surface-tension-driven self-alignment of microchips on black-silicon-based hybrid template in...

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 22, NO. 3, JUNE 2013 739

Surface-Tension-Driven Self-Alignment ofMicrochips on Black-Silicon-Based

Hybrid Template in Ambient AirAli Shah, Bo Chang, Sami Suihkonen, Quan Zhou, and Harri Lipsanen

Abstract—In this paper, we demonstrate self-alignment of mi-crochips on a simple-to-fabricate hybrid template with both wa-ter and UV-curing adhesive (EPO-TEK UVO-114). The hybridtemplate contains receptor sites with solid edges for droplet con-finement and large wetting contrast between the receptor sitesand the substrate for microchip manipulation. Nanostructuredblack silicon surface functionalized with fluoropolymer is used asa substrate, while protruded silicon dioxide patterns covered withfluoropolymer serve as receptor sites. A simple and fast processconsisting only of one pass of photolithography, cryogenic deepreactive-ion etching (RIE), and RIE steps is used to fabricate thehybrid template. The self-assembly tests are carried out in a hy-brid microassembly setup. Dummy microchips of sizes 200 μm ×200 μm × 50 μm are self-aligned on 200 μm × 200 μm recep-tor sites in ambient air with both water and adhesive. [2012-0178]

Index Terms—Black silicon, fluoropolymer, hybrid template,self-alignment, self-assembly, solid edge, wetting contrast.

I. INTRODUCTION

S ELF-ASSEMBLY is a highly promising technology foradvanced packaging applications, which brings together

the benefits of high throughput, high precision, and efficientparallel assembly. Self-alignment is a key process step inself-assembly to achieve the desired precise positioning ofmicrochips. Different forces can be utilized to achieve theattraction and alignment of microchips during self-alignment,i.e., gravitational forces [1], magnetic forces [2], [3], electro-static forces [4], and surface-tension forces [5]–[17]. Surface-tension forces have a favorable scaling law where the forcedecreases linearly with size and are ultimately dominant atmicroscale compared to the other forces [18]. Surface-tension-

Manuscript received June 26, 2012; revised January 10, 2013; acceptedJanuary 19, 2013. Date of publication February 21, 2013; date of current ver-sion May 29, 2013. This work was supported in part by the Academy of FinlandProgrammable and Spatially Multi-scale Self-assembly of Microcomponents(MUSA) (2010–2013) under Grant 134206 and in part by Tekniikan Edis-tämissäätiö under a personal grant for technology promotion. Subject EditorK. F. Bohringer.

A. Shah, S. Suihkonen, and H. Lipsanen are with the Department ofMicro- and Nanosciences, Aalto University, 00076 Aalto, Finland (e-mail:[email protected]; [email protected]; [email protected]).

B. Chang and Q. Zhou are with the Department of Automationand System Technology, Aalto University, 00076 Aalto, Finland (e-mail:[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2013.2243109

driven self-alignment is a promising self-assembly techniqueand allows massively parallel handling and self-alignment ofmicrocomponents [5], [19], [20].

Droplet confinement on receptor sites is a key process insurface-tension-driven self-alignment [7]. In surface-tension-driven self-alignment, the microchips are delivered to thematching receptor sites with confined droplets. Upon contact ofthe microchip with the droplet, a meniscus is formed betweenthe microchip and the receptor site. The meniscus will adjustits surface area to minimize the total system energy, which isusually reached when the microchip is perfectly self-aligned tothe receptor site.

Droplet confinement to receptor sites is commonly achievedby surface functionalization (hydrophobic/hydrophilic interac-tions) [5], [8], [9], [21], [22] or with a solid edge [10], [11],[23]. In the case of hydrophobic/hydrophilic interactions, wet-ting contrast between the substrate and the receptor sites con-fines the droplet on the receptor site. The nature of the receptorsites based on hydrophobic/hydrophilic interactions is influ-enced by the medium in which the self-alignment is performed,and receptor sites are made to be hydrophilic in air [24] orhydrophobic in liquid [19], [25]. Self-assembly in air is oftenpreferred as it has several advantages: freedom from liquid flowduring alignment and relatively easy elimination of the liquid,particularly water, in comparison with methods on/in the liquid[8]. Moreover, parallelization for self-assembly of microchipsin air can be enhanced by using mist-induced surface-tension-driven self-alignment [20].

