Measurement Technology for Opaque Multilayer Film Thickness by Photothermal Effect (2nd Report)

We propose a non-contact method for measuring the film thickness of various materials based on the photothermal effect. It involves irradiating a film surface with a nanosecond laser pulse. Various kinds of elastic waves that have a maximum frequency of several hundred MHz are excited by the photothermal effect. However, only those elastic waves which are dependent on the film thickness are sustained in the film, as a result of the resonance phenomena that occur between boundary surfaces having acoustic impedance. The film thickness can be calculated from the detected resonance frequency. Previously, we established a non-contact measurement technique which can measure the thickness of a single-layer film. In this report, we describe a measurement technique for measuring the thickness of a multilayer film. There are three kinds of modes excited by resonance. The frequencies of these resonance modes depend on the differences between the relative acoustic impedances of the materials in the film. We were able to measure the thickness of the layers in a multilayer film by analyzing these resonance modes in each layer. This was done by selecting the frequency analysis window for which the resonant wave was stable.