Single sequence phase shifting spectrally resolved interferometry for an in-line thin film thickness measurement using spectral reflectance and phase.

A novel, to the best of our knowledge, thickness measurement scheme based on a phase shifting spectrally resolved interferometry is proposed for a thin film measurement from spectral reflectance and phase in a single sequence. This purpose is realized by introducing a spectral reflectance phase surface (SRPS). The spectral phase, originally existing in the form of an imaginary term, is revealed as a real term with the spectral reflectance due to the proposition of SRPS. Therefore, the reflectance and phase are contained in a single parameter, SRPS, and are measured in a single sequence. The phase shift error compensation is presented to form the accurate SRPS and ensure the performance of the measurement. The thickness of the sample is determined as the measured SRPS, and the theoretical SRPSs are compared by the cross correlation method. The performance of the proposed system is validated by measuring several samples of the SiO2 on a silicon substrate and comparing the results with the commercial ellipsometer.

Single-sequence stable spectroscopic reflectometry using simultaneous measurement of incident light and reflected light: publisher's note.

This publisher's note contains corrections to Appl. Opt.60, 8915 (2021).APOPAI0003-693510.1364/AO.435321.

Single-sequence stable spectroscopic reflectometry using simultaneous measurement of incident light and reflected light.

We proposed a new scheme of spectroscopic reflectometry (SR) that improves the stability with respect to the intensity variations of the incident light and simplifies the measurement procedure. The proposed method enables this by simultaneously measuring the incident light and the reflected light. Using the multi-order retarder, we modulate the incident light intensity into a high-frequency signal at the spectral domain. Even though the spectrometer measures the sum of reflected light and modulated incident light, each signal can be separately obtained using the Fourier transform. The proposed method is verified through the measurement of SiO2 thin film on the Si substrate with various incident light intensities.

Coaxial spectroscopic imaging ellipsometry for volumetric thickness measurement.

We propose spectroscopic imaging ellipsometry that can measure spectral ellipsometric signals in the entire field of view simultaneously without areal scanning or operation of polarization devices. The proposed imaging ellipsometry is configured in a coaxial optical structure so that the high magnification objective lens is applicable and the spatial resolution is highly increased. Without the operation of polarization components and to efficiently obtain the spectral data in the object plane, the ellipsometric parameters are encoded into the high frequency in the spectral domain and are measured by an imaging Michelson interferometer. The volumetric thickness measurement by the proposed method was verified by comparing the thickness results of the SiO2/Si sample that has four different thicknesses with commercial ellipsometer results.

Co-axial spectroscopic snap-shot ellipsometry for real-time thickness measurements with a small spot size.

Spectroscopic snap-shot ellipsometry of co-axial structure is proposed to solve the large spot size and long measurement time issues of the conventional ellipsometer. By modulating the spectroscopic ellipsometry signal into high-frequency at the spectral domain and measuring the spectrum at the point of the back focal plane, the ellipsometry parameters(Δ, ψ) were measured in real-time with small spot size. Detailed analysis, calibration, and optimization process for the proposed methods are presented. The accuracy and precision of the proposed method were confirmed by comparing the thickness measurement result of SiO2/Si thin-film samples with a commercial ellipsometer.

Angle-resolved spectral reflectometry with a digital light processing projector.

We describe a novel approach for angle-resolved spectral reflectometry using a digital light processing (DLP) projector. Here, the DLP generates ring patterned images which are projected on the back focal plane of an objective lens. This way, the proposed method quickly changes the angle of incidence with ease based on the relation between the radius of the back focal plane and the angle of incidence. As a result, a detector captures the intensity of the image plane based on the angular and spectral axis. As the proposed method detects the interesting spot of a sample image, it can easily locate the measurement spot with viewing the full field of view, and the spot size is reduced by adopting the fiber. This method is verified by comparing the measurement output of the thin-film samples with a commercial ellipsometer. The result shows that our the proposed method enables the high accuracy of the thin-film inspection.

Simple method for volumetric thickness measurement using a color camera.

A simple volumetric thickness measurement method for in-line high-speed inspection is proposed. With a color camera alone, spectrally resolved reflectance in a spatial domain is obtained: a Bayer filter spectrally resolves the reflected signal, and a CMOS sensor acquires three multi-spectral reflectances from RGB data at a single shot. To determine an accurate thickness, a modified reflectance is derived to convert a conventional spectral reflectance throughout a wavelength domain into an adequate form in an RGB domain by considering the characteristics of wide-band multi-spectral acquisition. The proposed method is validated by the measurement of a uniformly deposited SiO2 film and a tapered SiNx film.

Residual vibration reduction of white-light scanning interferometry by input shaping.

White-light scanning interferometry is widely used for precision metrology of engineering surfaces. It needs a mechanical scanning for capturing an interferogram that determines where the surface of a measured sample is located. The residual vibration during the scanning procedure distorts the interferogram and it reduces the accuracy and the precision of the system. The residual vibration becomes bigger as the proportional gain gets higher for the fast response. So it is hard to achieve the fast and precise measurement simultaneously. In this study, input shaping which convolves a reference signal with the input shaper is investigated to reduce the residual vibration of the scanning system. The step response data is analyzed using Continuous Wavelet Transform (CWT) to design the input shaper. Using proposed method, the residual vibration of the white light scanning interferometry is reduced and it achieved both faster measurement speed and more accurate measurement.

Critical dimension measurement of transparent film layers by multispectral imaging.

An optical microscopy system as a non-destructive method for measuring critical dimension (CD) is widely used for its stability and fastness. In case of transparent thin film measurement, it is hard to recognize the pattern under white light illumination due to its transparency and reflectance characteristics. In this paper, the optical measurement system using multispectral imaging for CD measurement of transparent thin film is introduced. The measurement system utilizes an Acousto-Optic Tunable Filter (AOTF) to illuminate the specimen with various monochromatic lights. The relationship between spectral reflectance and CD measurement are deduced from series of measurement experiments with two kinds of Indium Tin Oxide (ITO) patterned samples. When the difference of spectral reflectance between substrate and thin film layers is large enough to yield a large image intensity difference, the thin film layer can be distinguished from substrate, and it is possible to measure the CD of transparent thin films. This paper analyzes CD measurement of transparent thin film with reflectance theory and shows that the CD measurement of transparent thin film can be performed successfully with the proposed system within a certain wavelength range filtered by AOTF.