Comparison and Optimization of Dispersion, and Losses of Planar Waveguides on Benzocyclobutene (Bcb) at THZ Frequencies: Coplanar Waveguide (Cpw), Microstrip, Stripline and Slotline

Abstract

Abstract-This paper proposes an investigation in the terahertz (THz) frequency range of the dispersion and an individual quantitative treatment of the losses of the most classical microwave waveguides (coplanar, slotline, microstrip and stripline) numerically led in three dimensions (3D). An original strategy has been used to quantify radiation losses associated with leaky modes. A very low THz permittivity polymer (benzocyclobutene (BCB)) was used as a very convenient substrate to be easily grafted as a THz environment of integrated passive or/and active devices. Direct comparisons of the losses and the dispersion have been performed following two criteria: a constant characteristic impedance Z c fixed at 100 Ω and a constant effective width W eff fixed at 30 µm. The best waveguides are microstrip (α T = 2.52 dB/mm for Z c = 100 Ω and for W/H = 35/50 µm (with W the strip width and H the substrate height) and α T = 2.29 dB/mm for W eff = 30 µm at 1 THz with H = 30 µm) and stripline (with quasi-null radiation losses and the best quality factor Q T = 63 for Z c = 100 Ω). The large dispersion and radiation losses of the slotline (SL) can be reduced with a thick BCB encapsulation to enhance the THz signal. The coplanar waveguide (CPW) remains in a medium position. Besides the parasitic mode (SL) and low Q T problems due to mainly ohmic losses, its major advantage is its planar geometry allowing to an easy circuit integration with THz sources, amplifiers and detectors based on semiconductor. Consequently, these THz studies on BCB microwave standard waveguides open to various perspectives to carry out a broad panel of integrated THz circuits.

Analysis and design of efficient passive components for the millimeter-wave and THz bands

“…Although only simulations are reported, conclusions give a useful general view of advantages and drawbacks of these waveguides. The stripline, which is the earliest transmission line, presents the lowest dispersion, and intermedium losses (theoretically 6 dB/mm at f = 1 THz [401]) compared with the other three. However, it requires strong symmetry and thereby presents difficulties in the circuit design, incorporation of elements, and bias circuitry.…”

“…At THz frequencies, however, only microstrip or CPW designs are found by several reasons. A deep analysis of these four waveguides for the THz regime can be found in [401]. Although only simulations are reported, conclusions give a useful general view of advantages and drawbacks of these waveguides.…”

“…This implies high confinement of the field, having, therefore, both, large conductor losses and large dielectric absorption. Benzocylobutene (BCB), a low permittivity, relative low-loss dielectric at THz frequencies (ε r = 2.42, tan δ = 0.007 [402]), is commonly used [401], [403], [159], also because of its easy grafting. Despite of the high-loss feature of the microstrip at THz frequencies, its easy integration and compactness has reserved some uses to it in the recent THz research.…”

“…Thus, dielectric losses are reduced and radiation losses are increased. Analysis and experiments of the last decades bibliography (see for instance [401], [415]- [409]) indicate that the second effect is much more influential at THz frequencies, despite of the high dielectric absorption. Furthermore, to achieve some confinement CPW requires the distance w (see Fig.…”

“…More recently, some works have gone again into deep analysis and fruitful experiments using BCB dielectric [401], [418], [409]. It is highlighted that planar waveguides characterization that fit the observed experimental results is hard at THz frequencies.…”