Zexiang Zhao

Alignment Method for Linear-Scale Projection Lithography Based on CCD Image Analysis

This paper presents a method to improve the alignment accuracy of a mask in linear scale projection lithography, in which the adjacent pixel gray square variance method is applied to a charge-coupled device (CCD) image to obtain the best position of the focal length of the motherboard and then realize the alignment of the focal plane. Two image positions in the focal plane of the CCD are compared with the traits overlap according to the image splicing principle, and four typical errors are corrected on the basis of the total grating errors. Simultaneously, the rotation error of the mask is used to summarize the grayscale variation function of the CCD image. Threshold functions are employed to express the factors including the wave crests of the amplitude, period error, and phase error, which govern the rotation accuracy and weight alignment accuracy expression of the established four error factors. Finally, in the experiment, the slope of the mask is corrected and adjusted to the same direction as the slide plate with the assistance of a dual-frequency laser interferometer. The effect of the alignment error on the lithography accuracy is discussed and verified in the static case, and it is found that the CCD maximum resolution pixel is 0.1 μm and accuracy of the scale is 0.79 μm in only a 200-mm-measurement range.

Investigation of grinding parameters and machine dynamic characteristics' effect towards brittle material subsurface damage

Elements performance is greatly affected by their surface and subsurface integrity, especially for the brittle material. Prediction model of the subsurface damage would provide the insight into the grinding parameters effects and better control of them. In this paper, based on the classic brittle solid crack theory, prediction model of brittle material subsurface damage induced by brittle mode diamond grinding was established. Firstly, contact area calculation was modeled to estimate the involved grits number using the grit density. Based on the prediction model, grinding parameters effects were investigated. Finally, grinding machine stiffness, accuracy and damping coefficients were introduced to quantitatively analyze their effects towards subsurface damage. The proposed model would promote more definite control of grinding induced subsurface damage and optimal design of the grinding machine dynamic characteristics.

Form error compensation of on-machine noncontact measurement of precision grinding for large and middle-diameter aspheric elements

Based on the cross grind ing mode for large-diameter aspheric, a high-precision profile error compensation method by using an on-machine noncontact measuring sensor is presented to improve the manufacturing accuracy and efficiency of large and middle-diameter aspheric elements. Profile errors arising from machine motion errors and tool offset errors are obtained through the measured data from on-machine noncontact measurement. By measuring a standard flat ruler, the motion errors of the measurement sensor from the machine positioning errors is calibrated, the grinding tool setting error could be calculated according to the on-machine coordinate to achieve the grinding tool offset quick eccentricity calibration. By comparing the measured profile and the ideal profile, the normal residual error of each grinding program point was calculated, and the new compensation path was generated thereafter. The 300-mm-diameter K9 mirror was ground to verify the proposed compensation grinding method. Results indicate that the profile error was reduced from 35μm to 10μm through the tool setting error elimination during semi-finish grinding stage. Using the compensation grinding path according to the normal residual error, the profile accuracy was improved from 10μm to 4μm in fine grinding stage. It could be concluded that the proposed compensation grinding method is effective to improve profile accuracy and manufacturing efficiency for the large and middle-diameter aspheric elements.