Tungsten Silicide Superconducting Nanowire Single-Photon Test Structures Fabricated Using Optical Lithography
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 (λ=248 nm) in 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 100 μm 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.