Research On The Method Of Improving The Thickness Measurement Accuracy Of Self-Supporting Thin Films Based On White Light Interference

Self-supporting thin-film devices are widely used in semiconductor,nuclear physics,aerospace and other fields.As a key parameter of thin-film devices,film thickness may have a decisive influence on the performance of thin-film devices.The development of thin-film thickness measurement technology is crucial for the improvement of fabrication precision of thin-film devices.Among the film thickness measurement technologies,white light interferometric measurement technology takes advantage of the weak coherence of white light,thus the absolute position measurement can be achieved by finding out the position of the center fringe in white light interferometric signal through proper algorithms.For different application scenarios,the light transmittance of the self-supporting film to be tested is uncertain,so the two-side thickness measurement method may be used for the measurement of transparent and opaque films.However,in the process of white light interferometry,the delay line scanning error will directly affect the measurement accuracy,and the center fringe recognition accuracy is also affected by the recognition algorithm;in addition,when measuring a transparent film with thickness less than the coherence length of white light,the obtained peaks in white light interferometric signal will overlap with each other,which seriously affects the measurement accuracy.In order to achieve high-precision thickness measurement of transparent and opaque self-supporting films in a wide range of thicknesses,this paper has carried out the following work respectively:(1)A white light interference signal resampling algorithm based on phase demodulation is proposed,a high-order fitting scheme based on the frequency domain analysis method is put forward;different center fringe recognition algorithms to process simulation signals are used;the influence of dispersion,uneven sampling of the optical delay line and different resampling algorithms on each demodulation schemes are compared and analyzed;(2)Based on the spectral analysis method for thickness measurement,a time-domain measurement method of transparent film thickness is proposed;the reliability and applicable range of different spectral fitting methods are analyzed;the influence of this method on improving the accuracy of transparent self-supporting film thickness measurement is discussed through numerical simulation and experimental testing;(3)The optical structures and measurement principles of different white light interferometers are introduced;the self-supporting film thickness measuring system is constructed;experimental white light interferometric signals are obtained;repeated thickness measurements of transparent films and opaque films are performed in order to analyze the accuracy and repeatability of thickness measurements;experimental results show that the measurement results are consistent with the calibration values given by the Chinese Academy of Metrology,and the standard deviation of repeated thickness measurements for opaque film is about 0.03 μm,which is improved by 7.1% compared to the traditional frequency domain analysis method and improved by 69% compared to the center of gravity method;the standard deviation of repeated thickness measurements for transparent self-supporting film is about 1μm,which is improved by 44% compared to the traditional frequency domain analysis method;the standard deviation of repeated thickness measurements for transparent coating using the spectrally resolved method based on time-domain signal is about 0.4 nm,while the thickness cannot be measured by the traditional frequency domain analysis method.