Faster, higher, more powerful, and strategically refined! Like in sports, electronics is driven by a public demand for faster communications, more powerful imaging modalities as well as complex and adaptive circuits and systems. Dedicated high-speed electronic technologies, complex system-on-chip solutions, and characterization facilities able to verify high-speed operation are essential for a leading position in this area. The Ferdinand-Braun-Institut (FBH) offers an excellent environment in this field and has additionally expanded its circuit technology facilities towards terahertz frequencies (THz).
The indium phosphide (InP) double heterojunction bipolar transistor (DHBT) transferred-substrate (TS) process, the InP-on-BiCMOS DHBT process, and the InP DHBT-TS-on-diamond process reach cut-off frequencies around 350 GHz today and are being extended to reach over 700 GHz soon. FBH has demonstrated mixed-signal nonlinear active integrated circuits (MMIC) up to 300 GHz as building blocks for system-on-chip solutions, using heterogeneous integration with silicon and diamond materials. For frequencies beyond 1000 GHz, FBH also explores plasmonic operation and develops the related interconnect and calibration techniques scalable to these frequencies.
Bridging the gap between fundamental research and applications
The broad spectrum of FBH activities encompasses chip design and fabrication. This way, FBH both advances the field of THz electronics and supports industry in developing applications that require THz electronics. FBH is cooperating with a large number of partners from research and industry, including a joint laboratory with the Goethe University Frankfurt am Main and foundry activities with the Leibniz-Institut für innovative Mikroelektronik (IHP). These FBH activities aim at filling the gap between the ongoing fundamental research and application-driven demand for a mature and stable technology along with a reliable support chain.
Terahertz (THz) electronics is used to generate and detect THz radiation or to amplify signals in this frequency range. THz radiation is referred to as that part of the electromagnetic spectrum which lies between microwaves and infrared. Recently, the THz spectrum has become accessible with the push in high-speed operation of electronic devices and circuits towards the speed limit of electrons in semiconductors. Access to the THz range with optoelectronic devices requires a device operation regime where photon energies approach the thermal energy, which means operation at the noise limit.
THz radiation is non-ionizing and considered harmless, it can penetrate through non-metallic barriers and is therefore useful to detect hidden objects including subcutaneous tumors and packaged items. It can be used for stand-off detection of arts objects and compound materials. Thus, applications are versatile and range from medicine, security, and border check to non-destructive testing and material classification.
FBH offers several microwave monolithic integrated circuits (MMIC) technologies reaching cut-off frequencies above 300 GHz. MMIC design at FBH is based on a MMIC design kit with active and passive elements and proprietary large-signal HBT device models including thermal effects. Design kits exist for stand-alone processes and for the heterointegrated InP-on-BiCMOS process, SciFab.
InP HBT technology offers high voltage operation at high frequencies with excellent phase-noise properties. Therefore, FBH focuses on signal generation and amplification circuits. These circuits are building blocks for transmitter modules in THz systems. FBH has realized fundamental oscillator signal sources at 100 GHz, 200 GHz, and 300 GHz with measured output power > 0 dBm and good phase-noise properties. These sources are augmented by frequency multipliers at 164 GHz, 180 GHz (doublers) with Pout > 3 dBm, and at 240 GHz (tripler) with Pout > 0 dBm. FBH has also realized power amplifiers in the range of 48 GHz - 180 GHz with Pout < 23 dBm and efficiencies > 20 %.
Novel approaches utilizing heterointegrated processes and plasmonic effects
Many of these MMICs have been combined with BiCMOS in a wafer-level heterointegrated process including an award winning InP-on-BiCMOS MMIC frequency doubler at 164 GHz and a tripler at 250 GHz (Best Paper Award EuMW 2013). Further circuits have been designed exceeding 600 GHz operation frequency.
FBH has further identified the region between 1 - 6 THz as particularly interesting for focal plane THz cameras and THz spectroscopy systems utilizing plasmonic operation of GaN HEMT MMIC detectors and signal sources in this frequency range. Arrays of such detectors for a focal plane THz camera operating in the frequency range 1 - 2 THz have been realized and exhibit state-of-the-art performance at 560 GHz.
T. Jensen, T. Al-Sawaf, M. Lisker, S. Glisic, M. Elkhouly, T. Kraemer, I. Ostermay, C. Meliani, B. Tillack, V. Krozer, O. Krueger, and W. Heinrich, "Millimeter-wave hetero-integrated sources in InP-on-BiCMOS technology", Int. J. Microwave Wireless Technolog., vol. 6, no. 3/4, pp. 225-233 (2014).
