In-situ Optical Characterization of Noble Metal Thin Film Deposition and Development of a High-Performance Plasmonic Sensor

Abstract

With ever-growing industry demand for more uniform and conductive metal or metal oxide thin film coatings, an improvement to the techniques that produce these materials must be done in parallel. Two techniques that have shown great utility in this regard are referred to as chemical vapour deposition (CVD) and atomic layer deposition (ALD). While both techniques offer highly modular reactor designs and robust precursor-substrate chemistries (e.g., the benchmark Al2O3 process), many processes result in non-uniformity issues, ultimately leading to poor device performance. One such challenging process has been the fabrication of high-purity and uniform noble metal thin films such as gold. Equally as challenging is the ability to characterize metal CVD and ALD processes when the metallic film is just beginning to nucleate. This “nucleation delay”, or induction process,is especially common in metal ALD processes and techniques such as ellipsometry lack sensitivity in the low-cycle regime of ALD (or pulsed CVD) processes. The present work addressed in this thesis introduces, for the first time, the use of tilted fiber Bragg grating (TFBG) sensors for accurate, real-time, and in-situ characterization of CVD and ALD processes for noble metals, but with a particular focus on gold due to its desirable optical and plasmonic properties. Through the use of orthogonally-polarized transverse electric (TE) and transverse magnetic (TM) resonance modes imposed by a boundary condition at the cladding-metal interface of the optical fiber, polarization-dependent resonances excited by the TFBG are easily decoupled. It was found that for ultrathin thicknesses of gold films from CVD (~6-65 nm), the anisotropic property of these films made it non-trivial to characterize their effective optical properties such as the real component of the permittivity. Nevertheless, the TFBG introduces a new sensing platform to the ALD and CVD community for extremely sensitive in-situ process monitoring. We later also demonstrate thin film growth at low (<10 cycle) numbers for the well-known Al2O3 thermal ALD process, as well as the plasma-enhanced gold ALD process. Finally, the use of ALD-grown gold coatings has been employed for the development of a plasmonic TFBG sensor with ultimate refractometric sensitivity (~550 nm/RIU).