Steve Semancik

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.

Surface state trapping models for SnO2-based microhotplate sensors

Abstract Due to their small size, SnO 2 -based microhotplate gas sensors can be used to develop a portable, sensitive, and low-cost gas monitoring system to detect, for example, leakage of hazardous gases. These devices, because of their low thermal mass, allow rapid temperature changes of the sensing material as a mode of sensor operation. To gain insight into the conductance response of microhotplate sensors, the basic physical and chemical processes involved in the sensing operation have been modeled. In this paper, intrinsic and extrinsic surface state trapping models are presented to describe the dynamic conductance responses of microhotplate gas sensors to argon and to air, respectively. These models relate the change in the conducting electron density to the change of the intrinsic/extrinsic surface state density based on potential barrier theory. Model parameters are estimated from one set of experiments, and then the models are used to predict output signals in a different set of experiments. Excellent agreement is achieved between the predicted and measured responses. The models can predict the fast temperature programmed sensing responses of microhotplate sensors on a time scale ranging from seconds to milliseconds. One interesting aspect of this modeling is that it correctly predicts that a transient conductivity response will occur when the temperature is cycled even if only argon is present. This paper also shows evidence for the effect of surface states on the conductance response of tin oxide films to these rapid temperature changes.

Optimization of temperature programmed sensing for gas identification using micro-hotplate sensors

Abstract Micro-hotplate chemical gas sensors, such as those being developed at the National Institute of Standards and Technology (NIST) by micromachining Si, can be operated in a temperature-pulsed mode, due to their small size and mass. In the temperature-pulsed mode of operation, different gases give different dynamic responses (i.e. signatures) depending on the temperature program used. In this paper a new methodology is presented to optimize the operation of micro-hotplate gas sensors for discriminating volatile organic compounds at a fixed concentration, while minimizing the detection time. The extension of the methodology to cases where concentrations vary is currently under investigation. The Wavelet Network method is applied to accurately predict the sensor’s response for a given temperature profile. Once a dynamic model is obtained, it is used for off-line optimization of the temperature profile, i.e. the maximization of the difference between two gas signatures. The difference between two response curves was initially measured by a metric based on the Euclidean distance. This metric was then modified using the Haar wavelet transformation. The methodology was implemented in a case study in which either methanol or ethanol had to be detected in air, but the methodology is generic, and it can be applied to any two gases.

Model studies of SnO2-based gas sensors: Vacancy defects and Pd additive effects

Abstract Surface analytical techniques including X-ray and UV photoemission spectroscopies (XPS, UPS), ion scattering spectroscopy (ISS) and in situ four-point conductance measurements have been used to carry out model studies of tin oxide (SnO2) based gas-sensing processes. Specific results presented here involve the interfacial properties of pure and Pd-dosed SnO2(110) crystals, both in vacuum and following adsorption of oxygen and hydrogen. Emphasis has been placed on experiments that isolate particular mechanistic effects and attempt to evaluate their relative importance in producing a sensing response.

Nanoparticle engineering and control of tin oxide microstructures for chemical microsensor applications

The use of metal nanoparticles as seed layers for controlling the microstructures of tin oxide (SnO2) films on temperature controllable micromachined platforms has been investigated. The study is focused on SnO2 due to its importance in the field of chemical microsensors. Nanoparticle seeds of iron, cobalt, nickel, copper and silver were formed by vapour deposition on the microhotplates followed by annealing at 500 °C prior to self-aligned SnO2 deposition. Significant control of SnO2 grain sizes, ranging between 20 and 121 nm, was achieved depending on the seed-layer type. A correlation was found between decreasing the SnO2 grain size and increasing the melting temperature of the seed-layer metals, suggesting the use of high temperature metals as being appropriate choices as seed layers for obtaining a smaller SnO2 grain structure. Smaller grain diameters resulted in high sensitivity in 90 ppm ethanol illustrating the benefits of nanoparticle seeding for chemical sensing. The initial morphology, particle size and distribution of the seed layers was found to dictate the final SnO2 morphology and grain size. This paper not only demonstrates the possibility of depositing nanostructured oxide materials for chemical microsensor applications, but also demonstrates the feasibility of conducting combinatorial research into nanoparticle growth using temperature controllable microhotplate platforms. This paper also demonstrates the possibility of using multi-element arrays to form a range of different types of devices that could be used with suitable olfactory signal processing techniques in order to identify a variety of gases.

