Abstract:
An RF current probe is encapsulated in a conductive housing to permit its placement inside a plasma reactor chamber. An RF voltage probe is adapted to have a long coaxial cable to permit a measuring device to be connected remotely from the probe without distorting the voltage measurement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 13/052,280 filed Mar. 21, 2011 entitled IN-SITU VHF VOLTAGE SENSORS FOR A PLASMA REACTOR, by Hiroji Hanawa et al., which claims the benefit of U.S. Provisional Application Ser. No. 61/392,121, filed Oct. 12, 2010 entitled IN-SITU VHF VOLTAGE/CURRENT SENSORS FOR A PLASMA REACTOR, by Hiroji Hanawa, et al. 
     
    
     BACKGROUND 
       [0002]    Accurate real-time measurements of RF voltage and RF current at selected locations in a plasma reactor are highly sought after, in the processing of workpieces such a semiconductor substrate or wafer. The RF frequencies involved in such measurements depend upon the type of plasma reactor, and may lie in the very high frequency (VHF) range from 30 MHz to 300 MHz, for example, or any other RF frequency range. Such measurements are essential for process control, tool maintenance and process design in semiconductor product fabrication of ultra large scale integrated circuits, solar panels, plasma displays, photolithographic masks, and the like. Processes in which such measurements are needed include reactive ion etching of dielectric materials, conductive or semi-conductive materials and organic materials such as photoresist. Other processes where such measurements are needed include plasma-enhanced chemical vapor deposition, plasma-enhanced physical vapor deposition, and the like. 
         [0003]    Such measurements may be accomplished using an RF current probe. An RF current probe typically includes a pick-up coil connected across a primary winding. A secondary winding provides an output voltage representative of the measured RF current near the pick-up coil. The RF current probe includes or is coupled to a signal measuring device. The signal measuring device is connected across the secondary winding. Such a signal measuring device may include signal-conditioning or analog-to-digital converter circuits, for example. Alternatively, or in addition, the signal measuring device may include an oscilloscope. The probe cannot be placed inside the plasma reactor chamber without compromising or damaging its components from exposure to plasma during processing. Therefore, permanent location of such an RF current probe is typically confined to locations outside of the chamber or on exterior chamber surfaces. 
         [0004]    Alternatively or in addition, such measurements may be accomplished using an RF voltage probe. An RF voltage probe includes a floating electrode serving as a conductive sensor head connected to a passive network of capacitors, that is, a capacitive voltage divider network. The capacitive voltage divider network is connected at an output node to a signal measuring device. Such a signal measuring device may include signal-conditioning or analog-to-digital converter circuits, for example. Alternatively, or in addition, the signal measuring device may include an oscilloscope. The voltage of the sensor head reflects the local RF electric field near the sensor head, as desired. Unfortunately, it is highly sensitive to the load impedance of the signal measuring device and of the signal path (e.g., a cable) connected from the output node to the signal measuring device. In order to avoid distortion of the measured voltage due to the load impedance of the signal path, the measuring device must be placed very close to (e.g., next to) the capacitive voltage divider network, to minimize the signal path length. Typically, the capacitive voltage divider network is sufficiently close to the conductive sensor head so that they separated by less than centimeter. The measuring device and the capacitive voltage divider network typically may be within two centimeters of one another, to minimize the signal path length and thereby minimize the distortion of the voltage on the sensor head. Distortion arises because the scope end of the coaxial cable is best terminated in a 50 Ohm termination resistor to avoid reflection of the RF signal at this end of the cable. This set up renders the input impedance of the coaxial cable so low as to distort the voltage on the sensor head. Therefore, the combination of the voltage probe and the measuring device constitute an assembly that is not separable. Unfortunately, the measuring device adds such bulk to the entire assembly. As a result, the RF voltage probe and assembly (including the measuring device) cannot be placed inside the plasma reactor chamber. Thus, there has seemed to be no way in which to obtain precise accurate RF measurements inside a plasma reactor chamber. 
       SUMMARY 
       [0005]    In accordance with one embodiment, an RF voltage probe has a coaxial cable and a circuit including a sensor head or conductive electrode, an output terminal along with an amplifier having very high input impedance and a very low output impedance near the characteristic impedance of said coaxial cable. The said sensor head is coupled to said input of said amplifier, and said output of said amplifier is coupled to said inner conductor of said coaxial cable. The circuit is contained within a conductive housing, said conductive housing having a front opening facing said conductive electrode, an RF-transparent window covering said front opening, and a rear opening receiving the near end of the coaxial cable. The remote end of the coaxial cable may be connected to a remote measuring device. The outer conductor of said coaxial cable is in electrical contact with said conductive housing. 
