IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015
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Fabrication of Nb Superconducting Nanowires by Nanoimprint Lithography Lu Zhao, Yirong Jin, Jie Li, Hui Deng, Hekang Li, Keqiang Huang, Limin Cui, and Dongning Zheng
Abstract—Nanoimprint lithography (NIL) is considered to be an attractive nonconventional lithographic technique in the fabrication of nanostructures with many advantages including low cost, high throughput, and high resolution on relatively large areas. In this paper, NIL was used to pattern superconducting nanowires with meander structures based on ultrathin (∼4 nm) Nb films deposited by dc-magnetron sputtering at room temperature. A combination of thermal-NIL and UV-NIL was exploited to transfer the meander pattern from the imprint hard mold to Nb films. The hard mold, etched into a Si wafer, was defined by e-beam lithography (EBL), which was nonexpendable due to the application of IPS as a soft mold to transfer the pattern to the imprint resist in the NIL process. Superconducting properties such as transition temperature T c and critical current density J c were measured on the NIL-made Nb nanowires. The results are compared with those of EBL-made nanowires. Index Terms—Meander structure, nanoimprint lithography, nanowires, single photon detectors.
I. I NTRODUCTION
S
UPERCONDUCTING nanowire single-photon detector (SNSPD) is a promising candidate for near-infrared photon detection in the quantum optics and quantum information field [1]. The operation principle of the SNSPD is based on a model of a supercurrent-enhanced resistive transition due to the generation of an unstable hotspot by the incident photons on a quasione dimensional superconducting nanowire with meander structure [2]–[4]. A number of different superconducting materials have been used to make the nanowires, such as Nb [5], [7], NbN [2], [8], NbTiN [9] and WSi [10]. In order to reach high detection efficiency, the thickness of superconducting layer is generally limited to less than 10 nm. The meander [1]–[11] structure of nanowires (usually less than 100 nm wide) is used in order to maximize the active area for photon absorption. Parallel structures with two or more meander nanowire SNSPDs connected in parallel were also proposed to have photonnumber resolving capability [12]. Manuscript received August 12, 2014; accepted December 4, 2014. Date of publication December 18, 2014; date of current version February 6, 2015. This work was supported in part by the National Basic Research Program of China (973 Program) under Grants 2011CBA00106, 2014CB921401, and 2014CB921202; by the National Natural Science Foundation of China under Grants 11104333 and 91321208; and by the Strategic Priority Research Program of the Chinese Academy of Sciences under Grant XDB07000000. The authors are with the Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China (e-mail:
[email protected];
[email protected]; dzheng@ iphy.ac.cn). 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.2014.2382976
Presently, the main fabrication technology for SNSPD is electron-beam lithography (EBL) [1]–[11] although some alternative method such as focused ion beam (FIB) [13] and local oxidation with an atomic force microscope (AFM) [14], [15] were also explored. A common disadvantage of these technologies is the low throughput, especially for making large area or multi element devices. Furthermore, to meet the requirement of future large scale production, techniques with low cost and high yield are highly desirable. Among various nanofabrication techniques, nanoimprint lithography (NIL) has attracted more and more interest due to its ability in making nanostructures with high-throughput, low cost, and yet high resolution (down to 5 nm) [17]–[21]. Unlike EBL and FIB, which achieves pattern definition through point by point exposure, NIL is a parallel lithography process in which many nanostructures on a film can be patterned simultaneously. An imprint resist is shaped by a NIL mold and, subsequently hardened by heating (themal-NIL) or UV light exposure (UV-NIL) to replicate patterns. Based on the mechanical deformation of the soft polymer resist, NIL can achieve high resolution without the limitations set by light diffraction or electron beam scattering that are encountered in conventional lithographic techniques. Whereas the resolution of pattern by NIL relies on the quality of the imprint hard mold defined by conventional lithography such as EBL, FIB or photolithography. Furthermore, hard molds can be used repeatedly to replicate nanostructures. So far, NIL technique has been commonly used in many areas such as microelectronics, photonics and biotechnology [18]. In this study, we apply the NIL technique to the fabrication of superconducting meander nanowires. We demonstrate that meander-structured nanowires of about 100 nm wide could be made from ultra-thin Nb films by a combination of thermal-NIL and UV-NIL approaches. The final meander structures were characterized by scanning electron microscopy (SEM). Patterns with nanowire width less than 100 nm and high uniformity were observed. Superconducting transition and current–voltage (I–V ) characteristics of the nanowires were measured. The results were compared with the data obtained by the EBL patterning. In this work, we choose Nb because its films are readily available in our laboratory. Although Nb has lower transition temperature and longer electron-phonon relaxation time than the most used NbN for SNSPDs, it has lower kinetic inductance and could potentially achieve higher count rate [7]. Moreover, we believe that the NIL technology we exploit here could be transformed to other materials without any change of parameters.
