WO2005048669A1

WO2005048669A1 - Flexible devices

Info

Publication number: WO2005048669A1
Application number: PCT/IB2004/052373
Authority: WO
Inventors: Giovanni Nisato
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-11-14
Filing date: 2004-11-10
Publication date: 2005-05-26
Also published as: TW200522810A; GB0326537D0

Abstract

A device-processing carrier plate (2) for processing a flexible substrate (1), for example for display and electronics applications, is described. The carrier plate (2) comprises a rigid plate (3) and an adhesive elastomer (4) attached to a surface of the rigid plate (3) which, in use, is proximal to the flexible substrate (1). The adhesive elastomer (4) has a surface energy sufficient to allow releasable adhesion of the flexible substrate (1) to the adhesive elastomer (4) and a sufficient elastic modulus to maintain the contact to between the flexible substrate (1) and the adhesive elastomer (4) during processing.

Description

DESCRIPTION

FLEXIBLE DEVICES This invention relates to flexible devices, particularly to the fabrication of flexible devices by processing a flexible substrate carried on a carrier plate.

Displays and electronic components formed on flexible substrates allow the fabrication of thin and conformable (rollable) devices providing new applications for the display and electronics sectors. The processing of flexible substrates, which may be based on polymers or glass microsheets (=200 μm), does, however, raise a number of difficulties primarily in the handling of such substrates, e.g. during photolithographic exposure, aligning, coupling, printing, spin coating, drying and transporting of substrates. At present the equipment and apparatuses for handling substrates for display and electronics applications are designed and optimised for rigid substrates made from glass plates and silicon wafers. Several solutions have been proposed for the fabrication of flexible substrates. One solution is to process free-standing flexible substrates. This approach is, however, quite inconvenient being time consuming and requires many ad hoc modifications to existing equipment and apparatuses. Alternatively the flexible substrate may be adhered (glued) to a rigid carrier plate, which is typically glass, fused silica or silicon. The flexible substrate may then be processed using standard procedures. However, the use of glues has a number of drawbacks. The glue has to form a sufficiently strong bond with the carrier plate to withstand the processing of the flexible substrate, which involves the use of solvents and elevated temperatures, whilst allowing the flexible substrate to be peeled off the surface of the carrier plate. These two requirements are generally incompatible since strong adhesion to the carrier plate typically results in difficulties in removing the fabricated flexible device. This is particularly problematic where the flexible device is a display since displays must remain transparent and colour neutral after processing. Solutions to these problems have been proposed using UV and heat exposure to release the flexible substrate. However, this constrains the processing parameters for the flexible device which often involve UV exposure and elevated temperatures. Another serious drawback in adhering the flexible substrate to the carrier plate is the mismatch in the (linear) thermal expansion coefficient (CTE) of glass (linear CTE 1 to 10 x 10^"6/K) and polymer- based flexible substrates (linear CTE 40 to 95 x 10^"6/K) which may result in breaking of the carrier plate or flexible substrate, or delamination of the flexible substrate from the carrier plate during processing. Another solution is provided in US 5,232,860 which describes a method for manufacturing a flexible photovoltaic device in which the device is bound weakly to a supportive substrate using an inorganic layer, e.g. lead, tin indium, zinc, silver, silicon or tin oxide, which allows the device to be removed from the substrate. However, the inorganic layer does not adhere well to the flexible substrate and limits the processing steps to those which do not affect the inorganic layer.

Accordingly the present invention provides a device-processing carrier plate for processing a flexible substrate comprising a rigid plate and an adhesive elastomer attached to a surface of the rigid plate which, in use, is proximal to the flexible substrate, wherein the adhesive elastomer has a surface energy sufficient to allow releasable adhesion of the flexible substrate to the adhesive elastomer and a sufficient elastic modulus to maintain the contact between the flexible substrate and the adhesive elastomer during processing. This provides a flexible -substrate carrier having a polymer layer which holds the flexible substrate in place during processing, is able to flex with the flexible substrate and is able to release the fabricated device. The present invention also provides an apparatus for processing a flexible substrate comprising carrier plate as defined above, a carrier plate support and a processing station as well as a method for processing a flexible substrate comprising the step of carrying the flexible substrate on a rigid plate wherein a surface of the rigid plate which is proximal to tie flexible substrate has an adhesive elastomer attached thereto which has a sufficient surface energy to allow releasable adhesion of the flexible substrate to the adhesive elastomer and a sufficient elastic modulus to maintain the contact between the flexible substrate and the adhesive elastomer during processing. This method uses a thin-film carrier which holds the flexible substrate in place during processing whilst allowing easy release of the flexible electronic device fabricated around the flexible substrate.

