Measurement Technology for Opaque Multilayer Film Thickness by Photothermal Effect (1^st Report)

—Resonance Wave Measurement with Single Layer Film and High-frequency Non-contact Measurement Technology for Rough Surface—

Abstract

We propose the non-contact film thickness measurement method for various materials based on photothermal effect. We illuminate nanosecond pulse laser to the film surface. The various kinds of elastic waves whose maximum frequency is several hundred MHz are excited by photothermal effect. And then, only elastic wave which depends on the film thickness remains selectively, because the resonance phenomena occur between boundary surfaces of the acoustic impedance. The film thickness can be calculated from this detected resonance frequency. In this report, we verified the generation mechanism of resonance wave by photothermal effect. We propose the collector optics type Twyman-green interferometer as the non-contact measurement method for high-frequency vibration of rough surface. In this interferometer, we introduce the objective lens to collect diffused light from rough surface and detect changes of light intensity by high-frequency response photodiode. Using the single layer film whose thickness is 1mm±0.05mm, we can detect the resonance frequency that is 3.1MHz, and the calculated film thickness is 1.035mm.