Templated self-assembly of block copolymer, based on topographic templates defined by electron-beam lithography (EBL), is an attractive candidate for next generation high-resolution lithography. Templated self-assembly has two advantages compared with other lithography methods: first, the resolution can be scaled down to 5 nm, which cannot be achieved by optical lithography; second, the throughput can be increased by several folds compared with EBL. In our previous study, complex sub-20-nm patterns were fabricated with 45.5 kg/mol poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) block copolymer ^[1] .

Here, we demonstrate high throughput sub-10-nm fabrication by using templated self-assembly of block copolymer. To achieve 10-nm resolution, the dimensions of a block copolymer and a topographic template were scaled down to 10-nm-length scale. We used 16 kg/mol PS-b-PDMS block copolymer, which yields 9-nm half-pitch PDMS cylinders. To control the orientation of 9-nm half-pitch PDMS cylinders, rectangular lattices of posts with height of 19 nm, diameter of 8 nm, and various periods were fabricated and annealed with the block copolymer. As a result, PDMS cylinders formed a long-range ordered region when the post array satisfied the commensurate condition. By varying the periods of posts, a broad range of block copolymer lattice orientation angles was achieved (Figure 1).

On a lattice with the period larger than 72 nm, PDMS cylinders lost long-range order. To further decrease the density of the posts and therefore increase the throughput without losing long-range order, a sparse lattice of dashes was tested. As a result, a region of well-aligned PDMS cylinders with width of 708 nm was achieved (Figure 2d). The dashes occupy only 1/66 of the final PDMS line pattern. This result suggests that if instead of writing the complete pattern, EBL is used to create template arrays and the pattern is then completed by a block copolymer, the throughput of EBL could be increased dramatically.

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.

Semiconductor quantum dots (QDs) are electronically-quantized systems with promising applications in optoelectronic devices ^[1] . A key aspect of such systems is the fine control of optical transitions in the synthesis process ^[2] . These QDs are predominantly used in thin-film arrangement, deposited by spin casting or dip coating. Single QD patterning is one of the major challenges to designing a system that takes advantage of individual properties of QDs. Here we present a template self-assembly technique to control the position of individual QDs through electron-beam lithography (EBL). This optimized top-down lithographic process is a step towards the integration of individual QDs in optoelectronics systems for industrial applications.

The fabrication process of templated QDs is illustrated in Figure 1a. A poly(methylmethacrylate) (PMMA) resist was spin coated on a silicon substrate, followed by the fabrication of a mask through EBL. The size of the resulted PMMA templates was minimized by varying development temperature ^{[3] [4]} . Figure 1b shows the optimized PMMA patterning, with minimum template (i.e., hole) size of 8 nm for development at 6 °C. After defining the PMMA templates, a solution of QDs (6-nm-diameter CdSe) was spin casted and the remaining resist was removed by dissolution in acetone. This process resulted in QD clusters attached on the substrate. By optimizing the QD solution concentration, resist thickness, and feature size, we fabricated clusters with 1 to 10 QDs. One figure of merit in this process is the pattern yield, which is the ratio of yielded structures to the patterned templates. Figure 2 shows QD clusters with 87% pattern yield, with an average of 3 QDs in each cluster. Control of QD placement will be further optimized and integrated into photonic devices.

1. A. P. Alivisatos, “Semiconductor clusters, nanocrystals, and quantum dots,” Science, vol. 271. no. 5251, pp. 933-937, Feb. 1996.
2. S. A. Empedocles, D. J. Norris, and M. G. Bawendi, “Photoluminescence spectroscopy of single CdSe Nanocrystallite quantum dots,” Phys. Rev. Lett. vol. 77, pp. 3873-3876, Oct. 1996.
3. W. Hu, K. Sarveswaran, M. Lieberman, and G. H. Bernstein, “Sub-10 nm electron beam lithography using cold development of poly(methylmethacrylate),” J. Vac. Sci. Technol. B vol. 22, pp. 1711-1716, June 2004.
4. B. Cord, J. Lutkenhaus, and K. K. Berggren, “Optimal temperature for development of poly(methylmethacrylate),” J. Vac. Sci. Technol. B vol. 25, pp. 2013-2016, Dec. 2007.

Superconducting nanowire single-photon detectors (SNSPDs) ^[1] perform single-photon counting in the near‑infrared with outstanding performance. The main limitations of standard SNSPDs, based on ~ 4-nm-thick, 100-nm-wide NbN nanowires, are: (1) fragility with respect to constrictions; and (2) substantially reduced sensitivity beyond 2 µm wavelength (λ). We developed SNSPDs based on ultra-narrow (30- to 10-nm-wide) superconducting nanowires ^[2] , which showed improved robustness to constrictions and higher sensitivity to near-infrared photons with respect to standard SNSPDs.

