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
.
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.
ISLAND DISCS USED ON EXISTING SLURRY LAPPER MACHINES
Island discs having “soft” diamond abrasive agglomerates can be used to polish wafers at very low speeds on the many existing slow-speed abrasive slurry lapping machines. The fixed-abrasive island discs can be attached to the slow speed platens with PSA applied to the disc backing substrate.
The “soft” agglomerates also can erode easily at very low abrade forces. Soft agglomerates can be produced by adding foam glass beads to the abrasive particle and synthetic glass powder mixture before vitrification in a furnace. Also, soft agglomerates can be produced by mixing a solvent-based adhesive with diamond particles and molding very small “Cubes” that erode easily.
VACCUM ATTACH DISCS ON SLURRY LAPPER MACHINES
Island discs can be easily and quickly attached (and interchanged) with vacuum on existing slurry lapper machines. This allows discs with smaller abrasive particles to be sequentially used for polishing wafers. A circular stainless steel vacuum plate is attached with an adhesive to the lapping machine platen. The stainless plate has a pattern of shallow vacuum grooves on its top surface. Also, a circular soft foam rubber O-ring recessed in a circular groove at its periphery is used to provide a vacuum seal for the island disc.
A vacuum port hole at the stainless plate center is coupled through a platen vertical hollow rotary drive shaft to a rotary union attached at the lower end of the drive shaft. Vacuum supplied to the stationary rotary union is coupled with the distributed shallow grooves on the plate surface. The vacuum strongly bonds the flexible island abrasive disc to the platen flat surface.
​
To remove the stainless plate from the platen, water is applied to the rotary union and injected in the layer between the platen and the island disc. A pattern of shallow surface grooves on the bottom surface of the stainless plate can help distribute the water over the whole island disc backing surface to separate the disc from the platen.
​
Additives such as soap can be added to the water or solvents such as acetone can be injected under the adhesive bonded island discs to separate them from the platen.
VITRIFIED DIAMOND AGGLOMERATE PRODUCTION
The vitrified diamond agglomerates are produced by mixing diamond abrasive powder with a synthetic glass powder and water to form a thick paste. The abrasive paste mixture is then spread out to form thin slabs on 0.001” thick pre-oxidized stainless-steel strips placed on a stiff stainless plate. The stainless strips are pre-oxidized at about 600 C to form a thin brown coating on them. Another 0.001” thick pre-oxidized stainless steel thick strip is placed on the abrasive paste mixture slab top surface. A heavy steel block is placed on top and squeezed flat to contact 0.045 diameter steel gap wires that are placed on both sides of the slabs. The squashed slabs typically have a uniform 0.045” thickness and are about 2” wide and 3” long.
The abrasive mix /stainless sandwich with the steel block weight is then placed in a furnace and heated to about 700 C to melt the glass powder that then encapsulates the diamond particles. The synthetic glass powder typically melts at a temperature as low as 500 C. The stainless strips seal the top surface of the abrasive mixture and prevent contact of the hot 700 C furnace air with the encapsulated diamond particles present at the surface of the mixture. Diamonds will degrade thermally when directly exposed to oxygen above 500 C but the top stainless strip shield prevents this contact.
When the glass has melted and encapsulated the diamond particles, the furnace is cooled to about 350 C and the vitrified stainless steel abrasive slab sandwiches are quenched in water. Here, thermal stresses due the differential shrinkages of the stainless and the slabs separates the slabs from the stainless. The oxide coating on the stainless prevents bonding of the glass in the vitrified slabs to the stainless strips.
After the thin and rigid diamond abrasive slabs are dried, they are placed in small plastic bags that are placed in flat-surfaced contact with a metal plate. The slabs are then rough-split into small vitrified agglomerate pieces by using sharp pointed tools that penetrate the top surface of the plastic bags and are forced partially into the slab surfaces. The bag restrains the large-sized agglomerates as a single layer as the rigid slabs are split.
