Abstract:
An RF plasma reactor is provided for depositing semi-conductive layers on to very large glass areas. The RF plasma reactor includes a vacuum chamber, a reactor chamber, RF power supply, a matching network, first and second metallic plates located inside the vacuum chamber and a plasma-discharge region defined between the first and second metallic plates. The RF plasma reactor further includes a feed line and an impedance-transformation circuit both of which are electrically connected to the first metallic plate. The impedance-transformation circuit further includes a blocking-tuneable capacitor that transforms an impedance of the reactor.

Description:
[0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/627,784 filed on Nov. 12, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates in general to RF capacitive coupled plasma reactors for processing a very large area display. More specifically, the present invention relates to improvements in the coupling efficiency of the RF power delivered to plasma typically at a frequency of 13.56 MHz or less.  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention is based on problems and requirements that have arisen in depositing semi-conductive layers on very large glass areas for the display and solar manufacturing industries. The resulting solution, however, can be applied to other applications. Thus, even though the present invention will be described relating to plasma reactors for Plasma Enhanced Chemical Vapor Deposition (PECVD) systems for very large area display processing, the present invention can also be applied to other applications relating to plasma reactors. Further, the development of PECVD for very large area display processing is disclosed in U.S. Pat. No. 6,281,469, the contents of which are herein incorporated by reference.  
         [0004]      FIG. 1  shows a conventional capacitively-coupled, RF-plasma reactor system  10  for a PECVD system. The reactor system  10  includes an RF power supply  12 , a matching network  14 , a reactor chamber  16 , and a vacuum chamber  18 . The reactor chamber  16  includes two metallic plates  20 ,  22  arranged in parallel enclosed in a metallic-reactor casing  24 . The first metallic plate  22  is electrically connected to the RF power supply  12  via a feeding element  26  and the matching network  14  and the first metallic plate  22  is, thus, a live electrode. The second metallic plate  20  is connected to ground and is, thus, a ground electrode. During the deposition process a substrate is placed on the second metallic plate  20  for processing. The feeding element  26  is shielded with a grounding shield  28  and can be any type of electrical feeding element known in the art, such as an RF stripline, RF ribbon, or a triplate stripline. A plasma-discharge region  30  is defined in between the two metallic plates  20 ,  22 . As shown in  FIG. 1 , the RF power supply  12  and matching network  14  are located outside the vacuum chamber  18  and the reactor chamber  16  and feeding element  26  are located inside the vacuum chamber  18 . Thus, during the deposition process, the RF power supply  12  and the matching network  14  are under atmospheric conditions and the reactor chamber  16  and RF feed line  28  are under vacuum conditions. A typical gas used for forming a plasma in the PECVD process is a silicon nitride SiN gas. Other gases commonly known in the art, however, may be used in this type of application such as organometallics, hydrides and halides.  
         [0005]      FIG. 2  shows a simplified equivalent circuit of the conventional PECVD system during the deposition of SiN and will be used to illustrate the disadvantage of the conventional RF-plasma reactor system  10 . The dotted line boxes represent a portion of the conventional RF-plasma reactor system  10  as indicated by the reference numbers. During the deposition process, in the conventional RF-plasma reactor, several kilowatts of RF power at a radio frequency of 13.56 MHz must be delivered to the plasma in order to achieve the necessary deposition rate and maintain a reasonable throughput. A disadvantage to this process in large area parallel plate reactors is that a very large parasitic-reactor capacitance C R , typically greater than 5000 pF, forms between the live electrode  22  and the grounded reactor casing  24 . Thus, the feeding element  26  must be capable of handling very large RF currents I F , typically greater than 300 A. The large RF currents require a very wide stripline design, which leads to a second parasitic-feed-line capacitance C F  between the live wire of the feeding element  26  and the grounding shield  28 . The feed-line capacitance C F  is typically greater than 3000 pF. The reactor C R  and feed-line C F  capacitance transform a plasma impedance Z P  to a feed-through impedance Re(Z F ) having a very small value, typically less than 0.05 ohms. The feed-through impedance Re(Z F ) is the impedance as seen at the entrance of the vacuum chamber  18  where the feeding element  26  enters the vacuum chamber  18 . The feed-through impedance Re(Z F ) in turn creates a larger RF current I F , typically greater than 400 A, which now must be accommodated by the matching network  14  and the feeding element  26 . As a result, most of the power delivered by the RF power supply is dissipated in the lossy elements R M , R F  of the matching network  14  and the feeding element  26  respectively. Thus, the efficiency of the system is low, typically ηs&lt;0.3. Therefore, very large and expensive RF power supplies are required in order to achieve the necessary plasma power density and deposition rate. Further, as the size of the glass increases the efficiency of the plasma power coupling efficiency decreases to values less than 20% at an RF frequency of 13.56 MHz.  
         [0006]     There have been several solutions to address the above mentioned problem but each have additional disadvantages. For example, the parasitic capacitance C R  and C F  could be reduced by increasing the gap between the live parts, i.e. the live electrode  22  and feeding element  26 , and the grounded parts, i.e. the reactor casing  24  and the ground shield  28 . The disadvantage to this solution, however, is that the plasma between the gaps could ignite. Another solution is to water cool the reactor. This, however, is difficult in a vacuum system and water cooling does not significantly enhance the plasma coupling efficiency.  
