Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1998-08-13
2001-01-23
Turner, Archene (Department: 1775)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S408000, C428S500000, C428S524000, C428S704000
Reexamination Certificate
active
06177184
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of simultaneously making multiple microelectronic ceramic substrates by stacking the substrates prior to firing with an intervening thermally degradable interface layer, and to a method of making the interface layer. The invention also relates to the microelectronic ceramic substrates and to the interface layers made by the methods of the invention.
2. Description of Related Art
In the ceramic electronic industry, multilayer ceramic (MLC) technology is typically used to create three-dimensional circuitry in ceramic substrates for microelectronic devices such as integrated circuits and ceramic capacitors. The three-dimensional circuitry in the ceramic substrate is made by applying a conductive material in a circuit pattern on a ceramic/polymer composite sheet. The ceramic/polymer composite sheet is known as a “green sheet” and may have a number of via holes punched in it to allow vertical connection between the conductive material on adjacent sheets. The green sheets are stacked in a designated order and laminated together under appropriate heat and pressure to form a laminate which can be handled as a unified structure.
To produce the final ceramic material, the laminated ceramic/polymer composite is fired. The firing includes heating to remove the polymer, followed by heating to a higher temperature to sinter and densify the ceramic.
The laminate may make up a single large electronic component, but more often, it is a repetition of multiple components located adjacent to one another which need to be cut apart into individual substrates for subsequent use. This cutting may be done in the unfired stated, in the fired state or at some point therebetween.
Cutting laminates in the unfired state is the easiest, based on a criteria of laminate hardness, However, debris is generated during the cutting which will remain on the fired units. This debris is detrimental to electrical conductivity and may result in defective substrates.
Cutting laminates in the fired state can eliminate the debris problem, but the ceramic is now very hard and cutting efficiency is greatly reduced. An alternative method is to partially cut the laminate when unfired. This produces less debris. After firing, the laminate can be broken in a controlled manner along the partial cuts. However, this method also has disadvantages because of the debris generated prior to firing and the fracturing and cracking that occurs during the breaking process.
It would be desirable to provide some method of protecting the surface of the substrates against debris contamination so that cutting could be done in the unfired state. It would also be desirable to provide some means of allowing the individual substrates to be completely cut apart prior to firing, to avoid the disadvantages of fracturing when breaking them apart, while still holding the substrates together as a unitary structure prior to firing for convenient handling of the laminate.
The number of individual units produced by the methods described above is limited by the physical size of the laminate that can be produced. For a fixed maximum laminate size, more individual units can be produced if they are smaller. Given a fixed laminate size and fixed cycle times for the ceramic sintering process the output of relatively large individual units is low.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of increasing the number of units which can be produced during each firing cycle.
It is another object of the present invention to reduce the problems resulting from debris contamination.
A further object of the present invention is to eliminate or reduce the problems with fractured surfaces when partially cut laminates are broken apart after firing.
Still other objects and advantages of the invention will be in part obvious and in part apparent from the specification.
SUMMARY OF THE INVENTION
The above and other objects and advantages, which will be apparent to those skilled in the art, are achieved in the present invention which is directed to, in a first aspect, a process for making multiple microelectronic ceramic substrates in which a thermally degradable interface layer is used to separate the substrates during firing. In a second aspect, the present invention is directed to the microelectronic ceramic substrates made according to the process of the invention. In other aspects, the invention is directed to the interface layer and to the method of making the interface layer used with the process of making the substrates.
In the process for making multiple microelectronic ceramic substrates, first and second pluralities of green sheets are stacked with a thermally degradable interface layer between them. The stack is then laminated, preferably under heat and pressure, to form a laminate suitable for subsequent handling. The laminate is then fired to degrade the interface layer and sinter the first and second pluralities of green sheets, thereby forming individual ceramic substrates. With a single interface layer separating two layers of green sheets, two layers of substrates are simultaneously fired, resulting in twice the rate of output.
The thermally degradable interface layer preferably includes a thermally degradable binder and a separating material. Most preferably, the thermally degradable binder is a polymer, such as ethylcellulose or methylcellulose, and the separating material is an inorganic material, such as boron nitride or graphite.
In one aspect, the process includes firing the laminate to an initial low temperature to degrade the interface layer and then raising the temperature to a higher temperature to sinter the first and second pluralities of green sheets and degrade the separating material.
To degrade the interface layer completely, it is desirable to fire the laminate in a reducing atmosphere, such as hydrogen or cracked ammonia. The process may include cutting the laminate fully apart, prior to firing, or providing a second thermally degradable interface layer below the first and second pluralities of green sheets. The laminate may then be partially cut prior to the step of firing by cutting through the first plurality of green sheets, cutting through the first interface layer and cutting through the second plurality of green sheets, but only partially cutting through the second interface layer. This allows the laminate to be handled as an integral unit prior to firing. During firing, the individual ceramic substrates are released and become fully separated as the second interface layer degrades, avoiding the necessity for breaking them apart.
The principal components of the interface layer of the present invention are a binder and a separating material. The binder is a material, such as ethylcellulose or methylcellulose, that thermally degrades at a temperature above room temperature and below the firing temperature for the microelectronic ceramic substrates being made.
The separating material is preferably a material, such as an inorganic, that acts to separate the first and second microelectronic ceramic substrates during firing, but which does not bind with them or detrimentally affect them at the sintering temperature of the green sheets forming the substrate.
The binder and the separating material are formed into a sheet for lamination between the green sheets which are to become the microelectronic ceramic substrates after firing.
In the preferred method of making the interface layer of the present invention, the separating material is mixed with a sufficient quantity of the binder and a solvent to bind the separating material. Mixing proceeds until a homogeneous mass has been formed, which is then cast in sheet form on a carrier to form the interface layer.
In the preferred method, the binder is ethylcellulose or methylcellulose and the solvent is a mixture of methanol and methyl isobutyl ketone. A plasticizer, such as dipropylene glycol dibenzoate, is also added, an
Casey Jon A.
Cohn Michael A.
Cropp Michael E.
Sullivan Candace A.
Sullivan Robert J.
Blecker Ira D.
DeLio & Peterson LLC
International Business Machines - Corporation
Peterson Peter W.
Turner Archene
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