Considerations on space charge effect for high current variable shaped beam forming

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Abstract
Variable shaped beam schemes are very promising for advanced VLSI pattern generation. In such a device, beam blurring caused by the space charge effect becomes greater as the beam current increases and the focal length of the electron lens is varied as the beam current changes. In order to keep the beam blurring within a permissible amount, refocusings are required whenever beam sectional areas are changed dramatically. However, it is often difficult to complete refocusing in negligible short time in comparison with the shot time of an element pattern and total exposure time is influenced considerably. Investigating the results of computer simulations and fundamental experiments, it is found that beam blurring caused by defocusing is almost proportional to the deviation of the beam current from that at the optimum focusing point and is kept under the permissible value in certain intervals of beam current. Considering this characteristic, the defocusing problem can be improved significantly by rearranging the exposure sequence of pattern elements so that the frequency of dramatic shape change becomes minimum.

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