Multi-reference fringe locking technique for interference lithography

Large-area gratings play a critical role in various fields such as astronomical observations, laser fusion, and precision measurements, with an increasingly urgent demand for the fabrication of meter-scale gratings. Interference lithography (IL) offers the capability to produce high-quality gratings and holds significant potential for scaling up grating sizes. The stability of the exposure light field significantly affects the processing quality. Therefore, this paper proposes a fringe locking technique based on multiple reference gratings.

In the dual-beam interference lithography setup with large-aperture optics, a reference grating is used to monitor the exposure field. The reference fringes are recorded by a CCD camera, and the drift values are calculated using a cross-correlation method. These values are used to generate the control signals, which actuate the motion mechanisms to dynamically adjust the phase and period of interference field. However, relying on a single reference grating is insufficient to capture the conditions across the entire exposure field.

Therefore, we conducted an analysis of the errors across the entire exposure field and identified period error as the primary cause of this phenomenon. To address this, fringe patterns from two reference gratings are used to monitor periodic variations in the interference field. The feedback calculated by these variations is used to adjust the motion mechanism. altering the angle between the two beams to achieve periodic compensation. Experimental results show that after implementing periodic compensation, the fluctuation RMS of the interference fringes decreased from 0.24λ to 0.06λ, demonstrating significant improvement.