The Millimeter-wave Properties of Superconducting Microstrip Lines

We have developed a novel technique for making high quality measurements of the millimeter-wave properties of superconducting thin-film microstrip transmission lines. Our experimental technique currently covers the 75−130 GHz frequency range. The method is based on standing wave resonances in an open ended transmission line. We obtain information on the characteristic impedance, phase velocity, and loss of the microstrip. Our data for Nb/SiO/Nb lines, taken at 4.2 K and 1.6 K, can be explained by a single set of physical parameters, with a temperature-independent loss tangent of tan δSiO = 1.2±0.3×10 for our latest samples. The amplitude 1/e attenuation length is 30 cm to 40 cm. INTRODUCTION Superconducting microstrip lines are of major importance for tuning elements in SIS mixers. Microstrip lines have also been proposed for use in millimeter and submillimeter direct detection applications, such as antenna coupled bolometer arrays with on-chip band-pass filters. Such highly integrated architectures would allow for a very large number of pixels and would enable novel instruments, such as a multiband imaging polarimeter or an on-chip spectrometer. The transmission losses of superconducting microstrip lines are a key issue for the feasibility of such architectures. The properties of superconducting microstrip lines have been investigated previously. However, the method that we present here provides much higher quality data. EXPERIMENTAL METHOD Our method is based on standing wave resonances in an open ended microstrip stub. The entire circuit is fabricated on a thick silicon substrate (400μm), and the millimeter-wave radiation is coupled onto the chip quasi-optically using a silicon substrate lens. There are two Nb/Al-oxide/Nb SIS junctions on the chip which serve as direct detectors. One of the junctions is connected to an open-ended Nb/SiO/Nb microstrip stub (Figure 1). Having two SIS junctions on the chip allows us to calibrate out changes in the power that is coupled onto the chip from the external signal source. The ratio of the signal from the junction connected to the stub, to the signal from the “power reference” junction, gives us a precise relative power response, whose frequency dependence carries information about the properties of the microstrip stub. We use two harmonic generators as radiation sources. These harmonic generators are driven by two microwave signal generators (10−18 GHz), and produce a series of coherent millimeter wave frequencies Contact information for A. Vayonakis: Email: [email protected] that are exact integer multiples of the input signal frequencies. The two microwave signals differ in frequency by a small value (typically 10 MHz), and this difference is multiplied with harmonic number. The two SIS junctions mix these signals and convert the millimeter waves to signals at various RF beat frequencies. By detecting the output signals from the junctions, that are locked to a specific beat frequency, we select out a unique harmonic number and therefore measure the microstrip response at the corresponding frequency. Figure 2(a) shows representative data taken at two different temperatures.