ORIGINAL ARTICLE

Finite element simulation and experimental studyon the through-mask electrochemical micromachining(EMM) process

Wang Li & Wang Quandai & Hao Xiuqing &

Ding Yucheng & Lu Bingheng

Received: 29 September 2009 /Accepted: 5 March 2010 /Published online: 13 April 2010# Springer-Verlag London Limited 2010

Abstract In this work, the influence of electrode surfacepatterning on shape evolution in through-mask electro-chemical micromachining (EMM) process is investigatednumerically and experimentally. With finite element method,the anodic dissolution process is predicted and the etchingbehavioral trends under various parameters involving spacingand opening ratio, mask aspect ratio, and the etched featureaspect ratio have been identified and interpreted in which theetched depth non-uniformity is mostly ascribed to the currentaggregate effect due to surface patterning on substrate. Thevalidation experiment is conducted and the procedure ofisland formation and disappearing gradually along withetching progress in the center of the etched region is observed,which is in good agreement with the numerical calculationprediction. With the combination of experiments and numer-ical simulation, the optimized electroetching conditions andthe formulation of the electrolyte in through-mask EMM areachieved, with which the ordered microstructures with afeature size down to 15μm and smooth etched surface in largescale are obtained.

Keywords Through-mask electrochemicalmicromachining . Current aggregate effect .

Finite element simulation . Shape evolution

1 Introduction

Electrochemical micromachining (EMM) is a controlledanodic dissolution process and has been considered to be a

promising technique for applications involving patterning,shaping, and finishing in the fabrication of ultraprecisionparts [1]. It has seen a resurgence of industrial interestwithin the last decade due to its significant advantages suchas no tool wear, stress-free, high throughput, smoothsurfaces, and the ability to machine complex shapes inmaterials regardless of their hardness or whether they areheat-resistant materials [2–4]. Microfabrication by EMMinvolves maskless or through-mask material removal. Thebreakthrough achieved in maskless EMM is the utilizationof ultrashort voltage pulses [5] with which structures with afeature size down to 90 nm were achieved successfully [6].However, technical application is still hampered for itslimited machining capability [7]. In through-mask EMM, aphotoresist patterned metal workpiece is made an anode inan electrochemical cell such that the exposed metal surfaceis removed by high-rate anodic metal dissolution by passingan external current. Compared with maskless EMM,through-mask EMM offers higher throughput and smallerpattern fabrication ability due to its parallel process innature and no superfine electrode requirement. A number ofresearch efforts on shape evolution [8], undercut [9], andinsulation mask fabrication [10] in through-mask EMMhave been conducted; however, the problem of how thecurrent aggregate effects due to anode surface patterning,the stray current (the current passing through the outersurface of the electrode), the mask characteristics involvingthe pitch, aspect ratio, feature size, and density distributionof the insulating patterns on workpiece surface influencethe etched results profile is far from being completelyanswered.

In this paper, much attention has been devoted to thegeneral problem of the strong effects that substratepatterning can exert on the anodic dissolution process, andboth simulation and experiments are conducted. The shape

W. Li (*) :W. Quandai :H. Xiuqing :D. Yucheng : L. BinghengState Key Laboratory for Manufacturing Systems Engineering,Xi’an Jiaotong University,Xi’an 710049, People’s Republic of Chinae-mail: wangli@mailst.xjtu.edu.cn

Int J Adv Manuf Technol (2010) 51:155–162DOI 10.1007/s00170-010-2610-x

evolution procedure involving the island formation anddisappearing gradually in the center of the etched area isobserved clearly in experiments, which is just in agreementwith the simulation result prediction. The aim of the workwas to predict and control the etched shape dimension andsubsequently obtain prescribed textured metal surface. Withthe optimized process, the ordered microstructures with afeature size down to 15 μm and smooth etched surface inlarge scale are obtained.

2 Model and numerical method

2.1 Model development

In through-mask EMM, the metal dissolution takes place atthe electrode surface that lies at the bottom of a cavitycreated by the photoresist mask. The current distribution atthe electrode is thus dictated by the photoresist artworkparameters. Therefore, in the calculation model, theemphasis is made on the mask parameters. Figure 1 showsthe scheme of simplified through-mask EMM process usedfor numerical calculation in which h is the thickness of thephotoresist mask, d is the etched depth, and wr and wm arethe width of the covered and exposed electrode surfaces,respectively. The following parameters that characterize theshape evolution during through-mask EMM is defined: (1)aspect ratio photoresist mask pattern ras ¼ h=wm; (2)spacing and opening ratio rso ¼ wr=wm; and (3) aspectratio of the etched feature ret ¼ d=wm.