The present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows a carrier plate in accordance with the present invention carrying a flexible substrate; Fig. 2 shows a carrier plate in accordance with the present invention having a handling facilitating area; Fig. 3 shows a carrier plate carrying a flexible substrate in which the adhesive elastomer is patterned; and Fig. 4 shows an apparatus incorporating a carrier plate in accordance with the present invention.

Fig. 1 shows a flexible substrate 1 attached to a carrier plate 2 which comprises a rigid plate 3, an adhesive elastomer 4 and a handling facilitating area 5. The rigid plate 3 and the adhesive elastomer 4 allow the flexible substrate 1 to be partially or fully processed and then the processed flexible substrate 1 is removed from the carrier plate 2 without leaving residues on the processed flexible substrate 1. The flexible substrate 1 used in the present invention is used for fabricating flexible devices for display or electronic applications and such flexible substrates and flexible devices are conventional in the art. The flexible device is typically a multilayer (sandwich) structure based on a core layer, i.e. the flexible substrate 1. The flexible substrate 1 is made from organic or inorganic polymers, metals or glass microsheets. Other layers, such as solvent-resistant layers, diffusion barriers, electrode layers, electronic thin-film circuits etc., are then assembled on or around the core layer. The fabrication of such additional layers involves processing steps using harsh conditions and reagents, such as high temperatures, UV exposure, etching, solvent treatments etc. Suitable organic polymers for the core layer include polyimide, polycarbonates, PET, PEN and PES. The solvent-resistant layers may be highly crosslinked acrylates or salt gels. The diffusion barriers may be Siθ2, alumina or SiN. The electrode layer is, for example, copper or ITO. The flexible substrate 1 preferably has a thickness of not more than 400 μm, more preferably not more than 200 μm and most preferably not more than 50 μm. The preferred thicknesses do, however, depend on the material of the flexible substrate 1. The thickness of a polymer substrate is preferably not more than 300 μm, more preferably not more than 100 μm and preferably not more than 20 μm. Metals preferably have a thickness of not more than 100 μm. Glass preferably has a thickness of not more than 200 μm, preferably not more than 150 μm and most preferably not more than 50 μm. The elastic modulus of the flexible substrate 1 is typically not more than 70 GPa. There is, however, no reason why a substrate having a greater thickness or elastic modulus could not be used in the present invention, although significantly thicker and less flexible substrates could be handled without the requirement of the rigid plate 3. The rigid plate 3 is a standard piece of equipment used in processing the flexible substrate 1 . The rigid plate 3 is sufficiently rigid to allow handling by conventional apparatuses used in processing the flexible substrate 1. The rigid plate 3 is typically made from glass, metal or plastics, e.g. polycarbonate. For certain processing steps, such as photolithography with back illumi nation, which could be with UV light, the rigid plate 3 will be transparent. For such an application, the rigid plate 3 is preferably glass, quartz, or plastics, such as cyclic polyolefins, e.g. polynorbonene. The carrier plate 2 is adapted for ease of handling and will often have a handling facilitating areas 5 typically at one or more of its edges as shown in Fig. 2. Such handling facilitating areas 5 allow the carrier plate 2 to be aligned correctly in the processing apparatus and for moving the carrier plate 2 from one processing area to another which is often carried out robotically. The rigid plate 3 is required to be able to resist the process steps used in fabricating a flexible substrate 1 , such as etching, solvent treatments, UV