Patent Publication Number: US-6222321-B1

Title: Plasma generator pulsed direct current supply in a bridge configuration

Description:
This a continuation of application Ser. No. 08/646,616, filed May 8, 1996 now issued as U.S. Pat. No. 5,917,286 and hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention pertains generally to power supplies and more particularly to pulsed DC power supplies that are used for generating plasmas that are used in thin film processing techniques for etching, deposition, etc. 
     2. Definitions 
     Alternating polarities means a current flow at any particular point in a circuit or plasma that changes direction, or a voltage at any particular point in a circuit that changes magnitudes around any desired neutral voltage. 
     Current connections means locations or points in a circuit that are coupled to electrodes of a plasma chamber. 
     Current controlled power source means a power source that is capable of maintaining a substantially constant current for a wide range of load impedances and has a low amount of capacitively stored energy. 
     Current reversing switches means any desired arrangement of switches that are capable of causing current to flow in different directions at a preselected location in a circuit. 
     Direct current means current that has a substantially constant magnitude. 
     Direction means the course of the flow of current on a conductor in a circuit. 
     Generating means initiating and/or maintaining. 
     Inductor means an electrical component that is designed to store energy in a magnetic field. 
     Plasma means a state of matter in which electrons and ions in a gas discharge are separated but together form a neutral assembly. 
     Plasma chamber means a device in which plasmas can be generated. 
     Power source means a device that is capable of supplying electrical energy. 
     Predetermined positions means either an opened or closed position of a switch. 
     Pulsed direct current means a current that flows at a particular point in a circuit that has a first substantially constant magnitude during a first period of time, and then has at least one additional substantially constant magnitude that is different from the first substantially constant magnitude during at least one additional subsequent period of time, and may repeat. 
     Substantially constant supply means a substantially constant magnitude. 
     DESCRIPTION OF THE BACKGROUND 
     Plasma processing techniques have found wide-spread use in industry for commercial processes such as plasma vapor deposition, sputtering, etc. These processes have become particularly useful in thin film applications. To generate a plasma, a power supply creates an electric potential between a cathode and one or more anodes that are placed in a plasma chamber containing the gases that are used to form the plasma. When using these processes for deposition, the plasma acts upon the material of a target placed in the plasma chamber that normally comprises the cathode surface. Plasma ions cause target material to be dislodged from the cathode surface. The target materials are then deposited on a substrate deposition surface to form a thin film. The thin film may constitute material sputtered by the plasma from the target surface, as disclosed above, or may be the result of a reaction between the target material and some other element included in the plasma. The materials and elements involved, as well as the specific applications of the plasma processing techniques vary greatly. Applications may range from coating architectural glass to deposition of thin film layers on microchips, or deposition of aluminum layers on compact disks. 
     In the past, high frequency voltage sources have been used to generate a high electrical potential that produces a plasma within a plasma chamber. These high-frequency voltage sources are expensive to construct and maintain, as well as dangerous to operate. Additionally, if the deposition material is formed by reaction with an element in the plasma, and further, is electronically insulating, the anode in the chamber can be coated with the insulator; this deposit can then prevent the anode from performing its function of collecting the electrons released from the plasma during the deposition process. 
     To overcome these disadvantages, pulsed DC voltage sources have been employed such as disclosed in U.S. Pat. No. 5,303,139 issued Apr. 12, 1994 to Mark, which is specifically incorporated herein by reference for all that it discloses and teaches. Mark discloses a constant voltage pulsed power supply that has alternating pulse polarities. The advantages of such a constant voltage pulsed power supply over the AC power supplies are that they are less expensive, easier to connect and set up, and overcome the problem of coating the anode if used with two target units. That is, the process of reversing polarities allows the electrodes to alternately act as anode and cathode; the sputtering process that occurs during the cathode phase cleans off any deposited insulating material and permits uninhabited operation of the electrode as an anode during the anode phase. Additionally, the process of reversing polarities allows both electrodes to alternatively act as a cathode so that both electrode surfaces are capable of providing target material. 
     Despite the advantages that constant voltage pulsed power sources provide, problems exist with regard to generation of excessive currents and spark discharges in the plasma chamber. As part of this problem, it has been found that as the current through a plasma increases, the resistance of the plasma decreases in an exponential manner to almost zero. Small changes in the voltage level of a voltage power source result in large changes in the current. Consequently, excessive current increases can be generated from only very small changes in the voltage level, and a high degree of accuracy is required for controlling voltage controlled power supplies to prevent excessive current increases. 
     To exacerbate the problem, it has been found that various benefits accrue including increases in efficiency as the plasma temperature is increased in the plasma chamber. It is therefore desirable to produce high temperature plasmas that have low resistances and that require the use of power supplies that operate in a controlled manner to prevent the generation of excessive currents. The high power required to produce the desired plasma temperatures places extreme demands on the power supply. For example, the power handling capabilities of switches and other electrical components must be increased to meet such high power specifications. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages the limitations of the prior art by providing a current controlled power supply that produces direct current pulses having alternating polarities to generate high temperature plasmas. A single power source, multiple power sources and/or multiple electrodes can be employed in accordance with the present invention. 
