This work reports on the design and implementation of the first integrated, balanced frequency quadrupler operating in the 480-530 GHz frequency range, with measured efficiency of 4.9% and corresponding output power of 17 mW at 518 GHz. The design is based on the cascaded balanced doubler topology, utilizing a 90-degree waveguide hybrid and two stages of frequency doublers. The design incorporates 18 varactor Schottky diodes, with each doubler comprising a balanced configuration of three diodes connected in series. The quasivertical diode structure is used to minimize series resistance and improve power handling and thermal performance.

Millimeter and submillimeter wave instrumentation operating above 100 GHz typically utilizes rectangular waveguide as the primary propagation medium and, consequently, is limited to operation over restricted bands bound by the cut-off frequencies for single- mode propagation. Diplexers provide an option for overcoming the band limitations imposed by conventional waveguide-based instruments. This paper reports, to our knowledge, the first implementation of a prototype “quasi-reflectionless” diplexer based on the reflectionless filter concept developed by Morgan, operating at millimeter-wave frequencies, and utilizing the integration of passive elements onto a silicon-on-insulator platform.

Reflectionless filters are two-port networks that ideally present matched terminations at both filter ports to all frequencies, within both the filter’s passband and stopband. Standard lossless filter structures reflect out-of-band power back to the input, which can profoundly impact the performance of heterodyne instruments and contribute to generation of spurious sidebands, unwanted gain compression, reduced dynamic range or instability. Reflectionless filters, conversely, direct out-of-band power to one of the filter’s two “internal” resistive terminations. If either of these internal resistors is converted into a port, then the resulting circuit forms a matched frequency diplexer This paper reports a prototype lowpass “quasi-reflectionless” filter and diplexer architecture that exploits integration of passive elements on a silicon-on-insulator platform. The design employs suspended high-impedance transmission lines for the filter inductive elements as well as integrated metal-insulator-metal (MIM) capacitors and thin-film titanium resistors on a 15 mm thick high-resistivity silicon substrate. As proof-of-concept implementations, a filter designed for operation in the 0—220 GHz band with 3 dB roll-off at 150 GHz and a diplexer architecture designed for the 0—220 GHz band with target cross-over frequency at 170 GHz are described. The filter and diplexer are implemented using a silicon-on-insulator (SOI) micromachining process that has been used broadly as a platform for integration. Fabrication utilizes frontside lithography, sputter deposition and electroplating processes to form thin-film titanium resistors (nominal impedance of 50 W, 2 mm10 mm, 50 nm thick), metal-insulator-metal capacitors (radii of 4.6 mm and 3.8 mm with SiO2 insulator thickness of 200 nm) and meandered high-impedance microstrip transmission lines (width of 3 mm, lengths of 180 mm and 90 mm) on a 15 mm thick, high-resistivity (>20 kW-cm) silicon membrane. After the topside features of the filter and diplexer have been formed, the SOI wafer is bonded topside-down to a temporary carrier to allow backside processing. Scattering parameter measurements of the filter and diplexer both show the desired lowpass characteristic from lowband input port to output, but with a softer roll-off than anticipated from electromagnetic finite-element analysis and circuit simulations of the structures. Modeling suggests this soft roll-off characteristic is associated with parasitic inductance and resistance contributed by the metallized ground via. These parasitic elements associated with the via-to-ground are not observed in the electromagnetic simulation of the filter or diplexer and the observation that they occur consistently in the model for each circuit node connected to ground suggest they are associated with the electrical interface of the circuits to the tests fixtures used to characterize them. Moreover, it is also hypothesized that these parasitic elements are associated with discontinuities between the filter/diplexer metallized ground plane (on the silicon backside) and the metallized measurement housing platform to which the circuits are affixed for measurement. Disruptions to ground currents flowing beneath the silicon-supported microstrip circuit and the suspended transmission lines (where the measurement housing itself serves as the ground) will contribute to parasitics associated with the ground connection, consistent with the observed measurements.

This work presents, to our knowledge, the first metal-to-metal diffusion-bonded Gallium Arsenide quasi-vertical Schottky diode heterogeneously integrated onto high resistivity silicon. Coplanar waveguide (CPW) fed quasi-vertical Schottky diodes were fabricated alongside a 25 mu m pitch on-wafer calibration kit and characterized with wafer probes and a vector network analyzer from 325-500 GHz. Diode parameters were extracted, and capacitance-voltage, and current-voltage were measured for a variety of diodes. Results indicate these diodes are suitable for high-frequency applications and have a high current density capacity, thus allowing their RF characterization under forward-bias conditions.

