Parallel pattern transfer of submicrometer-scale oxide features onto zirconium nitride thin films is reported. The oxidation reaction was verified by Auger microprobe analysis and secondary ion mass spectrometry. Oxide features of similar to70 nm in height can be formed and selectively etched in a dilute aqueous hydrogen fluoride solution. This provides an interesting route to potential new applications for high-melting point, biocompatible surfaces that possess small feature sizes with controlled geometries. (C) 2004 American Institute of Physics

Comer, J. R.

Dagata, J. A.

Evans, Edward A.

Farkas, N.

Ramsier, R. D.

Wight, S.

Zhang, G.

English

The University of Akron

The University of AkronIdeaExchange@UAkronChemical and Biomolecular Engineering FacultyResearchChemical and Biomolecular EngineeringDepartment12-6-2004Parallel Writing on Zirconium Nitride Thin Filmsby Local Oxidation NanolithographyN. FarkasJ. R. ComerG. ZhangEdward A. EvansUniversity of Akron Main Campus, evanse@uakron.eduR. D. RamsierSee next page for additional authorsPlease 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.; Comer, J. R.; Zhang, G.; Evans, Edward A.; Ramsier, R. D.; Wight, S.; and Dagata, J. A., "Parallel Writingon Zirconium Nitride Thin Films by Local Oxidation Nanolithography" (2004). Chemical and BiomolecularEngineering Faculty Research. 7.http://ideaexchange.uakron.edu/chemengin_ideas/7AuthorsN. Farkas, J. R. Comer, G. Zhang, Edward A. Evans, R. D. Ramsier, S. Wight, and J. A. DagataThis article is available at IdeaExchange@UAkron: http://ideaexchange.uakron.edu/chemengin_ideas/7Parallel writing on zirconium nitride thin films by local oxidationnanolithographyN. Farkas, J. R. Comer, G. Zhang, E. A. Evans, and R. D. Ramsiera)Departments of Physics, Chemistry, and Chemical Engineering, The University of Akron, Akron,Ohio 44325S. Wight and J. A. DagataSurface and Microanalysis Science Division and Precision Engineering Division, National Instituteof Standards and Technology, Gaithersburg, Maryland 20899(Received 20 August 2004; accepted 16 October 2004)Parallel pattern transfer of submicrometer-scale oxide features onto zirconium nitride thin films isreported. The oxidation reaction was verified by Auger microprobe analysis and secondary ion massspectrometry. Oxide features of ,70 nm in height can be formed and selectively etched in a diluteaqueous hydrogen fluoride solution. This provides an interesting route to potential new applicationsfor high-melting point, biocompatible surfaces that possess small feature sizes with controlledgeometries. © 2004 American Institute of Physics. [DOI: 10.1063/1.1833569]Nanoimprint technology is being intensively studied asan alternative to traditional optical lithography, especially fornew applications outside the microelectronics field such asoptoelectronics and biomedical templating. There are agrowing number of methods involving pattern transfer by astamp, template, or master using resist layers, direct me-chanical deformation, or electrical transfer methods.1–8Recently, Cavallini et al.7 and Yokoo8 have proposed amethod for parallel writing based on local oxidationnanolithography.9 The concept emerges from kinetic studiesof scanning probe oxidation in which a voltage generates anintense nonuniform electric field between the scanning probemicroscope (SPM) tip and a substrate.10,11 This field creates awater meniscus from the ambient water vapor and causes theelectrochemical generation of oxyanions, resulting in oxidegrowth on a scale determined by the size and shape of themeniscus.12 In their generalization of this view of the single-tip case to the nanoimprinting case, which uses a stamp withmany morphological features, the water meniscus is a com-pliant mediating material. Since the meniscus adjusts tovarying gaps across the stamp, narrow tolerances and goodreproducibility of