Measurement Technology for Opaque Multilayer Film Thickness by Photothermal Effect

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We propose the non-contact film thickness measurement method for various materials based on photo thermal 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 photo thermal 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. Currently, we have established the measurement technology which can measures the thickness of the single-layer film by non-contact. In this method, ArF pulse laser illuminates the film surface, and ultrasonic waves is caused by optical absorption at the film surface. A film thickness is measured from the resonance frequency of excited ultrasonic waves. This proposed method can measure extremely varied object. And this proposed method is a non-contact type measurement technology. In this report, we mention the technique of microvibration measurement for rough face by laser interferometer. And, we mention the measurement technique of the multilayer film thickness. There are three kinds of modes in exited resonance phenomenon. These resonance modes depend on the difference of relative acoustic impedance between each material. We applied it to the multilayer film thickness measurement by analyzing these resonance modes in each film. And we selected the frequency analysis window in which the resonance wave was in stable state.