Abstract:
A method of responding to voltage or current transients during processing of a wafer in a plasma reactor at each of plural RF power applicators and at the wafer support surface. For each process step and for each of the power applicators and the wafer support surface, the method includes determining an arc detection threshold lying above a noise level. The method further includes comparing each transient with the threshold determined for the corresponding power applicator or wafer support surface, and issuing an arc detect flag if the transient exceeds the threshold.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure concerns plasma reactors for processing semiconductor workpiece, and detection of arcing in such a reactor. 
       BACKGROUND 
       [0002]    Arcing in a plasma reactor during processing of a semiconductor workpiece or wafer can destroy the workpiece or make unusable, or contaminate the reactor chamber. Therefore, detection of arcing to stop a plasma reactor from processing further wafers is essential to avoid damage to a succession of wafers. In physical vapor deposition (PVD) plasma reactors, arc detection has been confined to arcing at the sputter target at the reactor ceiling. Such arc detection has been made by monitoring the output of the high voltage D.C. power supply coupled to the sputter target at the ceiling. Voltage or current transients can reflect arcing events. While this approach has provided reliable indication of arcing events occurring at or near the sputter target at the reactor ceiling, it has not provided a reliable indication of arcing at the wafer (wafer level arcing). Detection of wafer level arcing can be particularly difficult because of RF noise surrounding the wafer caused by RF power applied to the wafer support pedestal and, in some reactors, to RF power applied to an inductive coil on the chamber side wall. Another challenge is the large dynamic range of transients or noise caused by RF generator transitions called for by a process recipe, for example. Such transition-induced transients must be distinguished from transients caused by arcing at the wafer level. 
         [0003]    Plasma reactors typically have components within the reactor chamber that are consumed or degraded by their interaction with plasma. In a PVD reactor, the consumables may include the sputter target at the ceiling, an internal side wall coil and a process ring kit surrounding the wafer support pedestal including the electrostatic chuck (ESC). As such consumables degrade or are physically changed, they become more susceptible to arcing. The problem is how to determine when each consumable should be replaced before there is an arc. 
       SUMMARY 
       [0004]    A method is provided for monitoring arcing and providing control in a plasma reactor which may have plural plasma power applicators and plural power generators coupled to the power applicators and an electrostatic chuck with a wafer support surface. The method is performed while carrying out a plasma process in accordance with a process recipe having a succession of process steps. The method includes monitoring voltage or current transients at each of the power applicators and at the wafer support surface. For each process step and for each of the power applicators and the wafer support surface, the method includes determining an arc detection threshold lying above a noise level. The method further includes comparing each transient with the threshold determined for the corresponding power applicator or wafer support surface, and issuing an arc detect flag if the transient exceeds the threshold, the arc detect flag being associated with one of the power generators or the wafer support surface. The method further includes determining the location of the power applicator or wafer support surface for which a corresponding threshold value was exceeded by a transient, and displaying the location at a user interface. Power generators are turned off in response to the arc flag. In one embodiment, the displaying includes identifying a consumable in the chamber associated with the arc flag. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0006]      FIGS. 1A and 1B  depict a plasma reactor with bipolar and monopolar electrostatic chucks, respectively, having certain wafer level arc detection and automatic shutdown features. 
           [0007]      FIG. 2  is a schematic diagram depicting an RF current sensor circuit in the reactor of  FIG. 1A . 
           [0008]      FIG. 3  is a block diagram of a signal conditioner in the reactor of  FIG. 1A . 
           [0009]      FIG. 4  is a schematic diagram depicting an RF voltage sensor circuit in the reactor of  FIG. 1A . 
           [0010]      FIGS. 5A and 5B  are schematic diagrams of modifications of the embodiments of  FIGS. 1A and 1B , respectively, having a wafer level arc detecting circuit on an electrostatic chuck and employing a voltage sensor. 
           [0011]      FIGS. 6A and 6B  are schematic diagrams of modifications of the embodiments of  FIGS. 1A and 1B , respectively, having a wafer level arc detecting circuit on an electrostatic chuck employing a current sensor. 
           [0012]      FIGS. 7A and 7B  together constitute a flow diagram depicting the operation of a reactor controller in any of the foregoing embodiments. 
           [0013]      FIG. 8  depicts a retrofitting of the arc sensing and communication features of  FIG. 1A  into a reactor having a local area network. 
