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Research

Nanofabrication

Devices made from thin films patterned on a scale from microns to nanometers are the basis of modern electronics. We use magnetic thin film devices to study magnetism on the mesoscopic scale, and to explore the functionality of new material combinations.

Three steps are involved:
Deposition → Patterning → Characterization

Thin Film Deposition

Our thin film deposition is usually carried out by sputtering in the 'Shamrock' multichamber tool, which has several interconnected chambers devoted to metal or oxide films. The system is based on an industrial tool used to make spin valve stacks for read heads in the 1990s. In addition, e-beam and thermal evaporation (Temescal FC-2000; CRANN) and electrodeposition are also used to make oxide or metallic films, and pulsed-laser deposition (Prof James Lunney’s lab) is most useful for oxides. A separate UHV system is devoted to evaporating organic films and metals.

Shamrock sputtering system. The computer-controlled tool has separate, interconnected chambers for metals (6 DC sputter guns), oxides (3 rf sputter guns) and e-beam evaporation.

Patterning Thin Films into Devices

Patterning involves lithography and etching. We use the the OAI UV mask aligner or the Heidelberg DWL66 laser writer with a 405 nm laser for optical lithography and mask fabrication. A SUPRA scanning electron microscope with Raith software or the Elionix 7700 dedicated e-beam writer are used for electron-beam lithography (all in CRANN). The pattern written in a resist layer is transferred to the thin film stack by Ar-ion etching in a Millatron, otherwise the resist is dissolved in a lift-off process.

Device fabrication by photolithography and etching. The six-step process is illustrated.

Device Characterization

The crystal structures are determined by X-ray diffraction. Surface and magnetic imaging characterization is carried out with by AFM techniques. Magnetic properties are studied by SQUID or vibrating-sample magnetometry in a PPMS. Electical measurements are are usually of dc conductivity or Hall effect in an R:T rig. We also have facilities for rf characterization, up to 20 HHz.

Some thin film device structures. Spin valves for giant magnetoresistance and tunneling magnetoresistance, a double constriction peanut structure and a nanoconstriction produced by e-beam lithography, and a thin film SQUID.


Contact Nivetha Thiyagarajah

thiyagan@tcd.ie

Further Reading

Reduced low frequency noise in electron beam evaporated MgO magnetic tunnel junctions, Diao Z, Feng JF, Kurt H, and Coey J.M.D. Applied Physics Letters, 96, 202506 (2010).

Influence of growth and annealing condiions on low-frequency magnetic 1/f noise inMgO magnetic tunnl junctions, Jiafeng Feng, Zhu Diao, H. Kurt, N, R. Stearrett, A. Singh, E. R. Nowak and J. M. D. Coey. J. Appl. Phys. 112 093913 (2012).

Strategies for fabrication nanogap single-crystal organic transistors, A. Alborgetti, P. Stamenov, G. Fois and J. M. D. Coey, ISRN Nanotechnology 253246 (2012)