Title

An abrasive material, wheel and tool for grinding semiconductor substrates, and method of manufacture of same.

Field of the Invention

The present invention relates to techniques for increasing the quality and uniformity of the machining process of substrates. In particular, it relates to the reduction of scratches and chatter marks when machining substrate surfaces.

Background to the Invention.

Abrasive tools are commonly used for the precision grinding of hard brittle materials such as ceramic wafers. Various methods are used for the production of these grinding wheels and in the constituent components of the abrasive elements. Abrasive tools generally take the form of a wheel core attached to an abrasive rim of dense, metal bonded super abrasive segments by means of a thermally stable bond. However these grinding wheels with an abrasive rim comprised of segments with an face substantially parallel to its direction of travel i.e. the tangential direction, contributes substantially to the incidence of scratching and chatter marks on the surface of machined wafers.

In Japanese patent 60-118,469 Ishihara discloses a method of producing grinding wheels comprised of porous metal bonded grinding elements bonded to a hub. This is achieved by sintering the metal bond and abrasive mixed with granular salt, which was used as a sacrificial filler. After sintering, the grinding segments are submerged in water, which dissolves the salt, leaving a metal bond structure with open porosity behind. Ishihara recommends that the porosity level be kept below 50% in order to keep good structural strength in the metal matrix.

In US 6,755,729 and US 6,685,755 Ramanath et al. discloses a metal bond product with open porosity of between 50% and 80% that could be usefully employed to grind hard brittle materials. They also use the technique of including a sacrificial filler in the abrasive mix when sintering, and then removing it by the process immersion in a solvent. Typically, in the case of metal bond, salt is the sacrificial filler, and hot water,

the solvent. Furthermore, resin bond tools with open porosity levels of between 40% and 80% could also be usefully used, where sugar is used as the sacrificial filler.

JP60-118,469 discloses a method of forming pores by immersing the metal matrix and salt composite in water. This method was followed in US 6,685,755. However, this immersion technique has disadvantages. It is slow, and can be difficult to implement. Most especially, it has been found that the surface tension of the liquid solvent used, can make it difficult to produce matrices with small pores of less than 50 microns, and especially less than 25 microns. The presence of trapped air in the pores can stop the dissolving process. Furthermore, the immersion technique limits the depth from which the sacrificial filler may be removed to a few millimetres, in the case of pores of about 100 microns in size.

In addition bonds with porosity levels below 50% as disclosed in Japanese patent 60 118,469 can also display unfavourable characteristics and tend to behave in what is commonly called a 'hard' state, where scratching, vibration, and chatter can develop when grinding hard materials. Bonds with higher porosities as disclosed in US 6,685,755 can be very fragile.

Furthermore, they can behave with an overly soft characteristic, where wheel wear increases to an unsatisfactory level.

In US 3,925,035 Keat discloses the use of fillers, such as graphite, in metal bonded cup grinding wheels for dry grinding cemented carbides. He also discloses how useful brittle intermetallics bonds are in the grinding of such hard brittle materials. He particularly discloses a brittle bronze alloy based on the brittle intermetallics Cu3Sn, (equivalent to approximately 62% copper / 38% tin, by weight) and discloses how this gave longer life and lower power consumption than previous wheels when grinding cemented carbide. He discloses how using copper and tin powders give a low sintering temperature, with obvious economic advantages.

In US 6,093,092, the use of 10% to 35% by volume friable fillers, such as graphite, in a brittle bond are discussed. Bonds with % friable fillers in the range 15% to 35% as disclosed can display unfavourable performance characteristics. Under certain

performance conditions they behave in what is commonly called 'hard' state, where vibration, chatter and scratching can develop when grinding hard materials.

In US 4,042,347, Sioui discloses a metal-resin composite, which consists of two interlocking phases - one metal the other resin. It is manufactured by selecting materials with similar sintering temperatures and then sintering both phases together. The metal typically includes brittle intermetallics, and the resin is typically a high temperature resistant resin such as polyamide.

In US 6,063,148 Fischbacher also discloses a metal / resin tool where the metal and resin consist of two interlocking phases.

All of the above have certain limitations. Porosity of less that 50% does not give desirable grinding characteristics in many applications, however porosity levels of 50% to 80% leads to grinding segments that are fragile, and can break in handling or in operation. Additionally sintering both metal and resin phases together to form interlocking phases is very difficult to achieve in practice as the lowest temperature for sintering useful intermetallics is at the upper limit for sintering useful resin powders.

Summary of the Invention.

