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Thedirectnano-patterningofZnOusingnanoimprintlithographywithZnO-solandthermalannealing

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ImpactFactor:1.2·DOI:10.1016/j.mee.2009.03.078

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Microelectronic Engineering 86 (2009) 2228–2231

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Microelectronic Engineering

journal homepage: www.elsevier .com/locate /mee

The direct nano-patterning of ZnO using nanoimprint lithographywith ZnO-sol and thermal annealing

Ki-Yeon Yang a, Kyung-Min Yoon a, Kyung-Woo Choi b, Heon Lee a,*

a Department of Materials Science and Engineering, Korea University, 5-1 Anam-Dong, Sungbum-Ku, Seoul 136-701, South Koreab Korea Institute of Nuclear Safety, 19, Gusung-dong, Yu-sung-gu, Daejeon 305-338, South Korea

a r t i c l e i n f o

Article history:Received 25 November 2008Accepted 6 March 2009Available online 16 March 2009

Keywords:ZnO nano-patternZnO-solZnO-gelNanoimprint lithography

0167-9317/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.mee.2009.03.078

* Corresponding author. Tel.: +82 2 3290 3284; faxE-mail address: heonlee@korea.ac.kr (H. Lee).

a b s t r a c t

Nano-patterned ZnO layer was fabricated by ZnO-sol imprinting with a polymeric mold, followed byannealing. Instead of polymer based imprint resin, ZnO-sol was used as an imprint resin. During theimprinting process, the organic solvent in the ZnO-sol was absorbed into a polymeric mold and thus,ZnO-sol was converted to ZnO-gel. These patterns were subsequently annealed at 650 �C for 1 h in atmo-spheric ambient to form ZnO patterns. X-ray diffraction (XRD) and photoluminescence (PL) confirmedthat ZnO-gel was completely converted into ZnO by annealing. Using this ZnO-sol imprinting method,ZnO nano-patterns, as small as 50 nm, were fabricated on Si and oxidized Si wafer substrates. The ZnOnano-patterns were characterized using scanning electron microscopy (SEM) and Transmission electronmicroscopy (TEM).

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Zinc oxide (ZnO) is an attractive oxide semiconductor, with awide band gap (3.3 eV) [1] and large exitonic binding energy(60 mV) [2] that enable application to various fields such as thinfilm gas sensors [3], photo-detectors [4] and light emitting diodes[5] especially for UV region. Patterning of ZnO thin film usually re-quires at least three processes: deposition of ZnO layer, lithographyand etching of ZnO. To use ZnO nano-structures in various applica-tions, an inexpensive and simple ZnO patterning technique needsto be developed.

This paper presents the direct nano-patterning of ZnO, usingnanoimprint lithography (NIL) with ZnO-sol and thermal anneal-ing. By thermal imprinting of ZnO-sol with polymeric imprintingstamp such as PDMS, the organic solvent in the ZnO-sol was re-moved and ZnO-gel patterns were finally formed while the patternmorphology was maintained. Then, ZnO-gel pattern was convertedto ZnO pattern by thermal annealing. Compared to photolithogra-phy, NIL can effectively fabricate nano-scale patterns with simpleprocess, high throughput and relatively low cost [6]. By using theimprinting technique with the sol–gel process, the number of pro-cess steps can be decreased and the expensive processes such asphotolithography and reactive ion etching steps can be skipped.Therefore, ZnO nano-patterns can be made inexpensively and var-ious kinds of ZnO-based nano-devices can be realized using sol–gelnano-imprinting.

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ZnO nano-patterns were fabricated by direct imprinting ofN,N-dimethylformamide (DMF)-based ZnO-sol and subsequentannealing process. The ZnO patterns were characterized by scan-ning electron microscopy (SEM), Transmission electron microscopy(TEM), X-ray diffraction (XRD) and photoluminescence (PL).

