how does ellipsometry measure film thickness

ELLIPSOMETRY measures thin film thickness and properties using polarized light. HOW IT WORKS: you shine POLARIZED light at the film, and the light reflects off the top surface AND the film/substrate interface. The two reflected beams interfere, changing the POLARIZATION state of the light. You measure that change (two parameters, usually Ψ and Δ), and model it to extract THICKNESS, and often refractive index and roughness. WHY IT MATTERS: it's a fast, non-destructive, accurate way to measure film thickness — key for uniformity control (Lesson 53). AND THIS IS YOUR SPECIALTY — you were an SME on spectroscopic ellipsometry setup and qualification at Onto. Own it: 'I literally ran spectroscopic ellipsometry — it measures thickness from how polarized light changes when it reflects off the film stack.' This is a rare, direct match to the panel — lead with it.