Thermocompression bonding technology for multilayer superconducting quantum circuits

Abstract

Extensible quantum computing architectures require a large array of quantum devices operating with low error rates. A quantum processor based on superconducting quantum bits can be scaled up by stacking microchips that each perform different computational functions. In this article, we experimentally demonstrate a thermocompression bonding technology that utilizes indium films as a welding agent to attach pairs of lithographically-patterned chips. We perform chip-to-chip indium bonding in vacuum at 190 • C with indium film thicknesses of 150 nm. We characterize the dc and microwave performance of bonded devices at room and cryogenic temperatures. At 10 mK, we find a dc bond resistance of 515 nΩ mm −2 . Additionally, we show minimal microwave reflections and good transmission up to 6.8 GHz in a tunnel-capped, bonded device as compared to a similar uncapped device. As a proof of concept, we fabricate and measure a set of tunnelcapped superconducting resonators, demonstrating that our bonding technology can be used in quantum computing applications.

Substrate surface engineering for high-quality silicon/aluminum superconducting resonators

“…Each sample comprises a feed CPW transmission line capacitively coupled with coupling strength κ to nine meandered quarter-wave resonators each with a different resonance frequency f0 , in a multiplexed design (see the works in [42,44] for similar designs). We measure the feed-line transmission coefficient S 21 in the frequency range between 4 and 8 GHz and, for consistency, select one resonator from each sample with the same designed f0 = 4.5 GHz and κ = 40 kHz.…”

Qubit compatible superconducting interconnects

“…Cryogenic applications are fewer; bolometer arrays for submillimeter astronomy [10,11] and single flux quantum devices [12,13] have utilized this technology. Low resistance cryogenic bump bonds [14,15] and superconducting bump bonds that proximitize normal metals have also been fabricated [16]. Here we present a novel bump bond metal stack up consisting of all superconducting materials with the intent of achieving maximal flexibility in designing flux tunable qubit circuits where mA control currents are necessary.…”

Thin film metrology and microwave loss characterization of indium and aluminum/indium superconducting planar resonators

“…Sputtered Al/In films are deposited in situ in a sputter system from AJA International, Inc., model ATC-Orion 5 at the Toronto Nanofabrication Centre of the University of Toronto (deposition parameters can be found in Ref. 13).…”

“…The sputtered Al/In heated sample is processed in a custom-made vacuum chamber evacuated to 1 × 10 −2 mbar that is placed for a time of 100 min on a hot plate at 190 • C, above the In melting temperature (details on the vacuum chamber in Ref. 13).…”

“…6,7,[9][10][11] Alternatively, pairs of chips can be attached by means of thermocompression a) Author to whom correspondence should be addressed: matteo.mariantoni@uwaterloo.ca bonding of thick film In in ambient atmospheric pressure below the In melting temperature 12 or thin film In in vacuum above the In melting temperature. 13 Indium is thus becoming an important material to create compact, densely connected, and environment-protected quantum systems. A detailed characterization of loss mechanisms of In thin films is therefore an important step toward a medium-scale quantum processor.…”