The bag containing the single layer of large-sized loose agglomerates is then end-taped to the horizontal table of a motor driven X-Y slide. Then, a “gang” of multiple sharp-pointed carbide tipped scribe points are bonded together and attached to a vertical slide. The vertical slide is driven up and down by a gear motor where the scribe points penetrate the plastic bag top surface and partially into the slab thickness but do not contact the slide horizontal table surface. The relative speed of the vertical gear motor and the horizontal table slide motor are controlled to provide uniform sized agglomerates as the multiple scribe points split the loose large-sized agglomerates into the desired smaller sized agglomerates. After a “path” of agglomerates are split, the X-Y table is advanced sideways and the gang of multiple scribes spilt another parallel “path” of agglomerates.
Split agglomerates are then removed from the bag having the bag top surface shredded by the scribe points. The agglomerate slabs typically are 0.045” thick and are spilt into cross-sectional pieces that rage in size from 0.016” to 0.045” . These agglomerates are easily molded into raised islands, attached to a disc backing, that are 0.061” high and have a diameter of 0.186”.
Because the slabs are split into agglomerate pieces there is very little debris formed by the splitting action. Undersized agglomerates containing diamond particles can be collected and recycled in another batch of the vitrified abrasive slabs.
If the rigid vitrified agglomerate slabs are crushed by conventional techniques, they tend to break partially into agglomerate powder that is too small to perform well for abrading wafers. Splitting the vitrified slabs with the sharp-pointed tools avoids creating the undersized agglomerate debris.
“SOFT” VITRIFIED DIAMOND AGGLOMERATE PRODUCTION
“Soft” vitrified diamond agglomerates can be produced that easily erode at low abrading speeds and low abrading pressures. Here, small foam glass beads are mixed with diamond powder, synthetic glass powder and water to make a paste that is formed into thin slabs. The slabs are heated to 700 C in a furnace where both the beads and diamonds are encapsulated by the melted glass. The synthetic glass melts below 700 C but the high-melt-temperature foam glass beads remain intact. Because the foam glass beads are soft and fragile, the resultant split abrasive agglomerates are easily eroded at low abrade forces. Island discs with the “soft” abrasive agglomerates can then be used on the many existing slow-speed slurry lapping machines.
“CUBE” DIAMOND AGGLOMERATE PRODUCTION
Cube-shaped diamond abrasive agglomerates can be mold-formed from a mixture of diamond powder and an adhesive. A thin and flexible RTV silicon rubber mold with an array of small receptacle pocket holes can be level filled with a mixture of diamond powder and an adhesive which is solidified. After solidification, the small molded agglomerates are easily separated from the flexible mold as the cube agglomerate adhesive does not bond to the RTV silicon rubber. The agglomerate cubes typically have 0.040” X 0.040” X 0.040” sizes.
Using an epoxy adhesive, the agglomerate cubes can be almost as rigid as vitrified agglomerates. Foam glass beads can also be mixed with the diamond abrasive particles and the epoxy adhesive to provide “soft” cubes that erode easily for low-force wafer abrading.
The RTV mold can be easily made by cutting an X-Y pattern of posts in a polymer or metal plate and placing a thin edge dam around the X-Y plate. Then liquid RTV silicon rubber is poured into the post plate matrix of holes level with the edge dams to fill the gaps between the posts with a thin and uniform thickness above the posts. After curing the RTV rubber, the thin and flexible RTV agglomerate cube sheet is peeled of the polymer mold.
Solent-based adhesives can also be used to mold-form abrasive cubes that are porous. When the abrasive mixture in the RTV mold holes are heated, solvent evaporates from the adhesive solvent and the adhesive shrinks and forms voids between the diamond abrasive particles. The resultant porous cubes have high wafer cut rates and high erodibility when used at low abrading forces using existing slurry lapping machines.
RUBBER ISLAND-MOLD FONT SHEET
A rubber font sheet having a pattern of through-holes is used to mold-form the island structures with a mixture of abrasive agglomerates and a solvent-based adhesive. An array of holes in the rubber sheet are used to form an annular band of islands that are bonded to a disc backing substrate. The island structure heights are equal to the thickness of the rubber font sheet and the diameters of the islands are equal to the diameters of the font sheet holes.
