Session T48: Superconducting Fabrication, Packaging & Validation

CMOS-compatible wafer-scale MOCVD of superconducting TiN in Through Silicon Vias for 3D integration of qubits

Quantum Processors with large numbers of qubits demand CMOS-compatible, superconducting Through Silicon Vias (TSV) for realizing 3D integrated architectures. The superconductor TiN is a highly suitable material for conformal coating of TSV inner side walls. So far, used processes have mostly relied on customized tools or time-consuming atomic layer deposition (ALD).

In this study, we applied a standard production tool designed for 200 mm wafers to metalize superconducting TSVs using metal-organic chemical vapor deposition (MOCVD). First, we studied the temperature dependence of the two-step cyclic process, which involves thermal deposition followed by plasma annealing, on flat Si substrates. Deposition at 570K and below is reaction-limited (ALD mode) while at higher temperatures it becomes diffusion-limited (MOCVD mode). For optimized deposition at 690 K, we achieved T_c ~ 3 K, deposition rates > 2.0 nm/cycle, resistivities < 140 µΩcm and a thickness homogeneity of 15% across the wafer. To demonstrate the successful metallization of TSV sidewalls, we used Si ring structures with an aspect ratio of 1:6. Those were electrically analyzed at room temperature as well as cryogenic temperatures proving superconductivity below 0.35 K.   

Electrical characterization of Metal-Insulator-Metal contacts for superconducting qubits fabricated with CMOS production tools

Metal-insulator-metal (MIM) contacts are crucial for understanding Josephson junctions employed in superconducting qubits. In this work, we focus on comprehensive studies of Al-AlOx-Al MIM contacts with different sizes from 0.04 µm² to 1600 µm² fabricated on 200 mm silicon wafers with CMOS production tools. A vacuum breaking approach between sputtered bottom and top Al electrodes is examined.

We observed direct as well as trap assisted tunneling. For 0.35x0.35 µm² devices a mean resistance of 13 kΩ at room as well as cryogenic temperatures is achieved, which is the target value for qubits. It will be shown that besides SEM, TEM and EELS room temperature electrical characterization with current, capacitance, and voltage breakthrough measurements are useful.

Etching and different surface treatments (Ar ion etch) before oxidation strongly influence the overall device performance. For MIM stacks of 1 µm² size, depending on ion cleaning and corresponding surface roughness, mean resistance changes from 12 GΩ to 10 kΩ were observed. Furthermore, a significant current proportion is transported through the junction sidewalls, and not through the main overlapping area. Depending on the etching process the ratio varied between 1:25 and 1:1.25.   

Chip Packaging with Improved Microwave Performance for Superconducting Quantum Circuits

Chip packaging has become a limiting factor for achieving long lived coherent operation of superconducting quantum circuits. The local microwave environment represents a source for loss and decoherence that needs to be considered in the packaging design.[1] Quantum Machines and QDevil have developed a chip packaging solution called QCage, optimized for low loss and decoherence, reflection free transmission and free from resonances.[2,3] This solution has been adopted by several scientific laboratories who saw vast improvements compared to packaging solutions developed by themselves. In an extensive study of losses in superconducting resonators by the Houck lab at Princeton University, quality factors as high as 200 million were reached in the QCage.24 sample holder.[4] These results indicate that the residual microwave losses in the packaging are only contributing to relaxation on time-scales of tens of milliseconds, much longer than the best superconducting qubits that were demonstrated to date.[5] In another study of quasiparticle dynamics in hybrid semiconductor/superconductor Josephson junctions by Shabani lab at New York University,suppression of quasiparticle poisoning was observed.[6] This was in part attributed to the careful EMC shielding of the QCage design.   

Superconducting through-silicon vias as capacitive elements in quantum circuits

Superconducting qubits and their control components typically require large capacitances.  The dimensions of the capacitors can be large due to the relatively low capacitance density of 2D planar designs.  In this talk, I will discuss our recent work utilizing high-quality superconducting through-silicon vias (TSVs) as the capacitive element in both qubits and readout resonators.  The introduction of TSVs has led to a reduction in the size of the qubits and resonators, while maintaining a high qubit coherence [1].  I will additionally describe our work exploring the use of TSVs as part of a hybrid superconducting-semiconducting qubit platforms [2]. 

[1] T. M. Hazard et al., Appl. Phys. Lett. 123, 154004 (2023). 

[2] T. M. Hazard et al., Phys. Rev. Appl. 20, 034056 (2023).   

Hermetic packaging for cryogenic experiments

Realizing a universal quantum computer is an extremely complex task. Among the multiple challenges to be solved, extending the coherence times of qubits implemented in solid state nanodevices is of the utmost importance. Implementing qubits made of electrons trapped at the surface of superfluid helium with vacuum offers the opportunity to realize qubits in a noiseless environment, extending the qubit lifetime [1]. Furthermore, these qubits are compatible with standard circuit quantum electrodynamic (CQED) techniques for manipulation and readout [2].

In this talk we introduce our newest commercial packaging, used to perform experiments in completely sealed environments at cryogenic temperatures. The technology is based on our recently developed QCage chip carrier [3], which has been optimized for CQED experiments [4,5]. We demonstrate the superfluid helium tightness of this packaging by investigating how the resonance frequency of superconducting coplanar waveguide resonators is evolving as the chip cavity is filled with superfluid helium at cryogenic temperatures.   

