Field emission arrays (FEAs) are an attractive alternative to mainstream thermionic cathodes, which are power hungry and require high vacuum and high temperature to operate. Field emission of electrons consists of the following two processes: 1) transmission of electrons (tunneling) through the potential barrier that holds electrons within the material (workfunction φ) when the barrier is deformed by the application of a high electrostatic field and 2) supply of electrons from the bulk of the material to the emitting surface. Either the transmission process or the supply process could be the limiting step that determines the emission current of the field emitter (FE). Control of the transmission process (Fowler Nordheim) to produce high uniform current from FEAs is very challenging due to the physics of the field emission process. Due to the exponential dependence on the field factor and, hence, the tip radius, emission currents are extremely sensitive to tip radii variation; unfortunately, nanometer-sized tip radii in FEAs have a distribution with long tails that causes severe FEA underutilization. A better approach for achieving uniform emission from nanosharp FEAs is controlling the supply of electrons to the emitting surface. In a metal, the supply of electrons is very high, making the control of the supply challenging. However, in a semiconductor, where the local doping level and the local potential determine the concentration of electrons, it is possible to configure the emitter such that either the supply process determines the emission current.

We have designed and fabricated FEAs where each field emitter is individually ballasted using a vertical ungated field effect transistor (FET) made from a high aspect ratio (40:1) n-type silicon pillar. Each emitter has a proximal extractor gate that is self-aligned for maximum electron transmission to the anode (collector). Our modeling suggests that these cathodes can emit as much as 30 A.cm^-2 uniformly with no degradation of the emitters due to Joule heating; also, these cathodes can be switched at microsecond-level speeds. The design process flow, mask set and pillar arrays have been completed (Figure 1) with the self-aligned extractor gate to be completed by the spring of 2013. An ultra-high vacuum chamber has been designed and built to test the devices (Figure 2). The chamber can test full 150mm wafers with four high voltage probes at 10^-10 torr pressure.

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.

Previously we proposed a particle agglomeration model for chemical mechanical planarization (CMP) with the primary motivation of understanding the creation and behavior of the agglomerated slurry abrasive particles during the CMP process.  The particles are known to be a major cause of defectivity, poor consumable utility, and process variation. In this year’s work, we extend the model to include the effect of shear forces during CMP and to utilize a more sophisticated model of the double layer interactions between slurry abrasive particles in order to account for the charge effects of chemical additives. We also conducted fundamental experiments to observe agglomeration in non-commercial, experimentally created simple slurry mixtures (see Figure 1).

Our model considers the CMP slurry composition as a colloidal suspension of charged colloidal particles in an electrically neutral aqueous electrolyte. First, a theoretical relationship between the measurable chemical parameters of the slurry’s electrolyte (pH, conductivity/ionic strength), the surface potential of the abrasive particles, and subsequent zeta potential is established. Secondly, this potential is employed in a modified DVLO interaction potential model to determine the pair-wise particle interaction potentials due to both the attractive van Der Waals forces and repulsive electrostatic interactions as a function of the chemical parameters. Then, the total interaction potential created is used to define a stability ratio, which is combined with the shear rate of the process to calculate the orthokinetic aggregation rate constant. Finally, these rate constants are used in a discrete population balance framework to describe the growth of the slurry abrasive particle size distribution with respect to time and composition in a matter analogous to physical particle size distribution measurements.

We focus on the fundamental chemical and mechanical mechanisms by which slurry abrasive particles intended for planarization form agglomerates. We predict the formation of these agglomerates using measureable parameters and metrics of the slurry. The theoretical model described is empirically validated using literature and our own fundamental experimental data (shown in Figure 2).

Our model provides both a qualitative and quantitative description of agglomeration of slurry abrasive particles during CMP that will enable more accurate process control, increased consumable utility, and possible reduction of defectivity. Current work is focused on experimental development, validation, and extension of the theoretical model.

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.

Organic solar cells and photodetectors that feature singlet exciton fission materials have two additional exciton processes that traditional organic solar cells do not: singlet fission and triplet doublet annihilation. To maximize the usable power of the photovoltaic cell, we must understand how to optimize the gain from singlet fission and minimize the loss from doublet annihilation. We focus on the former here.

An organic photodetector is composed of thin layers of pentacene and PTCBI stacked repeatedly, as in Figure 1. This device structure is designed to enhance exciton dissociation at the donor/acceptor interface. The rapid dissociation of the singlet exciton in the photodetector competes with the singlet fission process, which is the formation of two triplet excitons from one singlet, and has been shown to be very fast and efficient ^{[1] [2]} . Modulation of the singlet fission rate by application of an external magnetic field changes the photocurrent by reducing the singlet fission rate relative to the rate of singlet dissociation into a charge.  The high field asymptotic value of the change in photocurrent is proportional to the fission yield ^[2] .