           [0014]      FIG. 9  depicts a retrofitting of the arc sensing and communication features of  FIG. 1A  into a reactor having a digital input/output network. 
           [0015]      FIG. 10  depicts a retrofitting of the arc sensing and communication features of  FIG. 1A  into a reactor having a D.C. safety interlock loop. 
       
    
    
       [0016]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings in the figures are all schematic and not to scale. 
       DETAILED DESCRIPTION 
       [0017]      FIG. 1A  depicts a PVD reactor having a system for intelligently sensing arcing at the wafer level. The reactor includes a chamber  100  defined by a cylindrical side wall  102 , a ceiling  104  and a floor  106 . Within the interior of the chamber  100  are provided a target  110  mounted on the ceiling  104 , an RF coil  112  mounted on the side wall  102  and a wafer support pedestal  114  extending upwardly from the floor  106 . A vacuum pump  116  evacuates the chamber  100  through a pumping port  118  in the floor  106 . A process gas supply  119  provides process gas (or gases) for introduction into the chamber  100 . 
         [0018]    The wafer support pedestal  114 , in one embodiment, may include an electrostatic chuck (ESC)  122  for holding a semiconductor wafer or workpiece  120  on a top surface of the pedestal  114 . The ESC  122  may consist of an insulating layer  124  resting on a conductive base  126 . In the embodiment of  FIG. 1A , the ESC  122  is a bipolar chuck, and there are two electrodes  128 ,  130  in the insulating layer  124 , and a conductive center pin  132  contacting the back side of the wafer  120 . A chucking voltage supply  134  imposes opposite but equal D.C. voltages between the center pin  132  and the electrodes  128 ,  130 .  FIG. 1B  depicts a variation of the embodiment of  FIG. 1A  in which the ESC  122  is a monopolar chuck and there is a single electrode  128  having a diameter corresponding, generally, to that of the workpiece or wafer  120 . In the embodiment of  FIG. 1B , the center pin  132  may not be present. Except for these differences, the embodiments of  FIGS. 1A and 1B  contain common structural features, and the description below of these common features with reference to  FIG. 1A  pertains to the embodiment of  FIG. 1B  as well, but are not repeated for the sake of brevity. 
         [0019]    Referring again to  FIG. 1A , D.C. power is applied to the sputter target  110  by a high voltage D.C. power generator  136 . Low frequency RF power is applied to the coil  112  through an RF impedance match  138  by an RF power generator  140 . The RF power generator  140  is connected to an RF input of the impedance match  138 . In one embodiment, the RF impedance match  138  may have a low power D.C. input (not shown) in addition to the RF input. In the embodiment of  FIG. 1A , RF bias power of a suitable frequency (such as low frequency and/or high frequency) is applied to the ESC electrodes  128 ,  130  through a bias impedance match  142  and blocking capacitors  144 ,  146  by an RF power generator  148 . An RF blocking filter  150  connected between the ESC electrodes and center pin  128 ,  130 ,  132  and the DC chucking voltage supply  134  isolates the chucking voltage supply  134  from RF power. In the embodiment of  FIG. 1B , RF bias power of a suitable frequency (such as low frequency and/or high frequency) is applied to the single ESC electrode  128  through the bias impedance match  142  and blocking capacitor  144  by the RF bias power generator  148 . 
         [0020]    Referring again to  FIG. 1A , a reactor controller  152  governs operation of all the active elements of the reactor. Specifically,  FIG. 1A  indicates the communication of ON/OFF commands from the controller  152  to each of the power generators  136 ,  140 , and  148 , to the gas supply  119 , to the vacuum pump  116  and to the ESC chucking voltage supply  134 . Although not shown in the drawing of  FIG. 1A , other active components of the reactor are likewise governed by the controller  152 , including coolant pumps, lid interlocks, lift pin actuators, pedestal elevation actuators, slit valve opening, wafer handling robotics, for example. 
