WO2004060600A2

WO2004060600A2 - Ablation methods and apparatus

Info

Publication number: WO2004060600A2
Application number: PCT/GB2003/005546
Authority: WO
Inventors: Andrew Gilbert
Original assignee: Cambridge Display Technology Limited
Priority date: 2003-01-03
Filing date: 2003-12-18
Publication date: 2004-07-22
Also published as: AU2003290283A8; GB0513245D0; AU2003290283A1; WO2004060600A3; GB0300105D0; GB2415090A; GB2415090B

Abstract

The invention generally relates to methods and apparatus for ablating material using a light source (214), and more particularly to laser ablation of materials for molecular electronic devices, such as organic light emitting diodes. A method of ablating material using a light source is described. The method comprises shaping a beam from said light source (214) such that on a surface of said material the beam has a shape with portions of different widths comprising a first width and a second narrower width; applying said beam to said surface to ablate material; and moving said beam relative to said surace such that material under a portion of said beam with second said narrower width receives more energy than material under a portion of said beam with said first width. The method, and corresponding apparatus, facilitates reduced debris laser ablation, particularly when ablating layered materials.

Description

ABLATION METHODS AND APPARATUS

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, PEDOT - polyethylene- dioxythiophene) 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, a PPV (poly(p-phenylenevinylene)) 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, PEDOT:PSS (polystyrene- sulphonate-doped polyethylene-dioxythiophene). 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 and 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 pixellated 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, somewhat similarly to a TV picture, 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 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 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 fiuence, 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 fiuence for ablations occur, defining a minimum fiuence, and the maximum fiuence 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 fiuence depends upon the type and thickness of material used in a display and can be detennined 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. 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 fiuence. 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, fiuence, 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, hi 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 3b 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 3c 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 be able to provide an improved ablation process, in particular a process which reduces the number of debris particles which adhere to the display area of the substrate and/or which removes particles deposited alongside a track left following laser ablation. It is further desirable to provide an improved laser ablation process which is suited to the special problems encountered in the ablation of layers of material, such as layers of a molecular electronic device, when some layers can be more easily removed by laser ablation than others.

According to a first aspect of the present invention there is therefore provided a method of ablating material using a light source, the method comprising shaping a beam from said light source such that on a surface of said material the beam has a shape with portions of different widths comprising a first width and a second narrower width; applying said beam to said surface to ablate material; and moving said beam relative to said surface such that material under a portion of said beam with second said narrower width receives more energy than material under a portion of said beam with said first width.

Thus, as the beam moves said portion of said beam with said first width overlaps material previously under said portion of said beam with said second width such that a region of said overlap receives a second energy and a region outside said overlap and under said portion of said beam with said first width receives a first energy, said second energy being greater than said first energy. In embodiments, material under the narrower portion of the beam, which receives more energy, is ablated in the normal manner. Material under a portion of the beam with the greater width however, in particular the portion lying outside the narrower width, receives less energy but nonetheless sufficient energy to reduce the quantity of debris left by ablation under the narrower width or central portion of the beam. It is believed that under the portion of the beam outside the narrower width incomplete or no ablation takes place but that the fiuence is nevertheless sufficient to reduce or remove particles of debris from the main ablation process.

Preferably the outer, wider portion of the beam is insufficiently powerful to remove the underlying polymer layers, but sufficiently powerful to remove any debris. Preferably the first and second widths are measured in a direction substantially perpendicular to a direction in which the beam is moved, in such a way that as the beam is translated across the surface of the material the wider width portion of beam follows the narrower width portion of the beam. In this way as the beam traverses the surface of the material debris created by the conventional ablation process at the leading portion of the beam is partially or completely removed by the wider, trailing portion of the beam.

Preferably the light source comprises a pulsed laser light source, although other intense light sources may also be employed. In this context the skilled person will understand that "light" includes radiation of wavelengths which are invisible to the human eye including, but not limited to, ultraviolet and near- and far- infra red radiation. The wavelength of the light source may be selected appropriate to the material to be ablated and, for organic molecular electronic devices such as polymer LEDs, a UV wavelength is often beneficial. However other wavelengths such as near-LR wavelengths or far-LR wavelengths, for example 10.6 microns from a carbon dioxide laser, may also be employed.

It will be appreciated that generally the ablation process will comprise repeatedly moving the beam relative to the target (whilst the beam is in an 'off portion of its pulsed duty cycle) and then applying the beam (that is a pulse of irradiation from the source) to the target to ablate the surface of the material of the target. Preferably the beam position is moved in a series of steps, and preferably the beam shape also comprises a step change between the first and second widths. In this way the stepped motion may be arranged such that the inner, narrower portion of the beam receives an integral multiple of the number of shots received by the outer, wider portion of the beam.

