Pneumatic Fittings Supply Air Or Vacuum To Each Chamber

Pneumatic fittings supply air or vacuum to each chamber

. The second piston includes a cylindrical side wall and an integral bottom plate.A retaining ring is attached to the lower edge of the annular piston. The retaining ring includes a peripheral groove for separating an outwardly extending flange from the main body of the ring. The underside of the ring includes one or more spiral grooves for circulating slurry about a wafer during polishing.

CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is "planarized" or smoothed one or more times during the fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.

The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is usually several times the diameter of a wafer and the wafer is kept off center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is rotated to prevent polishing a taper into the wafer.

The resilient layers on a polishing pad are assumed to provide a uniform pressure on a wafer. It is believed that this assumption is in error and one aspect of the invention addresses the problem of distortion in the polishing pad.

The wafer must be securely held but not damaged or contaminated. Polishing heads of the prior art typically use a wax-like material to attach the wafer temporarily to the carrier. The wax must be completely removable and must not affect the silicon to which it is attached. The polishing head cannot be so rigid that the wafer is chipped or damaged when the wafer engages the polishing pad or the head.

Whether the ring presses against the resilient polishing pad or not, there is inevitably an annular region about the periphery of the wafer where the polishing is not uniform, known in the art as "edge exclusion." Edge exclusion in the prior art is typically 5-10 mm. wide and reduces the area of the wafer from which good die can be obtained.

As a wafer is polished, chemical by-products locally change the composition, pH,particle size, and uniformity of the slurry. In the prior art, this problem was addressed by thoroughly mixing the slurry and by controlling the rate of flow to the polishing pad; specifically, by providing a sufficient flow to prevent large, local changes in composition, particle size, or pH. Even so, edge exclusion remains a problem.

The foregoing objects are achieved by this invention in which a polishing head includes a carrier plate having concentric, integral, cylindrical walls, an annular piston fitting within the outer of the cylindrical walls and a second piston fitting within the inner cylindrical wall and engaging the annular piston. Each piston defines a chamber with the carrier plate and the chambers are isolated from each other by a seal.

Pneumatic fittings supply air or vacuum to each chamber. The second piston includes a cylindrical side wall and an integral bottom plate. The bottom plate is thicker in the center than at the side wall and the underside of the plate is covered with a wafer adhering layer.

by: qiaorui

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