Either silicone rubber or nitrile rubber sheets can be used to make the island hole font sheet. Both rubbers are flexible and resist bonding of adhesives used in the island abrasive mixtures. To construct the abrasive islands, a solvent based adhesive, such as phenolic, is coated on the disc substrate abrasive-roughened surface and allowed to partially dry, which makes the phenolic tacky. The rubber hole sheet is placed in flat-surfaced contact with the tacky adhesive to lightly-bond the rubber to the disc substrate and to seal the bottoms of each of the island holes. A 0.031” thick epoxy fiberglass substrate provides limited flexibility, excellent adhesive bonding and good thermal stability for 250 F temperature curing of the phenolic adhesive without distortion.
A mixture of diamond abrasive agglomerates and the solvent-based phenolic is level-filled in all of the rubber font sheet holes with a squeegee. Here, the “wet” phenolic in the island abrasive mixture contacts the phenolic coating on the disc substrate that is exposed in each of the rubber holes. Upon curing of the phenolic adhesive, the islands are strongly bonded to the surface of the disc substrate. After the phenolic adhesive is cured, the flexible rubber font sheet can be easily peeled off both the disc backing substrate surface and from the mold-formed island structures bonded to the substrate.
Islands having a diameter of 0.186” and a height of 0.061” are formed when 0.186” diameter holes are punch-cut in 0.061” thick rubber sheets. The island through-holes can also be laser cut or water jet cut in the rubber sheets.
SOLVENT-ADHESIVE ISLAND CONSTRUCTION
Islands that are mold-formed using solvent-based adhesives require special procedures due to evaporation of the adhesive solvent. When a mixture of agglomerates, foam glass beads and the solvent adhesive is level-filled in the rubber mold sheet holes, the mixture is then heated by conduction from the island bottom. Heating the mixture evaporate the solvent in the adhesive which creates solvent-fume gas bubbles that expand within the island structures. As the gas bubbles expand within the islands, they tend to push the island abrasive mixtures up and out of the rubber island mold holes.
To restrain the abrasive mixtures within the rubber holes, a thin RTV silicone rubber flexible sheet having a X-Y pattern of small posts is placed on top of the island surfaces and the rubber mold sheet surface. The thin and flexible rubber posts conform to the island top surfaces. Then a layer of flexible foam rubber is placed on the RTV post sheet and a flat plate and a weight are placed on the foam rubber. Upon heating, the evaporated solvent fume bubbles push upward in the islands and break at the island surfaces while the RTV rubber posts retain the island mixture within the islands. The solvent gas bubbles burst at the island surfaces where the gas is routed outward from the islands by the open passageways between the posts on the RTV post sheet. Heating is continued until all of the solvent is exhausted and the adhesive is fully solidified.
Upon full curing of the island adhesive, the island structures are uniformly porous from top to bottom and the islands have flat exposed top surfaces.
RTV RUBBER “POST” SHEET FOR ISLANDS
The RTV rubber “post” sheet can be easily made by using a thin metal plate that is etched with a X-Y pattern of rectangular holes. This hole-plate can be attached to a flat plate and thin edge dams can be attached on the top surface of the hole-plate around the outer periphery of the hole-plate etched holes. The edge dam top surfaces extend a small distance above the hole-plate surface. Then liquid RTV silicon rubber is poured into the hole-plate matrix of holes and also level with the top of the edge dams. The RTV fills the holes and also forms a thin and uniform thickness layer above the holes to create a rubber sheet having integral posts that protrude above the sheet surface.
The “post” holes etched into the metal font hole-plate can form an annular pattern that “surrounds” the annular band of islands that are bonded to a disc backing substrate. The post holes extend both radially inward and outward from the annular band of islands to route the solvent gas fumes radially away from the islands as the solvent is evaporated. Large post screens are made for large island discs.
After curing the RTV rubber, the thin and flexible RTV agglomerate “post” sheet is easily peeled off the etched metal hole-plate.