     The present invention therefore may comprise an apparatus for generating a pulsed direct current having alternating polarities to be applied to a plasma chamber to generate plasmas comprising, a power source that generates a substantially constant supply of direct current, current connections for delivering the pulsed-direct current to the plasma chamber, and current reversing switches having at least two pre-determined positions, the current reversing switches coupled to the power source and the current connections that cause the substantially constant supply of the direct current to flow in a first direction through the current connections whenever the current reversing switches are set in a first pre-determined position, and in a second direction through the current connections whenever the current reversing switches are set in a second pre-determined position. 
     The present invention may also comprise a method of generating a source of pulsed current having alternating polarities for use in generating a plasma comprising the steps of, generating a substantially constant supply of current from a current controlled power source, and switching current flow direction of the substantially constant supply of current to be supplied to said plasma using flow reversing switches that produce the source of pulsed current having alternating polarities for generating said plasma. 
     The present invention may also comprise a method for causing two substantially constant direct currents to flow in a plasma chamber comprising the steps of, generating a first substantially constant direct current using a first current controlled power source, generating a second substantially constant direct current using a second current controlled power source, connecting the first current controlled power source to the plasma chamber to cause the first substantially constant direct current to flow through the plasma chamber in a first direction during a first pre-determined period, and connecting the second current controlled power source to the plasma chamber to cause the second substantially constant direct current to flow through the plasma chamber in a second direction during a second pre-determined period. 
     The present invention may also comprise a circuit for generating a direct current that flows between a plurality of electrodes in a plasma chamber comprising, a current controlled power source that generates a substantially constant supply of the direct current, a switch connected between the current controlled power source and each electrode of the plurality of electrodes, and inductors coupled between the electrodes of the plasma chamber and a common return of the current controlled power source that cause the direct current to flow in the plasma chamber when at least one of the switches is open and at least one other is closed. 
     A first advantage of the present invention is that the current controlled power source provides a device for accurately controlling the amount of current that is applied to the plasma chamber despite changes in the resistance of the plasma. The switches that control the flow of current through the plasma chamber can also be utilized to shunt current. Multiple electrodes can be used in conjunction with either a single power source or multiple power sources to increase the deposition capabilities of the plasma chamber. Multiple electrodes allow for multiple target surfaces when each of the electrodes is sequentially employed as the cathode target surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph of current versus voltage illustrating the current/voltage characteristics of a plasma. 
     FIG. 2 is a schematic circuit diagram of a current source employed in accordance wit hone embodiment of the present invention. 
     FIG. 3 is a schematic circuit diagram of a first embodiment of the present invention that uses a single power source. 
     FIG. 4 is a graph of the current pulses that can be produced in the plasma by the embodiment of FIG.  3 . 
     FIG. 5 is a schematic illustration of an alternative arrangement of current pulses that can be produced in the plasma that provide a predetermined duty cycle. 
     FIG. 6 is a schematic illustration of another embodiment of the present invention that uses a single current controlled power source with three electrodes. 
     FIG. 7 is a graph of the voltage on electrode  64  when various switches are closed. 
     FIG. 8 is a graph of the voltage on electrode  68  when various switches are closed. 
     FIG. 9 is a graph of the voltage on electrode  66  when various switches are closed. 
     FIGS. 10-12 schematically illustrate various conditions in a plasma chamber for a first, second and third states of operation. 
     FIG. 13 is a graph of the voltage on electrode  64  for the fourth, fifth and sixth states of operation. 
     FIG. 14 is a graph of the voltage on electrode  68  for the fourth, fifth and sixth states of operation. 
     FIG. 15 is a graph of the voltages on electrode  66  for the fourth, fifth and sixth states of operation. 
     FIGS. 16-18 schematically illustrate various conditions of the plasma in the plasma chamber during the fourth, fifth and sixth states of operation. 
     FIG. 19 is a schematic illustration of an embodiment of the present invention utilizing a single power source and four electrodes. 
     FIG. 20 is a schematic illustration of another embodiment of the present invention that utilizes two current controlled power sources. 
     FIG. 21 is a schematic illustration of the current pulses that can be produced by the embodiment of FIG.  20 . 
     FIG. 22 is a schematic illustration of additional current pulses that can be produced by the embodiment of FIG.  20 . 
     FIG. 23 is a schematic circuit diagram of an embodiment of the present invention utilizing three current controlled power sources coupled to three electrodes. 
     FIG. 24 is a schematic circuit diagram of another embodiment of the present invention utilizing four current control power sources coupled to four electrodes. 
     FIG. 25 is a schematic circuit diagram of another embodiment of the present invention that utilizes a single power source and two switches. 
     FIG. 26 is a schematic circuit diagram of another embodiment of the present invention that utilizes a single power source with three switches and three electrodes. 
     FIG. 27 is a schematic circuit diagram of another embodiment of the present invention that utilizes a single power source, four switches and four electrodes. 
     FIG. 28 is a schematic circuit diagram of a voltage controlled power source that is coupled to three electrodes. 
