Evaluation of spectroscopic ellipsometry from ultraviolet–visible–(near-infrared) to terahertz regions for the analysis of silicon photodiodes

Spectroscopic ellipsometry (SE) has been evaluated for the analysis of silicon photodiodes (PDs), employing instruments spanning the spectral regions of the ultraviolet-to-near-infrared (UV–Vis–NIR: 0.75–5.90 eV), near-infrared-to-mid-infrared (NIR–Mid-IR: 0.45–0.75 eV), and terahertz (THz: 0.10–1.02 THz). In this study, two PDs were chosen for their significantly different spectral responses, and the structural and optical characteristics of their components were determined by SE performed at multiple angles of incidence without contacting the devices. Applying a model of a uniform p+-layer at the front of the PD, analyses of SE data acquired individually over the UV–Vis–NIR SE and NIR–Mid-IR regions yield layer thicknesses and hole concentrations that are in good agreement. Differences in these characteristics can be attributed to variations in the shapes of the doping profiles for the two PDs and the depth sensitivities of the spectral regions. The most informative spectral region is the THz, which provides p+-layer hole concentration profiles and related hole mobilities, as well as the underlying lightly doped bulk Si thicknesses, electron concentrations, and mobilities. The hole concentration profiles from THz SE analysis identify the thickness from the UV–Vis–NIR SE analysis as the depth at which the concentration drops by factors of ∼(2–5) × 10−3. The p+-layer hole concentrations from the UV–Vis–NIR are consistent with depth-averaged values from THz SE to within ∼20% or better for these PDs. This comparison demonstrates that SE, even over the standard UV–Vis–NIR range, is applicable for the non-invasive evaluation of PD structures in manufacturing environments.