1. Field of the Invention
The present invention relates to a solution spray apparatus and solution spray method which spray a droplet or droplets onto a substrate.
2. Description of the Related Art
An organic EL (ElectroLuminescent) element is constituted by stacking an anode electrode, an organic EL layer which is formed from an organic material and emits light by internal flow of a current, and a cathode electrode. When a forward bias voltage exceeding a threshold is applied between the anode and cathode electrodes, the organic EL layer emits light. Such organic EL elements are arrayed as pixels in a matrix on a substrate to realize an organic EL display panel which displays an image by causing each organic EL element to emit light at a predetermined tonal luminance.
In an active matrix organic EL display panel formed by a plurality of pixels, one of the anode and cathode electrodes can be designed as a common electrode connected to all pixels. However, at least the other electrode and the organic EL layer must be patterned for each pixel. A method of patterning an electrode for each pixel can adopt a conventional semiconductor device manufacturing technique. More specifically, an electrode can be patterned for each pixel by properly performing the film formation step of an electrode material film by PVD, CVD, or the like, the mask step by photolithography or the like, and the shaping step of the electrode material film by etching or the like.
Organic EL layer formation methods can be roughly classified into dry vapor deposition and wet coating in accordance with conditions such as the material. In dry vapor deposition, a hard mask having an opening in a region where an organic EL layer is to be formed is interposed between a substrate and a vapor deposition source formed from an organic EL layer material. The organic EL layer material which is heated and vaporized is applied into a film in the target region on the substrate. In wet coating, an organic EL layer can be patterned for each pixel by applying an ink-jet technique, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-106278. That is, droplets of an EL solution prepared by dissolving as a solute a polymer organic EL material for an organic EL layer in a solvent are sprayed from a nozzle, and an organic EL layer is patterned for each pixel. In wet coating to which the ink-jet technique is applied, the film formation step and the patterning step for each pixel can be almost simultaneously performed. An organic EL layer need not be etched and patterned using a photoresist mask.
In order to provide an organic EL display panel which displays a high-resolution image, the organic EL layer must be micropatterned. The ink-jet method can micropattern an organic EL layer because the droplet diameter of the EL solution is very small. However, droplets run and spread till solidification after landing, and droplets of organic EL layers in adjacent pixels may mix. To prevent this, a matrix-like partition which isolates each pixel from the surrounding pixels is used. A droplet which lands in a region surrounded by the partition is stopped by the partition, preventing mixture of droplets in adjacent pixels. Particularly when the emission color is different between adjacent pixels, organic EL layer materials which are different in accordance with the emission color hardly mix. The color purity of the emission color is expected to improve.
The polymer organic EL material generally has a low solubility in the solvent. The use of a solvent the solubility of which is low, requires a large amount of solvent in order to fully dissolve the solute. The time taken to evaporate the solvent after droplets of the EL solution land is long, resulting in low productivity. In order to form an organic EL layer into a film thickness suitable for emission, spraying of droplets of a low-concentration EL solution from an ink-jet nozzle and evaporation of droplets after landing on a substrate must be repeated a plurality of number of times, also resulting in low productivity. If the droplet amount sprayed at once is increased to decrease the count of the droplet spray step, a droplet readily overflows over the partition. Droplets in adjacent pixels mix to degrade the image quality. If the partition is formed high so as to prevent overflow of droplets, film formation of the partition takes a long time, and an electrode formed on the organic EL layer may be cut off by the partition step.
As a solvent which easily dissolves the polymer organic EL material, an organic solvent such as xylene is sometimes employed. However, such organic solvent exhibits high volatility; it vaporizes in an ink-jet nozzle, and the polymer organic EL material segregates to clog the ink-jet nozzle.
It is therefore an object of the present invention to efficiently deposit a solution at only a target position with high precision.