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

2001-11-19

2003-11-18

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

C313S503000, C428S917000, C427S058000, C427S066000

Reexamination Certificate

active

06650046

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a thin-film EL device having at least a structure comprising an electrically insulating substrate, a patterned electrode layer 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. The EL devices work on a phenomenon in which a substance emits light at an applied electric field, viz., an electro-luminescence (EL) phenomenon.
The EL devices are divided into two types: dispersion type EL devices having a structure wherein electrode layers are provided on the upper and lower sides of a dispersion of light-emitting powder in an organic material or porcelain enamel, and thin-film EL devices having a thin-film light-emitting substance sandwiched between two electrode layers and two thin-film insulators on an electrically insulating substrate. 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 on account of 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 a long lifetime.
FIG. 2
shows the structure of a double-insulation type thin-film EL device typical of prior art thin-film EL devices. This thin-film EL device includes a transparent substrate
21
formed of a green glass sheet used for liquid crystal displays or PDPs, and a transparent electrode layer
22
formed of ITO or the like to a thickness of about 0.2 &mgr;m to 1 &mgr;m in a predetermined stripe pattern, a first insulator layer
23
in transparent thin-film form, a light-emitting layer
24
having a thickness of about 0.2 &mgr;m to 1 &mgr;m and a second insulator layer
25
in transparent thin-film form stacked on the substrate. Further, an electrode layer
26
is formed by patterning an Al thin-film or the like in stripes extending perpendicular to the transparent electrode layer
22
. The transparent electrode layer
22
and the electrode layer
26
together define a matrix, in which voltage is selectively applied to a selected area of light-emitting substance to allow the light-emitting substance of that specific pixel to emit light. The resultant light is extracted from the substrate side. Having a function of limiting current flow through the light-emitting layer, the thin-film insulator layers are able to inhibit the dielectric breakdown of the thin-film EL device, and contribute to the achievement of stable light-emitting properties. Thus, the thin-film EL device of this structure now finds 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 to a thickness of about 0.1 to 1 &mgr;m by sputtering, evaporation or the like.
Among light-emitting materials, 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, 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 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.
The light-emitting materials disclosed in Shosaku Tanaka, “the Latest Development in Displays” in Monthly Display, April, 1998, pp. 1-10, include 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
, Ca
2
Ga
2
S
4
:Ce, Sr
2
Ga
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 in a thin-film EL device having a blue light-emitting layer. 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. 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 production process, 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 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 with such thin-film insulators, JP-A07-50197 and JP-B07-44072 disclose a thin-film EL device using an electrically insulating ceramic substrate as a substrate and a thick-film dielectric material instead of the thin-film insulator located beneath the light-emitting substance. As shown in
FIG. 3
, this thin-film EL device has a structure having 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
stacked on a substrate
31
such as a ceramic substrate. Unlike the thin-film EL device shown in
FIG. 2
, the transparent electrode layer is formed at the top 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 several tens of nanometers to several hundreds of microns that 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 dielectric breakdown caused by pinholes formed by steps at electrode edges or dust, etc. occurring in the device fabrication process. Although the use of this thick- film dielectric layer leads to a problem that the effective voltage applied to the light-emitting layer drops, this problem can be solved or eliminated by using a high permittivity material for the dielectric layer.
However, the light-emitting layer formed on the thick-film dielectric layer has a thickness of barely several hundreds of nanometers 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 a volume shrinkage of about 30 to 40% upon dense sintering. However, ordinary ceramics are consolidated 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 only in the thickness direction or one-dimensionally. For this reason, the sintering of the thick-film dielectric layer does not proceed to a sufficient extent, yielding an essentially porous layer.
Since the consoli

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