W (tungsten) is a metal that is commonly used as a substrate in electronic and optical devices. Tungsten has a high melting point and excellent thermal stability, making it an ideal material for high temperature applications. Additionally, tungsten has a low thermal expansion coefficient and high electrical conductivity, making it a desirable material for substrate applications. Some common applications for tungsten substrates include the fabrication of transistors, optical devices, and high-temperature sensors. Tungsten can be deposited onto various substrates, including silicon and glass, by various methods such as chemical vapor deposition (CVD) or physical vapor deposition (PVD).