1. Field of the Invention
The present invention relates to a deposition device and a deposition method forming a light emitting element with a film containing an organic compound that emits light upon application of electric field (hereinafter referred to as organic compound layer), as well as an anode and a cathode. Specifically, the present invention relates to a manufacturing of a light emitting element of lower drive voltage than before and of longer lifetime. The term light emitting device in this specification refers to an image display device or a light emitting device that employs as an element a light emitting element. Also included in the definition of the light emitting device are a module in which a connector, such as an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package), is attached to a light emitting element, a module in which a printed wiring board is provided on the tip of a TAB tape or a TCP, and a module in which an IC (integrated circuit) is mounted directly to a light emitting element by the COG (chip on glass) method.
2. Description of the Related Art
A light emitting element is an element that emits light when electric field is applied. Light emission mechanism thereof is said to be as follows. A voltage is applied to an organic compound film sandwiched between electrodes to cause form the molecular exciton by recombination of electrons injected from the cathode and holes injected from the anode at the luminescent center in the organic compound layer and, when resultant molecular exciton returns to base state, it releases energy in the form of light emission.
There are two types of molecular excitons from organic compounds; one is singlet exciton and the other is triplet exciton. This specification includes both cases where singlet excitation causes light emission and where triplet excitation causes light emission.
In a light emitting element as above, its organic compound film is usually a thin film having a thickness of less than 1 μm. In addition, the light emitting element does not need back light used in conventional liquid crystal displays because it is a self-luminous element and the organic compound film itself emits light. The light emitting element is therefore useful in manufacturing a very thin and light-weight device, which is a great advantage.
When the organic compound film is about 100 to 200 nm in thickness, for example, recombination takes place within several tens nanoseconds since carriers are injected based on the mobility of the carriers in the organic compound film. Even the process from carrier recombination to light emission is taking into account, the organic light emitting element may be ready for light emission within an order of microsecond. Accordingly, fast response is also one of the features of the light emitting element.
Since the light emitting element is of carrier injection type, it can be driven with direct-current voltage and noise is hardly generated. Regarding drive voltage, a report says that a sufficient luminance of 100 cd/m2 is obtained at 5.5 V by using a very thin film with a uniform thickness of about 100 nm for the organic compound film, choosing an electrode material capable of lowering a carrier injection barrier against the organic compound film, and introducing the hetero structure (two-layer structure) (Reference 1: C. W. Tang and S. A. VanSlyke. “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, no. 12, 9)13-915 (1987)).
With those features, including thin/light-weight, fast response, and direct low voltage driving, light emitting elements are attracting attention as next-generation fiat panel display elements. In addition, for their being self-luminous and wide viewing angle, light emitting elements have better visibility and are considered as effective when used for display screens of electric appliances.
In the light emitting element disclosed in Reference 1, the carrier injection barrier is lowered by using a Mg:Ag alloy that is low in work function and is relatively stable for the cathode so that more electrons are injected. This makes it possible to inject a large number of carriers into the organic compound film.
Further, a single hetero structure, in which a hole transporting layer formed of diamine compound and an electron transporting light emitting layer formed of tris(8-quinolinolate)aluminum complex (hereinafter referred to as Alq3) are layered as the organic compound film, is adopted to improve the carrier recombination efficiency exponentially. This is explained as follows.
In the case of a light emitting element whose organic compound film consists of a single layer of Alq3, for example, most of electrons injected from the cathode reach the anode without being recombined with holes and the light emission efficiency is very low. In short, a material that can transport electrons and holes both in balanced amounts (hereinafter referred to as bipolar material) has to be used in order that a single layer light emitting element can emit light efficiently (i.e., in order to drive at low voltage), and Alq3 does not meet the requirement.
On the other hand, when the single hetero structure as the one in Reference 1 is adopted, electrons injected from the cathode are blocked at the interface between the hole transporting layer and the electron transporting light emitting layer and trapped in the electron transporting light emitting layer. Recombination of the carriers thus takes place in the electron transporting light emitting layer with high efficiency, resulting in efficient light emission.
