FIELD OF THE INVENTION

The invention describes a method of manufacturing a light-emitting device. The invention further describes such a light-emitting device. BACKGROUND OF THE INVENTION

Semiconductor light sources, in particular OLEDs (organic light-emitting devices) may be used in a wide variety of lighting applications. As will be known to the skilled person, an OLED comprises two electrodes - anode and cathode - between which one or more organic layers are sandwiched, at least one of which has a light- emitting capability. To cause the OLED to emit light, a voltage is applied across the electrodes, with the anode being at a higher potential. In a very basic construction, a 'hole transport layer' in contact with the anode facilitates the injection of holes from the anode, while an 'electron transport layer' in contact with the cathode is chosen for its ability to inject electrodes from the cathode. When the holes and electrons meet (usually in the electron transport layer, which is therefore often referred to as the 'emissive layer'), they recombine, causing energy to be released as visible light. Other types of OLEDs will be mentioned in the following. An OLED is characterised by light emission over the entire emissive layer.

However, in some lighting applications such as signage, in which a certain pattern or symbol is to be rendered by a light-emitting device, it may be desirable to have some regions of the light-emitting surface remain dark, while other regions are illuminated. An OLED could be used as such as device, for example by covering its light-emitting surface with a layer of suitable opaque material such as plastic, from which the pattern to be rendered has been cut out. In this way, only the desired parts of the pattern or symbol are illuminated through the cut-out areas, while the regions of the light-emitting surface behind the remaining material of the layer appear dark. However, this is not a very energy-efficient approach, since the OLED emits over its entire surface, and part of the emitted light is absorbed or blocked by the opaque material of the covering layer. In addition, this opaque layer is visible when the device is turned off, which may not be desirable.

In an alternative approach, regions of an electrode layer, corresponding to regions in a pattern or symbol that are to remain dark, could be excised or etched away, so that charge-carrier injection cannot take place in these regions. The etched or excised regions within the electrode layer result in different thicknesses throughout the electrode layer, so that the pattern or symbol may be discernable even when such an OLED is turned off.

It is therefore an object of the invention to provide an improved way of obtaining non-emitting areas in an OLED. SUMMARY OF THE INVENTION

The object of the invention is achieved by a method of manufacturing a light-emitting device according to claim 1, and by a light-emitting device according to claim 12.

The method according to the invention of manufacturing a light-emitting device, which light emitting device comprises a first electrode layer and a second electrode layer and a light-emitting layer stack arranged between the first electrode layer and the second electrode layer, which method comprises modifying an electrical property of the electrode material within a specific region of the first electrode layer such that an injection of charge carriers into the light-emitting layer stack is inhibited over the modified region of the first electrode layer to give corresponding dark region in an outcoupling surface of the light emitting device.

The term 'specific region' simply means a region of the first electrode which corresponds to a 'dark' or essentially non-illuminated region on the outcoupling surface (also referred to as the light exit area or external light-emitting surface) of the light-emitting device. By inhibiting the injection of charge carriers into the light-emitting layer stack throughout one or more such specific regions, a pattern, symbol or sign (simply referred to as 'pattern' in the following) can be rendered by the light-emitting device by effectively only illuminating those areas that are actually required. The term 'inhibiting' is to be understood to mean that the charge-carrier injection can be partially reduced, greatly reduced, or even essentially completely suppressed throughout the specific region. In this context, "essentially completely suppressed" or "essentially non- emitting" means that, even if a small degree of emission were to take place within a dark region, this would be so low as to be effectively unnoticeable by a viewer.

In contrast to prior art light-emitting devices which only cover parts of the light-emitting surface by an opaque material to block the emitted light in certain regions, the method according to the invention actually modifies the electrode itself within the specific region so that charge-carrier injection essentially does not take place, or only takes place to a very reduced extent.

In the method according to the invention, the body of the electrode itself is treated. Since only an electrical property of the electrode is altered or modified, the physical dimensions of the first electrode are left essentially unchanged, i.e. an essentially homogenous electrode thickness is maintained over the extent of the first electrode layer. This has the advantage that, in the off state of the light-emitting device, the pattern cannot be discerned since the specific regions defining the pattern are essentially physically indistinguishable from the rest of the first electrode layer.

