Patent Description:
A surface processing may be performed by means of dry chemo-mechanical grinding (CMG) (e.g., <CIT>). In a CMG process, a synthetic grindstone obtained by fixing abrasives (abrasive grains) with a resin binder such as a thermoplastic resin is used. The synthetic grindstone is pressed against a wafer while the wafer and the synthetic grindstone are rotated (e.g., <CIT>). By being heated and oxidized by friction with the synthetic grindstone, convex portions on the surface of the wafer become brittle, and are peeled off. In this manner, only the convex portions of the wafer are ground and planarized.

As a CMG process, for example, of a synthetic grindstone advances, abrasive grains (abrasives) gradually fall out from a surface (surface of action of a mirror surface processing) of a binder of the synthetic grindstone facing an object to be ground, making the surface of action of the synthetic grindstone smooth. This increases the opportunity of contact between the binder, which is, for example, a thermoplastic resin, and the object to be ground at the surface of action. While this results in a decrease in a contact pressure between the abrasive grains and the object to be ground and a decrease in processing efficiency, frictional heat between the surface of action of the synthetic grindstone and the object to be ground may become excessive if a dry processing is performed in an attempt to improve the processing rate, thereby possibly causing burns or scratches, caused by involution of polishing sludge, on the object to be ground.

Moreover, document <CIT> discloses a highly porous, bonded abrasive article having an interconnected pore structure. The article comprises abrasive particles, bond material and dispersoid particles. The bond material is a resin bond, a metal bond, etc., but not a non-woven fabric binder.

The present invention has been made to solve the above-described problem, and it is an object of the present invention to provide a synthetic grindstone, a synthetic grindstone assembly, and a method of manufacturing a synthetic grindstone capable of suppressing occurrence of excessive frictional heat at the time of performing, for example, a dry mirror surface processing.

According to claim <NUM> of the present invention, a synthetic grindstone for performing a surface processing includes abrasive grains with an abrasive grain proportion (Vg) higher than <NUM> vol. % and equal to or lower than <NUM> vol. %, and a nonwoven-fabric used as a binder of the synthetic grindstone, the binder having a binder proportion (Vb) equal to or higher than <NUM> vol. % and lower than <NUM> vol. The synthetic grindstone has a porosity (Vp) higher than <NUM> vol. % and equal to or lower than <NUM> vol.

As shown in <FIG>, a synthetic grindstone <NUM> is formed of abrasive grains (abrasives) <NUM> and a binder <NUM>. The synthetic grindstone <NUM> may further include pores <NUM>. In the synthetic grindstone <NUM> of the present embodiment, the abrasive grains <NUM> are dispersively retained in the binder <NUM>, and the pores <NUM> are dispersively disposed in the binder <NUM>.

If an object to be ground is silicon, it is preferable, for example, that a silica, a cerium oxide, or a mixture thereof be applied as the abrasive grains <NUM>; however, the configuration is not limited thereto. Similarly, if an object to be ground is sapphire, it is preferable that a chromic oxide, a ferric oxide, or a mixture thereof, etc. be applied. Other applicable abrasives that may be used depending on the type of the object to be ground include alumina, silicon carbide, or a mixture thereof.

In the present embodiment, an example will be explained in which the object to be ground is silicon, and a cerium oxide with an average grain size of, for example, approximately <NUM> is used as the abrasive grains <NUM>. The grain size of the abrasive grains <NUM> can be suitably set; however, it is preferable that it be, for example, smaller than <NUM>.

According to the invention, a nonwoven fabric is used as the binder <NUM>. Examples of the nonwoven fabric that can be used include polyester short fibers. Examples of the polyester short fibers that can be used include polyethylene terephthalate (PET) short fibers.

The synthetic grindstone (molded body) <NUM> is formed based on the flow (manufacturing method) shown in <FIG>.

