Only High-Speed Abrasive Polishing System
Keltech has the only high-speed abrasive polishing system in the world. Many attempts have been made by others in the industry but all have failed. Sapphire wafers are polished 10X faster than conventional liquid abrasive polishing systems. Our system is actively polishing sapphire at very high cut rates and very long abrasive life, as verified by multiple diamond abrasive tests. The magic is the use of abrasive islands to prevent workpiece hydroplaning and the use of vitrified diamond agglomerates to provide long abrade lives of annular-band abrasive discs.
The Keltech vitrified diamond agglomerate island discs are a new form of abrasive media that is now being introduced worldwide to the wafer polishing market.
Abrasive Disc, Lapper Machine System
To perform high speed abrasive polishing, both flexible vitrified diamond agglomerate filled island discs and a high-speed lapper machine are required. Together as a system, they provide the long-sought abrading speed increase that cannot be achieved with the slow conventional liquid abrasive slurry abrasive system.
Island Abrasive Prevents Hydroplaning
High speed abrasive lapping and polishing requires the use of abrasive islands to prevent hydroplaning of workpieces. Hydroplaning occurs when they are lifted by a water film, preventing contact with the abrasive. Island discs provide abrading contact of workpieces, even at high speeds of 500 to 3,000 rpm for a 12” disc. An analogy is where auto tire tread lugs (islands) prevent high speed hydroplaning on a wet road.
Vitrified Diamonds Provide Long Life
Vitrified diamond abrasive particles are encapsulated in the rigid glass agglomerates which strongly supports them while abrading sapphire workpieces. As each particle progressively wears down, its leading cutting edge is continually re-sharpened. Here, all the expensive diamond particles are fully utilized before being ejected and replaced by sharp new particles. These vitrified agglomerates provide both high cut rates and long disc abrade lives.
Layer Between Islands Carries Coolant Water
A erodible and porous layer is bonded between islands to carry water to the surfaces of the wafers during an abrading operation. Only a very thin film of water is spread on the wafer which allows contact of the abrasive particles to abrade the wafer surfaces. However, this film of water provides very effective cooling of the wafer surface to remove heat generated by abrading friction. The porous layer between the islands provides a very distinctive appearance to these high speed island abrading discs.
Annular Bands of Islands on Discs
Abrasive discs have a wide range of diameters with different radial widths of the annular bands of islands bonded in selected arrays on flexible polymer disc backings. Workpieces overhang both the inner and outer annular radii to provide uniform wear of both the workpieces and the abrasive. Abrasive discs and workpieces rotate in the same direction and rpm to provide equal-localized abrading speeds, and cut rates, at all areas of the workpieces.
Diamonds Required to Abrade Sapphire
Both aluminum oxide (sapphire) and silicon carbide (SiC) materials are so hard that they are commonly used as abrasive materials. However, diamond is harder and when used at high speeds cuts these materials and gallium nitride readily.
Island Discs Rough-Polish Wafers, Then CMP Polish
Sliced silicon and very hard sapphire, silicon carbide and gallium nitride wafers can be quickly flattened and polished in preparation for CMP polishing. First, the 10X faster fixed-abrasive vitrified diamond island discs are used at high speeds. Then, the loose-abrasive slurry particles are used to finish-polish the wafers using the slow CMP process.
This Two-Step process of using fixed-abrasives for polishing wafers is beginning to have widespread use worldwide. For example, 3M’s 9- and 3-micron diamond tile fixed-abrasive discs are used to first grind wafers. Then, loose-abrasive slurries are used to polish the wafers smooth by CMP. This is an excellent demonstration of the advantages offered in this new era of using fixed-abrasives for wafer polishing.
High speed sapphire abrading test data of Keltech vitrified diamond agglomerate island discs using 40-50 micron and 30 micron diamond discs have shown even faster cut rates than the small 9 and 3 micron abrasives. These vitrified agglomerates can also encapsulate 9 and 3 micron diamonds.
Wide Range of Abrasive Materials, Chemicals
A wide variety of abrasive particle materials including diamond, CBN, SiC, SiO2 and alumina can be encapsulated by glass in the vitrified agglomerates.
Also, different chemicals used for CMP polishing can be added to the island-disc coolant water to soften the wafer surfaces to increase the material removal rates. Liquid solutions containing silica and ceria particles can also be used.