         [0006]    In accordance with another embodiment, an RF current probe has a coaxial cable including an inner conductor and a cylindrical outer conductor and a circuit including a pick-up coil with a first center tap, a primary winding connected across said pick-up coil and having a second center tap connected to said first center tap, and a secondary winding having one end connected to said inner conductor and an opposite end coupled to said outer conductor. The circuit is contain in a conductive housing comprising a front opening facing said pick-up coil, an RF-transparent window covering said front opening, and a rear opening. The coaxial cable has a near end extending into said rear opening, and a remote end connectable to a measuring device, said outer conductor of said coaxial cable being in electrical contact with said conductive housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    So that the manner in which the exemplary embodiments of the present 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 appreciated that certain well known processes are not discussed herein in order to not obscure the invention. 
           [0008]      FIG. 1  is a diagram depicting an RF current probe in accordance with one embodiment. 
           [0009]      FIG. 2  depicts a perspective view of a housing assembly of the probe of  FIG. 1 . 
           [0010]      FIGS. 3A and 3B  depict use of the probe of  FIGS. 1 and 2  to measure RF current flowing through an RF coaxial conductor. 
           [0011]      FIG. 4  is a diagram depicting an RF voltage probe in accordance with another embodiment. 
       
    
    
       [0012]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the appended drawings illustrate only exemplary 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. 
       DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates an RF current probe in accordance with one embodiment. The probe is contained in a metal housing  80  and includes a pick-up coil  100  and a primary winding  105  connected across the pick-up coil  100 . The pick-up coil  100  has a center tap  110  connected to ground and the primary winding  105  has a center tap  115  connected to ground. The grounded center taps  110 ,  115  promote common mode suppression of RF electric field effects. A secondary winding  120  is inductively coupled to the primary winding  105  and has a first end  120 - 1  connected to an output node  125 , and a second end  120 - 2  that is connected to ground. A coaxial cable  130  has a center conductor  132  connected at one end to the output node  125  and an outer conductor  134  connected to ground. A signal measuring device  140  is connected to the opposite end of the coaxial cable  130 , at which the coaxial cable  130  may be terminated in a termination resistor  141 , such as a 50 Ohm resistor. As depicted in  FIG. 1 , the signal measuring device  140  includes signal processing or conditioning devices such as a signal conditioner  142 , an analog-to-digital converter  144  and a processor  146 . The measuring device  140  may include, in addition or alternatively, an oscilloscope  148 . 
         [0014]    The metal housing  80  may be of any suitable shape. A round shape may be preferable for use in an environment with a high RF electrical field. However, as depicted in  FIG. 2 , the metal housing  80  may be cylindrical in shape as defined by a cylindrical side wall  80 - 1 , and may be only a few centimeters in length and diameter. Referring again to  FIG. 1 , the housing  80  includes at its front end  80   a  a sensor opening  150  adjacent the pick-up coil  100 . The sensor opening  150  may be concentric with the cylindrical housing  80 . The sensor opening  150  is covered by a layer of material forming a window  155  that is transparent to electromagnetic radiation, such a high temperature glass material or the like. The housing further includes at its back end  80   b  an opening  160  for access by the coaxial cable  130 . The housing  80  may be grounded and connected to the outer conductor  134  of the coaxial cable  130 . For example, the edge of the opening  160  may be electrically connected to the outer conductor  134  of the coaxial cable  130 . As shown in  FIG. 2 , the back end  80   b  of the housing  80  may be tapered or conical in shape. 
         [0015]    The probe circuitry including the pick-up coil  100 , the primary and secondary windings  105 ,  120  and the output node  125  may be implemented as an integrated circuit, printed circuit board or surface mount structure or a combination of any of these or similar implementations. For example, the probe circuitry  100 ,  105 ,  120 ,  125  may be implemented on a single substrate  162  as an integrated circuit, printed circuit board or surface mount device. The substrate  162  may be planar and fit inside the housing  80  in the manner depicted in  FIG. 2 , with the pick-up coil  100  being adjacent the window RF-transparent  155 . 
