JPH046157Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a display device such as an EL light emitting element exhibiting good display characteristics, and more particularly to a display device equipped with a back electrode with higher contrast.

2. Description of the Related Art A typical structure of a thin film EL matrix type display panel as a display device will be described with reference to FIG. The right half of FIG. 3 is a sectional view in the X direction, and the left half is a sectional view in the Y direction orthogonal to the X direction. In FIG. 3, 1 is a glass substrate which is a transparent substrate, and 2 is a matrix type thin film EL element formed on the glass substrate 1. In FIG. In the thin film EL element 2, numeral 3 is a transparent electrode made of ITO or the like formed on the glass substrate 1 in the form of a large number of stripes at a constant pitch in the X direction by vapor deposition _; O ₃ , Y ₂
5 is a transparent first insulating layer formed by vapor deposition or sputtering of O ₃ , Ta ₂ O ₅ , etc.; 5 is a light emitting layer formed by vapor deposition of ZnS:Mn etc. on the first insulating layer 4; 6
7 is a transparent second insulating layer formed of Al ₂ O ₃ , Y ₂ O ₃ , Ta ₂ O ₅ or the like by vapor deposition or sputtering on the light emitting layer 5; 7 is a transparent second insulating layer formed on the second insulating layer 6; This is a back electrode made of an Al vapor deposited film formed in a number of stripes at regular pitches in the Y direction.

A thin film EL matrix type display panel is constructed by fixing a concave plate-shaped cover glass 8 onto the glass substrate 1 with an adhesive so as to surround the thin film EL element 2. In order to improve the moisture resistance of the thin film EL element 2, an insulating protective fluid such as silicone oil is sealed in the envelope consisting of the thin film EL element 1 and the cover glass 8 (Japanese Patent Application Laid-open No. 7086/1986).

In this thin film EL matrix type display panel, the transparent electrode 3 and the back electrode 7 intersect in a matrix to form a large number of pixels in a matrix.

When a driving voltage is selectively applied between the transparent electrode 3 and the back electrode 7, the pixel portion of the light emitting layer 5 selectively emits light, thereby displaying desired information in a dot matrix.

By the way, the thin film EL element 2 has a back electrode 7.
is formed with A vapor-deposited film, so if it is used in a place with strong external light such as sunlight, the external light will be reflected by the back electrode 7 and come out to the front of the panel.
There is a problem in that the contrast when lit is low.

Therefore, various back electrodes have been proposed to improve contrast. FIG. 4 shows a cross-sectional view of an example of a thin film EL panel equipped with a back electrode 9 made of a light-absorbing multilayer film. In the figure, the back electrode 9
The same parts as shown in FIG. 3 are designated by the same reference numerals and their explanations will be omitted.

This back electrode 9 consists of a dielectric film 11 such as aluminum oxide (Al ₂ O ₃ ), a metal film 10 and a metal film 12 with low reflectivity such as molybdenum (Mo), chromium (Cr), tantalum (Ta), etc. With a structure that holds the
It repeatedly absorbs external light. The metal film 12 is made of aluminum (Al), which is conventionally inexpensive and has a proven track record as a back electrode for thin-film EL display panels.
was often used (for example, JP-A-59-
Publication No. 146193).

A thin-film EL display panel equipped with a back electrode 9 made of such a light-absorbing multilayer film can absorb external light at the back electrode 9, so that the contrast is considerably improved.

Problems to be Solved by the Invention However, in the structure of the back electrode 9 in which the metal film 12 is made of aluminum (Al) as in the conventional example, sudden dielectric breakdown often occurs for reasons that are not clear. It was. In addition, the destruction mode was propagating, and the back electrode 9 was easily cut, which caused a problem in the reliability of the device.

Furthermore, since the metal film 12 is made of aluminum (Al), which has a high reflectance, black color cannot be obtained unless the film thicknesses of the metal semi-transparent film 10 and the dielectric film 11 are controlled very precisely. There were problems with color reproducibility and yield.

Means for Solving the Problems In a display device in which the back electrode is constituted by a light-absorbing multilayer film having a structure in which a dielectric film is sandwiched between a metal semi-transparent film having a low reflectance and a metal film, The transparent film is molybdenum (Mo) with a thickness of 50 Å to 300 Å, and the dielectric film has a thickness of 300 Å to 300 Å.
It is made of aluminum oxide (Al ₂ O ₃ ) with a thickness of 750 Å, and the top layer metal film is molybdenum (Mo) with a thickness of 300 Å or more.

Effect In this configuration, part of the external light is absorbed by the translucent molybdenum (Mo) film, and the transmitted part is repeated between the metallic molybdenum (Mo) film and the translucent molybdenum (Mo) film. In addition to being attenuated and absorbed,
The attenuation effect due to the interference effect of the light-absorbing multilayer film is also combined, and the reflectance of external light to the back electrode is significantly reduced, so it appears as black and the contrast is improved.

In addition, since the top layer metal film is also made of Mo, the reflectance decreases and the amount of light attenuation increases.Therefore, there is a margin for the amount of attenuation due to interference effects, so there is a tolerance for film thickness variation of the dielectric film. can be made larger, which is advantageous in manufacturing and reduces costs.

Furthermore, since the top layer metal film is made of Mo instead of Al, electrical breakdown voltage is improved, propagation type dielectric breakdown can be prevented, and disconnection of the back electrode can be prevented.

Embodiments Hereinafter, embodiments of this invention will be described with reference to the drawings.

