by Nancy Friedrich, Industry Solutions Marketing, 6G Thought Leadership
With initial 6G working groups launched by the Third Generation Partnership Project (3GPP), final research toward 6G standardization and deployment is underway. The initial research phase included two spectrum candidates: sub-terahertz (Sub-THz) frequencies from 90 to 300 GHz and the informally labeled frequency range 3 (FR3), covering 7 to 24 GHz. Although sub-THz frequencies may be used in future releases or generations, it is confirmed that 6G will use FR3.
The Wideband Multifunctional Software Defined Radio (WMSDR) project bridges the 5G-to-6G gap by covering up to 24 GHz using thin-film variable capacitors (varactors) developed by Professor Subramanyam at the University of Dayton in a project team also including the Air Force Research Laboratory (AFRL) and a new startup, CapV LLC. Guru Subramanyam, University of Dayton Professor, Electrical and Computer Engineering, explains that the project’s focus is on lab to fab transition by incentivizing key technologies to be matured very quickly for the benefit of the Department of Defense (DoD).
The Midwest Microelectronics Consortium (MMEC) funds this Microelectronics Commons Project, which is the U.S. DoD portion of the Creating Helpful Incentives to Produce Semiconductors (CHIPS) Act. MMEC is one of the eight hubs established across the country, located in Dayton, Ohio. As one of only five 5G/6G projects, the WMSDR is currently in its first year of the three-year program.
Lockheed Martin leads the WMSDR project team. In fact, Lockheed reached out to the University of Dayton in 2018, when Subramanyam was working with a company called Indiana Microelectronics on an Air Force-funded small business technology transfer research (STTR) phase 1 grant. Lockheed recognized the varactor’s potential to help them miniaturize broadband tunable filters. From there, they started an independent R&D (IRAD) program from 2018 through 2020, working for about two years on ultra-miniaturized tunable filters up to 18 GHz. These filters were roughly 11 x 11 mm2 in size, with chip scale integration being the overall program goal.
At the end of the Lockheed IRAD program, the team won the DARPA wideband adaptive RF protection (WARP) phase 1 program. The WARP tunable filters were designed for interference cancellation and other performance capabilities. Lockheed Martin, the University of Dayton (UD), Indiana Micro, and 3D Glass Systems (3DGS) were all part of those phase 1 and phase 2 programs. At the end of phase 2, CapV LLC was created for commercialization of thin-film varactor technology. The entire WARP team, along with Intel Corp., won the current WMSDR funding from the ME Commons program.
As a lab-to-fab transition company, CapV has developed AI-enabled varactor and varactor-based chips. Subramanyam lists the chips they make as one of the things he is most excited about in the current work. He says, “We make these thin-film varactor chips. A chip can be as simple as a single varactor, which can be used as a tuning element and integrated in a hybrid fashion with other chips. At the same time, we can also make multi-varactor based complex chips, such as analog phase shifters for precision phase control, tunable true time delay units, and reconfigurable antennas, which are all important for multiple input multiple output (MIMO) applications.”
6G Security and Performance
Looking beyond the chips to FR3 and 6G performance, Subramanyam anticipates improved latency, bandwidth, and connectivity, such as providing much faster data transfer and enhanced performance in terms of links and MIMO systems. He says, “What’s more exciting for us is, even at FR3 bands, you can easily switch the RF front-end modules from one center frequency to a completely different frequency using our technology. If there are some interferers, you can sense and move very quickly to a different band and still achieve a very high bandwidth and other performance features. That’s the capability provided by the varactor technology.”
Engineers can simply move up or down in frequency so that they can avoid interfering with signals anywhere nearby to effectively obtain the necessary bandwidth. This solution could have potential applications for privacy and security as well.
Design and Integration of Varactors
“There are a lot of applications for this technology,” Subramanyam notes. “Basically, all of them will be for chip-scale solutions. The challenges are in the integration of our chip-scale solutions onto existing RF circuits and systems. We have successfully demonstrated our chips for applications up to 18 GHz with no issues in terms of integrating our varactors and demonstrating tunable filters and other applications.”
Subramanyam believes that the varactor technology works well at higher FR3 frequencies as well. CapV has been working with Modelithics Corp. to develop models of varactor chips for integration with Keysight’s Advanced Design System (ADS) and Cadence’s AWR electronic design automation (EDA) tools. Modelithics has developed models for CapV’s varactor chips up to 67 GHz. These varactors can be designed for multiple form factors and integration requirements. Subramanyam says, “We can, for example, design a 50-ohm coplanar waveguide (CPW) transmission line multiple ways because the width of the signal line to the spacing between the ground lines determines the characteristic impedance. We can integrate our devices into multiple different existing designs.”
As a result, Subramanyam says everything is pretty much a custom design. This approach, for example, means they also have to design the biasing circuits for controlling the varactors precisely using a digital to analog converter (DAC). Subramanyam notes, “We also need to make sure that we are routing the biasing lines properly on existing boards and these will not affect the performance of the rest of the RF circuits.”
CapV is also developing a process design kit (PDK) for thin film varactor technology, with the goal of offering multi-project wafer (MPW) services by the last quarter of 2026. “Once we have the PDK,” Subramanyam states, “we can design and fabricate custom chips for any customer who wants to try our varactor technology. CapV will be the only U.S. entity to offer custom MPW chip fabrication service using the thin-film varactor technology.”
Testing for Confidence
The thin-film varactors are the key components for the WMSDR project for reconfigurable radio-frequency (RF) front end and interference cancellation. The team has tested the varactors all the way up to 67 GHz, so they are confident in their operation. Typically, they do electromagnetic (EM) simulations first to assess the performance of the chips. Subramanyam says, “Obviously, we try to model everything including the parasitics. We need to keep in mind the hybrid packaging, wire bonds, and the DC bias lines. We model everything very, very carefully and make sure that we can characterize each of the components of the chips using their drop-out test structures.”
Subramanyam states, “We have already supplied our varactor chips for the WMSDR project. Our team has integrated the varactor devices with the rest of the chips and built the WMSDR system. Demos are planned at the end of our first year. We are working toward solving the challenges in covering the FR3 bands and beyond in our next phase of the project.“
Meet the Expert
Subramanyam just completed his 27th year as a faculty at the University of Dayton. An electrical engineer by training, he got his MSEE and PhD in microelectronics from the University of Cincinnati. During his PhD, he worked with high-temperature superconducting thin films funded by NASA Glenn Research Center in Cleveland, Ohio. He worked closely with NASA Glenn researchers in the Communication Technology division during his graduate student years from 1986 to 1993. He credits the colleagues from NASA Glenn as the reason for his research in thin-film varactors. While serving as a summer faculty at NASA Glenn, he designed and developed ferroelectric thin-film-based tunable filters for satellite communications from 1997 to 2001.
Subramanyam says, “During my first year as a summer faculty fellow, I was introduced to thin-film ferroelectrics and started working on integrating thin-film varactors for Ku-band tunable filters. Throughout my career, it has been exciting to work with new materials and new devices. Even though I’m an EE, I enjoy solving many materials processing and materials integration issues for microwave and mmWave circuits and make them work well.”
Subramanyam has been collaborating with the AFRL Materials and Manufacturing Directorate and Sensors Directorate since he joined the UD in 1998. “I continue to collaborate with colleagues at AFRL. We have an excellent group of researchers at AFRL, UD, and CapV. We are excited to transition the technology that we have developed over the years.”
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