For surface ionization purposes, a study of the work functions of SiC and AIN, both refractory wide-gap semiconductors, has been undertaken. Work function measurements have been performed in the 300-1600-K range using the Shelton retarding field method. Surface cleaning was carried out by heating in uhv to a high temperature using of a cw CO 2 laser. Both n-andp-type 6H SiC single crystals with extreme bulk doping levels were investigated. For each doping type, the work functions have been found to be temperature independent. They exhibit only a slight variation from the n to thep type (4.75 and 4.85 eV, respectively) giving evidence, as in the case of Ge and Si, of Fermi level pinning at the surface by the intrinsic surface states. For an n-type AIN single crystal, the work function, measured in the high temperature range, remains constant as the temperature varies. The value of 5.35 e V obtained for AIN makes it a most attractive material for positive surface ionization applications.
A work function of 2.36 eV and a Richardson constant of 120 A/cm2 K2 have been measured for a hot-pressed porous lanthanum hexaboride rod cathode. It is shown that the LaB6 work function is highly susceptible to poisoning by molybdenum deposited at high temperatures during experiments. A stable work function of about 2.7 eV is then measured. After removal of the molybdenum by atomic iodine flux, the work function of clean LaB6 is restored.
An experimental study of the etching of tungsten with SF6 has been performed in a microwave multipolar plasma using an electron cyclotron resonance excitation with an independent low dc biasing. The anisotropy and etch rate of tungsten have been measured as a function of atomic fluorine concentration in the plasma and compared with silicon characteristics. The etching mechanisms of tungsten are analyzed in light of published data on the fluorine-tungsten interaction, and the results are explained in terms of the diffusion model for plasma etching developed for the Si-F system. Atomic fluorine adsorption on tungsten appears to be monolayerlike, whereas it is of multilayer type on silicon, and associative desorption of WF6 occurs from WF3 and/or WF4 adspecies in nearest-neighbor positions.
A plasma etching reactor is described which associates surface magnetic confinement, microwave discharge, and independently controlled substrate biasing. Preliminary measurements show that the reactor produces reactive plasmas allowing anisotropic fine line etching at low ion energy.
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