Abstract:
A system and method for the removal of deposited material from the walls of a plasma chamber is disclosed. The system may have two modes; a normal operating mode and a cleaning mode. During the cleaning mode, the plasma is biased at a higher potential than the walls, thereby causing energetic ions from the plasma to strike the plasma wall, dislodging material previously deposited. This may be achieved through the use of one or more electrodes disposed in the plasma chamber, which are maintained at a first voltage during normal operating mode, and a second, higher voltage, during the cleaning mode.

Description:
Embodiments of the present invention relate to methods and apparatus for improving implant quality in a plasma-based ion implantation system. 
     BACKGROUND 
     Semiconductor workpieces are often implanted with dopant species to create a desired conductivity. For example, solar cells may be implanted with a dopant species to create an emitter region. This implant may be done using a variety of different mechanisms. In one embodiment, shown in  FIG. 1 , an ion implant system  100  is used. This ion implant system  100  includes a plasma chamber  105  defined by several walls  107 , which may be constructed from graphite, silicon, silicon carbide or another suitable material. This plasma chamber  105  may be supplied with a source gas via a gas inlet  110 . This source gas may be energized by an RF antenna  120  or another mechanism to create plasma  150 . The RF antenna  120  is in electrical communication with a RF power supply (not shown) which supplies power to the RF antenna  120 . A dielectric window  125 , such as a quartz or alumina window, may be disposed between the RF antenna  120  and the interior of the plasma chamber  105 . The system  100  also includes a controller  175 . The controller  175  may receive input signals from a variety of systems and components and provide output signals to each to control the same. 
     Positively charged ions  155  in the plasma  150  are attracted to the substrate  160  by the difference in potential between the plasma chamber  105  (which defines the potential of the plasma  150 ) and the substrate  160 . In some embodiments, the walls  107  may be more positively biased than the substrate  160 . For example, the walls  107  may be in electrical communication with a plasma chamber power supply  180 , which is positively biased. In this embodiment, the substrate  160  is in communication with a platen  130 , which is in communication with bias power supply  181 , which is biased at a voltage lower than that applied by plasma chamber power supply  180 . In certain embodiments, the bias power supply  181  may be maintained at ground potential. In a second embodiment, the plasma chamber power supply  180  may be grounded, while the bias power supply  181  may be biased at a negative voltage. While these two embodiments describe either the substrate  160  or the walls  107  being at ground potential, this is not required. The ions  155  from the plasma  150  are attracted to the substrate  160  as long as the walls  107  are biased at any voltage greater than that applied to the platen  130 . 
     During operation, the ions and various forms of neutral particles in the plasma  150  may be deposited on the walls  107  of the plasma chamber  105 . In general, deposition layers are poor in electrical conductivity and may even be electrically insulating. As a result, the plasma  150  is not well-referenced, often causing plasma potential changes, non-uniformity of plasma (causing non-uniform doping on substrate  160 ), and plasma instability. This deposition of material may be uneven and may affect the conductivity of the walls  107 . Specifically, the deposition may be uneven, such that some portions of the walls  107  are coated, while other portions remain exposed. This non-uniform coating may affect the composition of parameters of the plasma, which may negatively impact the substrates being implanted. 
     In order to provide a reliable electrical reference for the plasma  150 , it is desirable to insure that no deposition coating exist on the walls  107 . However, removing this coating may require the application of large amounts of heat, which may not be practical. Alternatively, the ion implant system may need to be taken offline so that the coating can be removed, which reduces throughput and efficiency. 
     Therefore, a system and method to provide a reliable electrical reference to the plasma  150  for stable and repeatable doping process by reducing or eliminating coatings that are deposited in a plasma-based implantation system is needed. 
     SUMMARY 
     A system and method for the removal of deposited material from the walls of a plasma chamber is disclosed. The system may have two modes; a normal operating mode and a cleaning mode. During the cleaning mode, the plasma is biased at a higher potential than the walls, thereby causing energetic ions from the plasma to strike the plasma wall, dislodging material previously deposited. This may be achieved through the use of one or more electrodes disposed in the plasma chamber, which are maintained at a first voltage during normal operating mode, and a second, higher voltage, during the cleaning mode. 