According to the present invention there is provided a cup wheel for a grinding tool for use in grinding semiconductor substrates, the wheel comprising: a hub having a pair of opposing faces and a peripheral rim linking said faces; and at least one grinding segment mounted on and upstanding from at least one face of the hub; wherein in use, as the wheel is rotated, the face of the hub on which the at least one grinding segment is mounted is directed towards the substrate to be ground, to bring the at least one grinding segment into contact with the substrate, and wherein at least one pair of side faces of the at least one grinding segment intersect at a leading edge which substantially faces the direction of travel of the grinding segment as the wheel rotates.

Preferably the cup wheel comprises a plurality of grinding segments.

At least one of said plurality of grinding segments may have a triangular cross- section.

Alternatively at least one of said plurality of grinding segments may have a square cross-section.

Alternatively at least one of said plurality of grinding segments may have a rectanglar cross-section.

The invention further provides a cup wheel for a grinding tool for use in grinding semiconductor substrates, the wheel comprising: a hub having a pair of opposing faces and a peripheral rim linking said faces; and a plurality of cylindrical grinding segments mounted on and upstanding from at least one face of the hub.

It will be appreciated that the segments may be substantially cylindrical or have another curved-edged form.

The invention further provides a cup wheel for a grinding tool for use in grinding semiconductor substrates, the wheel comprising: a hub having a pair of opposing faces and a peripheral rim linking said faces; and at least one grinding segment mounted on and upstanding from at least one face of the hub, wherein said at least one grinding segments is an arcuate segment having at least one serrated side edge.

In each of the above and all embodiments of the invention, it will be appreciate that the leading profile (i.e. in the direction of travel in use) of the at least one segment is non planar. It will be appreciated that the leading profile could be curved or an apex of two faces. In other words, the leading face extends non-radially.

The invention further provides a peripheral wheel for a grinding tool for use in grinding semiconductor substrates, the wheel comprising: a hub having a pair of opposing faces and a peripheral rim linking said faces; and at least one grinding segment mounted on and upstanding from said peripheral rim of the hub; wherein in use, as the wheel is rotated, the rim of the hub is directed towards the substrate to be ground, to bring the at least one grinding segment into contact with the substrate, and wherein at least one side edge of the at least one grinding segment is serrated.

Preferably the plurality of grinding segments are positioned end-to-end around the circumference of the wheel to form a serrated grinding rim.

The invention further provides an abrasive material for use in grinding semiconductor substrates comprising: a metal bond matrix containing friable fillers and a network of open pores such that the open porosity level is from about 0% to about 50% by volume, and the friable filler content is from about 5% to about 65% by volume, and where the combined total of friable filler and open porosity is from about 50% to about 80% by volume, and where the balance is composed of a network of abrasive- containing metal.

Preferably the friable filler content is from about 50% to about 65% by volume.

Preferably, the friable filler is selected from the group consisting of graphite, hexagonal boron nitride, petroleum coke, volcanic stone.

The abrasive may be either diamond or CBN or a combination thereof, or any other suitable material.

Preferably the pores have been produced by a sacrificial filler method.

The sacrificial fillers may have been substantially removed by dissolving in a gas.

The invention further provides a grinding tool comprising the aforementioned abrasive material of the invention.

The at least one grinding segment of the cup wheel of the invention may comprises the aforementioned abrasive material according to the present invention.

The at least one grinding segment of the peripheral wheel of the invention may comprises the aforementioned abrasive material according to the present invention.

Either wheel may be embodied in a grinding tool.

The present invention further comprises a cup wheel substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

The present invention further comprises a peripheral wheel substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

The present invention further comprises an abrasive material substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

The present invention further comprises a grinding tool substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

The present invention further comprises a method of producing a cup wheel substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

The present invention further comprises a method of producing a peripheral wheel substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

The present invention further comprises a method of producing an abrasive material substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

The present invention further comprises a method of producing a grinding tool substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

The present invention discloses that segments, whose faces are at an angle substantially reduces the incidence of scratching.

The present invention discloses a grinding tool for grinding semiconductor substrates consisting of a hub with a one or more grinding segments attached so that the segments form an abrasive rim, with at least two faces with respect to the direction of travel during the machining operation, such that at least one of the rim faces is substantially not parallel to the direction of travel to its direction of travel during the machining process.

