Measuring and testing – Gas analysis – By vibration
Reexamination Certificate
1999-09-24
2001-04-10
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Gas analysis
By vibration
C073S023200, C073S03200R, C250S370010, C422S088000
Reexamination Certificate
active
06212939
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of chemical detection using cantilevered microelectromechanical structures (MEMS) devices and particularly to methods for detecting specific chemical entities using uncoated cantilevers by analysis of selectively excited adsorption-induced surface states upon exposure to photons having discreet energy levels.
2. Prior Art
The microcantilever sensor is a simple sensor concept with extreme high mass sensitivity or surface stress sensitivity. A sensitivity of sub-picograms has been demonstrated for microcantilever sensors and there exists the potential of achieving a mass sensitivity in the range of sub-femtograms. These microcantilevers can be made with a length ranging from one micron to a few hundred microns. The microcantilever resonance frequency changes due to adsorption-induced mass loading. The cantilever can also undergo bending due to adsorption-induced surface stress variation if the adsorption is confined to a single surface. The extreme mass sensitivity of the microcantilever is due primarily to its own extremely small mass. Microcantilever sensors based on mass detection were discussed in U.S. Pat. No. 5,445,008 to Wachter and Thundat. Microcantilever chemical sensors based on variation surface stress is discussed by Thundat and Wachter in U.S. Pat. No. 5,719,324.
The primary disadvantage to using microcantilevers, as well as other forms of mass sensor such as quartz crystal microbalances, surface acoustic wave devices, plate wave resonators, Lamb and Love wave sensors, is the inability to distinguish different chemical entities. As a result, to direct adsorption to a single surface, it has been known to apply coatings of various types to the cantilever. For example, coating the surface of a mass sensor with a thin layer of gold makes the sensor sensitive to mercury adsorption, but not selective because it also becomes sensitive to hydrogen sulfide and other sulfur compounds. Filters and other fixes to attain greater selectivity detract from the primary advantages of microcantilevers, small size and simplicity.
An alternative approach to chemical sensing is the use of arrays of mass sensors wherein each element is coated with a chemical coating selected to produce an unique and characteristic response. This requires many sensor elements and different chemical coatings. This also requires the use of neural networks for correct computation. Finding chemicals and unique combination of chemicals that can act as chemically selective or partially selective coatings is time consuming. Reliably applying the coatings to miniature sensors can be a challenging problem. Moreover, in a micromachined array, it is difficult to make all the individual cantilevers identical. Finally, the sensing based on a chemically modified single cantilever may not be reliable or reproducible due to a number of reasons including contamination. Therefore, chemical sensors based on chemically selective coatings do not offer a clear path to the development of reliable sensors that are miniature, easy to mass produce, and reliable.
BRIEF DESCRIPTION OF THE INVENTION
It is a first object of this invention to provide a method for detecting individual chemical species using uncoated microcantilevers.
It is a further object of this invention to utilize the chemistry of the cantilever itself as a principal determinant of selectivity.
It is a further object of this invention to use differences in the frequency of light as a discriminator in determining the identity of chemicals which are adsorbed on the surface of a cantilever.
These and other objects may be achieved by selecting an array of microcantilevers formed from semiconductor materials selected to have different band gaps and measuring the change in position of the cantilevers in the presence of one or more chemical entities while the surface structure of the cantilever is being scanned over a range of wavelengths of light lower than the band gap of at least one semiconductor cantilever in the array.
The invention may be used as the basis for a laboratory analytical instrument in the form of a spectrophotometer. It may be used industrially for process control. It may be used for environmental monitoring as a highly portable field instrument. Finally, its small size and low power requirement make the invention especially suitable as a personal warning device for soldiers potentially exposed to chemical and biological warfare agents.
REFERENCES:
patent: 4094608 (1978-06-01), Young
patent: 4942299 (1990-07-01), Kazmerski
patent: 5161147 (1992-11-01), Goldberg et al.
patent: 5440008 (1995-08-01), Wachter et al.
patent: 5464977 (1995-11-01), Nakagiri et al.
patent: 5719324 (1998-02-01), Thundat et al.
patent: 5770856 (1998-06-01), Fillard et al.
patent: 5918263 (1999-06-01), Thundat
patent: 5977544 (1999-11-01), Datskos et al.
Albrecht et al. Journal of Vacuum Science and Technology A 8 3386 (1990), Microfabrication of Cantilever Styli for the Atomic Force Microscope.*
Thundat et al. Microscale Thermophysical Engineering 1:185-199 (1997), Microcantilever Senors.*
Scott et al., Academic Press 1975, Surface Physics of Phosphors and Semiconductors, pp. v-xiv and pp. 162-175.
Cygan Michael
Hardaway/Mann IP Group of Nexsen Pruet Jacobs & Pollard, LLC
Larkin Daniel S.
Lockheed Martin Energy Research Corporation
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