     FIG. 29 is a schematic circuit diagram of a voltage controlled power source that is coupled to four electrodes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
     FIG. 1 illustrates the advantages of driving the plasma generator with a current source through current reversing switches. FIG. 1 shows the current/voltage characteristics of a typical plasma chamber. As shown in FIG. 1, as the voltage increases, the current through the plasma chamber rises exponentially. As is readily apparent from FIG. 1, once the slope of the current versus voltage curve of FIG. 1 exceeds 45 degrees, it is better to control the power source with current rather than with voltage. Accordingly, at the operating point of a current i 1 , a small change in voltage can produce a large change in current around i 1 . For example, although a small change in voltage from v 1  to v 2  only produces a relatively small change in current from i 1  to i 2  an equally small change in voltage from v 1  to v 3  causes a very large change in current from i 1  to i 3 . Thus, a voltage source driving the plasma chamber is very susceptible to arc discharges as a result of minor variations or instability in the voltage. 
     However, if the plasma generating chamber is driven by a current source  10  and the switch configuration  12  illustrated in FIG. 2, the current i 1  may be easily controlled even though the voltage v 1  may vary substantially about the operating point, because the current source  10  is capable of rapid voltage changes that may be required by rapid changes in the resistance of plasma chamber  14 . 
     FIG. 2 illustrates a preferred embodiment of the invention which includes a current source  10  driving a plasma chamber, as referred to above. In operation, current source  10  drives a current  18  that is applied to switches  20  and  22  that are connected in a parallel configuration. Switches  20  and  22  can be alternately and substantially simultaneously closed to apply current to current connections or nodes  24  and  26  as illustrated in FIG.  2 . The current connections or nodes  24  and  26  are coupled to electrodes  28  and  30  of plasma chamber  14 . Node  24  is connected to a switch  32 , which is in turn connected to the common return  36  of current source  10 . Similarly, node  26  is connected to switch  34 , which is in turn connected to the common return  36  of current source  10 . 
     Referring again to FIG. 2, the switch configuration  12  can be used as current reversing switches that operate in the following manner. In a first state of operation, switches  20  and  34  are closed and switches  22  and  32  are open. In this manner, current is caused to flow from electrode  28  to electrode  30  in plasma chamber  14 . Hence, the direction of flow of the current in the plasma chamber  14  is from electrode  28  to electrode  30 . 
     In a second state of operation, switches  20  and  34  are open and switches  22  and  32  are closed. This causes the current  18  from current source  10  to flow in the plasma chamber  14  from electrode  30  to electrode  28 . Hence, the direction of flow of current in the second state of operation in the plasma chamber  14  is from electrode  30  to electrode  28 . By operating the switch configuration  12  in this manner, pulses of direct current having alternating polarities can be generated in the plasma chamber  14 , as illustrated in FIG.  4 . 
     In a third state of operation, all four of the switches  20 ,  22 ,  34 ,  32  of switch configuration  12  can be closed so that no current flows through plasma chamber  14 . It may be desirable to place the switch configuration  12  in this state when an arc discharge or a potential arc discharge is detected in plasma chamber  14 . Additionally, this state of operation wherein all the switches are closed during a preselected time period may be desirable to modify the duty cycle of the substantially constant supply of direct current  18  being applied to plasma chamber  14 , such as illustrated in FIG.  5 . 
     FIG. 3 is a schematic circuit diagram of the manner in which the present invention is implemented with a power source  38 . As shown in FIG. 3, power source  38  also includes an inductor  40  which helps the power source  38  to function in a manner similar to an ideal current source, such as ideal current source  10  shown in FIG.  2 . Power source  38 , in conjunction with inductor  40 , is constructed in a manner to approximate the operation of an ideal current source within the practical limits of operation using available components. For example, a sudden decrease in the resistance in the plasma chamber  42  that results in an arc discharge between electrodes  44  and  46  will cause an instantaneous shift in impedance to inductor  40 . Power source  38  is designed to provide the desired impedance over longer durations. 
     In operation, the circuit of FIG. 3 operates in the same manner as described with regard to FIG.  2 . When switches  48  and  52  are closed simultaneously, current  39  flows through the plasma chamber  42  from electrode  44  to electrode  46 . In this case, the current  39  is applied to current connection, or node  56  via switch  48 , while electrode  46  is coupled to the common return  60  of power source  38  via current connection  58  and switch  52 . In a similar manner, switches  50  and  54  can be closed simultaneously to cause current to be applied to current connection or node  58 , and the common return  60  to be coupled to current connection, or node  56 , via switch  54 . In this case, direct current  39  flows from electrode  46  to electrode  44  in plasma chamber  42 . Hence, by alternately closing switches  48 ,  52  and  50 ,  54 , the current flow through the plasma chamber  42  produces direct current pulses having alternating polarities in the plasma chamber  42 , such as illustrated in FIG.  4 . Since the current  39  is produced by a power source  38  that generates a substantially constant supply of direct current, these pulses comprise pulsed direct current having alternating polarities in the plasma chamber  42 . 