In a preferred embodiment all the rims' faces are substantially not parallel to the direction of travel to its direction of travel during the machining process. The faces of the rims on the side from which the substrate is fed may not be parallel to the direction of travel to its direction of travel during the machining process. The substrate to be machined may be either the front side or the backside of a semiconductor wafer. In one embodiment the substrate to be machined is a prime wafer. The grinding tool of the present invention may be either a cup grinding wheel or a peripheral grinding wheel. The semiconductor substrate to be machined usually consists of one of the following materials GaAs, sapphire, silicon carbide, AlTiC, glass or GaN. In a preferred embodiment, the substrate consists of the device layer on a semiconductor wafer. Abrasives may be either diamond or CBN or a combination of both.

Pores are generally formed by the sacrificial filler method as in JP60-118,469 The present invention discloses that a more advantageous method is to use a dissolving gas to dissolve the sacrificial filler. The gas can penetrate even small-sized pores, and is not affected by trapped air bubbles in the pores. It can penetrate to a depth of 4mm or more, and so makes it possible to sinter large blanks in one mould. The gas is preferably used at a pressure of one atmosphere or above. The blanks can then undergo other production processes such as machining to thickness or further impregnation. The blank can then be machined into individual grinding segments ready for bonding to the machine hub.

Pores have drawbacks in certain grinding applications. They reduce the structural strength of the bond. Furthermore, if grinding soft, long-chipping materials such as metal, the pores can clog, leading to significant reduction in grinding performance. The present invention discloses that one method of avoiding this is to pump liquid through the porous network. As the grinding wheel usually operates in the presence of a cooling liquid, this coolant can be readily supplied via channels to the base of the grinding segments. The faces of the grinding segments can be sealed off with an epoxy to ensure the coolant flows to the grinding face. In one embodiment centrifugal force is sufficient to force the coolant through the pores, though additional pressure may be applied by using a pressurised water supply.

Brittle bronze bonds made from copper and tin and optionally copper phosphorous powders have been used since Keat disclosed them in US 3,925,035. The present invention discloses that using pre-alloyed bronze powder, particularly pre-alloyed 60/40 bronze powder, is advantageous as it reduces the number of powders to be kept in storage, and simplifies weighing and mixing of powders. It does however have the disadvantage that it has to be sintered at a higher temperature. A sintering temperature of approximately 600 ⁰ C is required for satisfactory sintering results. The present inventor has tested grinding wheels based on this bond with varying compositions, and has found that all offer improved manufacturing characteristics relative to those made from separate metal powders. These wheels included compositions containing from 30% up to 70% friable fillers, from 35% to 70% open porosity, and from 0% to 20% wear resistant fillers.

The friable fillers of the present invention may be selected from one of the following: graphite, hexaganol boron nitride, petroleum coke, volcanic stone etc.

The present invention discloses that using friable fillers in quantities between 35% and 75%, and most favourably between 50% and 70% is advantageous when grinding hard brittle materials. The higher % filler reduces the occurrence of scratching, chatter and vibration, and ensures satisfactory performance when grinding hard brittle materials such as glass, silicon, sapphire, silicon carbide and other ceramics.

It was discovered that a large gap in performance characteristics exists between those with low % fillers of US6,093,092 and high % open pores of US6,685,755. The present invention discloses that brittle metal bonds with friable filler content of from 50% to 70% help to bridge the gap in performance characteristics.

A preferred embodiment of the present invention uses a metal matrix with a combination of open pores and friable fillers is especially advantageous. A favourable combination of metal bond matrix is where the open porosity level is from 0% to 50%, and the friable filler content is from 5% to 65%, and where the combined total of friable filler and open porosity is from 50% to 80%. This region offers a continuum of performance characteristics from 'hard' through to 'soft' wheels, which is very useful to the wheel designer.

In practice it can be difficult to achieve open porosity at levels below 30% by simple powder metallurgy techniques. Micro machining holes in segments is technically feasible, and is becoming economically viable. In this case, an array of holes may be drilled, typically by laser.

Brief Description Of The Drawings

Various embodiments of the present invention will now be described with reference to the accompanying drawings in which:

FIG. 1 shows a conventional cup wheel with series of arcurate segments as common in industry and similar to those shown in US 6,685,755, 6,755,729, US 6,102,789 and US 6,093,092.

FIG 2 shows a conventional peripheral wheel. FIG.3 shows the rim formed with a series of substantially square shaped segments with their faces set at 45 degrees to the tangent. FIG 4 shows a face view of a quarter of the wheel rim.

Fig 5 shows a pattern made with individual segments, each with a serrated edge. Fig 6 shows a pattern made with triangular segments, where only one wheel rim edge is substantially not parallel to the direction of travel.

Fig 7 shows a peripheral wheel designed according to this invention. Fig.8 shows a rim consisting of more than one row of segments.

