Suspending superconducting qubits by silicon micromachining

Abstract

We present a method for relieving aluminum 3D transmon qubits from a silicon substrate using micromachining. Our technique is a high yield, one-step deep reactive ion etch that requires no additional fabrication processes, and results in the suspension of the junction area and edges of the aluminum film. The drastic change in the device geometry affects both the dielectric and flux noise environment experienced by the qubit. In particular, the participation ratios of various dielectric interfaces are significantly modified, and suspended qubits exhibited longer T 1 's than non-suspended ones. We also find that suspension increases the flux noise experienced by tunable SQUID-based qubits.The coherence times of superconducting qubits have steadily increased over the past decade due to careful engineering of the electromagnetic environment, better materials and fabrication methods, and improved device designs that minimize loss. State-of-the-art superconducting qubits with the longest lifetimes (T 1 ) make use of very low loss tangent dielectric substrates and have large separation between planar conductors to decrease the effect of dielectric loss in the interfaces between materials 1-3 . In particular, it has been shown that for aluminum 3D transmons on sapphire, T 1 times are limited by the various interfaces between the dielectric substrate, the superconducting metal, and vacuum 4 . This effect can be attributed to the larger electric fields near metallic surfaces and the higher concentration of two-level systems (TLS) at disordered interfaces 5-7 . At the same time, magnetic impurities at the surface of superconductors have been proposed as the cause of 1/f flux noise that limits the performance of SQUID based qubits and sensors [8][9][10] .In order to better understand these effects, one strategy is to drastically alter the geometry of materials and interfaces that contribute to qubit loss and decoherence. In this Letter, we present a procedure for removing the substrate and suspending aluminum Josephson junctions on silicon by micromachining. Silicon is a low-loss dielectric that offers several advantages for implementing the next generation of complex quantum circuits 11,12 . Its prevalent use in the semiconductor and MEMS industries have led to a large variety of fabrication techniques that are not available for sapphire 13 . Using silicon as a substrate material enables the development of novel devices and architectures in circuit QED, such as multilayer quantum circuits that incorporate micromachined superconducting enclosures and resonators 14,15 . Substrate micromachining has also been used to reduce dielectric loss and frequency noise in niobium titanium nitride coplanar waveguide resonators on silicon 16,17 . On the other hand, silicon has a more complex surface chemistry than sapphire; for example, it forms an amorphous oxide layer a) Electronic mail: yiwen.chu@yale.edu that may be host to a large number of TLS's and paramagnetic impurities 11,18,19 .We suspend our qubits with a simple, one-step dee...

Micromachined Integrated Quantum Circuit Containing a Superconducting Qubit

“…Alternative MMIQC designs are being developed that contain different seams and minimize the use of normal metals like gold. Extended qubit lifetimes can be achieved by removal of silicon substrate in the junction area and improved surface cleaning [18]. Furthermore, a wide range of coupling rates can be accessed by geometry modifications, some of which would require precise alignment and leveling control during wafer bonding.…”

Erratum: Micromachined Integrated Quantum Circuit Containing a Superconducting Qubit [Phys. Rev. Applied 7 , 044018 (2017)]

“…Extended qubit lifetimes can be achieved by removal of silicon substrate in the junction area and more directed surface cleaning. [18] Furthermore, a wide range of coupling rates can be accessed by geometry modifications, some of which would require precise alignment and leveling control during wafer bonding.…”

Overlap junctions for high coherence superconducting qubits

“…This non-linearity separates the two lowest energy levels from higher excitations, forming a two-level system as the physical qubit. Coherence times of superconducting qubits have been increased significantly in both 2D and 3D geometries (∼10-100µs) [1][2][3][4][5]. These relatively long coherence times, combined with fast, high-fidelity gate schemes, have enabled the demonstration of quantum error detection with superconducting devices [6][7][8].…”

“…In general, low participation ratios from both the Josephson junction and it's immediate surroundings are essential to the success of present-day superconducting qubits [4,9]. This goal is typically achieved by shrinking the junction size.…”