WO2008119949A1

WO2008119949A1 - A method of and an apparatus for laser removing coating material, the laser beam passing through the substrate towards the coating material; flat panel display with such removed coating material

Info

Publication number: WO2008119949A1
Application number: PCT/GB2008/001062
Authority: WO
Inventors: Paul Harrison; Jozef Wendland; Matthew Henry
Original assignee: Powerlase Ltd
Current assignee: Powerlase Ltd
Priority date: 2007-03-30
Filing date: 2008-03-27
Publication date: 2008-10-09
Anticipated expiration: 2009-09-30
Also published as: GB0706287D0

Abstract

The present application relates to a method of removing coating material (303) to pattern a coated substrate (301). The method comprises directing a laser beam (305) through the substrate (301) such that a region (307) of the coating (303) at the coating-substrate interface is ablated thereby producing confined vaporised coating material (303) which causes an overlying region of coated material to be removed from the substrate (301).

Description

A METHOD OF AND AN APPARATUS FOR LASER REMOVING COATING MATERIAL,

THE LASER BEAM PASSING THROUGH THE SUBSTRATE TOWARDS THE COATING

MATERIAL; FLAT PANEL DISPLAY WITH SUCH REMOVED COATING MATERIAL

This invention relates to a method and apparatus for removing coating material from a coated substrate. In particular, the invention relates to a method and apparatus for patterning thin films using laser techniques.

Patterned thin films are widely-used in Flat Panel Displays (FPDs) such as Liquid Crystal Displays and Plasma Display Panels. One patterned thin film that is common in Liquid Crystal Displays is "Black Matrix". Black matrix is a high optical density material which is patterned with apertures to provide a template for the colour elements constituting the pixels. Patterning of thin films, including "Black Matrix", is currently achieved using the technique of wet-etch lithography. However, wet-etching of "Black Matrix" does not produce patterns of satisfactory quality for some applications in terms of the quality of the edges of the patterned region. Furthermore, wet-etching is a very expensive process with high consumable costs and a demand exists for improved methods. Laser patterning is one such alternative but known approaches are inefficient and problematic, as also described in more detail below.

Methods are known for laser patterning Indium Tin Oxide (ITO) on glass for the manufacture of the front panels for Plasma Display Panels, for example. This ablation technique may replace more expensive wet-etch lithography techniques which use chemical means. However, "Black Matrix" has very different optical and material properties to ITO and much thicker coatings are typically used, Therefore, much more laser energy is required to pattern "Black Matrix" and the quality of the resulting pattern does not meet current industry standards.

According to one known approach of laser patterning thin films, a laser beam is fπri iςςprl to a fϊnp <?nr>t tn rqn cp nV>1 qtinii nf film m ateri al Tn nrήpr to ςpl prtivpl v remove material, the laser beam and film are moved relative to one another. By rapidly moving the laser beam across the substrate, it becomes possible to pattern the thin film quickly.

One such known method for direct laser ablation of thin films is shown in Fig. 1. Referring to Fig. 1 there is shown a substrate 103 coated with a thin film 101 and irradiated with laser light 107 delivered by scanning and focusing optics 105. The puipose of the substrate 103 is merely to support the thin film 101 and the laser light 107 is incident directly on to the thin film 101. By translating the laser beam 107 relative to the substrate 103, a pattern 109 corresponding with the ablated regions is formed.

According to another known approach, the beam is imaged onto the target by employing a mask to create a complex image plane at the workpiece. The technique of directly ablating a thin film using a laser in this way is well known - see, for example: Lunney, J. G., O'Neill, R. R., Schulmeister, K., (1991) Excimer laser etching of transparent conducting oxides, Journal of Applied Physics Letters, Vol. 59, No. 6 pp.647-649; Takai, M., Bollmann, D., Haberger, K., (1994) Maskless patterning of indium tin oxide layer for flat panel displays by diode-pumped Nd: YLF laser irradiation, Journal of Applied Physics Letters, Vol. 64, No. 19 pp.2560-2562; Henry, M., Harrison, P.M., Wendland, J., (2006) Laser direct write of active thin-films on glass for industrial flat panel display manufacture, Proceedings of the 4th International Congress on Laser Advanced Materials Processing, Kyoto, Japan; and US 5,024,968 and EP 0,834,191 Engelsberg). By moving the beam around relative to the workpiece, patterns can be rapidly built up. All of these disclosures are incorporated herein by reference.

