The development and application of a geometrically-based uniformity criterion is presented for film uniformity control in a radial-flow epitaxy reactor system. In this multi-wafer reactor system, individual wafers rotate on a susceptor in a planetary motion to reduce the effects of reactant depletion on deposition uniformity. The uniformity criterion developed for this system gives an unambiguous criterion for minimizing non-uniformity of any film property and gives physical insight into the reactor operating conditions that most influence uniformity. The uniformity criterion is used to demonstrate run-to-run uniformity control capabilities on a commercial system for SiC CVD.

Introduction ----------

The rapid evolution of material systems and continued tightening of quality control constraints for thin-film manufacturing processes in semiconductor and other (e.g., optical coating) industries pose a number of challenges to equipment design, giving rise to a wide range of reactor systems designed to reduce spatial nonuniformity of deposition thickness, composition, and microstructure. In some manufacturing processes, the use of substrate (wafer) rotation is integral to achieving acceptable film properties across the substrate. In Chemical Vapor Deposition (CVD) systems commonly used for semiconductor processing numerous reactor designs make use of wafer rotation, such as

1) cylindrical reactors, in which gas flows from a shower head over a wafer and exhausts out the bottom, where wafer rotation is used to eliminate any residual angular non-uniformities in the reactor design;

2) in cross-flow reactor designs, where gas flows through a tube or duct-shaped reactor chamber over a wafer and exhausts opposite the gas inlet, in which wafer rotation is used to reduce crossflow deposition non-uniformities and depletion effects in the direction of flow; and

3) in planetary reactors, where gas flows radially outward from a central feed point over the susceptor containing multiple wafers, each of which rotates on its individual axis. This design has the effect of eliminating reactor-induced angular non-uniformity generators through susceptor rotation, and wafer rotation is used to reduce the intrinsic (and completely unavoidable) effect of gas phase reactant decomposition and precursor depletion in the gas phase.

As a material system representative of the challenges to CVD reactor design and uniformity control, consider silicon carbide (SiC), a wide bandgap semiconducting material that has shown tremendous potential for developing advanced electronic devices. SiC posses superior physical properties, such as, large bandgap, high thermal conductivity, and high breakdown voltage. These properties and others have enabled fabrication of new and more efficient communication and radar systems technology.

In recent years, improvements in the growth of SiC by CVD have been studied by both experimental and computational methods. Common precursors used to grow SiC are silane (SiH2) and propane (C3H8), where hydrogen is the carrier gas. Fluid flow models that take into account heat, momentum, and mass transfer effects within CVD reactors have been detailed in several papers. Such models are routinely used to optimize the design and operating parameters to produce films of SiC with a spatially uniform thickness.

Recently, an approach to film uniformity control in planetary reactor systems was proposed by Adomaitis based purely on the geometry of radial flow reactors with this mode of 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 to not only film thickness but any distributed film quality for either process development or in a run-to-run control system.

This paper will present the theoretical underpinnings of the NUPP concept and will present some of the numerical challenges associated with computing the basis for the uniformity producing subspace. An interesting outcome of the theory and numerical implementation is the range of physical implications the construction of this subspace has on the practical design and control of thin film processing systems featuring substrate rotation. Current work on applying the NUPP based control criterion on an industrial SiC reactor system using a model-based run-to-run control methodology will be presented.