A collaboration of RLE and MTL investigators is creating the scientific and engineering knowledge for a compact coherent X-ray source for phase contrast medical imaging based on inverse Compton scattering of relativistic electron bunches. The X-ray system requires a low emittance electron source that can be switched at timescales of tens of femtoseconds or faster; the focus of our work has been the design, fabrication and characterization of massive arrays of a nanostructured high aspect-ratio silicon (Si) structures to implement low-emittance and high-brightness cathodes that can be triggered very fast using laser pulses to produce spatially uniform electron bunches. Si nanostructure arrays with highly uniform sub-10 nm tip radii have been fabricated via a combined optical lithography and diffusion limited oxidation technique. The fabrication process allows nanometer-level control over the dimensions of the electron emitter structures. Figure 1 shows an array of Si tips with 1.25 µm hexagonal pitch have an average radius of curvature of 6.2 nm and standard deviation of 1.1 nm (n=29); when the radius of curvature is changed to 21.6nm, the standard deviation remains approximately the same, i.e., 1.25 nm (n=69).

The tips are illuminated at a grazing incidence of roughly 84 degrees with a 1 kHz titanium sapphire laser (800 nm wavelength) with a pulse duration of 35 fs; the high electric field of the laser pulse is amplified by the silicon tips so the electrons can quantum tunnel from the tips into the vacuum. Experimental results using a time of flight spectrometer show electron beamlet array emission with 3-photon absorption. Work is ongoing to optimize the tip geometry for both low emittance and high current. We are also designing and building a new vacuum chamber to test the devices (Figure 2). The chamber will pump down to 10^-7 torr in ~15min with an anode bias up to 1100V.

Self-assembled block copolymer structures are useful in nanolithography applications, producing patterns with high resolution and throughput. We previously showed control over the direction of in-plane cylindrical microdomains formed by self-assembly of a block copolymer (BCP) using a variety of physical templates made from hydrogen silsesquioxane (HSQ) resist ^{[1] [2]} . The HSQ templates were fabricated by electron-beam lithography and then functionalized with a minority or majority block brush to interact with the BCP and direct the self-assembly (as shown in Figure 1). However, HSQ templates were not easily removed and remained as part of the final pattern. Remaining HSQ caused non-uniform pattern transfer due to dissimilar etch rates between the BCP and HSQ. In this study, we solved this issue by using a removable-resist template coated with an etchable-block brush. We fabricated two- and three-dimensional BCP patterns and then removed the templates. Examples (Figure 2) include three-dimensional bends, junctions and mesh-shaped structures, and the ability to change the BCP morphology through templating.

The negative-tone-post templates were made by electron-beam lithography of poly (methyl methacrylate) (PMMA) resist at high dose (100-600 pC/pixel). After development of patterns using methyl isobutyl ketone (MIBK) and acetone ultrasonication, the surface of the patterns was coated with hydroxyl-terminated polystyrene (PS) brush (1 kg mol^-1). Then poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) BCP (MW=45.5 kg mol^-1, f_PDMS=0.32, period 35 nm) was spun and solvent annealed with a mixture of heptane and toluene. CF₄ and O₂ reactive ion etch (RIE) was used to remove the top PDMS layer and the PS matrix.  The O₂ RIE not only removed not only the PS matrix but also removed the PMMA template in the same step. The final results were in-plane oxidized-PDMS cylindrical microdomain patterns in the form of two- and three-dimensional structures devoid of templates. This study provides a path to complex pattern formation for nanolithography with feature sizes below 20 nm.

1. Yang, J. K. W. et al. Nature Nanotechnology 5, 256-260, 2010.
2. Duan H. et al. Journal of Vacuum Science and Technology B 28 C6C58-C6C62, 2010.

The system detection efficiency of systems integrated with superconducting nanowire single photon detectors (SNSPDs) has been limited to 24% due to losses along the optical path ^[1] and the small active area of the detector (typically less than 100 µm²). SNSPDs are typically fabricated with a meandering structure to cover a large area; but the kinetic inductance, which limits device performance, increases with the nanowire length, limiting the ultimate area that can be covered. Integrating SNSPDs with antennas can lead to better detection of low photon flux IR sources, such as is required for VLSI circuit evaluation and long-wavelength astronomical observations, by increasing the area over which light is collected and concentrating it on the active area of the detector active.