feature sizes at the nanometer scale can beachieved. This allows patterns to be transferred from thestamp to the substrate simultaneously, or in other words, par-allel writing.These previous demonstrations7,8 used silicon as the sub-strate onto which oxide pattern transfer occurred. This islargely because most SPM oxidation kinetic studies havebeen performed using this material. However, almost all ma-terials can be oxidized under the extremely high fields,,108 V/m, generated by an electrically biased SPM tip.Many metallic, semiconducting, and insulating materialshave been patterned by scanning probe oxidation. In particu-lar, the group IV metals and their nitrides exhibit remarkablyenhanced kinetics, as first reported by Gwo et al.13 and, morerecently, in our work.14 For example, oxide feature heightsfor silicon tend to self-limit at 10 nm or less, whereas forZrN films, heights of several hundred nanometers have beenobtained. The motivation for the present study is to see ifthese enhanced feature heights obtained with a single SPMtip can be extended to the parallel writing case on group IVnitride films using a stamp.Thin films of zirconium nitride were sputter depositedonto silicon substrates by rf-magnetron sputtering with 120W power and a wide range of N2/Ar mixtures at The Uni-versity of Akron. N2 flow rates varied from 0 up to 10 sccm,as described previously.14 Film thicknesses were typically200 nm in the present study. Characterization of the thinfilms included x-ray diffraction, electrical resistivity, x-rayphotoelectron spectroscopy, and SPM oxidation kinetic stud-ies. The films are metallic below about 0.5 sccm flow rates ofnitrogen and become increasingly dielectric above this value.In this letter, we describe results for zirconium nitride filmswith N2 flow rates above 4 sccm.Master stamps were fabricated on silicon by standardphotolithography at NIST15 and coated with an,100-nm-thick layer of PdAu. The 1 cm34 cm stamps con-sist of sparse and dense arrays of isoareal geometrical fea-tures in which the lengths and widths are varied systemati-cally from 5 to 125 µm. Shapes include rectangular featureswith straight, inward/outward triangular, and inward/outwardhemispherical end caps and appear both in positive and nega-tive relief, i.e., as posts and wells. Feature heights for twosets of stamps were 550 and 730 nm, determined by siliconetching and verified by SPM topographic imaging. An opti-cal image of a portion of a stamp consisting of several arraysis shown in Fig. 1(a). These stamps were used to generateoxide patterns on the ZrN films. Pattern transfer was per-formed with dc voltages between 70 and 300 V in the pres-ence of isopropyl alcohol. An optical image of a ZrN filmfollowing pattern transfer is shown in Fig. 1(b).Consistent with oxidation being the chemical process re-sponsible for the observed pattern transfers, etching studiesusing a dilute 2%–5% HF solution show that zirconium ox-ide can be removed with good selectivity relative to zirco-nium nitride. This leads to the following pattern transferscheme: A SPM topograph of a portion of a stamp appears inFig. 2(a). The patterned features in this case are negativerelief, i.e., wells. The resulting oxide pattern formed on thesurface of a zirconium nitride film is shown in the SPMa)Electronic mail: rex@uakron.eduAPPLIED PHYSICS LETTERS VOLUME 85, NUMBER 23 6 DECEMBER 20040003-6951/2004/85(23)/5691/3/$22.00 © 2004 American Institute of Physics5691image in Fig. 2(b). Here the raised oxide features, appearingbright in the image, correspond to the upper surface plane.After etching in a dilute HF solution, the SPM image appearsreversed, as shown in Fig. 2(c). The right-hand side portionof Fig. 2 illustrates each step in the fabrication.SPM topographic images of more of these oxide patternsare shown in Fig. 3. Rectangular features are shown in theleft-hand