We report on the effects of mechanical stress on thermal microactuator performance. Packaging processes such as die
attach and lid sealing usually result in stresses on the die containing microsystems devices. While this phenomenon is
known, quantifying the effects systematically is difficult due to challenges in controlling the resultant stress resulting
from packaging. In this study, we use a four-point bending stage to apply loads of 12 lbf (53.4 N) in tension and
compression to 11.5 mm by 2.9 mm samples. Thermal microactuators and stress gauges were fabricated using the
Sandia 5-layer SUMMiT surface micromaching process and diced to fit in the bending stage. At each stress level, the
vernier scales on the thermal microactuator were imaged in order to determine the displacements. Thermal
microactuator displacements are reported as a function of applied current up to 35 mA at varying stress levels.
Increasing tensile stress decreases the initial displacement and flattens the thermal microactuator displacement versus
applied current curve. Raman spectroscopy and stress gauge measurements indicate that the stress range for the fourpoint
bending stage experiments extends from 200 MPa tensile to -250 MPa compressive. Numerical model predictions
of thermal microactuator displacement versus current are in qualitative agreement with the experimental results.
Quantitative information on the reduction in thermal microactuator performance as a function of stress provides
validation data for MEMS models and can guide future designs so that they will be more robust to stresses resulting
from packaging processes.
The response of microsystem components to laser irradiation is relevant to processes and applications such as laser processing, optical diagnostics, and optical microelectromechanical systems (MEMS) device design and performance. The dimensions of MEMS, which are on the order of several micrometers, are on the same order as infrared laser wavelengths, which results in interference phenomena when the parts are partially transparent. Four polycrystalline silicon structures were designed and irradiated with 532 and 808 nm continuous wave laser light to determine the effects of layers, air gaps, and the presence of a substrate via on the threshold laser power for damage. The presence of a substrate via resulted in lower damage thresholds, and a single-layer structure had the highest damage threshold for structures irradiated with infrared light. Structures irradiated with visible wavelength light exhibited less sensitivity to the underlying via. Optical interference calculations are carried out to evaluate the absorptance of these structures as a feasible explanation for the observed results.
Microsystems are potentially exposed to laser irradiation during processing, diagnostic measurements, and, in some cases, device operation. The behavior of the components in an optical MEMS device that are irradiated by a laser needs to be optimized for reliable operation. Utilizing numerical simulations facilitates design and optimization. This paper reports on experimental and numerical investigations of the thermomechanical response of polycrystalline silicon microcantilevers that are 250 μm wide, 500 μm long, and 2.25 μm thick when heated by an 808 nm laser. At laser powers above 400 mW significant deflection is observed during the laser pulse using a white light interferometer. Permanent deformation is detected at laser powers above 650 mW in the experiments. Numerical calculations using a coupled physics finite element code, Calagio, agree qualitatively with the experimental results. Both the experimental and numerical results reveal that the initial stress state is very significant. Microcantilevers deflect in the direction of their initial deformation upon irradiation with a laser.
Optical actuation of microelectromechanical systems (MEMS) is advantageous for applications for which electrical isolation is desired. Thirty-two polycrystalline silicon opto-thermal actuators, optically-powered MEMS thermal actuators, were designed, fabricated, and tested. The design of the opto-thermal actuators consists of a target for laser illumination suspended between angled legs that expand when heated, providing the displacement and force output. While the amount of displacement observed for the opto-thermal actuators was fairly uniform for the actuators, the amount of damage resulting from the laser heating ranged from essentially no damage to significant amounts of damage on the target. The likelihood of damage depended on the target design with two of the four target designs being more susceptible to damage. Failure analysis of damaged targets revealed the extent and depth of the damage.
KEYWORDS: Microelectromechanical systems, Actuators, Reliability, Interferometry, Power supplies, Oxides, Mirrors, Switches, Space based lasers, Prototyping
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.
