GB2182923A - Electrically conductive ceramic material - Google Patents

Abstract

A metal-loaded ceramic material is formed by casting a slurry containing a powdered ceramic such as powdered alumina, a powdered metal such as powdered tungsten, and one or more organic binders and solvents of the type used conventionally in the production of ceramic tapes. The slurry is cast in the form of a tape. When dry, the tape can be laminated and fired to form an electrically conductive monolithic structure can be laminated and co-fired with tape made of a pure ceramic material to form a monolithic structure containing any desired configuration of electrically conductive and non-conductive layers. One such product is a monolithic semi-conductor package having a body defining an inner chamber (16) for receiving a semi-conductor device. The chamber is closed by a lid. Conductive paths enabling electrical connection to the semi-conductor device in said chamber can be made from locations externally of the chamber by way of portions (20) formed of the metal-loaded ceramic material. <IMAGE>

Description

SPECIFICATION Electrically conductive ceramic material.

The present invention relates to ceramic materials and is concerned in particular with a means of achieving electrically conductive ceramic materials for use in monolithic ceramic structures.

Extensive use has been made of alumina within the electronics industry, a combination of high strength, good thermal conductivity and excellent electrical insulating properties making ita popular choice for multi-layer substrates and semi-conductor device packages.

In such devices, internal electrical connections are usually made by screen printing refractory metal paste, e.g. of tungsten, onto a green (i.e. unfired) ceramic surface, usually in the form of a ceramic tape. Electrical connections between the external and internal parts ofthe device are made by providing such screen-printed tapes within through-holes known as vias. Unfired printed conductortapesare laminated together and fired to obtain hermetic monolithic structures. Metal parts, such as lead frames, input/output pins, seal rings, heat sinks and the like, are brazed to exposed metallisation using a silver-copper eutectic mixture.

All exposed metal parts are subsequently plated, usuallywith gold.

As the requirement for advanced electronic packaging increases, so does the requirementfor improved circuit performance at low cost. Moreover, the higher packaging densities now demanded also necessitate the development of improved methods of heat dissipation.

It is thus a general object of the present invention to provide improved materials and a method of making samewhich meettheaforegoing requirements.

It is a more specific object of the invention to provide a means of incorporating electrically and thermally conductive planes internally and/or externally about a monolithic ceramic structure without recourse to screen printing or brazing procedures.

In accordancewiththe present invention in its broadest aspect, there is provided a metal-loaded ceramicformed by casting aslurrycontaining a powdered ceramic (such as powdered alumina), a powdered metal (such as powdered tungsten), and one or more organic binders and solvents. The organic binders and solvents can bethesameas used conventionally in the production ofthe known ceramic tapes described initially. Atypical solvent is 1,1,1 -trichloroethane. Atypical binder would be acrylic based polymericbindersuch as that sold by Dupontunderthetrade name.

Preferablyr the slu rry is cast in the form ofan elongate tape. When dry, the tape may be laminated and fired to yield an electrically conductive monolithic structure. It is found that the structure so-formed is not only electrically conductive but has superiorthermal conductivity properties to those of the pure ceramic.

Alternatively, the tape may be laminated and co-fired with tape made of a pure ceramic material, to yield a monolithic structure containing any desired configuration of electrically conductive and non-conductive layers.

Typical conductivityoftungsten-loaded alumina is 1 ohm cm and conductivity may be further improved bysubsequentelectroplating.

The principal advantage provided by this invention is the ability to co-fire a conductorwith an insulatorwithoutthe usual brazes, solders, metallisations, glazes and the liketo provide monolithic conducting/insulator structures.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings, in which:- Figures la, ib and it show diagrammatically three possible semi-conductor device package lids having flat, stepped and recessed configurations, respectively; and Figure 2 is a perspective view of one embodiment of a semi-conductor device package, without a lid.

The drawings show diagrammatically some examples ofthel:ype of usetowhichthe new material may be put.

It is the usual practice for semi-conductor device packages to be closed hermetically by means of a lid having a conductive surface layer applied theretofor the purposes of electrically screening the semi-conductor device housed within the package.

The necessity for such conductive layers can be obviated by the use of the new conductive ceramic/metal material. Thus, lids 8a, 8b, Sc such as those shown in Figures la, Ib and Ic can be manufactured at low cost simply by laminating layers of the ceramic/metal tape into the required shape and firing. In order to enable such lids to be fixec to the ceramic package body, they can be plated directly and brazed to metallisation on the body.

