GB2291259A

GB2291259A - Manufacture of thin-film electroluminescent elements

Info

Publication number: GB2291259A
Application number: GB9512980A
Authority: GB
Inventors: Hisato Kato; Tomoyuki Kawashima; Shinichi Nakamata; Takashi Tsuji; Yutaka Terao
Original assignee: Fuji Electric Co Ltd
Current assignee: Fuji Electric Co Ltd
Priority date: 1994-07-04
Filing date: 1995-06-26
Publication date: 1996-01-17
Anticipated expiration: 2015-06-26
Also published as: JPH0817574A; GB9512980D0; DE19524359A1; GB2291259B

Description

2291259 MANUFACTURE OF THIN-FILM ELECTROLUMiNESCENT ELEMENTS The present

invention relates to the manufacture of electroluminescent elements, and particularly concerns the formation of a luminous layer for a thin-film electroluminescent element, which has enhanced luminous brightness.

Thin-film eiectroluminescent elements that are totally solid are noted as flat display elements with high resolution and large display capacity.

Figure 7 shows a cross section ofa conventional thin-film electroluminescent element. The thin-film electroluminescent element has a double-insulation structure in which a transparent electrode 2, a first insulating layer 3, a luminous layer 4, a second insulating layer 5, and a back-plate electrode 6 are sequentially laminated onto a glass substrate 1. Zinc sulphide (ZnS) is a base material of the luminous layer in the thin-film electroluminescent element, to which a small amount of emission centre material, for example Mn, has been added.

Vacuum deposition methods, ALE methods classified into MID methods, and sputtering methods are currently used in the manufacture of luminous layers for thin-film electroiuminescent elements. Of these, sputtering methods allow uniform films with a large area to be formed at a very high speed, resulting in high productivity.

In the manufacture of luminous layers for thin-film electroiuminescent elements, using a sputtering method, however, it is difficult to provide a layer with sufficient luminous brightness (ACTA POLYTECHNICA SCANDINAVICA Applied Physics Series No. 170 'Sh International Workshop on Electroluminescence" pp 41 to 48).

Our research has shown that the different physical properties of zinc, sulphur, and manganese cause resultant films to have a composition which is significantly different from the target composition, and which deviates from the ideal stoichiometric composition of the film, resulting in reduced brightness of the electroluminescent element.

2 To solve this problem, the film formation techniques used to produce zinc sulphide based luminous layers have been studied, to determine how luminous brightness can be improved. Film formation at a substrate temperature of 300 to 35M (a conventional requirement) leads to improved luminous brightness, but also the generation (in the early phase of film formation) of dead layers with a thickness of about 10 nm, which are made up of regions with a small particle size. In addition, the luminous layers obtained have an average crystal particle size of 260 nm, which is so small that practical luminous brightness cannot be obtained. In other words, the presence of dead layrs and a small crystal particle size of the remainder of the film hinders the acceleration of electrons, causing them to excite dopants insufficiently, thereby reducing luminous brightness.

Figure 6 is a cross section showing the integral part of a conventional thin-film eiectroluminescent element. A dead layer 4A of small crystal size is seen in the luminous layer.

The present invention aims at solving the above problem, and takes as its objective the provision of a thin-film electroluminescent element with high luminous brightness. This is achieved by optimising the substrate temperature during reactive sputtering to prevent the generation of dead layers in the early phase of luminous layer generation and to increase the crystal particle size of the zinc sulphide during the formation of the luminous layer.

According to the present invention, the above objective can be achieved by manufacturing a thin-film electroluminescent element using as a luminous layer a zinc sulphide ZnS thin film with a dopant added as an emission centre, wherein a luminous layer is formed by a reactive sputtering method that uses a mixture of a sulphur-containing gas and a rare gas to sputter zinc and a dopant while the substrate temperature is maintained at 380 to 4800C.

In the manufacture of the above thin-film electroluminescent element, the dopant is effective when it is either manganese, manganese sulphide, a manganese halide, a rare-earth element, a rare-earth sulphide, or a rareearth halide.

3 Furthermore, the mixture of a sulphur-containing gas and a rare gas should preferably be composed of hydrogen sulphide and argon.

The melting points of zinc and sulphur are 4200C and 1200C, respectively. If, however, the substrate temperature is set at the melting point of zinc or higher, temperatures at which film formation is ordinarily considered difficult, the high temperature of the substrate increases the growing speed of crystals and causes any excess amounts of zinc and sulphur to evaporate quickly.

Embodiments of the invention will now be described in detail with reference to the drawings, in which:

Figure 1 is a schematic drawing showing a sputtering apparatus capable of producing a thin-film electroluminescent element according to an embodiment of this invention; Figure 2 is a target to be sputtered according to another embodiment of this invention, wherein Figure 2(a) shows the target in plan and wherein Figure 2(b) shows the target in cross section; Figure 3 is a cross section of another target to be processed according to the second embodiment of this invention; Figure 4 is a graph showing the dependency of luminous brightness on the substrate temperature of the thin-film electroluminescent element according to the first embodiment of this invention; Figure 5 is a cross section showing the integral part of the thin-film electroluminescent element according to the various embodiments of this invention; Figure 6 is a cross section showing the integral part of a conventional thin-film electroluminescent element; and Figure 7 is a cross section of the thin-film electroluminescent element.

