Fabrication of Highly Ordered Al 2 O 3 Nanohole Arrays As a Nanostructured Template Jie Gong, Bill Butler and Giovanni Zangari Materials Science Program University of Alabama at Tuscaloosa This work was funded by DOD grant no. DAAH04-96-1-0316 2002 MINT Fall Review, November, 2001
Introduction Porous Al oxide: a versatile nanostructured template Synthesis of ordered Al 2 O 3 porous structures Periodic nanoholes produced by ion milling of barrier layer of anodic porous alumina Sputtering of Cu on the nanohole side to produce a conductive support for further nanofabrication by electrodeposition Perspectives and opportunities in nanotechnology: magnetism electronics hybrid systems
Anodization of Aluminum Electrochemical oxidation: n- step anodization Morphology controlled by selection of the electrolyte film forming conditions Acid or alkaline electrolytes: porous films Electrolytes used: (COOH) 2 Pore size: 5 nm 1 mm, increases with ph and T Pore length up to 100 µm A Aluminum POWER SUPPLY + - Graphite 2Al + (3O 2- ) ox = Al 2 O 3 + 6e - H (O 2- ) aq = (O 2-2 O = 2H + + (O 2- ) aq ) ox Al = Al 3+ + 3e - C
Pore Ordering Process (1) Electropolishing: to flatten the Al surface Anodization Stripping Al 2 O 3 : in : in (COOH) 2 H 2 CrO 4 + H 3 PO 4 RMS roughness: 0.36 nm Imprinted pattern on Al after stripping
Pore Ordering Process (2) Anodization (2 nd ): in (COOH) 2 for 12 hours Stripping Al 2 O 3 : in H 2 CrO 4 + H 3 PO 4, 30 min Roughness still large Imprinted pattern on Al after stripping
Pore Ordering Process (3) Anodization (3 nd ): in (COOH) 2 for 5.5 hours Highly ordered porous alumina
Long-range Order (SEM observation) Planar view crosssection Ordering process slowed down at defects, impurities
Pore Ordering Process (4) Separation of alumite nanostructure and Al substrate: in HgCl 2 solution Periodic barrier U-shape caps can be discerned
Nanoholes on barrier side Ion milling on barrier side can open nano-holes Controllable hole dimension can provide nano-sized constrictions Upper: (left) ion milling 15 min; (right) 10 min Lower: 5 min
Conductive support on barrier side Fully blocked nanoholes 200 nm copper was sputtered on the nanohole barrier side Cu provides a full covered conductive support for further nanofabrication Upper: (left) ion milling 15 min; (right) 10 min Lower: 5 min
Attempts For Copper Electrodeposition Current Density (ma/cm 2 ) 0-5 -10-15 -20-25 0 300 600 900 1200 1500 1800 time (s) Current transient: Cu in holes Left: Current transient of DC potentiostatic electrodeposition of Cu into nanoholes with a conductive Cu layer. Right: cross-sectional SEM image of nanowires formed in nanoholes by electrodepsotion
Summary of Current Status Regular and uniform templates can be obtained Controllable hole dimension (15 ~ 40 nm) to form nanocontacts at the barrier hole position Versatile template for nanofabrication: spatially confined spin valves and magnetic constrictions 20-200 nm 20-200 nm Al 2 O 3 membrane Cu Al 2 O 3 membrane conductive support Co constrictions
Future Work Growth of isolated Cu nanowires by electrodeposition Growth of isolated Cu/FeCoNiCu/Cu multilayered nanostructure by electrodeposition Growth of Ni nanocontact Opportunities in (just a few examples): atom probe studies of interfaces nanoscale magnetism coupling molecular/metallic systems at the nanoscale