nickel electroplating for nanostructure mold...

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Copyright American Scientific Publishers

RESEARCH

Copyright copy 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 11 7006ndash7010 2011

Nickel Electroplating forNanostructure Mold Fabrication

Xiaohui Lin1 Xinyuan Dou1 Xiaolong Wang2 and Ray T Chen1lowast1Department of Electrical and Computer Engineering The University of Texas at Austin Austin TX 78758 USA

2Omega Optics Inc Austin TX 78759 USA

We demonstrated a practical process of fabricating nickel molds for nanoimprinting Dual-side pol-ished glass is chosen as the substrate on which nickel nanostructures are successfully electro-plated Photonic crystal structures with 242 nm diameters and other nanoscale pillars down to 9 nmdiameters are achieved over a large area The electroplating parameters are investigated and opti-mized This process extends the feasibility of electroplating process to nanoscale and shows greatpotential in nanoimprint mold fabrication with its low cost straightforward process and controllablepattern structures

Keywords Nickel Electroplating Electron Beam Lithography Nanostructure

1 INTRODUCTION

The fabrication of nanostructures has been a challengeover past decades Although electron beam lithography(EBL) has always been a possible solution every EBL runis time consuming and cost intensive Thus many orga-nizations do not have the luxury of using EBL and itis also not suitable from the mass production point ofview Therefore nano imprint lithography (NIL) has drawnmuch attention recently A successfully fabricated moldcan be used to replicate many devices thus reduces thecost of per unit noticeably Actually this low-cost andhigh-throughput technique has been successfully appliedto make sub-10 nm scale features for compact disks1

Photonic crystal structure has also been reported byBelotti et al to be successfully nanoimprinted via etchedglasssilicon mold with 10 nm minimum feature sizes2

In the light emitting diode (LED) industry nanoimprinttechnology is also booming34 When performing NIL pro-cess using ultraviolet two commonly fabricated molds arerigid quartz stamps5 and flexible stamps6 Rigid stamps aresuitable for small scales down to 20 nm or 10 nm whileflexible stamps (normally PDMS based) are suitable for alarge area with lower resolutionsElectroplating a low cost metal deposition method

is widely used in molding parts scaled from micronsto meters In micro machining electroplating is a keystep in LIGA and pseudo LIGA processes (German

lowastAuthor to whom correspondence should be addressed

acronym for LithographieLithography Galvanofor-mungElectroplating AbformungMolding) to replicatethe master photo resist mold with high aspect ratiostructures7 LIGA process is used to create high-aspect-ratio microstructures A typical LIGA process is composedof pattern exposure development electroforming resiststripping and replication steps with the purpose of fabri-cating various structure in micron scale8ndash11 Sub-micronand nanoscale electroplating has been demonstrated infabricating molds for NIL use Actually some groupshave performed related experiments to fabricate nanoim-print mold by electroplating Kuczko used template-basedmethod to electroforming nanowires12 Kouba et al fromGermany demonstrated a nanoimprint stamp for photoniccrystal13 In their research features were patterned insilicon stamps and were followed by nickel seed layerdeposition On top of the seed layer nickel mold waselectroplated and formed its own back support Anothergroup Chen et al presented their method of electroplat-ing using a seed layer pre-buried in the substrate Afterpatterning the electroplated metal grew only in zoneswhere the seed layer was exposed The final imprintingmold was also fabricated by overelectroplating14 Bureket al15 reported recently using electroplating and electronbeam lithography to fabricated gold and copper nanoscalespecimens for mechanical testing purposeCompared to other works our inspiration stemmed from

bringing the LIGA process to nanoscale We also use pre-buried seed layer to electroplate Ni on areas confinedby electron beam resist directly This method can be useto fabricate many nanoscale structures including photonic

