Patent ID: 6686587
Filing Date: 2004-02-03
Classification: G02B

Abstract:
A method of precision calibration of magnification of a scanning microscope with the use of a test object formed as a test diffraction grating, comprising the steps of positioning and orienting a test object on stage of microscope so that strips of a test diffraction grating are perpendicular to a direction along which a calibration is performed; scanning of a selected portion of the test object along axes X and Y by the scanning microscope; measuring values of a signal S by the scanning microscope versus coordinates x and y in a plane of scanning and storing of said values S (x, y) in a digital form as a two-dimensional digital array; transforming the two-dimensional array of signals S(x, y) into a two-dimensional array S (u, v) by turning of the axes so that a direction of a new axis u is perpendicular to the strips of the grating and a direction of a new axis v coincides with the strips of the grating; line-by-line mathematical processing of the array S(u,v) for each line S(u) by calculating of a Fourier spectrum SP(&ohgr;) of each line S(&ohgr;) in correspondence with the formula SP&it;(Ï‰)=12&it;1Ï€&it;&Integral;-âˆžâˆž&it;S&it;(u)&CenterDot;exp&it;â€ƒ&it;(&ii;u&it;â€ƒ&it;Ï‰)&it;â€ƒ&it;&dd;u,wherein &ohgr; is a coordinate in a reciprocal space which represents a space frequency; SP (&ohgr;) is the Fourier spectrum which is a complex spectrum density corresponding to the space frequency &ohgr;; S(u) is a function which describes a one-dimensional profile of a signal; and i={square root over ( )}âˆ’1 is an imaginary unit; converting of the one dimensional, complex function SP (&ohgr;) into a one-dimension spectrum of real values of a module |SP(&ohgr;)| by multiplying of each value SP (&ohgr;) by a complex-conjugate value; finding from the spectrum of the real values |SP(&ohgr;)| a greatest spectral maximum and determining its characteristic frequency &ohgr;h as an abscissa of a point with the maximum value |SP(&ohgr;)|; calculating an average value of a pitch Th of the diffraction grating in accordance with the formula: Th=1Ï‰hperforming the mathematical processing, the conversion, the determination and the calculation for subsequent lines S(u) with a new value of a coordinate v; statistically processing a thusly obtained set of values Th for all lines and determining an average value T and a standard deviation &Dgr;T over a whole frame; and determining a magnification Mu in accordance with the selected direction u in accordance with the formula: Mu=T&CenterDot;LT0&CenterDot;Nwhere L is a width of a medium of image in direction of the calibration, T0 is an independently obtained value of the pitch of the test object, and N is a number of pixels in the line along the direction u.