PROPERTIES OF CHEMICAL OXIDES FROM PRE-GATE CLEAN PROCESSES AND THEIR ROLE IN THE ELECTRICAL THICKNESS OF THERMALLY GROWN ULTRATHIN GATE OXIDES

摘要

Wet chemistry cleaning processes traditionally used prior to gate oxidation can result in the growth of chemical oxides of up to 1.5 nm as measured by ellipsometry. These chemical oxides have been considered to appreciably influence the subsequently prepared gate dielectric thickness. The issue is of particular concern for Sub-180 nm MOSFET design rules requiring gate oxides of less than 3.0 nm for traditional oxide-poly gate stacks, and interfacial oxides of less than 1 nm for high-K gate stacks. In this paper, the effect of various pre-gate clean strategies that generate chemical oxides of up to 1.2 nm on the electrical thickness of 2.5, 3.5 and 4.5 nm gate oxides was systematically studied. The growth kinetics of thermal oxides on wafers with different surface terminations at room and elevated temperatures was also investigated. It was found that the impact of chemical oxides on the thickness of thermally grown gate oxides > 2.5 nm is minimal. There is compelling evidence suggesting that the chemical oxide is very likely composed of aggregates of highly hydrated molecular silicic acid clusters formed in the aqueous oxidative medium, and that these silicic acid molecules are dehydrated and densified during thermal oxidation to form stoichiometric SiO{sub}2.