Abstract:
Here is disclosed a plasma generator using microwave wherein a plasma generating chamber is provided with a plurality of wave guide tubes extending in parallel one to another at regular intervals, each of the wave guide tubes being formed with a plurality of coupling ports arranged intermittently in an axial direction of the wave guide tube and dimensioned so that coupling coefficient thereof become gradually higher toward a distal end of the wave guide tube, and a plurality of dielectric windows provided through the plasma generating chamber in association with the respective coupling ports so that microwave electric power radiated through the coupling ports into the plasma generating chamber may be uniformized and thereby plasma of a large area may be generated with high and uniform density.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a plasma generator using microwave and more particularly to such plasma generator used, for example, to wash or etch surface of a workpiece such as metallic piece, semiconductor or insulator or to coat the surface of such workpiece with thin film.  
         BACKGROUND ART  
         [0002]    The plasma generator of RF (radio frequency power source) parallel-plate type as shown in FIG. 6 of the accompanying drawings has been most extensively used to generate processing plasma free from magnetic field.  
           [0003]    This plasma generator  10  of well known art comprises a plasma generating chamber  11  containing therein a pair of conductive parallel plates  12   a ,  12   b  adapted to be power-supplied from a high-frequency power source  13  so that plasma  14  may be generated between the parallel plates  12   a ,  12   b . Referring to FIG. 6, reference numeral  15  designates a vacuum pump and reference numeral  16  designates a gas charging line.  
           [0004]    The plasma generator  20  of RF induction type as shown in FIG. 7 has been also extensively used.  
           [0005]    This plasma generator  20  of well known art comprises a plasma generating chamber  21  provided therearound with a high frequency coil  22  adapted to be power-supplied from a high-frequency power source  23  so that plasma  24  may be generated within the plasma generating chamber  21 .  
           [0006]    Referring to FIG. 7, reference numeral  25  designates an auxiliary field coil, reference numeral  26  designates a vacuum pump and reference numeral  27  designates a gas charging line.  
           [0007]    An example of the conventional plasma generators using microwave is the plasma generator of surface wave excitation type as shown in FIG. 8.  
           [0008]    This plasma generator  30  of well known art comprises wave guide tubes  31 , a plasma generating chamber  32  and a dielectric plates  33  interposed between the wave guide tubes  31  and the plasma generating chamber  32  so as to define dielectric windows. With such construction, the surface wave is generated on the surfaces of the dielectric plates  33  facing the plasma generating chamber  32  as power Po in the form of microwave is supplied from the wave guide tubes  31  and this surface wave ionizes gaseous molecules and thereby generates plasma  34 .  
           [0009]    Referring to FIG. 8, reference numeral  35  designates a vacuum pump and reference numeral  36  designates a gas charging line.  
           [0010]    The plasma generator  10  of FIG. 6 is disadvantageous in that the plasma  14  has a diameter of 0.2 m or less, an electron density of 1×10 11  cm −3  or less and a plasma density homogeneity of ±5% or less. As a result, both the plasma area an the electron density are too low to be used for surface processing of the semiconductor and/or spray-coating with thin film.  
           [0011]    The plasma generator  20  of FIG. 7 is more advantageous than the plasma generator  10  so far as the electron density is concerned. However, it is impossible to obtain the plasma of homogeneous high electron density unless complicated adjustment is carried out by application of DC or high frequency auxiliary magnetic field by the auxiliary field coil  25 .  
           [0012]    The plasma generator  30  of FIG. 8 is necessarily accompanied with increased cost because a thickness of the dielectric plates  33  should be sufficiently increased to protect the plasma generating chamber  32  from ambient pressure if it is desired to enlarge the plasma area. Furthermore, it is difficult for this plasma generator  30  to ensure the desired homogeneity of the plasma density.  
           [0013]    In view of the situation as has been described above, it is a principal object of the present invention to provide a plasma generator constructed so that the microwave energy may be used to improve the homogeneity of plasma density as well as the electron density and to ensure plasma having a large area.  
         SUMMARY OF THE INVENTION  
         [0014]    The object set forth above is achieved, according to the present invention, by a plasma generator adapted to supply microwave energy into a plasma generating chamber to generate plasma within the plasma generating chamber, said plasma generator comprising a plurality of wave guide tubes arranged in parallel one to another to supply microwave energy, each of the wave guide tubes being formed with a plurality of coupling ports arranged intermittently in an axial direction of the wave guide tube and dimensioned so that coupling coefficient thereof become gradually higher toward a distal end of the wave guide tube, and a plurality of dielectric windows provided through the plasma generating chamber in association with the respective coupling ports of the wave guide tubes.  
