In general, this invention relates to an ablatable laminar imaging medium having an optimized exposure threshold, and in particular, one wherein said optimized threshold is resultant of the employment therein of an ablation enhancement layer with predefined absorption and reflection values.
Liquid crystal displays comprise a liquid crystal material sandwiched between two substantially transparent electrode assemblies. Touch screen displays of either the resistive or capacitive types comprise a display screen (for example, a cathode ray tube) having superposed thereover two substantially transparent electrode assemblies. In both types of displays, each of these electrode assemblies typically comprises a substrate on which is deposited a conductive layer thin enough to be substantially transparent.
The term xe2x80x9csubstantially transparentxe2x80x9d is used herein to mean that the electrodes transmit sufficient visible light so that the two superposed electrodes will not substantially obscure, nor substantially distort the color of, a liquid crystal display or touch screen display incorporating the two electrodes. Typically, commercial specifications require that the two superposed electrodes have a transmittance of at least 80% at 550 nm.
In liquid crystal displays the substrate is usually glass, whereas touch screen displays usually employ a synthetic resin (plastic) substrate for at least one electrode. The conductor is often formed from indium tin oxide or a similar metal oxide. The conductor is typically formed by depositing the oxide by sputtering or chemical vapor deposition at a high temperature, and then annealing, also at a high temperature. On glass substrates temperatures in excess of 300xc2x0 C. may be used to deposit and anneal the conductor; on plastic substrates, lower temperatures must be used, with resultant higher electrical resistance in the conductor.
Alternatively, both liquid crystal displays and touch screen displays may make use of thin film electrodes comprising a metallic conductive layer sandwiched between two layers having high refractive indices; these two layers usually being formed from metal oxides. The metallic conductive layer is patterned so as to divide it into a plurality of electrodes, and conductors are attached to each of these electrodes to enable formation of the desired patterns in the liquid crystal material.
Conventional prior art (i.e., photolithographic) processes for forming electrodes generally involve the deposition of layers of photoreactive and non-photoreactive thin films (typically, metal oxides) onto a substrate, the selective exposure of said layers through a mask (or like phototool) corresponding to the desired electrode pattern, and removal of either the exposed or unexposed portionsxe2x80x94depending on the nature of the photoreaction involvedxe2x80x94of the thin film layers.
Prior art processes for forming electrodes also often require the use of elevated temperatures of 200xc2x0 C. or more, which in practice requires the use of glass substrates or expensive high temperature plastics (polymers are known which have glass transition temperatures above 225xc2x0 C. and can thus withstand processing at such temperatures). There are many applications for liquid crystal displays (for example, in cellular telephones and other mobile electronic devices) where it would be advantageous to use less expensive plastic substrates having lower glass transition temperatures if thin film electrodes could be formed on such substrates.
While applicability of the above processes still remain practical and commercially viable, interest in the formation of electrode patterns by direct laser imaging (particularly, laser ablation processes) continues to develop at a relatively high ratexe2x80x94in part, because of laser imaging""s potentially higher accuracy, its faster production, and its independence from certain costly pre-imaging processes (e.g., mask preparation). Potential also exists in the use of lighter and cheaper plastic substrates.
The formation of LCD electrode patterns by laser ablation essentially involved the direct pattern-wise removal of portions layers of electrically conductive and insulating materials by exposing said portions to laser light of an intensity and quality (e.g., wavelength) sufficient to completely or partly decompose said material. The reaction can be fairly characterized as xe2x80x9cexplosivexe2x80x9d (i.e., on a microscopic level), producing vapor-like or gas-like streams consisting of fragments of the removed material. An example of such process is disclosed in U.S. patent application Ser. No. 09/009,391, filed by Hyung-Chul Choi, Yi-Zhi Chu, Linda S. Heath, and William K. Smyth on Jan. 20, 1998.
Overall, most laser ablation electrode-forming processes produce good results. Regardless, areas for further improvement exist.
For example, it has been observed that the process of ablating a metal and partial conductor sometimes results in the formation of ridges (like undesirable physical anomalies) as a result of the volcanic action of ablated metal from beneath the partial conductor. These ridges reduce boundary definition of ablated areas, and as such, can potentially compromise the pixel resolution in a finished flat panel display. While conventional post-patterning cleaning processes can be employed to address this problem, the debris on etched edges is difficult to remove, and requires vigorous steps that can damage the thin film structure.
