GB2522982A

GB2522982A - Post-synthesis processing of diamond and related super-hard materials

Info

Publication number: GB2522982A
Application number: GB1421809.3A
Authority: GB
Inventors: Mark Robin Mcclymont
Original assignee: Element Six Technologies Ltd
Current assignee: Element Six Technologies Ltd
Priority date: 2013-12-11
Filing date: 2014-12-08
Publication date: 2015-08-12
Anticipated expiration: 2034-12-08
Also published as: GB201421809D0; GB201321899D0; WO2015086544A1; GB2522982B

Abstract

A method of processing a super-hard material product 6, 100 having a Vickers hardness of no less than 2000 kg/mm2 comprises mounting the super-hard material product 6, 100 with a surface in contact with a surface of a processing wheel 2, loading the super-hard material product 6, 100 such that it is pressed against the surface of the processing wheel 2 with a loading force, rotating the processing wheel 2 and feeding an abrasive slurry 14 onto the surface of the processing wheel 2, the abrasive slurry 14 comprising a carrier fluid within which are disposed super-hard abrasive grit particles 104 having an average particle size of at least 1Âµm, which particles 104 roll between the surface of the processing wheel 2 and the surface of the super-hard material product 6, 100 in order to cause surface micro-cracking of the super-hard material product 6, 100 and removal of material from the surface of the super-hard material product 6, 100, the method further includes at least two processing periods: a first processing period during which the loading force 106 is applied to the super-hard material product 6, 100 in a non-central location of the super-hard material product 6, 100 and a second processing period during which the loading force 106 applied to the super-hard material product 6, 100 is reduced and centralised on the super-hard material product 6, 100 relative to the loading force 106 applied during the first processing period.

Description

POST-SYNTHESIS PROCESSING OF DIAMOND AND RELATED SUPER-HARD

MATERIALS

Background of Invention

The present invention relates to post-synthesis processing of diamond and related super-hard materials. In particular, the present invention relates to optimized lapping processes for diamond and related super-hard materials.

Summary of Invention

In the context of the present invention super-hard materials are defined as those materials having a Vickers hardness of no less than 2000 kg/mm2. These materials include a range of diamond materials, cubic boron nitride materials (cBN), sapphire, and composites comprising the aforementioned materials, For example, diamond materials include chemical vapour deposited (CVD) single crystal and polycrystalline synthetic diamond materials of a variety of grades, high pressure high temperature (HPHT) synthetic diamond materials of a variety of grades, natural diamond material, and diamond composite materials such as polycrystalline diamond which includes a metal binder phase (PCD) or silicon cemented diamond (ScD) which includes a silicon/silicon carbide binder phase.

In relation to the above, it should be noted that while super-hard materials are exceedingly hard, they are generally very brittle and have low toughness. As such, these materials are notoriously difficult to process into a product after the raw material is synthesized. Any processing method must be sufficiently aggressive to overcome the extreme hardness of the super-hard material while at the same time must not impart a large degree of stress or thermal shock to the material which would cause macroscopic fracturing of the material due to its brittle nature and low toughness. Furthermore, for certain applications it is important that surface and sub-surface damage at a microscopic scale, such as microcracking, is minimized to avoid deterioration of functional properties which may result from such surface and sub-surface damage including, for example, optical scattering, increased optical absorption, decreased wear resistance, and increased internal stress resulting in a decrease in coherence time for quantum spin defects near the processed surface, There is narrow operating window for achieving successful processing of super-hard materials and many available processing methods fall outside this operating window. For :i.

example, most processing methods are not sufficiently aggressive to process super-hard materials to any significant extent in reasonable time-frames. Conversely, more aggressive processing techniques tend to impart too much stress and/or thermal shock to the super-hard material thus causing cracking and material damage or failure.

Certain processing methods have operational parameters which can be altered so as to move from a regime in which no significant processing of a super-hard material is achieved into a regime in which processing is achieved but with associated cracking and damage or failure of the super-hard material. In this case, there may or may not be a transitional window of parameter space in which processing can be achieved without cracking and damage or failure of the super-hard material. The ability to operate within a suitable window of parameter space in which processing can be achieved without cracking and damage or failure of the super-hard material will depend on the processing technique, the size of any transitional operating window for such a technique, and the level of operation parameter control which is possible to maintain processing within the window of parameter space in which processing can be achieved without cracking and damage or failure of the super-hard material.

in light of the above, it will be appreciated that post-synthesis processing of super-hard materials is not a simple process and, although a significant body of research has been aimed at addressing this problem, current processing methods are still relatively time consuming and expensive, with processing costs accounting for a significant proportion of the production costs of super-hard material products.

Post synthesis processing may comprise one or more of the following basic processes: surface processing to remove material from the surface of the as-grown super-hard material in order to increase surface flatness, decrease surface roughness, remove surface defects, and/or attain a target thickness for the super-hard material; surface processing to achieve a fine surface finish where minimal material is removed from the super-hard product, i.e. polishing; and cutting of the super-hard material into target shapes and sizes for particular product application.

In principle there are two basic forms of mechanical surface processing: (i) a two-body process in which abrasive particles are embedded/fixed in one body which is moved against a second body to process the second body; and (ii) a three-body process in which one body is moved relative to a second body to be processed and free abrasive particles, constituting a third body, are disposed between the first and second bodies in order to achieving surface processing of the second body.