         [0007]     Another solution is adding an impedance-transformation circuit to the RF-plasma reactor system  10 . Power losses through the lossy elements R M , R F  in the matching network  14  and the feeding element  26  respectively can be reduced by decreasing the RF current I F . Reducing the RF current I F  while maintaining the plasma power can be accomplished with an impedance-transformation circuit, which increases the feed-through impedance Re(Z F ). In theory, connecting an inductor between the live electrode and ground will suffice as an impedance-transformation circuit. But an impedance-transformation circuit solely made of one inductor is impractical for several reasons. For example, the inductor needs to be a low-loss inductor, there is nothing to prevent the DC voltage from shorting to ground, and there is no tuning capability.  
         [0008]     Thus, what is desired is a practical impedance-transformation circuit for an RF capacitive coupled plasma reactor for processing very large substrates that overcomes the above mentioned disadvantages.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     In accordance with one aspect of the present invention, a plasma reactor is provided comprising, a vacuum chamber, a first metallic plate and a second metallic plate located inside the vacuum chamber, an RF power supply, a matching network, a plasma-discharge region containing plasma defined between the first and second metallic plates, a feeding element electrically connected to the first metallic plate, and an impedance-transformation circuit electrically connected to the first metallic plate.  
         [0010]     In accordance with another aspect of the present invention, a plasma reactor is provided comprising, a vacuum chamber, an RF power supply, a matching network, a first metallic plate and a second metallic plate located inside the vacuum chamber, a plasma-discharge region for containing plasma defined between the first and second metallic plates, a feeding element electrically connected to the first metallic plate, an impedance-transformation circuit electrically connected to the first metallic plate, comprising an isolation capacitor, later referred as blocking capacitor.  
         [0011]     In accordance with yet another aspect of the present invention a method of depositing semi-conductive layers in a vacuum is provided comprising the steps of, providing a plasma reactor further including an RF power supply, a vacuum chamber, a reactor chamber, having a reactor impedance, located inside the vacuum chamber, a first and second metallic plate located inside the vacuum chamber; a plasma-discharge region for containing plasma defined between the first and second metallic plates, a feeding element electrically connected to the first metallic plate, and an impedance-transformation circuit electrically connected to the first metallic plate, placing a substrate on the second metallic plate, delivering RF power to the plasma, transforming the reactor impedance to an intermediate impedance with the impedance-transformation circuit, and transforming the intermediate impedance to a feed-through impedance with the feeding element, whereby the feed-through impedance is increased.  
         [0012]     Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which it pertains upon a reading and understanding of the following detailed specification. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings that form a part of the specification.  
         [0014]      FIG. 1  is a schematic of a conventional capacitor-coupled, RF-plasma reactor system.  
         [0015]      FIG. 2  is an equivalent circuit of the reactor system of  FIG. 1 .  
         [0016]      FIG. 3  is a schematic of a capacitor-coupled, RF-plasma reactor system with an impedance-transformation circuit in accordance with the present invention.  
         [0017]      FIG. 4  is an equivalent circuit of the reactor system of  FIG. 3 .  
         [0018]      FIG. 5  is a graph showing the comparison of the impedance transformation between the conventional circuit of  FIG. 2  and circuit with the impedance-transformation circuit of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Referring now to  FIGS. 3 and 4 , a more practical impedance-transformation circuit is shown in the schematic in  FIG. 3  and the electrical equivalent circuit in  FIG. 4 . All components described in  FIGS. 1 and 2  above that are the same in  FIGS. 3 and 4  will not be repeated.  
         [0020]      FIG. 3  shows a capacitively-coupled, RF-plasma reactor system  40  (hereinafter “transformed RF-plasma reactor system”) for a PECVD system having an impedance-transformation circuit  42 , shown in  FIG. 4 , in accordance with the present invention. The impedance-transformation circuit  42  includes a transformation circuit feeding element  44  with a grounding shield  46  and a tuneable-blocking capacitor C BT .  
         [0021]     Referring to  FIG. 4 , the second feeding element  44  is represented in the equivalent circuit as having parasitic capacitance C T , a lossy element R T , and an low-loss inductor L T . The transformation circuit feeding element  44  is located inside the vacuum chamber  18  and is connected to ground via the tuneable-blocking capacitor C BT . Thus, with the addition of the impedance-transformation circuit  42 , the transformed RF-plasma reactor system  40  now includes the feeding element  26  and the transformation circuit feeding element  44  both of which are electrically connected to the first metallic plate  22 . The tuneable-blocking capacitor C BT  is located outside the vacuum chamber  18  and can be integrated into the matching network  14 , resulting in amended matching network  14 ′ ( FIG. 3 ). The tuneable-blocking capacitor C BT  can increase the feed-through impedance Re(Z F ′) thereby decreasing the total RF current I F ′ during the deposition process. Further, the tuneable-blocking capacitor C BT  can balance the current between the two feeding elements  26 ,  44  without venting the system.  
         [0022]     The following equations illustrate the effect of the impedance-transformation circuit  42  relating to power loss. First, in the conventional RF plasma reactor system  10 , the current I F  flowing out of the matching network  14  and through the feeding element  26  with no impedance-transformation circuit  42 , as shown in  FIGS. 1 and 2 , is approximately: 
 
