M.P. de Boer

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.

High-performance surface-micromachined inchworm actuator

This work demonstrates a polycrystalline silicon surface-micromachined inchworm actuator that exhibits high-performance characteristics such as large force (/spl plusmn/0.5 millinewtons), large velocity range (0 to /spl plusmn/4.4 mm/sec), large displacement range (/spl plusmn/100 microns), small step size (/spl plusmn/10, /spl plusmn/40 or /spl plusmn/100 nanometers), low power consumption (nanojoules per cycle), continuous bidirectional operation and relatively small area (600 /spl times/ 200/spl mu/m/sup 2/). An in situ load spring calibrated on a logarithmic scale from micronewtons to millinewtons, optical microscopy and Michelson interferometry are used to characterize its performance. The actuator consists of a force-amplifying plate that spans two voltage-controlled clamps, and walking is achieved by appropriately sequencing signals to these three components. In the clamps, normal force is borne by equipotential rubbing counterfaces, enabling friction to be measured against load. Using different monolayer coatings, we show that the static coefficient of friction can be changed from 0.14 to 1.04, and that it is load-independent over a broad range. We further find that the static coefficient of friction does not accurately predict the force generated by the actuator and attribute this to nanometer-scale presliding tangential deflections.

Interferometry of actuated microcantilevers to determine material properties and test structure nonidealities in MEMS

By integrating interferometric deflection data from electrostatically actuated microcantilevers with a numerical finite difference model, we have developed a step-by-step procedure to determine values of Youngs modulus while simultaneously quantifying nonidealities. The central concept in the methodology is that nonidealities affect the long-range deflections of the beams, which can be determined to near nanometer accuracy. Beam take-off angle, curvature and support post compliance are systematically determined. Youngs modulus is then the only unknown parameter, and is directly found. We find an average value of Youngs modulus for polycrystalline silicon of 164.3 GPa and a standard deviation of 3.2 GPa (/spl plusmn/2%), reflecting data from three different support post designs. Systematic errors were assessed and may alter the average value by /spl plusmn/5%. An independent estimate from grain orientation measurements yielded 163.4-164.4 GPa (the Voigt and Reuss bounds), in agreement with the step-by-step procedure. Other features of the test procedure include that it is rapid, nondestructive, verifiable and requires only a small area on the test chip.

Mechanics of microcantilever beams subject to combined electrostatic and adhesive forces

One of the most important issues facing the continued development and application of microelectromechanical systems (MEMS) is that of adhesion and friction between microstructures intended to transfer force. In this work, we develop modeling approaches for studying adhesion (i.e., stiction) using the observed shape of microcantilevers under electrostatic loading. Analytical models for an idealized configuration are presented first. The solutions reveal the regimes over which the cantilever deflections are sensitive to adhesion versus applied loading. Also, the energy release rate and hence the cantilever adhesion value is shown to be independent of the curvature of the initially freestanding beam. Second, with a finite-element modeling approach, we quantify the slight sensitivity of the cantilever deflections to the surface force law assumed and show that with Angstrom scale resolution of beam deflections, cohesive zone law information can in principle be deduced. We also use this approach to model the nonuniform electrostatic loading force used in our experiments and the effect of support post compliance. We then demonstrate how adhesion values are obtained along the length of a microcantilever.

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.

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.

Effect of adhesion on dynamic and static friction in surface micromachining

We measure friction of monolayer-lubricated microelectromechanical systems surfaces under both static and dynamic conditions while continuously controlling the applied normal load at positive or negative values (i.e., compression or tension). The dynamic friction experiment methodology we have devised enables fitting to the complete one-dimensional equation of motion. We observe friction at zero applied load, and quantitatively attribute this to interfacial adhesion. Within error, the adhesive force is the same under static and dynamic conditions.

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.

An investigation of sidewall adhesion in MEMS

Adhesion or stiction is a key problem in surface micromachining technology, affecting the reliability of most MEMS. To date, the quantitative analysis of the phenomenon has been limited to in-plane adhesion. Since many micromechanisms involve contacts between sidewalls, we have designed a microinstrument to measure sidewall adhesion. Here, we describe the design, modeling, results and problems encountered with this first generation of sidewall adhesion devices.