GB2352086A

GB2352086A - Patterning conductive layers

Info

Publication number: GB2352086A
Application number: GB9916658A
Authority: GB
Inventors: Julian Carter; Stephen Karl Heeks
Original assignee: Cambridge Display Technology Ltd
Current assignee: Cambridge Display Technology Ltd
Priority date: 1999-07-15
Filing date: 1999-07-15
Publication date: 2001-01-17
Also published as: GB9916658D0

Abstract

A method for forming an electroluminescent device, comprising: forming a first electrode layer 22, forming a profiled layer 25 over part of the first electrode layer 22, forming a light-emitting layer 26 over the first electrode layer 22, the light-emitting layer 26 comprising a light-emitting organic material; and forming a second electrode layer over the profiled layer 25 and the light-emitting layer; and removing at least part of the second electrode layer overlying the profiled layer. The profiled layer 25 may be formed from SiO<SB>2</SB>.

Description

2352086 PATTERNING CONDUCTIVE LAYERS This invention relates to patterning

conductive layers, for example electrodes, especially in light-emitting devices.

One specific class of display devices is those that use an organic material for light emission. Light-emitting organic materials are described in PCTNV090/13148 and US 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emitting organic layer, for instance a film of a poly(p-phenylenevinylene) ("PPV"), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. In PCT/VV090/13148 the organic light-emitting material is a polymer. In US 4,539,507 the organic light-emitting material is of the class known as small molecule materials, such as (8hydroxyquinoline)aluminium ("Alq3"). In a practical device one of the electrodes is typically transparent, to allow the photons to escape the device.

Figure 1 shows the cross-sectional structure of a typical organic lightemitting device ("OLED"). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent anode electrode 2 of a material such as indium-tinoxide ("ITO") that is suitable for injecting positive charge carriers. Such coated substrates are commercially available. This ITO-coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and a final layer forming a cathode electrode 4 of a material that is suitable for injecting negative charge carriers. The cathode electrode is typically of a metal or alloy. Other layers can be included in the device, for example to improve charge transport between the electrodes and the electroluminescent material.

2 Figure 2, in which like parts are numbered as for figure 1, shows a similar device having a plurality of independently operable lightemitting regions. In the device of figure 2 the cathode electrode 4 is patterned so that it comprises a plurality of unconnected areas in the form of parallel strips. The anode electrode 2 is patterned so that it comprises a plurality of unconnected areas also in the form of parallel strips, which run perpendicular to the strips of the cathode. By applying a suitable voltage between a selected cathode strip and a selected anode strip the regions of light-emifting material that are located between the selected strips can be made to emit light whilst the other regions of the light-emifting material remain quiescent. This is a passive matrix addressing scheme. The regions of lightemitting material that are located between the selected strips thus form independently actuable pixels of the device.

One problem in manufacturing devices such as the one shown in figure 2 is the formation of the patterned electrodes. This is a particular problem if the electrode that is formed over the light-emifting region has to be patterned because the organic light-emifting region is normally especially prone to damage from processes that take place after it has been deposited. Two general routes have been tried for forming a patterned electrode over an organic light-emifting region, and both have disadvantages.

The electrode could be deposited as a planar sheet of material and then parts of the sheet could be removed to give the desired electrode pattern. Methods proposed for removal of the electrode after deposition include selective etching using a photolithog rap hical ly defined mask and laser scribing. However, these methods are inherently destructive and have generally been found to carry a significant risk of damaging the underlying organic layer(s). This is generally because in order to pattern the electrode into unconnected regions the electrode must be cut through entirely. This typically exposes some of the underlying organic material which can then be damaged either by the cutting process itself or by a subsequent process, or by exposure to reactive components in the atmosphere. Furthermore, in order to avoid damage to the underlying organic material or even the layers of the device below that the 3 depth of the cutting process must be very precisely controlled. This generally means that the process is not capable of eroding a large area of the device in a short time, so these cutting processes are generally unsuitable for forming devices of relatively large area such as display boards for public areas.

