Use of electric probes in silane radio frequency discharges

Abstract

The use of electrostatic probe techniques for measurement of the electron temperature kTe and the electron density ne in pure and BF3 doped silane radio frequency (13.56 MHz) discharges is demonstrated to be feasible even though amorphous silicon is being deposited on the probe surfaces. The required conductivity of such deposited silicon surface layers is examined and compared with the measured photoconductivity and thermally induced conductivity of amorphous silicon. An equation is derived for the double probe current–voltage (I–V) curve which includes the first order effect of ac fields and can be used to determine the electron density. During typical depositions of thin films of amorphous silicon from such discharges, values of kTe =2–2.5 eV and ne =(1–1.5)×109 cm−3 were obtained. These values are consistent with those obtained by other workers using microwave and optical techniques. In BF3 doped silane an abrupt step is observed in the Langmuir I–V curves which indicates the presence of a group of fast, anisotropic electrons. In diborane doped silane, probe measurements become unstable and irreproducible. For this reason we investigate a rf probe technique. The advantages and constraints inherent in using this method in our rf discharges are examined and analyzed.