Abrasive Lapper Machine
The lapper machine is used to abrasively flat-lap or polish workpieces at high abrading speeds with water mist cooling. Vacuum is used for quick attachment of interchangeable flexible abrasive island discs to a precision-flat air bearing platen. The platen is operated from 0 to 3,000 rpm with 12” to 18” abrasive discs having coarse, medium and fine sized abrasive particles.
Single or multiple workpieces are quickly attached with vacuum to the lapper wafer head that presents the workpieces in flat-surfaced contact with the platen abrasive. Abrading pressure can be adjusted during an abrading operation. Multiple wafer heads can be used simultaneously on the same lapper platen abrasive disc. The lapper wafer head has a high-speed air bearing rotary union that supplies air pressure to the abrade internal chamber and vacuum for wafer attachment.
The lapper machine allows a wide range of abrasive discs to be performance tested at many different abrading speeds and abrading pressures. It is used to optimize abrading procedures and the selection of abrasive discs for different workpieces.
Lapper Wafer Head
The rotatable wafer head has workpieces that are held in abrading contact with abrasive discs mounted on the lapper machine platen. A precision and rigid high speed wafer head spindle is mounted on the lapper machine vertical slide. The wafer head is raised to vacuum attach workpieces and then lowered for abrading contact.
A spherical bearing in the head positions workpieces in floating flat-surfaced contact with the platen abrasive. The wafer head also has non-floating rigid mode operation. Workpiece abrading pressure is controlled by adjusting air pressure in an internal wafer head chamber. The abrade pressure is applied uniformly across the workpiece surfaces during an abrading operation.
Multiple Workpiece Polishing
Multiple workpieces can be abrasively polished simultaneously at high speeds by simply attaching them as a group to a flexible disc for vacuum mounting to the lapper wafer head.
Multiple Non-Equal Thickness Workpieces
Multiple workpieces are often abraded together in groups that contact a flat abrasive surface. With groups of non-equal thickness workpieces, just the thickest are abraded but adjacent thinner ones are not contacted. To polish all the workpieces simultaneously, the group of workpieces are first rough ground with a coarse diamond abrasive where all the surfaces are in a common plane. Then, small abrasive particles are used to polish the workpieces together.
Double-Sided Wafer Abrading
A first flexible low-tack adhesive coated polymer sheet disc is attached to a thin and flexible wafer holder having wafer receptacle holes where the adhesive is exposed in each hole. Wafers are then inserted in the holes where the exposed wafer surface protrudes above the wafer holder. The wafer holder is quickly attached with vacuum to a wafer head and the protruding top surfaces of the wafers are abrasively polished by an island disc attached to a platen.
The wafer holder is then removed from the wafer head and a second low-tack adhesive coated polymer disc is attached to the polished protruding wafer top surfaces. Also, the first adhesive disc is peeled off the wafer bottom surfaces. Both the wafers and the second adhesive disc is pushed downward until the disc adhesive contacts the wafer holder. Then the wafer holder is attached to the wafer head and the wafer protruding bottom surfaces are polished with the abrasive disc.
This abrasive polishing procedure is sequentially repeated with coarse, medium and fine diamond abrasive particles using the same wafer holder.
Sample Island Disc User Wafer Abrade Test Data
Keltech can supply to users a 6” vitrified diamond agglomerate abrasive island disc that can be used to abrade a 2” sapphire, SiC or GaN wafer at high abrading speeds.
This test can quickly and accurately provide wafer cut-rate data and island wear data using a very simple abrade test apparatus. A standard 6” abrasive flat-surfaced disc hub can be attached to a variable speed DC motor where the island disc is attached with a low-tack adhesive to the disc hub. Another small DC motor having a flat-surfaced wafer hub is mounted to a vertical slide where a wafer is attached with an adhesive to the wafer hub. The wafer is then positioned in flat-surfaced contact with the abrasive islands. An air cylinder attached to the vertical slide is used to control the wafer abrading pressure. Both the wafer and the water spray cooled abrasive island disc are rotated at the same high speed and in the same direction for a selected time during the abrading test.
The pattern of vitrified diamond islands is bonded in an approximate 1.75” wide annular band to the polymer flexible disc. The 2” wafer overhangs the island band to provide uniform wear of both the wafer and the island abrasive.
