Detection and Optimization of Weak Photoelectric Signal in Intelligent Optical Film Deposition System

Abstract

For the thin-film thickness monitoring(TFTM) system, a new technology based on a double-frequency modulation equipment, a photoelectric four-light path system, a comprehensive digital processing-controlling system and a dual-lock-phase circuit system, has been successfully developed by optimizing the circuitry and data processing to improve the precision of the photoelectricityextremum technology. The novel technique includes analog circuits of multi-stage amplifier, symmetrical dual-lock-phase amplifier, antidisturbance circuit technique, digital division, anti-pulse-disturbance digital filter, linearization of digital display, removal singular data, and extremum judgment with ΔR algorithm. Experiments show that the detection and optimization methods on weak photoelectric signal take advantages of higher stability and better static and dynamic precision in controlling the thin-film thickness. The static drift ratios of two output electric signals are equal to or less than 7%/h, as well as the analog signal-to-noise ratios of reference or measurement paths are more than 500. The digital display sensitivity responding to reflectivity is higher, the linear regression factor of the digital display data is 0.979, and the display resolution of low reflectivity is equal to 0.02%. Compared with the conventional TFTM system, the uncertainty of monitoring signal in the new technology declines one order of magnitude, consequently, the drift ratio of monitoring signal is approximately equal to 0.