Bonding a conductor to a substrate

Abstract: A hermetic seal is provided for a conductive feedthrough through a thin ceramic component by a platinum or palladium lead by sealing the gap between the lead and the ceramic with a copper-copper oxide eutectic. The lead may have a copper coating on it prior to and subsequent to formation of the copper-copper oxide eutectic.

Description: The present invention relates to bonding a conductor to a substrate and, for example, to electronic packages. The invention will be discussed herein by way of example to electronic packages but is not limited to such packages. For many applications, it is either desired or necessary to provide a hermetically sealed package for electronic devices. In the case of vacuum tubes, a hermetic seal is essential to the proper operation of the device. With semiconductor devices, a hermetically sealed package is considered necessary or desirable for semiconductor devices for which high reliability is required or which will be exposed to hostile environments. One of the problem areas in providing a hermetically sealed package is provision of hermetic seals between leads which extend from within the hermetically sealed portion of the package to outside the package enclosure. Over the years, many different lead and seal combinations have been developed in an attempt to provide hermetically sealed packages. These efforts have included the development of special metal alloys whose thermal coefficients of expansion are similar to that of the electrically insulating material through which the lead must pass along with the development of special sealing materials having substantially matched coefficients of thermal expansion such as special composition glasses for sealing the gap between the lead and the insulating body of the package. A direct bond copper process which may be used to form a copper foil-to-ceramic seal or bond is described in U.S. Patents 3,744,120, 3,854,892 and 3,911,553 to Burgess et al.; U.S. Patents 3,766,634 and 3,993,411 to Babcock et al.; U.S. Patents 3,994,430 and 4,129,243 to Cusano et al.; U.S. Patent 4,409,278 to Jochym; and U.S. Patent 4,563,383 to Kuneman et al. Each of these patents is incorporated herein by reference. These direct bond patents teach that copper-copper oxide, nickel-nickel oxide, cobalt-cobalt oxide, iron-iron oxide and copper-copper sulfide eutectics can be used to form bonds of this type. This bond is formed by a copper-copper oxide eutectic mixture which wets both metallic copper and ceramic materials such as alumina and beryllia and which bonds the members together upon solidification. This copper-copper oxide eutectic process has been in use for many years in the electronic arts. According to one aspect of the invention there is provided in combination an electrically insulating substrate which is wettable by a copper-copper oxide eutectic, said substrate having an aperture therein; an electrically conductive member comprising platinum, palladium or mixtures thereof disposed in said aperture, said substrate and said conducting member having closely matched temperature coefficients of expansion; and a copper-copper oxide eutectic layer bonding said conductive member to the substrate walls of said aperture. According to the same aspect of the invention there is also provided an hermetically sealed electronic package comprising a ceramic member having an aperture therein; a lead comprising platinum, palladium or mixtures thereof disposed in said aperture, said ceramic member and said lead having closely matched temperature coefficients of expansion; and a copper-copper oxide eutectic layer sealing said aperture between said lead and said ceramic member. According to another aspect of the invention there is provided a method of hermetically sealing an aperture in a ceramic member comprising disposing a conductive body comprising platinum or palladium or mixtures thereof in said aperture, said ceramic member and said conductive body having closely matched temperature coefficients of expansion; providing a precursor to a copper-copper oxide eutectic adjacent to said body, said precursor comprising a copper collar disposed around said body external to said aperture; providing an oxygen source; heating said ceramic member, said conductive body and said precursor to a temperature at which a liquid copper-copper oxide eutectic forms, wets said body and fills any gaps between said conductive body and said ceramic member; and allowing said ceramic member, conductive body and eutectic to cool so that said eutectic solidifies and seals said aperture. The invention, both as to organization and method of practice, may be better understood by reference to the following description taken in connection with the accompanying drawings in which: Figure 1 shows an illustrative ceramic package component and lead embodying the present invention prior to sealing the lead to the ceramic; Figure 2 shows an illustrative ceramic package component and lead embodying the present invention after hermetic sealing of the lead to the ceramic; and Figure 3 illustrates an uncoated platinum or palladium lead disposed in an aperture in a ceramic with a copper collar in place for providing the eutectic to seal the lead to the ceramic. In Figure 1, a lead 20 ready for sealing in a ceramic package component 12 is illustrated in a perspective cross-section view. A ceramic substrate 12 which may be alumina, bervllia or other appropriate materials may preferably be of the order of 635 microns (25 mils) thick and may have an aperture 14 which is 610 microns (24 mils) or 1.12 mm (44mils) in diameter therein, extending through the ceramic component 12 between its major surfaces. The aperture diameter may be made larger or smaller as may be considered desirable. The lead 20 has a platinum core 22 which may be either 508 microns (20 mils) or 1.02 mm (40 mils) in diameter having a thin copper plating 24 which may be approximately 25.4 microns (1 mil) in thickness sheathing the platinum core. It is preferred to provide a copper collar 26 which is a press fit on the lead 20 and has a cylindrical shell thickness of, for example, 254 microns (10 mils) and a height of 635 microns (25 mils) and is disposed adjacent the ceramic component 12. The copper collar 26 is preferred in order to ensure the formation of sufficient eutectic material to seal the gap between the lead 20 and the ceramic 12 surrounding the aperture 14. With the components disposed in the manner illustrated, the combined structure is passed through a copper direct bond thermal profile in accordance with the process taught in the above-identified direct bond copper process patents. In particular, the copper collar and plated platinum rod are heated in air at about 340°C for several minutes to oxidize the copper to provide the oxygen necessary for the formation of the copper-copper oxide eutectic. The alumina plate 12 is preferably fired in air at about 940°C for 10 minutes prior to assembly with the lead 20 and collar 26 in order to ensure that any unwanted residues from drilling and cleaning processes are removed prior to performing the bonding process. Following the preparatory steps, the assembled structure is heated to between 1,065°C and 1,083°C whereby a liquid, copper-copper oxide eutectic 28 forms, flows into the gap between the lead 20 and the ceramic 12 and fills that gap. The combined structure is then cooled to less than 1,065°C to solidify the copper-copper oxide eutectic, thereby effecting a hermetic seal between the platinum rod and the ceramic. The combined structure is then cooled to ambient temperature for assembly into the final desired package. The platinum rod has a thermal coefficient of expansion of substantially 8.6 x 10⁻⁶/°C, while the alumina substrate (96% alumina) has a thermal coefficient of expansion of 6 to 8 x 10⁻⁶/°C. Thus, the platinum and alumina are closely matched in thermal expansion coefficients with the result that the lead-to-ceramic seal should have a long, useful life, even in the presence of substantial thermal cycling. This same process has been carried out using a solid copper lead in place of the copper plated platinum lead. The resulting seal is not hermetic. The copper has a thermal coefficient of expansion of 17 x 10⁻⁶/°C which differs substantially from that of alumina. Consequently, the failure of the all-copper lead to produce a hermetic seal is believed to be at least partially due to contraction of the lead during cooling with a resulting fracturing of any hermetic seal initially formed. Palladium, whose thermal coefficient of expansion is 11 x 10⁻⁶/°C, may also be used as the core of the lead. A lead 20' is illustrated ready for bonding in a ceramic substrate 12 in Figure 3 in accordance with an alternative embodiment of the invention. In this embodiment, the lead 20 is solid platinum or palladium and is free of copper coatings of the type present in the Figure 1 structure. The copper collar 26 is disposed in direct contact with the platinum or palladium lead 20, but no copper is present on the surface of the portion of the lead which extends into the aperture 14 in the ceramic 12. With appropriate pre-oxidation of the copper collar or the presence of an appropriate oxidizing atmosphere during passage of this structure through a direct bond temperature profile, the collar 26 produces a sufficient quantity of the copper/copper oxide liquid eutectic for that eutectic to flow into and fill the gap between the lead 22 and the ceramic substrate 12. Upon solidification, this copper/copper eutectic forms a hermetic seal between the lead 22 and the ceramic substrate 12. Once the lead is sealed to the ceramic with this eutectic, the lead and substrate appear very similar, if not identical, to the final structure when made from a copper coated lead. This is because the copper/copper oxide eutectic has a high affinity for platinum and palladium and, therefore, normally coats the surface of the platinum or palladium lead. While the specific examples have been discussed in terms of either a platinum or a palladium lead, it will be understood that leads which are a mixture of platinum and palladium may be used as may leads which have a sufficient platinum or palladium content to provide good adherence by the copper/copper oxide eutectic in combination with a suitably low coefficient of thermal expansion. Thus, other lead materials comprising more than just platinum or palladium or mixtures thereof may be used in accordance with this invention. This hermetic seal is unique because of the small thickness (630 microns) of the alumina substrate. It is further highly advantageous since for a 20 mil (50 micron) diameter lead, leads may be placed 30 mils (762 microns) on center, thereby providing the ability to provide extremely closely spaced hermetic feedthroughs in a package lid or other component.

