This paper describes a modified Sagnac interferometer with a self-referenced polarization and phase-shifting technique for real-time thickness measurement of single- and double-layer transparent thin films. The proposed interferometric setup generated outstanding rotating linearly polarized light with a degree of polarization (DOP) of 99.40%. A beam splitter placed at the interferometer output separated the beam into two identical linearly polarized beams. One of the beams served as a reference, while the other served as a sensing arm. The output linear polarizer set at 45° relative to a reference plane was positioned anterior to the photodetectors to get rotating light intensities for phase shift measurement; hence, the intensities at various polarizations of 0°, 45°, and 90° were automatically acquired without any polarizing device adjustments. These intensities were then transformed into a phase retardation introduced by a sample, and the resulting phase shift was eventually converted into film thickness. The samples were properly prepared, with pure BK7 substrate being deposited by WO3-, Ta2O5-, and WO3/Ta2O5 films of known thicknesses. The thickness measurement obtained from the proposed system yielded reading errors of 1.3%, 0.2%, and 1.3/2.5% for WO3-, Ta2O5-, and WO3/Ta2O5 films, respectively. The mathematical theory was effectively demonstrated and empirically confirmed. The experimental results show that the proposed setup has a lot of potential for real-time, non-destructive thickness assessment of transparent thin films without the need to modify polarizing device orientations.

Real-Time Double-Layer Thin Film Thickness Measurements Using Modified Sagnac Interferometer with Polarization Phase Shifting Approach

The new scheme of a non-contact thickness measurement of multi-layer thin films with improving the sensitivity, accuracy, and performance of the measurement techniques is reported in this paper. Single- and double-layer film thickness measurement using different polarization states were studied. An alternative architecture using triangular cyclic interferometer with simultaneous measurements of reference and sample arms by splitting the beam at the output of interferometer is mathematically achieved. The polarization phase shift using two steps algorithm is observed, with steps shift of n/2 rad for a double layer sample. This shows that the polarization state has an impact on the thickness measurement of different thin- film materials.

Thickness Measurement of Double Layer Transparent Material Using Triangular Path Cyclic Interferometer

This paper presents the comparison of three-, four- and five-step techniques for measuring transparent thin-film thickness of Ta2O5 and WO3 deposited on BK-7 substrates. The Sagnac interferometer was modified with phase shifting approach for the determination of thin-film thickness. The input light beam was split into reference and testing beams. Before the output light reaching the balanced photodetectors, the real-time signal detection was performed to obtain the output intensities of both beams using three-, four-, and five-polarization settings of an analyzer. The thicknesses could then be efficiently translated from the measured intensities. Thicknesses from three-, four- and five-stepping algorithms were compared with ones from the conventional FE-SEM measurement, and it was discovered that the former performed better with less errors. The findings show that the phase-shifting technique using three-polarization settings is more suitable for the thickness measurement of the transparent thin film.

The comparison of multi-stepping algorithms for real-time thickness measurement of transparent thin films using polarization settings

An alternative polarization phase-shifting technique is proposed to determine the thickness of transparent thin-films. In this study, the cyclic interferometric configuration is chosen to maintain the stability of the operation against external vibrations. The incident light is simply split by a non-polarizing beam splitter cube to generate test and reference beams, which are subsequently polarized by a polarizing beam splitter. Both linearly polarized beams are orthogonal and counter -propagating within the interferometer. A wave plate is inserted into the common paths to introduce an intrinsic phase difference between the orthogonal polarized beams. A transparent thin-film sample, placed in one of the beam tracks, modifies the output signal in terms of the phase retardation in comparison with the reference beam. The proposed phase-shifting technique uses a moving mirror with a set of “fixed” polarizing elements, namely, a quarter-wave retarder and a polarizer, to facilitate phase extraction without rotating any polarizing devices. The measured thicknesses are compared with the measurements of the same films acquired using standard equipment such as the field-emission scanning electron microscope and spectroscopic ellipsometer. Experimental results with the corresponding measured values are in good agreement with commercial measurements. The system can be reliably utilized for non-destructive thickness measurements of transparent thin -films.

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Tossaporn Lertvanithphol

Papers

Utilization of the cyclic interferometer in polarization phase-shifting technique to determine the thickness of transparent thin-films