JP2018037671A - Method for polishing silicon wafer - Google Patents

Abstract

Provided is a method for polishing a silicon wafer in which the wafer surface is highly planarized and a silicon oxide film having a predetermined thickness is secured on the back surface of the wafer. A silicon wafer polishing method in which a natural oxide film is formed on the front surface and a silicon oxide film is formed on the back surface. Polishing of a silicon wafer is a double-sided polishing process using a polishing solution containing a water-soluble polymer additive in an alkaline aqueous solution, the natural oxide film on the surface of the silicon wafer is removed and mirror-polished, and on the back surface of the silicon wafer This is performed so that the formed silicon oxide film remains. The polishing liquid does not contain abrasive grains, the first polishing cloth that polishes the surface of the silicon wafer used in the double-side polishing process is a polishing cloth on which the abrasive grains are fixed, and the second polishing cloth that polishes the back surface of the silicon wafer is abrasive. A polishing cloth with no grains fixed, and a water-soluble polymer additive is included in an alkaline aqueous solution so that the difference in frictional resistance between the front and back surfaces of the silicon wafer is alleviated. [Selection] Figure 1

Description

The present invention relates to an improvement of a method for polishing a silicon wafer in a method for manufacturing a bonded SOI wafer.

As an SOI (Silicon on Insulator) wafer, a bonded SOI wafer is known in which an active layer wafer is bonded to a supporting wafer via a BOX (Buried Oxide) layer which is a silicon oxide film having high electrical insulation. This bonded SOI wafer is heat-treated to form an oxide film on the active layer wafer and the supporting wafer that supports the active layer wafer, and then bonded to ensure the bonding strength. It has been manufactured by grinding to the BOX layer , single-side grinding so that the surface of the active layer wafer is reduced to a desired thickness, and then polishing the ground surface of the active layer wafer.

  Conventionally, polishing of the surface of the active layer wafer of the bonded SOI wafer that has been ground on one side is performed by fixing the supporting wafer on the back surface of the bonded SOI wafer by wax holding, vacuum holding, waxless template holding, etc. One-side polishing was performed by a so-called single-side polishing apparatus that mirror-polished only the surface of the wafer for polishing with a polishing cloth (for example, see Patent Document 1).

  On the other hand, in the manufacture of polished wafers used as semiconductor wafers, both front and back surfaces of the wafer are simultaneously polished so as to remove processing strain (processing damage layer) introduced on the wafer surface by lapping or grinding. Processing has been done. Although the double-sided polishing process is a polishing process that does not have a reference surface, it has excellent flatness. However, since both sides of the wafer are mirror-polished simultaneously, there is a problem that it is difficult to distinguish the front and back surfaces. For this reason, a technique has been proposed in which only one side of a wafer is mirror-polished using a double-side polishing apparatus. For example, in order to polish one side of a wafer with a double-side polishing machine that contains abrasive grains in the polishing liquid, an oxidation protective film is formed only on the back side of the wafer before polishing, or it naturally occurs due to contact with air on the surface of the wafer When the natural oxide film on the polishing surface side is removed in advance with hydrofluoric acid or the single-side polishing is performed with a double-side polishing machine using a fixed abrasive polishing cloth without containing abrasive grains in the polishing liquid In other words, the fixed abrasive polishing cloth is disposed only on one side for polishing, and polishing is performed (see, for example, Patent Document 2).

JP 2004-022839 A (paragraphs [0021] to [0023], FIG. 6) JP 2002-016025 A (paragraphs [0002] to [0006], [0032] to [0044])

  However, in the conventional single-side polishing disclosed in Patent Document 1 above, the wafer is held in wax, vacuum, and waxless template to hold the back side of the polished surface of the wafer as a reference surface during polishing. It is difficult to achieve high flatness because the template shape is transferred to the wafer, the place where the vacuum is not held, and the wafer is polished while the wafer is stressed so that the wafer is wavy when the holding is released.

On the other hand, in the single-side polishing by the double-side polishing apparatus shown in the above-mentioned conventional patent document 2, an oxidation protective film is formed only on the back surface of the wafer in order not to polish one side of both sides of the wafer before polishing by the double-side polishing apparatus. or, Inado Do Rana unless removing a native oxide film on the polished surface side of the wafer surface is generated by the cleaning process and the air contacted in advance hydrofluoric acid, could not be performed is conveniently polished.

