Computerized optimization of electron-beam lithography systems

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Abstract
An optimization program has been developed for designing combined focusing and dual‐channel deflection systems with low aberrations for electron beam lithography. The program can handle any combination of magnetic and electrostatic lenses and deflectors, and uses the damped least squares method for minimizing a weighted sum of squares of aberrations, subject to specified physical constraints. The program has been used to optimize the design of several types of lithography system. The results show that, in many cases, the program can yield designs with extremely low aberrations. For example, a system with one magnetic lens and two deflectors has been found which, at the corners of a 5 mm scan field with 5 mrad aperture, has only 0.19 μm overall aberration before dynamic corrections; by adding an electrostatic retarding field, this can be reduced to 0.12 μm. By adjusting the strengths and positions of the deflectors, several of the aberrations can be individually eliminated, including field curvature. It is also shown that electrostatic subfield deflectors can be added without significantly increasing the aberrations, and a pure electrostatic focusing and deflection system for ion beam lithography has been designed which is limited almost entirely by axial chromatic aberration.

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