WO2005118210A1

WO2005118210A1 - Apparatus and method using a gas vortex for extracting debris during laser ablation

Info

Publication number: WO2005118210A1
Application number: PCT/GB2005/002087
Authority: WO
Inventors: Ashley Larthwell
Original assignee: Cambridge Display Technology Limited
Priority date: 2004-05-28
Filing date: 2005-05-26
Publication date: 2005-12-15
Also published as: GB2428398A; GB2428398B; TW200615071A; TWI364340B; GB0623390D0; GB0412000D0

Abstract

A method and apparatus (401) for ablating material (406a) from a substrate (406) using a light source (402) is provided, the method comprising irradiating an area (406a) of said substrate (406) with a beam (405) from said light source (402) to ablate substrate material from said irradiated area, whilst maintaining a vortical gas curtain (404a) around said irradiated area (406a), said curtain (404a) being formed of gas directed vortically onto said substrate (406) around said irradiated area (406a) and producing within said curtain (404a) a vortex (404b) directed away from said substrate (406), whereby said ablated material is contained by said gas curtain (404a) and is entrained in said vortex (404b) and thereby carried away from said substrate (406).

Description

APPARATUS AND METHOD USING A GAS VORTEX FOR EXTRACTING DEBRIS DURING LASER ABLATION

This invention generally relates to methods and apparatus for ablating material using a light source, and more particularly to laser ablation of materials for molecular electronic devices, such as organic light emitting diodes.

Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, colourful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic LEDs may be fabricated using either polymers or small molecules in a range of colours (or in multi-coloured displays), depending upon the materials used. Examples of polymer- based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so called small molecule based devices are described in US 4,539,507.

A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP) or a light emitting low molecular weight material, and the other is a layer of a hole transporting material such as a polythiophene derivative (for example, polyethylene-dioxythiophene (PEDOT)) or a polyaniline derivative. Fabrication of an OLED device by commonly employed techniques involves deposition of layers of the materials, followed by selective removal of the materials from areas where they are not wanted. In such a layered construction, the PEDOT layer has been found to be more difficult to remove than the LEP layer, and WO 01/39287 discloses a method of removing PEDOT by plasma etching.

A cross-section through a basic structure 100 of a typical organic LED is shown in Figure la. A glass or plastic substrate 102 supports a transparent anode layer 104 comprising, for example, indium tin oxide (ITO) on which is deposited a hole transport layer 106, an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, poly(p-phenylenevinylene) (PPV) and the hole transport layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise, for example, polystyrene-sulphonate- doped polyethylene-dioxythiophene (PEDOT:PSS). Cathode layer 110 typically comprises a low work function metal such as calcium and may include an additional layer immediately adjacent electroluminescent layer 108, such as a layer of lithium fluoride, for improved electron energy level matching. Contact wires 114 and 116 to the anode the cathode respectively provide a connection to a power source 118. The same basic structure may also be employed for small-molecule devices.

In the example shown in Figure la, light 120 is emitted through transparent anode 104 and substrate 102. Such devices are referred to as "bottom emitters". Devices which emit through the cathode may also be constructed, for example by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent.

Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixelated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image.

Figure lb shows a cross-section through a basic passive matrix OLED display 150, in which like elements to those of Figure la are indicated by like reference numerals. In the passive matrix display 150, the electroluminescent layer 108 comprises a plurality of pixels 152 and the cathode layer 110 comprises a plurality of mutually electrically insulated conductive lines 154, running into the page in Figure lb, each with an associated contact 156. Likewise, the ITO anode layer 104 also comprises a plurality of anode lines 158, of which only one is shown in Figure lb, running at right angles to the cathode lines. Contacts (not shown in Figure lb) are also provided for each anode line. An electroluminescent pixel 152 at the intersection of a cathode line and anode line may be addressed by applying a voltage between the relevant anode and cathode lines.

Figure lc shows a simplified cross-section through a practical passive matrix OLED display in which, for simplicity, individual pixels are not shown. Again, like elements to those of Figures la and lb are indicated by like reference numerals. The substrate 102 typically comprises 0.7mm or 1.1mm thick glass and an anode contact layer 105 is provided above ITO layer 104, for example comprising a layer of aluminium sandwiched between layers of chrome. Since the OLED materials, and in particular the light emitting polymer material and the cathode, are susceptible to oxidation and moisture, the device is encapsulated in a metal can 111 which is attached by glue 113 onto contact layer 105, small glass beads within the glue preventing the contacts being shorted out.