     In one embodiment, an ion implant system is disclosed, which comprises a plasma chamber defined by a plurality of chamber walls; an electrode disposed within the plasma chamber and electrically isolated from the chamber walls; a plasma chamber power supply for providing a first voltage to the chamber walls; an electrode power supply for providing a second voltage to the electrodes, wherein the first voltage and the second voltage are different during a cleaning mode. 
     In a second embodiment, a method of operating an ion implant system is disclosed, which comprises: creating a plasma within a plasma chamber, defined by a plurality of chamber walls; providing an electrode within the plasma chamber, electrically isolated from the chamber walls; biasing the chamber walls and the electrode at the same voltage during a normal operating mode; and biasing the electrode at a first voltage, higher than a second voltage applied to the chamber walls during a cleaning mode. 
     In a third embodiment, a method of operating an ion implant system is disclosed. This method comprises creating a plasma within a plasma chamber, defined by a plurality of chamber walls; providing an electrode within said plasma chamber, electrically isolated from the chamber walls; biasing the chamber walls and the electrode at a first voltage during a normal operating mode; biasing a substrate at a voltage less than the first voltage; implanting ions from the plasma into the substrate during the biasing at the voltage less than the first voltage during the normal operating mode; biasing the electrode at a second voltage higher than a third voltage applied to the chamber walls after the implanting, wherein ions from the plasma are attracted toward the chamber walls to clean the chamber walls during a cleaning mode. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG. 1  shows an embodiment of an ion implant system according to the prior art; 
         FIG. 2  shows an embodiment of an ion implant system that can be used in one embodiment; 
         FIG. 3  shows an enlarged view of an electrode used in the ion implant system of  FIG. 2 ; 
         FIG. 4  shows an embodiment of an ion implant system that can be used in another embodiment; 
         FIG. 5  shows a flowchart that may be used during the cleaning mode. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a representative diagram of the ion implant system  290  according to one embodiment. Components which retain their function from  FIG. 1  are given the same reference designators and are not described again. 
     As described above, the positive ions  155  in the plasma  150  are attracted to the substrate  160  by the difference in potential between the chamber walls  107  (and/or the plasma  150 ) and the platen  130 . The ions  155  from the plasma  150  are attracted as long as the walls  107  are biased at a voltage greater than that applied to the substrate  160 . Thus, the bias provided by plasma chamber power supply  180  is greater than the bias provided by bias power supply  181  in order to attract ions  155  toward the substrate  160 . 
     The plasma  150  is typically referenced to the voltage of the walls  107 . Thus, if the walls  107  are positively biased, such as by plasma chamber power supply  180 , the plasma  150  is likewise positively biased, for example, at a potential that may be slightly higher than the potential of the walls  107 . Ions  155  from the plasma  150  gain some energy through the plasma potential, which may be typically 10-30 V, and deposit onto the chamber walls  107 . Since the walls  107  may provide a cooler surface, ions  155  and various forms of neutrals from the plasma  150  may condense on the walls  107 . Thus, deposition may be a major contributor to coating on the chamber walls  107 . 
     To provide a stable doping condition, by providing a good electrical reference on the walls  107  for the plasma  150 , electrodes  200  are introduced into the interior of the plasma chamber  105 . As best seen in  FIG. 3 , these electrodes have a conductive outer surface  210 , which faces the interior of the plasma chamber  105 . The conductive outer surface  210  may be constructed of the same material used to make the chamber walls  107 , such as graphite, silicon, silicon carbide, etc. In some embodiments, a heating element  230 , such as a conductive coil, may be disposed within the electrode  200 . In these embodiments, an insulating material  220  is disposed inside the electrode  200  to electrically, but not thermally, isolate the outer surface  210  from the heating element  230 . The outer surface  210  and the heating element  230  are each in communication with a respective power supply  240 ,  250 . In some embodiments, the electrode power supply  240  may be a DC power supply providing a DC bias of between −1000 and +1000 V. The heater power supply  250  may provide an AC waveform having an amplitude of between 10-100 V. This heater power supply  250  may be referenced to the electrode power supply  240 . 
