Abstract:
The present invention aims to provide a ZnO thin film deposited substrate and a thin film deposition system exhibiting a specific resistance sufficiently reduced to be useful for transparent electrodes of a liquid crystal display, characterized in that Zn material evaporated and oxidized by microwave oxygen plasma to the compound ZnO which is, in turn, deposited on the substrate and thereby the thin film is formed, and the ZnO thin film deposited on the substrate is exposed to microwave hydrogen plasma so as to reduce a specific resistance of the ZnO thin film and thereby to modify this ZnO thin film to electrically conductive thin film.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a thin film substrate having thin film of ZnO deposited on a substrate surface under oxidizing effect of oxygen plasma and then exposed to hydrogen plasma so as to be converted to electrically conductive thin film and a deposition system such thin film. 
       BACKGROUND ART 
       [0002]    Conventionally, various types of deposition system for thin film of zinc oxide (ZnO) are available, one example of which is illustrated by  FIG. 6  of the accompanying drawings. 
         [0003]      FIG. 6  is a diagram schematically illustrating a RF deposition system. 
         [0004]    As illustrated, this RF deposition system is adapted to energize a coil antenna  12  provided within a chamber  11  from a radio frequency power source RF in the order of  13 . 56  MHz and thereby to cause the coil antenna  12  to generate oxygen plasma  13 . 
         [0005]    This RF deposition system is further provided in a lower region within the chamber  11  with an evaporation source (i.e., evaporation means)  14  adapted to evaporate zinc (Zn) material as the material to be deposited in the form of thin film. 
         [0006]    The evaporation source  14  is provided in the form of a conductive container loaded with Zn material so that this container is resistance heated and evaporates the Zn material loaded therein as the conductive container is supplied with electric current. 
         [0007]    The chamber  11  further includes therein an inverted L-shaped column support  15  adapted to hold a glass substrate  17  in a horizontal position above the oxygen plasma via a retainer  16  provided on a distal end of the column support  15 . 
         [0008]    The glass substrate  17  is evenly heated by a substrate heating device  18  provided in an upper region of the chamber 
         [0009]    Such RF thin film deposition system is usually operated in a manner that the chamber  11  is depressurized by a vacuum pump  19  while the chamber  11  is supplied with gaseous oxygen from O 2 -cylinder  20 . 
         [0010]    In the RF thin film deposition system, Zn material evaporated under resistance heating by the evaporation source  14  is oxidized by the oxygen plasma  13  and deposited on the glass substrate  17  in the form of ZnO so that transparent thin film is formed on the glass substrate  17 . 
         [0011]    The transparent thin film has been effectively utilized to produce various devices such as surface wave device, piezoelectric thin film and band-pass filter. 
         [0012]    Optically, the ZnO thin film formed in the manner as has been described above has a relatively high light transmission in the order of 81%. Electrically, however, such ZnO thin film is an insulator (for example, having a specific resistance of 1×10 13  Ω/cm 2  and a film thickness of 200 μm). 
         [0013]    In view of this, such ZnO thin film is certainly useful as the electrically insulating transparent thin film but has been unable to be utilized as electrically conductive material in the form of transparent thin film (for example, transparent electrodes in liquid crystal display). 
         [0014]    As a typical measure conventionally taken to overcome the restriction as has been described above, ZnO thin film containing Al (aluminum) or Ga (gallium) has been developed so as to reduce the specific resistance of this ZnO thin film. However, a processing equipment to obtain the desired ZnO thin film containing Al or Ga has become excessively complicate and the number of parameters used to control this equipment has correspondingly increased. Consequentially, the cost for making the thin film deposition has become too high to be practically accepted. 
         [0015]    In view of the current condition as has been described above, it is a principal object of the present invention to provide a substrate for ZnO thin film deposition having a low electric resistance and a thin film deposition system allowing the thin film to be easily deposited on the substrate. 