Self-alignment in air using water on hydrophilic receptorsites has been reported using patterned gold coated with self-assembled monolayers on both microchip and substrate [12],hydrophilic glass patterns surrounded by hydrophobic back-ground [8], and aluminum pattern on substrate coated withTeflon [13]. Feeding of the microchips can be done usingstochastic manner assisted by mechanical anchors [14] or us-ing deterministic robotic pick and place [7], [8]. In additionto hydrophilic receptor sites on hydrophobic substrate, self-alignment using adhesive in air has been reported on oleophilicreceptor sites on oleophobic substrate [15], where a topograph-ical microstructure of porous ormocer was functionalized witha fluorinated trichlorosilane for the oleophobic substrate, andgold patterns were used for the oleophilic receptor sites.

Using receptor sites with a solid edge, such as a pro-truded pattern, avoids the use of extra treatments, as no wet-ting contrast is needed for droplet confinement. The droplet

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740 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 22, NO. 3, JUNE 2013

confinement is due to geometrical and surface energy param-eters, which offer resistance to advancing droplet at sharpedges [23]. The fabrication of template becomes comparativelystraightforward and can easily be done by trenching, tapinga film, casting the structure, and other microfabrication tech-niques [10]. Self-alignment on such receptor sites has beenproven reliable [7]. Reliable solutions for bonding the mi-crochips onto the receptor sites after alignment have also beendemonstrated [9], [16], [26].

Both types of receptor sites, solid edge based andhydrophobic/hydrophilic interaction based, offer reliabledroplet confinement and ultimately lead to comparatively lowcost assembly alternative to conventional assembly. However,previous studies only addressed either solid edge orhydrophilic/hydrophobic interactions todrive theself-alignment.

In this paper, we present the fabrication of a hybrid tem-plate and its application in self-alignment. The hybrid templatecontains protruded receptor sites for droplet confinement andfluoropolymer-coated black silicon superhydrophobic substratefor microchip manipulation. Black silicon is a nanostructuredsilicon surface where light gets completely trapped inside thenanoscale cavities and the surface appears completely black.The fabrication is relatively simple, comparable to the pro-cess of making either protruded or hydrophilic/hydrophobicreceptor sites. The coated substrate surface was superhydropho-bic and therefore exhibited large wetting contrast with thehydrophobic receptor sites. Superhydrophobicity, in general,can be achieved using a wide range of techniques [27]. Ourprevious study [28] and other research efforts [29], [30] sug-gest that the low-surface-energy fluoropolymer-coated blacksilicon is a comparatively low cost and simple alternative toachieve superhydrophobicity on silicon.

On the hybrid template, surface-tension-driven self-alignment of microchips was carried out with both water andUV-curing adhesive (EPO-TEK UVO-114). Solid edge of thereceptor sites confined the water and UVO-114 droplet, andthe surface tension of droplets provided the required forcefor self-alignment of microchips. Reliable and high-accuracyself-alignment of 200 μm × 200 μm × 50 μm microchipson 200 μm × 200 μm receptor was observed in both cases.Additionally, the superhydrophobicity of the substrate enabledreliable dewetting of water and, thus, the manipulation (dragand release) of microchips between individual receptor sites.

The novelty of the proposed method lies in its simple andfast fabrication sequence for a hybrid template that facilitatesreliable self-alignment with both adhesives and water, andmicrochip manipulation.

II. FABRICATION AND TEST SETUP

The process steps for fabricating the receptor sites andthe black silicon substrate are shown in Fig. 1. A 100-mmsingle-side-polished silicon (100) wafer was used as a startingsubstrate. As shown in Fig. 1(a), plasma-enhanced chemicalvapor deposition was used to deposit a silicon dioxide hardmask. Standard photolithography with positive tone photoresist(AZ5214) was applied to pattern the resist on the top of thesilicon dioxide layer. Fig. 1(b) shows the silicon dioxide etching

Fig. 1. Process steps for fabricating the receptor sites and black siliconsubstrate.