A plurality of such OLED display devices may be fabricated on a single, large glass ITO substrate 160 as shown in Figure Id. This substrate is patterned using a photoresist and organic layers 106, 108 are then deposited by spin coating before cathode layer 110 is applied. However, since the spin coating technique is non- selective, that is it deposits a thin film of organic material uniformly across the substrate, material must afterwards be removed from where it is not wanted. In particular, the spin coated organic material must be removed from areas where the encapsulating can 111 will be attached to the substrate, and also from areas where electrical connections will be made to the devices. In Figure Id, horizontal and vertical strips or scribe lines 162 indicate where material is to be removed for attaching the can 111. The organic material may be removed mechanically, by scraping, or by using a wet chemical photolithographic process (relatively long and expensive) but the preferred method for removing the organic material is by laser ablation.

The detailed mechanics of laser ablation processes are not well understood but the ablation of polymers appears to involve two main physical mechanisms, photothermal vaporisation and photochemical dissociation, although it is not known which mechanism dominates. Photothermal vaporisation occurs when coupling of laser light into the surface of a material causes the local temperature and internal pressure to rise, causing violent ejection of material and vaporisation of part of the surface. Photochemical effects occur when the energy of an incident photon exceeds that required to break a bond within the material, again resulting in vaporisation of the material. Laser ablation process parameters are generally defined in terms of fluence, that is energy delivered per unit area, and shots per area (SPA) since the ablating lasers are generally pulsed. In an ablation process, there is generally a threshold fluence for ablations occur, defining a minimum fluence, and the maximum fluence is determined by the desire for clean and efficient ablation of the polymer layers without significant damage to the underlying ITO. The preferred range of fluence depends upon the type and thickness of material used in a display and can be determined by routine experiment.

In a preferred arrangement, an excimer laser with a wavelength of 248 nm is employed. At this wavelength, the LEP is strongly absorbing but the PEDOT is a poor absorber. It therefore appears that a single shot per area may be sufficient to ablate the LEP layer, which is typically one quarter to one half a wavelength in thickness, but a further three shots per area may be needed to remove the PEDOT, which has a typical thickness of 60- 120mm.

Figure 2a shows an outline diagram of laser ablation apparatus 200 which may be employed for scribing lines in the organic layers of a spin coated substrate for an OLED display. Figure 2b shows a diagram of the apparatus and Figure 2c shows, in simplified form, an optical arrangement for the apparatus. The features of Figures 2a to 2c, which are described further below, are generic but a preferred excimer laser based process tool is the M5000P laser process tool available from Exitech of Oxford, UK. For further details of this particular tool reference may be made to the manufacturer's specifications.

Referring to Figure 2a, apparatus 200 comprises a process cell 202 supporting a vacuum chuck 204 for holding a workpiece. The process cell allows a clean air or gas flow over the workpiece during laser ablation for debris removal and is moveable to facilitate loading of a substrate onto the chuck (in the Figure it is shown in its loading position). The apparatus also includes an optical input 206 and optics 208, and a dual axis mask stage 210 for holding a metal plate mask. The size and shape of the ablating laser beam can be adjusted by interchanging metal masks mounted on the mask changer, and is thus controllable by a computer programme. Once a substrate has been loaded and aligned using fiducial markers, laser ablation proceeds according to pattern data input from a data file, under control of a computer.

