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Templated Self-assembly of Block Copolymer for High Throughput Sub-10-nm Fabrication

• Authors: J. Chang, J. Son, C. A. Ross, K. K. Berggren
• Sponsorship: ONR, NRI

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.



Figure 1: Scanning electron microscope (SEM) images of 16 kg/mol PS-b-PDMS block copolymer annealed (a) without (b)~(l) with dot-arrays. Each figure is annotated horizontal pitch, vertical pitch of template array. Scale bar = 100 nm.



Figure 2: SEM images of PDMS microdomains templated by the dash-arrays with various horizontal pitches. The length of the dashes was 60 nm and the vertical pitch of the dashes was 100 nm. The horizontal pitch of the dashes was (a) 18 nm (b) 21 nm (c) 198 nm (d) 708 nm. Scale bar = 100 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. [↩]

Caroline A. Ross

• Title: Professor
• Department: Department of Materials Science and Engineering

Collaborators

• H.I. Smith
• C.V Thompson
• E.L. Thomas
• Y. Shao-Horn
• K.K. Berggren
• A. Alexander-Katz
• L.C. Kimerling (MIT)
• G. Dionne (Lincoln Lab)
• J. Borchers (NIST)
• G.J. Vancso (U. Twente, NL)
• M. Pardavi-Horvath (George Washington U.)
• A. Adeyeye, W.C. Choi (Nat. Univ. Singapore)
• J.Y. Cheng (IBM)
• M. Vazquez (Madrid)

Postdocs/Visitors

• Dr. Hyun Suk Kim
• Dr. Chunghee Nam
• Dr. Carlos Garcia
• Dr. Jeong Gon Son
• Dr. Youngman Jang
• Dr. Dong Hun Kim

Graduate Students

• Yeon Sik Jung
• Lei Bi
• Kevin Gotrik
• Mark Mascaro
• Adam Hannon
• Nicholas Aimon
• Frank Liu
• Mehmet Onbasli
• Jean Anne Currivan

Support Staff

• G. Joseph, Administrative Assistant II

Publications

Filip Ilievski and C. A. Ross, Graphoepitaxy of block copolymers using selectively removable templates, J. Vac. Sci. Technol. B 28(1) 42-44 (2010).

Hyun-Suk Kim, Lei Bi, Han-Jong Paik, Dae-Jin Yang, Yun Chang Park, Gerald F. Dionne and Caroline A. Ross, Self-assembled Single-Phase Perovskite Nanocomposite Thin Films, Nano Lett. 10 (2), pp 597–602 (2010).

R. Sanz, D. Navas, M. Vazquez, M. Hernandez-Velez, C.A. Ross, Preparation and Magnetic properties of cylindrical NiFe films and antidot arrays, J. Nanoscience and Nanotechnology 10 6776-6782 (2010).

Y. Ren, A.O Adeyeye, C. Nam and C.A. Ross, Effects of Interlayer Coupling in Elongated Ni₈₀Fe₂₀/Au/Co Nanorings, IEEE Trans. Magn. 46 1906-9 (2010).

J. L. Sánchez Llamazares, B. Hernando, C. García, and C. A. Ross, Kinetic arrest of direct and reverse martensitic transformation and exchange bias effect in Mn49.5Ni40.4In10.1 melt spun ribbons, J. Appl. Phys. 107, 09A956 (2010).

Joel K. W. Yang, Yeon Sik Jung, Jae-Byum Chang, Caroline A. Ross, and Karl K. Berggren, Complex self-assembled patterns using sparse commensurate templates with locally varying motifs, Nature Nanotechnology 5, 256-260 (2010).

B. C. Choi, E. Girgis, C. A. Ross, Th. Speliotis, Y. K. Hong, G. Abo, D. Niarchos, and H. Miyagawa, Incoherent interaction of propagating spin waves with precessing magnetic moments, Phys. Rev. B 81 092404 (2010) p1-4.

Lei Bi, Hyun-Suk Kim, Gerald F. Dionne and C. A. Ross, Structure, magnetic properties and magnetoelastic anisotropy in epitaxial Sr(Ti_1-xCo_x)O₃ films, New J. Physics 12 043044 (2010).

Yeon Sik Jung, J. B. Chang, Eric Verploegen, Karl K. Berggren and C. A. Ross, A path to ultra-narrow patterns using self-assembled lithography, Nano Letts. 10 (3), 1000–1005 (2010).