7006 J Nanosci Nanotechnol 2011 Vol 11 No 8 1533-48802011117006005 doi101166jnn20114236

RESEARCH

Lin et al Nickel Electroplating for Nanostructure Mold Fabrication

crystal structures gratings In our work by controllingthe electroplating condition precisely we have successfullyachieved nickel mold with nanoscale pillars down to 90 nmin diameter and a photonic crystal structure mold withinputoutput waveguides as well as the line defect Thismethod featured by its straightforward process showsgreat potential in fabricating nanoimprint molds that makelow cost device manufacturing possible

2 EXPERIMENTAL DETAILS

Figure 1 sketches the entire idea and process of devicefabrication using the nanoimprint method starting fromthe mold fabrication After the mold fabrication it canbe used in an imprinting machine to define patterns onother polymers or resist layers that served as etch maskfollowed by etching step to get functional devices Thedetails are discussed belowSquare dual-side polished 1primeprime times1primeprime glass slices were used

as the nanostructure mold substrate They are carefullycleaned with acetone and IPA then transferred to piranhasolution (H2O2H2SO4 = 12 by volume) for 20 min toremove organic matter Electronbeam evaporation was usedto coat a uniform 50 nm nickel film on the glass surfacewhich serves as the pre-buried seed layer for electroplat-ing The diagonal resistance is around 14 ohmsThe glass substrate coated with Ni film was then

spin coated with adhesion promoter hexamethyldisilazane(HMDS) and high resolution positive electron beam resistZEP520A The spin speed and time were carefully con-trolled so that a resist thickness of 400 nm is maintainedJEOL 6000 E-Beam system was employed to expose theresist After e-beam writing the sample was developed andthen followed by isopropanol alcohol rinseIn order to make a contact for the electroplating process

we used Oxford Reactive-Ion-Etching to open a contactarea of 330 mm2 The area containing nanoscale fea-tures was protected from to ion beam exposure duringthe etching process The ZEP520A resist served as theconfinement The electroplating environment is critical for

Fig 1 Fabrication process Steps (1)sim(3) shows the mold fabricationprocess using nanoscale electroplating Steps (4)sim(6) shows applying themold in a nanoimprint process

nanoscale features to be grown In our experiment wemaintained the bath temperature at 45 C and pH valueat 4 The current density was set to 01 mAmm2 Electro-plating time varied from 3 to 60 minutesThe ZEP520A resist was then removed by a solvent

stripper called NANOtrade REMOVER PG after electroplat-ing to expose the nanostructure To finalize the mold forimprinting process dicing saw was employed to cut out asquare area of around 8 mmtimes 8 mm in order to removethe edge defect during fabrication After dicing the samplecan be used as a nanoimprinting moldThe main advantage of using a pre-buried seed layer

before patterning features is that the electroplated nickelwill only grow on areas where the seed layer is exposedChenrsquos et al14 stated that a seed layer will also help reducevoids formed during the electroplating process comparedpost-patterning seed layer sputtering In the process out-lined above a pre-buried layer is necessary as the entiresurface is not over-electroplated for back support Insteadthe back support of the presented nickel mold is the dual-side polished glass substrate Furthermore no extra stepis necessary to polish the back support of the mold forhandling purpose after dissolving the resist

3 RESULTS

We started from fabricating pillar array to test the feasibil-ity of electroplating nickel in sub-micron and nanometerscale With optimized nickel parameters electroplatingwere executed on holes arrays with 500 nm diameter(1000 nm period) Figure 2(a) shows the results of electro-plating with current density of 01 mAmm2 for 60 min Asshown in the picture a uniform nickel cylinder array wassuccessfully fabricated across the entire electroplated areaThe inset picture in Figure 2(a) shows the detailed zoomedin view of a single pillar The top surface is relativelyrougher than the substrate due to immature electroplat-ing termination control However from the device pointof view this roughness is acceptable during the imprintingprocessThe cross-section image of the 500 nm pillar array is

