Ultra-low stress SiO₂ coatings by ion beam sputtering deposition

High mechanical stress can affect the performance of multilayer thin film optical coatings, causing wavefront aberrations. This is particularly important if the multilayer stack is deposited onto thin substrates, such as those used in adaptive optics. Stress in thin film coatings is dependent on the deposition process, and ion beam sputtering (IBS) thin films are known to have high compressive stress. In the present work, we show that stress in IBS $ {{\rm SiO}_2} $SiO₂ thin films can be reduced from 490 MPa to 48 MPa using high-energy $ {{\rm O}_2} $O₂ assist ion bombardment during deposition while maintaining high optical quality. A comparison of the reduction of stress in $ {{\rm SiO}_2} $SiO₂ deposited from oxide and metal targets is provided.