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  Methods for patterning indium tin oxide filmsUSPTO Application #: 20080061030Title: Methods for patterning indium tin oxide films Abstract: A method of patterning an indium tin oxide film includes the steps of forming a cap layer over the indium tin oxide film and subjecting exposed areas of the indium tin oxide film to a water plasma. (end of abstract)
Agent: Duane Morris LLPIPDepartment (tsmc) - Philadelphia, PA, US Inventors: Sheng-Chieh Liu, Tzu-Yang Wu, Sheng-Liang Pan, Yuan-Bang Lee, U. H. Lin USPTO Applicaton #: 20080061030 - Class: 216 41 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20080061030. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF INVENTION [0001]This invention relates to semiconductor fabrication. More particularly, this invention relates to methods for patterning indium tin oxide films. BACKGROUND OF THE INVENTION [0002]Indium tin oxide (ITO) is an electrically conductive material which, when used as a thin film (e.g., between about 100 .ANG. to about 2200 .ANG. in thickness), is also optically transparent. Because of these characteristics, ITO is used in various applications including, but not limited to, optical microelectromechanical systems (MEMS), flat panel displays, solar cells, touch screens, camera lenses, and surface heater sensors. [0003]ITO may be formed by doping Indium oxide (In.sub.2O.sub.3) with tin (Sn), which replaces the In.sup.3+ atoms of the In.sub.2O.sub.3. Thin films of ITO may be deposited on a surface using one or more of a variety of techniques including, but not limited to, electron beam evaporation, physical vapor deposition, sputtering, or pulsed laser deposition. [0004]Electrically conductive and optically transparent ITO structures are typically made by depositing a thin film of ITO on a desired substrate, forming a patterned photoresist layer on the ITO film, and etching areas of the ITO film which are exposed by the patterned photoresist layer to pattern the ITO film into a desired structure. [0005]ITO films are currently etched using dry and wet methods. One commonly used ITO film dry etching method is reactive ion etching (RIE). The RIE method uses a plasma that typically comprises a major proportion of chloroform (CHCl.sub.3) gas, which supplies a polymer, and a minor proportion of a polymer suppressant gas such as boron trichloride (BCl.sub.3) or chlorine (Cl.sub.2). The ion bombardment of the BCl.sub.3/Cl.sub.2 mixture performs the patterning process. The high ion ratio bombardment of the RIE process is an effective method to pattern the ITO film. The RIE process, however, produces an ITO pattern edge with an inclined or tapered edge profile, rather than a vertical edge profile, which limits critical dimension reductions. This process control problem is due to inadequate ITO etch selectivity, wherein the ion bombardment starts to etch the edges of the photoresist pattern, thereby causing the inclined or tapered edge profile of the ITO pattern. [0006]In an effort to improve ITO etch selectivity in RIE, methane (CH.sub.4) hydrogen (H.sub.2) gas mixtures have been used to pattern ITO films, however, this gas mixture is potentially explosive and is therefore, unsuitable without relatively expensive gas exhausting equipment operating continuously to remove any build-ups of this gas mixture. [0007]Wet chemical etching is a commonly used wet etching method for patterning ITO films. ITO films may be patterned with a hydrofluoric solution (HF) such as HF:H.sub.2O.sub.2:10H.sub.2O or more commonly with a hydrochloric (HCl) solution such as HCl:H2O. Etching rates using the HF:H.sub.2O.sub.2:10H.sub.2O solution are very high at between about 100 .ANG. (angstroms)/second to about 150 .ANG./second, and are often uncontrollable. [0008]The HCl:H2O solution in undiluted form containing 36% HCl by volume corresponding to a molar solution, has an etch rate of about 2500 .ANG./minute. When patterning with the HCl solution, a certain amount of ITO varying from about 0.5 um to about 1.5 um is often etched away from underneath the photoresist etch mask. Hence, the edge profile of the ITO pattern is not vertical, instead being undercut and in severe cases, thin traces of the photoresist may remain on the substrate. [0009]The non-vertical edge profiles of the ITO pattern reduce the sharpness and resolution of the ITO pattern, which in turn, increases the probability of photo-alignment rejection in further processing. In addition, the less than sharp edge profile of the ITO pattern limits further reductions in line width and critical dimension. Thus, in ITO display applications where panel sizes continue to become smaller, it is desirable to increase pixel size by reducing the space between the pixels. The inability to fabricate ITO patterns with reduced line width and critical dimension, limits the size of the pixels in small displays. [0010]Accordingly, an ITO patterning method is needed which allows further reductions in ITO pattern line width and critical dimension. SUMMARY [0011]A method of patterning an indium tin oxide film is disclosed herein. The method comprises the steps of forming a cap layer over the indium tin oxide film and subjecting exposed areas of the indium tin oxide film to a gas phase etchant comprising water. BRIEF DESCRIPTION OF THE DRAWINGS [0012]FIG. 1 schematically depicts an exemplary plasma process chamber that may used in the method. [0013]FIG. 2 shows a flowchart of an embodiment of a water plasma ITO patterning method. [0014]FIGS. 3A-3D are cross-sectional views illustrating a substrate after performing certain steps of the water plasma ITO patterning method. [0015]FIGS. 4A and 4B are cross-sectional scanning electron microscope photographs which compare the edge profiles of thin films of ITO patterned using a prior art RIE process (FIG. 4A) and the water plasma ITO patterning method (FIG. 4B). [0016]FIGS. 5A-5C are scanning electron microscope photographs at different magnifications of a thin film of electrically conductive, optically transparent ITO patterned for 240 seconds using the water plasma ITO patterning method. [0017]FIGS. 6A-6C are scanning electron microscope photographs at different magnifications of a thin film of electrically conductive, optically transparent ITO patterned for 300 seconds using the water plasma ITO patterning method. [0018]FIG. 7A is test pattern used in WAT spacing testing under the control rules of a generic IC fabrication process. [0019]FIG. 7B is a graph showing the results of WAT spacing testing under the control rules of a generic IC fabrication process. DETAILED DESCRIPTION OF THE INVENTION Continue reading... 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