       Wafer Level Arc Detection: 
       [0021]    Detecting plasma arcing at the wafer is difficult because of the presence of RF noise and harmonics and because of the large dynamic range of voltage or current transients at the wafer caused by non-arcing events (power generator transitions) and by arcing. These problems are overcome by sensing current or voltage changes at the RF bias power input to the ESC  122 . An RF sensor  154  is placed at (or connected to) an RF conductor  155  (e.g., the inner conductor of a 50 Ohm coaxial cable) running between the RF bias generator  148  and the RF bias impedance match  142 . In one embodiment, the RF sensor  154  is contained inside the impedance match  142  and is located at an internal coaxial output connector (not shown) to which the coaxial cable  155  is connected. The RF sensor  154  is capable of sensing an RF current or an RF voltage and generating a voltage signal proportional to the sensed current (or sensed voltage). The voltage signal is processed by a signal conditioner  156  to produce an output signal that has been filtered and peak-detected and scaled to a predetermined range. An arc detect comparator  158  compares the magnitude of the output signal to a predetermined threshold value. If this threshold is exceeded by the output signal, the arc detect comparator outputs an arc flag to the reactor controller  152 . The reactor controller  152  responds to the arc flag by shutting down active components of reactor such as the power generators  136 ,  140  and  148 . 
         [0022]    Referring now to  FIG. 2 , the sensor  154  may be an RF current sensor. In this embodiment, the sensor  154  includes a ferrite ring  160  encircling the RF conductor  155  and a conductive (e.g., copper) coil  162  wrapped around a portion of the ferrite ring  160 . One end  162   a  of the coil  162  may be allowed to float electrically, while the other end  162   b  is the output terminal of the sensor  154 . One advantage of the structure of the ferrite ring  160  and coil  162  is that the current through the coil  162  is weakly coupled to the RF current through the RF power conductor  155 . Therefore, the voltage induced on the coil  162  is attenuated and accordingly has a smaller dynamic range in response to transients or spikes in the current through the conductor  155 . A related feature is that the weakly coupling limits the amount of power or current drawn by the sensor  154  from the RF power in the conductor  155 . As a result, the sensor  154  places only a negligible load on the RF current in the conductor  155 . 
         [0023]      FIG. 3  depicts the different functions in the signal conditioner  156 . The signal conditioner  156  includes a peak detector  164 , an RF filter  166  for removing noise and providing a cleaner signal, a scaling circuit  168  for providing a predetermined range and a high impedance transducer  170  for controlling the signal amplitude range while providing a high impedance isolation between the output of the signal conditioner  156  and the sensor  154 . One embodiment of the signal conditioner  156  is illustrated in  FIG. 2 . In the embodiment of  FIG. 2 , the peak detector  164  is depicted as including a diode rectifier  164   a  and a capacitor  164   b . In other embodiments, the peak detector may include other circuit elements that provide an output level indicative of a true peak value. The RF filter  166  is depicted as a pi-network including a pair of shunt capacitors  166   a ,  166   b  and a series inductor  166   c . The scaling circuit  168  is depicted in  FIG. 2  as voltage divider consisting of a pair of resistors  168   a ,  168   b , whose output voltage is scaled down by the ratio of the resistance of the resistor  168   a  to the total resistance of the resistors  168   a  and  168   b . The transducer  170  is depicted in  FIG. 2  as including an operational amplifier  171  that provides an output signal within a range (e.g., 0-10 V) determined by the amplifier gain. The gain may be controlled by a variable feedback resistor  172  connected between the amplifier input and output. The amplifier  171  provides a high input impedance that isolates the signal conditioner  156  from a load placed on the signal conditioner output. 
         [0024]      FIG. 4  depicts an embodiment of the sensor  154  for sensing an RF voltage on the RF conductor  155 . The sensor consists of a resistor divider  154   a ,  154   b  connected directly between the conductor  155  and ground, the series resistance of the resistor divider  154   a ,  154   b  being very high (on the order of megOhms). This prevents any significant power diversion to ground. The resistor  154   a  is 10-100 times less resistive than the resistor  154   b , so that the voltage sensed by the peak detector  164  is very small compared to the voltage on the RF conductor  155 . This provides the sensor  154  with a high input impedance to avoid drawing an appreciable current from the RF conductor  155 . The signal conditioner  156  described above with reference to  FIGS. 2 and 3  may also be employed to condition the output signal of the RF voltage sensor  154  of  FIG. 4 . 