First, a mixed material (mixed powder) is obtained by mixing abrasive grains <NUM> with a short-fiber binder <NUM> for forming a nonwoven fabric at volume proportions shown in <FIG>, to be described below (step ST1). At this stage, the binder <NUM> is observed, not under magnification, in an approximately powder form.

Subsequently, the mixed material is filled into a metallic mold for forming the mixed material into a final shape of the synthetic grindstone <NUM> (step ST2). At this time, the fibers can be integrated using dry, wet, or other methods. The synthetic grindstone <NUM> is pressure-molded (hot-pressed) at <NUM> for <NUM> minutes, and is molded into a molded body (step ST3). Thereafter, the molded body in the metallic mold is removed from the mold (step ST4).

<FIG> shows a three-phase diagram showing "three aspects" (i.e., an abrasive grain proportion (Vg), a binder proportion (Vb), and a porosity (Vp)) of the above-described synthetic grindstone <NUM> produced with a nonwoven fabric.

<FIG> show experiment results (<NUM> products) obtained in an attempt to produce a synthetic grindstone <NUM> by suitably adjusting three aspects (an abrasive grain proportion (Vg), a binder proportion (Vb), and a porosity (Vp)) of the synthetic grindstone <NUM>. Through the experiments, a boundary indicating whether or not a synthetic grindstone <NUM> can be created has been formed. It has been found that a product with a composition falling inside the boundary shown in <FIG> is usable as a synthetic grindstone <NUM>.

Of the <NUM> products in total, <NUM> products were formed into a synthetic grindstone <NUM> durable for use. The <NUM> products were formed with an abrasive grain proportion (Vg) of abrasive grains falling in a range from <NUM> vol. % to <NUM> vol. %, with a binder proportion (Vb) equal to or higher than <NUM> vol. % and lower than <NUM> vol. %, and with a porosity (Vp) of pores higher than <NUM> vol. % and lower than <NUM> vol. It can be seen that a product shown in <FIG> with an abrasive grain proportion of <NUM> vol. % was formed into a synthetic grindstone <NUM>. Since a synthetic grindstone <NUM> whose abrasive grain proportion is <NUM> vol. % does not contain abrasive grains <NUM>, the abrasive grain proportion of such a synthetic grindstone <NUM> becomes higher than <NUM> vol. % in actuality. For the synthetic grindstone <NUM> of the present embodiment, the abrasive grain proportion (Vg) of the abrasive grains <NUM> is determined first, and then the binder proportion (Vb) of the binder <NUM> is set.

<FIG> shows an electron scanning microscope image in which one of the <NUM> synthetic grindstones <NUM> that are durable for use is magnified by <NUM> times. <FIG> shows the presence of a nonwoven-fabric binder <NUM> in the synthetic grindstone <NUM>. <FIG> shows the presence of, as well as a nonwoven-fabric fibrous resin (in an elongated shape), which is the binder <NUM>, granular abrasive grains <NUM> adhering to the fibrous resin.

The <NUM> products found to be usable as a synthetic grindstone <NUM> were subjected to a durometer hardness measurement (ASTM D <NUM>-<NUM> Type DO). <FIG> shows hardness measurements of the synthetic grindstone <NUM> at respective points shown in <FIG>. As shown in <FIG>, it can be seen that the synthetic grindstone <NUM> becomes relatively soft as the porosity increases, and becomes relatively hard as the porosity decreases.

Hereinafter, the six products that were not formed into a synthetic grindstone <NUM> will be referred to as "molded bodies". Of the <NUM> products in total, the remaining six products had a composition beyond the boundary shown in <FIG>, and were molded bodies not formed into a synthetic grindstone <NUM>. Of the molded bodies beyond the boundary shown in <FIG>, those in a region indicated by an arrow α had a high porosity and a low fill density. It can thus be presumed that corners and surfaces of such molded bodies crumbled greatly because of insufficient binding of the binder. Of the molded bodies beyond the boundary shown in <FIG>, those in a region indicated by an arrow β had a low porosity and a sufficiently high fill density. It can be presumed that such molded bodies had a powdered surface because of a low binding rate of the binder. It can be presumed that, of the molded bodies beyond the boundary shown in <FIG>, those in a region indicated by an arrow γ had an excessively low porosity and an excessively high fill density. It can be seen that such molded bodies did not have specified dimensions at the time of molding.