The latter three-body approach to surface processing is known as lapping and it is this approach which is conventionally used to remove macroscopic quantities of surface material from super-hard materials. Three-body lapping, as opposed to a two-body surface processing technique, is preferred for removing macroscopic quantities of surface material from super-hard materials as it has been found that lapping is more efficient at removing surface material from a super-hard material without imparting a large degree of stress or thermal shock to the material which would cause macroscopic fracturing of the material due to its brittle nature and low toughness. In contrast, when it is desired to achieve a fine surface finish without removing macroscopic quantities of material then a two-body processing technique may be utilized. As such, conventionally hipping is used to remove material from the surface of an as-grown supcr-hard matcrial in ordcr to incrcasc surfacc flatncss, dccrcasc surf'acc roughness, remove surface defects, and/or attain a target thickness for the super-hard material. Subsequently, if a fine surface finish is required, the super-hard material is polished and this may be performed using a two-body surface processing technique in which abrasive material is fixed in a polishing wheel such as via resin bonding. Polishing may also be achieved using an iron or steel wheel which is diamond impregnated and this is known as scaife polishing. Although scaife polishing generally utilizes free diamond abrasive particles these are of a small size relative to pores within the iron or steel wheel and are thus embedded/fixed into the wheel thus effecting a two-body processing as opposed to a true three body lapping process.

The present invention is primarily concerned with lapping super-hard materials using relatively coarse diamond powders or grits (or other super-hard powders or grits) to remove material from the surface of the as-grown super-hard material in order to increase surface flatness, decrease surface roughness, remove surface defects, and/or attain a target thickness for the super-hard material, Lapping of super-hard materials is known in the art. The type of abrasive which should be utilized will be dependent on the type of material which is to be lapped. Obviously, when lapping super-hard materials a super-hard abrasive will be required.

Accordingly, for lapping of diamond materials a diamond abrasive is utilized. In this regard diamond lapping pastes and slurries are widely available comprising a diamond powder or grit disposed within a carrier fluid. Diamond lapping slurries may have a variety of different concentrations of diamond powder/grit and varying particle sizes of diamond powder/grit.

Furthermore, lapping machinery for performing the lapping process is also widely commercially available (e.g. from Kemet'TM, Logitech, and Peter Wolters'TM).

A review article summarizing various surface processing techniques for use on chemical vapour deposited (CVD) diamond films and substrates is provided in Diamond and Related Materials 8 (1999) 1198-1213. This review article includes a description of mechanical lapping of diamond films using coarse diamond powders greater than 1.0 m in size (i.e. diamond grit) disposed in a binder fluid such as olive oil. It is described that this type of mechanical processing removes diamond material via a micro-chipping or micro-cleavage mechanism, It is further described that after lapping using coarse diamond abrasive particles finer diamond powers can be used for final polishing.

The Handbook of Lapping and Polishing (edited by Joan D. Mannescu, Eckhart Uhlmann, & Toshiro K. Doi, CRC Press, 2007) indicates that lapping incorporates three types of abrasive mechanism: rolling abrasive, sliding abrasive, and microcutting abrasive. Furthermore, the WikipediaT" entry for lapping indicates that lapping produces microscopic conchoidal fractures as abrasive rolls about between two surfaces and removes material thus suggesting that a rolling abrasive mechanism is an inherent feature of standard lapping processes.

WO2009/059384 also discloses a lapping method for processing diamond material and indicates that the process can be controlled to ensure that diamond abrasive particles roll over the surface of the processing wheel and the diamond being processed.

In fact, the aforementioned rolling mechanism is dominant when using larger abrasive grains in a coarse lapping technique as suggested in the Diamond and Related Materials review article. Such a coarse lapping process can be performed to remove material from the surface of an as-grown super-hard material in order to increase surface flatness, decrease surface roughness, remove surface defects, and/or attain a target thickness for the super-hard material. Subsequently, finer abrasive powders can be used for final polishing which are then more prone to be mechanically immobilized in pores of a processing wheel thus providing more of a microcutting processing mechanism.

Despite the fact that processing of super-hard materials using rolling, sliding, and microcutting abrasive mechanisms is known in the art as discussed above, the present inventors have found that there are still some difficulties in achieving lapping of super-hard materials in an oplimized manner. Accordingly, it is an aim of certain embodiments of the present invention to provide lapping configurations and techniques which allow super-hard materials to be processed efficiently and which a high degree of precision and controllability.

That is, while lapping is a standard processing technique, the difficulties in processing super-hard materials have led the present inventor to adapt standard lapping processes as described previous in the art so as to be able to more efficiently lap super-hard materials with a high degree of precision and controllability.

Summary of Invention

Lapping methods for processing a super-hard material product having a Vickers hardness of no less than 2000 kg/mm2 according to embodiments of the present invention comprising: mounting the super-hard material product with a surface of the super-hard material product in contact with a suitce of a processing wheel with an intertice region disposed between the surface of the super-hard product and the surface of the processing wheel; loading the super-hard material product such that the super-hard material product is pressed against the surface of the processing wheel with a loading force; rotating the processing wheel; and feeding an abrasive slurry onto the surface of the processing wheel, the abrasive slurry comprising super-hard abrasive grit particles disposed in a carrier fluid, wherein the super-hard abrasive grit particles of the abrasive slurry have a particle size of at least 1 gin and roll between the surface of the processing wheel and the surface of the super-hard material product within the interface region in order to cause surface micro-cracking of the super-hard material product and removal of material from the surface of the super-hard material product.

As-grown super-hard material products such as polycrystalline CVD diamond wafers often have variable thickness and may, for example, have a wedge shaped cross-sectional profile due to asymmetries in the growth process. In order to reduce this variable thickness and increase parallelism of front and rear surfaces of the as-grown super-hard material product it is possible during the aforementioned lapping process to apply a loading force to the super-hard material product in a non-central location of the super-hard material product. The loading force will generally be applied to a thicker portion of the as-grown super-hard material product, such as at a thicker side of a wedge shaped wafer of super-hard material, This asymmetric loading of the super-hard material product causes preferential processing of the thicker portion of the super-hard material product thus reducing the variation in thickness and increase parallelism of front and rear surfaces of the as-grown super-hard material product.