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015
TABLE I S PUTTERING PARAMETERS FOR N IOBIUM F ILMS
Fig. 2. Schematic of the fabrication of the NIL hard mold based on Si. (a) Meander lines with a width of about 85 nm covering an area of 10 × 10 μm2 were patterned at the central part of a Si wafer by EBL with PMMA resist and a RIE process to etch ∼55 nm down into Si. (b) A large square terrace pattern (50 × 50 μm2 ) of S1813 photoresist was defined by a UV photolithographic process, which was used to protect the meander area. RIE was then used to produce the surface relief structures for NIL.
Fig. 1. X-ray reflectivity (XRR) measurements performed on ultrathin Nb films. Experimental results (black solid line) of all the specimens were fit (short dash) by a multilayer model including Nb, NbOx, and Nb2 O5 from bottom to top with the fitted Nb thicknesses shown.
II. E XPERIMENTAL A. Ultra-Thin Film Deposition Ultra-thin Nb films were deposited on SiO2 /Si (100) or R-plane sapphire substrates using a custom ultra-high vacuum dc-magnetron sputtering system with a base pressure lower than 10−9 mbar. The Nb target has a purity ≥99.95%. Substrates were cleaned in a clean room and then baked inside the sputtering chamber at ∼400 ◦ C for about 3 hours to de-gas. Because Nb is a reactive metal, its superconducting properties could be degraded by even very small amount of oxygen. In order to reduce oxygen contamination coming from outgassing of the tubing and impurities in the Ar gas source, a customized builtin Non-Evaporable Getter (NEG) [23] purifier was inserted into the Ar gas pipe line to prevent active gas contaminations. Sputtering was performed on a room temperature substrate, and the conditions are listed in Table I. The film thickness was determined by several methods, including AFM, crosssectional SEM and X-ray reflectivity (XRR). The XRR results measured on ultrathin Nb films are shown in Fig. 1. Nb film thicknesses from 3.4 nm to 8.9 nm with about 0.5 nm surface roughness and 2–3 nm NbOx top layer were deduced from fitting an XRR model [24]. B. NIL Mold Fabrication NIL Hard Mold: The imprint hard mold was etched into a Si wafer, as illustrated in Fig. 2. First, a meander wire with a line width of 85 nm covering an area of 10 × 10 μm2 (together with alignment marks) was defined at the central part of a Si wafer by means of EBL. Then a RIE process was followed using CF4 /O2 gas mixture to transfer the pattern onto the Si wafer. There is
Fig. 3. (a) SEM image of the Si meander nanowire. (b) SEM image of the enlarged Si meander of (a). The width of meander line is about 85 nm.
a sensitive balance between high resolution in EBL and high aspect ratio in RIE process. On the one hand, we would like the electron beam resist (3% PMMA 495) as thin as possible to achieve fine patterning resolution. On the other hand, a thin resist layer cannot withstand a long etch time and may result in low aspect ratio. In this work, we found that a resist thickness around 100 nm gave optimal results. The nanowire patterns were written on a Raith 150 EBL system with an acceleration voltage of 20 kV and an area dose of 260 μC/cm2 . During the RIE process, 55 nm deep trenches were etched into Si. In the next step, a large square terrace pattern of S1813 (positive tone) photoresist (50 × 50 μm2 ) was defined by UV-photolithographic process, which was used to protect the meander area. RIE was then performed again to produce the surface relief structures required for NIL. Fig. 3 shows the SEM images of the fabricated Si mold. NIL Soft Mold: The hard mold can be directly used for pattern transfer, but this will cause some irreversible damage to the mold after a few imprint processes. Since the hard mold is expensive to fabricate, a soft mold is first used to copy the pattern from the hard mold. As it is a hard-soft imprint process, the hard mold is minimally damaged after many imprint cycles. Thermal-NIL process was used to define the soft mold on a nano-imprinter system (Eitre3 Nano-Imprinter from Obducat). A specialized polymer, IPS was used as the substrate of the soft mold. IPS is a transparent polymeric thermoplastic material
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Fig. 4. Schematic of the fabrication of meander lines based on ultrathin Nb films. (a) The soft mold was defined by the hard mold using thermal NIL. (b) By UV-NIL, meander patterns on TU2 resist were formed. RIE based on a SF6 /Ar gas mixture was then used to remove the resist residue and unwanted Nb, to form the meander-type nanostructure. TABLE II NANOIMPRINT C ONDITIONS
