Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package
Reexamination Certificate
1998-12-21
2001-12-04
Potter, Roy (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
C257S415000, C257S417000, C257S419000
Reexamination Certificate
active
06326682
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to semiconductor transducers and more particularly, to a reduced size, hermetically sealed semiconductor transducer and methods for fabricating the same.
BACKGROUND OF THE INVENTION
Semiconductor piezoresistive transducers have been widely known for many years and are used in a great variety of applications and harsh environments which require the transducer to be protected from the environment. In addition, in order to provide absolute pressure measurements, the transducer sensing elements must often be contained within a cavity of a known pressure, usually a vacuum, to provide a pressure reference. This vacuum cavity must of course be hermetically sealed to maintain the vacuum and protect the sensing elements. Such transducers are termed “absolute” pressure transducers, and may be fabricated as extremely small devices for use in the medical field, as pressure sensors in automobiles, and so on. The smaller the transducer can be manufactured for the given pressure range, the wider variety of applications that are possible and the cheaper the price.
Techniques for hermetically sealing semiconductor piezoresistive transducers from hostile environments have generally limited how small the transducers could be made since additional lateral space has been required to accommodate a hermetically sealing cover structure. Essentially, the piezoresistive transducer employs silicon resistive elements, the resistance of which varies according to the intensity or magnitude of an applied force upon an associated diaphragm. Such resistors comprise serpentine or tortuous line patterns. It is these resistor elements which must be hermetically isolated from the external environment to ensure proper sensing performance.
An example of such hermetically sealed environmentally protected absolute piezoresistive transducers is disclosed in U.S. Pat. No. 5,002,901 entitled “Method of Making Integral Transducer Structures Employing High Conductivity Surface Features” issued on Mar. 26, 1991 to A. Kurtz et al. and assigned to the assignee herein. The piezoresistive elements in that patent are formed over the central region of a dielectric layer which overlays a silicon diaphragm. The elements are arranged to form a Wheatstone bridge where the four circuit nodes of the bridge are configured as four p+ silicon electrical contact posts disposed on the peripheral corners of the device. Electrical interconnections, also comprised of p+ silicon, interconnect the contact posts with the piezoresistive transducer elements. A bias voltage is brought to two of the contacts, while the voltage is measured between the other two contacts. This is accomplished by bonding external wires to each contact, which wires are run to an external voltage source and potentiometer.
As the silicon diaphragm deflects in response to an applied force or pressure, the resistive of the piezoresistive transducer elements changes, thereby changing the measured voltage. The actual applied force or pressure can then readily be determined from the measured voltage.
The hermetic seal for this device was provided by fabricating a peripheral flange on the device's outer periphery beyond the contact posts. In this way an absolute cavity can be made which will provide a vacuum reference. A glass sheet cover is then bonded to the top of the flange to create the hermetic seal. The glass sheet is also bonded to the outside of the contact posts, while openings are left atop the contact posts to enable subsequent wire bonding thereto. A major drawback of this configuration is that the peripheral flange undesirably increases the size of the overall device. Moreover, the contact posts must be enlarged to provide adequate surface area for bonding to both the glass sheet and to the external wire bonds. This likewise increases the size of the device.
Accordingly, it would be desirable to reduce the size of this type of transducer by eliminating the above described additional space allocated to the hermetically sealing structure of the semiconductor transducer, and the required enlargement of the contact posts.
It is therefore an object of the present invention to provide a reduced size, hermetically sealed semiconductor transducer with a vacuum cavity reference in which the hermetically sealing structure contributes only minimally to the overall size of the device.
It is another object of the present invention to provide an improved method for fabricating such a reduced size, hermetically sealed semiconductor transducer.
SUMMARY OF THE INVENTION
The present invention is directed towards a reduced size, hermetically sealed semiconductor transducer and methods for fabricating the same. In a preferred embodiment, the transducer comprises a transducer wafer including a diaphragm with a dielectric layer disposed thereon, which diaphragm deflects upon the application of a force thereto. At least one piezoresistive element and two electrical contacts are disposed on a top surface of the dielectric layer, with the electrical contacts coupled to the piezoresistive element and extending to a peripheral portion of the dielectric layer. A cover member is provided that is dimensioned to surround the semiconductor element. The cover member is provided with corner apertures which will be congruent with the contact posts when the cover member is aligned with the transducer wafer. A peripheral glass frit bond is formed between the cover member and the transducer wafer. A central aperture is formed in a top portion of the cover member, positioned above a region bounded by the peripheral glass bond. This central aperture functions to allow the glass frit bond to be formed at atmospheric pressure which prevents air gap formation in the peripheral glass frit bond. A sealing member is used which covers the central aperture, where the sealing member may be bonded to the cover member in a vacuum environment to hermetically seal the aperture. This results in a vacuum being maintained between the transducer element and the cover and sealing members, the transducer element thereby being hermetically sealed from the external environment (with a vacuum reference) while at least a portion of the electrical contact remains exposed to enable subsequent wire bonding thereto.
The present invention is also directed towards a method for fabricating a hermetically sealed transducer of the type having a transducer wafer including a diaphragm and at least one semiconductor element disposed on a top surface of the transducer wafer. The method comprises the steps of: forming at least one electrical contact on the top surface of the transducer wafer, coupled to the semiconductor element and extending from the semiconductor element to an outer portion of the top surface; providing a cover member dimensioned to surround the semiconductor element and having an aperture formed therein; forming a peripheral glass frit bond between the cover member and the transducer wafer, and between the cover member and at least a portion of the electrical contact; and, hermetically sealing the aperture in a vacuum, so that a vacuum is maintained between the at least one semiconductor element and the cover member whereby the semiconductor element is hermetically sealed from the external environment while at least a portion of the electrical contact remains exposed.
REFERENCES:
patent: 3764950 (1973-10-01), Wallia
patent: 4815845 (1989-03-01), Kurtz
patent: 5614742 (1997-03-01), Gessner et al.
patent: 5656846 (1997-08-01), Yamada
Kurtz Anthony D.
Ned Alexander
Duane Morris & Heckscher
Kulite Semiconductor Products
Plevy Arthur L.
Potter Roy
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