As shown in Figure 1, at λ = 1550 nm our 30 nm nanowire‑width SNSPDs could be biased far from the device critical current (I_C) with minimal loss in detection efficiency (η), so even heavily‑constricted devices could reach the same efficiency as constriction‑free ones.

As shown in Figure 2 a, varying λ from 700 to 2100 nm, the η vs bias current (I_B) curves of 30 nm nanowire‑width SNSPDs kept a sigmoidal shape, with the cut-off current (I_co, taken to be at the inflection point of the η vs I_B curves) increasing from 0.31 I_C to 0.41 I_C. For 90 nm nanowire‑width detectors (shown in Figure 2 b), I_co increased from 0.64 I_C at λ = 500 nm to 0.89 I_C at λ = 1400 nm. This behavior indicates that ultra-narrow-nanowire SNSPDs are more sensitive to low-energy photons than standard devices and suggests that their sensitivity may extend to mid-infrared wavelengths.

The MIT Lincoln Laboratory portion was sponsored by the Department of the Air Force under Air Force Contract #FA8721-05-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by the United States Government.

1. G. N. Gol’tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, “Picosecond superconducting single-photon optical detector,” Applied Physics Letters, vol. 79, no. 6, pp. 705-707, 2001.
2. F. Marsili, F. Najafi, E. Dauler, X. Hu, M. Csete, R. Molnar, and K. Berggren, “Single-photon detectors based on ultra-narrow superconducting nanowires,” Nano Letters, vol. 11, no. 9, pp. 2048-2053, 2011.

Collaborators

• A. Alexander-Katz, MIT
• V. Avant, Photonspot
• M. Bawendi, MIT
• J. Buongiorno, MIT
• I. Chuang, MIT
• M. Csete, U. of Szeged
• E. Dauler, Lincoln Laboratory
• S. Gradecak, MIT
• A.J. Kerman, Lincoln Laboratory
• R.P. Mirin, NIST
• R. Molnar, Lincoln Laboratory
• M. Rooks, Yale
• C.A. Ross, MIT
• H.I. Smith, MIT
• M.J. Stevens, NIST
• E. Thomas, MIT
• A. Vladar, NIST
• F. Wong, MIT
• Y.K.W. Yang, IMRE

Visiting Faculty

• R. Peterson, U. of the South
• S. Warisawaa, U. of Tokyo

Postdoctoral Associates

• F. Marsili, EECS
• A. Mortensen, EECS
• S. Schulz, EECS
• S. Strobel, EECS
• J. Yang, EECS

Graduate Students

• A. McCaughan, Res. Asst., EECS
• C. Herder, Res. Asst., EECS
• M. Snella, EECS
• D. Meyer, Res. Asst., EECS
• F. Najafi, EECS
• S. Nicaise, EECS
• H. Korre, Res. Asst., EECS
• J-B. Chang, Res. Asst., Mat. Sci
• X. Hu, Res. Asst., EECS
• J. Leu, Res. Asst., EECS
• D. Winston, Res. Asst., EECS

Undergraduate Students

• D. Aude, UROP
• V. Ramasesh, UROP
• K. Harry, U. of Kansas
• W. Kalb, UROP
• S. Warnock, UROP

Visiting Students

• F. Bellei, Politecnico di Torino
• A. Tavakkoli, Ntl. U. Of Singapore

Support Staff

• A. Akins, Adm. Assistant
• J. Daley, Project Technician, NSL
• A. Dane, Res. Associate, QNN
• G. Mackay-Smith, Adm. Assistant
• M. Mondol, Facility Manager, SEBL

Publications

Jung, Y.S., J.B. Chang, E. Verploegen, K.K. Berggren, and C. A. Ross, “A Path to Ultranarrow Patterns Using Self-Assembled Lithography,” Nano Letters, vol. 10, pp.1000-1005, 2010.

Yang,  K. W.,Y.S. Jung, J.B. Chang, R.A. Mickiewicz, A.K. Alexander, 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, 2010.

Zhong, T., X. Hu, F.N.C Wong, K. K Berggren, T.D. Roberts, and P. Battle, “High quality fiber-optic polarization entanglement distribution at 1.3µm telecom wavelength,” Optic Letters, vol. 35, pp. 1392-1394, 2010.

