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Technical digest - International Electron Devices Meeting/Technical digest

CMOS-Fabricated Ring Surface Ion Trap with TSV Integration

Peng Zhao, Yu Dian Lim, Hongyu Li, J.-P. Likforman, Luca Guidoni, Lilay Gros Desormeaux, Chuan Seng Tan

Published Dec 9, 2023• 3 citations
Publisher

Abstract

We present the design, fabrication, and test of ring surface trap on 12-inch wafers with a CMOS process. The design is based on Through Silicon Vias (TSV) interconnects. Up to 200 ions were loaded and cooled; preliminary compensations of electrostatic potential imperfections show that rotational symmetry can be partially restored.
DOI
10.1109/iedm45741.2023.10413875

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In this study, we report the first Cu-filled through silicon via (TSV) integrated ion trap. TSVs are placed directly underneath electrodes as vertical interconnections between ion trap and a glass interposer, facilitating the arbitrary geometry design with increasing electrodes numbers and evolving complexity. The integration of TSVs reduces the form factor of ion trap by more than 80%, minimizing parasitic capacitance from 32 to 3 pF. A low RF dissipation is achieved in spite of the absence of ground screening layer. The entire fabrication process is on 12-inch wafer and compatible with established CMOS back end process. We demonstrate the basic functionality of the trap by loading and laser-cooling single 88Sr+ ions. It is found that both heating rate (17 quanta/ms for an axial frequency of 300 kHz) and lifetime (~30 minutes) are comparable with traps of similar dimensions. This work pioneers the development of TSV-integrated ion traps, enriching the toolbox for scalable quantum computing.
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Further miniaturization of surface electrode ion trap is restricted by wire bonding connection. To resolve this restriction, integration of conventional surface electrode ion trap with through silicon via (TSV) interconnects is proposed and preliminarily demonstrated. Located directly underneath the DC and RF electrodes, TSVs can transmit the electrical signal from interposer to the electrodes on the top. In this work, electrostatic simulation and fabrication processes are introduced to justify the feasibility of this integration. From the electrical characterization (I-V and C-V tests) results, TSV-integrated ion trap exhibits smaller leakage current and lower capacitance than its wire bonding counterparts due to the elimination of large surface area.
Surface electrode ion trap with through-silicon-via (TSV) integration enables 3D stacking of ion trap chip on an interposer to eliminate the wire-bonds on the surface electrodes and also addresses the challenge of the ever increasing complexity of surface electrode design with low-parasitic and high-density interconnect requirements. In this work, we demonstrate the design and fabrication of TSV integrated surface electrode ion trap on a 300-mm Si wafer platform. By designing the TSV arrays directly underneath the surface electrodes, the surface electrode foot print is reduced and the TSV traps show better RF performance compared to the planar traps with wire-bonding pads.
Laser-cooled trapped ions platform is one of the best candidates for the development of future quantum computing. This has generated a worldwide research effort for scaling and integrating trapping devices. As part of this effort, we are developping cylindrically symmetrical linear Paul traps manufactured in collaboration with Nanyang Technology University and the Microelectronics Institute of Singapore. The electrical contacts of these traps electrodes are made through the Silicon substrate (TSV: through silicon vias), allowing not to break the desired symmetry with surface connections. The trapping and laser cooling of ions in ring traps was demonstrated at Garching [2] and subsequently extended to microfabricated traps at Sandia National Labs [3] and Berkeley [4]. In the absence of defects, these devices enable rotational symmetry for the trapped ions. They are also candidates for manipulating the rotational quantum state of trapped ion ensembles [4].We will present our results concerning the performance of TSV surface ring traps manufactured in Singapore. In particular, we loaded these traps with Doppler-cooled 88Sr+ ions and we estimate the defects of the trapping potential with respect to perfect rotational symmetry. The application of DC voltages to a set of segmented electrodes makes it possible to compensate for most of these defects for observing sub milikelvin free motion.[1] P. Zhao et al., Appl. Phys. Lett. 118, 124003 (2021).[2] I. Waki et al.,Phys. Rev. Lett. 68, 2007 (1992).[3] B. Tabakov et al., Phys. Rev. Applied 4, 031001 (2015). [4] E. Urban et al., Phys. Rev. Lett. 123, 133202 (2019)