Surprisingly, it has been found that if the load force is subsequently reduced and centralised on the super-hard material product during the final stages of the lapping method this reduces edge rounding and lapping damage of the super-hard material product for a given abrasive grit size. Furthermore, it has been found that by using this approach and carefully controlling thc lapping proccss at this stagc it is also possiblc to rcducc or climinatc thc rcquircmcnt for lapping with fine abrasive particles and the super-hard material product can be transferred straight to a polishing process or sold without polishing which saves cost and time.

In light of the above, one aspect of the present invention utilizes a lapping process as defined above wherein the method of processing includes at least two processing periods including a first processing period during which the loading force is applied to the super-hard material product in a non-central location of the super-hard material product and a second processing period during which the loading force applied to the super-hard material product is reduced and centralised on the super-hard material product relative to the loading force applied during the first processing period.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure I shows a two stage lapping process for processing a surface of a super-hard material product; Figure 2 shows a cross-sectional view of a standard lapping configuration; Figure 3 shows a plan view of a standard lapping configuration; Figure 4 shows a side view of a constraining ring used to mount a super-hard material product on a processing wheel as illustrated in Figures 2 arid 3; Figure 5 shows a cross-sectional view of a lapping configuration using an underfeed arrangement for feeding abrasive slurry onto a lapping wheel; Figure 6 shows a plan view of a lapping configuration using an underfeed arrangement for feeding abrasive slurry onto a lapping wheel; and Figure 7 shows a cross-sectional view of a carrier substrate with a super-hard material product mounted thereto for processing.

Detailed Description

As described in the summary of invention section, embodiments of the present invention are directed to a lapping process for super-hard materials including a first processing period during which a loading force is applied to a super-hard material product in a non-central location of the super-hard material product and a second processing period during which the loading force applied to the super-hard material product is reduced and centralised on the super-hard material product relative to the loading force applied during the first processing period. As further described in the summary of invention section, providing a loading force applied to a super-hard material product in a non-central location of the super-hard material product is useful for preferentially processing a thicker portion of the as-grown super-hard material product thus reducing thickness variations and increasing parallelism between front and rear surfaces of an as-grown super-hard material product which has non-uniform thickness, Furthermore, it has been surprisingly found that if the load force is reduced and centralised on the super-hard material product during the final stages of the lapping method this reduces edge rounding and lapping damage of the super-hard material product for a given abrasive grit size. Furthermore, it has been found that by using this approach and carefully controlling the lapping process at this stage of processing it is also possible to reduce or eliminate the requirement for lapping with fine abrasive particles and the super-hard material product can be transferred straight to a polishing process without an intermediate fine lapping step or otherwise sold without polishing which saves cost and time.

The aforementioned lapping process is illustrated schematically in Figure 1. Figure 1(a) illustrates a first lapping step in which a super-hard material product 100 is lapped on a processing wheel 102 with abrasive particles 104. In the first lapping step the as-grown super-hard material product 100 has a wedge-shaped cross-section and a loading force 106 is applied at a non-central, thick portion of the super-hard material product 100. This causes preferential processing of the thick portion of the super-hard material product 100 to increase thickness uniformity and parallelism of front and rear surfaces of the super-hard material product 100. After the first lapping step, a second lapping step is performed as illustrated in Figure 1(b) in which the loading tbrce 106 is reduced in magnitude and centralized on the super-hard material product 100. As previously indicated, this has been found to reduce edge rounding and lapping damage of the super-hard material product 100 for a given abrasive grit size and by using this approach and carefully controlling the lapping process at this stage of processing it is also possible to reduce or eliminate the requirement for lapping with fine abrasive particles.

In relation to the above, according to certain embodiments during the final stages of lapping the loading force is reduced and moved such that it is located through the centre of mass of the super-hard material product being processed. If the super-hard material product is symmetrical, such as a square or round piece of super-hard material, then this may corresponding to locating the loading force through the centre of symmetry of the super-hard material product, i.e. through the geometric centre of the super-hard material. However, perfect centralization is not a strict requirement and some benefits can still be obtained if the loading force is moved to a more central location during the second lapping period relative to the first lapping period while not necessarily being perfectly centralized in the final period of lapping. That said, a central or substantially central location for the loading force in the final period of lapping is preferred.

The loading force may be applied by locating weights on the super-hard material product.

For example, during a first processing period a first weight is located on the super-hard material product and during the second period a second weight, which is smaller in mass than the first weight, is located on the super-hard material product. During the first processing period the first weight is positioned over the super-hard material product in a non-central location relative to the centre of mass of the super-hard material and during the second processing period the second weight is positioned in a different location to the first weight which is closer to the centre of mass of the super-hard material, In this regard, the first and second weights may be smaller in lateral size than the super-hard material product such that they can be readily moved around to different locations on the super-hard material product.

Further still, according to certain embodiments the loading force may be moved from a more peripheral location to a relatively more central location in a single step, in multiple steps in which the loading is moved sequentially towards a more central location, or otherwise moved gradually to a more central location in a continuous manner.

Yet further, it is also possible to move the location of the loading force over a surface of the super-hard material during the first and/or second lapping periods. In this case, the location of the loading force during a lapping period may be calculated as the time-averaged loading location and in this case it will be the time-averaged loading location which is moved to a more central location in the second lapping period relative to the first lapping period.

Furthermore, it may also be noted that super-hard materials are generally very rigid and the loading force is not intended to significantly bend the super-hard material product but rather provide a loading which enables the grit particle to process the surface of the super-hard material. If the loading is located in a non-central location then this will tend to cause preferential processing of the surface of the super-hard material by the abrasive grit particles at the non-central location.

