Project C02 - Beam-steerable Heterointegrated Dynamic Res-onant THz Transceivers

Principal Investigators: Prof. Dr. Nils Pohl, RUB / Prof. Dr. Nils Weimann, UDE

Achieved results and methods

Dynamic mobile sensing and localization at THz frequencies necessitates high RF power and beam forming capability, and low noise detection, to achieve high range and low integration time. Oscillator sources, either fundamental or with low multiplication factor, are power-efficient at THz frequencies. Above the transistor’s f_max, power amplifiers do not work, and traditional frequency multiplier chains are very inefficient. The electronic THz sources based on InP RTD are at the core of C02 since they exhibit high scaling potential, both in terms of output power and maximum system frequency. The RTD operates as a fundamental self-oscillator, and could be demonstrated up to 2 THz, unique compared to any other solid-state technology [1]. This is possible due to high resonant tunnelling current flowing through the quantum well structure under bias resulting in a highly nonlinear IV curve (see Fig. 1). A recent demonstration of a chip containing mutually coupled RTDs resulted in an output power of 12 mW at 450 GHz, from a very compact aperture [2].

Initial questions and progress

Drawbacks of free-running oscillator sources are high phase noise and frequency instability. In the 1^st phase of C02, precise control of oscillation frequency, narrow linewidth, and phase stabilization of power-efficient THz oscillators by subharmonic injection locking could be demonstrated for the first time, on both InP RTD and InP HBT circuits, also incorporating results from the merged project C11. Limited frequency tunability could be achieved. In the 2^nd phase of C02, the InP chip technology could be improved and stabilized. The much-increased availability of functioning chips allowed for the development of packaging techniques including flip‑chip bump mounting, and the addition of a collimating silicon lens as well as an integrated DC bias Tee. This led to the realization of functioning compact and mobile electronic fundamental THz oscillator sources [3], which could be tested in plant science applications [4]. In thorough design of experiments involving new RTD semiconductor heterostructures, the tuning range and output power of chips fabricated at UDE could be vastly improved. Subharmonic control circuits with integrated antenna for near-field radiative coupling of the control signal into the RTD could be developed [5]. Heterogeneous integration of the InP RTD chip containing a planar THz antenna with the SiGe control circuit could be successfully demonstrated and is under publication.

Detailed progress at the RTD device level (WP1 of C02 in the 2^nd phase) The InP RTD with integrated antenna is realized in monolithic planar technology in UDE’s lab, starting with an InP substrate and epitaxial RTD layers. Fig. 2a shows an RTD with 0.6 µm² junction area embedded in a planar slot line antenna. The oscillator core with antenna has a size of 200 x 400 µm², commensurate with  in a phased array. Single pixel RTDs now exhibit output power up to 300 µW at frequencies up to 450 GHz, with 20 – 40 mW DC power consumption. For rapid prototyping, a new free-space measurement setup and measurement routine was established allowing on-wafer characterization before chip packaging [6, 7]. An enhanced modelling technique based on S-parameter measurements up to 500 GHz with a Multiline-Through-Reflect calibration method was developed [8, 9], along with a novel technique to extract parasitic elements from the RTD at those extremely high frequencies. The compact RTD model will be available in compiled form and as open access code pending publication. In the design of RTD heterostructures, the quantum mechanical tunnelling process needs to be combined with the semiconductor equations. Newly developed code allows for quick iterative exploration of the multivariate parameter space, switching to detailed full simulations based on commercial tools in promising areas. The simulated data are found to be consistent with experimental results [10, 11] describing the large signal current-voltage (I-V) behavior of the RTD and small-signal parameter extraction (work done in the framework of the fellowship extended to the post-doctoral researcher from Ukraine). A quantum mechanical model for electron transport employing the Bohmian formulation was created in collaboration with Universitat Autònoma de Barcelona to calculate the response of the RTD (quantum capacitance) at THz frequencies [12]. The use of RTD as zero-bias detectors was investigated, and led to promising results in terms of responsivity and noise performance [13].

Detailed progress at the THz circuit level (WP2 and WP3 of C02 in the 2^nd phase). For complex applications, e.g. beam forming in arrayed configurations, the RTD oscillation needs to be controlled in phase and frequency by an injection locking signal. We developed Monolithic Microwave Integrated transistor Circuits (MMICs) operating at the subharmonic frequency for this purpose. In the 1^st phase of C11 and 2^nd phase of C02, two topologies of InP Heterojunction Bipolar Transistor (HBT) push-push Voltage-Controlled Oscillator (VCO) circuits could be demonstrated with a record DC-to-RF efficiency of 1.7% at a frequency of 421 GHz, with an output power of -2.4 dBm (ref. [14], see Fig. 2b). This circuit itself could be subharmonically injection-locked at f₀/2, with a locking range of 3.6 GHz at an injection power of ‑3.9 dBm. This InP HBT VCO would serve as an intermediate stage, “amplifying” a lower frequency control signal supplied by a SiGe BiCMOS circuit, and delivering sufficient output power around 400 GHz to subharmonically drive an RTD oscillator at 1.2 THz.

