Patent Publication Number: US-2005121423-A1

Title: Heating in a vacuum atmosphere in the presence of a plasma

Description:
FIELD OF THE INVENTION  
      The present invention relates to a method of heating in a vacuum atmosphere in the presence of a plasma. From a more general aspect the invention also relates to a method of avoiding arcing in a vacuum atmosphere in the presence of a plasma.  
     BACKGROUND OF THE INVENTION  
      Heating in a vacuum atmosphere is often required, by way of a first example, for heating a substrate in a vacuum deposition system. Continuing this first example, the substrate is wound from an unwinding supply roll in a vacuum chamber and is guided through subsequent deposition or coating steps before being wound on a winding roll in the vacuum chamber. After being unwound but before being coated, it is often preferred to preheat the substrate in order to obtain a good coating quality. A second example is the batch heat processing of silicon discs in vacuum. In ordinary vacuum conditions conduction or convection techniques do not work efficiently. This is the reason why radiation is used. This can be done by infrared lamps. However, heating by means of infrared lamps has some severe limitations. The electrical voltage over the infrared lamps is limited to values of about 55 Volt to 65 Volt. Increasing the value of the voltage above these values, leads to formation of secondary plasmas and arcing. As a result, the heating power is limited. As a result also, the speed of the substrate to be heated is also limited. The heating power can also be increased by providing more infrared lamps. This increased number of lamps, however, requires more space and requires more feed-throughs and higher currents through the walls of the vacuum chamber. It is hereby understood that, in general, the less the number of feed-throughs through the walls of a vacuum chamber the better since this simplifies the construction and maintenance and reduces the risk for loss of vacuum.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to increase the heating power when heating in vacuum.  
      It is another object of the present invention to avoid arcing when heating in vacuum.  
      It is also an object of the present invention to increase the speed of a moving substrate to be heated in vacuum.  
      It is still an object of the present invention to limit the number of infrared lamps when heating in vacuum.  
      It is still another object of the invention to heat a substrate in vacuum to higher temperatures.  
      According to the invention there is provided a method of heating in a vacuum atmosphere in the presence of a plasma. The method comprises the following steps: 
          a) providing infrared radiation means in a vacuum chamber;     b) providing a first electrical conductor to the infrared radiation means;     c) providing a second electrical conductor from the infrared radiation means;     d) putting an electrical voltage over the infrared radiation means;     e) preventing said first conductor and the second conductor from having an electric voltage above +55 Volt.        

      Preferably, the first conductor and the second conductor are prevented from having a positive electric voltage.  
      Preferably, the first conductor or the second conductor, and most preferably both, are kept electrically negative.  
      The invention is not limited to deposition systems such as sputtering systems but can be applied to all types of vacuum atmospheres where plasmas, i.e. ionized gases, are present. For example, the invention is applicable to plasma assisted chemical vapour deposition techniques, used e.g. for deposition of amorphous silicon.  
      Within the context of the present invention, the term “vacuum” refers to a pressure lower than 100 Pa (=100 mbar), e.g. lower than 10 Pa, e.g. lower than 1 Pa, e.g. 0.005 Pa. . . . .  
      The advantageous mechanism of the invention can be explained as follows. By keeping the first conductor and the second conductor electrically negative, it is avoided that the electrons, which are present in the plasma, are attracted to these conductors. As a consequence, electron clouds or secondary plasmas can no longer be built up around the conductors and arcing is avoided. Accordingly, the voltage put over the radiation means may be increased without substantially increasing the risk for arcing.  
      In a preferable embodiment of the present invention, a first feed-through is provided through which the first conductor enters the vacuum chamber. The second conductor is electrically grounded together with the walls of the vacuum chamber. This grounding avoids the need for another feed-through for the second conductor.  
      In another preferable embodiment of the present invention, the first conductor and the second conductor are double isolated. In addition thereto, a metal shield is wrapped around the first conductor and the second conductor. This shield is connected to earth. This avoids a charge build up from the plasma on the first and second electrical conductor.  
      According to a general and broader aspect of the invention, there is provided a method of avoiding arcing in a vacuum atmosphere in the presence of a plasma. The method comprises the following steps: 
          a) providing a vacuum chamber;     b) providing a plasma;     c) providing an electrial power to or from a device in a vacuum chamber;     d) providing a first electrical conductor to said device;     e) providing a second electrical conductor from said device;     f) preventing said first and second electrical conductor from being loaded above +55 Volt so that electrons are not attracted in mass.       

