Christopher L. Muhlstein

The critical role of environment in fatigue damage accumulation in deep-reactive ion-etched single-crystal silicon structural films

The importance of service environment to the fatigue resistance of n/sup +/-type, 10 /spl mu/m thick, deep-reactive ion-etched (DRIE) silicon structural films used in microelectromechanical systems (MEMS) was characterized by testing of electrostatically actuated resonators (natural frequency, f/sub 0/, /spl sim/40 kHz) in controlled atmospheres. Stress-life (S-N) fatigue tests conducted in 30/spl deg/C, 50% relative humidity (R.H.) air demonstrated the fatigue susceptibility of silicon films. Further characterization of the films in medium vacuum and 25% R.H. air at various stress amplitudes revealed that the rates of fatigue damage accumulation (measured via resonant frequency changes) are strongly sensitive to both stress amplitude and, more importantly, humidity. Scanning electron microscopy of high-cycle fatigue fracture surfaces (cycles to failure, N/sub f/>1/spl times/10/sup 9/) revealed clear failure origins that were not observed in short-life (N/sub f/<1/spl times/10/sup 4/) specimens. Reaction-layer and microcracking mechanisms for fatigue of silicon films are discussed in light of this empirical evidence for the critical role of service environment during damage accumulation under cyclic loading conditions.

Fatigue failure in thin-film polycrystalline silicon is due to subcritical cracking within the oxide layer

It has been established that microelectromechanical systems created from polycrystalline silicon thin films are subject to cyclic fatigue. Prior work by the authors has suggested that although bulk silicon is not susceptible to fatigue failure in ambient air, fatigue in micron-scale silicon is a result of a “reaction-layer” process, whereby high stresses induce a thickening of the post-release oxide at stress concentrations such as notches, which subsequently undergoing moisture-assisted cracking. However, there exists some controversy regarding the post-release oxide thickness of the samples used in the prior study. In this letter, we present data from devices from a more recent fabrication run that confirm our prior observations. Additionally, new data from tests in high vacuum show that these devices do not fatigue when oxidation and moisture are suppressed. Each of these observations lends credence to the “reaction-layer” mechanism.

High-cycle fatigue of micron-scale polycrystalline silicon films: fracture mechanics analyses of the role of the silica/silicon interface

It is known that micron-scale polycrystalline silicon thin films can fail in room air under high frequency (40kHz) cyclic loading at fully-reversed stress amplitudes as low as half the fracture strength, with fatigue lives in excess of 1011 cycles. This behavior has been attributed to the sequential oxidation of the silicon and environmentally-assisted crack growth solely within the SiO2 surface layer. This ‘reaction-layer fatigue’ mechanism is only significant in thin films where the critical crack size for catastrophic failure can be reached by a crack growing within the oxide layer. In this study, the importance of the bimaterial (e.g., Si/SiO2) interface to reaction-layer fatigue is investigated, and the critical geometry and stress ranges where the mechanism is a viable failure mode are established.

Notch Root Oxide Formation During Fatigue of Polycrystalline Silicon Structural Films

This paper investigates the fatigue behavior of n+-type 2-mum- thick polycrystalline silicon films that exhibit an initially thin (~2-3 nm) native oxide layer. The testing of kilohertz-frequency resonators provided accurate stress-life fatigue data at 30 and 50% relative humidity (RH) in the low (< 106)and high (up to 1011) cycle regimes. Long fatigue life specimens were associated with larger decreases in the natural frequency of the resonator and very smooth failure origins (at the notch) that encompassed several grains. Additional testing at various humidity levels highlighted the critical influence of humidity on the fatigue damage accumulation rate, which was measured via changes in the natural frequency. Finally, Auger electron spectroscopy (AES) characterized the formation of a nanometer-scale oxygen-rich reaction layer during cyclic loading. Although AES revealed a thin 2-3-nm initial oxide layer on a control specimen, measurements on a long-life fatigued specimen revealed an increased oxygen concentration over the first 10 nm of the material at the notch root. These findings demonstrate that the reaction-layer fatigue mechanism for silicon structural films operates even when reaction layers are initially very thin.

Design, Fabrication, and Performance of a Piezoelectric Uniflex Microactuator

Microactuators provide controlled motion and force for applications ranging from radio frequency switches to microfluidic valves. Large amplitude response in piezoelectric actuators requires amplification of the small strain, exhibited by the piezoelectric material, used in the construction of such actuators. This paper studies a uniflex microactuator that combines the strain amplification mechanisms of a unimorph and flexural motion to produce large displacement and blocking force. The design and fabrication of the proposed uniflex microactuator are described in detail. An analytical model is developed with three connected beams and a reflective symmetric boundary condition that predicts actuator displacement and blocking force as a function of the applied voltage. The model shows that the uniflex design requires appropriate parameter ranges, particularly the clearance between the unimorph and aluminum cap, to ensure that both the unimorph and flexural amplification effects are realized. With a weakened joint at the unimorph/cap interface, the model is found to predict the displacement and blocking force for the actuators fabricated in this work.

