T. H. P. Chang

Publications (2)2.68 Total impact

• Article: Minimum emission current of liquid metal ion sources

  J. C. Beckman, T. H. P. Chang, A. Wagner, R. F. W. Pease
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  ABSTRACT: The inability of liquid metal ion sources (LMIS) to operate at low dc emission currents limits their performance. We briefly describe a model that explains why LMIS have a minimum dc emission current (I_min) and also predicts I_min as a function of the temperature and the properties of the liquid metal. The model predicts that I_min=217+0.744T( nA ) for gallium LMIS, where T is the temperature (K). Measurements of I_min for gallium LMIS between 30 and 890 °C are in reasonable agreement with the model. A better fit to this data, however, is given by I_min =1187  exp (-0.026/kT)( nA ) where k is Boltzmann’s constant (eV/K). Below 30 °C, I_min drops precipitously—values as low as 380 nA have been measured at temperatures as low as 25.8 °C. This drop is attributed to a supercooling effect that is not accounted for in the model. I_min is also calculated for 17 pure-elemental LMIS at their melting points, and found to vary from 10 nA for mercury to 1.0 μA for aluminum. I_min is measured to be much lower for bismuth LMIS than for gallium LMIS, as predicted by the model, although difficulties with the bismuth LMIS have allowed only an upper limit of I_min≤77  nA to be measured. The model also suggests possibilities for improving ion sources by reducing or eliminating I_min. © 1997 American Vacuum Society.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 12/1997; · 1.34 Impact Factor
• Article: Energy spread in liquid metal ion sources at low currents

  J. C. Beckman, T. H. P. Chang, A. Wagner, R. F. W. Pease
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  ABSTRACT: In previous work, attempts to lower the energy spread in liquid metal ion sources (LMIS) by resorting to low current operation have rarely been successful. We have found the energy spread of gallium LMIS to be ≊4.5 eV full width at half‐maximum for emission currents from 0.45 μA down to 13 nA. Our experimental results suggest that this occurs because emission ≪0.45 μA is formed by pulses of height 0.45 μA and an appropriate duty cycle. Several observations support this hypothesis: (1) For sources that had a minimum current, below which they could not be made to operate, the minimum current was always 0.45 μA at room temperature, regardless of apex radius or surface roughness. (2) Emission ≤0.45 μA from blunt sources at room temperature always showed pulses of height 0.45 μA, and a clear transition to dc at 0.45 μA. The pulse height and transition current rose with source temperature. (3) Emission ≤0.45 μA from sharp sources always showed pulsing from the lowest currents up to the current at which the amplifier bandwidth of 1 MHz was exceeded. (4) The angular intensity of emission on axis was in constant proportion to emission current up to 0.45 μA, and in declining proportion at higher currents. © 1996 American Vacuum Society
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 12/1996; · 1.34 Impact Factor