Stevens, M.J., B. Baek, E.A. Dauler, A.J. Kerman, R.J. Molnar, S.A. Hamilton, K.K. Berggren, R.P. Mirin, and S.W. Nam, “High-order temporal coherences of chaotic and laser light,” Optic Express, vol. 18, pp. 1430-1437, 2010.

Huigao Duan and Karl K. Berggren, “Directed Self-Assembly at the 10 nm Scale by Using Capillary Force-Induced Nanocohesion,”  Nano Letters, Vol. 10, pp. 3710-3716, 2010.

Mickiewicz, R.A., J. K. W. Yang, A.F. Hannon, Y.S. Jung, A. Alexander-Katz, K. K. Berggren, and C. A. Ross, “Enhancing the potential of block copolymer lithography with polymer self-consistent field theory simulations,” Macromolecules, Vol. 43, No. 19, pp. 8290-8295, 2010.

Duan, H. 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-10nm scale,” Journal of Vacuum Science and Technology B, Vol. 28, No. 6, pp. H11-H17, 2010.

Strobel, S., C. Kirkendall, J.B Chang, and K.K Berggren, “Sub-10-nm Structures on Silicon by Thermal Dewetting of Platinum,” Nanotechnology, Vol. 21, 2010

Peterson, R.S., K.K. Berggren, and M. Mondol, “The Scanning Electron Microscope as an Accelerator for The Undergraduate Advanced Physics Laboratory,” CAARI 2010: 21st International Conference on the Application of Accelerators in Research and Industry, Denton, TX; AIP.

Wang, S.X., Y.Ge, J. Labaziewicz, E. Dauler, K. K. Berggren and I.L Chuang, “Superconducting microfabricated ion traps,” Applied Physics Letters, Vol. 97, 2010.

Duan, H., D. Winston, J.K.W. Yang, B.M. Cord, V.R. Manfrinato, and K.K. Berggren, “Sub-10-nm Half-Pitch Electron-Beam Lithography by Using PMMA as a Negative Resist,” Journal of Vacuum Science and Technology B, Vol. 28, pp. C6C58-C6C62, 2010.

Korre, H., C.P. Fucetola, J.A Johnson, and K.K Berggren, “Development of a simple, compact, low-cost interference lithography system,” Journal of Vacuum Science and Technology B, Vol. 28, pp. C6Q20, 2010.

Self-organized macromolecular materials can provide an alternative pathway to conventional lithography for the fabrication of devices on the nanometer scale. In particular, the self-assembly of the microdomains of diblock copolymers within lithographically-defined templates to create patterns with long range order has attracted considerable attention, with the advantages of cost-effectiveness, large-area coverage, and compatibility with preestablished top-down patterning technologies. Previously, we showed that spherical morphology poly(styrene-b-dimethylsiloxane) (PS-PDMS) block copolymers, which have a large interaction parameter and a high etch-contrast between two blocks, can be templated using an array of nanoscale topographical elements that act as surrogates for the minority domains of the block copolymer ^[1] . Recently, we showed that complex nanoscale patterns can be generated by combining the self-assembly of block-copolymer thin films with minimal top-down templating. A sparse array of nanoscale HSQ posts was used to accurately dictate the assembly of a cylindrical PS-PDMS diblock copolymer into a wide assortment of complex, unsymmetrical features, as shown in Figure 1 ^[2] . To extend the feature sizes to the sub-10-nm range, we demonstrated the formation of highly ordered grating patterns with a line width of 8 nm and period of 17 nm from a self-assembled PS-PDMS diblock copolymer and fabricated sub-10-nm-wide tungsten nanowires from the self-assembled patterns using a reactive ion etching process, as shown in Figure 2.

1. I. Bita, J. K. W. Yang, Y. S. Jung, C. A. Ross, E. L. Thomas, and K. K. Berggren, “Graphoepitaxy of self-assembled block copolymers on two-dimensional periodic patterned templates,” Science, vol. 321, pp. 939-943, 2008.
2. J. K. W. Yang, Y. S. Jung, J.-B. Chang, 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, 2010.

A commercially-available scanning-helium-ion microscope of high source brightness ^[1] has been modified for operation with neon gas. This neon system had been evaluated for nano-machining ^[2] , but not for resist-based lithography, as has been done with helium systems ^{[3] [4]} . The neon system may enable a lithography process with higher resolution than any scanning-particle system to date. This possibility is due to the combination of the high-brightness source and the expected reduction of secondary-electron (SE) range relative to electrons or helium ions. In addition, the expected increase in SE yield relative to electrons or helium ions may lead to a lithography process with high sensitivity. This high sensitivity could allow critical doses below substrate-damage thresholds. Figure 1 presents preliminary data on the point-spread function (PSF) of neon compared to helium.