  Also, when a fixed abrasive polishing cloth is installed on only one side and polished with a double-side polishing machine, the wafer is held by the difference in frictional resistance between the fixed abrasive polishing cloth and the wafer surface, and the other polishing cloth and the back surface of the wafer. When the wafer is pressed against the carrier plate, the outer edge of the wafer may be deformed, or abnormal noise or noise may occur during polishing.In severe cases, the wafer may come off the carrier and damage the wafer or the double-side polishing machine. It was.

An object of the present invention is to provide a method for polishing a silicon wafer in which a wafer surface is highly planarized and a silicon oxide film having a predetermined thickness is secured on the back surface of the wafer .

The present inventors have made intensive studies in order to achieve the above object, recalled that polishing the SOI wafer bonded with both sides polishing machine, by adding the polymer to the polishing liquid, bonded SOI The inventors have found that the active layer side wafer surface of the wafer is mirror-polished and a silicon oxide film having a predetermined thickness can be secured on the back surface of the support side wafer. Specifically, the present invention has been completed with the following knowledge.

Generally, it is known that when a double-side polishing process is performed using a polishing liquid containing abrasive grains, the silicon oxide film on the surface of the silicon wafer is removed by a mechanical polishing action by the abrasive grains in the polishing liquid. Yes. According to the experiments by the present inventors, when a polymer is added to a polishing liquid containing abrasive grains, a layer with a thickness of about several nanometers such as a natural oxide film is removed, but a bonded SOI wafer is obtained. The thickness level of the silicon oxide film , which is a thermal oxide film formed on the exposed surface of the bonded SOI wafer by the bonding strengthening heat treatment performed in the manufacturing process, is several μm to several tens μm, and it is removed even if it undergoes a polishing process. It has been found that the amount of silicon oxide film to be obtained is very small, and the thickness of the silicon oxide film can be sufficiently secured.

Further, when double-side polishing is performed using a polishing liquid that does not contain abrasive grains, the natural oxide film formed on the surface of the active layer side wafer is not removed. For this reason, it is possible to remove the natural oxide film by using a polishing cloth to which abrasive grains are fixed. However, the natural oxide film is removed from the active layer side wafer surface, and the oxide film on the back surface of the support side wafer remains. Since polishing proceeds in this state, the difference in frictional resistance between the front and back surfaces of the wafer becomes large, and it has become clear that the double-side polishing process itself cannot be performed. According to the experiments by the present inventors, by adding a polymer to a polishing liquid that does not contain abrasive grains, the difference due to the upper and lower frictional resistance is alleviated, and the wafer surface is mirror-polished by a fixed abrasive polishing cloth and supported. It has been found that a silicon oxide film having a predetermined thickness can be secured on the rear surface of the side wafer.

A first aspect of the present invention is a polishing method for a silicon wafer in which a natural oxide film is formed on the front surface and a silicon oxide film is formed on the back surface. The characteristic configuration is that the polishing of the silicon wafer is water-soluble in an alkaline aqueous solution. Is a double-sided polishing process using a polishing liquid containing a conductive polymer additive so that the natural oxide film on the surface of the silicon wafer is removed and mirror-polished, and the silicon oxide film formed on the back surface of the silicon wafer remains. The first polishing cloth for polishing the surface of the silicon wafer used in the double-side polishing process is a polishing cloth to which the abrasive grains are fixed, and the second polishing for polishing the back surface of the silicon wafer. a polishing cloth polishing cloth is not fixed abrasive, alkali to the water-soluble polymer additive, the difference in frictional resistance in the front and rear surfaces of the silicon wafer is reduced It is to be included in the aqueous solution.

According to a second aspect of the present invention, in the first aspect, the characteristic configuration is to polish the surface of the silicon wafer with the first polishing cloth with the surface of the silicon wafer facing downward.

According to a third aspect of the present invention, in the first or second aspect, the characteristic configuration is that the surface of the silicon wafer after double-side polishing is mirror-polished by single-side polishing.

