Kuo Shen Chen

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.

Micro - Heat Engines, Gas Turbines, and Rocket Engines - The MIT Microengine Project -

This is a report on work in progress on microelectrical and mechanical systems (MEMS)-based gas turbine engines, turbogenerators, and rocket engines currently under development at MIT. Fabricated in large numbers in parallel using semiconductor manufacturing techniques, these engines are based on micro-high speed rotating machinery with the same power density as that achieved in their more familiar, full-sized brethren. The micro-gas turbine is a 1 cm diameter by 3 mm thick SiC heat engine designed to produce 10-20 W of electric power or 0.050.1 Nt of thrust while consuming under 10 grams/hr of H 2 . Later versions may produce up to 100 W using hydrocarbon fuels. A liquid fuel, bi-propellant rocket motor of similar size could develop over 3 lb of thrust. The rocket motor would be complete with turbopumps and control valves on the same chip. These devices may enable new concepts in propulsion, fluid control, and por table power generation.

Viscoelastic Characterization and Modeling of Polymer Transducers for Biological Applications

Polydimethylsiloxane (PDMS) is an important polymeric material widely used in bio-MEMS devices such as micropillar arrays for cellular mechanical force measurements. The accuracy of such a measurement relies on choosing an appropriate material constitutive model for converting the measured structural deformations into corresponding reaction forces. However, although PDMS is a well-known viscoelastic material, many researchers in the past have treated it as a linear elastic material, which could result in errors of cellular traction force interpretation. In this paper, the mechanical properties of PDMS were characterized by using uniaxial compression, dynamic mechanical analysis, and nanoindentation tests, as well as finite element analysis (FEA). A generalized Maxwell model with the use of two exponential terms was used to emulate the mechanical behavior of PDMS at room temperature. After we found the viscoelastic constitutive law of PDMS, we used it to develop a more accurate model for converting deflection data to cellular traction forces. Moreover, in situ cellular traction force evolutions of cardiac myocytes were demonstrated by using this new conversion model. The results presented by this paper are believed to be useful for biologists who are interpreting similar physiological processes.

Intrinsic stress generation and relaxation of plasma-enhanced chemical vapor deposited oxide during deposition and subsequent thermal cycling

This paper discusses thermo-mechanical behavior of plasma-enhanced chemical vapor deposited oxide films during and after post-deposition thermal cycling and annealing. A series of thermal cycling experiments were conducted with various types of oxide and nitride films to elucidate the control mechanism of intrinsic stress generation and to develop engineering solutions for improving reliability of microelectromechanical system fabrication processes. Tensile intrinsic stress generation was observed during thermal cycling and the depletion of hydrogen and the shrinkage of micro voids existing in the oxide films was postulated as a major control mechanism for the stress generation and was modeled by an energy-based formulation. Subsequent experiments indicated that annealing at high temperature could reduce this intrinsic tensile stress. Both stress generation and relaxation were modeled to guide the development of engineering solutions to maintain structural integrity and improve fabrication performance.

Viscoelastic mechanical behavior of soft microcantilever-based force sensors

Polydimethylsiloxane (PDMS) microcantilevers have been used as force sensors for studying cellular mechanics by converting their displacements to cellular mechanical forces. However, PDMS is an inherently viscoelastic material and its elastic modulus changes with loading rates and elapsed time. Therefore, the traditional approach to calculating cellular mechanical forces based on elastic mechanics can result in errors. This letter reports a more in-depth method for viscoelastic characterization, modeling, and analysis associated with the bending behavior of the PDMS microcantilevers. A viscoelastic force conversion model was developed and validated by proof-of-principle bending tests.

Command-Shaping Techniques for Electrostatic MEMS Actuation: Analysis and Simulation

Precision positioning of microelectromechanical systems (MEMS) structures using electrostatic actuation has been widely used for optical and radio-frequency MEMS. How to achieve fast switching without exciting excessive residual vibration or structural impact is an important issue for these applications. This paper presents the analysis and simulation of applying command-shaping techniques for controlling MEMS electrostatic actuation. According to the nature of application fields, electrostatic actuators are classified into three categories: 1) lateral linear actuation; 2) vertical nonlinear actuation; and 3) pull-in actuation. Their corresponding linear or nonlinear command-shaping schemes are developed and presented. Both lumped element and continuous models of typical MEMS electrostatic actuated structures are simulated using Simulink and the finite-element method, and results indicate that the shaped command would yield a much superior response than that by the unshaped commands. Essential sensitivity studies are also conducted to examine the robustness of these shaping schemes, and results shows that within a certain level of parameter variation, these shapers are robust enough to retain the performance.

Micro hydraulic transducer technology for actuation and power generation

The paper introduces a novel transducer technology, called the solid-state micro-hydraulic transducer, currently under development at MIT. The new technology is enabled through integration of micromachining technology, piezoelectrics, and microhydraulic concepts. These micro-hydraulic transducers are capable of bi-directional electromechanical energy conversion, i.e., they can operate as both an actuator that supplies high mechanical force in response to electrical input and an energy generator that transduces electrical energy from mechanical energy in the environment. These transducers are capable of transducing energy at very high specific power output in the order of 1 kW/kg, and thus, they have the potential to enable many novel applications. The concept, the design, and the potential applications of the transducers are presented. Present efforts towards the development of these transducers, and the challenges involved therein, are also discussed.

Structural Design of a Silicon Micro-Turbo-Generator

The probabilistic structural analysis and design of a silicon micro-turbo-generator rotor are presented. This rotor was designed to have a tip speed of 500 m/s. Three-dimensional finite element analysis, fracture strength characterization of single crystal silicon, and structural failure probability calculations were performed. The results show that the design of micro-turbine-generator rotor, although highly stressed, is feasible. However, it is important to note that this feasibility is very dependent on the etching process achieving a high surface quality. The overall approach and tests employed are equally applicable to other highly stressed microelectromechanical systems.

A Novel Semianalytical Approach for Finding Pull-In Voltages of Micro Cantilever Beams Subjected to Electrostatic Loads and Residual Stress Gradients

Beam structures are widely used in microelectromechanical systems (MEMS) sensors and actuators, and modeling of pull-in behavior of beams subjected to electrostatic force is essential for MEMS actuators. However, from a fabrication perspective, MEMS microbeams are usually curled due to residual stress gradients, and this causes difficulties to accurately estimate the pull-in voltages. As a result, the characteristics of microbeams subjected to both residual stress gradients and electrostatic forces must be investigated to provide accurate information for the design of sensors and actuators. In this paper, a novel semianalytical formulation for computing the pull-in voltage of a curled cantilever beam due to residual stress gradients is proposed. By assuming an admissible deformation shape and using the energy method to determine the coefficients of the shape functions, it is possible to find the pull-in characteristics of the curled cantilevers. Detailed parametric studies are subsequently performed to quantify the influence of various geometry and processing parameters on the pull-in characteristics of those microbeams. Finally, we present a fitted formula for MEMS engineers to estimate pull-in voltages for beams with residual stress gradients for design optimization. The proposed method can also be extended for handling bilayered curled cantilever beams due to thermomechanical mismatches. Therefore, the method and results presented in this paper should be useful in micro sensor and actuator design.