GB2144269A - Electroluminescent device: method and product - Google Patents

Abstract

A method of electroluminescent panel manufacture in which a doped zinc chalcogenide phospher film-for example manganese doped zinc sulphide, is deposited upon an electrode bearing substrate in the presence of an hydrogen enriched atmosphere - for example a 90%: 10% argon:hydrogen atmosphere. This is followed by rapid anneal treatment, the substrate being raised quickly to a temperature of 450 DEG C, or greater, and cooled rapidly. It is preferable that, prior to film deposition, the substrate is pretreated by baking in the hydrogen enriched atmosphere. An additional current density limiting film may be applied - a film of low resistance cermet material - for example silica/nickel 20% Ni in SiO2, or a film of amorphous silicon.

Description

SPECIFICATION Electroluminescent device; method and product This invention concerns electroluminescent devices, especiallythin film electroluminescent panels operable under conditions of AC or DC drive.

For some considerable time much interest has been shown in electroluminescent devices based on doped zinc chalcogenide phosphor material, in particular manganese-dopedzincsulphide material, for use in large-area complex displays. A number of different approaches to fabricating efficient devices of this type have been tried using either powder orthin film phosphors. See for example:- Vecht et al, J Phys D, 2 (1969) 671 and Inoguchi et al, SID Int Symp Dig, 5 (1974) 84. For many applications, however, as in head-up cockpit displays, car dashboard displays and the like, the brightness, life or cost of such devices, has not yet proved wholly satisfactory.

Thin polycrystallinefilm manganese doped zinc chalcogenide phosphors have been prepared by radio-frequency (rf) sputtering. In the conventional application ofthistechnique, the phosphor is deposited upon a heated substrate in an rfelectricfield using either a powder or a solid hot-pressed powder target ofthe phosphor material in a low pressure inert atmosphere -- usually of argon gas. Radio-frequency (rf) sputtering has considerable commercial attractions as a method for depositing thin films. However, it has been established that for the production of efficiently luminescentZnS:Mn thin films rf sputtering is satisfactory only if followed by a high temperature annealing process.For example (see Cattell et al, Thin Solid Films 92(1982)211-217) it has recently been shown that, under cathodoluminescent excitation, the saturation brightness of conventionally prepared rf sputtered thin film phosphors on silicon substrates may be enhanced by a post-deposition anneal treatment.Asthere reported, a number of different phosphor samples were treated by raising the sample substrate temperature to one of several different peak temperatures 400,500,600 and 700"C respectively and maintaining each sample at peaktemperature for a prolonged period oftime, usually 1/2 hour, before allowing each sample to cool naturally. This was done in a resistively heated tube furnace in a continuously flowing argon atmosphere.The reported results show thatwith this post-deposition anneal treatment, the saturation brightness is increased progressively with increased peaktemperature attained, at least up to a temperature of 700"C, appreciable increase in brightness being attained fortemperatures in the range 600-700"C.

Unfortunately, however, such post-deposition heat treatment is not readily applicable to electroluminescent panel manufacture. Such panels incorporate transparent electrode structu res -- eg electrodes of tin-oxide, indium tin-oxide, or of cadmium stannate material. These electrode materials may become increasingly unstable when subjected to high treatmenttemperatures, ie, temperatures above 400"C, for prolonged periods; and indeed with some substrates the glass softening temperature may be such as to limit heattreatmentto 450"C.

Asolution to fabrication of a low cast high lumines cent efficient ZnS:Mn film is not in itself sufficent for the fabrication of a succegssful low cost electroluminescent device. Such a device requires the nondestructive passage of high currents (--/A/cm2, low duty cycle pulsesfor example) through the luminescentfilm and the background art consists of numerous partially successful schemes for providing this. In many, the solution has been to incorporate copper into the ZnS material but the inherent instability of CuxS attemperatures above 60"C has led to undesirable long term degradation effects.In others, copper has been avoided by automatically limiting the destructiveness of high currents by the use of capacitative coupling wherein the active ZnS: Mn film is supplied with current th rough encasing insulator layers. These insulators pass only displacement currents and these die away before the breakdown of the ZnS film becomes destructive. This capacitative coupling technique (commonly referred to as 'AC') requires the use of an inconveniently high alternating drive voltage which leads to high cost.

