Spectroscopic Mueller Metrix Polarimetry Based on Spectral Modulation and Division of Amplitude Demodulation
Thin film and nanostructure measurement technologies have played an important role in production process monitoring in industries such as integrated circuit manufacturing,flat panel displays,and solar cells.Many optical based measurement techniques have emerged to meet the industrial needs of high-speed and non-destructive measurement.Spectroscopic Mueller Metrix Polarimetry(SMMP)is a typical representative of these techniques and has become an important direction in the research and development of thin film and nanostructure measurement technology.It uses a Polarization State Generator(PSG)to convert a certain spectral range of polychromatic light into fully polarized light and project it onto the surface of the sample to be tested,and uses the Polarization State Analyzer(PSA)to detect the polarization state of the reflected or transmitted light on the surface of the sample for obtaining all 16 Mueller matrix elements of the sample as a function of wavelength,and then analyzes and extracts their characteristic parameters such as complex dielectric constant,carrier structure,and film thickness.SMMP can be divided into frequency modulation type and time modulation type according to its working principle.The polarization state generator and polarization state analyzer of the former are both composed of components that can change modulation parameters over time and fixed linear polarizers,such as rotary compensators,liquid crystal phase delay devices,and photoblastic modulators.When measuring in a wide spectral range,the SMMP with dual rotation compensators is the most common:the compensators of PSG and PSA rotate at a certain rate to produce different time modulation frequencies,and then use Fourier transform demodulation to obtain all 16 Mueller matrix elements of the sample,which takes a long measurement time and is not suitable for situations where Mueller matrix elements change rapidly over time;the PSG and PSA of the latter are both composed of two high-order phase delay devices configured with a certain thickness and fixed fast axis direction,as well as a fixed linear polarizer.All 16 elements of the measured Mueller matrix are modulated to 37 different frequency channels,and the spectra of all 16 Mueller matrix elements can be obtained by channel filtering and Fourier transform.As the system does not contain moving components,static real-time measurement can be achieved.However,when the light source or the measured Mueller matrix has sharp characteristic peaks,serious channel crosstalk will occur,which affects measurement accuracy and accuracy.According to the principle of Fourier transform spectroscopy,a large channel bandwidth corresponds to high restoration spectral resolution.Due to the limited total channel bandwidth,an increase in the number of channels will reduce the bandwidth required to restore the Muller matrix spectrum.Therefore,the spectral resolution of the measured Mueller matrix elements is much smaller than the spectral resolution of the spectrometer.Therefore,it is only suitable for situations where the measured Mueller matrix slowly changes with wavelength.To overcome these limitations,we presented a SMMP based on frequency modulation and division of amplitude demodulation.Compared with the spectroscopic Mueller polarimetry based on time modulation or frequency-temporal modulation,it has no moving components and electronic devices,and can achieve real-time measurement of the spectra of all 16 Mueller matrix elements of the sample.Compared with the spectroscopic Mueller polarimetry based on frequency modulation,it has higher spectral resolution and lower the probability of channel crosstalk generation.According to the research results,the selection of high-order retarders and spectrometers can further expand the spectral range of measurement.By optimizing the calibration method,the accuracy and precision of optical measurement can be further improved.This article has certain scientific significance and potential application prospects in the research and development of high-speed,high-precision,and wide spectral band generalized spectral ellipsometry technology in the field of non-destructive testing technology.
Frequency modulationMueller matrixSnapshotEllipsometryPolarimetery
NETL
NSTL
万方数据
