Dielectric loss of low-loss resins in the millimeter-wave region

Abstract

Low-loss resins are widely used as insulating materials in millimeter-wave (30–300 GHz) devices. The dielectric loss tangent of such resins is of significant interest, as their physical properties greatly affect device performance. For highly accurate measurements (on the order of 10^–4) in this frequency range, the resonator method combined with a vector network analyzer (VNA) is typically employed. However, this method provides values only at discrete resonant frequencies, requiring multiple measurements to obtain spectral information.
In contrast, terahertz time-domain spectroscopy (THz-TDS) can acquire broadband spectral data in a single measurement, enabling reduced measurement time. In this study, we developed a high-precision THz-TDS system optimized for characterizing low-loss resins in the millimeter-wave region.
Our system is based on a conventional THz-TDS configuration with a parallel THz beam. A triggering sensor is integrated into the optical delay line to synchronize data acquisition. By sweeping the delay stage over the required time window in a single motion using the trigger signal, we significantly reduced the scanning time. The shortened acquisition time also minimizes system fluctuations. The developed system achieves a 200-ps time window measurement in just six seconds. Additionally, we employed a spiral antenna with improved efficiency in the millimeter-wave band, enhancing the dynamic range to approximately 8.5 orders of magnitude.
We present the dielectric loss tangent of a cycloolefin polymer measured using the developed THz-TDS system. Despite some residual fluctuations, we successfully measured a dielectric loss tangent on the order of 10^–4 in the millimeter-wave band. This result is in good agreement with previously reported values using the Fabry–Pérot method with a VNA, which showed an average of 5×10^–4 in the 220–325 GHz range. To further improve measurement accuracy, it is necessary to reduce the remaining fluctuations. Accordingly, future work will focus on enhancing system stability and performance.
In summary, we developed a high-precision THz-TDS system and demonstrated its capability to measure the dielectric loss tangent of low-loss resins in the millimeter-wave region, with results consistent with existing literature. Further improvements are planned to enhance measurement accuracy.