Surface reconstructions of oxygen deficient SnO2(110)

Abstract The reconstruction of an ion-bombarded SnO 2 (110) surface has been studied with LEED, AES, XPS and UPS. In general, reconstructions of the ion-bombarded surface are associated with an extreme surface oxygen deficiency. Emission is observed throughout the band gap up to the Fermi level with UPS, and is associated with the presence of three-fold and four-fold coordinated Sn cations. Structural models have been proposed for the c(2 × 2) and (4 × 1) reconstructions based on disorder due to oxygen vacancies with a tendency toward alternation, and the formation of a SnO(101) overlayer, respectively.

A comparative study of signal processing techniques for clustering microsensor data (a first step towards an artificial nose)

Microsensor technology has progressed to the point where it is now feasible to place several hundred sensors on a computer chip. Such a sensor array can potentially be used in many applications including detecting hazardous chemical emissions, food processsing, and fire detection. This paper addresses an important aspect involved in microsensor applications, namely how the sensor signals are processed. The problem treated involves classifying whether a sensed signal is generated by one of four chemicals. Two broad approaches to processing the sensor signals are discussed, one based on classical signal processing approaches, and one based on a model of how the olfactory system in animals functions. The classical approaches used include: Gram Schmidt orthogonalization, fast Fourier transforms, and Haar wavelets. For the experimental signals treated, the classical approaches give superior results compared to those produced by the olfactory model.

Coupling Nanowire Chemiresistors with MEMS Microhotplate Gas Sensing Platforms

Recent advances in nanotechnology have yielded materials and structures that offer great potential for improving the sensitivity, selectivity, stability, and speed of next-generation chemical gas sensors. To fabricate practical devices, the “bottom-up” approach of producing nanoscale sensing elements must be integrated with the “top-down” methodology currently dominating microtechnology. In this letter, the authors illustrate this approach by coupling a single-crystal SnO2 nanowire sensing element with a microhotplate gas sensor platform. The sensing results obtained using this prototype sensor demonstrate encouraging performance aspects including reduced operating temperature, reduced power consumption, good stability, and enhanced sensitivity.

Coadsorption of water and sodium on the Ru(001) surface

The coadsorption of water and sodium on a Ru(001) surface has been studied as a model system of the interaction of adsorbed water with electropositive ions. A variety of surface sensitive methods were employed: electron stimulated desorption ion angular distributions (ESDIAD), thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The coadsorption results for temperatures above 80 K have been compared and contrasted to results obtained when water and Na are adsorbed separately on Ru(001). For Na coverages less than a critical value (0.25 monolayer), water adsorbs and desorbs in molecular form. However, even for low Na coverages, the presence of adsorbed Na strongly influences the desorption kinetics and causes an increase in the binding energy of water to Ru; in addition, the local bonding geometry of water is altered. The H+ ESD yield from adsorbed water monomers increases substantially in the presence of Na. For Na coverages greater than 0.25 monolayer, the adsorbed two-dimensional Na structures observed by LEED at 80 K become disordered when water is adsorbed, and water dissociates even at 80 K to form a hydroxide-like species with the OH molecular axis perpendicular to the surface. The hydroxide decomposes upon heating, and leads to hydrogen desorption between 350 and 600 K. The striking dependence of the water surface chemistry on the coverage of preadsorbed Na has been associated with the electropositive properties of the Na-Ru layer. The influence of Na on the adsorption of water on Ru(001) is important for understanding processes occurring at the metal-electrolyte interface in an electrochemical cell as well as the catalytic promotion of transition metal catalysts by adsorbed alkalis.

Low temperature ordering of sodium overlayers on RU(001)

Abstract The adsorption of alkali metals on transition metals can produce several technologically important effects, but only limited results have been reported on the geometrical structure of such adlayers, especially for adsorption temperatures below 300 K. We have examined the adsorption of Na on Ru(001) as a function of coverage and temperature using LEED to determine the adlayer structure and thermal desorption spectroscopy to characterize binding kinetics and relative Na coverages. The only Na LEED pattern observed following adsorption at 300 K was that of ( 3 2 × 3 2 ) structure which occurred near saturation of the first layer. However, Na adsorbed at 80 K produces a progression of distinct, ordered LEED patterns with increasing coverage which does not include the ( 3 2 × 3 2 ) pattern. These patterns result from increasingly compressed, hexagonal arrangements of adsorbate atoms which are uniformly spaced due to mutually repulsive interactions. The order-disorder transition temperature for each structure was also determined by LEED and used to develop a 2D phase diagram for Na on Ru(001). Ordered structures were observed only when Na thermally induced motion was sufficiently limited and the repulsive Na-Na interaction could force the uniform spacing of Na atoms. Thus, low coverage structures only developed where Na mobility was limited by low temperature. High coverage structures were stable to much higher temperatures since motion was inhibited by the high Na density.