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.

Description:
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 Å to about 2200 Å 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 2 O 3 ) with tin (Sn), which replaces the In 3+  atoms of the In 2 O 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 3 ) gas, which supplies a polymer, and a minor proportion of a polymer suppressant gas such as boron trichloride (BCl 3 ) or chlorine (Cl 2 ). The ion bombardment of the BCl 3 /Cl 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 4 ) hydrogen (H 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 2 O 2 :10H 2 O or more commonly with a hydrochloric (HCl) solution such as HCl:H2O. Etching rates using the HF:H 2 O 2 :10H 2 O solution are very high at between about 100 Å (angstroms)/second to about 150 Å/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 Å/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 
       [0020]    A method is disclosed for patterning an electrically conducting, optically transparent thin film of amorphous indium tin oxide (ITO) on a surface, using a cap layer (operative as a hard mask) and a plasma comprising water as an etchant species. The areas of the ITO thin film not covered and protected by the cap layer react with water plasma under high and are removed from the surface. 
         [0021]    The method may be performed in any suitable plasma process chamber including, but not limited to, a conventional resist strip chamber, a plasma etch reactor.  FIG. 1  schematically depicts an exemplary plasma process chamber  100  that may used in the method. The plasma process chamber includes a housing  110  that defines the plasma process chamber  100 . A platform  120  is provided inside the chamber  100  for mounting a substrate. A showerhead-shape gas inlet nozzle  130  is disposed above the wafer platform  120 . Reaction gases are routed into the chamber  100  via a gas inlet  140 , which communicates with the inlet nozzle  130 . An exhaust outlet  160  connected to a vacuum pump  170  is used to evacuate the process chamber  100 . Electric field generating means (not shown) are used to generate an electric field in the chamber  100  of a sufficient magnitude such that a process fluid flowing in the chamber  100 , breaks down and becomes ionized. A plasma may be initiated by releasing or discharging free electrons inside the chamber  100  using, for example, field emission from a negatively biased electrode within the chamber  100 . 
         [0022]    Referring now to  FIG. 2 , which shows a flowchart of an embodiment of the method, the method commences in step  200  with a substrate  180  having a dielectric layer  182  formed thereon and a thin film  184  of electrically conductive, optically transparent amorphous ITO formed on the dielectric layer  182 . In other embodiments, the dielectric layer may be omitted so that the thin film of ITO  184  is formed directly on the substrate  180 . 
         [0023]    The substrate  180  may comprise an optically transparent glass material or any other suitable substrate material, depending upon the application. The dielectric layer  182  is optically transparent and has a sufficiently high index of refraction so that it operates as an anti-reflective coating (ARC). The optical transparency of the dielectric layer  182  generally depends upon the thickness of the layer. Thicker dielectric layers provide less light scattering but reduce the optical transparency, stress and the adhesion of the layer. The exact thickness of the dielectric layer  182  depends upon the thickness of the ITO. Dielectric materials having suitable optical and mechanical properties include, but are not limited to, niobium oxide (Nb 2 O 5 ), tellurium dioxide (TeO 2 ), tantalum oxide (Ta 2 O 3 ), and alumina (Al 2 O 3 ). The dielectric layer  182  may be deposited 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. 
         [0024]    The thin ITO film  184  should be deposited to a thickness which provides the ITO film with good electrically conductivity, i.e., less than 20 ohm/square and good optical transparency, i.e., higher than about 90 percent light transmission. In some embodiments, the thin film  184  of ITO may be formed to a thickness ranging between about 100 Å to about 2200 Å. The thin film of ITO  184  may be deposited 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. 
         [0025]    In step  210  a cap layer  186  is deposited on the thin film  184  of ITO, as shown in the cross-sectional view of the substrate  180  shown in  FIG. 3A . In some embodiments, the cap layer  186  may comprise an oxide film, such as SiO 2 , deposited to a thickness greater than 100 Å by plasma enhanced chemical vapor deposition or any other suitable method. 
         [0026]    In step  220  of the flowchart shown in  FIG. 2 , a layer  188  of photoresist is deposited on the hard mask layer  186  and patterned to expose selected areas  186   a  of the hard mask layer  186 . The photoresist layer  188  may be deposited and patterned using conventional photolithographic methods. The cross-sectional view of  FIG. 3B  shows the substrate  180  after completion of step  220 . 
         [0027]    In step  230  of the flowchart shown in  FIG. 2 , the exposed areas  186   a  of the hard mask layer  186  are removed to pattern the hard mask layer  186  into a desired pattern. The exposed areas  186   a  of the hard mask layer  186  may be removed using conventional dry or wet etching methods. The cross-sectional view of  FIG. 3C  shows the substrate  180  after completion of step  230 . Upon completion of the hard mask patterning step, the patterned photoresist layer  188  may be removed using conventional photoresist removal methods. 
         [0028]    In step  240  of the flowchart shown in  FIG. 2 , the substrate  180  mounted on the wafer platform  120  inside the plasma process chamber  100  ( FIG. 1 ) and a process gas  150  containing one or more chemical species is introduced under pressure into the plasma process chamber  100 , via the gas inlet  140  and inlet nozzle  130 . The one or more chemical species are ionized by the electric field generated within the chamber. 
