Research on noise suppression in phase unwrapping for Ronchi shearing interferometry

Abstract

Lithography lies at the heart of integrated circuit manufacturing, and its projection lens is a key part that directly determines the device’s resolution through imaging quality. This imaging quality is typically characterized by wavefront aberration, and high-precision measurement of wavefront aberrations is an essential means of fabricating high-resolution optical systems. Ronchi shearing interferometry is an interferometer with simple structure and wavefront self-reference for wavefront aberration detection. Compared to other tools such as the Fizeau interferometer, Shack-Hartmann sensor, and point diffraction interferometer, its common-path, anti-interference makes it more suitable for in-situ wavefront aberration measurement in lithography projection lenses. In Ronchi shearing interferometry, the smaller the shear ratio, the richer the wavefront information you can obtain — but the trade-off is that more noise is introduced. This causes a low phase signal-to-noise ratio, which interrupts the phase unwrapping path. Furthermore, mask alignment errors can also cause the unwrapping path to break. Therefore, to achieve highly robust unwrapping with Ronchi shearing interferometry at small shear ratio, this paper compares several filtering algorithms and experimentally verifies their optimization performance. This paper proposes a joint filtering method based on adaptive median filtering and guided filtering. It effectively removes isolated spikes and abnormal residuals while preserving fine wavefront structures. Experimental results demonstrate that, compared with conventional Gaussian filtering, the proposed method reduces the residual root mean square (RMS) from 5.321 nm to 3.755 nm (a 41.7% reduction), and the peak-to-valley(PV) value from 196.397 nm to 78.374 nm. The residual RMS in the X-direction is further reduced by 62.9%, while the extreme residual values are compressed from approximately 160 nm to within 50 nm. Wavefront reconstruction results show that, at λ = 365 nm, the RMS achieved by the proposed method is 29.07 nm (0.08λ), with a smoother and more stable central profile. The QWLSI cross-validation result (30.28 nm) shows strong agreement with the proposed method, further confirming its effectiveness and reliability. Obviously, this method effectively alleviates the issue of unwrapping path interruptions in noise-complex interferograms, demonstrating excellent robustness. This suggests promising potential for future deployment in complex inspection environments.