Detailed Description of the Drawings In prior art and in common industrial application as in Figure 1 conventional cup wheels 1 comprise of a series of arcurate segments 101 bonded to a metal core 102. In Figure 1, these segments are discontinuous, however in other embodiments, they may form a continuous rim as shown on the peripheral wheel 2 in Figure 2. The continuous rim may comprise one segment or at least two segments, separately sintered and then mounted on the core 202. Discontinuous rims as in Figure 1 are manufactured from at least two segments 101 and the segments are separated by slots or gaps 103 in the rim. These gaps 103 in the rim facilitate the removal of waste product which could scratch the work piece surface, particularly useful in low speed surface finishing operations.

In a preferred embodiment of the present invention, shown in Figure 3 a conventional cup wheel 3 is shown, but the discontinuous rim 302 is formed from a series of substantially square shaped segments 301 with their faces set at 45° to the tangent. For ease of reference, only one segment in figures 3 to 8 is shown as porous, however, it should be understood that all remaining segments on the rim are also porous. These faces which are at an angle, substantially reduce the incidence of scratching. The segments are spaced along the rim 302. Preferably the segments are not uniformly spaced as shown by the gaps 304 a and b which are different dimensions. However, it will be appreciated that uniformly sized gaps are also possible.

These segments 301 are composed of a metal bond matrix containing friable fillers and a network of open pores. A preferred embodiment of the present invention, has an open porosity level from 0% to 50% by volume and the friable fillers content of 5% to 65% by volume, resulting in a combined total of friable filler and open porosity from 50% to 60% by volume, and where the remainder is composed of a network of abrasive containing metal. The friable filler in the present invention is selected from a group consisting of graphite, hexagonal boron nitride, petroleum coke, or volcanic stone. The abrasive used is either diamond or CBN or a combination thereof.

The pores 303 have been produced by the sacrificial filler method.

Pores 303 are typically formed by sintering the metal bond and abrasive mixed with a sacrificial filler such as granular salt. In one embodiment of the present invention, gas is used to dissolve the sacrificial filler. The gas can penetrate even small-sized pores, and is not affected by trapped air bubbles in the pores. It can penetrate to a depth of 4mm or more, and so makes it possible to sinter large blanks in one mould. The blanks can then undergo other production processes such as machining to thickness or impregnation. In the embodiment shown in Fig 3. the blanks have been machined into individual grinding segments ready for bonding to the machine hub.

Using a metal matrix with a combination of open pores and friable fillers is especially advantageous. A particular combination of metal bond matrix that is favoured is where the open porosity level is from 0% to 50%, and the friable filler content is from 5% to 65%, and where the combined total of friable filler and open porosity is from 50% to 80%. This offers a continuum of performance characteristics from 'hard' through to 'soft' wheels, which is very useful to the wheel designer. In practice it can be difficult to achieve open porosity at levels below 30% by simple powder metallurgy techniques. Micro machining holes in segments is technically feasible, and is becoming economically viable. In this case, an array of holes may be drilled, typically by laser.

A preferable embodiment of the present invention includes compositions containing from 30% up to 70% friable fillers, from 35% to 70% open porosity, and from 0% to 20% wear resistant fillers.

The segments can take various forms such as squares 301 in Fig 3 or rectangles 401 in Fig 4. The rim 402 is composed of substantially rectangular segments set to form a rim which has a jagged face at an angle to the direction of travel as indicated by the arrow in Fig. 4. These faces which are at an angle substantially reduce the incidence of scratching.

Alternatively in a further embodiment as shown in Fig 5, each segment can be formed with a number of serrated faces.

In an alternative embodiment, the segments may be triangular in shape as in figure 6. In the embodiment in Figure 6 the segment face 1062 is parallel to the direction of travel. Segment face 1601 is not. Segments on a grinding rim my be substantially the same and set in a regular geometric pattern as shown in Figure 7 where the segments 701 substantially abut each other. Figure 7 shows an arrangement where the grinding wheel is a peripheral wheel with the outer circumference of the wheel having a smooth surface.

Segments may however vary in size, geometry, orientation on the rim, or spacing from each other along the rim. The variances may be either consistent or random. Wheels with random variances have been tested, and found to work satisfactorily. It is suspected that random variations in the rim may have beneficial effects in helping to suppress resonant vibrations, regardless of the segment composition. Additionally, as shown in Figure 8 the rim may consist of more than one row of segments allowing a configuration useful for wide peripheral wheels. In Figure 8, three rows 801a, 801b and 801c are visible with segments spaced out at intervals along these rows.

The words "comprises/comprising" and the words "having/including" when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for

brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.