Expanding this idea of carrier blocking function, it is possible to control the carrier recombination region. To give an example, there is a report of success in making a hole transporting layer to emit light by inserting a layer that can block holes (hole blocking layer) between the hole transporting layer and an electron transporting layer and trapping the holes in the hole transporting layer. (Reference 2: Yasunori KIJIMA. Nobutoshi ASAI and Shin-ichiro TAMURA, “A Blue Organic Light Emitting Diode”, Japanese Journal of Applied Physics, vol. 38. 5274-5277 (1999)).
It can be said that the light emitting element in Reference 1 is characterized by separation of functions of the hole transporting layer and the electron transporting light emitting layer in which the former layer is assigned to transport holes and the latter layer is assigned to transport electrons and emit light. The idea of separating functions has been expanded to a double hetero structure (three-layer structure) in which a light emitting layer is sandwiched between a hole transporting layer and an electron transporting. (Reference 3: Chihaya ADACHI, Shizuo TOKITO. Tetsuo TSUTSUI and Shogo SAITO, “Electroluminescence in Organic Films with Three-layered Structure”, Japanese Journal of Applied Physics, Vol. 27, No. 2′ L-69-L271 (1988)).
An advantage of this separation of functions is an increased degree of freedom in molecule design and the like, for the separation of functions saves one organic material from bearing various functions (such as light emission, carrier transportation, and injection of carriers from electrodes) simultaneously (for instance, the separation of functions makes the effort to find a bipolar material unnecessary). In other words, high light emission efficiency can easily be obtained by simply combining a material excellent in light emission characteristic with a material excellent in carrier transportation ability.
Because of these advantages, the idea itself of laminate structure described in Reference 1 (carrier blocking function or separation of functions) continues to be utilized widely.
Also, in the case of manufacturing these light emitting elements, in particular, in the case of a mass production process, when a hole transport material, a light emitting layer material, an electron transport material, and the like are laminated by vacuum evaporation, an in-line system (multi-chamber system) film formation apparatus is used so as not to contaminate respective materials. Note that FIG. 15 is a top view of the film formation apparatus.
According to the film formation apparatus shown in FIG. 15, formation of a three layers structure (double heterostructure) of a hole transport layer, a light emitting layer, and an electron transport layer, vapor-deposition of a cathode, and sealing processing may be performed on a substrate having an anode (such as ITO).
First, the substrate having the anode is loaded to a loading chamber. The substrate is transferred to an ultraviolet ray irradiation chamber via a first transferring chamber and ultraviolet irradiation is performed in a vacuum atmosphere to clean the surface of the anode. Note that, when the anode is an oxide such as ITO, oxidation processing is performed in a pretreatment chamber.
Next, a hole transport layer is formed in an evaporation chamber 1501, light emitting layers (three colors of red, green, and blue in FIG. 15) are formed in evaporation chambers 1502 to 1504, an electron transport layer is formed in an evaporation chamber 1505, and a cathode is formed in an evaporation chamber 1506. Finally, sealing processing is performed in a sealing chamber and a light emitting element is obtained from an unloading chamber.
A feature of such an in-line system film formation apparatus is to perform evaporations of respective layers in different evaporation chambers 1501 to 1505. Therefore, in general, it is sufficient to provide one evaporation source (1511 to 1515) in each of the evaporation chambers 1501 to 1505. Note that, when the light emitting layers are formed in the evaporation chambers 1502 to 1504 by pigment doping, there is a case where two evaporation sources are required to form an coevaporation layer. In other words, the apparatus is constructed such that almost no mixing of respective layer materials with one another will occur.
A structure of a light emitting element manufactured using the film formation apparatus described in FIG. 15 is shown in FIGS. 16A and 16B. In FIGS. 16A and 16B, an organic compound layer 1604 is formed between an anode 1602 and a cathode 1603, which are formed on a substrate 1601. Here, with respect to the formed organic compound layer 1604, different organic compounds are formed in different evaporation chambers. Thus, laminate interfaces between a first organic compound layer 1605, a second organic compound layer 1606, and a third organic compound layer 1607 thus formed are clearly separated.