Preferably the steps in which the beam moves are equal in length to an integral fraction of a length of the beam portion having the first (wider) width, such as 1/1, 1/2, 1/3, 1/4 and the like. Preferably the integral fraction is equal to unity and the beam steps along so that adjacent regions of the material irradiated by the wider beam portion abut one another. It will be recognised that one or more laser shots may be applied to the material at each beam position. However in embodiments with such a stepped beam shape and such a stepwise beam motion it is preferable that a ratio of the length of the wider portion of the beam to the total length of the beam (in the direction of motion of the beam) is substantially equal to a ratio of energy received by material under the wider portion of the beam to energy received by the material under the centre portion of the beam.

The method is particularly advantageous when applied to a material comprising two or more layers, in which one of the layers is more easily ablated than another of the layers. As mentioned above, this type of layered structure introduces particular problems which embodiments of the method are especially suited to addressing. The method is particularly advantageously applied in cases where the less easily ablated layer lies below the more easily ablated layer, for example between the more easily ablated layer and the substrate, although the method may also be used in the converse situation. Thus the method is particularly advantageous for ablating layers of different organic materials in a molecular electronic device such as an OLED. In this context, 'organic' includes organo-metallic.

In another aspect the invention provides apparatus for ablating material using a light source, the apparatus comprising: means for shaping a beam from said light source such that on a surface of said material the beam has a shape with portions of different widths comprising a first width and a second narrower width; means for applying said beam to said surface to ablate material; and means for moving said beam relative to said surface such that material under a portion of said beam with said second narrower width receives more energy than material under a portion of said beam width with said first width.

This apparatus may be further configured to provide additional functions as set out in relation to the above described method. Corresponding advantages may be obtained.

In a related aspect the invention also provides an aperture mask plate for use with the above apparatus comprising an opaque plate with an aperture having a shape with portions of different widths comprising a first width and a second narrower width, and a step change in width from said first to said second width.

In a further aspect the invention provides a method of reduced debris laser ablation of a workpiece, the method comprising repeatedly applying a pulse of laser irradiation to said workpiece and translating said workpiece relative to said laser irradiation, wherein the method further comprises configuring said laser irradiation such that when translated said irradiation at least partially removes debris left by an earlier pulse.

The laser irradiation may comprise a single laser beam or one or a pair of a lower intensity laser beams to one or both sides of a main beam, the lower intensity beams being sufficiently powerful to remove adhered particles of debris but insufficiently powerful to remove the underlying polymer layers. However preferably a single laser beam is employed shaped so that portions of the material to be ablated receive different quantities of energy from the beam than do other portions of the material, as the beam is translated across the material, specifically material under outer or edge portions of the beam receiving less energy than material under central beam portions. In this way material under the central beam may be ablated whilst a boundary portion of the ablated region, that is material to one or both sides of the ablated region under the outer or edge portions of the beam, may be cleaned without substantial ablation. This may be achieved by appropriate shaping of the beam, for example by shaping the beam into a 'L' shape or 'T' shape. Again the method is particularly advantageous when applied to a layered material in which different layers have different ablation efficiencies. The invention further provides laser ablation apparatus configured to operate in accordance with this method, for example through use of an aperture mask plate as described above.

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures 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 3 c;

Figures 4a to 4d show, respectively, an aperture mask in accordance with an embodiment of an aspect of the present invention, a top view of stepwise scanning of a laser beam shaped using the mask of Figure 4a across a surface of a workpiece, a magnified view of an ablated line formed by the process illustrated by Figure 4b, and a cross-section through a via formed by a laser ablation process according to an embodiment of an aspect of the present invention; and

Figures 5a to 5f show various alternative aperture shapes for an aperture mask plate for use with embodiments of the present invention. It has been recognised that were it feasible to controllably taper the beam profile illustrated in Figure 3b to provide a gradual fall off in energy at the edges of the beam this could reduce the quantity of debris and defects produced by a laser ablation process.

However this is difficult and expensive to achieve using standard optical techniques and the applicants have recognised that a conventional laser ablation tool may be used to provide a reduced debris ablation process simply by changing the aperture mask plate and the scanning the beam appropriately.

