Precision alignment in semiconductor lithography demands nanometer-scale accuracy, as even minor misalignments between the mask and wafer can drastically impact chip yield. However, existing optical measurement techniques, which rely on coherent light sources and grating structures, face significant limitations. These methods require the detection of a vast number of photons to achieve sufficient signal-to-noise ratio through statistical averaging, leading to prolonged measurement times and constraints in real-time, high-speed applications such as multi-patterning lithography. Additionally, the physical size and complexity of conventional optical systems hinder their integration into modern lithography tools for in situ metrology.
In a breakthrough published in Light: Science & Applications, a collaborative team led by Prof. Din Ping Tsai at City University of Hong Kong, along with Prof. Shumin Xiao from Harbin Institute of Technology (Shenzhen) and Prof. Lijian Zhang from Nanjing University, has developed a novel lateral displacement sensing method compatible with contemporary semiconductor manufacturing processes. The key innovation lies in the integration of a geometric-phase metasurface with two-photon quantum interference.
The team designed and fabricated a metasurface capable of efficiently converting incident orthogonally polarized photons into circularly polarized light along specific paths. When photon pairs—generated via spontaneous parametric down-conversion—pass through this metasurface, they undergo quantum interference, effectively doubling the quantum Fisher information per photon pair compared to classical coherent light. This enables the extraction of the same nanoscale positional information using only a fraction of the photons.
Experimentally, the researchers demonstrated that their system could achieve a precision standard deviation between 4.28 and 7.95 nm—comparable to classical single-photon methods—while reducing the required number of detected photons by approximately 97%. On average, only about 3% of the photons typically needed in conventional methods are sufficient to achieve the same accuracy. The two-photon coincidence counting mechanism also inherently suppresses ambient noise and filters out non-informational photons, further enhancing signal quality.
Moreover, the system operates effectively across a dynamic range of 20 to 5000 nm/s, covering typical wafer stage speeds used in industrial lithography processes. This capability was validated through continuous motion tests using a piezoelectric stage followed by fast Fourier transform analysis, confirming the method's suitability for high-speed precision alignment.
"This isn't just better precision; it's a paradigm shift in efficiency," the authors note. "By harnessing quantum interference in a metasurface-based platform, we significantly accelerate measurement while maintaining nanometer-level accuracy."
Looking forward, the team aims to further improve the system's resolution and precision by optimizing optical integration to reduce jitter, employing higher-order photon states, enhancing photon indistinguishability, and potentially shortening the operating wavelength. This technology not only offers a powerful solution for next-generation semiconductor lithography but also holds promise for other fields requiring ultra-precise metrology, such as quantum sensing, vibration compensation, and nanofabrication.