The electrode could be deposited in a patterned state, for example by masking areas of the substrate whilst the electrode is being deposited, or by forming greatly raised portions of the substrate where the electrode strips are unwanted and then depositing the electrode over the whole substrate so that the steps at the edges of the raised portions of the substrate give rise to discontinuities in the electrode layer. However, using a shadow mask is unsuitable for geometries required for high resolution displays and the latter method involves adding complex structures to the substrate.

There is therefore a need for an improved process of forming patterned electrodes in organic light-emitting devices.

According to the present invention there is provided a method for forming an electroluminescent device, comprising: forming a first electrode layer; forming a spacing layer over part of the first electrode layer; forming a light-emitting layer over the first electrode layer, the light-emitting layer comprising a light-emitting organic material; and forming a second electrode layer over the spacing layer and the light-emitting layer; and removing at least part of the second electrode layer overlying the profiled layer.

The said at least part of the second electrode layer overlying the profiled layer is suitably raised or otherwise projecting relative to parts of the second electrode layer that do not overlie the profiled layer, preferably by virtue of the thickness of the profiled layer. The second electrode layer is preferably deposited to a substantially uniform thickness. The second electrode layer may be an anode or a cathode 4 Preferably the profiled layer extends the width of the second electrode and the said at least part of the second electrode layer overlying the profiled layer is removed so as to electrically separate the regions of the second electrode layer on either side of the profiled layer. The profiled layer and/or the said at least part of the electrode layer may be linear. The said at least part of the electrode layer may be of all or part of the width of the profiled layer.

The method of removal of the said at least part of the second electrode layer is suitably mechanical, most preferably by means of a cutting or abrasive action. Where the removal is by a cutting action it is suitably performed by the engagement of a cutting tool with the second electrode layer overlying the profiled layer and its movement relative to the second electrode layer along the path of the said at least part of that layer to be removed. Where the removal is by an abrasive action it is suitably performed by the engagement of a planar abrasive tool with relatively raised or otherwise projecting parts of the second electrode layer. The plane of the tool is suitably substantially parallel to a general plane of the device structure, for example to a plane of a substrate underlying the other layers of the device.

The light-emitting layer is suitably formed over the spacing layer. In that case, the said removal step comprises removing at least part of the light-emitting layer over the spacing layer. Alternatively the spacing layer may be formed over the lightemitting layer.

The spacing layer is suitably electrically non-conductive. The spacing layer suitably comprises Si02 or a polymer (e.g. polyimide). The thickness of the spacing layer is suitably greater than the total thickness of the light-emitting layer and the second electrode layer.

The spacing layer may be configured in the form of a set of parallel (and suitably spaced apart and non-contiguous) strips suitably running in the said general plane of the device. Where the first electrode is also configured in the form of a set of parallel (and suitably spaced apart and non-contiguous) strips running in the said general plane of the device the strips of the spacing layer are preferably perpendicular to the strips of the first electrode.

The said step of removing suitably comprises mechanically removing the said part of the second electrode layer, preferably removing some or all raised, upstanding or otherwise projecting parts of the second electrode layer.

The step of removing may be a planar removal process, preferably a planar abrasive process such as a polishing process.

The step of removing may comprises engaging upstanding parts of the second electrode layer with a removal tool and causing relative movement of the removal tool across the second electrode layer to remove the said part of the second electrode layer. The tool may be a cutting tool such as a blade or an ablative jet of material which could be gas or liquid or solid particles entrained in a gas or liquid flow.

Where the spacing layer is formed in strips, in order to perform the removal process the device is preferably moved in a direction parallel with the strips of the spacing layer whilst the removal tool is held stationary to remove the said part of the second electrode layer. The direction of motion is preferably the direction of motion of the device through a manufacturing process that irfcludes deposition of one or more of the layers set out above. It is also preferred that the said step of removing comprises engaging upstanding parts of the second electrode layer with a plurality of removal tools, the removal tools being spaced apart so that each removal tool engages one strip of the spacing layer.

Some preferred materials for components (where present) of the lightemissive unit are as follows:

One of the electrodes preferably has a work function of greater than 4.3 eV.