Mark D. Mascaro, Chunghee Nam, and C. A. Ross, Interactions between 180 and 360 degree domain walls in magnetic multilayer stripes Appl. Phys. Letts. 96, 162501 (2010) p1-3.

D.A. Navas, C. Nam, D. Velazquez, C.A. Ross, Shape and Strain-induced magnetization reorientation and magnetic anisotropy in thin film CoCrPt/Ti rings and lines, Phys. Rev. B 81 224439 (2010) p1-11.

C. García , J. M. Florez, P. Vargas and C. A. Ross, Asymmetrical Giant Magnetoimpedance in NiFe/IrMn thin films, Appl. Phys. Letts. 96, 232501 (2010) p1-3.

Chunghee Nam, M. D. Mascaro and C. A. Ross, Magnetostatic control of Vortex chirality in Co thin film rings, Appl. Phys. Letts. 96, 012505 (2010) p1-3.

Fabrication of diverse metallic nanowire arrays based on block copolymer self-assembly, Yeon Sik Jung and C. A. Ross, Nano Letts. 10 3722-6 (2010).

Enhancing the potential of block copolymer lithography with polymer self-consistent field theory simulations, Rafal A. Mickiewicz, Yeon Sik Jung, Joel K. W. Yang, A. Alexander-Katz, Karl K. Berggren, and C. A. Ross, Macromolecules 43 (19), 8290–8295 (2010).

AC and DC Current-Induced Motion of a 360˚ Domain Wall, M.D. Mascaro, C.A. Ross, Phys. Rev. B 82, 214411 (2010).

Hierarchical Nanostructures by Sequential Self-assembly of Styrene-Dimethylsiloxane Block Copolymers of Different Periods, Jeong Gon Son, Adam F. Hannon, Kevin W. Gotrik, Alfredo Alexander-Katz and Caroline A. Ross, Adv. Mater. 23 634-39 (2010).

Magnetization states in coupled Ni80Fe20 bi-ring nanostructures, Y Ren, S Jain, A O Adeyeye and C A Ross, New J. Phys. 12 093003 p1-11 (2010).

Effect of the exchange bias coupling strength on the magnetoimpedance of IrMn/NiFe films, C. Garcia, J. M. Florez, P. Vargas, and C. A. Ross, J. Appl. Phys. 109, 07D735 (2011).

Spectral Origins of Large Faraday Rotation at 1.5-mm Wavelength from Fe and Co in SrTiO₃ Films, Gerald F. Dionne, Lei Bi, H.-S. Kim and C.A. Ross, J. Appl. Phys. 109 07B761 (2011).

Lei Bi, Juejun Hu, Lionel Kimerling and C. A. Ross, Fabrication and characterization of As₂S₃/Y₃Fe₅O₁₂ and Y₃Fe₅O₁₂/SOI strip-loaded waveguides for integrated optical isolator applications, Proc. SPIE vol 7604 760406 (2010).

C.A. Ross, Y.S. Jung, V.P. Chuang, J.G. Son, K.W. Gotrik, R.A. Mickiewicz, J.K.W. Yang, J.B. Chang, K.K. Berggren, J. Gwyther, I. Manners, Templated self-assembly of Si-containing block copolymers for nanoscale device fabrication, Proc. SPIE vol 7637 76370H (2010).

Monolithic Integration of Chalcogenide glass/Iron Garnet Waveguides and Resonators for On-chip Nonreciprocal Photonic Devices, Lei Bi, Juejun Hu, Gerald F. Dionne, Lionel Kimerling and C. A. Ross, Proc. SPIE (2011).

Magnetoelastic Effects in Nanostructures, J.I. Arnaudas, A. Badia-Majós, L. Berbil-Bautista, M. Bode, F.J. Castaño, M. Ciria, C. de la Fuente, J.L. Diez-Ferrer, S. Krause, B.G. Ng, R.C. O’Handley, C.A. Ross and R. Wiesendanger, Solid State Phenomena Vols. 168-169 (2011) pp 177-184.

Self-assembly of Triblock Terpolymers

• Authors: J. G. Son, C. A. Ross (in collaboration with I. Manners, J. Gwyther), J. G. Son, C. A. Ross (in collaboration with I. Manners, J. Gwyther)

Triblock terpolymers are interesting because they can form a much wider diversity of 3D structures than diblock copolymers, including rings and square-symmetry patterns, which may be useful in nanolithography. Two examples of pattern formation in triblock terpolymers have been investigated, showing square arrays of dots from PI-PS-PFS ^[1] and close-packed arrays of rings from and PS-PFS-P2VP ^[2] . Square patterns are of particular interest for structures such as arrays of vias. In the PI-PS-PFS triblock terpolymer, the minority blocks (PI and PFS) form cylinders alternating with square symmetry. Oxygen etching removes the PI and PS, leaving oxidized PFS arrays with a period of 44 nm. On a smooth substrate, the correlation length of the square pattern is increased dramatically to several microns by the use of brush layers and specific solvent annealing conditions. The interaction between the square pattern and nanoscale topographical trenches and posts can be controlled by substrate functionalization, templating the structure.