         [0016]      FIGS. 3A and 3B  illustrate how to measure RF current in a component of a plasma processing reactor chamber, such as a coaxial cable  175 . The coaxial cable  175  has a center conductor  170 , an outer conductor  180  and an interior dielectric sleeve  185  separating the inner and outer conductors  170 ,  180 . For purposes of the measurement, an opening  190  is formed in the outer conductor  180  adjacent the probe&#39;s RF-transparent window  155 , exposing a small portion of the interior dielectric sleeve in the vicinity of the probe&#39;s RF-transparent window  155 . The probe housing  80  is inserted toward (or partially into) the opening  190 . The RF-transparent window  155  may be adjacent or contacting the exposed outer surface of the dielectric sleeve  185 . The sensor head  100  is thereby inductively coupled to the inner coaxial conductor  170  through the interior dielectric sleeve  185 . As depicted in  FIG. 3B , the RF magnetic field of the coaxial cable  175  is parallel to the axis of the pick-up coil  100 . The RF E-field is perpendicular to the coil axis. 
         [0017]    Preferably, the RF current probe of  FIGS. 1 and 2  is placed outside the vacuum chamber of an RF plasma reactor and operated at atmospheric pressure. However, the housing  80  may be hermetically sealed to allow the probe to be used inside the vacuum chamber. The grounded conductive housing  80  enables the RF voltage probe of  FIG. 1  to be inserted into the interior of a plasma reactor chamber during processing by protecting the probe circuitry  160  from the effects of exposure to plasma. 
         [0018]      FIG. 4  illustrates a voltage probe in accordance with another embodiment. The voltage probe of  FIG. 3  is implemented as a circuit  200  implemented as a substrate within a metal housing  80 ′ similar to the housing  80  of  FIG. 2 . The circuit  200  includes a floating electrode embodied within a sensor head  205 , a capacitive voltage divider network adjacent the sensor head  205  and consisting of at least one voltage divider capacitor  207  connected between the sensor head  205  and the housing  80 ′ (or RF ground). The circuit  200  further includes an impedance transformation buffer  210  having an input connected to the sensor head  205 . The output of the impedance transformation buffer  210  is connected at an output node  215  to the center conductor  219  of a long external coaxial cable  220 . The coaxial cable  220  has an outer conductor  221  connected to the metal housing  80 ′. The opposite end of the coaxial cable  220  is coupled to a remote measuring device  225 . The coaxial cable  220  is sufficiently long so that, in the case in which the probe housing  80 ′ is located on the interior side of a reactor chamber wall, the coaxial cable  220  may pass through an opening in the chamber wall to reach the remote measuring device  225 , which may be located at any convenient location external of the chamber regardless of distance without loading the sensor head  205  or distorting the RF voltage measurement. This ability to tolerate such a long signal path is made possible by the high input impedance of the impedance transformation buffer  210 , which will be discussed in greater detail below. 
         [0019]    In order to prevent the relatively low impedance of the long coaxial cable  220  from loading the sensor head  205  or distorting the voltage sensed at the sensor head  205 , the impedance transformation buffer  210  presents a very high load impedance to the sensor head  205  on the order of hundreds of 100 MegaOhms to GigaOhms (or greater). At the same time, the impedance transformation buffer  210  presents a very low input impedance to the coaxial cable  220  (e.g., within a factor of ten of the characteristic impedance of the coaxial cable  220 ). The advantage of such a high load impedance on the sensor head  205  presented by the impedance transformation buffer  210  is that the coaxial cable  220  and the measuring device  225  do not draw current from or otherwise load the sensor head  205 , and therefore do not distort the RF voltage sensed by the sensor head  205 . 