       FIG. 2  also shows an insulating material  260  disposed between the outer surface of the electrode  200  and the chamber walls  107 . This may alleviate operation problems because the voltages applied to the walls  107  and the electrodes  200  may differ in certain conditions, as described in more detail below. 
     In addition, a controller  175  is shown. The controller  175  can be or may include a general-purpose computer or network of general-purpose computers that may be programmed to perform desired input/output functions. The controller  175  can also include other electronic circuitry or components, such as application specific integrated circuits, other hardwired or programmable electronic devices, discrete element circuits, etc. The controller  175  may also include communication devices, data storage devices, and software. The controller  175  is in communication with a non-transitory medium. This storage element contains instructions, which when executed by the controller  175 , perform the steps and operations described herein. 
       FIG. 4  shows a second embodiment of the ion implant system  300 . In this embodiment, the walls  107  include a recess into which the electrodes  200  are disposed. In this way, the interior of the plasma chamber  105  may be smooth, with no penetrating components, as was shown in  FIG. 2 . The electrodes  200  may be identical to those described in conjunction with  FIG. 3 . 
     In each of these embodiments, two electrodes  200  are shown as being disposed on two opposite walls  107  of the plasma chamber  105 . However, other embodiments are also possible. For example, more or fewer electrodes may be used, and their location can be modified as needed. For example, there may also be single electrode, surrounding the entire plasma chamber  105 . 
     Having defined the structure of the ion implant system  290 ,  300 , its operation during both normal operating mode and cleaning/conditioning mode will be described. The steps described herein for the normal operating mode and the cleaning/conditioning mode are initiated by controller  175 , which is in communication with a storage element, having instructions, which when executed, perform this sequences. 
     During normal operating mode, the voltage applied by electrode power supply  240  is the same as that being applied by plasma chamber power supply  180 . Thus, the electrodes  200  will be at the same voltage as the walls  107 . As described above, the voltage applied by plasma chamber power supply  180  to the walls  107  is greater than that applied by bias power supply  181  to the platen  130 . In some embodiments, this voltage may be in the range of 100-10,000 V, while the voltage applied by bias power supply  181  may be electrically ground. In other embodiments, the voltage applied by power supplies  180  and  240  may be ground, while the voltage applied by bias power supply  181  to the platen  130  may be negative, such as between −100 and −10,000 volts. Other configurations of voltages are also possible and within the scope of the disclosure. 
     By maintaining the electrodes  200  at the same bias voltage as the walls  107 , the electrodes  200  have minimal effect on the operation of the ion implant system  290 ,  300  during normal operation. This configuration is used to implant ions  155  into the substrate  160 , as is traditionally done. As described above, deposition tends to occur within the plasma chamber  105  during the normal operating mode. Therefore, in some embodiments, a heating element  230  is disposed within the electrodes  200 . This heating element  230  may be activated during normal operating mode. By heating the electrodes  200 , the hot electrodes  200  may be more resistant to deposition during normal operating mode. These electrodes  200  may be heated to temperatures, such as 400° C. or greater. In other embodiments, the heating element  230  is not utilized. In these embodiments, deposition is more likely to occur on the electrodes  200  during normal operating mode. Thus, over time, deposition will form on the walls  107 , and optionally on the electrodes  200  (depending on whether a heating element  230  is employed). 
     To remove this coating and provide a reliable electrical reference to the plasma  150  for stable and repeatable doping process, a cleaning/conditioning mode is employed. During the cleaning/conditioning mode, the walls  107  may be cleaned, the plasma chamber  105  may be conditioned, or both operations may occur. It is not necessary for both events to occur during the cleaning/conditioning mode. A cleaning mode effectuates removal of deposited material from the walls  107 . As described above, the plasma  150  is typically referenced to the highest voltage proximate to the plasma  150 . During normal operation, this voltage is that used to bias the chamber walls  107 . However, in cleaning/conditioning mode, the electrodes  200  are biased to a higher voltage than the walls  107 . In other words, electrode power supply  240  supplies a greater voltage than plasma chamber power supply  180 . This voltage may be 100 or more volts greater than that applied by plasma chamber power supply  180 . In this way, the plasma  150  is referenced to the voltage supplied by electrode power supply  240 . Since this voltage is higher than that applied to the walls  107 , the walls  107  effectively become cathodes and energetic positive ions  155  are attracted to the walls  107 . These ion collisions may contain sufficient energy to clean or otherwise remove the deposition coating that had built up on the walls  107  during normal operating mode. 