°Microelectromechanical systems (MEMS) have enormous potential to contribute in diverse fields such as automotive, health care, aerospace, consumer products, and biotechnology, but successful commercial applications of MEMS are still small in number. Reliability of MEMS is a major impediment to the commercialization of laboratory prototypes. Due to the multitude of MEMS applications and the numerous processing and packaging steps, MEMS are exposed to a variety of environmental conditions, making the prediction of operational reliability difficult. In this paper, we investigate the effects of operating temperature on the in-use adhesive failure of electrostatically actuated MEMS microcantilevers coated with octadecyltrichlorosilane (OTS) films. The cantilevers are subjected to repeated temperature cycles and electrostatically actuated at temperatures between 25°C and 300°C in ambient air. The experimental results indicate that temperature cycling of the OTS coated cantilevers in air reduces the sticking probability of the microcantilevers. The sticking probability of OTS coated cantilevers was highest during heating, which decreased during cooling, and was lowest during reheating. Modifications to the OTS release method to increase its yield are also discussed.
Due to the relatively high compliance, large surface-to-volume ratio, and small separation distances, micromachined polycrystalline silicon (polysilicon) structures are susceptible to high adhesion forces including van der Waals, electrostatic, and capillary forces. Since these forces depend on the surface separation distance, it is essential to understand the microtribological properties, especially the surface roughness. In this study, four types of polysilicon microhinged flaps were designed to characterize the surfaces. The flaps enable access to both the top and bottom surfaces of the structural polysilicon layers. Multiple locations are scanned for each surface type using a Digital Instruments 3100 atomic force microscope (AFM). The results indicate that the top surface is much rougher than the bottom surface for structural layers and the roughness is influenced by the adjacent layers. Since the base polysilicon layer (poly0) is six times rougher than a base silicon nitride layer, depositing the MEMS devices on the poly0 layer rather than directly on silicon nitride will reduce adhesion. An adhesion model was used to analyze the effect of roughness parameters on stiction force between structural layers and substrate. Since the tip condition impacts the accuracy of AFM measurement, a wear test of silicon tips was also performed.
A technique for repairing stiction-failed, surface- micromachined polycrystalline silicon cantilevers using pulsed lasers has been developed and demonstrated. Microcantilevers, which adhered to the substrate during the chemical sacrificial layer etch and drying processes, were irradiated with either a 800 nm Ti:sapphire or a 1064 nm Nd:YAG laser and successfully freed from the substrate. The feasibility of the laser process was first shown with a Ti:sapphire laser system irradiating undoped, polycrystalline silicon microcantilevers. The repair yields for the Nd:YAG laser process were up to 100 percent for 10 micrometers wide beams as long as 1 mm for laser fluences of 70 mJ/cm2. The sample age and initial release quality affect the laser process yield, with newer samples and those with high quality initial releases having the highest number of microcantilevers being repaired.
Micro-Nano Technology Visualization (MNTV) is critical to studies in MEMS reliability. The ability to see and characterize the microstructures and interfaces with high resolution at the microscale and nanoscale is invaluable. In this paper we present the motivation, paradigm and examples of visualization techniques applied to several aspects of surface micromachined polysilicon structures. High resolution cross-section imaging, using both a FIB/SEM and FIB/STEM, is used to acquire information on profile differences between fabrication facilities and grain size and orientation. The AFM is used to compare surface roughness on both sides (top and bottom surfaces) of thin film polysilicon after release etching. The data gathered will be extremely useful feedback for fabrication facilities in terms of process characterization and quality assurance. The data will also be useful for MEMS CAD tools where device and process models must be validated.
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$DEGC and 300$DEGC. The supercritical CO2 dried devices showed an overall decrease in sticking probability as the actuation temperature was raised to 300$DEGC. After a distinct improvement in the failure rate between the first and second actuation temperatures, arrays released using laser-irradiation did not exhibit a consistent trend. Samples coated with an OTS monolayer had large increases in their sticking probability as the temperature was raised. However, at temperatures above 200$DEGC, a decrease in in-use stiction was observed which continued through most of the cooling cycle.
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