Fig. 2. Hybrid microassembly setup.

using reactive-ion etching (RIE) with CHF3 plasma and argonions. The photoresist removal step was carried out by acetonein an ultrasound bath. At this stage, the silicon dioxide receptorsites are formed on the silicon substrate. In the next step shownin Fig. 1(c), black silicon was formed using the method in [31].An optimized cryogenic deep RIE (DRIE) recipe with induc-tively coupled plasma was used to fabricate the desired type ofblack silicon as a substrate. The relationship between processparameters and different black silicon types has been reportedearlier [28]. Fig. 1(d) shows the final step, the deposition of40-nm low-surface-energy fluoropolymer. The fluoropolymerwas deposited using plasma polymerization with pure CHF3 asa precursor in RIE. The black silicon and receptor site surfaceswere characterized by scanning electron microscopy (SEM).

The self-alignment tests were carried out using a hybridmicroassembly setup consisting of an assembly system (pickand place), a dispensing system, and an imaging system asshown in Fig. 2. The assembly system contained a custom-built microgripper mounted on a motorized linear stage (PhysikInstrumente M111.1DG) for vertical movement and a samplecarrier mounted on a short-range motorized linear stage (PhysikInstrumente M-122.2.1DD) for motion in x-axis and a long-range motorized linear stage (Physik Instrumente M-404.8PD)for motion in y-axis between assembly site and dispensing site.The dispensing system consisted of a water droplet dispenserand an adhesive droplet dispenser, where water dispenser wasmounted in 45◦ angle at assembly site and adhesive dispenser

SHAH et al.: SELF-ALIGNMENT OF MICROCHIPS ON HYBRID TEMPLATE IN AMBIENT AIR 741

Fig. 3. Self-alignment procedure for microchip self-assembly. (a) Liquiddroplet is dispensed on the receptor site. (b) Microchip is brought to the receptorsite. (c) Microchip is released with the initial bias. (d) Microchip is self-alignedto the receptor site. (e) Final bonding is created.

was mounted vertically at dispensing site. The water dropletdispenser was a noncontact dispenser (Gesim/PicPIP) actuatedby a piezoelectric diaphragm and could dispense droplets in adistance of a few millimeters. The dispenser for adhesive was acontact air-powered dispenser (EFD Mikros pen system), whichcould deposit as small as 0.18-mm droplet in diameter. Thedeviations of the amount of liquid were about 30 and 100 pLfor water and adhesive, respectively. The imaging system con-sisted of a top-view microscope (Edmund/VZM1000i) and aside-view microscope (Edmund/VZM1000i) mounted at theassembly site and a top-view microscope (Edmund/VZM300i)and an angled-view microscope (Edmund/VZM300i) mountedat the dispensing site. In this paper, we used water and low-surface-tension (32.6 mN/m) UV-curing adhesive (EPO-TEKUVO-114) droplets for self-alignment. The contact angles(CAs) were measured with a traditional sessile drop methodusing the hybrid microassembly setup.

Two types of self-alignment tests were performed withSU-8 microchips on receptor sites: 1) self-alignment usingwater and 2) self-alignment using adhesive. The sizes of re-ceptor sites and SU-8 microchips were 200 μm × 200 μm and200 μm × 200 μm × 50 μm, respectively. The same fabricationprocess was used for the SU-8 microchips as demonstratedin [7]. The typical self-alignment procedure consisted of ini-tial robotic positioning and self-alignment steps, shown as aschematic in Fig. 3.