However, when the known methods described above are applied to "Black Matrix", the edges of the patterned area (viewed in plan) are of poor quality. Furthermore, effects such as "over-cut" and "under-cut" are observed wherein the ablated region (viewed in section) has a trapezoid shape rather than a rectangular one. This is shown in more detail in Fig. 2.

Fig. 2 shows in cross-section the geometry of patterned regions that can form in "Black Matrix" using prior art methods. Fig. 2 A shows an exposed portion 201 in a thin film of "Black Matrix" 203 supported on a substrate 205 wherein the wall 207 of the exposed portion 201 is tapered giving rise to a trapezoid shape. As shown in Fig. 2 A, the width B of the exposed portion 201 varies with the depth of film 203 since the top (as shown in Fig. 2A) of the exposed area has been preferentially removed. Typically, the ablated region is 70 x 270 μm, the thickness of film is 1-2 μm and the taper distance A is 2 to 5 μm.

Fig. 2B shows an alternative case which may result from wet-etch lithography wherein material on the substrate-side of the exposed portion 211 has been preferentially removed. As in Fig. 2 A, the width of the exposed portion 211 varies with the depth of the film 213 but the side walls are tapered in the opposite direction.

Fig. 2C shows a further alternative case which may result from wet-etch lithography wherein the resultant sidewalls of the exposed portion 221 are parallel.

In practice, the cross-section of the apertures as shown in Figs 2 A, 2B and 2C give rise to problems in some applications. For example, an array of these apertures may be used in the flat panel display industry as an optical filter wherein each aperture corresponds to a pixel. When illuminated from above or below, as orientated in Figs 2, the taper causes variable light transmission, edge blurring and/or diminished edge contrast. This adversely affects the size of the pixel and the contrast. As well as edge taper, the surface roughness of the side wall is important since this also affects edge contrast. Prior art laser methods for producing apertures in "Black Matrix" produce side walls with unsatisfactory surface roughness and tapering.

Furthermore, the shapes shown in Figs 2A, 2B and 2C can affect the mechanical stability of the film at the edge and/or the resolution of the resultant FPD such that existing methods for laser patterning ITO are inadequate for "Black Matrix".

The invention is set out in the claims. The claimed invention provides an improved technique for rapid laser patterning (also known as Laser Direct

Writing) of a thin film. The claimed invention significantly enhances the quality of thin film removal from transparent substrates by comparison to known techniques. In particular, because of the improved material removal mechanism, it is found that the side walls of the resulting apertures show significantly reduced roughness and tapering. Furthermore, residue and debris in the region of ablation is reduced as a result of the processing.

Embodiments of the invention will now be described by way of example with reference to the figures wherein:

Fig. 1 is a schematic of a known technique for laser ablation of thin films;

Figs. 2A, 2B and 2C show cross-sections of apertures in "Black Matrix" that may result using prior art methods;

Fig. 3 shows a first embodiment of the present invention wherein a small region of the thin film at the film-substrate interface is vaporised;

Fig. 4 is a schematic of the material removal mechanism according to embodiments of the present invention;

Fig. 5 shows a schematic of the translation means according to embodiments of the present invention; Fig. 6 shows a second embodiment of the present invention wherein the thin film is held in contact with a fluid; Fig. 7 shows a third embodiment wherein fluid is sprayed onto the film; Fig. 8 is a schematic of a system for processing the laser beam; and Fig. 9 shows a patterned substrate.

In overview, the present invention provides a technique for removing coating or deposited material, for example by selective laser-ablation from a coated substrate comprising, for example, a thin film supported by a transparent substrate by directing a laser beam through the substrate toward the coating or deposited material, for example, onto the substrate-film interface. The film absorbs the laser energy and forms a hot gas by vaporisation which expands and ejects or drives the remaining depth of film away from the substrate leaving an exposed area of substrate which is found to have better edge quality and less tapering.

