IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015
Tungsten Silicide Superconducting Nanowire Single-Photon Test Structures Fabricated Using Optical Lithography Andrew D. Beyer, Matthew D. Shaw, Francesco Marsili, M. Shane Allman, Adriana E. Lita, Varun B. Verma, Giovanni V. Resta, Jeffrey A. Stern, Richard P. Mirin, Sae Woo Nam, and William H. Farr
Abstract—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
Manuscript received August 12, 2014; accepted November 25, 2014. Date of publication December 22, 2014; date of current version March 12, 2015. This work was supported in part by a contract with the National Aeronautics and Space Administration and by DARPA. A. D. Beyer M. D. Shaw, F. Marsili, and W. H. Farr are with the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA (e-mail: [email protected]
). M. S. Allman, A. E. Lita, V. B. Verma, R. P. Mirin, and S. W. Nam are with National Institute of Standards and Technology, Boulder, CO 80305 USA. G. V. Resta was with the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA (e-mail: [email protected]
). J. A. Stern, deceased, was with the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2014.2378232
this technological advance should scale and enable fast and rapid production of kilo-pixel arrays in the future. Index Terms—Nanolithography, optical detectors, superconducting detectors, superconducting materials.
I. I NTRODUCTION
UPERCONDUCTING Nanowire Single Photon Detectors (SNSPDs) are key components enabling technological advancements in deep space optical communication and quantum information and communication applications. At optical communication wavelengths near 1550 nm, SNSPDs count singlephotons with near unity system detection efficiency (demonstrated up to ∼93%), high timing resolution from ∼30 ps to ∼150 ps, low intrinsic dark count rates of ∼1 cps or less, fast reset times between 1ns to 60 ns, and broad responsivity from visible to mid-infrared wavelengths –. The most common superconducting materials used include nitrides, such as niobium nitride (NbN) with bulk superconducting transition temperature TC = 10 K, and amorphous tungsten silicide (WSi), with TC = 4.9 K. The detectors made from these materials perform best when the superconductor is thin, approximately 5 nm thick. In a thin film format, the value of TC is reduced compared to the bulk TC . For example, WSi at 5 nm thick has TC ∼3.1 K, and the operation of such a WSi SNSPD occurs below ∼2K . The typical detector architecture consists of a superconducting nanowire, with nominal width ranging from 10 nm to 250 nm depending on material used, meandering across an active area and connected to an electrical circuit such as shown schematically in Fig. 1. The nanowire is current biased below its critical current IC , and photons create a hotspot that causes the current bias to divert through an impedance Z in parallel to the nanowire, creating a detectable electrical pulse. The uncertainty of the response time, due to the distribution of response times to a photon, is known as the jitter, and in NbN it is typically 50 ps or less and has been measured down to 15 ps , while WSi demonstrates jitter of about 150 ps . The inductance of the nanowire, dominated by the kinetic inductance of the material, affects the reset time, or minimum time between detection of subsequent pulses . The inductance of the SNSPD must be chosen so that the hotspot does not self-heat when generated and latch in the normal state . Minimum reset times, to avoid latching, empirically vary depending on the materials used. Light is coupled to the detector using an optical fiber or
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015
communication and information applications can also benefit from a large increase in the number of pixels, such as increased information per photon by using spatial modulation schemes (see  for details on spatial modulation). Therefore, optical lithography of SNSPDs can greatly improve the time to fabricate larger numbers of SNSPDs which can benefit all these applications. II. M ETHODS A. Motivation
Fig. 1. (Top left) A scanning electron microscope (SEM) image of a superconducting nanowire single photon detector (SNSPD) covering an active area with 15 μm diameter. (Top right) A zoomed in SEM of the nanowires with a sketched hotspot generated by a photon. (Bottom) The SNSPD can be thought of as a superconducting switch in parallel with the hotspot resistance RHS and having inductance L dominated by the kinetic i.nductance of the nanowire. When a hotspot is generated by a photon, the switch opens and the bias current Ibias , which is less than the nanowire critical current IC , is partially diverted to the impedance Z where the electrical pulse may be read out.
