Automatic Focusing and Astigmatism Correction Method based on Fourier Transform of Scanning Electron Microscope Images

A new method for automatic focusing and astigmatism correction of electron optics has been proposed. The method consists of the following 6 steps: (1) Obtain scanning electron microscope (SEM) images i1(x,y), i2(x,y) under two focusing conditions. (2) Obtain Fourier transformed images of i1(x,y) and i2(x,y), I1(kx,ky) and I2(kx,ky). (3) Obtain a characteristic function R(kx,ky)=ln(I1(kx,ky)/I2(kx,ky)). (4) Repeat the above steps under different focusing conditions. (5) From the change in R(kx,ky), calculate the change in beam sharpnesses in different directions. (6) Correct the astigmatism and adjust the focus. R(kx,ky) is independent of the sample shape, and is independent of the beam shape in the case of a character beam. Therefore, this method can be applied both to a SEM and to an electron beam writer with a character beam.