nano/micro structure fabrication of metal surfaces using the combination of nano plastic forming,...
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Nano/micro structure fabrication of metal surfaces using the combination of nano plastic
forming, coating and roller imprinting processes
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2009 J. Micromech. Microeng. 19 125028
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IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING
J. Micromech. Microeng. 19 (2009) 125028 (11pp) doi:10.1088/0960-1317/19/12/125028
Nano/micro structure fabrication of metalsurfaces using the combination of nanoplastic forming, coating and rollerimprinting processesW Kurnia and M Yoshino
Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama,Meguro-ku, Tokyo 152-8552, Japan
E-mail: [email protected] and [email protected]
Received 8 July 2009, in final form 25 September 2009Published 16 November 2009Online at stacks.iop.org/JMM/19/125028
AbstractThe fabrication technique using the combination of nano plastic forming, coating and rollerimprinting processes to transfer nano/micro patterns onto the surface of metallic materials isdeveloped. In this process, nano plastic forming (NPF) of pure aluminum (Al 99%) using adiamond tool is combined with a coating technique to produce a nickel die. The nickel die isthen used for the double plate roller imprinting process to transfer the pattern onto the surfaceof metallic material. Experimental works with 0.1 N to 1.0 N indentation load variations and2.5 μm to 7.5 μm pitch variations using a knife edge diamond tool are conducted to study thefeasibility of this technique. Comparisons between the master mold, nickel die and rollingresult in terms of geometries show good agreement, thus validating the proposed approach forhigh throughput, low cost, low emission and flexible fabrication technique. The capability ofthe proposed method is demonstrated in the fabrication of cross, net and brick patterns.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Advancement of technology has caused a rapid increase inthe demands of nano/micro products for microelectronics,medical, automotive and telecommunication industries. Inorder to match these global demands, techniques such aslithography processes have been commonly used to fabricatenano/microstructures with different variation of shapes on thematerial surfaces. However, these techniques are not alwayssuitable due to limits on work materials, cost, machining time,stringency in methods and chemical disposal which leadsto fundamental environment problems. In addition, thoseprocesses are based on two-dimensional fabrication methods,which are not essentially suitable for three-dimensionalstructures required in more sophisticated systems of microdevices. An alternative approach is required to overcome theselimitations and realize high throughput fabrication process ofnano/microstructures.
One interesting approach to achieve a high throughputprocess is to transfer the fabricated nano/microstructures ontothe surface of other materials by utilizing conventional manu-facturing techniques such as imprinting and rolling. Researchon this direction was proposed by Chou et al [1] to fabricatemetal patterns with a feature size of 25 nm using nano imprintlithography. Their study indicated that the ultimate resolutionof sub-10 nm with high repeatability and durability is possible[2]. Hirai et al [3] utilized the imprinting process to replicatefine patterns as small as 250 nm in width and 270 nm in heightfrom nickel molds onto the PMMA polymer without cracksor voids. They subsequently reported the imprinting processof fine grating on a glass surface using a Si3N4/SiO2/Si moldand a low Tg glass [4]. Diamond molds were also fabricatedusing a fine dry etching process to imprint the PMMApolymer [5]. Trusskett et al [6] reviewed the applications ofthe imprinting process in biological fields. The fabricationof optically encoded micro particles (diffractive barcodes)
0960-1317/09/125028+11$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK
J. Micromech. Microeng. 19 (2009) 125028 W Kurnia and M Yoshino
(a) Nano Plastic Forming
(b) Nickel plating
(c) Nickel die
(d ) Roller imprinting
(e) End product
Diamond tool
Pure Al (99 %)
Pure Al (99 %)
Nickel
Roll
Roll
Pure Al (99%)Indentation
Nickel die
Figure 1. Schematic diagram of the fabrication processes.
by the imprinting process of polymer using UV-lithographyfabricated stamps has also been studied [7].