Preliminary finite element simulations were performed to determine the coupling between a single NbN nanowire and a log-periodic gold antenna in the near IR range based on a previously reported design ^[2] , shown in Figure 1, using the RF module of COMSOL Multiphysics. The log-periodic design is broadband, but the resonance properties of the antenna are influenced by the electrical properties of the metal and the presence of the metallic superconducting nanowire, as shown in Figure 2. The preliminary design showed an increase in the absorption of light in the nanowire by a factor of 2.4 compared to the absorption in the nanowire without the antenna. Further simulations will be performed to optimize the structure of the nanoantenna for coupling to a boustrophedonic SNSPD structure in the mid-IR range.

1. X. Hu, T. Zhong, J.E. White, E.A. Dauler, F. Najafi, C.H. Herder, F.N.C. Wong, K.K. Berggren, “Fiber-coupled nanowire photon counter at 1550 nm with 24% system detection efficiency,” Optics Letters, vol. 34, pp. 3607-3609, Nov. 2009.
2. F. J. Gonzales and G.D. Boreman, “Comparison of dipole, bowtie, spiral and log-periodic IR antennas,” Infrared Science and Technology, vol. 46, pp. 418-428, June 2005.

We are interested in the application of arrays of electron field emitters, which can be achieved from a variety of materials, for the preparation of compact and coherent X-ray sources via inverse Compton scattering.  Field emission of electrons is commonly achieved by applying a static electric field or optical illumination to sharp metal tips. Sub-wavelength nanostructures can provide geometry-dependent electric field enhancement for both methods. For applications in coherent X-ray sources, the field emitter arrays should be able to emit short electron pulses, typically on the femtosecond timescale, which is difficult using conventional electrical circuits. Therefore, optical triggering, whereby a femtosecond laser is used to stimulate electron emission, has been considered.

We have simulated optical fields around various field emitter structures using COMSOL finite element software.  Among them, we are particularly interested in the “bullet” structure illustrated in Figure 1.  Conical tip structures are widely used to achieve both electrostatic and optical field enhancement in field emitters; however, uniform conical structures pose significant challenges in nanoscale fabrication due to their tapered geometry.  Arrays of metallic “bullet” structures, as shown in Figure 1, may be fabricated with a high areal density via positive-tone electron beam lithography with a PMMA resist.

We have designed a fabrication process for the preparation of arrays of optical-field emitters, based on the metallic “bullet” structure shown in Figure 2 (a).  The thin SiO₂ layer shown in Figure 2 (a) is used to prevent electron emission from the bulk silicon substrate by acting as an electrically insulating barrier.  The thickness of the SiO₂ layer has yet to be optimized. As a proof of concept, we have fabricated an array of Au nanoparticle emitters with an aspect-ratio of 1 (see Figure 2 (b)).  Further optimization of the fabrication process is currently underway.

1. D. Temple, “Recent progress in field emitter array development for high performance applications,” Materials Science and Engineering: R: Reports, vol. 24, pp. 185-293, Jan. 1999.
2. O. J. F. Martin and C. Girard, “Controlling and tuning strong optical field gradients at a local probe microscope tip apex,” Applied Physics Letters, vol. 70, pp. 705-707, Feb. 1997.
3. L. Novotny, R. X. Bian, and X. S. Xie, “Theory of nanometric optical tweezers,” Physical Review Letters, vol. 79, pp. 645-648, July 1997.

Figure 1: a). Schematic of a 2-bit position array by inductive current splitting. The detectors are biased at I_B. L_S is the choke inductor. L_K is the total kinetic inductance of the nanowires, including the active nanowires and the nanowire-inductors. R_K is the on-chip Ti/Au resistor. L_B, C, and Z₀ are the inductance of bonding wire, coupling capacitor, and input impedance of the readout circuit, respectively. The orange arrows show the current distribution when the second detector D2 is fired. The width of each arrow is proportional to the amplitude of the current. b). The simulated waveforms of the differential output, V_L-V_R. The position information can be clearly read by the amplitude of the pulses. c). The current fluctuation in the unfired detectors when the second detector D2 is fired. All the detectors are biased at 0.8 I_B/I_C. The maximum overshot is 3% of I_C showing the array can work steadily without being affected much by the fired detector.

Single pixels of superconducting nanowire single photon detector (SNSPD) have high detection efficiency, low dark-count rate, and low timing jitter. To achieve a large active area, spatial resolution, or photon-number resolution, an SNSPD array is a promising approach for applications in quantum-information processing, single-photon imaging, and deep-space communication. However, designing a readout scheme is a serious challenge for multi-pixel detector arrays. Two groups have previously used single flux quantum (SFQ) to multiplex the array and have reported some experiment results on single detector ^{[1] [2]} . This method is useful for large-scale SSPD array but Josephson junctions are needed to be fabricated on the same chip and increase the complexity of the fabrication.