A light-emitting device according to the invention comprises a first electrode layer and a second electrode layer, and a light-emitting layer stack arranged between the first electrode layer and the second electrode layer, and wherein the first electrode layer comprises a specific region in which an electrical property of the electrode material has been modified such that an injection of charge carriers into the light-emitting layer stack is inhibited in that modified region to give a corresponding dark region in an outcoupling surface of the light emitting device.

The dependent claims and the subsequent description disclose particularly advantageous embodiments and features of the invention.

The term 'light-emitting layer stack' is to be understood to a plurality of layers, in which each layer may serve a particular purpose. A layer stack generally comprises one or more layers of suitable semiconductor materials, e.g. a hole transport layer, a hole injection layer, an electron transport layer, etc., any of which can comprise a suitable organic material to give an OLED. In addition, the properties of these layers may be chosen, for example, to match the energetic levels within the device, to tune the efficiency of the device, etc. A specific layer may act, for example, as a hole-blocking layer, while another layer may act as an electron-blocking layer. A hole injection layer may be included, for example, to improve device reliability and to ensure a more homogeneous illumination. Latest advances in OLED technology have led to the development of polymer based OLEDs and SMOLEDs (small molecule OLEDs). The hole and electron transport functionalities of the materials used can also be combined in a single layer. In the following, for the sake of simplicity, but without restricting the invention in any way, the term 'light-emitting layer stack' is to be understood to mean any specific combination of layers in the light-emitting device.

Since a semiconductor light-emitting device with a relatively large light- emitting surface is generally realized for technical reasons as an organic light-emitting device (OLED), the term OLED will be used in the following to refer to a light-emitting device, without however restricting the invention in any way. Generally, the light- emitting surface of the OLED is given by the substrate (often glass) and transparent anode, while the cathode can be opaque or coated with a reflective material so that the light generated in the layer stack must exit through the anode. Of course, either anode or cathode can be transparent, or even both (particularly when the layer stack also comprises transparent materials) so that such an OLED device can emit on either side or even on both sides, depending on the choice of electrode material. Preferably, therefore, in a light-emitting device according to the invention, the materials of the first electrode layer and the second electrode layer comprise transparent materials such as transparent conducting oxide (TCO), and the layer stack also comprises transparent materials, so that the light emitted by the light-emitting layers is outcoupled through the first electrode layer and the second electrode layer. Such an OLED device, comprising only transparent materials for substrate, electrodes and layer stack, would be particularly advantageous, since it would be completely transparent in the off-state, while any symbol or pattern given by the modified specific regions, would only be rendered when the device is turned on.

The methods described herein for altering an electrical property of a specific region of the first electrode layer are particularly suitable for carrying out on the electrode layer that is attached to the substrate, and before the light-emitting layer stack and the second electrode layer are applied. Usually, this 'first' electrode layer is the anode of the OLED device, and generally comprises a transparent conducting oxide (TCO) such as indium tin oxide (ITO). Therefore, in a particularly preferred embodiment of the invention, the first electrode layer comprises the anode of the OLED. While it is possible to manufacture OLEDs in which the cathode is attached to the substrate, and the anode comprises the outcoupling surface, such devices are not as widespread. Therefore, for the sake of simplicity, but without restricting the invention in any way, the term 'anode' is used in the following to refer to the 'first electrode layer', the charge-carriers to be inhibited comprise holes, and a layer of the light-emitting layer stack adjacent to the first electrode layer serves to transport holes. Evidently, the same technique applies in principle for a first electrode comprising the cathode, in which case the layer of the light- emitting layer stack adjacent to the cathode would comprise an electron transport layer.

In the method according to the invention, a pattern is created by altering any suitable electrical property which would serve to reduce or inhibit the injection of charge carriers from the anode into the adjacent layer stack. 'Good' charge carrier injection into the layer stack is obtained by matching or aligning the work function of the relevant part of the layer stack to that of the anode (the same applies to relevant part of the layer stack and the cathode). The work function is the minimum energy required to release a charge carrier (hole/electron) from the electrode (anode/cathode) so that it can propagate into the layer stack. To allow for charge carrier injection, the work functions of the anode and cathode must be matched, or aligned, to the relevant parts of the layer stack. In state of the art OLEDs, materials are chosen whose work functions give an optimal charge carrier injection. In a preferred embodiment of the invention, therefore, the electrical property comprises the work function of the electrode material, and the step of modifying the electrical property within the specific region of the first electrode layer comprises 'misaligning' the work function of the electrode material in the specific region of the anode to the work function of the adjacent layer of the layer stack. A misaligning of the work function in this sense means that the work function of the modified region is 'worse' compared to the work function in an unmodified, emitting region. A misalignment of the work function of an electrode layer with respect to the work function of its adjacent light-emitting layer stack dramatically reduces the propagation of charge carriers from the electrode into the semiconductor material. The amount of current flowing in such regions will be lower by orders of magnitude compared to non-treated regions. As the light emitted by an OLED is proportional to the current that flows, the resulting contrast between dark (inhibited or non-emitting regions) regions and bright (emitting) regions will be very favourable.

