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Ion energy distributions (IEDs) at a radio frequency (RF) driven substrate electrode were measured in an rf (13.56 MHz) bias sputtering system with a phase shifting unit for regulating the two rf voltages applied to the target (upper) and the substrate (lower) electrodes. Without the regulation, the IED was of a centrally peaked shape, not of a bimodal shape which is a characteristic of ions accelerated across an RF sheath, while the IED with the regulation showed a strongly asymmetric bimodal saddle shape. Moreover, it was found that the energy spread and asymmetry of IEDs and the ion current varied with the phase angle difference, and were correlated with the measured sheath thickness variation. These results were discussed with a simple collisionless oscillating sheath model.

Mass and energy measurements of ions and neutrals impinging on a substrate surface were performed during radio frequency (rf) bias sputter deposition of cubic boron nitride (cBN) thin films in a pure Ar discharge. The sampling system was rf driven to measure the correct energy of ions impinging to the rf driven substrate. The ion energy distributions showed asymmetric bimodal shapes and the energy spreads varied with the masses of ions and the negative substrate bias voltage (Vs). Ar+ was the most dominant ion, and the average energy and energy spread changed with Vs from 90 to 310 eV and from 40 to 140 eV, respectively. The flux ratio of Ar+ to B (Φi/ΦB) increased from 1.0 to 2.3 with a decrease in the target power input. cBN could be deposited above a threshold of the total momentum transfer per depositing boron atoms; 138 (eV amu)1/2, which is comparable to the values reported in the ion beam assisted depositions. Therefore, argon ions with an energy of 135 to 380 eV are considered to be equally contributory for cBN formation. Moreover, nucleation of cBN depends on Φi/ΦB, namely, the flux ratio of ion to boron, and requires accumulation of a certain surface state caused by ion bombardment effects, such as defects generation or densification.