ORIGINAL ARTICLE

Fabrication of PDMS micro through-holesfor electrochemical micromachining

Ningsong Qu & Xiaolei Chen & Hansong Li & Di Zhu

Received: 18 July 2013 /Accepted: 6 February 2014 /Published online: 19 February 2014# Springer-Verlag London 2014

Abstract Surface texture has played a fundamental role in thedevelopment of many advanced fields. Through mask elec-trochemical micromachining (TMEMM) is a feasible alterna-tive for fabricating surface textures. In this paper, polydimeth-ylsiloxane (PDMS) is firstly employed as a mask in TMEMMdue to its chemical resistance, low cost, flexibility and highmoulding capability. A simple method for fabricating PDMSmicro through-holes is proposed. A vacuum-aided processwas introduced to fill a PDMS gel into an SU-8 mould, andthe PDMS gel was solidified in an oven. Then, the curedPDMS micro through-holes were peeled off of the SU-8mould. PDMS micro holes with a minimum diameter of50μmand a thickness of 200μmwere obtained. Furthermore,the PDMS micro through-holes were then used as a mask toprepare a micro dimple array by TMEMM. Experiments wereconducted to verify the feasibility of the proposed approach,and the effect of applied voltage and machining time on thediameter and depth of the micro dimple was investigated.Finally, an array of micro dimples 109 μm in diameter and9.7 μm deep was successfully fabricated by using PDMS.

Keywords Surface texture . Electrochemicalmicromachining . PDMS .Micro through-holes .Microdimple array

1 Introduction

Surface phenomena, particularly at the micro and nanometerscales, have played a fundamental role in the development ofmany advanced fields, such as energy, machining, optics,tribology, biomedicine etc. [1]. A large number of studieshave focused on specifically designing surfaces to provide aparticular function. Silk et al. [2] investigated the impact ofthree structured surfaces (cubic pin fins, pyramids and straightfins) on spray cooling and found that each of these surfacesimproved the evaporation efficiency compared to a flat sur-face. Such structured surfaces on a cutting tool could decreasethe contact length and reduce rake face friction, alter the chipcompression factor and normal forces and hence increase thetool’s lifetime. Hintze et al. [3] developed structured surfaceson a cutting tool for difficult-to-machine materials, such astitanium alloys and low-sulphur steels. Structured surfacesalso play a significant role in optics. Numerous applicationshave been developed, ranging from solar collectors to self-adhesive wide-angle rearview lenses for automotive applica-tions [4]. Careful construction of the surface structure can beused to control friction. Wakuda et al. [5] compared a lappedsmooth surface to a structured surface with a dimple size ofapproximately 100 μm at a density of 5–20 % and found thatthe dimple surface reduced the friction coefficient from 0.12 to0.10. Fujisawa et al. [6] reported that structured polymericsurfaces produced more rapid healing than non-structuredsurfaces in animal tests, which have the potential to causethromboembolic complications in blood-contact applications.

A number of techniques are available for fabricating microdimple patterns, such as milling, electrical discharge machin-ing, chemical etching, laser beam machining, abrasive jetmachining and electrochemical machining (ECM). ECM is aprocess used to selectively remove materials by an electro-chemical reaction at the anode workpiece in an electrolyticcell with an appropriate combination of machining parameters

N. Qu (*) :X. Chen :H. Li :D. ZhuJiangsu Key Laboratory of Accuracy and Micro-ManufacturingTechnology, Nanjing University of Aeronautics and Astronautics,Nanjing 210016, People’s Republic of Chinae-mail: nsqu@nuaa.edu.cn