First, a droplet of water or adhesive is dispensed at the centerof the receptor site [Fig. 3(a)]; next, an SU-8 microchip isbrought above a receptor site by a microgripper [Fig. 3(b)], andthen, the microchip is released with an initial bias [Fig. 3(c)],defined as the distance between the releasing position and thedesired alignment position in x-, y-, and z-axes. After that,the microchip is self-aligned with the receptor site driven by thesurface-tension force [Fig. 3(d)]. In the final step, the permanentfixing is achieved by curing the adhesive in UV [Fig. 3(e)].In the case of water-droplet-based self-alignment, the dropletevaporates, leaving the microchip aligned but not permanentlyfixed.

The amount of dispensed liquid was controlled by the dis-penser to be just sufficient to fully wet the receptor site. Thevolumes of water and adhesive needed for self-alignment wereestimated to be around 0.9 and 0.4 nL, respectively. The finalalignment accuracy was inspected with an optical microscope.The alignment accuracy was calculated by measuring the dif-ference between the geometry centers of the chip and the pat-tern after the self-alignment. An image-analysis-based methodwas implemented using Matlab for detection of the geometrycenters. The measurement of the alignment accuracy was lim-

Fig. 4. SEM images of a hybrid black silicon template. (a) Top-view imageshowing receptor sites and black silicon substrate with a fluoropolymer coating.(b) Side-view image showing the height of the sharp solid edge and blacksilicon needles.

ited by the resolution of our optical microscope, which wasabout 1 μm.

III. RESULTS, ANALYSIS, AND DISCUSSION

Fig. 4 shows top- and side-view SEM images of the fabri-cated template with a fluoropolymer coating.

In order to avoid charging in SEM, the surface was coveredwith a 10-nm layer of gold by e-beam evaporation. In Fig. 4(a),the receptor sites are seen as dark square patterns. The magni-fied view image “A” in Fig. 4(a) of a receptor site shows that thereceptor site is protruded and has a well-defined boundary linewith the substrate. The substrate surface consists of periodicnanoscale texture, which composes the surface roughness [see“B” view in Fig. 4(a)].

The side-view image of a fluoropolymer-coated receptorsite [Fig. 4(b)] shows the geometry of the sharp solid edgeand its height from the base of needles (< 4 μm). The solidedge height is a combination of etched silicon, silicon dioxidethickness on the protruded receptor site, and fluoropolymerthickness.

A. Wettability Characteristics

The wettability of a surface can be expressed in terms of CAof a droplet. Smaller CAs (< 90◦) correspond to higher wetting


Fig. 5. Advancing CA measurements of (a) water droplet on a receptor site(118◦), (b) adhesive droplet on a receptor site (55◦), (c) water droplet onsubstrate (179◦ ± 1◦), and (d) adhesive droplet on substrate (110◦).

(hydrophilic), whereas higher CAs (> 90◦) result in reducedwetting (hydrophobic). Surfaces with very high CA (> 150◦)are called superhydrophobic surfaces [27].

Advancing CA measurements with water and adhesive areshown in Fig. 5. We measured water CAs of 118◦ [Fig. 5(a)] onthe receptor sites and 179± 1◦ [Fig. 5(c)] on the substrate andadhesive CAs of 55◦ [Fig. 5(b)] on the receptor sites and 110◦

[Fig. 5(d)] on the substrate. This is shown in SEM and opticalmicroscope images in Fig. 5.

High CA or low wettability of plane and homogeneousreceptor sites [Fig. 5(a)] is attributed to the fluoropolymer coat-ing. The low-surface-energy fluoropolymer coating changes thechemical composition of the surface [32], which makes it hardfor the water droplet to spread on the surface, and therefore,the droplet exhibits high CA. With adhesive, the receptor sitesare more wettable [Fig. 5(b)]. Higher wettability with adhesiveis due to the lower surface tension of adhesive (32.6 mN/m)compared to water (72 mN/m).

