Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-10-29
2002-12-31
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S099000, C524S104000, C524S114000, C524S464000
Reexamination Certificate
active
06500885
ABSTRACT:
FIELD OF USE
This invention relates to the formation of polycarbonate films, including the formation of apertures through polycarbonate films.
BACKGROUND ART
Polycarbonate is a colorless thermoplastic polymer, i.e., polycarbonate softens when heated and hardens when cooled. Polycarbonate is commonly used in applications which take advantage of its outstanding impact resistance and toughness, such as molded helmets, battery cases, bottles and packaging, and in applications which also demand optical transparency, such as bullet-proof and safety glass, eyewear, compact discs and automobile lenses. In thin-film form, polycarbonate is used for a variety of applications ranging from precision filters to electron-emitting devices.
Polycarbonate membranes used as commercial filters are described in the 1990 Nucleopore® Laboratory Products Catalog, Costar Corp., 1990, pp. 3, 8 and 9. The membranes are created by subjecting stretched, crystalline polycarbonate film to irradiation, followed by etching to form pores. The Costar process is similar to that disclosed in Price et al., U.S. Pat. No. 3,303,085. The thickness of commercial membrane filters is typically 6 to 11 &mgr;m.
Bassiere et al., PCT Patent Publication WO 94/28569, disclose how thin polycarbonate layers are used in manufacturing electron-emitting devices. In one embodiment, Bassiere et al. provide a polycarbonate layer over a sandwich consisting of an upper conductor, an insulator and a patterned lower conductor. The multi-layer structure is irradiated with heavy ions to create radiation tracks through the polycarbonate layer. The tracks are etched to form pores through the polycarbonate layer down to the upper conductor. Using suitable etchants, the pore pattern in the polycarbonate layer is transferred to the upper conductor and then to the insulator, after which conical electron-emissive elements are formed in the resulting openings in the insulator.
Bassiere et al. indicate that the thickness of their polycarbonate layer is approximately 2 &mgr;m. This is significantly less than the thickness of the commercial polycarbonate membrane filters in the Costar product catalog. While Bassiere et al. specify that the polycarbonate layer in their structure can be created by spin coating, Bassiere et al. do not provide any further information on how to make the polycarbonate layer.
Macaulay et al., PCT Patent Publication WO 95/07543, disclose a similar fabrication technique in which electron-emissive features in an electron-emitting device are defined by way of charged-particle tracks formed in a track layer. Polycarbonate is one of the materials that Macaulay et al. consider for the track layer. The thickness of the track layer in Macaulay et al. is 0.1 to 2 &mgr;m, typically 1 &mgr;m. Consequently, the thickness of the track layer in Macaulay et al. is typically less than that of the polycarbonate layer in Bassiere et al. by a factor of up to twenty.
Kanayama et al, European Patent Specification 500,128 B1, application published Aug. 26, 1992, describes a polycarbonate resin utilized in forming a solid polycarbonate film. The polycarbonate resin consists of copolycarbonate formed with repetitions of two different carbonate repeat units. The polycarbonate film is created by dissolving the copolycarbonate in a non-halogenated solvent such as toluene, xylene, or ethylbenzene, forming a liquid film of the resulting solution over a substrate, and drying the liquid film.
The solid polycarbonate film of Kanayama et al may have enhanced mechanical strength. However, the film does not appear particularly suitable for receiving a fine pattern of small generally parallel apertures created by etching along the tracks of energetic charged particles that pass through the film. For example, the carbonate (CO
3
) groups in the repeat units do not appear to have significant free radical stabilization which would facilitate etching along the charged-particle tracks.
As film thickness is reduced, it becomes progressively more difficult to make high-quality polycarbonate films. Controlling and maintaining the uniformity of film thickness and other properties, such as density, becomes harder. Structural and compositional defects also become more problematic in very thin polycarbonate films. It would be desirable to have a method for making a thin polycarbonate film whose thickness and other physical properties are highly uniform, especially a thin polycarbonate film in which a fine pattern, such as a group of small generally parallel apertures, is to be formed. It would also be desirable to have a method for providing small parallel apertures through the film, particularly for use in defining openings in the gate layer of a gated electron emitter.
GENERAL DISCLOSURE OF THE INVENTION
The present invention involves the preparation and usage of polycarbonate films. More particularly, the invention furnishes properties and compositions for a polycarbonate-containing liquid chemical formulation from which a thin polycarbonate film of highly uniform thickness can be made. The invention also furnishes processing techniques for making the polycarbonate film. Apertures are created through a so-prepared polycarbonate film by etching along substantially parallel charged-particle tracks. The aperture-containing polycarbonate film is typically employed in fabricating a gated electron-emitting device.
The liquid chemical formulation of the invention is formed from polycarbonate material dissolved in a suitable liquid, preferably one capable of dissolving the polycarbonate material to a concentration of at least 1% by mass of the liquid formulation at 20° C. and 1 atmosphere. The liquid preferably contains a principal solvent consisting of at least one of pyridine, a ring-substituted pyridine derivative, pyrrole, a ring-substituted pyrrole derivative, pyrrolidine, a pyrrolidine derivative, chlorobenzene, and cyclohexanone. The liquid may include a cosolvent, different from the principal solvent, for modifying one or more properties of the liquid formulation.
Aside from the liquid and the polycarbonate material, the present liquid chemical formulation may be provided with one or more other constituents such as a water scavenger. To the extent that any other such constituent is present in the liquid formulation, each other such constituent is normally a minor component compared to the polycarbonate material. That is, the polycarbonate material is normally present in the liquid at a higher mass fraction than any other constituent present in the liquid.
The polycarbonate material typically includes copolycarbonate whose molecules each contain two or more different monomeric carbonate repeat units. Each carbonate repeat unit is formed with a carbonate (CO
3
) group and another group, normally a hydrocarbon group. The copolycarbonate normally constitutes at least 5%, typically more than 50%, by mass of the polycarbonate material.
Use of copolycarbonate leads to a polycarbonate film having properties that are highly advantageous when apertures are created in the polycarbonate film by etching along tracks formed by energetic charged particles. Each charged-particle track consists of a zone of damaged polycarbonate material in which the energy of one of the particles causes the polycarbonate molecules along the particle's path to cleave (undergo scission). A polycarbonate molecule typically cleaves along certain of its carbonate groups as decarboxylation occurs. Carbon dioxide is released from the molecule during decarboxylation. Apertures are created along the charged-particle tracks by removing the damaged polycarbonate material with etchant that attacks the remnants of the cleaved polycarbonate molecules much more strongly than the uncleaved polycarbonate molecules.
Each polycarbonate molecule in the damaged polycarbonate material need not be cleaved into a large number of small parts for apertures to be created in the polycarbonate film by etching along the charged-particle tracks. Etchants are available which can selectively remove remnants of polycarbonate mol
Porter John D.
Simmons Stephanie
Skinner Michael P.
Cain Edward J.
Candescent Technologies Corporation
Meetin Ronald J.
Skjerven Morrill LLP
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