For the PS-PFS-P2VP, the bulk structure consists of P2VP core-PFS shell cylinders in a PS matrix. Removing the P2VP and PS leaves ring-shaped PFS features. The cylinders can be oriented perpendicular to the top surface of the film by controlling the film thickness and annealing conditions. However, the cylinders typically lie in plane at the substrate-film interface, and the ring pattern therefore cannot be transferred directly by etching an underlying film. Pattern transfer was achieved instead by imprinting using the PFS rings as an imprint stamp. These examples show some of the diversity of geometries that can be achieved using triblock terpolymers. A wide range of geometries remains to be investigated, including lines and spaces of unequal widths, lines with width modulation, or tiling patterns, and self-assembly of these patterns may facilitate the fabrication of new types of devices.



Figure 1: (a) Ring shape structures of PFS domains from PS-PFS-P2VP triblock terpolymer films. (b) A square-array of PFS posts from a PI-PS-PFS triblock terpolymer blended with 18% homopolymer PS. This blended film deposited on substrates patterned with shallow trenches, after a chloroform solvent anneal. The orientation of the array depends on the functionalization of the substrate.



Figure 2: A highly ordered square-array of PFS posts from a PI-PS-PFS triblock terpolymer blended with 18% homopolymer PS on neutral substrates. (a) The PFS square arrays were highly aligned along the HSQ sidewalls coated with PFS brush, which prefer selective wetting with PFS domain, while the PEO brush-coated Si substrate formed neutral condition for three domains and formed highly ordered square arrays of PFS posts. (b) The locations of the PFS microdomains also can be templated using HSQ dot posts. The posts are attractive to the PFS block, while the flat substrate is coated with a PEO brush.




1. V. P. Chuang, J. Gwyther, R. A. Mickiewicz, I. Manners, and C. A. Ross,  “Templated self-assembly of square symmetry arrays from an ABC triblock terpolymer,” Nano Lett., vol. 9, pp. 4364-9, 2009. [↩]
2. V. P. Chuang, C. A. Ross, J. Gwyther, and I. Manners, “Self-assembled nanoscale ring arrays from a polystyrene-b-polyferrocenylsilane-b-poly(2-vinylpyridine) triblock terpolymer thin film,” Adv.Mater., vol,  21, pp. 3789-93, 2009. [↩]

Device Fabrication Using Block Copolymer Lithography

• Authors: J. G. Son, K. W. Gotrik, C. A. Ross

Block copolymers can be used to make a variety of functional electronic or magnetic devices. We have developed pattern transfer processes to produce metal, silicon, oxide, or polymer patterns using a mask made from a PS-PDMS (polystyrene-polydimethylsiloxane) block copolymer.

Metal films are patterned using a “damacene” process in which a metal film is deposited over a pattern made from the oxidized PDMS microdomains. Reactive ion etching planarizes and slowly removes the metal until the oxidized PDMS is exposed. The oxidized PDMS etches much faster than the metal, and termination of the etch process at this point leaves a metal pattern that is the reverse contrast of the block copolymer pattern. This process has been used to make magnetic nanostructures such as dot, line, and antidot arrays, as well as narrow metallization lines.

We have also deposited films of block copolymers on top of materials such as conductive polymers or graphene, and then used the block copolymer pattern as an etch mask to form structures such as arrays of conducting polymer nanowires or graphene nanoribbons. Other pattern transfer includes the formation of high aspect silicon nanowires by catalytic etching of a Si wafer using a BCP-patterned gold catalyst and the formation of inverted pyramid arrays by anisotropic etching of silicon through a block copolymer mask.



Figure 1: Metal wires made using block copolymer pattern and damascene process and PEDOT polymer wires for the gas sensors using block copolymer patterns.



Figure 2: Silicon wires made by catalytic etching process and array of inverted pyramids in Si with a 40-nm period made using a block copolymer mask.




