Abstract:
An apparatus includes a vacuum chamber, an electrical transformer that surrounds the vacuum chamber to induce an electromagnetic field within the vacuum chamber, and an ignition circuit. The electrical transformer induces an electromagnetic field within the vacuum chamber. The transformer includes a primary winding and a plasma loop coupled to the vacuum chamber to perform as a secondary winding. The ignition circuit is coupled to an ignition core section of the vacuum chamber to ignite the vacuum chamber.

Description:
The present application is a continuation of application Ser. No. 10/355,618, entitled “Transformer Ignition Circuit for a Transformer Coupled Plasma Source”, filed Jan. 31, 2003, now U.S. Pat. No. 6,822,396 and claims priority thereof. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of remote plasma sources; more particularly, the present invention relates to the ignition of transformer coupled plasma (TCP) sources. 
   BACKGROUND 
   Transformer coupled plasmas are routinely used in a number of different applications including materials processing, production of activated gases, pollutant abatement and many others. In such devices, the magnetic core of the excitation transformer is placed in close proximity to, around or within a vacuum chamber. When the primary winding of this transformer is excited with radio frequency (RF), the electromagnetic fields induced around the core sustain a gas plasma discharge within the vacuum apparatus. 
   One of the main advantages of TCPs is that the power capacitively coupled into the plasma is minimized, resulting in a decrease in chamber wall damage through ion bombardment and, consequently, in extended lifetimes for the plasma chamber. However, this advantage poses a problem for plasma ignition since the capacitive fields are needed to start a plasma discharge. 
   One of the solutions to this problem has been the introduction of an auxiliary capacitive discharge used to generate enough free charges within the plasma chamber so that an inductively coupled plasma can be established. This additional capacitive discharge is usually driven by an external high voltage circuit. 
   SUMMARY 
   According to one embodiment, an apparatus is described. The apparatus includes a vacuum chamber, an electrical transformer coupled to the vacuum chamber, and an ignition circuit. The electrical transformer induces an electromagnetic field within the vacuum chamber. The transformer includes a primary winding and a magnetic core. In addition, the transformer includes a secondary winding, to which the circuit used to ignite the vacuum chamber is coupled. The ignition circuit is used to ignite the vacuum chamber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only. 
       FIG. 1  illustrates one embodiment of a plasma source coupled to an RF power generator; 
       FIG. 2  illustrates an example of a chamber configuration; 
       FIG. 3  illustrates one embodiment of a chamber configuration; 
       FIG. 4  illustrates an electrical representation of one embodiment of a plasma source, with an ignition circuit, coupled to a RF power generator; 
       FIG. 5  illustrates an electrical representation of another embodiment of a plasma source, with an ignition circuit, coupled to a RF power generator; 
       FIG. 6  illustrates an electrical representation of yet another embodiment of a plasma source, with an ignition circuit, coupled to a RF power generator; 
       FIG. 7  illustrates an electrical representation of still another embodiment of a plasma source, with an ignition circuit, coupled to a RF power generator; and 
       FIG. 8  illustrates an electrical representation of still another embodiment of a plasma source, with an ignition circuit, coupled to a RF power generator. 
   

   DETAILED DESCRIPTION 
   According to one embodiment, a transformer ignition circuit is described. In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention. 
   Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
     FIG. 1  illustrates one embodiment of a plasma chamber  150  coupled to an RF power generator  110 . In one embodiment, the RF power is coupled to the plasma discharge within the plasma chamber  150  via one or more transformers that utilize at least one magnetic core. Only one transformer  130  is shown here to avoid obscuring the present invention. 
   The excitation transformer  130  includes the primary winding, which is coupled to the RF power supply  110 , and a magnetic core, and the secondary winding  170 , which along with the resistance Rp represent the plasma discharge. Note that resistance Rp does not appear until plasma source  130  has been ignited. The transformer  130  also includes a secondary winding  180 . 
   According to one embodiment, an ignition circuit  140  is coupled to plasma chamber  150  and to excitation transformer  130 . Ignition circuit  140 , driven by secondary winding  180  of one of the excitation transformers of the plasma chamber, produces and controls the ignition of the plasma within the plasma chamber  150 . 
     FIG. 2  illustrates an exemplary plasma source chamber configuration. The chamber includes a transformer with a primary winding, a magnetic core, and a dielectric break. The chamber also includes an ignition input that receives an ignition voltage Vi in order to ignite the chamber. 