2. Experimental section

For the experiment, 0.5 M ZnO-sol was prepared by dissolving0.05 mol of zinc acetate 2-hydrate (Zn(CH3COO)2 � 2H2O) in 0.1 lof DMF and dropping diethanolamine (DEA) into the solution asa sol stabilizer while maintaining the molar ratio of DEA/Zn2+ at1:1 [7]. Due to the high wettability of the DMF solvent, this ZnO-sol can be spin-coated on the various substrates, including Si wa-fer. As the organic solvent in the ZnO-sol must be removed duringthe imprinting process in order to form the ZnO-gel patterns fromZnO-sol, a poly-dimethylsiloxane (PDMS)-based polymer moldwas chosen due to its capability to absorb the solvent withoutdeformation [8]. The PDMS mold was fabricated by spin-coatinga 10:1 ratio mixture of Sylgard 184 (from Dow Corning) A and Bon the anti-adhesion-treated Si master mold, which has nano-dotarray patterns with a diameter of 250 nm and a height of 250 nm[9], at 500 rpm for 10 s and then curing process was followed at80 �C for 1 h. The h-PDMS mold was fabricated with two steps. Inthe first step, h-PDMS layer is formed by spin-coating of mixturevinyl PDMS prepolymer from Gelest, platinum divinyltetramethyl-disiloxane from Gelest as a catalyst, 2,4,6,8-tetramethyltetravinyl-cyclotetrasiloxane from Sigma–Aldrich as a modulator and hydros-ilane prepolymer from Gelest and toluene. The spin-coating of the

K.-Y. Yang et al. / Microelectronic Engineering 86 (2009) 2228–2231 2229

mixture was done at 3000 rpm for 30 s and baking process was fol-lowed at 60 �C for 1 h. In the second step, a 10:1 ratio mixture of

Fig. 1. (a) Schematic procedure of direct ZnO patterning process usi

Fig. 2. SEM, TEM micrographs of (a) ZnO-gel patterns on Si wafer after imprintin

Sylgard 184 A and B from Dow corning was spin-coated on theh-PDMS coated mold at 500 rpm for 10 s and baked at 80 �C for

ng NIL and ZnO-sol and (b) diagram of imprinting process flow.

g process and (b) ZnO patterns on Si wafer after annealing process at 650 �C.

Fig. 4. X-ray diffraction spectra of patterned and un-patterned ZnO layer on quartzsubstrate.

2230 K.-Y. Yang et al. / Microelectronic Engineering 86 (2009) 2228–2231

1 h. After curing of the PDMS, the h-PDMS/PDMS mold was de-molded from the Si master mold [10]. The thickness of both PDMSand h-PDMS/PDMS molds was about 1.2 mm.

Fig. 1 shows (a) a schematic of the overall procedure of directZnO nano-patterning method and (b) a diagram of imprinting pro-cess flow. The ZnO-sol was spin-coated on the pre-cleaned sub-strate at 2000–4000 rpm for 60 s. Then, the PDMS-basedpolymeric mold was placed on the substrate and imprinting pro-cess was done using a pressure vessel type imprint system, de-scribed elsewhere [11]. This process can be divided into twostages. In the first stage, spin-coated ZnO-sol was pressed withthe polymeric mold at a pressure of 500 kPa in order to fill thenano-sized cavities of PDMS stamp with ZnO-sol. In the secondstage, the temperature was raised to 200 �C while maintainingthe pressure. During this second stage, organic solvent in theZnO-sol solution was removed by diffusion through PDMS poly-meric mold. As a result, ZnO-sol was converted to ZnO-gel. Thenano-sized patterns of ZnO-gel were stable. After imprinting pro-cess, the ZnO-gel patterns were annealed at 650 �C for 1 h in atmo-spheric ambient using Rapid Thermal Annealing (RTA) instrumentin order to be converted into ZnO.

3. Results and discussion

SEM and TEM micrographs of the ZnO-gel patterns and ZnO pat-terns on the Si wafer are shown in Fig. 2. As shown in Fig. 2a, ZnO-

Fig. 3. SEM micrographs of ZnO patterns using h-PDMS mold on Si wafer substrate. (a)50 nm sized ZnO nano-patterns on Si wafer and (c) residual thickness of ZnO patterns a

gel patterns with a diameter of 240 nm and a height of 145 nmwere fabricated by thermal imprinting of ZnO-sol. These ZnO-gelpatterns were then converted into ZnO patterns by annealing at650 �C for 1 h. After annealing, the dimensions of ZnO were re-

ZnO patterns using h-PDMS mold, derived from the same master mold of Fig. 2, (b)s a function of spin-coating rpm.

Fig. 5. Photoluminescence spectra of un-patterned and patterned ZnO layer on quartz substrate. Patterned ZnO layer consists of array of 200 nm diameter ZnO islands.

K.-Y. Yang et al. / Microelectronic Engineering 86 (2009) 2228–2231 2231

duced to a diameter of 210 nm and a height of 130 nm. Comparedto the size of master mold, the dimensions of ZnO patterns weredecreased by 16% and 48% in the x- and z-directions, respectively.Fig. 2c is the cross-sectional TEM micrograph of directly patternedZnO using PDMS mold. According to Fig. 2c, ZnO nano-patterns wasformed by direct patterning method and the directly patternedZnO layer has very slight residual layer. Fig. 2b and c also show aslightly tapered ZnO pattern profile due to the slight deformationof the PDMS mold during imprinting.

