16. Design and operating characteristics of low pressure plasma systems

Abstract

Glow discharges in low pressure gases are commonly employed in surface etching and film deposition processes. The operating pressures and glow discharge characteristics are briefly reviewed for both physical and chemical procedures. Three basic conditions are then discussed: (a) the nature of substrate bombardment in sputtering systems, (b) the development of bias potentials at substrate surfaces and potential differences across insulating films and substrates and (c) the effects of the interaction of active gas and ion energy on the chemical sputtering of carbon. It is contended that employing crossed electric-magnetic field sputtering systems (e.g. magnetron), which deflect energetic electrons from the substrate also can enhance surface damage by back-scattered neutrals and sputtered atoms as these suffer less energy loss in the gas before reaching the work-holder. The operating conditions of diode sputtering without a magnetic deflecting field results in a high product of pressure, P , and target-work gap, d , and although a high dP lowers the deposition rate it also helps to reduce the energy of particles reaching the substrate. The floating potential V f attained by a substrate with respect to the plasma can influence the local bombardment conditions and differ from the potential of an anode or grounded rf electrode serving as a support. It is shown that the potential drop V o across an insulator resting on a biased probe is given by V o = V a + V f - V r , where V a is the applied potential between probe and anode reference electrode, and V r , the anode potential with respect to the plasma; if the anode is abnormally small then the relation can become V o = V a + V f + V r . In conclusion the results of etching carbon specimens at target and ground potentials in Ar- and Ar + O 2 -plasmas excited by dc and rf power supplies are summarized. It is shown that in terms of power input dc discharges are as effective as rf for etching carbon by physical and chemical means. Ion bombardment in O 2 -plasmas has been found to be particularly effective for removing carbon occlusions from metals.