Patent Application
20090200543
This invention generally relates to electronic device fabrication and more particularly relates to a method for mounting a flexible substrate to a carrier and forming an electronic device on the substrate.
Thin-film transistor (TFT) devices are widely used in switching or driver circuitry for electro-optical arrays and display panels. TFT devices are conventionally fabricated on rigid substrates, typically glass or silicon, using a well-known sequence of deposition, patterning and etching steps. For example, amorphous silicon TFT devices require deposition, patterning, and etching of metals, such as aluminum, chromium or molybdenum; of amorphous silicon semiconductors; and of insulators, such as SiO2 or Si3N4, onto a substrate. The semiconductor thin film is formed in layers having typical thicknesses ranging from several nm to several hundred nm, with intermediary layers having thicknesses on the order of a few microns, and may be formed over an insulating surface that lies atop the rigid substrate.
The requirement for a rigid substrate has been based largely on the demands of the fabrication process itself. Thermal characteristics are of particular importance, since TFT devices are fabricated at relatively high temperatures. Thus, the range of substrate materials that have been used successfully is somewhat limited, generally to glass, quartz, or other rigid, silicon-based materials.
TFT devices can be formed on some types of metal foil and plastic substrates, allowing some measure of flexibility in their fabrication. However, problems such as chemical incompatibility between the substrate and TFT materials, thermal expansion mismatch between substrate and device layers, planarity and surface morphology, and capacitive coupling or possible shorting make metal foil substrates more difficult to employ in many applications.
The fabrication process for the TFT may require temperatures in the range of 200-300 degrees C. or higher, including temperatures at levels where many types of plastic substrates would be unusable. Thus, it is widely held, as is stated in U.S. Pat. No. 7,045,442 (Maruyama et al.), that a TFT cannot be directly formed on a plastic substrate. In order to provide the benefits of TFT devices mounted on a plastic substrate, the Maruyama et al. '442 disclosure describes a method that forms the TFT on a release layer that is initially attached to a carrier substrate. Once the TFT circuitry is fabricated, the release layer is then separated from its carrier substrate and can be laminated onto a lighter and more flexible plastic material.
While there have been some solutions proposed for forming TFT components on flexible substrates, there are still a number of significant technical hurdles. Lamination of a release layer that is populated with TFT devices, as described in Maruyama et al. '442 requires additional fabrication steps and materials and presents inherent alignment difficulties. The use of higher-performance plastics still leaves difficulties with thermal expansion (expressed in terms of Coefficient of Thermal Expansion, CTE) and requires additional layers and processes in order to protect the plastic. Solutions using pulsed excimer lasers do not provide the full breadth of capabilities of more conventional TFT fabrication techniques and thus have limited utility. None of the known methods just discussed provides a flexible substrate that truly serves to replace glass or other silicon-based substrate, since the TFT must be formed either on a release layer or on some intermediate layer that must be formed on top of the flexible substrate.
TFT fabrication onto flexible substrates generally requires that the substrate be held on a carrier of some type during the various stages of layer deposition. One of the more important functions of such a carrier is providing dimensional stability to the flexible substrate. Thus, for example, a rigid glass carrier is conventionally provided. As described in Japanese Patent Publication Number JP 7-325297 A2 (Ichikawa), TFT devices can be formed onto a plastic substrate that is temporarily held to a glass carrier by means of an adhesive layer.
The use of a glass carrier, however, imposes some constraints on the types of flexible substrate materials that can be used. Some types of plastics are compatible with the use of a glass substrate, but can be impractical because they exhibit glass transition, Tg, temperatures near the region of temperatures used for deposition. Thus, plastic substrates can tend to soften somewhat, allowing unwanted expansion during a fabrication cycle. Metals do not have this disadvantage. However, metallic materials are not as dimensionally “forgiving” with change in temperature. A significant difference in coefficient of thermal expansion (CTE) between metals and glass results in excessive stress that can shatter glass or can cause a metal substrate to release from a glass carrier prematurely, losing its dimensional stability.
Thus, it can be seen that although there has been great interest in developing and expanding the use of both plastics and metals as flexible substrates, compatibility with a conventional glass carrier imposes some constraints on substrate material type.
An object of the present invention is to provide a method for forming an electronic device comprising steps of conditioning the surface of a carrier to form a holding area for retaining a flexible substrate; applying a contact surface of the flexible substrate against the carrier with a binding intermediate material applied between at least the holding area of the carrier and the corresponding area of the contact surface; removing entrapped gas between the flexible substrate and the carrier; processing the substrate to form the electronic device thereon; and removing the flexible substrate from the holding area.
Another object of the present invention is to provide an electronic device fabricated onto a flexible substrate. The range of flexible substrates available using embodiments of the present invention can include various types of metal, including some types of metal foil, and other very thin substrates.
An advantage of the present invention is that it adapts a glass or other similar carrier for processing a flexible substrate at high temperatures.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.
While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings.
It is to be understood that elements not specifically shown or described in the following detailed description may take various forms well known to those skilled in the art. The figures given in this application are representative of overall spatial relationships and arrangement of layers for deposition onto a substrate and may not be drawn to scale.
As the term is used in the present description, “plastic” refers to a material having a high polymer content, usually made from polymeric synthetic resins, which may be combined with other ingredients, such as curing agents, fillers, reinforcing agents, colorants, and plasticizers. A “resin” is a synthetic or naturally occurring polymer. Plastic is solid in its finished state, and, at some stage during its manufacture or processing into finished articles, can be shaped by flow. Plastics are typically formed using a curing process in which a solvent is evaporated at a suitable rate. Plastic includes thermoplastic materials and thermosetting materials. The term “flexible” refers generally to sheet materials that are thinner than about 1.5 mm.
