Richard E. Cavicchi

Tin oxide gas sensor fabricated using CMOS micro-hotplates and in-situ processing

A monolithic tin oxide (SnO/sub 2/) gas sensor realized by commercial CMOS foundry fabrication (MOSIS) and postfabrication processing techniques is reported. The device is composed of a sensing film that is sputter-deposited on a silicon micromachined hotplate. The fabrication technique requires no masking and utilizes in situ process control and monitoring of film resistivity during film growth. Microhotplate temperature is controlled from ambient to 500 degrees C with a thermal efficiency of 8 degrees C/mW and thermal response time of 0.6 ms. Gas sensor responses of pure SnO/sub 2/ films to H/sub 2/ and O/sub 2/ with an operating temperature of 350 degrees C are reported. The fabrication methodology allows integration of an array of gas sensors of various films with separate temperature control for each element in the array, and circuits for a low-cost CMOS-based gas sensor system.<<ETX>>

Microhotplate Platforms for Chemical Sensor Research

Abstract This paper describes the development and use of microdevices and microarrays in chemical sensor research. The surface-micromachined “microhotplate” structure common within the various platforms included here was originally designed for fabricating conductometric gas microsensor prototypes. Microhotplate elements include functionality for measuring and controlling temperature, and measuring the electrical properties of deposited films. As their name implies, they are particularly well-suited for examining temperature-dependent phenomena on a micro-scale, and their rapid heating/cooling characteristics has led to the development of low power sensors that can be operated in dynamic temperature programmed modes. Tens or hundreds of the microhotplates can be integrated within arrays that serve as platforms for efficiently producing processing/performance correlations for sensor materials. The microdevices also provide a basis for developing new types of sensing prototypes and can be used in investigations of proximity effects and surface transient phenomena.

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.

Anti-HER2 IgY antibody-functionalized single-walled carbon nanotubes for detection and selective destruction of breast cancer cells.

BackgroundNanocarrier-based antibody targeting is a promising modality in therapeutic and diagnostic oncology. Single-walled carbon nanotubes (SWNTs) exhibit two unique optical properties that can be exploited for these applications, strong Raman signal for cancer cell detection and near-infrared (NIR) absorbance for selective photothermal ablation of tumors. In the present study, we constructed a HER2 IgY-SWNT complex and demonstrated its dual functionality for both detection and selective destruction of cancer cells in an in vitro model consisting of HER2-expressing SK-BR-3 cells and HER2-negative MCF-7 cells.MethodsThe complex was constructed by covalently conjugating carboxylated SWNTs with anti-HER2 chicken IgY antibody, which is more specific and sensitive than mammalian IgGs. Raman signals were recorded on Raman spectrometers with a laser excitation at 785 nm. NIR irradiation was performed using a diode laser system, and cells with or without nanotube treatment were irradiated by 808 nm laser at 5 W/cm2 for 2 min. Cell viability was examined by the calcein AM/ethidium homodimer-1 (EthD-1) staining.ResultsUsing a Raman optical microscope, we found the Raman signal collected at single-cell level from the complex-treated SK-BR-3 cells was significantly greater than that from various control cells. NIR irradiation selectively destroyed the complex-targeted breast cancer cells without harming receptor-free cells. The cell death was effectuated without the need of internalization of SWNTs by the cancer cells, a finding that has not been reported previously.ConclusionWe have demonstrated that the HER2 IgY-SWNT complex specifically targeted HER2-expressing SK-BR-3 cells but not receptor-negative MCF-7 cells. The complex can be potentially used for both detection and selective photothermal ablation of receptor-positive breast cancer cells without the need of internalization by the cells. Thus, the unique intrinsic properties of SWNTs combined with high specificity and sensitivity of IgY antibodies can lead to new strategies for cancer detection and therapy.

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.

Fast temperature programmed sensing for micro-hotplate gas sensors

We describe an operating mode of a gas sensor that greatly enhances the capability of the device to determine the composition of a sensed gas. The device consists of a micromachined hotplate with integrated heater, heat distribution plate, electrical contact pads, and sensing film. The temperature programmed sensing (TPS) technique uses millisecond timescale temperature changes to modify the rates for adsorption, desorption, and reaction of gases on the sensing surface during sensor operation. A repetitive train of temperature pulses produces a patterned conductance response that depends on the gas composition, as well as the temperature pulse width, amplitude, and specific sequence of pulses. Results are shown for the vapors of water, ethanol, methanol, formaldehyde, and acetone.

Optimized temperature-pulse sequences for the enhancement of chemically specific response patterns from micro-hotplate gas sensors

Abstract Microfabricated solid-state gas sensors have been of continuing interest because of the potential for arrays of devices with low power consumption. Devices based on a micromachined ‘hotplate’ offer the additional advantage of a wide operating temperature range with a rapid thermal time constant of order 1 ms. By operating the device in a temperature-programmed mode, reaction kinetics on the sensing film surface are altered, producing a time-varying response signature that is characteristic of the gas being sensed. Approaches to optimizing such temperature programs to maximize the differences in response signatures for gases of interest or to enhance the sensitivity of the device are discussed.

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.

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.