Ultrahigh Vacuum Rapid Thermal Chemical Vapor Deposition of Epitaxial Silicon onto (100) Silicon: I . The Influence of Prebake on (Epitaxy/Substrate) Interfacial Oxygen and Carbon Levels

Abstract

This investigation is concerned with the influence of a vacuum prebake on oxygen and carbon levels at epitaxial silicon/silicon (100) interfaces. The epitaxial layers are deposited in an ultrahigh vacuum, rapid thermal reactor using chemical vapor deposition techniques. Secondary ion mass spectroscopy (SIMS) is used to evaluate carbon and oxygen levels at the epitaxy/substrate interface. We show that a vacuum prebake can be effectively used following a standard ex situ clean that consists of an RCA clean, a dilute (5%) HF dip, and a rinse in deionized water. The results show that if epitaxial deposition is initiated by introducing the reactive gases into the chamber at the prebake temperature, oxygen and carbon levels below the sensitivity limits of secondary ion mass spectroscopy are obtained at the epitaxy/substrate interface. This result can be reproducibly achieved with a low thermal budget prebake of 750~ s even after a relatively long rinse (-300 s) in deionized water. We propose that the mechanism responsible for cleaning is thermal desorption of oxygen and hydrocarbons from the (100) surface of silicon. We show that the surface obtained with this ex situ clean is very stable and, hence, the wafer can be left in a clean ultrahigh vacuum environnient for many hours without detectable changes in the oxygen and carbon levels. On the other hand, results indicate that when the prebake is terminated by cooling the wafer to the ambient temperature of the reactor, carbon is readsorbed on the silicon surface at a peak concentration of 3 to 6 x 10 TM cm -3. We also show that when a small amount of hydrogen is introduced into the reactor during the prebake, a higher thermal budget is required to remove oxygen from the surface. This observation is attributed to a higher H20 background associated with the presence of hydrogen. It is concluded that vacuum prebake is an attractive surface preparation technique which effectively reduces oxygen and carbon levels on a silicon (100) surface below the SIMS sensitivity limits.

Defects Induced by Carbon Contamination in Low-Temperature Epitaxial Silicon Films Grown with Monosilane

“…3) Thermal cleaning before epitaxy is the most important process of removing the contaminants in the epitaxial process. 4,5) Although oxide contamination can be removed during such thermal cleaning process below the relatively low temperature of 850 C, silicon-carbon bonds, once formed, are very difficult to break unless the surface is heated above 1200 C. 6,7) Such silicon-carbon bond contaminations can result in the threedimensional growth of silicon epitaxial layers leading to surface roughness. However, little is known about the formation mechanism of defects induced by such carbon and carbide contaminations during low-temperature silicon epitaxy.…”

Low Thermal Budget Surface Preparation for Selective Epitaxy. A Study on Process Robustness

“…The DI water rinse times were varied from 10 s (typical DI water rinse time in our S0013-4651(98)05-084-4 CCC: $7.00 © The Electrochemical Society, Inc. ex situ cleaning) to 1000 s. Note that there is a very slight increase in the oxygen peak when the duration of the rinse was varied from 10 to 1000 s. Sanganeria et al showed that the formation of these bonds is almost instantaneous when a wafer is dipped in DI water. 19 Graf et al demonstrated that the oxygen on the silicon surface at this point is in the form of OH bonds attached to the hydrogen on the surface. 21 Figure 2 shows the carbon concentration at the epitaxy/substrate interface for different waiting times in DI water after a dilute HF dip.…”

“…A detailed comparison of gas switching vs. temperature switching and its impact on interfacial carbon can be found in a previous publication from this laboratory. 19 Figure 13 illustrates the oxygen concentration at the epitaxy/substrate interface for two different partial pressures of oxygen. With an oxygen partial pressure of 1 ϫ 10 Ϫ6 Torr in the cleaning ambient, a residual oxide was observed on the surface even after an 800ЊC/20 s in situ clean.…”

“…When the wafers are passivated with hydrogen, they can be kept in vacuum at room temperature without any contamination for extended periods of time; Sanganeria et al showed that no additional oxygen or carbon contamination was observed on hydrogen terminated silicon surfaces for up to 18 h. 19 This is especially important for single-wafer cluster tools in which a cassette of wafers might be loaded into the reactor after ex situ clean and there may have been a lapse of time between processing of wafers. However, when the hydrogen passivation is removed from the surface, silicon surface is at a highly reactive state and it is prone to contamination from the ambient.…”

Ex Situ and In Situ Methods for Complete Oxygen and Non-Carbidic Carbon Removal from (0001)SI 6H-SiC Surfaces

“…Following Si technology, where surface cleaning and preparation are critical first step in all processes [1], a continued reduction in defects in SiC should be expected as a result of improved SiC wafer surface cleaning techniques. In Si technology for example, improper removal of surface contamination and oxides prior to Si homoepitaxy has been shown to result in an increase in the density of line and planar defects in epitaxial films from < 10 4 /cm 2 to > 10'°/cm 2 [2][3][4][5] and an associated drop in device yield [2]. Typically SiC ex situ cleaning consists of solvent degreasing and RCA cleaning with the last step usually a 5-10 min.…”