The presence ofthe same ceramic in both lid and body ensures minimum thermal mismatch between the two components.

Items of a more intricate or complex configuration may be manufactured by the application ofwire or spark eroding techniques to blocks ofthefired ceramic/metal composite material.

Figure 2 shows a semi-conductor device package of typical overall shape formed byaflat, rectangular block 10 carrying monolithically on its uppersurface 12 an upstanding rectangularwall 14which defines an inner area 1 6for receiving the semi-conductor device itself (not shown). A lid, of shape such ar; shown in any of Figures 1 a, 1 b or 1 c isfitted hermetically on thetop surface ofthewall 14to enclose the semi-conductorwithin the package.

When such a device package is made by conventional techniques, connections are madeto the semi-conductor device by way of vias formed in the block 10 and containing screen printed refractory metal paste, as described initially. In the present case, the block 10 and wall 14 can be fabricated by laminating and co-firing layers ofthe electrically conductive metal/ceramic tape and conventional ceramictape into the required configuration. A preferred ceramicforthe metal/ceramictape and to e pure ceramic tape is alumina.

In Figure 2, the conductive areas are cross-hatched and the non-conductive areas are plain. The vias of the known structure are replaced by conductive tape layers 20 which are laid down during the lamination process. A central conductive area 16 on the upper surface of the block 10 disposed within the region bounded bythewall 14 is adapted to receivethe semi-conductor device itself(not shown). Since the conductive area 1 forms the upper surface of a continuous block of electrically conductive alumina formed by tape layers 20, the block itselffulfilsthe necessary electrical screening and earth connection roles.

Thus, by co-firing the layers ofthe new conducting material with windowed and stepped layers of the conventional alumina, a hermetic package is obtained wherein the use of vias between the device attachment area and the earth plane has become redundant. It is found also that heat dissipation is superiortothatobtained with conventional alumina blocks containing vias.

As indicated at 22 in Figure 2,the new ceramic/metal tape may also be used in the lid attachment area, so reducing the amount of post-firing metal lisation required.

The invention can be achieved using a wide variety of materials and is not limited to the use of any particular metals or ceramics. The relative quantities ofthe materials is capable of wide variation, dependent upon the conductive (electrical and thermal) qualities required and is chosen to suitthe characteristics desired in the end product.

Claims (11)

1. A metal-loaded ceramic material formed by casting a slurry containing a powdered ceramic, a powdered metal, and one or more organic binders and solvents.

2. A metal-loaded ceramic material as claimed in claim 1, wherein the ceramic is powdered alumina.

3. A metal-loaded ceramic material as claimed in claim 1 or 2, wherein the metal is powdered tungsten.

4. A metal-loaded ceramic material as claimed in claim 1,2 or 3, wherein the solvent is 1,1, 1 -trichloroethane.

5. A metal-loaded ceramic material as claimed in any of claims 1 to 4, wherein the binder is an acrylic based polymeric binder.

6. A metal-loaded ceramic material as claimed in any of claims 1 to 5,wherein the slurry is cast in the form of an elongate tape.

7. An electrically conductive monolithic structure formed by casting, in the form of an elongate tape, a metal-loaded ceramic material as claimed in any of claims 1 to 5, drying the tape and laminating and co-firing layers of said tape to form an electrically conductive monolithicstructure.

8. A monolithic structure formed by casting, in theform of an elongatetape,a metal-loaded ceramic material as claimed in any of claims 1 to 5, drying the tape and laminating and co-firing layers of said tape with tape made of pure ceramic material to form a monolithic structure containing a predetermined configuration of electrically conductive and non-conductive layers.

9. A monolithic structure as claimed in claim 8, in theformofa packagewhich includesaclosed chamberforhermeticallycontaining a semi-conductor device, electrically conductive paths for enabling electrical connection to the semi-conductor device enclosed in said chamberto be made at locations externallyofthe package being achieved by portions of the monolithic structure formed of said electrically conductive metal-loaded ceramic material.

10. A monolithic structure as claimed in claim 9, wherein a central area ofthe innerfloor ofthe chambercontainingsaidsemi-conductordeviceis electrically conductive and is defined by the upper surface of a continuous block of electrically conductive material formed by laminated layers of said metal loaded ceramic material,the latter block thereby providing the facility for electrical screening and earth connection of the semi-conductor device.

11. A monolithic structure su bstantia I Iy as herein before described with reference to and as illustrated in the accompanying drawings.