In addition to the above, it has also been found to be useful to control the flatness of the processing wheel during the second processing period (i.e. during the final stages of lapping) such that the processing wheel deviates from a perfectly flat surface by no more than 20 jim, or more preferably no more than 10 jim, at least over a portion of the processing wheel which is utilized to process the super-hard material product. In this regard, it may be noted that it is difficult to provide a highly flat processing wheel for the entire lapping process and over multiple uses as processing of super-hard materials products using super-hard abrasive grit causes significant wear of the processing wheel leading to non-uniformities in the surface profile of the processing wheel, It has been found that it is possible to use a less flat processing wheel in a rough early stage lapping process and then move to a flatter processing wheel during the final stages of lapping.

As such, during the second processing period a flatness of the processing wheel may be controlled to have a greater degree of flatness than during the first processing period, at least over a portion of the processing wheel which is utilized to process the super-hard material product. This may be achieved either by re-processing of the processing wheel between the first and second processing periods to increase the flatness of its surface profile or otherwise by re-placing the processing wheel with another processing wheel has a greater degree of flatness between the first and second processing periods.

It is also known that the average particle size of super-hard abrasive grit particles will affect the material removal rate from a super-hard material product and the quality of surface finish.

In this regard, as previously indicated, it has been found that by carefully controlling the final stages of lapping by centralizing and reducing applied force it is possible to achieve a good quality surface finish without the need to a fine lapping stage using fine super-hard abrasive grit particles. The average particle size of the super-hard abrasive grit particles during the first processing period will still generally be equal to or greater than the average particle size of the super-hard abrasive grit particles during the second processing period. However, the average particle size of the super-hard abrasive grit particles during the second processing period may still be relatively large while still achieving a good quality surface finish by using the methodology as described herein. For example, the average particle size of the super-hard abrasive grit particles during the second processing period may be at least 1 tm, 5 tm, p.m, 20 m, 30 m, or 35 pm and/or no more than 60 pm, 50 pm, or 45 am and!or in a range having end points selected from any combination of the aforementioned lower and upper values. Using this approach, it is possible to move directly to a two body polishing process afler the second processing period of lapping without any intermediate three body surface processing step which uses super-hard abrasive grit particles having a smaller average particle size than the super-hard abrasive grit particles used during the second processing period.

The loading force used to press the super-hard material product against the surface of the processing wheel during the lapping method may be in a range 0.05 g/mm2 to 1.5 g/mm2 or 0.1 g/mm2 to 0.6 glmm2, at least during at least during the second processing period.

According to certain applications, during the first processing period a loading force towards an upper end of these ranges is used while during the second processing period a loading forced towards a lower end of these ranges is used. The loading force may be applied directly on the super-hard material product. Alternatively, a carrier substrate can be mounted to the super-hard material product and the loading force applied to the super-hard material product via the carrier substrate. In this case, the carrier substrate may have a surface flatness better than 20 tm, 10 lIm, 5 tm, or 1 tm.

Other processing parameters which have been found to be useful include: rotating the processing wheel at a speed in a range 0.2 ms1 to 7.5 ms1 or 1.5 ms1 to 5,5 ms', at least during the second processing period; use of an abrasive slurry comprising a concentration of super-hard abrasive grit particles in a range 50 to 150 carats per litre (10 grams/litre to 30 grams/litre), at least during the second processing period; feeding the abrasive slurry onto the surface of the processing wheel at a feed rate in a range 0.1 litres/hour to 2 litres/hour or 0.2 litres/hour to 0.7 litres/hour, at least during the second processing period.

Figures 2 to 4 illustrate a lapping apparatus on which the presently described processing method can be utilized. The apparatus comprises a rotatable processing wheel 2 on which a super-hard material product to be processed is mounted. The mounting configuration comprises a carrier substrate 4 to which the super-hard material 6 is adhered. The carrier substrate 4 on which the super-hard material 6 is adhered is arranged such that a surface of the super-hard material is in contact with a surface of the processing wheel 2 with an interface region disposed between the surface of the super-hard material 6 and the surface of the processing wheel 2. A weight 8 is provided on the carrier substrate 4 such that the super-hard material 6 is pressed against the surface of the processing wheel 2 with a suitable loading force.

The carrier substrate 4 and super-hard material product 6 is mounted on the processing wheel within a constraining ring 10 which constrains a location of the super-hard material product 6 over the processing wheel 2. The constraining ring 10 comprises a number of slots 11 for allowing the passage of abrasive fluid therethrough as illustrated in Figure 4. The constraining ring 10 has an internal diameter which is larger than the diameter of the super-hard material product 6 and carrier substrate 4. Furthermore, both the constraining ring 10 and the super-hard material product 6 are mounted so as to rotate on the surface of the processing wheel 2 driven by rotation of the processing wheel. In the illustrated configuration the constraining ring 10 is rotatable mounted on the processing wheel 2 via constraining arm 12.

in use, an abrasive slurry comprising super-hard abrasive particles within a carrier fluid is dripped onto the surface of the processing wheel 2 from above. In the illustrated configuration the abrasive slurry 14 is housed in a slurry container 16 and a tube 18 runs from the slurry container 16 to a position above the processing wheel 2 for dripping abrasive slurry 14 onto the processing wheel 2. A pump, such as a peristaltic pump, may be provided to control the drip rate of abrasive slurry onto the processing wheel 2. Generally, the abrasive slurry is dripped onto the processing wheel 2 near a central region thereof and the abrasive slurry moves radial outwards across the processing wheel 2 during rotation of the processing wheel in use.

For a rough lapping process where a significant amount of material is to be removed from a surface of the super-hard material product, the super-hard abrasive particles may be relatively large in size, e.g. having a particle size of greater than I am. These abrasive particle are larger than pores within the surface of the processing wheel 2 and thus roll between the surface of the processing wheel 2 and the surface of the super-hard material product 6 within the interface region in order to cause surface micro-cracking of the super-hard material product 6 and removal of material from the surface of the super-hard material product 6.

