Effect of system parameters on substrate bias during D.C. and R.F. sputter deposition

When an electrically isolated substrate holder is immersed in a gas discharge, as in sputter deposition, the holder and substrates are exposed to bombardment by energetic secondary electrons which may result in large substrate bias voltages. The induced substrate potential is affected by many system parameters. In this investigation, we have measured the variation of bias at an electrically floating substrate as a function of gas pressure and target to substrate separation for both d.c. and r.f. sputtering discharges. In the case of d.c. sputtering the voltages in question may attain negative potentials of up to 1 kV. Operating under these conditions will cause severe heating of the substrates and the growing deposits. R.F. sputtering discharges on the other hand, induce relatively small negative (less than -100 V) and positive voltages at the substrate. These small positive biases have been shown to markedly affect the properties of r.f. sputtered metal films. In the case of r.f. sputter deposited chromium, electrically floating the substrates resulted in deposits which were only approximately 70 percent dense. (7 figs, 19 refs) (U.S.)