High resolution electron beam systems include scanning electron microscopes, defect detection tools, VLSI testing tools and mask making and wafer exposure tools. In general, these systems include an electron source and electron optics for accelerating the electrons into an electron beam and focusing the electrons onto a target. These systems require an electron source with a high brightness and a small source size. Field emission electron sources have frequently been used in high resolution electron beam systems.
Negative electron affinity (NEA) photocathode electron sources have been proposed in the prior art. A negative electron affinity photocathode includes a semiconductor, usually a III-V compound such as gallium arsenide, heavily p-doped so as to raise the conduction band relative to the Fermi level. The semiconductor surface is coated with an activation layer, a few monolayers thick which lowers the work function so that the conduction band in the bulk is above the vacuum level, a condition of negative electron affinity. When electrons are excited by light energy into the conduction band within a diffusion length (typically a few micrometers) of the surface, many of them will diffuse to the surface where they will have a high probability of escaping into the vacuum, as described by A. Herrera-Gomez and W. E. Spicer in SPIE Vol. 2022, Photodetectors and Power Meters (1993), p. 51.
Most previous work on NEA photocathode electron sources has been performed on reflection mode photocathodes. In this mode, a light beam is incident on the emitting surface of the cathode, and, due to the need for electron optics above the cathode surface for accelerating and focusing the emitted electrons, the final lens used to focus the light cannot be positioned close to the photocathode. This necessitates low numerical aperture imaging, which results in large spot sizes and emission areas, typically at least a few tens of micrometers.
C. Sanford, as described in Doctoral Thesis, Cornell University Dept. of Electrical Engineering (1990), constructed free-standing membrane transmission mode photocathodes, as well as NEA photocathodes with integrated gate electrodes. In the former case, the final lens of the light optics was over 10 centimeters from the cathode. The emission area was estimated to be approximately 15 micrometers in diameter. In the second case, a flood illumination was used, and the emitting areas were greater than 100 micrometers in diameter.
A scanned electron beam system, including an electron beam source using an NEA activated photoemitter as the cathode, is disclosed in U.S. Pat. No. 4,820,927 issued Apr. 11, 1989 to Langner et al. The disclosed electron source is operated in the transmission mode and is stated to have a typical cathode emitting diameter of 18.75 micrometers. U.S. Pat. No. 4,970,392 issued Nov. 13, 1990 emphasizes the need for high brightness electron sources, but contains no discussion of the importance of emission area. Electron beam sources utilizing photocathodes are also disclosed in U.S. Pat. No. 4,460,831 issued Jul. 17, 1984 to Oettinger et al; U.S. Pat. No. 5,039,862 issued Aug. 13, 1991 to Smith et al; and U.S. Pat. No. 4,906,894 issued Mar. 6, 1990 to Miyawaki et al. The disclosed electron sources and systems do not provide for the achievement of the small spot sizes achievable by the present invention, and thus, for reasons explained below, cannot provide the high brightness necessary for high-performance electron beam instrumentation.
Achieving a small spot size at the emission surface of an NEA photocathode is crucial for several reasons. First, a small spot size may be used to maximize quantum efficiency and source brightness by minimizing the number of electrons trapped in surface states. Second, and most relevant for electron beam lithography, a reduction of the emission spot size allows for a reduction in the system demagnification necessary to achieve 0.1 micrometer feature sizes. Less demagnification allows nearly 100% of the current generated by the source to reach the wafer when 0.1 micrometer diameter emission areas are used, in sharp contrast to current electron beam instrumentation, which typically uses less than 1% of the emitted current. Since all current generated by the source contributes to space charge blurring of the beam, it is important to maximize the fraction of the current delivered to the wafer by restricting the emission area. See, for example, Schneider et al., "Semiconductor on Glass Photocathodes as High Performance Sources for Parallel Electron Beam Lithography", J. Vac. Sci. Technol., Part B, Vol. 14, No. 6, pages 3782-3786, November-December 1996.
A technique for achieving NEA activation on patterned gallium arsenide surfaces is disclosed by E. Santos et al in "Selective Emission of Electrons from Patterned Negative Electron Affinity Cathodes", IEEE Trans. on Electron Devices. Vol. 41, No. 3, March 1994, pages 607-611. The authors describe photocathodes onto which large scale electrode structures were fabricated.