Terry A. Michalske

Slow fracture model based on strained silicate structures

Slow crack growth data, molecular‐orbital calculations, and vibrational spectroscopy results are used to develop an atomistic model for environmentally controlled fracture of silica glass. The model is based on chemically active bond defects generated by the strain field of the crack tip. Molecular‐orbital results suggest that bond angle deformations are most effective in increasing the chemical activity of the Si–O–Si bond. Vibrational spectra identify silica polymorphs containing highly strained bonding configurations. A comparison of strained bond reactivity with crack growth results shows that strained silica polymorphs can be used to model the crack tip chemical reactions controlling slow fracture. Based on model complexes, a two‐dimensional fracture model involving kink site nucleation and motion is developed. The model shows that the stress intensity dependence of the crack growth rate is controlled by the energy required to form chemically active defects in the silica structure. The absolute rate ...

Accurate method for determining adhesion of cantilever beams

Using surface micromachined samples, we demonstrate the accurate measurement of cantilever beam adhesion by using test structures which are adhered over long attachment lengths. We show that this configuration has a deep energy well, such that a fracture equilibrium is easily reached. When compared to the commonly used method of determining the shortest attached beam, the present method is much less sensitive to variations in surface topography or to details of capillary drying.

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.

Kinetics of dissociative chemisorption on strained edge-shared surface defects on dehydroxylated silica

Abstract Fourier transform infrared (FTIR) spectroscopy is used to study the kinetics of reactions between water, ammonia, methanol, and methylamine and the edge-shared tetrahedral surface defects in dehydroxylated silica. Results show that all reactant gases tested undergo a dissociative chemisorption reaction on strained SiO bonds in the edge-shared ring. The reaction kinetics and observed product distributions indicate that two types of edge-shared rings are present, one which contains silanol groups and one which does not. The silanol-containing defects are less reactive than the silanol-free defects, with reaction rates which are more sensitive to the basicity of the gas phase reactant. The rate of reaction between water and defects is at least 100000 faster than the hydrolysis of unstrained SiO bonds. The relative reactivities of strained and unstrained Si-O bonds show that bond strain promotes bond rupture reactions which are important in phenomena such as the stress corrosion cracking of silica.

Adhesion hysteresis of silane coated microcantilevers

We have developed a new experimental approach for measuring hysteresis in the adhesion between a free standing thin film and a substrate. By accurately measuring and modeling the deformations in micromachined cantilever beams that are subject to combined interfacial adhesive and applied electrostatic forces, we determine adhesion energies for advancing and receding contacts. We examined adhesion hysteresis for silane coated cantilevers and found no hysteresis at low relative humidity (RH) conditions. The dominant contribution to interfacial energy at low RH is van der Waals attraction between portions of the surfaces that are separated by nanometer asperities. In contrast, significant hysteresis was observed for surfaces that were exposed to high RH conditions. Atomic force microscopy studies of these surfaces showed spontaneous formation of silane mounds that have irreversibly transformed from initially uniform hydrophobic surface layers. Contact mechanics considerations show that the compliance of the mounds can reasonably allow microcapillaries in surrounding hydrophilic areas to bridge at high RH as the surfaces are forced into contact by an externally applied load, leading to the adhesion hysteresis.

Spatially-resolved analysis of nanoparticle nucleation and growth in a microfluidic reactor

Microfluidic systems provide a unique platform for investigation of fundamental reaction processes, which is critical to understanding how to control nanostructure synthesis on a production scale. We have examined the synthesis of cysteine-capped CdS quantum dot nanocrystals (CdS-Cys) between two interdiffusing reagent streams in a continuous-flow microfluidic reactor. Using spatially resolved photoluminescence imaging and spectroscopy of the microreactor, we have acquired kinetic and mechanistic data on the CdS-Cys nanoparticle nucleation and growth, and observed a binary shift in the particle emission spectrum from a higher (2.9 eV) to lower (2.5 eV) energy emission peak within the first second of residence time. Several reactor models have been tested against the spatially and spectrally resolved signals, which suggest that homogeneous reaction and particle nucleation are diffusion-limited and occur only at the boundary between the two laminar streams, while a slower activation process occurs on a longer (seconds) time scale. The results provide direct insight into the rapid processes that occur during crystallization in microfluidic mixing channels, and demonstrate the potential of using controlled microfluidic environments with spatially resolved monitoring to conduct fundamental studies of nanocrystal nucleation and growth.

The Role of Interfacial Properties on MEMS Performance and Reliability

We have constructed a humidity-controlled chamber in which deflections of polysilicon cantilever beams are observed by interferometry, resulting in in-situ adhesion measurements within a fracture mechanics framework. From adhesion energy measurements for uncoated hydrophilic beams, we demonstrate an exponential dependence of adhesion on relative humidity (RH). We can explain this trend with a single-asperity model for capillary condensation. For coated hydrophobic beams, adhesion is independent of RH up to a threshold value which depends on the coating used. However, we have found that exposure to very high RH (greater than or equal to 90%) ambients can cause a dramatic increase in adhesion, surprisingly with a stronger effect for perfluorodecyltrichlorosilane (FDTS, C10H4F17SiCl3) than octadeycltrichlorosilane (ODTS, C18H37SiCl3). Newly developed computational mechanics to measure adhesion in the presence of an applied load allow us to explore how the adhesion increase develops. We believe that water adsorption at silanol sites at the FDTS/substrate interface, possibly exacerbated by coupling agent migration, leads to water islanding and the subsequent adhesion increase at very high RH levels.

The mechanical response of gold substrates passivated by self-assembling monolayer films

Interfacial force microscopy has been used to show that a single layer of self-assembling molecules adsorbed on a gold substrate can prevent adhesion between gold and a tungsten probe. The passivated gold is able to elastically support large repulsive loads, with plots of load versus deformation closely following the Hertzian model. The gold shear-stress threshold for plastic deformation is determined to be ∼1 gigapascal, which is in agreement with the theoretical value for the intrinsic gold-lattice stability.

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.

A micromechanical flow sensor for microfluidic applications

We fabricated a microfluidic flow meter and measured its response to fluid flow in a microfluidic channel. The flow meter consisted of a micromechanical plate, coupled to a laser deflection system to measure the deflection of the plate during fluid flow. The 100 /spl mu/m square plate was clamped on three sides and elevated 3 /spl mu/m above the bottom surface of the channel. The response of the flow meter was measured for flow rates, ranging from 2.1 to 41.7 /spl mu/L/min. Several fluids, with dynamic viscosities ranging from 0.8 to 4.5/spl times/10/sup -3/ N/m, were flowed through the channels. Flow was established in the microfluidic channel by means of a syringe pump, and the angular deflection of the plate monitored. The response of the plate to flow of a fluid with a viscosity of 4.5/spl times/10/sup -3/ N/m was linear for all flow rates, while the plate responded linearly to flow rates less than 4.2 /spl mu/L/min of solutions with lower dynamic viscosities. The sensitivity of the deflection of the plate to fluid flow was 12.5/spl plusmn/0.2 /spl mu/rad/(/spl mu/L/min), for a fluid with a viscosity of 4.5/spl times/10/sup -3/ N/m. The encapsulated plate provided local flow information along the length of a microfluidic channel.