Plastic and nonmetallic article shaping or treating: processes – Formation of solid particulate material directly from molten... – By impinging or atomizing with gaseous jet or blast
Reexamination Certificate
1995-06-07
2001-08-28
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Formation of solid particulate material directly from molten...
By impinging or atomizing with gaseous jet or blast
C071S064120, C071S064120, C071S064120
Reexamination Certificate
active
06280662
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to forming ceramic wafers, and more particularly to forming dense ceramic wafers from pyroelectric and bolometric material for use in fabricating hybrid thermal detectors.
BACKGROUND OF THE INVENTION
One common application for thermal sensors is a thermal (infrared) imaging device such as night vision equipment. One such class of thermal imaging devices includes a focal plane array of infrared detector elements or thermal sensors having pyroelectric material. The focal plane array and its associated thermal sensors are often coupled to an integrated circuit substrate with a corresponding array of contact pads and a thermal isolation structure disposed between the focal plane array and the integrated circuit substrate. The thermal sensors define the respective picture elements or pixels of the resulting thermal image.
One type of thermal sensor includes a thermal sensitive element formed from pyroelectric material which exhibits a state of electrical polarization and/or change in dielectric constant dependent upon temperature changes of the pyroelectric material in response to incident infrared radiation. An infrared absorber and common electrode assembly are often disposed on one side of the thermal sensitive elements. A sensor signal electrode is generally disposed on the opposite side of each thermal sensitive element. The infrared absorber and common electrode assembly typically extends across the surface of the focal plane array and is attached to the thermal sensitive elements. Each thermal sensitive element generally has its own separate sensor signal electrode. Each infrared detector element or thermal sensor may be defined in part by the infrared absorber and common electrode assembly and the respective sensor signal electrode. The common electrode and the sensor signal electrode constitute capacitive plates. The pyroelectric material constitutes a dielectric or insulator disposed between the capacitive plates.
For some thermal sensors barium strontium titanate (BST) may be used to form the thermal sensitive element for the resulting thermal sensors. Various dopants may be added to the BST depending upon the desired operating characteristics for the resulting thermal sensors. The starting place for fabricating such thermal sensitive elements is typically a wafer of barium strontium titanate or other suitable pyroelectric material having a diameter of approximately one hundred (100) millimeters and an approximate thickness of three (3) millimeters. Various grinding and/or polishing processes are frequently used to reduce the thickness of the BST wafer to approximately twenty-five (25) microns or less for the finished, bonded device.
SUMMARY OF THE INVENTION
In accordance with the present invention, the disadvantages and problems associated with forming previous ceramic wafers used in fabricating hybrid thermal detector systems have been substantially reduced or eliminated. The present invention allows production of a dense ceramic wafer from various pyroelectric and/or bolometric materials for further fabrication of thermal sensitive elements used in hybrid thermal detector systems.
One aspect of the present invention may include a method for fabricating a dense ceramic wafer using cold isostatic pressing techniques along with one or more rigid substrates to create the desired flat surface on the resulting ceramic wafer. The associated flat surface is useful in subsequent processing of the wafer. One of the technical advantages resulting from forming ceramic wafers in accordance with one aspect of the present invention includes the ability to densify the resulting ceramic wafer to produce the desired pyroelectric characteristics and/or bolometric characteristics required by the thermal sensitive elements formed from the ceramic wafer.
Important technical advantages of one embodiment may include substantially reducing the cost and waste associated with forming a dense, ceramic wafer from pyroelectric and/or bolometric material. The resulting ceramic wafer may be used to provide a large number of thermal sensitive elements. Such thermal sensitive elements may be used for fabricating thermal sensors for a focal plane array which may be coupled with an integrated circuit substrate to form a hybrid thermal detector system.
Another aspect of the present invention may include the use of cold isostatic pressing techniques (sometimes referred to as hydrostatic pressing) to form a ceramic wafer of pyroelectric material such as barium strontium titanate (BST). For one embodiment the resulting wafer may be compatible with silicon processing techniques associated with fabricating very large scale integrated circuits. Also, a flexible container or membrane may be used during isostatic pressing to reduce problems related to unevenness in the ceramic powders.
An additional technical advantage of another embodiment may include placing selected powder material used to form the ceramic wafer in a vacuum bag to remove any air trapped in the powder prior to hydrostatically pressing. By removing such air from the powder, pressing flaws in the resulting ceramic wafer are substantially reduced.
Further technical advantages of one embodiment may include using cold isostatic pressing techniques to avoid problems associated with excessive shear forces when a wafer is ejected from a conventional rigid ram and die set. Also, the use of isostatic pressing techniques eliminates the cost associated with ram and die sets capable of handling the large forces associated with pressing ceramic wafers having a diameter of one hundred to one hundred twenty-five (100-125) millimeters. For some applications, the present invention may be used to form ceramic wafers with a diameter of one hundred fifty (150) millimeters and larger. The relatively large surface area of such wafers would require a large, heavy duty ram and die set to withstand the associated loads.
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R.A. Wood, et al., “HIDAD-A Monolithic, Silicon, Uncooled Infrared Imaging Focal Plane Array,” 16.5/Wood/HIDAD, pp. 579-581.
“Treatise on Materials, Science and Technology”, vol. 9, Ceramic Fabrication Processes, Edited by Franklin F.Y. Wang, pp. 135-151; 199-215; and 331-337.
Beratan Howard R.
Gist Ida M.
Owens Tony A.
Baker & Botts L.L.P.
Raytheon Company
Silbaugh Jan H.
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