Thin-film EL device, and its fabrication process

Details Thin-film EL device, and its fabrication process Thin-film EL device, and its fabrication process

2001-05-30

2003-06-10

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

C313S509000, C427S066000

Reexamination Certificate

active

06577059

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Art Field
This invention relates to a thin-film EL device having at least a structure comprising an electrically insulating substrate, a patterned electrode layer stacked on the substrate, and a dielectric layer, a light-emitting layer and a transparent electrode layer stacked on the electrode layer.
2. Background Art
EL devices are now practically used in the form of backlights for liquid crystal displays (LCDs) and watches.
An EL device works on a phenomenon in which a substance emits light at an applied electric field, viz., an electro-luminescence (EL) phenomenon.
The EL device is broken down into two types, one referred to as a dispersion type EL device having a structure wherein electrode layers are provided on the upper and lower sides of a dispersion with light-emitting powders dispersed in an organic material or porcelain enamel, and another as a thin-film EL device using a thin-film light-emitting substance provided on an electrically insulating substrate and interposed between two electrode layers and two thin-film insulators. These types of EL devices are each driven in a direct or alternating voltage drive mode. Known for long, the dispersion type EL device has the advantage of ease of fabrication; however, it has only limited use thanks to low luminance and short service life. On the other hand, the thin-film EL device has recently wide applications due to the advantages of high luminance and very long-lasting quality.
The structure of a typical double-insulation type thin-film EL device out of conventional thin-film EL devices is shown in FIG.
2
. In this thin-film EL device, a transparent substrate
21
formed of a green glass sheet used for liquid crystal displays or PDPs is stacked thereon with a transparent electrode layer
22
comprising an ITO of about 0.2 &mgr;m to 1 &mgr;m in thickness and having a given striped pattern, a first insulator layer
23
in a transparent thin-film form, a light-emitting layer
24
of about 0.2 &mgr;m to 1 &mgr;m in thickness and a second insulator layer
25
in a transparent thin-film form. Further, an electrode layer
26
formed of, e.g., an Al thin-film patterned in a striped manner is provided in such a way as to be orthogonal with respect to the transparent electrode layer
22
. In a matrix defined by the transparent electrode layer
22
and the electrode layer
26
, voltage is selectively applied to a selected given light-emitting substance to allow a light-emitting substance of a specific pixel to emit light. The resultant light is extracted from the substrate side. Having a function of limiting currents flowing through the light-emitting layer, such thin-film insulator layers make it possible to inhibit the dielectric breakdown of the thin-film EL device, and so contribute to the achievement of stable light-emitting properties. Thus, the thin-film EL device of this structure has now wide commercial applications.
For the aforesaid thin-film transparent insulator layers
23
and
25
, transparent dielectric thin films of Y
2
O
3
, Ta
2
O
5
, Al
3
N
4
, BaTiO
3
, etc. are formed at a thickness of about 0.1 to 1 &mgr;m by means of sputtering, evaporation or the like.
For light-emitting materials, ZnS with yellowish orange light-emitting Mn added thereto has mainly been used due to ease of film formation and in consideration of light-emitting properties. For color display fabrication, the use of light-emitting materials capable of emitting light in the three primary colors, red, green and blue is inevitable. These materials known so far in the art, for instance, include SrS with blue light-emitting Ce added thereto, ZnS with blue light-emitting Tm added thereto, ZnS with red light-emitting Sm added thereto, CaS with red light-emitting Eu added thereto, ZnS with green light-emitting Tb added thereto, and CaS with green light-emitting Ce added thereto.
In an article entitled “The Latest Development in Displays” in “Monthly Display”, April, 1998, pp. 1-10, Shosaku Tanaka shows ZnS, Mn/CdSSe, etc. for red light-emitting materials, ZnS:TbOF, ZnS:Tb, etc. for green light-emitting materials, and SrS:Cr, (SrS:Ce/ZnS)
n
, Ca
2
Ga
2
S
4
:Ce, Sr
2
Ga
2
S
4
:Ce, etc. for blue light-emitting materials as well as SrS:Ce/ZnS:Mn, etc. for white light-emitting materials.
IDW (International Display Workshop), ′97 X. Wu “Multicolor Thin-Film Ceramic Hybrid EL Displays”, pp. 593-596 shows that SrS:Ce out of the aforesaid materials is used for a thin-film EL device having a blue light-emitting layer. In addition, this publication shows that when a light-emitting layer of SrS:Ce is formed by an electron beam evaporation process in a H
2
S atmosphere, it is possible to obtain a light-emitting layer of high purity.
However, a structural problem with such a thin-film EL device remains unsolved. The problem is that since the insulator layers are each formed of a thin film, it is difficult to reduce to nil steps at the edges of the pattern of the transparent electrode, which occur when a large area display is fabricated, and defects in the thin-film insulators, which are caused by dust, etc. occurring in the process of display production, resulting in a destruction of the light-emitting layer due to a local dielectric strength drop. Such defects offer a fatal problem to display devices, and produce a bottleneck in the wide practical use of thin-film EL devices in a large-area display system, in contrast to liquid crystal displays or plasma displays.
To provide a solution to the defect problem with such thin-film insulators, JP-A 07-50197 and JP-B 07-44072 disclose a thin-film EL device using an electrically insulating ceramic substrate as a substrate and a thick-film dielectric material for the thin-film insulator located beneath the light-emitting substance. As shown in
FIG. 3
, this thin-film EL device has a structure wherein a substrate
31
such as a ceramic substrate is stacked thereon with a lower thick-film electrode layer
32
, a thick-film dielectric layer
33
, a light-emitting layer
34
, a thin-film insulator layer
35
and an upper transparent electrode
36
. Unlike the thin-film EL device shown in
FIG. 2
, the transparent electrode layer is formed on the uppermost position of the device because the light emitted from the light-emitting substance is extracted out of the upper side of the device facing away from the substrate.
The thick-film dielectric layer in this thin-film EL device has a thickness of a few tens of &mgr;m to a few hundred &mgr;m or is several hundred to several thousand times as thick as the thin-film insulator layer. Thus, the thin-film EL device has the advantages of high reliability and high fabrication yields because of little or no dielectric breakdown caused by pinholes formed by steps at electrode edges or dust, etc. occurring in the device fabrication process. The use of this thick-film dielectric layer leads to another problem that the effective voltage applied to the light-emitting layer drops. However, this problem can be solved or eliminated by using a high dielectric constant material for the dielectric layer.
However, the light-emitting layer stacked on the thick-film dielectric layer has a thickness of barely a few hundred nm that is about {fraction (1/100)} of that of the thick-film dielectric layer. For this reason, the thick-film dielectric layer must have a smooth surface at a level less than the thickness of the light-emitting layer. However, it is still difficult to sufficiently smooth down the surface of a dielectric layer fabricated by an ordinary thick-film process.
To be more specific, a thick-film dielectric layer, because of being essentially constructed of ceramics using a powdery material, usually suffers from a volume shrinkage of about 30 to 40% upon closely sintered. However, ordinary ceramics are closely packed through a three-dimensional shrinkage upon sintering whereas a thick-film ceramic material formed on a substrate does not shrink across the substrate because the thick film is constrained to the substrate; its volume shrinkage occurs in th

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