Referring now to Figure 2b, this shows a simplified block diagram of apparatus 200. A vacuum chuck and workpiece 204, 205 is supported by a mount 212 which allows the workpiece to be moved in perpendicular x- and y- directions and to be rotated. An excimer laser 214, such as an LPX210 laser from Lambda Physik of Gottingen, Germany, provides a 248 nm 400 mJ pulsed output via a shutter (not shown) to an attenuator 216 to control the fluence. The beam is then shaped by a mask within mask changer 210 before being delivered to the workpiece via a lens elevator 218, which has a sensor output and control input to provide automatic focus adjustment to focus the laser beam onto the workpiece. A fluorescent screen 220 and a CCD camera 222 provide an input to a beam profiler to allow the beam profile at the mask plane to be imaged and analysed. The apparatus is controlled by a computer system 226 incorporating a processor, programme and data memory, input/output interfaces, and user input/output interface devices such as a screen, keyboard and mouse. Computer 226 controls attenuator 216, lens elevator 218, and stage 212 and interfaces to a variety of sensors and actuators via a programmable logic controller (PLC) 228. Computer 226 also provides a trigger output to laser 214. In use, control computer 226 loads pattern data defining a pattern for ablation by the laser beam, apertures of a mask within mask charger 210 to be employed, fluence, shot number and overlap, laser repetition rate and other parameters, and moves the workpiece and controls operation of the apparatus to achieve ablation in the defined pattern.

A particular number of shots per area may be achieved by moving the chuck piece at a set speed, and with a set laser repetition rate, relative to the stationary laser beam. Since the repetition rate or number of pulses per second is generally low for excimer lasers (for example a maximum of 100Hz for the LPX 210) it is normal to refer to the energy rather than power output of the laser. Moreover since the average pulse length is short, for example around 20 ns for LPX 210, each shot effectively sees the substrate as stationary. The XY stages move a step distance after each pulse so that successive pulses overlap to achieve a desired SPA.

Figure 2c shows a simplified diagram of the beam relay optics comprising a laser input 230 and optical system 232 focussing the beam onto a mask 234 adjacent a field lens (not shown), the shaped beam then being focussed onto workpiece 205 by a projection lens 236.

Figure 3 a shows an example of a known mask 234 having a central square aperture. In general an aperture mask plate may have a plurality of apertures of different shapes and sizes, to facilitate changing an aperture during the laser ablation process. In the M5000P machine mentioned above the beam delivery system incorporates a four times size reduction in both dimensions so that the focussed beam used for ablation is scaled down to one quarter of the size of the aperture.

Figure 3 b shows a graph of energy delivered to the work piece against distance along a line through the beam, illustrating an idealised, "top hat" profile (in practice the corners of this profile will be slightly rounded). Figure 3c shows a top view of the work piece, illustrating successive positions 302a, b, c, d, e of a square beam formed using the aperture of Figure 3a and having a profile as illustrated in Figure 3b. The ablation apparatus is controlled, for example, to provide a signal shot or pulsed output from the excimer laser 214 at each beam position 302a-e. As illustrated in Figure 3 c the beam is stepped along by one quarter of the length of a side of the square beam each shot and thus, after an initial 3 shots start up, the ablated material receives 4 shots per area (4 SPA).

Figure 3d shows, schematically, a magnified view of a surface of an OLED substrate after LEP and PEDOT layers have been removed by such a conventional laser ablation procedure. In Figure 3d, region 304 indicates the scribed line and regions 306 the remaining spin coated LEP and PEDOT layers to either side of the line (the cathode is deposited after laser ablation). Although the laser ablation apparatus will generally include a debris removal system such as a pumped flow cell, inspection of the substrate following ablation shows the presence of debris and surface defects, which are shown diagrammatically in Figure 3d. Thus, spots 308 can be observed to either side of the scribed line, possibly comprising ablated PEDOT and LEP material, and these can develop into black spots within the display after encapsulation. Also, a number of string-like structures 310, apparently less than 50 microns in length, can be observed as a wispy effect at line edges. These may result from melted or incompletely ablated material. Faint lines 312 can also be observed corresponding to the step movements of the beam, possibly comprising residual material and/or regions of over-ablation. These defects, and in particular the debris, have a deleterious effect on device lifetime and yield.

It is therefore desirable to provide an improved ablation method and apparatus, in particular a method and apparatus which reduces the number of debris particles adhering to the display area of the substrate and/or removes debris particles before they are deposited alongside a track or scribed line left following laser ablation.

Accordingly, a first aspect of the present invention provides a method of ablating material from a substrate using a light source, the method comprising irradiating an area of said substrate with a beam from said light source to ablate substrate material from said irradiated area, whilst maintaining a vortical gas curtain around said irradiated area, said curtain being formed of gas directed vortically onto said substrate around said irradiated area and producing within said curtain a vortex directed away from said substrate, whereby said ablated material is contained by said gas curtain and is entrained in said vortex and thereby carried away from said substrate.