column, features with inward triangular end capsare shown in the middle column, and those with outwardhemispherical end caps are shown in the right-hand column.The top row of images (a) shows posts, with cross sections ofthese features shown in row (b). Row (c) shows cross sec-tions of corresponding wells formed by the etching proce-dure, which are taken from the images shown in row (d). Thesize and shape of the transferred patterns are well-maintainedduring the etching process. Cross-sectional analysis indicatesthat the height/depth reversal is close to 70 nm. Secondaryion mass spectrometry (SIMS) depth profiling of the oxygenion signal (not shown) confirms that the oxide-patterned re-gions continue well below the surface of the unpatternedfilms. Our conclusion is that the total oxide thickness isroughly twice the oxide height, in good agreement with ourheight-to-depth ratios obtained from single-tip SPM oxida-tion and HF etching studies of ZrN films.An explanation of our use of isopropyl alcohol is re-quired: In their work with silicon substrates, Cavallini et al.7used water as the source of oxidizing species. However, oxy-gen is present in these ZrN films already, which reduces theneed for an external supply of oxidizing species. Neverthe-less, a coupling medium capable of ionic breakdown is stillnecessary. In fact, our initial attempts to use water, whileproducing the desired patterns, caused considerable damageto the entire film. A search for a less-reactive medium led usto isopropyl alcohol, prompted by the recent work of Telloand Garcia.16FIG. 1. Optical micrographs of (a) feature arrays on the reference stamp and(b) oxide patterns formed by the stamp on a zirconium nitride film. Featureheights of the etched silicon stamp are 550 nm.FIG. 2. Scanning probe topographic images of (a) a portion of the stamp, (b)oxide features patterned on a zirconium nitride thin film, and (c) features asthey appear after etching in dilute HF. A schematic of the sequence is de-picted on the right.FIG. 3. Scanning probe topographic images of three types of oxide featurespatterned on a zirconium nitride thin film before (a), (b), and after (c), (d),etching. The height difference from dark to bright is ,70 nm, as seen in thecross sections.5692 Appl. Phys. Lett., Vol. 85, No. 23, 6 December 2004 Farkas et al.Finally, chemical analysis by Auger microprobe of thepatterned features and unpatterned areas of the films con-firms the conversion of ZrNx→ZrOy at stamp-substrate con-tact locations, as shown in Fig. 4. These data are important,since they clearly identify the replacement of metal–nitrogenbonds by metal–oxygen bonds, and oxidation as the primarymechanism responsible for these patterns. Previously wehave argued that this conversion takes place, enhancing theheight of the nanostructures as compared to other materialssystem.14Group IV metal thin films and their oxides and nitrideshave many unique properties including hardness, dielectricconstant, corrosion resistance, high melting temperatures,biocompatibility, and biocatalytic activity. Although coatingtechnology for these films is mature, the difficulty of struc-turing these materials, especially on the nanoscale, has thusfar limited their application. The results reported here enablepromising applications for patterned group IV films for thefabrication of templates for directed cell growth studies.17For example, Galli et al.18 have investigated protein orderingof actin filaments by SPM oxide lines patterned on titaniumfilms, and we have reported that zirconium oxide layers maymitigate bacterial growth.19 The ability to produce oxide pat-terns on ZrN surfaces by local oxidation nanolithographywith a stamp will allow us to fabricate large-area biotem-plates on the order of 100 cm2. In addition, the process re-ported here is quick, simple and robust. These qualitiesshould dramatically