Swift and accurate measurement of overlay errors has long been imperative for ensuring throughput and yield in integrated circuit (IC) manufacturing. At present, image-based overlay (IBO) remains the predominant method for overlay metrology, relying on Linnik scanning white light interferometry (LSWLI) to guarantee rapid and precise focus assessment. Nevertheless, the focal plane position determined by LSWLI often does not align with the optimal contrast focal plane for imaging in the IBO system. This paper proposes a method to meticulously calibrate the systematic error in focus measurement. Initially, the Fourier transform method is employed to analyze the acquired LSWLI interference curve and extract the coherence envelope, from which the center of gravity is computed to ascertain the LSWLI focal plane position. Subsequently, the gradient RMS means of the images near the LSWLI focal plane are calculated and a weighted polynomial is fitted to these values to obtain the focal plane position imaged by the IBO system. Finally, by repeating these steps and averaging the results of multiple measurements, the inherent system focus offset (SFO) is obtained. This calibration can be conducted during the equipment test and calibration stage, ensuring that even in challenging working conditions, the IBO system can swiftly and accurately determine the final imaging focal plane position by solely completing the LSWLI focus measurement and supplementing it with the SFO. This calibration method is an important reference for the practical engineering application of LSWLI in IBO focus measurement system.
A high-resolution simultaneous polarization imager based on off-axis three-mirror telescopic objective detects polarization information to enhance contrast and identify target. Stray light can increase system noise, reduce image contrast, and affect imaging quality. It is necessary to analyze and suppress stray light in order to ensure the polarization measurement accuracy of the instrument. The three-dimensional solid model and optical properties of the instrument were established by stray light analysis software Tracepro. The stray light of channel 1 was simulated and analyzed. The important surface of the system and the first-order stray light paths were found through forward trace and backward trace. The main baffle and vane, secondary mirror baffle and third mirror light barrier were designed by using CAD drawing software. Then the PST (Point Source Transmittance) curve of stray light is given under off-axis angle is within ± 50 °. When the off-axis angle is 30 °, PST is less than 10-10. The VGI (Veiling Glare Index) is 1.47%, which was obtained by fitting the PST curve and integrating. The influence of the VGI on the polarization accuracy was 0.020@p=0.3. The results show that the stray light suppression system have obvious effects, and the influence on the polarization accuracy is acceptable.
In order to meet the application requirements of a space borne polarizing radiometer infrared band, a high-precision on-orbit temperature control scheme for the infrared detector combining active temperature control and passive temperature control is proposed. The infrared detector is installed on the heat sink copper block, and the temperature of heat sink copper block is controlled at -20°C~-30°C through the method of auxiliary cold plate + heat pipe thermal conduction. Combined with the infrared detector built-in three-level thermoelectric cooler, the photosensitive surface temperature of the infrared detector is cooled to below -60°C by a method of constant current driving. In order to ensure the measurement accuracy of infrared radiation polarization, the short-term temperature fluctuation of the photosensitive surface of the infrared detector is required to be less than 0.03°C/s. This article has designed the infrared detector temperature control scheme verification test, and actually measured the stability of infrared detector temperature and dark current. The results of the simulation and tests show that the range of infrared detector heat sink temperature is - 25±5°C, the range of infrared detector photosensitive surface temperature is -65°C ~ -75°C,the rate of short-term temperature change of the infrared detector photo-sensitive surface is better than 0.01°C/s, and the dark current fluctuation is less than 1.3pA. Satisfying the on-orbit high-precision polarization measurement requirements.
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