A better solution isto use direct coupling and to combat the inherent tendency ofthe ZnS to break down destructively. Hanak (Japan J Appl Phys Suppl 2, Pt 1 (1974) 809-812) has shown that the use of a high resistance current limiting rf sputtered high resistance cermet film intermediate the phosphor film and the backing electrode enhances stability at the price of considerable 12R losses in the limiting layer which leads again to examine drive voltage and loss of efficiency.

The invention disclosed hereinbelow is intended as an improvement in phosphorfilm deposition techni queapplicabletothe manufacture ofthin film electroluminescent panels wherein provision is made for the deposition of efficient phosphorfilms without recourseto excessive annealing temperatures. Furthermore, structures produced according to the method have an inherent tolerance to high current pulses which allows the use of lower current limiting materials and consequent reduction in drive voltage and increase in efficiency.

According to the invention there is provided a method of electroluminescent panel manufacture in whichadopedzincchalcogenidephosphorfilm is deposited upon the surface of a suitable prepared transparent electrode bearing substrate, wherein this deposition is performed in an hydrogen enriched atmosphere, and,following film deposition,thesubs- trate is raised quickly to an elevated temperature of 450"C or above in a suitable atmosphere, and, once such temperature is attained, cooled immediately at a relatively rapid rate, a rate being neither so slow as to result in a degradation ofthe attainable brightness, nor so fast as to result in thermal shock damage to the panel structure.

It has here been foundthata panel, produced bythe above method, exhibits an increase in the brightness that is attainable under operating conditions. Evi dence ofthis improvement is setforth in the description thatfollows below.

The deposition may be performed, for example, by rfsputtering using, as target, doped zinc chalcogenide material in powder or hot pressed powder form.

Alternatively, targets of zinc chalcogenide and of chalcogenides of manganese and/or rare earth ele ments may be used simultaneously.

The optimal rate for cooling, as aforesaid, is dependent upon the species of phosphor material as also upon the size and material ofthe supporting substrate. Forthe manufacture of a manganese doped zinc sulphidethin film panel, a panel incorporating a supporting substrate ofquartzorborosilicate glass material, a cooling rate in excess of 5"C per minute, and usually in the range 10 to 20"C per minute, would normally prove acceptable.

It is observed that prolonged post-deposition heat treatment, such as is typical of conventional anneal treatment would result in a degradation of the improved saturation brightness attained using the inventive method. The heat treatment, as used in the above inventive method, however, is effected so rapidly that such degradation is avoided, whilst at the sametimeitallowssufficientconsolidation of the film to effect improvement in panel brightness and stability.

For a practical device operating whih high dc pulses, an additional current density limiting film is required.

This film may be of low resistance cermet material, for example rf sputtered silica/nickel or alternatively it may be of dc or rf sputtered amorphous silica.

Forthe purposes of illustrating the performance of this inventive method, reference will be made now to an electroluminescent panel of which a simplified section is shown in Figure 1, the accompanying drawing.

This panel comprises a transparent substrate 1 bearing a pairofconnection lands3each having a low resistance contact 5. The substrate 1 supports a transparent electrode structure 7 which is overlaid by a thin film 9 of phosphor material. The electrode structure 7 liesincontactwith one of the two connection lands 3 and the overlying phosphorfilm 9 is backed by an overlaid thin film 11 of resistive material and a further electrode structure 13. This latter electrode structure 13 extends to, and makes contact with, the other one of the connection lands 3.

This panel is manufactured by carrying out the stages detailed below: (a) Aclean substrate 1 oftransparent material, for example quartz or borosilicate glas, is provided with a spaced pair of metallic connection lands 3. These lands 3 each have low resistance contacts 5 which are formed by soldering or bonding. Asuitable land can beformed byfirstdepositing a chrome seeding layer 150 Athickfollowed by a gold layer 0.5 to 1 thick.

Here the gold deposition is phased in before the chrome deposition is terminated, so that a well bonded structure is formed.

(b)An opticallytransmitting electrode7ofhigh electrical conductivity material isthen deposited upon the substate 1 so as to partially overlap and make contact with one of the connecting lands 3. Although this electrode 7 can be of any material possessing suitable electrical and optical characteristics, one such material which as been found to possess the prop erties required is cadmium stannate when deposited and optimised by the methods described in United Kingdom Patent Specification GB 1,519,733-Im- provements in or Relating to Electrically Conductive Glass coatings. A layer thickness of 3500 A of cadmium stannate is suitable.