X. Chene-mail: chxl8687@126.com

H. Lie-mail: hsli@nuaa.edu.cn

D. Zhue-mail: dzhu@nuaa.edu.cn

Int J Adv Manuf Technol (2014) 72:487–494DOI 10.1007/s00170-014-5702-1

[7]. Compared with other methods, ECM is a promisingmachining technique, with advantages such as high machin-ing efficiency, independence of material hardness and tough-ness, the absence of a heat-affected layer, a lack of residualstresses, cracks, tool wear and burrs and low production cost[8–11]. Through mask electrochemical micromachining(TMEMM) is a common method used to generate microdimple arrays with controlled size, location and density. Theconventional TMEMM process involves bonding a sheet ofinert photoresist on the metal anode workpiece. Lithography,which includes the procedures of spin coating, prebaking,exposure, development and post baking, is employed to patternthe photoresist. With this method, Madore et al. [12] fabricatedarrays of 30-μm-diameter hemispherical cavities on titanium.Wang et al. [13] fabricated three-dimensional cylindrical mi-crostructures with feature sizes as small as 40 μm. Londoltet al. [14] presented a new development in the TMEMM oftitanium using a laser-patterned oxide film. The film patterningwas achieved by local irradiation using a long-pulse XeCleximer laser. Electrochemical dissolution at irradiated lineson the oxide film yielded well-defined grooves, comparableto those resulting from electrochemical micromachiningthrough a photoresist mask. However, the photoresist andoxide film are only one-time masks andmust be removed from

the anode workpiece after machining. Therefore, mass produc-tion would be expensive for this method. Several studies havebeen conducted to overcome the defects of conventionalTMEMM and to improve machining efficiency. Zhu et al.[15] developed a modified TMEMM technique to preparemicro dimple arrays in which a sheet of insulation layer, coatedwith conductive metal layer and perforated with through-holes, was used as a mask to electrochemically etch micro-structures; the mask was closely attached to the workpiecesurface rather than being bonded to the workpiece. Comparedto conventional TMEMM, the modified process offers uniqueadvantages, such as a short lead time and low cost, because themask can be reused. Roy et al. [16] presented a concept ofmaskless electrochemical micro fabrication, in which the an-ode is placed in an electrochemical reactor in close proximityto the cathode with a resist micro pattern. Although the anodeis etched over its entire surface, it is etched at a higher rate inthe areas opposing the exposed regions of the cathode, hencereproducing a pattern on the anode.

PDMS, because of its chemical resistance, low cost,flexibility and high moulding capability, is widely usedin many areas, especially in lab-on-a-chip research. Thiswork focuses on the fabrication of PDMS microthrough-holes.

(a) The substrate1 with SU-8 micro-pillar array (b) SU-8 mould aligned on substrate2

(c) The PDMS gel fills the channel (d) The cured PDMS removed from the mould

Fig. 1 Fabrication process of theSU-8 mould with a micro pillararray. a The substrate 1 with SU-8micro pillar array. b SU-8 mouldaligned on substrate 2. cThePDMSgel fills the channel. d The curedPDMS removed from the mould

(a) SU-8 micro mould (b) PDMS micro through-holes layer

Fig. 2 SU-8 moulds andcorresponding PDMS layer. aSU-8 micro mould. b PDMSmicro through-holes layer

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Several attempts have been made to fabricate PDMSmicrothrough-hole layers. Park et al. [17] developed a simple meth-od for fabricating a PDMS layer with through-holes by spincoating a PDMS gel at a height smaller than that of a patternedSU-8 mould. However, the undesired thin PDMS films cov-ering the opening of through-holes are generally cured on topof the SU-8 mould, making the spin-coating method ineffi-cient. To complete the opening of the micro through-holes,additional labour or expensive equipment is required to re-move the undesired PDMS films, such as manual air blowing,plasma etching, reactive ion etching or wet etching. Yang et al.[18] presented a fabrication process for patterning microthrough-holes in a PDMS layer by combining a method ofcapillary micro moulding and a surface treatment ofatmospheric-pressure CH4/He RF plasma. In this method,the PDMS master mould for the micro through-holes was castfrom a corresponding SU-8 mould, and the surfaces of boththe PDMS master mould and a glass slide as a substrate weretreated by atmospheric-pressure CH4/He RF plasma to pre-vent the PDMS gel from sticking as it cured. Then, theprotruded patterns of the PDMS mould adhered closely tothe glass slide, and the empty space between the PDMSmould

and the glass slide was filled with PDMS gel by capillaryaction. When the PDMS cured, the PDMS through-hole layerwas smoothly peeled off from the PDMS mould and the glassslide. However, this process is complex because two mouldsare required and the PDMS master mould must be treated byatmospheric-pressure CH4/He RF plasma.