On the other hand, the roller imprinting process whichoffers continuity, hence higher throughput as compared toimprinting, is steadily gaining more attention. Makelaet al [8] studied the rolling process of submicron structureson inherently conductive polyaniline-dodecylbenzenesulfonicacid (PANI-DBSA) film. Recently, Ting et al [9] reportedthe fabrication of an antireflective optical film with sub-wavelength structures on the flexible PET substrate using theroll-to-roll micro-replication process. DUV lithography anddry etching were combined with electroplating to fabricate thenickel molds. Additionally, Ahn et al [10] reported roll-to-roll nano imprint lithography (R2RNIL) of ETFE molds ontoPDMS and epoxysilicone surfaces.
Unfortunately, the above-mentioned methods rely onlithography to fabricate the molds, hence high fabrication cost.In addition, polymer was mainly used as the rolling target.Little work has been done on metal as the rolling target.Only recently, work on metal was reported by Yamamotoet al [11] to fabricate microgrooves onto austenitic stainlesssteel (SUS303) and aluminum alloy (A5056) surfaces.However, the structure sizes were 10 μm in height and 0.5 mmin width. More work is required for smaller structures.
In this paper, a fabrication technique to manufacturenano/microstructures with high throughput, low cost, lowemission, flexibility in geometries and flexibility in workmaterials is developed. The combination of nano plasticforming (NPF), coating and roller imprinting processes,abbreviated as NPF-CRI, is used to fabricate the master mold,
die and end products respectively. As compared to previouslyreported nano imprinting [1–7] and roller imprinting [8–10],the proposed method eliminates the need for lithographyprocesses to fabricate the mold and replaces it with theNPF process, which allows various kinds of materials tobe fabricated without any need to change the tool. Hence,the fabrication cost and chemical usage can be significantlyreduced. In addition, the structure shapes and geometriesthat can be fabricated rely solely on the shape and geometryof the diamond tool which makes the fabrication of three-dimensional shapes possible.
Experimental works were conducted and nano/micropatterns were successfully replicated onto the metallic materialsurface. The effects of the indentation load, pitch variations,coating methods and rolling parameters were analyzed. Basedon these results, the feasibility of this method to replicatemicrostructures on metallic materials will be validated.
2. Experimental setup
The experimental work was carried out in three-step processes.First, the NPF process was used to fabricate a master moldusing a mirror-finished aluminum plate. Aluminum was useddue to its formability and because it can be easily removed.Second, the master mold was coated with nickel and thealuminum was then removed to acquire a nickel die. Third,the nickel die was used for double plate roller imprintingto transfer the patterns onto the surface of a pure aluminumplate. The schematic diagram of the whole process is shown infigure 1.
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J. Micromech. Microeng. 19 (2009) 125028 W Kurnia and M Yoshino
(a) Overall view (b) Schematic diagram
Z-axis
Load Cell
Diamond tool
Y-axis
X-axis
Tilt-stage
Diamond tool
Figure 2. Nano plastic forming equipment.
Table 1. Experimental parameters.
Specimen 15 mm × 15 mm × t0.5 mm pure aluminum (99%)Diamond tool Knife edge tool (edge angle: 60◦, width: 600 μm, tip radius: <50 nm)Indentation load 0.1–1.0 N (0.1 N interval)Pitch 2.5 μm, 5.0 μm and 7.5 μmMachining area 10 mm × 10 mm
2.1. Nano plastic forming
The nano plastic forming (NPF) process was first developedby Yoshino et al [12] to fabricate fine patterns on hardbrittle material surfaces. Various types of diamond toolssuch as point tool, knife edge, I-type, T-type and multi-nanostructures are available to fabricate various kinds ofnano/microstructures [13–15]. The application of thistechnique has been shown in the fabrication of the DNAmicro array chip [16]. This process is more advantageousas compared to the lithographic processes due to its lowercost, no usage of dangerous chemicals, less stringency inthe fabrication process and flexibility in work materials. Ascompared to other conventional machining processes usingthe diamond tool such as micro lathe and micro milling, NPFis able to fabricate smaller structures since the size limit isequal to the tool’s size. As for micro lathe and micro milling,the diamond tool has to be scratched for some distance tomake a groove, resulting in structures larger than the tool’ssize. In addition, NPF is also superior in terms of flexibility ingeometry and patterning speed.