It can be presumed that an excessively high abrasive grain proportion also resulted in crumbling of the molded bodies because of failure of binding. It is preferable that the abrasive grain proportion be, for example, higher than <NUM> vol. % and equal to or below <NUM> vol. %, as described above.

Thus, it can be seen that a synthetic grindstone <NUM> that uses a nonwoven fabric as a binder cannot be molded unless each of the abrasive grain proportion, the binder proportion, and the porosity is set to a volume proportion within a predetermined range.

In the present embodiment, it is assumed that the synthetic grindstone <NUM> is formed in a disc shape and used in a dry chemo-mechanical grinding (CMG) processing in which the synthetic grindstone <NUM> is treated by both a mechanical action and a chemical-component-based composition action. That is, the synthetic grindstone <NUM> exerts a dry chemo-mechanical grinding action on a surface of a wafer W, which is an object to be ground, and performs a surface processing on the wafer W to be ground. Thereafter, a synthetic grindstone assembly <NUM> is formed by fixing the synthetic grindstone <NUM> to a grindstone retaining member (substrate) <NUM> with a double-sided tape, an adhesive, or the like, and is then attached to a CMG device <NUM> shown in <FIG> and used for a surface processing of the wafer W, which is an object to be ground. The grindstone retaining member <NUM> may be of any material which has a suitable stiffness that is resistant to a CMG processing, which has a heat resistance up to a temperature that may be increased by use of the synthetic grindstone <NUM>, and which is not thermally softened, and examples of such a material include an aluminum alloy material.

The wafer W, which is an object to be ground, is pressed against the synthetic grindstone <NUM> while the synthetic grindstone assembly <NUM>, which includes the grindstone retaining member <NUM> and the synthetic grindstone <NUM>, and the wafer W are rotated in an arrow direction shown in <FIG>. At this time, the synthetic grindstone <NUM> is rotated at a circumferential velocity of, for example, <NUM>/min, and the wafer W is pressed at a processing pressure of <NUM>/cm<NUM>. This allows the synthetic grindstone <NUM> and the surface of the wafer W to slidably move. After the processing starts, the synthetic grindstone <NUM> and the surface of the wafer W slidably move, and an external force acts on the binder <NUM>. Through continuous action of the external force and advancement of the CMG process, abrasive grains (abrasives) gradually fall out from a surface (surface of action of a mirror surface processing) of the binder <NUM> of the synthetic grindstone <NUM> facing the surface of the wafer W to be ground. Through a chemo-mechanical grinding action of fixed abrasive grains <NUM> retained in a nonwoven fabric, which is the binder <NUM>, or abrasive grains <NUM> dislodged out of the nonwoven fabric, which is the binder <NUM>, the surface of the wafer W is ground. By being heated and oxidized by friction with the synthetic grindstone <NUM>, convex portions on the surface of the wafer W become brittle, and are peeled off. In this manner, through grinding of only the convex portions on the surface of the wafer W, the surface of the wafer W is planarized.

According to the invention, a nonwoven fabric is used as the binder <NUM> instead of using a thermoplastic resin material (e.g., ethyl cellulose) as a binder. Accordingly, an elastic deformation amount of the binder <NUM> can be made great compared to the case where a thermoplastic resin material is used as a binder. With such a configuration, the synthetic grindstone <NUM> according to the present embodiment is excellent in trackability to the surface of the wafer W, which is an object to be ground (processed object).

In the case of using a thermoplastic resin material as a binder, if heat accumulates between the synthetic grindstone and the wafer W, the thermoplastic resin material used as the binder is softened, thus causing elution, etc. at the surface of the synthetic grindstone. If the thermoplastic resin material used as the binder melts and adhesion to the surface of the wafer W, referred to as "sticking", occurs, a grinding resistance of the synthetic grindstone suddenly increases, possibly causing surface roughness and scratches of the wafer W.