Thus Figure 4a shows one example of a mask plate 400 incorporating such an aperture which, as illustrated, has a 'T'-shape. When used in the apparatus of Figure 2 this results in an ablating laser beam of substantially the same shape impinging upon the surface of the material to be ablated. In effect the aperture of mask 400 is imaged directly onto the substrate. The aperture shape, and hence beam shape, comprises a first portion 402 having a first width and a second portion 404 having a second, narrower width. The shape may be considered as a central portion 406 to which is attached two outer ears 408a, b. As will be described later, in use the shaped laser beam is translated in a direction indicated by arrow 410. The precise width of portion 404 is not particularly important in the context of embodiments of the invention and depends upon the width of the line (or other feature) which it is desired to ablate. The width of portion 402, that is the width of the ears 408, is again not critical but should be sufficient to reduce the debris to a distance either side of the ablated line. This distance may be determined by a routine experiment, for example by conventional ablating as described with reference to Figures 3 a to 3 c and then measuring the approximate distance to which debris is found either side of an ablated line, as illustrated in Figure 3d. The ratio of lengths (left to right dimension) of portions 402 to 404 is determined by the number of shots per area used - for example, for 4 shots per area the ratio is 1 :3, whilst for 8 shots per area the ratio is 1 :7.

Figure 4b shows how a beam having a shape corresponding to the aperture in the mask of Figure 4a is scanned across the surface of a workpiece to be ablated. In the example shown scanning takes place as a series of steps in direction 410, and the position of the beam at each step is shown by beam outlines 412a, b, c and d. It can be seen that in this example the beam is stepped along by the length of an ear 408, although in other embodiments of the method the beam may be stepped along by a fraction of this length.

With the illustrated beam shape ears 408 have a length of one quarter of the total length of the beam (in the direction of beam movement). Thus the beam is stepped along by one quarter of its length on each movement. If, say, the laser is configured for one shot at each beam position it can be seen from inspection of Figure 4b that the material underlying the central portion of the beam, that is between lines B and B' receives 4 shots per area whilst the material between lines A and B and between lines A' and B' receives only 1 shot per area. Moreover the surface of the material receiving only 1

SPA receives its single shot after the material between lines B, B' has received 3 shots.

Figure 4c illustrates diagraminatically the appearance of a line laser ablated by the process by Figure 4b when visually inspected, under magnification, from above. More specifically Figure 4c shows the appearance of a spin-coated polymer LED structure as described with reference to Figure 1 after ablation to form lines 162 shown in Figure Id. By comparison with Figure 3d, broadly speaking central ablated line 414 and adjoining substantially non-ablated regions 416 are substantially free from debris. It has also been noted that ablated line 414 appears marginally wider than a conventionally ablated line such as line 304 or Figure 3d.

It has been experimentally observed that the debris and effects contributing to yield and lifetime reductions are substantially reduced, although residual lines 418 within ablated line 414, resulting from the stepped motion of the beam, which are not believed to significantly affect device yield, may still be present. The precise mechanism by which the debris is removed is not fully understood but, broadly, it is believed that debris that is deposited by ablation using the central portion of the beam (between lines B B', at 4 SPA) is removed by the 1 SPA ablation under the ears of the beam.

hi Figure 4b the direction of motion 410 will generally correspond to an X- or Y- scan direction of the apparatus of Figure 2, although this is not essential. Generally the optical axis of the laser beam will be perpendicular and directed into the paper in Figure 4b. In more detail, in ablation of one example of an OLED device similar to that in Figure lc, it is believed that debris that is deposited to the sides of a laser ablated scribe line is a consequence of ablation of the PEDOT layer; the light emitting polymer layer appears to vaporise and leave very little debris. The LEP layer generally absorbs more strongly at the laser wavelength used and is hence easier to remove. PEDOT is less absorbing at the wavelengths used and is hence more problematic to remove and may require more shots and/or a higher fiuence for complete removal. A ' mask aperture is used to define the (KrF) excimer laser beam which is the projected onto the PEDOT and LEP coated display held on a x-y stage. The x-y stage then moves at a calculated rate such that the middle area of the image is overlapped to achieve the desired dose of 4 shots per area (SPA) and the "ears" of the 'T' deliver a dose of 1 SPA. Thus the "ears" subsequently ablate the debris that is deposited by the middle area and as the PEDOT does not readily ablate at 1 SPA no debris is left on the substrate. Less energy may be required to remove the PEDOT debris as this is in particulate form rather than a continuous film; it may also be that the debris is more absorbing of 248nm radiation having undergone a fairly energetic process. Some softening or melting under the ears may also occur.

