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Development of Integrated Superconducting Balanced Mixers for THz Focal Plane Arrays

Sina Fathi

Published Nov 9, 2018• 0 citations
OpenAlex

Abstract

This thesis work covers the development of integrated superconducting balanced mixers for heterodyne (resolution> 10^7) THz focal plane arrays. The aim was to develop on chip integrated balanced superconductor-insulator-superconductor (SIS) mixers at 800-1100 GHz and in addition at the Intermediate Frequency (IF) 4-12 GHz integrated \n180° IF hybrids both for the CHAI (CCAT Heterodyne Array Instrument) receiver for the Cerro Chajnantor Atacama Telescope (CCAT) observatory. A planar 180º hybrid ring \ncoupler, two separate slotline antennas, two separate slotline to CPW transitions to employ for the 800-1100 GHz balanced SIS mixer, based on NbTiN circuitry, were designed and all were integrated on one 6 μm silicon substrate. \nAt the IF side of the integrated balanced mixers, two different configurations of superconducting \nintegrated 180º IF hybrids were designed to combine the two generated IF outputs at either Σ or Δ port depending on the choice of the input port of the hybrid. These 4-12 GHz IF hybrid coupler designs were based on planar lumped elements to miniaturize the size to about 0.5×2.2 μm^2. The designs were simulated for 500 μm and 9 μm silicon substrates. The latter is studied in order to integrate it at a later \nstage with the RF part of the mixer on one substrate. The lithography mask and subsequently the micro-fabrication of the IF hybrids were made. The measurement of the Nb based IF hybrid on a 500 μm silicon with Vector Network Analyzer (VNA) in liquid helium is reported. The measured results are analyzed and compared to the IF hybrid with the implemented superconducting Nb in the simulation. \n \nThese developments and designs will be available and applicable for future receivers of the CCAT-prime observatory. Because the original envisioned CCAT-observatory project was discontinued due to the lack of funding, it was decided that my 800-1100 GHz designs would not be further developed into working mixers for the time being and that other projects would get priority. \n \nMy work continued with the the development of integrated 1.9 THz balanced Hot Electron Bolometer (HEB) mixers. This development is relevant for 1.9 THz focal plane array receivers for the airborne Stratospheric Observatory for the Far Infrared (SOFIA), for which the mixers are developed in our group. The LO power consumption of the array presently limits its usable RF bandwidth. This power consumption could be reduced by a factor of 5 by using integrated balanced mixers. The mixers are based on superconducting NbN HEBs and Au is applied as normal metal to wire the mixers. The development contained design, micro-nanofabrication, DC and cryogenic high frequency measurements. All designed planar elements consisted of an 1 octave broadband 180º RF hybrid-ring coupler, two slotline tapered antennas, two slotline to \nCPW transitions, two or four 120 Ω HEB bridges, two low pass filters (LPFs) and two DC/IF blocking capacitors integrated on one 3 μm silicon substrate and simulated using CST suite. A further integrated 1.9 THz HEB mixer was designed to support the two separate IF outputs at one side of a 3 μm silicon substrate with employing a CPW-CPW crossover. \n \nThe micro-nanofabrication of the 1.9 THz balanced mixers was done in house using E-beam and photo-lithography techniques. Almost all DC measured devices contained HEB bridges that were sufficiently similar in normal state resistance and transition temperature. The average resistance of the devices, in the one available wafer for measurements, was about 17% too low. The transition temperature of the devices was around 7.9 K, in comparison to 8.5 K for our state of the art single pixel mixers. The two machined 1.9 THz balanced waveguide blocks for testing of the devices were machined in the institute's in house workshop. I have tested the transmission of the blocks, and thereby the workmanship, with a THz time Domain Spectrometer, showed about 2.4 dB of loss per mm waveguide length at room temperature, for a split block waveguide made of the oxygen free copper tellurium alloy (CuTe). \n \nThe cryogenic RF measurements showed a very similar LO-power coupling to both mixers in the balanced device, from both input ports of the mixer. By using an in house built Quantum Cascade Laser (QCL) LO as a local oscillator and a severely attenuated Virginia Diode multiplier chain (VDI) LO as a signal source I have shown proper balanced mixer behavior, thereby validating the design. Signal suppression up to 38 dB was shown by inverting the bias voltages on one of the mixers in the balanced device, which added a 180º phase change to the IF of one of the mixers.

