A REVIEW OF FLUIDIC SELF ASSEMBLY
Towards the fulfillment of course requirement for EE5171 Under Prof. Stephen Campbell Electrical and Computer Engineering University of Minnesota, Twin Cities

By: Manan M Dedhia ID # 4279398 M.S. Graduate student

Electrical and Computer Engineering University of Minnesota, Twin Cities

THE INTEGRATION of microelectromechanical systems(MEMS) sensors and actuators with other classes of microcomponents—electronic, optical, and fluidic—onto a single substrate has the potential to create powerful and complex microsystems. To increase device performance and thus realize the potential of microsystems, it is now accepted that the integration must occur at the micro scale. There are two possible routes: either the micro-components are fabricated at their desired locations in a single process, or they are fabricated separately and then positioned using micro assembly techniques. Since many classes of micro components cannot be co-fabricated effectively due to materials and process incompatibilities, several research groups are developing micro assembly approaches. In the micro assembly route, different classes of micro components are fabricated in separate processes, removed from their substrates, and assembled onto a target substrate of choice. For most applications, sub micrometer positioning and methods for establishing high-quality mechanical and electrical connections to the substrate are required. As in co fabrication, small-area electrical connections are desirable as they provide interfaces with low parasitic capacitances, resulting in greater measurement sensitivity of the devices. One advantage of using micro assembly, rather than pursuing co-fabrication, is the fabrication process for each micro component can be optimized separately. Secondly, expensive materials can be used efficiently; the layout of costly micro parts on the donor wafer may be dense while the distribution of these parts on the target substrate is sparse. In addition, the yield losses from long integrated processes can be avoided. However, the yield of the assembly steps now becomes an important consideration. Self-assembly, which is defined as the ―spontaneous organization of molecules or objects into stable aggregates under equilibrium conditions‖ [2], is an intriguing route to micro assembly. In nature, spontaneous processes efficiently assemble ordered structures with up to millions of individual components. At the molecular scale, self-assembly is due to non-covalent forces (e.g., hydrogen bonds, van der Waals interactions, and hydrophobic interactions) which arrange molecules into ordered arrays of varying complexity [4]. Self-assembly can be either a reversible or irreversible process, depending on the level of background energy. For particles greater than molecular size, researchers have employed selfassembly principles to organize nanometer- and micrometer-size beads into two- and three-dimensional closepacked arrays [2]. In this research, a variety of forces cause the beads to aggregate: capillary, electrostatic, and magnetic forces, as well as molecular recognition and binding between complementary strands of DNA. The beads used in these demonstrations are a simple type of synthetic structure, as they have spherical symmetry and no surface patterning. New strategies are under study which applies selfassembly to position arbitrarily shaped micro-components on a substrate. In these processes, the microstructures are first freed from the substrates on which they were fabricated. The majority of processes being developed use fluidic transport of the micro-components because surface interactions can be controlled more easily in a liquid environment than in the gas phase. Using photolithographic techniques, binding sites are prepared on the micro-parts, and receptor sites are made on the target substrate where the parts are to assemble. Essentially, the substrate contains potential energy wells for the parts at the desired locations. A carrier fluid transports the micro-parts over the target substrate, and when a binding site on a micro part interacts with a receptor site on the substrate, there is a certain probability of attachment. If the micro device does not ―fall‖ in the energy well, it is carried away by the fluid and may be transported to another site. Through this process, devices eventually occupy all of the substrate sites. To design the binding and receptor sites for self-assembly, two important points must be considered: the shape of the potential energy well and the level of kinetic energy supplied to the system [2]. The desired assembly state should represent the minimum in potential energy and any energy barriers should be small compared to the driving potential energy difference. A lubrication method may be necessary so that the micro parts can reach their minimum energy configurations. The random kinetic energy needed to move the parts over the substrate can be supplied

