Technology
High-Speed Lapping
Only 10X Faster High-Speed Abrasive Wafer Polishing System
Keltech has the only high-speed abrasive polishing system in the world.
Sapphire wafers are polished 10X to 50X faster than by conventional liquid abrasive slurry polishing systems.
From 10 to 50 slurry machines can be replaced by a single island disc machine.
Abrasive islands prevent water-spray cooled wafers from hydroplaning at high abrading speeds. Vitrified diamond abrasive agglomerates in the islands provide high sapphire cut rates and long abrade life of the discs.
Multiple abrading tests of sapphire wafers have verified the high speed performance of the island discs.
The Keltech vitrified diamond agglomerate island discs are a new form of abrasive media.
Tests Verify High Cut Rates, Long Disc Abrade Life
Many high-speed sapphire abrading tests of Keltech vitrified diamond agglomerate island 12” discs on the lapper machine using 40-50 micron diamond particles have shown very high cut rates.
Hundreds of abrading tests using 6” island discs to abrade 2” sapphire wafers with 30 micron diamonds have verified both high wafer cut rates and long disc abrade life. See the DATA section for more info.
Abrasive Discs, Lapper Machine System
Vitrified diamond agglomerate island discs with 12” and 18” diameters are quickly mounted with vacuum on a lapper machine platen that rotates up to 3,000 rpm.
Wafers are also attached with vacuum to a 6” wafer head that applies controlled abrading pressure when the rotating wafers contact the rotating abrasive island disc.
Water spray is applied to the rotating island disc to cool the wafers while they are abraded.
Abrasive Islands Prevent Hydroplaning
High speed abrasive lapping and polishing requires the use of abrasive islands to prevent hydroplaning of workpieces.
Hydroplaning can occur at high speeds where a thin water film lifts the wafers, preventing contact with the abrasive.
One analogy is hydroplaning that lifts a boat up in the water at high speeds.
Another analogy is where bald car tires hydroplane at high speeds on a wet road but where tire tread lugs (disc islands) prevent hydroplaning.
Porous Abrasive Islands Provide Controlled Wear-Down
The abrasive agglomerate filled islands must be porous to provide a controlled rate of eroding due to wafer wear.
This eroding action allows wafer high and consistent cut rates to occur as the islands slowly wear down.
Solvent-based phenolic adhesives are used to strongly bond the abrasive agglomerates together in the island structures. As the solvent evaporates, voids are formed, making the islands porous.
Vitrified Diamond Agglomerates Provide High Cut Rates
Vitrified diamond abrasive particles are encapsulated in rigid glass agglomerates which strongly supports them while abrading wafers.
To produce the agglomerates, diamond particles are mixed with glass powder and heated to 700 degrees C in a furnace to fuse them together. The diamonds are protected from oxygen in the furnace as they carburized and evaporate above 500 degrees C. After cooling the fused mix is fractured into small agglomerates.
As each diamond particle progressively wears down, its leading cutting edge is continually re-sharpened by abrading action. Here, all the expensive diamond particles are fully utilized before they are replaced by new particles in the island agglomerates.
These vitrified agglomerates provide both high wafer cut rates and long disc abrade lives.
Flat-surfaced agglomerates are shown on the surface of worn islands.
Porous Layer Between Islands Carries Coolant Water
A porous layer of adhesive joint-bonded foam-glass beads between islands carries water to the wafers during abrading operations. A very thin film of water on the wafer allows the abrasive particles to contact and abrade the wafer surface.
The film of water on the wafer surface removes heat generated by abrading friction.
Also, the porous layer between the islands provides a very distinctive appearance to these high speed island abrading discs.
Annular Bands of Islands Required on Discs
Annular bands of abrasive islands are required.
Wafers overhang both the inner and outer annular radii to provide uniform wear-down of both the wafers and the abrasive islands.
Annular bands of abrasive are used both for CMP liquid abrasive slurry polishing and for the abrasive island discs.
Abrasive discs have a wide range of diameters (up to 60” or more) with different radial widths of the annular bands of islands.
Diamonds Required to Abrade Sapphire, SiC, GaN
Sapphire, SiC and GaN are almost as hard as diamond.
However, diamond is harder and when used at high speeds, cuts these materials readily.
Fully Utilized Individual Diamond Particles
Only some of the abrasive particles in an abrasive slurry are fully utilized. They are continually supplied in a liquid stream during the abrading operation.
Spent slurry, containing old and new particles and abrading debris, is continually washed off the abrading surface of the platen.
