Wafer-level hetero-integrated InP DHBT/SiGe BiCMOS technology for mm-wave and sub-THz applications

The mm-wave and sub-THz region from 100 to beyond 500 GHz holds many promises for emerging applications such as ultra-wideband wireless communications, high-resolution radar, spectroscopic sensing, or medical and industrial imaging, just to name a few. While complex RF frontends can be realized in silicon at microwave frequencies, the RF output power of any silicon-based technology is limited at mm-wave frequencies and beyond: the silicon substrate introduces losses, and the breakdown voltages are reduced as SiGe heterobipolar transistors – the silicon technology of choice for RF analog applications – are scaled to sub-100 nm dimensions. However, InP transistors are well suited for RF power generation in this frequency range: their electron mobility and velocity is higher than in silicon, and the breakdown fields are exceeding silicon’s due to the wide InP bandgap. 

On the other hand, III-V semiconductor technologies cannot match silicon’s complexity – the silicon industry has invested trillions over the last 60 years to perfect manufacturing equipment and processes. Thus, a wafer-level  combination of silicon and InP, where only critical high-frequency building blocks  are realized in InP and anything that can be done in silicon is left to silicon, is advantageous in terms of reduced chip  and packaging cost, system size and weight reduction, and test and assembly expenses. This combination is opening up commercial opportunities in the mm-wave and sub-THz region, which are currently underused because parts are not available at reasonable prices.

FBH and IHP’s monolithically integrated InP DHBT / SiGe BiCMOS process is now available in a foundry mode: third-party chip developers can submit their own designs for fabrication through IHP’s foundry interface. A complete process design kit (PDK) includes schematic and layout cells for seamless co-design. The heterointegrated technology has been developed between FBH and IHP over the last four years. The design kit development was supported under a recent Leibniz competitive project “SciFab”. The state of the technology was presented in a recent workshop on III-V-silicon heterointegration for RF, which was held at the international microwave symposium in San Francisco, CA (IMS 2016).

Heat removal from the InP transistors is particularly difficult when integrating these on top of a full-stack silicon BiCMOS wafer. Moreover, the InP transistors are embedded in BCB, an organic dielectric with very good RF properties, but also featuring near-zero thermal conductivity. The generated heat is removed through the transistor’s contacts. The pertaining problem is to connect the RF-carrying contacts to a heat sink without degrading the transistor’s performance due to capacitive loading or resistive losses. FBH’s solution to this problem is the integration of a diamond heat spreading layer into the InP HBT interconnect stack. The diamond layer is located on top of the completed InP MMIC stack. The hot side of the HBT, the collector, can be connected thermally to the diamond layer with a metal-filled floating via, with minimal impact on the HBT’s performance. The integration of the diamond heat spreader reduced the transistor’s thermal resistance by a factor of three or more, enabling higher power dissipation in the device and hence higher RF output power. In addition, multifinger transistor designs which previously suffered from thermal instability can now be realized, strongly reducing the need for corporate power combining of multiple individual transistors, which always reduces circuit efficiency and bandwidth. At IMS 2016, FBH has presented two circuit applications, highlighting the advantage of this technology: one RF power amplifier outputting 200 mW around 90 GHz, and an efficient active broadband quadrupler which can produce 330 GHz signals with -7  dBm output power.

Publications

M. Hossain, K. Nosaeva, N. Weimann, V. Krozer and W. Heinrich, „A 330 GHz Active Frequency Quadrupler in InP DHBT Transferred-Substrate Technology“, IEEE MTT-S Int. Microw. Symp. Dig., San Francisco, USA, May 22-27, Th1C-6 (2016).

T. Al-Sawaf, K. Nosaeva, N.G. Weimann, V. Krozer, and W. Heinrich, „A 200 mW InP DHBT W-band Power Amplifier in Transferred-Substrate Technology with Integrated Diamond Heat Spreader“, IEEE MTT-S Int. Microw. Symp. Dig., San Francisco, USA, May 22-27, THIF2-5 (2016).

N. Weimann, „Wafer-level heterointegrated InP DHBT / SiGe BiCMOS technology for mm-wave and sub-mm-wave applications“, IEEE MTT-S Int. Microw. Symp., San Francisco, USA, May 22-27, WMG-7 (2016).