In the method according to the invention, the work function of the anode is misaligned deliberately to reduce the efficiency of hole injection. The degree or extent of misalignment necessary to reduce or even prevent hole injection will depend on the properties of the layer adjacent to the anode. For example, reducing the work function of the anode from a value of -4.5 eV to a smaller value of -3.0 eV can be sufficient to inhibit hole injection When the OLED is turned on, therefore, holes are prevented from propagating into layer stack and towards the cathode, as they do in the unmodified electrode regions. The modified specific regions therefore appear darker than the unmodified regions in the surface of the OLED when turned on, but essentially cannot be distinguished form the unmodified regions when the OLED is turned off.

There may be a number of ways of altering the work function in a region of the electrode to the desired degree. For example, in a further preferred embodiment of the invention, the work function of the electrode material in the specific region of the first electrode layer is misaligned by modifying the surface stoichiometry of the first electrode layer in that specific region. The 'surface stoichiometry' is to be understood as the particular composition of elements or compounds in the surface of the electrode layer. By altering the stoichiometry in a certain manner and to a certain degree within a specific region of the electrode layer, the work function is effectively misaligned with respect to the light-emitting layer stack, so that the release of charge-carriers into the light-emitting layer stack can be reduced or inhibited throughout that region. Alteration of the surface stoichiometry in a specific area can be done by doping with a suitable material or reducing some components of the material of the first electrode layer, as appropriate.

Alternatively, a modification of the morphology of the first electrode layer may be effected in specific regions. For example, the surface of the first electrode layer in those regions can be altered using a laser beam in such a way that an electrical property of the electrode is modified sufficiently to inhibit charge-carrier injection from those regions into the layer stack.

It shall be stressed at this point that the modification of the surface stoichiometry or morphology in a specific region of the first electrode layer only affects a relatively thin sub-layer or stratum within the body of the electrode in that specific region. The Outward conductivity' is affected in this region, since the injection of charge- carriers is inhibited only in the outward direction, i.e. in the direction of the other electrode (the term 'outward' is to be understood to mean in a direction essentially perpendicular to the surface of the first electrode layer, e.g. the direction in which current would flow from anode to cathode). However, below this stratum, the lateral conductivity of the electrode is maintained. Therefore, even if an unmodified region of the first electrode layer is completely enclosed by a modified region, the OLED will still emit in this unmodified region since the electrode at that region is still connected electrically to the power supply owing to the lateral conductivity of the electrode 'underneath' the modified stratum.

In a further preferred embodiment of the invention, the electrical property comprises the electrical conductivity of the electrode material, and the step of modifying the electrical property within the specific region of the first electrode layer comprises reducing or eliminating the outward electrical conductivity of the electrode material in the specific region of the first electrode layer. This is also referred to as 'passivation' of the electrode, in this case only in a certain specific region of the electrode, and also has the result that the injection of charge carriers into the light-emitting layer stack, in the direction of the other electrode in a completed OLED device, is somewhat reduced, greatly reduced, or even entirely suppressed in that region.

Again, passivation of the first electrode layer, or indeed any other conceivable modification of an electrical property of the electrode material, is preferably only carried out within a stratum or sub-layer below the surface of the electrode while maintaining the lateral conductivity through the body of the electrode, so that emitting regions of the finished OLED device can be surrounded or enclosed by non-emitting regions. For a transparent electrode of ITO, for example, with a thickness of about 150 nm, the modified stratum might have a thickness of up to only a few tens of nanometers in the specific regions adjacent to the layer stack, so that the lateral conductivity throughout the body of the electrode is maintained in the underlying layer adjacent to the substrate. Evidently, the depth of the modified stratum within the body of the electrode layer will largely depend on the technique(s) applied to modify the specific regions, and on the desired effect.