exposure, heati ng etc. When the rigid plate 3 is made from a plastics material, the plastics material may need to be coated with a chemical or solvent-resistant coating. Such coatings include silica, SiN and salt gels, e.g. ORMOCER® by Fraunhofer. Preferably the rigid plate 3 is made from a material with substantially the same thermochemical properties (particularly linear CTE) as the flexible substrate 1 and more preferably from the same material as the flexible substrate 1. This is advantageous since the flexible substrate 1 and the carrier plate 2 are necessarily subjected to the same processing conditions. Significantly different thermochemical properties, such a CTE, may result in the breaking of the flexible substrate 1 or rigid plate 3, or delamination of the flexible substrate 1 from the carrier plate 2 during processing. The adhesive elastomer 4 is attached to the rigid plate 3 and provides releasable adhesion to the flexible substrate 1. Although not wishing to be bound by theory, it is believed that the adhesive elastomer 4 adheres to the flexible substrate 1 via non-covalent van der Waals interactions. The adhesive elastomer 4 requires a sufficient surface energy to adhere to the flexible substrate 1 while allowing the flexible substrate 1 to be removed from the carrier plate 2 after processing without leaving any significant residue on the flexible substrate 1. Preferably the surface energy (?) measured at 20°C is not more than 30 mJ/m², more preferably not more than 25 mJ/m². Lower surface energy provides better adhesion but it is preferred that the surface energy measured at 20°C is not less than 1 mJ/m², more preferably not less than 5 mJ/m². The adhesive elastomer 4 also requires a sufficient elastic modulus to allow deformation of the adhesive elastomer 4 to maximise the contact area between the adhesive elastomer 4 and the flexible substrate 1. Deformability of the adhesive elastomer 4 is required since the flexible substrate 1 may flex during processing and it is advantageous for the adhesive elastomer 4 to flex (i.e. deform) with the flexible substrate 1. Deformability also allows any point defects, such as dust particles, to be absorbed which could otherwise damage the flexible substrate 1. That is, the sufficient elastic modulus maintains the contact area between the adhesive elastomer 4 and the flexible substrate 1 during processing, despite flexing of the flexible substrate 1 and the presence of dust particles. Preferably the elastic modulus is not more than 2500 MPa, more preferably not more than 1000 MPa, more preferably not more than 50 MPa and most preferably not more than 5 MPa. It is also preferred that the elastic modulus is not less than 0.1 MPa, more preferably not less than 0.3 MPa. The elastic modulus should be measured for the bulk material since the surface of the adhesive elastomer 4, as with most materials, will interact with the atmosphere and may have a surface oxide layer with a different elastic modulus to the bulk material. The surface energy and elastic modulus are parameters which would be understood and be readily measurable by those skilled in the art. See, for example, "Physical Properties of Polymers Handbook", edited by J.E. Mark, AlP Press, New York, 1996 and "Polymers at Surfaces and Interfaces" by R.A.L. Jones and R.W. Richards, Cambridge University Press, 1999, pages 36 and 313. The adhesive elastomer 4 according to the present invention may be any organic polymer, including organosiloxanes, provided they have the necessary surface energy and elastic modulus. Examples include a silicone or fluorosilicone polymer, a wax, or a fluorinated polymer. A preferred polymer is polydiorganosiloxane, wherein the organo group is alkyl, cycloalkyl, aryl, aralkyl and/or alkenyl, optionally substituted with fluorine. Examples include poly (dimethylsiloxane) and poly [methyl (nonafluorohexyl) siloxane] with polydimethylsiloxane being particularly preferred. Other suitable polymers include poly