The gas curtain is produced in the shape of a funnel, from one or more apertures of a gas pipe or gas conduit system disposed at the wider end of the funnel, towards the substrate disposed at the narrower end of the funnel. Thus, the gas flow has an axial component towards the substrate and a radially inward component. The gas is further directed by the aperture(s) to impart a tangential component to the gas flow of the curtain. The combined axial, radially inward and tangential components of the gas flow afford a vortical ('twist') movement of gas to the gas curtain. Preferably, the aperture(s) are formed as or arranged in a circle so as to provide a funnel having a continuous curved gas wall, i.e. a frustoconical shaped funnel with the substrate defining the narrow end surface thereof. The invention will be further described by reference to the preferred funnel shape of a frustocone for the gas curtain. However, variants of the preferred frustoconical shape for the gas curtain are possible according to the invention, and some of the gas may escape outside the gas curtain, for example after impinging on the substrate, provided that the shape enables the gas curtain to contain within its enclosed volume the ablated material produced during the ablation process and to comprise the flow components necessary to afford a vortical property to the gas curtain.

The frustoconical gas curtain impinges on the substrate as a circle or oval, preferably a circle, that surrounds within its enclosed area the target area of the substrate to be ablated by the laser or other light beam. As is known, the target area may be any desired shape, for example square, and the cross-sectional shape of the light beam, which preferably is a laser beam, will be shaped accordingly by suitable masking or other methods. The gas curtain should at least enclose all of the target area on the substrate, and preferably by a margin of at least 0.3 mm, more preferably at least 0.5 mm, in order to contain all or substantially all of the ablated material within the confines of the gas curtain. However, the maximum margin around the target area should be kept to a minimum, preferably no more than 1.0 mm, in order to minimise the amount of non-target substrate area exposed to ablated material within the confines of the gas curtain.

We prefer that the gas curtain frustocone axis is both concentric with the light beam axis and normal to the substrate surface, although a degree of tilting of up to 10° of the frustocone with respect to the light beam axis, or the normal of the substrate surface, or both, may be desirable for certain applications. The half angle of the frustocone apex (projected), i.e. between the frustocone curved surface and the frustocone axis, is preferably in the range from 30° to 70°, more preferably in the range from 40° to 55°, and most preferably is 45°. The relative velocities of the respective gas flow components in the apical and tangential directions in the gas curtain may be varied by varying the angle of the gas pipe(s) at the entry point(s) of gas to the gas curtain. The velocities of the apical and tangential gas flow components preferably are in a ratio in the range from 2:3 to 2:1, more preferably 3 :4 to 3 :2, and most preferably are in a ratio of about 4:3.

On hitting the substrate, e.g. downwards, the gas curtain is reflected and produces within the curtain a vortex directed away from said substrate in the reverse axial direction, e.g. upwards. The vortex entrains ablated material present within the confines of the gas curtain and carries the material away from the substrate before it is deposited on the alongside the track left by the ablation of the irradiated area and/or removed any ablated material that has deposited, thus reducing the number of debris particles that might otherwise adhere to the display area of the substrate. The entrained ablated debris is conveyed (e.g. up) through the gas curtain funnel by the expanding vortex, and is extracted or otherwise removed via one or more gas outlets disposed circumferentially beyond (e.g. above) the gas injection aperture(s) producing the gas curtain. Because the entrained ablation debris is concentrated radially outwards by the vortex, and therefore away from the vortex axis, the ablation debris is removed from the path of the light beam. Thus, any interference of ablation debris with the light beam and optical components is avoided or minimised.

The housing, which comprises the gas injection aperture(s) that produce the gas curtain, is preferably frustoconical with its narrow end at the level of the gas injection aperture(s) so as to allow the vortex to expand radially within the housing before the gas and entrained ablation material is removed via the gas outlet(s).

Suitable gasses to constitute the gas curtain include inert or reducing gasses such as nitrogen and argon, or air or oxygen-enriched air (in which the O content preferably is less than about 30 vol% to avoid combustion), and mixtures thereof. Preferably, the gas used is nitrogen gas.

