Charles J. Taylor

A monolithic CMOS microhotplate-based gas sensor system

A monolithic CMOS microhotplate-based conductance-type gas sensor system is described. A bulk micromachining technique is used to create suspended microhotplate structures that serve as sensing film platforms. The thermal properties of the microhotplates include a 1-ms thermal time constant and a 10/spl deg/C/mW thermal efficiency. The polysilicon used for the microhotplate heater exhibits a temperature coefficient of resistance of 1.067/spl times/10/sup -3///spl deg/C. Tin(IV) oxide and titanium(IV) oxide (SnO/sub 2/,TiO/sub 2/) sensing films are grown over postpatterned gold sensing electrodes on the microhotplate using low-pressure chemical vapor deposition (LPCVD). An array of microhotplate gas sensors with different sensing film properties is fabricated by using a different temperature for each microhotplate during the LPCVD film growth process. Interface circuits are designed and implemented monolithically with the array of microhotplate gas sensors. Bipolar transistors are found to be a good choice for the heater drivers, and MOSFET switches are suitable for addressing the sensing films. An on-chip operational amplifier improves the signal-to-noise ratio and produces a robust output signal. Isothermal responses demonstrate the ability of the sensors to detect different gas molecules over a wide range of concentrations including detection below 100 nanomoles/mole.

Group IVB metal oxides high permittivity gate insulators deposited from anhydrous metal nitrates

The electrical performance of column IVB metal oxide thin films deposited from their respective anhydrous metal nitrate precursors show significant differences. Titanium dioxide has a high permittivity, but shows a large positive fixed charge and low inversion layer mobility. The amorphous interfacial layer is compositionally graded and contains a high concentration of Si-Ti bonds. In contrast, ZrO/sub 2/ and HfO/sub 2/ form well defined oxynitride interfacial layers and a good interface with silicon with much less fixed charge. The electron inversion layer mobility for an HfO/sub 2//SiO/sub x/N/sub y//Si stack appears comparable to that of a conventional SiO/sub 2//Si interface.

Chemical warfare agent detection using MEMS-compatible microsensor arrays

Microsensors have been fabricated consisting of TiO/sub 2/ and SnO/sub 2/ sensing films prepared by chemical vapor deposition (CVD) on microelectromechanical systems array platforms. Response measurements from these devices to the chemical warfare (CW) agents GA (tabun), GB (sarin), and HD (sulfur mustard) at concentrations between 5 nmol/mol (ppb) and 200 ppb in dry air, as well as to CW agent simulants CEES (chloroethyl ethyl sulfide) and DFP (diisopropyl fluorophosphate) between 250 and 3000 ppb, are reported. The microsensors exhibit excellent signal-to-noise and reproducibility. The temperature of each sensor element is independently controlled by embedded microheaters that drive both the CVD process (375/spl deg/C) and sensor operation at elevated temperatures (325/spl deg/C-475/spl deg/C). The concentration-dependent analyte response magnitude is sensitive to conditions under which the sensing films are grown. Sensor stability studies confirm little signal degradation during 14 h of operation. Use of pulsed (200 ms) temperature-programmed sensing over a broad temperature range (20/spl deg/C-480/spl deg/C) enhances analyte selectivity, since the resulting signal trace patterns contain primarily kinetic information that is unique for each agent tested.

Low Temperature Chemical Vapor Deposition of ZrO2 on Si(100) Using Anhydrous Zirconium (IV) Nitrate

Anhydrous zirconium(IV) nitrate was used as a volatile, carbon-free precursor for the low pressure chemical vapor deposition of thin ZrO 2 films on silicon (100) substrates. Depositions were performed at substrate temperatures between 300 and 500°C at total reactor pressures between 0.25 and 1.1 Torr. During deposition the N 2 carrier gas (flow rates = 20 or 100 sccm) was diverted through the precursor vessel which was maintained between 80 and 95°C. Under these conditions typical growth rates reached 10.0 nm/min. The polycrystalline films were predominantly monoclinic ZrO 2 with compositions very near the ideal value. Cross-sectional transmission electron microscopy and medium energy ion scattering established that an interfacial layer of SiO 2 separates the silicon substrate from the ZrO 2 . Electrical measurements made on capacitors constructed of 58 nm thick films of ZrO 2 with a platinum top electrode suggest that charge trapping occurs in the Si/ZrO 2 interfacial region.

Correlating Polymer-Carbon Composite Sensor Response with Molecular Descriptors

We report a quantitative structure-activity relationships QSAR study using genetic function approximations to describe the activities of a polymer-carbon composite chemical vapor sensor using a novel approach to selecting a molecular descriptor set. The measured sensor responses are conductivity changes in polymer-carbon composite films upon exposure to target vapors at partsper-million concentrations. The descriptor set combines the basic analyte descriptor set commonly used in QSAR studies with descriptors for sensing film-analyte interactions. The basic analyte descriptors are obtained using a combination of empirical and semiempirical quantitative structure-property relationships methods. The descriptors for the sensing film-analyte interactions are calculated using molecular modeling and simulation tools. A statistically validated QSAR model was developed for a training data set consisting of 17 analyte molecules. The applicability of this model was also tested by predicting sensor activities for three test analytes not considered in the training set.