(heptadecafluorodecyloxymethylstyrene), poly (pentadecafluorooctyl acrylate), poly (pentadecafluorooctyl methacrylate), poly

[heptadecafluorooctylsulfonamido (propyl) ethyl acrylate], poly (hexafluoropropylene, poly (tetrafluoroethylene), poly [methyl (trifluoro) siloxane], poly (trifluoroethylene), poly (vinylidene fluoride), poly (vinyl fluoride), poly (1 , 2 -butadiene), poly (isobutylene), poly (vinyl butyral) and paraffin wax, with paraffin wax being preferred. For further details regarding paraffin wax see the Materials Handbook, fourteenth edition, G.S. Brady et al., McGraw-Hill 1997, page 635 et seq. The polymer may be used alone or in the presence of a cross-linker. When a crosslinker is used, the silicone or fluorosilicone polymer requires terminal or pendant functional (i.e. crosslinkable) groups, for example, ethylenically unsaturated-, hydroxy- or epoxy-terminated or pendant functional groups. The adhesive elastomer 4 may also contain high molecular weight lower functionality polymeric components (gums); highly branched components (resins), such as silicate resins; and/or inorganic fillers, such as silica or surface-treated silica, in order to optimise the physical properties of the adhesive elastomer 4. Suitable adhesive elastomers 3 are commercially available, such as STLGARD® 182 and 184 from Dow Corning. Commercially available compositions of this type typically contain a functionalised polyorganosiloxane pre-polymer, such as vinyl-substituted polydimethylsiloxane and a crosslinker, such as polymethylhydrosiloxane, together with a curing catalyst, such as a platinum complex, and silica. The adhesive elastomer 4 is attached to the rigid plate 3 as a discrete layer and should not flow from the surface of the rigid plate 3, i.e. the elastic modulus G' should be greater than the loss modulus G" at any given frequency, e.g. 1 Hz, and preferably ten times greater, more preferably 100 times greater. Such flow properties are typically provided by a degree of crosslinking of the polymer of not more than 10%, preferably not more than 3% and most preferably not more than 0.1 %. The adhesive elastomer 4 may be applied to the rigid plate 3 by any conventional technique. The adhesive elastomer 4 may, for example, be applied in the form of a pre-polymer and cured on the surface of the rigid plate 3. Alternatively the polymer may be spin coated onto the surface of the rigid plate 3. In order to reduce the viscosity of the pre-polymerised material, a low molecular weight silicone oil having a low vapour pressure may be added. This silicone oil does not hinder polymerisation and simply evaporates from the surface of the rigid plate 3. Suitable silicone oils include OS-10, OS-20 and OS-30 Fluids from Dow Corning. The presence of small molecules, such as unreacted oligomers, in the adhesive elastomer 4 may be detrimental to some process steps as they can diffuse out of the bulk material, for example, during vacuum depositions steps. These potential contaminants may be extracted by heating the samples and outgassing them in vacuum. The adhesive elastomer 4 may be a continuous layer or, alternatively, as shown in Fig. 3, the layer may be a discontinuous layer having a pattern of channels 6 therein. The channels 6 facilitate the removal of air from the adhesive elastomer 4. The removal of air reduces the number of air pockets in the adhesive elastomer 4 which improves the adhesion between the adhesive elastomer 4 and the flexible substrate 1. The patterned surface of the adhesive elastomer 4 also assists in the removal of the flexible substrate after processing since presenting a discontinuous adhesive elastomer 4, i.e. islands of adhesive elastomer 4, reduces the peeling force required to remove the flexible substrate. Since the flexible substrate 1 may be removed from the carrier plate 2 without significant amounts of the adhesive elastomer 4 remaining attached to the flexible substrate 1 (in contrast to conventional glues) the carrier plate 2 may be reused. This provides a significant cost saving. The processing of the flexible device may be carried out in any suitable apparatus. Fig. 4 shows an apparatus for processing a flexible substrate into a flexible device. The apparatus will typically have a carrier plate support 7 for holding the carrier plate 2 in position at a processing station 8. Other modifications of the present invention will be apparent to those skilled in the art. EXAMPLE

A polydimethylsiloxane (PDMS) liquid mixture (SYLGARD® 184 from Dow Corning) was poured onto a clean, UV ozone treated 6" x 6" glass plate placed in a sample holder. The PDMS was then cured in an oven for 4 hours at 60°C. In order to achieve a better contact and remove air pockets, the glass plate and cured coating were held under vacuum (10^"3 Torr) for 1 hour. A 4" x 6" foil was then simply positioned on top of the glass plate. The glass plate having the cured coating and foil were then subjected to the following tests: Temperature of 200°C for 2 hours UV exposure in a Karl Suss aligner MA8 having a 1 kW Hg lamp. ITO etch using an HCI/H₂0 1 :1 and 6 wt% FeCI₃ solution at 40°C for up to 5 minutes Hot plate at 120°C for 30 minutes Stripping of photoresist Spin coating of 6" x 6" square flexible substrates up to 3000 rpm, e.g. of photo resist: JSR1400G Photoresist developer (e.g. PD523 for 2 minutes) Photoresist stripping (e.g. Microstrip 2001 60°C and 40°C) for up to 20 minutes, acetone Vacuum deposition of reactive metals (e.g. calcium and barium) on top of the flexible substrate No loss of adhesion of the foil was found.

