Hot Gas and Cool Jets

Looking for a way to get around the problems of probing CVD chemistry, Mountziaris' research group in 1990 hit on a novel idea. Taking a page from combustion research, they adapted a concept called counterflow jets for use in a new reactor to study CVD chemistry. First, they developed a computer model. Their two-dimensional model described the gas flow, heat transfer and mass transfer inside the reactor along with the chemistry -- a complicated model that required supercomputing. "To identify the regions of optimal operating conditions took many runs," says Mountziaris, "and in a workstation environment, the development time would make this project impractical. The memory requirements are also much higher than typical workstations can handle."

After supercomputer modeling told the researchers they were on to something promising, they built a prototype. To create a reaction zone that doesn't contact the reactor walls, the reactor uses two colliding jets of gas -- one of them a heated carrier gas, usually hydrogen, the other a cool jet of reactant gas diluted in the carrier. Like two streams of water splashing into each other in mid-air, the two jets collide and form a "stagnation flow pattern." "At the point where they collide," explains Mountziaris, "the hot gas heats the reactant, and reactions occur in a thin zone around the stagnation point. By manipulating the flow rates, we make that reaction zone quite thin, so it's far away from the walls."

By controlling the flow rates, the researchers also limit the residence time so that no secondary reactions occur. They can then measure reactant and product concentrations from fundamental one-step decomposition. With this data, they turn back to their computer model and determine rate constants that describe the reaction under varying conditions. "We can actually obtain the rate of that decomposition," says Mountziaris, "and this is one step towards solving the puzzle of the chemistry underlying the deposition process."

Research with the new reactor to date has focused primarily on tertiary-butyl-arsine, a potential substitute for arsine that -- because it is liquid at room temperature -- is much safer to handle. "We have seen essentially what the model predicts," says Mountziaris, "which indicates that the technique works. We're now developing the methods to fit our observations with the computational model, which will give us the rate constants. This reactor has the potential of becoming a major research tool for chemical kinetics."

Schematic of the counterflow jet reactor. The gases enter the reaction zone from the top and bottom, through two vertical tubes. They exit by flowing radially outward through a horizontal, disk-shaped region.

The temperature and concentration profiles are from computer modeling of the counterflow jet reactor and show what happens in a rectangular cross-section in the center of the reactor. Red represents maximum values of temperature and concentration and blue minimum.

The temperature profile shows the hot zone of the carrier gas merging with the cool jet carrying tertiary-butyl-arsine (TBA). The TBA concentration profile shows diminishing concentration as a reaction occurs in the reaction zone (blue and green area). The concentration of the reaction products is highest in the reaction zone, where they are formed.

"The reaction zone is confined to the middle," explains Mountziaris, "away from the walls." Viewing the results of simulations in this format on a graphic workstation lets the researchers see how the reaction zone varies using different gas flow rates, temperature and pressure. "This allows us to produce the reaction zone exactly where we want it to be."

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