(c) The substrate 1 is then placed in a sputtering chamber pumped by a liquid nitrogen trapped diffusion pump capable of achieving a base pressure in the region of 3 x 10-7Torr. It is then bakedfor30 mins at 400"C using quartz-iodine lamp heaters. Whilst this stage ofthe process may be conducted under vacuum, it is found preferable to introduce an hydrogen enriched atmosphere, prior to baking. Thfsr itisfound, enhances the reproduceabilityofthis process, and thus affords further improvement in yield. It is convenient, therefore, to introduce the sputtering atmosphere, as described below, atthis earlier stage ofthe process.An electroluminescentfilm 9 is then deposited by radio frequency sputtering so as to overlay the electrode film 7, whilstthe substrate 1 is maintained at a temperature of 200"C. The sputtering targetfrom which thin film 9 is deposited is one of high purity zinc sulphide doped with 0.6 Mol% Manganese, hot pressed to a density of around 3.3 grams per cc and bonded to a metal upon a water-cooled target.

The sputtering atmosphere used is a 90%/10% Argon/Hydrogen mixture at a pressure of 4.4 to 4.6 x 1 0-3. Torr. The thickness ofthis film 9 is chosen to suit working voltage requirements. A typical value for this thickness is 1 p, and is formed ata deposition rate in the range 80-100A/min. Although the phosphor ZnS(Mn) is embodied in the device described, neither the device geometry nor the processing steps precludethe use of othersuitable zinc chalcogenide phosphors or of rare-earth dopants.

Stoichiometry ofthe growing phosphorfilm and its dopant level is determined by recombination effects at the substrate and is critically related to substrate temperatu re. The film composition can also be affected by target surface temperature and steps should be taken to control this parametertata given power level, by ensuring that the back of the targetis kept atthe cooling water temperature. For constant and improved thermal conductivity over the whole of the interfacial area between targetand.water-cooled target electrode it may be necessa ry to use a two component resin bonding ages correctlyformu- lated for vacuum use, between th e ta rget and electrode faceplate. Afigure for ZnStargetdensity has been given already. However, it should be stressed that a figure of greaterthan 90% of theoretical density is always to be preferred in order to reduce the effects, reactive or otherwise, of a large target gas content.

(d) Following deposition of the phosphor layer 9, its stability and luminescent properties are further optimized by a post-deposition heattreatment. This heattreatmentis carried out in a tubularfurnace of lowthermal capacity so asto achieve relatively rapid heating and a relatively rapid cooling rate in the range lOto 20"C per minute. Cooling is assisted by increasing the argon flow over the substrate 1. The procedure is essentially that of raising the substrate to a selected temperature followed by immediate rapid cooling. The selected temperature is deter mined by factors relating to substrate material and prior processing, however a typical value is 450"C.

Alternatively, the heat treatment may be carried out in other inert or non-reactive atmospheres or in vacuo immediatleyfollowing deposition of the phos phor film 9 so as to reduce production time.

(e) After heat treatment, the substrate 1 is coated in selected areas with a cermetfilm layer 11. In the device described, the cermet layer 11 is of silica/nickel material and is deposited from a composite sputter ing target ofsilica and nickel, in which the surface area of the target comprises 20% nickel. The thick ness of the cermet layer 11 is chosen according to the performance characteristics desired. A typical thick ness is 8000 A, deposited at a rate of 120-180 A per minute. An added advantage of this choice of cermet material is that it is black in colour, so providing a high optical contrastto the light emitting areas of the phosphor layer 9.The form ofthe device does not however preclude the use of cermets of other compositions or proportions, as long as the voltage dropped at - 1 A/cm2 does not exceed ~ 1 OmV.

(f) To completethe device a metal film 13, which can conveniently be of aluminium in the thickness range 2000-6000 A, is vacuum deposited so as to overlap the cermetfilm and to makecontactwith the remaining connection land 3.

In the foregoing process, a film of amorphous silicon may be deposited in place of the cermet film 11. This likewise may be deposited by dc or rf sputtering.

Manganese doped zinc sulphide phosphorfilms deposited by rf sputtering in an hydrogen enriched argon atmosphere have been tested using pulsed cathodoluminescence excitation. The results found aretabulated below and are compared with results found for annealed films deposited byrfsputtering in a conventional argon atmosphere. In all cases the films were deposited upon a single-crystal silicon substrate.