In the analysis presented here, the reaction rate distribu-tion is assumed to be governed by an electric potential. Theassumptions are made as follows: (1) The current densitydistribution at the anode is determined solely by the Ohmiceffects and the electrodes are defined as equipotentialsurfaces. Thus, a primary current distribution is assumed.This assumption is justified as the EMM is at relativelyhigh current densities involving low Wagner numbers [8].(2) The concentration gradient in the bulk electrolyte isnegligible due to the ultrasonic agitation during the wholeprocess. (3) The conductivity of electrolyte, k, is uniform.(4) The current efficient is constant during the EMMprocess.

According to the electric field theory, the electricpotential distribution in electrolyte is governed by Laplace’sequation:

r2f ¼ 0 ð1Þ

where Φ is the electric potential in electrolyte flow field.Boundary conditions are as follows:

f ¼ Va at the anodeð Þ ð2Þ

f ¼ 0 ðat the cathodeÞ ð3Þ

where the Va is the voltage between the anode and thecathode (the kinetic resistances at the electrodes is neglected).

@f@n

¼ 0 ðalong the insulators and symmetry linesÞ ð4Þ

where n is the surface normal.The current density, I, is given by Ohm’s law:

I ¼ �krf: ð5Þ

The rate, r, at which the anodic surface recedes isdetermined by Faraday’s law:

r ¼ MhmFr

I ð6Þ

where M is the molecular weight of the metal used asanode, m is the metal dissolution valence, ρ is the density ofthe anodic metal, F is Faraday’s constant (F=96,485 C/mol),and η is the current efficiency of the metal dissolution, whichis assumed to be constant at 100%.

2.2 Method of solution

Finite element method (FEM) which is suitable forproblems involving irregular geometries is used as thenumerical technique for solution of the shape evolutionproblem. The commercial FEM software ANSYS isemployed in which planar elements used in electric fieldanalysis include plane67, plane223, and plane 230. Besidesthe element type, element size and computational timecycle also affect the accuracy of the computed results [11,12]; hence, various parameters mentioned above areadopted and the results compared with each other, whichshow that there is little variance in computed results withdifferent element types while the element size and timeinterval present appreciable variance in values of thecomputed results. However, as the mesh size and the time

Fig. 1 Schematic diagram of the simplified through-mask EMMprocess

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interval are reduced sufficiently small, the variancesbecome negligible. The operation conditions have beendetermined by trial and error as follows: Element plane67 isemployed and the computing domain is divided into aseries of small triangular elements; the time step is 0.1 s. Asthe gradient of calculated value in the area near patternedanode surface is comparatively large, hence the meshes inthe region have been refined locally. The typical meshingmodel near the patterned surface is as shown in Fig. 2.

The parameters of the materials are as follows: Theresistivity of the anodic metal copper is 1.7×10−8 Ω m, theresistivity of the cathodic metal aluminum is 2.9×10−8 Ω m,the resistivity of the photoresist is 1.0×108 Ω m, theresistivity of the electrolyte is 133 Ω m, and Va=2 V.

3 Experiments

3.1 Sample preparation

Mirror polished copper disc (30-mm diameter, 3-mmthickness) were used. All samples were rinsed with acetone(10 min) followed for 10 min with ethanol in an ultrasonicrinser and then copious rinsing with deionized (DI) water.After that, the samples were baked in an oven (100°C) for30 min to remove residual water and cooled and thenenclosed in dry airtight container ready for use.

3.2 Sample surface patterning

Prior to coating with photoresist, samples were dehydratedin an oven (70°C) for 10 min. After cooling, a thin layer(about 5 μm) of positive tone photoresist EPG 533(Everlight Chemical Industrial Co., Taiwan, ROC) wasspin-coated and baked (70°C) for 10 min, and then the UVlithography was performed with an exposurer (W-SP3648A, Samwo, China). The developing of the resistwas accomplished in 1 min by immersing the exposedsample into the developer (5 wt.‰ NaOH) and agitation.After rinsing in DI water and drying by N2 gas, the post-baking of the resist was carried out at 100°C for 30 min to

harden the etching mask, and then samples with firm resistpatterns on its surface were obtained.