     The device of FIG. 3 can also be operated in a third state of operation such as described with respect to FIG.  2 . In the same manner as described above, all four switches,  48 ,  50 ,  52 ,  54  can be closed so that direct current  39  is shunted around the plasma chamber  42 . The direct current  39  does not pass through the plasma chamber  42 . The switches  48 ,  50 ,  52 ,  54  comprise current reversing switches that are therefore capable of reversing the flow direction of current in the plasma chamber  42 , and also shunting the current  39  so that no current flow through plasma chamber  42 . Hence, in the third state of operation of switches  48 ,  50 ,  52 ,  54 , pulses such as those illustrated in FIG. 5 can be generated so that a predetermined duty cycle of the operation of plasma chamber  42  can be produced. Also, in the same manner as described above with regard to FIG. 2, the third state of operation can be initiated when an arc discharge is detected, or the potential for an arc discharge is detected, to minimize damage caused in the plasma chamber  42 . 
     The power source  38  of FIG. 3 is also designed so that only a small amount of capacitive storage is provided across its outputs. This allows power source  38  to function as nearly as possible as an ideal current supply. 
     FIG. 4 illustrates a source of pulsed direct current having alternating polarities that is applied to a plasma chamber to generate plasmas. As described above, switches  48  and  52  are closed while switches  50  and  54  are substantially simultaneously opened during a first state of operation that produces a pulse of direct current  62  for a predetermined period in the plasma. At the end of such a predetermined period, a second state of operation is produced when switches  48  and  52  are opened while switches  50  and  54  are substantially simultaneously closed. During this second state of operation, a pulse of direct current  64  is produced for a second predetermined period. This process can be repeated to produce a series of direct current pulses having alternating polarities such as illustrated by pulses  66 ,  68 ,  70 ,  72 ,  74 , and so on. FIG. 4 therefore illustrates the manner in which the switches  48 ,  50 ,  52  and  54  can be operated alternatively between a first and second state to produce a series of direct current pulses having alternating polarities in a plasma chamber. 
     FIG. 5 illustrates the manner in which switches  48 ,  50 ,  52  and  54  of FIG. 3 can be alternatively operated in three different states. As shown in FIG. 5, switches  48 ,  50 ,  52  and  54  can be operated in a first state to produce a direct current pulse  76 . The switches can then be operated in a third state by closing all of the switches  48 ,  50 ,  52 ,  54  to produce output  78  during a second predetermined period. In the third state of operation, no current flows through the plasma chamber  42  as shown at output  78  of FIG.  5 . During a third predetermined period, the switches can be operated in a second state to produce a current pulse  80  in the plasma chamber  42 . During a fourth predetermined period, the switches can again be operated in a third state to produce output  82  during which no current flows through the plasma chamber  42 . This process can be repeated to produce outputs  84 ,  86 ,  88 ,  90 ,  92 , and so on. The series of alternating polarity current pulses illustrated in FIG. 5 provide a predetermined operating duty cycle of the plasma chamber  42  that is dependent upon the length of the operation of the switches in the third state. 
     FIG. 6 illustrates another embodiment of the present invention which utilizes a single power source  62 . Power source  62  generates a substantially constant supply of direct current  74  that is applied to three electrodes  64 ,  66 ,  68  in a plasma chamber  70 . As shown in FIG. 6, power source  62  includes an inductor  72  that allows power source  62  to approximate the operation of a current source that is capable of providing a substantially constant supply of direct current  74 . As shown in FIG. 6, power source  62  has a current output  73  that is coupled to parallel switches  76 ,  78  and  80 . Similarly, switches  82 ,  84 ,  86  are connected in parallel to common return  75  of power source  62 . Switch  76  is coupled to connection or node  88 , which is in turn connected to electrode  68 . Switch  78  is connected to connection or node  90 , which is in turn connected to electrode  66 . Switch  80  is connected to connection or node  92 , which is in turn connected to electrode  64 . 
     The device of FIG. 6 has six different states of operation that are illustrated in FIGS. 7 through 18. FIGS. 7-9 and  13 - 15  all illustrate voltage waveforms for electrodes  64 ,  66  and  68 . These voltage waveforms illustrate the difference in voltage between these various electrodes and the plasma chamber, and also earth ground since the plasma chamber is usually connected to earth ground. The power sources, however, may float with respect to earthground and the plasma chamber. 
     FIG. 7 illustrates the voltage on electrode  64  during three states of operation. During the first state of operation  94  the voltage  102  on electrode  64  is negative. The first state of operation  94  occurs during the time period from times t 1  to t 2 . Referring to FIG. 6, switches  82 ,  78  and  76  are closed and switches  80 ,  84 ,  86  are open during the first state of operation  94 . As can be seen from FIG. 6, the direct current  74  is applied to node or connections  88  and  90 , that causes current to flow from electrodes  66  and  68  to electrode  64 . 
     FIG. 10 illustrates the condition of the plasma chamber  70  during the first state of operation  94 . As shown in FIG. 10, electrode  64  comprises a cathode while electrodes  66  and  68  comprise anodes. A plasma  96  is generated proximate to cathode  64 , as illustrated in FIG.  10 . Ions  98  are attracted to cathode  64 , while electrons  100  from plasma  96  are attracted to anode  66  and anode  68 . 
     FIG. 7 also illustrates the voltage  102  on electrode  64  during a second state of operation  104  that occurs from times t 2  to t 3 . As shown in FIG. 7, the voltage  102  on electrodes  64  is slightly positive during the second state of operation  104 . 
