Peggy J. Clews

Chemical Vapor Deposition of Fluoroalkylsilane Monolayer Films for Adhesion Control in Microelectromechanical Systems

We have developed a new process for applying a hydrophobic, low adhesion energy coating to microelectromechanical (MEMS) devices. Monolayer films are synthesized from tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (FOTS) and water vapor in a low-pressure chemical vapor deposition process at room temperature. Film thickness is self-limiting by virtue of the inability of precursors to stick to the fluorocarbon surface of the film once it has formed. We have measured film densities of {approx}3 molecules nm{sup 2} and film thickness of {approx}1 nm. Films are hydrophobic, with a water contact angle >110{sup o}. We have also incorporated an in-situ downstream microwave plasma cleaning process, which provides a clean, reproducible oxide surface prior to film deposition. Adhesion tests on coated and uncoated MEMS test structures demonstrate superior performance of the FOTS coatings. Cleaned, uncoated cantilever beam structures exhibit high adhesion energies in a high humidity environment. An adhesion energy of 100 mJ m{sup -2} is observed after exposure to >90% relative humidity. Fluoroalkylsilane coated beams exhibit negligible adhesion at low humidity and { 90% relative humidity. No obvious film degradation was observed for films exposed to >90% relative humidity at room temperature for >24 hr.

Adhesion of polysilicon microbeams in controlled humidity ambients

The authors characterize in-situ the adhesion of surface micromachined polysilicon beams subject to controlled humidity ambients. Beams were freed by supercritical CO{sub 2} drying. Consistent adhesion results were obtained using a post-treatment in an oxygen plasma which rendered the microbeams uniformly hydrophilic. Individual beam deformations were measured by optical interferometry after equilibration at a given relative humidity (RH). Validation of each adhesion measurement was accomplished by comparing the deformations with elasticity theory. The data indicates that adhesion increases exponentially with RH from 30% to 95%, with values from 1 mJ/m{sup 2} to 50 mJ/m{sup 2}. Using the Kelvin equation, the authors show that the data should be independent of RH if a smooth interface is considered. By modeling a rough interface consistent with atomic force microscopy (AFM) data, the exponential trend is satisfactorily explained.

The Influence of Coating Structure on Micromachine Stiction

Stiction and friction in micromachines is commonly inhibited through the use of silane coupling agents such as 1H-, 1H-, 2H-, 2H-perfluorodecyltrichlorosilane (FDTS). FDTS coatings have allowed micromachine parts processed in water to be released without debilitating capillary adhesion occurring. These coatings are frequently considered as densely-packed monolayers, well-bonded to the substrate. In this paper, it is demonstrated that FDTS coatings can exhibit complex nanoscale structures, which control whether micromachine parts release or not. Surface images obtained via atomic force microscopy reveal that FDTS coating solutions can generate micellar aggregates that deposit on substrate surfaces. Interferometric imaging of model beam structures shows that stiction is high when the droplets are present and low when only monolayers are deposited. As the aggregate thickness (tens of nanometers) is insufficient to bridge the 2 μm gap under the beams, the aggregates appear to promote beam–substrate adhesion by changing the wetting characteristics of coated surfaces. Contact angle measurements and condensation figure experiments have been performed on surfaces and under coated beams to quantify the changes in interfacial properties that accompany different coating structures. These results may explain the irreproducibility that is often observed with these films.

Microscale c-Si (c)PV cells for low-cost power

We are exploring fabrication and assembly concepts developed for Microsystems/MEMS technology to reduce the cost of solar PV power. These methods have the potential to reduce many system level costs of current PV systems including, among others, silicon material costs, module assembly costs, and installation costs. We have demonstrated a direct c-Si material reduction of approximately 20X (including wire-saw kerf loss and polishing loss). The cells have achieved efficiencies of almost 9% and Jsc of 30 mA/cm2. We are currently using integrated-circuit (IC) fabrication tools that will lead to higher efficiencies and improved yield. These advantages and the material reduction are expected to reduce the current module manufacturing costs.

Fully integrated switchable filter banks

Fully integrated switchable filter have been successfully demonstrated using a ra CMOS SOI process in conjunction with an a (AlN) microresonator process. Single pole-mul were developed in the CMOS SOI process th multi-project wafer runs while the filters were aluminum nitride based microresonators. Each concurrent design cycles and was demonstrated to integration. After design improvements to bo full monolithic integration was implem microresonator filters with the CMOS switc compatibility of the two technologies. A four ch switchable bank of ∼7MHz bandwidth filters demonstrated exhibiting approximately 8 dB of 60dB of stop band rejection.