In the present manuscript, a simple process for fabricating aPDMS layer with micro through-holes is proposed. In thismethod, the PDMS gel is rapidly filled into the channel be-tween the SU-8 mould and the substrate using a vacuum-aidedprocess (VAP). After the PDMS is cured, the PDMS layer canbe smoothly peeled off from the SU-8 mould and the substrate.

2 Materials and fabrication procedures

2.1 Materials and equipment

An SU-8 2050 negative photoresist and a propylene-glycol-methyl-lether-acetate (PGMEA) developer (MicroChemCorp., MA, USA) were used to fabricate the SU-8 mould.PDMS gel (Sylgard 170, Dow Corning Corp., USA) was usedto replicate the patterns of the mould. Polished 800-μm-thickstainless steel (1Cr18Ni9Ti) substrates were cut into 2-in.-diameter discs to fit the photomasks. The SU-8 mould, PDMSthrough-holes and micro dimples were scanned by a scanningelectron microscope (S3400N, Hitachi, Japan). The profile ofthe micro dimples was examined using a three-dimensionalprofilometer (DVM5000, Leica, Germany).

2.2 Fabrication of PDMS micro through-holes

In this study, the PDMS micro through-holes are constructedin two fabrication steps: lithography and moulding. Figure 1

Fig. 3 TMEMM schematic

PDMS Workpiece

Fig. 4 PDMS mask on top of the workpiece

Table 1 Machining parameters

Electrolyte (wt %) 10 % NaNO3+10 % NaCl

Electrolyte temperature (°C) 30

Workpiece material Stainless 304

Tank pressure (MPa) 0.1

Interelectrode gap (mm) 2

Applied voltage (V) 6,8,10,12

Machining time (s) 4, 6, 8, 10

Thickness/hole diameter of mask (μm) 200/50, 200/100

Table 2 Sylgard 170 silicone elastomer curing schedules

Curing temperature (°C) 70 100 150

Curing time(min) 30 11 7

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presents a schematic view of the fabrication process fora PDMS layer with micro through-holes. Firstly, thesubstrate 1 with the micro pillar array is fabricated withSU-8 photoresist by coating, softbake, exposure anddevelopment, as shown in Fig. 2a. Secondly, substrate1 with the micro pillar array is aligned with the flatsubstrate 2 and the mould for the micro pillar arraychannel is finished. Thirdly, the mould is filled withPDMS gel by vacuum-aided process, and then thePDMS is solidified. At last, the cured PDMS through-hole layer is smoothly peeled off of the SU-8 mouldand the flat substrate 2. The fabricated PDMS layerwith micro through-holes is shown in Fig. 2b.

2.3 Fabrication of micro dimple array

The PDMS layer with micro through-holes was subject-ed to TMEMM to fabricate micro dimples, as illustratedin Fig. 3. The PDMS layer with micro through-holeswas placed directly on the anode workpiece to act asthe mask, as shown in Fig. 4. The electrolyte wasflowed onto the surface of the PDMS layer to fill in

the holes of the layer. The anode regions that areexposed to the electrolyte dissolve when a sufficientvoltage is applied. The machining parameters are listedin Table 1. A major advantage of this method is that thePDMS mask, as a polymer material, can be reusedbecause it is not damaged in the TMEMM process.

