Low-temperature deposition of high-quality silicon dioxide by plasma-enhanced chemical vapor deposition

Abstract

Thin films of high-quality silicon dioxide have been deposited at low temperatures by plasma-enhanced chemical vapor deposition. A deposition rate much lower than that used in conventional plasma-enhanced processes is found to be crucial in obtaining material with reproducible, good properties. Controlled, slow deposition is achieved by using very low flow rates of reactive gases, together with a much higher flow of inert carrier gas to ensure uniformity. Films deposited at usual high deposition rates (∼500 Å/min) exhibit irreproducible and poor electrical properties and are porous. Those deposited slowly (∼60 Å/min) have very reproducible properties, are relatively dense and exhibit very good electrical integrity. Oxides deposited using a substrate temperature of 350 °C compare favorably with those deposited at 700°C using atmospheric-pressure chemical vapor deposition and can be deposited routinely over a wide range of oxide thickness. Deposition at 275 °C results in similar properties but with increased electron capture associated with deep bulk traps. Thicker layers can be deposited onto polycrystalline metal (sputtered films on glass substrates) without any deterioration in film properties. The as-deposited Si:SiO2 interface state density is quite high (∼1012 eV-1 cm-2), although it is reduced to very respectable levels (in the mid 1010-1011 eV-1 cm-2 range) by conventional postmetallization anneals. The suitability of these low-temperature oxides for thin film transistor applications is discussed.