1. Field of the Invention
The present invention relates to the art of vacuum vapor deposition of electroluminescent materials on a substrate to form an electroluminescent device. More specifically, the present invention relates to an apparatus and a method designed to accomplish the vacuum vapor deposition of the electroluminescent materials and also to an organic electroluminescent device resulting therefrom.
2. Description of the Related Art
To perform the vacuum vapor deposition, a vacuum deposition system is widely utilized, which generally includes a vacuum chamber having a vapor source and a substrate disposed in face-to-face relation with each other within the vacuum chamber. With this vacuum deposition system, after the vacuum chamber has been evacuated, the vapor source is heated to fuse and vaporize, or sublimated to vaporize, to thereby produce a gaseous phase of the vapor source which is subsequently deposited on one surface of the substrate to form a deposited film. For heating the vapor source, numerous methods have been employed such, for example, as electron beam radiation in which electrons are accelerated towards the vapor source to heat it, resistance heating in which an electric power is supplied to a resistance element constructed in the form of a boat made of a high melting point metal such as tungsten or molybdenum while the vapor source is placed on the boat, and so on. While vapor molecules emanating from the vapor source as the latter is heated are effused rectilinearly in a direction normal to the vapor source, the space into which the vapor molecules are effused is kept evacuated and, therefore, the vapor molecules will travel a mean free pass of a few tens of meters with the rectilinearly travelling vapor molecules subsequently deposited on a substrate that is disposed in face-to-face relation with the vapor source.
Since the vapor molecules are effused rectilinearly in a direction normal to the vapor source as discussed above, the vapor molecules travel rectilinearly in such a direction as to scatter in all directions as shown by arrows in FIG. 14. If the substrate shown by 3 has a flat surface, the distance d1 from the vapor source 2 to a center area of the substrate 3 differs from the distance d2 from the vapor source 2 to an end area of the substrate 3 and, therefore, the amount of deposition of the vaporized material on that center area of the substrate 3 and that on the end area of the substrate 3 differ from each other, resulting in a problem in that the thickness of a film formed on the substrate 3 tends to become uneven over the surface of the substrate 3.
Also, since the vapor molecules from the vapor source 2 travel rectilinearly in such a direction as to scatter in all directions, there is a substantial amount of the vapor molecules that do not travel towards the substrate 3 and do neither deposit on nor participate in formation of the film on the surface of the substrate 3, resulting in waste of the material. Accordingly, problems have been encountered in that the efficiency of utilization of the material used in the vapor source 2 is low, that is, the yield is low and in that the deposition rate at which the film is formed on the surface of the substrate 3 is also low.
Some of the vapor molecules that are not deposited on the substrate 3 are deposited on an inner wall surface of the vacuum chamber. If vacuum vapor deposition is carried out using the same vacuum chamber, but by changing the material from one over to another, vacuum vapor deposition of the different material would results in re-heating and, hence, re-vaporizing the material that has been previously deposited on the inner wall surface of the vacuum chamber, which would eventually be mixed in and, hence, contaminate the film being then vacuum vapor deposited, resulting in reduction in purity of the resulting film.
In addition, if the material is deposited on the inner wall surface of the vacuum chamber in the manner described above, such a problem has been arisen that release of the vacuum chamber to the atmosphere permits moisture and gaseous components of the outside air to be occluded in the deposited material to such an extent as to hamper maintenance of the vacuum chamber under vacuum. Also, there has been a possibility of such a problem occurring that occlusion of the atmospheric moisture and gaseous components in the deposited material results in eventual separation of the deposited material from the inner wall surface of the vacuum chamber in the form of finely divided flaky debris which would constitute a cause of contamination of the substrate and the crucible such as the boat.
Considering that an organic material such as an organic electroluminescent material requires a relatively low heating temperature to be used during vacuum vapor deposition, some of the vapor molecules that are not deposited on the substrate 3 are apt to be deposited on the inner wall surface of the vacuum chamber and, moreover, the vapor molecules deposited on the inner wall surface of the vacuum chamber are susceptible to evaporation. Accordingly, there is a high possibility of problems occurring that are associated with deposition of such some of the vapor molecules on the inner wall surface of the vacuum chamber.
