Patent ID: 6664532
Filing Date: 2003-12-16
Classification: H01J

Abstract:
A method of precision calibration of magnification of a scanning microscope with the use of test diffraction grating, comprising the steps of positioning and orientation of a test object on a microscope stage so that strips of a test diffraction grating are perpendicular to a direction on which a calibration is performed, scanning of a selected portion of the test object along axes X and Y; measuring values of a signal S versus coordinates X and Y in a plane of scanning and storing 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) including: separation from it of a one-dimensional array-line S(u) which contains a profile of an image of periodically repeating strips of a test-object; multiplication of the line S(u) by shifting of its copy relative to an original by an integer number of periods, adjustment of a value of the shift by the method of least squares in accordance with a criterion of the best coincidence of the overlapping portions of the original and the copy of the line S(u), calculation of average values S(u) in the zone of overlapping as a semisum of values of the original and copy S(u) in each point of the zone; transformation of the multiplied array S(u) into an array P(w) in accordance with the formula P&af;(w)=&Sum;j=1N/2&it;â€ƒ&it;[S&af;(j)*S&af;(j+w)]where N is a number of members in the multiplied array S(u); determination of coordinates w1, w2, w3, w4 . . . of successive maximums of the function P(w) wherein w4>w3>w2>w1>0; determination of an average period Tâ€² of the test grating in pixels for the selected line S(u) in accordance with the formula Tâ€²=1n-1&it;&Sum;i=1n-1&it;â€ƒ&it;(wi+1-wi)where n is a number of maxima in the function P(w); moving to a next line S(u) with a new value of coordinates v and performing for it the same line-by-line mathematical processing; performing statistic processing of obtained set of values Tâ€² corresponding to various lines v with calculation of an average period Tav for all lines and calculating a magnification Mu along the selected direction u in accordance with the formula Mu=Tav&CenterDot;LTo&CenterDot;Nwherein L is width of a medium of the image in direction of calibration, T0 is an independently attested value of a pitch of the test object, N is a number of pixels on a line along the direction u.