           [0015]    According to the present invention, the coupling ports are dimensioned so that these ports may have coupling coefficients gradually increasing toward the distal ends of the respective wave guide tubes and thereby microwave energy radiated through the respective coupling ports of the respective wave guide tubes may be uniformized.  
           [0016]    Accordingly, by appropriately selecting the intervals of the plural wave guide tubes as well as the intervals of the coupling ports, plasma of high and uniform density can be generated by the microwave energy radiated through the coupling ports and the associated dielectric windows into the plasma generating chamber.  
           [0017]    Area of the plasma generated in the manner as has been described above can be enlarged by providing the respective wave guide tubes with a plurality of the coupling ports.  
           [0018]    The dielectric windows can be separated one from another in association with the respective coupling ports and therefore the size of the windows can be limited. This feature is advantageous also from the view point of the manufacturing cost.  
           [0019]    The wave guide tubes are provided with a plurality of the coupling ports of circular or square shape and diameters of these circular or square ports formed through each of the wave guide tubes are gradually enlarged so as to have the coupling coefficient gradually increasing as closer to the distal end of the wave guide tube the coupling ports are.  
           [0020]    The plasma generating chamber is formed at positions corresponding to the respective coupling ports of the respective wave guide tubes with openings communicating with the respective coupling ports so that these openings are blocked off by the dielectric plates and thereby the plasma generating chamber is sealed hermetically.  
           [0021]    Furthermore, there are provided a plurality of wave guide tubes, each of these wave guide tubes being formed with a plurality of coupling ports adapted to uniformize the microwave energy radiated through these ports and thereby plasma of a large area is generated with high and uniform density.  
           [0022]    The invention allows the dielectric windows to be separated one from another and the dielectric windows as thin as possible can be used. In consequence, the plasma generator using microwave can be easily implemented at a rational manufacturing cost. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a structural diagram schematically illustrating a typical embodiment of the plasma generator according to the invention;  
         [0024]    [0024]FIG. 2 is a structural diagram schematically illustrating this plasma generator in a section taken along a line A-A in FIG. 1;  
         [0025]    [0025]FIG. 3 is a diagram illustrating placement and function of wave guide tubes constituting the plasma generator and of coupling ports of these wave guide tubes;  
         [0026]    [0026]FIG. 4 is a fragmentary diagram illustrating the plasma generator;  
         [0027]    [0027]FIG. 5 is a graphic diagram plotting plasma characteristics based on a result of experiments conducted on this embodiment;  
         [0028]    [0028]FIG. 6 is a diagram schematically illustrating a RF parallel-plate type plasma generator of prior art;  
         [0029]    [0029]FIG. 7 is a diagram schematically illustrating a RF induction type plasma generator of prior art; and  
         [0030]    [0030]FIG. 8 is a diagram schematically illustrating a plasma generator of prior art using microwave. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    Details of the present invention will be more fully understood from the description of its typical embodiment given hereunder in reference with the accompanying drawings.  
         [0032]    [0032]FIG. 1 is a structural diagram schematically illustrating the plasma generator in a section taken along axis of one of the wave guide tubes and FIG. 2 is a structural diagram schematically illustrating this plasma generator in a section taken along a line A-A in FIG. 1.  
         [0033]    For convenience of description, it is assumed in this embodiment that X-axis represents a direction in which the wave guide tubes are arranged in parallel one to another and Y-axis represents a direction in which axes of the respective wave guide tubes extend.  
         [0034]    As illustrated, this plasma generator  40  comprises a plasma generating chamber  41 , three wave guide tubes  42 ,  43 ,  44  arranged on upper surface of the plasma generating chamber  41  so as to extend in parallel one to another in the direction X, each of these wave guide tubes  42 ,  43 ,  44  being provided with three coupling ports arranged in a direction of the tube axis (i.e., in the direction Y).  
         [0035]    The coupling ports are circular radiation ports adapted to radiate microwave electric power Po into the plasma generating chamber  41  and, as will be seen in FIG. 1, each of the wave guide tubes, for example, the wave guide tube  42  is formed with three coupling ports  42   a ,  42   b ,  42   c  arranged in the axial direction of this wave guide tube  42 .  
         [0036]    These coupling ports  42   a ,  42   b ,  42   c  respectively have diameters gradually enlarged from a supply side of the microwave electric power Po toward a distal end  42 T of the wave guide tube  42  so that a coupling coefficient to the microwave electric power Po may gradually increase.  
         [0037]    Such arrangement is in view of the fact that he microwave electric power Po entering from a power source into the wave guide tube  42  gradually decreases toward the distal end  42 T of the wave guide tube  42 .  