This can be further understood with reference to a first embodiment of the present invention shown in Fig. 3. Laser pulse 305 is directed in the direction of arrow A, for example by being focussed or imaged, through substrate 301 onto the interface 307 between the thin film 303 and the substrate 301.

The first embodiment is shown in operation in Figs 4A, 4B and 4C. Referring to Fig. 4A there is shown a focused laser pulse 405 travelling in the direction of arrow A through substrate 401. The opposite face of substrate 401 is coated with a thin film of "Black Matrix" 403. The thin film may be have been coated, deposited, grown or applied on the substrate by any suitable method. Substrate 401 is, at least, partially transparent at the laser wavelength and so beam 405 is incident upon thin film 403 at the interface between the substrate 401 and film 403. The laser energy is absorbed within a thin region 407 of film 403 at the interface.

Referring to Fig. 4B, the temperature of thin region 407 is raised beyond the vaporisation point of the film 403 by laser pulse 405 such that a high pressure region 411 comprising trapped gas is confined between the remaining depth of film 403 and substrate 401. This pressure build-up eventually causes the remaining depth of film 403 to be ejected 415 away from the substrate, as shown in Fig. 4C, to leave behind an ablated region 417. The majority of film material is not vaporised and, instead, is driven off by the expansion of the underlying gas. Thus, more of the film is removed than would be expected from laser absorption alone. In particular, the thickness of coating removed is greater than the absorption depth of the coating at the laser wavelength.

The high pressure region 411 at the interface between substrate 401 and film 403 is vaporised resulting in a region of high pressure gas constrained between the substrate 401 and film 403. This region 411 expands explosively against the area of least resistance (i.e. the region of thin film 403 between the region 411 and atmosphere) and so the thin film is removed explosively. Thus, the material removal mechanism is different to, for example, the mechanism shown in Fig. 1.

The laser, substrate, film and coating may take any appropriate form. A pulsed laser is most suitable, for example, the Starlase AO4 laser from Powerlase Ltd, Crawley, UK which produces radiation at a wavelength of 1064 nm. The nominal power is 400 W with a pulse width of 20-200 ns (Full-wave Half Maximum) and a maximum repetition rate of 3-50 KHz.

Any substrate with a degree of transparency at the laser wavelength is suitable for this application. In most applications, such a flat panel display processing, it will be important to ensure the substrate is not damaged by the laser pulses.

In these cases, any substrate with an adequate Laser-Induced Damage

Threshold is suitable. For example, the substrate can be soda-lime glass with a thickness of 0.25 to 5.00 mm as available from Coming or Asahi Glass, for example. Any thin film capable of absorbing sufficient laser radiation is suitable. As the skilled reader will understand, suitability will depend on many properties of the thin film such as the melting temperature and latent heat of vaporisation of the composite material. Any thin film which absorbs a portion of the laser pulse and can thereby form a high pressure gas may be suitable. For example, the thin film can be 0.05 μm to 10 μm thick or, more preferably, 1 to 2 μm thick. As discussed above, the coating can be "Black Matrix". Alternatively, the coating can be a metal, such as aluminium or magnesium, or a semi-conductor such as a metal oxide like Indium Tin Oxide (ITO) having a thickness in the range 0.02 to 0.50μm. Alternatively, the coating may be a plastic or another type of organic material.

Any appropriate patterning / translation technique can be adopted in order to remove coating material from an extended area or from multiple areas of the coating, for example, by moving the laser relative to the substrate or moving the substrate relative to the laser. For example, the substrate may be mounted on an X-Y translation stage or the laser beam may be moved in between pulses by translating the imaging optics. According to embodiments of the present invention, a pair of beam-steering mirrors are used to achieve beam translation as shown in Fig. 5. Referring to Fig. 5, there is shown a laser beam 501 entering a scanner 503 comprising two beam-steering mirrors 505 to controllable adjust the path of the laser beam. A flat-field lens 507 focuses the translated beam onto a target 509 to produce an ablation pattern 511.

