Gas Implantation in Sputter Targets as a Factor Determining the Target Voltage in Reactive Sputtering

摘要

The absolute target voltage (ATV) in reactive sputtering depends essentially on the degree of ionization of the plasma. In turn, this ionization depends, on the one hand, on the sputter gas parameters such as pressure and ionization energy. On the other hand, it depends on the ion induced secondary electron emission (ISEE) of the target. Increasing the sputter gas pressure normally enhances the degree of ionization in the plasma, which results in a decrease of the ATV (in constant power or constant current mode). This effect is observed in many systems if they change from metallic sputtering towards poisoned mode sputtering under the influence of a pressure increase of the reactive gas. However, depending on the combination target material╚Dreactive gas―the ATV can increase as well and this effect must be attributed to a decrease of the ISEE of the target upon poisoning. It is widely accepted that either chemisorption or compound growth at the target surface are responsible for this change of the ISEE value when the system becomes poisoned. It is shown that the complex behavior of the ATV is also influenced by the implantation of reactive gas in the target during the sputter process. By using an ion gun it was possible to implant N_2~+ ions into a silver target without breaking the vacuum; these targets were used as sputter tar gets in a sputtering experiment. It was observed that the implantation of N_2~+ ions results in an increase of the ATV when such targets are used in a normal sputtering process. Consequently it follows that ion implantation of unreacted nitrogen leads to a reduction in ISEE.