In order to form the ZnO patterns with a vertical profile, thehigh rigidity h-PDMS/PDMS mold was used, instead of conven-tional soft PDMS mold. Fig. 3 presents SEM micrographs of theZnO patterns, fabricated by imprinting with h-PDMS/PDMS moldand annealing. As shown in Fig. 3, these ZnO patterns also shrankby about 20% and 40% in the x- and z-directions, respectively, com-pared to the dimension of master mold. The profile of the ZnO pat-tern, derived by imprinting with the h-PDMS/PDMS mold, wasmore vertical than that of ZnO patterns, imprinted with the PDMSmold. As shown in Fig. 3b, ZnO pattern, with 50 nm of width and75 nm of height, was successfully fabricated using direct pattern-ing with the h-PDMS/PDMS mold.

The thickness of the residual layer of the imprinted patternsstrongly depends on the initial thickness of the spin-coatedimprinting resist and the density and height of the pattern. Thesurface area of hole array on the polymeric mold, used in thisstudy, is about 42%. Fig. 3c shows that the residual thickness ofthe ZnO patterns after imprinting and annealing as a function ofthe spin-coating speed. According to Fig. 3c, the residual thicknessof the ZnO patterns was decreased with increasing the spin-coatingspeed. Thus, the thinner residual layer was obtained with fasterspin speed since initial thickness of spin-coated ZnO-sol layer be-comes thinner at higher spin speed. In case that low viscosity metaloxide sol resin is used, the thickness of residual layer can be ad-justed by imprinting pressure. However, in this study, only500 kPa of imprinting pressure was applied to prevent the defor-mation of the polymeric mold and thus, reflow of the excessiveZnO-sol toward the outside of the mold by pressing force is limited.In this research, a residual layer thinner than 10 nm was obtainedafter imprinting and post-imprinting annealing. Such a thin resid-ual layer can be easily removed by plasma etching process andeach ZnO-dot patterns can be isolated [12].

ZnO nano-patterns, formed by ZnO-sol imprinting, were charac-terized by XRD and PL analysis. Blanket ZnO layer was prepared asa reference sample using ZnO-sol imprinting to compare with pat-terned ZnO layer. The identical conditions of spin-coating, bakingand annealing processes were applied to make blanket ZnO layer.XRD patterns of the patterned and un-patterned ZnO layer onquartz wafer are shown in Fig. 4. For both layers, diffraction peaksof (100), (002) and (101) planes of ZnO were appeared at 31.8�,34.5� and 36.2�, respectively. The diffraction peaks from thenano-patterned ZnO were weaker and broader due to the nano-scale pattern size. The XRD patterns confirmed the nano-patterned

ZnO material was obtained by imprinting and annealing processesand the phase of ZnO is polycrystalline.

Fig. 5 shows the PL spectra of the patterned and un-patternedZnO layers on the quartz wafer. The patterned ZnO layer exhibiteda dot pattern with a diameter of 200 nm and a height of 110 nm.Luminescence emissions at 376 nm and 378 nm were observedfrom the un-patterned and patterned ZnO layers, respectively, bothof which corresponded to the band gap energy of ZnO (�3.3 eV).The emission intensity from the patterned ZnO layer was aboutfive times stronger than that from the non-patterned ZnO layer,which was attributed to the influence of the periodical dot patternsthat suppress the total internal reflection at the interface betweenZnO and air ambient.

4. Summary

In this study, ZnO nano-patterns were fabricated by ZnO-solimprinting and annealing. The ZnO-gel patterns were fabricatedby imprinting of ZnO-sol with PDMS-based polymeric mold andsubsequently converted to ZnO by annealing at 650 �C. By optimiz-ing the spin-coating speed of ZnO-sol, ZnO-gel patterns with nearzero residual layer (thinner than 10 nm) was obtained. Accordingto the XRD and PL characterization, the patterned ZnO layer, fabri-cated by imprinting of ZnO-sol and subsequent annealing, waspolycrystalline with a band gap of 3.3 eV.

Acknowledgments

This work was supported by Korea Health 21 R&D Project(Grant No. A050750) from Ministry of Health and Welfare, basic re-search program from Korea Science and Nano R&D programthrough the Korea Science and Engineering Foundation funded bythe Ministry of Education, Science and Technology (2008-04501).

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