Electrical resistors – Resistance value responsive to a condition – Ambient temperature
Reexamination Certificate
1998-12-07
2001-02-06
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Ambient temperature
C338S023000, C073S204250, C257S467000
Reexamination Certificate
active
06184773
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to thermal sensors of fluids, such as fluid flow or property sensors implemented on silicon in microstructure form. For convenience sake the term “flow sensor” will be used generically hereinafter for such thermal sensors. The reader will appreciate that such sensors may be utilized to measure primary properties such as temperature, thermal conductivity and specific heat; and that the flows may be generated through forced or natural convection. The invention relates more specifically to a sensor package of the microbridge or membrane type flow sensor having a central heating element and surrounding sensors which are capable of handling high pressure and have very low susceptibility to environmental damage or contamination.
2. Description of Related Art
Open microbridge structures such as detailed in U.S. Pat. No. 4,501,144, to Higashi, are well suited for measurements of clean gases, with or without large pressure fluctuations, since the microbridge structure is burst-proof. However, due to the open nature of the microbridge structure, condensates from vapor can be uncontrollably retained in the microbridge structure leading to uncontrolled changes in its thermal response, or output, making the structure susceptible to output error and poor stability. Also, in the typical microbridge structure, the silicon die is wire bonded at the top surface to a header, or substrate, carrying further electrical leads and/or electronics. Typically, such wire for the wire bonds would be a one mil gold wire. This wire has a further tendency to retain liquid condensates, increase undesirable turbulence, shift flow response. Due to its thinness, the wire is susceptible to damage in a high mass flux environment, such as liquid flow, and upon attempts to clean the sensor.
Membrane-based sensors such as shown in U.S. Pat. No. 5,705,745, to Treutler et al., overcome some of the problems of the microbridge structure because there is no opening between the bridge and the underlying thermal isolation cavity or air space. However, because the membrane is sealed over the isolation air space membrane based sensors have limited application in constant, near-atmospheric pressure, because the membrane can deform or burst as pressure differences increase above 100 PSI. The top surface of the membrane sensors is also typically wire bonded, leaving the problem of the wire in the flow path accumulating debris and possible breakage during cleaning attempts.
It would therefore be desirable to develop a flow sensor which is not susceptible to the above problems of vapor accumulation beneath the microbridge, poor ruggedness under high pressure capability of the membrane sensors, and exposed bonding wire near the heating and sensing elements. The design of such a structure would enable high pressure thermal property sensing over wide ranges at a reasonable cost and provide trouble free operation in heretofore hostile environments.
SUMMARY OF THE INVENTION
The present invention details a microstructure flow sensor having a silicon microsensor die with a micromembrane or microbridge sensing structure and through-the-wafer electrical contacts as well as a wide pressure range support structure consisting of a micromachined, back-etched honeycomb structure. A flat, passivated, top surface overlying the heater and sensor elements is featured on the silicon die. The silicon die, with its through-the-wafer electrical contacts, eliminates the need for bonding wires with their attendant problems as discussed above.
The die is attached to a substrate having a suitably matched coefficient of thermal expansion (CTE) by thermocompression bonding, solder bumping, adhesives or the like, preferably containing through-the-substrate electrical contacts terminating in the necessary electrically conductive runs for attachment to further electronics of the sensor.
The substrate may further have a glazing layer at the mating surface with the silicon die in order to provide a fluid barrier to the bottom of the die and back fill seals to prevent access to the back-side contacts and open ends of the honeycomb or micromembrane. Both silicon oxide and silicon nitride layers are used in the construction of the die. The present invention will benefit the user by trouble free and reliable service in all fluid flow applications as well as being easily fabricated and easily subjected to periodic maintenance.
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Ohnstein et al; “Environmentally Rugged Wide Dynamic Range Microstructure Airflow Sensor”, IEEE Solid-State Sensor and Actuator Workshop, Hilton Head, SC, Jun. 1990,Technical Digest,p. 158.
Kim et al; “Thermocompression Bonding Effects on Bump-Pad Adhesion”,IEEE Transactions on Components, Packaging, and Manufacturing Technology—Part B, vol. 18, No. 1, Feb. 1995.
Frampton et al; “Gas Mass Flow Sensor Proof of Concept Testing for Space Shuttle Orbiter Flow Measurement”,SAE Technical paper 961335,Int'l. Conf. Env. Systems, Monteray, CA, Jul. 1996.
Bonne; “New Developments in Natural Gas Transducer Technology”, 8th IGT Symposium on Gas Quality and Energy Measurement, Orlando, FL, Feb. 1997.
Bonne Ulrich
Padmanabhan Aravind
Abeyta Andrew A.
Easthom Karl D.
Honeywell Inc.
Roz and W. Norris
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