Energy spreading in the hydrogen field ionization source

Abstract

The application of field ionization as a tool for routine submicron fabrication will depend critically upon the characteristics of the source; the angular intensity and the energy spread of the ions. The chromatic aberration coefficient of the ion optical system, the angular aperture, and the energy spread combine to set the limit of spot size and current density for very-high-resolution probes with sources of energy widths even as low as 1 eV. It is known that the various molecular species produced by hydrogen field ionization are indentifiable by the energy distribution, and because the mass of the ion will determine the particle range in resists at any given energy, energy measurements carry further importance in the development of the probe-forming system. In order to characterize the source-energy widths and mass species as a function of beam current, angular intensity, and surface characteristics of the emitter tip, we have designed and operated a unitized, positionable intermediate image filter lens with a demonstrated resolution of 0.1 eV at 3.5 kV and acceptance half-angles of less than 9 mrad. At angular intensities less than 10 μA/sr, 90% of the field ions from bright site emission of fields of 1.5 V/Å occur within an energy width of 1.2–1.9 eV. This narrow spread shows that only H2+ is being produced. The beam current and angular intensity from a single bright site can be increased by raising the source gas pressure and the applied field (5% increase in voltage). That energy broadening occurs at high intensities is demonstrated by the fact that at 3.5 μA/sr [0.5 nA emitted through 50 μm aperture subtending a half-angle (ϑ1/2) of 0.0067 rad], 90% of ions appear within 1.9 eV (0.94 eV FWHM), while at 18 μA/sr (2.5 nA, ϑ1/2 of 0.0067 rad), 90% of ions appear within 2.2 eV (0.98 eV FWHM) where a tail develops on the low-energy side causing a broadening of the energy distribution. The impact of field emitter surface configuration on source angular intensity and reliability are discussed.