Nanomechanical and nanotribological characterizations for reliability design of MEMS resonators (NARDEMS)
Project description
Scope of project

The main goal of project is mechanical and tribological characterizations of microelectromechanical systems (MEMS) for their reliability design considering theoretical approaches, finite element analysis, optimization with Genetics Algorithms and advanced experimental investigations. Applications include but are not limited to microsystems, material characterization tribological investigations etc.

Scientific context and motivation

The correct material selection criteria are essential when designing MEMS structures. Many MEMS designers still do not take into account the implications that the scale difference and inaccurate material properties have on reliability when designing new devices. Even if people are willing to take into account all the available information on the failure modes and mechanisms of MEMS, they discover that not so much is known at all. At present, the MEMS researchers are not in a position to predict quantitatively when or where most of the different failure mechanisms will occur. This project investigates some of failure mechanisms of MEMS and gives rules of thumb to avoid many of them considering correct selection of materials and adequate geometrical design of structures. The material influence on the failure mechanism at micro and nano scale is investigated in order to increase reliability and the lifetime of MEMS.
MEMS devices need to be designed to perform their expected functions with short duration, typically in milliseconds to picoseconds timescale. This accuracy in response of MEMS is influenced by the material properties. The MEMS reliability can be divided as System Reliability - that is related to the lifetime properties of whole device or system, the Component Reliability - that is referred to the failure modes of the sub-components and provides to the designer the information to predict them, and the Material Reliability - that is focused on the material collapse under certain conditions. Some works present in the literature propose a failure model at the system level, examples are provided for complicated micro-engines, for wireless strain sensing systems with a high reliability package, for the thermo-mechanical reliability of technological packaging etc. This project focuses on the component and material reliability considering mechanical fatigue, mechanical strength and tribological failures. The correlation between materials and MEMS structures including optimization are performed during project development in order to improve the accuracy in response, the reliability and their lifetime.
The basic mechanical elements composing MEMS resonators are micromembranes, microcantilevers and microbridges (Fig.1). One way to characterize the performance of these flexible microcomponents is by defining their relevant stiffness.

Stiffness is a fundamental criterion of elastically-deformable mechanical flexible microcomponents. The stiffness and modulus of elasticity are needed to predict deformation in the elastic regime. The stress and strain are necessary to anticipate the acceptable operating limits. The resonant frequency and amplitude of vibrations have a direct influence on oscillating mode of MEMS resonators.
Mechanical fatigue of MEMS materials and structures is a typical problem in MEMS under cyclic actuations. The experimental estimation of the fatigue stress in flexible mechanical components will predict the MEMS lifetime in different applications. Two of the major failure causes in MEMS which operate under adequate conditions and which cannot be predicted from the initial designing steps are: stiction when the flexible plate collapses to substrate and fatigue which can manifest itself as the loss in the accuracy of response under an acting signal (loss of stiffness) or as fracture under cyclic loadings.
MEMS reliability deals with materials but also with the reliability of their mechanical components. Moreover MEMS materials are generally used as thin films and this make mechanical characterization more complicate and at present no standard procedures are available for the extraction of fatigue properties and for the detection of all factors influencing fatigue behaviour. The mechanical fatigue collapse can be divided into three separate phases for simplicity: the crack nucleation, the crack stable propagation and the final failure.
Tribological failures of MEMS material including stiction, wear and friction affect the MEMS lifetime. If the length of a device decreases from 1mm to 1Ám, the area decreases by a factor of million and the volume decreases by a factor of billion. As a result, surface forces such as adhesion, friction, meniscus forces, viscous drag forces and surface tension that are proportional to area, become a thousand times larger than the forces proportional to the volume, such as inertial and electromagnetic forces.
2011 - 2014
Proiect finantat de UEFISCDI 
Program Resurse Umane - Proiecte tip TE