For a medium sized SNSPD array, we propose a simple design using only an on-chip integrated inductor and resistor network. The positions and number of detectors that have received a photon are read by the combination of difference and sum signals of the pulses in the two outputs. The maximum size of the array is limited by the amplitude of the differential output signal, the electric noise, the sensitivity of the readout circuit, and the circuit element variability. To overcome these limitations and increase the size of the array, we made improvements over conventional superconducting nanowire single photon detectors in two ways. First, we redesigned the detector to operate by a multi-stage avalanche mechanism, which increases the output signal dramatically. Second, we added a cryogenic amplifier with a low-loss connection to the readout circuit, increasing the signal-to-noise ratio by a factor of ~4. Additionally, to construct the array, we have designed and tested on-chip resistors. We also simulated and analyzed the array in a lumped circuit element and electro-thermal model. Based on simulation results, the size of an array resulting from this approach could be over 100 elements.

1. S. Miki, H. Terai, T. Yamashita, K. Makise, M. Fujiwara, M. Sasaki, and Z. Wang, “Superconducting single photon detectors integrated with single flux quantum readout circuits in a cryocooler,” Applied Physics Letters, vol. 99, no. 11, pp. 111 108, 2011.
2. T. Ortlepp, M. Hofherr, L. Fritzsch, S. Engert, K. Ilin, D. Rall, H. Toepfer, H. G. Meyer, and M. Siegel, “Demonstration of digital readout circuit for superconducting nanowire single photon detector,” Opt. Express, vol. 19, no. 19, pp. 18 593-18 601, Sep. 2011.

Figure 1: Detection efficiency (η) of a 30 nm width SNSPD as a function of normalized bias current (IB/Isw) measured at wavelength λ = 1.4 µm (blue curve), λ = 3.7 µm (red curve), and λ = 5 µm (black curve).

We report on the detection of single-photons in the middle-infrared (mid‑IR) range using superconducting nanowire single-photon detectors (SNSPDs). In 2011 our group managed to detect single-photons at a wavelength λ = 1.55 µm using SNSPDs based on ultra-narrow nanowires (30‑nm‑width) with a saturated detection efficiency of η = 20 % at a lower bias current than is possible for wider NbN nanowires[1] (I_B = 0.7 I_sw, where I_sw is the bias current after which the detectors switch to the normal state). These results suggest that ultra-narrow SNSPDs are suitable for detection of photons at longer wavelengths. Figure 1 shows the detection efficiency η measured as a function of the normalized bias current (I_B/I_sw) for λ = 1.4 µm (blue curve), λ = 3.6 µm (red curve), and λ = 5 µm (black curve). The detection efficiency measured at λ = 1.4 µm does not reach η = 20 % because the polarization of the photons could not be optimized to enhance the absorption in the nanowires as in reference ^[1] . The detection efficiency reaches a value of η = 4 % at λ = 3.7 µm, which is two orders of magnitude higher than what has been reported in the literature so far for this type of detector at this wavelength. The figure also shows that the detection efficiency reaches an appreciable value of η = 2 % at λ = 5 µm.

The authors thank E. Dauler and R. J. Molnar from Lincoln Laboratory for technical support and scientific discussions.

1. F. Marsili, F. Najafi, E. Dauler, F. Bellei, X. Hu, M. Csete, R.J. Molnar, and K.K. Berggren, “Single-photon detectors based on ultra-narrow superconducting nanowires,” Nano Letters, vol. 11, pp. 2048-53, May 2011.

Topographic templates can be used for guiding the self-assembly of block copolymers to produce complex nanoscale patterns. In our previous work, sub-20-nm bends or meander structures were achieved based on topographic templates using 45.5 kg/mol polystyrene polydimethylsiloxane (PS-PDMS) block copolymer ^[1] . To template the self-assembly of 16 kg/mol PS-b-PDMS with a sub-10-nm periodicity, so as to scale down the resolution to sub-10-nm, the diameter of the templates must also be scaled down to the sub-10-nm range. However, the fabrication of sub-10-nm posts is challenging even with state-of-the-art lithography.