    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described into more detail with reference to the accompanying drawings wherein  
       FIG. 1  shows an electrical circuit of a first embodiment of the invention;  
       FIG. 2  shows an electrical circuit of a second embodiment of the invention;  
       FIG. 3  shows an electrical circuit of a third embodiment of the invention;  
       FIG. 4  and  FIG. 5  show the wave form of the electrical voltage at various spots in the electrical circuit of  FIG. 3 .  
       FIG. 6 ,  FIG. 7  and  FIG. 8  all show electrical circuits of preferable embodiments of the invention;  
       FIG. 9  shows an embodiment of an electrical circuit which is an alternative to the second embodiment of  FIG. 2 ;  
       FIG. 10  shows an embodiment of an electrical circuit where a diode bridge is integrated with a power controller;  
       FIG. 11  shows an electrical circuit of an experimental set-up.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION  
       FIG. 1  shows an electrical circuit of a first embodiment of the invention. In a vacuum chamber  10  a sputter target  12  is installed. The sputter target functions as a cathode and is negatively biased through an electrical source  14 . Substrate  15  is to be coated with the material of the target  12 . Before or during the coating step, substrate  15  is heated by means of an infrared lamp  16 . A first conductor  18  and a second conductor  20  supply electrical energy to the infrared lamp  16 . Both the first conductor  18  and the second conductor  20  are electrically double isolated. In addition hereto, a metal shield is wrapped around the double isolated conductors  18 ,  20  and this metal shield is connected to earth (not shown). First electrical conductor  18  enters the vacuum chamber  10  through an isolated feed-through  22  and second electrical conductor  20  enters the vacuum chamber through another isolated feed-through  24 . A DC power source  26  supplies electrical energy to the infrared lamp  16  and puts the infrared lamp under an electrically negative voltage. Electric conductors  18  and  20  are negative so that no electrons are attracted.  
       FIG. 2  shows an electrical scheme of a second embodiment of the invention. The difference with  FIG. 1  is that in  FIG. 2  an AC voltage source is used. The AC voltage is applied to the vacuum system via a transformer  28 . The feed-throughs  22  and  24  and electric conductors  22  and  24  are not grounded. As a result the AC voltage over the infrared lamp  16  is floating. Suppose that the AC voltage is 100 V. This means that there is a maximum voltage of 141 V over the infrared lamp  16 , i.e. between the electical conductors  18  and  20 . The absolute voltage on the conductors is not determined, having regard to the floating nature. This can be 0 V and +141 V, or −141 V and 0 V, or −70.5 V and +70.5 V. Despite such a relatively high level of positive voltage, no arcing problems occur. This can be explained as follows. If one of the conductors becomes electrically positive, it will attract electrons.  
      These electrons cannot flow away, since there is no grounding. The whole secondary circuit becomes negative and prevents other electrons from being attracted. So this negative loading by the electrons prevents the conductors from having a high positive voltage. And this absence of a high positive voltage prevents a concentrated stream of electrons and thus prevents arcing. This has been confirmed in experiments, the results of which are summarized in Table 1 below.  
       FIG. 3  shows another electrical scheme for implementing a third embodiment of the invention. The difference with  FIG. 2  is that diodes  30  and  32  filter now away the positive peaks.  
      The bold line curve  34  in  FIG. 4  gives the voltage at the second conductor  20 . The bold line curve  36  in  FIG. 5  gives the voltage at the first conductor  18 . Curve  36  has a 180° phase shift with respect to curve  34 .  
       FIG. 6 ,  FIG. 7  and  FIG. 8  all illustrate embodiments where a diode bridge  40  and a thyristor controller  42  are used. The thyristor controller  42  regulates the power of the heating element.  
      In the embodiment of  FIG. 6  two feed-throughs  22 ,  24  are still used.  
