Title: Sputtered thin film diffusion barriers for ohmic contact to silicon by platinum electrodes
Description: Microstructure plays a fundamental role in thin film technology, especially regarding the phase stability of a multi-layer ohmic contact stack. Preventing the solid-state reaction between layers is possible if a suitable diffusion barrier material can be inserted between them that is compatible with the manufacturing process, typically involving deposition by magnetron sputtering and patterning by selective chemical etching. However, the microstructural characteristics of sputtered films facilitate interdiffusion at temperatures far below the scale at which bulk materials begin to react. Diffusivities along grain boundaries and other extended defects dominate at lower temperatures, and in thin films, such defects are typically abundant. A polycrystalline barrier layer fails to prevent grain boundary diffusion and the onset of unwanted reactions because the barrier layer's microstructure, as well as its thermodynamic compatibility with the stack, determines its effectiveness. This dissertation discusses the development and implementation of a sputter deposition process for improving high temperature phase stability in a platinum electrode contact metallization to p-doped silicon.

Experiments on stacks with a polycrystalline Ti diffusion barrier scheme yield proof that this barrier fails in non-oxidizing ambient when inhomogeneous eruptions of PtSi form at pinhole centers of rapid interdiffusion at 525 ◦C. Fabrication and testing of stacks with two new barrier schemes follows under competing microstructural engineering strategies: (1) Addition of a small layer of C adjacent to the Ti barrier could clog its grain boundaries and suppress diffusion along them, and (2) exchanging Ti with amorphous Ta-Si-N might curb interdiffusion by removing the presence of grain boundaries from the fundamental structure of the barrier material. The addition of C was not found to improve the phase stability of the stack, but the Ta-Si-N barrier inhibits interdiffusion of the electrode and substrate up to 700 ◦C in Ar-H2 ambient. However, when interdiffusion is suppressed in the stack, additional phenomena limit phase stability. Morphological degradation in both the Pt and PtSi layers occurs.

More critically, catastrophic delamination of the Pt electrode ensues in oxidizing ambient. An adhesion layer of Ti mitigates the delamination problem, but bubbles form between the barrier and electrode layers. The delamination problem associated with Ta-Si-N cannot be suppressed due to the inherently strong oxidative kinetics of the material.