On the other hand, if a nonwoven fabric is used as the binder <NUM> as in the synthetic grindstone <NUM> according to the invention , even if heat accumulates in the binder <NUM>, elution at the surface of the synthetic grindstone <NUM> does not occur. It is thereby possible to prevent the binder <NUM> from being melted even if heat accumulates between the synthetic grindstone <NUM> and the wafer W. With such a configuration, the synthetic grindstone <NUM> according to the present embodiment can maintain stable processing properties for a longer period of time. It is thereby possible to prevent unintended scratches from occurring on the surface of the wafer W, which is an object to be ground. Thus, by using the nonwoven-fabric binder <NUM> according to the present embodiment, the object to be ground (worked surface) can be ground softly compared to the case where a thermoplastic resin material is used as a binder, thereby contributing to a decrease in damage to the object to be ground.

Behind this, the present inventors have made every effort to prevent occurrence of excessive frictional heat at the time of performing, for example, a dry mirror surface processing, and discovered that a synthetic grindstone <NUM> formed to satisfy the three aspects of the grindstone of the above-described three-phase diagram achieves excellent processing properties on the object to be ground. That is, a synthetic grindstone <NUM> preferable for performing a dry surface processing includes abrasive grains <NUM> with an abrasive grain proportion (Vg) higher than <NUM> vol. % and equal to or lower than <NUM> vol. %, includes a nonwoven-fabric binder <NUM> with a binder proportion (Vb) equal to or higher than <NUM> vol. % and lower than <NUM> vol. %, and has a porosity (Vp) higher than <NUM> vol. % and equal to or lower than <NUM> vol. By using the synthetic grindstone <NUM> according to the present embodiment, it is possible, at the time of performing, for example, a dry mirror surface processing, to suppress excessive frictional heat from occurring between the synthetic grindstone <NUM> and the object to be ground, through the employment of a chemical solid-phase reaction that locally occurs under a high temperature and a high pressure between the synthetic grindstone <NUM> and the object to be ground. By performing, for example, a dry mirror surface processing on the object to be ground using the synthetic grindstone <NUM> according to the present embodiment, it is possible to achieve a processing (mirror surface processing) with extreme flatness, with a surface roughness of the object to be ground on the sub-nanometer order.

According to the present embodiment, it is possible, at the time of performing, for example, a dry mirror surface processing, to provide a synthetic grindstone <NUM>, a synthetic grindstone assembly <NUM>, and a method of manufacturing the synthetic grindstone <NUM> capable of suppressing occurrence of excessive frictional heat.

In the present embodiment, a range of volume proportions within which a synthetic grindstone <NUM> can be formed has been set with respect to the example of using PET short fibers as a nonwoven fabric of the binder <NUM>. For the nonwoven fabric used as the binder <NUM>, polyamide (PA) short fibers or polypropylene (PP) short fibers, as well as polyester short fibers, may be used. For the nonwoven fabric, one or more of polyester short fibers, polyamide (PA) short fibers, and polypropylene (PP) short fibers may be selectively used. Even if such fibers are used as the nonwoven fabric of the binder <NUM>, the above-described range of volume proportions including the abrasive grain proportion (Vg), the binder proportion (Vb), and the porosity (Vp) may be set similarly to the case of using PET short fibers. The configuration of the nonwoven fabric used as the binder <NUM> is not limited thereto. For example, a long-fiber nonwoven fabric may be used. Examples of the long-fiber nonwoven fabric that may be used include polyester long fibers, polypropylene long fibers, etc., and a mixture thereof.

A short-fiber nonwoven fabric refers to a fabric using cut fibers, and a long-fiber nonwoven fabric refers to a fabric using an endless fiber. For a short-fiber nonwoven fabric that uses cut fibers, the length of fibers can be suitably set. The length of fibers of the short-fiber nonwoven fabric is on the order of microns. The long-fiber nonwoven fabric uses fibers with connected length equal to, for example, a winding length. For example, if the winding length is <NUM>, a single fiber is approximately <NUM> in length.