through fluidic agitation or ultrasonic vibration. There must be sufficient kinetic energy in the system so that parts can overcome energy barriers. There are both advantages and challenges to using self-assembly in contrast to a wafer-to-wafer transfer approach. Because of the difficulties in scaling up transfer techniques due to topography and wafer curvature, self-assembly may prove to be more scaleindependent. Furthermore, if the substrate receptor sites are defined without photolithography, they could be stamped or printed onto very large, or even curved, substrates. Second, decoupling donor- and targetsubstrate layouts is inherent in self-assembly because the components must be freed from the substrate anyway. This allows the layout of expensive donor substrates to be optimized to use the least area. A third consideration is yield. In self-assembly techniques which depend on one force to drive the assembly, defects such as vacancies and misaligned parts are inevitable. However, vacancies can be filled by increasing the assembly time, and misalignments can be minimized by supplying levels of agitation energy sufficient to correct or disassemble incorrectly assembled pieces [2]. In theory, self-assembly can give higher yields than wafer-to-wafer techniques since defective elements may be discarded before they are released from the donor substrate, and defective receptor sites on the target. Significant challenges in developing production-level self-assembly techniques include gaining the mastery of methods that can provide both mechanical and electrical connections to the substrate. Metal evaporation and photolithographic patterning techniques become difficult if the surface topography is greater than 5 m. Pico- to nano liter volumes of adhesives may be needed to establish strong mechanical connections. Further, given the importance of surface forces at the micro-scale, lubrication might be required so that the parts can reach minimum energy configurations on the substrate.

Due to scaling effects, forces that are insignificant at the macro scale become dominant at the micro scale and vice-versa. For example, when parts to be handled are less than 1mm in size, adhesion forces between a gripper and object can be significant compared to gravitational forces. Adhesion forces arise primarily from surface tension, Van der Waals, and electrostatic forces, and can be a fundamental limitation to part handling. While it is possible to fabricate miniature versions of conventional robot grippers, overcoming adhesion effects for the smallest parts has proved to be difficult.

Figure 1 :Magnitude of the different components of adhesion forces versus distance. [5]

Electrostatic attraction Fel is based on the Coulomb force between electrically charged objects. The force between a charged sphere and a dielectric plane can be quantified by :

The Van der Waals force Fvdw is an intermolecular force caused by momentary movements of electrons. Equation (2) shows the approximate expression for the Van der Waals force between a sphere of diameter d and a plane at distance z, where H is the Hamaker constant,

Rough surfaces increase the effective distance z and therefore reduce Fvdw. The last and most important type of adhesion is the capillary force Fcap that arises From a thin liquid film between two objects, which can originate from the air‘s humidity. For a hydrophilic sphere with diameter d close to a plane, the relation can be written as,

Hydrophobic surfaces and a dry atmosphere reduce the capillary force. It is important to note that adhesion forces are surface forces and their magnitude is therefore proportional to contact area. [5]

The Capillary Bond. A ―bond‖ is an interaction between two objects that holds them together. In principle, many different types of interactions : electromagnetic, hydrophobic, fluidic, photonic, capillary, and gravitational could be used to form bonds. In most of our work, we have used capillary interactions to cause self-assembly. We suggest that the capillary interactions between objects in our systems can be viewed as bonds that are loosely analogous to chemical bonds. This analogy is not an exact mapping from the molecular to the macroscopic, but rather an aid to the imagination. There are at least three similarities between capillary and chemical bonds. (i) Both are reversible at some level of agitation or temperature. (ii) Both interact through overlaps: of menisci for capillary bonds, and of orbitals for chemical bonds. (iii) Both are directional. There are, of course, fundamental differences between capillary and chemical bonds. (i) Capillary forces are described by classical mechanics; chemical bonds require quantum mechanics. (ii) Capillary and chemical bonds are described by potential curves with different forms. (iii) Capillary forces can act over distances up to a millimeter; chemical bonds act over distances of less than 1 nm.