M. Hossain, T. Kraemer, I. Ostermay, T. Jensen, B. Janke, Y. Borokhovych, M. Lisker3, S. Glisic, M. Elkhouly, J. Borngraeber, B. Tillack, C. Meliani, O. Krueger, V. Krozer, and W. Heinrich, "A 246 GHz Hetero-Integrated Frequency Source in InP-on-BiCMOS Technology", IEEE Microwave Wireless Compon. Lett., vol. 24, no. 7, pp. 469-471 (2014), in print.
M. Bauer, A. Lisauskas, S. Boppel, M. Mundt, V. Krozer, H.G. Roskos, S. Chevtchenko, J. Würfl, W. Heinrich, and G. Tränkle, "Bow-Tie-Antenna-Coupled Terahertz Detectors using AlGaN/GaN Field-Effect Transistors with 0.25 Micrometer Gate Length", Proc. 8th European Microwave Integrated Circuits Conf. (EuMIC 2013), Nuremberg, Germany, Oct. 6-8, pp. 212-215 (2013).
The SciFab foundry process newly offered by FBH and IHP combines the advantages of two high-frequency semiconductor technology worlds in a unique heterogeneous wafer-level integration high complexity BiCMOS (IHP) paired with high-power InP DHBT (FBH) technology. The wafer-level integration leads to a reduction in size, weight, and dissipated power as compared to existing assembly techniques. The combined technology includes high-power InP DHBT devices with fT & fmax above 320 GHz at 20 mA collector current and BVCEO = 4 V breakdown voltage. Typical applications for this process are integrated mm- and sub-mm-wave RF sources. The SciFab library includes all necessary models and layout cells of transistors, capacitors, resistors, coils, interconnects, and line models.
Indium phosphide (InP) heterobipolar transistors (HBT) are well suited for radio frequency (RF) power applications in the sub-THz region (100 - 1000 GHz) of the electromagnetic spectrum. Owing to the electronic properties of the InP semiconductor material, electrons are accelerated under an applied electric field more than three times faster than in silicon (Si), while the breakdown field in InP is 50% higher compared to Si. For sub-THz operation, besides electrons having a short travel time through the transistor structure, the device cross-section area needs to be minimized in order to suppress unwanted parasitic capacitances. FBH's fabrication process results in reduced parasitic capacitances through exposure of the collector side and InP wafer removal, allowing for the definition of very small area collectors as compared to the usual top-to-bottom processing. InP HBTs fabricated in FBH’s process with an emitter size of 0.8 x 5 µm² exhibit current gain and unilateral power gain cut-off frequencies (ft and fmax) of around 350 GHz at 20 mA collector DC current. The RF output power per emitter area amounts to 6500 W/mm² at 96 GHz. The circuit integration includes three gold interconnect layers embedded in low-k benzocyclobutene. RF connections are designed as microstrip waveguides, displaying low loss up to 300 GHz.
Heterointegration of InP HBT onto SiGe BiCMOS wafers
In FBH’s process, the active InP layers can be transferred to any suitable host substrate including fully processed silicon wafers. In collaboration with the research partner IHP, FBH has developed a wafer-scale process which monolithically integrates InP HBT, SiGe HBT, and CMOS devices. IHP’s high performance 0.25 µm SiGe BiCMOS MMIC process (SG25H1) includes HBT transistors with fT & fmax of 180/220 GHz and digital circuits with millions of CMOS transistors. Within the joint SciFab project it is used as substrate for combined MMICs. This technology is geared towards the realization of complex sub-mm-wave RF sources, offering reduced weight, size, and cost as compared to other module assembly techniques. The SciFab process is now open to external customers in a foundry mode.
Transistor cut-off frequency can be increased when reducing device dimensions. FBH is now introducing electron beam lithography to define the critical layers emitter, base, and collector. First transistors with 0.4 µm and 0.2 µm wide emitters are operational. With optimized dopant profile it is expected to reach an fmax of 700 GHz. In FBH’s process, the InP HBTs are embedded in BCB without thermal substrate connection, resulting in only moderate thermal device impedance (Rth = 3000 K/W). A 10 µm thick nanocrystalline (NC) diamond heat sinking layer is added on top of the layer stack in an additional wafer bonding step. With thin-film NC-diamond exhibiting high thermal conductivity (400 W m-1 K-1) and a very low loss tangent (< 10-4) FBH has reduced the HBT’s Rth
Active semiconductor device layers are located on top of a substrate, which usually consists of the same material, to facilitate handling during processing steps. The close proximity of the substrate makes it an integral part of the device—worsening the device’s high frequency properties through dielectric losses and fringing capacitances. At FBH, the indium phosphide (InP) semiconductor substrate is removed following a wafer bonding process to a host substrate. A layer made of benzocyclobutene (BCB), a material with low dielectric constant and low loss tangent, is used as a wafer bond adhesive. This layer electrostatically separates the active device layers from the substrate. As a host substrate, low-loss aluminum nitride (AlN) is used for the highest-frequency InP HBT circuits. The transferred-substrate process also works with fully processed BiCMOS wafers, in turn enabling InP-silicon heterointegration.
