In our work, the plasma of interest is an Electron Cyclotron Resonance (ECR) microwave plasma.
The plasma appears cylindrical due to the presence of a strong magnetic field oriented perpendicular to the growth substrate. The magnetic field acts confine the volume of the plasma by constraining the electrons to circular orbits. When the frequency of (or time for) the electron orbits is matched to the microwave a resonance condition is met, and power can be efficiently coupled into the plasma even at pressures of less than one-hundred millienth of an atmosphere.
The intense emission of light from the plasma makes normal spectroscopic determinations of the concentrations of chemical species difficult, at best. Consequently, we have chosen to develop a mass spectrometric approach to the problem.
Supersonic Pulse, Plasma Sampling
In supersonic pulse, plasma sampling, a short pulse of high pressure noble gas (typically argon) is released into the virtual vacuum of the ECR plasma chamber. As the noble gas pulse expands, it cools condensing any atomic or molecular species in its path into the supersonic pulse.
The Mass Spectometric Apparatus
From left to right, a pulse of gas is released from the piezo-electric actuated pulse valve(A). The gas then expands through the region between the ECR Plasma source(B) and growth substrate(C). After expansion, the pulse passes throught two stages of differential pumping (D&E) and emerges into the UHV Chamber(F). Once in this chamber, the composition of the pulse is measured in the quadrupole mass analyser(G).
Evidence for Supersonic Expansion of the Pulse
Simultaneous arrival of all species, regardless of mass, and a flight time 30% shorter than would be expected of thermal argon atoms, indicates that a supersonic expasion of the argon gas pulse has taken place. Simple conservation of energy arguments dictate that for the pulse to travel faster (have more energy in translation), it must lose an equal amount of energy in some other way. This loss of energy is observed to occur as a cooling of the gas within the pulse. Species present in the path of the rapidly cooling pulse become incorporated into the pulse through numerous near-thermal energy collisions with argon atoms. Experimental observation of significant signals of argon dimers and trimers, even after ionization with 80 eV electrons, suggest that the incorporated species may actually be condensed in large argon clusters.
Under normal (either molecular or viscous) gas flow conditions, molecules of different masses traverse the distance to the quadrupole mass analyser with transit times inversely proportional to the square root of the mass. This is the principle of a normal time-of-flight mass sepctometry, and is clearly not applicable to our experiment.
Uniform Sensitivity is Observed for All Hydrocarbons
By flowing gas through the plasma source with the microwave power off, the probability of incorporation into the pulse may be measured experimentally. The data clearly indicate a linear relationship between the pressure of the hydrocarbon in the plasma chamber and the signal measured. Further, the similar slopes of each curve indicate a uniform sensitivity, with the slight differences in slope most likely due to ionization cross-sections in the quadrupole.
Direct Observation of Chemistry in a Plasma
The "apparent" cracking patterns of a 2%-ethylene in hydrogen gas mixture as the microwave power (and consequently the plasma) is turned off (red) and on (yellow) demonstrates the significant changes that are observed between simple gas flows and plasmas. Having already measured the experimental cracking pattern for each individual hydrocarbon, one may deduce the concentration of each species in the plasma by fitting the "apparent" cracking pattern of the plasma to a sum of cracking patterns for the individual hydrocarbons. For more information of the analysis of diamond plasmas click on Diamond Growth Below
Chemistry of Diamond Film Growth
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