3.3 Through-mask EMM process

The setup for the electrochemical etching used in theexperiments is illustrated in Fig. 3. The sample to be etchedserves as anode and is attached to an insulation holder. Analuminum plate is used as a counter electrode. A directcurrent voltage is applied between the anode and thecounter cathode. The applied voltage is 2 V for all etchingconditions. The current density is 10 A/dm2. Both electrodesare facing each other with a gap of 40 mm in the electrolyte(500 ml, 25°C), which is a neutral salt solution with thecompositions of 10 wt.% NaNO3+10 wt.% NaCl. Thesolution is stirred with an ultrasonic agitator duringelectrochemical etching, which facilitates the concentrationgradient alleviation and continuous renewal of the electrolyteat the critical dissolution areas.

4 Results and discussions

4.1 Numerical simulation

Since the gap between electrodes is much more larger thanthe height of patterns on substrate and additionally the

Fig. 2 Typical meshing model in FEM: model (a); discretized finite element (b)

Fig. 3 Sketch of the through-mask EMM setup

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current density is approximately uniform in the electrolytearea about 100 μm away from photoresist mask, therefore,to view the current distribution map near the patternedsurface in more detail, in the following subsections aboutsimulation, only the adjacent area of the anode in thepictures of calculated results is intercepted.

4.1.1 Effect of the aspect ratio of the photoresist mask pattern

Figure 4 shows the current distribution vector diagram withinthe electrolyte fluid field with several typical mask aspectratios. From the current distribution vector diagram, it can beseen that beyond a certain distance from the surface, the linesof current are evenly spaced and close to the electrodesurface; the lines of current must bend and “crowd” into thespaces between the photoresist islands. This current aggre-gation effect due to surface patterning results in non-uniformity of current distribution at the initial metal surface,and the current density at the mask edge is much larger thanthat at the feature center. When the aspect ratio is small, thecurrent distribution is highly non-uniform. For ras≥1, thecurrent distribution at the anode surface is fairly uniform.

The improvement in the current distribution uniformityunder the insulating mask with a higher aspect ratio isprobably attributed to the enhancement of the currentscattering effect of the mask edge step. Higher mask aspectratio facilitates even current distribution and subsequenteven etched results; however, too deep mold feature goesagainst mass transportation involving removal of reactionproducts and the electrolyte renewal at the critical region.Besides, when the feature size is large (up to hundreds ofmicrometers), achieving a high aspect ratio requires theapplication of a thick photoresist process. Therefore, theoptimum aspect ratio of mask is determined to be 1.

4.1.2 Effect of the spacing and open ratio of the patternson substrate

In applications involving through-mask EMM, the patternson substrate can have a wide range of spacing and open

ratios at different areas on the sample surface depending onthe prescribed functions, and hence, it is significant tounderstand the influence of variance of the spacing andopen ratio on etched results.

Figure 5 shows the model and current distribution ofworkpiece with various spacing and open ratios on itsdifferent surface regions. Theoretically, at patterned anodesurface, with the pattern density increase, the effectivecurrent will increase due to the current aggregation effect,and on the other hand, the active surface will decrease,

Fig. 4 Current density vector diagram at various mask aspect ratios: a ras=0.1; b ras=0.5; c ras=1.0 (unit of current density, A/dm2)

Fig. 5 Effect of variance of the spacing and opening ratio at electrodesurface on current distribution: a computational model; b currentdensity cloud image (unit of current density, A/dm2); c normalizedcurrent density

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resulting in the local ohmic resistance increase whichtends to reduce the current density. Simulation resultsshow that in general, regions that are more denselypopulated with resist tend to crowd in a higher currentdensity, which indicates that the aggregation effect plays amajor role in current space distribution at the patternedelectrode. From the curve of current density at theelectrode surface shown in Fig. 4c, it can been seen thatthe current density at the region with a high spacing andopen ratio is several times that at the region with a lowspacing and open ratio, which will result in a highlyuneven etched profile and unwanted performance varia-tions device to device. Since the pattern space distributionon the electrode cannot be changed artificially due to thefunction requirements, the dummy artwork process seemsto be a possible technique to solve this problem in whichby introducing a dummy artwork, the large feature isconverted into a series of small features, and the sharp

variance of the spacing and open ratio at different electroderegions tend to be quenched [14???]. For successful applica-tion, care should be taken that the dummy artwork dimensionsmatch the sizes of the etched features so that the dummyartwork can be removed upon completion of the metal etchprocess.

4.1.3 Shape evolution

The discussion so far has been limited to the currentdistribution at the initial electrode surface. In through-maskEMM, it is essential to consider the current distributionwithin the evolving cavity. Therefore, in this section, resultsare given and discussed for the current distribution atvarious stages of the evolving cavity.