In a first aspect of the present invention, in a Turkey to use pads abrasive grains are fixed to the double-side polishing apparatus, a natural oxide film of the active layer wafer side of the wafer surface is removed is mirror polished, and polishing since the silicon oxide film formed on the wafer back surface of the supporting wafer side of the bonded wafer by containing a water-soluble polymeric additive to the liquid is left, the natural oxide film is removed active layer wafer side wafer the surface is chemically polished by the polishing liquid, the wafer back surface of the supporting wafer side with the silicon oxide film is the silicon oxide film is not chemically polishing becomes a protective film, the SOI wafer bonded at side polishing apparatus It becomes possible to perform single-side polishing in which the wafer surface on the active layer wafer side is highly planarized. In particular, by adding a water-soluble polymer additive to the polishing liquid that does not contain abrasive grains, the difference due to the upper and lower frictional resistance is mitigated, and the wafer surface is mirror-polished with a fixed abrasive polishing cloth, and the support side wafer back surface is polished. A silicon oxide film having a predetermined thickness can be secured.

In the first aspect of the present invention, a water-soluble polymer additive is added to the polishing liquid to account for the difference in stress applied to the silicon wafer due to the difference in the material of the polishing cloth on the wafer front and back sides of the double-side polishing apparatus. Thus, it becomes possible to perform polishing without imposing a burden on a double-side polishing apparatus that polishes even a silicon wafer that is reduced and polished.

In the second aspect of the present invention, the silicon oxide film surface on the back surface of the supporting side wafer is touched with an electrostatic chuck or a vacuum chuck without touching the surface of the active layer side wafer used as the product region formed in the first aspect. By doing so, the bonded SOI wafer can be taken out from the double-side polishing apparatus. That is, after the double-side polishing process, the bonded SOI wafer of the first on the polishing cloth, the wafer back surface of the supporting side is hydrophilic surface silicon oxide film remained, the wafer surface of the mirror-polished surface of the active layer side is water-repellent surface It is in the state of. Therefore, when the bonded SOI wafer is taken out from the first polishing cloth, the mirror-polished active wafer surface is easily peeled off from the first polishing cloth, and the supporting wafer back surface ( silicon oxide film surface) is adsorbed. By pulling upward, the bonded SOI wafer can be easily taken out from the double-side polishing apparatus.

In the second aspect of the present invention, the bonded SOI wafer can be taken out from the double-side polishing apparatus with an electrostatic chuck or a vacuum chuck without touching the highly planarized surface of the wafer formed in the first aspect. It becomes like this.

In the third aspect of the present invention, a higher flat surface can be obtained by mirror-polishing the wafer surface on the active layer wafer side after the double-side polishing treatment in the first or second aspect by single-side polishing treatment. It becomes like this.

It is the figure which represented typically an example of the manufacturing method of the bonding SOI wafer of embodiment of this invention. It is the figure which represented typically the whole cross section of an example of the double-side polish apparatus used for grinding | polishing of the bonding SOI wafer of this invention embodiment. It is the figure which represented typically the cross section of the part expanded centering on the wafer in the double-side polish apparatus shown in FIG. It is the figure which represented typically the cross section of the part expanded centering on the wafer in an example of the double-side polish apparatus shown in FIG. 3 when not including a water-soluble polymer additive. It is the figure which compared the back surface oxide film grinding | polishing amount of the bonding SOI wafer of Example 1, 2 and the comparative example 2. FIG. It is the figure showing the measurement point of the back surface of the wafer shown in FIG. It is a figure showing the flatness measurement result of the bonding SOI wafer of Examples 1, 2 and Comparative Examples 1, 2. It is a figure showing the surface (a) and the cross section (b) which visualized the flatness of the bonding SOI wafer of Example 1. FIG. FIG. 9 is a diagram corresponding to FIG . 8 of the second embodiment. FIG. 9 is a diagram corresponding to FIG . 8 of Comparative Example 1. FIG. 9 is a diagram corresponding to FIG . 8 of Comparative Example 2.

  Next, an embodiment for carrying out the present invention will be described with reference to the drawings.

The present invention, as shown in FIG. 1 (a) ~ (f) , the SOI wafer 100 bonded to prepared laminating a silicon wafer to form a silicon oxide film, at the end of the active layer side wafer surface steps It is an improvement of the method of polishing and flattening.

