Chemical-mechanical planarization
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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.
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[edit] Description
The process uses an abrasive and corrosive chemical slurry (commonly a colloid) in conjunction with a polishing pad and retaining ring, typically of a greater diameter than the wafer. The pad and wafer are pressed together by a dynamic polishing head and held in place by a plastic retaining ring. The dynamic polishing head is rotated 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. For example, this might be necessary in order to bring the entire surface within the depth of field of a photolithography system, or to selectively remove material based on its position. Typical depth-of-field requirements are down to Angstrom levels for the latest 65 nm technology.
[edit] How it works
The process of material removal is not simply that of abrasive scraping, like sandpaper on wood. The chemicals in the slurry also 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 from the 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 vigorous scraping does the candy dissolve away.
Another analogy is the act of brushing one's teeth. The toothbrush is the mechanical part and the toothpaste is the chemical part. Using either the toothbrush or the toothpaste alone will get one's teeth somewhat clean, but using the toothbrush and toothpaste together makes a superior process.
[edit] Usage in semiconductor fabrication
Before about 1990 CMP 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 impurities. Since that time, the integrated circuit industry has moved from aluminium to copper conductors. This required the development of an additive patterning process, which relies on the unique abilities of CMP to remove material in a planar and uniform fashion and to stop repeatably at the interface between copper and oxide insulating layers (see Copper-based chips for details). Adoption of this process has made CMP processing much more widespread. In addition to aluminum and copper, CMP processes have been developed for polishing tungsten, silicon dioxide, and (recently) carbon nanotubes.[1]
[edit] Key metrics
Key metrics that are important for CMP are the following:
- Rate of removal: How quickly can the material be removed?
- Uniformity of removal: How uniform is the removal across the die and the wafer?
- Planarity: How planar/flat is the surface after the removal process is complete?
- Defects: How many defects and of what size are left behind on the wafer?
