A robust focusing and astigmatism correction method for the scanning electron microscope—Part III: An improved technique

Abstract

Summary:As described in a previous work, a new technique has been developed to perform automatic focusing and astigmatism correction on any general scanning electron microscope (SEM) sample ranging from gold-on-carbon to integrated circuit (IC) tracks. In this work, various improvements made to this technique are reported. They include the implementation of direct control of the SEM and the development of three new algorithms, namely the adaptive fast Fourier transform (FFT) algorithm, the coarse focusing algorithm, and the fine focusing algorithm. Direct control reduces the communication time with the SEM while the three new algorithms are integrated with the existing technique to make it even more robust to noise and to extend it to correct images with any degree of defocus and astigmatism. The enhanced focusing and astigmatism correction algorithm is able to perform the correction in a shorter time while maintaining the accuracy of the original algorithm even under noisy conditions.

Contrast-to-gradient method for the evaluation of image resolution taking account of random noise in scanning electron microscopy

“…In the FFT model, on the other hand, the resolution is interpreted as an inverse of the maximum frequency of the Fouriertransformed image [7][8][9][10][11][12]. The superposition diffractogram method using two sequentially recorded copies of the same image is one effective method for discrimination of signals from noise.…”

Image sharpness measurement in the scanning electron microscope—Part III

“…A Fourier transform technique capable of high-resolution operation, was applied by Postek et al (1996). Procedures based on the Fourier transform technique can also be found in Ong et al (1997Ong et al ( ,1998. Since an SEM image is composed of a two-dimensional (2-D) array of data, the 2-D Fourier transform generates a 2-D spatial frequency spectrum.…”

Low-Dosage Maximum-A-Posteriori Focusing and Stigmation

“…One weakness of existing implementations for electron microscopy (Nicolls et al, 1997; Ogasawara et al, 1998; Baba et al, 2001) is that the estimation of the coefficients from two images acquired with a known change in defocus between them uses the ratio of their Fourier spectra. While this was an improvement over earlier implementations based on comparing thresholded Fourier spectra (Ong et al, 1998), this, as does any ratio-based approach, amplifies noise in regions with weak signal and is therefore ill-suited for low-dose imaging.…”