HK1207682B - Method of patterning electrically-conductive film on flexible substrates - Google Patents

Description

Method for patterning conductive film on flexible substrate

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/632,236, filed January 20, 2012, the contents of which are incorporated herein by reference.

Technical Field

The present invention generally relates to methods for forming conductive electrodes. Specifically, the present invention relates to methods for forming conductive electrodes on flexible substrates. More specifically, the present invention relates to methods for forming patterned electrodes by controlling the fracture of a conductive film (e.g., indium-tin-oxide (ITO)) layer disposed on a flexible substrate.

Background Art

Typical flat panel displays, such as LC (liquid crystal) displays and plasma displays, utilize conductive indium-tin-oxide (ITO) thin films vacuum deposited on rigid glass substrates to control various operating functions of the display. That is, during the production of the flat panel displays, the ITO films are patterned into optically transparent electrodes using conventional photolithography techniques. In addition, the production of flat panel displays using photolithography requires precise and accurate bonding of the electrodes to the display's drive circuitry, which can be expensive. However, recently the flat panel display industry has attempted to replace the use of rigid glass substrates with flexible substrates (such as those formed from flexible plastics), while still maintaining the use of ITO or other conductive polymers, using improved printing and photolithography techniques to form the transparent electrodes.

In addition, although ITO has the desired optical and electrical properties required for many electronic applications (including, for example, flat panel displays and photovoltaic devices), ITO is brittle and easily breaks when the substrate on which the ITO film is carried is bent or flexed. Therefore, electronic devices using ITO tend to be fragile and require careful handling, and in some cases, can lead to reduced productivity of the electronic devices. In contrast, conductive polymers, which are alternatives to ITO, have the advantage of being more flexible than ITO and able to be used in electronic device manufacturing processes using printing and photolithography techniques. However, conductive polymers have various disadvantages compared to ITO, including reduced conductivity and reduced light transmittance.

Therefore, there is a need for a low-cost method for patterning a conductive film, such as indium-tin-oxide (ITO), into a conductive electrode on a flexible substrate. Furthermore, there is a need for a method for patterning a conductive film, such as indium-tin-oxide (ITO), into a conductive electrode on a flexible substrate that is compatible with a continuous roll-to-roll manufacturing process. Furthermore, there is a need for a method for patterning a conductive film, such as indium-tin-oxide (ITO), into a precisely defined conductive electrode. Furthermore, there is a need for a method for patterning a conductive film, such as indium-tin-oxide (ITO), onto a flexible substrate that is easy to perform and eliminates the need for expensive and environmentally harmful materials and solvents.

Summary of the Invention

According to the above, a first aspect of the present invention provides a method for patterning a conductive film, the method comprising: providing a flexible substrate having a conductive film disposed thereon to form a combined layer, bending the flexible substrate around a radius of curvature, and moving the radius of curvature along the combined layer to form a plurality of crack lines in the conductive film, wherein each pair of the plurality of crack lines defines a conductive portion therebetween and electrically insulates the conductive portion.

Another aspect of the present invention is to provide a method for patterning a conductive film, the method comprising: providing a flexible substrate having a conductive film disposed thereon so as to form an elongated combined layer; providing first and second substantially parallel plates, the plates being separated by a gap; attaching a portion of the substrate to each of the first plate and the second plate so that the combined layer is bent across the gap at a radius of curvature; and sliding one of the first plate and the second plate relative to the other so as to bend the combined layer at the radius of curvature so as to form a plurality of crack lines in the conductive film, whereby each pair of the plurality of crack lines defines a conductive portion therebetween and electrically insulates the conductive portion.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings, in which:

FIG1A is a front view showing a composite layer formed of a PET substrate and an indium-tin-oxide (ITO) film disposed thereon according to the concept of the present invention;

FIG1B is an elevation view showing a combined PET/ITO layer being bent about a radius of curvature to form a crack line in the ITO film in accordance with the concepts of the present invention;

