Microelectromechanical Systems Systems Research Articles

Superconducting Micro-Bearings

This paper presents research into superconducting Micro-Bearings for MEMS systems. Advanced silicon processing techniques developed for the Very Large Scale Integration (VLSI) industry have been exploited in recent years to enable the production of micro-engineered moving mechanical systems. These devices commonly known as Micro-ElectroMechanical Systems (MEMS) have many potential advantages. In many respects the effect of scaling a machine from macro-sized to micro-sized are either neutral or beneficial. However in one important respect the scaling produces a severely detrimental effect. That respect is in the tribology and the subsequent wear on the high speed rotating machines. This leads to very short device lifetimes. This paper presents results obtained from a MEMS motor supported on superconducting bearings. The bearings are self-positioning, relying on, the Meissner effect to provide a levitation force which moves the rotor into position and flux pinning to provide stability thereafter. The rotor is driven by a simple electrostatic type motor in which photo resist is used to pattern the motor poles directly onto the rotor.

Studies on three-dimensional moulding, bonding and assembling of low-temperature-cofired ceramics for MEMS and MST applications

This paper broadly illustrates the preparation of three-dimensional shapes in low-temperature-cofired ceramics (LTCC). Test structures are fabricated by moulding single-layer green tapes into cylindrical form of different dimensions in order to investigate the penetration of cracks and the limiting value at which they appear. The factors responsible for the propagation of cracks have been analysed. Another important aspect which has been exemplified is the bonding of processed LTCC modules to metal parts with a dissimilar coefficient of thermal expansion. This is important from the point of view of attachment of LTCC-based micro-electromechanical systems (MEMS) or micro-system technology (MST) devices to a functional mechanical element. A methodology has been discussed for a smooth transition in the coefficient of thermal expansion of ceramic and metallic part in order to minimise stresses and avoid micro-cracks in the interconnections. An assembly of different post-fired modules by a fastening mechanism of individual modules for realizing a complex system are also described. Based on the above studies, a preliminary design to fabricate, shape, bond and post-process the LTCC substrate for the realization of a complex system together with the crucial processing issues is presented. Efforts have been made to implement this design and process steps on a real-time ball-bearing to prove the suitability of LTCC material for MEMS, MST and other complex systems.

A Preliminary Multiscale Modeling of Material Failure

Microelectromechanical systems (MEMS) and Nanoelectromechanical systems (NAMS) are on the revolutionary frontier of science and engineering. The development of MEMS and NAMS requires appropriate analysis, design, and fabrication techniques that enable the features such as reliability, repeatability, and high yield, etc., in general systems. To achieve this goal, a better understanding of the materials and mechanics at the micro- and nanodimension levels is imperative. In this paper, a preliminary material strength model, for dealing with the resistance to crack propagation at opposite extremes of material representation, continuum solid and atomic lattice, is described. The development of this model is in two parts: (1) The crack of continuum solids - using a quadric surfaces function to establish the piecewise failure envelope for incorporating the material anisotropy, and the final failure envelope of the quadric surfaces criterion is closed in order to accommodate the physical justification of materials with finite strengths. (2) The failure of atomic lattice - representing solids by many-body assemblages of point atomic masses linked by spring bonds, this simplified mass-spring model will be used to express the sequential bond-rupture picture of brittle fracture, in terms of an interatomic cohesive-force function. In this simplified mass-spring model, the solids are regarded as a simple array of parallel uncoupled linear chains, subject only to longitudinal components of forces, the lattice constraint effects are ignored. By addressing the failure mechanisms and establishing failure theories of not only macroscopic anisotropic composite materials, but also the metal lines at the micro level, results of this preliminary study provide some reference guidelines to do the reliable engineering designs of MEMS and NAMS systems.

Stiction and anti-stiction in MEMS and NEMS

Stiction in microelectromechanical systems (MEMS) has been a major failure mode ever since the advent of surface micromachining in the 80s of the last century due to large surfacearea-to-volume ratio. Even now when solutions to this problem are emerging, such as self-assembled monolayer (SAM) and other measures, stiction remains one of the most catastrophic failure modes in MEMS. A review is presented in this paper on stiction and anti-stiction in MEMS and nanoelectromechanical systems (NEMS). First, some new experimental observations of stiction in radio frequency (RF) MEMS switch and micromachined accelerometers are presented. Second, some criteria for stiction of microstructures in MEMS and NEMS due to surface forces (such as capillary, electrostatic, van der Waals, Casimir forces, etc.) are reviewed. The influence of surface roughness and environmental conditions (relative humidity and temperature) on stiction are also discussed. As hydrophobic films, the self-assembled monolayers (SAMs) turn out able to prevent release-related stiction effectively. The anti-stiction of SAMs in MEMS is reviewed in the last part.

Thermal issues in MEMS and microscale systems

Transduction mechanisms involving thermal phenomena play a central role in a wide range of microelectromechanical systems (MEMS) applications. An overview of a subset of thermal issues in MEMS technology is presented, including a discussion of traditional and emerging applications for microscale thermal systems. Issues relating to fundamental limitations and opportunities in thermal microsystems are presented. The use of thermal phenomena in three specific microsystems is reviewed, namely microhotplate chemical sensors, microfluidic systems, and electrothermal micromotors. Future directions in microscale and nanoscale thermal systems are presented.

Computer-aided generation of nonlinear reduced-order dynamic macromodels. I. Non-stress-stiffened case

Reduced-order dynamic macromodels are an effective way to capture device behavior for rapid circuit and system simulation. In this paper, we report the successful implementation of a methodology for automatically generating reduced-order nonlinear dynamic macromodels from three-dimensional (3-D) physical simulations for the conservative-energy-domain behavior of electrostatically actuated microelectromechanical systems (MEMS) devices. These models are created with a syntax that is directly usable in circuit- and system-level simulators for complete MEMS system design. This method has been applied to several examples of electrostatically actuated microstructures: a suspended clamped beam, with and without residual stress, using both symmetric and asymmetric positions of the actuation electrode, and an elastically supported plate with an eccentric electrode and unequal springs, producing tilting when actuated. When compared to 3-D simulations, this method proves to be accurate for non-stress-stiffened motions, displacements for which the gradient of the strain energy due to bending is much larger than the corresponding gradient of the strain energy due to stretching of the neutral surface. In typical MEMS structures, this corresponds to displacements less than the element thickness, At larger displacements, the method must be modified to account for stress stiffening, which is the subject of part two of this paper.

Micro and nanotechnologies: a challenge on the way forward to new markets

The main objective of space activities in the middle to long term it is to perform economies of scale on spacecrafts’ or space instruments’ life cycle costs using microelectromechanical systems (MEMS). This means integration of functions, materials, processes and devices of millimeter, submillimeter to micron size, leading to microinstruments and ultralight spacecrafts. MEMS technologies offer many advantages: (a) drastic reduction of requirements such as mass, volume and power, impacting directly on spacecrafts, tests facilities and launch costs; (b) replacement of several discrete components or devices by microsystems; (c) production in batch process on mass production lines (low cost per unit); (d) high system reliability through built-in design or integration of several microsystems; (e) access to microinstruments with a real improvement of system-performance-to-cost ratio. Obviously, there are some limitations to microsystems’ space applications, but some of them have already been solved, such as radiation hardening, single event upset or latch-up tolerances. Miniaturization would result in smaller and lighter spacecrafts, which could be close to a ‘mother’ platform, or tiny platforms in constellations, networks or swarms for earth or planetary missions. MEMS and specific system design studies will lead to more innovative research in space missions and present, in the event, many spin-off and business opportunities not restricted to ‘space only’ applications.