Dielectric Loss Reduction In Superconducting Transmission Lines

Abstract

ABSTRACT The dominant loss mechanism is the conductor loss in planar MIC waveguides on low-loss substrates. Superconductors can dramati­ cally reduce the conductor loss, so further reductions in the total loss must come in the dielectric or radiation loss. This paper inves­ tigates the use of ~90 K superconductors to reduce the conductor loss. A rigorous mode-matching procedure is used to analyze two microstrip-like transmission lines for their dielectric loss. Total loss reductions are on the order of 10 to 100 times at 10 GHz when using superconductors and dielectric loss reduction techniques. Reductions at other frequencies below 100 GHz should be similar.1. INTRODUCTIONLosses in waveguides are generated from three sources: conductor loss, dielectric loss, and radiation. For the typical planar transmis­ sion lines used in microwave integrated circuit (MIC) and monolithic microwave integrated circuit (MMIC) applications, the conductor loss is dominant when low-loss substrates are used. Efforts to reduce the total loss have concentrated on conductor loss through two avenues of research: novel structures, and structural modifications to typical waveguides.Novel structures that have low conductor loss include image line [1] dielectric strip waveguide [2], nonradiative waveguide [3], and Micro- slab 14]. These structures reduce the conductor loss by replacing metallic confinement with dielectric confinement, so they typically work best at mid-to high-millimeter- wave frequencies. The primary disadvantage of these waveguides is their non-planar nature and the resulting high cost of high-volume manufacturing.From the manufacturing point-of-view, planar waveguides that can be fabricated using standard MIC technology [5] are highly de­ sirable. Structural modifications generally compatible with pho­ tolithographic production techniques have been shown to result in lower conductor losses. In microstrip, conductor-loss reduction has been shown by reducing the strip metallization thickness [6]. To­ tal loss reductions have been shown for inverted microstrip [7] and suspended-substrate microstrip [8]. Inverting and suspending the substrate induces wider lines, which have lower conductor loss. In­ verted microstrip might allow the use of SrTiO^ substrates, where the high dielectric constant (300-1800) and high dielectric loss tan­ gent (0.001 - 0.02) [9] would not as severely affect the performance since the fields lie primarily in the air gap between the strip and the ground plane. For coplanar waveguide, etching or ion milling to