1. Field of the Invention
The present invention relates to a film formation apparatus and a film formation method, which are used when a film formation material is purified by sublimation to form a film using the purified material in forming an EL element over a substrate.
2. Description of the Related Art
In recent years, there has been an active study of a light emitting device which has an EL element as a self light emitting element, and in particular, a light emitting device using an organic material as an EL material has attracted attention. This light emitting device is also called an organic EL display (OELD) or an organic light emitting diode (OLED).
Note that the EL element has a layer containing an organic compound in which electro luminescence is generated by applying electric field (hereinafter referred to as EL layer), an anode, and a cathode. The electro luminescence in the organic compound includes luminescence produced in returning from a singlet excitation state to a ground state (fluorescence) and luminescence produced in returning from a triplet excitation state to a ground state (phosphorescence). A light emitting device manufactured by a film formation apparatus and a film formation method according to the present invention can be applied to the case where either luminescence is used.
A light emitting device has a characteristic that a problem with respect to an angle of a field of view is not caused because it is a self light emitting type which is different from a liquid crystal display device. That is, it is more suitable than the liquid crystal display device when a display is used in the outdoors, and thus various ways of use are proposed.
The EL element has a structure in which an EL layer is interposed between a pair of electrodes. The EL layer generally has a laminate structure. Typically, there is a laminate structure called “hole transport layer/light emitting layer/electron transport layer”, which is proposed by Tang, et al. of Eastman Kodak Company. Light emitting efficiency is very high in this structure, and thus, this structure is adapted to most of the light emitting devices which have been researched and developed at present.
In addition, a structure in which a hole injection layer, a hole transport layer, a light emitting layer, and an electron transport layer are laminated in this order on the anode is preferable. Also, a structure in which a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer are laminated in this order on the anode is preferable. The light emitting layer may be doped with a fluorescent pigment or the like. These layers may be formed using a low molecular system material or a polymer system material.
Note that all layers provided between the cathode and the anode in this specification is generically called the EL layer. Thus, the hole injection layer, the hole transport layer, the light emitting layer, the electron transport layer, and the electron injection layer, which are described above, are all included in the EL layer.
Also, in this specification, a light emitting element formed of a cathode, an EL layer, and an anode is called the EL element. There are two types, that is, a type in which an EL layer is formed between two kinds of stripe-shaped electrodes provided to intersect each other (simple matrix type) and a type in which an EL layer is formed between a pixel electrode which is connected with a TFT and arranged in matrix and a counter electrode (active matrix type).
The most serious problem in putting the EL element to practical use is the insufficiency of the element life. Also, deterioration of the element is recognized in the form that a non light emitting region (dark spot) is expanded with light emission for a long period of time due to deterioration of the EL layer which eventually becomes a problem.
An EL material for forming the EL layer is deteriorated by an impurity such as oxygen, water, or the like. Also, when other impurity is included in the EL material, there arises a fear that the deterioration of the EL layer is adversely influenced.
Conventionally, when film formation is performed by an evaporation method, an evaporated material is used without being processed . However, it is considered that an impurity is mixed into the evaporated material at evaporation. That is, there is a possibility that oxygen, water, and other impurity are mixed as one reason for the deterioration of the EL element.
Also, when the evaporated material has been purified in advance, the purity can be increased. However, there is a possibility that an impurity is mixed during a period until the evaporation is completed.