Abstract:
A long linear-type microwave plasma source using a variably-reduced-height rectangular waveguide as the plasma reactor has been developed. Microwave power is fed from the both sides of the waveguide and is coupled into plasma through a long slot cut on the broad side of the waveguide. The reduced height of the waveguide is variable in order to control the coupling between microwave and plasma so that the plasma uniformity can remain a high quality when extending the length of the linear-type plasma source.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention generally relates to a microwave plasma source and, more particularly, to a microwave plasma source using a variably-reduced-height waveguide as a linear-type plasma reactor. 
       BACKGROUND OF THE INVENTION  
       [0002]    As the throughput of silicon-based solar cells increases, continuous plasma enhanced chemical vapor deposition that is widely used in the making of anti-reflecting layers has to be significantly changed. In other words, the employed plasma source has to be extended linearly along the direction perpendicular to the conveyor belt to achieve enhanced throughput. 
         [0003]      FIG. 1A  and  FIG. 1B  are side views of a conventional long linear-type microwave plasma source from different viewing angles. The long linear-type microwave plasma source  100  is disclosed in Germany Patent DE19812558A1. In  FIG. 1A  and  FIG. 1B , the long linear-type microwave plasma source  100  comprises a reaction chamber  110 , a quartz tube  120  and a cylindrical waveguide  130 . The cylindrical waveguide  130  is disposed inside the quartz tube  120 . The quartz tube  120  is disposed inside the reaction chamber  110 . 
         [0004]    Therefore, when microwave  50  is applied to the cylindrical waveguide  130 , the microwave  50  travels inside the cylindrical waveguide  130  and then leaks out of the surface of the cylindrical waveguide  130  to pass through the quartz tube  120  to excite plasma  60 . The plasma  60  reaches the surface of the silicon substrate  140  to form a thin film. 
         [0005]      FIG. 1C  shows the plasma density profiles of a long linear-type microwave plasma source in  FIG. 1B , wherein the longitudinal axis denotes the plasma density and the traversal axis denotes the position. Referring to  FIG. 1B  and  FIG. 1C , the plasma density n 1  is distributed from the plasma generated due to the microwave power  50 ′ applied on the left and decreases along the traversal axis. On the contrary, the plasma density n 2  is distributed from the plasma generated due to the microwave power  50 ″ applied on the right and increases along the traversal axis. Therefore, the actual plasma density n in the reaction chamber  110  is the sum of the plasma density n 1  and the plasma density n 2 . 
         [0006]    However, to achieve enhanced throughput, the size of the long linear-type microwave plasma source  100  has to be enlarged to improve the rate of thin film deposition. As a result, both the microwave power  50 ′ applied from the left and the microwave power  50 ″ applied from the right may leak and decline so that the actual excited plasma distribution is as shown in  FIG. 1D , wherein the plasma density n as a sum of the plasma density n 1  and the plasma density n 2  is not uniform. More particularly, the plasma density is higher on both ends and lower at the center. 
         [0007]    Even though the aforesaid problem can be overcome by increasing the input microwave power, the increased cost is proportional to orders of the increased microwave power. Therefore, the high-power microwave generator is very costly, which makes the plasma processing less competent due to high cost. 
         [0008]    Referring to  FIG. 1A , and  FIG. 1B , however, the quartz tube  120  is surrounded by the plasma  60 , which causes deposition on the quartz tube  120  and even etching on the quartz tube  120 . This results in poor efficiency and poor uniformity of plasma intensity of the plasma  60  excited by the microwave power so that the film quality on the silicon substrate  140  is degraded. 
         [0009]    Therefore, the quartz tube  120  has to be renewed periodically to enhance the efficiency of plasma  60  excited by the microwave  50 . However, the replacement of the quartz tube  120  is not very easy, which causes lower throughput of the long linear-type microwave plasma source  100 . This increases the manufacturing cost of the solar cells. 
       SUMMARY OF THE INVENTION 
       [0010]    In view of the above, the present invention provides a long linear-type microwave plasma source using a variably-reduced-height waveguide so as to adjust microwave leakage and achieve enhanced throughput by generating long linear-type uniform plasma. 