shown in Figure 2(b) demonstrating the height of elec-troplated nickel to be 400 nm which is identical to thethickness of the spin coated e-beam resist Electroplat-ing conditions shall be carefully controlled to prevent thenickel over-electroplating Otherwise all the pillars willstart to have mushroom like shape and finally connectto each other and cover all the zones The cross sectionpicture also reveals the pre-buried Ni seed layer under-neath the structure which is measured to be 48 nm thickEmploying nickel as seed layer can help eliminate theinternal stress that may weaken the adhesion strength iftwo different materials are used We noticed that in thenickel mold the measured diameters of those pillars arearound 600 nm which are larger than designed This is

J Nanosci Nanotechnol 11 7006ndash7010 2011 7007

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Nickel Electroplating for Nanostructure Mold Fabrication Lin et al

(a) (b)

Fig 2 (a) Nanoscale pillar array with 500 nm in diameter with a pillar zoomed in (b) Cross section view of electroplated Ni pillar array

Table I Thickness of electroplated Ni insied nanoscale holes versustime

Electroplating time (minute) 5 15 30 45 60Electroplated nickel thickness(nm) 45 113 254 362 400

due to the electron scattering effect in the electron beamwriting system causing larger areas to be exposed to elec-trons Thus ZEP520A electron beam writing and develop-ment yielded a larger size of holes that were later filled byelectroplated nickel This difference can be eliminated byoffset the design dimension and optimize the EBL param-eters (ie dosage current and expose time) to account forelectron scattering effectsElectroplating speed varies according to different envi-

ronmental settings In the present work the relationbetween electroplating thickness and time was investi-gated Table I contains data indicating the nickel thickness

Fig 3 Photonic crystal pattern and electroplated Ni mold (a) Overview of the device template (b) zoomed in hexagonal array of photonic crystalstructure template (rotate by 90 scale bar 200 nm (c) light inputoutput coupling area scale bar 1 m

deposited versus time under fixed electroplating conditionsby setting the electroplating bath temperature to 45 Cthe pH value to 4 and the current density to 01 mAmm2

after stabilizing the current source The relation is veryclose to linear and the speed is roughly estimated tobe 6ndash8 nmmin Because of the nanoscopic thicknessesinvolved in the investigation care must be taken in extrap-olating to microscale thicknesses as this linear may notholdWith the success in the 500 nm diameter pillar we

continued to try smaller sizes integrated with practicalphotonic crystal design Figure 3 shows a mold for hexag-onal array of photonic crystal structures with a line defectelectroplated on a Ni coated glass substrate The sub-picture (a) shows the overview of the entire photonic crys-tal Ni template area under scanning electron microscope(SEM) after developing and removal of the e-beam resistThe length of the photonic crystal area is 5803 m The

7008 J Nanosci Nanotechnol 11 7006ndash7010 2011

RESEARCH

Fig 4 Electroplated Ni pillars with 90 nm pillars scale bar 200 nm

sub-picture (b) shows the topography of the area wherelight is coupled inout the photonic crystal area The sub-picture (c) shows the 90 rotated view of hexagonal arrayof the nickel pillars inside the photonic crystal area Thedevice design includes two straight waveguide areas witha photonic crystal area in-between As can be seen clearlyfrom above pictures the lithography patterns were suc-cessfully transferred into a nickel mold with remarkableuniformity The electroplating condition is optimized to007 mAmm2 at 45 C accordingly The air holes havebeen designed to have diameters of 212 nm and periodof 465 nm to let through light ranged from 1515 nm to1565 nm wavelengthThe fabricated mold is featured with both good bulk

mechanic properties and surface chemical properties Theglass substrate is pretty solid while not as brittle as siliconsubstrate High Youngrsquos module of pure nickel using LIGAprocess has been tested and reported to be over 1000 GPa16

which is also our estimation that needs to be confirmed inour future work Besides the adhesion force between glassand nickel is high enough to sustain the temperature up to200 C As an imprinting mold the surface needs to behydrophobic where the presented sample is well featuredwith a hydrophobic surface that renders easy de-moldingprocess when imprintingIn fact electroplating as a low-cost fabrication tech-

nique is able to deposit metal into even smaller structuresFigure 4 shows the nanoscale features with 90 nm diam-eters that are successfully electroplated in glass substratewith nickel coating

