ULTRAHIGH-VACUUM RAPID THERMAL CHEMICAL-VAPOR-DEPOSITION OF EPITAXIALSILICON ONTO (100)SILICON .1. THE INFLUENCE OF PREBAKE ON (EPITAXY SUBSTRATE) INTERFACIAL OXYGEN AND CARBON LEVELS

This investigation is concerned with the influence of a vacuum prebake on oxygen and carbon levels at epitaxial silicon/silicon (100) interf aces. 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 le vels at the epitaxy/substrate interface. We show that a vacuum prebake can be effectively used following a standard ex situ clean that consi sts of an RCA clean, a dilute (5 %) KF dip, and a rinse in deionized w ater. The results show that if epitaxial deposition is initiated by in troducing the reactive gases into the chamber at the prebake temperatu re, 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 p rebake of 750 degrees C/15 s even after a relatively long rinse (simil ar to 300 s) in deionized water. We propose that the mechanism respons ible for cleaning is thermal desorption of oxygen and hydrocarbons fro m 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 i n a clean ultrahigh vacuum environment for. many;hours without detecta ble changes in the oxygen and carbon levels. On the other hand, result s indicate that when the prebake is terminated by cooling the wafer to the ambient temperature of the reactor, carbon is readsorbed on the s ilicon surface at a peak concentration of 3 to 6 x 10(18) cm(-3). We a lso show that when a small amount of hydrogen is introduced into the r eactor during the prebake, a higher thermal budget is required to remo ve oxygen from the surface. This observation is attributed to a higher H2O background associated with the presence of hydrogen. It is conclu ded 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.