         [0038]    While FIG. 1 merely illustrates the wave guide tube  42  and its coupling ports  42   a ,  42   b ,  42   c , it will be obviously understood that the wave guide tube  43  is provided with coupling ports  43   a ,  43   b ,  43   c  (coupling ports  43   a  and  43   c  are not shown in FIG. 2) and the wave guide tube  44  is provided with coupling ports  44   a ,  44   b ,  44   c  (coupling ports  44   a  and  44   c  are not shown in FIG. 2).  
         [0039]    The plasma generating chamber  41  is formed through its upper wall with circular ports communicating with the coupling ports, respectively, and these circular ports of the plasma generating chamber  41  are blocked off by dielectric plates (e.g., plates of quartz or alumina) having peripheral edges thereof tightly engaged with peripheral edges of the associated ports to form dielectric windows  45  adapted to block the plasma generating chamber  41  off from outside air.  
         [0040]    These dielectric windows  45  are preferably as thin as possible so far as these windows  45  can resist the ambient pressure.  
         [0041]    Referring to FIG. 1, reference numeral  46  designates a vacuum pump and reference numeral  47  designates a gas charging line.  
         [0042]    [0042]FIG. 3 is a diagram illustrating placement of the coupling ports as have been described above.  
         [0043]    While the placement of these coupling ports is illustrated with respect only to the wave guide tube  42 , it should be understood that such placement is true also with respect to the other wave guide tubes  43 ,  44 .  
         [0044]    In FIG. 3, X 1  designates a position at which a longitudinal axis of the wave guide tube  42  extends, Y 1 , Y 2 , Y 3  respectively designate positions at which this wave guide tube  42  is formed with the coupling ports  42   a ,  42   b  and  42   c , and YY designates a short-circuit plate defining the distal end of this wave guide tube  42 .  
         [0045]    A dimension by which the coupling ports  42   a ,  42   b  are spaced from each other is set to Y 2 −Y 1 =(2n+1)·λg/2, a dimension by which the coupling ports  42   b ,  42   c  are spaced from each other is set to Y 3 −Y 2 =(2n+1)·λg/2 and a dimension by which the coupling port  42   c  and the short-circuit plate YY are spaced from each other is set to YY−Y 3 =λg/4.  
         [0046]    In these equations, kg represents a wave length within the tube and n represents integer.  
         [0047]    With the coupling ports  42   a ,  42   b ,  42   c  arranged as has been described above, the microwave is radiated through these coupling ports and the associated dielectric windows  45  into the plasma generating chamber  41  in a manner as will be described on the assumption that the microwave electric power Po is supplied from the side adjacent the coupling port  42   a  and the coupling ports  42   a ,  42   b ,  42   c  respective have coupling coefficients K 1 , K 2 , K 3 .  
         [0048]    The coupling coefficients depend on the sizes of the respective coupling ports and therefore K 1 &lt;K 2 &lt;K 3 .  
         [0049]    More specifically, the microwave electric power (free-traveling wave electric power plus reflective wave electric power) is radiated through the respective coupling ports  42   a ,  42   b ,  42   c  and the associated dielectric windows  45  in a manner as follows:  
         [0050]    through the coupling port  42   a:    
           Po·K   1 [1+(1 −K   1 )(1− K   2 ) 2 (1− K   3 ) 2 ]  (1)  
         [0051]    through the coupling port  42   b:    
           Po·K   2 (1 −K   1 )[1+(1 −K   2 )(1 −K   3 ) 2 ]  (2)  
         [0052]    through the coupling port  42   c:    
           Po·K   3 (1 −K   1 )(1 −K   2 )[1+(1 −K   3 )]  (3)  
         [0053]    The wave reflected on the short-circuit plate YY is phase-shifted from the free-traveling wave by 180° and, in consequence, the free-traveling wave and the reflective wave are in phase at the respective coupling ports. That is, these different types of wave electric power are combined with each other and radiated through the respective coupling ports into the plasma generating chamber  41 .  
         [0054]    The reflective wave is expressed by:  
           Po (1 −K   1 ) 2 (1 −K   2 ) 2 (1 −K   3 ) 2   (4)  
         [0055]    In order that the microwave electric power Po can be radiated through the respective coupling ports  42   a ,  42   b ,  42   c  into the plasma generating chamber as equally as possible, the coupling coefficients of the respective coupling ports may be set, for example, to K 1 =0.3, K 2 =0.4 and K 3 =0.5. In this case, the equations (1)˜(4) will result in:  
         [0056]    through the coupling port  42   a:  0.32Po  
         [0057]    through the coupling port  42   b:  0.32Po  
         [0058]    through the coupling port  42   c:  0.32Po  
         [0059]    by reflection: 0.04Po.  
