Real-Time Spectroscopic Ellipsometry Study of the Thermal Cleaning Process for Silicon Epitaxial Growth by UHV-CVD
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ABSTRACT Real-time spectroscopic ellipsometry (RTSE) method was applied to study thermal cleaning process of silicon surfaces for epitaxial growth by ultra-high vacuum chemical vapor deposition (UHV-CVD). For the first time, in-situ observation of oxide decomposition process under Si2H6 ambience was carried out. The substrates with thin oxide formed by wet chemical treatment were heated up by infrared heater under UHV or under Si2 H6 ambience in an UHV-CVD chamber and the oxide decomposition processes were observed by RTSE. Ellipsometric parameters Psi and Delta increase with the progress of oxide decomposition process and become constant with the completion of the decomposition. It was found that the oxide decomposition process consists of two phases and rate-determing processes are different in each phase. It was also found that Si2 H6 lowers the activation energies of oxide decomposition process in each phase. INTRODUCTION Epitaxial growth technique of silicon and group IV alloys on silicon substrates have recently received considerable interest for fabricating devices such as SiGe heterojunction bipolar transistors [I]. Cold-wall UHV-CVD is one of the most suitable methods to grow group IV alloys. Thermal cleaning process to remove surface oxide on substrates is necessary to grow high quality crystal by cold-wall UHV-CVD. However, this thermal cleaning process requires higher temperature than that of typical epitaxial growth, then causes dopant diffusion and pattern deformation. Temperature lowering and proper control of thermal cleaning process are important subjects for fabricating devices with using epitaxial growth. The oxide decomposition process during vacuum annealing has been studied by several characterization methods, such as thermal desorption spectroscopy (TDS) [2, 5], transmission electron microscopy (TEM) [3, 4], scanning electron microscopy (SEM) [3, 4] and so on. These studies have revealed that silicon oxide decomposes by the reaction: Si+SiO 2-- 2SiO(g) 1 [2] and the decomposition of oxide is spatially inhomogeneous [3, 4]. However, these characterization methods are not suitable for in-situ monitoring to control actual device production or difficult to use under ambience of reactive gasses, such as Si 2H6 . Although Si2H6 is known to be effective in shortening thermal cleaning [6], in-situ study of oxide decomposition process under Si2H6 ambience has not been reported. Real-time spectroscopic ellipsometry (RTSE) method is a nondestructive characterization technique and can be used under ambience of reactive gasses. Furthermore, RTSE system can be easily installed in an epitaxial growth system. Therefore, RTSE is suitable for not only investigation into the oxide decomposition process but also controlling thermal cleaning for epitaxial growth. 83 Mat. Res. Soc. Symp. Proc. Vol. 569 0 1999 Materials Research Society
In the present work, RTSE method was applied to study the oxide decomposition process during the thermal cleaning both under UHV and under Si2H6 ambience. SEM was also used to obs
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