Background
Lapping and Polishing
Diamonds Cut Sapphire, SiC and GaN
Sapphire, SiC and GaN are almost as hard as diamond. However, when diamond abrasive particles are used to abrade these hard materials at very high abrading speeds, the diamonds provide very high wafer cut rates.
Precision-Flat, Smoothly Polished Wafers Required
Logs (boules) of single-grain grown sapphire, SiC and GaN are rough-sliced into wafers that must be precisely flattened and polished..
Cut Rates Increase With Speed
Wafer cut rates increase directly with increased abrade speeds.
Conventional slow-speed liquid abrasive slurry abrading has very low wafer cut rates. It can not be speeded-up because of wafer hydroplaning.
High-speed fixed-abrasive island discs provide very high wafer cut rates without hydroplaning.
Vitrified Diamond Abrasive Agglomerates, Long Disc Life
Vitrified diamond abrasive agglomerates consist of diamond particles encapsulated in glass. The glass rigidly supports individual diamond particles against abrading forces.
The agglomerates are molded into island shapes that are bonded to flexible disc backings. The islands prevent hydroplaning.
These high-speed island discs are durable, have large cut rates and last a long time abrading sapphire and other hard materials.
High Speeds Cause Hydroplaning
When a wafer contacts a liquid abrasive slurry coated abrading surface moving at high speeds, the wafer hydroplanes. Upon hydroplaning, the wafer is forced by hydraulic pressure away from the abrading surface with a reduction in cut rates.
One analogy is high speed hydroplaning of a boat.
Another analogy is high speed hydroplaning of bald car tires on a water-wetted highway. Car tires with tread lugs (disc islands) do not hydroplane.
Abrasive island discs do not hydroplane at high speeds. The islands prevent the formation of thick layers of water that separate the wafers from the abrasive surface.
Platens Require Annular Bands of Abrasive
The radial width of the annular band of a platen coated with abrasive is slightly less than the width of the abraded wafers in abrading contact with the platen abrasive. This allows the wafers to overhang both the inner and outer radii of the abrasive during abrading.
Rotating both the platen and wafers in the same direction and at the same speed allows both the wafers and the platen abrasive to wear down uniformly and maintain their flatness.
Both liquid slurry and island disc abrading systems use annular bands of abrasive.
Non-Flat Worn Platens
With the conventional liquid abrasive slurry polishing system, abrasive particles are mixed with oil or water and applied to a rotating platen that is in flat-surfaced contact by abraded wafers. As the abrasive slurry is positioned between the wafers and the platen, both the wafers and the platen surfaces are abraded, making the platen surface non-flat.
Wafers are contained in fixed-position conditioning rings that continually abrade the platen surface to maintain its flatness.
The flatness of the platen surface is periodically measured. When out-of-spec, the platen surface is re-ground to make it flat again. The typical non-flat variations of a platen surface far exceeds the sizes of the micron-sized abrasive particles used to polish the wafers.
By comparison, the precision-flat platens used with the flexible island discs remain flat during extensive abrading action. Here, the surface of the platen is protected by the vacuum attached abrasive discs.
Re-Flatten Disc Island Surfaces
When the planar surface of the disc islands becomes non-flat, it can quickly and easily be re-conditioned.
Here , the wafer disc that is attached with vacuum to the wafer head is replaced with a standard aluminum oxide abrasive disc having the same diameter as the wafer disc. The rotating abrasive disc on the wafer head is then used to abrade flat the rotating disc islands.
Abrading Friction Heats Wafers
Heat produced by abrading friction must be removed to prevent distortion of the wafer surfaces.
Little heat is generated by slow-moving abrasive slurry. The heat is removed by the liquid abrasive slurry.
More friction heat is generated by the high speed abrading. However, this heat is very effectively removed from the wafer surfaces by use of a water spray applied to the moving disc surface. Very high heat transfer occurs with evaporation of the water film spread on the wafer surface by the porous bead filler located between the islands.
Change Abrasive Particle Sizes
The sizes of the abrasive particles are typically changed three times when abrading wafers. Large-sized particles are used to initially flatten the wafer. Medium-sized particles are used to rough-polish them and small particles provide a smooth surface.
Changing particle sizes with liquid abrasive slurries is messy and time consuming. All of the previous large-sized particles must be completely removed from both the wafers and the platen before the smaller-sized particles can be used. Contamination of the small particles with stray large-sized particles disrupts the polishing action.
Island discs, having fixed-abrasives, that are vacuum attached to a platen can be quickly changed with little clean-up required. The wafers remaining attached to the wafer head can be simply washed with water to remove stray large particles before the next phase of polishing.
