We report large area patterning of sputter-deposited FeN thin films by a high-voltage parallel writing technique that was recently developed to modify ZrN surfaces. Systematically patterned 15-100-nm-thick FeN films consisting of features with well-defined sizes and shapes are obtained by applying high dc voltages between a stamp and the samples. During the process the oxide dissolves, exposing the substrate beneath. This controlled breakdown eliminates the need for any postexposure etching. The single-step imprinting method presented here provides an emerging route to fabricate isolated FeN geometrical structures on silicon substrates for magnetic applications. (c) 2006 American Vacuum Society

Dagata, J. A.

Ehrman, J. D.

Evans, Edward A.

Farkas, N.

Ramsier, R. D.

English

The University of Akron

The University of AkronIdeaExchange@UAkronChemical and Biomolecular Engineering FacultyResearchChemical and Biomolecular EngineeringDepartment7-1-2006High-voltage Parallel Writing on Iron Nitride ThinFilmsN. FarkasJ. D. EhrmanEdward A. EvansUniversity of Akron Main Campus, evanse@uakron.eduR. D. RamsierJ. A. DagataPlease take a moment to share how this work helps you through this survey. Your feedback will beimportant as we plan further development of our repository.Follow this and additional works at: http://ideaexchange.uakron.edu/chemengin_ideasPart of the Chemistry CommonsThis Article is brought to you for free and open access by Chemical and Biomolecular Engineering Department atIdeaExchange@UAkron, the institutional repository of The University of Akron in Akron, Ohio, USA. It has beenaccepted for inclusion in Chemical and Biomolecular Engineering Faculty Research by an authorized administratorof IdeaExchange@UAkron. For more information, please contact mjon@uakron.edu, uapress@uakron.edu.Recommended CitationFarkas, N.; Ehrman, J. D.; Evans, Edward A.; Ramsier, R. D.; and Dagata, J. A., "High-voltage Parallel Writing onIron Nitride Thin Films" (2006). Chemical and Biomolecular Engineering Faculty Research. 14.http://ideaexchange.uakron.edu/chemengin_ideas/14High-voltage parallel writing on iron nitride thin filmsN. FarkasDepartment of Physics, The University of Akron, Akron, Ohio 44325 and Department of Chemistry,The University of Akron, Akron, Ohio 44325J. D. EhrmanDepartment of Physics, The University of Akron, Akron, Ohio 44325E. A. EvansDepartment of Chemical Engineering, The University of Akron, Akron, Ohio 44325R. D. RamsieraDepartment of Physics, The University of Akron, Akron, Ohio 44325 and Department of Chemistry,The University of Akron, Akron, Ohio 44325J. A. DagataPrecision Engineering Division, National Institute of Standards and Technology, Gaithersburg,Maryland 20899-8212Received 4 October 2005; accepted 21 November 2005; published 22 June 2006We report large area patterning of sputter-deposited FeN thin films by a high-voltage parallel writingtechnique that was recently developed to modify ZrN surfaces. Systematically patterned 15–100-nm-thick FeN films consisting of features with well-defined sizes and shapes are obtained byapplying high dc voltages between a stamp and the samples. During the process the oxide dissolves,exposing the substrate beneath. This controlled breakdown eliminates the need for any postexposureetching. The single-step imprinting method presented here provides an emerging route to fabricateisolated FeN geometrical structures on silicon substrates for magnetic applications. © 2006American Vacuum Society. DOI: 10.1116/1.2167074I. INTRODUCTIONThere is an increasing demand for structured magneticmaterials for magnetoelectronics,1,2 magnetic data storageand recording,3–5 and biomagnetic6,7 applications where theability to control the areal density, separation, and size of theindividual features is essential. Scanning probe microscopySPM-based lithographic techniques have been successfullyused to fabricate magnetic nanostructures and devices.8–11 Toobtain arrays of magnetic structures, the existing methodsoften require multiple processing steps and special instru-mentation, making the fabrication time consuming andcostly. The ultimate motivation for the work presented hereis to develop an alternative benchtop technique