       SUMMARY OF THE INVENTION 
       [0016]    The object set forth above is achieved, according to a first aspect of the present invention, by a thin film deposition system characterized in that a transparent thin film deposited substrate having compound ZnO deposited thereon is exposed to hydrogen plasma so as to reduce a specific resistance of the transparent thin film and thereby to modify this transparent thin film to the electrically conductive thin film. 
         [0017]    According to a second aspect, the present invention provides the thin film deposition system characterized in that the transparent thin film deposited substrate is exposed to microwave hydrogen plasma and thereby modified to the electrically conductive thin film. 
         [0018]    According to a third aspect, the present invention provides the thin film deposition system characterized in that Zn material is exposed to the microwave oxygen plasma to form the transparent thin film deposited substrate having compound ZnO deposited thereon and subsequently this transparent thin film deposited substrate is exposed to microwave hydrogen plasma so as to modify this transparent thin film to the electrically conductive thin film. 
         [0019]    According to a fourth aspect, the present invention provides a thin film deposition system characterized in that a transparent thin film deposited substrate having compound ZnO thereon is exposed to hydrogen plasma to reduce a specific resistance of the transparent thin film and thereby to modify this transparent thin film to the electrically conductive thin film. 
         [0020]    According to a fifth aspect, the present invention provides the thin film deposition system characterized in that the transparent thin film deposited substrate is obtained by steps of evaporating Zn material as material of the thin film, and exposing the evaporated Zn material to microwave oxygen plasma thereby to deposit compound ZnO on the substrate. 
         [0021]    According to a sixth aspect, the present invention provides a thin film deposition system comprising an oxygen plasma generating means serving to supply a depressurized chamber with microwave power and gaseous oxygen to generate microwave oxygen plasma, hydrogen plasma generating means serving to supply the depressurized chamber with microwave power and gaseous hydrogen to generate microwave hydrogen plasma, evaporation means provided with in the depressurized chamber to evaporate Zn material used as material for thin film deposition, and substrate positioned within the depressurized chamber so as to be heated by the microwave oxygen plasma, the thin film deposition system being characterized in that microwave oxygen plasma is generated so that the Zn material evaporated and oxidized by the microwave oxygen plasma to the compound ZnO may be deposited on the substrate and thereby forms the thin film, and microwave hydrogen plasma is generated as an alternative to the oxygen plasma to which the ZnO thin film deposited on the substrate is exposed so as to reduce a specific resistance of the ZnO thin film and thereby to modify this ZnO thin film to electrically conductive thin film. 
         [0022]    According to a seventh aspect, the present invention provides the thin film deposition system characterized in that the plasma generating means comprise the microwave windows provided within the depressurized chamber on its bottom and the evaporation means provided within the depressurized chamber on its bottom at the same level as the microwave windows or within the depressurized chamber at a level higher than the microwave windows. 
         [0023]    As has been aforementioned, the thin film deposited substrate according to the first aspect of the present invention is obtained by exposing the transparent thin film deposited substrate having the compound ZnO deposited thereon to the hydrogen plasma to reduce the specific resistance of this transparent thin film. 
         [0024]    In this way, the ZnO thin film can be made electrically conductive under a surface modifying effect of the hydrogen plasma and thereby the conductive transparent thin film deposited substrate can be obtained. 
         [0025]    In view of such modification, the transparent thin film deposited substrate obtained according to the present invention is useful as electrodes and/or circuits, for example, in the liquid crystal display. 
         [0026]    When the thin film deposited substrate is used as a transparent component such as a transparent electrode, the substrate on which the compound ZnO is to be deposited must be transparent. 
         [0027]    To form the thin film deposited substrate according to the present invention, microwave hydrogen plasma may used as in the second aspect or microwave oxygen plasma may be used together with microwave hydrogen plasma as in the third aspect. 
         [0028]    Although ITO electrode has conventionally been used extensively as such transparent electrode, this ITO electrode disadvantageously contains indium which is not only toxic but also scarce substance. Consequentially, the ITO electrode has necessarily required a cost which is practically acceptable. This problem can be satisfactorily solved by the transparent thin film deposited substrate as has been described above. 