A characteristic configuration of the present invention is in the polishing step of FIG. When adopting a double-side polishing apparatus in the polishing step of FIG. 1 (f), a natural oxidation generated in the active layer wafer 101 by using a pad on which abrasive grains are fixed or by including abrasive grains in the polishing liquid. The silicon oxide film formed on the supporting wafer 102 after the film is removed and mirror-polished can be left by adding a water-soluble polymer additive to the polishing liquid. In addition, by adding a water-soluble polymer additive to the polishing liquid, the difference in stress due to the difference in the material of the polishing cloth in contact with the front and back sides of the wafer of the double-side polishing apparatus is reduced, and polishing is also performed on the wafer to be polished. It is possible to perform polishing without imposing a burden on the double-side polishing apparatus. Thus, only the surface of the bonded SOI wafer 100 can be easily flattened with a double-side polishing apparatus, and the bonded SOI wafer 100 can be stably manufactured with reduced damage and failure of the double-side polishing apparatus and wafer. be able to.

  The steps of the present invention will be described in detail with reference to FIG.

FIG. 1A shows an example of a process for forming a silicon oxide film on a silicon wafer. The active layer wafer 101 and the supporting wafer 102 are mirror-finished from an ingot through a general manufacturing process. The active layer wafer 101 becomes a later SOI layer 103. The silicon oxide film 104 is formed by heat treatment in an atmosphere of hydrogen and oxygen mixed gas. This silicon oxide film 104 becomes a later buried oxide film layer 105 (BOX layer ). The film thickness of the buried oxide film layer 105 can be set to several μm to several tens of μm. FIG. 1 (a) shows a case where a silicon oxide film is formed only on the supporting wafer 102, but it follows the silicon crystal plane generated by heat treatment at high temperature and high oxygen concentration in the bonding heat treatment step. In order to prevent the occurrence of slip, which is a defective, both substrates may be protected by silicon oxide films. Silicon oxide film thickness of the silicon oxide film thickness and the supporting wafer 102 of the active layer wafer 101 set in this step is the film thickness of the buried oxide film layer 105.

FIG. 1B is a diagram showing an example of a process of superposing and bonding the active layer wafer 101 and the supporting wafer 102 together. Both silicon wafers on which the silicon oxide film 104 is formed are cleaned with a cleaning liquid, rinsed with pure water, and then cleaned with a hydrofluoric acid organic acid to terminate the surface of the silicon wafer with hydrogen. Thereafter, both silicon wafers are superposed and pressed to adhere.

FIG.1 (c) is a figure which shows an example of the process of bonding heat processing. The bonding heat treatment increases the bonding strength between the active layer wafer 101 and the supporting wafer 102 that have been hydrogen bonded. The bonding heat treatment is performed by holding the silicon wafer bonded in the overlapping process at 1200 ° C. for 60 to 180 minutes in a hydrogen and oxygen mixed gas atmosphere.

FIG. 1D is a diagram illustrating an example of an outer edge grinding process for creating the terrace 106 on the outer edge of the SOI layer 103. In the outer edge portion where the active layer wafer 101 and the supporting wafer 102 are bonded together, an area where bonding is not sufficient due to chamfering of the silicon wafer or the like is formed. The outer edge that is the region is ground before reaching the bonding surface. Thereafter, the terrace 106 is formed by etching the residual silicon at the outer edge with KOH (potassium hydroxide) or the like. Incidentally, the formation of the terrace 106 may be omitted or may be performed to form the processing of terrace 106 after one-side grinding process illustrated in Figure 1 (e) to be described later.

  FIG. 1E is a diagram showing an example of a surface grinding process in which a grinding process is performed to reduce the thickness of the active layer wafer 101 in order to form the SOI layer 103. With a single-side grinding apparatus, a process for reducing the thickness is performed by adding a margin in a subsequent polishing step so that the SOI layer 103 becomes 0.02 to 300 μm depending on a device to be used.