In one particularly preferred embodiment of the invention, the electrical property of the electrode material in a specific region of the first electrode layer - e.g. the electrical conductivity or the surface stoichiometry - is modified in a plasma-printing process, for example a dielectric barrier discharge process under atmospheric pressure, in which the first electrode layer is used as one of two electrodes, and a dielectric material is sandwiched between the first electrode layer (already applied to the substrate of the future OLED device) and a further 'apparatus electrode' required for the plasma- printing process. The dielectric material is preferably previously structured according to the pattern which is to be rendered by the finished OLED. For example, the dielectric can comprise a relief or bas-relief of the pattern which is to be rendered. The dielectric can then be sandwiched between the first electrode layer and the apparatus electrode, and the gaps or cavities given by the relief coincide with the specific regions in which the work function of the first electrode layer is to be misaligned. When a voltage is applied across the electrodes of the apparatus, a plasma is ignited in the cavities. Depending on the composition of a gas within the cavities of the dielectric, and depending on process parameters, the electrode material can be subject to certain modifications within the specific regions. For example, the surface stoichiometry of the first electrode layer can be altered to misalign the work function in those specific regions. The plasma-printing process is completed to produce a substrate with first electrode layer modified for the pattern to be rendered by the future OLED device. In a subsequent manufacturing step, which can be carried out directly or later, as appropriate, this substrate with its modified first electrode layer is further processed in the usual manner to apply the organic layer stack and the second electrode to give a finished OLED device for rendering the pattern.

In a further preferred embodiment, the electrical property of the electrode material in the specific region of the first electrode, for example the electrical conductivity, is modified in a diffusion process, preferably under suitable atmospheric conditions. Such a diffusion process may be a locally induced process, for example an oxygen diffusion process, in which the substrate anode is locally heated to promote a diffusion of species into the material, and is preferably carried out under specific suitable atmospheric conditions. For example, an oxygen gas atmosphere can be used, resulting in oxygen diffusion into the anode material. The diffused oxygen in the modified specific regions can satisfactorily inhibit charge-carrier injection from an ITO anode.

As outlined above, the rendering of a 'pattern' to be illuminated should be as accurate as possible, with well-defined contours. Therefore, in a further preferred embodiment of the invention, the locally induced diffusion process is supplemented by a local heating of the specific region of the first electrode layer. By applying heat to the specific region, or a part of the specific region, it may be ensured that the diffusion process is carried out to a greater degree of accuracy. Heat may be applied in any suitable way. However, in a preferred embodiment, the specific region of the first electrode layer is locally heated using a beam of laser light, which may be directed very accurately at the specific region or at a part of the specific region. The area or specific region heated by the laser may in this way be made more susceptible to the diffusion. In another approach, the specific regions of the electrode can be heated using a plasma printing process or a dielectric barrier discharge process with a structured dielectric, in a similar manner as already outlined, and wherein the cavities of the structured dielectric contain a specific gas composition including oxygen, which is caused by the ignited plasma to diffuse into the regions of the first electrode layer coinciding with the cavities, so that the electrical conductivity in those regions of the first electrode layer is considerably reduced.

The techniques described above - namely modification of the surface stoichiometry or reduction of the electrical conductivity through diffusion - could be applied exclusively, but may also conceivably be combined in the treatment of an electrode layer. For example, the work function of the anode may be misaligned in certain specific regions in a first step, and then the same specific regions or other specific regions may be further treated with a laser-augmented diffusion process or a plasma- printing diffusion process.

Using the method according to the invention, OLEDs with novel illumination characteristics can be obtained. For example, it is easy to arrive at an OLED which can illuminate a pattern in a 'grey-scale' manner, i.e. with different regions illuminated to different intensities, depending on the extent to which the electrical properties of the specific regions have been modified. Some regions may block charge- carrier injection completely, while other regions may be modified to only partially block charge-carrier injection. In this way, interesting patterns of dark, bright and intermediate regions can be obtained using such an OLED. Combined with the advances in OLED size, it may be possible, using the method according to the invention, to create large- scale OLEDs for rendering patterns in applications such as glazing for windows, room dividers, screens, etc.

Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the

accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic cross-section through a prior art OLED;

Fig. 2 shows a schematic cross-section through, and a plan view

of a prior art substrate and anode manufactured to

include metallization structures;

Fig. 3 shows a number of signs including pictograms;

Fig. 4 shows a schematic cross-section through an OLED manufactured using the method according to the invention;

Fig. 5 shows a schematic representation of relevant elements used in a plasma printing process;

Fig. 6 shows a schematic rendering of a laser-augmented diffusion process.

In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 shows a very simplified cross-section through an OLED 10. A substrate 11 , which can be made of glass or any other suitable material such as polyethylene terephthalate (PET), is coated with a transparent electrode 12, in this case the anode 12, which can be a transparent conductive material such as indium tin oxide (ITO). In this very simple example, the light-emitting layer stack 13, 14 between anode 12 and cathode 15 comprises an organic hole transport layer 13 and an electron transport layer 14. The materials of the anode 12 and hole transport layer 13 are matched to optimise hole injection, while the materials of the cathode 15 and electron transport layer 14 are matched to optimise electron injection. An additional hole- injection layer (not shown in the diagram) could be included between the substrate 11 and the anode 12 to further optimize the performance of the device 10. When the substrate is made of PET or another plastic, an additional barrier layer (diffusion barrier) may be included between the substrate and the anode. During operation, a DC power supply 6 applies a higher potential difference between anode and cathode. This causes holes to be transported from the anode 12 in the direction of the cathode 15, and electrons to be transported from the cathode 15 in the direction of the anode. Where the holes and electrons meet, energy is released in the form of visible light radiation, which is emitted through the external surface 16 of the OLED 10. Here, with an opaque or reflective cathode 15, the generated light is outcoupled through the transparent anode 12 and substrate 11. Evidently, with a transparent cathode 15, the OLED 10 could emit both through the anode 12 and through the cathode 15.

Use of the latest OLED technology may allow large-scale OLED devices to be incorporated into larger areas such as glass doors or windows. However, such large scale OLEDs may require additional metallization structures to improve conductivity during operation.

Fig. 2 shows a simplified cross-section through the substrate 11 and anode 12 of such an OLED, in this case a large-scale OLED with substrate size exceeding 6". Here, additional metallization structures 21, 22 have been applied to the electrode 12 to compensate for the relatively poor conductive properties of the electrode 12, usually made of a transparent material such as indium tin oxide (ITO). A 'frame' 22 surrounds the entire area to ensure that all regions of the electrode 12 are optimally connected to the power supply (not shown), and metallization strips 21 ensure an even distribution of the potential across the entire electrode area. As can be seen in the diagram, the metallization structures 21, 22 have a certain height, in the region of about 200 nm up to several μm depending on type of application process. The frame, which surrounds the entire light-emitting surface of the OLED, can have a width in the region of several millimetres, while the metallization strips can be very narrow, for example about 60 μm, and therefore effectively invisible to the naked eye. In large-area lighting devices, the metallization structures 21, 22 play an important role in improving the conductivity throughout the anode 2, particularly in the case of a transparent conducting oxide such as ITO, whose conductivity is not optimal over large areas, as the skilled person will know. This allows OLEDs to be used to good effect in large areas such as windows, glass screens, displays, glass doors, etc.

Fig. 3 shows a selection of well-known signs, namely "no smoking" and "exit" signs, chosen here for their simplicity. The signs or symbols can be rendered so that the informative pattern is illuminated while the background remains dark, or vice versa. State-of-the-art devices to render these such symbols may be stand-alone devices for mounting on a wall. The signs or symbols can be illuminated continuously or only as required. Such devices usually comprise a glass or plastic covering in which the pattern or symbol is rendered by means of transparent and opaque areas in the covering, which is back- lit using a conventional light source such as an incandescent bulb. However, even when turned off, the symbols rendered by these devices are still visible.

