Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1997-02-28
2001-01-30
Morgan, Kriellion S. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S104000, C524S099000, C524S114000, C524S464000
Reexamination Certificate
active
06180698
ABSTRACT:
FIELD OF USE
This invention relates to the formation of thin 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.
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. In fabricating electron emitters, it would be desirable to have a process for making a thin polycarbonate film whose thickness and other physical properties are highly uniform, especially for film thicknesses in the range of 0.1 to 2 &mgr;m.
GENERAL DISCLOSURE OF THE INVENTION
The present invention centers around the preparation of polycarbonate films. More particularly, the invention provides 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 polycarbonate films, along with intermediate structures used in creating polycarbonate films.
In accordance with the invention, a liquid chemical formulation suitable for making a polycarbonate film is formed with polycarbonate material dissolved in an appropriate liquid. In addition to the liquid and the polycarbonate, the present liquid chemical formulation may have one or more other constituents such as a surfactant or an adhesive or adhesion promoter, with each other such constituent typically being present in a lower mass fraction than the mass fraction of the polycarbonate material. The polycarbonate-containing liquid chemical formulation of the invention can be constituted in various ways.
In accordance with the invention, the polycarbonate-dissolving liquid used in the present liquid chemical formulation has the following properties. The minimum solubility of the polycarbonate material in the liquid is 1% by mass at a temperature of 20° C. and a pressure of 1 atmosphere. The boiling point of the liquid is at least 80° C. at 1 atmosphere. Also, the boiling point of the liquid is preferably no greater than (T
g
+15° C.) at a pressure of approximately 0.001 atmosphere, where T
g
is the glass transition temperature of the polycarbonate material. The protonated form of the liquid is normally a sufficiently strong Bronsted acid, i.e., a proton donor, in aqueous solution that the acid dissociation constant of the protonated liquid in aqueous solution exceeds 10
−8
at 20° C. and 1 atmosphere.
The polycarbonate can form up to 50% by mass of the liquid chemical formulation. The molecular weight of the polycarbonate is usually at least 10,000, preferably 30,000 to 35,000. At 20° C. and 1 atmosphere, the liquid formulation of the invention typically has a kinematic viscosity of no more than 100 centistokes, preferably 2 to 25 centistokes when the solution is used for spin coating.
By arranging for the polycarbonate, the polycarbonate-dissolving liquid and the liquid chemical formulation to have the foregoing properties, the liquid formulation is especially suitable for making polycarbonate films whose thickness is 0.1 to 2 &mgr;m.
Various polycarbonate solvents may be used to form the polycarbonate-dissolving liquid in the liquid chemical formulation of the invention. For example, the liquid can be formed with pyridine or/and a ring-substituted pyridine derivative. Alternatively, or additionally, the liquid may include pyrrole, a ring-substituted pyrrole derivative, pyrrolidine, or/and a pyrrolidine derivative. Furthermore, the liquid may alternatively or additionally include chlorobenzene or/and cyclohexanone. When constituted with any of these polycarbonate solvents, the liquid preferably has the polycarbonate-solubility, minimum boiling-point and Bronsted acid characteristics given above.
The manufacture of a polycarbonate film in accordance with the invention is accomplished by first providing a polycarbonate-containing liquid chemical formulation of the above type. A liquid film of the formulation is formed over a substructure to create an intermediate structure in accordance with the invention. The liquid film is typically produced by spin coating. The liquid film is further processed to remove volatile components. The material remaining after such processing is the solid polycarbonate film. Depending on the constituency of the liquid chemical formulation, the polycarbonate film may include, as minor components, one or more other non-volatile constituents of the liquid formulation and/or their reaction products.
In utilizing the invention's teachings to produce polycarbonate films of thickness in the range of 0.1 to 2 &mgr;m, the film thickness is highly uniform. For example, comparing film thicknesses at any two points on a planar substructure separated by up to 10 cm, the total percentage deviation in thickness from a perfectly uniform film is normally less than 10% (i.e., less than ±5%) and is typically less than 5% (i.e., less than ±2.5%). This amounts to a maximum point-to-point thickness variation of less than ±50 nm, typically less than ±25 nm, for a polycarbonate film of 500-nm mean thickness. The invention thus provides a substantial technological a
Crane Scott J.
Johnson Anthony W.
Oberg Stephanie J.
Porter John D.
Candescent Technologies Corporation
Cole B.
Meetin Ronald J.
Morgan Kriellion S.
Skjerven, Morrill, MacPherson, Franklin and Friel LLP
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