Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-06-21
2004-01-20
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S843000, C029S854000, C029S857000, C029S878000, C029S879000, C174S262000, C174S260000, C174S263000
Reexamination Certificate
active
06678949
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to carriers for electronic device assembly and, more particularly, to a carrier incorporating a polyimide “thin film” structure on the carrier.
BACKGROUND OF THE INVENTION
In a multi-chip module (MCM), a matrix of electronic devices, typically chips, are generally mounted on a multi-level ceramic (MLC) carrier. One of the main functions of such an MLC carrier is to interconnect the chips or other electronic devices mounted to the carrier (“top-to-top” connections) and to interconnect such chips to the input-output or I/O connections of the carrier board (“top-to-bottom” connections).
If the number of chips or chip connections increases, there is a corresponding need to increase the number of layers in the MLC carrier. Such additional layers are needed because there is only so much area available on each layer for the lines and vias which are formed within each ceramic level. For example, one typical MLC module uses line widths of approximately four mils (1 mil=0.001 inches=0.0254 mm), with five mils spacing between the lines, and vias occupying about nine mils. The current art with 100 chips on the carrier requires an MLC carrier of more than 50 layers.
For a variety of performance as well as manufacturing reasons, including cost and yield concerns, it is desirable to limit the number of MLC layers. One solution of the current art is to use “thin films,” generally made of polyimide, polymeric material, or other organic material having a low dielectric constant and copper wiring. The polyimide-copper thin film generally can have conductive lines and vias patterned on the thin film at a higher density than typical MLC carriers. The finer line widths and via sizes which can be patterned on thin films therefore allow each layer of a thin film to replace many corresponding layers of an MLC carrier. A typical thin film (TF) has a one-mil line width, a one-mil spacing, and one-mil vias; widths can be as small as one-half a mil, that is, about 13 microns. The higher densities made possible by thin film technology contain or reduce the complexity of MLC carriers. Typically, the thin film is secured to one of the planar surfaces of the MLC carrier and appropriate interconnections are made between the opposing, “mating” surfaces of the thin film and the MLC carrier.
The use of thin films also enhances the ability to repair certain defects in the resulting MLC carrier. Furthermore, the polyimide material and copper lines and vias of a thin film interconnect generally produce better electrical performance than the typical MLC carrier. As such, thin film technology has been a critical part of high-performance interconnect carriers for almost all MCMS.
In spite of its importance to MCM performance and manufacturability, thin film processing has traditionally been a painstaking, slow process with an associated long cycle time. Because the MCM generally comprises a ceramic (MLC) carrier with a thin film processed on top of the carrier, the MLC carrier must be built first, before the thin film processing can begin. Under traditional manufacturing techniques, the MLC carrier must have the surface destined to mate with the thin film properly finished and prepared, especially because thin film processing of the MLC carrier requires a substantially flat, smooth, and pin-hole free mating surface.
Another drawback to current thin film manufacturing processes is that, when a thin film is found defective, additional manufacturing steps, including lap and polish, must be undertaken to remove the thin film from the MLC carrier and to prepare the MLC carrier to receive a replacement thin film. Further, large format, multi-up processing of MLCs with thin film is impractical due to the weight and bulk of the ceramic substrates and the unavailability of large-format MLC substrates.
The prior art has attempted to address the aforementioned drawbacks and disadvantages, but has achieved mixed results. For example, in order to reduce thin film processing cycle time and costs, a parallel processing manufacturing technique has been devised. According to this technique, the thin film is processed on a “sacrificial carrier,” and is then transferred to the MLC carrier after electrical testing. Parallel processing means that thin film processing can occur separate and apart from the MLC substrate to which the thin film will ultimately be bonded. Parallel processing often also improves the yield of both of the MLC carriers and the thin films, as only those which pass the electrical test will be used together. Furthermore, the use of the sacrificial carrier improves the ability to process thin films in the “multi-up” format.
Although parallel processing may appear to address some of the drawbacks of thin film manufacturing technology, it is nonetheless characterized by several major drawbacks, and is generally less than ideal. One such drawback is, for example, that certain parallel processing techniques often use passive rather than active connections. An example of such a process is found in U.S. Pat. No. 4,812,191 issued to Ho. In such a process, the thin film is formed on an aluminum sacrificial carrier. The thin film is then laminated to an organic board. Connection from thin film to board is made by drilling and through-hole plating. The chip or electronic device is connected by wire bonding to the thin film. It can be appreciated by those skilled in the art that such a connection arrangement is less than ideal, limiting the performance and connection density.
The prior art processes of achieving active via connections with thin film also have drawbacks and disadvantages. In particular, when the thin film has been completely formed on a sacrificial carrier, typically glass according to prior art techniques, the thin film is released from the carrier in a free-standing form by using suitable laser techniques. In techniques disclosed in U.S. Pat. Nos. 4,812,191; 5,170,931; 5,258,236; and 5,534,094, the thin film is held after release in its free-standing form by a ring or frame which engages the edges of the thin film. The free-standing thin film is then laminated to an MLC carrier or module with its vias aligned with those of the MLC. The via connection between the thin film and the MLC carrier is achieved by gold, thermo-compression bonding.
Many of the drawbacks and disadvantages of this approach relate to the fact that the thin film is released from its sacrificial carrier in a free-standing form before lamination joining with the MLC. When the thin film is thus released, its internal stress generally will cause it to shrink, often by over 0.2% depending on the number of levels of the thin film. The shrinkage of the thin film makes it difficult to align the vias and other electrical connections of the thin film to the corresponding MLC carrier. Such shrinkage, and even distortion, are exacerbated if the thin film is released from its sacrificial carrier too soon in the manufacturing process. The solution of holding the thin film around its edges with a ring or frame after release has the additional disadvantage of reducing the active thin film area.
As a further disadvantage, once the thin film is held in its free-standing form described above, subsequent handling and cleaning of the thin film are difficult. Forces exerted against the thin film while it is being handled or cleaned often result in further distortion of the thin film, rendering its subsequent alignment with the corresponding MLC carrier all the more difficult. Similarly, tests of the thin film when it is in its free-standing form create pressure which may affect not only the results of the tests, but subsequent performance of the thin film itself. Should the thin film become contaminated, it likewise becomes difficult to remove such contamination without applying solution or forces, either of which may damage or distort the thin film.
GOLD COMPRESSION BONDING
Additional drawbacks to thin film manufacturing techniques of the prior art relate to the use of gold compression bondin
Canull Richard L.
DiGiacomo Giulio
Giri Ajay P.
Goldmann Lewis S.
Kelly Kimberley A.
Arbes Carl J.
Blecker, Esq. Ira D.
Nguyen Tai
RatnerPrestia
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