(375b) Application of a Geometrically-Based Uniformity Criterion for Film Uniformity Optimization in a Planetary Gallium Nitride Cvd System

Gallium Nitride (GaN) growth chemistry can be visualized as consisting of two competing reaction routes. The upper route is more commonly referred to as the adduct formation pathway, whereas, the lower route refers to the thermal decomposition pathway of TMG. Each pathway is responsible for producing an array of chemical species that may eventually participate in GaN deposition. The primary gas phase reaction is the spontaneous interaction between commonly used precursors, trimethylgallium and ammonia, to form stable Lewis acid-Lewis base adducts. Adduct formation is a ubiquitous problem during MOVPE of GaN and has been widely studied. Upon formation, these adducts may condense on cold surfaces inside the reactor system. For this reason, the formation of these adducts is believed to degrade film quality, uniformity, and consume the feed stream of organometallic sources.

Consequently, numerous research groups have designed reactor systems, in particular gas delivery systems, with the intent to minimize precursor interactions. The most common approach is to use separate injectors to reduce any premature mixing of the precursors. Reactor systems of this type have been developed by SUNY/Sandia/Thomas Swann researchers to illustrate a connection between gas phase reactions and film-thickness uniformity. It should be noted that while these designs can suppress reactions in the gas delivery system, complete mixing of the precursors must take place close to the wafer surface to achieve uniform film thickness. Hence, these studies and others reinforce the critical role chemistry holds in designing efficient MOVPE reactors.

In conjunction with reactor design, numerous studies have focused on developing simulation tools aimed towards optimizing film-thickness uniformity. Fluid flow models that take into account heat, momentum, and mass transfer effects within both horizontal and vertical MOVPE reactors have been detailed in several papers. Many of these models incorporate large sets of chemical reactions and the model predictions ultimately are tied to the specific reactions chosen by the research group. Such models are routinely used to optimize the design and operating parameters to produce thin films of GaN with a spatially uniform thickness. A novel approach to film uniformity control is developed for planetary CVD based purely on the geometry of radial flow reactors with planetary wafer rotation. In this approach, a sequence of stalled-wafer (non-rotating) deposition profiles are identified that, when rotated, produce perfectly uniform films. Then, a deposition profile, produced either by simulation or by an actual CVD process is projected onto this sequence of uniformity-producing profiles to compute the ?Nearest Uniformity Producing Profile? (NUPP), which under rotation would produce a uniform film. Thus, it becomes clear that one would want to drive the current profile to the?nearest? profile, NUPP, giving an unambiguous optimization criterion. Most importantly, the NUPP provides the process engineer with physical insight on how reactor operating conditions should be modified to drive the current profile towards the NUPP to improve uniformity. This technique is extremely powerful because it can be applied not only to film thickness but any distributed film quality for either process development or in a run-to-run control system.

In this paper, the NUPP approach is applied to a gallium nitride radial-flow chemical vapor deposition system with planetary wafer rotation. The results reveal three key points: (1) the influence of reactor geometry on gallium nitride deposition chemistry; (2) controllability of the competing nature (upper vs. lower pathways) of gallium nitride chemistry, and (3) utilization of the NUPP uniformity criterion to optimize deposition uniformity by adjusting reactor operating conditions, in particular, susceptor temperature.