Bonds: Capillarity, the Hydrophobic Effect, Surface Tension, and Interfacial Free Energy : Capillary interactions have five attractive characteristics. (i) The characteristic decay length for lateral capillary forces is in the right rangesfrom nanometer to millimeter, depending on the dimensions of the objects to be useful with objects of the size (micrometer to centimeter) that we wished initially to explore. (ii) The strength of the capillary interactions can be adjusted to be comparable to the strength of the shear forces that are used to disrupt the aggregation. (iii) Capillary interactions can be used to assemble either 2D or 3D arrays. (iv) Capillary interactions are well understood conceptually (although their numerical analysissusing the Laplace equations is often intractably complicated). (v) Designs of the surfaces that generate the menisci allow the capillary interactions to be made directional. It is useful to understand the molecular basis of capillary interactions. Capillarity, surface tension, and the hydrophobic effect all reflect the same phenomenon that is, the tendency of a system containing water or other liquids to minimize its interfacial free energy, usually by minimizing the area of the interface with the highest free energy (Figure 2). Molecules of a polar liquid or solid at an interface with a less polar phase (air, non- polar polymer, hydrophobic molecular interface) are less stable than those in the bulk. A system will thus tend to minimize the area of its exposed, high-energy interfaces. The spreading of a drop of water on a carboxyl-terminated SAM covers the surface with the highest surface free energy that of the SAM), albeit at the expense of increasing the area of the water/air interface (Figure 2a). Coalescence of drops of benzene suspended in water, association of a hydrophobic ligand with a hydrophobic active site, and rise of water in a capillary all reflect changes in areas of interfaces that minimize interfacial free energies (Figure 2 b-d).

Figure 2 . Six examples of systems that minimize the area of a high energy surface. (a) Water spreading on a SAM terminating in carboxylic acid groups. (b) Two drops of benzene in water coalescing into one drop. (c) A receptor with a hydrophobic area interacting with a hydrophobic ligand. (d) Water rising in a capillary to coat the high-energy glass surface. (e) Two objects floating at the PFD/H2O interface interacting through lateral capillary forces to minimize the surface area at the interface. (f) Two objects with one face hydrophobic and coated with a hydrophobic liquid or liquid metal and suspended in water coming into contact. [6] Another challenge is that of specifying the orientation of the assembled parts, as the parts are in random orientations in the carrier fluid. Despite these obstacles, several groups are developing micro scale self-assembly techniques using a variety of forces to achieve attraction and binding of microscopic parts onto a substrate. Current self-assembly techniques for micro scale parts are based on two major mechanisms. One is capillary-driven self-assembly and the other is shape directed self-assembly. To ensure self-assembly, the key techniques include the enhancement of capillary force using micro part shapes, strong bonding of micro parts and binding sites, external agitation to control the mixing and assembly speed, and shape recognition. Rotationally symmetric micro parts, such as squares or circled disks, have been the most popular for non-directional alignment by capillary forces by the external agitation method, self-assembly mechanism, micro part shape and size, yield rate and assembly speed, for parts of rotational symmetries . Square micro parts, mechanical/hydrodynamic agitations and shape recognition have been applied with very high yields in most related studies. Furthermore, gravity as a driving force and agitation help the micro parts to move across the substrate until they fall into recesses or wells. Van der Waals and capillary forces between the parts and the substrate help to hold the parts in place once they have been assembled. However, rotationally symmetric parts cannot be uniquely orientated easily. Numerous structures have been proposed to align the micro parts in unique orientations by exploiting specially designed asymmetric structures. These structures can be self-aligned in a unique orientation mainly by geometric recognition.

COMPARISON OF MILLIMETER-SCALE AND MOLECULAR SELF-ASSEMBLY [7] : Although the broad principles of self-assembly are similar for components that range in size from molecular to macroscopic, the details differ: • Size and number of components. Mesoscale systems contain 10–104 mm-scale components, while molecular scale systems might involve 1020 molecules. • Fabrication/synthesis of components. Chemical synthesis—the method used for ―fabrication‖ of components for molecular self-assembly—is a parallel process: many identical molecules can be synthesized simultaneously. Molecular synthesis is a highly developed art. Fabrication of μm-tommsized components can, in principle, be either a parallel or a serial process, but is much less efficient—and at this time, usually much more cumbersome—than simple molecular synthesis. The development of small components incorporating sophisticated function is just beginning. • Function. Components with sizes in the μm-to-mm range can be self-assembled into structures with optical or electrical functionality. Molecules cannot be positioned and connected into aggregates with such functionality.

