Role of organic contamination in gate dielectric degradation: Kinetics and mechanisms

Abstract

As semiconductor devices get faster and more compact, the equivalent gate dielectric thickness is reduced aggressively. This makes them highly sensitive to contamination. Contamination from organics can degrade the performance of ultra-thin silicon oxide gate dielectric films used in current generation devices. The current understanding of how organic contaminants cause defects in gate oxides is limited. The objective of this research is to perform a fundamental investigation of the kinetics and mechanisms of the interactions of organic contaminants on silicon wafers during thermal oxidation for the growth of ultra-thin gate oxide. The role of moisture, a universal contaminant, in attracting organic impurities is also studied. The adsorption properties of butyl hydroxy toluene (BHT) and isopropyl alcohol (IPA), representing high and low-molecular weight polar organic compounds respectively, on wafer surfaces were characterized. A new experimental system that allowed pre-gate oxidation cleaning of silicon wafers, controlled exposure to organic contaminants and thermal oxidation was developed. A method based on catalytic oxidation of organics was also developed for detection of the kinetics of outgassing of organic contaminants during thermal oxidation. Gate oxide quality was determined by a combination of surface and electrical analytical techniques such as Auger depth profiling, Tunneling Atomic Force Microscopy and Gate Oxide Integrity. Processing conditions such as the type of pre-gate oxidation cleaning, the ambient used for ramp-up to the oxidation temperature, the ramp rates as well as the nature of the organic molecule were found to be important factors affecting the quality of ultra-thin gate oxides. Theoretical models were proposed to determine the kinetic constants and activation energies governing the interactions of contaminants on wafer surfaces under various conditions. Air-borne molecular contamination can be expected to cause similar problems for high-k gate dielectrics expected to replace silicon oxide. The energetics and kinetics of the adsorption of trace-level moisture and organic contaminants on zirconium oxide, a promising high-k candidate, were investigated and compared with that on silicon oxide. Zirconium oxide was found to have a much greater attraction for moisture as well as polar organic impurities. This can be a concern for its adaptation as the gate dielectric.