Quarter-micron lithography with a gapped Markle-Dyson system

Abstract

0.25-µm lithography has previously been demonstrated using an ungapped prototype Markle–Dyson system, in which the wafer was held in soft contact with the mask. However, such an in-contact scheme would be inappropriate for semiconductor fabrication, and so a second prototype has been designed and constructed, in which a gap of 25 µm is introduced between the mask and the wafer. The two major technical problems to be overcome in implementing the gap are the measurement and setting of the gap itself, and correction for the spherical aberration which it introduces. In the new prototype, the gap is set by a piezo-electric actuator and measured using a capacitance gauge, and the spherical aberration is corrected by using a mirror which deviates slightly (by 0.33 µm) from sphericity. The system has been tested lithographically, using chromium reflective masks. It has demonstrated a resolution of 0.25 µm in Shipley XP89131 photoresist.