         [0020]    The impedance transformation buffer may be realized as an operational amplifier or a differential amplifier. In one example, the impedance transformation buffer  210  was realized as a differential amplifier having unity gain. Hereinafter, the impedance transformation buffer  210  may be referred to as a differential amplifier. Any suitable combination of amplifiers with suitable gain may be employed as long as the input impedance is kept high. In the illustrated example, the sensor head  205  is connected to the positive differential amplifier input  210   a,  and feedback from output  210   b  of the differential amplifier  210  is connected as feedback to the negative differential amplifier input  210   c.  Alternatively, a differential amplifier consisting of two or three operational amplifiers with a high input impedance can be adopted, in which its positive and negative inputs are connected to the sensor head  205  and the housing  80 ′, respectively 
         [0021]    In order to provide the required positive and negative bias voltages to operate the impedance transformation buffer or differential amplifier  210 , an external 5-volt D.C. power supply  240  outside of the housing  80 ′ has its +5 volt DC output node  240   a  coupled through an RF suppression or choke inductor  245  to the coaxial cable center conductor  219 . The negative or D.C. return terminal of the D.C. power supply  240  is connected to the coaxial cable outer conductor  221  or to ground. In this way, a +5 volt DC bias voltage is available on the substrate or circuit  200  from the D.C. power supply  240  via the cable center conductor  219  and via the output node  215 . This +5 volt DC bias voltage is coupled to a positive bias voltage supply terminal  210   d  of the differential amplifier  210  through an RF-choke inductor  250 . The RF-choke inductor  250  is connected between the output node  215  and the supply terminal  210   d.  The +5 volt D.C. bias voltage (received through the choke inductor  250 ) is also coupled to a D.C. inverter  255 . The D.C. inverter  255  has a −5 volt DC output  255   a  connected to a negative 5 volt DC bias voltage supply terminal  210   e  of the differential amplifier  210 . A high pass D.C.-blocking capacitor  260  prevents the D.C. voltage superimposed on the inner coaxial cable conductor  219  from reaching the differential amplifier  210  or the sensor head  205 . 
         [0022]    A ripple suppression capacitor  264  is connected between the differential amplifier positive bias supply terminal  210   d  and ground. Another ripple suppression capacitor  266  is connected between the differential amplifier negative bias supply terminal  210   e  and ground. The combination of the RF choke inductor  250  and the ripple suppression capacitors  264 ,  266  prevents or minimizes coupling of RF voltage on the coaxial center conductor  219  to the D.C. power supplied to the differential amplifier  210 . 
         [0023]    The impedance transformation buffer  210  may be implemented using an operational amplifier having a broad band response, or a 3 dB roll-off bandwidth of 200 MHz or more, and having a high input impedance (100 MegaOhms to GigaOhms or above) and a low output impedance (within a factor of ten of 50 Ohms). The impedances referred to here are the magnitude of the complex impedance. 
         [0024]    As depicted in  FIG. 4 , the far end of the coaxial cable  220  is connected to a signal measuring device  225 , and is terminated in a 50 Ohm termination resistor  141 . The signal measuring device  225  includes signal processing or conditioning devices such as a signal conditioner  142 , an analog-to-digital converter  144  and a processor  146 . The measuring device  140  may include, in addition or alternatively, an oscilloscope  148 . 
         [0025]    The metal housing  80 ′ may be of any suitable shape. A round shape may be preferable for use in an environment with a high RF electrical field. For example, the metal housing  80 ′ may be cylindrical in shape as defined by a cylindrical side wall  80 ′- 1 , and may be only a few centimeters in length and diameter. The housing  80 ′ includes at its front end  80 ′ a  a sensor opening  150  adjacent the sensor head  205 . The sensor opening  150  may be concentric with the cylindrical housing  80 ′. The sensor opening  150  is covered by a layer of material forming a window  155  that is transparent to electromagnetic radiation. The material may be a high temperature glass material or the like. The housing  80 ′ further includes at its back end  80 ′ b  a cable opening  160  for access by the coaxial cable  220 . The housing  80 ′ may be grounded and connected to the outer conductor of the coaxial cable  220 . For example, the edge of the opening  160  may electrically contact the outer conductor  221  of the cable  220 . The back end  80 ′ b  of the housing  80 ′ may be tapered or conical in shape. 
         [0026]      FIG. 4  illustrates how to measure RF voltage in a component of a plasma processing reactor chamber, such as a coaxial cable  175 . The coaxial cable  175  has a center conductor  170 , an outer conductor  180  and a dielectric sleeve  185  between them. For purposes of the measurement, an opening  190  is formed in the outer conductor  180  adjacent the probe&#39;s RF-transparent window  155 , exposing a small portion of the interior dielectric sleeve  185  in the vicinity of the probe&#39;s RF-transparent window  155 . The probe housing  80 ′ is inserted toward or into the opening  190 . The RF-transparent window  155  may be adjacent or contacting the exposed outer surface of the dielectric sleeve  185 . The sensor head  205  is thereby capacitively coupled to the inner coaxial conductor  170  through the dielectric sleeve  185 . 
         [0027]    While the foregoing is directed to embodiments of the present 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.