A proof-of-concept “quasi-reflectionless” filter integrated on a micromachined silicon-on-insulator (SOI) substrate platform is described. The filter is designed to have a low-pass response with 3 dB roll-off at 150 GHz. Suspended high-impedance transmission lines are incorporated as inductive elements, in addition to thin-film resistors, and metal-insulator-metal capacitors to realize the complete filter architecture. Through-silicon vias provide access to the circuit ground plane. Measurements of the filter are conducted using a broadband on-wafer measurement setup with dual-band probes covering the dc-to-220 GHz frequency band. The filter exhibits a return loss greater than 15 dB over the measured frequency range and to the authors' knowledge is the first reflectionless filter prototype operating over 100 GHz and implemented on micromachined SOI.

Nb-based circuits have broad applications in quantum-limited photon detectors, low-noise parametric amplifiers, superconducting digital logic circuits, and low-loss circuits for quantum computing. The current state-of-the-art approach for superconductor-insulator-superconductor (SIS) junction material is the Gurvitch trilayer process based on magnetron sputtering of Nb electrodes with Al-Oxide or AlN tunnel barriers grown on an Al overlayer. However, a current limitation of elemental Nb-based circuits is the low-loss operation of THz circuits operating above the 670 GHz gap frequency of Nb and operation at higher temperatures for projects with a strict power budget, such as space-based applications. NbTiN is an alternative higher energy gap material and we have previously reported on the first NbTiN/AlN/NbTiN superconducting-insulating-superconducting (SIS) junctions with an epitaxially grown AlN tunnel barrier. One drawback of a directly grown tunnel barrier compared to thermal oxidation or plasma nitridation is control of the barrier thickness and uniformity across a substrate, leading to variations in current density (Jc). Semiconductor barriers with smaller barrier heights enable thicker tunnel barriers for a given Jc. GaN is an alternative semiconductor material with a closed-packed Wurtzite crystal structure similar to AlN and it can be epitaxially grown as a tunnel barrier using the Reactive Bias Target Ion Beam Deposition (RBTIBD) technique. This work presents the preliminary results of the first reported high-quality NbTiN/GaN/NbTiN heterojunctions with underdamped SIS I(V) characteristics.

High quality Nb-based superconductor-insulator-superconductor (SIS) junctions with Al oxide (AlO$_x$) tunnel barriers grown from Al overlayers are widely reported in the literature. However, the thin barriers required for high critical current density (J$_c$) junctions exhibit defects that result in significant subgap leakage current that is detrimental for many applications. High quality, high-J$_c$ junctions can be realized with AlN$_x$ barriers, but control of J$_c$ is more difficult than with AlO$_x$. It is therefore of interest to study the growth of thin AlO$_x$ barriers with the ultimate goal of achieving high quality, high-J$_c$ AlO$_x$ junctions. In this work, 100\%\ O$_2$ and 2\%\ O$_2$ in Ar gas mixtures are used both statically and dynamically to grow AlO$_x$ tunnel barriers over a large range of oxygen exposures. In situ ellipsometry is used for the first time to extensively measure AlO$_x$ tunnel barrier growth in real time, revealing a number of unexpected patterns. Finally, a set of test junction wafers was fabricated that exhibited the well-known dependence of J$_c$ on oxygen exposure (E) in order to further validate the experimental setup.

This work presents a micromachined dual-band probe for ultra-broadband single-sweep measurements to 220 GHz. The probe features both WR-5.1 waveguide and 1.0 mm coaxial inputs that are combined in an integrated passive diplexer. The probe shows a maximum loss in the crossover region of 6-7 dB and a minimum output return loss of 10 dB.

Software-Defined Radio (SDR) technology has been driven by advances in RF integrated circuits and is generally available up to 40 GHz. State-of-the-art SDRs used with open software frameworks and interfaced to commercially available up/down-converter hardware provide a means for accessing the submm-THz spectrum. This makes it possible to realize measurement systems and communications channels that exploit the flexibility and processing capabilities of SDR backends. This work demonstrates the first use of SDR to implement a 1 THz FM communications channel using commercial off-the-shelf (COTS) SDRs, general RF components, and submillimeter-wave frequency extenders.