free-space coupling, and the system detection efficiency (SDE) of a particular detection architecture measures how many input photons are actually detected. A common architectural feature is to embed the nanowires into an optical cavity comprised of stacks of dielectrics to ensure the photons are detected and not reflected away, enhancing SDE. For example, we use SiO2 and TiO2 dielectrics in our optical cavities for near infrared photons in WSi SNSPDs. The end usage, architecture, and performance of SNSPDs vary with application. To date, the most efficient SDE has been reported in a single-pixel, fiber-coupled WSi SNSPD . In other applications, such as deep-space optical communications, one would like to reduce the reset time of a detector to reduce blocking losses and increase the active area to avoid astronomical seeing effects. With larger active areas, free space coupling also becomes desirable over fiber coupling. One way to achieve larger active area and shorter device reset times is to interleave nanowires together over a large active area. In this multi-pixel scheme, when an initial photon event produces a pulse in one nanowire pixel, subsequent photons are not missed or blocked, but may be detected by the other nanowire pixels. Such a detector scheme using WSi SNSPD devices with 12-pixels has recently demonstrated downlink data rates of 79Mbps 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) . Four detectors based on NbN were able to achieve downlink rates of up to 622 MBps as part of the LLCD because of lower jitter values than the WSi detector and the fact that there are 4 detectors instead of one . As fabrication capabilities are improved, larger active area and faster device reset times will enable better performance of SNSPD detection schemes for deep space optical communication. Future SNSPDs will need to be comprised of thousands of pixels and to cover 0.1 mm2 to 1 mm2 active areas for deep space optical communications. Advanced quantum
While nitride SNSPDs have demonstrated superior jitter and reset times, we employ WSi for our SNSPDs because the material is more uniform, has higher yield, lends itself to simpler fabrication techniques, and allows for a range of operating biases that makes producing larger active area, multipixel detectors easier than in nitride-based SNSPDs . Much of the advantages of WSi stem from it being an amorphous, homogeneous superconductor that is not as sensitive to grain boundaries as the nitride superconductors . Another advantage is that the wire widths needed for efficient detection at near infrared wavelengths near 1550 nm are approximately between 120 nm to 220 nm , . To date nanowires in NbN and WSi have been fabricated using electron-beam lithography (EBL), because EBL can easily define nanowires from 10 nm to 250 nm. However, the widths necessary for WSi are also amenable to optical lithography procedures to define the nanowires instead of electron-beam lithography (EBL). The use of optical lithography offers an advantage over EBL because the nanowires may be written in parallel, as opposed to being written serially as in EBL. Thus, optical lithography reduces the time to fabricate devices and typically reduces the cost to fabricate SNSPDs. Below, we describe the use of optical lithography to define 64-pixel SNSPDs with nominal 160 nm wire widths. B. Detector Scheme Example We fabricated 64-pixel SNSPDs in order to investigate spatial position modulation and pulse position modulation  for studies on the information capacity of a SNSPD when detecting photons. A picture of an 8 × 8 SNSPD is shown in Fig. 2(a), which employs a row/column contact scheme described in  to reduce the number of wires connecting to the SNSPD for readout. Fig. 2(b) shows a zoomed in image of one of the pixels, showing a resistor, an inductor to control cross-talk, and the 15 μm × 15 μm active area nanowire for one pixel. The inductor controls cross-talk by minimizing current diverted to adjacent pixels when one SNSPD is hit by a photon . This detector scheme was fabricated using optical lithography of WSi nanowires in the Microdevices Laboratory (MDL) at the Jet Propulsion Laboratory to explore the feasibility of optical lithography for SNSPDs. C. Fabrication Our SNSPDs consist of metal wire traces to connect to the row and column contact points, along with alloyed Pd/Au films to serve as resistors. We used Au/Nb traces to connect
BEYER et al.: TUNGSTEN SILICIDE SUPERCONDUCTING NANOWIRE SINGLE-PHOTON TEST STRUCTURES
Fig. 2. (a) An 8 × 8 SNSPD used for spatial and pulse position modulation studies of photon information capacity. (b) A zoomed in image of one of the pixels drawn schematically showing the nanowire (NW), inductor, and resistor. We chose this architecture to demonstrate optical lithography because of its large active area of nanowires per die, as well as large inductors that needed to be patterned simultaneous to the nanowires.
