A need often arises to grind or polish flat workpieces such as disk-shaped articles, e.g., silicon wafers, sapphire disks, optical elements, glass or aluminum substrates for magnetic recording devices, and the like, such that the two major surfaces are both parallel and free from significant scratches. Such grinding or polishing operations, differing in the rate of material removal and final surface finish, may be referred to collectively as lapping. A typical machine used for finishing the disks includes two superposed platens respectively disposed over and under one or more of the disks, so that opposing surfaces of the disks can be ground or polished simultaneously. Moreover, the lapping machine may include carriers that position and retain the disks during the grinding or polishing operation. Such carriers may be adapted to rotate relative to the platens. For example, the lapping machine may also include an outer ring gear, disposed around an outer periphery of the platens, and an inner gear, that projects through a hole formed in a center of the platens. The carriers can have a toothed outer periphery, which engages with the teeth or pins of the outer ring gear and the teeth or pins of the inner gear. Rotation of the inner gear and outer gear in opposite directions, for example, thus causes the carrier to rotate globally around the inner gear, and about an axis of the carrier.
Typically, the manufacturer of the single- or double-sided finishing machine will polish the surfaces of the platens using a lapping technique, prior to the polishing machine being shipped to the end user. It is conventionally believed that the lapping technique provides the platens with a relatively flat and planar surface suitable for most polishing operations.
To polish the workpieces, a polishing slurry is provided on a surface of the disks. The platens are brought together to exert a predetermined pressure upon the workpieces, and the carriers and workpieces are rotated, thus planarizing, polishing and/or thinning the surfaces of the workpieces.
Recently, fixed abrasive articles disposed over the working surfaces of the platens have been employed to reduce maintenance costs and the accompanying unproductive time associated with periodic dressing of the platens to the necessary degree of flatness and coplanarity.
It has further been observed that during the polishing of glass disks, for example, that the teeth of the carriers tend to wear prematurely. In fact, the teeth can become so worn that they will shear off from the carrier, causing the lapping machine to become inoperative (i.e., a so-called mid-cycle crash). As will be appreciated, since the carriers are relatively expensive, a long life is desirable. Moreover, mid-cycle crashes require that the polishing machine be removed from service for an extended period of time, thus reducing throughput and increasing the cost of operations.