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jun 14, 2006•5 citations
This thesis work covers the development of integrated superconducting balanced mixers for heterodyne (resolution> 10^7) THz focal plane arrays. The aim was to develop on chip integrated balanced superconductor-insulator-superconductor (SIS) mixers at 800-1100 GHz and in addition at the Intermediate Frequency (IF) 4-12 GHz integrated \n180° IF hybrids both for the CHAI (CCAT Heterodyne Array Instrument) receiver for the Cerro Chajnantor Atacama Telescope (CCAT) observatory. A planar 180º hybrid ring \ncoupler, two separate slotline antennas, two separate slotline to CPW transitions to employ for the 800-1100 GHz balanced SIS mixer, based on NbTiN circuitry, were designed and all were integrated on one 6 μm silicon substrate. \nAt the IF side of the integrated balanced mixers, two different configurations of superconducting \nintegrated 180º IF hybrids were designed to combine the two generated IF outputs at either Σ or Δ port depending on the choice of the input port of the hybrid. These 4-12 GHz IF hybrid coupler designs were based on planar lumped elements to miniaturize the size to about 0.5×2.2 μm^2. The designs were simulated for 500 μm and 9 μm silicon substrates. The latter is studied in order to integrate it at a later \nstage with the RF part of the mixer on one substrate. The lithography mask and subsequently the micro-fabrication of the IF hybrids were made. The measurement of the Nb based IF hybrid on a 500 μm silicon with Vector Network Analyzer (VNA) in liquid helium is reported. The measured results are analyzed and compared to the IF hybrid with the implemented superconducting Nb in the simulation. \n \nThese developments and designs will be available and applicable for future receivers of the CCAT-prime observatory. Because the original envisioned CCAT-observatory project was discontinued due to the lack of funding, it was decided that my 800-1100 GHz designs would not be further developed into working mixers for the time being and that other projects would get priority. \n \nMy work continued with the the development of integrated 1.9 THz balanced Hot Electron Bolometer (HEB) mixers. This development is relevant for 1.9 THz focal plane array receivers for the airborne Stratospheric Observatory for the Far Infrared (SOFIA), for which the mixers are developed in our group. The LO power consumption of the array presently limits its usable RF bandwidth. This power consumption could be reduced by a factor of 5 by using integrated balanced mixers. The mixers are based on superconducting NbN HEBs and Au is applied as normal metal to wire the mixers. The development contained design, micro-nanofabrication, DC and cryogenic high frequency measurements. All designed planar elements consisted of an 1 octave broadband 180º RF hybrid-ring coupler, two slotline tapered antennas, two slotline to \nCPW transitions, two or four 120 Ω HEB bridges, two low pass filters (LPFs) and two DC/IF blocking capacitors integrated on one 3 μm silicon substrate and simulated using CST suite. A further integrated 1.9 THz HEB mixer was designed to support the two separate IF outputs at one side of a 3 μm silicon substrate with employing a CPW-CPW crossover. \n \nThe micro-nanofabrication of the 1.9 THz balanced mixers was done in house using E-beam and photo-lithography techniques. Almost all DC measured devices contained HEB bridges that were sufficiently similar in normal state resistance and transition temperature. The average resistance of the devices, in the one available wafer for measurements, was about 17% too low. The transition temperature of the devices was around 7.9 K, in comparison to 8.5 K for our state of the art single pixel mixers. The two machined 1.9 THz balanced waveguide blocks for testing of the devices were machined in the institute's in house workshop. I have tested the transmission of the blocks, and thereby the workmanship, with a THz time Domain Spectrometer, showed about 2.4 dB of loss per mm waveguide length at room temperature, for a split block waveguide made of the oxygen free copper tellurium alloy (CuTe). \n \nThe cryogenic RF measurements showed a very similar LO-power coupling to both mixers in the balanced device, from both input ports of the mixer. By using an in house built Quantum Cascade Laser (QCL) LO as a local oscillator and a severely attenuated Virginia Diode multiplier chain (VDI) LO as a signal source I have shown proper balanced mixer behavior, thereby validating the design. Signal suppression up to 38 dB was shown by inverting the bias voltages on one of the mixers in the balanced device, which added a 180º phase change to the IF of one of the mixers.
