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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Fabricating WSi based superconducting microwire single photon detectors with laser lithography

Maximilian Protte, Varun Verma, Jan Philipp Höpker, Richard P. Mirin, Sae Woo Nam, Tim J. Bartley

Published Jun 21, 2021• 0 citations
Publisher

Abstract

To measure light down to the single photon level, superconducting nanowire single photon detectors (SNSPDs) have become the go-to devices, due to their near unity detection efficiency [1] , high-speed and low noise. An important figure of merit is the internal detection efficiency (IDE). To achieve a saturated IDE in the infrared, wire widths of the order 100 nm are typically required. Nevertheless, recent experiments have shown that functional detectors can be fabricated with wire widths up to 3 µm [2] – [4] . A higher silicon content of the films combined with a smaller film thickness enables wider wires. Increasing the wire width allows for bigger fabrication tolerances which enables the fabrication with optical lithography.

DOI
10.1109/cleo/europe-eqec52157.2021.9542704

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•Jun 9, 2017•0 citations
To measure light down to the single photon level, superconducting nanowire single photon detectors (SNSPDs) have become the go-to devices, due to their near unity detection efficiency [1] , high-speed and low noise. An important figure of merit is the internal detection efficiency (IDE). To achieve a saturated IDE in the infrared, wire widths of the order 100 nm are typically required. Nevertheless, recent experiments have shown that functional detectors can be fabricated with wire widths up to 3 µm [2] – [4] . A higher silicon content of the films combined with a smaller film thickness enables wider wires. Increasing the wire width allows for bigger fabrication tolerances which enables the fabrication with optical lithography.
Abstract We demonstrate the fabrication of micron-wide tungsten silicide superconducting nanowire single-photon detectors on a silicon substrate using laser lithography. We show saturated internal detection efficiencies with wire widths ranging from 0.59 µm to 1.43 µm under illumination at 1550 nm. We demonstrate both straight wires, as well as meandered structures. Single-photon sensitivity is shown in devices up to 4 mm in length. Laser-lithographically written devices allow for fast and easy structuring of large areas while maintaining a saturated internal efficiency for wire widths around 1 µm.
Superconducting nanowire single photon detectors (SNSPDs) are the highest-performing technology for time-resolved single-photon counting from the UV to the near-infrared. The recent discovery of single-photon sensitivity in micrometer-scale superconducting wires is a promising pathway to explore for large active area devices with application to dark matter searches and fundamental physics experiments. We present 8-pixel 1 mm2 superconducting microwire single photon detectors (SMSPDs) with 1 μm-wide wires fabricated from WSi and MoSi films of various stoichiometries using electron-beam and optical lithography. Devices made from all materials and fabrication techniques show saturated internal detection efficiency at 1064 nm in at least one pixel, and the best performing device made from silicon-rich WSi shows single-photon sensitivity in all eight pixels and saturated internal detection efficiency in 6/8 pixels. This detector is the largest reported active-area SMSPD or SNSPD with near-IR sensitivity, and it extends the SMSPD to an array format. By further optimizing the photolithography techniques presented in this work, a viable pathway exists to realize larger devices with cm2-scale active area and beyond.
We report on the fabrication and characterization of single-photon-sensitive WSi superconducting detectors with wire widths from 1 μm to 3 μm. The devices achieve the saturated internal detection efficiency at a wavelength of 1.55 μm and exhibit maximum count rates in excess of 105 s−1. We also investigate the material properties of the silicon-rich WSi films used for these devices. We find that many devices with active lengths of several hundred micrometers exhibit critical currents in excess of 50% of the depairing current. A meandered detector with a 2.0 μm wire width is demonstrated over a surface area of 362 × 362 μm2, showcasing the material and device quality achieved.