TABLE: RF Atmosphere Anneal Saturation Temperature Brightness ("C) (Relative units) Argon/Hydrogen - 1 Argon 700 1 600 0.53 500 0.37 400 0.22 0.1 As can be seen from an inspection ofthese results, the saturation brightness found forthe film is a factor x10 up on thatfor conventional sputtered film as deposited, and is comparable to thatfound upon annealingto 700"C.

It is notedthatfilm samples, obtained byrf sputtering in an hydrogen enriched atmosphere as above, show a severe decrease in attainable brightness if annealed for extended periods attemperatures in excess of 200"C. Provided, however, any heat treatment is ofthe relatively rapid form described above, this severe decrease may be avoided.

An illustration ofthe improvements in efficiency, brightness and life, attained for panels produced by this inventive method, is given below: Sample378:ZnS:Mn 1 uthickuponacadmium stagnate electrode bearing substrate, heated to a maximum temperature of 550"C and rapidly cooled.

Selected areas coated with a cermetfilm (nominal 20% Ni in SiO2) 0.8 ;ithick; Al top electrodes.

Continuous DC operation (cermetfree areas): 80 ft L at 96 V, 8 mA/cm2. 0.02% efficiency (Wat/Watt). Pulsed operation (simulated 100 row matrix, cermet included: 27 ft L at 98 V, 400 mA/cm2, 1% duty cycle 1 Ops pulses.

Lifetest (under above pulsed conditions, cermet included) 27ftLto 13ftLin 1000 hours.

Claims (18)

1. A method of electroluminescent panel manufacture is which a doped zine chalcogenide phosphor film is deposited upon the surface of a suitable prepared transparent electrode bearing substrate, wherein this deposition is performed in an hydrogen enriched atmosphere, and following the deposition of the film, the film bearing substrate is raised quickly to an elevated temperature of 450"C or above in a suitable atmosphere, and, immediately such elevated temperature is obtained, cooled at a relatively rapid rate, a rate being neither so slow as to result in a degradation of the attainable brightness, nor so fast asto result in thermal shock damageto the structure of the panel.

2. A method, as claimed in claim 1, wherein the substrate is prepared by baking in an hydrogen enriched atmosphere.

3. A method, as claimed in either claims 1 or2, and wherein the deposition is performed in an hydrogen enriched argon atmosphere.

4. A method, as claimed in claim 3, wherein the proportions of argon and hydrogen are approximate ly 90% and 10% respectively.

5. A method, as claimed in any one of the preceding claims, wherein the zinc chalcogenide is zinc sulphide.

6. A method, as claimed in any one of the preceding claims, wherein the deposition is performed by rf sputtering using as target doped zinc chalcogenide material.

7. A method, as claimed in any one of the preceding claims 1 to 5, wherein the deposition is performed by rf sputtering using as target materials zinc chalcogenide and a chalcogenide of manganese or a rare earth element, as dopant source.

8. A method, as claimed in any one of the preceding claims, wherein the transparent electrode is of cadmium stannate material.

9. A method as claimed in any one of the preceding claims 1 to 7 wherein the transparent electrode is of tin oxide.

10. A method, as claimed in any one ofthe preceding claims 1 to 7, wherein the transparent electrode is of indium tin oxide.

11. A method, as claimed in any one ofthe preceding claims, wherein the film bearing substrate is cooled at a rate in excess of 5"C per minute.

12. A method, as claimed in claim 11 wherein the film bearing substrate is cooled at a rate of between 10 C and 20"C per minute.

13. A method, as claimed in any one of the preceding claims wherein the elevated temperature is in the range 450-550"C.

14. Athinfilm electroluminescent panel, includ- ing a film of doped zinc sulphide material, produced by any method as claimed in claims 1 to 13 above.

15. A panel, as claimed in claim 14, including a backing electrode structure and a current limiting resistive layer disposed between the film and this backing electrode structure.

16. A panel as claimed in claim 15 and wherein the resistive layer is of amorphous silicon material.

17. A panel as claimed in claim 15 wherein the resistive layer is of silica/nickel cermet film nominally 20% Ni in SiO2.

18. A panel constructed, adapted and arranged to perform, substantially as described hereinbefroe, with reference to and as shown in Figure 1,the accompanying drawing.