This thesis presents the work done in the field of development of low noise instruments for radio astronomy applications, and addresses some of the key components in the receiver. Part of this thesis describes the design and development of a Superconductor-Insulator-Superconductor (SIS) junction mixer covering RF frequencies from 163 to 211 GHz, reaching the quantum limited noise performance. The mixer uses an integrated approach, where an LO coupler and a resistive LO termination are fabricated on the same mixer chip.Another part of the thesis describes the work on the design and development of the Band 5 receiver cartridge for the Atacama Large Millimeter Array (ALMA) project. Details on the development of Band 5 receiver cartridge, sideband separation mixer, and a superconducting IF hybrid is presented.Additionally, this thesis presents first ever experimental demonstration of an SIS device as frequency multiplier, and presents successful off-chip detection of the multiplied signal. The experimental frequency multiplier was designed for the 182 to 192 GHz output frequency band, and uses an array of SIS junctions to boost the total output power. Dependence of multiplier output power on the DC bias voltage and pumping power are investigated. The measurements successfully confirm x2 multiplication, where extremely sharp multiplied signal was detected with no observable spurious signals. The SIS multiplier shows no deterioration of the linewidth of the original pumping signal.
Abstract We developed a low-noise, compact, balanced superconductor–insulator–superconductor (SIS) mixer, operating in the 787–950 GHz radio frequency range. A waveguide mixer block was designed to integrate all the key components, such as a radio frequency (RF) 90° hybrid coupler, two identical SIS mixer chips, bias-tees, and an intermediate frequency power-combiner. The RF waveguide 90° hybrid coupler consists of branch lines with wide slots optimized by numerical simulation, for ease of fabrication. The balanced mixer was installed into a cartridge type receiver, originally developed for the Atacama Large Millimeter/submillimeter Array Band 10 (787–950 GHz). The receiver demonstrated double sideband noise temperatures of approximately 200 K for most of the band, without any correction for loss in front of the receiver. The local oscillator noise rejection ratio was estimated to be more than 15 dB within the measured frequency range.
This thesis investigates the prospects of some different superconducting mixers integrated with planar antennas for the millimeter and submillimeter regime. Two kinds of Superconductor/Insulator/Superconductor (SIS) tunnel junctions for use in mixers have been fabricated; Nb/PbBi edge junctions and Nb trilayer junctions. One of the very high quality trilayer mixers is currently operating in a 350 GHz radio-telescope heterodyne-receiver. The reduced sensitivity of SIS mixers for frequencies above the gap frequency (?700 GHz) of the superconductors has directed the interest towards another type of superconducting mixer device, the superconducting Hot Electron Bolometer (HEB). Various types of this device have been studied; Nb devices for scale-measurements at 20 GHz to more easily verify some of the theoretical background of the HEB, NbN waveguide devices for 100 GHz, and NbN devices integrated with planar antennas for a quasi-optical mixer at 350 GHz. Two types of planar antennas have been used; the endfire "Broken" Linearly Tapered Slotline Antenna (BLTSA) on a thin dielectric membrane, and the double dipole antenna on a quartz substrate lens. The beam pattern of the BLTSA has been examined theoretically and measured at 40 GHz with antennas supported by Kapton foil, and at 350 and 800 GHz with antennas on thin dielectric SiOxNy membranes. Measurements of the 20 GHz Nb HEB show a mixer conversion loss close to 0 dB for IF frequencies below 40 MHz. The maximum IF bandwidth given by the electron temperature relaxation time corresponds to a frequency of about 80 MHz. A DSB mixer noise temperature of only a few hundred Kelvin has been measured. Corresponding measurements of NbN devices at 100 and 350 GHz give a mixer conversion loss of about 6-10 dB and an IF bandwidth of about 700 to 1000 MHz. The output noise is found to be dominated by electron temperature fluctuations with an equivalent noise temperature of about 100 K, and a frequency dependence equal to that of the IF signal. Phenomenological expressions for conversion loss, noise, and IF impedance have been compared measurements and shows good agreement. The quasioptical NbN HEB mixers are still under development with respect to film quality and coupling to radiation, why the mixer performance of these is expected to improve.