  FIG. 1F is a diagram illustrating an example of a polishing process for highly planarizing the surface of the SOI layer 103. The double-side polishing apparatus 200 polishes only the surface of the active layer wafer 101 without having a reference surface. For polishing, the abrasive solution 201 with abrasive grains contains a water-soluble polymer additive as an additive and the polishing cloth does not contain fixed abrasive grains. A polishing cloth containing a water-soluble polymer additive may contain a fixed abrasive and be polished.

  FIG. 2 is a view showing an example of a double-side polishing apparatus 200 used in the polishing step of FIG. An internal gear 203 is installed around the lower surface plate 202, and a sun gear 204 is installed in the center. The lower surface plate 202 on which the first polishing cloth 205 is laid is sandwiched between the internal gear 203 and the sun gear 204 and set on the carrier plate 206. Depending on the size of the double-side polishing apparatus 200, 4 to 8 wafers are arranged. The upper surface plate 207 is provided with a second polishing cloth 208 on the surface in contact with the wafer and a supply hole 209 for supplying the polishing liquid 201. A supply pipe 210 is provided above the supply hole 209 to supply the polishing liquid 201. Above the sun gear 204, a shaft 211 that transmits rotation from a power source (not shown) that rotates the sun gear 204 is installed. The upper surface plate 207 is installed so that the second polishing cloth 208 is in contact with the wafer, and the sun gear 204 is rotated by a power source (not shown) while being pressed by the upper surface plate 207. The carrier plate 206 on which the wafer is installed revolves around the sun gear 204 while rotating as the sun gear 204 rotates.

< Reference form>
Next , a reference embodiment will be described first with reference to FIG. 3 focusing on a polishing step having a characteristic configuration of the present invention.

  FIG. 3 schematically shows a cross section of an enlarged portion centered on the wafer of the double-side polishing apparatus 200 used for polishing the bonded SOI wafer 100 when the polishing liquid 201 contains a water-soluble polymer additive and abrasive grains. FIG. The bonded SOI wafer 100 is placed on the first polishing cloth 205 that is spread on the upper surface of the lower surface plate 202 with the active layer wafer 101 to be polished facing down, and the second surface of the bonded SOI wafer 100 that is expanded on the lower surface of the upper surface plate 207. The polishing cloth 208 is placed in contact with the supporting wafer 102. The double-side polishing apparatus 200 revolves around the sun gear 204 while rotating the sun gear 204 and rotating the carrier plate 206 while supplying the polishing liquid 201 from the supply pipe 210. In this embodiment, the use of the sun gear type double-side polishing apparatus has been described. However, the present invention is not limited to this, and a sun gear type or non-revolution type double-side polishing apparatus may be used.

  The main component of the polishing liquid 201 is an alkaline aqueous solution. As for content of the alkaline agent in alkaline aqueous solution, 100-1000 ppm is preferable. If it is less than 100 ppm, the etching of the surface of the silicon wafer with an alkaline agent is not sufficient, and it takes a long time to polish the silicon wafer. If it exceeds 1000 ppm, it becomes difficult to handle the polishing liquid 201 and the etching reaction is caused. This is because the surface becomes rough due to the excess.

  As an alkaline agent (pH adjuster) of an alkaline aqueous solution, an alkaline aqueous solution or an alkaline carbonate aqueous solution to which any of a basic ammonium salt, a basic potassium salt, or a basic sodium salt is added, or an alkaline aqueous solution to which an amine is added is used. In addition, aqueous solutions of hydrazine and amines can be employed. From the viewpoint of increasing the polishing rate, it is preferable to use an alkali excluding ammonia, particularly an amine.

  In the alkaline aqueous solution, abrasive grains such as colloidal silica abrasive grains, diamond abrasive grains, and alumina abrasive grains are mixed. The average particle size of the abrasive grains is preferably 30 to 200 nm, particularly 50 to 150 nm. If the average particle size is less than 30 nm, defects such as micro scratches due to agglomeration of the abrasive grains tend to occur, and if it exceeds 200 nm, colloidal dispersion becomes difficult and concentration variation occurs.