         [0029]    The transparent thin film deposited substrate is useful also for the other fields such as anti-clouding window glass of car, ETC radio shielding plate for fast highway, anti-theft plate glass, electrodes and/or circuits of flat display, car antenna, solar cell and LED. 
         [0030]    The thin film deposition system according to the fourth aspect of the present invention is similar to the system according the fifth aspect of the present invention in that the Zn material is evaporated and exposed to microwave oxygen plasma to produce the compound ZnO which is, in turn, deposited on the substrate. 
         [0031]    However, it should be understood that the transparent thin film deposited substrate can be also formed by the FR thin film deposition system previously described above as an example of the prior art without relying on the thin film deposition system according to the fifth aspect of the present invention. 
         [0032]    As the substrate on which the compound ZnO is deposited, materials such as glass and synthetic resin can be used. 
         [0033]    The thin film deposition system according to the sixth aspect of the present invention adopts the process comprising steps of depositing the compound ZnO on the substrate and modifying the deposited ZnO thin film to be electrically conductive. 
         [0034]    Specifically, in the case of this thin film deposition system, microwave power and gaseous oxygen are supplied to the depressurized chamber to generate microwave oxygen plasma. 
         [0035]    Subsequently, the Zn material to be deposited in the form of thin film is evaporated and exposed to oxygen plasma to produce the compound ZnO which is, in turn, deposited on the substrate. 
         [0036]    Then, supply of gaseous oxygen is shut off and thereupon gaseous hydrogen is supplied to the depressurized chamber to generate microwave hydrogen plasma within this depressurized chamber. 
         [0037]    As a result, the ZnO thin film is converted from the insulating thin film to electrically conductive thin film and thereby the electrically conductive substrate can be obtained. 
         [0038]    The thin film deposition system according to the seventh aspect of the present invention is provided with the evaporation means located at the level same as or higher than the microwave windows so that the evaporated Zn material and/or the compound ZnO might not directed to the microwave windows (e.g., made of quartz glass). 
         [0039]    In other words, the microwave windows (e.g., made of quartz glass) is reliably protected from the evaporated Zn material and/or the compound ZnO. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1  is a diagram schematically illustrating a thin film deposition system according to a first embodiment of the present invention. 
           [0041]      FIG. 2  is a characteristic graph plotting the irradiation time of hydrogen plasma versus a change in the specific resistance of the ZnO thin film. 
           [0042]      FIG. 3  is a characteristic graph plotting the crystallinity of the ZnO. 
           [0043]      FIG. 4  is a diagram schematically illustrating a thin film deposition system according to a second embodiment of the present invention which is adapted to continuously deposit thin film on a plurality of glass substrates. 
           [0044]      FIG. 5  is a diagram schematically illustrating a thin film deposition system according to a third embodiment of the present invention. 
           [0045]      FIG. 6  is a diagram schematically illustrating a thin film deposition system of prior art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0046]    Details of the present invention will be more fully understood from the description of the particular embodiments given hereunder in reference with the accompanying drawings. 
         [0047]      FIG. 1  is a diagram schematically illustrating a thin film deposition system according to a first embodiment of the present invention. 
         [0048]    This thin film deposition system is provided on one side of a chamber  21  with a microwave window  22  through which microwave power P is supplied into the chamber  21 . 
         [0049]    According to this embodiment, the microwave window  22  made of quartz glass and microwave power P oscillated from a microwave source is guided by a wave guide tube through the microwave window so as to irradiate the inside of the chamber  21 . 
         [0050]    This embodiment further includes a vacuum pump  23  used to depressurize the inside of the chamber  21 , an O 2  cylinder  24  from which gaseous oxygen is supplied into the chamber  21  so as to generate oxygen plasma  25  within the chamber  21  and a H 2  cylinder  26  from which gaseous hydrogen is supplied into the chamber  21  so as to generate microwave hydrogen plasma within the chamber  21 . 