to fabricatelarge arrays of discrete magnetic structures with differentgeometrical shapes for magnetic resonance imaging MRIapplications. Our recent study,12 which demonstrates that en-hanced single-tip oxidation on ZrN thin films can be success-fully extended to parallel writing,13,14 provides a startingpoint to create arrays of well-defined features on FeN thinfilms.II. EXPERIMENTAL METHODSIron thin films were deposited on boron-doped p-typeSi111 and Si100 wafers. Each substrate was ultrasonicallycleaned in acetone and isopropyl alcohol IPA for 10 minsequentially. Radio-frequency magnetron sputtering was per-formed at a power of 125 W with a constant argon flow rateof 5 SCCM standard cubic centimeter per minute. Nitrogenwith a flow rate of 5 SCCM was introduced into the sputter-ing plasma to prepare FeN thin films. The substrate holder,positioned 8 cm above the sputtering source, was groundedand there was no temperature control during deposition. Thepurity of the Fe target was 99.98% SCI Engineered Materi-als. Prior to deposition, the vacuum system was pumpeddown to 610−4 Pa using a turbomolecular pump, and theresulting film thickness was measured with a quartz crystalmicrobalance.SPM images were taken under ambient conditions at theNational Institute for Standards and Technology NIST witha TopoMetrix Accurex II operating in contact mode withuncoated silicon cantilevers from Silicon-MDT Ltd. Mag-netic force microscope MFM imaging was performed usinga Digital Instrument NanoScope IV operating in lift modewith Co/Cr-coated silicon tips from Veeco. The composi-tions of the surfaces of the films were obtained by x-rayphotoelectron spectroscopy XPS. The XPS analyses wereperformed in fixed analyzer transmission mode under high-vacuum conditions at pressures of 610−6 Pa. We used aKratos ES 300 electron spectrometer with an aluminum an-ode operated at 12 kV and 10 mA.III. RESULTS AND DISCUSSIONTo extend the high-voltage parallel writing technique toFe/FeN, silicon master stamps fabricated by standard photo-lithography at NIST Ref. 15 are used. The 14 cm2aAuthor to whom correspondence should be addressed; electronic mail:rex@uakron.edu1340 1340J. Vac. Sci. Technol. A 24„4…, Jul/Aug 2006 0734-2101/2006/24„4…/1340/4/$23.00 ©2006 American Vacuum Societystamps consist of sparse and dense arrays of 730 nm highfeatures that appear both in positive and negative reliefs, i.e.,as posts and wells. Shapes having three different lengths andwidths include rectangular features with straight, inward/outward triangular, and inward/outward hemispherical endcaps as shown in Fig. 1.A schematic of the high-voltage parallel writing proce-dure is depicted in Fig. 2A. High dc voltages of 200–400 Vare applied between the stamp and the substrate for 30 s.Isopropyl alcohol is used as a compliant medium instead ofwater,16 which causes damage to the films during patterntransfer. Many materials can be oxidized under these condi-tions, but the resulting patterns will differ based on the gov-erning oxidation mechanism. Zr oxidation is driven by oxy-gen migration,17 whereas Fe oxidizes by metal iontransport.18 In our previous study we demonstrated that ro-bust 70-nm high oxide features are generated on ZrN thinfilms.12 An example of SPM images of raised oxide featuresformed on ZrN under the extrusion of the stamp is shown inFig. 2B. In the case of FeN presented in Fig. 2C, theoxide dissolves during the pattern transfer process, revealingthe silicon substrate under the raised portion of the stamp.The formation of holes and dielectric breakdown underextremely high electric fields using single SPM tips has beenobserved for several materials systems, such as Cr, Al, Fe,and Ti.19–23 The majority of these metals oxidize by cationdiffusion through the growing oxide layer. Dielectric break-down is often associated with the uncontrolled nature of thesingle-tip oxidation using very high voltages. Here we dem-onstrate that high-voltage parallel writing on FeN utilizes thebreakdown of the oxide in a controlled and useful way as iteliminates any postexposure etching step.To verify that the iron oxide formed under the stamp dis-solves entirely down to the substrate, FeN films are sputteredon Si100. As the imprinted