         [0025]      FIG. 5A  depicts a modification of the embodiment of  FIG. 1A  in which arc detection is performed at the ESC electrodes  128 ,  130 . The RF bias power generator  148  and RF bias impedance match  142  of  FIG. 1A  are not shown in the drawing of  FIG. 5A , although they may be present if RF bias power is applied to the ESC electrodes  128 ,  130 . Alternatively, no RF bias power is applied to the ESC electrodes  128 ,  130 . The sensor  154  of  FIG. 1A  is replaced in  FIG. 5A  by a voltage sensor  174 . The voltage sensor  174  is connected across the ESC center pin  132  (that is in contact with the semiconductor workpiece or wafer  120 ) and a reference point. The reference point may either be ground or one of the ESC electrodes  128  or  130 . The voltage sensor  174  is a differential amplifier, with its differential inputs connected to the center pin  132  and the reference point (e.g., ground). Voltage transients on the wafer  120  appear as a large difference between the inputs of the differential amplifier  174 . The output of the amplifier is proportional to this difference, and is furnished to the signal conditioner  156 . The output of the signal conditioner is tested by the arc detect comparator  158  by comparison with a predetermined threshold, as in the embodiment of  FIG. 1A . 
         [0026]      FIG. 5B  depicts a similar modification to the embodiment of  FIG. 1B , in which the sensor  154  of  FIG. 1B  is replaced in  FIG. 5B  by the differential amplifier  174 . In the embodiment of  FIG. 5B , the inputs to the differential amplifier  174  are connected to the single ESC electrode  128  and a suitable voltage reference such as ground. The RF bias power generator  148  and RF bias impedance match  142  of  FIG. 1B  are not shown in the drawing of  FIG. 5B , although they may be present if RF bias power is applied to the ESC electrode  128 . Alternatively, no RF bias power is applied to the ESC electrode  128 . Voltage transients on the wafer  120  appear as a large difference between the inputs of the differential amplifier  174 . The output of the amplifier is proportional to this difference, and is furnished to the signal conditioner  156 . The output of the signal conditioner is tested by the arc detect comparator  158  by comparison with a predetermined threshold, as in the embodiment of  FIG. 1A . 
         [0027]      FIG. 6A  illustrates a variation of the embodiment of  FIG. 5A  in which a current sensor  176  replaces the voltage sensor  174 . The current sensor  176  includes a ferrite ring  178  around the center conductor  132  and a conductive winding  180  around the ring  178 . One end  180   a  of the winding  180  is the output of the current sensor  176  and is connected to the input of the signal conditioner  156 .  FIG. 6B  illustrates a similar variation of the embodiment of  FIG. 5B  in which a current sensor  176 ′ replaces the voltage sensor  174 . The current sensor  176 ′ includes a ferrite ring  178 ′ around the conductor connected to the single ESC electrode  128 , and a conductive winding  180 ′ around the ring  178 . One end  180   a ′ of the winding  180 ′ is the output of the current sensor  176 ′ and is connected to the input of the signal conditioner  156 . 
         [0028]    Referring again to  FIG. 1A , a second RF sensor  184  is coupled to an RF power conductor  185  connected between the RF generator  140  and the RF impedance match  138  for the side wall coil  112 . The second RF sensor  184  may be an RF current sensor, as in  FIG. 2 , or an RF voltage sensor, as in  FIG. 4 . The output of the second RF sensor  184  is applied to a second signal conditioner  186  that may be the same type of circuit as the signal conditioner  156  of  FIGS. 2 and 3 . A second arc detect comparator  188  compares the output of the signal conditioner with a certain threshold value to determine whether an arc has occurred. If an arc has occurred, the comparator  188  generates an arc flag that is sent to the controller  152 . A third sensor  190  is coupled to the output of the D.C. power generator  136 . The output of the third sensor  190  may be applied to a third signal conditioner  192 . A third arc detect comparator  194  compares the output of the signal conditioner  192  with a certain threshold to determine whether an arc has occurred. Its output, an arc flag, is transmitted to the process controller  152 . 
         [0029]    The controller  152  may include a memory  152   a  for storing a sequence of instructions and a microprocessor  152   b  for executing those instructions. 
         [0030]    The instructions represent a program that may be downloaded into the controller memory  152   a  for operating the reactor. In accordance with one feature, the program requires the controller  152  to shut off the power generators  136 ,  140 ,  148  in response to receipt of an arc flag from any of the arc detect comparators  158 ,  188  or  194 . This program will be discussed in greater detail in a later portion of this specification. 