A proof-of-concept demonstration of on-wafer electronic calibration in the submillimeter-wave band (325-500 GHz) is presented. A GaAs Schottky diode shunting a coplanar transmission line is employed as an electronic standard that is tuned by bias applied through wafer probes. Error-corrected scattering parameter measurements, based on a Thru-Reflect-Line (TRL) calibration, are used to characterize the on-wafer diode and establish it as a standard for electronic calibration. Subsequently, ensembles of measurements from a multiline TRL calibration and the diode calibration standard are performed and compared to assess the uncertainty associated with the two approaches. It is found that the error coefficients estimated using the electronic standard are in good agreement with those found from the multiline TRL calibration. Moreover, the electronic standard exhibits a significant improvement in precision compared with the TRL standards, consistent with previous work showing that error in probe placement between measurements was a principle source of uncertainty for TRL-based calibration.

Nb-based superconducting alloys, such as NbN and NbTiN, can have a superconducting energy gap near twice that of elemental Nb, thus, being ideal candidates for low-loss superconducting circuits operating above the gap frequency of Nb. We are particularly interested in these materials for THz-frequency superconducting-insulating-superconducting (SIS) mixers, kinetic inductance detectors, and traveling wave kinetic inductance parametric amplifier devices. We recently reported the use of an alternative synthesis technique, reactive bias target ion beam deposition (RBTIBD), used to realize high-quality NbTiN films and high-energy gap NbTiN/AlN/NbTiN SIS heterostructures. In this paper, the effect of post-deposition annealing on the electrical and structural properties of room temperature deposited RBTIBD NbTiN was investigated. Our room temperature RBTIBD NbTiN films were annealed using rapid thermal annealing from 200 to 1000 $^\circ$C. The change in structural and electrical properties was characterized by X-ray diffraction, atomic force microscopy, and four-point probe measurements. The annealed films demonstrated an enhancement in transition temperature (T$_c$), increased crystallinity, larger grain size, and significantly reduced intrinsic stress.

An approach for one port on-wafer electronic calibration at submillimeter wavelengths is described. Quasi-vertical GaAs Schottky diodes integrated onto silicon serve as the electronic calibration standard. The S-parameters of the diode standards are characterized over the WM-570 (325-500 GHz) band as a function of bias and subsequently used as the standard for one-port calibration. Comparisons of the error coefficients derived using the diode standard are shown to be in good agreement with those obtained from a conventional set of standards consisting of coplanar delayed short circuits.

This paper reports on the design and fabrication of quasi-vertical Schottky diodes for submillimeter-wave applications. Use of the diodes to implement an integrated 160 GHz frequency quadrupler are described.

Thermal management and design have been understood, for many years, as critical factors in the implementation of submillimeter-wave Schottky-diode-based circuits and instruments. Removal of heat is particularly important for frequency multipliers, as these circuits generally exhibit low-to-modest conversion efficiencies, and are usually driven with high-power sources to achieve usable output power in the submillimeter-wave region of the spectrum. Elevated diode junction temperature due to inadequate heat sinking is known to degrade performance, accelerate aging effects (for example, due to electromigration, ohmic contact deterioration, or thermally-induced stress), and can ultimately lead to device failure. The relatively-low thermal conductivity of GaAs (the predominant material technology for submillimeter-wave diodes), coupled with restrictions on diode anode size and geometry needed to minimize parasitics and achieve the device impedances required for high-frequency operation, present significant challenges and trade-offs between electrical and thermal designs of these devices. Recognition that heating is a major factor limiting efficiency and output power has prompted a number of approaches to mitigate excessive temperature rise in the junction of planar Schottky diodes, including the use of AlN or diamond as low-loss substrates that act as heat spreaders. A new diode topology, based on a quasi-vertical geometry that is realized through heterogeneous integration of GaAs with high-resistivity silicon, was recently developed at the University of Virginia for submillimeter-wave applications. Unlike planar diodes, the device structure of the quasi-vertical diode consists of a metal contact that underlies the diode's anode and epitaxial mesa, thus providing a large-area ohmic cathode contact that also serves as an integrated heat sink. Measurement of high-efficiency multipliers based on this technology suggest the quasi-vertical architecture provides an effective approach for heat removal and thermal management in Schottky diodes. This paper presents the first results reporting thermal performances of terahertz quasi-vertical GaAs Schottky diodes integrated on silicon. The devices are characterized using a thermoreflectance measurement technique, a method based on the change in refractive index, and therefore surface reflectivity, with changes in temperature. Heating and cooling temperature profiles and 2-D temperature maps are obtained for 3.5 micron and 5.5 micron diameter diodes. From these measurements, the device thermal resistances, junction temperatures, and thermal time-constants are extracted. Equivalent thermal circuit and finite element models are developed to study the device geometry, and extract material thermal parameters. The devices are also characterized using an electrical transient method, and the temperature and cooling transients found from this technique are found to be comparable to those obtained from thermoreflectance measurements. The quasi-vertical diodes studied in this work are shown to demonstrate a faster transient thermal response compared to flip-chip bonded terahertz diodes reported in the literature.