The skilled person will realise that any pattern can be created by controlling the movement of the laser relative to the target. For example, the pattern could comprise an entire area such that material is removed to obtain a predetermined geometry or shape such as holes or edges.

As will be apparent to the skilled reader, the amount of coating material removed is related to the size of the image plane. The maximum area of the removed portion is only limited by the available laser power density. The laser power density must be sufficient to produce a region of vaporised film material with a pressure sufficient to eject the full depth of film away from the bulk film.

Referring to Fig. 6 there is shown a second embodiment of the present invention. The system comprises a planar substrate 603 coated on a first face with an absorbing film 601 wherein the film 601 is in contact with a containment fluid 605 such as liquid. A laser beam 607 is directed through the non-coated second face of substrate 603 onto the substrate-film interface 609. Film is ejected from the substrate in accordance with the first embodiment.

The fluid acts in two ways which further improve the material removal process. Firstly, the fluid significantly reduces or eliminates debris and residue by solvating some of the ablated material and flushing away particulate. This reduces the amount of vaporised film that re-condenses on other parts of the film and/or substrate. Furthermore, the fluid constrains the lateral material removal as the film is ejected away from the bulk, resulting in much reduced edge taper and edge variation. In particular, this leads to a significant reduction in the roughness of the side walls of the exposed area. Since the film is irradiated through the substrate, embodiments according to the present invention gives rise to the added advantage that the laser beam does not have to pass through the fluid. This reduces energy loss owing to absorption by the water and minimises variations in at the image plane owing to lensing effects and local variations in refractive index within the water.

The skilled person will be aware that the fluid could also be sprayed across the film during machining in any appropriate manner. For example, this can be achieved using approaches disclosed in: US20070000875A1 Coherent; WO03028943A1 Lambda Physik; WO03028941A1 Scaggs; and US20030062126A1 Scaggs. WO05120763A2 also Exitech/Sony also discloses a method of laser processing of a substrate underwater in a turbulent flow to enhance material removal and improve the quality of the resulting features. Here, the substrate is contained within a vessel containing turbulent water. However, this means that the laser beam has to pass through the fluid before reaching the film. All of these disclosures are incoiporated herein by reference.

In a further embodiment shown in Fig. 7, a pressurised liquid spray creates a local flow across the surface of the substrate which further reduces debris and residue. This is because the fluid acts to flush the vapour and particulate away from the substrate. Moreover, a solvating fluid may be used to enhance removal of the vaporised material.

Referring to Fig. 7 there is shown a laser beam 707 directed through the uncoated side of substrate 703 onto the thin film 701. A device 711 sprays liquid 705 onto the interface 709 between substrate 703 and film 701. This leaves a region 713 where the coating material has been removed from the substrate.

In this embodiment, a Powerlase A04 laser, having a laser fluence of 1 J/cm" in 35 ns pulse, was used to create an array / pattern of 70 x 270 μm rectangular apertures in a 1 μm thick coating of "Black Matrix" supported on a soda-lime substrate. The array / pattern was created by adjusting the beam delivery optics during processing.

The extent of the tapering may be measured by the linear distance A shown in Fig. 2A. With a 1 μm thick film of "Black Matrix" the tapering distance A achieved by the prior art laser method shown in Fig. 1 is 2-5 μm and a laser fluence of 6 J/cm² at 35 ns would be required to achieve this. By way of comparison, using the improved method according to embodiments of the present invention, a laser fluence of 0.1 to 50 J/cm² in 1 to 200 ns achieves a tapering distance of 0.55 μm +/- 0.10 μm (Standard Deviation) with a straightness of +/- 0.25 μm (standard deviation from a straight line) as measured over a length of 290 μm. More preferably, the laser fliience is 0.5 to 2.0 J/cm² and the pulse duration is 20 to 50 ns.

As the skilled reader will understand, the laser and substrate may be oriented with either the laser coming down on the workpiece with the water jet from beneath with substrate held vertically or in any other orientation.