• Interactions. Different types of interactions are used to cause self-assembly of molecules and mm-sized objects. The intermolecular interactions used in molecular self-assembly are much weaker and shorter range than those used in mesoscale self-assembly. More types of forces can be used in the self-assembly of μm-to-mm-sized systems than in molecular systems. • Sensitivity to wall effects. In both molecular and mesoscopic systems, wall effects extend over dimensions of 10–100 component layers from the walls. The absolute distance over which wall effects are important is thus enormously different (by 106–107) in molecular and mesoscopic systems, although the scaled length may be similar. • Temperature and agitation. Molecular systems are agitated thermally, and the distribution of kinetic energies follows the Boltzmann equation. Systems containing μm-to-mm-sized components are sufficiently massive (relative to the molecules making up the solutions in which they are suspended) that they are unaffected by thermal motion; when suspended in an unstirred liquid, mm-scale objects are effectively at 0 K. The mm-scale systems require external agitation (e.g., by fluid shear or by mechanical agitation) to cause encounters between the components. We have not characterized the distribution of energies among the components under the conditions used in our experiments. • Reversibility or “adjustability”. In both molecular and mm-scale self-assembled systems, the interactions between the components must be reversible (or the components able to adjust their relative positions within an aggregate) in order to achieve highly ordered structures. If components stick irreversibly upon encounter, the resulting aggregate is usually a glass rather than a crystal.

Applications and Examples:
Following are some examples of how components are being assembled using fluidic self assembly. The major force involved in these papers is usually capillary force. It is sometimes coupled with some other force like magnetic fields to bring about the desired structure/function. 1. U.Srinivasan et al [8] have demonstrated the fluidic self-assembly of micro machined silicon parts onto silicon and quartz substrates in a preconfigured pattern with sub micrometer positioning precision. Self-assembly is accomplished using photo lithographically defined part and substrate binding sites that are complementary shapes of hydrophobic self-assembled mono-layers. Figure 3 (a) Schematic of fluidic self-assembly technique. (b) Substrate adhesive-coating procedure, adapted from (c) Self-assembly using capillary forces of the adhesive. [8] 33.pdf

2. Gracias et al [9] describe the development of a ―bottom-up‖ strategy to fabricate functional, patterned, metallic nano-wires that can interact with each other, and coalesce in a fluidic medium as a result of the minimization of interfacial free energy, to form well-defined 3D structures. Their strategy utilizes surface tension based self-assembly that involves the modification of the surface energy of components using molecules (hydrophilic / hydrophobic), polymerizable adhesives or high surface energy alloys (solder) and subsequent agitation in a fluidic medium. Figure 4(a) A schematic diagram of the process used to fabricate wires consisting of all Au or Au-Ni-Au segments by electrodeposition in a nano-porous alumina template. (b) Selfassembly of Au wires results in the formation of 3D bundles. (c) Self-assembly of Au-Ni-Au wires results in the formation of 2D networks.[9]

3. Chen et al [10] new method for the self-assembly of a carbon nanotube (CNT) using magnetic capturing and fluidic alignment has been developed and characterized in this work. In this new method, the residual iron (Fe) catalyst positioned at one end of the CNT was utilized as a self-assembly driver to attract and position the CNT, while the assembled CNT was aligned by the shear force induced from the fluid flow through the assembly channel. Figure 5. Procedure for fluidic alignment of the CNT by magnetically capturing the Fe catalyst. (a) Fabrication of the Ni pattern on Au/Ti electrodes, (b) solution containing CNTs is guided to the Ni pattern by a PDMS microchannel, (c) Fe catalyst at the end of the CNT is magnetically attracted to the Ni pattern, and (d) the CNT is aligned parallel to the flow direction. [10]

4. The investigation by Lin et al [11] develops a novel design of a two-dimensional modified alignment mark of a tear-drop/elliptical hole with a tip angle of 60◦. TDE-1 and TDE-2 pattern shapes are adopted to increase the recovery angle and reduce the energy barrier to uni-directional micropart alignment.