The wear of both the wafer and the disc islands can be accurately measured periodically with a digital micrometer while both remain attached to their hubs. The wafer high cut rate and the slow vitrified diamond agglomerate island wear can be easily established by a user with this simple test apparatus.
Mount Bowed Wafers Stress-Free On Wafer Disc
Warped or bowed wafers can be mounted stress-free on a wafer disc prior to abrading with the vitrified diamond island discs. A wax-coated bowed wafer is positioned to make three-point contact with a wafer disc having wax drainage grooves on its surface. The excess melted wax is drained off the disc via the surface grooves. After solidification of the wax, all localized areas of the stress-free wafer are rigidly supported by the wax and can be abraded to remove the wafer high spots. When flipped over and the opposed wafer surface abraded, the stress-free flat wafer has parallel abraded surfaces .
Vitrified Island Disc vs Vitrified Grinding Wheel
A comparison of our fixed-abrasive vitrified diamond agglomerate flexible island discs with vitrified grinding wheels is a little like comparing apples and oranges, as they are so different. Grinding wheels typically concentrate abrading at a narrow annular ring of vitrified diamond abrasives at their outer periphery. Here, the narrow ring of abrasive is placed in contact with a silicon or SiC wafer, rotated at high speeds and slowly translated across the surface of the rotating wafer with very small micron-sized depths of cuts. This process creates grooves in the wafer surface. It is repeated many times to flatten and polish the wafer. The rotating speeds, the advancement speeds and depth of cut are all optimized to prevent clogging of the grinding wheel abrasive, optimizing the wafer surface roughness and preventing wafer surface overheating.
By contrast, our island discs, having wide annular bands of abrasive islands, are positioned in flat-surfaced contact with a wafer and both are rotated in the same direction and same high speed to abrade the whole large area of the wafer simultaneously. There are no translating grooves cut into the surface of the wafer.
The island discs have a wide range of diamond particle sizes available in the agglomerates, so large, then medium and finally, fine particles are progressively used by quickly changing the island disc when polishing the wafers. Abrasive wheels having the required progressively smaller-sized diamond particles can not be quickly changed.
Another important difference is the concentrated abrading pressure present with the narrow band of abrasive elements on the vitrified grinding wheels that can cause wafer sub-surface damage. By comparison, these concentrated localized abrading pressures are not present with our island discs as they are held in flat-surfaced contact with the whole wafer surface.
Platen Conditioning Rings Not Required
Conditioning rings are used for liquid abrasive slurry wafer polishing. These large, slow-rotating annular rings are
positioned in flat-surfaced contact on the platens to maintain a flat platen abrading surface. Because loose abrasive particles are present between the wafers and the platens, both the platen surface and the wafer surfaces are worn down. Platens lose their required precision-flat surface due to the annular wear-paths caused by abrading contact with the wafers.
Non-flat platens result in non-flat wafers.
Conditioning rings are not required for the high-speed abrasive polishing system. The flexible fixed-abrasive island discs attached with vacuum to the platen protect the platen surface from abrading action. The protected precision-flat platen surfaces stay flat during abrading operations.
Island Top-Surface Conditioning Disc
If non-flat areas occur on the surface of a platen abrasive disc, they can be easily and quickly removed with the use of a conditioning disc. A flexible abrasive disc is attached to the lapper wafer head in place of the workpiece disc. Both the wafer head and the platen are rotated in abrading contact until the precision flatness of the platen abrasive disc is reestablished.
Air Bearing Platen Spindles
Large air bearing spindles are used on both the lapper and disc grinder machines to provide precision-flat platen surfaces and high rotational speeds. The 19” diameter platens are flat to 0.0001” and operate from 0 to 3,000 rpm. The very stiff air-purged spindles repel abrasive debris and have spherical mount bases to provide perpendicular platen alignment with the lapper wafer head spindle. Vacuum mounted abrasive discs protect the platen surface to continually maintain its required precision flatness.
Lapper: Air Pad Supported Platen
A precision-flat platen can be supported by air bearing pads distributed around the circumference of the platen to provide friction-free high-speed operation. Use of a single plate having shallow vacuum grooves on the platen surface allows quick and strong attachment of abrasive discs and easy removal of abrading debris.
