JPH088147B2 - Thin film EL device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film EL element used for displaying characters and figures, and more particularly to a thin film EL element having stable light emission characteristics over a long period of time.

Thin film EL elements are thin and have excellent display visibility.
It is most suitable as a terminal display for OA equipment.

2. Description of the Related Art Conventionally, a thin film EL display panel in which a thin film EL element has an XY matrix structure has been known. This panel has horizontal parallel electrode groups and vertical parallel electrode groups arranged on both sides of a laminated thin film of a first dielectric layer / a phosphor layer / a second dielectric layer so as to be orthogonal to each other, and is connected to each electrode group. A power supply line applies a signal through the switching device to cause the phosphor layer at the intersection of both electric poles to emit light (the light emitting surface of this intersection is called a picture element), and depending on the combination of the picture elements that emit light, a character symbol, a figure Is displayed.

In the thin film EL display panel, a transparent parallel electrode group made of indium oxide doped with tin is formed on a transparent substrate usually made of glass, and a first dielectric layer is formed on the transparent parallel electrode group.
A phosphor layer and a second dielectric layer are sequentially laminated, and a back parallel electrode group generally made of Al metal is laminated on the phosphor layer and the second dielectric layer so as to be orthogonal to the transparent parallel electrode group.

As the phosphor layer, generally, a ZnS matrix doped with Mn or a rare earth element as a light emitting center, or a CaS or SrS matrix doped with Ce, Eu or the like as a light emitting center is used.

For the first and second dielectric layers, Y ₂ O ₃ , SiO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ , Sm ₂ O ₃ ,
A dielectric thin film selected from Si ₃ N ₄ , BaTiO ₃ , PbTiO ₃ , BaTa ₂ O _{6 and the} like is used.

The dielectric layer functions as a current limiter that controls the current flowing in the phosphor layer, is important for preventing electrical breakdown of the EL display panel and maintaining the withstand voltage reliability.

Furthermore, the dielectric layer has a great influence on the temporal change of the brightness-voltage characteristic (B / V characteristic), and if the B / V characteristic changes with the driving time, the image is disturbed and the contrast characteristic deteriorates. As far as possible, it is necessary to select a dielectric thin film whose change with time is small.

Problems to be Solved by the Invention When a thin film EL element having a matrix-like electrode is line-sequentially driven by a simultaneous inversion method (see Japanese Patent Publication No. 55-27354), and light is emitted twice in one scanning period, a transparent electrode is used. In each picture element sandwiched between the back electrode and the back electrode, the time from the application of the positive pulse to the application of the reverse pulse and the application of the reverse polarity pulse depend on the location of the picture element. The time from the start of application of the positive pulse differs. When the thin film EL element is driven for a long time by the conventional technique due to the drivability in which the phases of the positive and reverse pulses are different as described above,
The picture element that emits light according to the display information has a problem that the light emission starting voltage fluctuates by about 10 V as compared with the picture element that does not emit light.

An object of the present invention is to solve the above-mentioned problems, and to drive a thin film E having stable B / V characteristics for a long time even when driven by AC pulses having different phases or AC pulses having different amplitudes in the forward and reverse directions.
It is to provide an L element. At the same time, another object of the present invention is to provide a thin film EL device having high adhesion between thin films and excellent withstand voltage characteristics.

Means for Solving the Problems The present invention provides a transparent electrode, a first dielectric layer, E on a porous substrate.
In a thin film EL device in which an L phosphor layer, a second dielectric layer, and a back pole are sequentially laminated, Al ₂ in which the first dielectric layer and the second dielectric layer are AlN on the light emitting layer side O ₃ and
A two-layer composite dielectric layer of AlN is used.

It is considered that the fluctuation of the light-emission start voltage is due to the formation of new trap levels at various depths at the interface between the EL phosphor layer, the dielectric layer, and the dielectric layer. By making AlN the dielectric thin film that contacts the EL phosphor layer and forms an important interface,
It is considered that the formation of new trap levels was suppressed and stable B / V characteristics were exhibited for a long time.

