Plasma-Surface Reactions at a Spinning Wall

We report a new method for studying surface reactions and kinetics at moderately high pressures ($<10\mathrm{\text{Torr}}$) in near real time. A cylindrical substrate in a reactor wall is rotated at up to 200 000 rpm, allowing the surface to be periodically exposed to a reactive environment and then analyzed by a triple-differentially pumped mass spectrometer in as little as $150\mu \mathrm{s}$ thereafter. We used this method to study oxygen plasma reactions on anodized aluminum. When the substrate is spun with the plasma on, a large increase in ${\mathrm{O}}_2$ signal at $m/e=32$ is observed with increasing rotation frequency, due to O atoms that impinge and stick on the surface when it is in the plasma, and then recombine over the $\sim 0.7$ to 40 ms period probed by changing the rotation frequency. Simulations of ${\mathrm{O}}_2$ signal versus rotation frequency indicate a wide range of recombination rate constants, ascribed to a range of O-binding energies.

Figure 1

Side view of the hollow chamber wall containing the rotating cylindrical substrate and conical skimmers.

Figure 2

Chopper-open (open triangles and circles) and chopper-closed (solid triangles and circles) mass spectrometer signals at $m/e=32\mathrm{amu}$ as a function of substrate rotation speed at a pressure of 5 mTorr ${\mathrm{O}}_2$ for: (a) plasma off, (b) plasma on (600 W), no grids, (c) $D_b^{\mathrm{on}}$, the net ${\mathrm{O}}_2$ desorption signal.

Figure 3

$D_b^{\mathrm{on}}$ for ${\mathrm{O}}_2$ vs reaction time (reciprocal of twice the rotation frequency) for a 5 mTorr ${\mathrm{O}}_2$ plasma, at 600 and 100 W of plasma power. No grids were present between the sample and the plasma.

Figure 4

Simulations of $D_b^{\mathrm{on}}$ vs reaction time (reciprocal of twice the rotation frequency) at $\theta =\pi$. Lines with symbols: simple second-order recombination mechanism, $s=1$, $f=1\times {10}^{17}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$, $[S{]}_0=1\times {10}^{15}{\mathrm{cm}}^{-2}$. Thick line without symbols: multisite model [18] (multiplied by 10 before plotting).