One problem the present inventors have found with the aforementioned lapping configuration is that the lapping process can be difficult to control in order to achieve high rates of material processing without causing undue damage to the surface of the super-hard material being processed. Furthermore, another problem the present inventors have found with the aforementioned lapping configuration is that the lapping process can be difficult to obtain uniform processing across large areas of super-hard material. In accordance with the presently described method, one way to alleviate these problems is to vary the location and magnitude of the loading force during the lapping process, In particular, the weight 8 in the aforementioned lapping configuration can be located in a non-central location on the super-hard material to preferentially process thicker regions of the super-hard material and then the weight 8 can be reduced in magnitude and centralized on the super-hard material in the latter stages of lapping to reduce edge rounding and lapping damage of the super-hard material while still achieving good rates of material removal. In an alternative configuration a pneumatic arrangement can be utilized to apply the loading force in place of the weight 8.

In additional to controlling the final stages of the lapping process as described above, another way of achieving high rates of material processing without causing undue damage to the surface of the super-hard material being processed arid to obtain uniform processing across large areas of super-hard material is to utilize a processing wheel which has one or more feed ports disposed therein and at least a portion of the abrasive slurry is fed directly from the one or more feed ports into the interface region between the surface of the processing wheel and the super-hard material product being processed. For example, the abrasive slurry may be fed onto the surface of the processing wheel though the one or more feed ports from underneath the processing wheel rather than dripped onto the surface of the processing wheel from above. In this way, the abrasive slurry can be fed onto the surface of the processing wheel though the one or more feed ports in a continuous stream.

Figures 5 to 7 illustrate a lapping apparatus re-configured according to the aforementioned arrangement. The apparatus shares many common features with that illustrated in Figures 2 to 4 and like reference numerals have been used for like parts to highlight common features, As in the previously described arrangement, the apparatus comprises a rotatable processing wheel 2 on which a super-hard material product to be processed is mounted, The mounting configuration may comprise a carrier substrate 4 mounted to the super-hard material 6 such that a loading force is applied to the super-hard material 6 via the carrier substrate 4. For example, the super-hard material 6 may be bonded to the carrier substrate 4. Alternatively, the super-hard material may be retained in a free-standing configuration which is not bonded to a carrier substrate, In the illustrated embodiment, the super-hard material 6 is adhered to a carrier substrate 4 and arranged such that a surface of the super-hard material is in contact with a surface of the processing wheel 2 with an interface region disposed between the surface of the super-hard material 6 and the surface of the processing wheel 2. A weight 8 is provided on the carrier substrate 4 such that the super-hard material 6 is pressed against the surface of the processing wheel 2 with a suitable loading force. In an alternative configuration a pneumatic arrangement can be utilized to apply the loading force in place of the weight 8, As in the previously described arrangement, the carrier substrate 4 (if present) and the super-hard material product 6 can be mounted on the processing wheel within a constraining ring which constrains a location of the super-hard material product 6 over the processing wheel 2. The constraining ring 10 may comprise a number of slots for allowing the passage of abrasive fluid therethrough as previously illustrated in Figure 4 although it is possible to utilize a constraining ring which does not comprise slots. The constraining ring 10 has an internal diameter which is larger than the diameter of the super-hard material product 6 and carrier substrate 4. Furthermore, both the constraining ring 10 and the super-hard material product 6 are mounted so as to rotate on the surface of the processing wheel 2 driven by rotation of the processing wheel. In the illustrated configuration the constraining ring 10 is rotatable mounted on the processing wheel 2 via constraining arm 12. In certain configurations the constraining ring 10 and/or the super-hard material product 6 are rotatably driven independently of the processing wheel 2 and this can be desirable to provide a controlled rotation of the constraining ring 10 and/or the super-hard material product 6 rclativc to thc processing wheel 2. In this case, thc constraining arm 12 may comprise drivcn wheels for rotating the constraining ring 10 and/or the super-hard material product 6.

Alternatively, a rotating force may be applied from directly above the super-hard material product 6, e.g. via an upper surface of the super-hard material product 6, the carrier substrate 4, the weight 8, and/or via a pneumatic loading configuration if present.

One of the major differences between the apparatus of Figures 5 to 7 and that illustrated in Figures 2 to 4 is that the apparatus of Figures 5 to 7 is configured to provide an under-feed arrangement for the abrasive slurry. As illustrated in Figure 5, abrasive slurry is fed upwards through a rotational post 20 as illustrated by arrow 22. The processing plate 2 is adapted to provide a plurality of feed ports 24 disposed in the surface thereof such that in use an abrasive slurry is fed through the feed ports 24 onto the surface of the processing wheel from underneath the processing wheel as illustrated by arrows 26. The abrasive particles then move radial outwards from the feed ports 24 across the surface of the processing wheel 2 and roll through the interface region between the super-hard material product 6 and the processing wheel 2 in order to cause surface micro-cracking of the super-hard material product and removal of material from the surface of the super-hard material product.

The plurality of feed ports 24 can be radially distributed across the surface of the processing wheel such that at least a portion of the abrasive slurry is fed directly from the feed ports into the interface region between the surface of the processing wheel and the surface of the super-hard material product being processed.

Surprisingly, the present inventors have found that higher rates of material processing can be achieved in a much more controllable manner using a lapping configuration in which the surface of the processing wheel has one or more feed ports disposed therein and the abrasive slurry is fed through the feed ports during processing of the super-hard material product onto the surface of the processing wheel from underneath the processing wheel rather than dripped onto the surface of the processing wheel from above as is done in a more standard lapping configuration. A better surface finish is also achieved, especially for large polycrystalline CVD diamond wafers when compared with a top feed approach. While not being bound by theory, it is believed that the under-feed configuration is advantage over the top-feed configuration for the following reasons.