On scaling superconducting qubits with 3D-integration modules in an advanced 300mm-fabrication environment

Superconducting qubits have emerged as one of the prime contenders for quantum computing architectures, owing to their potential for scalability and controllability. The most advanced superconducting qubit processors to date have predominantly used lift-off techniques, especially in the fabrication of Josephson junctions and for flip-chip 3D integration. However, it's important to note that while lift-off techniques have been successful, they present compatibility challenges when integrated into standard foundry-manufacturing processes. We report on the realization of highly coherent transmon qubits without the need for shadow evaporation and lift-off, and 3D-integration modules that are promising to integrate with such qubits in a 300mm fab environment. With these demonstrators, our work paves a way towards upscaling of superconducting qubits in advanced manufacturing environments.   

Recent results on manufacturing spin and superconducting qubits in advanced 300 mm fabrication environments

In this talk, we will report on recent progress in manufacturing spin and superconducting qubits using advanced 300 mm manufacturing lines at imec. Both progress on Si MOS qubits and demonstrations of Si/SiGe spin qubits will be discussed, as well as the realization of high coherence superconducting transmon qubits using subtractively etched junctions, bypassing the need for shadow evaporation.

We will comment on the challenges and benefits of moving such qubits to a 300 mm fab environment, and the challenges and opportunities offered with respect to upscaling of the technology.

 

    

Realizing Scalable Quantum Processing Units (QPUs): Achievements and Prospects from IQM Finland Oy

The pursuit of large-scale quantum computing systems has emerged as a paramount objective in the contemporary scientific and technological landscape. Within this context, IQM Finland Oy, an avant-garde hardware quantum computing enterprise, has positioned itself at the forefront of this transformative endeavor. Our 200mm wafer-level production for our Quantum Processing Units (QPUs) encompasses various critical aspects that contribute to the realization of large-scale quantum computers. Focusing on the development of quantum processors based on superconducting circuits, we present our latest achievements in the field. High yield, extended qubit coherence time, exceptional uniformity, and advanced 3D integration techniques have all played integral roles in our journey. This significant achievement not only demonstrates our commitment to scalability but also paves the way for cost-effective mass production of quantum processors.   

Qubit frequency tuning via electron-beam irradiation

We present a technique to tune the frequency of a superconducting qubit via electron-beam irradiation of the Josephson junction element. The tuning of the junction modifies the resistance of the junction and thereby the frequency of the qubit is modified. This technique can be used to selectively alter qubit frequencies to improve quantum processor unit (QPU) frequency allocation.   

Self-heating of qubit input-output lines: implications for quantum computing

 At present, superconducting qubits represent a standard solution for quantum computers among academic and industrial developers alike [1,2]. In this architecture, state preparation, state manipulation and state readout are all performed using microwave transmission lines. In this talk, using measurement data combined with qubit noise models [3], we systematically assess the impact of I/O lines self-heating in cryogenic quantum computing for different attenuation schemes, qubit specifications and chip geometries.  Specifically, we describe how the qubit  and  time-constants T₁ and T₂ are governed by the I/O thermal noise and can be limited at varying operating conditions, i.e., under different attenuation schemes, and noise spectral densities. Our results provide a useful diagnostic tool for a realistic evaluation of power versus noise trade-off in a realistic quantum computing cryogenic set-up.    

[1] S. Krinner, et al., EPJ Quantum Technology (2019) 6:2

[2] Sergey Bravyi , et al., Journal of Applied Physics 132, 160902 (2022)

[3] A. Vaaranta, M. Cattaneo, and R. E. Lake, Physical Review A 106, 042605 (2022)   

Qubit as a reflectometer

Microwave pulse distortion due to signal reflections is a major source of gate errors in superconducting qubits. We extend previous pulse-sequence techniques to characterize both in-phase and quadrature components of such reflections [1]. Through analytical modeling of the sequences, we identify that microwave reflections with a relatively long round trip time lead to a long tail of the microwave pulse causing gate error. We then demonstrate that by pre-distorting the waveforms to compensate for the reflection-induced tail, we significantly reduce the coherent error of single qubit gates. Our characterization method is general and can be applied to other types of signal distortion.

[1] Gustavsson et al., Phys. Rev. Lett. 110, 040502 (2013)   

Transmon-based single-photon generator

Superconducting circuits enable strong coupling between artificial atoms (transmon qubits) and transmission lines, making them an interesting platform for studying light-matter interactions in the microwave frequency range. We are developing a device consisting of a superconducting transmon qubit that is strongly coupled to a transmission line that we plan to use in several quantum technology applications and for calibration purposes. This device can be used as an absolute power-meter and it can calibrate and characterize transmission line attenuation, microwave components, or other devices inside a dilution refrigerator. Since the transmon can be approximated as a two-level system, it absorbs and emits only single microwave photons, making it an on demand microwave single-photon source. To demonstrate the quantum nature of the emission state we reconstructed its Wigner function and measured the second-order correlation function.

To perform the second order correlation function we use a 90-degree hybrid coupler that enables single photons to be emitted at two outputs with equal probability. The design of the source and the use of Josephson parametric amplifiers (JPAs) enables us to improve the efficiency of these millikelvin microwave measurements.