Fig. 4 shows an OLED 1 manufactured using the method according to the invention and used to render a pattern such as one of the symbols shown in Fig. 3. Here, a glass substrate 11 is coated with a transparent anode 2, which is followed by a light- emitting layer stack 13, 14 and a cathode 15. Light emitted by the OLED 1 exits through a surface emitting area 16 given by the substrate 11 and transparent anode 2. The OLED 1 exhibits regions Ri, R2 of the anode 2 in which an electrical property of the anode has been modified or altered, so that hole injection from the anode 2 into the adjacent layer 13 of the layer stack is reduced or inhibited over that region when a voltage is applied across the electrodes, resulting in dark areas Di, D ₂ in the light-emitting surface 16 of the OLED 1 corresponding to the modified regions Ri, R ₂ . The diagram shows that the anode 2 remains unchanged with respect to thickness. This makes the method according to the invention particularly suitable for the processing of an electrode on a large-scale substrate to which metallization structures can later be applied, as described using Fig. 3 above, since these structures can only be applied to an electrode that is essentially flat. Furthermore, the modified regions Ri, R ₂ of the anode 2 do not extend through the entire depth of the anode 2, so that the lateral conductivity through the body of the anode 2 is maintained. Therefore, 'closed' regions such as the regions 30, 31 shown in Fig. 3 can be easily obtained, since the region of the anode 2 'underneath' the modified stratum Ri, R ₂ is still electrically connected to the power supply 6.

Optically, an OLED 1 according to the invention will be essentially indistinguishable - in the off state - from a state of the art OLED that emits over its entire area. This is because the modification of the electrical property in the specific regions Ri, R ₂ of the first electrode layer 2 does not involve any physical alteration to the dimensions of the electrode layer 2, nor to the transparency of the electrode layer 2. This makes the OLED 1 according to the invention particularly attractive in applications in which a pattern or symbol should be rendered only when desired, and which should be essentially invisible otherwise.

Fig. 5 shows, in a very simplified way, how the anode 2 of Fig. 4 may be manufactured in a plasma-printing process. An additional apparatus electrode 4, required for the plasma-printing procedure, is covered with a structured dielectric material 3 such as a suitable ceramic, glass or polymer. The dielectric material could also be applied to the apparatus electrode 4 in a photoresist or photolithography technique. The 'structure' corresponds to the pattern or symbol that is to be rendered by the finished OLED device 1, and is given by cavities Ci, C ₂ in the body of the dielectric 3, for example as a type of relief or bas-relief pattern. The apparatus electrode 4, dielectric 3 and anode 2 are combined to give a layered structure, wherein the cavities are filled with a certain gaseous composition, for example a reductive gas or simply oxygen, depending on whether a diffusion, a reaction or even a deposition within the monolayer thickness regime is desired. Current is caused to flow through this structure under specific atmospheric conditions, for example at atmospheric pressure. To this end, a suitable power supply with a high voltage and alternating current is connected across the anode 2 and the apparatus electrode 4. Under suitable conditions, the ensuing dielectric barrier discharge causes a plasma to be ignited in the cavities Ci, C ₂ , which in turn acts on the material of the anode 2 to alter the electrical property of the anode 2 in regions Ri, R ₂ corresponding to the cavities Ci, C ₂ . For example, with appropriate choice of anode material, the work function of the anode 2 may be altered by oxygen diffusion into those regions Ri, R ₂ to significantly reduce or even inhibit hole injection in the completed OLED device.

Once the anode 2 has been treated in this manner, manufacture of the OLED 1 can continue with deposition steps to deposit the organic layers 13, 14 and the cathode 15. The modified regions Ri, R2 of the anode 2 allow the desired pattern to be rendered by the finished OLED 1.

Fig. 6 shows, in a very simplified way, how the anode 2 of Fig. 4 may be manufactured in a diffusion process. Here, the substrate 11 and anode 2 are suspended in a diffusion apparatus 63, and a diffusion gas 62 with a composition specifically chosen to alter the surface stoichiometry or the electrical conductivity of the anode 2, for example oxygen is released into the apparatus 63 through a gas inlet 61. The regions Ri, R2 of the anode 2 that are to be treated in order to alter an electrical property are specifically heated by a laser beam L directed at the anode 2, for example from a laser source 60 located outside the diffusion apparatus 63 (which can be a glass-enclosed chamber 63 or a chamber 63 with a glass window through which the laser light beam L can be directed), so that the laser light beam L strikes the anode 2 at the surface. The application of laser energy for a limited time will stop the slowly progressing diffusion process, resulting in a limited depth of transformation. The desired modification therefore only takes place in those regions Ri, R2 that are heated by the laser light beam L, so that the electrical property of the anode 2 is only altered in those desired specific regions Ri, R ₂ .

Although the present invention has been disclosed in the form of a number of preferred embodiments, it is to be understood that additional modifications or variations could be made to the described embodiments without departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. A "unit" or "module" can comprise a number of units or modules, unless otherwise stated.