IE20080376U1 - An abrasive material, wheel and tool for grinding semiconductor substrates, and method of manufacture of same - Google Patents

Abstract

ABSTRACT A cup wheel for a grinding tool for use in grinding semiconductor substrates is presented herein. 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 tace of the hub; wherein in use, as the wheel is rotated. the face otthe 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 ofthe at least one grinding segment intersect at a leading edge which substantially faces the direction oftraxrel ofthe grinding segment as the wheel rotates.

Description

The present invention relates to techniques for increasing the qiiality 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 ofhard 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 lace 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 ofmachined wafers.

In Japanese patent 60-1 18,469 lshihara 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. lshihara 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 ofincluding a sacrificial filler in the abrasive mix when sintering, and then removing it by the process immersion in a solvent. Typically, in the case ofmetal bond, salt is the sacrificial filler, and hot water, lEG80?376 T he invention further provides a grinding tool comprising the aforementioned abrasive material ofthe invention‘ The at least one grinding segment of the cup wheel ofthe 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 ofthe 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 ofthe 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 ofproducing a peripheral wheel substantially as described herein with reference to and as shown in any one or more of the accompanying drawings. lE080376 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 oftwo 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 polyamidei In US 6,063,148 Fischbacher also discloses a metal / resin tool where the metal and resin consist oftwo interlocking phases.

All ofthe 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 ofopposing laces and a peripheral rim linking said laces: and at least one grinding segment mounted on and upstanding from at least one face olthc 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 oftravel ofthe grinding segment as the wheel rotates.

IE Q 8 E) 7'? 7 6 Preferably the cup wheel comprises a plurality ofgrinding segments.

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

Alternatively at least one ofsaid plurality of grinding segments may have a square Cross-section, Alternatively at least one ofsaid plurality ofgrinding 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 ofopposing faces and a peripheral rim linking said faces; and a plurality of cylindrical grinding segments mounted on and upstanding from at least one face ofthe 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; at 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 lace olthe hub, wherein said at least one grinding segments is an arcuate segment having at least one serrated side edge.

In each ofthe above and all embodiments ofthe invention, it will be appreciate that the leading profile (i.e. in the direction of travel in use) ofthe 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. l\J tJ.

|ECi80?€76 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 ofthc hub; wherein in use, as the wheel is rotated, the rim ofthe 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 ofthe wheel to form a serrated grinding rim.

The invention limher provides an abrasive material for use in grinding semiconductor substrates comprising: a metal bond matrix containing friable fillers and a network ol‘ 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 ofgraphite. 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 tillers may have been substantially removed by dissolving in a gas. to U: lE080376 3 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 offorming pores by immersing the metal matrix and salt composite in water. This method was followed in US 6,635,755. However, this immersion technique has disadvantages. It is slow. and can be difficult to implement.

Most especial ly. it has been found that the surface tension ofthe liquid solvent used. can make it difficult to produce matrices with small pores ofless than 50 microns. and especially less than 25 microns. The presence oftrapped air in the pores can stop the dissolving process. Furthennore, the immersion technique limits the depth from which the sacrificial tiller may be removed to a few millimetres, in the case of pores ofabout microns in size.

In addition bonds with porosity levels below 50% as disclosed in Japanese patent 60 1 I 8,469 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 .685,755 can be very fragile. l-‘urtliermore. 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 offillers, such as graphite, in metal bonded cup grinding wheels for dry grinding cemented carbides. He also discloses how useful brittle inlermetallics bonds are in the grinding of such hard brittle materials. Hc particularly discloses a brittle bronze alloy based on the brittle intermetallics C u3Sn. (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 l0% to 35% by volume friable fillers, such as graphite. in a brittle bond are discussed. Bonds with % friable fillers in the range l5% to 35% as disclosed can display unfavourable performance characteristics. Under certain lE2Ti85)T376 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 ofproducing 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 ofa 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 ofthe rim faces is substantially not parallel to the direction oftravel to its direction oftravel during the machining process. ln :1 preferred embodiment all the rims’ faces are substantially not parallel to the direction of travel to its direction oftravel 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 oftravel to its direction oftravel during the machining process. The substrate to be machined may be either the front side or the backside ofa 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 ofthe 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. lEt380S76 3 Pores are generally formed by the sacrificial filler method as in JP60—l 18,469 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 ol‘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 ofthe 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 ofavoiding 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 ofthe grinding segments. The faces ofthe grinding segments can be sealed off with an epoxy to ensure the coolant flows to the grinding face. In one embodiment centriliugal loree is sufficicnt 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 olapproximately 600°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. to um /“ . ‘V,’ 9 l E e 8 0 ca 7 6 The friable fillers ofthe present invention may be selected from one ofthe 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 % lillcrs 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 % to 70% help to bridge the gap in perfomiance 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 ofperfonnance 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: IN) ‘J: lE:;8L>375 10 FIG. I shows a conventional cup wheel with series of areurate segments as common in industry and similar to those shown in US 6.685.755, 6,755,729, US 6, I 02,789 and US 6,093,092.