with a thickness of about 200 μm. It has good strength and toughness at room temperature, while becoming highly elastic above 120 ◦ C. The hard mold was pressed to IPS at 160 ◦ C and under a pressure of 40 bar for 1 minute to transfer the pattern to the IPS soft mold. After cooling down, the soft mold needed to be stripped away. IPS is a good non-adhesive material due to the fact that it contains fluorine, and can be easily removed. C. Fabrication of Nb Meander Nanowires The IPS soft mold was used to transfer the pattern to Nb films in a UV-NIL process, as illustrated in Fig. 4. Nb films of about 4 nm thick were spin coated with a UV curable resist TU2-60 and then pre-baked at 95 ◦ C for 3 min to evaporate the solvent in the resist. The soft mold was then positioned above the Nb film, contacting with the pattern side. Next, a parallel UV light was applied through the transparent IPS to cure the TU2 resist at 80 ◦ C under a pressure of 30 bar for 5 min. Detailed conditions of the SNSPD fabrication using NIL are listed in Table II. After lifting off the soft mold, the patterned resist layer was used directly as an etching mask in the subsequent RIE step using SF6 /Ar gas mixture to remove unwanted Nb. The SEM images of a fabricated Nb meander line with the width and the filling factor of about 90 nm and 50% respectively are shown in Fig. 5. The nanowire structure is almost identical to that on the hard mold. Fabrication of Large Features: In principle, large structures like electrodes can be transferred simultaneously with small
Fig. 5. (a) Optical micrograph of an SNSPD device, including the Ti/Au contact pads, a series resistor of about 20 Ω, and the meander nanowire. (b) Local zoom of (a) showing the shunt resistor. (c) SEM image of the meander structured Nb nanowire fabricated by the NIL process. The mean width of the nanowire is about 90 nm. (d) SEM image of the meander structured Nb nanowire (on SiO2 /Si) fabricated by the NIL process.
structures like nanowires in a single NIL process. However, NIL experiences difficulties in replicating large features [18]. In this study, we combine NIL with photolithography to solve this problem. We used a separate UV-lithographic process to pattern the large features. In a SNSPD, the nanowire needs to be coupled through a low loss transmission line to form a large bandwidth connection to room temperature readout electronics. Usually, the electrodes were designed as 50 Ω CPW structure. However, Nb detectors are significantly more susceptible than NbN to thermal instability (latching) at high bias. To avoid this problem, the detectors could be stabilized by reducing the input resistance of the readout circuit [7]. Taking this into account, a series resistance of about 20 Ω was designed to connect the nanowire on-chip. The electrodes and resistor were E-beam evaporated with 5 nm Ti and 100 nm Au, and then patterned by a lift-off process. Images of the electrodes and shunt resistor are shown in Fig. 5. III. S UPERCONDUCTING P ROPERTIES The superconducting transition temperatures (Tc ) of Nb films with the thickness from 3 nm to 100 nm on R-sapphire substrates were systematically measured in a PPMS (physical property measurement system, from Quantum Design Inc.). Fig. 6 shows Tc and the transition width (ΔT ) dependence of the film thicknesses t. A dramatic drop of Tc and increase of
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015
Fig. 6. Superconducting critical temperature (Tc ) and transition width (ΔT ) dependence on the Nb film thickness (t) for films grown on sapphire substrates. (The solid lines are a guide to the eye.)
Fig. 8. Current–voltage characteristics of the meander wire with 50% filling factor by NIL at 2 K. Ic is about 1.5 μA, and Jc was evaluated to be 4.2 × 105 A/cm2 .
Fig. 7. Resistance vs. temperature (R–T ) dependence of the original Nb film (∼4 nm thick) (blue triangles), meander structured nanowire by EBL (red solid circles) and by NIL (black open circles).
critical current density (Jc ) can then be evaluated to be about 4.2 × 105 A/cm2 . The hysteresis is likely due to heating. As the temperature increases, the hysteresis shrinks and disappears. In total, we have made 6 samples with three on sapphire substrates and three on SiO2 /Si. For all the samples, the Tc is similar and Jc is on the order of 105 A/cm2 . As shown in Fig. 7, Tc is similar for samples made by NIL and EBL. The Tc value is also close to that reported in [7] in which Nb nanowires 100 nm wide and 7–8 nm thick show Tc around 4.3–4.5 K. In contrast to Tc , we notice that the Jc of NIL samples with both substrates is considerably lower than that of EBL samples on SiO2 /Si substrates, by a factor of 4–8. The reduced value of Jc appears to contradict the Tc data which are very close for all samples made by either NIL and EBL techniques. We proposed two possible explanations. Firstly, the hard mold used in the NIL method was made in-house by EBL with PMMA as mask. The PMMA mask was kept thin to obtain fine 100 nm wide meander structure on the hard mold. After RIE, the trenches of the meander structure are only 55 nm deep. Consequently, when transferring the pattern onto the Nb films, the mask was also very thin. This means that some of the Nb on the top part of nanowires may have been etched away during RIE. In other words, the thickness is further reduced and thus the Ic becomes lower. The second possible reason is that the defects along the edges of the nanowires will reduce the width of the nanowires, and thus decrease Ic . As shown in Fig. 5(d), there are some defects clearly visible along the edges of the nanowire for the sample made by NIL on a SiO2 /Si substrate. The SEM micrograph shown in Fig. 5(c) appears to suggest that sample on a sapphire substrate has fewer defects.
