Real-Time Monitoring of GaAs(100) Etching by Surface Photoabsorption
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151 Mat. Res. Soc. Symp. Proc. Vol. 406 ©1996 Materials Research Society
of the current study is to demonstrate the utility of SPA for monitoring the relative Ga- or Asrichness of the surface as the etching conditions are changed dynamically to induce changes in the surface stoichiometry. In these studies, the changes in surface composition have also been determined via mass spectrometric detection of the relative fluxes of Ga- and As-containing products. The direct correlation between the SPA results and the integrated difference in the etching product fluxes suggests that changes in surface composition, as opposed to changes in surface structure, are the dominant contribution to the measured changes of the SPA signal as a function of etching conditions. EXPERIMENT The experimental arrangement is shown schematically in Figure 1. A supersonic molecular beam of HC1 (beam energy = 0.9 eV) was formed by expanding a 5% HC1/95% He gas mixture through a graphite nozzle with a nozzle diameter of 0.127 mm. The beam was collimated with a skimmer and then chopped with a mechanical shutter to produce a pulsed flux of HCl at the GaAs surface. In these studies, the beam was chopped to produce a train of square wave pulses with equal on/off times of 50 sec. This time scale was chosen to allow the surface etching reaction to decay to zero and then return to steady state during each period of the modulation.
(a)
Side View
Differentially Pumped Supersonic Molecular Beam Source
(b)
ITop View
Beam Source Photodiode L
Sample
Len
fl~Y:Y~~ 7P
Mechanical Shutter
Molecular
[011 II
Sample Polarizer
~650
TrpyDfental TriplYumpDifferentiallYss
Spectrometer showing Ionization Region
Lens
Figure 1. a) Schematic of the experimental set-up indicating the important geometrical parameters and optical elements in the SPA experiment. (The mass spectrometer has been omitted for clarity.) b) Orthogonal view of the experimental set-up showing the molecular beam scattering geometry. (The optical elements used in SPA have been omitted for clarity.) The HC1 beam was incident onto the GaAs(100) surface at 650 from the surface normal (beam diameter = -1 cm along the major axis of the ellipse on the sample surface) and the etching products were detected via a triply differentially-pumped mass spectrometer located in the specular scattering plane and oriented at 250 from the surface normal towards specular reflection. Prior studies have shown that the product distribution is independent of the angle of detection, so the results at 250 are indicative of the angle-integrated product yield from the surface [9]. The relative product fluxes have been calibrated according the procedure in Ref.
[9].
152
Prior to insertion into the vacuum chamber, the wafer was degreased in chlorofrom, immersed in a 1:1 NH4OH:H20 solution for 20 min, and rinsed with copious amounts of deionized (18.2 MQ)-cm) water. It was then immersed in 4:1:1 H2 SO4 :H2 02:H20 solution for 10 min., again rinsed with de-ionized water, and dried under nitrogen. The sample was m
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