4 CONCLUSION

We have demonstrated the process of fabricating Ni moldfor the purpose of nanoimprinting Arrays of nanoscaleNi pillars in photonic crystal patterns were successfullyachieved with remarkable uniformity Smaller mold fea-ture dimensions of 90 nm were demonstrated and thesmallest size we can achieve now were mainly limitedby the electron-beam lithography process but subsequentdesigns can be tailored to achieve intended dimensions

Mold thickness was accurately characterized over a widerange of nanoscopic thicknesses ongoing work on elec-troplating termination shall decrease roughness in futuremoldsThe main advantage of this process includes (1) It

is a simple fabrication process (2) Compared with othermethods the supporting substrate is not formed by elec-troplating thus no postpolish step is necessary (3) Afterelectroplating REMOVER PG is used to dissolve thee-beam resist and no further mold release step is neces-sary (4) The Ni seed layer is pre-buried in the substrateenabling tighter control of features Electroplating Ni onNi also minimizes the internal stress that would be inducedif using two different materialsFuture work shall include but not limited to the improve-

ment of the adhesion strength between different layersmore precise thickness control over the entire mold bettercontrolled electroplating termination for improved sur-face roughness and more investigation into the imprint-ing process such as surface energy reduction moldprotection etc

References and Notes

1 S Y Chou P R Krauss W Zhang L J Guo and L ZhuangJ Vac Sci Technol B 15 2897 (1997)

2 M Belotti M Galli D Bajoni L C Andreani G GuizzettiD Decanini and Y Chen Microelectron Eng 73ndash74 405 (2004)

3 W M Zhou G Q Min Z T Song J Zhang Y B Liu and J PZhang Nanotechnology 21 (2010)

4 R Ji M Hornung M A Verschuuren R van de LaarJ van Eekelen U Plachetka M Moeller and C Moormann Micro-electron Eng 87 963 (2010)

5 M Bender M Otto B Hadam B Vratzov B Spangenberg andH Kurz Microelectron Eng 53 233 (2000)

6 M Bender U Plachetka J Ran A Fuchs B Vratzov H KurzT Glinsner and F Lindner J Vac Sci Technol B 22 3229 (2004)

7 M J Madou Fundamentals of Microfabrication The Science ofMiniaturization 2nd ed CRC Press Boca Raton (2002) p 723

8 C K Chung K L Sher Y J Syu and C C Cheng MicrosystTechnol 16 1619 (2010)

9 S C Shen C J Lee M W Wang Y C Chen Y J Wang andY Y Chen Advanced Manufacture Focusing on New and EmergingTechnologies 594 132 (2008)

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10 L Gu Z Z Wu F Wang R Cheng K W Jiang and X X Li9th International Conference on Solid-State and Integrated-CircuitTechnology (2008) Vols 1ndash4 p 2349

11 J Jahns T Seiler J Mohr and M Borner Micro-Optics 7716 734(2010)

12 A Huczko Appl Phys a-Mater 70 365 (2000)13 K J and et al Journal of Physics Conference Series 34 897 (2006)

14 A Chen B Z Wang S J Chua O Wilhelmi S B MahmoodB T Saw J R Kong and H O Moser Int J Nanosci Ser 5 559(2006)

15 M J Burek and J R Greer Nano Lett 10 69 (2010)16 G He and G C Shi 4th IEEE International Conference on

NanoMicro Engineered and Molecular Systems (2009) Vols 1and 2 p 758

Received 2 November 2010 Accepted 24 January 2011