For generation of the initial control signal, a SiGe MMIC was developed, featuring a Colpitts VCO operating at 120 GHz, followed by a frequency doubler, and a power amplifier to generate the 240 GHz locking signal [5]. Frequency synthesis was implemented including offset phase locked loop (PLL) components such as an auxiliary VCO, an offset-mixer, and additional frequency dividers. The SiGe MMIC can directly “drive” the InP RTD via an included on-chip patch antenna through near-field coupling. It features a tuning range of about 27 GHz at an output power of 0 dBm and includes DC feed through to the RTD. From a system view, therefore, the individual parts of the chain consisting of SiGe source → InP VCO → InP RTD, which had been sketched as a concept within the proposal of the 2^nd phase of C02, could be realized and validated in hardware. Also, a first iteration of phase modulator circuits was designed (ref. [15, 16], see Fig. 3), as well as ultra-wideband signal generation MMICs [17]. Moreover, the generation of THz frequencies using SiGe circuits was researched [18].

Detailed progress at the THz module level (WP4 of C02 in the 2^nd phase) In the 1^st phase, the chips could only be measured on wafer probing stations and thus were not “mobile”. During the 2^nd phase, several mounting and packaging schemes suitable for THz assemblies containing InP and SiGe chips were investigated and developed successfully, including chip-to-chip hetero-integration of the dissimilar wafer materials InP and silicon. The chip-to-chip integration process resulted in a number of compact, integrated THz assemblies (see Fig. 4). At first, stud bumps made from gold wire, or deposited in a gold electroplating process, are placed onto the InP chip’s bond pads. The InP chip is then mounted head first using a precision flip-chip aligner, where the metal interfaces are formed by compression bonding at controlled pressure and temperature compatible with the InP heterostructure. In the next step, a hyper-hemispherical THz lens made from highly resistive silicon is placed and glued onto the assembly in the same machine. The repeatable and accurate placement of the lens ensures efficient outcoupling of THz radiation from the chip. Very compact (footprint of 2 x 2 cm²) THz RTD modules could be realized and tested in a lab environment in both Tx and Rx operation to measure water content of plant leaves, and the THz response of dielectric and metallic honeybee replicas [4]. A battery-powered module could be demonstrated (see Fig. 4b), running off a 3 V lithium coin battery, which can be operated in cw mode for around 30 minutes, emitting an output signal at 330 GHz with a broadside radiated power of 39 µW [19].

Chip-to-chip hetero-integration was developed using the same assembly steps of thermocompression gold bumps. InP RTDs with integrated THz slot antenna could be successfully mounted onto SiGe local oscillator chips. The subharmonic injection locking signal is coupled in a near-field regime between opposing planar antennas on the SiGe and on the InP chip. The InP-SiGe assembly was mounted on a PCB, which provided off-chip PLL stabilization of both VCOs and tunable bias supply to the InP and SiGe chips. The RTD was placed face-down again on the SiGe chip, radiating through the backside of the InP substrate (facing upward in the integration), as shown in Fig. 5a, which also includes a hyper-hemispherical silicon lens on top of the assembly. In Fig. 5b, measured spectra vs. BiCMOS VCO tuning voltage are shown for fixed RTD bias (and thus fixed RTD natural oscillation frequency). Outside of the locking range, the RTD oscillates at around 245 GHz (BiCMOS VCO tuning voltage below 1 V). When the BiCMOS tuning voltage is increased above 1 V, the frequency and output power of the VCO is within the locking range of the InP RTD, the coupled oscillators both assume the VCO’s oscillation frequency. In this regime, only one tone is observed, which can be tuned by changing the VCO’s control voltage.