1. Y. S. Jung and C. A. Ross, “Fabrication of diverse metallic nanowire arrays based on block copolymer self-assembly,”Nano Lett.  vol. 10, pp. 3722-3726, 2010.
2. S.-W. Chang, V. P. Chuang, S. T. Boles, C. A. Ross, and C. V. Thompson, “Densely-packed arrays of ultrahigh-aspect-ratio silicon nanowire fabricated using block copolymer lithography and metal-assisted etching,” Adv. Functional Materials vol. 19, pp. 2495-2500, 2009.
3. Y. Sik Jung and C. A. Ross, “Well-ordered thin film nanopore arrays formed using a block copolymer template,” Small, vol. 5, pp. 1654-1659, 2009.
4. Y. S. Jung, W. Jung, H. L. Tuller, and C. A. Ross, “Nanowire conductive polymer gas sensor patterned using self-assembled block copolymer lithography,” Nano Lett., vol. 8, pp. 3776-3780, 2008.

Templated Dewetting of Metal Films

• Authors: R. Esterina, Y. J. Oh, K. W. Gotrik, C. A. Ross

Magnetic metal nanoparticle arrays have attracted considerable interest for applications in patterned magnetic recording media as well as catalyst arrays for growing carbon or semiconductor nanotubes. These applications ideally require large-area, low-cost nanoparticle arrays with controlled long-range order. Well-ordered nanoparticle arrays are typically made by “top-down” lithographic planar processing methods, but the formation of sub-50-nm features becomes increasingly difficult as the dimensions of the particles are reduced. As an alternative, chemical synthesis can be used to obtain magnetic nanoparticles with dimensions of a few nm and above, but the long-range order of arrays formed from such particles is limited.

Nanoparticle arrays may also be made by the spontaneous dewetting (or agglomeration) of a poorly wetting metal film on a substrate such as silica upon annealing at temperatures of several hundred ˚C. These arrays consist of islands with a range of sizes and spacings, and the islands lack long-range order. Giermann et al. reported the fabrication of regular Au nanoparticle arrays by dewetting a thin metal film on a topographically patterned Si substrate at elevated temperatures ^[1] . The templating effect was attributed to the local curvature of the film over the topography, which leads to grooving of the film at the edges of the pits, and its eventual separation into nanoparticles located within the pits. We applied this technique to thin Co films on oxidized silicon substrates that were topographically prepatterned with an array of 200-nm period pits. The Co nanoparticle size and uniformity are related to the initial film thickness, annealing temperature, and template geometry. One particle per 200-nm period pit is formed from a 15-nm film annealed at 850˚C; on a smooth substrate, the same annealing process forms particles with average interparticle distance of 200 nm. Laser annealing enables templated dewetting of 5-nm thick films to form one particle per pit. Although the as-deposited films exhibit a mixture of hcp and fcc phases, the ordered cobalt particles are predominantly twinned fcc crystals with weak magnetic anisotropy. Templated dewetting is shown to provide a method for forming arrays of nanoparticles with well-controlled sizes and positions. Recent work includes the use of block copolymers to fabricate inverted pyramid arrays with a 40-nm period and analysis of the dewetting behavior on such substrates. We are also examining dewetting of films patterned with small holes made using a block copolymer process. ^[2]



Figure 1: Dewetted Co film on a smooth substrate, which forms Co islands with a range of sizes and shapes.



Figure 2: Dewetted Co film on an array of inverted pyramids, forming a regular arrangement of fcc Co particles.




1. A. Giermann and C.V. Thompson, “Solid-state dewetting for ordered arrays of crystallographically oriented metal particles,” Appl. Phys. Lett., vol. 86, p. 121903, 2005. [↩]
2. Y.-J. Oh, C. A. Ross, Y. S. Jung, Y. Wang, and C. V. Thompson, “Cobalt nanoparticle arrays made by templated solid-state dewetting,” Small, vol. 5, no. 7, pp. 860–865, 2009. [↩]

Magnetic Ring Devices for Memory and Logic

• Authors: M. D. Mascaro, C. Nam, C. A. Ross, S. Lnu, A. Adeyeye

We are investigating the fabrication and magnetic properties of rings (Figure 1) for magnetic logic and memory devices. The magnetic multilayer rings show giant magnetoresistance, in which the resistance is a function of the relative orientation of the magnetization directions in the magnetic layers. These small structures have potential uses in magnetic-random-access memories (MRAM), magnetic logic devices, and other magneto-electronic applications. Unlike that of conventional MRAM devices, the ring-shaped geometry of these devices allows for a complex response with multiple stable resistance states. This capability can be used for multi-bit memory and for programmable, non-volatile memory.

