Moinuddin Ahmed

Temperature Sensor in a Flexible Substrate

This paper presents the fabrication and measured performance of temperature sensors embedded in flexible polyimide substrates. The sensing material used for the temperature sensor was undoped amorphous silicon which was fabricated on a 35-m-thick layer of polyimide that serves as the flexible substrate. Another flexible polyimide superstrate layer of 35-40 m was spin-coated on top of the sensors to make sure the temperature sensors lie on a zero stress plane. The temperature coefficient of resistance was measured over the temperature range of 200- 360 K. The maximum temperature coefficient of resistance at 30 C was measured to be 0.0288 K-1. The effect of the flicker noise and voltage dependence of the voltage noise power spectral density exhibited by the sensor was evaluated. The normalized flicker noise coefficient was found to be 1.2 × 10-11.

MEMS Force Sensor in a Flexible Substrate Using Nichrome Piezoresistors

MEMS force sensors embedded in flexible polyimide substrates are reported, motivated by the need to monitor force and pressure on nonplanar surfaces for structural health monitoring. Details of the fabrication, measured preliminary results, and figures of merit are described. The sensors showed an average piezoresistive gauge factor of 1.75. The average value of normalized Hooge noise coefficient K1/f was found to be 1.89×10-10. The sensitivity was measured to the range from 0.266 to 2.248 V/N with an average sensitivity of 1.25 V/N. The noise equivalent force was calculated to the range from 8.83 to 46.56 μN with an average value of 20.47 μN for the force sensors in a noise frequency bandwidth from 1 to 8 Hz.

MEMS absolute pressure sensor on a flexible substrate

This paper describes the fabrication and characterization of micromachined, piezoresistive, absolute pressure sensors sandwiched between a flexible polyimide. The sensors are designed for structural health monitoring in aerospace applications. A suspended aluminum oxide diaphragm is utilized where nichrome (Ni-80%/Cr-20%) piezoresistive sensors are placed in a half Wheatstone bridge geometry to ensure a linear response and thermal stability. The sensitivity was found to be 1.51 nV/Pa with an average dynamic range of 8.31 MPa.

MEMS relative pressure sensor on flexible substrate

The fabrication and characterization of micromachined, piezoresistive, relative pressure sensors sandwiched between flexible polyimide layers is described. This work is motivated by the need to monitor force and pressure on nonplanar surfaces for structural health monitoring. 35-nm-thick nichrome (Ni-80%/Cr-20%) is used as a sensing material for the pressure sensor. The average value of the gauge factor of the nichrome was found to be 1.75.

MEMS sensors on flexible substrates towards a smart skin

This paper reviews the most recent results on the fabrication and characterization of a variety of microelectromechanical (MEMS) based sensors fabricated on flexible polymer substrates without the loss of performance, towards the development of a smart skin. These sensors include amorphous silicon based temperature sensors; nichrome based piezoresistive, absolute pressure sensors, tactile sensors and force sensors; and capacitive accelerometers. The sensors are sandwiched between a polyimide substrate and superstrate, placing them at a low stress plane, thus packaged at device-level. The flexible sensor package is grown during the device fabrication process. This allows the sensors to be integrated directly with flexible circuit boards to maintain overall flexibility. Some notable features are the incorporation of a sealed vacuum cavity for the absolute pressure sensors and the formation of a hermetically sealed cavity to contain the accelerometers in a constant atmosphere.

Low-profile, self-packaged uncooled microbolometer on a flexible substrate towards an infrared radiation sensitive skin

This paper describes fabrication and characterization of self-packaged microbolometers on a flexible substrate. Amorphous silicon (a-Si) was used as the sensing material and the entire detector was device-level packaged in a vacuum cavity with an aluminum oxide packaging layer which can withstand up to 10 atm pressure. The flicker noise coefficient K1/f was calculated to be 1.23 ×10-13 for the a-Si detecting material from the measured noise spectra.

Flexible Conformal Micromachined Absolute Pressure Sensors

The fabrication and characterization of micromachined piezoresistive absolute pressure sensors in a flexible substrate is presented. A suspended aluminum oxide diaphragm containing nichrome (Ni-80%/Cr-20%) piezoresistive sensors backed by a vacuum cavity was utilized to form the sensor. The piezoresistors were placed in a half-Wheatstone bridge geometry to provide a linear response and thermal stability. The average value of the gauge factor of nichrome was measured to be 1.95. The average normalized Hooge coefficient K1/f was found to be 4.64×10-11 for the nichrome piezoresistors. The pressure sensors displayed an average sensitivity of 1.25 nV/Pa and average value of noise equivalent pressure (NEPr) of 7.44 kPa in the bandwidth of 1-10 Hz in the 1/f -noise limited regime. In the Johnson noise-limited regime, the NEPr was found to be 10 Pa in a 1-Hz bandwidth.

Design and Simulation of Device-Level Vacuum-Packaged Microbolometer With Integrated Optical Filter

In this paper, we have reported the design and simulation of uncooled microbolometer infrared radiation detectors. We have demonstrated several improvements in design over the conventional microbolometers such a double-layer absorption structure instead of single-layer absorber; a nanomesh radiation absorber to improve thermal isolation of the thermometer structure; device-level vacuum packaging which serves to limit thermal convection away from the detector; the monolithic integration of an optical filter into the device-level vacuum package to reduce the absorption of visible radiation and thus reduce the photo-generation of carriers in the amorphous silicon thermometer.

Pyroelectric modified lead titanate thin films for uncooled infrared detection

Polycrystalline ferroelectric modified lead titanate thin films have been deposited by pulsed laser deposition and annealed for pyroelectric infrared detection. The deposited thin films have been characterized for the temperature dependence of their pyroelectric parameters.

The flicker noise in amorphous silicon based temperature sensors in flexible substrates

This paper presents the flicker noise or 1/f-noise measurement of RF sputtered amorphous silicon temperature sensors. The temperature sensor was fabricated between a polyimide substrate and superstrate to place it on a zero stress plane. The effects of flicker noise and voltage dependence of noise voltage power spectral density of the sensor were evaluated. The average value of flicker noise coefficient or normalized Hooge parameter K1/f was found to be 1.2×10−11.