to the rows, and Au traces to connect to the columns. The Nb was included to attempt to reduce parasitic resistances. Additionally, the mirrors underneath the devices were Au/Nb as well. The WSi wire layers were sandwiched between layers of dielectrics to form optical cavities for optimal free-space coupling to the devices. We typically use SiO2 and TiO2 as dielectrics for our optical stacks. The definition of the photoresist for patterning of the nanowires was accomplished using either a Canon EX3 or a Canon EX6 KrF (λ = 248 nm) excimer laser, optical lithography stepper. A brief schematic of direct illumination of chrome on glass masks in a stepper is shown in Fig. 3(a). The critical dimension (CD) resolution for a stepper in such a configuration is given by CD =
k1 λ NA
where λ is the wavelength of light in the stepper, NA = n sin θ and is the numerical aperture of the focusing lens, and k1 describes process related factors. Here, n is the index of the lens and θ is the angle of the light captured by the projection lens. The resolution of the tools is approximately 229 nm using k1 ≈ 0.6 and NA = 0.65 for the steppers. Equation (1) describes the resolution of a stepper assuming direct illumination, but there are also so-called resolutionenhancement technologies (RETs) commonly employed in the semiconductor fabrication industry to push the CD even lower. The two RETs used to increase resolution are off-axis illumination and phase-shift masks. We did not employ phase-shift masks due to prohibitively high cost, and more details of this technique may be found in . Off-axis illumination is the technique of illuminating the photolithography mask at an oblique angle, commonly by using an annular or dipole illumination pattern as illustrated in Fig. 3(b) and (c). Such illumination patterns are available commercially for insertion into the EX3 and EX6 steppers. The
Fig. 3. (a) On-axis illumination is shown schematically for the case of a periodic grating on a mask. The light is incident on the mask after passing through a condenser lens, generating diffraction orders m = 0, ±1. At least one diffraction order other than m = 0 is necessary to reproduce the periodicity of the grating pattern on the wafer. As more orders are captured by the lens, the higher the fidelity of the image becomes. Here, fidelity is a measure of how well the actual pattern matches the designed pattern. The angle, θ, which is used to define the numerical aperture NA = n sin θ is also shown. The term n describes the index of refraction of the lens. (b) An illustration of off-axis illumination showing that the order m = 0 may be captured at the edge of the lens instead of in the middle, and orders m ≥ 1 are now captured by the lens, instead of just m = ±1. Thus, θ is effectively doubled and the critical dimension resolution improved. (c) In this image, schematics of conventional on-axis illumination and some examples of off-axis illumination, including annular and dipole illumination, are illustrated. Please see .
simplest way to consider how the resolution is enhanced by off-axis illumination is to consider the light going through a diffraction grating. When light glances off the grating or mask at an oblique angle, the rays of the diffraction pattern are rotated such that higher orders pass through the projection lens in a stepper. Instead of just capturing the orders m = 0, ±1 for a grating at the CD, the orders m = 0, 1 are now captured, as well as some of the m > 1 orders as shown in Fig. 3(b). The fidelity, or measure of how well the actual photoresist pattern matches the designed pattern, is improved for this setup. The angle θ in NA is then effectively doubled, improving the ultimate CD resolution as well. The meandered nanowire patterns in our SNSPDs are similar to a diffraction grating, so the example of the grating illumination is appropriate here. In order to improve the reliability of optical lithography for nanowire fabrication, backside anti-reflection coatings (BARCs) and optical proximity correction (OPC) features are also necessary to improve the mask-to-photoresist-transfer on a wafer. BARC layers are used to prevent the unwanted reflection of light from other features on a wafer when exposing the desired pattern. Such reflections can create constrictions or bulges in the nanowires due to unwanted light impinging upon the nanowire pattern. We employed BARC layers in our fabrication process to prevent such errors in our nanowire patterning. More details about BARC layers and errors in patterning may be found in . OPC features are additional features added to the desired pattern to counteract pattern edge effects or the effect of missing diffraction orders in the projection lens. Proximity corrections are well known in EBL. We did not use OPC features for this work, but we found that the nanowires at the edge of our nanowire pattern had ∼10% narrow widths than
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015
Fig. 5. A single pixel SNSPD test structure fabricated using optical lithography to enable an end-view of the photoresist profile achieved using the techniques described here. The whole structure is shown on the left, while the nanowire ends are viewed on end on the right image. As shown, a boxcar like structure with rounded top is achieved. Given the short etch times for 5nm films, this structure easily reproduces the designed meander structure in WSi before resist etch through.