accelerate progress in the bioattachmentand biotemplating research areas.In this work, we have demonstrated an emergingnanoimprint technology that is a promising approach to pat-terning group IV thin films. Although important issues suchas ultimate resolution, alignment method, and exposure uni-formity across centimeters remain to be addressed, these arechallenges which are largely generic to most nanoimprintschemes. Furthermore, a detailed understanding of local oxi-dation kinetics has provided considerable guidance in estab-lishing the optimal conditions for parallel writing, as Caval-lini has shown for silicon,7 and as we have shown here forZrN.The authors thank Greg Gillen (NIST) for SIMS analy-sis; J.A.D. acknowledges support from Jack Martinez andSteve Knight of the NIST Office of Microelectronics Pro-grams; N.F. acknowledges support from NIST-MEL; J.R.C.acknowledges support from NSF/NIST-SURF; R.D.R. ac-knowledges support from NIH-NIBIB Grant No. EB003397-01, and from the donors of the American Chemical SocietyPetroleum Research Fund.1S. Y. Chou, P. R. Krauss, and P. J. Renstrom, Appl. Phys. Lett. 67, 3114(1995); Science 272, 85 (1996).2A. Kumar, H. Biebuyck, and G. M. Whitesides, Langmuir 10, 1498(1994).3S. W. Pang, T. Tamamura, M. Nakao, A. Ozawa, and H. Masuda, J. Vac.Sci. Technol. B 16, 1145 (1998).4E. Schaffer, T. Thun-Albrecht, T. P. Russell, and U. Stiener, Nature (Lon-don) 403, 873 (2000).5H. O. Jacobs and G. M. Whitesides, Science 291, 1763 (2001).6S. Hoeppener, R. Maoz, and J. Sagiv, Nano Lett. 3, 761 (2003).7M. Cavallini, P. Mei, F. Biscarini, and R. Garcia, Appl. Phys. Lett. 83,5286 (2003).8A. Yokoo, J. Vac. Sci. Technol. B 21, 2966 (2003).9J. A. Dagata, J. Schneir, H. H. Harary, C. J. Evans, M. T. Postek, and J.Bennett, Appl. Phys. Lett. 56, 2001 (1990).10J. A. Dagata, F. Perez-Murano, G. Abadal, K. Morimoto, T. Inoue, J. Itoh,and H. Yokoyama, Appl. Phys. Lett. 76, 2710 (2000).11E. Dubois and J.-L. Bubendorff, J. Appl. Phys. 87, 8148 (2000).12R. Garcia, M. Calleja, and H. Rohrer, J. Appl. Phys. 86, 1898 (1999).13S. Gwo, C.-L. Yeh, P.-F. Chen, Y.-C. Chou, T. T. Chen, T.-S. Chao, S. F.Hu, and T.-Y. Huang, Appl. Phys. Lett. 74, 1090 (1999); F. S.-S. Chien,J.-W. Chang, S.-W. Lin, Y.-C. Chou, T. T. Chen, S. Gwo, T.-S. Chao, andW.-F. Hsieh, ibid. 76, 360 (2000).14N. Farkas, J. C. Tokash, G. Zhang, E. A. Evans, R. D. Ramsier, and J. A.Dagata, J. Vac. Sci. Technol. A 22, 1879 (2004); N. Farkas, G. Zhang, K.M. Donnelly, E. A. Evans, R. D. Ramsier, and J. A. Dagata, Thin SolidFilms 447-448, 468 (2004); N. Farkas, G. Zhang, E. A. Evans, R. D.Ramsier, and J. A. Dagata, J. Vac. Sci. Technol. A 21, 1188 (2003).15J. A. Dagata and J. S. Suehle (unpublished).16M. Tello and R. Garcia, Appl. Phys. Lett. 83, 2339 (2003).17L. Kam and S. G. Boxer, J. Biomed. Mater. Res. 55, 487 (2001).18C. Galli, M. Collaud Coen, R. Hauert, V. L. Katanaev, P. Groning, and L.Schlapbach, Colloids Surf., B 26, 255 (2002); C. Galli, M. Collaud Coen,R. Hauert, V. L. Katanaev, M. P. Wymann, P. Groning, and L. Schlapbach,Surf. Sci. 474, L180 (2001).19B. W. Buczynski, M. M. Kory, R. P. Steiner, T. A. Kittinger, and R. D.Ramsier, Colloids Surf., B 30, 167 (2003).FIG. 4. Scanning Auger microprobe (JEOL 7830F) analysis of three areas ofa zirconium nitride film, as indicated in the inset. Nitrogen is replaced byoxygen at stamp-substrate contact locations during pattern transfer. The sur-face was etched for 90 s with a 3 keV Ar+ ion beam to remove surfacecontamination before these spectra were recorded.Appl. Phys. Lett., Vol. 85, No. 23, 6 December 2004 Farkas et al. 5693

Parallel Writing on Zirconium Nitride Thin Films by Local Oxidation Nanolithography

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Parallel Writing on Zirconium Nitride Thin Films by Local Oxidation Nanolithography

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