A system and process for forming optical quality, protective, relatively thick, thin film coatings on workpieces such as detectors or solar cells. The apparatus includes a rotary cylindrical sputtering system which incorporates separate deposition devices and at least one chemical reaction device for simultaneously (1) depositing materials which form tensile and compressive oxides and (2) oxidizing the deposited materials. The system also includes a stressometer system, preferably a cantilevered beam stressometer system which monitors the stress of the depositing film in-situ. The monitored stress levels are used to control the relative amounts of compressive and tensile materials which are deposited and, thus, control stress in the thin film coatings. In a preferred embodiment for forming protective covers on solar cells, the deposition devices are linear magnetron sputter cathode devices having silicon and aluminum targets, and the reaction device is a linear magnetron ion source oxidizer device. Film stress in the thin film coating is controlled by controlling power to the silicon and aluminum targets.