Locking could be observed over a wide range from 220 to 240 GHz. In this (world’s first) demonstration of injection locking between a hetero-integrated BiCMOS LO and an InP RTD, fundamental locking was applied for simplicity; an extension to subharmonic injection locking is planned for the 3^rd phase of C02. A first module integration scheme was developed in the 2^nd phase of C02 for an injection-lockable InP RTD array with 2 x 2 elements (see Fig. 4c), to investigate the effects of cross-coupling and mutual injection locking, and to gauge multiple RTD locking behavior in the presence of realistic process tolerances. In this first iteration of the module, an external injection locking signal could be applied from the backside of the housing; subsequent development will include LO signal generation and distribution inside of the package. Bias-dependent measurements of the combined array’s oscillation frequency are shown in the red trace (“locked”) of Fig. 5c. The oscillation is tunable between 210 and 213 GHz. First injection locking experiments were also carried out with an external source, which was set to 212 GHz. To verify the locking range, the RTD’s bias was varied, which slightly altered the complex impedance of the diode and thus changed its natural oscillation frequency. Locking could be observed in a range of 1 GHz, along with the characteristic phenomenon of frequency pulling near the locking range, proving the successful first realization of a multi-pixel portable RTD array module with injection locking capability.

Selected project-related publications

• Clochiatti, R. Schmidt, E. Mutlu, M. Dieudonne, W. Prost, D. Schreurs and N. Weimann, "On-Wafer Characterization and Modelling of InP Resonant Tunnelling Diodes up to 500 GHz," 2022 52nd European Microwave Conference (EuMC), Milan, Italy, 2022, pp. 282-285. [DOI: 10.23919/EuMC54642.2022.9924370]
• Mutlu, W. Li, B. Sievert, R. Kress, S. Clochiatti, A. Rennings, A. Grygoriev, W. Prost, D. Erni, and N. Weimann, “Investigation of RTD THz Oscillator with Wide Frequency Tuning Capability,” in 2023 48th Intern. Conf. on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), 17-22 September 2023, Montreal, Quebec, Canada, pp. 1–2. [DOI: 10.1109/IRMMW-THz57677.2023.10299140]
• Mutlu, A. Grygoriev, S. Clochiatti, R. Kress, C. Preuss, A. Possberg, W. Prost and N. Weimann (2023), Experimental and Theoretical Investigation of Collector Spacer and Doping Profile on Triple-Barrier Resonant Tunneling Diodes. Phys. Status Solidi A, 2300575. [DOI: 10.1002/pssa.202300575]
• Villani, S. Clochiatti, W. Prost, N. Weimann and X. Oriols, "There is Plenty of Room for THz Tunneling Electron Devices Beyond the Transit Time Limit," in IEEE Electron Device Letters, vol. 42, no. 2, pp. 224-227, Feb. 2021. [DOI: 10.1109/LED.2021.3049229]
• Clochiatti; R. Kress; E. Mutlu; F. Vogelsang; M. van Delden; N. Pohl; W. Prost; N. Weimann, "InP RTD Detector for THz Applications,"2023 18th European Microwave Integrated Circuits Conference (EuMIC), Berlin, Germany, 2023, pp. 1-4. [DOI: 10.23919/EuMIC58042.2023.10288943]
• Possberg, F. Vogelsang, N. Pohl, M. Hossain, H. Yacoub, T. K. Johansen, W. Heinrich and N. Weimann, "An Injection-Lockable InP-DHBT Source Operating at 421 GHz with −2.4 dBm Output Power and 1.7% DC-to-RF Efficiency," 2022 IEEE/MTT-S International Microwave Symposium (IMS 2022), Denver, CO, USA, 2022, pp. 336-339. [DOI: 10.1109/IMS37962.2022.9865468]
• Bott, F. Vogelsang and N. Pohl, "A D-Band Phased-Array Chain Based on a Tunable Branchline Coupler and a Digitally Controlled Vector Modulator," in IEEE Journal of Microwaves. [DOI: 10.1109/JMW.2023.3318528]
• Wittemeier, M. A. Yildirim and N. Pohl, "Compact and Digitally Controlled D-Band Vector Modulator for Next-Gen Radar Applications in 130 nm SiGe BiCMOS," in IEEE Journal of Microwaves, vol. 3, no. 2, pp. 815-826, April 2023. [DOI: 10.1109/JMW.2023.3250340]
• Vogelsang, D. Starke, J. Wittemeier, C. Bredendiek, K. Aufinger and N. Pohl, "Ultra-Wideband Signal Source Tuneable From 86 GHz to 142 GHz in SiGe Technology," 2023 Sixth International Workshop on Mobile Terahertz Systems (IWMTS), Bonn, Germany, 2023, pp. 1-5. [DOI: 10.1109/IWMTS58186.2023.10207779]
• Starke, J. Bott, F. Vogelsang, B. Sievert, J. Barowski, C. Schulz, H. Rücker, A. Rennings, D. Erni, I. Rolfes, and N. Pohl, “A compact and fully integrated 0.48 THz FMCW radar transceiver combined with a dielectric lens,” International Journal of Microwave and Wireless Technologies, pp. 1–12, 2023. [DOI: 10.1017/S1759078723001368]