The gas is injected from the gas inlet aperture(s) at high pressure, at a sufficient pressure to provide a curtain that can withstand, absorb or counteract the ablation Shockwaves and explosions and thus contain within its confines the ablation debris, without the gas unduly affecting or damaging the substrate that it impacts. The gas pressure will also be chosen so as to provide a vortex that is sufficiently strong to carry away the ablated debris. Suitable pressures are in the range from 0.5 to 3 bar.

In a preferred embodiment, the target area to be irradiated is a 4 mm x 4 mm square, the gas curtain impinges on a substrate in a circle having a radius of 3.3 mm, the housing is disposed 1 mm above the substrate and from gas inlets at its lower end provides a gas curtain in a frustoconical shape having a half angle of 45°, the gas is nitrogen at a pressure of 0.5 to 3 bar, the light source is a pulsed laser operated at a fluence in the range from 200 to 300 mJ moving at 1 mm increments with respect to the substrate to form an ablated track of 4 mm width.

The method and apparatus according to the invention is suitable for ablating organic layers that have been deposited, for example by spin-coating or dip-coating, onto a device substrate, in order to expose tracks of the underlying substrate for subsequently glueing to an encapsulating can that provides a barrier for the organic and other functional layers against oxidation and moisture. Suitable organic layer(s) for ablation in accordance with the invention include hole transport layers (also referred to as hole injection layers) formed of PEDOT:PSS, as disclosed for example in WO 98/05187 or EP 0901176, and electroluminescent layers of the small-molecule type, such as (8- hydroxyquinoline) aluminium ("Alq3") as disclosed in US 4,539,507, of the dendrimer type, such as disclosed in WO 90/13148, or of the polymer type such as disclosed in WO 90/13148, in particular of the conjugated polymer type such as polyfluorenes as disclosed in EP 0842208 or WO 99/54385.

The method and apparatus is particularly suitable for ablating the organic hole transport and/or electroluminescent layers on the substrate of an OLED device, so that an encapsulating can may be glued to the exposed substrate tracks. Accordingly, in a preferred embodiment, the substrate is an OLED substrate having one or more organic layers as hole transport and/or electroluminescent layers, and the substrate material to be ablated is the hole transport and/or electroluminescent layer(s). However, it will be appreciated that the method is also suitable for other devices comprising a substrate having coated on it one or more organic layers to be ablated to expose tracks of the underlying substrate. Accordingly, in another embodiment, the substrate is a photovoltaic device substrate having one or more organic layers as hole transport and/or photon capture layers, and said substrate material to be ablated is the organic hole transport and/or photon capture layer(s).

Thus, in a second aspect, the present invention provides an ablation apparatus configured to operate in accordance with the method described above. In accordance with this aspect, the present invention further provides an apparatus for ablating material from a substrate using a light source, the apparatus comprising: a light source; means for irradiating an area of said substrate with a beam from said light source to ablate substrate material from said irradiated area; means for directing gas vortically onto said substrate around said irradiated area to provide a vortical gas curtain around said irradiated area and within said curtain a vortex directed away from said substrate; means for removing ablated material contained by said gas curtain and entrained in said vortex and thereby carried away from said substrate.

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying drawings in which:

Figures la to Id show, respectively, a cross-section through an OLED structure, a cross- section through a pixelated OLED display, a cross-section through a practical, encapsulated OLED display device, and a substrate for fabrication of a plurality of devices as shown in Figure lc;

Figures 2a to 2c show, respectively, an outline diagram of a laser ablation apparatus, a block diagram of the laser ablation apparatus of Figure 2a, and an optical arrangement for the laser ablation apparatus of Figure 2a;

Figures 3a to 3d show, respectively, a conventional aperture mask plate, a beam profile obtained using the mask of Figure 3 a, a top view of the surface of a workpiece as a beam formed by the aperture of Figure 3 a is scanned across the workpiece, and a magnified, schematic view of an ablated line and debris left by the beam scanning of Figure 3c;

Figure 4 shows a side cross-sectional representation of part of a laser flow cell housing for an apparatus in accordance with an embodiment of the present invention; and

Figure 5 shows a top view of a vane plate for a laser flow cell body as shown in Figure 4.