High energy gap Nb-based superconducting alloys with low normal state resistivity are fundamentally important for realization of low-loss high frequency circuits operating above the typical ~670 GHz gap frequency of elemental Nb. NbTiN has been shown to have a superconducting energy gap nearly twice that of elemental Nb, and is emerging as an important material for various detector and superconduc...

This paper presents the first thermal characterization of terahertz quasi-vertical GaAs Schottky diodes integrated on silicon. The devices are characterized using a thermoreflectance measurement technique. Heating and cooling temperature profiles and 2-D temperature maps are obtained for 5.5-mu m diameter diodes. From the measurements, we extracted the device thermal resistances, anode temperatures, and thermal time constants. Equivalent circuit and finite-element models are developed to study the device geometry, and extract unknown material thermal parameters. The devices are also characterized using an electrical transient method, and the temperature and cooling transients found from both techniques are compared. The quasi-vertical diodes show comparable thermal resistance and a faster transient response compared to other terahertz Schottky diodes reported in the literature.

High-quality Nb-based superconductor-insulator-superconductor (SIS) junctions with aluminum oxide tunnel barriers grown from Al overlayers are broadly reported in the literature. However, as the current density increases, aluminum oxide tunnel barriers yield significant leakage current due to the pinholes and defects in the tunnel barrier. In this work, the growth dynamics of AlN are studied through the inductively coupled plasma (ICP) nitridation of an Al overlayer at an increased sample-to-ICP distance with the modification of the University of Virginia (UVA) trilayer deposition system. The new ICP geometry follows the previous UVA optical spectroscopy investigation of the plasma at the wafer, but now, it also allows spectroscopy measurement in the vicinity of the ICP source via an additional quartz window. We report a significantly slower nitridation growth rate, while maintaining the relative dissociation, at a larger growth distance. High-quality, low leakage SIS I(V) characteristics with current densities exceeding 30 kA/cm 2 and better control of R n A are obtained.

This paper demonstrates the first differential on-wafer probe with integrated balun operating in the WR-3.4 (220 - 330 GHz) waveguide band. The probe employs integrated balun circuitry to convert the single-ended signal from the waveguide output of a VNA into differential stimuli at the on-wafer transmission line output. The design approach, fabrication method, and measured results are described in this paper.

We are developing a new superconducting amplifier technology for radio astronomy instruments called the Traveling-Wave Kinetic Inductance Parametric (TKIP) amplifier. Invented at Caltech/JPL, recent laboratory demonstrations have resulted in near quantum-limited noise performance over more than an octave of microwave bandwidth and operating temperatures as high as 3 Kelvin. These amplifiers have the potential to be used as front-end replacements for ALMA's mm/sub-mm SIS receivers and intermediate frequency (IF) amplifiers, and for multiplexing faint signals from focal-plane arrays of single-photon detectors on space telescopes such as NASA's Origins Space Telescope (OST). The enhanced observational capabilities that would be enabled by TKIP front-end amplifiers on ALMA would tremendously benefit ALMA science across all bands.

An integrated frequency quadrupler operating at 160 GHz, producing 100 mW of output power, and achieving peak efficiency of 25.5% is described. The quadrupler design is based on prior art and consists of GaAs Schottky diodes with epitaxy transferred to a micromachined silicon carrier forming a heterogeneously-integrated chip. A newly-developed fabrication process that eliminates high temperature annealing and utilizes SU-8 for adhesive bonding was employed to realize the circuit. The new process improves device yield and reliability compared to previous implementations.