Vertically aligned Ni magnetic nanowires fabricated by diblock-copolymer-directed Al thin film anodization

Transcription

1 Vertically aligned Ni magnetic nanowires fabricated by diblock-copolymer-directed Al thin film anodization Researcher: Kunbae (Kevin) Noh, Graduate Student, MAE Dept. and CMRR Collaborators: Leon Chen, Research Scientist, MAE Dept. and CMRR Edward Choi, Postdoctoral Researcher, MAE Dept. and CMRR Stanley Kim, Graduate Student, MAE Dept. and CMRR Advisor: Sungho Jin, Professor, MAE Dept. and CMRR Center for Magnetic Recording Research & MAE Department University of California, San Diego 1

2 Outline Introduction - Nano patterning by diblock copolymer - Nano patterning by anodized aluminum oxide Experimental Results and Analysis - Schematic process illustration - Diblock copolymer formation - Pattern transfer by reactive ion etch - Al guided anodization using patterned diblock copolymer - Ni nanowires (NWs) electrodeposition into vertical pores Future Plans - Strategies to improve pattern density and periodicity - Fabrication of composite soft/hard magnetic nanowires Summary 2

3 Introduction What are block copolymers? Comprise two or more homopolymer subunits linked by covalent bonds A B ex) PS-b-PMMA (Polystrylene-block-poly(methyl methacrylate)) Diblock copolymer Due to its self ordering nature at certain conditions, it can be used as a building block for nanofabrication A B A(C) Advantages Simple, fast and parallel fabrication process Triblock copolymer Higher pattern density compared to other self ordered templates such as AAO Tailorable pore size and pitch distance Drawbacks Small thickness of BCP template Low selectivity as an RIE etch mask Lack of long range ordering if not guided Bockstaller et al., Adv. Mat 17, 1331 (2005) 3

4 Among these Polystyrene-b-poly(4-vinylpyridine) micelle structure [expressed as PS-b-P4VP] in solvent such as toluene and ethanol is one of the frequently studied materials. Hexagonally close packed arrangement as being cast on Si substrate Further reorganized when being annealed at specific solvent vapor atmosphere toluene spin coat PS P4VP PS in corona; P4VP in core Self assemble of monolayer 4

5 Anodized Aluminum Oxide Approach Versatile Platform Because of: -. Self-ordered nature -. Economic method -. Simple process -. High aspect ratios -. Tailored pore size

6 Aluminum Anodizing Process Anodization set-up Self-ordered - + guided Pt Electrolyte Al Masuda and Fukuda, Science 268, 1466 (1995) Masuda et al. Appl. Phys. Lett. 71, 2770 (1997) 6

7 Pattern Creation by e-beam Lithography Guided AAO Processing for Periodic, Square Patterns Self-ordered region 25nm E-beam guided + AAO nanopatterned region obtained at UCSD 7

8 Experimental Details & Current Status 8

9 Experimental details Schematic fabrication process for vertically aligned Ni nanowires P4VP PS Au Al Si [Spin coating] [PS removal by O 2 plasma] [Al anodization] Al 2 O 3 Al Ni [Solvent annealing] [Al etching by RIE] [Ni electrodeposition] [Surface reconstruction] [P4VP residual removal by RIE] 9

10 RMS value (nm) Aluminum Film Deposition Flatness of Al film is an important factor for high-quality AAO formation, especially with thin film AAO applications. Roughness control of sputter deposited film on Si substrate evaluated at various T s temperature (77K to RT) o C deposited o C o C -115 o C deposited 25degC 500nm Al film thickness (um) -90degC 500nm 10

11 PS-b-P4VP porous film on Al thin film/si substrate As-spun PS-b-P4VP Low mag. of solvent annealed PS-b-P4VP Solvent annealed PS-b-P4VP Interpore distance, D int = 48.3nm ± 2.7nm 11

12 Pattern Transfer by RIE RIE I: Residual P4VP etching located bottom of the pores RIE II: Selective Al etching exposed to the surface Dark spots inside bright rims reveal bare Al indents created by RIE process Bright spots reveal Al underneath porous PS-b-P4VP film SEM Image obtained after certain amount of time under SEM beam illumination, revealing RIEinduced indented Al surface after DBCP polymer is degraded. 12

13 Guided Al anodization under the optimized condition -- More periodic vertical AAO pores formed Top view Bird eye s view Interpore distance, D int = 48.3nm ± 2.7nm For pattern transfer, applied voltage, V a = D int [V] / 2.5 [V/nm] ~ 19.4V 13

14 Ni NWs (~30 nm dia) electrodeposition into vertical AAO nanoholes Ni NWs liberated from AAO by etch removal of AAO Top view of Ni NWs into AAO 14

15 Future Plans Guided assembly of DBCP Pattern doubling by tuning anodizing process for higher density application D int = D int / 3 ~ 27.9 nm, therefore V a = D int /2.5 ~ 11.2 V Nanoimprint mould fabrication by guided assembly of DBCP Pattern doubling by using NIL master mould for BPM application 15

16 Pattern Doubling Triangular pre-patterns + guided AAO processing. Pattern doubling has been created. 50 nm 200 nm Pattern doubling 16

17 NIL-guided AAO process Master mould having significantly reduced dimension (e.g. ~20nm diameter and ~40nm pitch) can be utilized as a pre-patterning medium for directed AAO assembly 2µm Noh K, Choi C, Kim JY, Oh Y, Brammer KS, Loya MC, and Jin S, J. Vac. Sci. Technol. B. 28(6), C6M88, 2010 Long-range ordered aluminum oxide nanotubes by nanoimprint-assisted aluminum film surface engineering 17

18 Summary Spontaneous self-assembly of PS-b-P4VP diblock copolymer has been explored as an RIE mask for nanofabrication process --- to form more periodic and smaller diameter AAO nucleation. DBCP-guided Al thin film anodization has been demonstrated for a high aspect ratio (>~ 30) nanoporous template, which could be useful for higher-density nanowire array structures. Example magnetic Ni NWs were electrochemically deposited into AAO template to demonstrate its versatility as a template for 1-D NWs. Highly dense, long-range-ordered AAO templates will be obtained with the pattern doubling concept, which is desirable for magnetic memory media application. 18

19 Acknowledgements Financial Support by CMRR at UCSD, Iwama fund, NSF Various other UCSD and outside collaborators. 19