Further information regarding FBH's transferred-substrate process
I. Ostermay, A. Thies, T. Kraemer, W. John, N. Weimann, F.-J. Schmückle, S. Sinha, V. Krozer, W. Heinrich, M. Lisker, B. Tillack, O. Krüger, "Three-dimensional InP-DHBT on SiGe-BiCMOS integration by means of Benzocyclobutene based wafer bonding for MM-wave circuits", Microelectron. Eng., vol. 125, pp. 38-44 (2014).
T. Kraemer, I. Ostermay, T. Jensen, T.K. Johansen, F.-J. Schmueckle, A. Thies, V. Krozer, W. Heinrich, O. Krueger, G. Traenkle, M. Lisker, A. Trusch, P. Kulse, and B. Tillack, "InP-DHBT-on-BiCMOS Technology With fT/fmax of 400/350 GHz for Heterogeneous Integrated Millimeter-Wave Sources", IEEE Trans. Electron Devices, vol. 60, no. 7, pp. 2209-2216 (2013).
M. Lisker, A. Trusch, A. Krüger, M. Fraschke, P. Kulse, S. Marschmeyer, J. Schmidt, C. Meliani, B. Tillack, N. Weimann, T. Kraemer, I. Ostermay, O. Krüger, T. Jensen, T. Al-Sawaf, V. Krozer, and W. Heinrich, "Combining SiGe BiCMOS and InP Processing in an on-top of Chip Integration Approach", ECS Trans., vol. 64, no. 6, p. 177, 2014.
Flip-chip connections offer shorter interconnection paths as compared to wire bonding, thus enabling higher bandwidth. The use of front-end semiconductor fabrication techniques allows for the accurate definition of thin-film waveguide structures leading to the flip-chip connection, resulting in predictable EM behavior up to 500 GHz. Recently, FBH has successfully developed flip-chip mounts operating up to 250 GHz, including a stripline-to-coplanar waveguide RF transition. The on-chip RF connections were realized as gold (Au)/benzocyclobutene (BCB) stripline waveguides, with an approximately 10 µm wide Au signal line sandwiched between top and bottom Au ground planes. BCB was used as a low-k interlayer dielectric. The aluminum nitride submount featured gold-plated coplanar waveguides; interconnecting structures were realized with 10 µm diameter AuSn microbumps consisting of a multilayer metal stack with eutectic Au80Sn20 composition. Finally, the chips were placed onto the submount with the help of a semiautomatic flip-chip aligner with ±1 µm lateral alignment accuracy, within the design margin of ±2 µm. In the flip-chip bonding process, the chips and the substrate are briefly heated to above 300°C, leading to the alloying of the Au/Sn microbumps into the substrate’s Au contacts. This forms an electrical and mechanical connection between the chip and the substrate. Small-signal RF measurements of back-to-back flip-chip transitions showed an insertion loss below 0.5 dB per interconnect, and a return loss of more than 10 dB from DC up to 250 GHz. So far, these are the best reported figures for flip-chip mounts in this frequency range.
Further information on flip-chip mounting (research news, 06/2013)
Electronic devices and circuits at THz frequencies can be tested at FBH using an automatized on-wafer measurement system up to 500 GHz (extendible to 1100 GHz). The equipment features precise semi-automatic probing (accuracy < 3 µm) and can map entire wafers without manual assistance. FBH develops low-loss interconnects (insertion loss < -0.5 dB @ 200 GHz) as well as calibration standards and methods with predictable performance up to 500 GHz. FBH also cooperates with NIST, PTB, and industrial partners on calibration hardware and software, leading to verifiable high-accuracy results. Key focus of these activities is to overcome the inherent multi-mode waveguide propagation along with radiation and coupling effects on wafers at frequencies above 100 GHz. FBH further operates a large-signal characterization setup for power amplifier and mixer characterization up to 750 GHz, currently being extended beyond 1100 GHz.
Further information regarding FBH's THz characterization facilities
V. Krozer, R. Doerner, F.-J. Schmückle, N. Weimann, W. Heinrich, A. Rumiantsev, M. Lisker, B. Tillack, "On-Wafer Small-Signal and Large-Signal Measurements up to Sub-THz Frequencies", IEEE Bipolar / BiCMOS Circuits and Technology Meeting 2014, San Diego, Sept. 2014, available soon.