  As the water-soluble polymer, anionic and amphoteric and nonionic polymers and monomers are used. Specifically, hydroxyethyl cellulose and polyethylene glycol are suitable. Hydroxyethyl cellulose easily forms a polymer film on the surface of the silicon wafer, so it has a high effect of suppressing the etching reaction due to alkali, and the polymer film is removed because the polishing solution is less swept away by the polishing cloth at the outer edge of the silicon wafer. This is preferable because it has an effect of reducing the amount of roll-off, which is a phenomenon that the outer edge of the silicon wafer is sagless. Therefore, a water-soluble polymer having an effect of promoting etching of a silicon wafer with an alkaline aqueous solution is not suitable. One or more water-soluble polymers may be used. The concentration of the water-soluble polymer in the polishing liquid is set in the range of 1 ppm to 200 ppm, and particularly preferably 100 ppm or less.

  This is because the content of the abrasive grains and the concentration of the water-soluble polymer are within the range in which the polishing rate is not extremely lowered and the native oxide film is removed and the oxide film 104 of the supporting wafer 102 remains. Further, instead of the water-soluble polymer, a surfactant (polyoxyethylene alkyl ether or the like) or an aliphatic alcohol (polyvinyl alcohol) may be employed.

As the polishing cloth, both the first polishing cloth and the second polishing cloth do not follow the shape of the outer edge portion of the silicon wafer, and a polishing cloth that is hard enough not to cause polishing scratches on the silicon wafer surface reduces the roll-off. preferable. Specifically, a polyester or polyurethane polishing cloth is desirable, and the Shore A hardness specified by JIS K 6253-1997 / ISO 7619 is 70 ° to 90 °, the compression ratio is 0.5 to 5% , particularly 2 to 2. It is desirable to use 4% abrasive cloth.

Polishing rate of the silicon wafer is set in view of preventing the surface roughness of the shortening and the silicon wafer polishing time 0.05 to 1 [mu] m / min. The rotation speed of the double-side polishing apparatus 200, the rotation speed of the carrier plate 206, and the polishing thickness are within the above polishing rate range, that is, the rotation speed of the double-side polishing apparatus 200, the rotation speed of the carrier plate 206 is 5 to 100 rpm, and the polishing pressure is 30 to 30. Set to 500 g / cm2. The polishing amount in the double-side polishing step may be set according to the processing strain depth introduced by the single-side grinding process performed as the pre-processing, and a polishing amount of approximately 20 μm or less is sufficient.

<Implementation of the form>
Next, an embodiment will be described with reference to FIG. 3 focusing on a polishing step having a characteristic configuration of the present invention. In this embodiment, the double-side polishing apparatus 200 of FIG. 3 is used, a polishing liquid in which a water-soluble polymer is added as an additive to a polishing liquid 201 that does not contain abrasive grains is used, and an elastic substrate is used as the first polishing cloth 205. A fixed abrasive polishing cloth in which abrasive particles having a particle diameter (average particle diameter) of 2 to 8 μm were dispersed and fixed (for example, kneaded and solidified) was adopted. As the material for the abrasive grains, for example, silica (including fumed silica) can be employed. In addition, colloidal silica may be used. As a material for the elastic substrate, for example, a polyether-based resin, a polyester-based resin, a polyurethane-based resin, or the like can be used. The active layer wafer 101, which is a polishing surface, was placed facing the lower surface plate 202 on which the first polishing cloth 205 was spread. Other conditions are the same as those in the reference embodiment.

Here, referring to FIG. 4 and described a unique effect on the form of implementation. When the polishing liquid 201 does not contain a water-soluble polymer, the first polishing cloth 205 contains fixed abrasive grains, so that the friction resistance of the first polishing cloth 205 is less than the friction resistance of the second polishing cloth 208. Then, the friction on the second polishing cloth 208 side of the wafer increases when rotating. The wafer applies pressure to the carrier plate 206 due to the friction. The wafer bends to the first polishing cloth 205 side that releases the pressure, and the outer edge of the wafer is sagted to the first polishing cloth 205 side. This pressure is considered to cause abnormal noise during rotation, or if the pressure is further increased, remove the wafer, or damage the carrier plate 206, the gear of the double-side polishing apparatus 200, etc. by the removed wafer. It is considered that by adding a water-soluble polymer to the polishing liquid 201, the difference due to the upper and lower frictional resistance is alleviated and the above phenomenon does not occur.

