Noel C. MacDonald

Five parametric resonances in a microelectromechanical system

The Mathieu equation governs the forced motion of a swing, the stability of ships and columns, Faraday surface wave patterns on water,, the dynamics of electrons in Penning traps, and the behaviour of parametric amplifiers based on electronic or superconducting devices. Theory predicts that parametric resonances occur near drive frequencies of 2ω0/n, where ω0 is the systems natural frequency and n is an integer ⩾1. But in macroscopic systems, only the first instability region can typically be observed, because of damping and the exponential narrowing of the regions with increasing n. Here we report parametrically excited torsional oscillations in a single-crystal silicon microelectromechanical system. Five instability regions can be measured, due to the low damping, stability and precise frequency control achievable in this system. The centre frequencies of the instability regions agree with theoretical predictions. We propose an application that uses parametric excitation to reduce the parasitic signal in capacitive sensing with microelectromechanical systems. Our results suggest that microelectromechanical systems can provide a unique testing ground for dynamical phenomena that are difficult to detect in macroscopic systems.

SCREAM I: A single mask, single-crystal silicon, reactive ion etching process for microelectromechanical structures

A single-crystal slhcon, high aspect ratlo, low-temperature process sequence for the fabrlcatlon of suspended rmcroelectromechamcal structures (MEMS) usmg a smgle hthography step and reactwe Ion etching (RIE) IS presented The process IS called SCRJZAM I (single-crystal reactwe etchmg and metalhzatmn) SCREAM I IS a bulk mlcromachmmg process that uses RIE of a s~hcon substrate to fabricate suspended movable smgle-crystal s&on (SCS) beam structures Beam elements wth aspect ratios of 10 to 1 and widths rangmg from 0 5 to 4 0 Frn have been fabricated All process steps are low temperature (<3OO “C), and only conventronal sd~con fabrlcation tools are used photohthography, RIE, MIE, plasma-enhanced chemxal-vapor deposrtlon (PECVD) and sputter deposlhon SCREAM I IS a self-ahgned process and uses a smgle lithography step to define beams and structures srmultaneously as well as all necessary contact pads, electrIcal mterconnects and lateral capaators SCREAM I has been specifically deslgned for integration with standard Integrated cmxnt (IC) processes, so MEM deuces can be fabricated adjacent to prefabricated analog and dIgItal carcuitry In this paper we present process parameters for the fabncatlon of discrete SCREAM I devices We also discuss mask design rules and show micrographs of fabncated deuces

Capacitance Based Tunable Micromechanical Resonators

We present actuators which tune the resonant frequency of micromechanical oscillators. Experimental results show resonant oscillations from 7.7% to 146% of the original resonant frequency. Numerical results substantiate these results. Two failure modes have been identified which limit

Optimal shape design of an electrostatic comb drive in microelectromechanical systems

Polynomial driving-force comb drives are synthesized using numerical simulation. The electrode shapes are obtained using the indirect boundary element method. Variable-gap comb drives that produce combinations of linear, quadratic, and cubic driving-force profiles are synthesized. This inverse problem is solved by an optimization procedure. Sensitivity analysis is carried out by the direct differentiation approach (DDA) in order to compute design sensitivity coefficients (DSCs) of force profiles with respect to parameters that define the shapes of the fingers of a comb drive. The DSCs are then used to drive iterative optimization procedures. Designs of variable-gap comb drives with linear, quadratic, and cubic driving force profiles are presented in this paper.

A RIE process for submicron, silicon electromechanical structures

A reactive ion etching (RIE) process is used for the fabrication of submicron, movable single-crystal silicon (SCS) mechanical structures and capacitor actuators. The process is called SCREAM for single crystal reactive etching and metallization process. The RIE process gives excellent control of lateral dimensions (0.2 mu m approximately 2 mu m) while maintaining a large vertical depth (1 mu m approximately 4 mu m) for the formation of high aspect ratio, freely suspended SCS structures. The silicon etch processes are independent of crystal orientation and produce controllable vertical profiles. The process also incorporates process steps to form vertical, 4 mu m deep, aluminum, capacitor actuators. Using SCREAM, the authors have designed, fabricated and tested two-dimensional x-y microstages and circular SCS structures. For the x-y stage they measured a maximum displacement of +or-6 mu m in x and y with 40 V DC applied to either x or y, or both x and y actuators. The process technology offers the capability to use a structural stiffness as low as 10-2 N m-1.

Independent tuning of linear and nonlinear stiffness coefficients [actuators]

Using a combination of electrostatic actuators, we present a method to independently tune the linear and nonlinear stiffness coefficients of a uniaxial micromechanical device. To demonstrate the methods capability, we investigated the tuning of an oscillator with linear and cubic restoring forces. We successfully tuned the cubic stiffness from 0.31/spl times/10/sup 11/ to -5.1/spl times/10/sup 11/ N/m/sup 3/ without affecting the resonant frequency or the linear stiffness. Numerical results are presented which characterize the actuators and indicate important design parameters. Finally, issues such as actuator design, quadratic stiffness, and stability are discussed.

SCREAM MicroElectroMechanical Systems

A process called SCREAM for Single Crystal Reactive Etching And Metallization is used to make MicroElectroMechanical Systems (MEMS). The SCREAM process yields high-aspect-ratio (>50:1) released, single crystal silicon structures with micrometer-scale minimum features and a suspension span of greater than 5 millimeters. Such structures include picofarad sensing capacitors and high force actuators that generate milli-Newton forces (100mN/cm^2 at 40 Volts) and controllable, three dimensional motion and displacements. Examples of SCREAM devices and microinstruments include a materials testing or loading instrument, and micro-scanning tunneling microscopes.

A micromachined, single-crystal silicon, tunable resonator

We present a fully integrated, single-crystal silicon (SCS), micromachined tunable resonator with a natural resonant frequency of nominally 1 MHz and a room temperature quality factor approaching 10,000 in vacuum. The microelectromechanical systems (MEMS) process used to create the SCS tunable resonators is fully compatible with VLSI technology. These resonators can be electrostatically excited and turned simultaneously or independently in both the x and y dimensions (in-plane), and can be further enhanced with a third dimension excitation (out-of-plane, z) and with reference marks (tips) for position detection in sensor applications. The dimensions of our microbeam tunable resonators are sub-micrometer in width and a few micrometers in height, with variable lengths ranging from 10 mu m to a few hundred mu m to achieve the desired natural (un-tuned) resonant frequencies. The prototype first generation tunable resonator has a measured out-of-plane (z) natural resonant frequency of 0.96 MHz and a quality factor of 4370. This resonator has a linear tuning range (with respect to the transverse (z) tuning force) of 60 kHz with a maximum required dc tuning voltage of 35 V. We describe the structure, the fabrication process, the frequency tuning, and the non-linear frequency response of the tunable resonators.

Chaos in MEMS, parameter estimation and its potential application

In this paper we present theoretical analysis and experimental results on the dynamic behavior of a bistable microelectromechanical systems (MEMS) oscillator, demonstrate the existence of a strange attractor in the MEMS device, and perform model verification using the experimental data. Secure communication schemes based on synchronized chaos are also applied to the device and successfully performed in the simulation.

Microelectromechanics-based frequency signature sensor

An acoustic filter array of microelectromechanical beams each having a characteristic resonance frequency response to mechanical and/or acoustical vibration. The array divides incoming acoustic signals into a plurality of discrete spectral components, each of which may be separately detected and converted into corresponding electrical signals. The acoustic filter may be integrated onto a single crystal silicon substrate with electrical circuity for performing acoustic signal processing functions required for applications such as speech processing and simulating the physiological function of the ear.