Encapsulated Abrasive Particles
Small abrasive particles are encapsulated in large erodible porous ceramic beads.
The abrasive beads are mixed with a phenolic adhesive and coated on the tops of
the islands that are integral to flexible and tough precision-thickness polycarbonate
discs. Diamond particles encapsulated in the beads range in sizes from 45 microns
to sub-micron. Large 30 or 45 micron abrasive particles are used to initially
develop the wafer flat surfaces. Then, medium sized 10 micron particles start the
polishing action. Finally, 3 micron and sub-micron particles provide the desired
wafer smooth surface finishes.
Abrasive Particle Materials
Diamond abrasive particles are used to quickly remove material from very hard
wafer materials such as sapphire. The material removal rates (MRR) are very high
for sapphire wafers when even harder diamond particles are used at high abrading
speeds of 5,000 to 10,000 sfpm (56 to 113 mph). Other bead-encapsulated
abrasive materials can include BC, SiC, aluminum oxide, CBN and even silica
(for final polishing of sapphire wafers).
Abrasive Bead Sizes
The large porous ceramic erodible beads used here are cousins of the same abrasive beads used successfully for years on diamond abrasive lapping films used to polish hard materials and fiber optics devices. However, our large .015" equal-sized spherical beads are much larger than conventional .002” non-equal sized beads. With beads having the same diamond composition, a .015" bead contains 400X as much abrasive as conventional .002” beads. This provides long abrading life and full utilization of all the expensive diamond particles encapsulated in each bead.
Industry Standard Abrasive Bead Production
Abrasive beads are typically made by mixing abrasive particles with a water solution containing suspended particles of nanometer sized silica particles. The liquid mixture is formed into ligament streams (by a high-speed rotary wheel) that are ejected into hot air, breaking them into segments. Surface tension forces create spherical shapes from the ejected liquid segments before they are solidified by the hot air. The sizes of the broken segments are variable, so the diameters of the formed beads are nonequal. Solidified spherical beads are dried in an oven and sintered in a furnace at 500 C to form a porous ceramic matrix that supports individual abrasive particles.
Equal-Sized Abrasive Beads
Our equal-sized abrasive beads are made by use of a screen having equal-sized openings that are level-filled with a mixture of diamond particles and a colloidal water solution of silica particles. The liquid abrasive mixture in the holes is ejected as lumps which form spherical shapes due to surface tension before they land in a surface-solidifying environment. These equal-sized beads are then dried in an oven before finally being sintered in a furnace.
Erodible Abrasive Beads
It is well established that the bead’s porous ceramic matrix erodes by abrading action at a rate that releases new sharp particles when dull abrasive particles are ejected. All the expensive fixed-abrasive diamond particles encapsulated in the beads are progressively utilized as the beads are worn down by abrading action. As the individual diamond particles slowly wear, they continually develop sharp cutting edges at the leading edges of the particles.
Compare Beads with Slurry
Abrasive particles are encapsulated in beads that are adhesively coated on the island top surfaces of flexible discs. Abrasive slurries by contrast, have loose abrasive particles suspended in oil or water. Typically, only a fraction of the individual abrasive particles in a liquid abrasive slurry are fully utilized. Only the individual slurry particles that become embedded on a moving platen high spot have abrading contact with a wafer. Other particles that “float” in the non-flat gaps between the platen surface and the wafer have no abrading function. Abrasive slurry is continually fed to the platen surface and often leaves that surface without utilizing all of the particles and these unused particles are not recovered for reuse. By comparison, fixed-abrasive discs hold all of their abrasive beads in place, which ensures that all beads on the disc are used to abrade the workpiece. This means that nearly all of the diamond used on a fixed abrasive disk is fully utilized.
Compare Beads with Pyramids
Spherical abrasive beads allow a more efficient use of diamond particles than coated abrasives having pyramid shapes. Most of the volume of abrasive particles contained in the beads is located well above the abrasive disc backing surface where it is in abrading contact with a wafer. Only a few abrasive particles in a bead are located close to the backing surface. Conversely, most of the volume of abrasive material contained in a pyramid is located very close to the backing surface. The small pyramid tops are quickly worn down and often the backing is contacted by the wafer before the main bulk of particles within the pyramid are used for abrading action.
Lapping Film Beads
Conventional abrasive non-island lapping films with large 30 micron (.001”)
diamond particles do not use beads. This is because it is difficult to
encapsulate large abrasive particles in small beads. Here, conventional
abrasive beads are only 50 microns (.002”) in diameter, or much smaller,
compared to the relatively large size of the 30 micron (.001”) particles.
Instead, these relatively large particles are coated in a monolayer directly
on thin .003” polyester backings. The lower portions of the single layer of
particles are covered by the binder adhesive leaving only the top portions
of the particles exposed for abrading. Dimensional variations of the
flatness of conventional platens can easily approach this particle exposed
height. This results in only localized wafer abrading contact with “high
portions” of the coated abrasive disc attached to the rotating platen when
abrading at high speeds.