Abstract: A thin membrane having a thin film optical coating thereon is formed from multiple layers of different materials in which the overall stress of the thin film is not more than 15 MPa. Such films can be formed through thermal evaporation with ion assist, by directing an electron beam on a source and evaporating material from the source onto a thin flexible membrane while directing an ion stream onto the membrane. The current of the source of the ion stream should be sufficient to provide a thin film coating that has substantially no porosity. Successive applications at constant current can be deposited, while varying the voltage of the ion stream. The stress of the thin films deposited under each different voltage can be evaluated and the voltage at which the stress is acceptably low can be determined.