 I   F   =sqrt[P   F   /Re ( Z   F )]  (1) 
 
 for an RF signal having a wavelength greater than the diameter of the electrode or longer than the feeding line and where P F  is the power at the output of the matching network  14 , which is dissipated through the lossy elements R M  and R P  in the feeding element  26  and the plasma. The power lost through the lossy elements R M  and R F  in the matching network  14  and feeding element  26  is defined by the following equation: 
 
 P   Loss1   =I   F   2 ( R   M   +R   F )  (2) 
 
 The efficiency of the matching network  14  is defined by: 
 
 η MB   =Re ( Z   F )/[ Re ( Z   F )+ R   M ]  (3) 
 
 Further, the efficiency of an L-type or T-type matching network is given through: 
 
η= Q   U /( Q   U   +Q   L )  (4) 
 
 where Q U  is the unloaded quality factor of lumped elements and Q L  is the loaded quality factor of the lumped elements. 
 
         [0023]     With the addition of the impedance-transformation circuit  42 , as shown in  FIGS. 3 and 4 , the current I F ′ flowing out of the matching network  14  and through the feeding element  26  is partially compensated by the current I T  flowing through the transformation circuit element  44  and tuneable-blocking capacitor C BT . The optimal balance between I F ′ and I T  can be adjusted through the tuneable-blocking capacitor C BT  and depends on the balance between the power losses of the lossy elements (R F ′+R M ) of the feeding element  26  and matching network  14  and the lossy element R T  of the transformation circuit feeding element  44 . The total losses in the matching network  14  and the feeding element  26  and transformation circuit feeding element  44  is defined as: 
 