Now, a region 1608 near an interface between the first organic compound layer 1605 and the second organic compound layer 1606 is shown in FIG. 16B. From this drawing, it is apparent that impurities 1610 are mixed into an interface 1609 between the first organic compound layer 1605 and the second organic compound layer 1606. In other words, in the case of a conventional film formation apparatus shown in FIG. 15, the respective layers are formed in separate film formation chambers. Therefore, when the substrate is moved between the film formation chambers, the impurities 1610 are adhered onto the surface of the substrate and thus mixed into the interface 1609. Note that the impurities as described here specifically refer to oxygen, water, and the like.
Since the laminate structure described above is formed by a junction among different types of materials, an energy barrier is necessarily generated in the interface. If the energy barrier exists, movement of a carrier in the interface is hindered and thus the following problems result.
First, one problem is that the energy barrier becomes a hindrance to a further reduction in a drive voltage. Actually, it is reported that, in terms of a drive voltage of a current light emitting element, an element having a single layer structure using a conjugate polymer is superior and it attains top data (note that comparison for light emission from a singlet excitation state is performed) in power efficiency (unit: [lm/W]) (Reference 4: Tetsuo Tsutsui. “The Japan Society of Applied Physics, Organic Molecule and Bioelectronics Division”, Vol. 11, No. 1, P. 8 (2000)).
Note that the conjugate polymer described in reference 4 is a bipolar material and a level equal to that attained in the laminate structure can be achieved with respect to recombination efficiency of a carrier. Thus, as far as equal recombination efficiency of the carrier can be attained without using a laminate structure, by using a bipolar material or the like, actually lower drive voltage is attained with the single layer structure having fewer interfaces.
There is a method of inserting a material for relaxing the energy barrier in an interface with, for example, an electrode to thereby improve injection efficiency of the carrier and thus reduce the drive voltage. (Reference 5: Takeo Wakimoto, Yoshinori Fukuda, Kenichi Nagayama. Akira Yokoi, Hitoshi Nakada, and Masami Tsuchida, “Organic EL Cells Using Alkaline Metal Compounds as Electron Injection Materials”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 44. NO. 8, 1245-1248 (1997)) In Reference 5, Li2O is used for the electron injection layer to achieve the reduction in the drive voltage.
However, the mobility of the carrier in an interface between organic materials (for example, an interface between the hole transport layer and the light emitting layer, and hereinafter referred to as an organic interface) is still an unresolved issue, which is considered as important in attaining a low drive voltage achieved in the single layer structure.
Further, the influence on the element life of the light emitting element is considered as a problem resulting from the energy barrier. That is, there is a reduction in luminance due to storage of a charge resulting from hindered carrier mobility.
A clear theory with respect to this deterioration mechanism is not yet established. However, there is a report that, by inserting the hole injection layer between the anode and the hole transport layer and by performing ac drive by rectangular waves instead of dc drive, the reduction in the luminance can be suppressed. (Reference 6: S. A. VanSlyke. C. H. Chen, and C. W. Tang, “Organic electroluminescent devices with improved stability”, Applied Physics Letters. Vol. 69. No. 15, 2160-2162 (1996)) It is said that this is an experimental support such as the storage of a charge is prevented by the insertion of the hole injection layer and the ac drive and thus the reduction in the luminance can be suppressed.
Thus, with respect to the laminate structure, it has an advantage of easily improving recombination efficiency of a carrier and of extending the range of choice of materials in view of functional separation. On the other hand, since a large number of organic interfaces are produced, mobility of the carrier is hindered, which negatively affects the reduction in the drive voltage and the luminance.
Also, in the case of a conventional film formation apparatus, when the hole transport material, the light emitting layer material, the electron transport material, and the like are laminated by vacuum evaporation, evaporation sources are separately provided in separate chambers so as not to contaminate respective materials and different layers are separately formed in different chambers. However, in the case of such an apparatus, when forming the above laminate structure, there is a problem in that not only the organic interfaces are clearly distinguished but also that an impurity such as water or oxygen is mixed into the organic interfaces when the substrate is moved between the chambers.