The superhydrophobic fluoropolymer-coated black siliconsubstrate shows excellent rolling and dewetting characteristicsfor water droplets, demonstrated later with its application forchip manipulation. The reason is that the substrate surface isnonhomogeneous and contains periodic nanoscale black siliconneedles. With the introduction of such roughness, the surfacebecomes inhomogeneous, and the Young’s equation [33] cannotbe directly applied in this situation. In the case of inhomoge-neous surface, two other models are normally used to explainthe wetting behavior, i.e., Wenzel [34] or Cassie–Baxter [35].Based on these models, the droplet characteristics are shown inFig. 6. On an intrinsically hydrophilic surface, the introductionof roughness leads to increase in hydrophilicity [Fig. 6(a)],whereas on hydrophobic surface, the roughness increases hy-drophobicity, and the droplet usually assumes Cassie–Baxteror composite state [Fig. 6(b)] [29]. Therefore, for a superhy-drophobic nanostructured substrate, the Cassie model is moresuitable. In this case, the apparent CA (θCB) can be estimatedby [35]

cos θCB = f(cos θY + 1)− 1 (1)

Fig. 6. Water droplet behavior on rough surface. (a) Wenzel state(b) Cassie–Baxter state.

Fig. 7. CA and solid protrusion angle.

where θY is the Young’s CA on smooth surface, which is 118◦,and f is the fraction of the solid surface in contact with thewater droplet, which is very small for black silicon, i.e., 0.01%of total droplet footprint [30]. Therefore, based on (1), theapparent CA on the substrate should be 180◦, which is exactlywhat we observed in the experiments. For the adhesive, theYoung’s CA is much smaller (55◦) on smooth surface, whichleads to much larger f . Based on the observed apparent CA, weget f = 0.42 for the adhesive droplet on substrate.

B. Droplet Confinement

The receptor sites compose a protruded solid edge with thesubstrate. The droplets are dispensed on top of receptor sites,and then, the liquid front proceeds toward the edges of thereceptor site. The behavior of the advancing droplet on pro-truded edges can be described with the Gibb’s inequality [36]

θY < θ < (180− θP ) + θY (2)

where θ is the droplet CA on the edge, θY is the Young’s CA,and θP is the angle of two surfaces forming the protrusion(Fig. 7).

As shown schematically in Fig. 7, the CA θ may extendover a range of angles based on (2). Studies conducted in [23]assume that the upper limit of θ on the edge that a droplet canattain can be written as

θc = (180− θP ) + θY (3)

where θc is the critical angle. Once the droplet reaches θc,the possibility that the droplet will keep the contact line or itwill tumble along the protrusion is related with θP [23]. Atsmaller scales, where the effect of gravity is negligible, if θP ≤θY so that θc ≥ 180◦, the droplet volume can be increased


Fig. 8. Large droplet confinement on a receptor site with (a) 29 nL of waterand (b) 22 nL of adhesive.

indefinitely, and θ will increase until 180◦. The droplet will stayconfined to a point where the effect of gravity is appreciable,and in that case, it will cross the edge and roll over theprotrusion. On the other hand, if θP > θY so that θc < 180◦,increase in the droplet volume after the droplet reaches θc willcause the droplet to move over the edge or to sudden spreaddown the protrusion.

The protruded receptor sites in this work have a protrusionangle (θP ) of approximately 104◦. The advancing CA of watermeasured on the receptor site θY = 118◦ is greater than the pro-trusion angle (θP < θY ), and consequently, the critical angle isgreater than 180◦ (θc > 180◦), as demonstrated in Fig. 8. Thissituation dictates that an increase in the volume of water dropletwill keep the droplet confined inside the receptor site unlessgravity becomes appreciable.

On the other hand, with adhesive, θY = 55◦; we have θP >θY , and θc according to (3) is approximately 131◦ (less than180◦). In this case, an increase in the droplet volume after θcwill fail the droplet confinement, and the droplet will eithermove over the edge or slide along the protrusion.

The volume of water droplet used for self-alignment (0.9 nL)in this work (demonstrated later) and the size scale of operationare very small. Therefore, the possibility of entering a regimewhere gravity becomes appreciable is negligible, and reliableconfinement could be realized even at larger droplet volume.With adhesive, the droplet volume needed for self-alignmentwas even smaller (0.4 nL) than that with water. In order tocross θc and fail the alignment, the droplet volume should growenormously compared to the one needed for self-alignment.Therefore, a small volume needed for self-alignment negatesthe possibility of reaching θc and therefore corresponds toreliable confinement. Fig. 8 shows large droplet volumes ofwater [29 nL in Fig. 8(a)] and adhesive [22 nL in Fig. 8(b)]confined on a protruded receptor site. The volume of confineddroplets in Fig. 8 is over 20 times larger than what was used forself-assembly.