  Note that the surface of the active layer wafer 101 is mirror-polished again by the single-side polishing process so as to improve the surface roughness of the active layer wafer 101 with respect to the bonded SOI wafer 100 subjected to the double-side polishing process. It is desirable.

  Since the bonded SOI wafer 100 thus obtained has a completely isolated structure between elements, high-speed CMOS, high-integrated digital IC, analog-digital mixed IC, in which high-speed operation, low power consumption, and high withstand voltage are expected, It can be used as a substrate for an automotive power IC or the like.

Next, examples of the present invention will be described in detail together with comparative examples. In addition, in this specification and drawings, Example 1 is not an example but a “reference example”, and Example 2 is simply an “example”.

<Example 1>
Two silicon wafers having a diameter of 200 mm and a thickness of 725 μm are used as the active layer wafer 101 and the supporting wafer 102, the buried oxide film layer 105 has a thickness of 2000 nm, the silicon oxide film 104 of the supporting wafer 101 has a thickness of 2000 nm, and the SOI layer 103 has a thickness of A bonded SOI wafer 100 having an active layer thickness of 15 μm by a single-side grinding process was prepared. Then, after the cleaning process was performed on the bonded SOI wafer, the surface of the SOI layer 103 was observed, and it was confirmed that a natural oxide film of about 10 mm was formed on the surface of the SOI layer 103 after the cleaning process. It was. Next, the double-sided simultaneous polishing process was performed on the bonded SOI wafer 100 after cleaning. The condition of the polishing liquid 201 used for double-side polishing was that the alkali agent in the alkaline aqueous solution was TMAH (tetramethylammonium hydroxide) and the content of the alkali agent was about 500 ppm. In this alkaline aqueous solution, about 5% by mass of colloidal silica abrasive grains having an average particle size of 30 to 40 nm was contained, and hydroxyethyl cellulose was set to a concentration of 100 ppm as a water-soluble polymer. In addition, a polishing cloth made of polyurethane was used as the polishing cloth for the first polishing cloth 205 and the second polishing cloth 208. The polishing amount in this double-side polishing step was adjusted so that the surface of the active layer wafer 101 was removed by 10 μm. The wafer polished by the double-side polishing apparatus 200 under the above conditions was used as the bonded SOI wafer 100 of Example 1.

<Example 2>
Double-side polishing using a bonded SOI wafer 100 under the same conditions as in Example 1, using a polishing liquid 201 that does not contain abrasive grains, and using a fixed abrasive polishing cloth with abrasive grains fixed as the first polishing cloth 205 Thus, a bonded SOI wafer 100 of Example 2 was obtained. Specifically, fixed abrasive grains in which colloidal silica is dispersed and fixed in polyurethane-based resin as the first polishing cloth 205 using a polishing liquid under the same conditions as the polishing liquid used in Example 1 except that the abrasive grains are not included. A polishing cloth was used. Other conditions are the same as those in Example 1.

<Comparative Example 1>
A wafer obtained by polishing a bonded SOI wafer 100 under the same conditions as in Example 1 with a single-side polishing apparatus was used as a bonded SOI wafer 100 in Example 2.

<Comparative example 2>
A bonded SOI wafer 100 of Comparative Example 2 was defined as a wafer polished by the double-side polishing apparatus 200 under the same conditions as in Example 1 except that the polishing liquid 201 did not contain a water-soluble polymer.

<Contrast of Examples and Comparative Examples>
The polishing amount of the backside silicon oxide film 104 was compared between Examples 1 and 2 and Comparative Example 2. The polishing amount of the silicon oxide film 104 on the back surface for Examples 1 and 2 and Comparative Example 2 is shown in FIG. FIG. 6 is a diagram illustrating measurement points of the bonded SOI wafer 100. The figure shows the back surface of the bonded SOI wafer 100. The reference point, Notch, is 2, and measurement points 1, 2, 3, 4, and 5 are assigned, respectively. The measurement points correspond to the measurement points in FIG. For the measurement, a visible light reflection interference type film thickness measuring device (Nanometrics Japan Co., Ltd., model Nanospec) was used. Since Comparative Example 1 is polishing by a single-side polishing apparatus, the silicon oxide film 104 on the back surface is not polished, and thus is excluded from comparison.