The NPF setup (see figure 2) consists of a linear motioncontroller, data acquisition system and diamond tool. Thelinear motion controller is controlled by the computer for itsXY-axes and Z-axis. The resolution of those stages is 10 nm,and the strokes are 20 mm and 40 mm for the XY-axes andZ-axis, respectively. The XY-axes are mounted on a tilt stagewith 0.015◦ resolution. As for the data acquisition system,a high-resolution load cell (Kistler 9205) is mounted on theZ-axis to measure the indentation load.
The experimental parameters used are shown in table 1.In this experiment, the knife edge tool was selected amongother types of diamond tools because the resulting structurescan be easily observed by cross sectioning and its deformationcharacteristics are apparent based on plasticity theory. During
the NPF process, the knife edge tool traversed the surface ofthe specimen until contact occurred. Then, the indentation wasperformed while maintaining the indentation load according tothe load parameters. When the required indentation load wasachieved, the diamond tool was retracted from the specimen’ssurface. The process was then continued to the next processpoint or stopped when the number of indentations requiredhad been reached. The whole process was done inside aclean chamber to control the machining environment. ACCD camera was used to observe the NPF process in realtime.
2.2. Coating process
Two kinds of nickel coating methods were examined to coatthe fabricated master mold. Nickel was chosen as the coatingmaterial due to its hardness and smoothness of the platingresults.
2.2.1. Nickel electroless plating process. Prior to theelectroless plating process, ultrasonic cleaning followed byzincating were used as the pre-treatment of the aluminumplate. Zincating is an electrochemical exchange between zinccomplexes in solution and aluminum, which deposits zinccrystallites at the expense of aluminum [17]. The next step waselectroless nickel plating using a commercially available Ni-801 electroless plating solution to achieve a nickel thickness of400 μm. The rolling die was then acquired from nickel platingby dissolving the aluminum using NaOH. The parameters usedin these processes are shown in table 2.
2.2.2. Combination of gold sputter coating and nickelelectroplating. Prior to sputter coating, the master mold wascleaned using an ultrasonic cleaner. The master mold was then
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J. Micromech. Microeng. 19 (2009) 125028 W Kurnia and M Yoshino
Motor
Rolls
Amplifier
Data recorder
Rolling gap control
Strain gages
Figure 3. Roller imprinting equipment.
Table 2. Nickel electroless plating parameters.
No Process Composition Temperature Current Time
1 Zincating ZnCl2:NaOH:H2O = 2 g:15 g:100 ml Room temperature 0.5 A 5 s2 Nickel electroless plating Ni-801:H2O = 1:4 80 ◦C – 8 h3 Aluminum removal NaOH:H2O = 15 g: 100 ml Room temperature – ∼24 h
Table 3. Sputter coating and nickel electroplating parameters.
No Process Remarks Current Time
1 Sputter etching – 4 mA 4 min2 Sputter coating Target: gold; thickness: 5 nm 6 mA 1 min3 Nickel electroplating N-100ES:H2O = 2:3 (room temperature) 0.01 A ∼48 h
subjected to sputter etching to remove contamination layersfrom the master mold surface. After that, a thin layer of gold(5 nm) was sputtered onto the surface of a master mold. Theprocess was then continued by nickel electroplating using acommercially available electroplating solution (N-100ES) toachieve a nickel thickness of 400 μm. The parameters used inthese processes are shown in table 3.