High Speed Agglomerate Abrasive Island Discs
Vitrified diamond abrasive agglomerates are molded into islands bonded in annular bands on flexible discs. The island shapes prevent hydroplaning of water-cooled wafers at high speeds.
The diamond-particle agglomerates provide high cut rates of sapphire wafers and long abrade lives of the island discs.
A porous-bead material between individual islands provides a water film that cools the wafer during abrading operations.
Discs range in size from 6”, 12”, 18” and up to 60”, or more.
Porous Abrasive Islands Provide Controlled Wear-Down
The abrasive agglomerate filled islands must be porous to provide a sustained and controlled rate of eroding during wafer abrading. The agglomerates and their diamond particles are continually worn down and replaced.
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 during production of the discs, voids are formed, making the islands porous.
Vitrified Diamond Abrasive Agglomerates
Vitrified agglomerates contain diamond particles strongly bonded by glass to resist abrading forces.
During abrading, the glass erodes and exposes individual diamond particles that are continually re-sharpened until each particle is slowly worn away.
This allows full utilization of each expensive diamond particle.
Fully-Utilize 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.
Just the largest particles present on the high-spot areas of the platen are in abrading contact with the wafers. Many of the typical tiny 2.5 micron (0.0001”) diamond particles residing in the platen low-spot areas are not utilized.
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.
Percent Diamond Particles In Agglomerates
A wide range of abrading cut rates and disc abrade life can be affected by many variables.
These include: the percent of diamond particle content in the vitrified abrasive agglomerates, the quantity of agglomerates in the islands, the porosity of the islands, the size of the diamond particles, the island disc and agglomerate manufacturing processes, the abrade speeds and the abrade pressures.
Percent Diamond Particles In Agglomerates
A wide range of abrading cut rates and disc abrade life can be affected by many variables.
These include: the percent of diamond particle content in the vitrified abrasive agglomerates, the quantity of agglomerates in the islands, the porosity of the islands, the size of the diamond particles, the island disc and agglomerate manufacturing processes, the abrade speeds and the abrade pressures.
High Speed Abrasive Lapper Machine
Flexible abrasive island discs are quickly attached to a platen by vacuum to allow wafers to be polished at high abrading speeds. An air-bearing platen having a 0.0001” precision-flat surface allows abrasive discs up to 18” diameters to be used interchangeably at speeds up to 3,000 rpm.
The wafers are quickly attached with vacuum to a wafer head to provide flat-surfaced conformal contact to the platen abrasive surface. Both the wafers and the platen are rotated in the same direction and about the same rpm to obtain uniform abrading of the workpieces and uniform wear of the disc abrasive.
Verified Island Disc Sapphire Abrade Test Data
High speed sapphire abrading test data of Keltech vitrified diamond agglomerate island discs using 40-50 micron and 30 micron diamond discs have shown 10X to 50X faster cut rates.
The test data and test conditions are described in the TECHNOLOGY and DATA sections.
Use Abrasive Island Discs WITH CMP Slurry System
Sliced silicon and very hard sapphire, SiC and GaN wafers can be quickly flattened and polished with Keltech’s abrasive island discs at high abrading speeds in preparation for CMP slurry polishing.
Then, the loose-abrasive slurry particles can be used to finish-polish the wafers using the slow CMP process.
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.
Use of 3M Fixed-Abrasive Diamond Tile Discs
The use of 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 polish wafers at lower abrading speeds.
One significant disadvantage of the 3M Diamond Tile discs is that they do not have annular bands of abrasive. Wafers can not overhang the abrasive to provide uniform wear-down of both the wafers and the disc abrasive during abrading operations.
Keltech’s fixed-abrasive vitrified diamond island discs having annular bands of islands can be used at abrading speeds of from 500 to 3,000 rpm to provide huge cut rates of sapphire wafers.
Keltech’s island discs can have diameters of more than 60”.
These island discs have 50 micron down to 3 micron) diamond abrasive particles
Vitrified Diamond Agglomerate Abrasive Wheels
The vitrified diamond agglomerates can be mixed with a solvent-based adhesive and coated on a wheel hub.
As the solvent evaporates, voids are formed in the mixture to form the required porous adhesive layer.
The adhesive strongly supports each individual agglomerate and the agglomerates support the individual diamond abrasive particles.
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.
Patents, Trade Secrets
Keltech has a large portfolio of issued and filed patents. Many are focused on the use of the vitrified diamond agglomerate island discs.
Also, Keltech has developed many proprietary (Trade Secret) process techniques that are used to efficiently make the high performance island discs.
