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European Microelectronics and Packaging Conference

Investigation of Aluminum and Gold Flip-Chip Bonding for Quantum Device Integration

Imants Cirulis, Uwe Zschenderlein, Silvia Braun, M. Radestock, R. Pantou, Klaus Vogel, Franz Selbmann, S. Kurth, Bernhard Wunderle, Harald Kühn

Published Sep 11, 2023• 4 citations
Publisher

Abstract

The majority of qubit chip integration is realized in a two-dimensional (2D) architecture. Whereas 3D architecture enables more advantages like efficient interconnect routing, allowing for more compact qubit coupling geometries, reducing form factor, and increased connectivity beyond nearest-neighbor interactions. Flip-chip (FC) assembly has been demonstrated to enable 3D architecture connecting qubit chip, interposer, and readout in a sandwich-like structure. Moreover, 3D integration allows the fabrication of hosting chip circuitry without degrading the qubit performance [1]–[4]. Different material considerations have to be taken into account since qubit operational frequency is in the gigahertz range and operating temperature is in the mK range to avoid thermal excitation. Materials that possess superconducting characteristics like Indium (In), Titanium Nitride (TiN), Tantalum Nitride (TaN), and Niobium (Nb) have been discussed as potential interconnect between building blocks [3], [5]. The In bumping on Aluminum (Al) redistribution layers require under-bump metallization (UBM) layers thus introducing multiple fabrication steps before the bonding process. An alternative approach would be to use the existing Al surface to electrochemically grow Al bumps and bond the chip using thermosonic bonding (TSB) at below 150°C to form a homogeneous metal-metal interface [6], [7].

