Rf sputtering–voltage division between two electrodes

Abstract

In a dc sputtering system only the cathode is bombarded by positive ions. However, in a two-electrode rf sputtering system, both electrodes are bombarded. In both sputter deposition and sputter etching this bombardment must be controlled for control of contamination and film properties. The ratio of ion bombardment energies at the two electrodes has been studied here by an indirect method using measurements of the rf and induced dc voltages between the electrodes. The behavior of this voltage ratio, and of the corresponding ion energy ratio, has been studied as a function of the electrode area ratio (varied between 0.3 and 1), the electrode material (SiO2 and stainless steel), the rf voltage on the electrodes (varied from 300 to 2000 Vpp), the gas (He, Ne, Ar, Xe, and CF4), and its pressure (from 0.1 to 100 Pa). The results show that while it is easy to obtain roughly equal ion bombardment of the rf electrodes, large ion bombardment energy ratios are difficult to obtain and require careful system design. In particular the dependence of the ion bombardment ratio on the fourth power of the electrode area ratio, which has been widely assumed in the past, has not been confirmed. The analysis also describes the effect of the dc bias voltage and the rf peak-to-peak voltage on target etch rates.