Leslie M. Phinney

A thermomechanical model for adhesion reduction of MEMS cantilevers

Presents a thermomechanical model that describes adhesion reduction in MEMS structures using laser heating. A fracture mechanics model is developed where the interface between the stiction-failed microcantilever and the substrate is treated as a crack, and the energy release rate is calculated using elastic theory. In order to include the effect of a temperature difference between the microcantilever and the substrate, an associated thermal strain energy is included in the fracture model. If the free length is longer than the critical buckling length, the beam buckles decreasing the strain energy of the system. For surface-micromachined polycrystalline silicon cantilevers with an initial crack length of 400 /spl mu/m, the model predicts that a temperature difference of 100 K repairs microcantilevers as long as 1300 /spl mu/m. The peeling of adhered beams from the substrate after laser irradiation is experimentally shown with measured crack lengths within 15% of predicted values indicating that the proposed model establishes the mechanism of adhesion reduction by laser irradiation.

Thermal contact conductance of adhered microcantilevers

The thermal contact conductance G for polycrystalline silicon cantilever beams that are adhered to an underlying substrate is examined using two different optical techniques. Using time-domain thermoreflectance, we measure G=9±2 MW m−2 K−1 at 25 °C and G=4±1 MW m−2 K−1 at 150 °C. The room temperature value is confirmed using a modified Angstrom method, which establishes a lower limit of G>5 MW m−2 K−1. This contact conductance is a factor of 10–105 greater than values reported for metal–metal and ceramic–ceramic interfaces. The large interfacial conductance is consistent with the presence of a thin layer of water trapped between the cantilever and the substrate. The thermal conductivity Λ of the phosphorus doped polysilicon cantilever is nearly isotropic with Λcross plane=65 W m−1 K−1, and Λin plane=70 W m−1 K−1 at room temperature.The thermal contact conductance G for polycrystalline silicon cantilever beams that are adhered to an underlying substrate is examined using two different optical techniques. Using time-domain thermoreflectance, we measure G=9±2 MW m−2 K−1 at 25 °C and G=4±1 MW m−2 K−1 at 150 °C. The room temperature value is confirmed using a modified Angstrom method, which establishes a lower limit of G>5 MW m−2 K−1. This contact conductance is a factor of 10–105 greater than values reported for metal–metal and ceramic–ceramic interfaces. The large interfacial conductance is consistent with the presence of a thin layer of water trapped between the cantilever and the substrate. The thermal conductivity Λ of the phosphorus doped polysilicon cantilever is nearly isotropic with Λcross plane=65 W m−1 K−1, and Λin plane=70 W m−1 K−1 at room temperature.

Surface roughness measurements of micromachined polycrystalline silicon films

The characteristics of the materials and surfaces in microelectromechanical systems (MEMS) and microsystems technology (MST) profoundly affect the performance, reliability, and wear of MEMS and MST devices. It is critical to measure the properties of surfaces that are in contact during microstructure movement, such as the underside of a MEMS gear and the underlying substrate. However, contacting surfaces are usually inaccessible unless the MEMS device is broken and removed from the substrate. This paper presents a nondestructive method for characterizing commercially fabricated surface micromachined polycrystalline silicon (polysilicon) devices. Microhinged flaps were designed that enable access to the upper surface, the part of a structural layer deposited last; the lower surface, the part of a structural layer deposited first; and the underlying substrate. Due to the susceptibility of surface-micromachined MEMS to adhesion failures, the surface roughness is a key parameter for predicting device behavior. Using the microhinged flaps, the RMS surface roughness for polycrystalline surfaces was measured and indicated that the upper surfaces were 3.5–6.4 times rougher than the lower surfaces. The difference in the surface roughness for the upper surface, which is easily accessed and the one most commonly characterized, and that for the lower surface reveals the importance of characterizing contacting surfaces in MEMS and MST devices.

