Uthara Srinivasan

Microstructure to substrate self-assembly using capillary forces

We have demonstrated the fluidic self-assembly of micromachined silicon parts onto silicon and quartz substrates in a preconfigured pattern with submicrometer positioning precision. Self-assembly is accomplished using photolithographically defined part and substrate binding sites that are complementary shapes of hydrophobic self-assembled monolayers. The patterned substrate is passed through a film of hydrophobic adhesive on water, causing the adhesive to selectively coat the binding sites. Next, the microscopic parts, fabricated from silicon-on-insulator wafers and ranging in size from 150/spl times/150/spl times/15 /spl mu/m/sup 3/ to 400/spl times/400/spl times/50 /spl mu/m/sup 3/, are directed toward the substrate surface under water using a pipette. Once the hydrophobic pattern on a part comes into contact with an adhesive-coated substrate binding site, shape matching occurs spontaneously due to interfacial free energy minimization. In water, capillary forces of the adhesive hold the parts in place with an alignment precision of less than 0.2 /spl mu/m. Permanent bonding of the parts onto quartz and silicon is accomplished by activating the adhesive with heat or ultraviolet light. The resulting rotational misalignment is within /spl sim/0.3/spl deg/. Using square sites, 98-part arrays have been assembled in less than 1 min with 100% yield. The general microassembly approach described here may be applied to parts ranging in size from the nano- to milliscale, and part and substrate materials including semiconductors, glass, plastics, and metals.

Alkyltrichlorosilane-based self-assembled monolayer films for stiction reduction in silicon micromachines

We have investigated the potential of self-assembled monolayer (SAM) coatings for the purpose of adhesion reduction in microelectromechanical systems (MEMS). Two types of SAM coatings, derived from the precursor molecules octadecyltrichlorosilane [CH/sub 3/(CH/sub 2/)/sub 17/SiCl/sub 3/, OTS] and 1H,1H,2H,2H-perfluorodecyltrichlorosilane [CF/sub 3/(CF/sub 2/)/sub 7/(CH/sub 2/)/sub 2/SiCl/sub 3/, FDTS], were applied to polycrystalline silicon microstructures in a liquid-based process. Due to the hydrophobicity of these coatings, the water capillary forces responsible for the phenomenon known as release-related stiction are eliminated, and SAM-coated cantilever beams 2 /spl mu/m thick, 2 /spl mu/m above the substrate, and up to 2 mm in length emerge dry and free standing when removed from the final water rinse. The effects of SAM coating on adhesion encountered during device operation, termed in-use stiction, were characterized using arrays of cantilever beams of varying lengths. Structures made with a polycrystalline silicon of 3-nm rms roughness gave apparent works of adhesion of 30 and 8 /spl mu/J/m/sup 2/ for the OTS and FDTS SAM coatings, respectively, in comparison to 56 mJ/m/sup 2/ for standard oxide-coated structures. These results demonstrate that OTS coating reduces adhesion by more than three orders of magnitude over the conventional process and that the fluorinated SAM can lessen it further by four times. With regard to thermal stability, both SAM coatings can withstand heat treatment for 5 min at 450/spl deg/C in an N/sub 2/ ambient. In air, the OTS film begins to degrade at 150/spl deg/C while the fluorinated coating remains intact up to 400/spl deg/C. Therefore, both types of SAM coatings are compatible with several MEMS packaging techniques, with the FDTS monolayers exhibiting superior stiction and thermal stability properties to those derived from OTS. Furthermore, the FDTS formation does not require any chlorinated solvents such as carbon tetrachloride, which has been banned from industrial use, making the latter coating an industrially viable antistiction treatment.

Fluidic self-assembly of micromirrors onto microactuators using capillary forces

The authors discuss the application of self-assembly techniques for positioning microscopic components onto a substrate in a desired configuration. The basis is a fluidic self-assembly technique in which capillary forces assemble microparts with submicrometer alignment precision. A heat-curable acrylate-based adhesive is used to provide the capillary forces for assembly and is then polymerized in a bath of water at 80/spl deg/C for 16 h with continuous nitrogen bubbling. The application we describe is self-assembly of flat silicon micromirrors; onto surface-micromachined actuators for use in an adaptive-optics mirror array. Photolithography defines shapes of hydrophobic self-assembled monolayers for self-assembly. Mirrors with fill factors up to 95% were assembled. Mirrors 464 /spl mu/m in diameter and assembled onto actuators remain flat to within 6 nm rms. This mirror quality would be difficult to attain without the process decoupling afforded by microassembly. The general self-assembly approach described here can be applied to parts ranging in size from the nanometer to the millimeter scale and to a variety of part and substrate materials.

