The substrate is the foundation on which the thin film components are built. The substrate properties have significant influence on the performance of the device. Alumina is still the most common substrate material due to its surface quality, cost, and high frequency performance. For power applications, Aluminum Nitride and BeO substrates are still the choices. Fused silica and quartz are used at very high operating frequencies. Sapphire gets called on for optical window use or when high mechanical strength is a must.