         [0011]    Moreover, the present invention provides a long linear-type microwave plasma source, comprising a reaction chamber, a variably-reduced-height waveguide, a long linear-type coupling window and a moving mechanism. The variably-reduced-height waveguide is disposed on the reaction chamber and comprises a frame portion, a long linear-type coupling frame, a first moving portion and two second moving portions. The frame portion has a first wide side adjacent to the reaction chamber. The long linear-type coupling frame is disposed on the first wide side of the frame portion. The first moving portion and the two second moving portions are disposed inside the frame portion, wherein the first moving portion is disposed between the second moving portions. The long linear-type coupling window is disposed on a linear slot. The moving mechanism is capable of adjusting the distance between the first moving portion and the first wide side and the distance between the second moving portions and the first wide side. 
         [0012]    In the long linear-type microwave plasma source of the present invention, the distance between the first moving portion and the first wide side is adjusted to control the input microwave leakage so that the waveguide power input at one end vanishes before it reaches the other end. As a result, the standing wave ratio (SWR) is reduced and the microwave power efficiency is enhanced to maintain the plasma uniformity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The objects and spirits of the embodiments of the present invention will be readily understood by the accompanying drawings and detailed descriptions, wherein: 
           [0014]      FIG. 1A  and  FIG. 1B  are side views of a conventional long linear-type microwave plasma source from different viewing angles; 
           [0015]      FIG. 1C  shows the plasma density profiles of a long linear-type microwave plasma source in  FIG. 1B ; 
           [0016]      FIG. 2A  is a cross-sectional view of a long linear-type microwave plasma source according to one embodiment of the present invention; 
           [0017]      FIG. 2B  is a cross-sectional view of the long linear-type microwave plasma source in  FIG. 2A ; and 
           [0018]      FIG. 2C  is a 3D exploded view of the long linear-type microwave plasma source in  FIG. 2A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    The present invention can be exemplified by but not limited to the embodiments as described hereinafter. 
         [0020]      FIG. 2A  is a cross-sectional view of a long linear-type microwave plasma source according to one embodiment of the present invention;  FIG. 2B  is a cross-sectional view of the long linear-type microwave plasma source in  FIG. 2A ; and  FIG. 2C  is a 3D exploded view of the long linear-type microwave plasma source in  FIG. 2A . Please refer to  FIG. 2A  to  FIG. 2C , wherein the long linear-type microwave plasma source  200  comprises a reaction chamber  210 , a variably-reduced-height waveguide  220 , a long linear-type coupling window  230  and a moving mechanism  240 . The variably-reduced-height waveguide  220  is disposed on the reaction chamber  210 . The variably-reduced-height waveguide  220  may be a rectangular waveguide. The variably-reduced-height waveguide  220  comprises a frame portion  222 , a long linear-type coupling frame  224 , a first moving portion  226  and two second moving portions  228 . The frame portion  222  has a first wide side  222   a  adjacent to the reaction chamber  210 . The long linear-type coupling frame  224  is disposed on the first wide side  222   a  of the frame portion  222 . The first moving portion  226  and the two second moving portions  228  are disposed inside the frame portion  222 , wherein the first moving portion  226  is disposed between the second moving portions  228 . The long linear-type coupling window  230  is disposed on the long linear-type coupling frame  224 . The moving mechanism  240  is capable of adjusting the distance h 1  between the first moving portion  226  and the first wide side  222   a  and the distance h 2  between the second moving portions  228  and the first wide side  222   a .    
         [0021]    Therefore, when the microwave power  70  is applied to the variably-reduced-height waveguide  220  at its two ends, the microwave power  70  travels inside the variably-reduced-height waveguide  220 . According to the waveguide theory, the microwave power  70  leaks out of the variably-reduced-height waveguide  220  through the long linear-type coupling window  230  toward the reaction chamber  210  to excite plasma  80  and further perform film deposition on the substrate  250  that is carried by the conveyor belt  260 . 
         [0022]    Generally, the distance h 1  between the first moving portion  226  and the first wide side  222   a  (or referred to as the height of the waveguide) can be used to determine the leakage of the microwave power  70 . In other words, the higher the height h 1  of the waveguide, the lower the leakage of the microwave power  70 . The leakage of the microwave power  70  can be controlled by adjusting the distance h 1  so that the microwave power  70  input at any end of the waveguide  220  leaks into the reaction chamber before it reaches the other end without increasing the microwave power  70  to change the height of the waveguide  220 . Therefore, uniform long linear-type plasma  80  can be generated. 
         [0023]    Moreover, the leakage of the microwave power  70  depends on the pressure in the reaction chamber  21  and the width of the long linear-type coupling frame  224 . Therefore, in the present invention, the moving mechanism  240  is capable of adjusting the height h 1  of the waveguide to achieve the optimal microwave leakage rate to generate uniform long linear-type plasma  80 . 
