The terahertz region (0.1 – 3 THz) is located in the electromagnetic spectrum below the optical frequencies. To date, the terahertz band is mostly unused for a lack of suitable electronic components, which are commercially available only up to around 100 GHz. It is the aim of this project to establish a monolithic microwave integrated circuit (MMIC) technology to fill the THz gap.
In order to reach these high frequencies, both intrinsic and extrinsic parasitic capacitances need to be reduced, and semiconductor materials with high electron mobility need to be used. For this purpose, we developed a transfer-substrate Indium Phosphide (InP) Hetero-Bipolar Transistor technology. Besides high electron mobility, the InP material system offers a high breakdown field due to its large energy gap, enabling higher output power at THz frequencies than any other semiconductor material. Capacitances are effectively reduced with the transfer-substrate approach.
InP HBTs with an emitter size of 0.5 × 5 µm2 are defined by electron beam lithography, demonstrating an fmax of more than 450 GHz at a breakdown voltage of BVCEO = 4.5 V. Monolithically integrated circuits such as amplifiers, mixers, and oscillators operating in the frequency range from 100 GHz to over 300 GHz have been fabricated and tested.
The cutoff frequency of ultra-high frequency transistors is increased with geometrical device scaling. The cooling of these transistors becomes ever more important as the power density increases with shrinking device dimensions. The heat can be efficiently extracted from the transistors with the integration of an electrically isolating diamond heat spreading layer, without having to compromise the high frequency performance. The thermal resistance of diamond-integrated InP HBT could be reduced by more than a factor of three compared to standard InP HBT, reaching a value below 1 K/mW. The RF output power of analog amplifier circuits operating at around 100 GHz could be doubled with the inclusion of the diamond heat sink.
System integration of integrated terahertz circuits requires a suitable mounting technology, foremost to connect the terahertz circuit to an antenna structure. The required mounting and connection technology needs to be sufficiently broadband, should not incur significant RF losses, must be reproducibly manufacturable from the initial electromagnetic design, and needs to be low cost. Classic bond wire connections are difficult to implement beyond 100 GHz due to manufacturing tolerances. A flip-chip mounting technology based on gold-tin with 10 µm design rule was developed, including multilayer passive submount substrates with shielded transmission lines, which were manufactured in a process sequence similar to the InP DHBT process flow. Passive and active InP HBT circuits were mounted onto these submounts and measured. A bandwidth from DC to 450 GHz could be demonstrated. The insertion loss of the flip-chip transitions was less than 1 dB even at the highest frequencies.
Based on the transferred-substrate concept, we developed a wafer-scale 3D integration approach of InP DHBT technology onto Silicon Germanium Bipolar-CMOS (SiGe BiCMOS) wafers using face-to-face adhesive wafer bonding, subsequent InP substrate removal, and formation of vertical RF interconnects between the InP DHBT and SiGe BiCMOS subcircuits. In this complementary approach, we combine the advantages of both technologies: highly complex BiCMOS analog and digital circuits are augmented with the high bandwidth and output power of InP DHBT amplification and mixing stages. Signal sources including a BiCMOS VCO and InP mixing and power amplification stages operating at up to 330 GHz were demonstrated.
The combined InP/SiGe technology platform "SciFab" is developed in cooperation with the Leibniz Institute IHP Frankfurt (Oder) and is available to external partners in a foundry mode.