To predict the shape of the evolving trench, the boundarieswere progressively updated. At each computational substep,the electrode surface was displaced proportionally to the

Fig. 6 Simulation results of shape evolution process: a current density cloud image (unit of current density, A/dm2); b normalized etching rate atvertical and lateral directions; c shape of the evolving cavity

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current density according to Faraday’s law (Eq. 6), and theetched profile was in turn used as the initial surface ofthe geometric model for current distribution computation inthe next substep. Figure 6 shows part of the simulationresults on the shape evolution at a patterned metal surface inwhich the rNx and rNy are the normalized etch rate in lateraland vertical directions, respectively.

The results show that the current distribution at the initialmetal surface is highly non-uniform and the maximum ofthe current density occurs at the intersection of the metalsurface and the mask. Since the metal removal rate is

proportional to the local current density, the maximumvertical displacement of the metal surface is away from thecenter of the feature. As the etching proceeds, theredistribution of the current density within the evolvingcavity makes the maximum in the current density shiftsaway from the edge of the feature toward its center. Fromthe results, it can be deduced that the island formation at thecenter of the feature will occur if the aspect ratio of etchedfeature, ret, is small. However, if the etched depth is large,namely, the ret is sufficient enough, the problem of islandformation will be solved.

Fig. 7 Experimental observation results of shape evolution process; the etching time is 60, 80, and 100 min in a–c, respectively

Fig. 8 Comparison of the SEMpicture and cross-sectioncontour (scanned with a profiler)of etched metal surfacemorphology before (a, b) andafter (c, d) process optimization

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From etch rate curves at the electrode surface shown inFig. 6b, it can be seen that the etch rate in vertical directionis much larger than that in the lateral direction at the initialstage, and as the metal removal progressed, the etch rate inthe two directions are approaching each other gradually.This behavior is important as it leads to a certain degree ofanisotropy during EMM process and thus improves the etchfactor, indicating that the EMM process provides advantageover wet chemical etching process in terms of machiningdirectionality.

4.2 Experimental results

4.2.1 Simulation results verification experimentally

To validate the simulation results, the shape evolutionprocess is observed experimentally in which the samplewith a cross lithographic pattern with a width of 200 μm, a

length of 600 μm, and a thickness of 5 μm on coppersubstrate was etched and the etched results at variousetching stages were observed. Figure 7 gives several SEMmicrographs of the etched results.

The experimental results show that the island has beenformed at the center of the feature and has shrunk graduallyuntil it has disappeared completely in the end, indicatingthat the current has mostly aggregated at the edge of themask at the initial dissolution stage, resulting in the currentdensity at the center of the feature being not up to thecritical value for electrochemical reaction and the maximumin the current density within the evolving cavity movestoward its center as etching proceeds, which is qualitativelyin good agreement with the simulation results mentioned inSection 4.1.3, and to some extent, the validity of thesimulation conclusions is verified.

4.2.2 Etching conditions and electrolyte optimization

With a combination of simulation conclusions and experi-ments, the electrochemical conditions have been optimizedto improve the etched surface roughness and etcheduniformity. Proceeding from the base conditions, we varyone parameter at a time to learn how the system behaves,which yields the optimal electrolyte recipe as mentioned inSection 3.3. With each combination of etching conditions,the morphology of the etched result is observed throughimage observation system on electron beam lithographysystem (CABL-9000C, Crestic Co., Japan) and profiler(XP-2, AMBios Technology, USA) to investigate theetched results quality. The typical morphology of theetched surface and the cross-sectional view of the etcheddepth on copper substrates are as shown in Fig. 8. It can beseen that since the width of the pattern is not so large likethat in Section 4.2.1, the island in the middle of the featurehas almost disappeared, which agrees with the simulationresults in Section 4.1.3. The surface roughness and

Fig. 9 Etched results with the optimized process: line array on coppersubstrate, width of the line top is about 15 μm and the etch depth isabout 8 μm

Fig. 10 Fabrication results bytwo-step etching usingthrough-mask EMM: a rectanglearray; b square cavity array atbottom of grooves

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uniformity of the etched results have been improvedobviously after process optimization. However, since maskwith optimum aspect ratio (1 as proposed in Section 4.1.1)using the EPG 533 photoresist cannot be obtained(maximum EPG 533 photoresist thickness of 5 μm andfeature width of 30 μm, resulting in an aspect ratio of lessthan 0.2), the uniformity of the etched surface is not quitesatisfied. If more even morphology is expected, the thickphotoresist process is required. With the optimized process,the lines with even space in large scale are obtained asshown in Fig. 9.