       Operation of the Process Controller: 
       [0031]    Operation of the process controller  152  of  FIG. 1A  is depicted in the flow diagram of  FIGS. 7A AND 7B . The process recipe may be downloaded into the controller memory  152   a  (block  300  of  FIGS. 7A AND 7B ). The reactor component history (e.g., the number of use hours for each consumable in the reactor) may also be loaded into the controller memory  152   a  (block  302 ). The controller then starts the process in the reactor (block  304 ). For the current process step, the controller  152  notes the RF power settings called for by the recipe (i.e., the RF power applied to the ESC  122  and the RF power applied to the coil  112 . From these power settings, the controller  152  predicts an RF noise level encountered by each of the sensors  154 ,  184  and  190 . For each sensor, the controller  152  determines from the noise level an appropriate arc detection comparison threshold for each of the comparators  158 ,  188  and  194  (block  306  of  FIGS. 7A AND 7B ). For the particular sensor, a sensed voltage (or current) level exceeding the assigned threshold is considered to be an arc event. Optionally, the controller  152  may also define a warning level threshold that is below the arc detection threshold. 
         [0032]    After the comparison threshold has been defined for each of the sensors  154 ,  184 ,  190 , each threshold is revised in accordance with the age of the associated reactor consumable components (block  308 ). This may be done in accordance with historical data representing typical lifetimes of each consumable component in the reactor. For example, the sensor  154  detects arc events closest to the wafer  120 . These are most likely affected by the condition of consumable components closest to the wafer, such as a process ring kit surrounding the ESC (not shown in  FIG. 1A ), for example. Accordingly, the arc detect comparison threshold for the sensor  154  are revised depending upon the age of the process ring kit. Likewise, the sensor  184  detects arc events at the side wall coil  112 . Therefore, the thresholds chosen for the sensor  184  are revised based upon the age of the coil  112 , for example. Typically, this revision causes the threshold to increase with consumable age or hour usage, because as the consumable wears and its surface becomes rougher during exposure to plasma, it tends to experience or promote more RF noise and harmonics. This revision of the arc detection threshold based upon age may be performed based upon empirical data representing the histories of a large sample of the consumable component. 
         [0033]    In one embodiment, the next step is to determine whether the upwardly adjusted threshold is at or too near the expected voltage or current level of a real arc event (block  310 ). If so (YES branch of block  310 ), this fact is flagged (block  311 ) to the user and/or to the process controller  152 . In one embodiment, this flag may cause the process controller  152  to shut down the reactor. The location of the sensor whose threshold has become excessive in this way is identified, and the reactor consumables closest to that detector are identified to the user as being due for replacement. 
         [0034]    Provided the adjusted thresholds are not excessive (NO branch of block  310 ), they are then sent to the arc detection comparators  158 ,  188  and  194  for use during the current process step (block  312 ). From the process recipe, the controller  152  can identify the specific times of occurrence of process-mandated transients (block  314 ), such as the activation or deactivation of an RF power generator. During the performance of the current process step, the controller  152  monitors each of the arc detection comparators  158 ,  188  and  194  for arc flags (block  316 ). Each of the comparators  158 ,  188 ,  194  constantly compares the output of the respective signal conditioner  156 ,  186 ,  192  with the threshold received from the controller  152  for the current process step. Whenever the signal conditioner output exceeds the applicable threshold, the comparator transmits an arc flag to the controller  152 . The controller  152  may sample the comparator outputs at a rate of 30 MHz, for example. For each sample of each comparator  158 ,  188 ,  194 , a determination is made whether an arc flag has issued (block  318 ). If no flag is detected (NO branch of block  318 ), then the controller  152  determines whether the current process step has been completed (block  320 ). If not (NO branch of block  320 ), the controller  152  returns to the monitoring step of block  316 . Otherwise (YES branch of block  320 ) the controller  152  transitions to the next process step in the recipe (block  322 ) and loops back to the step of block  306 . 
         [0035]    If an arc flag is detected (YES branch of block  318 ), then it is determined whether the sensed voltage or current transient merely exceeded the warning threshold or whether it exceeded the arc threshold level (block  324 ). This is determined from the contents of the flag issued by the particular comparator. If it was a warning level only (YES branch of block  324 ), then the controller  152  records the event and associates the event with the current wafer (block  326 ). The controller  152  then determines whether the number of warnings for the current wafer is excessive (block  328 ). If the number of warnings for that wafer exceeds a predetermined number (YES branch of block  328 ), a flag is issued (block  330 ) and the controller  152  may shut down the reactor. Otherwise (NO branch of block  328 ), the controller returns to the step of block  316 . 