In the present embodiment, an example has been explained in which the synthetic grindstone <NUM> is provided in a disk shape. The synthetic grindstone <NUM> may be formed in a pellet shape, an elongated cuboid shape, or another shape. The synthetic grindstone assembly <NUM> is formed in a suitable shape that retains the synthetic grindstone <NUM>.

An example has been explained in which the synthetic grindstone <NUM> according to the present embodiment is used in a dry processing; however, it may also be used in, for example, a wet processing using grinding water (e.g., pure water).

A case will be explained where a synthetic grindstone <NUM> according to the present modification contains, as a first filler, coarse particles with a suitable size.

It is preferable that the first filler be, for example, in a spherical shape; however, the first filler need not necessarily be in a spherical shape, and may be of a massive form that may include irregularities and/or deformations. The first filler is, for example, silica, and is dispersively fixed by a binder <NUM> formed of a nonwoven fabric. It is preferable that the first filler contain silica with a grain size larger than that of the abrasive grains <NUM>, and silica with a smaller grain size fixed to the periphery of the silica with the larger grain size. It is preferable that the grain size of the silica with the smaller grain size be smaller than that of the abrasive grains <NUM>. A volume proportion of the first filler in the synthetic grindstone <NUM> is set by a correlation with a binder proportion (Vb) of the binder <NUM> based on, for example, an abrasive grain proportion (Vg) of the abrasive grains <NUM>. That is, for the synthetic grindstone <NUM> of the present modification, an abrasive grain proportion (Vg) of the abrasive grains <NUM> is determined first, and then a binder proportion (Vb) of the binder <NUM> and a volume proportion of the first filler are set based on a correlation between the binder <NUM> and the first filler. It is preferable that the first filler be larger than <NUM> vol. % and equal to or smaller than <NUM> vol.

The abrasive grains <NUM>, which are formed of a cerium oxide, have a hardness equivalent to or lower than a wafer W to be ground, and are composed mainly of silicon or an oxide thereof. As compared to the abrasive grains <NUM>, the first filler, which is formed of silica, has a hardness equivalent to or lower than the wafer W, or an oxide thereof.

The synthetic grindstone <NUM> including the abrasive grains <NUM>, the nonwoven-fabric binder <NUM>, and the first filler is manufactured as explained in the above-described embodiment.

Since the average grain size of the first filler is larger than that of the abrasive grains <NUM>, the synthetic grindstone <NUM> and the wafer W are, during a processing, brought in near contact with each other via vertexes of the particles of the first filler. That is, since the first filler is present between a matrix (i.e., the abrasive grains <NUM> and the nonwoven-fabric binder <NUM>) of the synthetic grindstone <NUM> and the wafer W, the matrix and the wafer W are not brought in direct contact, and a certain clearance occurs.

If a processing is started with the first filler being in contact with the wafer W, an external force acts on the matrix. Through continuous action of the external force, the abrasive grains <NUM> are dislodged out of the matrix. The dislodged abrasive grains <NUM> are present at a processing interface in a state of adhering to the first filler in the clearance between the synthetic grindstone <NUM> and the wafer W. Accordingly, the abrasive grains <NUM> and the wafer W are, during the processing, brought in near contact with each other via vertexes of the particles of the first filler. Thereby, an actual contact area between the abrasive grains <NUM> and the wafer W becomes significantly small, thus increasing a working pressure at the point of processing. This advances the grinding processing with a high processing efficiency.

The clearance promotes replacement of air in the neighborhood of the surface of the wafer W with fresh air, thereby cooling the worked surface. Also, the sludge caused by the abrasive grains <NUM> is discharged from the wafer W to the outside via the clearance, thereby preventing the surface of the wafer W from being damaged. As a result, it is possible to prevent burns, scratches, etc. on the surface of the wafer W caused by frictional heat.