That layer may comprise a metallic oxide such as indium-tin oxide ("ITO") or 6 tin oxide ("TO"). The other electrode preferably has a work function less than 3.5 eV. That layer may suitably be made of a metal with a low work function (Ca, Ba, Yb, Sm, Li etc.) or an alloy comprising one or more of such metals together optionally with other metals (e.g. Al). At least one of the electrode layers is suitably light transmissive, and preferably transparent, suitably at the frequency of light emission from one or more of the light-emissive regions.

There may be one or more charge transport layers between the lightemissive material and one or both of the electrodes. The or each transport layer may suitably comprise one or more polymers such as polystyrene sulphonic acid doped polyethylene dioxythiophene ("PEDOT- PSS") and/or poly(2,7-(9,9-di-noctylfluorene)-(1,4-phenylene-(4- imino(benzoic acid))1,4-p henylene-(4imino(benzoic acid))- 1,4- phenylene)) ("BFA") and/or polyaniline and/or PPV.

The or each organic light-emitting material may comprise one or more individual organic materials, suitably polymers, preferably conjugated or partially conjugated polymers. Suitable materials include poly(p phenylenevinylene) ("PPV'), poly(2-methoxy-5(2-ethyl)hexyloxyphenylenevinylene) ("MEH-PPV), a PPV-derivative (e.g. a di-alkoxy or di-alkyl derivative), a polyfluorene and/or a co-polymer incorporating polyfluorene segments, PPVs and/or related co-polymers, poly (2,7-(9,9-di-noctylfluorene) (1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene)) ("TFB"), poly(2, 7 (9,9-di-n-octylfluorene) (1,4-phenylene-((4-methylphenyl)imino)-1,4- phenylene-((4 methylphenyl)imino) - 1,4-phenylene)) ("PIFIVI"), poly(2, 7 - (9,9 di-n-octylfluorene) - (1,4-phenylene-((4-methoxyphenyl)imino)-1,4phenylene- ((4-methoxyphenyl)imino)-1,4-phenylene)) ("PFMO"), poly (2,7-(9,9-di-n octylfluorene) ("FW) or (2,7-(9,9-di-n-octylfluorene)-3,6Benzothiadiazole) ('7813T"). Alternative materials include organic molecular light-emissive materials, e.g. solution processable small molecule materials such as spiro compounds (see EP 0 676 461 A), and other solution processable small molecule or conjugated polymer electroluminescent materials.

The present invention will now be described by way of example, with reference to the accompanying drawings, in which:

7 figure 3 shows a cross-sectional view of an organic light-emitting device structure at a first stage of its formation; figure 4 shows a cross-sectional view of the organic light-emitting device structure of figure 3 after a second stage of its formation; figure 5 shows a cross-sectional view of the organic light-emitting device structure of figure 3 after a third stage of its formation; figure 6 shows a cross-sectional view of another organic light-emitting device structure at a first stage of its formation; figure 7 shows a cross- sectional view of the device structure of figure 6 during a second stage of its formation; figure 8 shows a cross-sectional view of the device structure of figure 6 after a second stage of its formation; figure 9 illustrates a roller cutting process for forming an organic lightemitting device; and figure 10 is a plan view of the device of figure 9 after the cutting procedure.

In figures 3 to 10 like reference numbers indicate like parts. References to any orientation of the device refer to its orientation as shown in the figures and not necessarily to its orientation in manufacture or use.

When they are completed, the devices of figures 3 to 10 compriise individual top electrode regions which are electrically unconnected from each other, by means of which selected light-emitting regions of the device may be addressed and driven to emit light. In the device structure of figures 3 to 5 these regions (shown as 20 in figure 5) are created from a continuous layer by a damascene process in which exposed upper portions of that layer are removed - for example by polishing - to leave the individual top electrode regions themselves. In the device structure of figures 6 to 8 the individual top electrode regions 20 are created by a scoring process in which exposed portions of a continuous layer are incised to form non-conductive gaps 40 between the said regions.

