Method of patterning electrically conductive polymers

Details Method of patterning electrically conductive polymers Method of patterning electrically conductive polymers

2001-10-25

2003-11-18

Duda, Kathleen (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Making electrical device

C430S311000

Reexamination Certificate

active

06649327

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inexpensive and simple method to selectively pattern conducting polymer film on a substrate.
2. Discussion of the Background
Most electronics devices such as liquid crystal displays (LCDs) and organic light emitting devices (OLEDs) require electrically conductive and transparent electrodes. Typically, In
2
O
3
:Sn (ITO) is used as electrode since it is highly conductive and transparent in the visible region (400~800 nm). ITO can be vapor-deposited, sputtered or chemically or pulsed laser vapor deposited (CVD) onto glass or plastic substrates. ITO films with a surface resistance of less than 500 &OHgr;/square and a high transparency of >80% can be easily obtained using these deposition methods. However, the production of ITO must be carried out under vacuum and is very costly. Also, an additional multi-step photolithographic process is necessary to provide the substrate with the desired pattern geometry for the specific application. The photolithographic patterning method employs various chemicals including cleaning solutions, photoresist/solvent, acidic etchant, developer and stripper, most of which are environmentally toxic and hazardous.
It is known that polymers can be electrically conductive when they are in a polyconjugated form. However, due to a relatively large band gap of 1.5~4 eV, they are intrinsically insulating or have very low conductivity. The conductivity of these semiconductor polymers can be dramatically increased, by doping them with the proper oxidizing agent such as I
2
, FeCl
3
, sulphonates, perchlorates, AsF
5
and SO
3
The positive charges generated on the polymer are compensated by the negatively charged counter ions. The doped conducting polymers are generally soluble in organic solvents and some are even soluble in water depending on the substituents and polymerization methods. The surface resistance of the conducting polymeric films is found to be in the range of ~100-200 &OHgr;/square which is very comparable to values reported for metal oxides such as ITO. They can be processed by various methods such as spin or spray coating and inkjet printing onto various rigid and flexible substrates. Due to their high conductivity and ease of processability, these conducting polymers have attracted much attention as potential electrodes for display devices such as LCDs and OLEDs, interconnection in integrated chips and printed circuit boards. Other electronic applications include antistatic and electromagnetic shielding. It has been shown that the light emitting devices made with a transparent conducting polymer anode yield improved performance over a conventional ITO anode. The improvement is due to the ability of the polymer anode to make a reproducibly clean interface that substantially slows the device degradation.
However, as with conventional anode materials, the conducting polymer needs to be patterned with a suitable geometry design for a specific device. For example, a 80 mm×20 mm size passive matrix OLED with 256×64 pixels, the conducting layer is typically patterned in a series of stripes with a ~100 &mgr;m width and ~100 &mgr;m gap between the stripes. Therefore, it is extremely important to explore new cost effective methods for patterning materials used for devices. In addition, conducting polymers that maintain their intrinsic properties such as conductivity, optical transparency and physical characteristics are highly desirable.
Conventional photolithographic-techniques may be employed to pattern the conductive polymers. However, these processes were originally developed for metal oxide patterning in which the intrinsic electrical and physical properties of the conductive layer are not affected by the relatively harsh treatment and complicated wet process. This patterning approach is not suitable for conducting polymers, since it dramatically increases their surface resistance. The geometry of the conductive layer can also be detrimentally modified or etched from the substrates. A mild, simple and economical technique to pattern conducting polymers is necessary for their realization in electronic, electro-optic and opto-electronic devices.
A method of patterning an electrically conductive polymer, using a thiophene monomer, 3,4-ethylenedioxythiophene (EDOT), has been patented. See U.S. Pat. Nos. 5,976,284 and 5,447,824. EDOT is mixed with a solution of an oxidizing agent, tris(toluenesulphonate)Fe(III), and the base, imidazole, in 1-butanol. After spin coating the solution, the layer is exposed to deep UV light (&lgr;<300 mm) in accordance with the desired pattern. Subsequently, the layer is heated to initiate a polymerization reaction. In the unexposed area, the conductive polymer retains its electrical properties such as a low square resistance while an insulating polymer is formed in the areas exposed to deep UV light. The polymer layer may then be extracted with methanol, water or 1-butanol to reduce the oxidizing agent. However, this technique requires precise control of the amount of base to lower the polymerization reaction rate on the substrate. In addition, the reaction mixture is unstable and spontaneously reacts to polymerize even at room temperature or higher. Further, it is impossible to completely remove the unreacted oxidizing agent and requires a large quantity of solvent. This method suffers from a major drawback in which the conductivity difference in the conducting area and the non-conducting area is not large enough such that electric current continues to flow through the non-conducting part. It is claimed that the conductivities of the conducting and non-conducting parts are ~300 S/cm and 10
−2
S/cm, respectively. The conductivity of the non-conducting part is still quite high, making this method unsuitable for most applications including those for electronic and semiconductor devices. Additionally, cross talk may occur between adjacent electrodes when used, for instance, in a passive matrix device.
A similar method of patterning a conductive polymer uses a mixture of a conjugated polymer, polyaniline, a photo-acid generator (generates free acid upon UV irradiation) and a solvent that are spin coated onto a substrate. See patent EP-A-399299. After heating, UV irradiation through a photo-mask and further heating, the exposed areas of the polymer layer become electrically conductive, whereas the non-exposed areas remain non-conductive. Unfortunately, this method provides a specific conductivity on the order of 0.01~0.1 S/cm, which is too low for practical use in electronic and semiconductor devices.
An example of the multi-step prior art procedure follows. The prior art involves the use of a photo-resist and chemicals to strip the resist. The resist is spincoated onto a substrate containing a conductive polymer, followed by pre-exposure baking of the substrate, polymer, and resist at 100 degrees Celsius for three minutes. The substrate is then exposed to light with a wavelength of 380 nm, followed by post-exposure baking. The substrate is then developed or 40 seconds in a 20% solution of AZ303 developer. Immediately after development, the substrate must be rinsed with DI water for 40 seconds. The substrate is then dried at 90 degrees Celsius for 1-3 minutes, followed by full-plane exposure. The resist is now stripped using methoxypropanol for 30 seconds, followed by a DI water rinse. After the water rinse, the substrate is again dried at 90 degrees Celsius for 1-3 minutes.
These established methods have several disadvantages including complicated processing, cross talk, and leak current issues. In addition, the optical transparency of the organic monomers, oligomers or polymers changes upon UV irradiation due to photo-induced reactions such as photo-degradation, photo-polymerization and photo-cross-linking. This results in an optical or spectral shift as well as a change in the transparency of the conducting polymer between the exposed part and the unexposed part, that is obvious to the naked eye. Even slight

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