1. Field of the Invention
The present invention relates to a wiring board with an electronic device, and a method for manufacturing the same.
2. Description of Related Art
In recent years, technical development has been proceeded for decreasing a cost by forming a film of a predetermined pattern on a substrate by using an ink jet method. Particularly, in an organic thin film transistor using an organic semiconductor, not only a semiconductor film but also all of constituent materials such as a gate dielectric film, a gate electrode, a source electrode, and a drain electrode have been provided by coating materials applicable to the ink jet method.
However, the ink jet method has a limit for a control of a lateral size of an elongate film pattern and this hinders, for example, improvement for a switching performance of an organic transistor.
On the other hand, a study for realizing a fine wire pattern necessary for transistor performance while controlling a spreading of a coating material by use of a dam-like stepped structure referred to as a bank has proceeded (for example, refer to Document 1 (Japanese Patent Laid-open No. 2007-158003)). In this case, since a width of an elongate trench has a great concern with a finished size of a pattern width necessary for transistor performance, control for the amount of the coating material filled in the trench is difficult. Further, in the ink jet method, filling into the trench narrower than an impact area is also difficult in view of a landing accuracy. Accordingly, a portion of the trench is defined as a bank of an impact area size. However, in a case where an entire length of the trench is longer, a flowing distance of the coating material from the bank for the impact area is long to sometimes make the pattern width instable due to dry-up or deposited obstacles in the trench during flowing. As a countermeasure for the problem, it has been proposed to promote the flow by static electricity from an external electrode (for example, refer to Document 2 (Japanese Patent Laid-open No. 2006-7061)).
However, in a case of constituting, for example, the organic thin film transistor with a coating pattern, it is in a multi-layered structure. That is, since an elongate film is formed over a region where patterns for an insulation layer, a semiconductor layer and a conductor layer are disposed in a lower layer, flow interference tends to occur in the trench. Then, for the static electricity applied from the external electrode, interference of the lower layer pattern has to be taken into consideration and the electronic circuit by the application of the organic thin film transistor has to be formed while undergoing such restriction. As described above, while there have been great expectations to electronic circuits by applying electronic devices that are constituted with coating materials for realizing saving of the cost and improvement for a throughput comparable with those of printing by the technical development based on a printing technique, the problems described above produce bars.
On the other hand, in a coating material for a conductor in which metal nano-particles are dispersed in a solvent using an organic protective colloid, a compactness of a conductor structure obtained finally is low due to an effect upon decomposition of an organic protection film and a specific resistivity is increased remarkably compared with that of a bulk metal, so that lowering of a conductor resistance is essential.
For attaining a fine conductor wire by the coating material, it is necessary to increase a film thickness by a stack coating upon formation of a fine wire or metal plating after formation of the fine wire for decreasing the resistance of the conductor. Stack coating method is desirable with a reason of reducing installation cost and a shortening of steps. However, in the stack coating method, flow interference in the trench tends to occur more frequently in the second coating step, which tends to deteriorate a uniformity of a wire width. Further, when the conductor film is formed at a bottom of the trench due to a first coating, promotion of flow by static electricity becomes difficult.
For the wiring pattern width required, for example, in a wiring board, since direct drawing is possible by the ink jet system, the problem does not occur during formation of a multi-layer structure. However, for attaining an electronic circuit using the organic thin film transistor that requires the multi-layer structure as described above, a method for forming a fine wire pattern conformable to the multi-layering using the coating material is necessary.
A problem is present in a countermeasure to the flow interference in the trench. The proposal described above is applicable to the formation of the fine wire pattern for the lowermost layer. However, a pattern fabricated substrate from a substrate with a conductor layer can also be utilized in a case of forming a fine wire conductor for the lowermost layer, and the formation of the fine wire pattern for the layer thereabove is more important than the formation of the lower layer pattern. Further, there is also a problem of increasing a thickness of the fine wire by the stack coating method.
Increase for the thickness of the fine wire by the stack coating method requires repeating a coating step and a baking step. In a flexible substrate or the like, local undulation tends to occur due to the difference of the thermal expansion coefficient between the substrate and the pattern material and it is important to form a pattern by supplying the coating material at a high speed for attaining a low cost and high throughput comparable with those of printing.
Particularly, in a case of the organic thin film transistor, it is necessary to attain a gate electrode providing a low resistance at a gate length size of a fine wire width, as well as attain a gap between a source electrode and a drain electrode with a size approximate to the gate length to lower a resistance and to lower a parasitic capacitance for each of electrodes. Accordingly, while increase in the thickness and saving of the area are intended for the electrode, there is a problem that the coating layer covers the gap upon the stack coating. Since a performance of the transistor is deteriorated, it is difficult to increase the thickness of the dielectric layer over the gate.
An object of the present invention is to provide a wiring board having an organic thin film electronic device of good characteristics at a reduced cost.