Dept. of Applied Physics
Oregon Graduate Center
This work was undertaken to determine the feasibility of using a field ionization (FI) source to produce fine focus ion beams. Operating parameters for a FI source have been measured and a source sensitivity of ~ 5 x 10[superscript-5] A sr[superscript -1] torr[superscript -1] was found for both H[subscript 2] and Ar at 77 K. The source is gas phase, differentially pumped with typical operating pressures of 1 - 30 x 10[superscript -3] torr at 77 K, resulting in a maximum source brightness â 10[superscript 8] A cm[superscript -2] sr[superscript -1] and angular intensity of â 10[superscript -6] A sr[superscript -1] with beam energies of 10 - 20 keV. Angular distributions were measured and found to be uniform near Î¸ = 0Â°, with the beam confined to Â±20Â°. A scanning ion microscope (SIM) was built to further evaluate the source. The SIM has been operated with currents on the specimen of 10[superscript -11] â 10[superscript -10 amperes in the secondary electron mode with contrast provided primarily by the sec (Î¸) dependence of the secondary electron yield, where Î¸ is the angle between the beam and the specimen normal. Secondary electrons are detected and amplified with a channeltron multiplier and images generated as with a conventional SEM. All electrostatic optics are used in a doublet arrangement, and with this configuration current is independent of working distance which is 3 - 4 cm. A current of 5 x 10[superscript -11] A was focused into a spot of â 6500 Ã with H[subscript 2] gas, the resolution being limited by chromatic aberration caused by the â 4 eV energy spread of the two component (H[superscript +] H[superscript +, subscript 2]) beam. Signal to noise ratio measurements on the source made at the specimen position show that the bulk of the noise power spectrum falls below f = 30 Hz.
Orloff, Jonathan Harris, "Scanning ion microscope with a field ionization source" (1976). Scholar Archive. 30.