The polishing amount of the silicon oxide film 104 on the back surface is 360 to 450 mm in Example 1, 0 to 27 mm in Example 2, 1500 to 2250 mm in Comparative Example 2, and the silicon oxide film 104 on the back surface in Examples 1 and 2 is It was confirmed that a sufficient oxide film thickness remained without being removed.

Next, for each of the bonded SOI wafers 100 obtained by polishing in Examples 1 and 2 and Comparative Examples 1 and 2, the flatness is measured with respect to the back surface, and the maximum and minimum widths of the wafer surface are measured. Back Ideal Range). For the GBIR measurement, an ADE flatness measuring device (Ultra Gage 9500) was used. The result of GBIR measurement is shown in FIG.

  Example 1 was 0.2626 μm, Example 2 was 0.2976 μm, Comparative Example 1 was 0.3440 μm, and Comparative Example 2 was 0.8055 μm. Compared to Example 1, Example 2 was about 10%, Comparative Example 1 was about 30%, and Comparative Example 2 was about 3 times as bad as GBIR.

  Next, the surface of the wafer was visualized three-dimensionally based on the data measured by the flatness measuring device, and the cross section in the direction of the arrow of the wafer was visualized. 8 to 11 show three-dimensional and cross-sectional views corresponding to Examples 1 and 2 and Comparative Examples 1 and 2, respectively. Further, a diagram visualized three-dimensionally is shown in each (a), and a diagram visualized in a cross section is shown in each (b).

  In Example 1, although there were small irregularities on the entire wafer, the outer edge of the wafer did not drop and the resulting flatness was satisfactory. In Example 2, although the overall unevenness is small, it is considered that the flatness was inferior to that in Example 1 because there was a slight drop in the outer edge on one side of the wafer. In Comparative Example 1, it is considered that the flatness was inferior because the entire wafer was uneven and the outer edge on one side of the wafer was depressed. In Comparative Example 2, it is considered that the flatness was greatly deteriorated because the entire wafer was uneven and there was a large drop at the outer edge on one side of the wafer.

From the above results, it was confirmed that a silicon oxide film remained on the back surface of the support wafer, and a natural SOI film was removed from the surface of the active layer wafer to obtain a bonded SOI wafer with excellent mirror-polished GBIR characteristics. .

The bonded SOI wafer subjected to the double-side polishing treatment of the present invention is used as a substrate for high-speed CMOS, high-integrated digital IC, analog / digital mixed IC, automotive power IC, etc. that are expected to operate at high speed, low power consumption and high withstand voltage. Can be used.

DESCRIPTION OF SYMBOLS 100 Bonded SOI wafer 101 Wafer for active layer 102 Wafer for support 104 Silicon oxide film 200 Double-side polishing apparatus 201 Polishing liquid 202 Lower surface plate 205 First polishing cloth 208 Second polishing cloth

Claims (3)

In a method for polishing a silicon wafer in which a natural oxide film is formed on the front surface and a silicon oxide film is formed on the back surface,
Polishing of the silicon wafer, a double-side polishing treatment using a polishing solution containing a water-soluble polymeric additive in an aqueous alkaline solution, wherein said natural oxide film on the surface of the silicon wafer is removed by mirror polishing, and the silicon It is performed so that the silicon oxide film formed on the back surface of the wafer remains,
The polishing liquid does not contain abrasive grains, the first polishing cloth for polishing the surface of the silicon wafer used in the double-side polishing process is a polishing cloth to which abrasive grains are fixed, and the second polishing pad for polishing the back surface of the silicon wafer. The polishing cloth is a polishing cloth in which abrasive grains are not fixed,
Wherein the water-soluble polymer additive, a polishing method of a silicon wafer, comprising the inclusion in said alkaline aqueous solution such that the difference in frictional resistance in the front and rear surfaces of the silicon wafer is reduced. The method for polishing a silicon wafer according to claim 1, wherein the surface of the silicon wafer is polished downward by the first polishing cloth with the surface of the silicon wafer facing downward. 3. The method for polishing a silicon wafer according to claim 1, wherein the surface of the silicon wafer after the double-side polishing process is mirror-polished by a single-side polishing process.