Christopher B. Montgomery

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.

A High-Quality Speech and Audio Codec With Less Than 10-ms Delay

With increasing quality requirements for multimedia communications, audio codecs must maintain both high quality and low delay. Typically, audio codecs offer either low delay or high quality, but rarely both. We propose a codec that simultaneously addresses both these requirements, with a delay of only 8.7 ms at 44.1 kHz. It uses gain-shape algebraic vector quantization in the frequency domain with time-domain pitch prediction. We demonstrate that the proposed codec operating at 48 kb/s and 64 kb/s out-performs both G.722.1C and MP3 and has quality comparable to AAC-LD, despite having less than one fourth of the algorithmic delay of these codecs.

Enhanced mass transport in ultrarapidly heated Ni/Si thin-film multilayers

We investigated multilayer and bilayer Ni/Si thin films by nanodifferential scanning calorimetry (nano-DSC) at ultrarapid scan rates, in a temperature-time regime not accessible with conventional apparatus. DSC experiments were completed at slower scan rates as well, where it was possible to conduct parallel rapid thermal annealing experiments for comparison. Postexperimental characterization was accomplished by x-ray diffraction, and by transmission electron microscopy (TEM) and energy-filtered TEM of thin cross sections prepared by focused ion beam milling. We found that rate of heating has a profound effect on the resulting microstructure, as well as on the DSC signal. After heating to 560 °C at 120 °C/s, the general microstructure of the multilayer was preserved, in spite of extensive interdiffusion of Ni and Si. By contrast, after heating to 560 °C at 16 000 °C/s, the multilayer films were completely homogeneous with no evidence of the original multilayer microstructure. For the slower scan rates, we...

An Interlaboratory Comparison of Sizing and Counting of Subvisible Particles Mimicking Protein Aggregates

Accurate counting and sizing of protein particles has been limited by discrepancies of counts obtained by different methods. To understand the bias and repeatability of techniques in common use in the biopharmaceutical community, the National Institute of Standards and Technology has conducted an interlaboratory comparison for sizing and counting subvisible particles from 1 to 25 μm. Twenty-three laboratories from industry, government, and academic institutions participated. The circulated samples consisted of a polydisperse suspension of abraded ethylene tetrafluoroethylene particles, which closely mimic the optical contrast and morphology of protein particles. For restricted data sets, agreement between data sets was reasonably good: relative standard deviations (RSDs) of approximately 25% for light obscuration counts with lower diameter limits from 1 to 5 μm, and approximately 30% for flow imaging with specified manufacturer and instrument setting. RSDs of the reported counts for unrestricted data sets were approximately 50% for both light obscuration and flow imaging. Differences between instrument manufacturers were not statistically significant for light obscuration but were significant for flow imaging. We also report a method for accounting for differences in the reported diameter for flow imaging and electrical sensing zone techniques; the method worked well for diameters greater than 15 μm.

Restructuring tungsten thin films into nanowires and hollow square cross-section microducts

We report on the growth of nanowires and unusual hollow microducts of tungsten oxide by thermal treatment of tungsten films in a RF H2/Ar plasma at temperatures between 550-620 °C. Nanowires with diameters of 10-30 nm and lengths between 50-300 nm were formed directly from the tungsten film, while under certain specific operating conditions hollow microducts having edge lengths ~ 0.5 μm and lengths between 10-200 μm were observed. Presence of a reducing gas such as H2 was crucial in growing these nanostructures as was trace quantities of oxygen necessary to form a volatile tungsten species. Preferential restructuring of the film surface into nanowires or microducts appeared to be significantly influenced by the rate of mass-transfer of gas phase species to the surface. Nanowires were also observed to grown on tungsten wires under similar condition. A surface containing nanowires, annealed at 500 °C in air, exhibited capability of sensing trace quantities of nitrous oxides (NOx).

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.

Particle shape effects on subvisible particle sizing measurements.

Particle analysis tools for the subvisible (<100 μm) size range, such as light obscuration, flow imaging (FI), and electrical sensing zone (ESZ), often produce results that do not agree with one another, despite their general agreement when characterizing polystyrene latex spheres of different sizes. To include the effect of shape in comparison studies, we have used the methods of photolithography to create rods and disks. Although the rods are highly monodisperse, the instruments produce broadened peaks and report mean size parameters that are different for different instruments. We have fabricated a microfluidic device that simultaneously performs ESZ and FI measurements on each particle to elucidate the causes of discrepancies and broadening. Alignment of the rods with flow causes an oversizing by FI and undersizing by ESZ. FI also oversizes rods because of the incorrect edge definition that results from diffraction and imperfect focus. We present an improved correction algorithm for this effect that reduces discrepancies for rod-shaped particles. Tumbling of particles is observed in the microfluidic ESZ/FI and results in particle oversizing and breadth of size distribution for the monodisperse rods.

Demonstration of fast and accurate discrimination and quantification of chemically similar species utilizing a single cross-selective chemiresistor.

Performance characteristics of gas-phase microsensors will determine the ultimate utility of these devices for a wide range of chemical monitoring applications. Commonly employed chemiresistor elements are quite sensitive to selected analytes, and relatively new methods have increased the selectivity to specific compounds, even in the presence of interfering species. Here, we have focused on determining whether purposefully driven temperature modulation can produce faster sensor-response characteristics, which could enable measurements for a broader range of applications involving dynamic compositional analysis. We investigated the response speed of a single chemiresitive In2O3 microhotplate sensor to four analytes (methanol, ethanol, acetone, 2-butanone) by systematically varying the oscillating frequency (semicycle periods of 20–120 ms) of a bilevel temperature cycle applied to the sensing element. It was determined that the fastest response (≈ 9 s), as indicated by a 98% signal-change metric, occurred for a period of 30 ms and that responses under such modulation were dramatically faster than for isothermal operation of the same device (>300 s). Rapid modulation between 150 and 450 °C exerts kinetic control over transient processes, including adsorption, desorption, diffusion, and reaction phenomena, which are important for charge transfer occurring in transduction processes and the observed response times. We also demonstrate that the fastest operation is accompanied by excellent discrimination within a challenging 16-category recognition problem (consisting of the four analytes at four separate concentrations). This critical finding demonstrates that both speed and high discriminatory capabilities can be realized through temperature modulation.

Analog BIST Functionality for Microhotplate Temperature Sensors

In this letter, we describe a novel long-term microhotplate temperature-sensor calibration technique suitable for built-in self-test (BIST). The microhotplate thermal resistance (thermal efficiency) and the thermal voltage from an integrated platinum/rhodium thermocouple were calibrated against a polysilicon temperature-sensor calibration curve which drifts over time. Both of these temperature sensors, which cannot be directly calibrated, exhibit excellent long-term temperature stability and are appropriate for BIST functionality.

Microsensors for Mars: Trace Analyte Detection in a Simulated Martian Environment

Chemiresistive microsensor arrays are being developed and tested in a simulated Martian environment. Target analyte species include trace small molecules that may indicate current geological or, potentially, biological activity on Mars. The sensing films are based upon robust, nanostructured metal oxide materials, including doped and undoped tin oxide, tungsten oxide and indium oxide. In combination with dynamic operating temperatures and advanced signal processing, we show in this presentation the capabilities of the microsensor arrays for analyte discrimination and quantification at target levels below 500 nmol/mol in a simulated Martian environment. The simulated environment uses a carbon dioxide-rich background and low temperature and pressure.