Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1999-08-03
2002-10-08
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06460411
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thermal semiconductor flow sensors with direction recognition for liquids and gases and, in particular, to a silicon flow sensor component for such flow sensors.
Silicon technology-based sensors have promising properties for many measurement applications. They have, for example, small dimensions, they can be mass-produced at a reasonable price and they permit the monolithic integration of read-out electronics. Silicon flow sensors additionally offer the advantage of short response times and of a low power consumption. In the production process of the silicon sensors, integration of the electronics is, however, only possible if process steps, which are compatible with the production methods of microelectronics, are used for this integration of the electronics.
2. Description of Prior Art
Silicon technology-based flow sensors are known, which operate on the basis of pressure-difference measurements. Such flow sensors are described e.g. in S. T. Cho, K. Najafi, C. E. Lowman and K. D. Wise: An ultrasensitive silicon pressure-based microflow sensor, IEEE Transaction on Electron Devices, Vol. 39, No. 4 (1992) 825-835.
J. Braneberg, O. J. Jensen, N. G. Laursen, O. Leistiko and H. Soeberg: A micromachined flow sensor for measuring small liquid flows, Proc. 6th Int. Conf. Solid-State Sensors and Actuators (Transducers 1991) San Francisco, Calif., USA, Jun. 24 to 27, 1991, pp. 41-44, describe flow sensors operating on the principle of transit-time measurement with thermal marking. Furthermore, flow sensors are known operating on the basis of the thermal principles according to the hot-film anemometer principle. Most of the sensors of the type described hereinbefore have been used for the flow measurement of gases up to now.
In the following, known solution approaches, which are similar to the hot-film or hot-wire anemometer principle, will be described. The hot-film anemometer principle is based on the cooling down of heated structures by flowing media which are in contact with these structures. The cooling down of the heated structures depends on the flow rate of the medium.
First silicon flow sensors were already described between 1970 and 1980. R. W. M. van Riet and J. H. Huijsing: Integrated direction-sensitive flowmeter; Electronic Letters, 1976, Vol. 12, No. 4, pp. 647-648, describe a flow sensor in the case of which two transistors, which serve as temperature sensors, are arranged on a silicon substrate before and after a transistor, when seen in the direction of flow, the central transistor serving as a heating element. In this flow sensor, the signal difference between the two temperature sensors, which are arranged before and after the heating element, when seen in the direction of flow, is a measure of the flow rate of a medium flowing past the flow sensor.
A similar set-up making use of Wheatstone bridges of ion-implanted resistors is described in A. F. P. van Putten and S. Middlehoek: Integrated silicon anemometer; Electronic Letters, Vol. 10 (1974), pp. 425-426. With regard to more extensive examinations of such sensors, reference is also made to A. F. P. van Putten: An integrated silicon double bridge anemometer. Sensors and Actuators, Vol. 4 (1983), pp. 387-396; and J. I. Huijsing, J. P. Schuddemat and W. Verhoef: Monolithic integrated direction sensitive flow sensor; IEEE Trans. Electron Devices, Vol. ED-29 (1982) No. 1, pp. 133-136.
In such known flow sensors, the sensitivity was low, since a thermally good contact exists between the heating element and the temperature sensors via the silicon chip which has a good thermal conductivity. The sensitivity depends also strongly on the structural design. Hence, structures have been developed which provide a better thermal insulation between the two sensors and the heating element. The detection of the flow-dependent percentage of the heat entering the medium can be improved in this way.
R. G. Johnson and R. E. Higashi: A highly sensitive silicon chip microtransducer for air flow and differential pressure sensing applications; Sensors and Actuators, Vol. 11 (1987), pp. 63-72, discloses a flow sensor with a freestanding silicon nitride bridge which is anisotropically undercut on the front side thereof. On this silicon nitride bridge, heating elements and temperature sensors are arranged, which are based on metal resistors. A disadvantage of such an undercut structure is, however, that it is sensitive to e.g. dust and oil particles.
M. Stenberg, G. Stemme and G. Kittisland: A silicon sensor for measurement of liquid flow and thickness of fouling biofilms, Sensors and Actuators, 13 (1988), 203-221, disclose a solution approach in which the thermal insulation between heating elements and temperature detection elements is accomplished by means of a polyimide. In this silicon sensor a thin tongue is etched out of the back of a chip; with the exception of the conductor tracks and an oxide layer on the back, the tip of the tongue is separated from the tongue by means of a front etching process. The resultant V-shaped indentation is filled with a polyimide for the purpose of stabilization. The tip has provided thereon a heating element and a diode for temperature measurement. A disadvantage of such a system is the dependence of the thermal conductivity on outer influences, e.g. humidity, acting on the polyimide.
A flow sensor provided with a diaphragm formed by back-etching from a block of fully oxidized porous silicon and making use of platinum resistors, is described in O. Tabata: Fast response silicon flow sensor with an on-chip fluid temperature sensing element; IEEE Trans. Electron. Devices, Vol. ED-33 (1986), pp. 297-302. The structure described in this publication has a smooth surface and offers therefore no direct hold for particles or an impact pressure; it does, however, not permit an integration of microelectronic components in the diaphragm.
In addition flow sensors are known, which are constructed with the aid of freestanding polysilicon bridges on a chip surface. The layer used as a sacrificial layer below the polysilicon bridge is e.g. phosphorus silicate glass. In the central area of the polysilicon bridge, there is a lightly doped region, which serves as a flow-sensitive heated resistor, the rest of the bridge being heavily doped and acting as an electric conductor. A flow sensor consisting of a silicon diaphragm, which is freely suspended from four arms so that it is thermally insulated, is disclosed in B. W. van Oudheusden, A. W. van Herwaarden: High-sensitivity 2-D flow sensor with an etched thermal insulation structure; Sensors and Actuators, A21-A23 (1990) 425-430. Temperature difference measurement is here carried out via a thermopile.
E. Yoon, K. D. Wise: An integrated mass flow sensor with on-chip CMOS interface circuits; IEEE Transactions on Electron. Devices, Vol. 39, No. 6 (1992) 1376-1386; R. G. Johnson and R. E. Higashi: A highly sensitive silicon chip microtransducer for air flow and differential pressure sensing applications; Sensors and Actuators, Vol. 11 (1987), pp. 63-72; as well as T. R. Ohnstein, R. G. Johnson, R. E. Higashi, D. W. Burns, J. O. Holmen, E. A. Satren, G. M. Johnson, R. Bicking and S. D. Johnson: Environmentally rugged, wide dynamic range microstructure airflow sensor; IEEE Solid-State Sensor and Actuator Workshop Tech. Digest (1990), Hilton Head Island, S.C., describe monolithically integrated flow sensors for gases provided with the necessary electronics. For reasons of robustness, a diaphragm formed by back-etching and consisting of oxide has been used as a flow-sensitive element also in these flow sensors. The metallization of the chip consists of gold and chromium. Also the temperature-sensitive elelements are formed by Au—Cr thin-film resistors.
Deviating from the above-described sensors, the publication O. Tabata: Fast response silicon flow sensor with an on-chip fluid temperature sensing element; IEEE Trans. Electron. Devices, Vol. 33 (1986), pp. 297-302, describes a flow sensor in the case
Kersjes Ralf
Mokwa Wilfried
Vogt Holger
Zimmer Günter
Dougherty & Clements LLP
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung
Fuller Benjamin R.
Thompson Jewel V.
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