     Referring to FIG. 6, the second state of operation occurs when switches  78 ,  80  and  86  are closed, and switches  76 ,  82 ,  84  are open. When switches  78  and  80  are closed, current is applied to electrodes  64  and  64  through nodes or connections  90  and  92 , respectively. When switch  86  is closed, electrode  68  is coupled to the common return  75  of the power source  62 . 
     FIG. 11 illustrates the condition of operation of the plasma chamber during the second state of operation  104 . As shown in FIG. 11, electrode  68  functions as a cathode, while electrode  64  and  66  function as anodes. A plasma  106  is generated proximate to the cathode  68 . Positive ions  108  are attracted to the cathode  68 , while negative electrons are attached to anodes  64  and  66 . 
     FIG. 7 additionally illustrates the voltage  102  on electrode  64  during a third state of operation  110  that occurs from time t 3  to t 4 . As shown in FIG. 7, the voltage  102  on electrode  64  during the third state  110  is slightly positive. 
     Referring to FIG. 6, the third state of operation occurs when switches  76 ,  80  and  84  are closed and switches  78 ,  82  and  86  are open. When switches  76  and  80  are closed, direct current  74  is applied to electrodes  68  and  64  via nodes  88  and  92 , respectively. By closing switch  84 , electrode  66  is connected to the common return  75  of power source  62 . 
     FIG. 12 illustrates the condition of the plasma chamber  70  during the third state of operation  110 . Electrode  66  functions as a cathode, while electrodes  64  and  68  function as anodes. A plasma  112  is generated proximate to cathode  66 . Plasma  112  generates ions  114  that are attracted to cathode  66  and electrons that are attracted to anodes  64  and  68 . 
     Referring again to FIG. 7, the first state of operation is again repeated between times t 4  and t 5 , so that a negative voltage pulse is produced on electrode  64 . Similarly, the second state of operation  104  is repeated from times t 5  and t 6 . These three states of operation can be repeated in the order shown, or any desired order of operation of the switches  76  through  86 . FIG. 8 illustrates the voltages  116  produced on electrode  68  during the three states of operation. As shown in FIG. 8, the voltage  116  on electrode  68  is positive during the first state of operation  94 , is negative during the second state of operation  104 , and is positive again during the third state of operation  110 . The voltages  116  on electrode  68  are illustrated in FIGS. 10-12. 
     FIG. 9 illustrates the voltages  118  produced on electrode  66  during the three states of operation. As shown in FIG. 9, the voltage  118  on electrode  66  is positive during the first state of operation  94  and the second state of operation  104 . The voltage  118  on electrode  66  is negative during the third state of operation  110 . It is possible and reasonable to operate the system of FIG. 6 in only these three first states of operation or to operate additionally with states wherein more than one element at a time acts as a cathode. 
     FIGS. 13-15 illustrate the voltages on the electrodes  64 ,  68  and  66  during the fourth state  120 , fifth state  122  and sixth state  124 . 
     FIG. 13 illustrates the voltage  126  on electrode  64  during the fourth state  120 , fifth state  122  and sixth state  124 . As shown in FIG. 13, the voltage  126  on electrode  64  is negative during the fourth state  120  and fifth state  122 . The voltage  126  on electrode  64  is positive during the sixth state  124 . As also illustrated in FIG. 13, the various states  120 ,  122 ,  124  can be repeated in order or, can be repeated in any desired order to produce the desired conditions on the electrodes  64 ,  66  and  68 . 
     Referring to FIG. 6, the fourth state of operation  120  occurs when switches  76 ,  82  and  84  are closed, and switches  78 ,  80  and  86  are open. When switch  76  is closed, current is applied to electrode  68  via connection or node  88 . By closing switches  82  and  84 , electrodes  64  and  66  are connected to common return  75  of power source  62  through connection or nodes  92  and  90 , respectively. 
     FIG. 16 illustrates the condition of the plasma chamber  70  during the fourth state of operation  120 . Electrode  68  comprises an anode, while electrodes  64  and  66  comprise cathodes. Plasma  128  is generated proximate to cathode  64 . Ions from plasma  128  are attached towards cathode  64 , while electrons from plasma  128  are attracted towards anode  68 . A plasma  130  is generated proximate to cathode  66 . Ions from plasma  130  are attracted to cathode  66 , while electrons from plasma  130  are attracted to anode  68 . 
     FIG. 14 illustrates the voltage  132  on electrode  68  during the fourth state  120 , fifth state  122  and sixth state  124 . As shown in FIG. 14, the voltage  132  on electrode  68  is slightly positive during the fourth state  120 , and negative during the fifth state  122  and sixth state  124 . 
     FIG. 17 illustrates the condition of the plasma chamber  70  during the fifth state  122 . As shown, electrodes  64  and  68  comprise cathodes, while electrode  66  comprises an anode. A plasma  134  is generated proximate to cathode  64 . Ions from plasma  134  are attracted to cathode  64 , while electrons from plasma  134  are attracted to anode  66 . Similarly, a plasma  136  is generated proximate to cathode  68 . Ions from plasma  136  are attracted to cathode  68 , while electrons from plasma  136  are attracted to anode  66 . 
     FIG. 15 illustrates the voltage  138  on electrode  66  during the fourth state  120 , fifth state  122  and sixth state  124 . During the fourth state  120 , the voltage  138  on electrode  66  is negative. During the fifth state  122 , the voltage  138  on electrode  66  is slightly positive. During the sixth state  124 , the voltage  138  on electrode  66  is negative. 
