Exploiting the Optical Response of Matter for Fun (Science) and Profit (Manufacturing)
Spectroscopic optical properties of a material, the complex index of refraction (N = n + ik) or dielectric function (Î µ = Î µ1 + iÎ µ2 ), contain information about order (crystallinity or degree of disorder), chemistry (reflected in infrared active vibrational bonding modes), and electrical transport (when free carrier absorption is observed directly or through correlations with features in Î µ). Spectroscopic ellipsometry measurements are sensitive to optical properties, but also to film thicknesses in multilayer stacks, composition (voids, relative material fractions in mixtures), and interfacial formation between layers. I will discuss ellipsometric measurements from 38 µm to 140 nm (0.037 to 8.85 eV), spanning the infrared to vacuum ultraviolet (FTIR to VUV), and from 2.7 mm to 210 µm (0.45 to 6.3 meV, THz) used to determine electronic structure (near infrared to VUV), vibrational modes (FTIR), and free carrier absorption (near infrared, FTIR, THz). From these non-contacting probes a variety of information is obtained for individual materials and interfaces within multilayer samples, such as thin film devices. Over the near infrared to ultraviolet (currently 1700 to 190 nm or 0.75 to 6.5 eV) ellipsometric spectra are acquired quickly (< 1 second), allowing for monitoring changes occurring during processing or quickly scanning samples for quality control and assessing uniformity. In situ real time spectroscopic ellipsometry (RTSE) monitoring of film growth, mapping spatial uniformity, and ex situ extended spectral range techniques have been applied to study layers in thin film (Si:H, CdTe, CIGS, CH3 NH3 PbI3 ) photovoltaic devices. The ultimate application of these studies are to not only uncover fundamental science related to the materials and their roles in devices but also to enable detection of variations in material during manufacturing"”all using the same tools.