This simple design allows a robust and inexpensive lapper machine to be constructed. Air pads are positioned directly under multiple wafer heads to provide stiff support of wafers being abraded. Low pressure air supplied by a blower to a controlled-leakage chamber at the platen bottom surface prevents contamination by abrasive debris.
Lapper: Tapered Roller Supported Platen
Multiple tapered rollers can be spaced around the periphery of a platen to provide rigid support to the platen as abrading forces are imposed on the platen by multiple wafer head abrading stations. Each skid-free roller is tilted at an angle to have flat surfaced contact with the platen bottom surface. High-speed spherical-action, self-aligning roller bearings in each tapered roller allows the use of dimensionally stable, non-precision flat machine bases.
A sandwich construction platen using top and bottom plates separated by a spacer provides a very stiff but low inertia platen that is easily accelerated and decelerated. Shallow vacuum groves in the platen top surface allows the quick and strong attachment of abrasive discs.
Slide-Housing Lapper Wafer Head
A wafer head with shallow vacuum grooves on the surface of the wafer plate allows quick and strong attachment of wafer discs. Single or multiple rigid wafers can be mounted of the flexible wafer discs.
The wafer head has a slide housing that is concentric with a hollow drive housing and the wafer head contains a pressure chamber. Controlled air pressure applies a uniform abrading pressure across the full surface of the wafers during abrading.
A self -aligning roller bearing allows the wafers to assume flat surfaced contact with the surface of an abrasive disc attached to a platen, even when there is slight misalignment of the wafer head and the platen
Air Bearing Rotary Union
A friction-free air bearing rotary union is used to provide high-speed 3,000 rpm continuous operation. Standard sliding-contact rotary unions cannot operate at high speeds without over-heating due to friction.
The air bearing rotary union supplies vacuum to attach the wafer discs to the wafer head. It also supplies controlled-pressure air to the wafer head chamber to provide selected abrading pressures to wafers during an abrading operation.
Disc Grinder Machine
The disc grinder machine is a heavy industrial machine that provides precision flat surfaced abrading of island discs or to pre-grind groups of workpieces attached to a disc. A large 0 to 3,000 rpm air bearing spindle having a 19” platen with 0.0001” flatness is used to vacuum attach single or multiple workpieces. The diamond grinding wheel spindle assembly is mounted on a heavy horizontal carriage that provides only 0.0001” vertical variation as it translates across the rotating workpieces.
Production of Agglomerates, Discs and Machines
Vitrified diamond agglomerates and island discs are all produced on-site at Keltech. In addition, both the lapper machine and the disc grinder were developed and built here. This shows that this high speed abrading technology can be implemented anywhere.
Vitrified Diamond Agglomerate Grinding Wheels
Vitrified diamond agglomerates can be coated or molded onto the surface of grinding wheel hubs to produce high quality and durable grinding wheels. Bonding the agglomerates with industry-standard solvent based phenolic adhesives provides a very simple method of wheel construction.
By comparison, the conventional system is to mold a mixture of glass powder and diamond particles on the wheel surface and placing the whole wheel in a furnace to melt the glass during vitrification to bond the individual diamond particles to the wheel. Special materials are added to the glass mixture form gasses at high temperatures that create the required void spaces within the abrasive layer.
Using agglomerates that have been independently pre-vitrified in a furnace allows the grinding wheels to be produced at room temperatures and fully cured in an oven. Evaporation of the solvent from the adhesives during curing produces the void spaces in the abrasive layer used for improved liquid cooling of the wheel and workpiece.
Vitrified Diamond Agglomerate Abrasive Discs
Vitrified diamond agglomerates can be coated with adhesives on the surface of polymer discs or on continuous webs, Abrasive sheets, strips or discs can be cut from the webs.
Pre-vitrified diamond abrasive agglomerates are simply coated on the polymer backing discs (or webs) using conventional coating processes and solvent-based adhesives. Evaporation of solvent from the adhesives during curing produces the void spaces between individual agglomerates that provide high cut rates of hard materials and effective water cooling of workpieces.
The agglomerates provide long disc abrade life and support individual expensive diamond particles as they slowly wear down and continually resharpen before they are ejected and replaced with new sharp particles.
A wide range of abrasive particle sizes and abrasive materials can be encapsulated by the solidified glass during vitrification.