C. Self-Alignment With Water

Fig. 9 shows optical microscope images of the self-alignment process on the hybrid template with water droplet(0.9 nL). The process was recorded both from top [shownin Fig. 9 (a)–(c)] and from side [shown in Fig. 9(d)–(f)]. InFig. 9(a) and (d), the microchip is moving toward the receptorsite and is released in Fig. 9(b) and (e). The surface tension ofthe water droplet drives the microchip, and it is self-aligned tothe receptor site as seen in Fig. 9(c) and (f).

Fig. 9. Self-alignment sequences with water. (a) and (d) SU-8 microchip isready to be released with the predefined initial biases. (b) and (e) Microchip isreleased. (c) and (f) Microchip is aligned with the receptor site.

Fig. 10. Measurement of the self-alignment accuracy with water. (a) SU-8microchip is released to a receptor site, with a large initial bias (60 μm inx-axis, 60 μm in y-axis, 50 μm in z-axis, and 49% overlap). (b) Self-alignedmicrochip with an accuracy of 1.3 μm in x-axis and 2.8 μm in y-axis. The edgesof the receptor sites are highlighted with blue, and the edges of the microchipare highlighted with red lines. The centers of the receptor site and microchipare marked with a yellow cross and a red circle, respectively.

TABLE ISELF-ALIGNMENT RESULTS WITH WATER DROPLET

Due to the hydrophobicity of the receptor sites (advancingCA of 118◦), a small amount of pressure is exerted withmicrogripper before the release of SU-8 microchips to push thewater droplet against the receptor sites. The external pressureimproves the wetting of the receptor sites with water. Anotherway to improve the wetting is to use larger water dropletvolume; however, it makes the self-alignment duration muchlonger due to the longer evaporation time [37].

The alignment accuracy was calculated by measuring the dif-ference between the geometry centers of the microchip and thereceptor sites after the self-alignment was finished. An image-analysis-based method was implemented using Matlab for thedetection of the geometry centers. The detection included thefollowing steps.

1) Manually detect four edges of the receptor site whenthe chip is ready to be released, and this is shown inFig. 10(a), where the edges of the receptor site are high-lighted with blue lines and the center of the receptor siteis marked with a yellow cross.


Fig. 11. (a)–(g) Top-view and (h)–(n) side-view microscope images of chip manipulation and self-alignment with water, assisted by the dewetting behavior ofthe substrate.

2) Manually detect four edges of the microchip after themicrochip is self-aligned with the receptor site, and this isshown in Fig. 10(b), where the edges of the microchip arehighlighted with red lines and the center of the microchipis marked with a red circle.

3) Compare and calculate the difference between the centerof the receptor site and the center of the microchip.

Multiple self-alignment tests were carried out to investigate thereliability of self-alignment. During the tests, the same initialbias (60 μm in x- and y-axes, 50 μm in z-axis, and 49%overlapping area) and volume of water (0.9 nL) were used. Theinitial bias was defined as the difference between the releasingposition and target position, which included the bias in x-, y-,and z-axes. The tests were repeated for five times. The resultsare summarized in Table I, where 5/5 tests were successful withthe 49% overlap between the microchip and the receptor site.The alignment accuracy shown in Table I was estimated withthe mean and the standard derivation (std) of the five tests.