In this study, we demonstrate aligned sub-10-nm block copolymer patterns, based on the understanding of the interaction between the block copolymer cylinders and posts with a range of diameters and heights. Figure 1 shows how the orientation of the block copolymer cylinders depended on the post diameter for a given spacing, L_x = 48 nm and L_y = 32 nm. The commensurate orientation of the 16 kg/mol PS-b-PDMS block copolymer cylinders for this array period is depicted in Figure 1(b), giving a cylinder orientation of 53.1˚ with respect to the x-axis. The area fraction of the block copolymer oriented along the commensurate orientation was over >75% when templated by posts with a height of 16 nm and diameters from 8 nm to 12 nm. The cylinders were not aligned in a preferential orientation when templated by posts with a diameter of 6 nm. However, the cylinders were aligned parallel to the y-axis when templated by large (diameter > 13 nm) or tall (height > 24 nm) posts.

Based on the preceding result, we demonstrated every possible commensurate condition for the 18-nm period PS-b-PDMS. As shown in Figure 2, the orientation of the block copolymer cylinders could be varied between 0^o and 90^o with respect to the x-axis by using posts with a height of 19 nm and a diameter of 10 nm.

1. J. K. Yang, Y. S. Jung, J. Chang, R. A. Mickiewicz, A. Alexander-Katz, C. A. Ross, and K. K. Berggren, “Complex self-assembled patterns using sparse commensurate templates with locally varying motifs,” Nature Nanotechnology, vol. 5, pp. 256-260, Mar. 2010.

Templated assembly of biomolecules can create complex nanostructured devices with precisely tailored chemical or biological responses, with applications in, for example, nanoscale patterning for electronics, biomedical devices, or environmental sensors. In this research project, we focus on developing methods for creating complex molecular top-down templating of assembled structures of protein that will be relevant to a range of devices.

We examined a range of EM staining techniques for cortexillin, which is coiled-coil protein and forms a parallel homodimer as an actin-binding domain. The protein constructs have the addition of single cysteine residues at either the N- or C- terminus that would facilitate binding gold metal surfaces, as Figure 1(A) shows. Weakly staining the rod shape of proteins in uranyl acetate resulted in observation of structures 20 nm in length and 4~5 nm in width, which roughly matches the expected protein model from crystallographic structure, as in Figure 1(B). We also tested tagging of the cortexillin homodimer using gold nanoparticles, which resulted in gold points colocalized to the proteins. This provides a straightforward way to visualize single protein molecules using TEM and SEM.

Figure 1: (A) The structure of cortexillin (B) TEM images of cortexillin stained with 2% uranyl acetate and inset images (right) showing morphology of protein such as fat rods in higher magnification. (C) Single molecular protein array in the project, presents gold tagging to His-tag in protein and Cys residue facilitates binding to gold posts. (D) SEM images of protein array in the gold post pattern with 20-nm pitch.

We are also been developing a method to position proteins into the patterned surfaces with sub-10-nm resolution. We have functionalized an e-beam patterned surface with gold, resulting in two-post arrays with a pitch varying from 10 to 50 nm. The cysteine of the cortexillin homodimer can then be used to direct the attachment of the protein to the posts via thiol-gold attachment chemistry. The cortexillin proteins also contain a C-terminal hexa-histidine tag, which allows for the attachment of Ni-NTA gold nanoparticles (NTA: nitrilotriacetic acid) via the coordination of the charged histidines around the nickel ion (Kd ≅ 10-6 M). Figure 1(C) illustrates the assembly scheme of single protein array on a gold post pattern, where the cysteine residue in a coiled-coil protein that allows binding to surfaces of gold posts and the hexa-histidine sequence allows attachment of Ni-NTA gold nanoparticles. By performing multiple step protocol of: e-beam patterning of poly methyl methacrylate (PMMA) resist, gold post generation on PMMA pattern, PMMA development, cleaning by Plasma etching, and incubation in the Cys-modified protein solution, we achieved results suggesting the formation of single-molecule protein arrays around the gold posts of the pattern as observed by SEM. SEM imaging of the pattern after incubation with protein reveals that there is a relatively low density of protein binding, as indicated by small gold nanoparticles around the gold post patterns, as Figure 1(D) shows. Here, we could develop the method using electron-beam lithography to create a gold post pattern with sub-10-nm resolution pitch close to biomolecular scale, which could then be used to template novel systems including DNA, protein, and other biomolecules.

1. D. Klinov, K. Atlasov, A. Kotyar, B. Dwir, and Kapon E., “DNA nanopositioning and alignment by electron-beam-induced surface chemical patterning,” Nano Letters, vol. 7, p. 3583, 2007.
2. M. R. Diehl, K. Zhang, H. J. Lee, and D. A. Tirrell, “Engineering cooperativity in biomotor-protein assemblies,” Science, vol. 311, p. 1468, 2006.
3. N. Zizlsperger, V. N. Malashkevich, S. Pillay, and A. E. Keating, “Analysis of coiled-coil interactions between core proteins of the spindle pole body,” Biochemistry, vol. 47, p. 11858, 2008.

