Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2003-04-09
2004-11-16
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S498000
Reexamination Certificate
active
06819044
ABSTRACT:
TECHNICAL FIELD
This invention relates to a thin-film EL device having at least a structure comprising a lower electrode layer having a predetermined pattern, a lower insulating layer, a light emitting layer, and an upper electrode layer of a transparent conductive material stacked on an electrically insulating substrate. It also relates to a composite substrate for use in thin-film EL devices and various other display devices.
BACKGROUND ART
EL devices are on commercial use as backlight in liquid crystal displays (LCD) and watches.
The EL devices utilize the phenomenon that a material emits light upon application of an electric field, known as electroluminescent phenomenon.
The EL devices using inorganic phosphors include dispersion type EL devices of the structure that a dispersion of powder phosphor in organic material or enamel is sandwiched between electrode layers, and thin-film type EL devices in which a light emitting thin film sandwiched between a pair of insulating thin films and further between a pair of electrode layers is disposed on an electrically insulating substrate. For each type, the drive modes include DC voltage drive mode and AC voltage drive mode. The dispersion type EL devices are known from the past and have the advantage of easy manufacture, but their use is limited because of a low luminance and a short lifetime. On the other hand, the thin-film EL devices are currently on widespread use on account of a high luminance and a long lifetime.
FIG. 2
shows the structure of a dual insulated thin-film EL device as a typical prior art EL device. This thin-film EL device has a structure comprising a lower electrode layer
3
, a lower insulating layer
4
, a light emitting layer
5
, an upper insulating layer
6
, and an upper electrode layer
7
stacked on an electrically insulating substrate
2
. The substrate
2
is transparent and constructed, for example, of a soda-lime glass customarily used in liquid crystal displays and plasma display panels (PDP). The lower electrode layer
3
is a layer of indium tin oxide (ITO) having a thickness of about 0.2 to 1 &mgr;m. The lower and upper insulating layers
4
and
6
are thin films deposited by sputtering, evaporation or the like to a thickness of about 0.1 to 1 &mgr;m and usually formed of Y
2
O
3
, Ta
2
O
5
, Al
3
N
4
, BaTiO
3
or the like. The light emitting layer
5
has a thickness of about 0.2 to 1 &mgr;m. The upper electrode layer
7
is formed of a metal such as Al. The lower and upper electrode layers
3
and
7
are patterned into orthogonally extending stripes so that they constitute column and row electrodes, respectively. In this electrode matrix, the intersections between column and row electrodes make pixels. The matrix electrodes are controlled to apply an AC voltage or pulse voltage to a selected pixel whereby the light-emitting material at that site emits light which comes out from the substrate
2
side.
In this thin-film EL device, the lower and upper insulating layers
4
and
6
have a function of restricting the current flow through the light emitting layer
5
in order to restrain breakdown of the thin-film EL device and act so as to provide stable light-emitting properties. Thus thin-film EL devices of this structure find widespread commercial use.
Among phosphor materials of which the light-emitting layer
5
is made, Mn-doped ZnS exhibiting yellowish orange light emission has mainly been used for ease of film formation and light-emitting properties. For color display fabrication, it is inevitable to use light-emitting materials capable of emitting light in the three primary colors, red, green and blue. These materials known so far in the art, for instance, include Ce-doped SrS and Tm-doped ZnS exhibiting blue light emission, Sm-doped ZnS and Eu-doped CaS exhibiting red light emission, and Tb-doped ZnS and Ce-doped CaS exhibiting green light emission.
Shosaku Tanaka, “the Latest Development in Displays” in Monthly Display, April, 1998, pp. 1-10, discloses ZnS, Mn/CdSSe, etc. as red light-emitting materials, ZnS:TbOF, ZnS:Tb, etc. as green light-emitting materials, and SrS:Cr, (SrS:Ce/ZnS)
n
, CaGa
2
S
4
:Ce, SrGa
2
S
4
:Ce, etc. as blue light-emitting materials. Such light-emitting materials as SrS:Ce/ZnS:Mn are also disclosed as white light-emitting materials.
International Display Workshop (IDW), 1997, X. Wu, “Multicolor Thin-Film Ceramic Hybrid EL Displays”, pp. 593-596 discloses that among the aforesaid materials, SrS:Ce is used as a blue light-emitting layer in a thin-film EL device. In addition, this article discloses that when a light-emitting layer of SrS:Ce is formed, an electron beam evaporation process in a H
2
S atmosphere enables to form a light-emitting layer of high purity.
However, for these thin-film EL devices, a structural problem remains unsolved. When a large area display is fabricated, steps appear on the lower insulating layer
4
at the edges of the pattern of the lower electrode layer
3
, and dust and debris occurring during the process introduce defects into the lower insulating layer
4
. Since the lower insulating layer
4
is a thin film, it is difficult to reduce to nil such steps and defects, resulting in a destruction of the light-emitting layer
5
due to a local dielectric strength drop. These problems are fatal to display devices, and become 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 associated with such thin-film insulating layers, JP-B 07-44072 discloses an EL device using an electrically insulating ceramic substrate as the substrate
2
and a thick-film dielectric layer instead of a thin-film insulating layer as the lower insulating layer
3
. Since the EL device of the above patent is constructed such that light emitted by the light emitting layer
5
is extracted from the upper side remote from the substrate
2
as opposed to prior art thin-film EL devices, a transparent electrode layer is used as the upper electrode
7
.
Further, in this EL device, the thick-film dielectric layer is formed to a thickness of several tens to several hundreds of microns, which is several hundred to several thousand folds of the thickness of the thin-film insulating layer. This minimizes the potential of breakdown which is otherwise caused by steps in the lower electrode layer
3
and pinholes formed by debris during the manufacturing process, ensuring advantages of high reliability and high manufacturing yields. Meanwhile, the use of such a thick-film dielectric layer entails a problem of reducing the effective voltage applied across the light emitting layer
5
. For example, the above-referred JP-B 7-44072 overcomes this problem by constructing the thick-film dielectric layer from a lead-containing complex perovskite high-permittivity material.
However, the light emitting layer formed on the thick-film dielectric layer has a thickness of several hundreds of nanometers which is merely about {fraction (1/100)} of that of the thick-film dielectric layer. This requires that the thick-film dielectric layer on the surface be smooth at a level below the thickness of the light emitting layer. However, a conventional thick-film procedure is difficult to form a dielectric layer having a fully smooth surface.
Specifically, the thick-film dielectric layer is essentially constructed of a ceramic material obtained by sintering a powder raw material. Intense sintering generally brings about a volume contraction of about 30 to 40%. Unfortunately, although customary ceramics consolidate through three-dimensional volume contraction upon sintering, thick-film ceramics formed on substrates cannot contract in the in-plane directions of the substrate under restraint by the substrate, and is allowed for only one-dimensional volume contraction in the thickness direction. For this reason, sintering of the thick-film dielectric layer proceeds insufficiently, resulting in an essentially porous body. Moreover, since the surface roughnes
Ookoba Minoru
Ootsuki Shirou
Shirakawa Yukihiko
Takizawa Masatoshi
Patel Vip
TDK Corporation
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