Effect of silicon wafer cleaning on pre and post thermal oxidation charges

A controlled study is reported in which a correlation between pre and post oxidation charge is established. This study utilizes a Surface Charge Analyzer (SCA) to measure electrically active contamination on the surface of silicon wafers. The SCA is a new surface photovoltage technique using low intensity light chopped at high frequencies to characterize the electrical properties of a silicon wafer surface. The method does not require any additional processing. By measuring charge values after pre oxidation cleaning, gate oxidation, and anneal it is shown that the contamination left on bare wafers is highly dependent on the last steps of the cleaning process. It is also shown that this contamination has a significant impact on the quality of a subsequent gate oxidation. A higher temperature nitrogen anneal reduces the oxide charge variation observed, but is not sufficient to compensate for charge induced at cleaning.