Figure 6. Schematic diagram and alignment result of the tear-drop alignment shape employed in micropart self-assembly in the uni-direction. (a) Capillary-driven self-assembly process. (b) Overlap ratio with alignment angles for tear-drop 60◦ and rectangular shape. [11]

5. The paper by Jacobs et al [12] introduces a method for self-assembling and electrically connecting small (20–60 micrometer) semiconductor chiplets at predetermined locations on flexible substrates with high speed (62500 chips/45 s), accuracy (0.9 micrometer, 0.14°), and yield (>98%). The process takes place at the triple interface between silicone oil, water, and a penetrating solderpatterned substrate. The assembly is driven by a stepwise reduction of interfacial free energy where chips are first collected and preoriented at an oil-water interface before they assemble on a solder-patterned substrate that is pulled through the interface. Patterned transfer occurs in a progressing linear front as the liquid layers recede.

Figure 7. Procedure of surface tension-directed self-assembly at a liquid– liquid–solid interface employing an energy cascade to (i) move components from a suspension to the interface (55 mJ∕m2), (ii) preorient the components within the interface to face in the right direction (90 mJ∕m2), and (iii) assemble the components on molten solder through dipping (400 mJ∕m2). [12]

6. Whitesides et al [13] [14] show the selfassembly of millimeter-scale polyhedra, with surfaces patterned with solder dots, wires, and light-emitting diodes, generated electrically functional, three-dimensional networks. The patterns of dots and wires controlled the structure of the networks formed; both parallel and serial connections were generated.

Figure 8. The procedure used to form electrical networks in 3D by self-assembly . (A) An array of the basic pattern of copper dots, contact pads, and wires was deÞned on a ßexible copper-polyimide sheet using photolithography and etching (B) These pattern elements were cut out along the dotted line, (C) glued on the faces of the polyhedron, and (D) LEDs were soldered manually onto the contact pads. (E) The copper dots and wires on the TOs were coated with solder, and self-assembly occurred in hot, isodense, aqueous KBr solution. [13] [14]

12 and 14 pdf

7. The self-assembly process by Jacobs et al [15] uses geometrical shape recognition to identify different components and subsequent bond formation between liquid solder and metal-coated areas to form mechanical and electrical connections. They applied this concept of shape recognition and subsequent formation of contacts to assemble and package Microsystems that contained non identical subunits. The self-assembly of three-component assemblies is demonstrated by sequentially adding device segments to the assembly solution including 200-um-sized light-emitting diodes. Six hundred AlGaInP–GaAs light-emitting diode segments self-assembled onto device carriers in 2 min without defects.

Figure 9. Fabrication strategy to assemble and package microsystems by shape- and solder-directed selfassembly. Chip-on-carrier assembly (a) and chip-encapsulation (b) are performed in an ethylene glycol solution at a temperature of 100 C where the solder is liquid. The illustrated device segment has two contacts: a small circular anode on the front and a large square cathode covering the back. The silicon carrier has a solder-coated area in a tapered opening to host a single semiconductor device segment. The encapsulation unit has five solder-coated copper areas inside a tapered opening to connect to corresponding contact pads on the device and carriers. The agitated components self-assemble in a twostep sequence and form a 3-D circuit path between device layers. (a) Chip-on-carrier assembly and (b) chip-encapsulation.[15]

8. In this article, Chung et al [16] introduce ‗railed microfluidics,‘ as an agile method to guide and assemble microstructures inside fluidic channels. The guided movement of microstructures in microfluidic channels was done by fabricating grooves (―rails‖) on the top surface of the channels and also creating complementary polymeric microstructures that fit with the grooves. Using the rails as guiding mechanism for the microstructures, they built complex 1D and 2D Microsystems without wasting a single microstructure.

Figure 10. Concept of railed microfluidics and guiding experiments. (a) Schematic diagram of a railed microfluidic channel. (b) Cross-section of the PDMS railed microfluidic channel and a finned microtrain cut at a-a` from (a). (c) 3D view of a fabricated microtrain on a rail. (d)-(f) Various microtrains

9. In this paper by Bohringer et al [17] they reported a novel capillary-force-driven self assembly technique which proceeds in an air environment. They demonstrated this technique for the self-assembly of piezoelectric driving elements (PZT) for diffuser valve micropumps: with the agitation of an orbital shaker, square PZT actuators self-align and mount to the hydrophilic trench binding sites with electric Connections by heat curable lubricant oil and conductive physical contact between the PZT actuators and the substrate.