The fluoropolymer-coated black silicon substrate is super-hydrophobic and shows strong dewetting behavior with water.The dewetting behavior of the substrate with water brings largetolerance and flexibility in assembly process. Water does notneed to be dispensed precisely at the center of the receptor site,and the microchips can be placed roughly near the receptorsite with low-precision and fast-speed robotic handling tool.We demonstrated that a microchip can align with a receptorsite even when part of the water droplet is in contact withthe substrate, which normally leads to failure in self-alignmentwith conventional protruded receptor sites. Fig. 11 shows oneexample of alignment assisted by the dewetting behavior ofthe substrate with water. The volume of water used in thiscase was 15 nL, comparatively larger due to water evaporationduring manipulation. First, an SU-8 microchip is placed abovea receptor site on a sample carrier [Fig. 11(a) and (h)]; then,a droplet of water is dispensed roughly onto the receptor sitewhere the water droplet is well confined [Figs. 10(b) and 11(i)].The sample carrier moves the receptor site along the x-axis,while a part of the water droplet remains in contact with thereceptor site [Fig. 11(c) and (j)]. The sample carrier keepsmoving, and the whole water droplet is in contact with thesubstrate [Fig. 11(d) and (k)]. The sample carrier moves towardanother receptor site, and a part of water droplet forms contactwith another receptor site [Fig. 11(e) and (l)]. The microchip is

Fig. 12. Self-alignment sequences with adhesive. (a) and (d) SU-8 microchipis ready to be released with the predefined initial bias. (b) and (e) Microchip isreleased. (c) and (f) Microchip is aligned with the receptor site.

released from the gripper [Fig. 11(f) and (m)]. The microchipis aligned with the second receptor site [Fig. 11(g) and (n)].

D. Self-Alignment With Adhesive

Permanent fixing of self-assembled microchips is necessaryfor final packaging of devices. This has been demonstratedwith adhesives [9], [16]. In ambient air environment, adhesiveconfinement to the receptor sites is more difficult than waterdue to their low surface tension. Consequently, self-alignmentwith adhesives is generally more challenging.

Fig. 12 shows optical microscope images of adhesive self-alignment of an SU-8 microchip on a receptor site. Similar tothe self-alignment with water droplet, the process was recordedfrom both top [Fig. 12(a)–(c)] and side [Fig. 12(d)–(f)]. The mi-crochips were delivered in a similar manner as in water-droplet-based self-alignment tests described earlier. The amount ofadhesive droplet was carefully controlled with the adhesivedispenser, and 0.4 nL adhesive has been used in the tests.

The accuracy of the self-alignment is seen in Fig. 13(a) and(b). The microchip is released with a large initial bias as shownin Fig. 13(a) (50 μm in x-axis, 40 μm in y-axis, 50 μm inz-axis, and 60% overlapping area). The alignment accuracy wasmeasured using the same method as in the water self-alignmenttests. The measured alignment accuracy is 1.2 μm in x-axis and2.3 μm in y-axis. The self-alignment process was completed inless than 110 ms.

Multiple tests were carried out to determine the reliabil-ity of the self-alignment process with adhesive. During the


Fig. 13. Measurement of the self-alignment accuracy with adhesive. (a) SU-8 microchip is released to the same-sized receptor site, with a large initialbias (50 μm in x-axis, 40 μm in y-axis, 50 μm in z-axis, and 60% overlap).(b) Self-aligned microchip with an accuracy of 1.2 μm in x-axis and 2.3 μmin y-axis.

TABLE IISELF-ALIGNMENT RESULTS WITH ADHESIVE DROPLET

Fig. 14. Permanent bonding of microchip on a receptor site after adhesivecuring.

tests, the same initial bias (50μm in x-axis, 40 μm in y-axis,50 μm in z-axis, and 60% overlapping area) and adhesivevolume (0.4 nL) were repeatedly used on different receptorsites of the same size. The results are summarized in Table II,where 5/5 tests were successful with the 60% overlap betweenthe microchip and the receptor site. The results indicate thatthe self-alignment is reliable with 60% overlap between themicrochip and the receptor site.

Curing of the adhesive with UV is needed for the finalbonding of self-aligned microchip. Fig. 14 shows the finalbonding of a microchip on a receptor site after self-alignmentand curing. The accuracy confirms the optical-vision-basedestimation methods.