films are placed in a 60 °Csolution of 83 ml tetra-methyl ammonium hydroxideTMAH and 17 ml IPA, the exposed silicon is etched selec-tively because the FeN pattern serves as an etching mask. Inaddition, cross-sectional analysis, an example of which isshown in Fig. 2C for a 15-nm-thick film, confirms that theheights of the resulting features are equal to the originalthicknesses of the FeN films.To investigate the effect of nitrogen content and thicknessof the films, the high-voltage parallel writing process wasapplied to several Fe and FeN films in a thickness range of15–200 nm. Figure 3 shows the surface and bulk XPS surveyFIG. 1. Optical image of a portion of the silicon stamp consisting of densearrays of features with synthetic shapes illustrated in the top row. The heightof the features is 730 nm.FIG. 2. A General representation of the parallel writing technique. BSchematic of pattern formation on ZrN and a corresponding SPM image of70 nm oxide structures bright. C Schematic of pattern formation on a15-nm-thick FeN film using the same stamp. The oxide dissolves during theprocess resulting in FeN features appearing bright in the SPM image sepa-rated by areas of exposed silicon. The film thickness is equal to the featureheight obtained from the cross sections.FIG. 3. XPS survey scans of 80-nm-thick iron films sputtered with 0 and 5SCCM flow rates of nitrogen. Argon sputtering is performed to obtain spec-tra of the bulk. Optical images of the corresponding patterns formed byapplying 300 V for 30 s are shown on the right.1341 Farkas et al.: High-voltage parallel writing on iron nitride thin films 1341JVST A - Vacuum, Surfaces, and Filmsscans of 80 nm films sputtered with 0 and 5 SCCM nitrogenflow rates along with the corresponding patterns formed ontheir surfaces. The XPS spectra verify the existence of nitro-gen throughout the FeN film grown with a 5 SCCM nitrogenflow rate. In addition to surface carbon that almost disap-pears as the samples are argon sputtered, the films containconsiderable amounts of oxygen. The presence of oxygenwithin the films is consistent with the need to replace waterwith alcohol as a coupling medium between the stamp andsubstrate. This is because the presence of oxygen in the filmsenhances the oxidation, requiring less oxygen from an exter-nal source.SPM oxidation kinetic studies report enhanced oxidationfor silicon and group-IV metal nitrides.24–28 Auger mi-croanalyses have been used to verify the nitrogen to oxygenreplacement for ZrN which is partially responsible for theenhanced growth as well.12 Most likely nitrogen reacts withhydrogen and the resulting ammoniac species leave the reac-tion zone suppressing the space-charge buildup. Similarly,oxidation becomes enhanced and more controllable on ni-trided iron films. The optical images on the right of Fig. 3show partial oxidation and considerable damage to the Fefilm, whereas a completely exposed silicon substrate withwell-defined features can be observed on the FeN film.Thickness dependence studies performed on FeN thin filmsreveal that the resulting iron oxides dissolve, leaving the un-derlying silicon uncovered when the films are up to 100 nmthick. In the case of thicker films the oxidation below thesurface does not reach the FeN/Si interface.Figure 4 displays more optical and SPM images alongwith cross sections of arrays of rectangular features withstraight and outward triangular end caps. This figure empha-sizes the well-controlled size and shape of the objects ob-tained on FeN and that the high-voltage parallel writing tech-nique is inherently simple to extend to different materialsystems. The lateral dimensions of the features are deter-mined by the stamp, but by changing the thickness of thefilms, the height and therefore the volume of the objects canbe precisely controlled.To be used for potential MRI applications, the patternedFeN films have to be magnetic in order to locally alter themagnetic field that will result in signal attenuation by short-ening the T2 relaxation time of the surrounding hydrogennuclei. Figures 5A and 5B show simultaneous SPM andMFM images of a patterned 20-nm-thick FeN film, respec-tively. This film was sputter deposited. The contrast in theMFM