In this manner, the wafer W is ground with the synthetic grindstone <NUM> to have a planar surface with a predetermined roughness.

With the synthetic grindstone <NUM> according to the present modification, it is possible to maintain a high processing efficiency by maintaining a sufficient contact pressure between the abrasive grains <NUM> and the wafer W even in an advanced stage of the processing, and to prevent a decrease in the quality of the wafer W and occurrence of scratches by suppressing direct contact between the binder <NUM> and the wafer W. In the present modification, with the heat generated between the synthetic grindstone <NUM> and the object to be ground, it is possible to suppress generation of excessive frictional heat, as explained in the above-described embodiment.

Examples of the first filler that may be applied include silica, carbon, silica gel (which is a porous body of them), activated charcoal, and a spherical resin. It is to be noted that a hollow balloon, which is used as a pore forming agent, is not appropriate, since it may burst during the processing and cause scratches.

A case will be explained where a synthetic grindstone <NUM> according to the present modification contains, as a second filler, an electrically conductive substance of a suitable size smaller than that of the first filler explained in the first modification. In the present modification, an example will be described in which an aluminum alloy material, for example, is used as a material of the grindstone retaining member <NUM> of the above-described CMG device <NUM> having an electrical conductivity and a suitable level of thermal conductivity.

Examples of the electrically conductive material include carbon nanotubes. Such substances have an average grain size smaller than that of the abrasive grains <NUM>. A volume proportion of the second filler in the synthetic grindstone <NUM> is set by a correlation with a binder proportion (Vb) of the binder <NUM> based on, for example, an abrasive grain proportion (Vg) of the abrasive grains <NUM>. That is, for the synthetic grindstone <NUM> of the present modification, an abrasive grain proportion (Vg) of the abrasive grains <NUM> is determined first, and then a binder proportion (Vb) of the binder <NUM> and a volume proportion of the second filler are set based on a correlation between the binder <NUM> and the second filler. It is preferable that the second filler be added at a volume proportion larger than <NUM> vol. % and equal to or smaller than <NUM> vol. By using, for example, carbon nanotubes as the second filler, the intensity of the synthetic grindstone <NUM> can be improved as a structure.

As the processing of the wafer W is started with the CMG device <NUM>, the synthetic grindstone <NUM> and the wafer W slidably move, thus causing an external force to act on the binder <NUM>. Through continuous action of the external force, the abrasive grains <NUM> are dislodged. The dislodged abrasive grains <NUM> slidably move in the clearance between the synthetic grindstone <NUM> and the wafer W. Through a chemo-mechanical grinding action of the abrasive grains <NUM>, the surface of the wafer W is ground.

If the surface of the wafer W is ground and a friction occurs, static electricity may occur on the surface of the wafer W. At this time, the second filler, which is electrically conductive, allows the static electricity on the surface of the wafer W to flow through the grindstone retaining member <NUM> (see <FIG>). Accordingly, by using the synthetic grindstone <NUM> according to the present embodiment, static electricity occurring on the surface of the wafer W can be discharged while grinding the surface of the wafer W. As a result, it is possible to prevent adhesion of dust, etc. to the surface of the wafer W.

In the present modification, the grindstone retaining member <NUM> has a high thermal conductivity compared to the synthetic grindstone <NUM>. If the surface of the wafer W is ground and a friction occurs, frictional heat occurs on the surface of the wafer W. At this time, the second filler absorbs the frictional heat, and the heat absorbed by the second filler is conducted to the grindstone retaining member <NUM>. Accordingly, by using the synthetic grindstone <NUM> according to the present modification, frictional heat occurring on the surface of the wafer W can be removed by grinding the surface of the wafer W. As a result, it is possible to prevent occurrence of burns on the surface of the wafer W caused by frictional heat between the surface of the synthetic grindstone <NUM> and the surface of the wafer W, and to prevent scratches. With the synthetic grindstone <NUM> according to the present modification, it is possible to provide a preferable surface processing of the wafer W, and to increase the lifespan of the synthetic grindstone <NUM>.

