• Acta Optica Sinica
  • Vol. 44, Issue 19, 1923002 (2024)
Zhifang Yao1,2, Fang Wu3,*, and Yang Bu1,2,**
Author Affiliations
  • 1Department of Advanced Optical and Microelectronic Equipment, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
  • 2Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
  • 3Zhangjiang Laboratory, Shanghai 201210, China
  • show less
    DOI: 10.3788/AOS240763 Cite this Article Set citation alerts
    Zhifang Yao, Fang Wu, Yang Bu. Deep-Ultraviolet Polarizer Based on Two-Dimensional Photonic Crystal[J]. Acta Optica Sinica, 2024, 44(19): 1923002 Copy Citation Text show less

    Abstract

    Objective

    Immersion ArF lithography machine is a mainstream lithography equipment for manufacturing integrated circuits with technology nodes below 28 nm. It adopts the filling of water between the projection lens group and the wafer, increasing the numerical aperture and improving the resolution of lithography. The feature size of the mask used in deep ultraviolet immersion lithography is already smaller than the wavelength of the light source, and the inclination angle of the imaging beam on the wafer is relatively large. Under such a situation, the polarization state of the imaging beam will affect imaging contrast, making polarization control particularly important. The polarization lighting system mainly consists of polarization purification components, polarization control components, and polarization conversion components, with the polarization purification component performing the polarizing function. For traditional deep ultraviolet polarizers, the incident angle must be equal to the Brewster angle when using a polarizing beam splitter stack; polarization gratings can achieve a high polarization extinction ratio but suffer severe energy loss and are commonly used for polarization state detection. Polarization prisms made of birefringent crystals are currently the mainstream solution, with the transmittance of about 60% and 70% respectively for its two polarization modes, and it is inclined to a slight energy loss. In addition, in the polarization detection optical path, the beam splitting angle of the polarization prism is limited by the birefringence index of its material, which makes the detection optical path too long to adjust. To produce a polarizing device with high transmittance and high extinction ratio in the polarization illumination system of an immersion lithography machine, a low-loss photonic crystal polarizer under deep ultraviolet wavelength is proposed.

    Methods

    Photonic crystals refer to materials with periodic distribution of dielectric constants, and their special dispersion relationship is similar to the band structure of electronic crystals, known as photonic bands. Similar to electronic crystals, there may be gaps between energy bands, and light at frequencies within the photonic band gap cannot form propagation modes in the photonic crystal, providing a new way to control light. Two-dimensional photonic crystals, due to their inherent symmetry, divide electromagnetic waves into two orthogonal polarization modes: transverse electric (TE) and transverse magnetic (TM) modes. If a two-dimensional photonic crystal has a band gap for one polarization state of light at the same frequency, and another can be transmitted through it, it can serve as a polarizer. The photonic crystal polarizer is made of non-destructive medium fused quartz in the deep ultraviolet band. The dielectric cylindrical photonic crystal with a triangular lattice has a band gap for TM polarization and an approximately straight equifrequency line for TE polarization, which indicates total reflection for TM polarization and self-collimation transmission for TE polarization. Based on the polarizer, a tilted interface designed based on the fundamental vector direction of the triangular lattice enables the photonic crystal to perform as a polarization beam splitter.

    Results and Discussions

    Based on the plane wave expansion (PWE) method, the band structure and equifrequency contour are calculated, and the structure with a lattice constant to wavelength ratio of 0.5 and a radius to lattice constant ratio of 0.26 meets the requirements of a photonic crystal polarizer. The reflectivity, transmittance, and self-collimation propagation direction of the modified photonic crystal are verified using the finite difference time domain (FDTD) method. The photonic crystal polarizer with a vertical interface has a TE polarization transmittance of over 99% and a polarization extinction ratio of over 70 dB. The tilted interface photonic crystal polarization beam splitter generates a beam splitting angle of 120°, as well as output efficiency and polarization extinction ratio of 97%, 70 dB, and 99%, 16 dB for TE and TM ports, respectively. In addition, the lattice of the polarization beam splitter has been optimized to 102 nm, whose lattice constant wavelength equals 0.53. The new structure has a better performance of about 95% self-collimation transmission within the incidence angle range of -5° to 15°, and the output light angle remains approximately 0°.

    Conclusions

    Firstly, the PWE method is used to calculate the band characteristics and equifrequency plots of triangular lattice photonic crystals, obtaining the structural parameter range of the photonic crystal polarizer. When the fixed filling rate is 0.26, a normalized frequency range of 0.49?0.53 exists; when the fixed normalization frequency is around 0.5, the filling rate range is 0.24?0.32. A photonic crystal polarizer structure is designed using the FDTD method, and numerical simulation results show that the polarization characteristics of TE polarization transmittance above 99% and TM polarization transmittance at the order of 10-7 can be maintained in the range of period 94.5?106 nm and dielectric column radius 23?30 nm, as the high transmittance structure basically covers the entire photonic bandgap range. Under this situation, a photonic crystal polarization beam splitter with a 120° angle beam splitting is achieved by introducing a tilted interface 30° away from the vertical direction. We analyze the self-collimation propagation effects of different incident light angles under different periodic structures and obtain the optimal self-collimation angle of approximately 4° in the normalized frequency range of 0.5?0.54. With the period of 0.53λ as part of the optimal photonic crystal polarization beam splitter design, the structure can achieve self-collimation propagation with a TE transmittance of 95% and a refractive angle of around 0°.