The Evolution of Silicon Wafer Cleaning Technology

The purity of wafer surfaces is an essential requisite for the successful fabrication of VLSI and ULSI silicon circuits. Wafer cleaning chemistry has remained essentially unchanged in the past 25 years and is based on hot alkaline and acidic hydrogen peroxide solutions, a process known as "RCA Standard Clean." This is still the primary method used in the industry. What has changed is its implementat ion with optimized equipment: from simple immersion to centrifugal spraying, megasonic techniques, and enclosed system processing that allow simultaneous removal of both contaminant films and particles. Improvements in wafer drying by use of isopropanol vapor or by "slow-pull" out of hot deionized water are being investigated. Several alternative cleaning methods are also being tested, including choline solutions, chemical vapor etching, and UV/ozone treatments. The evolution of silicon wafer cleaning processes and technology is traced and reviewed from the 1950s to August 1989. The importance of clean substrate surfaces in the fabrication of semiconductor microelectronic devices has been recognized since the early days of the 1950s. As the requirements for increased device performance and reliability have become more stringent in the era of VLSI and ULSI silicon circuit technology, techniques to avoid contamination and processes to generate very clean wafer surfaces have become critically important. Besides, over 50% of yield losses in integrated circuit fabrication are generally accepted to be due to microcontamination. Trace ibmpurities, such as sodium ions, metals, and particles, are especially detrimental if present on semiconductor surfaces during high-temperature processing (thermal oxidation, diffusion, epitaxial growth) because they may spread and diffuse into the semiconductor interior. Impurities must also be removed from surfaces before and/or after lower temperature steps, such as chemical vapor deposition, dopant implanting, and plasma reactions. Postcleaning after photoresist stripping is necessary for every mask level throughout the production process. Many wafer cleaning techniques have been tested and several are being used. The generally most successful approach for silicon wafers without metallization uses wetchemical treatments based on hydrogen peroxide chemistry. This process has remained essentially unchanged during the past 25 years, but important advances have been made in its technical implementation. The evolution of the cleaning technology for premetallized silicon wafers from its beginning to the present time will be traced in this