The effects of Xe on an rf plasma and growth of ZnO films by rf sputtering

Sputter deposition of thin films using Xe gas instead of Ar is believed to suppress damage to the films caused by ion bombardment. To investigate the effects of Xe on ionic species in the rf plasma, and on the growth of ZnO thin films, rf plasmas generated using Xe and Ar sputtering gases were analyzed using Langmuir probe methods and optical emission spectroscopy (OES). The Langmuir probe results showed that the electron temperature of the Xe plasma was nearly half of that of the Ar plasma. In addition, the ion density in the Xe plasma was found to be greater than the ion density in the Ar plasma. OES data showed that a significant fraction of the ionic species in the Ar plasma was oxygen, which is thought to be the primary cause of damage to growing films. Ionic molecular oxygen (O2+) was observed only in the Ar plasma, and the concentration of excited atomic oxygen (O*) in the Ar plasma was ten times as high as that in the Xe plasma, for an oxygen partial pressure of 20%. In addition, ions in the Xe plasma were found to have smaller kinetic energies than for the ions in the Ar plasma, and the main ionic species were different. Using Xe as the sputtering gas therefore resulted in higher quality ZnO thin films with smoother surfaces