A few weeks ago, Volume 13 of J.A. Woollam’s newsletters was published. It features interesting articles on for example a brand new product for in-line measurements, a status report on project phase II of the development of a THz ellipsometer and many more.
A prototype in-line SE system has been designed to measure films on glass panels as they move down a conveyer line. It consists of an M-2000 mounted near the surface of the moving substrate and can be mounted above or below the substrate. The prototype system shown here is mounted below the substrate for measurements through the glass to the coatings on the reverse side. The ellipsometer can also scan laterally to monitor uniformity over the entire substrate width.
Real-time feedback allows the ellipsometer to maintain the proper working distance within a few micrometers even if there is substrate warpage and vibration.
The J.A. Woollam Company, in collaboration with the University of Nebraska, has completed a phase II research contract devoted to THz ellipsometry. Dr. Craig Herzinger, at the Woollam Company, has developed and constructed a variable-angle, spectroscopic, rotating-element ellipsometer operational in 0.1 to 1.5 THz range. At UNL, Drs. Mathias Schubert and Tino Hofmann were instrumental in developing potential THz applications and demonstrating instrument performance.
The THz-VASE instrument utilizes a backward-wave oscillator (BWO) light source and a wide spectral range Golay cell as the detector. It scans wavelength-by-wavelength across various frequency ranges – operating from 0.11 to 1.45 THz via frequency multipliers added to the base tube (0.11 to 0.165 THz).
As with other Woollam ellipsometers, the THz-VASE includes a compensator to ensure accurate data for the full range of samples. A double polarization-modulation optical configuration provides both
isotropic and anisotropic sample measurements as fast as 1.3 seconds per wavelength. Advanced measurement capabilities include Mueller Matrix (12 of 16 elements) and generalized ellipsometry, as well as polarized transmittance/reflectance intensity.
Because of the long wavelength and highly coherent nature of the light, THz-transparent samples as thick as 6 mm behave as free-standing films, where reflections from the front and back of the sample produce coherent interference oscillations (figures 4 and 5). This is true even for visibly rough substrates, such as single-side polished silicon wafers.
THz applications include basic research of free carrier dynamics in semiconductors, novel sensing devices, and security systems. Not only can THz-VASE be used directly for research in these areas, there is also a need to characterize the optical properties of substrates, films and devices designed for THz applications. It will be exciting to watch as THz characterization develops for new applications.