Jennifer M. English

Wireless micromachined ceramic pressure sensor for high-temperature applications

In high-temperature applications, such as pressure sensing in turbine engines and compressors, high-temperature materials and data retrieval methods are required. The microelectronics packaging infrastructure provides high-temperature ceramic materials, fabrication tools, and well-developed processing techniques that have the potential for applicability in high-temperature sensing. Based on this infrastructure, a completely passive ceramic pressure sensor that uses a wireless telemetry scheme has been developed. The passive nature of the telemetry removes the need for electronics, power supplies, or contacts to withstand the high-temperature environment. The sensor contains a passive LC resonator comprised of a movable diaphragm capacitor and a fixed inductor, thereby causing the sensor resonant frequency to be pressure-dependent. Data is retrieved with an external loop antenna. The sensor has been fabricated and characterized and was compared with an electromechanical model. It was operated up to 400/spl deg/C in a pressure range from 0 to 7 Bar. The average sensitivity and accuracy of three typical sensors are: -141 kHz Bar/sup -1/ and 24 mbar, respectively.

Wireless micromachined ceramic pressure sensors

In high temperature applications, such as pressure sensing in turbine engines and compressors, high-temperature materials and data retrieval methods are required. The microelectronics packaging infrastructure provides well-developed, high temperature ceramic materials, processing tools, and processing techniques that have the potential for applicability in high temperature sensors. A completely passive wireless telemetry scheme, which relies on a frequency shift output, has been integrated with the sensors, thereby eliminating the need for contacts, active elements, or power supplies to be contained within the sensor. This simplicity of sensor design allows the sensor to be exposed to elevated temperatures. As a proof-of concept, a wireless micromachined ceramic pressure sensor has been designed, fabricated, tested, and, compared with theoretical models. Sensors have been operated up to 200/spl deg/C and in pressure ranges from 0-1 bar and from 0-100 bar. The measured sensitivity of a fabricated sensor, expressed in frequency shift per pressure difference, is 2.6 MHz/bar, which compares well to the theoretically determined sensitivity of 2.2 MHz/bar.

Small-area bends and beamsplitters for low-index-contrast waveguides

We explore the use of air trenches to achieve compact high efficiency 90 degrees waveguide bends and beamsplitters for waveguide material systems that have low refractive index and low refractive index contrast between the core and clad materials. For a single air interface, simulation results show that the optical efficiency of a waveguide bend can be increased from 78.4% to 99.2% by simply decreasing the bend angle from 90 degrees to 60 degrees . This can be explained by the angular spectrum of the waveguide mode optical field. For 90 degrees bends we use a micro-genetic algorithm (GA) with a 2-D finite difference time domain (FDTD) method to rigorously design high efficiency waveguide bends composed of multiple air trenches. Simulation results show an optical efficiency of 97.2% for an optimized bend composed of three air trenches. Similarly, a single air trench can be designed to function as a 90 degrees beamsplitter with 98.5% total efficiency.

A flexible, self-healing sensor skin

A skin structure exhibiting flexibility, self-healing and damage sensing has been designed, fabricated and tested. The skin is fabricated on a substrate of copper-clad polyimide sheets in a layer-by-layer technique using polyimide sheets and an ultraviolet (UV)-curable epoxy. The UV-curable epoxy is used as both a structural adhesive and as the self-healing fill material. The skin structure is integrated with an array of LC circuits, where each circuit is characterized by a unique resonant frequency. If the skin is damaged, the UV-curable epoxy is released and is cured by ambient sunlight. Further, damage affects one or more of the LC circuits, altering its resonant frequency. An integrated antenna coil is used to detect and locate the damaged portion of the skin. A proof-of-concept skin is presented which includes a 2 × 2 array of LC circuits. Tests indicate good performance with respect to self-healing of the skin and fault isolation.

High Temperature Characterization of Ceramic Pressue Sensors

This work reports functional wireless ceramic micromachined pressure sensors operating at 450 °C, with demonstrated materials and readout capability indicating potential extension to temperatures in excess of 600 °C. These devices are self-packaged and are operating in actual high-temperature environments, not in simulated hot-plate testbeds. A resonant readout technique is employed, in which a planar spiral inductor and a pressure-sensitive capacitor form a passive LC circuit, the resonance frequency of which is sensitive to the external applied pressure, and which can be read out using a simple external loop antenna.