 P   Loss2   =I   F ′ 2 ( R   M   +R   F ′)+ I   T   2   R   T   (5) 
 
 for an RF signal having a wavelength greater than the diameter of the electrode or longer than the feeding line. If the tuneable-blocking capacitor C BT  is adjusted such that the currents I F ′, I T  are equal then I F ′=I T =I F /2. Further, if the currents I F ′, I T  are equal and if the lossy elements R M , R F ′, R T , and R F  are also equal, thus, R M =R F ′=R T =R F . Thus, if I F ′, I T  are equal P Loss1  becomes: 
 
 P   Loss1   =I   F   2 ( R   M   +R   F )=2 I   F   2   R   F   (6) 
 
 and P Loss2  becomes: 
 
 P   Loss2   =I   F ′ 2 ( R   M   +R   F ′)+ I   T   2   R   T =(3/4) I   F   2   R   F .  (7) 
 
 Thus, the loss ratio between the conventional RF-plasma reactor system  10  (no impedance-transformation circuit  42 ) and the transformed RF-plasma reactor system  40  with the impedance-transformation circuit  42  is: 
 
 P   Loss1   /P   Loss2 =(2 I   F   2   R   F )/(3/4) I   F   2   R   F =8/3.  (6) 
 
 As these equations illustrate, the power lost through the lossy elements R M , R F  in the conventional RF-plasma reactor system  10  (no impedance-transformation circuit  42 ) are more than twice as much as the power lost through the lossy elements R M  R F ′, R T  in the transformed RF-plasma reactor system  40  with the impedance-transformation circuit  42 . Thus, because the power loss decreases in the transformed RF-plasma reactor system  40  with the impedance-transformation circuit  42 , the power delivered to the plasma to maintain the same deposition rate as the conventional RF-plasma reactor system  10  (no impedance-transformation circuit  42 ) can be reduced. Thus, a smaller RF power supply can be used to achieve the same deposition rate. On the other hand, if the same size RF power supply is used the deposition rate will increase thereby increasing throughput. 
 
         [0024]      FIG. 5  shows a graph that illustrates how the impedance-transformation circuit  42  transforms the feed-through impedance Re(Z F ′) to thereby decrease the power lost through the lossy elements. In the conventional RF-plasma reactor system  10  the plasma impedance Z P  is transformed by a reactor capacitance C R  and a reactor inductance L R  to a reactor impedance Z R  located at the end of the feeding element  26 . The feeding element  26  then transforms the reactor impedance Z R  to a feed-through impedance designated as Z F . In the transformed RF-plasma reactor system  40  the plasma impedance Z P  is transformed to the reactor impedance Z R  just as in the conventional RF-plasma reactor system  10 . The impedance-transformation circuit  42 , however, transforms the reactor impedance Z R  to an intermediate impedance Z R ′. The feeding element  26  then transforms the intermediate impedance Z R ′ to a feed-through impedance designated as Z F ′. As illustrated in  FIG. 5 , the feed-through impedance Z F ′ has both a higher resistive or real part and a higher inductive reactive or imaginary part than the feed-through impedance Z F . In other words, Re(Z F ′)&gt;Re(Z F ) and Im(Z F ′)&gt;Im(Z F ). More specifically, the real part of the feed-through impedance Re(Z F ′) is approximately 0.1 to 0.2 ohms whereas the real part of the feed-through impedance Re(Z F ) is approximately 0.0 to 0.1 ohms. The imaginary part of the feed-through impedance Im(Z F ′) is approximately 1 to 5 ohms whereas the imaginary part of the feed-through impedance Im(Z F ) is approximately −3 to 1 ohms. Further, the impedance-transformation circuit  42  is not intended to compensate for reactive impedance or cancel out phase shifts. The more inductive the feed-through impedance Z F , however, the less inductance is required in the matching network. As a result, the quality of the matching network can be enhanced even more because the RF power losses are mainly associated with lumped elements such as inductors made from copper.  
         [0025]     While specific embodiments of the invention have been described and illustrated, it is to be understood that these embodiments are provided by way of example only and that the invention is not to be construed as being limited thereto but only by proper scope of the following claims.