In this paper we present the first investigation of a batch method for folding of threedimensional micrometer-sized silicon nitride structures by capillary forces. Silicon nitride tubes have been designed and fabricated using DRIE at the center of the planar origami patterns of the structures. Water is brought to the structures by pumping the liquid through the wafer via those tubes. Isolated micro-structures were successfully folded using this method. The potential of this technique for batch self-assembly is discussed

Abelmann, L.

Berenschot, J.W.

Legrain, A.

Ma, K.

Sanders, R.G.P.

Tas, N.R.

English

Legrain, A.B.H.

Berenschot, Johan W.

Sanders, Remco G.P.

Ma, Kechun

Tas, Niels Roelof

Abelmann, Leon

University of Twente Research Information

ELASTOCAPILLARY FOLDING OF THREE DIMENSIONAL MICRO-STRUCTURES USINGWATER PUMPED THROUGH THE WAFER VIA A SILICON NITRIDE TUBEA. Legrain, J.W. Berenschot, R.G.P. Sanders, K. Ma, N.R. Tas and L. AbelmannTransducers Science and Technology Group, MESA+ Research Institute, University of Twente, P.O. Box 217, 7500AEEnschede, The NetherlandsAbstract — In this paper we present the firstinvestigation of a batch method for folding of three-dimensional micrometer-sized silicon nitride struc-tures by capillary forces. Silicon nitride tubes havebeen designed and fabricated using DRIE at the cen-ter of the planar origami patterns of the structures.Water is brought to the structures by pumping theliquid through the wafer via those tubes.Isolated micro-structures were successfully foldedusing this method. The potential of this techniquefor batch self-assembly is discussed.Keywords: Micro-fabrication, elasto-capillarity,self-assembly, origami, three-dimensional, micro-structuresI – IntroductionAn important part of the research in the field ofMEMS is nowadays focused on fabrication of three-dimensional (3D) systems [1]. It is well-known that theminiaturization of electronic elements becomes moreand more difficult approaching fundamental limits. Thethird dimension presents a promising option to achievemuch higher component densities.The fabrication of 3D MEMS is difficult becauseof the inherent two-dimensionality of projection photolithography [2]. Being primarily based on micro-machining, 3D structures are essentially an extensionof 2D patterns obtained by means of lithography. There-fore the height, and complexity, of 3D structures is verylimited. Self-assembly is seen to be an encouragingmethod to overcome these problems [1].As we recently showed [3, 4, 5], capillarity is a veryeffective mechanism to achieve the folding of microme-ter sized planar structures into 3D shapes. It was demon-strated that 3D micro-objects of silicon nitride couldbe fabricated starting from planar geometries on whicha tiny water droplet was deposited. Surface tension ofthe deposited liquid induces the out-of-plane rotation ofinfinitely stiff wings connected to each other by flexiblehinges.Microfabrication inspired on origami, the Japanesepaper folding art, can combine the conventional parallelmicro-fabrication with the creation of complicated 3Dstructures [6, 7, 8]. Structures assembled so far usingelasto-capillarity have been folded after deposition ofa tiny droplet of water using a manually controlledhollow fiber [3, 4, 5]. That method is adequate forexperiments carried out in a laboratory, but obviouslynot for potential industrial applications.One of the options to provide batch folding of struc-tures is to implement a central tube into the structuresand pump water parallel through the wafer towardsthe structures. By selectively adding or retracting thewater, one could nicely control the folding with thepump. Furthermore, the central position of the tubewould ensure a good distribution of the liquid over thestructures.The work introduced here aims at the first experimentof folding isolated micro-structures using liquidpumped from backside. The micro-machining ofsilicon nitride structures patterns along with thepreliminary folding experiments are presented in thispaper.II – Design and fabricationFigure 1 presents a cross-section of the micro-structures during their folding by pumping waterthrough the wafer. The structures are made with stiffflaps connected to each other by flexible hinges. Thecentral plate is connected to a tube which passes com-pletely through the wafer as represented in figure 1a. Allelements of the structure are made of silicon nitride.The size of the flap is in the 50-200µm range, theradius of the tube is 34µm. The folding occurs with theevaporation of water once the pumping has stopped, asrepresented in figure 1b and c. Figure 2 presents themain steps of the fabrication of the structuresFigure 1: Folding principle. Water is pumped through thewafer via the central tube (a). The liquid overflows over thestructures and the pumping is stopped. The water tends toreduce its liquid-air interface and induces the rotation of theflaps (b). Once the water is evaporated, the flaps stick togetherand the structure stays closed (c).In order to reduce the length of etching through thePage 250Figure 2: Main steps of