Referring now to the drawings, Figure 5 is a cross section showing the integral part of the thin-film electroluminescent element according to this invention. A transparent electrode 2, is formed from indium-tin oxide with a thickness of 1700 A. A first insulating film 3 with a thickness of 2100 A consisting of silicon oxide or silicon nitride, a luminous layer 4 with a thickness of 7000 A, a second insulation film 5 4 with a thickness of 2100 A consisting of silicon nitride or silicon oxide, and a back-piate electrode 6 with a thickness of 7000 A consisting of aluminium, were sequentially laminated in this order.

Figure 1 shows a sputtering apparatus for producing a luminous layer for a thin-film electroluminescent element according to this invention. In this Figure, a cathode 9 attached to a target 8 is connected to a 13.56 MHz RF power supply 11 via a matching circuit 121 An anode 10 is located opposite the target, and a substrate 1 is placed on the anode 10.

To form the luminous layer, zinc with manganese concentration [Mn/(Zn + Mn)] of 0.3 wr/G was selected as the target.

In a first example of the method, the sputtering gas was a mixture of a sulphur-containing gas and a rare gas, that is, argon gas with a hydrogen sulphide addition of 20 to 40%. The gas was introduced into a reaction chamber 14 from a gas inlet 13. Sputtering was carried out under a gas pressure of 10 mTorr, a substrate temperature of 350 to 5OWC, and a discharge power of 3 W1CM2. Under these conditions, the surface of the target was sulphurised to reduce the sputtering rate of zinc, resulting in a zinc-sulphide film with a composition close to the stoichiometric composition. The luminous layer thus formed was then thermally treated at 500%.

The luminous layer obtained in this manner at a substrate temperature within a wide range of 380 to 4800C, has a large crystal particle size of about 450 nm, and achieves a luminous brightness of 200 cd/M2or more. This luminous brightness is twice that of a conventional film formed at a substrate temperature of 35TC.

Figure 4 is a graph showing the relationship between luminous brightness and the substrate temperature during sputtering for the thin-film electroluminescent element according to this embodiment of the invention.

This graph shows that the luminous brightness is about 200 cd/M2 or more when the substrate temperature is maintained at 3800C or higher during sputtering. The elevated substrate temperature increases the growing speed of crystals and causes any excess amounts of zinc and sulphur to evaporate. Large crystals of about 450 nm thus grow from the surface of the first insulating layer to prevent dead layers from being generated thereon, thereby allowing the crystals to obtain a stoichiometric composition.

Figure 2 shows a target to be sputtered according to a second embodiment of this invention, wherein Figure 2(a) shows the plan and Figure 2(b) shows the cross section. Terbium fluoride TbF3 is spotted on the zinc target 41 at doped areas 42.

The apparatus used in this example was similar to the apparatus used for the first example. The target used was Zn with TbF3 appropriately spotted thereon, and the surface of the target was adjusted to have an average terbium concentration [Th/(Zn + Th)] of 2.5 WC/0. Sputtering was carried out using argon gas with a hydrogen sulphide addition of 40%, at a gas pressure of 10 mTorr, a substrate temperature of 4000C, and a discharge power of 3 WICM2. Once formed, the luminous layer was thermally treated at 6000C.

A luminous layer with a large crystal particle size resulted and a thinfilm electroluminescent element with high luminous brightness was obtained using this luminous layer.

Although Th was used as a rare-earth element in the examples, other rareearth elements including Sm, Tm, Pr, Ho, Er, Dy, Eu, and Ce can also be used as alternatives in combination.

Figure 3 is a plan view showing a different target to be sputtered using the method of the second example of this invention. This target is a mosaic target where Zn is dotted with Tb which is inlaid therein in the above-mentioned concentration.

This example is the same as example 1 except that a gaseous mixture of sulphur vapour and argon was used as the sputtering gas to form a film instead of a mixture of hydrogen sulphide and argon.

The luminous layer obtained in this manner at a substrate temperature within a wide range of 380 to 4801C, had a large crystal particle size of about 430 nm and a luminous brightness of 200 cd/M2 or more. This luminous brightness is twice that in conventional film formed at a substrate temperature of 3500C.

6 As an alternative to the above gases, sulphur hexafluoride may also be used as the sputtering gas.

According to this invention, a luminous layer is formed by a reactive sputtering method using a mixture of a gas containing sulphur elements and a rare gas at a substrate temperature of 380 to 4800C. A luminous layer, in which no dead layers are generated and which has a large crystal particle size, can thus be obtained, and the crystals can be controlled so that they retain a stoichiometric composition. The luminous layer, when incorporated in a thin-film electroluminescent element results in a luminous brightness twice that obtaMed by the prior art.

7

Claims

1. A method of manufacturing a zinc sulphide ZnS thin film, to which a dopant has been added as an emission centre, as a luminous layer of a thin-film electroluminescent element wherein the luminous layer is formed by a reactive sputtering method using a mixture of a sulphur-containing gas and a rare gas as a sputtering gas to sputter zinc together with a dopant while the substrate temperature is maintained at 380 to 4800C.

2. The method of Claim 1 wherein the dopant is either manganese, manganese sulphide, or a manganese halide.

3. The method of Claim 1 wherein the dopant is either a rare-earth element, a rare-earth sulphide, or a rareearth halide.

4. The method of Claim 1 wherein the mixture of a sulphurcontaining gas and a rare gas consists of hydrogen sulphide and argon.

5. A luminous layer for a thin-film electroiuminescent element formed by a method according to any of Claims 1 to 4.

6. A thin-film electroluminescent element including a luminous layer according to Claim 5.

7. A method substantially as described herein.