Design of a low-brightness highly uniform source for projection electron-beam lithography (SCALPEL)

The requirements for a projection electron-beam lithography source, such as one suitable for a system based upon the SCAL- PELR (scattering with angular limitation projection electron-beam lithography) technique, are significantly different from those of a conventional TEM, SEM, or direct write type of instrument. While high resolution imaging is still a primary goal, this must now be achieved at relatively high (1-200 (mu) A) beam currents in order to realize economically meaningful wafer throughput. Space-charge limitations considered over the entire system (not just the electron gun) lead to the use of relatively large illumination angles (approximately 0.5 mrad). Taken together with an illuminated mask area of approximately 1 mm2, this means that the electron gun axial brightness needs to be only 102 to 104 Acm-2sr-1, as compared with a value of 106 to 109 Acm-2sr-1 for a TEM. Similar considerations indicate that the source emittance must exceed 700 micrometer(DOT)mrad, which is more than an order of magnitude larger than that provided for a standard focused-beam system. Additionally, the uniformity of the illumination must be within 2% in order to ensure that the variation in printed feature size across the imaged area remains negligible. This type of source performance must be stable for extended periods of time in order to maximize the uptime of the lithography tool. In this paper we review the source built for our SCALPEL proof-of-concept system, discuss the impact of an interim modification, and then examine the potential of a further source redesign.