         [0036]    If the arc flag was for a full arc event in which the arc threshold was exceeded (NO branch of block  324 ), then a determination is made (block  332 ) as to whether it coincided with a power transition time identified in the step of block  314 . If so (YES branch of block  332 ), the flag is ignored as a false indication (block  334 ) and the controller loops back to the monitoring step of block  316 . Otherwise (NO branch of block  332 ), the arc flag is treated as valid. The controller  152  uses the contents of the arc flag to identify and record in memory the location of the sensor that sensed the arc event (block  338 ). The controller issues “OFF” commands to each of the power generators  136 ,  140  and  148  (block  340 ). The arc flag may embody digital information identifying the particular comparator that issued the arc flag. This information is output by the controller  152  to a user interface, which can correlate sensor location with consumable components (block  342 ). This feature can enable the user to better identify consumable components in the reactor chamber that need to be changed. For example, if the controller  152  determines that the arc flag was issued by the comparator  188 , then it identifies the consumable component closest to the RF power monitored by the comparator, namely the side wall coil  112 . For an arc flag issued by the comparator  158 , the closest consumable components are those surrounding the wafer, particularly the process ring kit, and the controller  152  would associate such an arc flag with the process ring kit, for example. For an arc flag issued from the comparator  194 , the relevant component is the ceiling target, and the controller would associate such an arc flag with the ceiling target. Thus, the controller  152  in one embodiment can provide the user different lists of possible candidate consumables for replacement for different arc flag events. 
         [0037]    The process depicted in  FIGS. 7A AND 7B  includes, in one embodiment, dynamic adjustment of the arc detection comparison threshold for each step in the plasma process. The threshold is further adjusted based upon consumable component age. The controller  152  updates the thresholds in each of the comparators  158 ,  188 ,  194  as often as necessary. By such dynamic adjustment of the comparison thresholds, the sensitivity of each comparator  158 ,  188 ,  194  is optimized by seeking the minimum threshold that can be used in the environment of a particular wafer process step. The threshold is adjusted downwardly whenever noise conditions (for example) improve, and is adjusted upwardly when noise level increases, due to an increase in RF power level, for example. Such a threshold increase avoids false arc indications that can arise when the noise level approaches the arc detection threshold level. In an embodiment, the process of  FIGS. 7A AND 7B  further includes performing arc location identification and corresponding identification of the likeliest consumable components involved in the arc event. The controller  152  communicates this information to the user, to facilitate easier management of consumables and more accurate selection of consumables needing replacement. 
         [0038]    In one embodiment, the process of  FIGS. 7A AND 7B  is embodied in software instructions downloaded into the controller memory  152   a . In this embodiment, therefore, all the intelligent actions are performed by the controller  152 , while the arc detection comparators simply perform a comparison function. However, in another embodiment, the arc detection comparators  158 ,  188 ,  194  may include their own internal processors and memories, enabling them to perform some of the functions in the process of  FIGS. 7A AND 7B . 
       Retrofit Control and Communication: 
       [0039]    The process of  FIGS. 7A AND 7B  involves frequent two-way communication between the controller  152  and each of the arc detection comparators  158 ,  188 ,  194 . The controller  152  periodically transmits updated comparison threshold values to particular ones of the comparators  158 ,  188 ,  194 , different values being downloaded to different comparators. The comparators  158 ,  188 ,  194  transmit arc flags whenever an arc is detected. The arc flag includes the identity of the individual comparator that issued it. The controller further transmits shutdown (ON/OFF) commands to the power generators  136 ,  140  and  148  in response to a valid arc flag from any of the arc detection comparators  158 ,  188 ,  194 . It is intended that the arc detection features of  FIG. 1A  (as implemented in the process of  FIGS. 7A and 7B ) be installed on plasma reactors already operating in the field. For reactors already installed in the field, installation onto each reactor of a custom communication network to meet each of the foregoing communication needs would be prohibitively costly. To reduce costs, communication systems already existing on such reactors are exploited. In some cases, the existing communication systems are able to meet and facilitate all of the communication needs of the arc detection features of  FIGS. 1 and 7 . 