2.3. Roller imprinting
The rolling mill used in this experiment is depicted in figure 3.The acquired nickel die from the coating process was used inthe flat roller imprinting (dry) where the die and work materialare butted and rolled together. It is important to note that thecylindrical roller imprinting process where the die is attachedaround the roller surface provides better uniformity, lowerforce and ability to replicate patterns onto a large surfacearea as compared to flat roller imprinting. This is becauseonly a line area is in contact at a time, which significantlyreduced the effects of thickness variation and dust during theroller imprinting process [18]. However, flat roller imprintingwas chosen in this experimental work due to its simplicitywhich concords with the objective of studying the feasibilityof the proposed technique. Using this method, the pattern wasreplicated onto the surface of a 15 mm × 15 mm × 0.5 mmmirror-finished aluminum plate. The roll diameter and rollinggap are 27 mm and 0.9 mm, respectively. The roller imprintingprocess was carried out by controlling the rolling load from5 N to 360 N. The parameters of the rolling mill are shown intable 4.
Table 4. Roller imprinting parameters.
Roll diameter 27 mm Maximum rolling force 46.67 kNRoll width 30 mm Maximum torque 123.48 N mRolling speed 850 mm Applied rolling force 0–360 N
min−1
3. Results and discussion
3.1. Analysis of nano plastic forming results
Figure 4 shows the master mold fabricated by the NPF processand the typical SEM observation result along with the crosssection. As shown in figure 4(a), a variety of grooves ofdifferent sizes are fabricated within the 10 mm × 10 mm areaof the master mold. The grooves are indented at differentindentation loads and pitch settings in order to examine theeffects of those conditions on structure geometries and transferratio. Observations on the cross section were carried out bycuring the master mold inside the epoxy resins. After curing,the master mold was cut and then polished to the middle of thegrooves. The cross sections are then observed using SEM andthe result is shown in figure 4(c).
Geometries for different indentation loads and pitchesare measured using atomic force microscopy (AFM) andthen confirmed with the cross section measurements. Fivereplications were used to ensure reliability and the results aredepicted in figure 5. The data show that patterns down to250 nm in depth were successfully fabricated on the mastermold surface. It was found that the geometries are highlyaffected by the load and pitch variations. For each pitch setting,the structure depths increase linearly with indentation load at
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J. Micromech. Microeng. 19 (2009) 125028 W Kurnia and M Yoshino
15 mm Aluminium
Epoxy
(a) Fabricated master mold (b) Top view (c) Cross section
15 m
m
Load=0.5 N; pitch=5 m
40 m3.33 m
Figure 4. Nano plastic forming results (Al 99%).
µ
µ
µ
Figure 5. Measurement results of the master mold (Al 99%).
Last indentation point
Surface profile 10 m
[ m]
[ m]
Figure 6. Effect of load and pitch variations on the structure’sprofile (load = 1.0 N, pitch = 5 μm).
low indentation load and start to reach a steady state at highindentation load. This is because at this critical point, thedeformation area is larger than the pitch setting. Hence,the material flow at a new indentation point starts to affectthe shape of the previous indentation, decreasing its width anddepth. This phenomenon is shown in figure 6 where the lastindentation point is clearly wider and deeper compared to theprevious indentation point. In addition, it also causes a roughsurface at the adjacent point between the two indentations.Therefore, it is necessary to study the critical indentationforce for a particular pitch setting in order to fully utilize thisprocess.
The plastic deformation during the NPF process usingthe knife edge diamond tool can be explained by plasticity
theory. As proposed by Hill et al [19], it is assumed thata perfectly rigid-plastic material is indented with a wedge-shaped indenter, causing a plain strain deformation. As shownin figure 7, the smooth symmetrical knife edge diamond toolof semiangle, β = 30◦, is indented normally into a semi-infinite medium surface. Only a plastic deformation zone isconsidered here and the slip line field corresponding to thatzone is represented by the triangles and centered fan locatedon the right side of the diamond tool.