We built photodetectors that utilize variably thick layers of pentacene to modulate the rate competing with singlet fission. The maximum the internal quantum efficiency (IQE) of 130% occurs for an 8-nm-thick pentacene layer. The trend in IQE is matched by the trend of an increasing change in photocurrent with applied magnetic field, as Figure 2 shows. We conclude that there is less competition between the singlet for performing fission and dissociating at the donor/acceptor interface.

The gain in IQE from the singlet fission process is largest for pentacene layers of 8 nm. The change in photocurrent suggests that the fission efficiency is even larger for thicker layers; however, we observe loss in IQE, which could be due to exciton diffusion.

1. M. W. B. Wilson, A. Rao, J. Clark, R. S. S. Kumar, D. Brida, G. Cerullo, and R. H. Friend, “Ultrafast dynamics of exciton fission in polycrystalline pentacene,” Journal of the American Chemical Society, vol. 133, no. 31, pp. 11830–11833, 2011
2. J. Lee, P. Jadhav, and M. A. Baldo, “High efficiency organic multilayer photodetectors based on singlet exciton fission,” Applied Physics Letters, vol. 95, p. 033301, 2009.

Natural biological surfaces have evolved to optimize their physicochemical properties and structures at the micro/nanoscale for a wide variety of functions, ranging from wettability to optical properties ^{[1] [2] [3]} . Microscopic studies of the textured surfaces commonly encountered on living organisms, e.g., lotus leaves, desert beetles, and moth eyes, have revealed complementary roles of material properties and texture on the surface functionalities that have been developed during adaptation to different environments. These studies have in turn inspired biomimetic surfaces emulating the self-cleaning ^[4] , water harvesting ^[5] , and anti-reflective ^[6] capabilities of functional surfaces found in nature.

Taking cues from nature, we use tapered conical nanotextures to fabricate the multifunctional surfaces; the slender conical features result in large topographic roughness while the axial gradient in the effective refractive index minimizes reflection through adiabatic index-matching between air and the substrate. Precise geometric control of the conical shape and slenderness of the features as well as periodicity at the nanoscale are all keys to optimizing the multi-functionality of the textured surface, but at the same time these demands pose the toughest fabrication challenges.

Here we report a systematic approach to concurrent design of optimal structures in the fluidic and optical domains and a fabrication procedure that achieves the desired aspect ratios, periodicities with few defects, and large pattern area ^[7] . Our fabricated nanostructures demonstrate structural superhydrophilicity or, in combination with a suitable chemical coating, robust superhydrophobicity. Enhanced polarization-independent optical transmission exceeding 98% has also been achieved over a broad range of bandwidth and incident angles. These nanotextured surfaces are also robustly anti-fogging or self-cleaning, offering potential benefits for applications such as photovoltaic solar cells.

1. W. Barthlott and C. Neinhuis, “Purity of the sacred lotus, or Escape from contamination in biological surfaces,” Planta, vol. 202, no. 1, pp. 1-8, 1997.
2. P. B. Clapham and M. C. Hutley, “Reduction of lens reflection by moth eye principle,” Nature, vol. 244, no. 5414, pp. 281-282, 1973.
3. W. J. Hamilton and M. K. Seely, “Fog basking by the Namib Desert Beetle, Onymacris unguicularis,” Nature, vol. 262, no. 5566, pp. 284-285, 1976.
4. A. Lafuma and D. Quéré, “Superhydrophobic states,” Nature Materials, vol. 2, no. 7, pp. 457-460, 2003.
5. A. R. Parker and C. R. Lawrence, “Water capture by a Desert Beetle,” Nature, vol. 414, no. 6859, pp. 33-34, 2001.
6. Y.-F. Huang, S. Chattopadhyay, Y.-J. Jen, C.-Y. Peng, T.-A. Liu, Y.-K. Hsu, C.-L. Pan, H.-C. Lo, C.-H. Hsu, Y.-H. Chang C.-S. Lee, K.-H. Chen, and L.-C. Chen, “Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures,” Nature Nanotechnology, vol. 2, no. 12, pp.770-774, 2007.
7. K.-C. Park, H. J. Choi, C.-H. Chang, R. E. Cohen, G. H. McKinley, and G. Barbastathis, “Nanotextured silica surfaces with robust super-hydrophobicity and omnidirectional broadband super-transmissivity,” ACS Nano, vol. 6, no. 5, pp.3789-3799, 2012.

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.