This dissertation presents a detailed study of the intermediate frequency (IF) properties of NbN hot electron bolometric (HEB) terahertz mixers, performed over the widest reported IF frequency range. A broadband technique to obtain the IF small signal impedance of HEB devices using network analyzer measurements is presented. The superconducting mixer impedance is measured for local oscillator (LO) frequencies ranging from 694 GHz to 1.9 THz and for a 1-10 GHz IF range. The results obtained are in agreement with the so-called Standard and NSGR models. Further, the variation of receiver noise, mixer gain, and output noise with IF frequency is experimentally estimated for several devices and for a wide range of dynamic conditions using LO frequencies of up to about 2 THz (fLO < 2 THz). Empirical results demonstrate near-quantum limited noise performance with wide receiver bandwidth. A record IF noise bandwidth of 8 GHz (measured for f LO = 694 GHz) and a record effective IF bandwidth of 5 GHz (measured for fLO = 1:6 THz) are demonstrated without using ferrite isolators. The stability of HEB mixers is measured for different IF bandwidths for the first time using the so-called Allan variance. Further evidence of intrinsic device stability limitations is presented. An algorithm to design integrated receivers based on HEB mixer elements is proposed and implemented. The measured impedance of the mixer is used to obtain consistent model parameters. The Standard model formalism for the mixer is used in combination with computer models for the IF amplifier to find theoretical estimates of the receiver performance. Good agreement between theoretical and empirical results is found. The direct and close integration of HEB mixers with InP microwave monolithic integrated circuit (MMIC) IF amplifiers is accomplished for the first time. Several compact down-converter prototypes for terahertz frequencies are demonstrated using two- and three-dimensional integration methods. Using the integration techniques described here, it has been possible to decrease the volume of our receivers by a factor of 20 (with a corresponding mass reduction of 15:1) while maintaining good electrical performance. Finally, a compact 3-element focal plane array (FPA) based on 2D integrated receivers is designed, constructed, and experimentally verified. This is the first heterodyne FPA to be reported for any frequency greater than 1 THz. It is hoped that the results presented here will serve as the basis for the development of large-format arrays with potentially hundreds of elements.
We report on the development of a waveguide-based balanced superconducting mixer for operation near 2.7 THz. The mixer employs a pair of NbN hot-electron bolometers defined on 6 μm-thick silicon substrate that follows a 90°hybrid coupler. To produce the critical structures of the coupler and waveguide embedding circuit, we have utilized silicon micromachining techniques based on deep reactive ion etching. Operating near 4.2 K bath temperature, we have measured a minimum uncorrected DSB receiver noise temperature of less than 2000 K using Callen-Welton formula and local oscillator sideband noise rejection better than 13±3 dB at 2.74 THz. The concept is suitable for building arrays, readily scalable for higher frequencies up 5 THz, and could accommodate other mixer technologies, such as room-temperature Schottky diode mixers.
The Atacama Pathfinder EXperiment (APEX) is a 12 m antenna operating at the Atacama Desert on the Chilean Andes at about 5000 m altitude. APEX would be equipped with a suit of single-pixel heterodyne receivers covering 211 - 1500 GHz frequency range. We present here a design of a sideband-separating superconductor-insulator-superconductor (SIS) mixer for the APEX, receiver Band 3, operating in 385 - 500 GHz band. The receiver uses quadrature scheme with the RF signal passing via a 90-degree waveguide 3 dB hybrid and the LO is divided by a waveguide E-plane Y-junction. The outputs of the waveguide hybrid are coupled to the mixer SIS junctions through an E-probe with integrated bias-T. For the LO coupler, conventional branch waveguide couplers are difficult to manufacture at this high frequency with required accuracy as the branch waveguides become extremely narrow. In order to solve this problem, we propose an on-chip LO injection, where the LO coupler is integrated onto the mixer chip and fabricated together with the SIS junction and the tuning circuitry. The on-chip LO coupler is made of superconducting lines, which gives almost a lossless solution and provides fabrication accuracy better than 0.5 μm by using optical lithography only. Furthermore, the mixer design includes a novel component, an ellipse termination for the idle LO port, made of thin-film resistive material with sheet resistance equal to the transmission line characteristic impedance, which gives very broadband performance using extremely compact area.