Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1996-02-22
2001-05-22
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S155000, C430S321000
Reexamination Certificate
active
06236445
ABSTRACT:
TECHNICAL FIELD
The present invention is in the field of production of topographic projections, in particular projections with dimensions in the micrometer range.
BACKGROUND OF THE INVENTION
Liquid crystal light valves (LCLVs) generally employ cells containing liquid crystal molecules. When the ordering of the liquid crystal molecules in regions of the cell is changed (for example, by means of a local electric field), the optical properties of those regions also change. Examples of LCLVs are disclosed in U.S. Pat. Nos. 4,728,174 and 4,826,293. Generally, a LCLV includes a layer of liquid crystal enclosed in a cell formed by insulating films on either side to provide electrical and chemical isolation. Optical images can thus be produced by a spatial voltage pattern applied to the device.
In LCLV displays, topographic projections (“bumps”) are used to keep neighboring planar surfaces at a fixed distance apart from one another. The liquid crystal thickness of LCLV, which is critical to the device performance, is traditionally maintained by using plastic shim spacers (cutout sheets of plastic) or evaporated dots of silicon monoxide “posts”. Each post is large in scale (typically one millimeter in diameter or larger) and restricted to use around the outer edges of the device. A thickness variation thus results if the surfaces of the device are not precisely flat. In an alternative spacing method, small glass beads (typically between 3 to 4 micrometers in diameter) are strewn onto a sublayer to serve as spacers. This method is deficient in that the locations where the glass beads finally land cannot be predetermined, so the distribution pattern of the beads cannot be controlled. Further, although the glass beads are relatively small, they tend to clump together into larger lumps which cast shadows, i.e. artifacts, onto the LCLV read-out.
A membrane probe is used for testing integrated circuits. An example of a membrane probe and the apparatus utilizing it are disclosed in U.S. Pat. Nos. 5,313,157 and 5,148,103, respectively. Such a probe comprises a flexible membrane having a pattern of electrically conductive traces formed on one side of the membrane, and a plurality of contact pads on selected ones of the traces to provide a temporary electrical connection to the circuit under test. Connector pads on the other side of the membrane are connected electrically to the traces to facilitate rapid detachable electric connection to a test fixture.
In membrane probe devices for testing integrated circuits, small projections (of about 3 to 6 &mgr;m in diameter, and 25 to 50 &mgr;m in height) are used to assure that a low resistance electrical contact is made at precise locations over distributed surfaces. There are several conventional ways to fabricate the projections on membrane probes. In one process disclosed in U.S. Pat. No. 5,197,184, the projecting contacts are produced as a mold on a metallic plate. Polyimide isolating layers and metal interconnecting layers are laminated and delineated to build up the test probe. This multilayer sheet is then demounted from the metal plate and attached to a stiffer substrate with the center removed to provide a tight membrane that can be deflected by air pressure or mechanically to contact the integrated circuits (ICs) under test. An alternative process has been used in which a photoresist mask is prepared with holes where the contact metal will be electroplated up to form the projection. These projections are generally flat-topped and make a relatively poor contact with the IC. These projections are difficult to prepare in diameters less than about 100 &mgr;m. Moreover, due to their small sizes, the adherence of the contact metal to the metal traces is poor. In yet another process, the contact pads on the signal traces are made by depositing a thick metal film and etching away the excess metal around the projection. This is a slow and costly process.
SUMMARY OF THE INVENTION
The present invention presents a method for producing topographic projections. The method is especially useful for producing many small projections of less than about 100 &mgr;m in diameter (and preferably from about 3 to 6 &mgr;m in diameter), and of less than about 100 &mgr;m (and preferably of about 25 to 50 &mgr;m) in height. Though the method can make projections of up to 100 &mgr;m or more in diameter, it has the advantage over the prior art in permitting the making of smaller projections, especially those of less than about 50 &mgr;m in diameter, in particular projections with heights that are proportionately greater than their diameters. The method allows for specific placement, in a predetermined pattern, of multiple projections with specific dimensions. The method utilizes optical exposure and processing of a photodefinable material, such as preimidized polyimide material. The height of each projection is controlled by the initial thickness of the photodefinable material. The size, shape, and location of each projection is determined by the use of a photomask to expose only the polyimide in the vicinity of the projection. The unexposed material is removed, e.g., by washing or etching, in the development process. The final shaping and hardening of the projection is preferably accomplished by a thermal curing process.
The projections have many uses. Their properties may be varied according to their intended uses. For example, they may be insulating or conducting. They may be used between two layers of materials to either insulate or allow electrical or heat conduction between the two layers. Electrically insulating projections can be used, for example, in LCLVs to separate and prevent the movement of charges/electrons from a photoactivated or electrically activated image input substrate to the counter electrode on the side of the LCLV display. In membrane probes, the projections may be made of a conducting material. If the projections are made of insulating material, it can be layered, covered or encapsulated with a conducting material, such as a metal. These conducting projections allow electrical conduction between the ICs and the membrane probe for testing of the functioning of the ICs.
The projections can also serve to physically separate one material from another. For example, they can be used to maintain two layers of materials at a desired distance from each other. As such, they find uses in LCLV and membrane probes as further described below.
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Multichip Module Design, Fabrication and Testing, J. J. Licari (McGraw-Hill, Inc., New York, 1995) No Date Provided.
Foschaar James A.
Garvin Hugh L.
Duong Tai V.
Duraiswamy V. D.
Hughes Electronics Corporation
Sales M. W.
Sikes William L.
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