Claims: In combination: an electrically insulating substrate which is wettable by a copper-copper oxide eutectic, said substrate having an aperture therein; an electrically conductive member comprising platinum, palladium or mixtures thereof disposed in said aperture, said substrate and said conducting member having closely matched temperature coefficients of expansion; and a copper-copper oxide eutectic layer bonding said conducting member to the substrate walls of said aperture. A hermetically sealed electronic package comprising the combination of claim 1, wherein the said substrate is a ceramic member having the said aperture therein; the said electrically conductive member is a lead, and the said copper-copper oxide eutectic layer seals said aperture between said lead and said ceramic member. The combination of claim 1 or the package of claim 2, wherein said substrate or respectively said ceramic member is alumina. The combination of claim 1 or claim 3 or the package of claim 2 or claim 3 wherein said conductive member or respectively said lead is platinum. The combination of claim 1 or claim 3 or the package of claim 2 or claim 3 wherein said conductive member or respectively said lead has a platinum core. The combination of claim 1 or claim 3 or the package of claim 2 or claim 3 wherein said conductive member or respectively said lead is palladium. The combination of claim 1 or claim 3 or the package of claim 2 or claim 3 wherein said conductive member or respectively said lead has a palladium core. A method of hermetically sealing an aperture in a ceramic member comprising: disposing a conductive body comprising platinum or palladium or mixtures thereof in said aperture, said ceramic member and said conductive body having closely matched temperature coefficients of expansion; providing a precursor to a copper-copper oxide eutectic adjacent to said body, said precursor comprising a copper collar disposed around said body external to said aperture; providing an oxygen source; heating said ceramic member, said conductive body and said precursor to a temperature at which a liquid copper-copper oxide eutectic forms, wets said body and fills any gaps between said conducting body and said ceramic member; and allowing said ceramic member, conductive body and eutectic to cool so that said eutectic solidifies and seals said aperture. The method recited in claim 8 wherein: said ceramic member is alumina. The method recited in claim 8 or 9 wherein: said conductive body comprises platinum or palladium or mixtures thereof having a copper coating thereon. The method recited in claim 8 or 9 wherein: said conductive body has a platinum or palladium or mixtures thereof exposed surface. The method recited in claim 11 wherein: said external surface of said collar has a coating of copper oxide thereon to provide said oxygen source.

Inventors: WEBSTER HAROLD FRANK, WEBSTER, HAROLD FRANK
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1990
Main classification: H01L23
IPC codes: H05K3, H01L21, C04B37, H01L23, H05K1