title: Combining gradient ascent search and support vector machines for effective autofocus of a field emission-scanning electron microscope.
abstract: Autofocus is an important issue in electron microscopy, particularly at high magnification. It consists in searching for sharp image of a specimen, that is corresponding to the peak of focus. The paper presents a machine learning solution to this issue. From seven focus measures, support vector machines fitting is used to compute the peak with an initial guess obtained from a gradient ascent search, that is search in the direction of higher gradient of focus. The solution is implemented on a Carl Zeiss Auriga FE-SEM with a three benchmark specimen and magnification ranging from x300 to x160 000. Based on regularized nonlinear least squares optimization, the solution overtakes the literature nonregularized search and Fibonacci search methods: accuracy improvement ranges from 1.25 to 8 times, fidelity improvement ranges from 1.6 to 28 times, and speed improvement ranges from 1.5 to 4 times. Moreover, the solution is practical by requiring only an off-line easy automatic train with cross-validation of the support vector machines.
authors.affiliation: FEMTO-ST Institute, AS2M Department, Université Bourgogne Franche-Comté, Université de Franche-Comté / CNRS / ENSMM, Besançon, France. Sounkalo.dembele@femto-st.fr.|FEMTO-ST Institute, AS2M Department, Université Bourgogne Franche-Comté, Université de Franche-Comté / CNRS / ENSMM, Besançon, France. Sounkalo.dembele@femto-st.fr.
authors.forename: S
authors.initials: S
authors.lastname: Dembélé
authors.affiliation: FEMTO-ST Institute, AS2M Department, Université Bourgogne Franche-Comté, Université de Franche-Comté / CNRS / ENSMM, Besançon, France.
authors.forename: O
authors.initials: O
authors.lastname: Lehmann
authors.affiliation: FEMTO-ST Institute, AS2M Department, Université Bourgogne Franche-Comté, Université de Franche-Comté / CNRS / ENSMM, Besançon, France.
authors.forename: K
authors.initials: K
authors.lastname: Medjaher
authors.affiliation: KUKA Robotics, Great Western Street, Wednesbury, U.K.
authors.forename: N
authors.initials: N
authors.lastname: Marturi
authors.affiliation: FEMTO-ST Institute, AS2M Department, Université Bourgogne Franche-Comté, Université de Franche-Comté / CNRS / ENSMM, Besançon, France.
authors.forename: N
authors.initials: N
authors.lastname: Piat
keywords: Autofocus; gradient ascent search; machine learning; normalized variance; scanning electron microscopy; support vector machines regression