扫描白光干涉技术在硅片检焦过程中的应用

Application of scanning white light interferometry in process of silicon wafer focusing

晶圆检测设备常采用机器视觉方法采集晶圆图像,但基于光谱共焦位移传感器的调焦系统测量精度低、鲁棒性差,基于三角测量原理的调焦系统虽精度足够高,但硅片表面图案与沟槽会影响探测光路从而导致系统无法工作。针对这些问题,提出一种基于扫描白光干涉技术的调焦系统,以实现对于不同工艺属性硅片焦面位置的快速精准测量。此检焦系统首先进行相机最佳焦面位置与PZT零光程差位置之间垂向距离的标定,使得在检焦测量时可将零光程差位置作为检出对象,极大地提高了检出速度,并且在零光程差位置定位时,根据干涉波形的形状自适应选择定位算法,提高了检焦系统的工艺适应性。实验结果表明,对于单峰波形和双峰且双峰半分离波形,重心法处理得到的焦面位置的平均绝对误差分别为20 nm和56.1 nm,而包络法则有38 nm和56.2 nm,并且重心法处理耗时在1 ms之内,包络法则需25 ms左右,故选用重心法作为零光程差定位算法。对于双峰且双峰完全融合波形和双峰且双峰完全分离波形,重心法处理得到的焦面位置的平均绝对误差分别为20.3 nm和17.8 nm,而包络法则有13.2 nm和15.8 nm,故选用包络法作为零光程差定位算法。模板匹配法对于4种波形的处理精度最高,在模板形状确定的情况下使用。文中所提系统的焦面测量结果最大平均绝对误差仅为57 nm,最小可达到8 nm,有较强的实际应用意义。

The machine vision method is often used to acquire wafer images for wafer inspection equipment.The focusing system based on spectral confocal displacement sensors has low measurement accuracy and poor robustness,while that based on triangulation principle is of high enough accuracy.However,the patterns and grooves on the wafer surface will affect the detection optical path,so the system fails to work normally.In view of the above,a focusing system based on scanning white light interferometry is proposed to achieve fast and accurate measurement of the focal plane positions of silicon wafers with different process properties.In this focusing system,the vertical distance calibration between the best focal plane position of the camera and the zero optical path difference position of the PZT are firstly carried out,so that the zero optical path difference can be used as the detection object during the focusing measurement,which greatly improves the detection speed,and adaptively selects the positioning algorithm according to the shape of the interference waveform when positioning the zero optical path difference position,so as to improve the process adaptability of the focusing system.The experimental results show that,for single⁃peak waveforms and bimodal and bimodal semi⁃separated waveforms,the average absolute errors of the focal plane positions obtained by the center⁃of⁃gravity method are 20 nm and 56.1 nm,respectively,while those are 38 nm and 56.2 nm in the envelope method.In addition,the processing time of the center⁃of⁃gravity method is within 1 ms,while that is about 25 ms in the envelope method,so the center⁃of⁃gravity method is chosen as the zero optical path difference positioning algorithm.For the bimodal and bimodal fully⁃fused waveforms and the bimodal and bimodal⁃fully separated waveforms,the average absolute errors of the focal plane positions obtained by the center⁃of⁃gravity method are 20.3 nm and 17.8 nm,respectively,while those are 13.2 nm and 15.8 nm in the envelope method,so the envelope method is chosen as the zero optical path difference positioning algorithm.The template matching method has the highest accuracy for the four waveforms and is used when the template shape is determined.The maximum average absolute error of the focal plane measurement results of the proposed system is only 57 nm,and the minimum can reach 8 nm,which is valuable in practical application.