In embodiments, the liquid is introduced using a jet or jets, or by immersion. There may, of course, be other ways, for example, turbulent flow in a vessel, flow of liquid across the surface, or lamina flow.

In a further embodiment, shown in Fig. 8, the laser beam is delivered by an optical fibre 801. The output 803 from the fibre 801 is collimated by a collimating lens 805 and passed through a homogeniser 807 and mask 809. Finally, a scanning lens 811 images the beam onto the target 813. This results in an ablated image at the target 813. In this embodiment, the laser beam illuminates an area encompassing at least the mask. As shown in Fig. 9, this can be used to create an array 901 of ablated images 903 onto the target.

Embodiments according to the present invention significantly enhance the quality of the thin film removal from transparent substrates by comparison with conventional rapid laser processing and other processes such as wet-etch lithography.

Embodiments of the present invention offer improvements to resulting patterned thin film quality in terms of: edge taper; edge variation; edge straightness; reduced debris; and residue. In the case of "Black Matrix" processing, such quality may be superior to that achieved by both wet-etch lithograph)' and conventional rapid laser processing. In particular, the transmissivity of the ablated region is more uniform over its area owing to better defined edges. Furthermore, these embodiments may allow the resolution of finer features on the thin film than is possible by other known techniques, for example, in the manufacture of high definition LCD displays.

Embodiments according to the present invention offer advantages particularly for relatively thick films of "Black Matrix". Using a Black Matrix film 1-2 μm thick, the optical absorption depth is much less than the film thickness but the full depth of film can be explosively-driven off by the method described with reference to Fig. 4. However, using the known approach as shown in Fig. 1 , either multiple pulses or very high energy pulses are required to remove all the thin film. Therefore, embodiments of the present invention require significantly less energy than the previously known method to remove the same volume of material. This leads to improved productivity and material removal rate.

Furthermore, embodiments of the present invention do not use harmful chemicals and so the process is environmentally benign. In embodiments described, the liquid may be deionised or otherwise cleaned water thereby easily recyclable and low cost.

Embodiments of the present invention are more energy efficient than for other known rapid laser processing and so offer productivity or cost improvements. Embodiments according to the present invention require approximately 1 J/cm² only per laser pulse to produce the high-quality pattern discussed in this application. By way of comparison, the known approach of directly ablating coatings of ITO typically requires 3 J/cm².

The process benefit is not derived from a chemical reaction and so does not affect the thin film or substrate. As the process is self-cleaning it may eliminate or reduce the need for subsequent cleaning stages in manufacture. This technique results in unprecedented quality achieved for Black Matrix processing and is applicable to patterning all thin films on nano- & micro-scale where quality enhancements are sought. It also proves to be more energy efficient than conventional laser processing in some cases - resulting in potential productivity/cost enhancements. The use of water adds to the attraction in that no chemicals are required and so this approach is environmentally preferable to patterning using wet-etch lithography.

The skilled reader will appreciate that embodiments of the present invention could be applied to any thin film on any transparent substrate. The method is applicable to any thin film patterning where the thin film is at least partially absorbing and the substrate is at least partially transmissive at the laser wavelength. The process may apply to thin films on both the nano- and the micro-scale. Furthermore, the process is applicable to any absorbing thin film, and so is highly flexible and may be applied to organics, semi-conductors and metals

Embodiments of the present invention are particularly suitable for patterning thin films for the Displays sector (for example, patterning Black Matrix on glass for LCD manufacture). However, it could be applied for any thin films on transparent substrates of any kind. It is also applicable to thin film processing on flexible substrates for next generation flexible displays. It would be relevant to all display types including LCD, PDP, OLED and SED. However, this technique can be applied to any thin film application with a transparent substrate, and so would be relevant to patterning thin films for solar cell applications and also semiconductors applications.

