When sourcing thin film substrates, the surface roughness (Ra) is the most critical parameter, as it directly impacts the adhesion and electrical properties of the deposited film; for high-frequency applications, an Ra < 0.05μm is often required. Additionally, evaluate the Thermal Coefficient of Expansion (TCE) to ensure it matches the thin film material to prevent cracking or peeling during thermal cycling. Common materials include 99.6% Alumina (Al2O3) for standard circuits, Aluminum Nitride (AlN) for high power dissipation, and Quartz or Glass for optical or high-frequency RF applications.