Also, in the conduct of conventional laser ablation processes, high threshold exposure energies are typically involved. Aside from the apparently high energy costs, use of intense ablation exposure produces correspondingly intense temperature elevations in and around the area of ablation. This limits selection of material used for the ablation medium to those having high thermal resistance. And, in the selection of substrates, this greatly constrains the use of plastic materials, which is often preferred over the more common glass substrates when lower costs, greater flexibility, and lighter weight are desired.
Finally, laser ablation produces residue, which needs to be washed off. Being essentially composed of the same material that ultimately becomes the finished electrode, such residue, if left in the valleys between electrodes, can cause an electrical short. The presence of such a short is of course undesirable, since it in effect turns the two adjacent electrodes into a single electrode and thereby adversely affects the quality of a liquid crystal or touch screen in which the electrode assembly is used.
In response to the above-identified issues, an ablatable laminar imaging medium is described herein, characterized by its incorporation of an ablation enhancement layer, the ablation enhancement layer being composed, configured, and located to effectively reduce the threshold energy requirement for effecting laser ablation. The use of the ablation enhancement layer dramatically improves laser ablation efficiency (i.e., faster scan speed, lower exposure energy threshold, and quality), and promotes xe2x80x9ccleanerxe2x80x9d ablation of the metallic layer(s) in the area of exposure (e.g., well ablated areas with few xe2x80x9cridgesxe2x80x9d, unablated residue, and/or other like ill-defined edges, surfaces, and boundaries).
In particular, the ablatable laminar imaging mediumxe2x80x94which is particularly useful in the manufacture of a substantially transparent electrode assemblyxe2x80x94can be defined as comprising: (a) a substrate; (b) a high-index metal oxide layer; (c) an ablatable metallic conductive layer; (d) a high-index conductive metal oxide layer; and (e) an ablation enhancement layer having an IR-absorption greater than the IR-absorption of said high-index conductive metal oxide layer and an IR-reflectivity less than the IR-reflectivity of said high-index conductive metal oxide layer.
The present invention also provides a new process for dealing with (cf., xe2x80x9ccleaningxe2x80x9d) remnant and/or residual ablation enhancement material left in the course of ablation, which for many applications and products is unwanted. The characterizing steps of the inventive method involve the execution of a xe2x80x9ccleaningxe2x80x9d operation in close connection with a xe2x80x9cpatterningxe2x80x9d operation, preferably using the same apparatus (e.g., the same laser) for both. The new process can be accomplished by either a sequential methodology (i.e., ablation patterning, followed by ablation cleaning) or a continuous methodology (i.e., ablation patterning conducted contemporaneously with ablation cleaning ).
In light of the above, it is a principal object of the present invention to provide an ablatable laminar imaging, useful in the manufacture of an electrode assembly for a flat panel display, having a low exposure threshold requirement.
It is another object of the present invention to provide an ablatable laminar imaging medium having incorporated therein an ablation enhancement layer, the IR-reflectivity and IR-absorptivity of the ablation enhancement layer specifically designed for favorable effect on the medium""s exposure threshold.
It is another object of the present invention to provide photo-ablation means for making an electrode assembly that requires less exposure energy, thus generating less heat, and thus allowing for use in manufacture of relatively inexpensive, heat-sensitive stacks and light-weight substrates (such as plastic substrates) that typically have less thermal resistance than glass.
It is another object of the present invention to provide a method for patterning an ablatable laminar imaging medium, wherein the patterning operation is executed in close association with a cleaning operation, ie., the cleaning operation being accomplished by laser ablation after the patterning operation, preferably by the same apparatus used therefor.
It is another object of the present invention to provide a method for patterning an ablatable laminar imaging medium, wherein the patterning operation is executed in close association with a cleaning operation, ie., the cleaning operation being accomplished by laser ablation contemporaneously with the patterning operation, preferably by the same apparatus used therefor.
Other objects of the present invention will become apparent from the detailed description below taken in conjunction with the accompanying drawings.