8 The device structure of figures 3 to 5 is formed on a glass substrate 21 which is coated with an anode electrode layer 22 of indium-tin oxide (ITO). Such ITOcoated glass substrates are commercially available. The glass sheet could be a sheet of sodalime or borosilicate glass of a thickness of, for instance, 1mm. Instead of glass other materials such as polyester could be used. The thickness of the ITO coating is suitably around 150nm and the ITO suitably has a sheet resistance of between 10 and 300/o, preferably around 150/o. The ITO is patterned into a set of parallel stripes 23 running parallel to the plane of figures 3 to 5. In conjunction with a corresponding set of orthogonal ly-d irected stripes (constituted by regions 20) later formed in the cathode electrode these allow lightemitting regions of the completed device to be addressed by a passive matrix addressing scheme. Instead of ITO other transparent conductive materials such as tin oxide (TO) or a thin layer of gold could be used for the anode electrode layer.

In the regions other than where the eventual individual cathode electrode regions 20 are desired to be, banks 25 are formed. The banks 25 are formed Of Si02. The Si02 25 can be spun on to the ITO in a uniform layer and then patterned by a method such as photolithography followed by etching with HF. The thickness of the banks 25 is suitably around 500nm.

Light-emitting material is deposited over the banks 25, and the regions of the anode that are exposed through the banks, to form a light-emitting layer 26. The light-emitting material could be any suitable lightemitting polymer, small molecule or oligomer material or the like, or a mixture of two or more of such materials together optionally with other materials. The layer could, for example, be formed by substituted poly(pphenylenevinylene) polymers. This could be deposited on top of layer 22 by, for example, spin-coating from an organic solvent. The lightemitting material may be mixed with components that assist charge transport within the light-emitting region. The thickness of the layer of light-emitting material is suitably around 90nm. The light-emitting layer conforms to the profile of the underlying layers, so that in the regions where the banks are not present the light- 9 emitting material is significantly closer to the substrate 21 than in the regions where the banks are present.

A cathode electrode is deposited over the light-emitting layer 26. The cathode layer 27 comprises a thin layer of calcium with adjacent to the light-emitting material and a thicker layer of aluminium over the calcium layer. These layers could be deposited by, for example, evaporation. The layers of the cathode electrode conform to the profile of the underlying layers, so that in the regions where the banks are not present the layers of the cathode electrode are significantly closer to the substrate 21 than in the regions where the banks are present. The cathode electrode could be formed of other materials, and as one or more than two layers.

In order to pattern the cathode the upper parts of the device structure as shown in figure 4 are removed by a damascene process. A fine polishing action is applied to the cathode side of the device, for example using an organic based slurry, so as to remove the uppermost parts of the device. The removal continues until all the parts of the cathode layer atop the banks have been removed. This leaves the structure shown in figure 5, in which light-emitting material and cathode material remain only in the recesses between the banks. The cathode material lying in each such recess is electrically insulated by the bank from the adjacent cathode material.

The cathode should be polished to the thickness of the bank to ensure electrical insulation. In that case it is also preferred that the total thickness of the lightemitting material 26 and the lower (calcium) layer of the electrode that is adjacent to it is less than the thickness of the banks, to reduce the risk that the calcium layer is exposed during the damascene process. Alternatively it may be preferred that the total thickness of the.light-emitting material 26 and the electrode layer 27 is less than the thickness of the banks 25, so that after the removal process the electrode material can lie protected in grooves between the banks.

In the device of figures 3 to 5 the banks are formed as strips running orthogonally to the anode strips and in the plane of the device. Thus, the resulting cathode strips between the banks are also strips that run orthogonally to the anode strips and in the plane of the device. (Analogous to figure 2). Therefore, the anode and cathode strips can be used to address individual regions of the emitting region where respective anode and cathode strips overlap. For example, region 29 (which could represent a pixel of the device) can be addressed by means of anode strip 22 and cathode strip 20a. Other configurations could be used. In other passive matrix schemes the anode and cathode strips could be other than orthogonal - although they may not be parallel - and the spacing between them could be the same or different. The cathode could be patterned by the above method to yield individual cathode portions for each pixel, to which individual connections could be made. The pixels or in general the individually controllable light-emitters of the device could be square, rectangular or another shape - for example dedicated shapes could be used for specific user interface indications.

