Patent No. 5750926 Hermetically sealed electrical feedthrough for use with implantable electronic devices

 

Patent No. 5750926

Hermetically sealed electrical feedthrough for use with implantable electronic devices (Schulman, et al., May 12, 1998)

Assignee: Alfred E. Mann Foundation for Scientific Research (Sylmar, CA)

Abstract

A thin hermetically sealed electrical feedthrough suitable for implantation within living tissue permits electrical connection between electronic circuits sealed within an hermetically sealed case and electrical terminals or contacts on the outside of the case. The hermetically sealed case is made by hermetically bonding a cover to an insulating layer. The hermetically sealed electrical feedthrough is made by depositing a conductive trace on the insulating layer and then depositing another insulating layer thereover, so that the conductive trace is hermetically encapsulated within the insulating layers. At least two spaced-apart openings are formed in the insulating layers before bonding the cover thereto, exposing the conductive trace. Additional conductive material is then inserted within each of the openings or holes so as to form conductive vias that make electrical contact with the conductive trace. The cover is then hermetically sealed to the insulating layer so that at least one conductive via resides inside of an hermetically sealed cavity formed under the cover, and the other conductive via resides outside of the hermetically sealed cavity. An electrical feedthrough is thus formed through the respective conductive vias and conductive trace so that electrical contact may be made between the outside and inside of the hermetically sealed cavity.

 

Notes:

BACKGROUND OF THE INVENTION

The present invention relates to hermetically sealed feedthroughs that allow electrical connections to be made with electronic circuitry or components which are hermetically sealed in housings or cases suitable for implantation within living tissue. More particularly, the invention relates to a hybrid ceramic extremely thin film hermetic seal that permits very thin hermetically sealed housings to be formed wherein electronic circuitry may me placed and protected, yet still allow electrical contact to be readily established with such electronic circuitry through the use of thin film hermetically sealed feedthroughs.

Hermetically sealed cases or housings are widely used to protect electronic components that may be susceptible to damage or malfunction from exposure to the surrounding environment. For example, a piezoelectric crystal and certain semiconductor devices need to be protected from the atmosphere, and are thus commonly hermetically sealed in a metal can. The hermetic seal is simply an airtight, durable seal that is long-lasting and physically rugged. Sometimes the interior of an hermetically sealed enclosure is filled with an inert gas such as helium, to further retard the deterioration of the component or components inside. As no seal is perfect, the tightness of the hermetic seal, referred to as the hermeticity, is typically measured or specified in terms of the leakage rate through the seal, expressed in cc/sec. Sometimes, for very low leakage rates, the hermeticity can only be measured by placing a radioactive gas within the enclosure and then using an appropriate radiation detector to "sniff"0 the seal for radioactive leaks.

Where the electrical component or components are to be implanted in body tissue, the hermetically sealed case (which must be made from a material that is compatible with body tissue, such as platinum or stainless steel or glass) serves a dual purpose: (1) it protects the electrical component or components from body fluids and tissue, which fluids and tissue could otherwise prevent the components from performing their desired function; and (2) it protects the body tissue and fluids from the electrical component or components, which component or components may be made at least in part from materials that may be damaging to body tissue, and which therefore could pose a significant health risk to the patient wherein they are implanted. It is thus critically important that the hermetic seal of an implanted device be especially long-lasting and physically rugged. For this reason, stringent requirements are imposed on the hermeticity of an implanted device, typically requiring a seal that provides a leakage rate of less than 10.sup.-8 cc/sec.

In recent years, the size of implanted medical devices has decreased dramatically. It is now possible, for example, to construct a simple stimulator device in a small hermetically sealed glass tube that can be implanted through the lumen of a needle. See, U.S. Pat. Nos. 5,193,539; and 5,193,540, incorporated herein by reference. With such a small size comes increased requirements for the tightness of the hermetic seal because there is less empty space inside of the sealed unit to hold the moisture that eventually leaks therethrough. The hermeticity requirements of such small devices may thus be on the order of 10.sup.-11 or 10.sup.-12 cc/sec. While the small size is thus advantageous, the stringent hermeticity requirements imposed for such small devices makes them extremely difficult to manufacture, and thus increases the cost of manufacture.

A significant problem associated with an hermetically sealed package, particularly where the package is implanted in living tissue, is the feedthrough mechanism used to allow electrical conductivity between the circuits sealed in the hermetically sealed package, and the environment surrounding the enclosure. Most implanted medical devices, such as a cardiac pacemaker, neural stimulator, biochemical sensor, and the like, require such a feedthrough in order to establish electrical contact between the appropriate circuitry sealed in the hermetically closed package and an external electrode that must be in contact with the body tissue or fluids outside of the sealed package. In a pacemaker, for example, it is common to provide such a feedthrough by using a feedthrough capacitor. A representative feedthrough capacitor is described in U.S. Pat. No. 4,152,540. Alternatively, a hermetic feedthrough is typically used to establish electrical connections between the appropriate electronic components or circuitry sealed in the hermetically closed package and an external control device, or monitoring equipment.

