단일 영상 분광 타원계측기 기반 다층 박막 분석 기술 연구

Abstract

Recently, precise film structures have become essential for integrated circuit (IC) and flat panel display (FPD) products because of their multi-functional purposes and compactness. In these products, thin films are deposited on a silicon or glass substrate with various materials and thicknesses for conductivity, insulation and packaging. For the successful operation and performance, film structure should be manufactured as designed. In addition, the measurement or inspection of the geometrical dimensions and material properties of film layers becomes an important issue in order to confirm their functional uniformity. Ellipsometry has been very important for the characterization of thin film structures as a non-destructive metrological tool. Comparing the measured quantities such as ellipsometric angles of ψ and Δ with their counterparts of theoretical model, ellipsometry can determine the thin film thicknesses with high sensitivity and even a spectroscopic ellipsometry (SE) provides the reliability and robustness of the measurement results with sufficient spectral data of ψ(λ) and Δ(λ). However, most of SE only measure film thicknesses at a single point of the interest and adopt polarization adjustment of the light by using rotation of polarizing optic elements or electrical phase modulation, which leads to time consumption. Imaging ellipsometry has capability of obtaining 3D or line profile of film thicknesses, but it still needs rotating mechanisms for the polarization changes. In this study, I propose two types of spectroscopic ellipsometers, a spatially phase-retarded spectroscopic ellipsometer and a polarization pixelated spectroscopic ellipsometer that can measure the multi-layered film thicknesses with a single image. The spatially phase-retarded spectroscopic ellipsometer can collect all information, necessary to characterize film structures, with a single image acquisition. Instead of using temporal phase retardation devices, a spatial phase retardation plate is used and an imaging spectrometer can obtain the intensity variations by polarization states and wavelengths. As an optical source, a broadband light is used and the light with a small beam size is linearly polarized by a linear polarizer. Then, the light is incident to a film specimen and reflected off with the polarization changes caused by the ratio of p-pol and s-pol waves. The reflected light passes through a beam expander and the large size of the light goes through spatial phase retardations by a spatial phase retardation plate (SPR), designed to provide the periodic phase retardation along a horizontal axis. The spatially phase retarded light passes through an analyzer and detected in an imaging spectrometer. Then, the imaging spectrometer only images one of the horizontal lines of SPR at the center and spectrally resolve this line. The horizontal axis of the image indicates the polarization changes caused by the periodic phase retardations while the vertical axis means the spectral axis. As the result, the intensity variations by various polarization states and wavelengths can be obtained at once in a single acquisition of the image. In the polarization pixelated spectroscopic ellipsometer, the ellipsometric parameters can be acquired by adopting a polarization-pixelated CMOS camera and an imaging spectrometer without any mechanical or electrical moving parts and their spectral and spatial profile are obtained with a single image. As a light source, a spatially coherent broadband light is used and the light beam size is enlarged by beam expander to fully illuminate the measurement line of a film specimen (S). The light beam goes through a 45° rotated polarizer and is incident to S with the semi-Brewster angle of S. The reflected beam from S passes through a 45° rotated quarter-wave plate and is incident to an imaging spectrometer through an imaging lens. Then, the spectral density is distributed along the vertical axis of the captured image while each spectral density set corresponding to the point on the measurement line is horizontally located at the image of a polarization pixelated CMOS camera (PCMOS). In PCMOS, an individual pixel of the image sensor has its own polarizer, oriented by 0°, 45°, 90°, and 135° while a (2⨯2 pixels) unit cell block is repeatedly arranged in whole imaging area. In this case, these four kinds of polarizers of the unit cell in PCMOS can act as a role of rotating an analyzer with a step of 45° under the assumption that the light obtained in the unit cell is from the same point on the measurement line and its wavelength is the same. Then, the whole image obtained by PCMOS can be split as four images according to polarizing angles and the ellipsometric information can be obtained by these polarization changes. Subsequently, the proposed system is equivalent to a number of rotating-analyzer type SE, and it can obtain the ellipsometric spectral data for the line at once with a single image only. In order to verify the spatially phase-retarded spectroscopic ellipsometer and the polarization pixelated spectroscopic ellipsometer, single and multi layered film structures were used as specimens, and it was confirmed that multi-layered film thicknesses could be successfully obtained at a point and a line, respectively, with each single image.