Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
1997-01-22
2001-04-24
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C427S372200, C427S379000, C427S126200, C427S383500
Reexamination Certificate
active
06221427
ABSTRACT:
TECHNICAL FIELD
The present invention relates to structures, and methods of fabrication thereof, having an interface region between metal and ceramic regions wherein the interface region has a temperature coefficient of expansion, intermediate of the metal and ceramic regions, which substantially prevents separating of the metal and ceramic regions during temperature cycling of the structure. More particularly, the interface region is a mixture of particles of the metal and particles of the ceramic. Most particularly, the present invention relates to a multilayer metal/ceramic electronic packaging substrate, and methods of formation thereof, containing metal patterns having an interface layer between the metallization pattern and the ceramic wherein the interface layer is a mixture or metal particles and ceramic particles which forms a hermetic seal between the metal patterns and the ceramic capable of withstanding thermal cycling during fabrication and use. Additionally, more particularly, the present invention relates to a method of fabricating the interface region wherein a combination of a metal paste in a green ceramic precursor is alternatively sintered in oxidative and reductive atmospheres.
BACKGROUND OF THE INVENTION
Multilayered ceramic circuit substrates contain patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as electrical insulators. These ceramic substrates are formed by laminating together thin green sheets of glass particles or an admixture of glass and crystalline particles mixed with binders and patterns of paste containing metal particles mixed with binders for forming conductors between, the ceramic greensheet. This green laminate is fired to burn off the binder materials, fired to coalesce the particles to a dense state further fired if necessary to crystallize the densified glass particles or to further crystallize the admixture of glass and crystalline particles to form an insulator and to coalesce the metal particles to form conducting metal lines.
Terms such as ceramic and glass ceramic are often used interchangeably in the art. To avoid confusion for the purpose of this application the following definitions will be used. The term ceramic has the following meaning: an aggregate of randomly oriented crystallines wherein the interstices between crystallines may contain uncrystallized material such as glass. The terms coalescence or densification refer to a heat treatment to reduce the density of pores in greensheets. The term crystallization refers to further heating after coalescence or densification or to heating if there is no coalescence or densification step to form crystallites from a glass. The term sintering refers to the heat treatment required to form the final ceramic. Sintering of a greensheet of an admixture of glass particles and crystalline particles is a heat treatment to coalesce or densify the greensheet plus a crystallizing heat treatment only if further crystallization is required. The term sintering temperature means, for a green sheet requiring crystallization, the crystallization temperature. The term sintering temperature means, for a greensheet not requiring crystallization, the coalescence temperature. Substrates made of ceramics requiring high temperatures for particle coalescence and densification, such as alumina, restrict the choice of co-sinterable conducting metallurgics to high melting point metals, for example refractory metals, such as molybdenum, tungsten, platinum, palladium or a combination of these with each other or certain other metals and precludes the use or preferable electrical conductors such as gold, silver and copper which have melting points less than the alumina sintering temperature. Alumina is a good insulator, has high thermal conductivity and has good strength. The dielectric constant of alumina is about 10.
Materials often referred to as glass-ceramics have been intensively studied in recent years for use as circuit substrates. These ceramics generally have low dielectric constant, a low thermal coefficient of expansion which is close in value to silicon and a low sintering temperature. The low sintering temperature permits the use of low melting point metals, such as copper and noble metals, for electrical conductors. Noble metals have low resistives comparable to copper. However, copper is less expensive and, therefore, its use substantially reduces manufacturing cost. When an oxidizable metal, such as copper, silver, molybdenum is used as the electrical conductor, it is necessary that thermoplastic organic binder materials contained within the green sheet used to form the ceramic and contained within the paste used to form the copper conductors be depolymerized and burned out in an atmosphere and at a temperature wherein the copper is not substantially oxidized.
Failure to burn out binders results in less than desirable ceramic properties. For example, if the binder is not fully burned out, residual carbon is left in the sintered ceramic which modifies the ceramic dielectric constant and inhibits complete densification. With only 0.1% residual carbon content the ceramic may be black having an apparent dielectric constant greater than 1000 and, rather than being an insulator, the fired ceramic will be a semiconductor. An oxidizing atmosphere is generally needed to burn out the binder.
Removal of the binder is complicated by the additional requirement that the burn-out ambient not excessively oxidize the oxidizable metal lines and planes. If the metal is excessively oxidized, the metal oxide diffuses into the ceramic and changes the dielectric properties of the ceramic. Also, when metal is excessively oxidized it expands causing stress within the green laminate which can result in delamination and cracking of the green laminate. Such cracks may not be removed by the sintering heat treatment thereby resulting in a ceramic weakened by cracks.
Quite surprisingly, applicants have discovered that by controlling the oxidation and reduction (dioxidation) of the metal lines this cracking can be avoided while providing substantial improvement in the interface region between the metal and ceramic as described herebelow.
Two references generally describing binder bum-out and the fabrication of ceramics are U.S. Pat. No. 4,234,367 to Herron et al. and U.S. Pat. No. 4,504,339 to Kamehara et al., the teachings of both of which are incorporated herein by reference.
Herron et al. U.S. Pat. No. 4,234,367 describes a method for forming sintered ceramic substrates containing multilevel, interconnected circuit patterns of copper-based conductor films obtained by heating the green laminate composed of particles of crystallizable glass in an ambient of hydrogen and water to a burn-out temperature of about 700° C. to 800° C. at a rate of 1° C. to 3° C. per minute. A lower burn-out temperature would take a prohibitively excessive amount of time for carbon removal. The binder burn-out time is about 11 hours as is apparent from
FIG. 4
of the Herron et al. patent. This time is needed: 1) to avoid bloating of the ceramic article caused by entrapped volatile products which cause the article to expand instead of contracting on sintering, 2) to fully oxidize carbon in the binders, 3) to avoid drastic volume changes in the copper conductor resulting from the copper-oxide formation, and 4) to maintain reducing to neutral conditions for copper. After the binder is burned out the laminate is sintered in a neutral atmosphere to form the ceramic material by first heating to coalesce the laminate to dense state and thereafter heating to form crystals from the densified glass particles. During binder burn-out Herron et al. encountered difficulties alternating air and forming gas for purposes of oxidizing copper and reducing any formed copper oxide to copper without drastic volume changes resulting from the copper oxide formation. Applicants have discovered that by alternating environments after burn out interfacial region of controlled thickness can be formed. U.S. Pat. No. 4,504,39
Wen Sheree Hsiao-Ru
Wood Carl Stephen
International Business Machines - Corporation
Morris Daniel P.
Talbot Brian K.
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