These devices are programmed using either a magnetic field or a current. We have shown that spin-polarized currents can reverse the devices by domain wall motion. We are currently investigating concatenation of multiple devices through stray field interactions and control of the behavior of individual devices through interlayer magnetostatic pinning interactions and manipulation of 360° domain walls with a spin-polarized current.

We are also investigating the effects of repeated cycling on rhombic ring devices. Stray field interactions between layers give rise to complex reversal paths, which depend on the field history. These complex reversal behaviors can be attributed to the formation of 360° domain walls and mirror domains in the structure. Understanding these interactions will facilitate the development of multilayer domain wall devices.



Figure 1: Magnetic force micrograph showing a pair of ring devices. Domain walls are visible as black and white highlights. Interactions between these domain walls allow one ring to select between two isoenergetic vortex states in the other ring in an otherwise stochastic reversal process.



Figure 2: Simulated hysteresis loop demonstrating multiple 360° domain wall formations in a rhombic multilayer ring with simulation images at selected fields. Complex mirror domain structures form due to the strong stray fields around the domain walls formed during the reversal process.




1. C. Nam, M. D. Mascaro, and C. A. Ross, “Magnetostatic control of vortex chirality in Co thin film rings,” Appl. Phys. Lett. vol. 96, pp. 012505:1-3, 2010.
2. Y. Ren, S.  Jain, A. O. Adeyeye, and C. A. Ross, “Magnetization states in coupled Ni80Fe20 bi-ring nanostructures,” New J. Phys. vol. 12, pp. 093003:1-11, 2010.­­­

360-degree Magnetic Domain Walls

• Authors: Y. Jang, M. Mascaro, C. A. Ross

Many domain wall (DW) devices require the presence of multiple DWs, each of which may need to be independently controlled and moved. As a result, we are investigating the interaction of domain walls in a single ferromagnetic nanowire. In particular, we are focusing on 360^o domain walls (360 DWs) in which the magnetization makes a full in-plane 360° turn in a localized region of the stripe, while the rest of the stripe is magnetized parallel to its edges.

Applied current, utilizing the spin-torque effect, is the most promising method for driving domain wall devices. Our simulations indicate that 360 DWs have a response to a current that is qualitatively different from the behavior of the constituent 180 DWs. The 360 DWs move at a velocity independent of applied magnetic fields and can be destroyed by a burst of applied current. The stability of the domain wall can be controlled by an externally applied field, as shown in Figure 1. These features make the 360 DW a potential candidate as a data token in novel domain wall logic devices. Additionally, we have observed and are currently investigating resonant behavior in the 360 DW, which can be tuned by an applied field, as in Figure 1(b)). Simultaneously, we experimentally demonstrate the controlled generation of 360 DWs in rings and in a nanostructure consisting of a circular pad attached to a curved wire, as shown in Figure 2. The response of a 360 DW to current is being characterized using anisotropic magnetoresistance.




Figure 1: (a) Regimes of behavior of a 360 DW in the presence of a current and an applied field. In the “spontaneous annihilation” regime, the wall is destroyed by the current. An applied field increases or decreases the annihilation current and thus the maximum velocity of the wall. (b) Resonant behavior of a 360° DW pumped by high-frequency current. Modest applied fields from -20 to 50 Oe shift the resonant frequency from 0.8 to 1.7 GHz.



Figure 2: (a) Magnetic force microscopy (MFM) image after saturation at 5000 Oe, showing generation of a 180 DW. (b) MFM image of 360 DW after injection of second 180 DW. The inset is an atomic force microscopy image of Co (6 nm) nanostructure.




1. M. D. Mascaro, C. Nam, and C. A. Ross, “Interactions between 180 and 360 degree domain walls in magnetic multilayer stripes,” Appl. Phys. Letts. Vol. 96, no. 162501, pp. 1-3, 2010.
2. C. Nam, M. D. Mascaro, and C. A. Ross, “Magnetostatic control of vortex chirality in Co thin film rings,” Appl. Phys. Lett., vol. 96, no. 012505, pp. 1-3, 2010.
3. M. D. Mascaro and C.A. Ross, “AC and DC Current-Induced Motion of a 360˚ Domain Wall,” Phys. Rev. B vol. 82, p. 214411, 2010.