Optimization Study of a Silicon-Carbide Micro-Capillary Pumped Loop

Wide bandgap semiconductors such as SiC and GaN are materials that are advantageous for high power electronic devices. High power devices generate large amounts of energy that must be removed, and traditional cooling methods are insufficient for maintaining the desired operating temperatures. Thus, thermal management methods for high power electronic devices need to be developed. A SiC micro-capillary pumped loop thermal management system is being evaluated to cool SiC high power devices. Mathematical models incorporating two-phase flow and capillary wicking have been developed to analyze capillary pumped loops or loop heat pipes. This investigation uses a model based on the methodology of Dickey and Peterson (1994). The model takes an energy balance on the condenser and evaporator regions, as well as a pressure balance across the meniscus. A parametric study has been performed on the micro-CPL to determine the best design for a p-i-n diode that is less than 1 cm square and which produces a heat flux at the junction of over 300 W/cm2 . The micro-CPL will be limited to a maximum size of 6.5 cm2 . The liquid and vapor line lengths, number of grooves, and groove dimensions are varied to determine optimal values. The results and trends of the optimization calculations are discussed.Copyright

Investigation of adhesion during operation of MEMS cantilevers

Reliability of MEMS is a major concern for the commercialization of laboratory prototypes. Surface adhesion or stiction strongly affects the reliability of MEMS devices which have sliding or rubbing contacts. Determination of adhesion energies, adhesion forces, and pull-off forces are important for predicting stiction in MEMS. We present an experimental technique to estimate the pull-off forces for MEMS surfaces. Polysilicon microcantilevers were electrostatically actuated using gradually varying voltages. A hysteresis was observed in the voltage at which the tip of the cantilevers made and broke contact with the substrate. Pull-off forces were estimated from the hysteresis in the voltage values using a strain energy formulation. The pull-off forces for microcantilevers dried out of isopropyl alcohol and repaired using laser irradiation were estimated to be in the range of 45-121 nN. The role of adhered length, variable external loading, and actuating signal on in-use stiction is also investigated. From our experimental results, we demonstrate an empirical approach to predict in-use stiction of microcantilevers.

Release processing effects on laser repair of stiction-failed microcantilevers

Undesirable adhesion in microelectromechanical systems (MEMS) is referred to as stiction and is a principal failure mechanism in surface-micromachined MEMS devices. Previous investigations demonstrated repairing stiction-failed polycrystalline silicon MEMS structures released from isopropyl alcohol (IPA) using Nd:YAG laser irradiation and predicting the laser repair process using a thermomechanical model. The current paper reports the effectiveness of the laser repair process and corresponding thermomechanical model predictions for microcantilevers that have failed during four release treatments: water, IPA, octadecyltrichlorosilane (OTS), and supercritical CO/sub 2/ drying. Model predictions and experimental measurements of the laser repair process are also provided for MEMS devices that failed due to contact during electrostatic actuation. The results indicate that the laser repair process is very effective for both failure modes and that the thermomechanical model predicts the laser repair of microcantilevers that failed during release much better than for microcantilevers that failed due to subsequent contact.

Effects of temperature on surface adhesion in MEMS structures

Techniques to predict the reliability of microdevices are necessary to facilitate the transfer of MEMS designs from the laboratory to the marketplace. An important reliability concern for microfabricated structures is in-use stiction, the operational failure of devices due to surface adhesion. The current study determines the temperature dependence of in-use stiction for polyscrystalline silicon microcantilevers subjected to three different release conditions: supercritical CO2 drying; laser-irradiation repair; and self- assembled monolayer post processing. The microcantilever beam arrays were electrostatically actuated at temperatures between 22

Use of thermal cycling to reduce adhesion of OTS-coated MEMS cantilevers

DEGC and 300

Surface characterization and adhesion analysis for polysilicon micromachined flaps

DEGC. The supercritical CO2 dried devices showed an overall decrease in sticking probability as the actuation temperature was raised to 300

Laser Repair Process Yields for MEMS Devices Adhered During Release or Operation

DEGC. After a distinct improvement in the failure rate between the first and second actuation temperatures, arrays released using laser-irradiation did not exhibit a consistent trend. Samples coated with an OTS monolayer had large increases in their sticking probability as the temperature was raised. However, at temperatures above 200