By comparison, with the island discs, each individual fixed-abrasive diamond particle encapsulated by glass in the agglomerates is fully utilized as it slowly wears down during abrading.
Wide Range of Disc Sizes, Abrasive Materials
Island discs are available in sizes ranging from 6” to 60”, or more.
A wide variety of abrasive particle materials including diamond, CBN, SiC, and aluminum oxide can be encapsulated by glass in the vitrified agglomerates.
Daimond abrasive particle sizes range from 3 microns to 50 micron.
Percent Diamond Particles In Agglomerates
The cut rates and disc abrade life can be affected by many variables.
These include: the percent and size of diamond particles in the vitrified abrasive agglomerates, the quantity of agglomerates, the porosity of the islands, and the abrade speeds and the abrade pressures.
Island Discs Used Before CMP Polishing
Rough-surfaced sliced sapphire, SiC and GaN wafers can be quickly flattened and polished with island discs in preparation for CMP polishing.
Then, the loose-abrasive slurry particles in the CMP system can be used to finish-polish the wafers.
Use CMP Chemicals With Island Discs
The chemical additives used to “soften” the surfaces of wafers for faster CMP abrading can also be used with the abrasive island discs.
These chemicals can be simply added to the coolant water that is sprayed on the island disc when abrading wafers.
3M Fixed-Abrasive Diamond Tile Abrasive Discs
3M’s Diamond Tile fixed-abrasive discs are being widely used as a replacement for liquid slurry polishing. These discs typically have either 9 micron or 3 micron diamond abrasive particles. They are used at lower abrading speeds
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One significant disadvantage of the 3M Diamond Tile discs is that they do not have annular bands of abrasive. Wafers cannot overhang the abrasive to provide uniform wear-down of both the wafers and the disc abrasive during abrading operations.
Keltech’s 12” vitrified diamond agglomerate island high speed discs can be used from 500 rpm up to 3,000 rpm without hydroplaning to provide very high cut rates of sapphire wafers.
Also, diamond particles in the island discs can have a range of from 3 microns to 50 microns. Cut rates increase directly with the particle size, and with abrading speed.
Abrasive Lapper Machine
The lapper machine abrasively polishes wafers 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 quickly attached with vacuum to the lapper wafer head contact the platen abrasive disc. Abrading pressure is adjusted during an abrading operation.
New Lapper Machine
This is a simple, robust and stiff lapper machine having a movable wafer head that is moved by a motor driven screw jack. The wafer disc is attached with vacuum to the raised wafer head. A low-friction limited spherical action ball bearing allows the rotating wafers to maintain flat-surfaced contact with the rotating island disc, even when the lapper machine components are distorted during abrading.
Controlled abrading pressure is applied uniformly across the surface of the wafer during abrading.
A concentric, slidable driven housing allows vertical motion as the wafers are worn down. The outer housing is rotated with a spline gear set where the inner gear has a slight spherical shape that allows the wafer plate to pivot as it is rotated.
High-speed rotary unions supply both vacuum and abrading air pressure to the wafer head and the platen.
The stiff and low inertia platen having precision-flat surfaces can be supported by porous carbon air pads at each wafer abrading station.
These lappers can be constructed by multiple machine builders using these free design concepts.
This lapper design is an updated and simplified version of the high speed lapper used for years on-site at Keltech.
Click to increase the size of the drawing.
Lapper Wafer Head
The rotatable wafer head has waferss 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 wafers and then lowered for abrading contact.
A spherical bearing in the head positions wafers in floating flat-surfaced contact with the platen abrasive. The wafer head also has non-floating rigid mode operation.
Wafer abrading pressure is controlled by adjusting air pressure in an internal wafer head chamber. The abrade pressure is applied uniformly across the wafer surfaces during an abrading operation.
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 on the flexible wafer discs.
The wafer head has a very stiff slide housing that is concentric with a hollow drive housing where 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 spherical bearing allows the wafers to assume flat surfaced contact with the surface of an abrasive disc attached to a platen.
High-Speed Rotary Union
A compact rotary union is used to provide high-speed 3,000 rpm continuous operation of the wafer head. Standard sliding-contact rotary unions cannot operate at these high speeds without over-heating due to friction.
The slide-tube 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.
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 rotate 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.
Multiple Wafers Polished
Multiple wafers 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-thickness wafers can be abraded together in groups that contact a flat planar abrasive island surface . With groups of non-equal thickness workpieces, the thickest wafers are abraded first, then the thinner wafers.