Oxidation of RuAl and NiAl Thin Films: Evolution of Surface Morphology and Electrical Resistance

RuAl and NiAl thin films on SiO2/Si were oxidized, and the results were compared to those from aluminum, ruthenium, and nickel films. Both aluminides are more oxidation resistant than nickel, aluminum, and ruthenium, and they form an outer layer of alumina after oxidation to 850 °C. The depth profiles differ for NiAl and RuAl, with alternating layers of alumina and a Ru-rich phase forming on RuAl, while a more complex structure forms on NiAl due to reaction with the substrate. The surface of RuAl after oxidation remains fairly smooth and reflective, whereas NiAl has a hazy appearance. However, the surface morphology changes at a slightly lower temperature in the case of RuAl (~ 500°C) . Both films remain conductive even after the surface begins to show signs of oxidation, with the NiAl remaining conductive to a higher temperature (after 1 h at 850 °C) than RuAl. The results show that NiAl and RuAl films can be used in an oxidizing atmosphere up to ~ 500°C (at least 1 h) for applications requiring a smooth reflective surface and to higher temperatures when the surface quality is less important but conductivity needs to be maintained (~ 800°C for RuAl and ~ 850°C for NiAl).

Utilizing On-Chip Testing and Electron Microscopy to Study Fatigue and Wear in Polysilicon Structural Films

Wear and fatigue are important factors in determining the reliability of microelectromechanical systems (MEMS). While the reliability of MEMS has received extensive attention, the physical mechanisms responsible for these failure modes have yet to be conclusively determined. In our work, we use a combination of on-chip testing methodologies and electron microscopy observations to investigate these mechanisms. Our previous studies have shown that fatigue in polysilicon structural thin films is a result of a ‘reaction-layer’ process, whereby high stresses induce a room-temperature mechanical thickening of the native oxide at the root of a notched cantilever beam, which subsequently undergoes moisture-assisted cracking. Devices from a more recent fabrication run are fatigued in ambient air to show that the post-release oxide layer thicknesses that were observed in our earlier experiments were not an artifact of that particular batch of polysilicon. New in vacuo data show that these silicon films do not display fatigue behavior when the post release oxide is prevented from growing, because of the absence of oxygen. Additionally, we are using polysilicon MEMS side-wall friction test specimens to study active mechanisms in sliding wear at the microscale. In particular, we have developed in vacuo and in situ experiments in the scanning electron microscope, with the objective of eventually determining the mechanisms causing both wear development and debris generation.

Cyclic Stabilization of Electrodeposited Nickel Structural Films

Tensile, room-temperature creep, and fatigue tests were conducted to determine the stability of the mechanical properties of electrodeposited films and to establish a unique strategy to reduce variation in micromachined devices made from them. Microcrystalline nickel films with columnar grains with a typical diameter of less than 1 μm and nanocrystalline films with 20-nm equiaxed grains were evaluated. While the tensile strengths of the films were higher than bulk forms of the material, the strength, strain to failure, and apparent elastic modulus were highly variable. Creep tests revealed that the films accumulated plastic strain rapidly at room temperature and that the apparent elastic modulus increased after exposure to stress. However, the response of the film was sensitive to the local deposition conditions. Fortunately, incremental-step fatigue tests demonstrated that the films cyclically hardened and that further changes in the elastic and plastic deformation responses do not occur after cyclic stabilization. As a result, cyclic stress strain curves and the transient stabilization behavior can be used to define a mechanical “burn-in” sequence for electrodeposited nickel films that will improve the stability and reproducibility of micromachined devices.

Reaction-layer fatigue: understanding the limitations of structural silicon

Previous research has attributed the fatigue susceptibility of silicon films to the sequential oxidation of the silicon and environmentally-assisted crack growth solely within the SiO2 surface layer. This “reaction-layer fatigue” mechanism is only significant in thin films where the critical crack size for catastrophic failure can be reached by a crack growing within the oxide layer. Fracture mechanics analyses can provide important insight into the limitations of structural silicon films. In this paper, our current understanding of the reaction-layer fatigue mechanism will be reviewed. Current results suggest that surface oxide layer thicknesses as low as 10-20 nm may induce reaction-layer fatigue when considering failure of the specimen for a crack reaching the silica/silicon interface. In contrast, 3-fold thicker surface oxide layers are required for failure due to a crack within the oxide layer.

Practical Implications of Instrument Displacement Drift during Force-Controlled Nanoindentation

The accuracy of instrumented indentation data relies heavily on the evaluation of experimental errors such as displacement drift. In spite of its importance, little attention has been given to the magnitude of an acceptable displacement drift rate, its relationship to a given set of test conditions, and how errors manifest themselves in force-displacement data. In this work we explored how drift rates that were acceptable for short-term tests caused artificial “abnormal” behavior that could have been interpreted as a true material response for a longer-term test. A critical review of the drift behavior of the nanoindentation system revealed that a useful metric for screening data quality was the nominal accumulated system drift as a fraction of the maximum penetration depth. Additionally, suggestions for drift management during nanoindentation tests were given.