Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2000-04-11
2002-04-30
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S277100, C430S275100, C430S935000, C204S471000, C204S478000
Reexamination Certificate
active
06379865
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to coating formulations and a method, useful in microelectronics applications, for isolating and protecting fine-pitch, electrically conducting circuit interconnects, and related structures. More particularly the invention provides coating materials for application to conductive elements using an electrophoretic deposition technique. The coatings provide protective, high resistivity, low dielectric constant, negative image bearing layers after exposure to radiation patterns of suitable wavelength, followed by development with mild aqueous acid solutions.
BACKGROUND TO THE INVENTION
Modern society relies upon the trouble-free conveniences provided by electrical and electronic devices. Since the earliest recognition that useful devices could be developed by combining electrical circuits, circuit combinations have become more complex, and the resulting devices more sophisticated in their capabilities. Effective circuit performance relies upon electrical current isolation within a particular circuit with no possibility of current leakage into a neighboring circuit. Any unintended current transfer between circuits of a multi-circuit, multi-function electrical device will ultimately cause an inconvenient malfunction of the device.
Isolation or insulation of circuits from each other represents an increasing challenge with the continuing emphasis on more complex printed circuit designs and increased functionality for electrical devices, especially miniature electronic devices. Progress in electrical device design has caused a transition from the interconnection of discrete electrical components, using pre-insulated wiring structures, to interconnection, with modern printed circuits, using conductive traces only microns wide. Protection and isolation of such narrow traces, from each other, demands materials that may be precisely placed over the elongate current carrying traces while leaving tiny contact points exposed for electrical connection to other circuits that form part of a particular device. For a significant period of time it was possible to essentially cover the printed circuit with a protective coating, leaving voids in the coating corresponding to the needed points of contact. More recently, however, the introduction of flexible printed circuits and multi-layer printed circuits has led to the need for coatings and processes capable of high precision in protective cover formation and placement. High precision techniques provide a cover-layer with essentially just sufficient insulation to protect a conductive trace without straying into other portions of a printed circuit substrate. Such coatings tend to be very thin and subject to attack by, e.g. solvents, moisture, or other potentially damaging environments. For this reason, precision coating of printed circuits must provide both insulative and environmental protection for electrical conductors.
A variety of coating methods exists for applying coatings, covercoats and the like as protective, insulating coatings to printed circuit patterns. The term covercoat refers to a dielectric coating, over the printed circuit basestock, applied after the conductive circuit pattern has been etched. The covercoat serves to protect the copper conductors from moisture, contamination and damage. Conventional coating methods include screen printing and application of continuous layers by methods such as knife coating, spin coating, extrusion coating, dip coating, curtain coating, and spray coating. Application of continuous coatings covers not only the leads but also the area in between the leads. This condition has several disadvantages when found in intricately structured printed circuits. For example, differences in expansion coefficients between a continuous cover-coat and a flexible printed circuit substrate may introduce stresses that cause the circuit to adopt an inconvenient curl-set. Segmentation of a cover-coat, into separate coated areas, is less likely to be subject to this condition.
Selective deposition processes, such as electrophoretic deposition, also known as “e-coat,” may achieve coating separation and precise positioning (details of this process may be found in the “Handbook of Electropainting Technology” by W. Machu, Electrochemical Publication Limited, 1978). Application of electrophoretic deposition techniques began at least three decades ago for painting automobiles and appliances. Electrophoretic deposition involves precise distribution of a layer of charged droplets over a conducting surface that represents an electrode of an electrolytic cell operating under direct current potential. Charged droplets migrate towards an oppositely charged electrode to be deposited thereon. Droplet deposition and layer formation may occur at either an anode or a cathode. Preferably the droplets are positively charged for deposition on a cathodic surface. Cathodic coatings do not suffer the oxidative corrosive processes associated with anodic deposition. Also, electrophoretic deposition of water-based compositions produces essentially void free and substantially non-polluting coatings.
Compared to conventional coating processes, such as screen printing, electrophoretic deposition selectively places a protective layer only on conductive portions of the printed circuit. Use of electrophoretic deposition should produce individually encapsulated conductors, whereas conventional techniques coat the entire printed circuit. Selective deposition also offers other advantages, such as the production of lighter weight circuits which is important for hard disk drive (HDD) flexible circuits applications.
The use of electrophoretic deposition is known for coating printed circuits with photoresists. U.S. Pat. Nos. 4,845,012; 5,055,164; 5,607,818; 5,384,229; 5,959,859; and 5,439,774 contain reference to the technique. Other U.S. Pat. Nos. 4,592,816 and 5,181,984 describe epoxy/acrylate compositions for electrophoretic deposition of solder mask/covercoat systems. Photoresist and solder mask materials are typically photosensitive and developable to a patterned polymer, covering selected (imaged) portions of the printed circuit. This provides evidence of photoimageable coatings, formed by electrophoretic deposition. Additionally, U.S. Pat. No. 4,832,808 teaches electrophoretic deposition of coatings of piperazine-containing polyimides. However, such coatings possess neither photosensitivity nor solubilization in aqueous acid developers.
The effective use of electrophoretically deposited, photoimageable coatings may depend upon the image resolution attainable with such systems. Printed circuits of increasing density require the use of photoresists of increasing image resolution. Image resolution depends upon radiation scattering within photosensitive layers and the variation of image characteristics, i.e. resolution, related to developers and development processes.
Polyimide-containing formulations provide potentially useful materials for photoimageable coatings produced by electrophoretic deposition. They also have the thermal and dielectric properties suitable for protecting and insulating electrical current carrying conductors. Image development of polyimide coatings, after exposure to an image pattern, may involve non-aqueous, solvent-based developers or aqueous-based developers. The use of solvent-based development systems applies to photoimageable polyimides that may use a benzophenone moiety as a built-in photo-crosslinker. U.S. Pat. Nos. 4,629,685; 4,656,116; 4,841,233; 4,914,182; 4,925,912; 5,501,941; 5,504,830; 5,532,110; and 5,599,655; and European Patent No. EP 0456463 A2 provide evidence of autosensitized polyimides. As indicated previously, these materials need organic solvents for image development. High volume use of solvent developers, in production operations, may cause environmental problems associated with solvent pollution and disposal. Aqueous developers provide a more environmentally friendly alternative to organic solvent developers. Some alkaline aqueous developers contain tetramethy
Eitouni Hany B.
Mao Guoping
Pocius Alphonsus V.
Scheibner John B.
Somasiri Nanayakkara L. D.
3M Innovative Properties Company
Ball Alan
Chernivec Gerald F.
Griswold Gary L.
Hamilton Cynthia
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