INDUSTRY MOCVD
injector. So there is a limited utility in delivering more reactant gases at the outer areas of a full- sized injector.
Figure 1. The chamber has flared profile, which alters the direction of gas flow
radial-horizontal or a vertical gas flow. If the former is employed, wafers are mounted on a rotating device to address the radial depletion of reactants. With the latter design, having an injector diameter the same as the wafer carrier, much of the gas is wasted as it flows over the fluid boundary layer above the wafer carrier and out of the exhaust.
In contrast, our design, US patent application 12/248,167, has a relatively small, multi-port injector mounted in a chamber with a curved top wall that flares out to the full diameter of the wafer carrier – more than 500 mm (see Figure 1). We adopted this design concept based on three observations. Our first of these was that many vertical-flow type reactors require high gas flows to suppress the rotation-induced recirculation zone that tends to form at the outer edge of a rapidly rotating wafer carrier. Much of this gas flows straight to the exhaust.
The VPE GaN-500 that was released in summer 2013 can accommodate 52 2-inch wafers
Our second observation was that gas arriving at the centre of the wafer carrier must migrate outward over the wafer carrier to reach the exhaust – because it can’t flow through it! So substrates towards the edge of the wafer carrier are shielded by this gas from fresh gas delivered vertically downward from outer areas of the
The third observation was that it is quite difficult to uniformly distribute gas flows from relatively small supply tubes through a very large diameter injector. But it is of extreme importance to achieve a flow front of highly-uniform velocity as the gases exit the injection surface. If there are velocity variations across the surface of the injector – as often occurs when flows from the tubes locally ‘punch-through’ areas of the injector – recirculation eddies form, compromising the laminar velocity profile of the reactor. This limits the ability to control the thickness uniformity over the wafer carrier, causes deposition of reaction by-products on the injection surface, and allows reaction by- products to incorporate into the growing layer.
We realized that these challenges could be addressed by using a small-diameter injector in combination with a curved reactor top-wall that eliminated the inefficient volume of a traditional cylindrical chamber. This design takes advantage of the natural tendency of the gas to expand as it flows down and outward over the wafer carrier. Our approach, then, was to develop a highly- symmetric, laminar reactor flow profile that allows reactants, injected through a smaller diameter injector, to reach the outer areas of the wafer carrier. The curved wall profile forces those gases into close proximity to the wafer carrier in order to squeeze out as much of the reactants as possible before they are ‘lost’ into the exhaust.
The result was a unique flared reactor design. Gas enters the chamber vertically, then, guided by the curved walls and rotating wafer carrier below it, becomes increasingly diverted in a more horizontal direction as it approaches the wafer carrier. As the gas progresses through the chamber, the curved top wall forces the gas closer to the wafer carrier. Fluid dynamics modelling results revealed that as the top, curved wall of the chamber approaches the horizontal surface of the wafer carrier, the convergence of the flow path- lines, combined with the increase of the tangential velocity of the wafer carrier with radius, produces differences in the radial and tangential velocity components of the gas when compared to a standard vertical reactor.
These differences indicate that a particular gas molecule will spend a greater amount of time in the reaction zone, allowing a higher proportion of material to be used for epitaxial growth. In addition, the profiled chamber dramatically reduces the proportion of gas flowing straight from the injector to the exhaust without interacting with the wafers.
A key decision for any designer of MOCVD tools is how to introduce the group III alkyl and group V hydride gas streams into the reactor. In our
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www.compoundsemiconductor.net August / September 2013
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