Referring to
The apparatus and methods of the present invention provide ways to fabricate electronic device 10 on flexible substrate 20 using a carrier 18. Using the apparatus and methods of the present invention, a range of flexible substrates 20 can be used, supported on carrier 18 without requiring a match between the CTE of the substrate and that of the carrier. Thus, for example, a metal substrate such as stainless steel having a CTE of about 17 ppm/degree C. could be supported on a glass carrier having a CTE in the range of 2-3 ppm/degree C. Unlike substrate mounting methods requiring closely matched CTE values, the present invention allows respective CTE values of the carrier 18 and substrate 20 to differ from each other by more than 2 ppm/degree C.
The perspective view of
Alternatively, such a holding area may be formed by conditioning the surface of carrier 22 around its perimeter, not illustrated, leaving an unconditioned central area opposite which an electronic device can be formed on a flexible substrate supported by the carrier. Also, a perimeter holding area may be formed on the contact surface of the flexible substrate by depositing adhesion-promoting material on the perimeter of the flexible substrate, thereby forming on the perimeter a stronger adhesive bond between the substrate and the carrier, than in the center. Or, an adhesion-reducing material may be applied in the central area of either the substrate, the carrier, or both, thereby also forming on the perimeter a stronger adhesive bond between the substrate and the carrier, than in the central area.
The cutaway side view of
In one embodiment, binding material 28 is a reflowable plastic binder material such as a Teflon coating. Flexible substrate 26 is a sheet of grade 304 stainless steel. The following basic sequence is used.
The steps given for this example admit any of a number of variations. For example, the thickness as well as the composition of binding material 28 can be suitably adjusted for substrate 26 and carrier 22 conditions. A relatively pure Teflon material can be used; however, a composition that includes Teflon with particulate, fibrous, or other filler materials could alternately be used, where the particulate additive provides improved behavior, temperature range, or other condition.
Where carrier 22 is glass having an etched, sandblasted, or deposited holding area 24, a Teflon or other reflowable plastic material provides a relatively strong bond to the roughened glass holding area. Outside of holding area 24, the reflowable plastic material acts as a barrier to minimize entrapment of air or other gases between the interfacing surfaces of substrate 26 and carrier 22. Reheating the Teflon intermediate binding material then allows removal of substrate 26 from carrier 22. In some embodiments, intermediate binding material 28 remains deposited on substrate 26 after its removal from the carrier 22 surface, serving as a dielectric layer for the fabricated device or circuit, for example.
Heat and pressure provide one type of lamination. Other lamination methods may use heat alone, pressure alone, or solvents or other materials as intermediate binding material 28. Where epoxy or other adhesive is used, heat or electromagnetic energy can be applied to weaken the epoxy bond sufficiently for substrate 26 removal following component fabrication. For example, a number of types of epoxy lose adhesive strength under higher temperatures. For example, Epo-Tek 353ND epoxy, available from Epoxy Technology, Inc., Billerica, Mass., and similar epoxies can have relatively low glass transition temperatures for extended periods of heat application and degradation temperatures at which bond strength significantly decreases.
De-lamination can be performed in a number of ways, using heat or chemicals for example. Alternately, peeling could be used for delamination, including methods that peel using a metal or metal wire skive, for example. Radiation over a band of wavelengths can be used to fix or soften intermediate binding material 28 between carrier 22 and substrate 26. Radiation wavelengths can be chosen so that either substrate 26 or carrier 22 is substantially transparent to the radiation energy.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above, and as noted in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, holding area 24 can be formed on the surface of carrier 22 in a number of ways. Methods for conditioning this surface to form holding area 24 can include the use of abrasive materials, such as sandblasting, or chemicals such as etchants, for example. These methods tend to roughen the surface by removing material. Alternately, methods for deposition of material onto the surface of carrier 22 can also be used. Methods such as sputtering can be used to add an area of material that is the same as is already used in carrier 22; alternately, a different material can be deposited onto the surface of carrier 22 to form holding area 24. Particulate material could be embedded in the surface of carrier 22 to form holding area 24. Various materials could be bonded to the surface of carrier 22 to form holding area 24. Deposition methods can be particularly advantageous where it is desirable to apply pressure, without heat, for mounting substrate 26 to carrier 22. The invention also includes providing a conditioned retaining area on the contact or underside of substrate 26, rather than on the upper surface of carrier 22, as already described. For example, the contact surface of substrate 26 could be chemically treated to facilitate adhesion of the plastic binder material. The surfaces of carrier 22 or substrate 26 also may be chemically pre-treated to facilitate separation from binding material 28 during de-lamination.
Thus, what is provided is a method for mounting a substrate to a carrier for forming an electronic device on a flexible substrate.
Reference is made to U.S. Ser. No. 11/461,080 by Kerr et al. entitled FLEXIBLE SUBSTRATE WITH ELECTRONIC DEVICES FORMED THEREON, filed Jul. 31, 2006; U.S. Ser. No. 11/538,173 by Kerr et al. entitled FLEXIBLE SUBSTRATE WITH ELECTRONIC DEVICES AND TRACES, filed Oct. 3, 2006; U.S. patent application Ser. No. ______ by Kerr et al. entitled METHOD FOR FORMING AN ELECTRONIC DEVICE ON A FLEXIBLE SUBSTRATE SUPPORTED BY A DETACHABLE CARRIER AND RESULTANT DEVICE, filed Feb. 8, 2008; and U.S. patent application Ser. No. ______ by Kerr et al. entitled METHOD FOR FORMING AN ELECTRONIC DEVICE ON A FLEXIBLE METALLIC SUBSTRATE AND RESULTANT DEVICE, filed Feb. 8, 2008.