         [0040]    In some reactors, a local area network (LAN) is provided in which the controller communicates via the LAN with every active device and sensor on the reactor.  FIG. 8  illustrates the structure of such a LAN in a reactor of the type depicted in  FIG. 1A . For each active device to be governed by the controller  152 , an interface device is coupled to it. The interface device converts received digital commands to actions that shut down the active device. For example, in  FIG. 8 , interface devices  355 ,  357 ,  359  are connected to respective power generators  136 ,  140 ,  148 . The interface devices are capable of shutting down the generators in response to received digital commands. A local area network (LAN)  360  is provided. The LAN is a multiple conductor communication channel or cable having multiple I/O ports  361 ,  362 ,  363 ,  364 ,  365 ,  366 ,  367 , which may be implemented as multiconductor connectors. Each device that is to communicate on the LAN  360  has a memory and limited processing capability that permits it to store and issue a unique address on the LAN  360 . Thus, each control interface  355 ,  357 ,  359  and each comparator  158 ,  188 ,  194  has conventional process circuitry that responds to LAN protocols and stores its own device address. Each device  158 ,  188 ,  194 ,  355 ,  357 ,  359  on the LAN responds only to received communications that are addressed to its device address. Furthermore, each device attaches its device address to its data transmissions on the LAN. Each of the devices  158 ,  188 ,  194 ,  355 ,  357 ,  359  is coupled to the LAN  360  at a unique one of the ports  361 ,  362 ,  363 ,  364 ,  365 ,  366 ,  367  via its own multiconductor cable  371 ,  372 ,  373 ,  374 ,  375 ,  376 ,  377 , respectively. On reactors already installed in the field having such a LAN, there may be no comparators  158 ,  188 ,  194 . Therefore, the arc detection system communication features of  FIG. 1A  are realized in such a reactor by identifying spare (unused) ports on the existing LAN  360  (e.g., the ports  363 ,  365  and  366 ) and connecting the comparators  158 ,  188 ,  194  to them in the manner depicted in  FIG. 8 . 
         [0041]    The device addresses of all the devices on the LAN  360  (i.e., the comparators  158 ,  188 ,  194  and the control interfaces  355 ,  357 ,  359 ) may be intelligently assigned by the controller  152  upon activation of the LAN  360 , using conventional techniques. In the system of  FIG. 8 , the controller  152  carries out the process of  FIGS. 7A AND 7B  by sending an individual communication addressed to an individual comparator with instructions to download a certain threshold value, for example. Each comparator responds to an arc event by transmitting a communication addressed to the controller  152  and containing the comparator&#39;s device address and a message signifying occurrence of an arc event. The controller  152  can respond to a valid arc event by transmitting a communication addressed to each of the power generator control interfaces  355 ,  357 ,  359  containing a command to shut down the corresponding generator. The location of the sensor that caused the arc flag to be issued is deduced by the controller  152  from the device address of the corresponding arc detect comparator. The controller  152  can provide this information to the user at a user interface  153  of the controller  152 . 
         [0042]    In other reactors, no LAN is provided or available, and instead a custom communication digital input/output (DI/O) network is provided as depicted in  FIG. 9 . In the DI/O network of  FIG. 9 , each device communicates with the controller  152  (and vice versa) over a communication channel dedicated to that device. The DI/O network on pre-existing reactors employs respective DI/O relays  401 ,  402 ,  404 ,  406  that individually communicate with the controller  152 , and monitor individual safety points. Specifically, for example, the DI/O relay  401  signals whenever a lid  101  of the chamber  100  is opened, the DI/O relay  402  signals whenever the RF power cable to the side wall coil is disconnected, the DI/O relay  404  signals whenever the RF bias power cable is disconnected and the DI/O relay  406  signals whenever the D.C. power cable to the ceiling target is disconnected. The controller  152  receives the signals from these relays at inputs A, B, D, and F, as indicated in the drawing of  FIG. 9 . The controller  152  transmits shutdown (ON/OFF) commands to each of the power generators  136 ,  148  and  140  via dedicated communication channels J, K and L, as indicated in  FIG. 9 . 
         [0043]    The communication features of  FIG. 1A  may be implemented in the DI/O network of  FIG. 9  provided there are three DI/O relays that can be spared for use with the arc detect comparators  158 ,  188  and  194 . As shown in the example of  FIG. 9 , pre-existing DI/O relays  403 ,  405  and  407  are appropriated for connection to the outputs of the arc detect comparators  188 ,  158  and  194 , respectively. Each time one of the arc detect comparators  158 ,  188  or  194  issues an arc flag, the DI/O relay attached to it signals the controller  152 . The controller  152  deduces the identity of the arc detect comparator that issued the arc flag from the location of the wire or channel carrying the signal. This information may be furnished to the controller&#39;s user interface  153  for use in managing consumable replacement. 
