Precision Lithography: Optimising Transverse Displacement Measurement via Quantum States

A novel quantum optical configuration has successfully executed **transverse displacement measurement** with equivalent precision to standard techniques but requires 97% fewer photons. This efficiency gain offers a direct pathway to accelerate semiconductor lithography processes without compromising mask-to-wafer alignment accuracy.

The Limits of Classical Transverse Displacement Measurement

Semiconductor fabrication operates on the edge of physical possibility. Precise alignment between the mask and the wafer is non-negotiable. Nanometre-level precision is the current standard. However, traditional lateral displacement metrology faces a rigid barrier. Classical methods rely on coherent states and gratings. These techniques demand high photon flux to overcome the shot-noise limit and achieve sufficient signal-to-noise ratios. High intensity creates two problems: it generates heat which can distort the wafer, and it requires longer integration times to gather data. Speed is sacrificed for accuracy. The industry requires a method that measures faster without losing resolution.

Solution: Two-Photon State Interference

The research team introduced a method utilizing a polarisation gradient metasurface. Instead of classical light sources, the study employed two-photon state interference. This quantum mechanical strategy alters how position data is encoded. The focus shifts from simple intensity measurements to quantum correlations. By feeding entangled photons through the metasurface, the system exploits the unique properties of quantum light. The metasurface creates a spatially varying polarisation profile. As the target moves transversely, the quantum state evolves in a predictable, highly sensitive manner.

Mechanism: Polarisation Gradient Metasurface

The core mechanism relies on the specific interaction between the entangled photons and the metasurface. The polarisation gradient creates a spatial dependency. Detecting these changes allows for precise calculation of position. Crucially, the signal-to-noise ratio improves due to the quantum nature of the light. The interference pattern shifts sensitively with transverse movement. This allows the system to gather necessary data points with a fraction of the optical input required by classical interferometry.

Impact: Speed and Integration

The experimental results are stark. The new method achieved equivalent precision to classical techniques while reducing the detected photons to approximately 3%. This reduction is not merely academic. It suggests that acquisition times could drop drastically. Faster measurements mean higher throughput in lithography machines. Furthermore, lower photon requirements reduce the risk of thermal expansion or damage to the wafer during alignment. This approach appears highly suitable for integration with existing semiconductor lithography processes. It promises to realise equivalent measurement precision within notably shorter acquisition durations, providing a robust solution for next-generation manufacturing requirements.