Intrinsic stress in dielectric thin films for micromechanical components

The film stress in coated micromechanical elements may cause bending of such elements and thus impair their performance. In these cases, stress reduction within a single layer by proper choice of deposition parameters or stress compensation within multilayer systems is necessary. In this paper, possibilities for stress reduction in high-reflection (Nb2O5/SiO2)(n) quarterwave multilayers for thin silicon laser mirrors have been investigated. Film deposition was performed by reactive direct-current (Nb2O5) and non-reactive radio-frequency magnetron sputtering (SiO2), respectively. The film stress was investigated as a function of process gas pressure, substrate temperature and ion bombardment of the growing film. At zero bias voltage, a total stress of about -30 MPa was obtained in the Nb2O5 films. Utilization of an additional electrode to reduce the plasma density in front of the substrate did change the stress to a small tensile value. SiO2 films show a compressive stress that could not be reduced below 100 FvLPa within the parameter range investigated. Complete stress compensation in the multilayer film systems was only possible by application of an additional tensile-stressed metal interlayer. Chromium films deposited prior to the growth of a (4 x 2) stack of Nb2O5 and SiO2 did compensate - within the error of measurement of +/-25 MPa - the average stress in the multilayer system to zero. (C) 1999 Elsevier Science S.A. All rights reserved.