An organic thin film element is generally formed by successively laminating and depositing organic thin films on a substrate so as to exhibit individual functions. When it comes to the organic electroluminescent (EL) device, the organic electroluminescent device generally represents a layered structure in which organic thin films are successively layered on a transparent electrode on the substrate. Such layered structure is fabricated by successively vacuum vapor depositing a plurality of organic materials. Since U.S. Pat. No. 4,769,292 issued Sep. 6, 1988 to Tang, et al. and assigned to Eastman Kodak Company has been published, a method of doping a small quantity of a fluorescent light emitting material into an organic host material during the vacuum vapor deposition is largely practiced. By way of example, for formation of a green color element, it is well known to dope quinacridone into the host material, which is used in the form of an Alq3 (tris-(8-hydroxyquinoline) aluminum) layer, by means of codeposition in which quinacridone is simultaneously vacuum vapor deposited. Also, in order to form a high efficiency organic EL device, it is also well known to codeposit a hole transporting organic material and an acceptor for enhancing the hole injecting efficiency from an anode and also to form an electron injected layer by codeposition of the electron transporting organic material and a donor such as Li.
In those film forming methods in which the organic materials are vacuum vapor deposited, it is a general practice to dispose two or more vapor sources within a single vacuum chamber to achieve the vacuum vapor deposition, but in performing such codeposition, the concentration of the dopant is an important factor that determines the nature of the resultant film. In other words, in the case of the organic materials, the temperature at which they evaporate is low and, even when a dual vapor deposition is carried out, it often occurs that the respective vaporizing temperatures of two vapor sources are close to each other. Where one of the organic materials is deposited on, for example, a wall of the vacuum chamber, reheating and consequent re-evaporation of the organic material deposited on the wall surface of the vacuum chamber will adversely affect the doping concentration within the eventually formed film. Accordingly, even in such case, there is a high possibility of occurrence of such problems associated with deposition of some of the vapor molecules, not deposited on the substrate, on the inner wall surface of the vacuum chamber.
The present invention has been developed to overcome the above-described disadvantages.
It is accordingly an objective of the present invention to provide a vacuum vapor deposition apparatus in which, when an organic material is to be vapor deposited, the organic material can be deposited at a high deposition rate to a uniform film thickness and at a high yield and in which vacuum vapor deposition can be performed at a high purity while minimizing deposition of a vaporized matter within a vacuum chamber and maintaining a high quality of vacuum within the vacuum chamber.
In accomplishing the above and other objectives, the present invention provides a vacuum vapor deposition apparatus which includes a vacuum chamber having a plurality of vapor sources and a heater for heating the vapor sources to achieve vacuum vapor deposition on a surface of at least one substrate within the vacuum chamber, wherein at least one of the vapor sources utilizes an organic material. A space in which the vapor sources and the substrate confront each other is enclosed by a hot wall, which is heated to a temperature at which the organic material will be neither deposited nor decomposed. The organic material is vapor deposited on the surface of the substrate by heating the vapor sources while the vapor sources and the substrate are moved relative to each other.
Preferably, the vapor deposition is effected on the surface of the substrate by heating the vapor sources while the vapor sources and the hot wall are moved relative to each other.
In order to accomplish the relative movement between the substrate and the vapor sources, the vacuum vapor deposition apparatus preferably includes a substrate transport device for moving the substrate relative to the vapor sources. This substrate transport device is capable of controlling a speed of movement of the substrate. Preferably, the substrate transport device may have a capability of causing the substrate to move along a circular path.
The substrate to be handled with the vacuum vapor deposition apparatus of the present invention may be a length of strip, which is transported from a supply roll towards a take-up roll.
In a preferred embodiment of the present invention, the vacuum chamber may have at least one auxiliary chamber fluid-connected therewith. This auxiliary chamber can be evacuated. In this design, one of the vapor sources is selectively loaded or unloaded between the vacuum chamber and the auxiliary chamber.
A top opening of the hot wall adjacent the substrate preferably has a width as measured in a direction perpendicular to a direction of movement of the substrate, which is within the range of a value 20% greater than a width of the substrate as measured in a direction perpendicular to the direction of movement of the substrate, to a value 20% smaller than the width of the substrate. Also, the hot wall preferably encloses 60 to 90% of the space in which the vapor sources and the substrate confront each other.
In another preferred embodiment, a surface of the hot wall confronting the space in which the vapor sources and the substrate confront each other is made of a material hard to react with the organic material.
The present invention also provides a method of vacuum vapor depositing a plurality of materials on a surface of at least one substrate by heating vapor sources within a vacuum chamber. This method includes: using an organic material for at least one of the vapor sources; heating a hot wall to a temperature at which the organic material will be neither deposited nor decomposed; heating the vapor sources while the vapor sources and the substrate, which confront each other, are enclosed by the hot wall; and moving the substrate and the vapor sources relative to each other. The organic material used as the vapor source may be an organic electroluminescent material.
The present invention furthermore provides an organic electroluminescent device formed by depositing a plurality of organic electroluminescent materials on a surface of a substrate by means of the vacuum vapor deposition method discussed above.