Sensitive determination of layer thickness by waveguide terahertz time-domain spectroscopy

Thin film sensing and layer thickness determination are important for various industrial processes. The terahertz (THz) frequency band (located between 100 GHz and 10 THz, corresponding to wavelengths between 3000 µm and 30 µm in free-space) efficiently penetrates most dielectrics. Thus, THz non-destructive testing can be used to measure the thickness of common plastics and paint layers by evaluation of pulse delay information. We demonstrate the determination of layer thickness on dielectrically coated metal cylinders using terahertz time-domain spectroscopy. A considerable sensitivity increase of up to a factor of 150 is obtained for layers down to 2.5 micron thickness by introducing an experimental geometry based on a 2-cylinder waveguide sensor. This approach uses concepts of adiabatic THz wave compression and the advantages of THz waveguides. The results are compared to measurements on free-standing layers. The experimental layout operates as an exchangeable sensor for a standard THz-TDS system. The film under investigation is wrapped on the surface of one metal cylinder. Together with the opposing metal cylinder (both 63 mm diameter) the THz optics are formed with the coated and the uncoated cylinders mounted in direct contact or with a preset gap defined by spacers. Using the 2-cylinder waveguide sensor, the approaching THz wave is spatially compressed from a wavelength dependent spot-size to a subwavelength line focus (less than 10 µm). After the THz wave propagates through the dielectrically filled gap the metal surfaces of the two facing cylin ders continue to act like a horn, as the expanding wave is guided out of the sensor. The high efficiency coupling is due to the slowly varying beam pattern. In this arrangement the THz wave has a much longer propagation length within the film compared to the single-pass case, thereby giving rise to a considerably increased delay. The geometry of commercial metal cylinders is reproducible, easy to align, and does not require additional optical components. The sensitivity increase given by the ratio of measured stretched delay of the 2-cylinder waveguide sensor to the theoretical single-pass delay reaches up to 150 for the 2.5 µm coating and the cylinders in contact, dropping to 66 for a 24.5 µm coating.