An optimized i
n s
i
t
u argon sputter cleaning process for device quality low-temperature (T≤800 °C) epitaxial silicon: Bipolar transistor and p
n junction characterization
Authors: Burger W. R. , Reif R. 
Published: July 2002

A novel in situ argon sputter cleaning process has been developed for the deposition of high-quality low-temperature (Tdep≤800 °C) epitaxial silicon films. A conventional in situ argon sputter cleaning process requires a room-temperature sputter followed by a high-temperature anneal in an ultrahigh vacuum system to prevent the deposition of a heavily dislocated epitaxial layer. We have found that by reducing the ion energy and flux by an order of magnitude to ∼100 eV and ∼4×1013 ions/cm2 s, respectively, and reducing the total ion dose by two orders of magnitude to ∼3×1016 ions/cm2, the sputter temperature may be increased to the deposition temperature without degrading the quality of the epitaxial film. Having a common sputter and deposition temperature is extremely important since it allows the in situ argon sputter cleaning process to be incorporated into a standard high-vacuum chemical vapor deposition process for the low-temperature deposition of epitaxial silicon layers. This paper will report the results of experiments designed to correlate the effects of the sputter temperature, and the ion energy, flux, and dose with the electrical and structural properties of subsequently deposited epitaxial films. We have found that an ion energy of either 200 or 300 eV inevitably leads to the severe degradation of device electrical characteristics and the generation of &gt;108 dislocations/cm2.

However, a 100-eV ion energy permits the deposition of high-quality epitaxial layers at temperatures at least as low as 750 °C. The optimum ion flux and dose for the subsequent deposition of dislocation-free epitaxial layers are strong functions of temperature. A mere doubling of either the optimum ion flux or dose at 750 °C leads to the generation of dislocations; increasing the temperature to 800 °C for these same sputter conditions permits the deposition of essentially dislocation-free epitaxial silicon layers (no dislocations were detected from measurements of the emitter-collector shunt density in bipolar transistors, which statistically requires the dislocation density to be &lt;100/cm2). The electrical quality of devices fabricated in the subsequently deposited epitaxial layers (extracted from dislocation-free devices) is only a weak function, if at all, of the ion flux and dose; the electrical quality is primarily determined by the deposition conditions. It is believed that the key to obtaining dislocation-free films when sputtering at elevated temperatures is to contain the damage created by the argon implantation to the top several monolayers where low-temperature surface annealing is effective. The sputter temperature then determines the amount of damage that can be continuously self-annealed, which in turn determines the temperature dependence of the optimum ion flux and dose. The low-temperature in situ argon sputter cleaning process has facilitated the deposition of essentially dislocation-free epitaxial silicon at 800 °C with electrical characteristics comparable or superior to bulk silicon. This was accomplished in a system utilizing only standard high-vacuum chemical vapor deposition technology.