Stephen D. Senturia

M-TEST: A test chip for MEMS material property measurement using electrostatically actuated test structures

A set of electrostatically actuated microelectromechanical test structures is presented that meets the emerging need for microelectromechanical systems (MEMS) process monitoring and material property measurement at the wafer level during both process development and manufacturing. When implemented as a test chip or drop-in pattern for MEMS processes, M-Test becomes analogous to the electrical MOSFET test structures (often called E-Test) used for extraction of MOS device parameters. The principle of M-Test is the electrostatic pull-in of three sets of test structures [cantilever beams (CBs), fixed-fixed beams (FBs), and clamped circular diaphragms (CDs)] followed by the extraction of two intermediate quantities (the S and B parameters) that depend on the product of material properties and test structure geometry. The S and B parameters give a direct measure of the process uniformity across an individual wafer and process repeatability between wafers and lots. The extraction of material properties (e.g., Youngs modulus, plate modulus, and residual stress) from these S and B parameters is then accomplished using geometric metrology data. Experimental demonstration of M-Test is presented using results from MITs dielectrically isolated wafer-bonded silicon process. This yielded silicon plate modulus results which agreed with literature values to within /spl plusmn/4%. Guidelines for adapting the method to other MEMS process technologies are presented.

Generating efficient dynamical models for microelectromechanical systems from a few finite-element simulation runs

In this paper, we demonstrate how efficient low-order dynamical models for micromechanical devices can be constructed using data from a few runs of fully meshed but slow numerical models such as those created by the finite-element method (FEM). These reduced-order macromodels are generated by extracting global basis functions from the fully meshed model runs in order to parameterize solutions with far fewer degrees of freedom. The macromodels may be used for subsequent simulations of the time-dependent behavior of nonlinear devices in order to rapidly explore the design space of the device. As an example, the method is used to capture the behavior of a pressure sensor based on the pull-in time of an electrostatically actuated microbeam, including the effects of squeeze-film damping due to ambient air under the beam. Results show that the reduced-order model decreases simulation time by at least a factor of 37 with less than 2% error. More complicated simulation problems show significantly higher speedup factors. The simulations also show good agreement with experimental data.

Microfabricated structures for the in situ measurement of residual stress, Young’s modulus, and ultimate strain of thin films

Two microfabricated structures for the in situ measurement of mechanical properties of thin films, a suspended membrane, and an asymmetric ‘‘released structure,’’ are reported. For a polyimide film on silicon dioxide, the membrane measurements yield a residual tensile stress of 30 MPa and a Young’s modulus of 3 GPa. The released structures measure the ratio of residual stress to Young’s modulus, and yield 0.011 at strains comparable to the suspended membranes, and 0.015 at larger strains. The ultimate strain as measured by both structures is approximately 4%.

A computer-aided design system for microelectromechanical systems (MEMCAD)

The authors describe the MIT microelectromechanical computer-aided design system (MEMCAD), in which selected commercial software packages are linked with specialized database and numerical programs to allow designers to quickly perform both mechanical and electrical analyses of structures either described directly, or derived from the design specification (mask data plus process flow). The system architecture, the various modules, and their present status are described, and present system performance is demonstrated with several examples. >

Dielectric Analysis of Thermoset Cure.

All dielectric measurements involve the determination of the electrical polarization and conduction properties of a sample subjected to a time-varying electric field. Section 2 addresses dielectric measurement methods, the various instruments and electrodes, and their calibrations. Section 3 examines the microscopic mechanisms giving rise in the observed microscopic dielectric properties, and Section 4 explores in detail the effects of temperature and cure on these properties. Finally, Section 5 contains a selected bibliography of applications of dielectric analysis to the study of thermoset cure.

Design and calibration of a microfabricated floating-element shear-stress sensor

A microfabricated floating-element shear-stress sensor for measurements in turbulent boundary-layers is reported. Using surface micromachining of polyimide, a 500- mu m*500- mu m probe has been fabricated incorporating a differential-capacitor readout circuit. A model for the sensor response is described and is used for the design of an element to measure shear stresses of 1 Pa in air. The sensor is packaged for calibration in laminar flow, and electrical results obtained match the expected response. >

CAD challenges for microsensors, microactuators, and microsystems

In parallel with the development of new technologies, new device configurations, and new applications for microsensors, microactuators, and microsystems, also referred to as microelectromechanical devices and systems (MEMS), there has arisen a growing need for computer-aided engineering and design systems. There is a wide range of design problems: process simulation, solid-body geometric renderings from photomasks and process descriptions, energetically correct simulations of behavior across multiple coupled energy domains, extraction of lumped low-order models of device behavior, optimization of geometry and process sequence, and design of full systems that include MEMS devices. Because of the computational demands of the modeling required to support full computer-aided design (CAD), there is a premium on fast and memory-efficient algorithms that help the designer, both by automating, where possible, complex sets of related tasks and by providing rapid computational prototyping at critical points in the design cycle. This paper presents an overview of the present state of the art in CAD for MEMS, with particular emphasis on the role of macromodels and test structures as part of the design environment.

Operation of microfabricated harmonic and ordinary side-drive motors

A variable-capacitance harmonic side-drive motor is presented and the operation of this motor and ordinary variable-capacitance side-drive motors without the need for air-levitation assist is reported. Native oxide formation on motor polysilicon surfaces, resulting from the clamping of the rotor to the shield beneath it, is identified as the cause of motor operational failure. With proper release and testing directed at minimizing this oxide formation, the motors can be readily operated. Operational characteristics of the micromotors, including the role of rotor electric shielding, speed, and frictional effects, are studied. For the side-drive motors, measurements of stopping and starting voltages indicate that the drive torque required to sustain motor operation is 5-7 pN-m, while that required to initiate motor operation after a 30 second rest is nearly twice as high.<<ETX>>

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.

Simulating the behavior of MEMS devices: computational methods and needs

Technologies for fabricating a variety of MEMS devices have developed rapidly, but computational tools that allow engineers to quickly design and optimize these micromachines have not kept pace. Inadequate simulation tools force MEMS designers to resort to physical prototyping. To realistically simulate the behavior of complete micromachines, algorithmic innovation is necessary in several areas.