As shown in Figure 4, a housing 401 for a laser 402 has gas injection apertures 403a defined by complementary angled surfaces 401c and 40 Id. that produce a frustoconical gas curtain 404a. The housing 401 is configured concentrically with axis of the laser beam 405 directed at the target area 406a of substrate 406 , and has a funnel-shaped interior with its narrow end at the level of the gas injection apertures 403 a. The housing 401 comprises a main body part 401a and a vane plate part 401b, defining gas inlet passages 403b fed via a gas ring 403 c from gas inlet pipes 403 d. High pressure nitrogen gas passing through gas inlet passages 403b is deflected by angled surface 401c of housing body part 401a and thus directed downwards towards the substrate 406 in a frustoconical gas curtain 404a, impacting with the substrate 406 in a circle around and outside of target area 406a. On impacting with the substrate 406, gas from gas curtain 404a is deflected upwards within the confines of the gas curtain 404a and the funnel- shaped interior of the housing 401.

Gas passages 403b are offset with respect to the central axis of the housing 401, as shown in Figure 5, whereby a tangential component is imparted to the flow of gas forming the gas curtain 404a. Thus, the radially inward, downward and tangential components of the gas flows from multiple gas injection apertures provide a vortical gas flow for the gas curtain 404a. On impacting with the substrate 406, gas from gas curtain 404a is flows upwards in a vortex 404b, whereupon any ablation debris created by ablation of the target area 406a is entrained and carried away from the vicinity of the substrate. The entrained ablation material is conveyed upwards by the gas vortex 404b within the gas curtain 404a and into the interior of the housing 401. Due to the expanding funnel shape of the housing interior, the vortex 404b expands within the housing interior and thus concentrates and directs the entrained ablation debris into gas outlets 407 for removal from the housing 401. The entrained ablation debris is concentrated radially outwards by the vortex 404b and away from the vortex axis, and thus is removed from the path of the laser beam 405, so as to prevent or minimise any interference with the laser beam and optical components of the laser 402.

No doubt other effective alternatives will occur to the skilled person and it will be understood that the invention is not limited to the described embodiments but encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims.

Claims

CLAIMS:

1. A method of ablating material from a substrate using a light source, the method comprising irradiating an area of said substrate with a beam from said light source to ablate substrate material from said irradiated area, whilst maintaining a vortical gas curtain around said irradiated area, said curtain being formed of gas directed vortically onto said substrate around said irradiated area and producing within said curtain a vortex directed away from said substrate, whereby said ablated material is contained by said gas curtain and is entrained in said vortex and thereby carried away from said substrate.

2. A method according to claim 1, wherein said light source comprises a pulsed laser.

3. A method according to claim 1 or claim 2, wherein said gas curtain is in the shape of a frustocone with said substrate defining the narrow end surface thereof.

4. A method according to claim 3, wherein said maintaining a vortical gas curtain comprises providing a gas flow comprising axial, radially inward and tangential components to produce said vortical gas curtain.

5. A method according to claim 4, wherein the axis of said gas curtain is concentric with said light beam and normal to the surface of said substrate.

6. A method according to claim 5, wherein said gas curtain has an apex half angle in the range from 40° to 55°.

7. A method according to claim 6, wherein said gas is nitrogen gas.

8. A method according to claim 7, wherein said gas curtain is formed from gas at a pressure sufficient to provide a curtain that can withstand, absorb or counteract Shockwaves and explosions caused by said ablation.

9. A method according to claim 8, further comprising removing said entrained ablated material via one or more gas outlets.

10. A method according to claim 1, wherein said substrate is an OLED substrate having one or more organic layers as hole transport and/or electroluminescent layers, and said substrate material to be ablated is said one or more organic layers.

11. A method according to claim 1, wherein said substrate is a photovoltaic device substrate having one or more organic layers as hole transport and/or photon capture layers, and said substrate material to be ablated is said one or more organic layers.

12. Laser ablation apparatus configured to operate in accordance with the method of any of claims 1 to 11.

13. Apparatus for ablating material from a substrate using a light source, the apparatus comprising: a light source; irradiating means for irradiating an area of said substrate with a beam from said light source to ablate substrate material from said irradiated area; directing means for directing gas vortically onto said substrate around said irradiated area to provide a vortical gas curtain around said irradiated area and within said curtain a vortex directed away from said substrate; removing means for removing ablated material contained by said gas curtain and entrained in said vortex and thereby carried away from said substrate.