According to the slip line model presented in figure 7, theshape of the plastic deformation zone depends only on the tooltop angle (β), which, in turn, determines the center fan angle,θ :
cos(2β − θ) = cos(θ)/(1 + sin θ), (1)
and the length of the affected surface, AB, is
AB = d (sin β + cos(β − θ)2 / cos θ. (2)
During the NPF process, after one indentation, the tool islifted, shifted and indented at the next indentation point inorder to create an array of structures, as shown by the arrowsin figure 7. Hence, the minimum pitch that can be fabricated onthe workpiece without affecting the previous indentation pointwill depend only on the indentation depth (d) and the tool topangle (β). Conversely, the maximum depth will depend onlyon the pitch settings and the tool top angle (β) according tothe following equation:
maximum depth = pitch
/{[sin β+ cos(β − θ)
cos β − sin(β − θ)
]+ tan β
}.
(3)
Therefore, for any particular pitch setting, the maximum depthbefore the material flow starts to affect the shape of the previousindentation point can be calculated. The correlation betweenmaximum depth and pitch setting for various tool top anglesis calculated and plotted in figure 8. Based on the calculationresults for a knife edge diamond tool (β = 30◦), the center fanangle is found to be θ = 17.34◦ and the maximum depths arefound to be 875 nm, 1749 nm and 2623 nm for 2.5 μm, 5 μmand 7.5 μm pitch settings, respectively.
The corresponding calculated maximum depths, thegeometries of the master mold and the SEM figures closeto and beyond the maximum depth lines are plotted togetherin figure 9. It was found that the experimental results areconsistent with the calculation results where the structure
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J. Micromech. Microeng. 19 (2009) 125028 W Kurnia and M Yoshino
Figure 7. Plain strain deformation of the workpiece surface by the knife edge diamond tool.
µ
µ
Figure 8. Correlation between maximum depth and pitch settingsfor various tool top angles.
geometries start to reach a steady state beyond the maximumdepth limit. Hence, it is confirmed that the material flowaffects the shape of the previous indentation point, decreasingits width and depth. Based on these results, the criticalindentation force which corresponds to the maximum depthis approximated to be 0.3 N, 0.5 N and 0.7 N for 2.5 μm,5.0 μm and 7.5 μm pitch settings, respectively.
10 m
10 m
µµµ
µµµ
Figure 9. Nano plastic forming results with calculated maximum depth (Al 99%).
The behavior of plastic deformation in the NPF processdepends on the material’s crystal orientation and contact withthe diamond tool. As shown in figure 7, the contact areadeformed in accordance with the shape of the diamond toolwhile the surrounding free surface area generates a pileupregion with an irregular surface due to the collision of slipdeformations at grain boundaries. After the NPF process,the thickness distributions over the entire master mold surfacewill change due to the pileup. By ignoring the effects of springback and volume changes under compression, the amount ofmaterial piling up after indentation will be equal to the volumeof the material indented under the diamond tool. Hence, theheight of the pileup can be expressed as hpileup = 0.219 h. Thethickness distribution of the pileup for every pitch setting willbe proportional to the depth of the master mold.
The tip radius of the diamond tool and surface roughnessof the master mold play a major role in determining theminimum obtainable size of the NPF process. However, thiscan be overcome by using advance manufacturing technologywhere a tip radius of less than 10 nm and sub-nanometersurface roughness are possible. In addition, the plasticdeformation of a metallic material under indentation isattributed to the dislocation behavior. Thus, the minimumsize that can be fabricated on the master mold surface will
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J. Micromech. Microeng. 19 (2009) 125028 W Kurnia and M Yoshino
Nickel
Epoxy
(a) Fabricated nickel die (b) Top view (c) Cross section
15 mm
15 m
m
Load=0.5 N; pitch=5 m
40 m 3.33 m
Figure 10. Nickel electroless plating result and SEM figures.
be restricted by the size of the dislocation mesh structuregenerated in the metallic crystal. In the case of a metallicmaterial, the Young modulus is large so the amount of plasticrecovery is only a small percentage of the indented volume.Therefore, this effect on the minimum obtainable size can beneglected. On the other hand, the aspect ratio generally relieson the geometry of the diamond tool. As a result, the maximumlimit depends on the fabrication technology used to fabricatethe tool.
