3.2 Cleaning and Etching of wafers

Surface of semiconductor wafer gets contaminated during device processing. The source of contaminants are ambient air, storage ambient, process gases, chemicals, materials, water etc which are used in the fabrication processes. Processing tools as well as personnel operating in the clean- rooms are also sources of contamination. The most prevalent contaminants are particles and they may cause a catastrophic failure during device manufacturing process. The measure of the air quality of a clean room is described in Federal Standard 209. Clean rooms are rated as Class 10K, where there exist no more than 10,000 particles larger than 0.5 microns in any given cubic foot of air; Class 1K, where there exists no more than 1000 particles; and Class 100, where there exist no more than 100 particles. These small particles are controlled in a clean-room by using High Efficiency Particulate Air (HEPA) filters.

Another type of contaminants which degrade the devices are metallic contaminants originating primarily from liquid chemicals, water and process tools. The most common metallic contaminants are iron (Fe), aluminum (Al), copper (Cu), nickel (Ni) as well as ionic metals such as sodium (Na) and calcium (Ca). Organic contaminants are present in ambient air, storage containers and can arise from photoresists. Organic compounds readily adsorbed on surfaces adversely affect device properties. Native oxides as well as moisture from the ambient air or wet processes adversely affect the devices and can be considered as a contaminant and its removal is a part of cleaning process.