Modeling of capillary forces and binding sites for fluidic self-assembly

Massively parallel self-assembly is emerging as an efficient, low-cost alternative to conventional pick-and-place assembly of microfabricated components. The fluidic self-assembly technique we have developed exploits hydrophobic-hydrophilic surface patterning and capillary forces of an adhesive liquid between binding sites to drive the assembly process. To achieve high alignment yield, the desired assembly configuration must be a (global) energy minimum, while other (local) energy minima corresponding to undesired configurations should be avoided. Thus, the design of an effective fluidic self-assembly system using this technique requires an understanding of the interfacial phenomena involved in capillary forces; improvement of its performance involves the global optimization of design parameters such as binding site shapes and surface chemistry. This paper presents a model and computational tools for the efficient analysis and simulation of fluidic self-assembly. The strong, close range attractive forces that govern our fluidic self-assembly technique are approximated by a purely geometric model, which allows the application of efficient algorithms to predict system behavior. Various binding site designs are analyzed, and the results are compared with experimental observations. For a given binding site design, the model predicts the outcome of the self assembly process by determining minimum energy configurations and detecting unwanted local minima, thus estimating expected yield. These results can be employed toward the design of more efficient self-assembly systems.

Self-assembled fluorocarbon films for enhanced stiction reduction

We have developed a fluorinated self-assembled monolayer (SAM) coating process for stiction reduction in polysilicon MEMS that does not use chlorinated solvents. Using this process, cantilever beams up to 2 mm in length emerge from the final water rinse dry and released. Beam arrays fabricated from two types of polysilicon were used to characterize in-use stiction. In the first set of structures (poly A, rms roughness /spl ap/12 nm), all beams out to the maximum length of 1 mm remained unstuck following actuation, giving an adhesion energy per apparent contact area of less than 2.4 /spl mu/J/m/sup 2/. Poly B structures (rms roughness /spl ap/3 nm) gave an apparent adhesion energy of 5.2 /spl mu/J/m/sup 2/ compared to 23 /spl mu/J/m/sup 2/ for the OTS SAM coating. The fluorinated SAMs survive heat treatment in both air and N/sub 2/ at 400/spl deg/C for 5 minutes and are thus compatible with several MEMS packaging processes.

Micromirrors for Adaptive-Optics Arrays

We present a micromachined mirror array for use in adaptive optics. Piston motions of more than 6 µm as well as tip/tilt motions of 11 mrad (0.65°) are demonstrated. Single-crystal-silicon mirrors are assembled onto electrostatic parallel-plate actuators. The actuators lift off the substrate after microstructure release as a consequence of residual stresses in nickel-polysilicon bimorph flexures. The assembled mirrors provide fill factors of 95%. Peak-to-valley surface variations are smaller than 30 nm over a 464 µm-diameter (vertex-to-vertex) mirror segment.

Lubrication of polysilicon micromechanisms with self-assembled monolayers

Here, the authors report on the lubricating effects of self-assembled monolayers (SAMs) on MEMS by measuring static and dynamic friction with two polysilicon surface- micromachined devices. The first test structure is used to study friction between laterally sliding surfaces and with the second, friction between vertical sidewalls can be investigated. Both devices are SAM-coated following the sacrificial oxide etch and the microstructures emerge released and dry from the final water rinse. The coefficient of static friction, {mu}{sub s} was found to decrease from 2.1 {+-} 0.8 for the SiO{sub 2} coating to 0.11 {+-} 0.01 and 0.10 {+-} 0.01 for films derived from octadecyltrichloro-silane (OTS) and 1H,1H,2H,2H-perfluorodecyl-trichlorosilane (FDTS). Both OTS and FDTS SAM-coated structures exhibit dynamic coefficients of friction, {mu}{sub d} of 0.08 {+-} 0.01. These values were found to be independent of the apparent contact area, and remain unchanged after 1 million impacts at 5.6 {micro}N (17 kPa), indicating that these SAMs continue to act as boundary lubricants despite repeated impacts. Measurements during sliding friction from the sidewall friction testing structure give comparable initial {mu}{sub d} values of 0.02 at a contact pressure of 84 MPa. After 15 million wear cycles, {mu}{sub d} was found to rise to 0.27. Wear of the contacting surfaces was examined by SEM. Standard deviations in the {mu} data for SAM treatments indicate uniform coating coverage.

Fluidic self-assembly of micromirrors onto surface micromachined actuators

We describe the fluidic self-assembly of ultra-flat, single-crystal silicon micromirrors onto a surface micromachined actuator array. Sub-micron precision self-alignment of the mirrors onto the actuator platforms is achieved by pattern matching hydrophobic binding sites on the underside of the mirror and on the platform.

Stroboscopic interferometer with variable magnification to measure dynamics in an adaptive-optics micromirror

Interferometry has proven a powerful tool to measure out-of-plane movements in MEMS with high accuracy, In this paper, we demonstrate a setup for stroboscopic interferometry that combines the precise data registration obtained by combining phase-shifting techniques with the high spatial resolution and aperture that are characteristic for an optical microscope.

MEMS: Some Self-Assembly Required

Fluidic self-assembly can provide the vital step in the fabrication of chip-sized optical MEMS for a wide variety of applications. Nature herself provides an inspirational model for fluidic self-assembly, as well as useful guidelines for processes that can be employed to build microsystems. In this review article, we describe some achievements in assembly that have already been demonstrated, including our own research on fluidic self-assembly of micromirrors for an adaptive-optics array. The results reported to date lead us to predict significant growth in the applications to microsystems of fluidic self-assembly techniques.