Figure 11. Pump fabrication processes (schematic diagrams not to scale). (a) pump structure after 2 DRIEs and laser drilling; (b)microscope image of the pump structure; (c) anodic boding of pyrex 7740 wafer to the substrate; (d) Cr/Au patterned on the pyrex for the trenched PZT binding sites [17] 20.pdf

10. This paper by Whitesides et al [18] describes a dynamic system that develops order only when dissipating energy comprising millimeter to centimeter scale gears that self-assemble into a simple machine at a fluid/air interface. The gears are driven externally and indirectly by magnetic interactions; they are made of poly(dimethylsiloxane) (PDMS) or magnetically doped PDMS, and fabricated by soft lithography. Transfer of torque between gears can take place through three different mechanisms: mechanical interaction, hydrodynamic shear, and capillarity/overlap of menisci.

Figure 12. Schematic diagram of a representative experiment, showing two ring gears positioned at the fluid/air interface. The fluid used in these experiments was perfluorodecalin (PFD). The ring gears depicted are diamagnetic; the gear on the left is driven by a ferromagnetic pinion inside the ring gear. The gears interact with each other through a balance of capillary (Fc), hydrodynamic (Fh), and magnetic forces (Fm). The gears are driven externally by a permanent bar magnet (length ) 5.6 cm), whose upper face is separated by approximately 3 cm from the fluid/air interface; the magnet rotates at an angular velocity ö below the dish (diameter ) 12 cm). The magnetic pinion also rotates at an angular velocity, ö (in units of revolutions per minute, rpm). The angular velocity of the ring gear is less than ö, but depends on various contributions to the drag on it. (Note: Dimensions are not drawn to scale.)

11. Saeedi et al[19] presented the use of self-assembly to integrate a large number of free-standing microcomponents onto unconventional substrates. The micro components are batch fabricated separately from different semiconductor materials in potentially incompatible microfabrication processes and integrated onto unconventional substrates such as glass and plastic.

Figure 13: The heterogeneous self-assembly process. Micro components are introduced over a template submerged in a liquid medium and moved with the fluid flow. Self-assembly occurs as microcomponents first fall into complementarily shaped wells and then become bound by the capillary forces resultant from a molten alloy [19]

ISSUES IN DESIGN OF FUNCTIONAL SELF-ASSEMBLING SYSTEMS:
In designing functional self-assembling systems, one must consider several intertwined issues: 1. Nature of the function The first consideration when designing a functional system is the size regime that is appropriate for the desired function. Not every function is possible in every size regime; this size restriction is imposed by the physical phenomenon providing the basis for the function, or by practical limitations concerning the fabrication of functional structures. Self-assembled aggregates that function as optical elements illustrate the phenomenological restriction. The spacing in aggregates that function as optical elements must be close to the wavelength of the radiation of interest: photonic band-gap materials active in the visible region require periodicity in the range of 200–700 nm, while materials optically active in the far IR require 100 times larger periodicity . Aggregates with electric connectivity or electronic function illustrate the restrictions imposed by the methods used for fabrication and connection of the components. Components with sizes between a few nanometers (e.g., organic molecules, carbon nanotubes, and semiconductor nanowires) and hundreds of microns (e.g., standard microelectronic components) can, in principle, be used to build an electronic device. While fabrication and manipulation of micron-sized components is well developed, wiring together nanoelectronic components remains a challenge . 2. Fabrication of components The application of self-assembly is often (in fact, usually) limited by the methods available for the fabrication of the individual components. Self-assembly seems to be a very general strategy; making components designed for optimal self-assembly may be difficult or impossible using currently available methods. There is no general pathway to fabricate small (nm–μm) 3D components, and the fabrication is even more complicated when the components have to carry electrical, magnetic, optical, or biological functionality. A possible strategy for fabrication of small, functional components is based on selfassembly:folding of 2D precursors into 3D shapes, or self-assembly of 2D components onto prefabricated 3D scaffolds. Current methods of microfabrication require a compromise between the size and the complexity of the components: smaller components result in an aggregate with higher 3D density, but with lower complexity per component; larger components are easier to fabricate and can incorporate more complex function, but give a smaller volumetric density of functionality. 3. Interactions between components Self-assembly occurs when components interact with one another through a balance of attractive and repulsive interactions. In the μm-to-mm size range, a broader range of forces is available for use in selfassembly than at the molecular scale . Self-assembling systems can be designed to employ one or several types of interactions; if several, these interactions can act simultaneously or sequentially. The choice of the type of interaction(s) between components in a self-assembling functional system is based on several factors: (i) The scale and magnitude of the interactions must be appropriate for the system being considered. (ii) The interactions must be compatible with the possible/desired environment for the self-assembly: for example, magnetic forces can act in both air and in water, while hydrophobic and biospecific interactions require aqueous environments. (iii) The operation of the interactions causing self-assembly must not affect the functionality of the components: for example, magnetic fields may not be suitable for self-assembly of sensitive electronic components, and high-temperature solders can not be used with delicate biological molecules.

