Control system design and alignment methods for electron lithography

The electron-beam exposure system (EBES) was designed to economically expose integrated circuit masks on a production basis and to directly pattern semiconductor substrates for special applications where a limited number of devices are required. The machine was designed to assure absolute pattern accuracy over the entire substrate so that masks made by EBES will be interchangable with those made by our optical mask shop or those made by later electron-beam pattern generators. This has been achieved by a combination of control system design and by automated periodic system alignment. The control system design utilizes a laser interferometer to determine precise table position and a feedback system that continuously corrects the writing-beam location for errors in table location. This combination, coupled with a small-angle electron-beam deflection system using an ultrastable sawtooth-scan generator to achieve repeatable beam positioning over a small writing area, allows exposure rates of 107 addresses per second with submicron absolute accuracies over a 10×10-cm area. A series of automated measurements have been implemented in order to maintain this accuracy for extended periods. Every five minutes beam-positioning errors resulting from external magnetic fields, temperature variations, and amplifier drifts are measured by using the electron column as a scanning electron microscope to locate a registration mark permanently affixed to the table. Several times a day, the deflection system is automatically adjusted under computer control to assure that the electron-beam deflection axes agree precisely with the interferometer-controlled table motion in both magnitude and direction. Periodically, a set of special test patterns are written by EBES and then, using EBES as a computer-controlled coordinate measuring machine, these patterns are measured and analyzed to provide an accurate assessment of the machine’s true writing performance. This alerts the mask shop to potential problems and allows service personnel to make the necessary corrections before the final product is materially affected.