Improvement of beam-adjustment accuracy by beam-intensity distribution measurement on a second shaping aperture in electron-beam writing systems

Abstract

We proposed a new adjustment method for the beam-intensity distribution. On the 2nd shaping aperture plate, a small aperture is positioned sufficiently apart from the aperture for variable shaping. The center of the 1st shaping aperture image is moved to the small aperture by adjusting an alignment coil. The beam current, which passes through the small aperture, is measured by the Faraday cup on the target while the 1st shaping aperture image is scanned over the small aperture by operating the shaping deflector. Using this method the beam-intensity distribution of the 1st shaping aperture image on the 2nd shaping aperture plane is obtained. The beam-intensity distribution obtained is ideally a series of concentric circles and the maximum value is reached at the center circle. The center is shifted when the alignment of the limit aperture at the illumination or at the 1st shaping aperture is incorrect. The position of the maximum in the beam intensity is adjusted to the center of the 1st shaping aperture image. At the beam-current-density of 20 A/cm 2 , the slope of the distribution is usually under 1 percent. To evaluate the adjustment accuracy, resist profile of drawn pattern is measured by AFM. The resist profiles in a beam shot coincide well with the beam intensity distribution.