3 Results and discussion

3.1 The influence of curing temperature on shrinkage ratio

The PDMS curing temperatures and corresponding timesare shown in Table 2. As shown in Table 2, when thetemperature rises, the curing time sharply decreases.Furthermore, the curing temperature impacts the PDMSshrinkage, which will affect the shrinkage ratio of themicro through-holes. In this paper, the two-dimensionalshrinkage ratio of the PDMS micro through-holes isinvestigated for different curing temperatures and layerthicknesses. Each specimen was measured at 20 sam-pling points, and the shrinkage ratio was calculated asfollows:

Shrinkage ratio %ð Þ ¼

X

i¼1

n

Di −X

i¼1

n

D;i

�����

�����X

i¼1

n

Di

� 100

D Diameter of the SU-8 pillarsD’ Diameter of the PDMS micro through-holesn Number of sampling points

The results are shown in Table 3 for varying temperaturesand PDMS thicknesses.

As shown in Fig. 5, the shrinkage ratio increases rapidlywith increasing curing temperature, and the maximum shrink-age ratio was observed to be 7.1 % at 150 °C. For the same

Table 3 Average diameter of PDMS holes under different PDMS curing conditions

Test number Thickness of PDMS (μm) Temperature (°C) Time (min) Average diameterof SU-8 pillars (μm)

Average diameterof PDMS holes (μm)

1 200 70 30 101.5 100

2 200 100 11 101.5 98.5

3 200 150 7 101.5 97.5

4 300 70 30 102.3 99.5

5 300 100 11 102.3 97.2

6 300 150 7 102.3 95

Fig. 5 The 2D shrinkage ratio with different temperatures andthicknesses

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temperature, the shrinkage ratio is higher for a thicknessof 300 μm in comparison to a thickness of 200 μm.When PDMS is cured at a high temperature, the PDMSmonomers are cross-linked and the total volume is re-duced [19], which would reduce the diameter of thePDMS micro through-holes. In addition to the increasedshrinkage ratio, some blind holes occur when the curedPDMS micro through-hole layer is peeled off the SU-8mould due to residual SU-8 pillars in the PDMS layers.According to our experimental results, the optimal cur-ing temperature is 70 °C.

3.2 The effect of process parameters on micro dimples

3.2.1 Effect of machining parameters on micro dimpledimensions

In this experiment, both applied voltage and machiningtime were varied to investigate their effects on microdimple formation. Factor experiments were performed,as presented in Table 1.

Figure 6 shows the effect of the increasing voltageand machining time on the diameter and depth of themicro dimple. As shown in Fig. 6a, with the voltage of6 V, the diameter and depth of the micro dimple

increased, and the minimum size was 109 μm in diam-eter and 9.7 μm deep. Figure 6b shows the change inthe voltage of 8 V, which had the similar tendency withFig. 6a. While the minimum size increased to 117 μmin diameter and 13 μm deep, in Fig. 6c, d, the increas-ing tendency was weaken. When the size reached to130 μm in diameter and 20 μm deep, the increasingsize became slower. It can be explained that with theincreasing diameter and depth, the removal of the elec-trochemical products became difficult which blocked theelectrochemical etching and the machining processslowed down.

In Fig. 6, it can be found that the position of thecurve in the coordinate rises with the increasing voltage.It meant that with the same machining time, a lowvoltage can be got the micro dimple with a smallerdiameter and depth. Therefore, when the other parame-ters were fixed, the applied voltage should be controlledstrictly to weaken the undercut of the micro dimpleduring electrochemical machining.

The experimental results indicate that, with differentmachining parameters, the micro dimples array withvarious diameter and depth can be effectively machinedon the metal workpiece. Figure 7a shows the array ofmicro dimples 109 μm in diameter and 9.7 μm deep

(a) Different machining times with 6 V (b) Different machining times with 8 V

(c) Different machining times with 10 V (d) Different machining times with 12 VFig. 6 The effect of different parameters on the diameter and depth of the dimple. a Different machining times with 6 V. b Different machining timeswith 8 V. c Different machining times with 10 V. d Different machining times with 12 V

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and profile of a single dimple generated with the PDMSmask of 100 μm in diameter. Moreover, with this meth-od, an array of micro dimples 60 μm in diameter and3 μm deep was successfully fabricated with the PDMSmask of 50 μm in diameter, as shown in Fig. 7b.

