Chemical Mechanical Polishing (CMP)

With every additional layer – isolation oxide, contact level, metal layer – the topography on the wafer surface keeps growing. This is a problem for lithography: the depth of focus of modern exposure systems is only a few hundred nanometers, so structures on surfaces of different height can no longer be imaged sharply at the same time. Subsequent deposition and etch steps also proceed more uniformly on a flat surface than across steps and trenches. Beyond a certain level of integration density, a globally planar surface is therefore no longer a matter of convenience but a precondition for the next process level.

In chemical-mechanical polishing (also chemical-mechanical planarization, CMP for short), the wafer surface is attacked chemically and removed mechanically at the same time – unlike with pure reflow techniques, this produces uniformity across the entire wafer rather than just local smoothing.

The wafer is held face-down in a chuck with vacuum suction (head) and pressed onto a polishing surface (pad, usually made of polyurethane) on the polishing table. The head and the polishing table rotate, while the head can additionally perform horizontal movements. A solution (slurry) of abrasive particles and chemically active substances serves as the polishing medium; under pressure it alters the surface and thereby supports the material removal. To distribute the slurry evenly and to condition the polishing cloth, the pad is roughened with a diamond-studded steel disc (dresser/conditioner) – either during polishing (in-situ) or before/after it (ex-situ).

The CMP process usually runs in two to three stages on pads with different surface properties and different slurries; the wafer is transferred to the next pad after each step. A cleaning step follows to remove particle and slurry residues.