Ultrathin Flexible Crystalline Silicon: Microsystems-Enabled Photovoltaics

We present an approach to create ultrathin (<;20 μm) and highly flexible crystalline silicon sheets on inexpensive substrates. We have demonstrated silicon sheets capable of bending at a radius of curvature as small as 2 mm without damaging the silicon structure. Using microsystem tools, we created a suspended submillimeter honeycomb-segmented silicon structure anchored to the wafer only by small tethers. This structure is created in a standard thickness wafer enabling compatibility with common processing tools. The procedure enables all the high-temperature steps necessary to create a solar cell to be completed while the cells are on the wafer. In the transfer process, the cells attach to an adhesive flexible substrate which, when pulled away from the wafer, breaks the tethers and releases the honeycomb structure. We have previously demonstrated that submillimeter and ultrathin silicon segments can be converted into highly efficient solar cells, achieving efficiencies up to 14.9% at a thickness of 14 μm. With this technology, achieving high efficiency (>;15%) and highly flexible photovoltaic (PV) modules should be possible.

Measuring and modeling electrostatic adhesion in micromachines

We measure deformations of electrostatically actuated cantilever beams adhered to a substrate and compare results to numerical simulations. Beams are peroxide treated and either supercritically dried or coated with octadecyltrichlorosilane (ODTS). In air with relative humidity (RH) of about 50%, measured deformations are consistent with numerical results when an effective insulator thickness of 14 nm is assumed. This value is a measure of the roughness of the substrate. Deformations are generally reversible for voltages up to 2 V. With RH of 75%, deformations of supercritically dried beams are dominated by capillary rather than electrostatic forces. The ODTS-coated beams exhibit less effect from the capillary forces, as expected for a hydrophobic coating.

Microfabrication of microsystem-enabled photovoltaic (MEPV) cells

Microsystem-Enabled Photovoltaic (MEPV) cells allow solar PV systems to take advantage of scaling benefits that occur as solar cells are reduced in size. We have developed MEPV cells that are 5 to 20 microns thick and down to 250 microns across. We have developed and demonstrated crystalline silicon (c-Si) cells with solar conversion efficiencies of 14.9%, and gallium arsenide (GaAs) cells with a conversion efficiency of 11.36%. In pursuing this work, we have identified over twenty scaling benefits that reduce PV system cost, improve performance, or allow new functionality. To create these cells, we have combined microfabrication techniques from various microsystem technologies. We have focused our development efforts on creating a process flow that uses standard equipment and standard wafer thicknesses, allows all high-temperature processing to be performed prior to release, and allows the remaining post-release wafer to be reprocessed and reused. The c-Si cell junctions are created using a backside point-contact PV cell process. The GaAs cells have an epitaxially grown junction. Despite the horizontal junction, these cells also are backside contacted. We provide recent developments and details for all steps of the process including junction creation, surface passivation, metallization, and release.

Thermal conductivity manipulation in single crystal silicon via lithographycally defined phononic crystals

The thermal conductivity of single crystal silicon was engineered to be as low as 32.6W/mK using lithographically defined phononic crystals (PnCs), which is only one quarter of bulk silicon thermal conductivity [1]. Specifically sub-micron through-holes were periodically patterned in 500nm-thick silicon layers effectively enhancing both coherent and incoherent phonon scattering and resulting in as large as a 37% reduction in thermal conductivity beyond the contributions of the thin-film and volume reduction effects. The demonstrated method uses conventional lithography-based technologies that are directly applicable to diverse micro/nano-scale devices, leading to potential performance improvements where heat management is important.

Back-contacted and small form factor GaAs solar cell

We present a newly developed microsystem enabled, back-contacted, shade-free GaAs solar cell. Using microsystem tools, we created sturdy 3 µm thick devices with lateral dimensions of 250 µm, 500 µm, 1 mm, and 2 mm. The fabrication procedure and the results of characterization tests are discussed below. The highest efficiency cell had a lateral size of 500 µm and a conversion efficiency of 10%, open circuit voltage of 0.9 V and a current density of 14.9 mA/cm2 under one-sun illumination.