IV. CONCLUSION

We have demonstrated the fabrication and use of a hybridblack silicon template for surface-tension-driven self-alignmentof microchips with water and UV-curing adhesive. Nanostruc-tured black silicon surface covered with fluoropolymer was

used as a substrate, while the silicon dioxide patterns coveredwith fluoropolymer were used as receptor sites. The fabricationof a hybrid template included only a single photolithography,cryogenic DRIE, and RIE steps. The solid edge between thereceptor sites and the substrate was used to confine the waterand adhesive droplet needed for surface-tension-driven self-alignment. Low-surface-energy fluoropolymer-coated receptorsites facilitated the droplet confinement and self-alignmentat small protrusions (height < 4 μm). With water, the self-alignment was reliable, and the dewetting property of the sub-strate enabled manipulation (release and drag) of microchipsafter feeding. With adhesive, alongside reliable self-alignment,permanent fixing of a microchip was also demonstrated bycuring the adhesive with UV. The tests were performed on a100-mm silicon wafer, but the method can be easily scaled upfor high-throughput industrial processes.

ACKNOWLEDGMENT

The authors would like to thank the provision of facilitiesand technical support at the Micronova Nanofabrication Centerof Aalto University.

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Ali Shah received the M.Sc. degree in micro- andnanotechnology from the School of Electrical Engi-neering, Aalto University, Espoo, Finland, in 2010,where he is currently working toward the Ph.D.degree in electrical engineering in the Department ofMicro- and Nanosciences.

His research interests include micro- and nanofab-rication for self-assembly, polymer microprocessing,and LED microprocessing.

Bo Chang received the M.Sc. degree in controlengineering from Tampere University of Technology,Tampere, Finland. She is currently working towardthe Ph.D. degree in the Department of Automationand Systems Technology, School of Electrical Engi-neering, Aalto University, Espoo, Finland.

Her research interests include micro- andnanorobotic assembly, self-assembly of microscopiccomponents, and hybrid microassembly.

Sami Suihkonen received the Ph.D. degree fromHelsinki University of Technology, Espoo, Finland,in 2008.

He is currently the III-N Group Leader with theSchool of Electrical Engineering, Aalto University,Espoo. His research interests include LEDs, nano-electronics, III-N materials, and defects in semicon-ductors. He has more than 40 publications in refereedinternational journals.

Quan Zhou received the M.Sc. degree in controlengineering and the Dr.Tech. degree in automationtechnology from Tampere University of Technology,Tampere, Finland.

He is an Adjunct Professor with the Departmentof Automation and Systems Technology, Schoolof Electrical Engineering, Aalto University, Espoo,Finland, leading the Micro- and Nanorobotics re-search team. He was a Professor with the School ofMechatronics, Northwest Polytechnical University,Xi’an, China. Since 1995, he has worked on nearly

20 academic, industrial, and European Union (EU) projects in micro- andnanorobotics and micromechatronics and authored more than 70 scientificpublications. Since 2000, he has been actively leading research teams atdifferent universities. His current research interests are to bring microroboticsand self-assembly together, including hybrid assembly, robotic assembly, andself-assembly, and their applications in radio-frequency identification assembly,handling of optoelectronic components, and 3-D integration of microsystems.He is also actively working on mobile microrobots and micro- and nanomecha-tronic systems and their industrial and biomedical applications. He is currentlythe Coordinator of EU Seventh Framework Programme project FAB2ASM. Heis also an Associate Editor of the Journal of Micro-Nano Mechatronics.

Harri Lipsanen received the Ph.D. degree from theDepartment of Technical Physics, Helsinki Univer-sity of Technology, Espoo, Finland, in 1994.

He is currently a Professor with the School ofElectrical Engineering and the Vice Director of theDepartment of Micro- and Nanosciences, Aalto Uni-versity, Espoo. He has more than 25 years of researchexperience in experimental micro- and nanosciencesparticularly in nanophotonics and nanoelectronics.His fields of interest include semiconductor nanos-tructures such as self-assembled quantum dots and

nanowires, nanofabrication, epitaxy, atomic layer deposition, and novel nano-materials such as graphene.

surface-tension-driven self-alignment of microchips on black-silicon-based hybrid template in...

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