         [0029]    In some embodiments, the one or more chemical species may comprise water (H 2 O) and N 2  based species. Of these species, the H 2 O based species is a reactive species that reacts with exposed areas  184   a  of thin film  184  of ITO, which are not covered by the cap layer  186 . The N 2  is non-reactive species. 
         [0030]    The pressure (partial pressure) exerted by the process gas  150  inside the plasma process chamber  100  before initiating a plasma is set to between about 0.5 Torr and about 5.0 Torr. The flow rate of the H 2 O based species of the process gas  150  is set between about 200 sccm (standard cubic centimeters per minute) and about 1500 sccm. The flow rate of the N 2  species in the process gas  150  is set between about 100 sccm and about 1000 sccm. The temperature of the chamber  100  is set between about 200° C. and about 300° C. In a preferred embodiment, the pressure exerted by the process gas  150  is set to 2.0 Torr, the gas flow rate of the H 2 O species is set to 500 sccm, the gas flow rate of the N 2  species is set to 200 sccm, and the chamber temperature is 245° C. 
         [0031]    An electric field is generated inside the chamber  100  by the electric field generating means. In one embodiment, the electric field used to excite the plasma may be in the microwave or RF frequency range and the power of such a field may be about 1400 watts. Free electrons are discharged inside the plasma process chamber  100  and travel through the process gas to generate a H 2 O plasma  190  in the chamber  100 . As the H 2 O plasma  190  stabilizes, the pressure exerted by the process gas  150  inside the plasma process chamber  100  is adjusted to between about 0.5 Torr and about 5.0 Torr, and preferably 2.0 Torr. The temperature of the chamber is maintained between about 200° C. and about 300° C., and preferably 245° C. 
         [0032]    The H 2 O plasma  190  is highly etch selective to the thin film of ITO relative to the hard mask layer  186  and the dielectric layer  182  (or the substrate  180  in embodiments not employing the dielectric layer  182 ). Consequently, as shown in the cross-sectional view of the substrate  180   FIG. 3D , the H 2 O plasma  190  reacts with the exposed areas  184   a  of the thin film  184  of ITO to remove same without substantially reacting with the cap layer  186  or the corresponding underlying areas  182   a  of dielectric layer  182  (or substrate  180  in embodiments not employing the dielectric layer  182 ). In some embodiments, the cap layer  186  is removed. In other embodiments, the cap layer  186  may remain. 
         [0033]      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 described above (FIG.  4 B). As can be seen, the RIE process produces an ITO pattern edge with an inclined or tapered edge profile, which limits line width and critical dimension reductions. In contrast, the superior ITO etch selectivity of the water plasma patterning method produces a substantially vertical edge profile which allows for further reductions in ITO line widths and critical dimensions. 
         [0034]      FIGS. 5A-5C  are scanning electron microscope photographs at magnifications of 40,000×, 8000×, and 40,000× of a thin film of electrically conductive, optically transparent ITO patterned for 240 seconds using the water plasma method.  FIGS. 6A-6C  are scanning electron microscope photographs at magnifications of 40,000×, 8000×, and 40,000× of a thin film of electrically conductive, optically transparent ITO patterned for 300 seconds using the water plasma method. In both examples, the exposed areas of the thin film of ITO were completely removed after reaction with the water plasma. 
         [0035]    WAT spacing testing under the control rules of a generic IC fabrication process, further confirmed the patterning performance of the water plasma thin film ITO patterning method. More specifically, a thin film of electrically conductive, optically transparent amorphous ITO was patterned into a test pattern, as shown in  FIG. 7A  using the water plasma method. The spacing result of the test pattern revealed no ITO residue remaining between the lines of ITO and the test pattern passed the control limits of the 1.0 um pattern design, i.e., from about 12 volts to about 20 volts (VF on the x-axis) and from about 0.15 to about 1 microamps (IF on the y-axis) and the long term testing value under the same condition was 17 volts, as shown in the graph of  FIG. 7B . 
         [0036]    The thermal crystallization temperature of the thin film  184  of amorphous ITO is slightly higher than 150° C. The growth of crystallites dispersed in the amorphous matrix may be suppressed by increasing the amount of H 2 O in the plasma, while sharply enhancing the nucleation of the crystallites. The amount of bonded hydrogen increases and that of oxygen vacancies decreases at the same time, with the introduction of inhomogeneity in the amorphous matrix. Specifically, the oxygen vacancies are effectively terminated by the —OH species generated by the added H 2 O in the plasma, which reduces the number of oxygen vacancies and suppresses the crystal growth with the H 2 O addition. After the crystallization is completed and the thin film  184  of ITO is patterned, the remaining ITO crystallites in the thin film  184  are minimal and small, i.e., less than 0.1 um. 
         [0037]    One of ordinary skill in the art will appreciate that the water plasma thin film ITO patterning method may be performed in-situ without additional equipment tools. Compared with the prior art etching methods, the water plasma patterning method provides better pattern edge profile control via superior ITO etch selectively. In addition, the water plasma method is suitable for processes which involve ITO patterning including, but not limited to, optical MEMS processes. 
         [0038]    While the foregoing invention has been described with reference to the above, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the appended claims.