FIG1C is an elevation view showing a crack line formed in an ITO film after the combined PET/ITO layer has been bent, in accordance with the concepts of the present invention;

FIG2 is a schematic representation of a transmission optical microscope image of a combined PET/ITO layer showing crack lines formed in the ITO film, in accordance with the concepts of the present invention;

FIG3 is a front view showing a combined PET/ITO layer being rolled between two flat glass plates separated by a predetermined distance to form a crack line in the ITO film, in accordance with the concepts of the present invention; and

4 is a schematic representation of a scanning electron microscope (SEM) image of a combined PET/ITO layer showing crack lines formed in the ITO film, in accordance with the concepts of the present invention.

DETAILED DESCRIPTION

A method for patterning a conductive film disposed on a flexible substrate to form a conductive electrode is proposed. Specifically, a thin conductive film 10, which may comprise indium-tin-oxide (ITO) or other suitable material, is coated on a flexible substrate 20 to form a composite layer 30, as shown in FIG1A . In one aspect, the flexible substrate 20 may comprise polyethylene terephthalate (PET) and any other suitable flexible material, such as, for example, plastic. The ITO film 10 may be coated or otherwise disposed on the flexible substrate 20 using any suitable process, such as, for example, sputtering or vacuum deposition. Once the ITO film 10 is disposed on the flexible substrate 20, stress is applied to the composite layer 30 to cause the ITO film 10 to break into electrically insulating conductive portions. Specifically, the ability to cause the ITO film 10 to break is due to its brittleness (i.e., its inability to sustain dimensional changes without breaking) and the ability of the substrate 20 to be flexed.

In order to crack the conductive ITO film 10 or otherwise pattern it, stress is applied to the ITO film 10 by mechanical bending or by application of thermal stress. Mechanical bending is achieved by bending the combined layer 30 in the manner to be discussed, with the radius of curvature of the bend thus inversely controlling the amount of mechanical stress applied to the substrate 20 and the ITO film 10. That is, the smaller the radius of curvature used to form the bend, the greater the amount of mechanical stress applied to the substrate 20 and the ITO film 10. Alternatively, in the case of heat treatment of the combined layer 30, the substrate 20 and the conductive ITO film 10 have different coefficients of thermal expansion so that changes in temperature will produce mechanical strain that is used to produce a crack line in the conductive ITO film 10.

Thus, after the ITO film 10 is disposed on the flexible substrate 20, a mechanical stress is applied to the combined layer 30 by bending the substrate 20 so as to have a radius of curvature designated as "R." In one aspect, the radius of curvature R can be generated by simply bending the combined layer 30 on itself, or by bending the combined portion around a generally cylindrical portion 32 (e.g., a rod) so that the combined layer extends about the radius of curvature R by a set amount, such as, for example, approximately 180 degrees, as shown in FIG. 1B . The bending stress applied to the conductive ITO film 10 forms a plurality of evenly spaced crack lines 40 in the ITO film 10, as shown in FIG. 1C and FIG. 2 . The crack lines 40 are formed in the ITO film 10 as lines extending in a direction generally perpendicular to the direction in which the combined layer 30 is being bent. Additionally, the spaced crack lines 40 define generally uniform conductive strips or portions 50 in the ITO film 10 therebetween. Furthermore, it should be appreciated that the crack lines 40 electrically isolate adjacent conductive strips or portions 50 from one another, thereby allowing the conductive portions 50 to function as conductive electrodes.

