Study On Differential Astigmatism Detection Technology

Laser direct writing machine is the key device to producing diffractive optical elements, which mainly consists of the following parts: He-Cd laser, Acousto-optic modulator, Servo control system, Photolithographic lenses, CCD camera and Control computer, etc.Defocus detection system is one of its core techniques, which provides the defocusing amount of work surface for the servo system, so that the writing beam could be precisely focused on the photoresist surface through servo control to ensure the quality of etching. And it is installed in the laser writing head, and move along with the head after being driven by servo mechanism. In order to avoid that the load is too large to affect the system's dynamical characteristics, the volume and weight of defocus detection system are required as small as possible.The purpose of the study on"Defocus Detection Sensor Based on Differential Astigmatic Principle"is to design a high accuracy, small size and good real-time defocus detection sensor. In this paper, after analyzing advantages and disadvantages of existing defocus detection method as well as the practical application background, differential astigmatic theory is determined. First, starting with the system's overall design, different critical sections are fixed, such as optical parameters, mechanical structure, photoelectric conversion, digital processing, and software platform design of testing system. Then, according to the characteristics of laser beams, a mathematical model of differential astigmatism optical system is established, and the simulation analysis is conducted by ZEMAX and MATLAB. Meanwhile, parameters of the optical system are optimized through comparing the impact of different light path parameters on the focusing error signal ( FES ). Furthermore, due to the size and simulation results of the various optical components, miniaturized and high-precision mechanical structures are designed and Bonding technology is introduced to realize the high-precision installation of different mechanical components.In addition, the system's hardware and software designs are optimized to achieve fast and high-precision information processing, respectively. The sections of hardware mainly include: photoelectric conversion, signal adjustment and analog filter, AD converter, CPLD to help DSP control etc. Software sections include: data acquisition, data processing and data communications, as well as using the IIR digital filter and median filter to reduce the signal noise, which also contains the optimization of software functions and performance. The experiment platform of defocus detection sensors is built up, especially to verify the effect of speed and detection accuracy of TMS320F2812DSP as well as the effect of expanding measuring range by using data interpolation methods. Experiment results show that the measurement sensitivity is 80nm and the range is 60μm, which satisfy the requirement of laser direct writing machine very well.