Conclusion : In this paper, I have tried to show how fluidic self assembly works and also some of the work that has been accomplished in the same. These show that fluidic self assembly is a probable direction for large scale fabrication and as an alternative to robotic pick and place assembly and also traditional lithography techniques, at the same time being cost effective and highly efficient. Fluidic self assembly is also very beneficial for the fabrication of MEMS / NEMS sensors in various media.

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[10] Joon S Shim, Yeo-Heung Yun, Michael J Rust1, Jaephil Do, Vesselin Shanov, Mark J Schulz and Chong H Ahn, "The precise self-assembly of individual carbon nanotubes using magnetic capturing and fluidic alignment", Nanotechnology 20 (2009) 325607 (7pp). [11] Cheng Lin, Fan-Gang Tseng and Ching-Chang Chieng, "Orientation-specific fluidic selfassembly process based on a capillary effect", J. Micromech. Microeng. 19 (2009) 115020 (12pp) [12] Robert J. Knuesel, and Heiko O. Jacobs, "Self-assembly of microscopic chiplets at a liquid– liquid–solid interface forming a flexible segmented monocrystalline solar cell", PNAS January 19, 2010, vol. 107, no. 3. [13] David H. Gracias, Joe Tien, Tricia L. Breen, Carey Hsu,George M. Whitesides, "Forming Electrical Networks in Three Dimensions by Self-Assembly", 18 AUGUST 2000 VOL 289, SCIENCE [14] Mila Boncheva, Derek A. Bruzewicz, and George M. Whitesides, "Millimeter-scale selfassembly and its applications", Pure Appl. Chem., Vol. 75, No. 5, pp. 621–630, 2003. [15] Wei Zheng,Jaehoon Chung, and Heiko O. Jacobs, "Fluidic Heterogeneous Microsystems Assembly and Packaging", JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 4, AUGUST 2006 [16] Su Eun Chung, Wook Park, Sunghwan Shin, Seung Ah Lee, and Sunghoon Kwon, "Guided Fluidic Self-Assembly of Microtrains Using Railed Microfluidics". [17] Jiandong Fang, Kerwin Wang, Karl F. Böhringer, "Self Assembly of micro pumps with high uniformity in performance", IEEE Journal of Microelectromechanical Systems 12(2), pp.117127, 2003. [18] Jessamine M. K. Ng, Michael J. Fuerstman, Bartosz A. Grzybowski,Howard A. Stone, and George M. Whitesides, "Self-Assembly of Gears at a Fluid/Air Interface", J. AM. CHEM. SOC. 2003, 125, 7948-7958 [19] Ehsan Saeedi, Samuel S. Kim, James R. Etzkorn, Dierdre R. Meldrum, and Babak A. Parviz, "Automation and yield of micron-scale self-assembly processes", Proceedings of the 3rd Annual IEEE Conference on Automation Science and Engineering Scottsdale, AZ, USA, Sept 22-25, 2007