It is also preferable that a heat dissipator such as heat radiation fins be provided on the grindstone retaining member <NUM>, which rotates together with the synthetic grindstone <NUM>, namely, it is preferable that the synthetic grindstone assembly <NUM> include a heat dissipator (heat transfer section). In this case, the heat dissipator is brought in contact with air, causing the heat of the synthetic grindstone <NUM> to be effectively dissipated.

It is also possible to arrange water piping for cooling water in the grindstone retaining member <NUM>, thereby cooling the grindstone retaining member <NUM> and the synthetic grindstone <NUM>.

In the present modification, an example has been explained in which the grindstone retaining member <NUM> has an electrical conductivity and a higher thermal conductivity than that of the synthetic grindstone <NUM>; however, the grindstone retaining member <NUM> may be of a material having at least one of an electrical conductivity or a thermal conductivity higher than that of the synthetic grindstone <NUM>. In the case of the grindstone retaining member <NUM> having an electrical conductivity, it is possible to remove the static electricity between the object to be ground and the synthetic grindstone <NUM>; in the case of the grindstone retaining member <NUM> having a thermal conductivity higher than that of the synthetic grindstone <NUM>, it is possible to effectively dissipate heat that may occur in the synthetic grindstone <NUM>.

In the first modification, an example has been explained in which the first filler is used, and in the second modification, an example has been explained in which the second filler is used. It is also preferable that the synthetic grindstone <NUM> include both the first filler and the second filler.

A case will be explained where a synthetic grindstone <NUM> according to the present modification contains, as a third filler, particles of a suitable size smaller than that of the first filler explained in the first modification.

Examples of the particles of the third filler include green carborundum (GC). Such particles have a hardness higher than the wafer W, which is an object to be ground. The particles of the third filler such as GC may be greater than or smaller than the average grain size of the abrasive grains <NUM>. As a matter of course, the particles such as GC may be of a size equivalent to the average grain size of the abrasive grains <NUM>.

The average grain size of the abrasive grains <NUM> based on a metal oxide such as an aluminum oxide (alumina), a zirconium oxide (zirconia), a cerium oxide (ceria), and a silicon oxide (silica) may be greater than, smaller than, or equivalent to that of GC. For example, average grain sizes of the alumina-based, zirconia-based, or ceria-based abrasive grains <NUM> are mostly greater than that of GC. For example, the average grain size of alumina-based abrasive grains <NUM> may be equivalent to the size of GC (smaller than <NUM>). If, for example, the particle size of GC, etc. is <NUM>, the average grain size of the abrasive grains <NUM> based on silica, etc. may be <NUM>. A volume proportion of the third filler in the synthetic grindstone <NUM> is set by a correlation with a binder proportion (Vb) of the binder <NUM> based on, for example, an abrasive grain proportion (Vg) of the abrasive grains <NUM>. It is preferable that the third filler be added at a volume proportion larger than <NUM> vol. % and equal to or smaller than <NUM> vol.

A technique (gettering effect) is known in which a gettering site such as fine flaws is formed on a back surface, opposite to a top surface, of the wafer W, and impurities are captured in the gettering site. GC, which has a hardness higher than the back surface of the wafer W, is used to intentionally make flaws on the back surface of the wafer W.

Claim 1:
A synthetic grindstone for performing a surface processing on an object to be ground, comprising:
abrasive grains having an abrasive grain proportion (Vg) higher than <NUM> vol.% and equal to or lower than <NUM> vol.%; and
a nonwoven-fabric which is used as a binder of the synthetic grindstone, the binder having a binder proportion (Vb) equal to or higher than <NUM> vol.% and lower than <NUM> vol.%,
wherein the synthetic grindstone has a porosity (Vp) higher than <NUM> vol.% and equal to or lower than <NUM> vol.%.