Among the various physical systems for quantum bit (qubit) implementation, trapped ion has drawn significant attention due to its long coherence time (in the second range) and high gate fidelity (>99.99% for single qubit gate). After years of careful development, the ion trap, from mechanically assembled linear trap to lithography-defined surface electrode ion trap, is compatible with CMOS fabrication, which opens the door for the high-resolution, high-repeatability and large-scale ion trap manufacturing. To date, the number of fully connected ion qubits in state-of-the-art ion trapping devices is ~10. However, to scale close to or even beyond the noisy intermediate-scale quantum regime (NISQ, ~100 ions), some challenges remain. One such challenge is to maintain the delicate control of multiple components over individual ion with increasing ion number. In such context, this thesis is committed to boost the scalability of ion trap by integrating advanced 3D interconnects (e.g., through silicon via) and other functional modules (e.g., silicon photonics and multilayer metallization). 3D interconnects are integrated in the half space underneath surface electrode, to replace conventional wire bonding that extends out of the surface. Silicon photonics (grating coupler and waveguide) in place of bulk optics and lenses are used for on-chip light routing and emission. Multilayer metallization is used to maintain the overall RF performance and enable high-density interconnection where necessary. Firstly, surface electrode ion traps on different substrates (namely high resistivity silicon, silicon with grounding plane and glass) are designed and fabricated. Standard CMOS back-end-of-line fabrication process on 12-inch wafer platform is employed. The electrical performances of different traps are compared in terms of leakage current (I-V), parasitic capacitance (C-V), on-chip RF loss (S-parameter) and post-packaging resonance. Though the leakage (at the order of 10-8 A) between neighboring electrodes is relatively high, ion trap on glass substrate demonstrates extremely small capacitance (<1 pF) and low RF loss (insertion loss of <0.05 dB at RF frequency of 50 MHz), as compared to the silicon counterparts. This is mainly due to excellent insulation property of glass. Following that, ion trapping test is performed on the glass trap and ions are successfully confined with an averaged lifetime of ~30 minutes (compatible with the 10-11 mbar vacuum level (10-13 Pa)). To simultaneously leverage the dielectric property of glass and the microfabrication compatibility of silicon, the integration of silicon ion trap and glass interposer is performed, between which TSV is used to build the vertical interconnection. Due to the incorporation of TSV, the original wire bonding pads as well as the connection circuits on the surface electrodes can be eliminated. As a result, the form factor of TSV integrated trap is reduced by ~10 times. Correspondingly, the parasitic capacitance between neighboring electrodes is reduced from >24 pF to 3 pF. Similarly, the insertion loss of TSV integrated trap drops to 0.11 dB, in sharp comparison to the previous traps on silicon substrates (up to 2.4 dB). Analytical models are built to respectively characterize the electrode capacitance and overall power loss, and a good agreement is achieved between the model and the measurement result. In addition, since TSV is located in the half space underneath the surface electrodes, the electrodes geometry design is enabled with high flexibility. Meanwhile, the laser obstruction issue due to the stick-out bonding wires has been resolved. In terms of the ion trapping performance, both the heating rate (17 quanta/ms for an axial frequency of 300 kHz) and the lifetime (~30 minutes) of TSV trap are comparable with non-cryogenic traps of similar dimensions. Moreover, a customized CPGA with patterned redistribution layer is used to locate TSV integrated trap directly in the absence of glass interposer, facilitating high-efficiency thermal management and upgrading the packaging flexibility. At the next stage, the co-integration of other functional modules such as silicon photonics and multilayer metallization with TSV trap is investigated. In silicon photonics module, waveguide and grating coupler are on-chip introduced in place of bulk optics to achieve localized light routing and emission. In multilayer metallization module, as a first step, a grounding plane is incorporated into TSV trap to shield the silicon substrate from RF signal, in which specific windows are patterned onto the plane to accommodate TSV and allow the transmission of lights. The compatibility between TSV/multilayer metallization and waveguide/grating coupler for electrical and optical (E/O) signal routing is demonstrated. The preliminary test result based on wafer frontside shows that a ~40% capacitance reduction is achieved, and the insertion loss further drops to 0.06 dB, close to the performance of glass trap. In addition, the simulation results on photonics components indicate that an overall coupling efficiency of <30 dB is obtained from the input fiber to the ion through the photonics circuit. However, the measured power loss for light with wavelength of 1092 nm is as high as 50 dB. The deviation between the simulation and measurement is largely due to the fabrication imperfection and the resultant sloped grating coupler profile. This thesis focuses on the scalability enhancement of ion trap by integrating TSV, silicon photonics and multilayer metallization into conventional ion trap. The results are promising and pave the way for the future large scale quantum computing based on trapped ion.