Surface roughness of aluminum oxide thin films deposited by DC and RF reactive magnetron sputtering

Abstract

Aluminum oxide thin films produced by reactive sputtering processes are widely used in optical and microelectronic applications. This work investigates the dependency of the films’ surface roughness (Ra) on the operating pressure, the sputtering power and the deposition technique. Amorphous aluminum oxide thin films were grown on silicon (100) wafers by the DC and RF reactive magnetron sputtering deposition processes. In this experiment, high purity Al target (99.999%) was used. The operating pressures were set to 3, 5 and 10 mTorr. The sputtering powers were adjusted to 150 W and 300 W. Oxygen was injected into the chamber to react with sputtered Al atoms and formed aluminum oxide. For DC reactive magnetron sputtering, it was found that the surface roughness increased with increasing operating pressure. For the sputtering power of 300 W, the surface roughness increased from 1.03 nm to 2.17 nm as the operating pressure changed from 3 mTorr to 10 mTorr, respectively. It was also found that lower sputtering power resulted in smoother films. At 150 W sputtering power and 3 mTorr operating pressure, the Ra value was measured to be 0.81 nm. Higher operating pressure and sputtering power resulted in higher rates of particle collisions and scatterings. This reduced energies of sputtered atoms, preventing them to form dense and smooth films. The film deposited by the RF reactive magnetron sputtering yielded the lowest Ra value. At 150 W sputtering power and 3 mTorr operating pressure, the Ra value of the film was found to be 0.49 nm.