4.2.3 Two-level structure machining

In this work, the application of EMM to the fabrication oftwo-level structures has been explored in which twoapproaches are adopted: (a) patterning and etching groovefollowed by another patterning process with the sample laidin the orthogonal direction relative to the placementdirection of the sample while patterning for the first timeand etching groove by which the rectangle patterns arefabricated conveniently in large scale as shown in Fig. 10awhile the datum for lithographic mask fabrication isreduced significantly and thus very cost-saving comparedwith that for rectangle array fabrication in single stepetching, (b) etching of large groove followed by theapplication of a resist and etching of square cavity at thegroove bottom etched previously, resulting in 3D structures.

In approach (a), by varying the placement direction ofthe sample while patterning for the second time, diamondpatterns with various angles will be obtained, which can beapplied in the investigation of the friction drag reductioneffect of surface texture with various geometrical patterns.Approach (b) is of interest for 3D micro-mold fabrication.Both approaches involve aligning between individuallayers. Alignment is carried out on an alignment systembased on computer micro-vision with an overlay accuracyof 1.5 μm developed for multilevel imprint lithography(detail on the alignment system is referred to the previouspublished paper [13]).

5 Conclusions

Through-mask EMM is a promising technique for surfacetexturing. To alleviate the influence of current aggregateeffect on the etched results uniformity, it is necessary tounderstand the strong effects that substrate patterning canexert on the anodic dissolution process. Therefore, in thispaper, the influence of electrode surface patterning on shapeevolution in through-mask EMM process was investigated

numerically and experimentally. Through finite elementsimulation, the etching behavioral trends under variousprocess parameters have been identified and the anodicdissolution procedure is predicted, which is in goodagreement with the experimental observation in which theisland is formed and disappeared gradually along withetching progress in the center of the etched area. Etchingconditions and electrolyte were optimized through acombination of experiments and simulation. With thedeveloped process, the ordered microstructures with afeature size down to 15 μm and smooth etched surface inlarge scale were obtained.

Acknowledgements The authors would like to thank the Xi’anApplied Materials Innovation Fund (No. XA-AM-200804) for fundingthis research.

References

1. Datta M (1998) Microfabrication by electrochemical metalremoval. IBM J Res Develop 42(5):655–669

2. Landolt D, Chauvy PF, Zinger O (2003) Electrochemical micro-machining, polishing and surface structuring of metals: fundamentalaspects and new developments. Electrochim Acta 48(20–22):3185–3201

3. Munda J, Bhattacharyya B (2008) Investigation into electrochemicalmicromachining (EMM) through response surface methodologybased approach. Int J Adv Manuf Technol 35(7–8):821–832

4. Kern P, Michler J (2007) New development in through-maskelectrochemical micromachining of titanium. J MicromechMicroeng17(6):1168–1177

5. Schuster R, Kirchner V, Allongue P, Ertl G (2000) Electrochemicalmicromachining. Science 289(5476):98–101

6. Trimmer AL, Hudson JL (2003) Single-step electrochemicalmachining of complex nanostructures with ultrashort voltagepulses. Appl Phys Lett 82(19):3327–3329

7. Schuster R (2007) Electrochemical microstructuring with shortvoltage pulses. Chem Phys Chem 8:98–101

8. Sshenoy RV, Datta M (1996) Effect of mask wall angle on shapeevolution during through-mask electrochemical micromachining.J Electrochem Soc 143(2):544–549

9. Cosse JT, López GP, Atanassov P, Bauer TM, Chaudhury ZA,Schwappach CD, Mosley LE, Brevnov DA (2007) Minimizationof undercutting in electrochemical micromachining of patternedaluminum–copper films. J Micromech Microeng 17(1):89–97

10. Zhu D, Qu NS, Li HS, Zeng YB, Li DL, Qian SQ (2009)Electrochemical micromachining of microstructures of micro holeand dimple array. CIRP Ann-Manuf Techn 58(1):177–180

11. Jain VK, Pandey PC (1981) Tooling design for ECM—a finiteelement approach. J Eng Ind 103(5):183–191

12. Jain VK, Kanetkar Y, Lal GK (2005) Stray current attack andstagnation zones in electrochemical drilling. Int J Adv ManufTechnol 26:527–536

13. Wang QD, Duan YG, Ding YC, Lu BH, Xiang JW, Yang LF(2009) Investigation on LIGA-like process based on multilevelimprint lithography. Microelectron J 40(1):149–155

162 Int J Adv Manuf Technol (2010) 51:155–162