Continuing with reference to Figures 1A-1B, the stress applied by mechanical bending of the combined layer 30, previously discussed, can be calculated according to the following method. It should be understood that for the purposes of the following discussion, the thickness of the conductive film 10 is typically several orders of magnitude thinner than that of the substrate 20. Therefore, when calculating the stress applied to the combined layer 30, the thickness of the conductive film 10 is neglected, as presented below. Specifically, referring to Figure 1B, in the case where the substrate 20 is formed of PET and is bent 180 degrees about a curved portion 32 having a radius of curvature "R," the length of the inner surface of the substrate 20 adjacent to the radius of curvature R of the curved portion 32 is equal to πR. The outer surface of the substrate 20 has a longer length than its inner surface, which allows the substrate 20 to accommodate the thickness of the substrate film 20, designated as " _T1 ." Furthermore, the length of the substrate 20 is equal to _πT1 , and the radius of curvature about which the substrate 20 is bent is increased to R + _T1 , resulting in a total length of the film of π(R + _T1 ). Therefore, stress can be expressed as the relative increase in length required for substrate 20 to accommodate bending, and is defined by the following equation: which simplifies to T ₁ /R. Therefore, the ratio of substrate thickness 20 to the radius of curvature R of curved portion 32 used to bend combined layer 30 defines the stress applied to combined layer 30, which ultimately results in the formation of crack line 40 in conductive ITO film 10.

In other words, as flexible substrate 20 bends around the radius of curvature R provided by curved portion 32, as shown in FIG1B , the outer surface of substrate 20 stretches and/or the inner surface of substrate 20 contracts by an amount defined by radius of curvature R and substrate thickness _T1 to accommodate the bend. The length of the outer surface of substrate 20 will be longer than the length of the inner surface of substrate 20 around the bend by π times the thickness _T1 of substrate 20, as shown in FIG1B . Therefore, the relative amount of stress induced by the bend is equal to the ratio of the thickness _T1 of substrate 20 relative to the radius of curvature R of curved portion 32 being used to bend combined layer 30. For example, in the case where substrate 20 comprises a substrate formed of PET having a thickness _T1 of approximately 7 mils, or approximately 0.17 mm, and is tightly bent around curved portion 32 having a radius of curvature R of 1 mm, the percent change in length of substrate 20 will be 17%, which is sufficient to generate crack line 40 in ITO film 10.

Alternatively, crack lines 40 can also be formed in the conductive ITO film by applying thermal changes to the combined layer 30 to change the dimensions of the substrate 20 relative to the dimensions of the conductive ITO film 10. That is, if the coefficient of thermal expansion of the flexible substrate 20 is significantly different from that of the conductive film 10, then crack lines 40 can be generated in the conductive film 10 by heating or cooling the combined layer 30. Additionally, if the coefficient of thermal expansion is isotropic for the flexible substrate 20, then temperature changes in the combined layer 30 will produce randomly aligned crack lines in the conductive film 10. Alternatively, it is possible to use a flexible substrate 20 that is uniformly stretched in one direction. Heating the combined layer 30 above the glass transition temperature will produce a large, uniform contraction of the substrate 20 in one direction, which can be used to produce uniform crack lines in the conductive film 10, similar to those produced by the bending techniques discussed previously.

Thus, the crack lines 40 formed in the conductive film 10 serve to destroy or at least greatly reduce the conductivity of the conductive film 10 and, therefore, to electrically insulate adjacent conductive portions or strips 50 of the conductive film 10. Thus, controlled fracturing of the conductive ITO film 10 allows for the formation of regularly spaced crack lines 40, which allows the process to be used to form electrode patterns in the conductive film 10 with precise shapes and dimensions. For example, uniform crack lines 40 separated by 5-10 microns can be produced by uniformly rolling a polyester PET substrate 20 coated with an indium-tin-oxide (ITO) thin film 10. Cracks in the ITO film 10 are formed as lines perpendicular to the bending direction.