     Referring to FIG. 6, switches  80 ,  84  and  86  are closed and switches  76 ,  78  and  82  are open during the sixth state. As shown in FIG. 6, the direct current  74  is applied to electrode  64  via connection or node  92 . When switches  84  and  86  are closed, electrodes  66  and  68  are coupled to the common return  75  of power source  62  via nodes or connections  90  and  88 , respectively. 
     The condition of the plasma chamber  70  during the sixth state  124  is illustrated in FIG.  18 . As shown in FIG. 18, electrode  64  comprises an anode, while electrodes  66  and  68  comprise cathodes. A plasma  140  is generated proximate to cathode  68 . Positive ions from plasma  140  are attracted to cathode  68 , while negative electrons are attracted to anode  64 . Similarly, a plasma  142  is generated proximate to cathode  66 . Ions from plasma  142  are attracted to cathode  66 , while negative electrons are attracted to anode  64 . Although not shown, all of the switches of FIG. 6 can be closed during the same time to generate a seventh state of operation in which no current flows through the plasma chamber  70 . It is possible to operate the system of FIG. 6 in any combinations of these states depending upon desired results. This seventh state of operation, as described above, may be utilized for arc discharge dissipation or to provide a duty cycle within the plasma chamber  70 . 
     FIG. 19 is a schematic circuit diagram of another embodiment of the present invention. The embodiment of FIG. 19 illustrates the use of a single current controlled power source  144  that is capable of generating a substantially constant supply of direct current  172  in combination with four electrodes  146 ,  148 ,  150  and  152  disposed in a plasma chamber  154 . FIGS. 6 and 19 illustrate the manner in which any desired number of electrodes can be placed in a single plasma chamber utilizing a singlecurrent control power source. Switches  156 ,  158 ,  160 ,  162 ,  164 ,  166 ,  168  and  170  can be opened and closed in any desired configuration to generate various states within the plasma chamber  154  as desired. 
     The advantages of using multiple electrodes in a plasma chamber are that the target surfaces which comprise the cathode can be changed from one electrode to another to provide additional cathode surfaces. Moreover, additional anode surfaces are provided in the plasma chamber  154  to attract negative electrons that enhances the generation of the plasma. Of course, any desired configuration of the electrodes can be used within the plasma chamber  154  other than that shown in FIG. 19, or any of the figures. 
     FIG. 20 is a schematic circuit diagram of another embodiment of the present invention. As shown in FIG. 20, a current controlled power source  174  generates a substantially constant supply of direct current  176 . An additional current controlled power source  178  generates a substantially constant supply of direct current  180 . Switches  182  and  184  are connected in a shunt configuration with power source  174  and power source  178 , respectively. A plasma chamber  190  is disposed in the circuit so that an electrode  186  is coupled to a common return  188  of power source  174 . Electrode  192  is similarly connected to a common return  194  of power source  178 . Power source  174  includes an inductor  196  that assists the power source  174  in functioning as an ideal current source. Similarly, inductor  198  of power source  178  assists the power source  178  in functioning as an ideal current source. 
     In operation, the embodiment of FIG. 20 has three different operating states. In a first operating state, switch  182  is closed and switch  184  is open. Direct current  176  from power source  174  is shunted to the common return  188  and does not pass through the plasma chamber  190 . However, direct current  180  from power source  178  passes through switch  182  to electrode  186  in plasma chamber  190 . The direct current  180  then passes through the plasma chamber  190  to electrode  192  to common return  194  of power source  178 . Hence, the direct current  180  passes through the plasma chamber  190  in a first direction from electrode  186  to electrode  192 . 
     FIG. 21 illustrates the flow of current through the plasma chamber  190 . As shown in FIG. 21, in a first state of operation, the direct current  180  passes through the plasma chamber  190  for a predetermined period while switch  182  is closed and switch  184  is open. In a second state of operation, switch  182  is open and switch  184  is closed. Direct current  180  from power source  178  is shunted to the common return  194  of power source  178  and does not pass through the plasma of plasma chamber  190 . However, direct current  176  of power source  174  flows through the switch  184  to electrode  192 . Direct current  176  flows from electrode  192  to electrode  186  that is connected to common return  188  of power source  174 . In this manner, direct current  176  passes through the plasma chamber  190  from electrode  192  to electrode  186 . As FIG. 21 shows, a pulse of direct current  176  passes through the plasma chamber  190  during a predetermined period when switch  184  is closed and switch  182  is open. FIG. 21 also shows a manner in which the switches  182  and  184  can be alternately opened and closed to allow the direct current  180  and direct current  176  to alternately pass through the plasma chamber  190  in a periodic fashion. FIG. 21 additionally illustrates that direct current  176  and direct current  180  are not necessarily equal. Of course, these direct currents from power sources  174  and  178  can be generated at any desired magnitude that is consistent with the operation of the plasma chamber  190 . FIG. 21 simply illustrates that the direct currents  176  and  180  need not necessarily be equal in magnitude. 