Figure 1 shows a cross-sectional view of a thin film EL display panel implementing this idea. In the figure, except for the back electrode 13 made of a light-absorbing multilayer film, the second
Since it is similar to FIG. 3 and FIG. 3, the same parts are given the same reference numerals and the explanation thereof will be omitted. This back electrode 13 has a thickness formed on the second insulating layer 6.
A metal semitransparent film 14 with a low reflectance made of molybdenum (Mo) with a thickness of 10 nm, a dielectric film 15 made of aluminum oxide (Al ₂ O ₃ ) with a thickness of 45 nm formed on the metal semitransparent film 14, and a dielectric material It is formed of a light-absorbing multilayer film in which a metal film 16 made of molybdenum (Mo) with a thickness of 200 nm is laminated on a film 15.

Curve A in FIG. 2 shows that the first and second insulating layers 4 and 6 are made of tantalum pentoxide (Ta ₂ ) with a thickness of 500 nm by sputtering.
This is a characteristic diagram of the thickness versus reflectance of the dielectric film 15 made of aluminum oxide (Al ₂ O ₃ ) in a thin film EL display panel formed with O ₅ (refractive index 2.10).
Characteristic curve B of a conventional thin film EL display panel whose metal film is made of aluminum (Al) is also shown. As is clear from FIG. 2, the reflectance of the device equipped with the back electrode 13 according to this invention is 7 to 8% lower than that of the conventional product. Also,
The first and second insulating layers 4 and 6 are made of aluminum oxide (Al ₂ O ₃ , refractive index
The characteristic curve C for the panel formed in 1.63) is also
It shows almost the same reflectance as curve A.

In other words, when the metal film is an aluminum (Al) film, it can be similarly blackened, but the light absorption efficiency is poor due to its high reflectance, and dielectric films, etc.
Even if the film thickness deviates slightly from the set value, the absorption rate will drop significantly. For example, when the metal film is an aluminum (Al) film, a deviation of ±30 Å in thickness of the dielectric (aluminum oxide) film is fatal, but when the metal film is a molybdenum (Mo) film, a deviation of about ±60 Å is acceptable. can do. Increasing the allowable amount of film thickness variation is extremely advantageous in manufacturing and has the effect of reducing costs. Furthermore, the higher the refractive index of the second insulating layer 6, the greater the difference in absorption efficiency between metal films, making a molybdenum (Mo) film advantageous. For example, if the second insulating layer 6 is made of aluminum oxide (Al ₂ O ₃ ) with a refractive index of 1.63, the reflectance of the entire device is about 6 to 8% under optimal conditions, but the second insulating layer 6 In the case of tantalum pentoxide (Ta ₂ O ₅ ) with a refractive index of 2.10, the refractive index is within 8% when the metal film is molybdenum (Mo), but when the metal film is made of aluminum (Al), even the optimum value exceeds 15%.

Also, electrically, the metal film is made of molybdenum (Mo).
Experiments have shown that this has better pressure resistance and the failure mode is pinhole type.

Further, as a result of experiments, it was found that optimizing the light absorption function depends on the refractive index of the second insulating layer 6, but blackening occurs under the following conditions.

Translucent molybdenum (Mo) film 14:50≦x≦300
(Å) Aluminum oxide (Al ₂ O ₃ ) film 15: 200≦x≦800
(Å) Metallic molybdenum (Mo) film 16: x≦300 (Å) Furthermore, in conventional thin-film EL display panels that have a back electrode with a metal film formed from aluminum (Al), they can be driven at twice the rated voltage. At this time, 1/4 to 1/5 of the total number of pixels did not emit light due to dielectric breakdown, but the back electrode 1 with the metal film 16 formed of molybdenum (Mo) according to this invention
In the thin film EL display panel having No. 3, the number of pixels that failed to emit light under the same driving conditions was zero.

In the above embodiment, the present invention was applied to a thin-film EL display panel, but a display device in which a display medium is sandwiched between a transparent electrode and a back electrode and a voltage is applied between the two electrodes is applicable. It can be implemented similarly.

Note that the upper limit of the thickness of the metal molybdenum (Mo) film 16 is determined from other viewpoints such as compatibility with the extraction electrode, manufacturing cost, and the like. Practically 5000Å
The limit is

In addition, the optimum range for the aluminum oxide film, which is a dielectric film, is 300°C, where the reflectance is approximately 10% or less, as shown in Figure 2.
Å to 750 Å is desirable.

Effects of the invention As explained above, this invention uses the back electrode as
It is composed of a light-absorbing multilayer film with a dielectric film sandwiched between a semi-transparent metal film with low reflectivity and a metal film made of molybdenum (Mo), so it can be used easily compared to conventional aluminum (Al).
Compared to a display device with a back electrode made of a light-absorbing multilayer film using a metal film, it not only improves the contrast, but also allows rough control of the film thickness, making it easier to mass-produce. Furthermore, a highly reliable display device that is less likely to be destroyed can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a thin film EL display panel implementing this invention, and FIG. 2 is a characteristic diagram of the reflectance versus thickness of the dielectric film of the back electrode in the thin film EL display panel. FIG. 3 is a sectional view of a conventional thin film EL display panel, and FIG. 4 is a sectional view of a thin film EL display panel with improved contrast. 13...Back electrode, 14...Semi-transparent metal film, 1
5...Dielectric film, 16...Metal film.

Claims (1)

[Claims for Utility Model Registration] A display device in which a display medium is sandwiched between a front transparent electrode and a back electrode, and a voltage is applied between the two electrodes, wherein the back electrode is made of a metal semiconductor with low reflectance. In a display device configured with a light-absorbing multilayer film having a structure in which a dielectric film is sandwiched between a transparent film and a metal film, the metal semi-transparent film is made of molybdenum (Mo) with a thickness of 50 Å to 300 Å, The dielectric film is 300Å~
A display device characterized in that the film is made of aluminum oxide (Al ₂ O ₃ ) with a thickness of 750 Å, and the uppermost metal film is made of molybdenum (Mo) with a thickness of 300 Å or more.