Samhita Dasgupta

A novel MEMS pressure sensor fabricated on an optical fiber

We describe the fabrication, and initial testing of a novel optically interrogated, microelectromechanical system (MEMS) pressure sensor in which the entire MEMS structure is fabricated directly on an optical fiber A new micromachining process for use on a flat fiber end face that includes photolithographic patterning, wet etching of a cavity, and anodic bonding of a silicon diaphragm is utilized. We have employed both 200- and 400-/spl mu/m-diameter multimode optical fibers. A pressure sensor fabricated on an optical fiber has been tested displaying an approximately linear response to static pressure (0-80 psi). This sensor is expected to find application in situations where small size is advantageous and where dense arrays may be useful.

Novel MEMS pressure and temperature sensors fabricated on optical fibers

We present the fabrication and initial testing of novel optically interrogated pressure and temperature sensors fabricated directly on optical fibers using microelectromechanical systems (MEMS) technology. A new micromachining process for use on a flat fiber end face that includes photolithographic patterning, wet etching of a cavity and anodic bonding of a silicon diaphragm is utilized. Two prototype pressure sensors, fabricated on 400 μm diameter multimode fibers, have been tested displaying an approximately linear response to static pressure (14–80 psi). A prototype temperature sensor, fabricated by anodically bonding an ultra-thin crystalline silicon onto a fiber end face, has been tested in the range 25–300 °C. A minimum detectable temperature variation of 6 °C is observed. Since these sensors are significantly miniaturized, they will find application in situations where small size is advantageous and where dense arrays may be useful.

Optically interrogated MEMS pressure sensors for propulsion applications

Pressure sensors suitable for propulsion applications that uti- lize interrogation by fiber optics are described. To be suitable for many propulsion applications, sensors should have fast response, have a con- figuration that can be readily incorporated into sensor arrays, and be able to survive harsh environments. Microelectromechanical systems (MEMS) technology is utilized here for sensor fabrication. Optically inter- rogated MEMS devices are expected to eventually be more suitable than electrically interrogated MEMS devices for many propulsion applications involving harsh environments. Pressure-sensor elements are formed by etching shallow cavities in glass substrates followed by anodic bonding of silicon onto the glass over the cavity. The silicon is subsequently etched to form the pressure-sensitive diaphragm. Light emerging from a fiber is then used to interferometrically detect diaphragm deflection due to external pressure. Experimental results for static and dynamic pres- sure tests carried out in a shock tube demonstrate reasonable linearity, sensitivity, and time response.

Raman scattering from rapid thermally annealed tungsten silicide

Raman scattering as a technique for studying the formation of tungsten silicide is presented. The tungsten silicide films have been formed by rapid thermal annealing of thin tungsten films sputter deposited on silicon substrates. The Raman data are interpreted by using data from resistivity measurements, Auger and Rutherford backscattering measurements, and scanning electron microscopy.

Two‐beam laser recrystallization of polycrystalline silicon on an insulating substrate

A novel two‐laser technique has been investigated for recrystallizing polycrystalline silicon deposited on an insulating substrate. An Ar+ laser and a CO2 laser have been scanned simultaneously to perform the recrystallization of unpatterned samples and samples patterned with antireflection stripes. The use of the two beams enabled recrystallization without the use of a substrate heater, and resulted in grain widths a factor of 2 larger than those obtained with the more conventional technique using an Ar+ laser only. A Raman microprobe analysis revealed that the stress in the two‐beam recrystallized material was about an order of magnitude less than found in Ar+ laser recrystallized material. The polarization dependence of the Raman scattered light indicated that the orientation of the recrystallized material between antireflection stripes was in a 〈100〉 orientation.

MOEMS pressure sensors for propulsion applications

Pressure sensors utilizing MEMS technology for fabrication of the sensing element, interrogation by fiber optics, and which are suitable for propulsion applications are described. Devices utilizing micro-opto-electro-mechanical systems (MOEMS) technology are often better suited for harsh environments than electrically interrogated MEMS devices, so with sturdy packaging these optical devices may be useful to many propulsion applications. MOEMS pressure sensors can also be incorporated into arrays for detailed spatial characterization along with inherent high speed temporal characterization. Such characterization is expected to be very useful for propulsion systems. This presentation will first review optical-MEMS pressure sensor configurations. We will then concentrate on configurations most suitable for high speed applications in harsh environments. Examples of experimental results for static pressure test as well as for dynamic pressure test carried out in a shock tube demonstrating good linearity, sensitivity and time response will then be presented. Hybrid and monolithic array configurations will be presented. A discussion of the use of wavelength division multiplexing for efficient accessing of array elements will also be included.

Design and fabrication of optical-MEMS pressure sensor arrays

Arrays of pressure sensors utilizing microelectromechanical systems (MEMS) technology for fabrication of the sensing element and interrogation by fiber optics are described. Optically interrogated MEMS devices are potentially more suitable for many propulsion applications involving harsh environments than electrically interrogated MEMS devices. Pressure sensor elements form a Fabry-Perot interferometer so that reflected light measures pressure. Rationale for the design of the geometry of sensor elements and array configurations will be presented. These devices are designed to provide sensitivity over a given pressure range, ease of fabrication, and array configurations useful for propulsion characterization. Sensor element and array fabrication will be discussed. These sensor elements are fabricated by etching shallow cavities in glass substrates followed by electrostatic bonding of silicon onto the glass over the cavity. The silicon is then etched to form the pressure sensitive diaphragm. Linear arrays having 6 elements will be described. Experimentally results for static pressure tests and dynamic pressure test carried out in a shock tube demonstrate reasonable linearity, sensitivity and time response.

Two-Beam Laser Recrystallization of Silicon on an Insulating Substrate

Laser recrystallization of silicon on an insulating substrate has been carried out by irradiating polysilicon with both an Ar + laser operating on all lines in the visible and a CO 2 laser operating at 10.6 microns. These experiments were carried out over a variety of laser power densities and substrate temperatures. The use of the two lasers allowed for independent spatial control of temperature in both the polysilicon and the SiO 2 layers and helped to reduce the strain at the polysilicon - SiO 2 interface. We report the successful recrystallization of polysilicon films without substrate heating for two different silicon-on-insulator structures.

Design of a smart optically controlled high-power switch for fly-by-light motor actuation systems

In avionic systems, data integrity and high data rates are necessary for stable flight control. Unfortunately, conventional electronic control systems are susceptible to electromagnetic interference (EMI) that can reduce the clarity of flight control signals. Fly-by-Light systems that use optical signals to actuate the flight control surfaces of an aircraft have been suggested as a solution to the EMI problem in avionic systems. Fly-by-Light in avionic systems reduces electromagnetic interference hence improving the clarity of the control signals. A hybrid approach combining a silicon photoreceiver module with a SiC power transistor is proposed. The resulting device uses a 5 mW optical control signal to produce a 150 A current suitable for driving an electric motor.

Design and simulation of a smart optically controlled high-power switch based on a Si/SiC hybrid device structure

In avionic systems, data integrity and high data rates are necessary for stable flight control. Unfortunately, conventional electronic control systems are susceptible to electromagnetic interference (EMI) that can reduce the clarity of flight control signals. Fly-by-Light systems that use optical signals to actuate the flight control surfaces of an aircraft have been suggested as a solution to the EMI problem in avionic systems. Fly-by-Light in avionic systems reduces electromagnetic interference hence improving the clarity of the control signals. A hybrid approach combining a silicon photoreceiver module with a SiC power transistor is proposed. The resulting device uses a 5 mW optical control signal to produce a 150 A current suitable for driving an electric motor.