         [0044]    Earlier model reactors currently installed in the field may have neither a LAN nor a DI/O network. In such reactors, the arc detection system of  FIG. 1A  may be implemented in a basic form by exploiting a 24 volt safety interrupt circuit provided on such reactors. This circuit ensures immediate shut down of the power generators whenever the chamber lid is opened or whenever a power cable connection to the chamber is interrupted. Referring to  FIG. 10 , the power generator  136  has an interlock  501 , the power generator  148  has an interlock  502  and the power generator  140  has an interlock  503 . Each generator  136 ,  148  and  140  can operate only if its interlock constantly senses a 24 Volt DC potential on a circuit conductor  504 . The circuit conductor  504  connects all of the interlocks  501 ,  502 ,  503  in series with a 24 VDC supply  506 . The series circuit conductor  504  is interrupted by several simple switch relays  510 ,  512 ,  514 ,  516 ,  518 ,  520  and  522 . Therefore, each relay by itself can sever the series connection of the 24 volt supply  506  to the generator interlocks  501 ,  502 ,  503 , thereby shutting down the reactor. The relay  510  opens its connection whenever the chamber lid  101  is opened. The relay  512  opens its connection whenever the RF power cable connection to the side wall coil  112  is interrupted. The relay  516  opens its connection whenever the RF power cable to the ESC  122  is interrupted. The relay  522  opens its connection whenever the RF power cable connection to the ceiling target is interrupted. The chamber  100  may be automatically shut down in response to arc detection by any of the three arc detect comparators  158 ,  188  and  194  provided there are three spare relays connected in series along the circuit conductor  504  and available to accept the outputs of respective ones of the comparators  158 ,  188  and  194 .  FIG. 10  shows that three such relays, namely the relays  514 ,  518  and  520 , may be connected to the outputs of the comparators  188 ,  158  and  194 , respectively. Whenever any one of the comparators  188 ,  158 ,  194  senses a voltage (or current) exceeding its predetermined threshold, it issues an arc flag in the form of a voltage that causes the corresponding relay ( 514 ,  518  or  520 , respectively) to open its connection. This interrupts the 24 volt circuit of the conductor  504 , causing each of the interlocks  501 ,  502 ,  503  to disable the associated power generator ( 136 ,  148  and  140 , respectively). 
         [0045]    While the embodiment of the RF impedance match  138  illustrated in  FIG. 1A  has a single RF input and an RF output, in another embodiment the RF impedance match  138  may have, in addition, a low power D.C. input (not shown in the drawings). In such a case, an additional arc sensor and threshold comparator of the type described above may be coupled to the unillustrated low power D.C. input of the RF impedance match  138 . 
         [0046]    While the reactor of  FIG. 1A  or  1 B has been described as detecting an arc based upon the output of a single one of the various sensors  154 ,  184 ,  190 , etc., the decision may instead be based upon the outputs of several (or possibly all) of the sensors. For example the controller  152  of the embodiment of  FIG. 1 ,  8  or  9  has been described as responding to an arc event based upon the output of any single one of the sensors  154 ,  184  or  190  through the corresponding comparator  158 ,  186  or  194 , respectively. However, in one embodiment, the controller  152  of  FIG. 1 ,  8  or  9  is programmed to combine the outputs of at least two (or more) of the threshold comparators  158 ,  186 , and make a decision based upon the combined signals. The output signals may be combined by the processor  152  through a linear, polynomial, or more complex mathematical function. In this case, the controller  152  would be programmed to respond to the combined signal to determine whether an arc was detected or to determine whether to shut down the reactor. In yet another embodiment, the individual outputs of the sensors  154 ,  184 ,  190  may be combined before being processed by a threshold comparator. The individual output signals from at least two of the sensors  154 ,  184 ,  190  may be combined through a linear, polynomial, or more complex mathematical function. The resulting combined signal is then fed to a single comparator (e.g., the comparator  186 ), and the output of that single comparator is fed to the controller  152 . 
         [0047]    While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.