and wafer polishing apparatus of the present invention, it is possible to suppress the occurrence of scratches on the surface of the wafer W when polishing the wafer W whose first surface W1 and second surface W2 have different polishability. It can be seen that it is possible to achieve high workability and accuracy.

Although the present invention has been described above based on examples, it goes without saying that the present invention is not limited to the above-mentioned examples, and can be applied with appropriate modifications without departing from the spirit thereof.

INDUSTRIAL APPLICATION This invention can be utilized for the manufacturing method and manufacturing apparatus of the semiconductor wafer utilized for electronic devices, such as an integrated circuit.

W...Wafer W1...First surface W2...Second surface 7...First polishing pad 9...Second polishing pad O1...First axis 1...First surface plate 3...Second surface plate O2...Second axis 5 ...Carrier 5a...Fixing part 10...Wafer polishing device

Claims (4)

a first surface plate having a first polishing pad extending in a direction perpendicular to the first axis and rotated around the first axis;
a second surface plate that extends in a direction perpendicular to the first axis, has a second polishing pad facing the first polishing pad, and is rotated around the first axis;
a fixing portion extending in a direction perpendicular to a second axis parallel to the first axis, facing the first polishing pad and facing the second polishing pad, and rotating around the second axis; Using a wafer polishing device equipped with a carrier,
A wafer is fixed to the fixing part so that a first surface faces the first polishing pad and a second surface, which is the back side of the first surface, faces the second polishing pad,
Under a predetermined surface pressure, the first surface and the first polishing pad are relatively moved to polish the first surface, and the second surface and the second polishing pad are relatively moved to polish the second surface. A wafer polishing method for polishing a surface,
The wafer is made of SiC in which the first surface is a Si surface and the second surface is a C surface,
The second surface of the wafer has a higher polishability than the first surface,
The first polishing pad and the second polishing pad are made of resin and have a base material in which a plurality of pores are formed, and abrasive particles held in the base material and housed in the pores,
The porosity of the base material in the first polishing pad is lower than the porosity of the base material in the second polishing pad,
The content rate of the abrasive particles in the first polishing pad is equal to or higher than the content rate of the abrasive particles in the second polishing pad,
A wafer polishing method , wherein the polishing efficiency of the second surface by the second polishing pad is 1.2 to 2.0 times the polishing efficiency of the first surface by the first polishing pad . 2. The wafer polishing method according to claim 1 , wherein the polishing efficiency of the second surface by the second polishing pad is 1.2 to 1.4 times the polishing efficiency of the first surface by the first polishing pad. interposing a polishing liquid between the wafer and the first polishing pad and between the wafer and the second polishing pad,
3. The wafer polishing method according to claim 1, wherein the polishing liquid does not contain polishing particles. a first surface plate having a first polishing pad extending in a direction perpendicular to the first axis and rotated around the first axis;
a second surface plate that extends in a direction perpendicular to the first axis, has a second polishing pad facing the first polishing pad, and is rotated around the first axis;
a fixing portion extending in a direction perpendicular to a second axis parallel to the first axis, facing the first polishing pad and facing the second polishing pad, and rotating around the second axis; and a carrier to be
A wafer is fixed to the fixing part so that a first surface faces the first polishing pad and a second surface, which is the back side of the first surface, faces the second polishing pad,
The wafer is made of SiC in which the first surface is a Si surface and the second surface is a C surface,
The second surface of the wafer has a higher polishability than the first surface,
The first polishing pad and the second polishing pad are made of resin and have a base material in which a plurality of pores are formed, and abrasive particles held in the base material and housed in the pores,
The porosity of the base material in the first polishing pad is lower than the porosity of the base material in the second polishing pad,
The content rate of the abrasive particles in the first polishing pad is equal to or higher than the content rate of the abrasive particles in the second polishing pad,
A wafer polishing apparatus characterized in that the polishing efficiency of the second surface by the second polishing pad is 1.2 to 2.0 times the polishing efficiency of the first surface by the first polishing pad .

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