The Stopping and Range of Ions in Matter (SRIM) is a popular, industry-standard tool for simulating the trajectories of incident ions in a target sample. However, SRIM does not simulate the trajectories of secondary electrons (SEs) produced by ion-sample interactions. SEs are responsible for exposure of resist and thus figure prominently in modeling of electron-beam lithography and proton-beam lithography. We developed a hybrid approach to modeling helium-ion lithography that combines the power and ease-of-use of SRIM with the results of recent work simulating SE yield in helium-ion microscopy ^[5] . This approach traces along SRIM-produced helium-ion trajectories, generating and simulating trajectories for these SEs using a Monte Carlo method. Figure 2 illustrates the utility of our software, which can also simulate electron beams.

1. B. W. Ward, J. A. Notte, and N. P. Economou, “Helium ion microscope: a new tool for nanoscale microscopy and metrology,” J. Vac. Sci. and Technol. B, vol. 24, pp. 2871-2874, 2006.
2. S. Tan, R. Livengood, D. Shima, J. Notte, and S. McVey, “Gas field ion source and liquid metal ion source charged particle material interaction study for semiconductor nanomachining applications,” J. Vac. Sci. and Technol. B, vol. 28, pp. C6F15-C6F21, 2010.
3. D. Winston, B. M. Cord, B. Ming, D. C. Bell, W. F. DiNatale, L. A. Stern, A. E. Vladar, M. T. Postek, M. K. Mondol, J. K. W. Yang, and K. K. Berggren, “Scanning-helium-ion-beam lithography with hydrogen silsesquioxane resist,” J. Vac. Sci. Technol. B, vol. 27, pp. 2702-2706, 2009.
4. V. Sidorkin, E. van Veldhoven, E. van der Drift, P. Alkemade, H. Salemink, and D. Maas, “Sub-10-nm nanolithography with a scanning helium beam,” J. Vac. Sci. Technol. B, vol. 27, pp. L18-L20, 2009.
5. D. Winston, J. Ferrera, L. Battistella, A. E. Vladar, and K. K. Berggren, “Modeling the point-spread function in helium-ion lithography,” submitted for publication.

Figure 1: SEM images of the ox-PDMS micro-domains guided by using HSQ template. White and grey colors represent HSQ and ox-PDMS. a) Line frequency doubling happened when using a PS brush and an HSQ template with the same period as the BCP. b) Rectangular lattice shape and dot doubling frequency were achieved by using the PS brush. (Insets: schematic illustrations of the images, in which grey and black colors represent HSQ and ox-PDMS).

In this study, we demonstrated a high-resolution method for doubling the spatial frequency of lines and dots of structures defined by electron-beam lithography, as well as a method for achieving a rectangular lattice in the case of dots. The method used an array of structures defined by electron-beam-lithography (EBL) to template block copolymers. The spatial frequency of the obtained results was half of the original BCP pitch. This method resulted in higher resolution and higher areal density in comparison to previously reported methods for spatial frequency multiplication using templated self-assembly of BCPs, which were limited by the BCP pitch.

In the first step, the templates were fabricated such that the periods of the lines or dots were the same as the period of the BCP. After fabrication of the templates by means of EBL exposure of hydrogen silsequioxane (HSQ) resist, the templates were chemically functionalized with a hydroxyl terminated polystyrene (PS) brush. Then, BCPs of polystyrene-b-polydimethylsiloxane (PS-PDMS) were spin cast onto the substrates with HSQ templates. Annealing of the BCP thin film was done using a cosolvent vapor anneal consisting of 5 parts toluene to 1 part heptane. An oxygen reactive ion etch (RIE) was used to remove the PS block and leave the oxidized-PDMS patterns on the substrate.

Figure 1a shows a scanning electron micrograph (SEM) image of line frequency doubling. The BCP used in this experiment was cylindrical morphology. This result indicates that by using PS functionalized posts; we can template linear spatial-frequency doubling. The period of the resultant lines was half of the period of the HSQ template and the BCP. Figure 1b shows an SEM image of a rectangular lattice obtained by using the above-mentioned method. The HSQ posts formed a centered rectangular lattice as seen in Figure 1b. The BCP used in this experiment was spherical morphology PS-PDMS.

Figure 1: (a) Instrument response function (IRF) of a SNAP based on two 30-nm-wide nanowires (2-SNAP). (b) Jitter of a 2-, 3-, and 4-SNAP based on 30 nm wide nanowires as a function of the normalized bias current (IB / ISW). (c) IRF asymmetry of the same devices as in (b).

