Chemical Mechanical Planarization

 

Commonly known as CMP, it is a technique for planarizing the top surface of an in-process wafer. This process uses abrasive and corrosive chemical slurry in conjunction with a polishing pad and retaining ring, which is normally of a greater diameter than the wafer. The smooth surface is necessary to maintain the photolithographic depth of focus for subsequent steps,

 

and also to ensure that the aluminum interconnects are not deformed over contour steps.
The dynamic polishing head is rotated with different axes of rotation (not concentric). It removes material and tends to even out any irregular topography, making the wafer flat or planar. This may be necessary in order to set up the wafer for the formation of additional circuit elements.