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Title: Absolute humidity sensors and methods of manufacturing humidity sensors
Claims: What is claimed is: 1. The method of manufacturing a humidity sensor comprising the steps of (a) providing a substrate having a microscopically smooth surface, (b) building up a uniform layer of Al on at least a portion of said substrate surface, said layer having a first surface in contact with said substrate surface and a second exposed surface, (c) forming an oxide over at least a major portion of the area of said Al layer to provide a porous Al.sub.2 O.sub.3 layer over said Al layer, (d) building up an electrically conductive layer, which is substantially permeable to water vapor, over at least a major portion of the Al.sub.2 O.sub.3 layer, and (e) building up an electrically conductive strip in contact with said permeable electrically conductive layer and extending beyond a border of the Al.sub.2 O.sub.3 layer, without electrical contact with the underlying Al, to an electrical contact location on said substrate. 2. The method as claimed in claim 1 wherein said step (a) comprises providing an Si wafer having a first microscopically smooth surface and treating said wafer to cause the growth of an SiO.sub.2 layer on said first microscopically smooth surface, said SiO.sub.2 layer having an exposed second microscopically smooth surface upon which said layer of Al is built up. 3. The method as claimed in claim 2 wherein said Si wafer is treated to provide an SiO.sub.2 layer having a thickness of the order of 5000A. 4. The method as claimed in claim 2 wherein said step (c) comprises forming an oxide on only a major portion of the area of said Al layer, the unoxidized area of said Al layer enabling electrical contact with said Al layer. 5. The method as claimed in claim 4 wherein said Al layer is shaped to have a major central region and a minor lobe projecting from said major central region, said unoxidized portion comprising said minor lobe. 6. The method as claimed in claim 1 wherein said step (d) comprises the vacuum deposition of a layer of Au having a thickness rendering the Au layer permeable to water vapor. 7. The method as claimed in claim 6 wherein said step (e) comprises the vacuum deposition of Au to form said electrically conductive strip. 8. The method as claimed in claim 7 wherein said steps (d) and (e) are performed simultaneously. 9. The method as claimed in claim 8 further comprising the step of building up at least one electrically conductive finger over said Au strip, said finger having a thickness greater than the thickness of said Au strip, thereby enabling electrical contact between said Au layer and the electrical contact location on said substrate even if there are discontinuities in said Au strip. 10. The method as claimed in claim 9 further including the step of providing an electrical contact location on said substrate in the form of an electrically conductive bonding pad spaced-apart along said substrate surface from said Al layer provided by step (b). 11. The method as claimed in claim 10 wherein said bonding pad is Al and is built up on said substrate simultaneously with the building up of said uniform layer of Al. 12. The method as claimed in claim 1 wherein said uniform layer of Al is of the order of 2000A thick, and the electrically conductive layer of step (d) is formed as a layer having a thickness in the range of about 100A to about 500A 13. The method as claimed in claim 12 wherein the layer formed in step (d) is a layer of gold. 14. The method as claimed in claim 1 wherein said step (b) comprises building up a uniform layer of Al on only a portion of said substrate surface, and wherein the method comprises the additional steps of providing heating means on said substrate and providing a temperature sensor on said substrate, thereby enabling operation at a temperature above the ambient temperature. 15. The method as claimed in claim 14 wherein in said step (b) said layer of Al is built up in a central portion of said substrate and wherein said temperature sensor is formed by depositing a first thin film strip, a material having a thermally dependent electrical resistance on said substrate in a pattern at the periphery of the structure provided by said steps (b) through (e). 16. The method as claimed in claim 15 wherein said heater is formed by depositing a second thin film strip of metal on said substrate in a pattern at the periphery of the structure provided by said steps (b) through (e). 17. The method as claimed in claim 15 wherein said heater is formed by the steps of (1) providing a thin strip of material, that generates heat when an electrical current is passed through it, on said substrate prior to said step (b) and in the region of said substrate where said layer of Al is to be built up and (2) providing an insulating layer over said thin strip. 18. The method as claimed in claim 17 wherein said strip is provided during said step (a), said step (a) comprising providing an Si wafer having a first microscopically smooth surface, doping to a predetermined depth in a pattern a portion of said Si wafer adjacent said first microscopically smooth surface with dopants rendering it conductive to said depths, treating said wafer to cause the growth of an SiO.sub.2 layer on said first microscopically smooth surface, said SiO.sub.2 layer having an exposed second microscopically smooth surface upon which said layer of Al is built up. 19. The method as claimed in claim 17 wherein said strip is provided during said step (a), said step (a) comprising providing an Si wafer having a first microscopically smooth surface; treating said wafer to cause the growth of an SiO.sub.2 layer on said first microscopically smooth surface; depositing a thin film of material in a pattern on said second microscopically smooth surface in the region where said uniform layer Al is to be built up said material being of the type that generates heat when an electrical current is passed through it; building up an insulating layer of material over said second microscopically smooth surface and said layer of material, said insulating layer having an exposed third microscopically smooth surface upon which said layer of Al is built up. 20. An absolute humidity sensor constructed according to the method of claim 14. 21. A humidity sensor constructed according to the method of claim 1. 22. The absolute humidity sensor of claim 21 wherein in said Al.sub.2 O.sub.3 layer the Al.sub.2 O.sub.3 content is no greater than about 0.0001 grams per square centimeter. 23. The method of manufacturing a humidity sensor comprising the steps of (a) providing a substrate having a microscopically smooth surface, (b) building up a uniform layer of Al on at least a portion of said substrate surface, said layer having a first surface in contact with said substrate surface and a second exposed surface, (c) forming an oxide of substantially the entirety of said Al layer to provide a porous Al.sub.2 O.sub.3 layer, (d) building up an electrically conductive layer, which is substantially permeable to water vapor, over at least a major portion of the exposed surface of said Al.sub.2 O.sub.3 layer, and (e) providing means for establishing electrical contact with said first surface of said Al.sub.2 O.sub.3 layer. 24. The method as claimed in claim 23 where said Al layer is built up to a thickness of no more than about 2500A. 25. The method of manufacturing a humidity sensor comprising the steps of (a) providing a low resistivity P-type silicon substrate having a microscopically smooth surface, (b) building up a uniform layer of Al on at least a portion of said substrate surface, said layer having a first surface in contact with said substrate surface and a second exposed surface, (c) forming an oxide of substantially the entirety of said Al layer to provide a porous Al.sub.2 O.sub.3 layer, (d) building up an electrically conductive layer, which is substantially permeable to water vapor, over at least a major portion of the exposed surface of said Al.sub.2 O.sub.3 layer, and (e) providing means for establishing electrical contact with said low resistivity P-type silicon. Other info: Inventors: Kovac, Michael G. (Sudbury, MA, US) Chleck, David J. (Brookline, MA, US) Goodman, Philip (Lexington, MA, US) Application Number: 764394 Filing Date: 1977-01-31 Publication_date: 1979-03-06 Assignee: Panametrics, Inc. (Waltham, MA) Primary Class(es): 427/79 73/335.05, 324/687, 338/35, 427/80, 427/102, 427/103 Other Classes: US Patent Ref:
Other Refs: Primary Examiner: Pitlick, Harris A. Assistant Examiner: Attorney: Kenway & Jenney |
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