         [0060]    In this way, the microwave electric power Po is absorbed in the vicinity of the respective coupling ports and equally radiated through the respective coupling ports into the plasma generating chamber  41  so that the plasma  48  may be homogenously generated within the plasma generating chamber  41 .  
         [0061]    The microwave interacts with electrons in the plasma generated in this manner to be subjected to Landau damping and the energy due to this Landau damping is consumed for acceleration of the electrons and contributes to ionization of gaseous molecules.  
         [0062]    The Landau damping effect is proportionately higher as higher the density of the electrons is.  
         [0063]    The plasma  48  generated within the plasma generating chamber  41  spreads by diffusion.  
         [0064]    Diffusion is a statistical phenomenon and dependence of the density on a range of diffusion results in Gauss distribution. In other words, the plasma generated in the vicinity of the coupling ports  42   a ,  42   b ,  42   c  present independent Gauss distributions, respectively.  
         [0065]    The density of the electrons in the plasma in the vicinity of the respective dielectric windows is proportionately higher as thinner the dielectric windows  45  are. The microwave is thus rapidly attenuated in the vicinity of the windows and consequently the plasma of higher density is generated in the vicinity of the windows. In this way, electromagnetic interference among a plurality of windows is eliminated, making it possible to propagate the microwave substantially free from reflection.  
         [0066]    In view of this, it is possible to make the respective Gauss distributions of plasma overlap one another and thereby to ensure the plasma  48  of a uniform density distribution by appropriately selecting the distance between each pair of the adjacent wave guide tubes  42 ,  43 ,  44  and the number as well as the intervals of the coupling ports.  
         [0067]    [0067]FIG. 4 is a diagram illustrating a specific example of the plasma generator  40 .  
         [0068]    [0068]FIG. 5 is a graphic diagram plotting plasma characteristics based on a result of experiments conducted on this embodiment  50 .  
         [0069]    Referring to FIG. 5, a characteristic curve  42 A indicates Gaussian plasma characteristic associated with the coupling port  42   a , a characteristic curve  42 B indicates Gaussian plasma characteristic associated with the coupling port  42   b  and a characteristic curve  42   c  indicates Gaussian plasma characteristic associated with the coupling port  42   c.    
         [0070]    It has been found from the experiment that each plasma  42 A,  42 B,  42 C achieves the plasma density homogeneity with deviation of ±5% or less over a width of 70 mm.  
         [0071]    A characteristic curve  42 Z indicates a composite characteristic of those exhibited by the plasma  42 A,  42 B and  42 C. According to this composite characteristic, combination of the plasma  42 A,  42 B and  42 C enables the homogeneity of plasma density to be ensured with deviation of ±5% or less over a width of 280 mm.  
         [0072]    With this plasma generator  50 , placement of the wave guide tubes  42 ,  43 ,  44  have been optimized and it has been found that the plasma having a large area of 0.3 m×0.3 m, an electron density of 1×10 12  cm −3  and a homogeneity with deviation of ±5% or less can be generated.  
         [0073]    While the plasma generator  50  is provided with the rectangular plasma generating chamber  41  taking account of the fact that the workpiece is often rectangular and of relatively large area, the plasma generating chamber  41  may be of any shape other than the rectangular shape.  
         [0074]    Obviously, it is essential not only to reinforce the plasma generating chamber  41  and thereby to prevent the plasma generating chamber  41  from being collapsed by ambient pressure but also to provide on one or both of the side walls of the plasma generating chamber  41  with load lock or locks  51  if continuous processing is intended.  
         [0075]    In the plasma generator as has been described above, gas such as Cl 2  or F 2  is charged through the gas charging line  47  for etching process and gas such as CH 4  or C 2 H 6  is charged through the gas charging line  47  for thin film coating process.  
         [0076]    As will be apparent from the foregoing description of the embodiment and the specific example thereof, it is unnecessary to make the dielectric windows  45  continuous one to another. In other words, it is possible to separate these windows  45  one from another so as to correspond to the associated coupling ports. In this way, a relatively thin dielectric plate can be used to define each of the windows.  
         [0077]    This is an important feature from the viewpoint of the manufacturing cost.  
         [0078]    Without departing the scope of the invention, the number as well as the intervals of the wave guide tubes and the number as well as the intervals of the coupling ports formed through the respective wave guide tubes can be appropriately selected. Furthermore, the shape of the coupling ports is not to circular shape and may be square to achieve the same effect.