Compressive stresses are a major problem in the field of optical coatings. As an example, excessive pressure can bend the base body or reduce the adhesion of the coating, which can ultimately lead to the coating detaching completely. There is currently no established method of reducing the mechanical compressive stresses in the coatings, especially for silicon oxide (SiO2), the most frequently used material in optics. Normally plasma-assisted deposition processes are used for producing optical films. Although these, as a consequence of bombardment with high-energy ions, can produce high-density layers (corresponding to a high layer stability), high compressive stresses are in most cases also created in the layer. At the Fraunhofer IST, a hot-wire chemical vapor deposition method (HWCVD) has for the first time been evaluated as an alternative production method for SiO2 layers as part of a DISCOVER project sponsored by Fraunhofer.
At the Fraunhofer IST the HWCVD process has already been developed for diamond, silicon, silicon carbide and silicon nitride materials for different applications up to industrial implementation. In addition, the HWCVD method allows for the deposition on plastics, with the capability of even depositing nitrides free from internal stresses, under virtually particle-free conditions, and at high rates. The production of oxide layers with this coating technology is however still largely unexplored and yet totally unknown in the optical industry. In the HWCVD process, electrically heated tungsten wires in the 1900 °C – 2100 °C range are used in a vacuum chamber to dissociate SiH4. From here oxygen is added to produce the metal oxide. The challenge now is to prevent the hot wires from becoming oxidized by the oxygen, which is essential for oxide formation.
Results of silicon oxide production
In the development of the silicon oxide layers a parameter study on quartz glass was carried out with the aid of a statistical design of experiment (DOE). This took gas composition, pressures, and temperatures into account. Investigation of highly transparent layers produced with low compressive stresses revealed the following layer properties: high transparency for d (SiO2) = 380 nm T250 nm > 89 % on quartz, low compressive stress for d (SiO2) 2,5 μm σ < 170 MPa (low internal stress), low surface roughness for d (SiO2) 2,5 μm < 6 nm, coating rates > 2 nm / s, tungsten contamination [W] ≤ 0.2 atom%, and no substrate or layer damage caused by ion bombardment.
Following the parameter study, a low-stress anti-reflective coating system on a surface area of 10 x 10 cm2 was produced at the Fraunhofer IST. It consists of four alternating layers of the high-index Si3N4 and of the low-index SiO2 layers developed (these being deposited by the HWCVD method). The anti-reflective coating has a very low residual compressive stress of less than 50 MPa, in other words, the risk of the coating detaching or of the base body bending can be excluded.
Due to the high coating rate coupled with a high optical quality and low internal stresses, final products with a significantly longer service life will be feasible in the future at an attractive cost of production. Some examples of their planned uses are in the production of cell-phone displays, or of automotive interior components, or applications in the field of solar and architectural glass, or also utilization as barrier or decorative coatings.