FIG 2 shows a conventional peripheral wheel.

F l(}.3 shows the rim fonned with a series of substantially square shaped segments with their faces set at 45 degrees to the tangent.

FIG 4 shows a face view ofa quarter ofthe 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 oftravel.

Fig 7 shows a peripheral wheel designed according to this invention.

Fig.8 shows a rim consisting of more than one row ofsegments.

Detailed Description of the Drawings In prior art and in common industrial application as in Figure l conventional cup wheels I comprise ofa series of arcurate segments 101 bonded to a metal core 102. In Figure l. these segments are discontinuous, however in other embodiments. they may form :1 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 ofthe 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 30! with their faces set at 45° to the tangent.

These faces which are at an angle, substantially reduce the incidence Ofscratching, 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 ofa metal bond matrix containing friable fillers and a network ofopen pores. A preferred embodiment of the present invention. has an IEUBG575 it open porosity level from 0% to 50% by volume and the friable fillers content of5% to 65% by volume, resulting in a combined total offriable filler and open porosity from 50% to 60% by volume, and where the remainder is composed of a network oi‘ 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 ofthe present invention, gas is used to dissolve the sacrificial filler. The gas can penetrate even small—sized pores, and is not allected by trapped air bubbles in the pores. It can penetrate to a depth 01' 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 % 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 diflicult 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 ofholes may be drilled. typically by laser.

A preferable em bodiment ofthe present invention includes compositions containing from 30% up to 70% friable fillers, from 35% to 70% open porosity, and from 0% to % 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 fonn a lEait80376 ,2 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 ofscratching.

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 160] 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 70l 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 ofsegments allowing a configuration useful for wide peripheral wheels. In Figure 8, three rows 801a, 80| b and 801 c 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 ofone 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.

Claims (27)

1. 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 ofside 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.

2. The cup wheel ofclaim I comprising a plurality ofgrinding segments.

3. The cup wheel of claim 2 wherein at least one of said plurality of grinding segments has a triangular cross-section.

4. The cup wheel ofclaim 2 wherein at least one ofsaid plurality ofgrinding segments has a square cross-section.

5. The cup wheel ofclaim 2 wherein at least one of said plurality ofgrinding segments has a rectangular cross-section.

6. A cup wheel for a grinding tool for use in grinding semiconductor substrates, the wheel comprising: a hub having a pair ofopposing 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. F0 in lEaa057e M

7. 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 ofthe hub. wherein said at least one grinding segments is an arcuate segment having at least one serrated side edge.

8. A peripheral wheel for a grinding tool for use in grinding semiconductor substrates. the wheel comprising: a hub having a pair ofopposing faces and a peripheral rim linking said faces; and at least one grinding segment mounted on and upstanding from said peripheral rim ofthe 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 ofthe at least one grinding segment is serrated.

9. The peripheral wheel ofclaim 8 comprising a plurality ofgrinding segments positioned end-to—end around the circumference ofthe wheel to form a serrated edged grinding rim.

10. 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 tiller and open porosity is from about 50% to about 80% by volume. and where the balance is composed ofa network ofabrasive-containing metal.

11. 1 I. The abrasive material ofclaim 10 wherein the friable filler content is from about 50% to about 65% by volume. 30 lEae03?6 5

12. The abrasive material ofclaim 10 or claim I I wherein the friable filler is selected from the group consisting of graphite, hexagonal boron nitride, petroleum coke. volcanic stone.

13. The abrasive material ofany ofclaims 10 to 12 wherein the abrasive is either diamond or CBN or a combination thereof.

14. The abrasive material ofany ofclaims 10 to 13 where the pores have been produced by a sacrificial filler method.

15. The abrasive material ofany ofclaim 14 wherein the sacrificial tillers have been substantially removed by dissolving in a gas.

16. A grinding tool comprising the abrasive material ofany ofclaims 10 to 15.

17. I7. The cup wheel ofany ofclaims 1 to 7 wherein the at least one grinding segment comprises the abrasive material ofany ofclaims 10 to 15.

18. The peripheral wheel of claim 8 or claim 9 wherein the at least one grinding segment comprises the abrasive material ofany ofclaims 10 to 15.

19. I9. A grinding tool comprising the wheel of any ofclaims 1-9, 17 or 18.

20. A cup wheel substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

21. 2]. A peripheral wheel substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

22. An abrasive material substantially as described herein with reference to and as shown in any one or more of the accompanying drawings.

23. A grinding tool substantially as described herein with reference to and as shown in any one or more ofthe accompanying drawings. I6

24. A method of producing a cup wheel substantially as described herein with reference to and as shown in any one or more ofthe accompanying drawings.

25. 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.

26. 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.

27. A method ofproducing a grinding tool substantially as described herein with reference to and as shown in any one or more ofthe accompanying drawings.