Claims

1. A device-processing carrier plate (2) for processing a flexible substrate (1) comprising a rigid plate (3) and an adhesive elastomer (4) attached to a surface of the rigid plate (3) which, in use, is proximal to the flexible substrate (1), wherein the adhesive elastomer (4) has a surface energy sufficient to allow releasable adhesion of the flexible substrate (1) to the adhesive elastomer (4) and a sufficient elastic modulus to maintain the contact between the flexible substrate (1) and the adhesive elastomer (4) during processing.

2. A carrier plate (2) as claimed in claim 1, wherein the adhesive elastomer (4) has a surface energy measured at 20 °C of not more than 30 mJ/m².

3. A carrier plate (2) as claimed in claim 1 or 2, wherein the adhesive elastomer (4) has an elastic modulus of 0.1 to 2500 MPa.

4. A carrier plate (2) as claimed in any of claims 1 to 3, wherein the adhesive elastomer (4) comprises a silicone or fluorosilicone polymer, a wax, or a fluorinated polymer.

5. A carrier plate (2) as claimed in claim 4, wherein the adhesive elastomer (4) comprises a silicone polymer and the silicone polymer is polydimethylsiloxane.

6. A carrier plate (2) as claimed in claim 4 or 5, wherein the adhesive elastomer (4) further comprises a gum, a resin and/or an inorganic filler.

7. A carrier plate (2) as claimed in any preceding claim, further comprising a handling facilitating area (5).

8. A carrier plate (2) as claimed in any preceding claim, wherein the surface of the adhesive elastomer (4) which, in use, contacts the flexible substrate (1) contains one or more channels (6).

9. A device-processing carrier plate (2) comprising a rigid plate (3) and an adhesive elastomer (4) which comprises polydimethylsiloxane.

10. An apparatus for processing a flexible substrate (1) comprising a carrier plate (2) as claimed in any preceding claim, a carrier plate support (7) and a processing station (8).

11. A method for processing a flexible substrate (1) comprising the step of carrying the flexible substrate (1) on a rigid plate (3) wherein a surface of the rigid plate (3) which is proximal to the flexible substrate (1) has an adhesive elastomer (4) attached thereto which has a sufficient surface energy to allow releasable adhesion of the flexible substrate (1) to the adhesive elastomer (4) and a sufficient elastic to maintain the contact between the flexible substrate (1) and the adhesive elastomer (4) during processing.

12. A method for processing a flexible substrate (1) as claimed in claim 11 , wherein the adhesive elastomer (4) has a surface energy measured at 20°C of not more than 30 mJ/m².

13. A method for processing a flexible substrate (1 ) as claimed in claim 11 or 12, wherein the adhesive elastomer (4) has an elastic modulus of 0.1 to 2500 MPa.

14. A method for processing a flexible substrate (1) as claimed in any of claims 11 to 13, wherein the adhesive elastomer (4) comprises a silicone or fluorosilicone polymer, a wax, or a fluorinated polymer.

15. A method for processing a flexible substrate (1) as claimed in claim 14, wherein the adhesive elastomer (4) comprises a silicone polymer and the silicone polymer is polydimethylsiloxane.

16. A method for processing a flexible substrate (1) as claimed in claim 14 or 15, wherein the adhesive elastomer (4) further comprises a gum, a resin and/or an inorganic filler.

17. A method for processing a flexible substrate (1) as claimed in any of claims 11 to 16, wherein the surface of the adhesive elastomer (4) which contacts the flexible substrate (1) contains one or more channels (6).

18. A method for processing a flexible substrate (1) as claimed in any of claims 11 to 17, wherein the rigid plate (3) and the flexible substrate (1) have substantially the same linear thermal expansion coefficient.

19. A method for processing a flexible substrate (1) as claimed in claim 18, wherein the rigid plate (3) and the flexible substrate (1) are made of the same material.