High numerical aperture reflective deep ultraviolet Fourier ptychographic microscopy for nanofeature imaging

A. Implementation of reflective DUV FPM

Figure 2 illustrates the experimental setup of the reflective DUV FPM equipped with the aperture scanning illumination system. The optics used for the ArF excimer laser light (193.3 nm wavelength, 0.3 nm linewidth, 10 ns pulse width, 200 Hz repetition rate, and 15 mJ maximum energy) employed in this system is designed for less energy concentration in the optical paths to avoid the creation of plasma, which would result in a shock wave and damage to the optical elements.2727. M. Thiyagarajan and J. E. Scharer, IEEE Trans. Plasma Sci. 36, 2512 (2008). https://doi.org/10.1109/tps.2008.2004259 The optical path is sealed and filled with nitrogen gas to prevent optical power loss caused by light absorption from oxygen in the air, which also generates ozone gas. The rectangular output beam of the 193 nm excimer laser is transformed to a circular beam with enlarged size by a cylindrical lens pair (CL) and shaped to an extended source by a rotating diffuser (RD) placed on a vibration isolation plate (IB), which also removes speckle patterns caused by the laser coherence.3232. E. Cho, T. Kim, Y. S. Bae, S.-S. Choi, B. M. Barnes, R. M. Silver, and M. Y. Sohn, Opt. Lasers Eng. 152, 106953 (2022). https://doi.org/10.1016/j.optlaseng.2022.106953 An effective source (ES) with 3 mm diameter is formed by a lens group (LES) to fit an effective incident NA of the aperture scanning illumination optics. A lens group (LCBFP), with a magnification of about 3.9, transfers the ES to the CBFP of diameter of about 11.7 mm, which is large enough to implement aperture scanning for discrete angular illumination. The aperture scanning illumination optics consists of a relay lens group (LR) and a catadioptric objective lens (OL). A divergent beam passing through the SA at the CBFP is transferred to the back focal plane of the OL with a magnification of 0.34 and is transformed to an angular illumination beam at the target plane by an angle defined in Eq. (1).4242. M. Y. Sohn, B. M. Barnes, and R. M. Silver, Optik 156, 635 (2018). https://doi.org/10.1016/j.ijleo.2017.11.206 An SA of 1 mm diameter mounted to a two-axis stage (TCBFP) is discretely scanned on the CBFP along circles with diameters 3.7, 7.1, and 10.1 mm to form 16 angular illuminations for each circular scan, yielding 48 angle-scanned images at the charge-coupled device (CCDI) plane with a non-uniform scan pattern that ensures stable convergence in the FP reconstruction.4444. K. Guo, S. Dong, P. Nanda, and G. Zheng, Opt. Express 23, 6171 (2015). https://doi.org/10.1364/oe.23.006171 A 193 nm catadioptric objective lens (Corning Tropel, microCAT Panther) with a working distance of 8 mm and an effective NA of 0.13–0.74 is used as OL for both illumination and collection.4545. J. E. Webb and L. Denes, Proc. SPIE 5377, 788–797 (2004). https://doi.org/10.1117/12.535358 The missing frequency components less than NA = 0.13 are recovered by extending frequency bands shifted with the illumination angles using the FP reconstruction algorithm. Angle-scanned beams reflected at the target plane are collected at the charge-coupled devices for imaging or Fourier planes (CCDI, CCDF) by a tube lens (LT) or a Fourier plane lens (LF) selectively using a flip mirror (FM). Images are captured by CCD cameras (Hamamatsu C8000) with 640 × 480 pixels, 14 µm pixel size, and a quantum efficiency of 60% at 193 nm. The light scattered at the target is imaged through the collection optics with a magnification of 350 because of which the pixel size and area of CCDI correspond to 40 nm and 25.6 × 19.2 µm2 at the target plane, respectively. The Fourier images collected at CCDF are used for the illumination angle calibration by placing a plane mirror at the target plane. A navigation microscope with low resolution and large field of view, consisting of a navigation objective lens (OLN), a visible charge-coupled device (CCDN), and a fiber-coupled visible light emitting diode (LED), is used to locate the target in the field of view of OL by the target stage (TTAR) with six axes, because the non-standard OL is fixed above the target plane with a small field of view. All imaging devices and stages are connected to and controlled by a processing computer for the FPM measurement procedure.Compared to aperture scanning methods in recent FPM illumination systems,37,46,4737. X. Ou, J. Chung, R. Horstmeyer, and C. Yang, Biomed. Opt. Express 7, 3140 (2016). https://doi.org/10.1364/boe.7.00314046. J. Chung, H. Lu, X. Ou, H. Zhou, and C. Yang, Biomed. Opt. Express 7, 4787 (2016). https://doi.org/10.1364/boe.7.00478747. L. Wang, Q. Song, H. Zhang, C. Yuan, and T.-C. Poon, Appl. Opt. 60, A243 (2021). https://doi.org/10.1364/ao.402644 the aperture scanning illumination scheme in this DUV FPM avoids light energy concentration that causes damage to nanoscale features. The aperture diameter is determined at 1 mm by empirically considering the optimized energy and spatial coherence of the illumination beam. The energy fluence from the laser source to the CCDI and CCDF is optimized while considering sufficient intensity for imaging as well as damage thresholds of the coatings of optical components and targets. The tile size for the FP recovery process is 30 pixels, which is determined by the spatial coherence length of 1.2 µm calculated from l = 1.22λz/w, where λ, z, and w correspond to the wavelength of light, propagation length, and diameter of the illumination source, respectively.1515. X. Ou, R. Horstmeyer, G. Zheng, and C. Yang, Opt. Express 23, 3472 (2015). https://doi.org/10.1364/oe.23.003472

The significant background intensity of each raw target image caused by stray lights is removed using the background image taken without the target. Subtracting the background image at each illumination angle from the raw target image generates the low resolution input images for the FP reconstruction process, ensuring convergence to and divergence from the local minima during the recovery.

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