Since AlN has difficulty in adhering to a transparent electrode mainly composed of indium oxide or an Al electrode, AlN in which an Al ₂ O ₃ thin film is further stacked on AlN
Adhesion between all thin films is excellent by forming the first and second dielectric layers with Al ₂ O ₃ composite dielectric layer and reduction of film defects such as pinholes by using the composite dielectric layer Thus, a thin film EL element with excellent withstand voltage characteristics could be created.

Example 1 FIG. 1 shows a cross-sectional structure of a thin film EL device according to the present invention. In the figure, 1 is a glass substrate, on which an indium-tin mixed crystal oxide thin film (hereinafter abbreviated as ITO thin film) is formed by an alloy targate by a DC sputtering method and processed into a stripe shape by a photolithography technique. Then, the transparent electrode 2 was obtained. A composite film of an Al ₂ O ₃ thin film ₄ and an AlN thin film 5 was formed thereon as a first dielectric layer 3. Al ₂ O
_The film thickness of the ₃ and AlN thin films is 1000Å each. Both were prepared by high frequency magnetron active sputtering method using Al metal target. The Al ₂ O ₃ thin film can be formed by the active electron beam heating vapor deposition method (EB vapor deposition method), but using the Al metal target as described above, simply sputtering gas is used.
It is convenient to use the active sputtering method, which can create a laminated film of Al ₂ O ₃ and AlN thin film by one pump down by switching from O ₂ to N ₂ . First, the conditions for forming the Al ₂ O ₃ thin film will be described. Active sputtering was carried out in an O ₂ + Ar atmosphere, targeting metallic Al. A suitable flow ratio of O ₂ and Ar is about 1: 3. Although a larger amount of O ₂ may be used, it is advantageous in terms of production to include the above-mentioned divided Ar in order not to delay the sputter rate. The sputtering pressure is 4 to 7 × 10 ⁻³ Torr, the substrate temperature is 150 to 400 ° C., and the power density is ² to 3 W / cm ² , and an Al ₂ O ₃ thin film suitable for the EL device of the present invention can be formed. Next, for the production of AlN, active sputtering was continuously performed using the same Al metal target in an N ₂ atmosphere containing 50% or less of Ar. Since the nitriding reaction may not be sufficient in an atmosphere containing more than 50% of Ar, it is preferable to set the content to 50% or less as described above. Sputtering gas pressure is 6 × 10 ^{-3 to} 6 × 10 ^-2 Torr, substrate temperature is 200 to 500
The desired AlN thin film can be formed at ℃ and power density of 2-3 W / cm ² .

Under the conditions described above, Al ₂ O ₃ and AlN thin films each having a thickness of 1000 Å were sequentially laminated, and then ZnS: Mn phosphor thin film 6 was formed to a thickness of 4000 Å by EB evaporation. The Mn content of the luminescence sensor was 0.8 atomic%.

After forming a thin film, 550 ℃, 1 in a vacuum atmosphere to increase brightness
Heat treatment was performed for an hour.

Next, the second dielectric layer 7 was formed on the phosphor layer 6.
In exactly the same manner as the first dielectric layer 3, simply formed by reversing the order of creating Al ₂ O ₃ and AlH films were arranged so that the AlN thin film is in contact with the fluorescent body layer 6. Similar to the case of the first dielectric layer
The film thickness was 1000Å, and the composite dielectric layer was 2000Å.

Finally, after attaching the Al thin film to the Al back electrode 8 by the EB vapor deposition method,
A thin film EL device was completed by processing it into a stripe shape orthogonal to the ITO electrode using photolithography technology.

The thin film EL device according to one embodiment of the present invention and the conventionally used dielectric thin films Y ₂ O ₃ , Ta ₂ O ₅ , and Al ₂ O ₃ were used for the first and second dielectric layers. As shown in FIG. 2, AC pulse voltages having different phases were applied to the device (thickness of each of the first and second dielectric layers was 2000Å). Applied voltage V _H is thin film EL
The light emission threshold voltage V _th of the device was further increased by 30 V. Since the EL element having the above-mentioned film thickness configuration, which is an embodiment of the present invention, has a light emission threshold voltage of 160V, _VH of 190V was applied. In general, the light emission threshold voltage largely depends on the film thickness of each thin film, and particularly on the dielectric constant of the dielectric thin film. The thinner the film thickness and the larger the dielectric constant, the lower the light emission threshold voltage.

