Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2000-07-19
2002-09-03
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S210000, C174S259000
Reexamination Certificate
active
06444297
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a metallization process for aluminum nitride ceramic substrates having a low thermal expansion and a high degree of thermal conductivity. More particularly, this invention refers to a method of producing a thick film oxide based metallization layer structure on an aluminum nitride ceramic by the addition of an intermediate buffer layer over the aluminum nitride substrate.
Traditionally, beryllium oxide ceramics (BEO), or other materials containing an oxide base, have been used as the substrate material to effectively remove heat and function as a major structural element for the production of active and passive microwave devices. Usually, there is a need to firmly fasten pins or terminal assemblies of various natures, for purposes of electrical connection, to these oxide based ceramics. Conventionally, a metallization is established so that a metallization layer can be created on the ceramic of the oxide based system. This metallization functions as an attached adhesive layer so that the aforementioned pins can be firmly attached to the oxide based ceramics. Readily available thick film oxide based materials are used for this metallization attachment. Pins or terminal assemblies can then be mounted on the metallization layer created on the oxide based ceramics. These assemblies are then soldered or brazed in place. Examples of the conventional systems have been disclosed in the Japanese Laid-Open patent application No.'s 79910/1978, 207691/1984, 57997/1985, U.S. Pat. No. 4,539,223, relating to thick film resistor compositions and U.S. Pat. 5,491,118, relating to thick film paste compositions.
More recently, however, there has been growing concern in the European communities about the toxicity and disposal of devices containing beryllium oxide. The beryllium oxide ceramic remains the best thermal heat conductor of all the various oxide based ceramics and has a very favorable dielectric constant for microwave applications. This material has been in high demand, in the past, for these reasons. In an effort to slowly replace the beryllium oxide ceramics and phase in a more environmentally favorable ceramic, aluminum nitride ceramics have been considered. The properties of aluminum nitride with respect to thermal expansion, thermal conductivity and dielectric constant are not quite as favorable as beryllium oxide. However, they are still very acceptable for the fabrication of microwave devices. As an example, aluminum nitride has three to eight times the thermal conductivity of previously used alumina ceramics. Unfortunately, standard thick film oxide metallizations do not react favorable with the active surface of the aluminum nitride ceramics. This is specific to aluminum nitride since it does not contain any appreciable lattice oxygen network necessary to form an oxide eutechnic bond with the metallic thick film oxide based material. Consequently, a conventional metallic thick film oxide layer directly on an aluminum nitride ceramic substrate can be broken loose from the aluminum nitride ceramic substrate. This leads to questionable reliability (bond integrity) at room temperature. If the substrate is stressed via thermal shock or thermal cycling and the coefficient of linear expansion is changing, there is little chance that the bond will remain intact. A method is needed to facilitate bonding of thick film metallization on aluminum nitride ceramic substrates.
SUMMARY OF THE INVENTION
The present invention provides means for applying useful metallization layer structures on aluminum nitride ceramics such that the previously described problems with metallization layer adherence integrity is overcome. The method of this invention produces a metallization layer structure having acceptable adhesive bond strength at room temperature and during accelerated environmental stress temperatures.
A specific object of the present invention is to provide a method of producing metallization layer structures on aluminum nitride ceramics that are capable of providing an improved device reliability with regard to fastening pins or terminal assemblies onto the surface.
The method of the present invention is achieved by applying an intermediate buffer layer on the aluminum nitride ceramic. This intermediate buffer layer contains silicon monoxide or silicon dioxide. A platinum, palladium/silver, or other precious metal thick film oxide based metallization layer is then applied over the aforementioned buffer layer. The metallization layer forms a heat resistant firmly adhered metallic layer capable of being soldered or brazed. Other resistive oxide based thick films also can be adhered on the buffer layer to provide additional reliable passive devices.
The aluminum nitride ceramic substrate employed is laser “diced”, thus creating through holes such that a snap action can be used to separate smaller chips from the larger substrate and to provide groups of larger holes for the purpose of creating a wrap around metallization layer that would connect both sides of the aluminum nitride ceramic electrically.
REFERENCES:
patent: 4539223 (1985-09-01), Hormadaly
patent: 4659611 (1987-04-01), Iwase et al.
patent: 4906514 (1990-03-01), Francis et al.
patent: 4939022 (1990-07-01), Palanisamy
patent: 5393558 (1995-02-01), Allison et al.
patent: 5491118 (1996-02-01), Hormadaly
Electro Technik Industries, Inc.
Lam Cathy
Larson James E.
Larson & Larson, PA
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