         [0051]    It should be noted that the plasma is generated in the form of microwave surface wave. 
         [0052]    In the accompanying drawings, reference numeral  27  designates an evaporation source (evaporation means) of well known art, reference numerals  28 ,  29  designate valves located in the supply conduit, reference numeral  30  designates a glass substrate, and reference numerals  31  and  32  designate a support column and a retainer, respectively, both for the glass substrate. 
         [0053]    The thin film deposition system as has been described above is operated in a manner that the inside of the chamber  21  is irradiated with the microwave power P and gaseous oxygen is supplied into the chamber  21  from the O 2  cylinder  24  while supply of gaseous hydrogen from the H 2  cylinder  26  is shut off so as to generate the microwave oxygen plasma  25  within the chamber  21 . 
         [0054]    Consequently, the Zn material evaporated from the evaporation source  27  is oxidized by the oxygen plasma  25  and compound ZnO generated as a result of such oxidization is deposited on the glass substrate  30 . In this manner, transparent thin film is formed on the glass substrate  30 . 
         [0055]    Subsequently, supply of gaseous oxygen from the O 2  cylinder  24  is shut off and gaseous hydrogen is supplied from the H 2  cylinder  26  into the chamber  21  so as to generate microwave hydrogen plasma within the chamber  21 . 
         [0056]    The transparent thin film of the compound ZnO deposited on the glass substrate  30  is now exposed to the hydrogen plasma and a specific resistance of the ZnO thin film is reduced under a surface modifying effect of the hydrogen plasma. In this way, the electrically conductive thin film deposited substrate is obtained. 
         [0057]      FIG. 2  is a characteristic graph plotting the irradiation time versus the change in specific resistance of the ZnO thin film. 
         [0058]    According to this embodiment, the ZnO thin film having a thickness in the order of 200 (μm) was formed. As will be apparent from the characteristic graph of  FIG. 2 , irradiation of the hydrogen plasma for about 1 minute (60 seconds) significantly decreased the specific resistance from 10 13  units down to 10 2  units. 
         [0059]      FIG. 3  is a characteristic graph plotting a result obtained by XRD (X-ray diffraction instrument) of the ZnO thin film formed according to this embodiment. 
         [0060]    In this characteristic graph, abscissa axis indicates the angle at which the X-ray irradiates the sample of ZnO thin film and ordinate axis indicates the peak intensity of the diffraction line. 
         [0061]    It was found from this characteristic graph that this ZnO thin film has three peaks of the diffraction line, in other words, this ZnO thin film is a polycrystalline thin film having three crystalline structures. 
         [0062]    It has been also found that the ZnO thin film produced according to this embodiment exhibits a light transmission as high as 97(%) and λ=550 (nm). 
         [0063]      FIG. 4  is a diagram schematically illustrating a thin film deposition system according to a second embodiment of the present invention. 
         [0064]    According to this second embodiment, a plurality of the glass substrates may be successively fed into the chamber to ensure that electrically conductive thin film is deposited successively on the respective glass substrates. 
         [0065]    The thin film deposition system according to this embodiment is provided on the side of an inlet  41   a  to the chamber  41  with a front auxiliary chamber  42  and on the side of an outlet  41   b  from the chamber  41  with a rear auxiliary chamber  43 . 
         [0066]    The chamber  41  is provided on its bottom with a plurality of microwave windows  44   a ,  44   b ,  44   c.  The microwave power P is supplied via waveguides  45   a,    45   b,    45   c  underlying the microwave windows  44   a ,  44   b ,  44   c , respectively, into the chamber  41  through these windows so that a wide range of the surface wave oxygen plasma  46  and then of the surface wave hydrogen plasma may be generated within the chamber  41 . 
         [0067]    The chamber  41  further includes a plurality of evaporation sources  47   a,    47   b  from which the Zn material is evaporated upward. 
         [0068]    The glass substrate  48  is fed into the chamber  41  by feed rollers  49  adapted to be to-and-fro rotated so as to ensure that the glass substrate  48  can be reciprocated over a short range before the ZnO thin film is completed. 