Using a top feed approach all abrasive particles entering an interface region between the surface of the processing wheel and the surface of the super-hard material must do so from an edge of the super-hard material. It has been ibund that this can lead to edge chipping, edge rounding, and/or groove formation across the surface of the super-hard material being processed. In contrast, if the abrasive slurry is under-fed then at least a portion of the slurry can be introduced directly under the super-hard material being processed in the interface region. As such, this abrasive material moves from a central region of the super-hard material to an edge region rather than from an edge region to a central region. It has been found that such a modified lapping technique reduces edge chipping, edge rounding, and grooving in the super-hard material being processed and thus can lead to a better surface finish. In addition, regardless of the direct under-feed to interface region configuration, it is also believed that generally an under-feed configuration allows the abrasive slurry to be introduced onto the surface of the processing wheel at a more controllable rate and optionally in a continuous stream.

In addition to the above, it is also possible using the modified lapping process to achieve more uniform processing across a large surface of a super-hard material such as a polycrystalline CVD diamond wafer. As previously indicated, standard lapping techniques involve dripping a suspension of diamond grit onto the lapping wheel from above. However, using such a technique requires grit to move into a peripheral region of the interface between the lapping wheel and a polycrystalline CVD diamond wafer and then propagate across the interfhce region in order to process the surface of the polyciystalline CVD diamond wafer.

The grit particles are broken down as they hit the peripheral region of the wafer and during propagation under the wafer. This can result in differential processing of peripheral and central regions of the wafer with central regions being processed by smaller particles of grit than peripheral regions. This problem is particular to processing of super-hard materials, such as diamond wafers, as other materials do not cause the diamond grit to be broken down into smaller particles. As previously described, in order to solve this problem the lapping apparatus has been modified to feed the suspension of diamond grit from an underside of the lapping wheel through holes in the lapping wheel at locations which result in the grit being fed directly into the interface region between the wafer and the lapping wheel. As such, using this arrangement it is possible to avoid differential processing of peripheral and central regions of the wafer. This underfeed configuration can thus be utilized in combination with a carefully controlled final stage lapping procedure as previously described in order to further reduce edge rounding, lapping damage, and differential processing of the super-hard material.

In the above described arrangement, abrasive slurry is fed onto the surface of the processing wheel though one or more feed ports from underneath the processing wheel. Furthermore, the abrasive slurry is preferably fed onto the surface of the processing wheel though the one or more feed ports in a continuous stream. In this regard, the lapping apparatus may further comprise a pump configured to feed abrasive slurry though the one or more feed ports in a continuous stream and the pump may be configured to vary a rate of flow of the abrasive slurry though the one or more feed ports in the processing wheel. The processing wheel may also comprise one or more grooves or slots disposed in the working surface thereof; e.g. radially disposed slots, concentrically disposed grooves, or spiral grooves. Such slots and grooves can aid in controffing the movement of abrasive slurry across the processing wheel and/or removal of abrasive slurry from the surthce of the processing wheel. The one or more feed ports disposed in the processing wheel may be configured to inject abrasive slurry into one or more slots or grooves on the surface of the processing wheel. Alternatively, the processing wheel may be planar without any such grooves. While slots or grooves are known to increase lapping rates in certain applications, the presence of such slots and grooves can make the re-conditioning of such processing wheels more problematic.

As an alternative to the illustrated configuration, it is also envisaged that the entire apparatus could, in principle, be inverted relative to the Earth. In that case, the super-hard material product would be mounted against a lower surface of the processing wheel and abrasive slurry would be fed through feed ports in the processing wheel from above the processing wheel to the lower surface with at least a portion of the abrasive sluny being fed directly from the one or more feed ports into the interface region between the surface of the processing wheel and the super-hard material product being processed.

In addition to the above, it has also been found that the lapping process as described herein is sensitive to the rotation speed of the processing wheel and that the rotation speed of the processing wheel may be selected to optimize processing rates while retaining a good surface finish. It has been found that the rotation speed of the processing wheel may advantageously be selected to be in a range 0.2 ms1 to 7.5 ms1 with certain embodiments optionally using a rotation speed in a range 1.5 ms1 to 5.5 ms1. For example, for a 350 mm diameter processing wheel this would equate to a rotational speed of about 10 rpm (revolutions per minute) to about 400 rpm with certain embodiments optionally using a rotation speed of about 80 rpm to 300 rpm. That said, for certain applications it has been found that higher rotational speeds can be utilized (e.g. up to 1000 rpm).

The present inventors have also found that the lapping process as described herein is sensitive to the particle size of the super-hard abrasive grit and that the particle size of the abrasive may be selected to optimize processing rates while retaining a good surface finish. It has been found that the particle size of the super-hard abrasive grit may advantageously be selected to be in a range 5 m to 100 jim with certain embodiments optionally using an abrasive particle size in a range 10 tm to 65 tm. It has been found that surprisingly good surface finishes with relatively low surface roughness can be achieved when lapping with relatively coarse grit.

Different grades of abrasives grit may be used either individually or sequentially to achieved particular surfaces finishes, For example, a coarse grit having an average particle size of approximately 50 jim to 60 jim, an intermediate grit having an average particle size of approximately 35 jim to 45 jim, and a fine grit having an average particle size of approximately 20 im to 30 m. Finer grits are more expensive and should only generally be used when a very high degree of flatness and smoothness are required. As previously indicated, it has been found that a low roughness finish can be achieved for diamond material using relatively coarse grits. For example, using a grit size of 20 to 25 am it is possible to achieve a surface roughness of better than 150 nm Ra. To achieve a comparable surface roughness on a metal wafer would require grit particles of approximately 2 im in size. One reason for this is that when processing a diamond surface the diamond grit particles roll across the surface, form microcracks in the surface, and then these microcracks intersect such that a portion of the diamond surface falls away from the wafer. In contrast, when processing a metal wafer the diamond grit particles gouge metal material out of the surface as they are dragged across the surface resulting in a less smooth finish.

