Research Of SEM Auto Focus System Based On FPGA

Scanning electron microscopy plays an important role in the field of modern scientific research. Manual focusing is time-consuming. It can improve the use efficiency of SEM to implement SEM autofocus. In this paper, the SEM autofocus algorithm is studied after the completion of related SEM circuits and software.Firstly, the scan acquisition and transmission section of the SEM dashboard is designed and implemented. It uses an FPGA as the master unit to control related circuits to obtain SEM image. FT232 is selected to communicate with PC via a USB interface. After the circuit and FPGA logical module are designed and implemented, software is written to test the SEM.Several image definition evaluation functions is tested and analyzed. It is proposed to use gray variance evaluation function over a wide range and wavelet sharpness evaluation function over small scale. After verifying commonly used search algorithm, an improved search algorithm is proposed. That is, using a linear fit to complete blurring focus, using golden search to complete fine focus. To implement algorithm on FPGA, the auto focus algorithm is divided into gray variance function, linear fitting, two-dimensional wavelet transform and Golden Search modules. With the help of HDL Coder, four modules are realized. To reduce use of resources, several techniques are used. A one-dimensional wavelet transform based on the structure of pipeline is presented. An improved two-dimensional wavelet transformation is proposed to reduce resource use. FPGA timing simulation is done to test the FPGA algorithm modules.After the online test, the circuit and software designed in the paper works well, it can complete SEM image acquisition and display. By hardware simulation, each FPGA module designed works correctly, completing the corresponding function.