A system for measuring complex dielectric properties of thin films at submillimeter wavelengths using an open hemispherical cavity and a vector network analyzer

Abstract

Quasi-optical (QO) methods of dielectric spectroscopy are well established in the millimeter and submillimeter frequency bands. These methods exploit standing wave structure in the sample produced by a transmitted Gaussian beam to achieve accurate, low-noise measurement of the complex permittivity of the sample [e.g., J. A. Scales and M. Batzle, Appl. Phys. Lett. 88, 062906 (2006); R. N. Clarke and C. B. Rosenberg, J. Phys. E 15, 9 (1982); T. M. Hirovnen, P. Vainikainen, A. Lozowski, and A. V. Raisanen, IEEE Trans. Instrum. Meas. 45, 780 (1996)]. In effect the sample itself becomes a low-Q cavity. On the other hand, for optically thin samples (films of thickness much less than a wavelength) or extremely low loss samples (loss tangents below 10(-5)) the QO approach tends to break down due to loss of signal. In such a case it is useful to put the sample in a high-Q cavity and measure the perturbation of the cavity modes. Provided that the average mode frequency divided by the shift in mode frequency is less than the Q (quality factor) of the mode, then the perturbation should be resolvable. Cavity perturbation techniques are not new, but there are technological difficulties in working in the millimeter/submillimeter wave region. In this paper we will show applications of cavity perturbation to the dielectric characterization of semi-conductor thin films of the type used in the manufacture of photovoltaics in the 100 and 350 GHz range. We measured the complex optical constants of hot-wire chemical deposition grown 1-μm thick amorphous silicon (a-Si:H) film on borosilicate glass substrate. The real part of the refractive index and dielectric constant of the glass-substrate varies from frequency-independent to linearly frequency-dependent. We also see power-law behavior of the frequency-dependent optical conductivity from 316 GHz (9.48 cm(-1)) down to 104 GHz (3.12 cm(-1)).

Sub‐THz complex dielectric constants of smectite clay thin samples with Na+/Ca++ ions

“…The top mirror is hemispherical and connected to two WR-10 waveguide couplers working as a transmitter and a receiver, the lower mirror is flat and smaller than the upper one. We measured the real part of refractive index of ∼1 mm thick glass substrate (borosillicate) to be 1.98 at 310 GHz which is the same as its published value [Rahman et al, 2013]. For details on the cavity and methodology see Rahman et al [2013].…”

“…At Colorado school of Mines (CSM) we use 3 mm wave (or sub‐THz) modalities to extract material properties: (1) a quasioptical system, [ Scales and Batzle , , ; Greeney and Scales , ] to study bulk properties; (2) a near‐field scanning system [ Weiss et al , ], to measure local properties; and (3) the open hemispherical cavity resonator [ Rahman et al , ; Dudorov et al , ], for samples that are too thin or too low loss for quasioptical techniques. In this work, using cavity resonance perturbation (Figure ), we extract the complex dielectric constants of clay‐thin films in 100–165 GHz and investigate electrical properties in the presence of Ca ++ /Na + ions.…”

“…So in this paper we look at monmorillionite using much higher-resolution CW methods based on harmonic multiplication of phase stabilized microwaves, electronically generated; these methods provide very low noise/high dynamic range out to about 1.4 THz at present. cavity resonator [Rahman et al, 2013;Dudorov et al, 2005], for samples that are too thin or too low loss for quasioptical techniques. In this work, using cavity resonance perturbation (Figure 1), we extract the complex dielectric constants of clay-thin films in 100-165 GHz and investigate electrical properties in the presence of Ca ++ /Na + ions.…”

Sub-THz thermally activated-electrical conductivity of CdS thin films

“…Rahman et al 15,16 describe the open cavity method for measuring complex dielectric properties of thin films in detail. In this experiment, we repeated the whole process (of putting the sample into the cavity and taking out) for five times to estimate the measurement uncertainy.…”

“…We use perturbation of an open hemispherical cavity resonator 15 to probe complex dielectric constants of CdS thin films, unannealed and annealed. This method is also able to observe laser-induced photoexcitation measurements.…”