Double-Sided Wafer Abrading
Wafers are first attached to a wafer-disc and abraded. To abrade the opposing side of the wafers, they are simply flipped over, transfer-attached to a second wafer-disc and abraded.
Island Top-Surface Abrasive Conditioning Disc
If non-flat areas occur on the surface of a island abrasive disc, they can be easily and quickly removed with the use of a conditioning disc.
A standard flexible abrasive disc is attached to the lapper wafer head in place of the wafer disc.
Both the wafer head and the platen are rotated in abrading contact until the precision flatness of the platen abrasive disc is reestablished.
6” Sample Island Disc Abrade 2” Wafers, Test Set-Up #1
Keltech can supply to users a 6” disc having a 1.75” radial-width annular band of vitrified diamond agglomerate abrasive islands to abrade a 2” sapphire, SiC or GaN wafer at high abrading speeds.
Wafer abrade tests operated by you can provide both wafer cut-rate and island wear data using a very simple abrade test apparatus.
First, a 6” diameter, flat-surfaced disc hub is mounted on a variable speed DC motor that is attached to a standard shop milling machine. The 6” sample island disc is attached with adhesive to the disc hub.
Then, a 2” wafer is bonded to a 2” hub having a shaft that fits into the mill head. The wafer is then positioned in flat-surfaced contact with the disc islands where the 2” wafer is centered on the 1.75” wide band of islands on the 6” island disc. The 2” wafer overhangs both the inner and outer diameters of the annular abrasive band to provide uniform wear-down of both the wafer and the island discs.
A section of aluminum angle is attached to the mill head pivot arm and a steel weight bar is clamped to the aluminum angle. The arm weight applies a selected and constant downward abrade pressure force on the rotating wafer while in abrading contact with the rotating disc island abrasive. The wafer abrading pressure can be calibrated by sliding the weight along the arm angle while contacting a force weight scale positioned on the mill table while having force contact of the mill head wafer head with the scale.
Both the wafer mill head and the abrasive island disc are rotated at the same high speed (typically 1,000 rpm) and in the same rotation direction for a selected time ( typically 1 or 2 minutes) during the abrading test, using a cell phone timer attached to the mill.
6” Sample Island Disc Abrade 2” Wafers, Test Set-Up #2
Coolant water is sprayed of the rotating island disc to cool the wafers by removing the heat generated by abrading friction. Water spray can be applied manually during an abrade test with the use of a water spray bottle or by a spray-control system.
For collecting wafer wear and island wear data for many independent abrade tests over a long period of disc and wafer abrade time, a fixed-position paint spray gun can be used. An air pressure regulator, a water jar and a water collection tray can be used to provide adjustable and uniform water coolant spray during all the series of tests.
For convenience, the wafer variable speed ½ hp dc motor and controller can be mounted to a 0.750” aluminum plate that is clamped in place by a standard milling machine vise attached to the mill X-Y table. The whole assembly can be easily removed and reinstalled with little alignments required. Also, positioning the wafer relative to the disc abrasive islands are easily and accurately replicated with use of the mill X-Y digital readout.
The wafer thickness can be accurately measured after each abrade test as the wafer hub is easily removed from the mill head. And, it is accurately repositioned in the mill head for the next abrade test. The wafer thickness is typically measured at 4 positions around the wafer circumference.
Measurements of the disc abrasive island heights are made less often as the disc island wear is so small. Six selected island thicknesses are typically measured at 6 ink-marked positions around the island disc circumference.
The front water guard is removable and held in place with magnets. Wastewater with sapphire debris is thrown radially off the rotating island disc. It impacts the water guard vertical walls and drips down the walls to enter the three angled folded-poly water troughs that progressively feed into each other. The water trough at the lower part of the back wall is used to direct all the water into a removable water collection tray.
The collected water typically has a milky-white color from the abraded sapphire debris.
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 stiff, solid or sandwich platen having shallow vacuum grooves on the platen surface allows quick and strong attachment of flexible abrasive island 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 line-contact with the platen bottom surface. High-speed, precision roller bearings in each tapered roller allows the use of dimensionally stable, non-precision flat machine bases. Epoxy-sandwich brackets allow each roller assembly to be precisely aligned, in common, with the platen precision-flat bottom surface.
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 grooves in the platen top surface allows the quick and strong attachment of abrasive discs.
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 having a porous abrasive layer.
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 added to the glass mixture form gasses at high temperatures that create the required void spaces within the abrasive layer.
Using diamond abrasive 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 workpiece cut rates and cooling by the porous wheel.
Vitrified Diamond Agglomerate Non-Island 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 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.
