S.M. Spearing

MEMS actuators and sensors: observations on their performance and selection for purpose

This paper presents an exercise in comparing the performance of microelectromechanical systems (MEMS) actuators and sensors as a function of operating principle. Data have been obtained from the literature for the mechanical performance characteristics of actuators, force sensors and displacement sensors. On-chip and off-chip actuators and sensors are each sub-grouped into families, classes and members according to their principle of operation. The performance of MEMS sharing common operating principles is compared with each other and with equivalent macroscopic devices. The data are used to construct performance maps showing the capability of existing actuators and sensors in terms of maximum force and displacement capability, resolution and frequency. These can also be used as a preliminary design tool, as shown in a case study on the design of an on-chip tensile test machine for materials in thin-film form.

Effect of process parameters on the surface morphology and mechanical performance of silicon structures after deep reactive ion etching (DRIE)

The ability to predict and control the influence of process parameters during silicon etching is vital for the success of most MEMS devices. In the case of deep reactive ion etching (DRIE) of silicon substrates, experimental results indicate that etch performance as well as surface morphology and post-etch mechanical behavior have a strong dependence on processing parameters. In order to understand the influence of these parameters, a set of experiments was designed and performed to fully characterize the sensitivity of surface morphology and mechanical behavior of silicon samples produced with different DRIE operating conditions. The designed experiment involved a matrix of 55 silicon wafers with radius hub flexure (RHF) specimens which were etched 10 min under varying DRIE processing conditions. Data collected by interferometry, atomic force microscopy (AFM), profilometry, and scanning electron microscopy (SEM), was used to determine the response of etching performance to operating conditions. The data collected for fracture strength was analyzed and modeled by finite element computation. The data was then fitted to response surfaces to model the dependence of response variables on dry processing conditions.

Power MEMS and microengines

MIT is developing a MEMS-based gas turbine generator. Based on high speed rotating machinery, this 1 cm diameter by 3 mm thick SiC heat engine is designed to produce 10-20 W of electric power while consuming 10 grams/hr of H/sub 2/. Later versions may produce up to 100 W using hydrocarbon fuels. The combustor is now operating and an 80 W micro-turbine has been fabricated and is being tested. This engine can be considered the first of a new class of MEMS device, power MEMS, which are heat engines operating at power densities similar to those of the best large scale devices made today.

Diamond and diamond-like carbon MEMS

Diamond and diamond-like carbon (DLC) thin films possess a number of unique and attractive material properties that are unattainable from Si and other materials. These include high values of Youngs modulus, hardness, tensile strength and high thermal conductivity, low thermal expansion coefficient combined with low coefficients of friction and good wear resistance. As a consequence, they are finding increasing applications in micro-electro-mechanical systems (MEMS). This paper reviews these distinctive material properties from an engineering design point of view and highlights the applications of diamond and DLC materials in various MEMS devices. Applications of diamond and DLC films in MEMS are in two categories: surface coatings and structural materials. Thin diamond and DLC layers have been used as coatings mainly to improve the wear and friction of micro-components and to reduce stiction between microstructures and their substrates. The high values of the elastic modulus of diamond and DLC have been exploited in the design of high frequency resonators and comb-drives for communication and sensing applications. Chemically modified surfaces and structures of diamond and DLC films have both been utilized as sensor materials for sensing traces of gases, to detect bio-molecules for biological research and disease diagnosis.

The role of fiber bridging in the delamination resistance of fiber-reinforced composites

Delamination cracks in composites may interact with misaligned or inclined fibers. Such interactions often lead to fiber bridging, which causes the nominal delamination resistance to increase as the crack extends. Substantial specimen geometry effects are also involved. An experimental investigation of the role of fiber bridging has been conducted for three different composites. The results are compared with fiber bridging models based on a softening traction law, leading to schemes for predicting trends in delamination resistance with specimen geometry and crack length. Implications for utilizing this effect to suppress the growth of delaminations are presented.

Microfabrication of a high pressure bipropellant rocket engine

A high pressure bipropellant rocket engine has been successfully micromanufactured by fusion bonding a stack of six individually etched single crystal silicon wafers. In order to test the device, an innovative packaging technique was developed to deliver liquid coolant and gaseous propellants to the rocket chip at pressures in excess of 200 atm at temperatures above 300°C. Testing continues on the 1.2 g devices, which have been run to date at a chamber pressure of 12 atm, generating 1 N of thrust, and delivering a thrust power of 750 W.

Fatigue damage mechanics of composite materials. I: Experimental measurement of damage and post-fatigue properties

A new approach for modelling the post-fatigue strength and stiffness of notched fibre composite laminates has been developed. It is based on the observation of notch tip damage which can be quantified by the extent of the individual failure processes, splitting in the 0° plies and delamination between the 0° ply and off-axis plies. The notch tip damage zone grows stably under tensile cyclic loading in a self-similar manner and the size and shape of this damage zone is dependent on laminate geometry and constituent properties of the fibre, matrix and interface. The post-fatigue strength and stiffness of the laminate can be related uniquely to the split length, which defines the extent of damage growth. In this first paper in a series of four, observation is made and measurements taken of the damage growth mechanisms that make up the damage zone in carbon fibre/epoxy laminates. Radiographs are used to characterise the notch tip damage zone and to establish a qualitative relationship between post-fatigue strength (or stiffness), cyclic stress, damage size and numbers of cycles.

Micro-Raman measurement of bending stresses in micromachined silicon flexures

Micron-scale characterization of mechanical stresses is essential for the successful design and operation of many micromachined devices. Here we report the use of Raman spectroscopy to measure the bending stresses in deep reactive-ion etched silicon flexures with a stress resolution of /spl sim/10 MPa and spatial resolution of /spl sim/1 /spl mu/m. The accuracy of the technique, as assessed by comparison to analytical and finite-element models of the deformation, is conservatively estimated to be 25 MPa. Implications for the use of this technique in microsystems design are discussed.

Ultra High Resolution Computed Tomography of Damage in Notched Carbon Fiber—Epoxy Composites:

This article presents the first use of synchrotron radiation computed tomography (SRCT) to achieve sub-micron resolution of damage in aerospace grade carbon fiber—epoxy composites. The structure and interaction of the damage can be visualized in 3-D on a scale not previously observed in practical engineering configurations. The ability to detect and accurately measure features down to individual fiber breaks provides a valuable platform for future research; from the rigorous evaluation of damage models to understanding the fundamental physical mechanisms governing crack growth in composites. In particular the key role of intra-laminar cracks and delaminations in localizing fiber fractures is unambiguously identified for the first time.

Matrix crack spacing in brittle matrix composites

A model describing the evolution of matrix cracks in undirectional continuous fiber, brittle matrix composites is developed. The approach involves calculation off the steady state strain energy release rate available for crack extension in terms of the constituent properties, the applied stress and the distances to the neighboring cracks. Interactions between cracks are found to occur when the crack spacing falls below twice the slip length. The model provides an analytical solution to the crack spacing for periodic arrays of cracks. Comparisons are conducted with predictions derived from computer simulations of random cracking. The effects of the matrix flaw density are briefly considered.