ΔT with reducing of t occurs when t < 10 nm, like those previously reported for Nb and for NbTi. This phenomenon could be caused by grain-size effect [23]. For films with thicknesses of about 4 nm, the Tc is around 4.6 K. We used films with the thicknesses below 10 nm for making meander structured nanowires. In Fig. 7, we show the temperature dependence of resistance for the part around the superconducting transition for Nb meander nanowires made by NIL and EBL techniques, respectively. Also in the figure, the data for a Nb film is presented. The line width was about 90 nm with filling factor of about 50%. It appears that the transition temperature is slightly lower for the nanowires than for the film. Nevertheless, for samples fabricated by both techniques, the transition temperature Tc is very similar, indicating that nanowires fabricated by NIL is comparable with that by EBL. The meander lines of about 90 nm width covering a 10 × 10 μm2 area with different filling factors (f = 45% and 33%) were also fabricated by NIL process using different Si hard molds. The sample with small filling factor (f = 33%) appears to show slightly higher Tc . The current–voltage (I–V ) characteristics of the nanowires were also measured and a typical result is shown in Fig. 8 for a sample made by NIL on sapphire substrate. The I–V curve shows hysteretic behavior, with Ic about 1.5 μA at 2 K. The
IV. C ONCLUSION In summary, we have demonstrated a fabrication technique that is a combination of thermal-NIL and UV-NIL, to fabricate meander-type nanowire samples on ultra-thin Nb films. Meander nanowires of about 90 nm width with different filling factors covering effective detection areas of 10 × 10 μm2 have been fabricated. The meander nanowires are uniform in width. Superconducting transition temperature and critical current density are measured and compared with the data obtained on
ZHAO et al.: FABRICATION OF Nb SUPERCONDUCTING NANOWIRES BY NANOIMPRINT LITHOGRAPHY
conventional EBL-made meander nanowire samples. The Tc is similar for all samples, whereas the Jc is considerably lower for NIL samples than for EBL samples. We attributed this discrepancy to the thin RIE mask in the NIL process and the presence of some defects along the edges of the nanowires.
[10] [11] [12]
ACKNOWLEDGMENT The authors would like to thank W. Peng for technical assistance with X-ray reflectivity and A. Z. Jin for helpful discussion of the NIL technique. R EFERENCES [1] R. H. Hadfield, “Single-photon detectors for optical quantum information applications,” Nat. Photon., vol. 3, no. 12, pp. 696–705, Dec. 2009. [2] G. N. Goltsman et al., “Picosecond superconducting single-photon optical detector,” Appl. Phys. Lett., vol. 79, no. 6, pp. 705–707, Aug. 2001. [3] A. D. Semenov, G. N. Gol’tsman, and A. A. Korneev, “Quantum detection by current carrying superconducting film,” Phys. C, Supercond., vol. 351, no. 4, pp. 349–356, Apr. 2001. [4] 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, Jun. 2012, Art. ID. 063001. [5] A. J. Annunziata et al., “Superconducting Nb nanowire single photon detectors,” in Proc. SPIE, Adv. Photon Counting Tech., 2006, vol. 6372, Art. ID. 63720V. [6] G. Fujii et al., “Fiber coupled single photon detector with niobium superconducting nanowire,” in Quantum Communication and Quantum Networking, A. Sergienko, S. Pascazio, and P. Villoresi, Eds. Berlin, Germany: Springer-Verlag, 2010, pp. 220–224. [7] A. J. Annunziata et al., “Niobium superconducting nanowire singlephoton detectors,” IEEE Trans. Appl. Supercond., vol. 19, no. 3, pp. 327– 331, Jun. 2009. [8] G. Goltsman et al., “Ultrafast superconducting single-photon detector,” J. Mod. Opt., vol. 56, no. 15, pp. 1670–1680, Sep. 2009. [9] S. Miki, T. Yamashita, H. Terai, and Z. Wang, “High performance fibercoupled NbTiN superconducting nanowire single photon detectors with
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