3.2.2 Effect of the PDMS mask on the profile of the microdimple

As shown in Fig. 7, the bottom of the micro dimplegenerated with PDMS is flat and the taper of wall issmall. Compared with the conventional TMEMM, thePDMS mask is a high-aspect-ratio structure. To studythe effect of the PDMS mask on the profile of themicro dimple on the workpiece, a mathematical model

is set up, as shown in Fig. 8, where H is the thick-ness of the mask, D is the diameter of the exposedanode surface and G is the distance between thecathode and anode.

The electric potential φ in the interelectrode gap can beapproximately described by Laplace’s equation:

∇2φ ¼ 0 ð1Þ

The corresponding boundary conditions are as follows:

φ Γ 1j ¼ UR at theanodesurfaceð Þ ð2Þ

φ Γ 5j ¼ 0 at thecathodesurfaceð Þ ð3Þ

(a) Micro dimples fabricated with the PDMS mask of 100 µm in diameter

(b) Micro dimples fabricated with the PDMS mask of 50 µm in diameter

Fig. 7 SEM image of the micro dimple array and profile of a single dimple. aMicro dimples fabricated with the PDMS mask of 100 μm in diameter. bMicro dimples fabricated with the PDMS mask of 50 μm in diameter

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∂φ∂n

Γ 2;3;7;8

�� ¼ 0 insulationboundariesð Þ ð4Þ

∂φ∂n

Γ 4;6

�� ≈0 virtualboundariesð Þ ð5Þ

The relationship between the current density i and theelectric potential is

i ¼ κ∇φ ð6Þ

where k is the conductivity of the electrolyte. FEM wasemployed to solve the boundary problem, and the currentdensity over the workpiece surface exposed to the electrolytewas calculated using COMSOL Multiphysics under the fol-lowing conditions: UR=6 V, σ=15 S/m, G=2 mm, D=100 μm. The normalized current densities, defined as i/imax

(i is the current density of every key point on the surface,where imax is the maximum current density), for differentthicknesses of the mask are shown in Fig. 9. The minimumcurrent density exists at the centre of the circle and increasesrapidly from the centre to the edge when the thickness of themask is 5 μm. Moreover, the current density distributionbecomes increasingly uniform with the increasing thickness

of the mask. When the thickness reaches 200 μm, the currentdensity distribution becomes uniform over the hole.

According to Faraday's law, the metal removal rate v can beexpressed as follows:

v ¼ ηωi ð7Þ

where η is the current efficiency and ω is the volumetricelectrochemical equivalent of the material.

As η and ω are constant, according to Eq. (7), it can be gotthat the metal removal rate on the workpiece is uniform whichleads to the micro dimple with a flat bottom when the PDMSmicro through-holes were used as a mask to prepare a microdimple array by TMEMM.

4 Conclusions

A simple fabrication process for patterning microthrough-holes in a PDMS layer has been demonstrated.Based on the experimental investigations, conclusionscan be summarized as follows:

1. PDMS micro through-holes can be well fabricated with aminimum hole diameter of 50 μm at thicknesses of200 μm.

2. The optimal curing temperature is 70 °C to reduce theinfluence of PDMS shrinkage ratio on the quality of themicro through-holes.

3. The machining parameters are studied. The result showsthat a low voltage can be used to get the micro dimplewith smaller diameter. With the mask of 50 and 100 μm indiameter, the micro dimple arrays of 59 and 109 μm indiameter were successfully fabricated.

4. The PDMSmask is reusable because no damage is causedto the mask during TMEMM due to the PDMS’s goodchemical resistance.

Acknowledgments The work described in this study was supported bythe Joint Funds of the National Natural Science Foundation of China andGuangdong Province (Grant No. U1134003).

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