the process. See text for description ofthe steps.wafer and make the future pumping easier, a thin (110)wafer is selected (380 µm thick). Then a foil is appliedon the bottom-side of the wafer (a). This foil is tem-porary needed for the subsequent DRIE step throughthe entire wafer (b). It acts as an etch stop and preventsleakage of the helium backside cooling, ensuring a sta-ble temperature control of the wafer during the etchingprocess. As a foil we use a DuPont MX500 polymerfilm [9]. The DRIE is performed using an ADIXENAMS100SE machine using a Bosch recipe.After removal of the foil using a reactive oxygenplasma, a 1 µm layer of silicon nitride is grown all overthe wafer, including inside the hole, by LPCVD (c).Then, a lithography step follows to define the locationof the flexible hinges (d). In order to protect the edgesat the intersection between the tubes and the structurespatterns, particular attention must be paid to the lithog-raphy steps of the process. Otherwise, this intersectionis attacked during the subsequent etching step and thesilicon nitride patterns are not tethered anymore.For this purpose, we used a relatively thin photo-resist (OiR 907-35). The wafer is spun at slow speed(2000 rpm) to provide a 5.2 µm thick layer of photo-resist. After verification, it appears that the outer limitsof the tubes are protected with about 1 µm of photo-resist as represented in step (d) of the process. This layeris thick enough to protect the edges during etching ofthe silicon nitride.Openings are created by a dry SiN etching step(RIE). After stripping the photo-resist in oxygenplasma and in HNO3, a 100 nm silicon nitride layeris deposited (e). This thin layer defines the flexiblehinges which enables the structures to fold. Thenanother lithography step is performed to define theoverall geometry of the structures. Once the edges arewell-protected, the silicon nitride can be etched using adry etching step (RIE) (f). At this time of the process,several origami-like patterns of structures (boxes,tetrahedrons, pyramids) are defined on top of the wafer.Finally, the last lithographic step is performed and theperipheral flaps are freed by under-etching the silicon indry SF6 gas etcher (g). The specific design parametersare given in figure 2 (g).III – Fabrication resultsFigure 3a represents a SEM photograph of anisolated tube surrounded by trenches in the first siliconnitride layer. This picture has been taken after thedefinition of folding locations, figure 2 (e). One can seein figure 3b the characteristic shape of the scallop dueto the Bosche recipe used for the creation of the tube.Moreover, there is no discontinuity between the planarpattern and the tube thanks to the spinning of a 907-35photo-resist layer at low rotation speed during thepreceding lithography step as discussed in the previoussection.Figure 3: SEM picture of a silicon nitride tube through thewafer after step (d) in figure 2. At the end of the process, thiswill become a pyramidal pattern (a). Close up of the centralpart (b).Figure 4a shows an origami pattern of a cube restingon its silicon nitride tube after the under-etching of thesilicon substrate (figure 2 step (f)). Figure 4b is a closeup view of the fixed central part. Thanks to the usePage 251of a thick enough photo-resist layer the pattern is stillattached to the tube. We can also clearly observe thethin hinges along with the thicker flaps. The tube has adiameter of 34µm, the length of the side of the flaps is60µm.Figure 4: SEM picture of a silicon nitride origami patternresting on a silicon nitride tube (a). Close up of the centralpart (b).Figure 5 represents an overall view of a group ofmicron-sized patterns of boxes. The structures areready to be folded at the same time.Figure 5: SEM picture of the frontside of an array of unfoldedstructures. The holes in the centre can just be distinguised.IV – Folding experimentsA. Experimental detailsWhen filling the tubes, the edge between the tube andthe planar pattern forms a barrier, since at this pointthe radius of curvature of the surface of the water isthe lowest. This induces a Laplace pressure, defined as∆p=Pl-Pv where Pl is the pressure of the liquid and Pvis the pressure of the vapor, in this case the air [10]. Theoverpressure ∆p needed to overcome the edge is givenby:∆p =2γR' 10 kPa (1)where γ=72 mN/m is the surface tension of water at 25°Cand R is the radius of the curvature of the surface atthe edge. Since the radius of the tube is in the sub-millimeter range, the radius of the curvature of thesurface can be considered in first approximation ofthe same order of magnitude as the radius of the tube(R' 34µm). A pump capable to apply this overpressureis therefore required for the experiments. Moreover,pumping must be precise to avoid the structure to besubmerged. Indeed, once the water gets over the edge,the Laplace pressure is much lower and the flow ofwater could become uncontrollable.Thus, a high precision syringe pump has beenused for our preliminary experiments. Small pulsesof pumping can be realized with this pump. A fiberconnects the pump to the bottom of a plate with