      In the embodiment of  FIG. 7  the positive pole  44  is connected to earth as well as is the first electrical conductor  18 . This embodiment has the advantage that only one feed-through  24  is required.  
      In the embodiment of  FIG. 8  an extra coil  46  is provided for securing semi-conductor parts from an arc between the two electrodes. The positive pole  44  is connected to earth by way of a resistor  48 .  
       FIG. 9  illustrates an electrical circuit which is a preferable alternative to the circuit of  FIG. 2 . The secondary winding of transformer  28  has three parts. A main part  59  which gives the voltage over infrared lamp  16 , and two auxiliary windings. A first auxiliary winding  60  is via a diode  62  over an impedance  64  connected to the ground. A second auxiliary winding  66  is via another diode  68  and over the same impedance  64  connected to the ground. The result is that a sinusoidal voltage is across the infrared lamp  16 , however, with both maximum and minimum values negative.  
       FIG. 10  shows an embodiment of an electrical circuit where a diode bridge is integrated with a power controller.  70  is a three-phase transformer. Thyristor bridge  72  is an integration of the diode bridge  40  of FIGS.  6  to  8  with thyristor controller  42  of FIGS.  6  to  8 . Thyristor bridge  72  comprises six thyristors  74  and transforms the three-phase AC input signal into a single phase output signal for the infrared heater. The temperature is measured continuously and a related signal  76  is fed back to a control circuit  78  which steers the thyristors  74 .  
       FIG. 11  illustrates an electrical circuit which was used for setting up some arcing experiments. A Variac  50  supplies variable voltages to the system. Part of the voltage goes over a transformer  52  and is put over a variable gap  54 . Another part of the voltage goes over another transformer  56  and is put over a 10-Ohm resistor  58 . Once an arc develops in the vacuum chamber  10 , it is safely dissipated in resistor  58 . An oscilloscope is connected to various points in the circuit for monitoring.  
      The experiments carried out consisted of adjusting the gap, pumping out the vacuum chamber, starting an Argon flow to achieve an Argon partial pressure of about 1 mTorr, starting the sputtering cathode, and subsequently increasing the Variac  50  until arcs became apparent.  
      Table 1 summarizes the results of the obtained data:  
                                           TABLE 1                       Exp   Gap           AC/       Arcing           No.   (cm)   Plasma   Voltage   DC   Grounded   (No or V)   Notes                                                                1   1.20   ON   85   AC   Y   85           2   1.20   OFF   85   AC   Y   N       3   1.20   ON   85   AC   N   N       4   1.20   ON   300   AC   N   N       5   2.50   ON   300   AC   N   N       6   2.50   ON   62   AC   Y   62       7   6.98   ON   Any   AC   N   N       8   9.52   ON   65-70   AC   Y   65       9   8.89   ON   Any   AC   N   N       10   8.89   ON   62   AC   Y   62       11   12.70   ON   Any   AC   N   N       12   12.70   ON   65   AC   Y   65       13   19.05   ON   275   AC   N   275       14   19.05   ON   76   AC   Y   65       15   27.94   ON   82   AC   Y   82       16   27.94   ON   Any   AC   N   N       17   19.05   ON   65   AC   Y   65       18   19.05   ON   330   AC   N   330       19   25.40   ON   260   AC   N   260       20   30.48   ON   275   AC   N   275       21   30.48   ON   72   AC   Y   72       22   38.10   ON   240   AC   N   240       23   38.10   ON   60   DC   Y−   60       24   38.10   ON   Any   DC   Y+   N   (*)       25   64.77   ON   220   AC   N   220       26   64.77   ON   221   DC   Y+   N   (*)                 (*) no arc at maximum voltage of 430 V             
 
      Not shown in the above Table 1 is that arcing occurs only when the ungrounded electrode is driven positive.  
      As may be derived from Table 1, in the absence of grounding (Grounded=N), the voltage where arcing occurs is much higher than in similar cases with grounding. For example, comparing experiment No. 5 with No. 6, there is no arcing at 300 V in the non grounded embodiment while there is already arcing at 62 V in the grounded embodiment.