Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
2000-08-09
2002-09-03
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S330000, C430S944000, C430S945000
Reexamination Certificate
active
06444400
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for making a electroconductive pattern in an organic polymer layer which is suitable as an electronic circuitry in an electric or semiconductor device.
BACKGROUND OF THE INVENTION
Electric or semiconductor devices such as flat panel displays, photovoltaic cells or electrochromic windows typically contain a substrate provided with an indium tin oxide (ITO) layer as a transparent electrode. The coating of ITO is carried out by vacuum sputtering methods which involve high temperature conditions up to 250° C., and therefore, glass substrates are generally used. The high cost of the fabrication methods and the low flexibility (pliability) and stretchability of such electrodes, due to the brittleness of the inorganic ITO layer as well as the glass substrate, limit the range of potential applications. As a result, there is a growing interest in making all-organic devices, comprising plastic resins as a substrate and organic electroconductive polymer layers as an electrode. Such plastic electronics allow to obtain low cost devices with new properties (Physics World, March 1999, p.25-39). Flexible plastic substrates can be provided with an electroconductive polymer layer by continuous roller coating methods (compared to batch process such as sputtering) and the resulting organic electrodes enable the fabrication of electronic devices characterised by a higher flexibility and a lower weight.
The production and the use of electroconductive polymers such as polypyrrole, polyaniline, polyacetylene, polyparaphenylene, polythiophene, polyphenylenevinylene, polythienylenevinylene and polyphenylenesulfide are well known in the art. EP-A-440 957 describes a method for preparing polythiophene in an aqueous mixture by oxidative polymerisation in the presence of a polyanion as a doping agent. In EP-A-686 662 it has been disclosed that highly conductive layers of polythiophene, coated from an aqueous coating solution, could be made by the addition of a di- or polyhydroxy and/or a carbonic acid, amide or lactam group containing compound in the coating solution of the polythiophene layer and by keeping the coated layer at elevated temperature, preferably between 100 and 250° C., during preferably 1 to 90 seconds.
Coated layers of organic electroconductive polymers can be structured into patterns using the known wet-etching microlithography techniques. WO97/18944 describes a process wherein a positive or negative photoresist is applied on top of a coated layer of an organic electroconductive polymer, and after the steps of selectively exposing the photoresist to UV light, developing the photoresist, etching the electroconductive polymer layer and finally stripping the non-developed photoresist, a patterned layer is obtained. A similar technique has been described in Synthetic Metals, 22 (1988), p. 265-271 for the design of an all-organic thin-film transistor. Such methods are cumbersome as they involve many steps and require the use of hazardous chemicals. Research Disclosure No. 1473 (1998) describes photoablation as a method suitable for patterning organic electroconductive polymer layers, wherein the selected areas are removed from the substrate by laser irradiation. Such photoablation processes are convenient, dry, one-step methods but the generation of debris still requires a wet cleaning step and may contaminate the optics and mechanics of the laser structuring device. Some prior art methods also induce a difference of the optical density between electroconductive and non-conductive areas of the patterned surface, which should be avoided.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a convenient, dry, one-step method of patterning an electroconductive organic polymer layer which does not require a wet cleaning step and which does not influence the optical density of the polymer layer. This object is realized by the method of claim 1. Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
All values of surface resistivity (SR) presented in this document are measured according to the following method. The sample is cut to obtain a strip having a length of 27.5 cm and a width of 35 mm. Electrodes are applied over the width of the strip at a distance of 10 cm from each other. The electrodes are made of a conductive polymer, ECCOCOAT CC-2 available from Emerson & Cumming Speciality polymers. A constant potential is then applied over said electrodes and the current flowing through the circuit is measured on a Pico-amperemeter KEITHLEY 485. SR in &OHgr;/square (&OHgr;/□) is calculated from the measured values of potential and current, taking into account the geometry of the measurement.
The polythiophene layer used in the method of the present invention is similar to those described in EP-A-686 662. However, contrary to the method of EP-A-686 662, the coated layer is not dried or treated at elevated temperature, so that the initial surface resistivity (SR
i
) is kept at a high value of at least 10
4
&OHgr;/□, or more preferably at least 10
6
&OHgr;/□, i.e. the conductivity of the layer is insufficient to be suitable as an electrode. By heating selected areas (paths), hereinafter referred to as ‘image-wise heating’, the SR of said areas is decreased to SR
i
/&Dgr;, wherein the factor &Dgr; is at least 10, preferably at least 10
3
or even more preferably at least 10
5
so as to obtain a pattern of conductive and non-conductive areas which can be used as an electronic circuitry. Said electronic circuitry can be a relatively low-resolution circuitry as typically used in printed circuit boards or a high-resolution microcircuitry as used in integrated semiconductor devices. More applications of the present invention are indicated below.
Simulation experiments, wherein the above mentioned polymer layer is overall heated in an oven, indicate that a treatment at 200° C. during 2 minutes suffices to reduce SR
i
by a factor &Dgr; between 10
4
and 10
5
. The thickness of the layer is not significantly influenced by such treatment which illustrates that the organic polymer layer can be rendered electroconductive by heat treatment without invoking ablation. So the method of the present invention is particularly distinguished from prior art methods in that no material is removed from the layer, i.e. the conducting and non-conducting areas are all located in the same plane which may be beneficial when used in devices wherein very thin layers are applied on the electrodes, e.g. organic light emitting polymer displays. Moreover, the method of the present invention allows to obtain an electroconductive pattern showing no substantial difference of optical density between conducting and non-conducting areas. As an alternative method, contrary to the method of the present invention, one may also obtain an electroconductive pattern by ablating selected areas of the polymer layer and then overall heating the remaining areas, thereby decreasing the SR value so as to render said remaining areas electroconductive. However, said alternative method also involves ablation and thus is characterized by the same problems as the prior art methods described above.
The polythiophene used in the method of this invention has preferably the following formula:
in which n is larger than 1 and each of R
1
and R
2
independently represents hydrogen or an optionally substituted C
1-4
alkyl group or together represent an optionally substituted C
1-4
alkylene group or an optionally substituted cycloalkylene group, preferably an ethylene group, an optionally alkyl-substituted methylene group, an optionally C
1-12
alkyl- or phenyl-substituted ethylene group, a 1,3-propylene group or a 1,2-cyclohexylene group.
The preparation of such a polythiophene and of aqueous dispersions containing such a polythiophene and a polyanion is descr
Andriessen Ronn
Cloots Tom
Louwet Frank
Thillo Etienne Van
Agfa-Gevaert
Breiner & Breiner L.L.C.
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