3.2. Analysis of the coating process
3.2.1. Nickel electroless plating process. Observations usingSEM are used to evaluate the master mold and the fabricatednickel die. It was found that the patterns were successfullytransferred from the master mold to the nickel die. Figure 10shows the fabricated nickel die and the typical SEM figurealong with the cross section. The pictures confirm that thenickel die is able to replicate the shape of the master moldreasonably well. However, the measurement using SEMshows that the tip radii are drastically changed from 0.05–0.1 μm to 0.3–0.5 μm (see figures 4(c) and 10(c)). The reasonbehind this phenomenon is the dissolution of an aluminumoxide layer during the zincating process according to thefollowing reaction: Al2O3 + 2NaOH + 3H2O ⇒ 2NaAl(OH)4.Hence, the top layer of the master mold was removed, alteringthe pattern’s edges and tip radii. Consequently, lower heightand wider structures resulted.
Quantitative comparisons between the master molds andnickel die are carried out using the AFM measurement resultsusing five replications. Figure 11 shows the plot betweenthe depth of the master mold and the height of the nickeldie for each pitch setting. The linearity of the plot showsthat the structures are uniformly transferred from the mastermold to the nickel die by the nickel electroless plating processregardless of structure sizes or pitch settings. The gradients ofthe regression lines also show that the transfer ratios (heightof the nickel die/depth of the master mold) are relatively high,ranging from 87% to 97%. However, even with high transferratios, the heights of the nickel die are still lower as comparedto the depth of the master mold, as shown by its offset from theideal transfer line. The offset values range from 118.9 nm to278.2 nm. Since the offset values are fairly constant for eachpitch setting, it is believed that these values are contributed bythe changes in tip radii during the nickel electroless platingprocess.
µ
µ
µ
Figure 11. Comparison between the master mold and the nickel die(electroless plating).
3.2.2. Combination of gold sputter coating and nickelelectroplating. The fabricated nickel die and the typical SEMfigure along with the cross section are shown in figure 12. Thepictures show that the resulting structures on the nickel die arein good agreement with the master mold structures with noapparent reduction on the tip radius and angle (see figures 4(c)and 12(c)).
Figure 13 shows the comparison of the AFM data betweenthe depth of the master mold against the height of the nickeldie for each pitch setting. As in nickel electroless platingresults, this plot also gives good linearity between the data.Hence, the structures are also uniformly transferred from themaster mold to the nickel die regardless of structure sizes orpitch settings. However, the gradients of the regression linesreveal that this coating method results in higher transfer ratios(92–97%) as compared to the nickel electroless platingprocess. In addition, the offset values are only 16.9 nm,17.6 nm and 0.3 nm for 2.5 μm, 5.0 μm and 7.5 μm pitchsettings, respectively, making it close to the ideal transferline. Consequently, the changes in tip radii during the coatingprocess are considerably small.
By comparing the analysis results of these two coatingtechniques, it can be concluded that the coating method usingthe combination of sputter coating and nickel electroplatingis able to provide better replication results compared to thenickel electroless plating method. In addition, the cohesionbetween the master mold and gold layer is considerably weakbecause sputter coating was used. Hence, the nickel die was
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J. Micromech. Microeng. 19 (2009) 125028 W Kurnia and M Yoshino
(a) Fabricated nickel die (c) Cross section (b) Top view
Nickel
Epoxy
15 mm
15 m
m
Load=0.5 N; pitch=5 m
40 m 3.33 m
Figure 12. Sputter coating–electroplating result and SEM figures.
