I first met Sorin Voinigescu in 1995, when – with a newly minted Ph.D. in hand — he was at the International Electron Devices Meeting (IEDM) presenting some of the early work on RF circuits crafted in CMOS technology.
Nearly 24 years later, Voinigescu is doing equally innovative pathway research in quantum computing, using the 22FDX® process from GLOBALFOUNDRIES (GF) to investigate how to integrate qubits with the RF control and readout circuits. And Voinigescu sees a type of Moore’s Law for quantum devices, in which scaled-down qubits and support circuits are able to operate at higher temperatures, perhaps obviating the need for the scarce helium consumed in today’s cryogenics.
Quantum devices today are largely Josephson Junction superconducting devices operating at milliKelvin temperatures, with wires connecting the qubits to the control and measurement electronics. Voinigescu’s lab at the University of Toronto is studying how to create semiconductor-type qubits that could be controlled with millimeter wave signals. The present-day superconducting qubits have quantum energy separation levels in the 5-10 GHz range. In order to operate the quantum gate, the microwave control signals need to be at that frequency, the 5-10 GHz range.
“All qubits, regardless of implementation, mimic a spin, and the control is performed with a signal that must resonate with the electron spin resonance frequency of that qubit,” Prof. Voinigescu explained. One way of thinking of it, he said, is that each quantum gate could require the equivalent of a 5G cellular signal, perhaps in the 60 GHz range. In fact, he was attracted to the field of quantum computing a few years ago, when he attended a session on quantum computing at IEDM and realized that his two decades of research in high-frequency circuits could play a role in the quantum computing field.
22FDX at 3.3 degrees Kelvin
There is what he calls a “trinity” in the search for higher-temperature quantum computing, where devices must be isolated from any heat or disturbances. The smaller the gate width of the transistor, the higher the frequency required to excite the qubit gate, and the higher temperature it can be operated at. Now, the 22FDX-based devices the Voinigescu lab is investigating have a gate width of 50 nm (the gate length is 18nm, and the channel thickness is 6-7nm). As the gate width is reduced, a somewhat higher temperature environment can be used for the qubits, control, and measurement circuits.
And the cool (excuse the pun) thing about the 22FDX process is something the Toronto lab and its partners recently discovered: at the extremely low temperatures required of quantum systems, the performance of the active and passive high-frequency devices actually improves.
The University of Toronto team, working with GF and industrial partners Lake Shore Cryotronics and Keysight Technologies, among others, reported at the 2019 RFIC conference, held in Boston in June, how the 22FDX process was used to create monolithically integrated double quantum dots with readout transimpedance amplifiers (TIAs), with the output matched to 50 Ω.
More importantly for circuit design, the researchers found that the high-frequency performance of all the active and passive devices created in a production-type 22nm 22FDX technology improved at 3.3 degrees K, with no variance of the polysilicon resistors and improved quality factor of the MOM capacitors.
“What is unique to FD-SOI is that at low and high frequencies, the circuits are not affected by de-ionization, as bulk MOSFETs are known to be affected. Because of that we essentially get significantly better performance at low temperatures, as measured down to 2 degrees Kelvin. In fact, we see significant improvements down to 60-70 degrees K, and below that performance essentially remains flat,” he said. Transconductance, mobility, and fmax all improved, and that has important implications for space, satellites, and other low-temperature environments as well.
At low temperatures, threshold voltages increase for n-MOSFETS and decrease for p-MOSFETs, regardless of the technology. With FD-SOI, the back gate can be used to adjust the Vts to the optimal operation point. Circuits can be designed at room temperature, and then at low temperatures can be “validated,” tuning the Vt’s with back-gate biasing. Circuits that find a “sweet spot” at room temperature, Voinigescu said, can maintain that current density down to 2 degrees Kelvin.
Smaller Dimensions Help Raise Temps
Jamie Schaeffer, the product offering manager for the 22FDX and 12FDX FD-SOI platforms at GF, said the qubits are created in the six or seven-nanometer active layer, providing confinement for Coulomb and spin blockade devices that are, in a sense, boxed in by the buried oxide. “We have to get the spin layers to interact, and with more advanced dimensions we can get those closer. As we are going from 22 to 12 FDX, the smaller dimensions serve the goal of higher temperature quantum computing,” Schaeffer said.
Nigel Cave, a technologist who works in the CTO office at GF, said as semiconductor-type qubits are scaled to smaller dimensions, it may be possible to bring the quantum system’s operating temperature above 4 degrees Kelvin, from the 10-100 milliKelvin in today’s systems. This would enable the use of a standard helium cryostat versus a dilution cryostat, thus reducing costs and also allowing 1-2 watts of total power to be removed from the system. “The ability to remove more power potentially paves the way to co-integrate the Qubits and their control circuitry in the same FDX based device” Cave said.
Schaeffer said IBM, Google, Intel, Microsoft, and others have large quantum research programs underway. “In our case, we believe we can contribute something that is enabling for our partners who are doing meaningful work in the quantum sciences. We have a toolset that is manufacturable and leveraging our process integration capabilities is a way to get to lower costs.”
Two Camps in Quantum Ecosystem
Ted Letavic, a vice president and senior fellow at GF, said from a ten-thousand-foot level, the quantum compute community can be divided into two camps: those who are pushing for ways to create thousands of qubits in order to increase the quantum compute power; and a camp that argues more attention needs to be paid to how to use the roughly 50-100 qubit systems that now exist in order to solve real-world problems.
“One faction says we need thousands of qubits, the other faction says we have 50-100 qubit systems now and don’t know what to do with them. One answer is to provide free access in consortia, and together we can best figure out how to use them, how to create economic value and advance our economy,” he said.
GF has “some key technologies that can help,” acting as a foundry for startups, universities, and others as they investigate different approaches. Letavic, along with Cave and John Pellerin, deputy CTO and vice president of worldwide R&D, provided input to the Department of Energy, which earlier this year put out a Request for Information regarding how best to organize the Quantum Information Science Centers (QISCs).
They argued that while current exploratory R&D is largely being done in non-standard university labs, GF could provide a process integration and early manufacturing effort for researchers, startups, and others participating in the QISCs. Working with foundries would ensure that “devices intended to unlock the promise of quantum systems can be fabricated in volume within existing manufacturing assets.”
Letavic pointed to the work being done with Prof. Voinigescu as one real-life example, where the FD-SOI devices proved advantageous for I/O at 4 degrees Kelvin, and hold promise as a source of qubit transistors confined in the very thin FD-SOI layer. The Toronto effort used wafer shuttles that were processed at GF’s Dresden, Germany fab.
GF also has a silicon germanium platform, as well as a silicon photonics capability, that could play a role in “unlocking the promise of quantum.”
“I do believe in quantum, but it is going to be additive to classical compute,” Letavic said. “The society that gets to a quantum compute infrastructure first will have a very large economic advantage over the rest. And whether you are in the camp of ‘let’s chase the maximum number of qubits,’ or the camp that says ‘let’s figure out how to best use quantum systems to the best of our ability,’ GF is playing in both.”