     In the embodiment where a heating element  230  is used, the heat deters deposition of material on the electrode  200 . Thus, this single step cleaning sequence is sufficient to remove deposition from all surfaces within the plasma chamber  105 , and provide a reliable reference potential for the plasma. However, in embodiments where a heating element  230  is not utilized, a two step cleaning/conditioning process may be used to provide reliable reference potential for the plasma inside the plasma chamber  105 . The first step may be as described above, which serves to remove deposition from the chamber walls  107 . In the second step, the voltage applied by plasma chamber power supply  180  is greater than that applied by electrode power supply  240 . In this way, ions  155  are attracted to the electrodes  200  and serve to remove any deposition from these surfaces. In other words, in one step, the plasma  150  is referenced to the electrodes  200 , and deposition is removed from the chamber walls  107 . In the second step, the plasma  150  is referenced to the walls  107 , and deposition is removed from the electrode  200 . These steps can be performed in any order. 
     In another embodiment, a heating element  230  is not utilized, but the single step cleaning sequence described above is used. This may be acceptable if the surface area of the electrodes  200  is small relative to the walls  107 . 
     The cleaning mode can be activated for a duration of time necessary to remove the deposition from the chamber walls  107 . For example, in some embodiments, the cleaning mode may be activated for several minutes after hours of normal operating mode. For example, cleaning mode may be used 5-30 minutes after every 5-20 hours of continuous normal operating mode. In other embodiments, the cleaning mode may be activated when substrates  160  are being exchanged to and from the ion implant system  290 ,  300 . 
       FIG. 5  shows a flow chart showing the operation of the ion implant system  290  during the cleaning mode. First, as described above, a first voltage is applied to the electrodes  200 , where the first voltage is set to a level greater than a second voltage being applied to the chamber walls  107 , as shown in step  500 . The second voltage applied to the chamber walls  107  may be the same as is used during normal operating mode. In other embodiments, the second voltage applied to the chamber walls  107  may be less than that typically used for normal operating mode. However, in all embodiments, the first voltage applied to the electrodes  200  is greater than the second voltage applied to the walls  107 . 
     In embodiments where a heating element  230  is utilized, this heating element is actuated, as shown in step  510 . This heating element may be a conductive coil which is heated by applying an alternating current thereto, such as by the use of heater power supply  250 . The heater power supply  250  may be referenced to the electrode power supply  240 . The electrode  200  is heated to a temperature sufficient to deter or prevent the deposition of material on the electrode  200 . The electrode  200  may also be heated during normal operation. 
     The electrode  200  remains at this elevated voltage for a sufficient time to remove the deposition from the walls  107  of the plasma chamber  105 , as shown in step  520 . In embodiments where a heating element  230  is used, the cleaning process terminates after step  520 . 
     In other embodiments, a second cleaning step is needed to remove deposition from the electrodes  200 . As described above and shown in step  530 , a third voltage is applied to the electrodes  200 , where this third voltage is less than a fourth voltage being applied to the walls  107 . In some embodiments, the voltage applied to the walls  107  is unchanged, and the electrode power supply  240  simply changes from a first voltage, greater than the plasma chamber wall voltage to another voltage less than the plasma chamber wall voltage. As explained above, in this mode, energetic ions  155  will strike the electrodes  200 , removing deposition from these components. 
     The electrode  200  remains at this depressed voltage for a sufficient time to remove the deposition from the electrodes  200 , as shown in step  540 . 
     After this step, the chamber walls  107  and the electrodes  200  are cleaned. After this sequence, normal operation may resume. 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.