DOI
10.23919/empc55870.2023.10418420

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The majority of qubit chip integration is realized in a two-dimensional (2D) architecture. Whereas 3D architecture enables more advantages like efficient interconnect routing, allowing for more compact qubit coupling geometries, reducing form factor, and increased connectivity beyond nearest-neighbor interactions. Flip-chip (FC) assembly has been demonstrated to enable 3D architecture connecting qubit chip, interposer, and readout in a sandwich-like structure. Moreover, 3D integration allows the fabrication of hosting chip circuitry without degrading the qubit performance [1]–[4]. Different material considerations have to be taken into account since qubit operational frequency is in the gigahertz range and operating temperature is in the mK range to avoid thermal excitation. Materials that possess superconducting characteristics like Indium (In), Titanium Nitride (TiN), Tantalum Nitride (TaN), and Niobium (Nb) have been discussed as potential interconnect between building blocks [3], [5]. The In bumping on Aluminum (Al) redistribution layers require under-bump metallization (UBM) layers thus introducing multiple fabrication steps before the bonding process. An alternative approach would be to use the existing Al surface to electrochemically grow Al bumps and bond the chip using thermosonic bonding (TSB) at below 150°C to form a homogeneous metal-metal interface [6], [7].
We have demonstrated superconducting interconnect technologies that enable a variety of flip-chip 3D integrated structures and packages compatible with high-coherence superconducting qubits. Superconducting indium micro-bumps and underbump metal (UBM) were used to join superconducting qubit chips to superconducting readout and control modules while maintaining high qubit coherence (T1, T2,echo> 20 μs) in the presence of capacitive and inductive coupling between the chips. Scanning electron microscope, X-ray, infrared and confocal microscopy were used to investigate the micro-structure, alignment accuracy, and parallelism of flip-chip qubits. The superconducting readout and control modules can accommodate both niobium and aluminum-based circuit and amplifier fabrication processes, including shadow-evaporated aluminum or Nb/Al-AlOx/Nb trilayer Josephson junctions (JJs). We present results for up to 16 active superconducting chips having trilayer junctions bonded to a passive superconducting module. The I-V characteristics and switching behavior were measured for flip-chip-connected JJ arrays with 40-20,000 JJs. Our approach maintained chip-level junction critical current and qubit coherence, demonstrating it to be a viable approach to building larger quantum computing systems. This paper also discusses packaging approaches to developing a quantum-to-classical interface in a cryogenic environment with multiple temperature stages.
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.
In this article, we present a technology development of a superconducting qubit device 3D-integrated by flip-chip-bonding and processed following CMOS fabrication standards and contamination rules on 200 mm wafers. We present the utilized proof-of-concept chip designs for qubit- and carrier chip, as well as the respective front-end and back-end fabrication techniques. In characterization of the newly developed microbump technology based on metallized KOH-etched Si-islands, we observe a superconducting transition of the used metal stacks and radio frequency (RF) signal transfer through the bump connection with negligible attenuation. In time-domain spectroscopy of the qubits we find high yield qubit excitation with energy relaxation times of up to 15 us.
In recent years the interest in building quantum computers with superconducting qubits has steadily increased. Indium is one of the chosen interconnect materials for bonding qubit microchips with Josephson junctions to microelectronic resonators because indium is superconducting and ductile. Indium bump bonding is a common technique for hybridizing pixel radiation sensors to readout chips for some time; however, such detectors operate usually at room temperature. The challenge is to transfer this bonding technique to the fabrication of devices that can operate and withstand very low mKtemperatures. This paper summarizes the current challenges for implementing indium bump bonding to stacked superconducting qubit chips with a particular focus on chip parallelism and superconductivity of the interconnects. A high precision of parallelism between bonded qubit and resonator microchips is essential for minimizing signal loss, and superconducting materials such as indium are required for signal transfer. Photolithography or shadow masking techniques are commonly used to deposit severalμ\muμm-thick indium bumps on microchips. This paper outlines initial experiments to verify and optimize the indium bump deposition and flip-chip bonding for superconducting qubit chips. Stacked chip parallelism is evaluated after flip-chip bonding, the tolerance of the parallelism between stacked chips will be determined with an optical scanner, microscope and Scanning Electron Microscopy (SEM). For the superconductivity of interconnects, daisy chain chips are designed, built and flip-chip bonded. The common Under-Bump Metallization (UBM) for the adhesion of indium bumps to pixel pads requires modification with superconductive material and is investigated with resistance measurements that are taken at room temperature with a probe tester and at mK-temperature in a helium dilution refrigerator.
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.
As one of the mainstream technique routes to implement universal quantum computing, superconducting qubit has been highly anticipated in recent years. However, the current widely-using 2D interconnect forms are not capable to meet the interconnect demand of high integration quantum processor in practical use due to wiring congestion, electromagnetic crosstalk, mechanical stress concentration, limited interior space of dilution refrigerator, etc. Therefore, quantum packaging will also need to be updated from 2D planar to 3D vertical architecture for higher integration density of superconducting qubits. In view of the current chiplet development trends in IC industry, we propose a 3D integration architecture based on the partition strategy of qubit chiplet and bus chiplet for superconducting quantum computing. This chiplet solution respectively utilizes the TSV and microbump structure in superconducting constituent material scheme to achieve low-loss intra-layer and inter-layer connection, which provides a possible technical route for modular large-scale quantum processor. To verify the feasibility of the above architecture, we design a kind of TiN-based TSV test vehicle including daisy chain structure to inspect the stabilization of DC resistance and transmission line structure for evaluation of DC/microwave signal transporting performance during the cool-down process. The TSVs in this test vehicle possess high aspect ratio (10:1,30μm30 \mu\mathrm{m}30μmdiameter) fabricated by DRIE, coating with ∼200nm TiN layer by ALD. Subsequently, Al/TiN planar wiring layer is patterned by secondary lithography and ICP process. Then morphology observation, R.T./cryogenic electrical tests are successively carried out to evaluate the performance of TSV test vehicle. SEM along with EDS reveal that the TiN coating is conformal and defect-free with nearly vertical sidewall profile. Electrical tests show that TSV structures possess satisfactory linear ohmic characteristics and a wide range of process uniformity and repeatability at R.T. Additionally, the DC resistance and S21loss amplitude both exhibit abrupt drop during the cooling process, indicating that certain degree of superconducting transition has probably occurred. Therefore, the TiN-based TSV interposer elaborated in this paper combined with indium-based flip-chip TCB methods provides the possibility to implement 3D quantum integration architecture based on chiplet design.