Special research capabilities are required for the detailed study of sensing materials and transduction mechanisms, as well as for the fabrication and performance evaluation of prototype sensing devices. Five custom research systems have been constructed in the Process Measurements Division of the Chemical Science and Technology Laboratory at NIST in order to support development of optimized materials and concepts for next-generation sensors. These facilities are the (a) Film Deposition and Characterization System, (b) Chemical Vapor Deposition System, (c) Surface Analytical Facility, (d) Scanning Electron Microscope, and (e) Sensor Response Test System. Specific objectives for research using this equipment include:
•Determining the basic properties of active materials•Developing practical methods for fabricating microstructure- controlled oxide and ultrathin metal films
•Characterizing the structure, morphology and composition of films and interfaces
•Understanding the molecular scale gas-solid interactions that lead to macroscopic electrical responses
• Modeling the performance characteristics of prototype sensors to efficiently optimize devices
• Developing databases for use in constructing pattern recognition schemes for gas sensor arrays
A brief description of each facility follows.
This system is a dual chamber, ultrahigh vacuum (UHV)-based system used to develop procedures for fabricating oxide and metal films of controlled microstructure. Oxide films (1-1000 nm think) are deposited by reactive sputter deposition and ultrathin metal overlayers (monolayer regime island structures) are deposited by evaporation. The films can be characterized, in situ, using 4-point conductance measurements and their structures can be studied by low energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED).
This system provides a multi-source capability for depositing films on temperature-elevated substrates by Chemical Vapor Deposition (CVD). The CVD system schematic and reactor cell of this system are shown above. At present the system is used primarily for depositing thin (10-200 nm) semiconducting oxide and ultrathin (1-10 nm) metal films. Capabilities exist for depositing films (at atmospheric and reduced pressures) on macrosamples, and on microsensor elements in array formats. In the latter case, self-lithographic processing is used to considerable practical advantage. Source-containing bubblers are temperature-controlled to assure growth uniformity.
SURFACE ANALYTICAL FACILITY
This multi-chamber UHV-based facility is equipped with a complement of analytical hardware for characterizing the outermost regions of samples (0.5-10 nm). In order to perform thorough examinations of sensor material properties, instrumentation is included for performing chemical, structural, electronic and electrical analyses by x-ray photoemission spectroscopy (XPS), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), secondary ion mass spectroscopy (SIMS), energy loss spectroscopy (ELS), ion scattering spectroscopy (ISS), low energy electron diffraction (LEED), electron stimulated desorption (ESD), scanning tunneling microscopy(STM), ultraviolet photoemission spectroscopy (UPS), measurement of work function change, and four-point conductance measurements. A variety of sample preparation and surface modification hardware (ion guns, evaporators, gas sources, quartz crystal monitors, etc.) critical to tailoring sensor materials is also mounted on the three chambers of this system. The image has labels for the two preparation chambers (Prep 1 and Prep 2), the STM chamber, the Analysis chamber, the x-ray gun for XPS, the ion gun for ISS, the UV lamp for UPS, the energy analyzer, and the four-point surface conductance probe.
SCANNING ELECTRON MICROSCOPE
The microscope is a Hitachi S-4100 Cold-Field-Emission scanning electron microscope. This microscope has an accelerating voltage range from 0.5kV to 30 kV. The capability to operate at low voltages enables the imaging of oxide surfaces without applying the conductive coatings usually required to minimize surface charging. This instrument also offers high magnification and high resolution, with 40-300,000x magnification and 1.5 nm resolution at 30 kV specifications, respectively. Electrical feedthroughs allow voltage contrast imaging techniques to be applied, as well as the in-situ observation of activated microsensor structures. Digitial image acquisition is accomplished through a Macintosh Power PC computer system, and photo quality Images are provided with a dye sublimation printer. The system also includes an Energy Dispersive X-ray Spectroscopy system for chemical analyses and spatial compositional mapping.
This fully-automated system operates under computer control to mix gases in desired ratios and flow them, at specified rates and pressure, to up to eight samples mounted within the test chamber. Electrical characteristics from the test devices are computer recorded with an update rate of up to 100 Hz. Efficient studies of adsorption/desorption kinetics and response functions of customized films can be performed in order to optimize active sensor interfaces. Prototype array devices can also be examined to acquire databases for development of pattern recognition/neural net algorithms. A FTIR spectrometer is used to calibrate component concentration levels in gas mixtures.
Additional facilities for processing semiconductors and fabricating micromachined devices are found in the Semiconductor Electronics Division of the Electronics and Electronics and Electrical Engineering Laboratory at NIST. We have close ties to the MicroElectroMechanical Systems (MEMS) Project and the IC Technology Group at NIST.