The present inventors have also found that the lapping process as described herein is sensitive to the concentration of super-hard abrasive grit particles within the abrasive slurry and that the concentration of super-hard abrasive grit particles within the abrasive slurry may be selected to optimize processing rates while retaining a good surface finish. It has been found that the concentration of super-hard abrasive grit particles within the abrasive slurry may advantageously be selected to be in a range 50 to ISO carats per litre (10 grams/litre to 30 grams/litre) for cost effective processing. Higher concentrations can work well and have a faster material removal rate but are not as attractive on a cost basis due to the cost of the super-hard abrasive grit, As such, higher concentrations (e.g. up to 500 carats per litre or too grams/litre) are generally only used for higher margin products where cost is less of an issue.

Very high concentrations can result in aggregation of grit particles which is not desirable, On the other hand, very low concentrations can cause processing damage.

The lapping process as described herein is also sensitive to the rate at which the abrasive slurry is fed onto the surface of the processing wheel and that the feed rate may be selected to optimize processing rates while retaining a good surface finish, It has been found that the feed rate may advantageously be selected to be in a range 0.1 litres/hour to 2 litres/hour with certain embodiments optionally using a feed rate in a range 0.2 litres/hour to 0.7 litres/hour, Furthermore, typically if the rate of grit supplied to the lapping wheel is doubled then the weight on the wafer being processed may also be doubled such that the load per grit particle remains reasonably constant except for the final stages of lapping when the loading force is reduced and centralized as previously described, A number of possible carrier fluids may be utilized for the abrasive grit particles. The carrier fluid should have a viscosity which is sufficient to suspend the abrasive grit particles during transport through the one or more feed ports to the surface of the processing wheel. The precise requirements for the carrier fluid will also depend on flow rate, grit size, suspension time, and grit concentration. Glycerine based carrier fluids may be utilized but even water based carrier fluids with various additives have been found to be suitable.

The processing parameters of rotation speed, loading force, grit size, grit concentration, and slurry feed rate are inter-related and preferably a combination of the aforementioned parameters is utilized to achieve the best lapping of super-hard products.

In addition to the use of an under-feed lapping configuration and optimized processing parameters as described above, a simple and effective mounting configuration has also been developed for ensuring that highly flat surface finishes can be achieved. In particular, it has been found to be advantageous to use an ultra-flat carrier substrate (e.g. one which has a surface which deviates from a perfectly flat surface by no more than 20 jim, 10 jim, S jim, or 1 jim), Furthermore, it has been found to be advantageous to use a very low viscosity adhesive to mount the super-hard material product to the carrier substrate, Use of a low viscosity adhesive avoids the adhesive drying with a non-uniform flatness which introduces non-uniformities in the reference surface to which the super-hard material product is adhered.

Such non-uniformities can result in non-uniformities in the flatness of the super-hard material being processed. That said, while low viscosity adhesives have been found to be useful for mounting super-hard material products for processing, another alternative is to leave the super-hard material product in an un-bonded state.

A suitable mounting configuration for the super-hard material to be processed is illustrated in Figure 7, The mounting configuration comprises a carrier plate 30 having an ultra-flat surface. The carrier plate 30 may be formed from a number of different materials including invar, silicon carbide, silicon cemented diamond, quartz, or borosilicate glass, Ideally, the carrier plate 30 should be made of a material which has a low thermal expansion coefficient and which is capable of being processed to a high degree of flatness. The carrier substrate may be cylindrical in shape and may comprise an 0-ring 32 disposed around a peripheral surface thereof. The 0-ring 32 is advantageous to protect the carrier in use where it abuts against a constraining ring as previously described. A super-hard material product plate 34 is mounted to the ultra-flat surface of the carrier plate 30, e.g. via a low viscosity adhesive 36.

While in Figure 7 the super-hard material product plate 34 has a diameter substantially equal to that of the carrier plate 30, in practice the diameter of the super-hard material product plate 34 may often be smaller than that of the carrier plate 30, One mounting technique involves pressing a wafer of super-hard material onto a carrier substrate (nucleation face down as this provides the smoothest, flattest reference face when compared with the growth face which is rough and comprises larger particles) and beading glue around the edge of the wafer. The glue is drawn into the interface between the wafer and the carrier substrate around a peripheral region by capillary action and sets to adhere the wafer to the carrier substrate, if the wafer or the substrate is not sufficiently flat then glue may be drawn into a more central region of the interface which is not desirable as flatness is compromised. What is desired is a very small volume of glue within the interface region only around a peripheral region. The glue must be a low viscosity adhesive in order to be drawn into the interface and set in the required manner, The glue must also be capable of withstanding the temperatures generated during lapping and polishing in order to retain adhesion during processing. After processing one surface, the substrate-wafer composite is hcatcd to soflcn thc gluc and rclcasc thc wafcr which is thcn turncd ovcr and rc-adhcrcd to the carrier substrate for processing the other surface of the wafer, In one preferred methodology for processing a polycrystalline CVD diamond wafer the growth face of the wafer is processed first before processing of the nucleation face, This is because the nucleation face is usually more smooth and flat than the growth face and thus it is preferably to mount the more smooth flat nucleation face to a carrier substrate and process the growth face prior to processing the nucleation face.

While the above described mounting configuration is suitable for processing flat super-hard material products, the lapping process as described herein may also be utilized for non-planar super-hard material products. In this case the working surface of the processing wheel may be curved and the carrier plate may also be curved to provide a suitable mounting configuration and processing surface for a non-planar super-hard material product such a dome or lens.