Expanding the capabilities of the JPL electronic nose for an International Space Station technology demonstration

An array-based sensing system based on polymer/carbon composite conductometric sensors is under development at JPL for use as an environmental monitor in the International Space Station. Sulfur dioxide has been added to the analyte set for this phase of development. Using molecular modeling techniques, the interaction energy between SO2 and polymer functional groups has been calculated, and polymers selected as potential SO2 sensors. Experiment has validated the model and two selected polymers have been shown to be promising materials for SO2 detection.

A microarray approach for optimizing localized deposition of carbon nanotubes using microhotplate arrays

A 340-element array of microhotplates was used to characterize the chemical vapour deposition growth of carbon nanotubes and nanofibres under a variety of process conditions. One dimension of the 17 by 20 element array was used to vary the thickness of a Ni catalyst layer. The second dimension was used for temperature control. Growth took place in an ambient temperature gas flow system, with processes only occurring on activated heaters. This allowed different process sequences to be defined on different columns of the array. Four parameters were varied: pre-anneal temperature of the catalyst, growth temperature of the carbon nanostructures, growth pressure, and growth time. Scanning electron microscope images of each array element revealed trends in microstructure as these parameters, together with the catalyst thickness, were varied.

MICROSTRUCTURE OF AL CONTACTS ON GAAS

The microstructure of Al films deposited on GaAs(100) 2×4 surfaces through chemical vapor deposition from dimethylethylamine alane in the 100–160 °C temperature range exhibits a dominant (111) texture which is not encountered in evaporated films. Such a texture has been associated with enhanced electromigration resistance in related systems. Growth of (111)-oriented grains is observed when the deposition rate is limited by the surface reaction of the impinging precursor molecules, while at higher temperatures (160–400 °C) only the conventional texture is observed.

Amorphous Mixed TiO 2 and SiO 2 Films on Si(100) by Chemical Vapor Deposition

Amorphous thin films of composition Ti x Si 1-x O 2 have been grown by low pressure chemical vapor deposition on silicon (100) substrates using Si(O-Et) 4 and either Ti(O- i Pr) 4 or anhydrous Ti(NO 3 ) 4 as the sources of SiO 2 and TiO 2 , respectively. The substrate temperature was varied between 300 and 535°C, and the precursor flow rates ranged from 5 to 100 sccm. Under these conditions growth rates ranging from 0.6 to 90.0 nm/min were observed. As-deposited films were amorphous to X-rays and SEM micrographs showed smooth, featureless film surfaces. Cross-sectional TEM showed no compositional inhomogeneity. RBS revealed that x (from the formula Ti x Si 1-x O 2 ) was dependent upon the choice of TiO 2 precursor. For films grown using TTIP-TEOS x could be varied by systematic variation of the deposition conditions. For the case of TN-TEOS x remained close to 0.5 under all conditions studied. One explanation is the existence of a specific chemical reaction between TN and TEOS prior to film deposition. TEOS was mixed with a CCl 4 solution of TN at room temperature to produce an amorphous white powder (Ti/Si = 1.09) and 1 HNMR of the CCl 4 solution indicated resonances attributable to ethyl nitrate.

Identification of Odor Blend Used by Caenorhabditis elegans for Pathogen Recognition

Abstract Animals have evolved specialized pathways to detect appropriate food sources and avoid harmful ones. Caenorhabditis elegans can distinguish among the odors of various species of bacteria, its major food source, but little is known about what specific chemical cue or combination of chemical cues C. elegans uses to detect and recognize different microbes. Here, we examine the strong innate attraction of C. elegans for the odor of the pathogenic bacterium, Serratia marcescens. This initial attraction likely facilitates ingestion and infection of the C. elegans host. Using solid-phase microextraction and gas chromatography coupled with mass spectrometry, we identify 5 volatile odors released by S. marcescens and identify those that are attractive to C. elegans. We use genetic methods to show that the amphid chemosensory neuron, AWCON, senses both S. marcescens-released 2-butanone and acetone and drives attraction to S. marcescens. In C. elegans, pairing a single odorant with food deprivation results in a reduced attractive response for that specific odor. We find that pairing the natural odor of S. marcescens with food deprivation results in a reduced attraction for the natural odor of S. marcescens and a similar reduced attraction for the synthetic blend of acetone and 2-butanone. This result indicates that only 2 odorants represent the more complex odor bouquet of S. marcescens. Although bacterial-released volatiles have long been known to be attractive to C. elegans, this study defines for the first time specific volatile cues that represent a particular bacterium to C. elegans.