Heretofore, an hermetic feedthrough for implantable packages has consisted of a ceramic or glass bead that is bonded chemically at its perimeter through brazing or the use of oxides, and/or mechanically bonded through compression, to the walls of the sealed package. A suitable wire or other conductor passes through the center of the bead, which wire or conductor must also be sealed to the bead through chemical bonds and/or mechanical compression. The feedthrough is thus circular, and the wire(s) or conductor(s) mounted within the bead are centered or mounted in a uniform pattern centrally positioned within the bead. Such centering is necessary due to the thermal coefficients required for the different expansion rates that occur when heating is made to either cause the compression seal or to cause the oxide or bronze bonding.

Other related art relating to methods for forming hermetically sealed cases having electrical feedthroughs and vias include U.S. Pat. No. 4,525,766 issued to Petersen, U.S. Pat. No. 4,861,641 issued to Foster et al., and U.S. Pat. No. 4,882,298 issued to Moeller et al. While these patents teach improvements in the art, such teachings are limited to use with semiconductor substrates and are not easily adaptable for use with microminiature devices implantable within living tissue.

As implantable devices have become thinner and thinner, the size of the ceramic or glass beads used for electrical feedthroughs has also become smaller and smaller. This means that the holes through the center of the glass beads have likewise become smaller and smaller, and/or that the distance between the center wire or conductor and the wall of the metal case or package has become smaller and smaller. A small distance between the conductor and the metal wall presents a problem in that an electrical short can easily occur therebetween. To prevent the possibility of such a short, which can occur, e.g, if water or other conductive fluid establishes a bridge between the wire and wall on the outside of the package, it is common to insulate the wire on the outside of the can or package with epoxy or other kinds of plastics or waxes. However, as the overall size of the components decreases, it becomes increasingly difficult to make an effective insulating seal in this manner. Further, although using ceramic packages or cases in place of metal packages or cases eliminates this problem (because the ceramic cases are non-conductive), ceramic cases are by their very nature brittle, and must thus be made thicker than metal walls. Hence, use of ceramic packages reduces the ability to make the case very thin. It is thus evident that what is needed is a way to provide a thin hermetically sealed metal package or case having an electrical feedthrough that eliminates or reduces the possibility of shorting between the feedthrough conductor and the metal wall of the package or case.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing a thin film hermetically sealed feedthrough. Such feedthrough may be used in a wide variety of applications, but typically is used with a very thin hermetically sealed case or housing suitable for implantation within living tissue, thereby permitting electrical connection between electronic circuits or components sealed within the case and electrical terminals on the outside of the case.

The hermetically sealed feedthroughs and case of the present invention comprise at least one insulating layer that encapsulates and hermetically seals a metal trace. For example, a conductive trace is deposited on an upper surface of a first insulating substrate or layer. The conductive trace is then covered by depositing another insulating layer over the conductive trace so that the conductive trace is effectively sandwiched between two insulating layers. Advantageously, as the insulation layer is deposited over the metal trace, the metal trace becomes hermetically sealed within the insulating layers. As the deposition of the insulating layer is carried out over the metal trace, at least two openings (or channels) are transversely formed therethrough so as to expose different ends or portions of the conductive trace. Additional conductive material is then placed within each of the openings or channels to form conductive paths or links (hereafter "vias") that make electrical contact with the ends or portions of the conductive trace at the respective locations of the vias. A cover is then hermetically sealed or bonded to one of the insulating layers so as to form an hermetically sealed cavity, with at least one of the vias residing inside of the hermetically sealed cavity, and with at least another one of the vias residing outside of the hermetically sealed cavity. Because the via in the hermetically sealed cavity is in electrical contact by way of the conductive trace with the via on the outside of the hermetically sealed cavity, an electrical feedthrough is thus advantageously provided that allows electrical contact to be made between the via on the inside of the hermetically sealed cavity and the via on the outside of the hermetically sealed cavity. Hence, electronic circuitry or components may be mounted within the hermetically sealed cavity, and electrical contact can be established with such circuitry or components from a location outside of the hermetically sealed cavity, as required, for a given application.

In accordance with one aspect of the invention, the insulating layers that hermetically sandwich the metal trace may be deposited on a thin metal substrate, thereby allowing an extremely thin feedthrough to be made.

In accordance with another aspect of the invention, the insulating layer(s) and/or substrate are made from aluminum oxide Al.sub.2 O.sub.3, or other suitable insulating material, such as magnesium oxide, zirconium oxide, or many types of glass, and may be deposited using conventional deposition techniques.

It is thus a feature of the present invention to provide an hermetically sealed thin film feedthrough.

It is another feature of the invention to provide such a hermetically sealed thin film feedthrough in combination with a thin hermetically sealed cavity wherein electronic components and/or circuitry may be housed, thereby allowing electrical connections to be established with the circuitry and/or components within the sealed cavity from a location outside of the cavity.

It is another feature of the invention to provide an extremely thin hermetically sealed housing, including hermetically sealed feedthroughs for allowing electrical contact to be established between the inside and outside of such housing, suited for protecting electronic circuitry and/or components from a hostile environment.

It is yet an additional feature of the invention to provide an extremely thin hermetically sealed housing, including hermetically sealed feedthroughs that permit electrical connections between the inside and outside of the sealing housing, especially suited for implantation in living body tissue, e.g., especially suited for implantation in animals or humans.

It is still another feature of the invention to facilitate the manufacture and use of tiny, thin hermetically sealed electrical circuits or components.