The field of terahertz integrated technology has undergone significant development in the past ten years. This has included work on different substrate technologies such as III–V semiconductors and silicon, work on field-effect transistor devices and heterojunction bipolar devices, and work on both fully electronic and hybrid electronic–photonic systems. While approaches in electronic and photonics can often seem distinct, techniques have blended in the terahertz frequency range and many emerging systems can be classified as photonics-inspired or hybrid. Here, we review the development of terahertz integrated electronic and hybrid electronic–photonic systems, examining, in particular, advances that deliver important functionalities for applications in communication, sensing and imaging. Many of the advances in integrated systems have emerged, not from improvements in single devices, but rather from new architectures that are multifunctional and reconfigurable and break the trade-offs of classical approaches to electronic system design. We thus focus on these approaches to capture the diversity of techniques and methodologies in the field. This Review Article examines the development of terahertz integrated electronic and hybrid electronic–photonic systems, considering, in particular, advances that deliver important functionalities for applications in communication, sensing and imaging.

Terahertz integrated electronic and hybrid electronic–photonic systems

Traditional terahertz (THz) equipment faces major obstacles in providing the system cost and compactness necessary for widespread deployment of THz applications. Because of this, the field of THz integrated circuit (THz IC) design in CMOS and SiGe HBT technologies has surged in the last decade. An interplay of advances in silicon process technology, design technique, and microelectronic packaging promises to narrow the gap between the requirements and the reality of system cost and performance of THz components. Furthermore, the scalability, reconfigurability, and signal processing features of silicon technology have initiated research in complex THz ICs that expand the functionality of THz systems; this has enabled new applications, methods, and algorithms. This paper reviews the progress in THz IC research and investigates several realizations of THz imaging and sensing applications with silicon-based components regarding their motivation, system performance, and challenges. THz computed tomography, broadband multicolor imaging, high-resolution FMCW radar imaging, subwavelength resolution near-field imaging, and compressed sensing are presented.

Terahertz Imaging and Sensing Applications With Silicon-Based Technologies

Transition-metal oxides exhibiting a bistable resistance state are attractive for non-volatile memory applications. The relevance of oxygen vacancies (VO) for the resistance-change memory was investigated with x-ray fluorescence, infrared microscopy, and x-ray absorption spectroscopy using Cr-doped SrTiO3 as example. We propose that the microscopic origin of resistance switching in this class of materials is due to an oxygen-vacancy drift occurring in close proximity to one of the electrodes.

Role of oxygen vacancies in Cr-doped SrTiO3 for resistance-change memory

Terahertz (THz)-band communications are a key enabler for future-generation wireless communication systems that promise to integrate a wide range of data-demanding applications. Recent advances in photonic, electronic, and plasmonic technologies are closing the gap in THz transceiver design. Consequently, prospect THz signal generation, modulation, and radiation methods are converging, and corresponding channel model, noise, and hardware-impairment notions are emerging. Such progress establishes a foundation for well-grounded research into THz-specific signal processing techniques for wireless communications. This tutorial overviews these techniques, emphasizing ultramassive multiple-input–multiple-output (UM-MIMO) systems and reconfigurable intelligent surfaces, vital for overcoming the distance problem at very high frequencies. We focus on the classical problems of waveform design and modulation, beamforming and precoding, index modulation, channel estimation, channel coding, and data detection. We also motivate signal processing techniques for THz sensing and localization.

An Overview of Signal Processing Techniques for Terahertz Communications

Highly scaled indium phosphide (InP) heterojunction bipolar transistor (HBT) technologies have been demonstrated with maximum frequencies of oscillation ( $f_{\max}$ ) of >1 THz and circuit operation has been extended into the lower end of the terahertz (THz) frequency band. InP HBTs offer high radio-frequency (RF) output power density, millivolt (mV) threshold uniformity, and high levels of integration. Integration with multilevel thin-film wiring permits the realization of compact and complex THz monolithic integrated circuits (TMICs). Circuit results reported from InP HBT technologies include: 200-mW power amplifiers at 210 GHz, 670-GHz amplifiers and fundamental oscillators, and fully integrated 600-GHz transmitter circuits. We review the state of the art in THz-capable InP HBT devices and integrated circuit (IC) technologies. Challenges in extending transistor bandwidth and in circuit design at THz frequencies will also be addressed.

InP HBT Technologies for THz Integrated Circuits

An experimental SiGe HBT technology featuring fT/fmax/BVCEO = 505 GHz/720 GHz/1.6 V and a minimum CML ring oscillator gate delay of 1.34 ps is presented. The improved speed compared to our previous SiGe HBT developments originates primarily from an optimized vertical profile, an additional decrease of the base and emitter resistance which is made possible by combining millisecond annealing with a low-temperature backend, and from lateral device scaling.

SiGe HBT with fx/fmax of 505 GHz/720 GHz

This work reports the bipolar resistive switching behavior of more than 100 back-end-of-line (BEOL) integrated 600×600nm2 TiN/HfO 2 /Ti/TiN MIM devices in a 4 kbit memory array. Reliable current-voltage switching characteristics were only observed for devices with a thickness ratio of 1 (10 nm HfO 2 /10nm Ti), indicating the importance of the interface chemistry of the Ti/HfO 2 interface. Moreover, the devices show good inter-cell uniformity and thus demonstrate promising prospects for embedded non-volatile memory (eNVM) applications.

Resistive switching behavior in TiN/HfO 2 /Ti/TiN devices

Modular integration of annular TSV structures filled with tungsten in a 0.25µm SiGe

Rapid thermal processing is one of the key thermal processes in semiconductor manufacturing. The temperatures and times at which they are carried out range from very low to extremely high temperatures for very short or long durations depending on the application. As the temperature–time cycle has changed radically over the years, manufacturing equipments need to be monitored for the stability of the manufacturing process. This paper examines and presents especially the complex monitoring process for low temperature processing by a defined implantation condition and an alloying method capable of monitoring processes at temperatures below 600 °C to ensure the process requirements of the manufacturing flow. Graphical abstract

Low-temperature monitoring with implantation and alloying

A methodology for fast detection of via faults is presented. Defective vias in large via chains are detected by subsequent use of a parametric tester for resistance measurements, a CD-SEM for voltage contrast inspection and defect localization, and focused ion beam (FIB) preparation for failure analysis. Short loop experiments were applied to fabricate large via chains in a short cycle time. The method was applied for optimizing the lithography process for via layers. As a result, the density of yield-limiting blob defects was reduced.

Detection and reduction of via faults

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