Motion-free TSOM using a deformable mirror

Abstract

Through-focus scanning optical microscopy (TSOM) is a model-based optical metrology method that involves the scanning of a target through the focus of an optical microscope. Unlike a conventional optical microscope that directly extracts the diffraction-limited optical information from a single in-focus image, the TSOM method extracts nanometer scale sensitive information by matching the target TSOM data/image to reference TSOM data/images that are either experimentally or computationally collected. Therefore, the sensitivity and accuracy of the TSOM method strongly depends on the similarities between the conditions in which the target and reference TSOM images are taken or simulated, especially the lateral instability during through-focus scanning. As a remedy to the lateral instability, we proposed the application of adaptive optics to the through-focus scanning operation and initially developed a closed-loop system with a tip/tilt mirror and a Shack-Hartmann sensor, with which we were able to keep the plane position within peak-to-valley (PV) 33 nm. We then further developed a motion-free TSOM tool reducing the instability down to practically zero by the replacement of the tip/tilt mirror with a deformable mirror that performs through-focus scanning by deforming its mirror surface. The motion-free TSOM tool with a × 50 (NA 0.55) objective lens could provide a scanning range of up to ± 25 µm with a minimum step of 25 nm at a maximum update rate of 4 kHz. The tool was demonstrated to have a recognition accuracy of < 4 nm for critical dimension (CD) values in the range of 60 ∼ 120 nm with a reference TSOM image library generated by a Fourier modal method matching various observations conditions.

A Survey on Adaptive Cameras

“…Adaptive Optics (AO, Sec. 6.3) uses this principle to image sky on astrophotographs 20 , be used in ophtalmology for accurate retinal imaging (Gill et al 2019) or even allows to capture microscopic images with nanometric accuracy (Lee et al 2020). Changes of curvature of more than 5 mm cause two different effects: a concavity magnifies a scene region, which can be useful for surveillance (Kuthirummal and Nayar 2007) or autonomous vehicles by detecting fatigue on driver's face while monitoring the surrounding road (Layerle et al 2008) ; a convexity, on the contrary, enlarges the FOV of the camera pointing to the mirror, allowing to capture a large group of people without stepping back or monitor a whole street even if the camera is in the middle of it (Kuthirummal and Nayar 2007).…”

Optical wafer defect inspection at the 10 nm technology node and beyond

“…Here we should mention that the potential improvement of defect sensitivity in the through-focus scanning method is obtained at the expense of more time. Although deformation-mirror [95], liquid lens [96], and spatial light modulator may enable motion-free vertical scanning [97], it is still more time-consuming than conventional brightfield microscopy.…”

MEMS High Aspect Ratio Trench Three-Dimensional Measurement Using Through-Focus Scanning Optical Microscopy and Deep Learning Method

“…The dimensional information of the target is extracted by matching the intensity information with the simulation results in the database. For improving the performance, great attention has been paid to minimizing the dissimilarities between the intensity information acquisition and simulation conditions [12][13][14][15][16][17][18][19]. In the meantime, model-based TSOM utilizes only the optical intensity range and the mean-square intensity information of the TSOM image; hence, large amounts of intensity distribution information correlated to the dimensional information of the targets is ignored.…”