         [0069]    The chamber  41  further comprises a vacuum pump  50  for depressurization and gas supply sources  51 ,  52  for gaseous O 2  and gaseous H 2 , respectively. 
         [0070]    The front auxiliary chamber  42  includes shutters  53   a,    53   b  provided at the in- and outlet for the glass substrate  48 , respectively, so that the glass substrate  48  may be fed into the front auxiliary chamber  42  with the shutter  53 b closed and with the shutter  53  opened. Specifically, the glass substrate  48  is fed into the front auxiliary chamber  42  by feed rollers  54  as indicated by a chain double-dashed line in  FIG. 4 . 
         [0071]    Immediately after the glass substrate has been fed into the front auxiliary chamber  42 , the shutter  53   a  is closed and then the front auxiliary chamber  42  is depressurized by the vacuum pump  55 . 
         [0072]    Then the shutter  53   b  is opened and the glass substrate  48  is fed from the front auxiliary chamber  42  into the chamber  41  as indicated by a solid line in  FIG. 4  to initiate a process for ZnO thin film deposition. 
         [0073]    This process for ZnO thin film deposition comprises steps of generating the oxygen plasma  46 , depositing the compound ZnO on the substrate  48  to form the ZnO thin film and then generating hydrogen plasma to make this ZnO thin film electrically conductive. 
         [0074]    The rear auxiliary chamber  43  is provided at the in- and outlets for the glass substrate  48  with shutters  56   a,    56   b , respectively, in the similar manner to that in the case of the front auxiliary chamber  42 . 
         [0075]    This rear auxiliary chamber  43  is depressurized by a vacuum pump  57  when the glass substrate  48  having the thin film deposited thereon is discharged from the chamber  41 . Then the shutter  56   a  is opened to feed the glass substrate  48  into the rear auxiliary chamber  43  by means of feed rollers  49 ,  58 . 
         [0076]    Immediately after the glass substrate  48  has been fed into the rear auxiliary chamber  43  as indicated by a chain double-dashed line in  FIG. 4 , the shutter  56   a  is closed and then the shutter  56   b  is opened to discharge the glass substrate  48  from the rear auxiliary chamber  43 . 
         [0077]    According to this embodiment, the glass substrate  48  is conveyed through the front auxiliary chamber  42 , the chamber  41  and the rear auxiliary chamber  43  so that the electrically conductive ZnO thin film may be deposited thereon. Such arrangement allows the thin film to be successively deposited on a plurality of the glass substrate, on one hand, and allows the thin film deposition to be achieved even on the glass substrate which has a relatively large area, on the other hand. 
         [0078]    By locating the evaporation sources at a level higher than the microwave windows as in this second embodiment, it can be reliably avoided that the evaporated Zn material and the compound thereof ZnO might cling to the microwave windows. 
         [0079]      FIG. 5  is a diagram schematically illustrating a thin film deposition system according to a third embodiment of the present invention. 
         [0080]    This embodiment is characterized in that, after a transparent thin film deposited substrate  70  having ZnO thin film  70 a thereon has previously been produced in the separate system, this transparent thin film deposited substrate  70  is positioned within a chamber  71  and the ZnO thin film is exposed to hydrogen plasma  72  to modify the thin film to the electrically conductive thin film. 
         [0081]    Referring to  FIG. 5 , reference numeral  73  designates a vacuum pump, reference numeral  74  designates a H 2  cylinder, reference letter P designates microwave power and reference numeral  75  designates microwave windows. 
         [0082]    While the preferred embodiments of the present invention have been described, it should be understood that the ZnO thin film deposition may be formed without utilization of the microwave oxygen plasma, for example, by the FR thin film deposition system. 
         [0083]    The hydrogen plasma used to modify the ZnO thin film to the electrically conductive thin film is not limited to the microwave hydrogen plasma and it is also possible to use, for example, the hydrogen plasma generated by supplying the ER thin film deposition system with gaseous H 2 .