Embodiments of the present invention may be applied to a range of super-hard materials including a range of diamond materials, cubic boron nitride materials (cBN), sapphire, and composites comprising the aforementioned materials. For example, diamond materials include chemical vapour deposited (CVD) single crystal and polycrystalline synthetic diamond materials of a variety of grades, high pressure high temperature (HPHT) synthetic diamond materials of a variety of grades, natural diamond material, and diamond composite materials such as polycrystalline diamond which includes a metal binder phase (PCD) or silicon cemented diamond (ScD) which includes a silicon/silicon carbide binder phase.

While this invention has been particularly shown and described with reference to embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appending claims.

Claims

Claims 1. A method of processing a super-hard material product having a Vickers hardness of no less than 2000 kg/mm2, the method comprising: mounting the super-hard material product with a surface of the super-hard material product in contact with a surface of a processing wheel with an interface region disposed between the surface of the super-hard product and the surface of the processing wheel; loading the super-hard material product such that the super-hard material product is pressed against the surface of the processing wheel with a loading force; rotating the processing wheel; and feeding an abrasive sluny onto the surface of the processing wheel, the abrasive slurry comprising super-hard abrasive grit particles disposed in a carrier fluid, wherein the super-hard abrasive grit particles of the abrasive slurry have an average particle size of at least 1 jim and roll between the surface of the processing wheel and the surface of the super-hard material product within the interface region in order to cause surface micro-cracking of the super-hard material product and removal of material from the surface of the super-hard material product, and wherein the method of processing includes at least two processing periods including a first processing period during which the loading force is applied to the super-hard material product in a non-central location of the super-hard material product and a second processing period during which the loading force applied to the super-hard material product is reduced and centralised on the super-hard material product relative to the loading force applied during the first processing period.
2. A method according to claim 1, wherein during the second processing period a flatness of the processing wheel is controlled to have a greater degree of flatness than during the first processing period, at least over a portion of the processing wheel which is utilized to process the super-hard material product.
3. A method according to claim I or2, wherein during the second processing period flatness of the processing wheel is controlled such that the processing wheel deviates from a perfectly flat surface by no more than 20 jim, at least over the portion of the processing wheel which is utilized to process the super-hard material product.
4, A method according to claim 3, wherein during the second processing period flatness of the processing wheel is controlled such that the processing wheel deviates from a perfectly flat surface by no more than 10 jim, at least over the portion of the processing wheel which is utilized to process the super-hard material product.
5. A method according to any one of claims 2 to 4, wherein during the second processing period the flatness of the processing wheel is controlled to have a greater degree of flatness than during the first processing period by re-processing of the processing wheel between the first and second processing periods.
6. A method according to any one of claims 2 to 4, wherein during the second processing period the flatness of the processing wheel is controlled to have a greater degree of flatness than during the first processing period by re-placing the processing wheel with another processing wheel have a greater degree of flatness between the first and second processing periods.
7. A method according toy preceding claim, wherein the average particle size of the super-hard abrasive grit particles during the first processing period is equal to or greater than the average particle size of the super-hard abrasive gilt particles during the second processing period.
8. A method according to any preceding claim, wherein the average particle size of the super-hard abrasive grit particles during the second processing period is at least 1 pm, 5 pm, 10 pm, 20 pm, 30 gin, or 35 pm.
9. A method according to claim 8, wherein the particle size of the super-hard abrasive gilt particles during the second processing period is no more than 60 gm, 50 gm, or 45 gin.
10. A method according to any preceding claim, wherein after the second processing period the super-hard material product is polished using a two body polishing process without any intermediate three body surface processing step which uses super-hard abrasive grit particles having a smaller average particle size than the super-hard abrasive grit particles used during the second processing period.
11. A method according to any preceding claim, wherein the processing wheel it rotated at a speed in a range 0.2 mi' to 7.5 mi' or 1.5 ms to 5.5 ms4, at least during the second processing period.
12. A method according to any preceding claim, wherein the loading force which presses the super-hard material product against the surface of the processing wheel is in a range 0.05 g/mm2 to 1.5 g/mm2 or 0.1 g/mm2 to 0.6 g/mm2, at least during the second processing period.
13. A method according to any preceding claim, wherein the abrasive slurry comprises a concentration of super-hard abrasive grit particles in a range 50 to 150 carats per litre (10 grams/litre to 30 grams/litre), at least during the second processing period.
14. A method according to any preceding claim, wherein the abrasive slurry is fed onto the surface of the processing wheel at a feed rate in a range 0.1 litres/hour to 2 litres/hour or 0.2 litres/hour to 0.7 litres/hour,
15. A method according to any preceding claim, wherein a carrier substrate is mounted to the super-hard material product and the loading force is applied via the carrier substrate,
16. A method according to claim 15, wherein the carrier substrate has a surface which deviates from a perfectly flat surface by no more than 20 tm, 10 tm, 5 tm, or I tm.
17, A method according to any preceding claim, wherein the surface of the processing wheel has one or more feed ports disposed therein aiid at least a portion of the abrasive slurry is fed directly from the one or more feed ports into the interface region between the surface of the processing wheel and the super-hard material product being processed.
18, A method according to claim 17, wherein the abrasive slurry is fed onto the surface of the processing wheel though the one or more feed ports from underneath the processing wheel.
19. A method according to claim 17 or 18, wherein the abrasive slurry is fed onto the surface of the processing wheel though the one or more feed ports in a continuous stream.

20, A method according to any preceding claim, wherein during the second processing period the loading force is located through the centre of mass of the super-hard material product, 22. A method according to any preceding claim, wherein the loading force applied during the first processing period is provided by a first weight and the loading force applied during the second period is provided by a second weight which is smaller in mass than the first weight, wherein during the first processing period the first weight is positioned over the super-hard material product in a non-central location relative to the centre of mass of the super-hard material, and wherein during the second processing period the second weight is positioned in a different location to the first weight which is closer to the centre of mass of the super-hard material.23. A method according to claim 22, wherein the first and second weights are smaller in lateral size than the super-hard material product.