Process adaptability for digital grating-based focusing and leveling sensors

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 measurements 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 lithography machine’s focusing and leveling sensor to measure the wafer surface height accurately. In the actual measurement, the unexpected patterns on the wafer surface interfere with the reflection imaging of the optical grating, then affect the image processing results. 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. The proposed method shows good stability when the large area pattern appears on the wafer substrate, and 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 significantly reduced and the method’s Z-direction error correction is within ±33 nm (±3σ=±41.2 nm) after reconstructing the light intensity. This method can enhance the process adaptability of the digital grating focusing and leveling sensor. Therefore, it can serve as a technical reference for the focusing and leveling method.