The banks could be formed of any suitable insulating material, or of a conductive material coated with insulating material. Suitable matedals other than Si02 include polymers such as polyimide, which could be deposited by spin-coating and patterned by etching, or - especially in relatively large displays - could be deposited by screen printing. Si02 and other hard materials may be especially preferred for some patterning processes since they are relatively incompressible and therefore resist deformation under compression.

The banks could be formed at another stage in the process. For example, they could be formed after the light-emitting layer but before the cathode layer, so that the light-emitting layer can be at least.generally planar. However, it is preferred that the banks are formed before the light-emitting layer - and most preferably before any particularly sensitive layers of the device such as other polymer layers - since that reduces the risk of damage to the device during manufacture. With the banks formed before the light-emitting layer, as in figure 3, the patterning of the banks can be done before the light-emitting layer is applied. Therefore, the 11 step of patterning the banks can be done without risk of damaging the lightemitting layer The damascene process is preferably performed in a glove box or other relatively inert environment in order to reduce the risk of damage to the device during that step.

Instead of a polishing process other planar removal techniques such as scraping or planing could be used. The damascene process is especially suitable for use on small-scale devices, but could also be used for larger devices.

Figures 7 to 9 illustrate another technique for forming a patterned cathode. Figure 7 illustrates the device prior to cathode patterning. The structure of the device at that stage is the same as that of the device of figure 4 and that device could be formed in the same way as that of figure 4.

Figure 8 illustrates the result of the patterning process. A linear removal technique is used to remove a strip of cathode material, and preferably the lightemitting material that underlies it, from over the banks. The removed strip runs in a direction along the length of each bank, and preferably for the full length of the bank. The removed strip is preferably along the centre of each strip. Once the strip of material has been removed in this way the adjacent regions of cathode are electrically unconnected and can be used as before in a passive matrix addressing scheme.

The structure of the banks allows for some imprecision in the removal technique. Provided the width of the removed strip is less than the width of the banks - as is preferred - the removed strip can deviate somewhat across the width of the bank. The thickness of the banks means that there is also some margin for error in the depth to which the linear removal technique must cut: it is not significant if the linear removal step cuts into the banks provided it does not cut fully through them 12 Numerous techniques are available for the linear patterning step. Examples include:

a cutter, scraper, blade or scorer that is drawn along the banks; a rotatable cuffing wheel, which may be driven to rotate or may simply roll along the banks, and which may optionally have peripheral teeth; jets of a cutting medium such as pressurised gas (e.g. air) or liquid, which may optionally contain abrasive particles and which may be delivered in a linear spread pattern along the bank, or in a concentrated pattern which is moved along the banks; laser scribing, which may be chemically enhanced.

One or more of the patterning tools may be provided. It is preferred that one tool is provided for each bank, so that the patterning of all the banks can be performed in a single step.

In order that the cutting tool is moved relative to the banks, the tool itself could be moved and/or the banks could be moved. In a continuous manufacturing process one preferred arrangement is for the banks to be formed parallel to the direction of movement of the substrate(s), and for a plurality of cutters to be fixed in location where they can engage the banks and perform the removal step as the substrates move past them. This provides a method that is especially efficient for manufacturing relatively large devices, but is also applicable to smaller devices. The technique is illustrated in figure 9, in which a device is sh own moving past an array of cutting wheels 50 that are located so as to remove material from over the bank of a sheet-form device as it passes under the cutters in a direction parallel to its banks. The sheet-form device may be intended for use as a single device or may later be cut into a number of devices. 'The wheels 50 may cut just to the surface of the banks 25 (as shown in figure 9), somewhat into the banks 25 in order to ensure isolation between adjacent cathode regions, or only part way into the light-emitting layer 26 provided the conductivity of the light-emitting layer is sufficiently low to resist significant current spread between cathode regions.

