Advantages of the "optical cavity substrate" for real time infrared spectroscopy of plasma-surface interactions

Infrared reflectance spectroscopy can be used to analyze surface reactions during the plasma processing of thin films via the characteristic absorption modes of chemical bonds. However, infrared absorption is a relatively weak effect: to detect a submonolayer quantity of adsorbed or near-surface species in real time, the optical sensitivity must be enhanced in order to produce a measurable reflectance change. This article presents the use of a optically resonant dielectric stack, called an optical cavity substrate, to provide this sensitivity enhancement, and compares this approach quantitatively to other substrate/beam combinations. The optical cavity substrate provides several advantages: (i) a large signal enhancement for both bulk and surface vibrational modes, which is nearly independent of the film refractive index n and thickness up to ∼20 nm, (ii) a large signal enhancement for interfaces buried under thick films, and (iii) a relatively constant baseline signal, which simplifies the data analysis. To demonstrate the power of the optical cavity substrate, we analyze the growth of hydrogenated amorphous silicon in real time.