During experimental evaluation, cracks 40 were formed in the conductive ITO film 10 by positioning the combined layer 30 between two thick, opposing flat glass plates 100 and 110 separated by a gap 112 having a predetermined distance, as shown in FIG3 . It should be understood that the plates 100, 110 can be formed from any suitable material, such as, for example, aluminum. In one aspect, the plates 100 and 110 can be separated by a rolling spacer 114, such as a cylindrical rod or other rollable object or roller, such as a ball bearing or a bead, to maintain the plates 100, 110 in a generally parallel arrangement to define the radius of curvature to be imparted to the combined layer 30 and to facilitate movement of the plates 100, 110 relative to each other. Specifically, the combined layer 30 was positioned so that one end of the flexible substrate 20 was attached to the inner surface 120 of the glass plate 100, while the other end of the flexible substrate 20 was attached to the inner surface 130 of the other glass plate 110, as shown in FIG3 . Next, the combined layer 30 is rolled between the two plates 100 and 110, thereby bending the ITO film 10 and substrate 20 by an amount defined by a radius of curvature R, which is determined by the size of the gap 112. On the one hand, in this configuration, the flexible substrate 20 is bent approximately 180 degrees. As a result, a crack line 40 is formed in the ITO film 10 along the bend or along the axis of curvature R to accommodate the stress applied by the radius of curvature r defined by the gap 112. For example, FIG. 4 shows an SEM (scanning electron microscope) image of a crack line 40 produced by bending the combined layer 30, where the plates 100 and 110 are spaced apart so that the gap 112 imparts a radius of curvature of 1 mm to the combined layer 30.

Thus, in one aspect, it is expected that the distance separating crack lines 40, which defines the width of conductive portion 50, increases with increasing radius of curvature R used to bend combined layer 30, with the amount of separation depending on a variety of other factors, including, but not limited to, the thickness of ITO film 10, and the flexibility and brittleness of ITO film 10 and substrate 20. For example, for a radius of curvature of 1 mm or less, crack lines are separated by approximately 2-8 microns, as shown in Figures 2 and 3. In another embodiment, a radius of curvature of approximately 2 mm can produce crack lines separated by approximately 10-20 microns.

Next, after the ITO film 10 has been bent to form a crack line 40 therein, the conductivity of the ITO film 10, when measured along the crack line 40, remains substantially unchanged, regardless of the curvature radius R used to complete the bending process. For bending with a tight or small curvature radius R of approximately 1 mm or less, the conductivity measured perpendicular to the formed crack line 40 decreases by more than an order of magnitude. Table 1 below shows the parallel resistance and vertical resistance (the inverse of conductivity) of the ITO film 10 as a function of the bending curvature radius R, showing that although the vertical conductivity (i.e., the conductivity in the direction perpendicular to the crack line 40) decreases with decreasing curvature radius R, it does not disappear.

Alternatively, the crack line 40 may be enlarged to increase the difference in conductivity measured parallel to and perpendicular to the crack line 40. For example, photochemical etching using dilute HCl may be used to remove any remnants of ITO remaining in the crack line 40, which would promote electrical conduction across the crack line 40. As a result, the conductivity of the crack line 40 is further reduced.

Additionally, to further reduce the conductivity of the crack line 40, the substrate 20 may be uniaxially stretched in a direction perpendicular to the crack line 40. Uniaxial stretching has been shown to increase the difference in conductivity by another order of magnitude to over 500:1.

Additionally, the method of forming conductive electrodes using the aforementioned ITO fracture process can be used to form polymer dispersed liquid crystal (PDLC) shutters or windows. Furthermore, the differential conductivity between the parallel and perpendicular directions of the crack lines 40 allows individual conductive portions 50 between the crack lines 40 to be used for addressing applications. This is achieved by charging the appropriate conductive portions 50 by forming electrical contacts only at both ends of a portion of the substrate 20. Because the electric field can conduct electricity across the crack lines 40, the non-addressed conductive portions 50 are charged to a lesser extent, depending on the frequency of the applied electric field. Furthermore, the differential conductivity allows for the use of much lower frequencies in such applications.

On the other hand, conductive portions 50 defined by the spaces between successive crack lines 40, which can be used as address lines for various electronic devices, such as LCDs (liquid crystal displays), can be formed by fracturing the conductive film 10 in intersecting directions to form multiplexed address lines for use in various electronic applications. For example, in one aspect, the separation between adjacent crack lines 40 can be formed to be approximately 5-20 microns, which also results in the width of the formed conductive portions or strips 50 being approximately 5-20 microns. Therefore, because the pixel size of currently commercially produced flat panel displays is approximately 100-200 microns, the bonding process used to couple the electrodes including the conductive portions 50 to the display's driver circuitry does not require the same highly accurate alignment that would be required when patterning ITO strips or sections using typical photolithography.

