Coating processes – Nonuniform coating
Reexamination Certificate
2002-10-10
2004-10-26
Pianalto, Bernard (Department: 1762)
Coating processes
Nonuniform coating
C427S335000, C427S336000, C427S337000, C427S385500, C427S421100
Reexamination Certificate
active
06808749
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a thin film forming method including arranging, on a substrate, droplets of a solution prepared by dissolving a thin film forming material in a solvent. The present invention also relates to a thin film forming method including ejecting the solution to arrange a plurality of droplets of the solution on a substrate, and evaporating a solvent from each droplet, thereby forming a thin film on the substrate.
2. Description of Related Art
Recently, attention has been focused on electronic devices employing an organic thin film (thin film made of an organic substance) as a functional thin film. An example one of such electronic device is an organic EL device. One example of an organic thin film for use as a light emitting layer in an organic EL device is a thin film of Alq3 (quinolinol-aluminium complex) formed by vacuum deposition, for example. When that thin film is formed by an ordinary vacuum deposition process, it is not obtained in a crystal state, but in an amorphous state.
In the related art, a crystalline thin film of Alq3 can be obtained with the vacuum deposition process by providing a layer of fulleren as an underlying layer (see, e.g., Japanese Unexamined Patent Application Publication No. 10-41070). This Publication also discloses that, by employing, as a light emitting source, a crystalline Alq3 thin film formed by the disclosed method, the light emission efficiency of an organic EL device can be enhance from that in the case employing, as a light emitting source, a crystalline Alq3 thin film formed by the ordinary vacuum deposition process.
Also, in the related art, a crystalline organic thin film can be formed using a liquid phase process. Depending on the kinds of materials, a crystalline organic thin film can be formed, for example, by a method of spin-coating a solution of an organic substance. Examples of available materials includes a-sex-ithiophene, hexadecafluorocopper phthalocyanine, and naphthalene tetracarboxyl diimide.
On the other hand, a functional thin film is patterned when used in many electronic devices. However, it is difficult to pattern a crystalline organic thin film by an ordinary patterning process including photolithography and etching because of low resist resistance of the organic substance. The related art regarding various crystalline organic thin films of the above-mentioned type do not pattern the crystalline organic thin films. Accordingly, a selectable range of functional thin film materials for practical use is restricted and any desired material cannot always be employed. Furthermore, although crystallinity directly affects physical properties, the related art does not provide details regarding that effect.
If it is possible to arrange a very small amount of solution of an organic substance in a predetermined position on a substrate by an ink jet method and to crystallize the arranged solution, a patterned crystalline organic thin film can be easily formed on the substrate. Also, this method is able to form a crystalline thin film from all kinds of materials that can be dissolved into a solution, and to produce a perfect crystal (single crystal) in principle.
A thin film formed in the related art by the ink jet method is made of a high-molecular compound. Such a thin amorphous film made of a high-molecular compound can be readily formed by dissolving the high-molecular compound in a solvent to obtain a solution, arranging the solution on a substrate by the ink jet method, and evaporating the solvent from the arranged solution.
However, when making an effort to form a thin film using a compound having a relatively low molecular weight and not classified into high-molecular compounds (hereinafter “low-molecular compound”) by the ink jet method similarly to the above-mentioned case using a high-molecular compound, a thin film made of the low-molecular compound is not formed and particles of the low-molecular compound are deposited on the substrate.
Such a result is attributable to the fact that, even with the substrate treated to become lyophilic, the cohesion among molecules of the low-molecular compound is much greater than the binding force between the low-molecular compound and the substrate. Thus, because a low-molecular compound has a great cohesion, it is a material having high crystallinity.
Further, when ejecting a solution by the ink jet method and arranging droplets on a substrate, if a partial pressure of a gas made up of the same components as those of a solvent for the solution forming the droplets is not uniform around each droplet, the droplet is distorted and tends to easily move toward the side in which the partial pressure is higher. That movement of the droplets is apt to particularly occur when a solution of a low-molecular compound having a weak binding force with respect to the substrate is ejected. Then, the movement of the droplets raises a difficulty in forming a thin film in a predetermined position on the substrate. As a result, the ununiformity of the partial pressure of the gas prevents the thin film from crystallized.
For example, when a plurality of droplets are formed along a line at small intervals between the droplets, the partial pressure around the droplet formed at the most end of the line is not uniform because the gas partial pressure is lower on the side where the adjacent droplet is not present and higher on the side where the adjacent droplet is present. The-reason that this occurs is because, around the droplet formed at the most end of the line, the gas partial pressure is higher on the side, in which the adjacent droplet is present, with the presence of solvent vapors vaporized from the adjacent droplet.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a method capable of crystallizing a very small amount of solution arranged in a predetermined position on a substrate so that a patterned crystalline thin film (particularly an organic thin film) can be easily formed on the substrate by the ink jet method.
To address or achieve the above, the present invention provides a thin film forming method including: arranging, on a substrate, a plurality of droplets of a solution prepared by dissolving a thin film forming material in a solvent, and evaporating the solvent from each droplet, thereby forming a thin film on the substrate. The gas partial pressure around each droplet formed on the substrate is kept uniform to prevent the above-mentioned movement of the droplet so that a thin film of a low-molecular compound can be formed in a predetermined position on the substrate.
Thus, to address or achieve the above, the present invention provides a thin film forming method including: arranging, on a substrate, droplets of a solution prepared by dissolving a thin film forming material in a solvent, controlling a partial pressure of a gas made up of the same components as those of the solvent in the vicinity of the droplets, thereby creating crystalline nuclei in the droplets, and then growing the crystalline nuclei to form a crystalline thin film.
Also, as a thin film forming method for practically implementing a method of controlling the partial pressure, the present invention provides a thin film forming method including: arranging, on a substrate, droplets of a solution prepared by dissolving a thin film forming material in a solvent, creating crystalline nuclei in the droplets by controlling a partial pressure of a gas made up of the same components as those of the solvent in the vicinity of each of the arranged droplets to a first partial pressure (e.g., a partial pressure equal to or substantially equal to the saturation vapor pressure) under which the solution forming the droplets is brought into a supersaturated state, and after creation of the crystalline nuclei, lowering the partial pressure of the gas in the vicinity of the droplets to a second partial pressure (e.g., {fraction (1/10)} to {fraction (1/100)} of the saturation vapor pressure) under which the crystalline nuclei are able to gr
Masuda Takashi
Morii Katsuyuki
Oliff & Berridg,e PLC
Pianalto Bernard
Seiko Epson Corporation
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