Applying AC pulse as shown in Fig. 2 to each thin film EL device,
The variation of the emission threshold voltage with time was examined. The results are shown in FIG. In the conventional device using Y ₂ O ₃ , Ta ₂ O ₅ and Al ₂ O ₃ as the dielectric, the emission threshold voltage decreased by about 7% in 100 hours (Fig. 3, a, b, c). In the thin film EL device of the present invention, it was 0.8% or less (Fig. 3d). The EL device of the present invention showed almost no decrease in the light emission threshold voltage after 5000 hours. The reason why the emission threshold voltage is shifted by the application of AC pulse voltage with different phases is considered to be that the distribution and density of the interface trap levels at the interface between the fluorescent layer and the dielectric layer changed with the driving time. But
It can be said that the interface between ZnS and AlN was very suppressed and stabilized. It is theoretically sufficient if the AlN thin film is in contact with both surfaces of the ZnS: Mn phosphor thin film to form an interface, and the AlN thin film may simply form the first and second dielectrics. However, in that case, the AlN thin film did not have sufficient adhesion to the ITO or Al electrode, and sometimes peeling occurred at the interface with these thin films during manufacturing.
As a method of preventing this and further improving the withstand voltage characteristic, the first and second dielectric layers are made of AlN and A as described in the examples.
The composite dielectric layer was an l ₂ O ₃ thin film. As a result, an EL element excellent in withstand voltage characteristics was obtained by eliminating the unstable element in manufacturing such as the variation of the light emission threshold voltage and the peeling between the thin films.

That is, each of the first and second dielectric layers has a thickness of 2000 Å
Compared with the case of AlN, a composite film of 1000 Å AlN and 1000 Å Al ₂ O ₃ thin film has a dielectric breakdown and pinhole density is 1/10.
The dielectric breakdown strength of the dielectric thin film itself was doubled from 6 MV / cm to 12 MV / cm. The thickness of the composite dielectric layer must be adjusted according to the thickness of the fluorescent layer,
It has been experimentally investigated that AlN occupies a thickness of at least 500Å in order to play an important part in determining the interface characteristics with ZnS: Mn in AlN and Al ₂ O ₃ as a whole. Therefore, the first and second dielectric thin films should have an A of at least 500Å in contact with ZnS: Mn.
Al ₂ O of appropriate thickness considering lN and necessary withstand voltage of EL element
You can combine _three .

The EL structure of the present invention is also effective for the EL light-emitting layer 6 even for ZnS phosphors containing, for example, an active substance of a rare earth element other than Mn, or for those containing an active substance of Ce or Eu in CaS or SrS. It was
It is considered that this is because the interface state is almost determined by the species of the phosphor matrix and the dielectric, and that the CaS and SrS matrices are similar to ZnS as the phosphor.

EFFECTS OF THE INVENTION As described above, according to the present invention, it is possible to manufacture a thin film EL element which is excellent in withstand voltage characteristics and has a very small variation in the threshold voltage for light emission even when driven for a long time. It can be widely used for shapes and high-quality displays, and has great practical value.

[Brief description of drawings]

FIG. 1 is a sectional view showing the structure of a thin film EL element according to the present invention, FIG. 2 is a diagram showing a driving voltage waveform of the thin film EL element, and FIG.
The figure is a diagram showing changes with time in the light emission threshold voltage. 1 ... Glass substrate, 2 ... Transparent electrode, 3 ... First dielectric layer, 4 ... Al ₂ O ₃ thin film, 5 ... AlN thin film, 6 ... Fluorescent material layer, 7 ... Second dielectric Body layer, 8 ... Al back electrode.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yosuke Fujita 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Takao Nita, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. (72) Inventor Atsushi Abe 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) Reference JP 62-85089 (JP, A) JP 61-237396 (JP, A) JP Sho 63-174298 (JP, A) JP-A 63-119196 (JP, A)

Claims (1)

[Claims] 1. A thin-film EL device in which a transparent electrode, a first dielectric layer, a phosphor layer, a second dielectric layer and a back pole are sequentially laminated on a translucent substrate, wherein the first dielectric layer. And the second
Al ₂ whose dielectric layer is always AlN on the side in contact with the phosphor layer
Thin film E characterized by being a two-layer laminated composite film of O ₃ and AlN
L element.