Figure 13. Comparison between master mold and nickel die(sputter coating–electroplating).
easily acquired by peeling the nickel from the coated layer,leaving the patterns on the master mold intact. As a result,the master mold can be used repeatedly to fabricate morenickel dies and the total fabrication time can be significantlyreduced. Based on this analysis, the fabricated nickel die usingthe combination of sputter coating and nickel electroplating isselected to fabricate the nickel die for the roller imprintingprocess.
3.3. Analysis of the roller imprinting process
Figure 14 shows the results of the roller imprinting processon a pure aluminum plate (99%) using different variations ofrolling load (0–360 N). It is clear from the figure that thetransferability of patterns increases with rolling load. Thetransfer ratio between the master mold and rolling result is
(a) 5 N (c) 120 N (d ) 260 N (e) 360 N (b) 60 N
Figure 14. Roller imprinting results (Al: 99%)
I : Indentation of nano and micro structures
II : Optimum rolling load
III : Rolling and elongation of work material
Figure 15. Average transfer ratio versus pitch settings for variousrolling loads.
then calculated using the AFM measurement data and shownin figure 15.
When the rolling load is increased to 100 N, the portionof nano- and microstructures indented on the work materialsurface is also increased, resulting in a linear increase of thetransfer ratio with the load, as denoted by area I in figure 15.Beyond this area, the transfer ratio becomes saturated after90% when the rolling load is between 100 N and 120 N.At this point, it is believed that all the structures are fullyembedded into the surface of the workpiece and then springback takes place after rolling, resulting in a slight reduction instructure sizes. Consequently, a transfer ratio of approximately92% results. This area is denoted as area II and consideredas the critical rolling load values where the transition fromindentation to rolling process occurs. When the rolling loadexceeds this area, there is only a minute change in the transfer
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J. Micromech. Microeng. 19 (2009) 125028 W Kurnia and M Yoshino
(a) Rolling load: 120 N (b) Rolling load: 360 N
Rolling direction
Figure 16. Distortion of fabricated structures during the rollerimprinting process.
ratio, as indicated in area III. This is because the deformationand elongation of the work material similar to the sheet metalrolling process have started. Due to this deformation andelongation, slipping occurs and the structures become distorted(see figure 16). Therefore, the rolling load must not belarger than the critical values in order to avoid this problem.The rolling load is controlled within 100–120 N in the nextexperiments to facilitate optimum roller imprinting results. Itshould be noted that the optimum rolling load investigated inthis work is only applicable for the nickel die’s size, structuregeometries and rolling material under consideration. It isexpected that the optimum rolling load would be proportionalto these parameters.
In figure 17, the roller imprinting result of 120 N rollingload and the typical SEM figure along with the cross sectionare depicted. The figures show that the patterns are uniformlytransferred onto the surface of the aluminum plate duringthe roller imprinting process. Comparison with the crosssection of the master mold (see figure 4(c)) and nickel die (seefigure 12(c)) reveals good consistency in terms of geometries.
Quantitative comparison of the master mold and rollingresults is shown in figure 18 for the 120 N rolling load. The
15 mm
15 m
m
(a) Roller imprinting result (c) Cross section (b) Top view
Load=0.5 N; pitch= 5 m
Aluminium
Epoxy
20 m 3.33 m
Figure 17. Roller imprinting result and SEM figures (rolling load: 120 N).
µ
µ
µ
Figure 18. Comparison between master mold and rolling results(rolling load: 120 N).
linearity of the plot shows that the structures are uniformlytransferred from the master mold to the rolling resultsregardless of structure sizes or pitch settings. Finally, thestatistical data in terms of the average transfer ratio betweeneach step are calculated and presented in table 5. Overallaverage transfer ratios between master mold and rolling resultsare found to be 90%, 92% and 92% for 2.5 μm, 5.0 μmand 7.5 μm pitches, respectively. Plausible reasons for thediscrepancy between the master mold and roller imprintingresults are the changes in the tip radius during the coatingprocess and spring back which occurs during the rollerimprinting process.