The skilled person will recognise that the various ablation parameters, such as the number of shots per area and the energy per area applied with each shot, may be selected in the same way as with a conventional laser ablation procedure. Thus typically the fiuence will be selected to be above the threshold energy for ablation but below a level which causes excessive damage to underlying material. With the laser process tool described above with reference to Figure 2, for example, the fiuence may be adjustable between 150 mJcm^"2 and 1000 mJcm^"2.

The above described ablation process is particularly suited to the ablation of lines, but may also be used for ablation of other features. For example, Figure 4d shows a via 420 in a polyimide insulating layer 424 over a contact or conductor layer 422 formed by a process similar to that described above. To achieve a reduced shots per area effect the stage carrying the substrate may, for example, be rotated rather than scanned. It will be appreciated that the shape of the aperture in the mask of Figure 4a may be varied according to the requirements of a particular application, and some variants are illustrated in Figures 5a to 5f. Figure 5a illustrates a variant in which the wide and narrow portions of the beam have equal lengths, and thus when used in a similar manner to that of Figure 4b (ears abutting) this will deliver 2 SPA to the material under the central portion of the beam and 1 SPA under the material at the edge (under the ears) of the beam. Figure 5b shows a similar arrangement but configured for debris removal on one side of the beam only. Figure 5c illustrates an alternative, less preferred configuration to Figure 5b. Figure 5d illustrates a beam shaped similarly to that shown in Figure 5c but delivering a greater energy to the edge portion of the beam (this may be appropriate in a layered material depending upon from which layer the debris originates, and depending upon the relative ease of ablation of material from different layers).

Figures 5e and 5f illustrate further variants of beam shape achieving, in effect, a graded fiuence outwards from the centre of the beam.

No doubt many 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 attached claims.

Claims

CLAIMS:

1. A method of ablating material using a light source, the method comprising shaping a beam from said light source such that on a surface of said material the beam has a shape with portions of different widths comprising a first width and a second narrower width; applying said beam to said surface to ablate material; and moving said beam relative to said surface such that material under a portion of said beam with said second narrower width receives more energy than material under a portion of said beam with said first width.

2. A method as claimed in claim 1 wherein said widths are measured in a direction substantially perpendicular to a direction of said moving.

3. A method as claimed in claim 1 or 2 wherein said light source comprises a pulsed laser.

4. A method as claimed in claim 3 wherein said beam shape comprises a step change from said first to said second width and said moving comprises a stepwise motion.

5. A method as claimed in claim 4 wherein said portion of first width has a first length and said stepwise motion comprises moving by a step substantially equal to an integral fraction of said first length.

6. A method as claimed in claim 5 wherein as said beam moves said portion of said beam with said first width overlaps material previously under said portion of said beam with said second width such that a region of said overlap receives a second energy and a region outside said overlap and under said portion of said beam with said first width receives a first energy, wherein said portion of second width has a second length, and wherein a ratio of said first length to a total of said first and second lengths is substantially equal to a ratio of said first energy to said second energy.

7. A method as claimed in any preceding claim wherein said material comprises first and second layers of material on a substrate, said first layer being more easily ablated than said second layer.

8. A method as claimed in any preceding claim wherein said material comprises organic or organo-metallic material for a molecular electronic device.

9. Apparatus for ablating material using a light source, the apparatus comprising: means for shaping a beam from said light source such that on a surface of said material the beam has a shape with portions of different widths comprising a first width and a second narrower width; means for applying said beam to said surface to ablate material; and means for moving said beam relative to said surface such that material under a portion of said beam with said second narrower width receives more energy than material under a portion of said beam width with said first width.

10. Apparatus as claimed in claim 9 further comprising a mask, the mask comprising an opaque plate with an aperture having a shape with portions of different widths comprising a first width and a second narrower width, and a step change from said first to said second width.

11. A method of reduced debris laser ablation of a workpiece, the method comprising repeatedly applying a pulse of laser irradiation to said workpiece and translating said workpiece relative to said laser irradiation, wherein the method further comprises configuring said laser irradiation such that when translated said irradiation at least partially removes debris left by an earlier pulse.

12. A method as claimed in claim 11 wherein said workpiece comprises at least two layers, an uppermost layer and a second layer, said uppermost layer being more easily ablated by said irradiation than said second layer, and wherein said method further comprises configuring said beam such that a boundary portion of an ablated region receives less energy than a non-boundary portion of said region.

13. A method as claimed in claim 11 or 12 wherein said workpiece comprises part of an organic electroluminescent display.

14. Laser ablation apparatus configured to operate in accordance with the method of claim 11 or 12.