13 Figure 10 shows the device structure following the process of figure 9, and especially illustrates the orthogonal arrangement of electrode strips suitable for passive matrix addressing. Devices formed using the processes described above may be operable according to other addressing schemes, for example active matrix addressing.

After the devices have been formed they are preferably packaged, for example in epoxy, for environmental protection.

The electrodes could be reversed, so that the cathode electrode is deposited first and the anode electrode is patterned by one or more of the processes described above. At their surfaces adjacent the light- emitting material the anode electrode preferably has a relatively high work function, e.g. greater than 4.3eV, and the cathode electrode preferably has a relatively low work function, e.g. less then 3.5eV. Either or both of the electrodes may be light-transmissive, or both electrodes may be opaque and light may escape through the side of the device.

There may be one or more charge transport layers located between the lightemitting material and one or both of the electrodes. Such charge transport layers may comprise conductive polymers such as polystyrene doped polyethylenedioxythiophene (PEDOT:PSS).

The present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof irrespective of whether it relates to the presently claimed invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

14

Claims

1. A method for forming an electroluminescent device, comprising: forming a first electrode layer; forming a spacing layer over part of the first electrode layer; forming a light-emitting layer over the first electrode layer, the light-emitting layer comprising a light-emitting organic material; and forming a second electrode layer over the spacing layer and the lightemitting layer; and removing at least part of the second electrode layer overlying the profiled layer.

2. A method as claimed in claim 1, wherein the light-emitting layer is formed over the spacing layer.

3. A method as claimed in claim 1 or 2, wherein the said removal step comprises removing at least part of the light-emitting layer over the spacing layer.

4. A method as claimed in claim 1, wherein the spacing layer is formed over the light-emitting layer.

5. A method as claimed in any preceding claim, wherein the spacing layer is electrically non-conductive.

6. A method as claimed in any preceding claim, wherein the spacing layer comprises Si02 or a polymer.

7. A method as claimed in any preceding claim, wherein the thickness of the spacing layer is greater than the total thickness of the lightemitting layer and the second electrode layer.

8. A method as claimed in any preceding claim, wherein the spacing layer is configured in the form of a set of parallel spaced apart strips.

9. A method as claimed in claim 8, wherein the first electrode is configured in form of a set of parallel spaced apart strips.

10. A method as claimed in claim 9, wherein the strips of the spacing layer are formed perpendicular to the strips of the first electrode.

11. A method as claimed in any preceding claim, wherein the said step of removing comprises mechanically removing the said part of the second electrode layer.

12. A method as claimed in any preceding claim, wherein the said step of removing comprises removing upstanding parts of the second electrode layer.

13. A method as claimed in any preceding claim, wherein the step of removing is a planar removal process.

14. A method as claimed in any preceding claim, wherein the said step of removing comprises an abrasive process.

15. A method as claimed in any preceding claim, wherein the said step of removing comprises a polishing process.

16. A method, as claimed in any of claims 1 to 12, wherein the said step of removing comprises engaging upstanding parts of the second electrode layer with a removal tool and causing relative movement of the removal tool across the second electrode layer to remove the said part of the second electrode layer.

17. A method as claimed in claim 16, wherein the tool is a cutting tool.

16

18. A method as claimed in claim 17, wherein the tool is a blade.

19. A method as claimed in claim 17, wherein the tool is an ablative jet.

20. A method as claimed in any of claims 16 to 19, as dependant directly or indirectly on claim 8, comprising the step of moving the device in a direction parallel with the strips of the spacing layer whilst the removal tool is held stationary to remove the said part of the second electrode layer.

21. A method as claimed in any of claims 16 to 20, as dependant directly or indirectly on claim 8, wherein the said step of removing comprises engaging upstanding parts of the second electrode layer with a plurality of removal tools, the removal tools being spaced apart so that each removal tool engages one strip of the spacing layer.

22. A method for forming an electroluminescent device substantially as herein described with reference to the accompanying drawings.