Thin-film deposition processes, in particular ion-beam sputtering (IBS), for multilayer optical coatings can cause stress in the coatings, which in turn can stress the underlying optic. If the optic is thin enough, the stress can cause the optic to warp enough to create aberrations. For example, some adaptive-optical systems have intentionally thin optics serving as deformable mirrors; here, warpage of the mirror due to coating fabrication reduces the operable range of the mirror for adaptive-optical wavefront correction. In response to this problem, researchers at Colorado State University (Fort Collins, CO) have developed an improved IBS process that can deposit high-optical-quality silicon dioxide (SiO2) thin films with residual stress reduced from 490 MPa to 48 MPa using high-energy O2-assist ion bombardment during thin-film deposition. The results are intended to improve the fabrication of high-reflectance tantalum pentoxide/silicon dioxide (Ta2O5/SiO2) multilayer stacks.
The researchers concentrated on the SiO2 deposition because stress in an IBS-deposited SiO2 film is typically about 5X higher than the stress in an IBS-deposited Ta2O5 film. Using dual IBS (DIBS), which has a primary and a secondary ion source with the secondary in this case being the O2 assist, the researcher produced SiO2 films deposited from an oxide target with 48 MPa residual stress and an optical absorption of less than 20 parts per million (ppm) at a 1064 nm wavelength, with less than 9 ppm sometimes achievable. The stress was shown to be lower at lower deposition rates. The researchers say that an optimum set of conditions for the deposition of low stress SiO2 would require a compromise between stress and deposition rate, especially when depositing a multilayer thin-film stack. Reference: A. Davenport, E. Randel, and C. S. Menoni, Appl. Opt. (2020); https://doi.org/10.1364/AO.380844.