Based on the foregoing, the advantages of the present invention are readily apparent. A primary advantage of the present invention is that a method is provided for patterning a conductive film, such as indium-tin-oxide (ITO), on a flexible substrate into electrically insulating, conductive portions that form electrodes for a variety of electronic device applications. Another advantage of the present invention is that the method for patterning a conductive film, such as indium-tin-oxide (ITO), on a flexible substrate into individual conductive portions of ITO is simple to perform and can be implemented on existing roll-to-roll processes, without requiring the complex photolithographic or printing processes currently used. A further advantage of the present invention is that the method for patterning a conductive film, such as indium-tin-oxide (ITO), on a flexible substrate allows electrodes to be bonded to display driver circuitry or other circuitry without requiring the precise alignment therebetween, as is required in conventional photolithographically patterned ITO strips or portions, which reduces the cost and manufacturing complexity of flat panel displays. Another advantage of the present invention is that the method for patterning a conductive film, such as indium-tin-oxide (ITO), on a flexible substrate can be implemented in a high-throughput and low-cost process. Another advantage of the present invention is that the method of patterning a conductive film such as indium-tin-oxide (ITO) on a flexible substrate uses a reliable and economical process to produce conductive ITO portions that are insulated from each other. Another advantage of the present invention is that the method of patterning a conductive film such as indium-tin-oxide (ITO) on a flexible substrate requires only controlled bending of the substrate carrying the ITO film thereon to form the conductive ITO lines. Another advantage of the present invention is that the method of patterning a conductive film such as indium-tin-oxide (ITO) on a flexible substrate immediately forms cracks when the substrate carrying the ITO film is flexed and/or stretched so that conductive portions are formed between the formed cracks. Another advantage of the present invention is that the method of patterning a conductive film such as indium-tin-oxide (ITO) on a flexible substrate allows for the formation of conductive electrodes in a manner that minimizes the inactive inter-electrode area and maximizes the active electrode area.

Thus, it can be seen that the objects of the present invention have been met by the structures and methods of use presented above. Although, in accordance with the patent statutes, only the best mode and preferred embodiments have been presented and described in detail, it should be understood that the present invention is not limited thereto or thereby. Therefore, for an appreciation of the true scope and breadth of the present invention, reference should be made to the following claims.

Claims (8)

1. A method for patterning a conductive film, the method comprising: A flexible substrate with a conductive film disposed thereon is provided to form an elongated composite layer; The composite layer is positioned in the gap formed between the spaced-apart first and second plates; The first portion of the composite layer is attached to the inner surface of the first plate near the gap; A second portion of the composite layer is attached to the inner surface of the second plate adjacent to the gap, wherein the attachment of the first and second portions of the composite layer to the plates forms a bend in the composite layer, which is positioned between the first and second plates across the gap with a radius of curvature; and The combined layer between the first and second plates is caused to move relative to the other, so as to roll around the radius of curvature. The moving step involves forming a plurality of slits in the conductive film, such that each pair of the plurality of slits defines a conductive portion therebetween and electrically insulates the conductive portion. 2. The method according to claim 1, wherein the thickness of the conductive film is less than the thickness of the substrate. 3. The method of claim 1, wherein the moving step is performed to bend the composite layer about 180 degrees around the radius of curvature. 4. The method according to claim 1, wherein the radius of curvature is about 1 mm. 5. The method according to claim 1, wherein the conductive film comprises indium tin oxide (ITO). 6. The method of claim 5, wherein the substrate comprises polyethylene terephthalate (PET). 7. The method of claim 1, wherein the first plate and the second plate are spaced apart by at least one roller. 8. The method of claim 1, wherein the moving step is performed to cause the composite layer to bend continuously.