PRISM at Discovery Park

Welcome to PRISM at Discovery Park

Target System

The focus of PRISM research is a contacting radio frequency (RF) capacitative MEMS, such as that shown. This type of device is widely used for contact actuators and capacitative switches, and involves metal-dielectric contact. A fixed-fixed metal membrane makes repeated contact with a dielectric contact pad. In the up state the switch presents a very small capacitance, which allows the signal to travel through the transmission line with very low loss. On the other hand, in the down state, the suspended bridge contacts the dielectric layer forming a large capacitance, which is an effective short circuit at high frequencies. Consequently, the incident signal is reflected and the switch presents a high isolation.

Failure Modes

Though the technology has much promise, RF MEMS switches are prone to a variety of failure mechanisms. Dielectric charging is caused by trapped charges inside the thin dielectric layer and leads to uncontrollable changes of the actuation voltage versus time, and eventually, complete device failure. Contact area damage and wear leads to contact resistance degradation even for relatively low power levels. This effect is probably due to local strain hardening and change of chemistry due to strong mechanical stresses at the contact area and is further exacerbated by switched RF currents. It is often manifested as contact resistance increase versus time that renders the switch unusable. MEMS switches are very sensitive to environmental contaminants and humidity. For example, while high humidity levels favor high adhesion forces that may lead to failure due to the stiction, nearly-zero humidity is also not ideal since it significantly increases the friction coefficients between the contacting surfaces, leading to premature failure. Mechanical failure mechanisms are tied to reductions in the physical size of the device, resulting in an interaction of microstructure with interfaces and free surfaces. Mechanical stress and creep development are important at very high or very low temperatures.Under conditions of shock loading, mechanisms related to materials microstructure, such as spalling, ductile and brittle fracture and delamination, become important.

Research Thrusts

The work to be undertaken by PRISM is divided into three science thrusts and two cross-cutting thrusts.

Thrust 1: Contact Physics.

The focus of Thrust 1 is failure mechanisms directly associated with metal-dielectric contact, including dielectric charging, current channeling, contact-area evolution and damage, and the influence of environmental factors such as humidity on contact. A significant focus of the center is the atomistic resolution of adhesion and stiction phenomena, and their influence on macroscale physics.

Thrust 2: Multiscale Modeling of MEMS response.

This thrust addresses the coupled electro-thermo-mechanical response of the MEMS structure. A critical element is the development of micromechanical materials models based on atomistic simulations for the description of single and polycrystalline plasticity, dislocation and defect evolution and the inclusion of size effects during plastic deformation.

Thrust 3: Multiscale Models for Aerodynamic Damping.

This thrust addresses the external fluid domain, and targets the aerodynamics of fluid damping. An important feature is the accurate characterization of squeeze film damping, particularly during the time-periodic transition between the continuum and the rarefied gas regimes as the gas gap opens and closes. Simulations spanning the continuum regime, described by the Navier-Stokes equations, to rarefied regimes, described by the Ellipsoidal-Statistical Bhatnagar Gross-Krook (ES-BGK) model, are proposed.

Cross-Cutting Thrusts: Uncertainty Quantification Science.

MEMS fabrication processes create uncertainties in critical physical dimensions. Electrical, thermal and mechanical properties depend on specific processing techniques and initial stress, micro-structural, and surface states, among others. Since these quantities form inputs to our simulations, the effect of their uncertainty on predictions must be quantified. Thrust 4 seeks to quantify this through the use of the generalized polynomial chaos (gPC) formalism. A particular challenge is the extension of this methodology to multiscale multiphsyics systems.

Cross-Cutting Thrust: Computational Science and Engineering.

The computational science and engineering thrust provides expertise to the science thrusts in the development of numerical algorithms, linear solvers and in large-scale computing.

Experimental Program

A significant experimental effort is planned to provide high-quality validation data. Our experiments start with a careful characterization of material and interface phenomena  to provide initial conditions on the microstructure and to validate important micro-scale models. The experiments reflect a three-tier structure, starting with single physics experiments focusing independently on the electrical, mechanical and thermal domains and graduating to multiphysics experiments that couple two or more domains - electro-mechanical, thermo-mechanical and electro-thermal, and culminating finally in system-level validation experiments.

Another unique aspect of our program is the development of validation data for uncertainty quantification in microsystems. Typical validation experiments include the measurement of fluid damping on microcantilevers and fixed-fixed membranes across a range of Knudsen numbers, gap-versus-voltage and pull-in voltage measurements in RF-MEMS, long-range creep measurements in RF-MEMS as well as the characterization of dielectric charging in dielectrics used in MEMS applications. The resulting database will be used to validate our uncertainty quantification effort, and will provide a first-of-its-kind view of how uncertainty is propagated in these microsystems.

Verification and Validation: Experimental Program

A significant experimental effort is planned to provide high-quality validation data. Our experiments start with a careful characterization of material and interface phenomena  to provide initial conditions on the microstructure and to validate important micro-scale models. The experiments reflect a three-tier structure, starting with single physics experiments focusing independently on the electrical, mechanical and thermal domains and graduating to multiphysics experiments that couple two domains - electro-mechanical, thermo-mechanical and electro-thermal, and culminating finally in system-level validation experiments.

About PRISM

PRISM leverages advances in nanoscale science and engineering to create innovative nanotechnologies addressing societal challenges and opportunities in computing, communications, the environment, security, energy independence, and health.


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Education


Contact

Matt Potrawski
Managing Director, PRISM Center
Purdue University
Birck Nanotechnology Center
1205 W. State St., Suite 2027
West Lafayette, IN 47907-2088