Surface Cleaning for Silicon Epitaxy Using Photoexcited Fluorine Gas
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ABSTRACT We studied surface cleaning using photoexcited fluorine gas diluted with hydrogen (UV/F 2/H2). We found that UV/F 2 /H2 cleaning selectively removes native Si oxides from thermal oxides without etching the bulk Si. After UV/F 2/H 2 cleaning, hydrogen atoms terminate almost all the dangling bonds on the Si surface, and fluorine atoms terminate the few remaining bonds. UV/F 2/H2 cleaning also flattens the Si surface. We applied UV/F 2/H2 cleaning to Si epitaxy and obtained single-crystal Si films with preannealing and growth temperatures as low as 600' C, 1500 C lower than for conventional methods. UV/F 2/H2 cleaning is a good dry precleaning method for various processes that include Si epitaxy.
INTRODUCTION
Si surface treatment is an important aspect of wafer processing for ultra-large-scale integration (ULSI). It includes removing oxide, carbon, or metallic impurities; reducing surface damage or controlling surface roughness; and forming surfaces to prevent contamination between processing stages. The removal of native Si oxide is a key process for fabricating highperformance devices [1], because it lowers the preannealing temperature for low-temperature Si epitaxy [2,31, helps form thin high-quality gate oxides [4], and decreases contact resistances. Low-temperature processes are also required to maintain sharp, compact dopant profiles, and for use with new materials which have high vapor pressures or high reactivity with adjacent materials. It is difficult to remove the stable Si oxides, including native Si oxide from Si surfaces. For example, preannealing at over 750*C is needed to remove the native oxide for Si epitaxy [5]. Many techniques have been proposed for the low-temperature removal of native Si oxide. Gases or solutions containing fluorine compounds have been used; examples include hydrogen fluoride (HF) gas [4,6], nitrogen fluoride (NF 3) gas [7], and a diluted aqueous HF dip [8,9]. Using HF gas [4], native oxides were selectively etched from other oxides, including thermal oxide. Following this cleaning, a high-quality gate oxide was grown. Silicon film has been grown after HF vapor cleaning [10]. An aqueous diluted HF dip has also been used to remove the native oxide ex situ. It was reported that most dangling bonds on a surface dipped in aqueous diluted HF were terminated by hydrogen atoms [11,12]. This surface is suitable for Si epitaxy because, at low temperatures, it is effectively protected from reoxidation and, at epitaxial growth temperatures, above 5000 C, H atoms on the Si surface are desorbed. The cleaned wafers were, however, passed to the epitaxial chamber through air, and the Si surface adsorbed some 269 Mat. Res. Soc. Symp. Proc. Vol. 318. ©1994 Materials Research Society
contaminants, including hydrocarbons. A combination of the aqueous HF dip and ultraviolet ozone cleaning (UVOC) [13] eliminated carbon contaminants [14,15]. The final step must remove the oxide formed during UVOC, so the wafer should be cleaned just before epitaxial growth or transferred through a less con
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