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摘要:
数字光栅位移测量技术将CMOS相机像素阵列当作一个“数字化”的光栅，通过构造光学光栅像和数字光栅的周期差，利用微米级的光学光栅像，实现纳米级的位移测量。可以应用于光刻机的调焦调平传感器中，结合倾斜入射的检测光路，对晶圆表面高度进行精确测量。在实际测量中，晶圆表面意外出现的图形会干扰光学光栅的反射成像，进而影响图像处理的结果。本文提出一种数字光栅位移测量的工艺适应性方法，以数字光栅周期为单位，对存在干扰图案时的CMOS图像进行光强的重建和光强曲线的恢复，能在晶圆基底出现较大面积图案时表现出很好的稳定性，且该方法可以适应多种表面缺陷，如划痕、颗粒、污渍和沟槽等。实验结果表明，图像光强的重建后，光强曲线的均方误差大幅减小，修正后的Z向测量误差在±33 nm（±3
σ =±41.2 nm）以内。该方法能增强数字光栅调焦调平传感器的工艺适应性，为调焦调平方法提供技术参考。Abstract:In the digital grating displacement measurement technique, the CMOS pixel array of the camera is regarded as a ‘digitized’ grating. The micron-scale grating images can realize nanoscale displacement measurement by constructing the period difference between optical grating and digital grating. Combined with the detection light path of oblique incidence, it can be applied to the focusing and leveling sensor of lithography machine to measure the wafer surface height accurately. In the actual measurement, the unexpected patterns on the wafer surface will interfere with the reflection imaging of the optical grating, then affect the results of image processing. In this paper, a process adaptability method for digital grating displacement measurement is proposed, which reconstructs the light intensity and recovers the light intensity curve from the CMOS image when interference patterns exist. It shows good stability when the large area pattern appears on the wafer substrate, which can adapt to multiple surface defects such as scratches, particles, stains and grooves. The experimental results show that the mean square error of the light intensity curve is greatly reduced and the Z-direction error correction by the method is within ±33 nm(±3
σ =±41.2 nm) after reconstructing the light intensity. This method can enhance the process adaptability of the digital grating focusing and leveling sensor, and provide technical reference for the focusing and leveling method.-
Key words:
- focusing and leveling /
- digital grating /
- displacement measurement /
- process adaptability
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图 2 数字光栅对光学光栅像的保留和去除
Figure 2. Digital grating retain and remove optical grating image
图 6 晶圆表面图案成像与光学光栅像位移不同步
Figure 6. Displacement out of sync between the wafer surface pattern imaging with optical grating imaging
图 9 标定前的一个数字光栅周期内的累计光强
Figure 9. The accumulated light intensity in one digital grating period before calibration
图 10 晶圆表面有图案时一个数字光栅周期内的累计光强
Figure 10. The accumulated light intensity in one digital grating period when wafer surface pattern
图 12 M个累计光强值的分布区间统计
Figure 12. Statistics of the distribution interval of M accumulated light intensity values
图 13 修正后的累计光强（一个数字光栅周期）
Figure 13. Corrected accumulated light intensity curve (in one digital grating period)
图 15 修正前后的光强曲线和对准点及其放大图
Figure 15. Light intensity curves and alignment points before and after correction
图 16 晶圆表面出现4种图案时对应的光学光栅像
Figure 16. The corresponding optical grating image when 4 patterns appear on the wafer surface
图 17 不同晶圆表面在Z轴在0μm到40μm不同位置对应的光强I曲线，黑色为洁净表面时对应的光强曲线，红色为表面存在图案时的光强曲线，蓝色为修正后的光强曲线（a、b、c、d分别代表圆孔、竖线、划痕和灰尘颗粒）
Figure 17. The light intensity I curves for different wafer surfaces during the z-axis from 0μm to 40μm, while the black curves is the light intensity curve when the surface is clean, the red is the light intensity curve when the pattern exists on the surface, and the blue is the corrected light intensity curve (a,b,c,d represent circular pattern, lines, scratches, and dust respectively)
图 18 不同晶圆表面在Z轴不同位置对应的光强I曲线拟合的均方误差((a)圆孔,(b)线条,(c)划痕,(d)颗粒)
Figure 18. Mean square error of fitting corresponding intensity I curves for different wafer surfaces during Z direction stepping ((a)circular pattern, (b) lines, (c)scratch, (d)dust)
图 19 基于数字光栅的调焦调平传感器测试平台
Figure 19. Testing platform of focusing and leveling sensor based on digital grating
图 21 步进过程中数字光栅测量的位移值
Figure 21. Displacement values measured by the digital grating during the step
表 1 类比数字光栅和游标卡尺
Table 1. Analogical to digital gratings and vernier calipers
数字光栅 游标卡尺 主尺 相机的像素阵列 机械主尺 游标尺 周期性光栅像 机械游标尺 原理 利用周期差对微小位移进行“放大” 方法 求解光强曲线零点 找刻度对准点读数 分辨率 高于周期差 两尺的周期差 
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