Magnetooptical and Magnetic Oxides for Optical Isolators and Magnetoelectronic Devices

• Authors: C. A. Ross, G. F. Dionne, L. Bi, D. H. Kim, P. Jiang, M. C. Onbasli

We maintain a thin-film laboratory that includes a pulsed-laser deposition (PLD) system and an ultra-high vacuum sputter system. The PLD is particularly useful for making complex materials such as oxides because it can preserve the stoichiometry of the target material.

We have been using PLD to deposit a variety of oxide films for magneto-optical devices such as isolators. The ideal material for an isolator combines high Faraday rotation with high optical transparency. Garnets, such as bismuth iron garnet (BIG, Bi₃Fe₅O₁₂), have excellent properties but do not grow well on silicon substrates, making it difficult to integrate these materials. One way to solve this problem is to develop new magneto-optical active materials, which can grow epitaxially on Si by using buffer layers. Through being doped with transitional metal ions, these materials can exhibit strong Faraday rotation as well as low optical loss. We have examined Fe and Co-doped SrTiO₃ thin films (Figure 1) ^{[1] [2]} , which show high magneto-elastic anisotropy ^[3] , strong magneto-optical properties, and lower optical absorption compared with iron oxide. We also demonstrated epitaxial integration of these films on silicon using a CeO₂/YSZ buffer layer. When Sr(Ti_0.6Fe_0.4)O₃ films are doped with Ga ions on the Ti site up to 50 at.%, the optical absorption of the material decreases by more than one order of magnitude at 1550 nm (Figure 2), while it still shows a Faraday rotation of ~400 deg/cm ^[4] . A high material figure of merit of 3~4 dB/cm was achieved in Sr(Ti_0.6Ga_0.4Fe_0.2)O₃. These films could be useful for waveguide isolators and other magnetoelectronic devices in which optical absorption losses are critical.



Figure 1: Faraday rotation at 1550-nm wavelength vs. applied field for Sr(Ti_0.6Fe_0.4)O₃ and Sr(Ti_0.7Co_0.3)O₃ films grown on LaAlO₃ (001) substrates.



Figure 2: Optical transmission spectrum of Sr(Ti_0.6-xGa_xFe_0.4)O₃ films on LSAT substrate. The inset shows the transmission spectrum of a 4-mm wide, 3.7-mm long As₂S₃(400 nm)/Sr(Ti_0.6Ga_0.4Fe_0.2)O₃(100 nm) strip-loaded waveguide at near infrared wavelengths.




1. H.-S. Kim, L. Bi, G. F. Dionne, and C. A. Ross, “Magnetic and magneto-optical properties of Fe doped SrTiO₃ films,” Appl. Phys. Lett., vol. 93, p. 092506, 2008. [↩]
2. L. Bi, H.-S. Kim, G. F. Dionne, and C. A. Ross, “Structure, magnetic properties and magnetoelastic anisotropy in epitaxial Sr(Ti_1-xCo_x)O₃ films,”  New J. Phys., vol. 12, p. 043044, 2010. [↩]
3. D. H. Kim, L. Bi, P. Jiang, G. F. Dionne, and C. A. Ross, “Magnetoelastic effects in transition metal-substituted Sr(Ti _1-x(Fe,Co or Cr)_x)O₃ epitaxial thin films,” submitted for publication. [↩]
4. P. Jiang, L. Bi, D. H. Kim, G. F. Dionne, and C. A. Ross, “Enhancement of the magneto-optical performance of Sr (Ti_0.6-xGa_xFe_0.4)O₃ perovskite films by Ga substitution,” submitted for publication. [↩]

Modeling and Theoretical Design Methods for Self-assembly of Block Copolymers

• Authors: A. F. Hannon, C. A. Ross, A. Alexander-Katz

Block copolymer self-assembly on nanolithographically-defined templates has great potential in fabricating patterned media and devices at the nanometer scale.  Current experimental work requires first that the templates be written lithographically and then that the block copolymer morphology around the template be observed and categorized.  An alternative approach to the problem of finding the correct template to get the desired morphology of block copolymers can be achieved through self-consistent field theory modeling (SCFT) ^[1] .  By reducing the problem of polymer interactions with the substrate to density and field interactions, the theory allows for quick computational exploration of phase diagrams for two-dimensional unit cell templates.  Large cell templates and three-dimensional unit cell templates can be modeled as well but at a cost of larger computation time.  However, these simulations can be used to predict or confirm proposed three-dimensional structures that are hard to observe using conventional planar microscopy methods, as seen in Figure 1 ^[2] .  In the theory, topographical constraints are modeled as hard potential barriers to the polymer, and chemical surface affinity effects are modeled as attractive potential fields, as schematically represented in Figure 2.  The traditional approach with the theory is to impose an initially randomly configured system of copolymers to a predefined template and have the system evolve through pseudo-dynamical relaxation schemes to reach saddle points in the system energy space, thus allowing for both metastable states and the thermodynamic equilibrium state to be observed.  Combining SCFT with surface energy analysis calculations of the observed equilibrium morphologies, one can develop a simple theory of where ordered structure transitions occur based on geometric commensurability conditions. Eventually, the theory should allow for taking desired block copolymer morphologies as the simulation input and determining what the minimum required template is to achieve that morphology.