3.4. Fabrication of cross, net and brick patterns
In order to demonstrate the capability of the proposed method,various kinds of patterns are fabricated using the knifeedge tool, flat tool, T-type tool and nanostructure tool (seefigure 19). Using similar procedures, the structures are
Table 5. Average transfer ratio between master mold, nickel die and rolling results.
Average transfer ratio
Pitch setting Master mold–nickel die Nickel die–rolling results Master mold–rolling results
2.5 μm 93% 97% 90%5.0 μm 96% 96% 92%7.5 μm 95% 96% 92%
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J. Micromech. Microeng. 19 (2009) 125028 W Kurnia and M Yoshino
Diamond tool Master mold
(fabrication time: 24~72 hours)
Nickel die
(fabrication time: 48 hours)
Rolling result
(fabrication time: 3 seconds)
Knife edge tool
Indentation load=0.5 x 0.8 N
Pitch setting=5 m
Depth=1.7 m x 2 m
Flat tool
Indentation load=2 N
Pitch setting=100 m
Depth=3.5 m
T-Type tool
Indentation load=0.1 N
Pitch setting=43 x 22.5 m
Depth=2 m
Nano structure tool
Indentation load=0.1 N
Pitch setting=25 m
Depth=3 m
600 m
50m
50 m
22.5
m
43 m
8.5
m
9.5 m
0.3 m
0.3
m
Figure 19. Cross, net, brick and nano-holes patterns results.
successfully transferred onto the nickel die and subsequentlyrolled onto the surface of the aluminum plate. During theroller imprinting process, the rolling time is measured and it isverified that this method significantly decreases the fabricationtime of nano/microstructures from 24–72 h (10 mm × 10 mmmachining area) using the NPF process to only 3 s using therolling process. It should be noted that the fabrication timeof the NPF process can be further improved for industrialmanufacturing purposes by increasing the surface indentationarea of the diamond tool. In addition, open loop control canbe used instead of closed loop control during indentation toincrease the indentation speed, provided that the specimen’sflatness tolerance is less than 1 μm and the alignment with theXY-stages is carefully controlled.
The fabrication results show very good consistency interms of geometries for each pattern, thus validating theproposed approach for the rapid fabrication process, low cost,low emission and flexible fabrication technique.
4. Conclusion
The NPF-CRI process has been studied experimentally. Inthe NPF process, the effect of material flow is investigatedin order to fully utilize this technique. Based on theexperimental and calculation data, critical indentation loadswere approximated to be 0.3 N, 0.5 N and 0.7 N for 2.5 μm,5.0 μm and 7.5 μm pitch settings, respectively. For the coatingprocess, comparisons between two coating methods reveal that
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J. Micromech. Microeng. 19 (2009) 125028 W Kurnia and M Yoshino
the combination of sputter coating and nickel electroplatingshows better results compared to the nickel electroless platingmethod. In addition, the master mold can be used to fabricatethe nickel die repeatedly, which significantly reduces thetotal fabrication time. In the roller imprinting process, it isshown that the fabrication time of nano/microstructures wasimproved from 24–72 h using the NPF process to only 3 s usingthe rolling process. The optimum rolling loads are found tobe within 100–120 N.
Overall average transfer ratios between the master moldand rolling results are found to be 90%, 92% and 92% for2.5 μm, 5.0 μm and 7.5 μm pitches, respectively. Theseresults show good agreement between the master mold, nickeldie and rolling results, thus validating the proposed approachfor the rapid fabrication process, low cost, low emission andflexible fabrication technique. The capability of the NPF-CRIprocess has been demonstrated by the fabrication of cross, netand brick patterns.
Acknowledgment
This research work is financially supported by Naito TaisyunScience and Technology Foundation.
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