a0.8mm hole at the center. The sample is placed on thissupport and an accurate positioning system permitsto perfectly align a structure with the fiber. A smallsilicone o-ring is placed between the sample and thefiber to ensure a watertight connection. The sample isfastened using a u-shaped clamping tool surroundingthe structure.B. ResultsAs expected, the water emerges out of the tube buta large amount of liquid comes out once the edge isovercome. However, even if the droplet is one orderof magnitude bigger than the structure, it still remainson the silicon nitride pattern. Thus, the water does notoverwhelm the substrate and the folding occurs afterevaporation of the liquid. Once the flow stops and thevolume of the droplet stabilizes, small volumes of water(5-10 nL) can be added by actuating the pump.Figure 6 shows a long structure which has beenfolded using this setup. This new method of assemblyis a considerable improvement for our research. Indeed,there is no deviation of the folding anymore due tothe manual deposition of the water since the holesare designed precisely in the center of the structures.Furthermore, from now on the folding sequence canbe studied more in depth by adding small volumes ofwater on demand.Figure 6: SEM pictures of a long symmetric structure foldedby pumping water through the tubes. An overview (a) and aclose-up of one of the extremities (b).V – ConclusionIt is possible to fabricate silicon nitride plates con-nected to a long silicon nitride tube which traverses thethickness of a wafer. One of the most critical processsteps is to protect the outer limits of the tubes with a rel-Page 252atively thin layer of photo-resist. This can be achievedby spinning the resist at low rotation speed.Isolated three-dimensional micron-sized structurescan be successfully folded using the surface tension ofwater pumped through the tubes from the backside ofthe wafer. As has been theoretically and experimentallyshown, the critical point of the assembly is to pump thewater out of the tubes to form droplet on the structure.The overpressure required to overcome this barrier cancause an uncontrolled flow once the droplet starts toform.VI – RecommendationsFuture work will have to address the issue of un-controlled flow. By reducing the capacitance of thepumping system and better control over the appliedpressure, unwanted flow might possibly be reduced.For batch fabrication, the pressure needs to be identi-cal in all tubes over the wafer, which will put stringentdemands on the fluidic system. Nevertheless, this newmethod of assembly is promising, also for all fields ofresearch where the behavior of liquid over surfaces isstudied. The fact that the water is brought exactly wherethe holes have been designed and that small volumeof water can be added on demand provides a strongadvantage.AcknowledgmentsWe wish to acknowledge the contribution of Joostvan Honschoten to this work. The main theme ofthis publication is based on his original ideas. Wecommemorate his passion for science and for thesubject of elasto-capillarity specifically.References[1] K. Ariga, J P. Hill, M V. Lee, A. Vinu, R. Charvet,and S. Acharya. Challenges and breakthroughsin recent research on self-assembly. Sci. Technol.Adv. Mater., 9:96 pp., 2008.[2] Madou. Fundamentals of Microfabrication. CRCPress, New York, 1997.[3] J. W. van Honschoten, J. W. Berenschot,T. Ondarçuhu, R. G. P. Sanders, J. Sundaram,M. Elwenspoek, and N. R. Tas. Elastocapillaryfabrication of three-dimensional microstructures.Appl. Phys. Lett., 97:0141031–0141033, 2010.[4] J. W. van Honschoten, J. W. Berenschot, R. G. P.Sanders, N. R. Tas, and M. Elwenspoek. Fabri-cation of three-dimensional microstructures usingcapillary forces. In Proc. of MicroMechanicsEurope (MME), 115, 2009.[5] J. W. van Honschoten, A. Legrain, J. W. Beren-schot, L. Abelmann, and N. R. Tas. Micro-assembly of three dimensional tetrahedra by cap-illary forces. In 24th IEEE International Confer-ence on Micro Electro Mechanical Systems, pages288–291, Cancun, Mexico, january 2011.[6] C. Py, P. Reverdy, L. Doppler, J. Bico, B. Roman,and C. N. Baroud. Capillary origami: Spontaneouswrapping of a droplet with an elastic sheet. PhysRev Lett, 98(15):156103, 2007.[7] T. G. Leong, A. M. Zarafshar, and D. H. Gracias.Three-dimensional fabrication at small size scales.Small, 6(7):792–806, 2010.[8] M. Boncheva, S A. Andreev, L. Mahadevan,A. Winkleman, D R. Reichman, M G. Prentiss,S. Whitesides, and G M. Whitesides. Magneticself-assembly of three-dimensional surfaces fromplanar sheets. Proc. Natl Acad. Sci., 102:3924–3929, 2005.[9] www.dupont.com. DuPont Mx5000 Series - DataSheet and processing information.[10] P. G. de Gennes, F. Brochard-Wyart, and D. Quéré.Capillarity and Wetting phenomena Drops, Bub-bles, Pearls, Waves. Springer, 2004.Page 253

Elastocapillary folding of three dimensional micro-structures using water pumped through the wafer via a silicon nitride tube

Elastocapillary folding of three dimensional micro-structures using water pumped through the wafer via a silicon nitride tube

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