Electron-beam lithography (EBL) readily enables the fabrication of sub-10-nm features ^[1] . However, the resolution limits of this technique at length scales for below 10 nm are not well understood. The known resolution limiting factors of EBL are: (1) electron scattering; (2) spot size; (3) development process; and (4) resist structure. We decided to minimize the influence of electron scattering by using 200-keV electrons. We used Si₃N₄membranes as the substrate to minimize backscattered electrons. To minimize the spot size, we chose an aberration-corrected scanning transmission electron microscope (STEM) as the exposure tool with 0.14-nm spot size. STEM exposures at 200 keV have been done in conventional resists before ^{[2] [3]} , resulting in feature sizes of 6 nm and resolution (i.e., pattern period) of 30 nm. However, the resolution-limiting factors were not systematically explored. In this work we did STEM exposures in 10-nm-thick hydrogen silsesquioxane (HSQ) at 200 keV. We developed the structures with salty development ^[1] and performed bright field TEM metrology ^[4] .

Figure 1 shows feature sizes from 1 to 3 nm and maximum resolution of 10-nm pitch, which represent the smallest structures written in conventional e-beam resists. The reduced spot size in the STEM was responsible for the minimum feature size achieved. In addition, we measured the point-spread function (PSF) at 200 keV, shown in Figure 2. The PSF at 200 keV is much narrower than the 30keV one in the small radius range, leading to smaller short-range proximity effect and thus higher resolution.

1. J. K. W. Yang and K. K. Berggren, “Using high-contrast salty development of hydrogen silsesquioxane for sub-10-nm half-pitch lithography,” Journal of Vacuum Science & Technology B, vol. 25, no. 6, pp. 2025-2029, Dec. 2007.
2. C. Vieu, F. Carcenac, A. Pépin, Y. Chen, M. Mejias, A. Lebib, L. Manin-Ferlazzo, L. Couraud, and H. Launois, “Electron beam lithography: Resolution limits and applications,” Applied Surface Science, vol. 164, pp. 111-117, Aug. 2000.
3. S. Yasin, D. G. Hasko, and F. Carecenac, “Nanolithography using ultrasonically assisted development of calixarene negative electron beam resist,” Journal of Vacuum Science & Technology B, vol. 19, no. 1, pp. 311-313, Jan. 2001.
4. H. Duan, V. R. Manfrinato, J. K. W. Yang, D. Winston, B. M. Cord, and K. K. Berggren, “Metrology for electron-beam lithography and resist contrast at the sub-10-nm scale,” Journal of Vacuum Science & Technology B, vol. 28, no. 6, pp. C6H11-C6H17, Dec. 2010.

In recent years, several ion-trapping groups have shown that microwave electrodes integrated into surface electrode ion traps can be used for state manipulation of trapped ions ^[1] . Our group has pursued integrating microwave electrodes into superconducting ion traps to perform microwave spectroscopy in both atomic and molecular ions. Large fields are required to perform state manipulation, and these fields can be locally amplified using resonators. This amplification comes at the expense of bandwidth on the spectroscopic signal.

We designed microwave-integrated ion traps and fabricated them with a niobium on A-plane sapphire process in the Nanostructures Laboratory at MIT. We deposited thin films between 50-200 nm of niobium on the sapphire with an in-house DC magnetron sputtering system. Once we completed the material stack, we patterned the front niobium layer in PR1-2000A resist using the Heidelberg uPG-101 pattern generator, developed it, and then reactive-ion etched it with CF4 and O2 to transfer the pattern into the niobium

We are currently testing to see if our integrated microwave signals can excite the microwave clock transition in ⁸⁷Sr⁺ ions. This experiment uses microwave signals to couple between two hyperfine levels in the ground state of ⁸⁷Sr⁺. A laser excites the atom from one of the ground states to an excited state; when the atom decays back to one of the ground states, photons are emitted, and they can be imaged. Without the applied microwaves, the ion would quickly shelve into a dark state.

1. Ospelkaus, C., Warring, U., Colombe, Y., Brown, K.R., Amini, J.M., Leibfried, D., Wineland, D.J., “Microwave quantum logic gates for trapped ions,” Nature, vol. 476, no. 7359, pp. 181-184, 2011.