Based on microscopic imaging, optical systems can effectively detect defects on the surface of laser gyro reflector without causing damage. However, the minimum detection size is limited by the resolution of the microscopic imaging system. To detect submicron-level defects on laser gyro reflector substrate, a surface scanning dual-source scattering measurement scheme based on scattering measurement technology is proposed. Utilizing the Finite Difference Time Domain (FDTD) method and the detection scheme, an electromagnetic scattering model of the laser gyro reflector substrate is established to simulate the characteristics of defects and the distribution of electromagnetic fields. An experimental platform for surface scanning dual-source scattering measurement is established, and polystyrene latex (PSL) spheres with a diameter of 200nm are deposited on the surface of the laser gyro reflector substrate to verify the effectiveness of the proposed method. Scattering imaging experiments in both bright and dark fields are conducted on the USAF 1951 standard resolution plate to obtain the directional characteristics of dark field scattering. Additionally, standard-sized rectangular line patterns, dots, and checkerboard patterns of 1-10μm are fabricated using reactive ion beam etching to create defect samples of photomask patterns, and scattering imaging experiments are conducted on these samples to obtain the detection distribution of bright field patterns. The results indicate that the system can achieve a detection resolution better than 175nm. This method provides a reference for the detection of substrate in inertial guidance systems.
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.
In recent decades, polarization imaging has garnered much interest and research focus, due to its wide range of applications in the fields of atmospheric remote sensing, astronomy, biomedical and target detection, etc. Linear division-of-focal plane (DoFP) polarimeters deploying metal wire grid micro-polarization array as the polarization state analyzer (PSA) possess the capacity to capture polarization properties of the scene target during a single snapshot and benefit from their rugged and compact designs. However, these systems acquire the polarization measurements through spatial modulation and inevitably lead to spatial resolution loss, which cause instantaneous field of view (IFOV) errors and degradation of polarimetric accuracy. In response to this challenge, various interpolation/demosaicking methods tailored to DoFP imaging to fill the missing polarization information have emerged in recent years. This survey aims to explore these methods and briefly describe their effectiveness in enhancing the image restoration performance, highlighting their advantages and disadvantages. Lastly, according to current progress, some suggestions are made for high accuracy polarimetric measurements, especially for polarization remote sensing applications at different scales.
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