Superconducting nanowire avalanche photodetectors (SNAPs) ^[1] are based on a parallel architecture that performs single-photon counting with higher signal-to-noise ratio (up to a factor ~4) than traditional superconducting nanowire single-photon detectors (SNSPDs) ^[2] . Although the understanding of the operation mechanism of SNAPs was recently improved ^{[3] [4]} , a comprehensive study of the timing performance is still lacking. In the only study reported so far ^[5] , SNAPs showed significantly higher timing jitter (>250 ps FWHM) than SNSPDs (~30ps FWHM ^[6] ).

We report a study of the SNAP timing performance for several device architectures and bias regimes. Our main findings were: (1) the instrument response function (IRF) shifted to longer delay times when the bias current was decreased (Figure 1.a); (2) when properly biased, SNAPs achieved the same jitter as SNSPDs (Figure 1.b); and (3) the IRF became more asymmetric when the bias current approached the avalanche current (Figure 1.c).

We simulated the electro-thermal dynamics of SNAPs using the model described in ^[3] . While we could not explain the origin of the IRF asymmetry, the IRF shift to longer delay times was shown to be caused by a change in the electro-thermal behavior of the detectors at decreasing bias currents. We conclude that SNAPs are suitable for timing-sensitive single-photon experiments while offering a higher signal-to-noise ratio than standard SNSPDs.

1. M. Ejrnaes et al., Appl. Phys. Lett. vol. 91, p. 262509, 2007.
2. G. N. Gol’tsman et al., Appl. Phys. Lett. vol. 79, p. 705, 2001.
3. F. Marsili et al., Appl. Phys. Lett. 98, p. 093507, 2011.
4. F. Marsili et al., Nano Lett., 2011, 11 (5), pp 2048–2053.
5. M. Ejrnaes et al., Appl. Phys. Lett. 95, p. 132503, 2009.
6. E. A. Dauler et al., IEEE Trans. Appl. Supercond. vol. 17, p. 279, 2007.

Over the last decade, quantum information experiments with trapped ions have demonstrated essential steps towards quantum computing and quantum simulation ^[1] .  Large fields are required to achieve strong coupling to the ions via dipolar interactions. To accomplish this coupling, we are integrating transmission line microresonators into the 2D trap structures already implemented at the Center for Ultracold Atoms at MIT ^{[2] [3]} . The resonators are superconducting niobium to minimize loss and maximize quality factors ^{[4] [5]} . Using the large fields locally generated from the accumulation of microwave photons in the resonator, we hope to demonstrate interaction with the trapped ions first by heating them and eventually to control the rotational states of the ions by coherently coupling to them.

We designed integrated resonator-ion traps and fabricated them on niobium on A-plane sapphire using optical lithography. We deposited the 200-nm niobium layers on the sapphire with an in-house DC magnetron sputtering system.  Tests showed that the niobium achieved superconductivity at the expected temperature of 9.2 K.  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.

Current results from our fabricated resonators show first-resonance quality factors of at least 10⁴ at 3.23 GHz at device temperatures of 3-4 K.  We have recently successfully demonstrated on-chip superconductivity while trapping crystallized ions, bringing us closer to our goal of coupling microwave photons to the ions.  In the coming months, we hope to integrate the high quality factor resonators into the trap design and use them incoherently heat the trapped ion cloud.  We also intend to shift our process from niobium to niobium nitride, allowing us a higher temperature threshold in which we may still be superconducting.

1. A. André, D. DeMille, J. M. Doyle, et al. “A coherent all-electrical interface between polar molecules and mesoscopic superconducting resonators,” Nature Physics, vol. 2, no. 9, pp. 636-642, 2006
2. J. Labaziewcz, Y. Ge, P. Antohi, D. Leibrandt, K. Brown, and I. L. Chuang, “Suppression of heating rates in cryogenic surface-electrode ion traps,” Phys. Rev. Lett. vol. 100, p. 130001, 2008
3. D. Stick, W. Hensinger, S. Olmschenk, et al. “Ion trap in a semiconductor chip,” Nature Physics, vol. 2, no. 1, pp. 36-39, 2005
4. M. Göppl, A. Fragner, M. Baur, R. Bianchetti, S. Filipp, J. Fink, et al. “Coplanar waveguide resonators for circuit quantum electrodynamics,” Journal of Applied Physics, vol. 104, no. 11, pp. 3904, 2008.
5. J. Gao, “The physics of superconducting microwave resonators,” PhD thesis, California Institute of Technology, Pasadena, 2008.