Planarization techniques for vertically integrated metallic MEMS on silicon foundry circuits

Various micromachining techniques exist to realize integrated microelectromechanical systems (MEMS), which include sensors, signal processing and/or driving circuits, and/or actuators in one small die. Post-processing techniques performed on foundry-fabricated circuits (e.g., MOSIS) are attractive since such an approach eliminates the need for an in-house integrated circuit fabrication line to produce integrated MEMS. A method based on the combination of metallic (e.g., electroplating) micromachining techniques with multichip module deposited (MCM-D) processes is a possible candidate to realize vertically-stacked integrated MEMS using the post-processing of integrated circuits (post-IC) approach. In order to realize such devices, planarization of the surface of foundry-fabricated circuit chips or wafers is often required. In such planarization layers, mechanical and chemical stability, as well as adhesion between the circuit-containing substrate and the micromachined devices, should be addressed. A PI/BCB/PI sandwich interlayer system, which utilizes both advantages of DuPont polyimide PI 2611 and Dow benzocyclobutene (BCB) Cyclotene 3022 series, was developed as a planarization interlayer for vertically integrated MEMS. The PI/BCB/PI interlayer system shows an over 95% degree of planarization (DOP) as well as passes the Method 107G Thermal Shock from the military standard MIL-STD-202F. A interlayer system was also developed as an alternative to the PI/BCB/PI system.

Micromachined viscosity sensor for real-time polymerization monitoring

This paper reports on low-frequency micromachined viscosity sensors intended for real-time polymerization monitoring. The viscosity sensors consist of micromachined membrane resonators featuring electrothermal excitation and piezoresistive detection of transverse membrane vibrations. In contrast to high frequency (above 1 MHz) sensors, e.g. thickness-shear mode sensors or Lamb wave sensors, the use of low-frequency (1-20 kHz) resonators for viscosity sensing, which more closely mimics conventional resonant viscometers, is investigated in this work. The viscous fluid loads the membrane resonator and changes the quality factor of its fundamental resonance. The change in the Q-factor is most pronounced in the viscosity range from 10/sup -3/ Pa/spl middot/s to 1 Pa/spl middot/s.

MEMS-Assisted Cryptography for CPI Protection

The authors present a concept for an anti-tamper system that dynamically generates a cryptographic key derived from microelectromechanical systems (MEMS) arrays encapsulated within a protected system in a single package. The system provides protection in active and passive states with no battery backup.

Ninety-deg. bends in low-index contrast waveguides

We discuss the use of multiple layer air trench and silicon strip structures to realize high efficiency 90° bends for low index contrast waveguides. We use a micro-genetic algorithm (mGA) coupled with a 2-D finite difference time domain method to perform rigorous electromagnetic optimization of multi-layer structures for single mode waveguides. We find that a 3-layer air trench structure can be designed for a 90° waveguide bend that exhibits 97.2% efficiency for TM polarized light at a wavelength of 1.55 μm. We are also able to design five- and six-layer silicon strip bends that have high efficiency for both TE and TM polarizations. For example, simulation results for a six-layer design show 95.2% and 97.2% for TE and TM polarizations, respectively. Moreover, the bend efficiency for each polarization state is greater than 90% over a broad wavelength range (1.5 μm to 1.7 μm).

Model of a MEMS sensor using a common gate MOSFET differential amplifier

The design, modelling and feasibility of a MEMS sensor based on a common gate metal-oxide-semiconductor field effect transistors (MOSFET) differential amplifier is presented. A pair of adjacent MOSFETs shares a common gate electrode that is suspended in a torsional configuration over the transistor gate regions. Rotation of the common gate alters the relative spacing between the gate tips and the substrate changing the gate capacitance of each transistor. Connection of the transistor pair in a differential readout configuration generates a differential current flow through the MOSFET pair. This sensor integrates transduction and amplification as movement of the MEMS gate modulates the relative gate capacitance and drain current of each transistor. The differential configuration provides a high common mode rejection ratio and the estimated sensitivity is 68 µA (fF−1) and the responsivity of the sensor is 2.2 mV/degree.