针对小特征尺寸连续位相板中频段成分分布广、误差梯度大的面形特点, 分析了离子束修形技术加工连续位相板过程中影响加工精度的几种因素, 包括扫描步距、材料去除方式、定位精度和材料去除量求解。分析指出: 根据采样定理确定去除函数的扫描步距可实现对不同尺寸特征单元的有效加工; 进一步优化材料去除方式能够确保修形过程中驻留时间的平稳运行, 实现全频段误差一致收敛。另外, 采用面形匹配方法对测量误差进行校正实验, 可获取准确的面形材料去除量; 而采用提高去除函数定位精度的方法可显著提升小尺寸特征单元的加工精度。基于研究结果, 在消除各种工艺误差的基础上, 采用离子束修形技术对特征尺寸小至1.5 mm, 面形峰谷值小于200 nm, 面形梯度高至1.8 μm/cm的连续位相板进行了高精度加工, 结果显示: 加工面形与理论面形的匹配精度达到8.1 nm (RMS), 证实了误差分析的准确性。

According to the surface characteristics of continuous phase plates with small feature structures on high surface gradients and extensive mid-to-high spatial frequencies, several factors influencing the machining accuracy of the phase plates are analyzed. These factors include raster scanning pitch, material removal programming, positioning precision and material removal. The analysis points out that the optimal pitch can be selected according to the Nyquist sampling theorem to achieve the effective imprinting of feature structures with different sizes. When the surface errors in all spatial frequency ranges are controlled effectively , the stable dwell time can be achieved during the raster scanning process by the optimized material removal programming. Furthermore, when the surface matching method is used to correct the measurement errors , the accurate material removal information can be obtained. When the positioning precision of the removal accuracy is increased, the machining accuracy of the element with small feature structures can be improved significantly. Finally, on the basis of eliminating of different process errors, the Ion Beam Fabrication(IBF) is adopted to imprint the complex phase structures with characteristic dimensions as small as 1.5 mm, surface peak-to-valley (PV) smaller than 200 nm and surface gradient as large as 1.8 μm/cm. The results show that the matching error between figured surface and desired surface has been controlled down to 8.1 nm (Root-Mean-Square, RMS) of design specifications. The experimental results verify the reliability of the error analysis.