image is due to the attractive or repulsive interactionbetween the magnetized tip and the different magnetic do-mains. The exposed silicon substrate appears dark in the to-pographic and bright in the MFM images. The magneticstructure of the FeN features is composed of small randomlyoriented ferromagnetic grains. Figure 5C presents a SPMimage of a pattern generated on a 20-nm-thick iron film de-posited by evaporation. In good agreement with the findingson sputtered iron films, high-voltage parallel writing onevaporated iron films is less controllable and reproduciblethan that on FeN. The matching MFM image of Fig. 5Ddemonstrates that the magnetic domain structures of thefilms depend on the deposition conditions. Consistent withMFM studies on ultrathin iron films,29 the evaporated ironfilms exhibit in-plane magnetization indicated by the cross-tie domain walls.IV. IMPLICATIONSIron oxide particles with biocompatible coatings are ex-tensively used as contrast agents in MRI.30–33 The iron con-tent of the particles affects the susceptibility-induced de-crease in the T2-weighted MRI signal. Not only the size butthe shape of the objects also influences the apparent imagedistortion. Since the size and iron content of particles showlarge variations, quantitative analysis of the detection sensi-tivity and resolution in MRI is rather difficult. These prob-lems could be addressed by imaging arrays of iron nitridefeatures with systematically designed shapes and separation.Controlling the volume of the objects would provide a meansof studying the relationship between the MRI signal decreaseand the iron content.V. SUMMARY STATEMENTSWe have demonstrated the versatility of the high-voltageparallel writing technique, developed to modify group-IVmetal surfaces, by extending it to magnetic thin films. Sys-tematically patterned FeN thin films consisting of differentgeometrical features with heights up to 100 nm are obtained.Using extremely high electric field conditions, the oxide dis-FIG. 4. A Optical image of two types of arrays next to each other generatedon an 80-nm-thick FeN film. Matching SPM images and cross sections ofB features with outward triangular and C straight end caps are alsoshown.FIG. 5. Simultaneous A SPM and B MFM images of a patterned 20-nm-thick FeN film deposited by sputtering. Matching C SPM and D MFMimages of a patterned 20-nm-thick iron deposited by evaporation. The ex-posed silicon substrate appears dark in both SPM images.1342 Farkas et al.: High-voltage parallel writing on iron nitride thin films 1342J. Vac. Sci. Technol. A, Vol. 24, No. 4, Jul/Aug 2006solves under the extrusion of the stamp, leaving the siliconsubstrate exposed between FeN structures. Consequently,there is no need for additional etching which makes the high-voltage parallel writing technique on FeN a single-stepmethod to obtain well-separated structures. TMAH etchingexperiments verify that the FeN features are isolated fromeach other. With precisely controlled height and methodi-cally designed lateral size, the FeN arrays are potential can-didates for MRI sensitivity and resolution measurements.ACKNOWLEDGMENTSOne of the authors I.A.D. acknowledges support fromJack Martinez and Steve Knight of the NIST Office of Mi-croelectronics Programs. Another author N.F. acknowl-edges support from NIST-MEI. Another author R.D.R.acknowledges support from NIH-NIBIB Grant No.EB003397-01.1M. Bal, A. Ursache, M. T. Tuominen, J. T. Goldbach, and T. P. Russell,Appl. Phys. Lett. 81, 3479 2002.2W. J. Gallagher, S. S. P. Parkin, Y. Lu, X. P. Bian, A. Marley, K. P. Roche,R. A. Altman, S. A. Rishton, C. Jahnes, T. M. Shaw, and G. Xiao, J. Appl.Phys. 81, 3741 1997.3X. M. Yang, A. Eckert, K. Mountfield, H. Gentile, C. Seiler, S. Brank-ovic, and E. Johns, J. Vac. Sci. Technol. B 21, 3017 2003.4S. M. Weekes and F. Y. Ogrin, J. Appl. Phys. 97, 10J503-1 2005.5B. D. Terris, D. Weller, L. Folks, J. E. E. Baglin, A. J. Kellock, H.Rothuizen, and P. 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High-voltage Parallel Writing on Iron Nitride Thin Films

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High-voltage Parallel Writing on Iron Nitride Thin Films

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