   The plasma chamber is ignited by an ignition circuit (not shown) that couples power to the plasma source via a capacitive discharge. This discharge generates free charges that are used to establish the inductive plasma discharge. Once established, the inductive discharge performs as a single turn secondary for the transformer. 
     FIG. 3  illustrates one embodiment of a plasma chamber  150 . The chamber includes four dielectric breaks, excitation transformers  130  and  135 , and ignition inputs  370  and  375 . Sections  300  and  310  of the plasma chamber are connected to ground. Sections  320  and  330 , denominated ignition cores are electrically isolated from ground by the dielectric breaks. 
   According to one embodiment, the ignition inputs  370  and  375  are connected to ignition cores  320  and  330 , respectively. According to another embodiment, the ignition inputs  370  and  375  can be connected to electrodes electrically isolated form the chamber.  FIG. 3  also shows the plasma loop  340 , which is established within the plasma chamber after the ignition circuit produces enough free charges to initiate the inductively coupled discharge. 
   Ignition inputs  370  and  375  are used to inject a voltage in order to start chamber  150 . The ignition inputs receive a high voltage from an ignition circuit, described below. According to one embodiment, controlled ignition voltage and current are provided at both inputs  370  and  375 . The voltage in inputs  370  and  375  can either be applied with respect to ground or be a differential voltage between the inputs. 
   In the first case, capacitive discharges are established between the ignition cores and the grounded sections of the chamber, whereas in the second case the capacitive discharge is established between the ignition cores. However, in other embodiments, the ignition voltage may be supplied to either of inputs  370  and  375  individually. 
     FIGS. 4 to 8  illustrate electrical representations of different embodiments of a plasma source with an ignition circuit driven by a secondary winding of the excitation transformer. In all cases, the ignition circuits include one or more switches S that are closed during ignition. As a result, an ignition voltage is applied to ignition input  370  and/or  375  to ignite chamber  150 . According to one embodiment, a switch is opened once chamber  150  is ignited. Consequently, the ignition current and voltage applied to chamber  150  are discontinued after the inductively coupled plasma is established. 
     FIG. 4  illustrates an electrical representation of one embodiment of an ignition circuit  140  coupled to excitation transformer  130 . Note that only one transformer is shown in order to avoid obscuring the present invention. Ignition circuit  140  includes the primary and secondary windings of transformer  450 , a switch (S), and a capacitor (C). 
   According to one embodiment, whenever a voltage from RF power generator  110  is applied to primary winding  160 , the secondary winding  180  generates a floating voltage at the primary of transformer  450 . When the switch S is closed, the secondary of transformer  450  provides the ignition voltage V I  through the coupling capacitor C. 
     FIG. 5  illustrates an electrical representation of another embodiment of an ignition circuit  140  coupled to excitation transformer  130 . In this embodiment, ignition circuit  140  includes a switch S that is coupled directly to secondary winding  180 . In addition, a capacitor and inductor are coupled in series with the switch. 
     FIG. 6  illustrates an electrical representation of another embodiment of an ignition circuit  140  coupled to excitation transformer  130 . In this embodiment, ignition circuit  140  includes a switch S coupled to the secondary winding  180  and a ballast  620 . Ballast  620  limits the maximum current through ignition circuit  140  so that the winding  180  is not shorted during ignition. 
     FIG. 7  illustrates an electrical representation of another embodiment of an ignition circuit  140  coupled to the secondary winding  180  of excitation transformer  130 . In this embodiment, ignition circuit  140  includes the switch S, the capacitor C, and an autotransformer  750  coupled to the capacitor. During ignition, when the switch S is closed, the autotransformer  750  generates a voltage that ignites the plasma chamber  150 . 
     FIG. 8  illustrates an electrical representation of yet another embodiment of an ignition circuit  140  coupled to excitation transformer  130 . In this embodiment, ignition circuit includes the switches S and the transformer  850 . The output of transformer  850  is connected to ignition inputs  370  and  375 . During ignition, when the switches S are closed, the secondary winding of transformer  850  generates a differential voltage between inputs  370  and  375  that ignites the plasma chamber  150 . 
   Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims which in themselves recite only those features regarded as essential to the invention. 
   Thus, a mechanism to ignite a transformer coupled plasma chamber has been described.