         [0024]    Moreover, the second moving portions  228  are used as a quarter wavelength impedance converter to achieve impedance matching. More particularly, the frame portion  222  has a second wide side  222   b  with respect to the first wide side  222   a , wherein the distance h 3  between the second wide side  222   b  and first wide side  222   a  is a constant. In order to achieve optimal impedance matching, the moving mechanism  240  is capable of adjusting the distance h 2  according to the height h 1  of the waveguide, where h 2 ≅√{square root over (h 1 ×h 3 )}. Moreover, the moving mechanism  240  is, for example, made of automatical or manual devices to lift or lower the first moving portion  226  and the second moving portions  228 . However, the present invention is not limited to the aforementioned example of the moving mechanism  240 . 
         [0025]    Referring to  FIG. 2A  to  FIG. 2C , to achieve higher microwave leakage, the microwave power  70  is input into the variably-reduced-height waveguide  220  at one end to reach the other end. Ideally, when the microwave power  70  is input into the variably-reduced-height waveguide  220  at one end and leak out of the waveguide  220  completely as it reaches the other end, the excited plasma exhibits the highest efficiency with lowest power reflection. Therefore, if the reflected power of the microwave power  70  is too large, the moving mechanism  240  may shorten the distance h 1  between the first moving portion  226  and the first wide side  222   a  to increase the leakage of the microwave power  70  and shorten the distance h 2  between the second moving portions  228  and the first wide side  222   a  to achieve dual-port impedance matching. In other words, the moving mechanism  240  is capable of adjusting the distance h 1  between the first moving portion  226  and the first wide side  222   a  and the distance h 2  between the second moving portions  228  and the first wide side  222   a  according to the reflected microwave power. 
         [0026]    In the present embodiment, the reaction chamber  210  is provided with a long linear-type coupling window  212 . The variably-reduced-height waveguide  220  is disposed on the long linear-type coupling window  212  of the reaction chamber  210 . The long linear-type coupling frame  224  is disposed corresponding to the long linear-type coupling window  212  with an O-ring at the top side to maintain a vacuum and two row of gas inlets at the bottom side to supply reaction gas for plasma. Moreover, the microwave plasma source  200  is also provided a conveyor belt  260  disposed inside the reaction chamber  210  under the long linear-type coupling frame  224 . The conveyor belt  260  is capable of carrying a substrate  270  so that film deposition can be performed with the plasma  80  on the substrate  270 . With an adjustable transport speed of the conveyor belt, the long linear-type plasma  80  provided in the present invention can be used to deposit uniform thin films on the substrate  270 . 
         [0027]    Therefore, there is an atmospheric pressure inside the variably-reduced-height waveguide  220 . The pressure inside the reaction chamber  210  is lower. The long linear-type coupling window  230  separates the variably-reduced-height waveguide  220  and the reaction chamber  210 . Beneath the long linear-type coupling frame  224 , two rows of gas inlets (not shown) introduce reaction gases (not shown) into the reaction chamber  210  so that the reaction gases are excited by microwave to generate plasma  80  to deposit a thin film on the substrate  270 . 
         [0028]    Moreover, the long linear-type microwave plasma source  200  can be being used for continuous or batch type plasma processes. The variably-reduced-height waveguide  200  is capable of receiving the microwave power  70  at one end or two ends to excite plasma. Moreover, the long linear-type coupling window  230  may be made of quartz glass, ceramic or other dielectric materials. 
         [0029]    It is noted that the generated plasma  80  is used for film deposition on the substrate  270 . However, the plasma  80  in the present invention is not limited thereto. For example, plasma can also be used to etch the substrate. Moreover, the substrate  270  may be a silicon substrate or a transparent glass substrate. The long linear-type coupling window  230  may be made of quartz glass or other dielectric materials. 
         [0030]    Accordingly, the microwave plasma reactor of the present invention controls the input microwave leakage by adjusting the distance between the first moving portion and the first wide side so as to enhance the length of the linear-type plasma without increasing the microwave power. 
         [0031]    Although this invention has been disclosed and illustrated with reference to particular embodiments, the principles involved are susceptible for use in numerous other embodiments that will be apparent to persons skilled in the art. This invention is, therefore, to be limited only as indicated by the scope of the appended claims.