Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-04-17
2003-12-09
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S851000, C029S846000, C029S825000, C427S097100, C427S099300
Reexamination Certificate
active
06658733
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a ceramic multilayer wiring board, in particular a ceramic wiring board using copper as a via interconnection, to a ceramic multilayer wiring board having a suitable post-sintering via microstructure, and to a copper paste for obtaining this microstructure.
Ceramic wiring boards having a multilayer structure are used in electrical devices where modular wiring substrates are required for high integration and high-speed processing, due to the need for making fine interconnections. Copper is the material of choice for these interconnections due to its low specific resistance.
As substrate used as a support for interconnections, an inorganic material having glass as its principal component is used as the glass can be sintered at the same time as the copper of the interconnections. A borosilicate glass suitable for substrates is described in detail in Japanese Patent Laid-Open No. Hei 8-333157. Fillers which may be added are disclosed in Japanese Patent Laid-Open No. Hei 8-18232.
Here, the method of manufacturing the substrate will be briefly described.
Generally, the inorganic material is supplied in the form substrate is manufactured by the well-known green sheet method. This method consists of the following steps.
(1) Making a slurry of the powdered inorganic material using an organic binder and a solvent.
(2) Forming this slurry into the shape of a sheet.
(3) Opening vias (through holes) in the sheet.
(4) Embedding an interconnection paste in the vias.
(5) Forming an interconnection or other pattern on the sheet surface with the interconnection paste.
(6) Laminating these sheets with interconnection patterns together under pressure.
(7) Heat treating the resulting laminate.
In the above-mentioned heat treatment process, the organic binder in the laminate and the organic substance in the interconnection paste decompose and are thus eliminated. At the same time, the inorganic material in the laminate which is in a powdered state of aggregation and the conducting metal in the conducting paste are sintered and become finer.
However, if the organic binder remains in the sintered compact, it will be converted to graphite, and the quality of the substrate and wiring after sintering will deteriorate. For this reason, sufficient binder removal time is generally allowed in the sintering step, followed by a sintering period which has the main purpose of increasing the fineness.
This classical type of heat treatment profile is disclosed in Japanese Patent Laid-Open No. Hei 8-18232, etc. The binder is removed in an atmosphere at about 800° C. for 15 hours, and the product is kept in an atmosphere at about 1000° C. for 2 hours for sintering. Water vapor etc. is usually added to the processing atmosphere during the above-mentioned binder removal.
However, when copper is used for the metallic material of the conductor, although sintering of the copper takes place starting from approximately 600° C., sintering of the glass ceramics itself begins at a higher temperature. This difference of sintering start times may causes serious problems in the substrate, particularly in the conductor or at the interface between the conductor and the ceramics, so in the case of copper paste, an attempt is often made to adjust the sintering start temperature of the ceramics.
As an example, a copper paste mixed with alumina of particle size 0.1 &mgr;m to 1 &mgr;m is disclosed by Japanese Patent Publication 2584911. Also, a copper paste comprising copper oxide and glass frit is disclosed by Japanese Patent Laid-Open No. Hei 8-279666.
SUMMARY OF THE INVENTION
In producing a multilayer wiring board using the above-mentioned green sheet method, in the case of an alumina and copper mixture, it is difficult to disperse fine alumina of particle size less than 1 &mgr;m in the copper paste. For this reason, it is difficult to obtain desired paste printing properties required for processes such as embedding interconnection paste in vias, or forming interconnections or other patterns.
Moreover, copper oxide tends to discharge copper ions in the glass, and may produce a fine copper colloid in the ceramics depending on the firing conditions. This causes deterioration of the insulating properties of the ceramics, and decreased hardness.
On the other hand, as the microinterconnections are formed and via diameters reach about 50 &mgr;m, a new problem may arise in addition to the above-mentioned difference of sintering start temperature. Specifically, if copper particles grow very large during their growth when the substrate is fired, they will reach a size of the same order as that of the via diameter. As a result, after sintering, vias will be formed of several enlarged copper particles, particle interfaces will break down due to the effect of subsequent heat cycles, and breaks will tend to occur in the via interconnections. Moreover, there is also the disadvantage that via interconnections may fall out of the via holes of the ceramic substrate.
As an example of one way of dealing with this copper particle diameter problem after sintering, a copper paste mixed with aluminum acid which generates alumina of sub-micron size in the sinter is disclosed in Japanese Patent Laid-Open No. Hei 8-17217. However, as water vapor is generated simultaneously during the alumina forming reaction, more voids than needed were produced in the copper interconnections.
This invention aims to overcome the disadvantages of the prior art by suppressing the size of copper particles in the via to 20 &mgr;m or less, thereby reducing breaks in interconnections due to fractures at interfaces of copper particles which grow during sintering, and reducing the risk of fractured vias separating from the ceramic substrate.
To achieve this objective, alumina having an average particle size of 1 &mgr;m to 4 &mgr;m was distributed in sintered copper at an interval of 7.4 &mgr;m or less in terms of the average distance between particle centers.
The reason why the copper particles grow large during sintering is that the copper particle boundaries migrate through the copper, fusing with the surface of the sinter body or with other copper particle boundaries, and this leads to a decrease of copper particle interfaces in the sintered copper.
By distributing alumina of average particle size 1 &mgr;m to 4 &mgr;m in the sintered copper at the aforesaid interval, the copper boundaries can no longer migrate, the copper interfaces do not decrease even at the high temperature of the sintering step, and the copper particles remain in a fine state of division. As a result enlargement of copper particles is prevented, and fractures of via interconnections do not occur.
The inventors experimentally verified that migration of particle boundaries in sintered copper was inhibited by alumina particles having the aforesaid size in restricted shapes such as vias. The details of these experiments will now be described.
The test substrate was an ordinary glass ceramic substrate having vias of diameter 60 &mgr;m and comprising 10-40 layers, these layers being laminated so that the vias were vertically connected with each other right through the substrate from one surface to the other. It should be noted that the number of layers in the substrate is not limited to the above, and it may comprise only one layer.
Next, after sintering this substrate under sintering conditions known in the art, it was cut so that the center line of the via appeared on the surface. The cut surface was polished by the ordinary method, and then etched so that the copper particle boundaries could be clearly seen.
Next, for 500 or more vias observed in this cut surface, the shapes of the copper particle boundaries therein were read by a computer, and these shapes were accurately traced so as to calculate the surface area of the copper particles.
The reason why, in evaluating the state of the via interconnections formed inside the vias, the surface area of the copper particles was used as a parameter instead of the diameter which has usually been used in the past,
Ami Norihiro
Horikoshi Mutsumi
Ishihara Shosaku
Okamoto Masahide
Tanaka Minoru
Antonelli Terry Stout & Kraus LLP
Arbes Carl J.
Hitachi, ltd.
Nguyen David
LandOfFree
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