     FIG. 22 schematically illustrates the manner in which switches  182  and  184  can also be operated in a third state. As shown in FIG. 22, direct current  180  passes through the plasma chamber when switch  182  is closed and switch  184  is open during a first state of operation. Both switches  182  and  184  can then be closed during a predetermined time period  200  so that no current passes through the plasma chamber  190  for example, whenever conditions are detected in the plasma chamber that could result in an arc discharge. Switch  182  can then be opened and switch  184  can remain closed so that direct current  176  passes through the plasma chamber  190 , as illustrated in FIG.  22 . These states can be periodically repeated, as shown in FIG. 22, to produce a predetermined duty cycle of pulses that are applied to the plasma chamber  190 . Of course, any desired order of states can be applied by switching the switches  182  and  184  in the positions to produce the desired state of operation. 
     FIG. 23 illustrates another alternative embodiment of the present invention that utilizes three power sources. Each of the power sources has an associated shunt switch and electrode. For example, power source  202  has an associated shunt switch  208  and electrode  214  that are both coupled to the common return  216  of power source  202 . Similarly, power source  204  has an associated shunt switch  210  and an electrode  218  that is connected to a common return  220  of power source  204 . Power source  206  has an associated shunt switch  212  and electrode  222  that are connected to common return  224  of power source  206 . Various states of operation can be generated employing the embodiment of FIG. 23 similar to the various states of operation of the embodiments of FIGS. 7-15 with the exception that separate currents can be generated by each of the power sources  202 ,  204  and  206 . For example, in one state of operation, switch  208  is closed, while switches  210  and  212  are open. In that case, the substantially constant supply of direct current  226  is shunted through switch  208  to common return  216  so that the substantially constant supply of direct current  226  does not pass through the plasma chamber  232 . However, the substantially constant supply of direct currents  228  and  230  from power sources  204  and  206 , respectively, pass through switch  208  and are applied to electrode  214 . Direct current  228  passes through plasma generated in the plasma chamber  232  from electrode  214  to electrode  218  to common return  220  of power source  204 . In a similar manner, direct current  230  passes from electrode  214  through the plasma chamber  232  to electrode  222 , and to the common return  224  of power source  206 . As can be seen, various states of operation can be generated by opening and closing the switches  208 ,  210 , and  212  at various predetermined times. Of course, if all of the switches are closed, no current passes through the plasma chamber  232 . 
     FIG. 24 illustrates the manner in which four power sources  234 ,  236 ,  238 ,  240  can be employed with four shunt switches  242 ,  244 ,  246  and  248  and four associated electrodes  250 ,  252 ,  254  and  256 , respectively. The embodiment of FIG. 24 can be operated in a manner similar to that disclosed with respect to the operation of the embodiment of FIG.  23 . FIG. 24 also illustrates that any number of power supplies can be used in conjunction with a similar number of electrodes and shunt switches. 
     FIG. 25 is a schematic circuit diagram of another embodiment of the present invention. As illustrated in FIG. 25, a power source  260  generates a supply of a substantially constant direct current  262 . Switches  264  and  266  are connected in parallel to the output  268  of the power source  260 . A plasma chamber  270  having electrodes  272  and  274  is connected to nodes  276  and  278  that are, in turn, connected to switches  264  and  266 , respectively. Inductors  280  and  282  are connected to nodes  276  and  278 , and the common return  284  of power source  260 . 
     In operation, the embodiment of FIG. 25 has several different states of operation. In startup mode, switch  264  may be closed while switch  266  is open. In this state of operation, current increases in inductor  280  for a predetermined period. In a second state of operation, during the startup phase, switch  264  is opened and switch  266  is simultaneously closed so that the direct current  262  flows to node  278 . Inductor  280  attempts to draw some of the current  262  through the plasma chamber  270  from electrode  274  to electrode  272 . Initially, inductor  282  provides a certain amount of impedance so that all of the current  262  cannot immediately flow through the inductor  282  when the switch  266  is first closed. These factors, in combination, cause the plasma to ignite under normal conditions so that a flow of current is established in the plasma chamber from electrode  274  to electrode  272 . The current  262 , however, increases on inductor  282  for a predetermined period. As the current increases on inductor  282 , the current through plasma chamber  270  and inductor  280  decreases. Switch  266  is then opened and switch  264  is substantially simultaneously closed. At that point, current flows from electrode  272  to electrode  274  to maintain the current in inductor  282 . In a similar manner, the current builds on inductor  280  while the current lessens on inductor  282  until the switches  264 ,  266  are switched again. Of course, both switches  264  and  266  can be closed to prevent the flow of current in the plasma chamber  270 . Although the embodiment of FIG. 25 utilizes inductors  280  and  282  that have substantially equal inductances, it is possible that the embodiment of FIG. 25 can be operated with inductors that do not have the same impedance. Additionally, the operation of switches  264  and  266  is dependent upon the ramping time of current inductors  280  and  282 , so that the switching period of switches  264  and  266 , as well as the efficiency of the system, is dependent upon the magnitude of the inductance of inductors  280  and  282 . With longer switching periods, the current flow in plasma chamber  270  may take the appearance of ramped pulses rather than square pulses. 
