Future Needs of Cleaning
Maria A. Lester, Associate Editor -- Semiconductor International, 7/1/2001
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Werner Kern, president of technical/scientific consulting firm Werner Kern Associates (Lakewood, N.J.), recently presented future requirements for the cleaning industry during his silicon wafer cleaning seminar at the SCP Global Technologies 8th International Symposium in Boise, Idaho. He addressed the practical and theoretical aspects of wafer cleaning and provided an overview of the technology, including the role of contaminants, wet cleaning processes and their underlying chemical and physical mechanisms, implementation in the fab, and wafer rinsing and drying.
Kern, previously a senior scientist at Lam Research Corp. before starting his own company, discussed the need for improved wafer cleanness for future VLSI processing in terms of particles, trace metals and alkaline elements; for ultraclean processing fabs to reduce contaminants and increase yields; and for improved purity of processing chemicals. Kern stated that, to control contamination of wafers more effectively, improved and more sensitive defect detection, measurement and analysis methods must be developed (Figure).| The cleaning roadmap shows the progressive steps to a single wafer wet clean. (Source: IMEC) |
Processing chemicals— Further reduction of total metal content in liquid chemicals to 1 ppb, particles to <10/mL; general reduction of liquid chemicals for processing and ecology reasons; a shift from liquid to gaseous reactants; and development of gaseous analogs to conventional liquid reactants in wafer cleaning operations required.
Cleaning equipment— Fully automated system needed for gas phase cleaning of one or several wafers, with entire cleaning sequence to be conducted in this system. The system should be integrated or clustered with processing equipment. Furthermore, cleaner fab equipment, in general, is required to reduce particle density.
Contamination control and analysis— Ultraclean semiconductor processing facilities with low particle generation equipment are needed along with improved contamination control in air, gases, chemicals and tools. A more widespread use of chelating reagents, surfactants, ozone and more diluted processing chemicals required. Also needed are defect reduction and metrology for bare and patterned VLSI wafers at increased inspection speed and with higher sensitivity levels.
Control of contamination from equipment, chemicals and processes— Wafer contamination must be avoided by reducing particles in equipment through scheduled maintenance, electrostatic charge removal and the use of high-purity, low-particulate processing chemicals, gases and DI water for critical process steps. Dispensing chemicals by bulk distributed delivery subsystems with point-of-use ultrafiltration also is necessary.
Control of contamination in tools and processes— Reduction of dust particles in processing equipment through scheduled maintenance and control of electrostatics by charge removal are essential. The use of high-purity, low-particulate processing chemicals, gases and DI water is mandatory for all critical process steps. The use of high-purity gas, water and chemical bulk distributed delivery subsystems with point-of-use ultrafiltration is needed. Guidelines include optimizing process and gas-flow systems to eliminate or reduce particle generation; transferring and storing wafers in a controlled microenvironment; monitoring impurities on wafers in-line (e.g., particle counting); and measuring ultratrace levels of contaminants on wafers by advanced instrumental analysis methods.
For additional information on clean processing, go to www.semiconductor.net/clean
