Peter G. Hartwell

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.

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

Capacitance based tunable resonators

We present four electrostatic actuators that tune the stiffness and hence the resonant frequency of a micromechanical oscillator. Using these actuators, resonant frequencies have been reduced to 7.7% and raised to 146% of the original values. These shifts correspond to approximately two orders of magnitude reduction in stiffness and a doubling in stiffness, respectively. Comparisons are drawn between these actuators based on functionality, area utilization efficiency, linearity and stability. Other issues discussed are asymmetries, nonlinearities and failure modes. With regard to the nonlinearities, near the limit of resonant frequency reduction, we show the ability to tune the system into a bistable state.

Single mask lateral tunneling accelerometer

A single-mask lateral tunneling accelerometer with integrated tip has been developed and characterized. High aspect ratio single-crystal silicon springs provide high resolution, wide operating bandwidth, and excellent isolation from off-axis stimuli. In this paper, we present the first such device implementing the SCREAM (1994) process technology. We focus on the advantages that this technology affords tunneling accelerometers and present a typical high-resolution accelerometer with 20 /spl mu/g/rt Hz performance at 100 Hz and 5.5 kHz resonant frequency.

Hewlett packard's seismic grade MEMS accelerometer

HP has recently made its plans to develop an ultrahigh-resolution seismic sensing solution public. This solution is being developed with Royal Dutch Shell Corporation for oil and gas exploration [1]. Central to delivering this system is HPs new single axis seismic grade MEMS accelerometer. The HP device uses both bulk micromachining methods and standard thin film technologies as well as HPs new three phase sensing technology. This sensor detects motion when an array of electrodes located on the proof mass moves relative to a stationary array of electrodes across a fixed gap. Initial characterization shows a flat noise power spectral density of < 100 nG/vHz over a bandwidth of DC-200 Hz. The device exhibits a linear response to +/− 150mG with a sensitivity of over 25 V/g within the specified bandwidth.

Integrated Multifunctional Environmental Sensors

We present the design, microfabrication, and characterization of ten sensors on one silicon die. We demonstrate simultaneous monitoring of multiple environmental parameters, including temperature, humidity, light intensity, pressure, wind speed, wind direction, magnetic field, and acceleration in three axes. Through an integrated design and fabrication process, these ten functions require only six photolithography mask steps. Temperature is measured redundantly using aluminum and doped silicon resistance thermal detectors and a bandgap temperature sensor. Humidity is transduced by the dielectric change of a polymer due to water absorption. Light intensity is measured with a p-n junction photodiode and doped silicon photoresistor. Pressure is transduced using piezoresistor strain gauges on a sealed membrane. Wind speed and direction are measured with two perpendicular hot wire anemometers. Magnetic field strength is measured with a doped Hall effect sensor. Acceleration in three axes is measured using electrostatic comb finger accelerometers, and an additional z-axis accelerometer uses piezoresistor strain gauges. We measured the cross-sensitivity of each function to all other environmental parameters and can use the chips multifunctional capabilities to compensate for these effects. Sensor integration can enable significant cost, size, and power savings over ten individual devices and facilitate deployment in novel applications.

A Method for Wafer-Scale Encapsulation of Large Lateral Deflection MEMS Devices

Packaging of microelectromechanical systems (MEMS) is a critical step in the transition from development to commercialized product. This paper presents a thin-film encapsulation process that allows varying trench widths suitable for MEMS devices with lateral deflections as large as 20 ¿m. The process involves the deposition and planarization of a sacrificial-oxide layer of up to 23 ¿m thick, the deposition of a 20 ¿m epitaxial-silicon sealing cap, the release of structures using hydrofluoric acid (HF) vapor, and the sealing of the structure at low pressure. Devices produced using this encapsulation method are capable of surviving standard backend processes such as wafer singulation and wire bonding. Among the numerous types of devices encapsulated, two different types of silicon MEMS resonators were fabricated. These functioning resonators demonstrate the ability of the process to successfully encapsulate devices, taking advantage of both large and small trench widths. Such a generalized fabrication platform greatly expands the possibilities of the wafer-scale encapsulation to numerous MEMS devices and retains the robustness necessary for backend processing.

Development of an SU-8 Fabry-Perot blood pressure sensor

This paper presents the fabrication method and testing of an interferometric pressure sensor designed for intravascular blood pressure measurements. A cap containing a pressure-sensing diaphragm was mounted onto the end of a fiber optic cable. The Fabry-Perot interferometer, formed between the reflective diaphragm and the fibers end, measured the diaphragm deflection. Microfabricated from the biocompatible polymer, SU-8, the device is fast, simple, and inexpensive to manufacture. Its small dimensions (<300/spl mu/m outer diameter) reduce the risk of infection and thrombosis and allow for its insertion into small vessels. The sensor showed a linear response to pressure from 0 to 125 mmHg with approximately 1-2 mmHg resolution. The use of an optical displacement transducer allowed a series of careful measurements of drift and hysteresis of SU-8 materials in different environments. This data may be valuable to other researchers working with SU-8.

Micro-G silicon accelerometer using surface electrodes

We present a new technology platform for silicon inertial sensors. The platform combines three technology features to set new performance and manufacturability standards for MEMS sensors. First, bonding three silicon wafers creates wafer level packaging and a homogenous stack of silicon material improving device temperature stability. Second, through-wafer etching is used to define the mechanical structure creating a proof mass with 1000x larger mass than a typical MEMS sensor. Finally, we use surface electrode technology to create a lateral capacitance-based transducer enabling large capacitance change per acceleration and allowing a large dynamic range without electrode contact. The large mass together with reduced damping of a lateral sensor result in substantially reduced thermal-mechanical noise. We present a two axis, in-plane, MEMS accelerometer having nG/√Hz noise performance, over 130 dB dynamic range, 300 Hz bandwidth, and a chip size comparable to other MEMS accelerometers. The platform is extensible to gyroscopes and single chip IMU.

SCREAM'03: A SINGLE MASK PROCESS FOR HIGH-Q SINGLE CRYSTAL SILICON MEMS

We present a single-mask single-crystal silicon (SCS) process for the fabrication of suspended MicroElectroMechanical devices (MEMS). This is a bulk micro-machining process that uses Deep Reactive Ion Etch (DeepRIE) of a silicon-on-insulator (SOI) substrate with highly doped device layer to fabricate movable single-crystal silicon MEMS structures, which can be electrically actuated without metal deposition. The buried oxide layer is not removed afterwards and no wet process release is involved in the whole process sequence, which makes this process different from others works based on SOI wafer. Several MEMS oscillators have been made using this process. Dynamic behavior is characterized using a laser vibrometer. Quality factor is improved by more than 1 order compared to the same oscillator made using SCREAM process.