Figure 1: SCFT results of 3D structures obtained by different topological boundary conditions. (a) Double layer of hexagonal spheres in a hexagonal lattice template and (b) cylinders assembled on a trench template.



Figure 2: Schematic view of field constraints used in simulations. The color-coding corresponds to regions with different field constraints.




1. R. A. Mickiewicz, 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. [↩]
2. J. G. Son, A. F. Hannon, K. W. Gotrik, A. Alexander-Katz, and C. A. Ross, “Hierarchical nanostructures by sequential self-assembly of styrene-dimethylsiloxane block copolymers of different periods,” Adv. Mater. vol. 23, pp. 634-639, 2011. [↩]

Templated Self-assembly of Block Copolymers for Nanolithography

• Authors: K. W. Gotrik, J. G. Son, A. Hannon, Y. S. Jung, J. B. Chang, J. K. W. Yang, K. K. Berggren, C. A. Ross

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.



Figure 1: SEM images of highly organized and asymmetric complex cylindrical BCP patterns with an array of HSQ posts (brighter dots) prepared by electron-beam patterning. (a) zig-zag pattern and (b) meander structure with sharp bends.



Figure 2: Sub-10-nm-wide PDMS pattern and their pattern transfer to sub-10 nm tungsten nanowires.




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. [↩]

Magnetic Domain Wall Logic

• Authors: J. A. Currivan, Y. M. Jang, F. Liu, M. A. Baldo, C. A. Ross

We are interested in using ferromagnetic materials to engineer more energy-efficient transistors and logic gates.  Transistors today are limited by the energy dissipated per switching operation, and this heat dissipation can potentially be greatly reduced by using a collective effect, such as the collective switching of magnetic moments.  In our research we are designing and fabricating a promising instantiation of magnetic logic, using the switching of a magnetic domain wall in a soft ferromagnet.  The state of the logic gate is read out using a magnetic tunnel junction.  This gate is nonvolatile and can have a fanout greater than one; additionally, each device is a universal NAND gate.  Figure 1 shows a cartoon of the device, for a soft ferromagnet with magnetic moments parallel to the plane of the wire.

The logic gate uses current-induced domain wall motion in a soft ferromagnetic wire such as NiFe or CoFeB to write the state of the device.  We ensure that there is only one 180° transverse domain wall by depositing an antiferromagnet such as IrMn on each end of the wire, creating an exchange bias that fixes the net magnetic moment of the ends.  The output current of the device will depend on the tunnel magnetoresistance (TMR) of the tunnel junction, using an insulating tunnel barrier such as MgO.  TMR values from 300% to 600% have been observed at room temperature ^[1] , allowing a possible fanout up to 6.

The device is fabricated using electron-beam lithography and UHV sputter deposition.  We are using micromagnetic simulations to understand the scaling of the device with size, and we are implementing the device in circuit designs.



Figure 1: (a) Soft ferromagnet, (b) Antiferromagnet, (c) Tunnel barrier, and (d) Synthetic antiferromagnet magnetic tunnel junction.



Figure 2: Device fabrication. The soft layer is sputtered NiFe, with the ends exchange biased by IrMn. A U-shape is used to easily set the magnetization of the ends with one step. The tunnel junction will be placed between the center contacts.




1. S. Ikeda, J. Hayakawa, Y. Ashizawa, Y. M. Lee, K. Miura, H. Hasegawa, M. Tsunoda, F. Matsukura, and H. Ohno. “Tunnel magnetoresistance of 604% at 300 K by suppression of Ta diffusion in CoFeB/ MgO/ CoFeB pseudo-spin-valves annealed at high temperature.” Applied Physics Letters, vol. 93, p. 082508, 2008. [↩]

Integrated Optical Isolators

• Authors: C. A. Ross, G. F. Dionne, L. Bi, D. H. Kim, P. Jiang (in collaboration with L.C. Kimerling)

We have experimentally demonstrated a monolithically integrated ultra-compact optical isolator on silicon. This device uses a different design strategy from any other previously reported optical isolators, which allows a significant device footprint reduction from the centimeter or millimeter scale down to the 10s-of-micrometers level. As a first experimentally demonstrated monolithic nonreciprocal optical component on silicon, this device can also be developed into a variety of integrated nonreciprocal photonic devices including optical circulators, modulators, or switches, which opens a new dimension of functionality for silicon photonics.