Square posts in a X-Y array pattern provide gas exhaust open channels between the posts. Typical RTV silicon rubber posts have cross-sectional sizes of 0.035” X 0.035” with gaps of 0.025” between posts and with post heights of 0.025”. The total thickness of the flexible post sheet is about 0.050”, including the heights of the protruding posts.
Adhesives in the abrasive agglomerate island mixtures will not bond to the RTV silicone rubber post sheets.
ADD SOLVENT TO PHENOLIC ADHESIVE
Commercially available phenolic adhesive having approximately 22% alcohol solvent is typically quite thick with high viscosity. When mixed with vitrified diamond agglomerates, the thick mixture is difficult to level-fill the island holes in the rubber hole font sheets. However, the viscosity of the mixture can be significantly lowered by adding denatured alcohol to the phenolic. Increasing the total alcohol in the phenolic adhesive to about 50% results in a very “liquid” abrasive agglomerate mixture that is easily used to level fill multiple island holes in the rubber font sheet with a squeegee.
APPLY POROUS FILLER BETWEEN ISLANDS
First, epoxy is coated on the disc backing substrata between the islands. This epoxy coating is used to provide a strong bond of the porous bead filler to the substrate surface. The epoxy coating between the islands can be done quickly and effectively by a double-transfer coating process using the rubber hole font sheet that was used to mold-form the island structures on the disc substrate.
First a thick coat of epoxy is applied to a flat plate. Next, the rubber font sheet is placed in flat-surfaced contact with the thick epoxy layer on the plate to wet the rubber sheet surface with the epoxy. Upon separating the rubber island sheet from the plate, the original epoxy layer thickness is split, applying an epoxy coating on the rubber hole sheets. The epoxy-wetted rubber sheet holes are aligned with the molded islands and the rubber sheet is pressed down around the islands to contact the disc substrate and wet the substrate between the islands with epoxy. Upon separation of the rubber font sheet from the disc substrate, very little epoxy is bonded to the sidewalls of the islands.
About 6 grams of epoxy is mixed with approximately 3 cubic inches of 0.50 to 1.0 mm foam glass beads where the epoxy wets the surfaces of the beads but leaves voids between individual beads. The epoxy/bead mix is placed on the 158 islands of a 6” diameter abrasive disc, spread over the islands and a piece of non-stick parchment paper is placed over the bead mass. The bead mix is then pressed down between the islands with a 1.5” diameter by 2” wide roller to squash the bead mix into contact with both the disc substrate surface and the sides of the islands. Excess bead mix is scraped off the disc substrate at both the inner and outer radius of the annular band of islands.
The epoxy in the bead mix is cured at room temperature to avoid distortions of the disc due to differential shrinkage of oven-heated, and cooled, disc materials having different coefficients of thermal expansion.
After the epoxy in the bead mix is cured, the bead mix between the islands is rigid but completely porous. The outer diameter of the island disc backing substrate is cut circular by a bandsaw or water jet.
An abrasive disc attached to a rotating wafer head is placed in abrading contact with the rotating island disc to grind both the island surfaces and the porous bead filler between the islands into a common flat plane, parallel to the island disc substrate bottom surface.
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.
Non-Precision-Flat Platens
Non-precision-flat platens can be used with the abrasive island discs. A island disc can be attached to a non-flat platen with vacuum and an abrasive disc attached to the workpiece (or wafer} holder and the islands ground flat into a common plane. The wafer abrading action will be totally smooth because all of the island surfaces are flat in the common plane that is perpendicular to the platen spindle axis of rotation.
After the disc islands are ground flat, both the disc and the platen are ink-marked at a common alignment location on the circumference of the platen. After the disc is marked, the disc can be removed and then re-attached to the platen at a later time where all of the island top surfaces will again be in a common plane. The re-mounted disc will provide smooth wafer abrading action. Multiple discs having different abrasive particle sizes can be ground flat to match the non-flat platen, alignment marked, and used interchangeably to progressively abrade wafers.
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.
