Secondary electron effect on power–voltage relationship in rf sputtering

Radio frequency power–voltage characteristics for a number of sputtering target-gas combinations are presented. The log–log plots of these data highlight a strong correlation between the target’s secondary electron emission coefficient and the exponent in the expression P∝Vn. Most metal targets sputtered with argon exhibit less than a square law (i.e., n=2) dependence. These target-gas combinations generally have low secondary electron yields for ions at the energies used. However, the addition of an oxidizing gas or the use of targets in the alkali halide and metal oxide families produces exponents greater than two. The increased number of secondary electrons due to ion bombardment accounts best for this higher degree of ionization in the plasma. There is also an electrode coupling effect due to fast electrons from the target surface which is shown in the anode power–voltage characteristics. We suggest a model of the rf power–voltage relationship which includes the energy dependence of the secondary electron yield.