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arXiv:1907.12882 · Technical digest - International Electron Devices Meeting/Technical digest

Silicon Hard-Stop Spacers for 3D Integration of Superconducting Qubits

Bethany M. Niedzielski, David K. Kim, Mollie E. Schwartz, Danna Rosenberg, Greg Calusine, R. Das, Alexander J. Melville, Jason J. Plant, Livia M. Racz, Jonilyn Yoder, Donna Yost, William D. Oliver

Published Dec 1, 2019• 1 citation
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Abstract

As designs for superconducting qubits become more complex, 3D integration of two or more vertically bonded chips will become necessary to enable increased density and connectivity. Precise control of the spacing between these chips is required for accurate prediction of circuit performance. In this paper, we demonstrate an improvement in the planarity of bonded superconducting qubit chips while retaining device performance by utilizing hard-stop silicon spacer posts. These silicon spacers are defined by etching several microns into a silicon substrate and are compatible with 3D-integrated qubit fabrication. This includes fabrication of Josephson junctions, superconducting air-bridge crossovers, underbump metallization and indium bumps. To qualify the integrated process, we demonstrate high-quality factor resonators on the etched surface and measure qubit coherence (T1, T2,echo> 40 μs) in the presence of silicon posts as near as 350 μm to the qubit.

DOI
10.1109/iedm19573.2019.8993515

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As designs for superconducting qubits become more complex, 3D integration of two or more vertically bonded chips will become necessary to enable increased density and connectivity. Precise control of the spacing between these chips is required for accurate prediction of circuit performance. In this paper, we demonstrate an improvement in the planarity of bonded superconducting qubit chips while retaining device performance by utilizing hard-stop silicon spacer posts. These silicon spacers are defined by etching several microns into a silicon substrate and are compatible with 3D-integrated qubit fabrication. This includes fabrication of Josephson junctions, superconducting air-bridge crossovers, underbump metallization and indium bumps. To qualify the integrated process, we demonstrate high-quality factor resonators on the etched surface and measure qubit coherence (T1, T2,echo> 40 μs) in the presence of silicon posts as near as 350 μm to the qubit.
© 2019 IEEE. As designs for superconducting qubits become more complex, 3D integration of two or more vertically bonded chips will become necessary to enable increased density and connectivity. Precise control of the spacing between these chips is required for accurate prediction of circuit performance. In this paper, we demonstrate an improvement in the planarity of bonded superconducting qubit chips while retaining device performance by utilizing hard-stop silicon spacer posts. These silicon spacers are defined by etching several microns into a silicon substrate and are compatible with 3D-integrated qubit fabrication. This includes fabrication of Josephson junctions, superconducting air-bridge crossovers, underbump metallization and indium bumps. To qualify the integrated process, we demonstrate high-quality factor resonators on the etched surface and measure qubit coherence (T1, T2,echo > 40 μs) in the presence of silicon posts as near as 350 μm to the qubit.
Quantum computers perform calculations using quantum bits, or qubits, which can be made from superconducting circuits. These superconducting qubits allow direct control over device parameters while leveraging conventional Si-based fabrication techniques. MIT Lincoln Laboratory is designing and building high performance superconducting qubit devices with increased connectivity and addressability by utilizing advanced process techniques and 3D integration. Our three-tier stack architecture combines separately fabricated qubit, interposer, and routing chips with indium bump bonding. This stack contains high performance qubits, resonators, and couplers, superconducting air bridge crossovers, hard-stop spacing control, superconducting high aspect-ratio through-Si vias, and planarized superconducting routing layers. I will discuss our most recent accomplishments in the fabrication of three-tier stack devices and the effects of additional processing on the uniformity and integrity of their individual components.
