Background

Lapping and Polishing

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 mirror-smooth to produce LEDs or semiconductors. All three materials are almost as hard as diamond. They are now polished with slow chemical mechanical polishing (CMP) liquid abrasive slurries that can not be speeded-up because of wafer hydroplaning.

Keltech’s fixed-abrasive island-disc prevent hydroplaning at very high abrading speeds and provide 10X faster polishing using diamond particles. Wafer cut-rate increases are proportional to abrade-speed increases.

Faster production means fewer polishing machines and operators with large time and cost savings.

High Speed Abrasive Polishing

High speed abrading is made possible with abrasive discs having annular bands of abrasive islands attached to a flexible disc backing. Hydroplaning of workpieces is avoided as the islands prevent a thick liquid film of coolant water from separating the workpiece surfaces from the abrasive. Vitrified diamond agglomerates allow abrasive particles to contact and abrade workpiece surfaces and provide high sapphire cut rates and long disc abrade lives.

Vitrified Diamond Abrasive Agglomerates

The use of vitrified diamond abrasive agglomerates provide a new era of high speed abrading of sapphire and other hard materials. Melted glass strongly bonds individual diamond particles that slowly wear down. High cut rates and long abrade life occurs with the abrasive agglomerates.

Agglomerates can be molded into island structures on flexible abrasive discs. They can also be coated with adhesives as a layer on a continuous polymer web that can be cut into sheets and discs.

Further, the agglomerates can be used to make grinding wheels. They are simply coated or molded on wheel hubs and bonded with an adhesive. This is a far easier way to make these grinding wheels compared to the conventional method of molding a glass-diamond mixture on a wheel and placing the whole wheel in a furnace to melt and fuse the glass around the diamond particles.

Vitrified abrasive agglomerates are formed by mixing glass powder and diamond particles, melting the mixture in a furnace and then fracturing the solidified mixture into small agglomerate shapes.

Vitrified Agglomerate Island Discs

Keltech recently-developed vitrified diamond abrasive agglomerates are formed into agglomerate islands that are molded in annular bands on flexible discs. The island shapes allow excess coolant water to be moved off their tops into the recessed channels between them to prevent hydroplaning at high speeds.

By vitrification, melted glass strongly supports encapsulated individual diamond particles and allows them to slowly wear down during abrading. The relatively large-sized vitrified agglomerates are easily supported by polymers when they are molded into agglomerate island shapes.

Vitrified agglomerates provide both the high cut rates and the required long abrading life of the abrasive discs. Multiple sapphire abrading tests have verified this performance.

Annular Bands of Islands on Discs

Agglomerate filled islands arranged in annular bands on flexible polymer discs allow the workpieces to overhang the band to provide uniform wear of both the workpieces and the annular abrasive. Both the discs and workpieces are rotated in the same direction and speeds.

Flexible discs are quickly attached to a platen by vacuum to allow the required change of abrasive sizes from coarse to medium to fine for progressive polishing of workpieces. Discs having different abrasive materials can be used interchangeably. The discs also prevent wear of the precision-flat platen surfaces.

An erodible porous layer is applied to the disc surface between islands to carry coolant water to workpiece surfaces during abrading.

Abrasive island discs can range in size from 12” to 60” , or more. 12” discs typically have 550 vitrified agglomerate islands.

Liquid Abrasive Slurry Polishing

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 workpieces. As the abrasive slurry is positioned between the workpieces and the platen, both the workpieces and the platen surfaces are abraded, resulting in non-flat platen surfaces. Platens are reconditioned periodically to regain flatness.

The platen is typically rotated at the very slow speeds of 50 to 100 rpm to prevent workpiece hydroplaning. This is where the liquid abrasive slurry forms a film that separates the workpieces from abrading contact with the platen abrasive. By comparison, fixed abrasive island discs are rotated from 500 to 3,000 rpm without hydroplaning.

Only some of the expensive abrasive particles in an abrasive slurry are fully utilized. Spent slurry containing old and new particles and abrading debris is continually washed off the abrading surface of the platen.

High Speed Abrasive Lapper Machine

The lapper is a heavy industrial machine with an air bearing platen having a precision flatness of 0.0001” and a speed range of 0 to 3,000 rpm. Either 12” or 18” diameter discs are quickly attached with vacuum for interchangeable use of discs with different sized abrasive or different abrasive materials.

Individual or groups of workpieces are mounted on a wafer disc and quickly attached with vacuum to a wafer head.
The wafer head has a spherical bearing which provides floating flat-surfaced contact of the workpieces to the platen abrasive surface.
Can also be operated in a non-floating rigid mode.
An air chamber in the wafer head allows the workpiece abrade pressure to be adjusted during an abrading operation.
Disc island surfaces can also be quickly and easily reconditioned by attaching a conventional abrasive disc to the wafer head with vacuum and grinding the islands.

Multiple wafer heads can be used on a lapper machine to simultaneously polish groups of wafers on the same rotating island abrasive disc.