Samuel Lee Miller

Physics of the ferroelectric nonvolatile memory field effect transistor

The operation of the ferroelectric nonvolatile memory field effect transistor is theoretically examined extensively for the first time. The ferroelectric transistor device properties are derived by combining the silicon charge‐sheet model of metal‐oxide‐semiconductor field‐effect transistor device operation with Maxwell’s first equation which describes the properties of the ferroelectric film. The model we present describes ferroelectric transistor I‐V and C‐V behavior when time‐dependent voltages are applied which result in hysteresis due to ferroelectric switching. The theoretical results provide unique insight into the effects of geometrical and material parameters on the electrical properties of the transistor. These parameters include the ferroelectric spontaneous and remanent polarization, the coercive field, and dielectric layer thicknesses. We have found that the conventional concept of threshold voltage is no longer useful, and that increasing the spontaneous polarization has only a minor impact ...

Modeling the anneal of radiation-induced trapped holes in a varying thermal environment

The anneal of radiation-induced trapped holes in MOS transistors is found to be thermally activated. A quantitative, physical model based on thermal emission and tunneling is developed. It accurately predicts the anneal of radiation-induced trapped holes in constant or time-varying thermal environments. Data are presented which quantitatively verify the accuracy of the model for temperatures between 25 and 160 degrees C. This model provides the basis for developing accurate quantitative screens for the rebound failure mechanism. >

Modeling ferroelectric capacitor switching with asymmetric nonperiodic input signals and arbitrary initial conditions

The switching behavior of ferroelectric capacitors experiencing arbitrary time‐dependent electric fields and arbitrary initial conditions is investigated both theoretically and experimentally. A general approach for modeling incomplete dipole switching in ferroelectric capacitors is used to derive equations describing the electrical behavior of a simple characterization circuit with arbitrary initial conditions and arbitrary time‐dependent applied voltages. The equations include four experimentally determined parameters: the remanent and spontaneous polarizations, the coercive field, and the ferroelectric dielectric constant. Once these model parameters are determined from a single high‐frequency sinusoidal hysteresis loop, model predictions are made with no adjustable parameters. The circuit behavior for both sinusoidal and trapezoidal input signals is computed, including asymmetric and nonperiodic signals as well as several different initial conditions. The accuracy of the model predictions is quantitat...

Shaped comb fingers for tailored electromechanical restoring force

Electrostatic comb drives are widely used in microelectromechanical devices. These comb drives often employ rectangular fingers which produce a stable, constant force output as they engage. This paper explores the use of shapes other than the common rectangular fingers. Such shaped comb fingers allow customized force-displacement response for a variety of applications. In order to simplify analysis and design of shaped fingers, a simple model is developed to predict the force generated by shaped comb fingers. This model is tested using numerical simulation on several different sample shaped comb designs. Finally, the model is further tested, and the use of shaped comb fingers is demonstrated, through the design, fabrication, and testing of tunable resonators which allow both up and down shifts of the resonant frequency. The simulation and testing results demonstrate the usefulness and accuracy of the simple model. Finally, other applications for shaped comb fingers are described, including tunable sensors, low-voltage actuators, multistable actuators, or actuators with linear voltage-displacement behavior.

The effect of humidity on the reliability of a surface micromachined microengine

Humidity is shown to be a strong factor in the wear of rubbing surfaces in polysilicon micromachines. We demonstrate that very low humidity can lead to very high wear without a significant change in reliability. We show that the volume of wear debris generated is a function of the humidity in an air environment. As the humidity decreases, the wear debris generated increases. For the higher humidity levels, the formation of surface hydroxides may act as a lubricant. The dominant failure mechanism has been identified as wear. The wear debris has been identified as amorphous oxidized silicon. Large slivers (approximately 1 /spl mu/m in length) of debris observed at the low humidity level were also amorphous oxidized silicon. Using transmission electron microscopy (TEM), we observed that the wear debris forms spherical and rod-like shapes. We compared two surface treatment processes: a fluorinated silane chain (FTS) process and supercritical CO/sub 2/ dried (SCCO/sub 2/) process. The microengines using the SCCO/sub 2/ process were found to be less reliable than those released with the FTS process under two humidity levels.

