Improving multi-wavelength overlay measurement time by the development of color-mixing light source

Abstract

This paper introduces an innovative approach to streamline the time-consuming process of multiple-wavelength measurement in semiconductor overlay metrology. To achieve this, a color-mixing light source was developed, which employed light sources of different wavelengths, each modulated with different sine wave frequencies. By aligning these sources and directing them simultaneously onto the test sample, we mixed all wavelength information into a single test signal. Leveraging the known modulation frequencies, we can accurately extract the amplitudes of each wavelength using a three-parameter sine wave fitting method. These amplitudes effectively represent the sought-after signal for each wavelength. A 4-wavelength mixed light source was developed to demonstrate this method's feasibility. Compared with the intensities received independently for each wavelength, the reconstructed amplitude error was below 5 %. Finally, in the test of multiple-wavelength overlay measurement, the overlay difference between this and traditional methods was only 0.078 nm. The experimental results confirm that this method enables a reduction in the measurement time for multiple wavelengths from 0.8 sec to 0.2 sec, i.e., a 75 % reduction).