Abstract As superconducting qubit circuits become more complex, addressing a large array of qubits becomes a challenging engineering problem. Dense arrays of qubits benefit from, and may require, access via the third dimension to alleviate interconnect crowding. Through-silicon vias (TSVs) represent a promising approach to three-dimensional (3D) integration in superconducting qubit arrays—provided they are compact enough to support densely-packed qubit systems without compromising qubit performance or low-loss signal and control routing. In this work, we demonstrate the integration of superconducting, high-aspect ratio TSVs—10 μm wide by 20 μm long by 200 μm deep—with superconducting qubits. We utilize TSVs for baseband control and high-fidelity microwave readout of qubits using a two-chip, bump-bonded architecture. We also validate the fabrication of qubits directly upon the surface of a TSV-integrated chip. These key 3D-integration milestones pave the way for the control and readout of high-density superconducting qubit arrays using superconducting TSVs.
Flip-chip 3D integration is a key enabling technology for overcoming the routing and layout constraints of planar superconducting quantum processors, but it requires accurate and uniform control of the inter-chip spacing while preserving qubit coherence. In conventional processes, thick indium bumps are deposited symmetrically on both chips, while dedicated spacers are introduced to limit the residual tilt. Here, we present a flip-chip bonding process based on an asymmetric indium architecture, combining 20 μm20\,μ\mathrm{m}20μm thick indium bumps on the control chip with 200 nm200\,\mathrm{nm}200nm thin indium pads on the qubit chip. Using electrical and microwave measurements, we demonstrate low resistance and low loss indium interconnects, and show that transmon qubits integrated with this process reach relaxation and coherence times of T1=(159±14) μsT_1=(159\pm14)\,μ\mathrm{s}T1​=(159±14)μs and T2,echo=(294±42) μsT_{\mathrm{2,echo}}=(294\pm42)\,μ\mathrm{s}T2,echo​=(294±42)μs, respectively, comparable to planar devices. Combined with a multi angle compression procedure that suppresses systematic tilt, this approach reduces the average post bonding tilt to (20±12) μrad(20\pm12)\,μ\mathrm{rad}(20±12)μrad. These results establish an asymmetric indium architecture combined with multi angle compression as a robust, coherence preserving route toward 3D-integrated superconducting quantum processors.
Abstract Three-dimensional device integration facilitates the construction of superconducting quantum information processors with more than several tens of qubits by distributing elements such as control wires, qubits, and resonators between multiple layers. The frequencies of resonators and qubits in flip-chip-bonded multi-chip modules depend on the details of their electromagnetic environment defined by the conductors and dielectrics in their vicinity. Accurate frequency targeting therefore requires precise control of the separation between chips and minimization of their relative tilt. Here, we describe a method to control the inter-chip separation by using polymer spacers. With the spacers, we measure a mean tilt of (76 ± 36) μrad, and a mean deviation of (0.4 ± 0.8) μm from the target inter-chip separation of 10 μm. We apply this process to coplanar waveguide resonator samples and observe chip-to-chip resonator frequency variations below 50 MHz ( ≈1%\approx 1\%≈1% ≈ 1 % ). We measure internal quality factors of 5×1055\times10^{5}5×105 5 × 10 5 at the single-photon level, suggesting that the added spacers are compatible with low-loss device fabrication.
Flip-chip integration offers a promising route toward scalable superconducting quantum processors and hybrid semiconductor-superconductor quantum devices. We develop a three-dimensional transmon architecture using electroplated indium in which the qubit electric field is shared nearly equally between two bump-bonded substrates while maintaining low participation at the indium-bump interface. The resulting geometry is well suited for future hybrid qubits, enabling the integration of distinct material platforms while minimizing sensitivity to bump-interface loss. Using this platform, we evaluate electroplated indium interconnects for superconducting quantum circuits. Flip-chip transmons incorporating electroplated indium bumps exhibit qubit quality factors around 10610^6106. In addition, a systematic study of coplanar-waveguide resonators is used to identify losses associated with the electroplating process. In particular, we find that surface losses associated with the gold-layer, used to enable good electric contact with the indium, is likely the primary contributor to the qubit decay rate. These results demonstrate the compatibility of electroplated indium technology with high-coherence superconducting circuits and establish a promising platform for three-dimensional hybrid quantum integration.