The optical isolator device is based on an optical resonator structure, as shown in Figure 1. A silicon racetrack resonator was first fabricated on silicon on insulator (SOI) substrate with part of the SiO₂ top cladding layer etched away to expose the underlying silicon waveguide. Then a magneto-optical oxide thin film layer was deposited on this patterned resonator structure. When an in-plane magnetic field is applied perpendicular to the light propagation direction in the exposed resonator region shown in the figure, a nonreciprocal magneto-optical phase shift is observed for TM polarized light in the resonator. The effective indices between forward and backward propagating TM mode lights are different, and the resonance wavelengths are non-degenerate. This effect is demonstrated in a sketch of the transmission spectra of forward and backward propagating TM polarized light near resonant wavelengths, shown in Figure 2 (a). The different resonant wavelengths allow different transparencies of the device at a wavelength range (shaded region) between forward (red curve) and backward (green curve) propagating light, therefore allowing the device to operate as an optical isolator. The maximum isolation ratio and the corresponding insertion loss are described by the perpendicular broken lines in Figure 2 (a). Experimental demonstration of the device performance was carried out by using an Y₃Fe₅O₁₂ buffered Ce₁Y₂Fe₅O₁₂ polycrystalline thin film as the magneto-optical oxide layer, fabricated by pulsed laser deposition ^[1] . The experimental and theoretical TM mode transmission spectra with oppositely applied magnetic fields near the resonance wavelength of 1549.5 nm are shown in Figure 2 (b). This device achieved an isolation ratio of 19.5±2.9 dB, an insertion loss of 18.8±1.1 dB, and a 10 dB isolation bandwidth of 1.6 GHz.



Figure 1: Schematic plot of an optical resonator based optical isolator on SOI.



Figure 2: (a) Sketch of the transmission spectrum of a nonreciprocal resonator shown in Figure 1. (b) The experimental and theoretical simulated TM mode transmission spectrum near resonance.




1. L. Bi, J. Hu, G. F. Dionne, L. C. Kimerling, and C. A. Ross, “Monolithic integration of chalcogenide glass/iron garnet waveguides and resonators for on-chip nonreciprocal photonic devices,” Proc. SPIE 2011, to be published. [↩]

Self-assembled Hetero-structured Oxides

• Authors: N. Aimon, H. K. Choi, D. H. Kim, L. Bi, C. A. Ross

We are studying BiFeO₃/CoFe₂O₄ oxide composite thin films, which have the remarkable characteristic of self-assembling into vertical nanostructures during their deposition on a single crystal substrate, typically SrTiO₃. Through the characterization of the structural (Figure 1), magnetic, and electric properties of samples deposited under varying conditions, we are studying the processes happening during deposition that govern the self-assembly of such thin films. A better understanding of the magnetoelectric coupling and strain relationships, due to the coherent interfaces between the three phases, will allow us to fine-tune their magnetic, ferroelectric and multiferroic properties for potential applications.

We demonstrated that these hetero-structured thin films can successfully be grown using a novel alternated-target deposition method, where two ceramic targets of single component BiFeO₃ and CoFe₂O₄ are successively moved under the laser beam for ablation for a given number of pulses (Figure 2). The ratio of pulses on the two targets allows good control of the composition of the film, whereas the total number of cycles controls the thickness of the film. In contrast to the commonly used deposition by ablation from a single target, this alternated method allows fine-tuning of the average composition of the film at different stages of the growth, without the need to fabricate a new target for each desired ratio of the two phases.



Figure 1: SEM of the surface of the film. The brighter rectangular structures are CoFe₂O₄ pillars; the darker background is the BiFeO₃ matrix. (inset: XRD pattern close to the substrate peak showing epitaxial relationship between the three components).



Figure 2: Schematic of the deposition process, in which two single component targets are alternatively ablated under the laser to give the desired composition.




Templated Self-Assembly Using Physical Templates with Majority-block Brushes

• Authors: A. Tavakkoli K. G., K. W. Gotrik, A. F. Hannon, C. A. Ross, K. K. Berggren



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.