MICRO:Extreme Silicon by Peng Sun, p.41 (April '99)

Peng Sun and Marty Adams, As the semiconductor industry ramps up to 300-mm wafers and faces the evolving defect density requirements of the SIA roadmap, nonmetallic ionic contaminants on wafer surfaces have become an increasingly important concern. As the semiconductor industry ramps up to 300-mm wafers and faces the evolving defect density requirements of the SIA roadmap, nonmetallic ionic contaminants on wafer surfaces have become an increasingly important concern. Because such impurities can degrade wafer surfaces, particularly in the presence of adsorbed water, they have detrimental effects on device fabrication. Unfortunately, methods for measuring nonmetallic ions at the ultratrace levels found on wafer surfaces lag behind the industry's demands. Recent research in this area has focused on using ion chromatography (IC) and capillary electrophoresis (CE). Figure 1: Schematic of the wafer-extraction system. A common approach is to use high-purity deionized (DI) water to extract the wafer surface (or a specific surface location) and then analyze the extraction solution using IC or CE. Through years of experience in surface contamination analysis, the current authors have found that sample preparation and handling are among the most critical processes for the successful measurement of surface ionic contaminants. Airborne gas-phase species such as ammonium, hydrofluoric acid, hydrogen chloride, and sulfur and nitric oxides can cause significant contamination problems during the surface extraction process, especially when it is done in an open environment in which such chemicals are used. This airborne contamination problem has also been reported elsewhere. Performing extractions in a Class 1 cleanroom cannot totally eliminate the problem, since ULPA filters are not designed to remove airborne molecular species. When a traditional extraction box method is used, an operator has to open the box to collect the extraction solution, and airborne ionic species can contaminate