High emittance source for the PREVAIL projection lithography system

In the PREVAIL proof of concept electron beam projection lithography system, 1 mm×1 mm sq reticle subfields are illuminated and imaged to a wafer with 4× demagnification. A relatively large beam semiangle (5-8 mr at the wafer) is required to optimize resolution at the beam currents (5-15 μA) needed for high-throughput lithography. A high emittance source and illumination system have been developed which can uniformly illuminate the reticle subfield with a beam semiangle up to 16.3 mr (1/e) at the wafer. The source utilizes a tantalum single crystal 10 mm in diameter. The crystal is heated by electron bombardment incident on the side opposite the emitting surface, which is a low work function crystal plane. A variation of the "critical Köhler" illumination scheme is utilized wherein the cathode surface is imaged approximately to a square shaping aperture, and the shaping aperture is conjugate to the reticle (and the wafer). The emission is temperature limited, so care must be taken to obtain a uniform temperature distribution across the emitter surface. Long term emission current stability better than 1% is provided by servo control of the bombardment heating power. Illumination uniformity has been measured at the wafer plane using a pinhole detector. The measured 3σ variation in current density is 3.2%. On the basis of these results, the tighter specifications required for a commercial PREVAIL projection system appear achievable. © 1999 American Vacuum Society.