Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2002-10-01
2004-06-01
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S209000, C428S210000, C428S702000
Reexamination Certificate
active
06743534
ABSTRACT:
BACKGROUND OF THE INVENTION
The co-sintering or firing of laminated ceramic tapes is a well-known module manufacturing method in the microelectronics industry. The term “low-temperature co-fired ceramic (LTCC)” refers to a technology for forming multilayered ceramic circuits. In this approach, a tape is formed from glass and ceramic powders uniformly dispersed in an organic binder. Typically, two or more layers of this tape are laminated together to form a circuit. To form electrical connections from one layer of tape to the next, via holes are punched through the tape and filled with a thick-film conductor paste, for example as described in U.S. Pat. No. 4,654,095 of Steinberg.
In the next step, thick-film pastes (dispersions of metallic, ceramic or glass powders in volatilizable organic vehicles) that will form components of electronic circuits, such as conductors or resistors, are then screen-printed onto the tape. After all of the layers of tape necessary to form the completed circuit have been prepared, the pieces of tape are aligned to ensure that the via connections from one layer will make contact with conductor traces or via holes on the next. The layers of tape are then laminated with a combination of heat and pressure to form a single green body, i.e., a structure that is held together by organic binders, such as polyvinyl butyral or acrylate materials.
In order to form the final ceramic body, the green body is fired in a firing profile that typically reaches a peak temperature of about 850° C. to 900° C. before returning to ambient temperature. In a range of temperatures between about 350° C. and 450° C., the organic binders that give the green body strength are volatilized or burned out. To give the volatilized gases sufficient time to escape, the ramp rate (change in temperature per unit time) of the profile is often reduced in this temperature range.
Above the burnout temperature, the ramp rate of the firing profile is increased and the part is heated until reaching the peak firing temperature. The LTCC tape typically contains a significant amount of a glass with which a glass softening point T
g
is associated. The glass and ceramic powders will begin to sinter into a dense body when the temperature is above the softening point of the LTCC glass, so the peak firing temperature of the tape is typically 100° C. to 200° C. above the T
g
. The thick-film conductor and resistor materials used in the circuit body will undergo a similar metamorphosis from organically bound powders into dense sintered structures. After allowing the parts to remain at the peak firing time to reach an adequately dense body, the parts are cooled to room temperature.
Manufacturing of LTCC tapes is typically performed using tape casting techniques, such as those described in U.S. Pat. No. 5,821,181 of Ursula, et al. In this method, ceramic slurry (a mixture of the inorganic and organic components of the tape before drying) is deposited on top of a polyester film or carrier using a doctor blade. One disadvantage of using tape casting techniques for tape manufacturing is the difficulty of thickness control as the tape becomes thinner and thinner. More specifically, thickness, accuracy and variance become uncertain when casting under 2 mils (50 microns), a measurement which refers to the gap between the blade and the backing as the wet slurry passes through. Therefore, control of the layer thickness, especially of inner layers, becomes difficult and often inaccurate.
While accurate casting of individual layers is achievable, the method described in U.S. Pat. No. 5,102,720 for drying the tapes individually and subsequently laminating them together is uneconomical. Thus, methods which involve drying individual layers and lamination with heat and pressure, or casting a subsequent layer on top of a dry layer, not only introduce significant costs to the manufacturing process, but also limit product yields.
Other manufacturing methods include dipping a moving carrier film in a slurry to create a meniscus between the carrier film and the slurry. However, the meniscus created by capillary forces between the wet organic binder and the film causes it to stick to the surface of the polyester film. As in other methods, drying one layer at a time and then casting a wet layer on top of a dry layer or subsequent heat lamination are needed. Because of the disadvantages with known methods for manufacturing LTCC tapes, there remains a need in the art for an improved, economical method for fabricating LTCC tapes which will maximize product yield and permit tight control of layer thickness.
The LTCC technology has advanced beyond the microelectronic circuit industry and is currently in use for a variety of applications. One important attribute of LTCC is the ability to create three-dimensional structures using multiple layers of tape. The bio-medical device industry, for example, uses LTCC for the manufacturing of cavities and channels for moving part pumps used in in-situ drug delivery systems. Biological test modules have been realized which facilitate the automatic testing of biological and chemical materials.
In the telecommunications industry, there is a need for integrated opto-electronic modules. LTCC offers the simplicity of being able to co-sinter optical fibers together with the driving electronics. The co-firing of meso-scale structures containing metallization, cavities, vias, and channels is thus an appealing feature of LTCC.
LTCC meso-systems are small packages capable of handling at least two media, such as electricity and fluids, by means of sensors, actuators, interconnection, control and/or signal processing. Miniaturization is one of the biggest drivers of this technology, thus allowing systems in package (SIP), in which several components are inserted into a monolith.
An attractive feature of LTCC tapes is the possibility for making cavities for the placement of integrated circuits within. For example, as shown in
FIG. 1A
, a single electronic module
15
contains a cavity
15
A, a metallic via
15
B, and a metallic line trace
15
C on the surface of the ceramic monolith.
FIG. 1B
shows a panel area
17
which contains an array of microelectronic modules
15
. The panel
17
is typically formed and processed as a whole and then cut into individual modules
15
.
Cavities allow a module to retain a low profile, while a lid may be placed on top for hermeticity. However, during surface or sacrificial constrained sintering, as explained below, the cavity walls exhibit a phenomenon called necking, a vertical curvature from the top surface interface to the bottom of the fired substrate surface. During sintering of sacrificially constrained structures, there is a stress distribution due to the shear and in-plane tensile stresses from top to bottom. It has been shown that stresses are significantly higher at the constrained interface. Moving along the z-axis towards the middle of the fired substrate, there are fewer constraining forces that counteract the in-plane tensile stresses. Therefore, there is significantly more densification in the middle of the monolith, which causes the vertical curvature. Furthermore, as a consequence of the higher stress distribution at the interface, delamination or buckling is usually present. The aforementioned properties are undesirable, especially when constructing cavities or other precision features in the ceramic structures.
Despite the numerous applications of LTCC technology, the LTCC process has several disadvantages. First, there are significant changes in the dimensions of the ceramic monolithic structure during sintering. More specifically, when the constituent powders of the LTCC structure densify during traditional unconstrained or free sintering, shrinkage occurs in all dimensions. Typically, the shrinkage of the tape across its width or length (the x- or y-directions) will be nearly identical and only slightly different from the shrinkage through the thickness of stack-up of tape layers (the z-direction). Usually, the dimensions of the structure after
Amaya Edmar M.
Hochheimer J. Thomas
Lautzenhiser Frans P.
Akin Gump Strauss Hauer & Feld & LLP
Blackwell-Rudasill G. A.
Heraeus Incorporated
Jones Deborah
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