Mr. Bo Liu Profile

Bo Liu

at Beijing Institute of Technology

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Publications (2)

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Proceedings Article | 21 August 2014 Paper

Vector simulation of pinhole diffraction behavior for high numerical aperture converging incident beam at deep ultraviolet wavelength

Bo Liu, Ke Liu, Meng Zheng, Lisong Dong

Proceedings Volume 9283, 92830B (2014) https://doi.org/10.1117/12.2069704

KEYWORDS: Wavefronts, Diffraction, Near field optics, Optical testing, Near field, Spherical lenses, Wavefront sensors, Calibration, Light wave propagation, Sensors

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For in situ calibration of high numerical aperture wavefront sensor, a small pinhole, whose diameter should be less than the diffraction limit of incident beam, is used to generate reference spherical wavefront. In this way, the calibration accuracy is determined by the quality of pinhole diffracted quasi-spherical wavefront. Combining scalar and vector diffraction theory, a pinhole diffraction model is established to determine the pinhole parameters which could generate high quality quasi-spherical wavefront. The model includes three steps. Firstly, scalar diffraction theory is utilized to propagate light from exit pupil of test optics to the pinhole’s incident near-field. Secondly, vector diffraction theory is used to propagate light from pinhole’s incident near-field to the pinhole’s emergent near-field. Finally, scalar diffraction theory is utilized again to propagate light from pinhole’s emergent near-field to the detection plane of wavefront sensor. The most important feature of this model is that the influence of test optics’ exit pupil aberrations on the quality of reference spherical wavefront can be taken into account quantitively. Based on the modeling approach above, we conduct a simulation on the pinhole diffraction behavior of light with wavelength of 193.3 nm, X-polarized, numerical aperture of 0.75. The results show that, in order to achieve reference spherical wavefront error of 2 mλ rms, pinhole mask made up of Chromium membrane with pinhole diameter of 170 nm and membrane thickness of 200 nm should be chosen. At the same time, the pinhole lateral alignment, pinhole sidewall and pinhole eclipse should be controlled in ±20 nm, ±30 nm and ±10 nm, respectively. Also the requirement on the exit pupil aberrations of test optics is given based on wavefront error and intensity uniformity on detection plane.

Proceedings Article | 18 December 2012 Paper

High numerical aperture Hartmann wavefront sensor with pinhole array extended source

Ke Liu, Yanqiu Li, Meng Zheng, Hai Wang, Bo Liu

Proceedings Volume 8550, 85503M (2012) https://doi.org/10.1117/12.980898

KEYWORDS: Sensors, Objectives, Photomasks, Calibration, Charge-coupled devices, Wavefront sensors, Spherical lenses, Radio propagation, In situ metrology, Light

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In situ aberration measurement of projection objective is necessary for lithography tool. For 90 nm technology node, aberration measurement accuracy of 1 nm rms is required. In this paper, a high numerical aperture Hartmann wavefront sensor with pinhole array extended source is proposed. The sensor uses source mask with pinhole array on the object plane of projection objective to filter the aberration of illumination optics as well as provide sufficient power required by Hartmann sensor. A coupling objective, which is installed at the confocal position of the projection objective under test, transforms the high numerical aperture spherical waves to plane waves. A null mask, which has similar structure with source mask, can be inserted at the image plane of projection objective. With the null mask installed and source mask uninstalled, the systematic measurement errors mainly caused by coupling objective can be calibrated by the relative measurement process. In this paper, some design considerations of source mask and null mask are presented. Using partial coherent light propagation and Fourier optics theory, the proper spacing and quantity of pinholes on either source mask or null mask are calculated. Finally, measurement accuracy of the sensor is evaluated using three-dimensional electromagnetic simulation of 193nm high numerical aperture converging beam propagation through pinhole with different pinhole parameters. Simulation results show that, measurement accuracy of the sensor is better than 0.5 nm rms in theory after systematic errors calibration.

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