Embodiments of the present invention use a laser operating at a wavelength of 1064 nni but the skilled reader will appreciate that any wavelength laser from vacuum ultraviolet to far infrared could be used so long as the thin film is at least partially absorbing and the substrate is at least partially transmissive. The laser described in the embodiments is unpolarised, however a polarised light source will work just as well. In embodiments, the beam is imaged using a homogeniser and mask. However, as the skilled reader will appreciate, this process may also work well using different beam shaping techniques or simple focussing. The skilled reader will also be aware that other fluids are equally suitable for containment and removing debris, for example, a common solvent such as methanol could be used instead of water or a pressurised gas such as an inert gas. Furthermore, additives can be added to the liquid to enhance the liquid performance, for example, to aid material removal or enhance debris solvation.

The skilled person will realise that the claimed invention is applicable to all coating materials, all substrates with a degree of transparency and all lasers.

Claims

1. A method of removing coating material to pattern a coated substrate comprising directing a laser beam through the substrate towards the coating material wherein a region of the coating at the coating-substrate interface is ablated by the laser beam to produce confined vaporised coating material which causes an overlying region of coated material to be removed from the substrate.

2. A method as claimed in claim 1 wherein the thickness of coating removed is greater than the absoiption depth of the coating material at the laser wavelength.

3. A method as claimed any preceding claim wherein the coating material is in contact with a containment fluid.

4. A method as claimed in claims 3 wherein the fluid is directed at the coating material.

5. A method as claimed in either of claims 3 or 4 wherein the fluid is turbulent.

6. A method as claimed in claim 3 wherein the fluid is static.

7. A method as claimed in any preceding claim wherein the coating is in the range 0.05 to 100 μm thick and preferably 1 to 2 μm thick.

8. A method as claimed in any preceding claim wherein the coating material is a visible light absorbing coating.

9. A method as claimed in any preceding claim wherein the coating is "Black Matrix"

10. A method as claimed in any preceding claim wherein the laser beam has an energy density of 0.1 to 50 J/cm² and preferably 0.5 to 2.0 J/cm² at the point of incidence with the coating material.

11. A method as claimed in any preceding claim wherein the one of the laser beam or substrate is translated relative to the other during processing.

12. A method as claimed in any preceding claim wherein the pattern is an array of apertures.

13. An apparatus for removing coating material from a coated substrate comprising: a laser; and a coated substrate support wherein the coating substrate support is arranged to support a coated substrate relative to the laser such that the laser beam is directed to pass first through the uncoated side of the substrate, and arranged such that in use a region of the coating, at the coating-substrate interface of a coated substrate supported by the coated substrate support, is ablated by the laser beam to produce confined vaporised coating material which causes an overlying region of coating material to be removed from the substrate.

14. An apparatus as claimed in claim 13 wherein the coating material is held in contact with a containment fluid.

15. An apparatus as claimed in any of claims 13 to 14 arranged such that fluid is directed at the coating material.

16. An apparatus as claimed in any of claims 13 to 15 wherein the laser beam has a pulse duration of 1 to 200 ns and preferably 20 to 50 ns.

17. An apparatus as claimed in any of claims 13 to 16 wherein the laser beam has an energy density of 0.1 to 50 J/cm² and preferably 0.5 to 2.0 J/cm² at the point of incidence with the coating.

18. An apparatus as claimed in any of claims 13 to 17 further comprising a translation stage for translating the laser beam and coated substrate support relative to one another.

19. An apparatus as claimed in claim 18 further comprising a controller for temporally interleaving a sequence of translations with a sequence of laser pulses.

20. An apparatus as claimed in any of claims 13 to 19 wherein the laser has a wavelength of 1064 nm.

21. A system for patterning a visible light absorbing coating for flat panel displays comprising the apparatus according to any of claims 13 to 20 and the method according to any of claims 1 to 12.

22. A system according to claim 21 wherein the visible light absorbing coating is "Black Matrix"

23. A Flat Panel Display comprising a visible light absorbing coating wherein the visible light absorbing coating has been patterned using a laser.

24. A Flat Panel Display according to claim 23 wherein the pattern comprises at least one aperture in the a visible light absorbing coating wherein the side walls of the aperture have a taper extending less than

2 μm

25. A Flat Panel Display according to either of claims 23 or 24 wherein the visible light absorbing coating is "Black Matrix".

26. A method or apparatus substantially described as herein with reference to the drawings.