Single-pixel fiber-coupled superconducting nanowire single-photon detectors (SNSPDs) operating at 1550 nm and utilizing amorphous superconducting tungsten silicide (WSi) films have proven ability to detect photons with: high system-detection efficiency (SDE) of up to 93%, low-jitter on the order of ∼150 ps, dark count rates of ∼1 kcps, and fast reset times on the order of tens of nanoseconds. Additionally, WSi SNSPD devices with 12-pixels have recently demonstrated downlink data rates of 79 Mbps between a terminal in orbit around the moon and a terminal on earth, as part of the Lunar Laser Communication Demonstration (LLCD) at the Lunar Lasercomm OCTL Terminal (LLOT). To further extend the performance of SNSPD devices for optical and quantum communication for terrestrial and space-based applications, the next generation of devices will need to incorporate hundreds to thousands of SNSPD pixels and to be free-space coupled. The wire widths necessary for optimal performance of WSi (∼120–220 nm) devices have to date been achieved using electron-beam lithography (EBL) to pattern photoresists for etch-back fabrication methods. The high cost and time to fabricate kilo-pixel arrays of SNSPDs using EBL will become prohibitive in producing such devices. Here, we report fabrication of a WSi SNSPD test structure with 64 pixels using optical lithography instead of EBL. Specifically, we used Canon EX3 and EX6 deep-UV (DUV) steppers with KrF excimer lasers$(\lambda=248\ \mbox{nm} ) intheMicroDevicesLaboratoryattheJetPropulsionLaboratorytofabricatethearray.Dieswith8×8pixelswith166−nm−widewireswereproduced,withpixelshavinga100in the Micro Devices Laboratory at the Jet Propulsion Laboratory to fabricate the array. Dies with 8 × 8 pixels with 166-nm-wide wires were produced, with pixels having a 100intheMicroDevicesLaboratoryattheJetPropulsionLaboratorytofabricatethearray.Dieswith8×8pixelswith166−nm−widewireswereproduced,withpixelshavinga100\mu\mbox{m} $pitch in the vertical and horizontal directions. Two improvements were observed: 1) the time to pattern the 8 × 8 SNSPD pixels on 3.5 mm × 3.5 mm dies filling a 4-in Si wafer required ∼24 hours using EBL while optical lithography wrote the same dies in approximately 15 minutes; and 2) the cost to write one 4-in wafer using EBL was comparable to the cost for one optical mask for use in the stepper to write many 4-in wafers. While fabrication times and costs will vary from facility to facility, the improvements in speed and cost for optical lithography versus EBL are apparent, and this technological advance should scale and enable fast and rapid production of kilo-pixel arrays in the future.
The optimization of superconducting thin-films has pushed the sensitivity of superconducting nanowire single-photon detectors (SNSPDs) to the mid-infrared (mid-IR). Earlier demonstrations have shown that straight tungsten silicide nanowires can achieve unity internal detection efficiency (IDE) up to λ = 10 μm. For a high system detection efficiency (SDE), the active area needs to be increased, but material nonuniformity and nanofabrication-induced constrictions make mid-IR large-area meanders challenging to yield. In this work, we improve the sensitivity of superconducting materials and optimize a high-resolution nanofabrication process to demonstrate large-area SNSPDs with unity IDE at 7.4 μm. Our approach yields large-area meanders down to 50 nm width, with average line-width roughness below 10%, and with a lower impact from constrictions compared to previous demonstrations. Our methods pave the way to high-efficiency SNSPDs in the mid-IR band with potential impacts on astronomy, imaging, and physical chemistry.
Superconducting nanowire single photon detectors (SNSPD) made from amorphous superconductors have showed great promise for achieving high fabrication yields, due to the highly uniform nature of the films. We present progress on the development of SNSPD based on amorphous MoSi with a critical temperature of around 5 K, which is ideal for detector operation at temperatures of 1 – 2.5 K, accessible with widely available cryogenic systems. First generation devices have achieved a saturated internal efficiency from visible to near-infrared wavelengths, which is the first requirement for high overall system efficiency. The broadband response has allowed us to make a robust study the energy-current relation in these devices, which defines the current required for a saturated internal detection efficiency for a given incident photon energy. Contrary to previous studies with other material systems, we find a nonlinear energy-current relation, which is an important insight into the detection mechanism in SNSPDs. The latest generation devices have been embedded into an micro-cavity structure in order to increase the system detection efficiency, which has increased to over 65% at 1550 nm. The efficiency is believed to be limited by fabrication imperfections and we present ongoing progress towards improving this characteristic as well as the yield of the devices. Efforts are also being made towards increasing the maximum operating temperature of the devices.