Design and Analysis of a Broadband SIS-Mixer for the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory

In this thesis the design and analysis of SIS-mixers (SIS: superconductor-isolator-superconductor) for the use as low-noise and broad-band detectors in the submm-region for astronomical observations is described. The SIS-mixers have been developed for the frequency band 2 (636-802 GHz) of the Heterodyne Instrument for the Far Infrared (HIFI) on ESA's space-observatory Herschel. The required performance baseline has been defined by the HIFI-consortium in terms of the estimated noise temperature contribution of the mixer to be 110 K at 636 GHz and 150 K at 802 GHz. In the frequency region between 80 GHz and 900 GHz Heterodyne SIS mixers are established as the best devices for low noise mixing with a high spectral resolution (mixing of the signal radiation with local oscillator, LO). For the HIFI band 2 mixers, the technology of Nb/Al-Al2O3/Nb-junctions has been used with junction areas of 0.5-1.0 um2, a target value for the current density jc of 15 kA/cm2 and a gap-voltage of 2.75-2.77 mV. For the compensation of the SIS-junction's intrinsic capacitance and the optimized power coupling to the tunnel junction, two types of matching circuits have been theoretically modelled and experimentally studied: (1) three-step transformer single junction devices and (2) double-junction devices. This has been done for two material combinations: for an all-superconductive micro strip NbTiN/SiO2/Nb, and for the superconductor/normal-conductor combination NbTiN/SiO2/Al. A numerical tool has been developed and applied in order to define the optimum set of fabrication parameters and strip line geometry for the lithography masks. From a large number of fabricated devices the SIS-junctions with the optimum noise performance have been selected to be integrated in the HIFI instrument as flight-mixers. The selection and characterization of devices has been done based on the DC I-V-curves, heterodyne hot-cold characterization and spectral analysis in a Fourier-transform-spectrometer at 4 K. For a comparison of the noise data with the HIFI requirements, the measurement setup has been characterized and its noise contributions have been subtracted. The single-junction devices with Nb top-wiring have shown the optimum noise performance. This is in accordance with the presented simulations. The optimum measured mixer noise contribution for a SIS-mixer has been determined to depend on the DC-quality Q; a higher Q is a measure for a low leakage current and an indication for a low device shot-noise. The measured noise performance of the flight-mixers is compliant with the HIFI baseline over 80% of the band, as has been confirmed in the HIFI integration tests. For the fabricated SIS-wafers a good absolute accuracy of the junction area AJ (below 5%) and a large current density jc = 13−14 kA/cm2 have been identified to be crucial for the optimum noise performance. AJ via the intrinsic capacitance and the normal resistance of the tunnel junction mainly determines the resonance frequency of the whole device. For a more complete theoretical analysis of a SIS-mixer a numerical tool has been developed which allows the calculation of the noise- and gain band-pass based on the three-frequency approximation of the Quantum Theory of Mixing. This tool has been used to verify the power design calculation, and to evaluate the optimum fabrication parameters for future designs of SIS-devices. The effect of four fabrication parameters (NbTiN-quality, gap-voltage VGap, current density jc and DC-quality Q) on the mixer performance has been evaluated. Based on the calculated data jc and Q have been identified as the most critical fabrication parameters for the mixer performance. A second result of the simulation is a significant reduction in mixer noise for an increased DC-quality up to Q = 8 − 10, but less improvement for a larger Q. And finally, for large micro-strip losses (30-40%) as for NbTiN/SiO2/Nb-microstrips around 800 GHz an increase in current density jc has resulted in a significant reduction of the calculated mixer noise contribution. This suggests a strong potential to reduce the noise temperatures in this frequency region with AlN-barriers, which are currently developed at KOSMA and promise current densities around and above 30 kA/cm2. The absolute calculated noise data deviate with a factor 2 from the measured data. But the calculated results show a good agreement with the measured noise data regarding the frequency-, bias-voltage and LO-power dependence. Thus the applied model is expected to give reliable estimates for relative variations in noise performance