     FIG. 26 is a schematic illustration of another embodiment of the present invention that utilizes a single power source  286  that produces a substantially constant supply of direct current  288  that is applied to a plasma chamber  290  having three electrodes  292 ,  294  and  296 . Three switches  298 ,  300  and  302  are coupled to the power source  286  for supplying the direct current  288  to electrodes  292 ,  294  and  296 , as well as to inductors  304 ,  306  and  308 , respectively. The embodiment of FIG. 26 operates in a manner similar to the embodiment of FIG. 25 by using the impedance of inductors  304 ,  306  and  308  to cause current to flow between the electrodes  292 ,  294  and  296  in plasma chamber  290 . 
     FIG. 27 illustrates another alternative embodiment of the present invention that utilizes a single power sources  310  that generates a substantially constant supply of direct current  312  that is applied to a plasma chamber  314  that has four electrodes  316 ,  318 ,  320 ,  322 . Switches  324 ,  326 ,  328  and  330  are connected to inductors  332 ,  334 ,  336  and  338 , and electrodes  316 ,  318 ,  320  and  322 , respectively, in a manner similar to that illustrated in FIG.  26 . FIG. 27 illustrates that the number of electrodes in a plasma chamber  314  can be increased utilizing a single power source  310  by increasing the number of switches and inductors that are connected in the manner shown. The advantages associated with the use of multiple electrodes, as described above, can be realized with the embodiments of FIGS. 26 and 27 while employing only a single power source. 
     FIG. 28 discloses another embodiment to the present invention that is similar to the embodiment of FIG. 6 with the exception that power source  340  comprises a voltage controlled power source. The embodiment of FIG. 28 operates in substantially the same manner as the embodiment of FIG. 6 with the exception that the power source  340  provides a substantially constant supply of voltage to electrodes  342 ,  344  and  346  of plasma chamber  348  by activation of switches  350 ,  352 ,  354 ,  356 ,  358 ,  360 . As indicated with respect to FIG. 1, when the slope of the current verses voltage curve is less than 45 degrees, it may be advantageous to use a voltage controlled power source rather than a current controlled power source since incremental changes in the voltage will produce smaller incremental changes in the current. FIG. 29 illustrates the manner in which the number of electrodes in a plasma chamber  364  can be increased utilizing a single voltage controlled power source  362 . The embodiment of FIG. 29 is similar to the embodiment of FIG. 19 with the exception that the power source  362  is a voltage controlled power source. The switches  366 ,  368 ,  370 ,  372 ,  374 ,  376 ,  378  and  380  can be operated to apply voltages to the various electrodes  382 ,  384 ,  386  and  388  to produce a plasma in plasma chamber  364 . 
     The present invention therefore provides various embodiments for generating a plasma in a plasma chamber using current controlled power sources that are capable of accurately controlling the amount of current delivered to a plasma chamber. High temperature plasmas have low resistances such that slight changes in voltages cause large changes in the amount of current delivered to the plasma chamber. Excessive increases in current increase the susceptibility of arc discharges in the plasma chamber. Since the present invention utilizes a current controlled source in association with high temperature plasmas, the amount of current is regulated utilizing a power source that resembles a current source. The present invention encompasses embodiments employing a single power source with multiple electrodes in the plasma chamber, as well as embodiments including both multiple power sources an multiple electrodes. The present invention also encompasses various switching arrangements to produce various states of operation. 
     The foregoing discussion of the advantages of current sourcing relates to the short times involved in the pulsing operation and in arc formation, detection, and quenching. In reactive sputter deposition of certain oxides, the impedance of the plasma drops as the target sputtering region is encroached by oxide formation on the target. If, as is true for many materials, the oxide has a higher secondary emission coefficient than the metal itself, the plasma impedance will drop as the oxide encroaches. This is so because as ions strike the insulating surface, secondary electrons are emitted with will be pulled into the plasma and increase its density, lowering the target voltage to a given power level. This is another way of stating that the plasma impedance will drop. Thus, if the power supply is set up to hold the voltage constant on the target, the power (and therefore the sputtering rate) will increase as the voltage drops. This will increase the metal available to react with the background gas, and inhibit to some extent the encroachment of insulator on the sputter area. Inhibiting the encroachment will make the process more stable and easier to control. 
     To be effective as a stabilizing approach, the holding of the voltage constant must be on the time scale of the chemical reactions of the background oxygen (or other reactive gas) with the target and the deposited film, which is measured in milliseconds. The requirement to be current sourced is on a time scale of the arcs and the pulsing, which is measured in microseconds. That is, the power source should appear to be a constant current source for the interpulse period. To permit this duality of control, one must set up a current-sourced power supply to be voltage regulated, which means that the value of the instantaneous current is to be continuously adjusted by the regulation loops of the power supply to maintain the voltage constant on a millisecond time scale. By this means, on a short time scale, measured in microseconds, the current is held constant, while on a longer time scale, measured in milliseconds, the power supply appears to hold the voltage constant. Additionally, the use of multiple electrodes in association with a single voltage controlled power source is an alternative embodiment of the present invention that may provide advantages in low temperature plasma processes. 
     The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise from disclosed and other modifications and variations may be possible in light of the above teachings. For example, various embodiments disclosed in the present application may be utilized with a voltage controlled power source which may have advantages in low resistance plasma applications. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.