MEMS Reliability: Infrastructure, Test Structures, Experiments, and Failure Modes

The burgeoning new technology of Micro-Electro-Mechanical Systems (MEMS) shows great promise in the weapons arena. We can now conceive of micro-gyros, micro-surety systems, and micro-navigators that are extremely small and inexpensive. Do we want to use this new technology in critical applications such as nuclear weapons? This question drove us to understand the reliability and failure mechanisms of silicon surface-micromachined MEMS. Development of a testing infrastructure was a crucial step to perform reliability experiments on MEMS devices and will be reported here. In addition, reliability test structures have been designed and characterized. Many experiments were performed to investigate failure modes and specifically those in different environments (humidity, temperature, shock, vibration, and storage). A predictive reliability model for wear of rubbing surfaces in microengines was developed. The root causes of failure for operating and non-operating MEMS are discussed. The major failure mechanism for operating MEMS was wear of the polysilicon rubbing surfaces. Reliability design rules for future MEMS devices are established.

The effect of frequency on the lifetime of a surface micromachined microengine driving a load

Experiments have been performed on surface micromachined microengines driving load gears to determine the effect of the rotation frequency on median cycles to failure. We did observe a frequency dependence and have developed a model based on fundamental wear mechanisms and forces exhibited in resonant mechanical systems. Stressing loaded microengines caused observable wear in the rotating joints and, in a few instances, led to fracture of the pin joint in the drive gear.

Friction in surface-micromachined microengines

Understanding the frictional properties of advanced Micro-Electro-Mechanical Systems (MEMS) is essential in order to develop optimized designs and fabrication processes, as well as to qualify devices for commercial applications. We develop and demonstrate a method to experimentally measure the forces associated with sliding friction of devices rotating on a hub. The method is demonstrated on the rotating output gear of the microengine recently developed at Sandia National Laboratories. In-situ measurements of an engine running at 18300 rpm give a coefficient of friction of 0.5 for radial (normal) forces less than 4 (mu) N. For larger forces the effective coefficient of friction abruptly increases, suggesting a fundamental change in the basic nature of the interaction between the gear and hub. The experimental approach we have developed to measure the frictional forces associated with the microengine is generically applicable to other MEMS devices.

Performance trade-offs for a surface micromachined microengine

An electromechanical model of Sandias microengine is developed and applied to quantify critical performance tradeoffs. This is done by determining how forces impact the mechanical response of the engine to different electrical drive signals. To validate the theoretical results, model- based drive signals are used to operate actual engines, where controlled operation is achieved for the following cases: 1) spring forces are dominant, 2) frictional forces are dominant, 3) linear inertial forces are dominant, 4) viscous damping forces are dominant, and 5) inertial load forces are dominant. Significant improvements in engine performance are experimentally demonstrated in the following areas: positional control, start/stop endurance, constant speed endurance, friction load reduction,and rapid actuation of inertial loads.

Extremal dynamics: A unifying physical explanation of fractals, 1/f noise, and activated processes

The properties of physical systems whose observable properties depend upon random exceedances of critical parameters are quantitatively examined. Using extreme value theory, the dynamical behavior of this broad class of systems is derived. This class of systems can exhibit two characteristic signatures: generalized activation when far from equilibrium and noise with a characteristic power spectrum (including 1/f ) when in quasiequilibrium. Fractal structures can also arise from these systems. It is thus demonstrated that generalized activation, noise, and fractals, in some cases, are simply different manifestations of a single common dynamical principle, which is termed ‘‘extremal dynamics.’’ Examples of physical processes governed by extremal dynamics are discussed, including data loss of nonvolatile memories and dielectric breakdown.