Chemical-mechanical planarization

Chemical-mechanical planarization or Chemical-mechanical polishing, commonly abbreviated CMP, is a technique used in semiconductor fabrication for planarizing the top surface of an in-process semiconductor wafer or other substrate.

The process uses an abrasive chemical slurry (commonly a colloid) in conjunction with a polishing pad, typically of a greater diameter than the wafer. The pad and wafer are pressed together and rotated at different rates, with different axes of rotation (i.e., not concentric). This 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, i.e. to bring the entire surface within the depth of field of a photolithography system.

Before about 1994 it was looked on as too "dirty" to be included in high-precision fabrication processes, since abrasion tends to create particles and the abrasives themselves are not without impuritites). Since that time, the integrated circuit industry has moved from aluminium to copper conductors, making CMP processing much more widespread. Thick copper features were produced by electroplating, which gives a less uniform and planar result than is possible with other thin-film deposition methods, and so requires some method of planarization. Once this planarization step became necessary, however, other process steps were further optimized to exploit it.

The process of material removal is not simply that of abrasive scraping, like sandpaper on wood. The chemicals in the slurry can react with and/or weaken the material to be removed. The abrasive accelerates this weakening process and the polishing pad helps to wipe the reacted materials off the surface of the semicondutor wafer surface. The process has been likened to that of a child eating a gummy candy. If the candy sits on the tongue without being scraped around, the candy becomes covered with a gel coating, but the majority of the candy is not affected. Only with a vigourous scraping does the candy dissolve away.