Fig. 4. (a) An SEM image of the nanowire photoresist pattern produced using optical lithography in a DUV stepper showing the inductor and nanowire regions. The width of the nanowires on the edges of the nanowire pattern is ∼10% narrower width than the nanowire width in the middle of the nanowire active area. This edge effect could possibly be fixed with optical proximity correction (OPC). For example, an electrically disconnected length of wire just outside the active area pattern could serve as a simple OPC feature to improve the fidelity of the desired pattern (shown schematically as a yellow dashed line for ease of differentiation from the actual pattern—the pattern would not be dashed but simply a disconnected nanowire). (b) An SEM image zoomed in on the edge of the nanowire pattern defined by optical lithography. The zoomed in region is from the region illustrated by the blue box in (a). The nanowire width in the middle is 166 nm, while the edge nanowire is about 10% narrower, which would restrict the IC and reduce the bias operating range for this device.
those nanowires in the middle, as described more fully below. This issue could be fixed by employing a simple OPC feature such as dummy, disconnected nanowires just outside the pattern as show in Fig. 4(a) for example.
ranges in WSi devices). Thus, it would be advantageous to eliminate this effect in order to maximize the operating current bias regime that will give maximal SDE. A dummy nanowire that is not connected electrically could be patterned in resist outside the pattern or a larger nanowire width for the nanowires on the edge of the pattern could be implemented as OPC features here. An example of the dummy nanowire scenario is shown in Fig. 4(a) as a dashed yellow line. An image of the photoresist profile achieved using the techniques described here is shown for an optically lithographed single pixel SNSPD test structure in Fig. 5. We imaged a single pixel test structure because its architecture enabled easier photoresist profile measurements in our SEM. Finally, the sought-after improvement in time and cost to manufacture the 64-pixel devices was demonstrated. The calculated time to populate a 4” wafer with 200 dies of the 64-pixel test structures with 3.5 × 3.5 mm die size was on the order of 24 hours using the EBL software suite at MDL. In contrast, optical lithography using a deep-UV (DUV) stepper successfully defined the nanowires in 15 minutes. IV. C ONCLUSION
III. R ESULTS The photolithography mask we used to fabricate the 64-pixels had nominal 160 nm wide nanowire designs. A photoresist and BARC layer were used along with off-axis illumination to reliably produce 166 nm nanowires, as shown in Fig. 4(a) and (b). The fidelity was not perfect given nominal 160 nm width and resulting 166 nm widths, but the results are extremely close to the design and could perhaps be improved with further refinements to the process. The optimum conditions to produce the 166 nm wide wires were obtained by varying the dose and focus offset of the stepper illumination system. Fig. 4(b) shows the results of the nanowire pattern in the photoresist after processing. As can be seen, the nanowire at the edge of the active area (leftmost nanowire in Fig. 4(b)) did not have the same width as the wires in the middle of the active region (wires on the right side of Fig. 4(b)). Any reduction in the width of the nanowire such as this narrowing or any constrictions will reduce the maximum operating bias current of the device (see  for operating bias
We have demonstrated the ability to fabricate SNSPD test structures with nanowire widths of ∼160 nm width using optical lithography instead of EBL. We are now investigating whether the yield and maximum active area that can be defined without defects is improved using optical lithography over EBL. This study and the subsequent one will provide useful information for planning the next detector architectures for deep space optical communication. Finally, experiments to confirm SNSPD operation of these devices are underway. R EFERENCES  F. Marsili et al., “Detecting single infrared photons with 93% system efficiency,” Nature Photon., vol. 7, pp. 210–214, Feb. 2013.  C. M. Natarajan, M. G. Tanner, and R. H. Hadfield, “Superconducting nanowire single-photon detectors: physics and applications,” Supercond. Sci. Technol., vol. 25, no. 6, Apr. 2012, Art. ID. 063001.  B. Baek et al., “Superconducting a-WxSi1—x nanowire single-photon detector with saturated internal quantum efficiency from visible to 1850 nm,” Appl. Phys. Lett. 98, Jun. 2011, Art. ID. 251105.  L. X. You et al., “Jitter analysis of a superconducting nanowire single photon detector,” AIP Adv., vol. 3, no. 7, Jul. 2013, Art. ID. 072135.
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