Patent Publication Number: US-2007119701-A1

Title: High-Power Pulsed Magnetron Sputtering

Description:
BACKGROUND OF THE INVENTION  
      Sputtering is a well-known technique for depositing films on substrates. Sputtering is the physical ejection of atoms from a target surface and is sometimes referred to as physical vapor deposition (PVD). Ions, such as argon ions, are generated and then directed to a target surface where the ions physically sputter target material atoms. The target material atoms ballistically flow to a substrate where they deposit as a film of target material.  
      Diode sputtering systems include a target and an anode. Sputtering is achieved in a diode sputtering system by establishing an electrical discharge in a gas between two parallel-plate electrodes inside a chamber. A potential of several kilovolts is typically applied between planar electrodes in an inert gas atmosphere (e.g., argon) at pressures that are between about 10 −1  and 10 −2  Torr. A plasma discharge is then formed. The plasma discharge is separated from each electrode by what is referred to as the dark space.  
      The plasma discharge has a relatively constant positive potential with respect to the target. Ions are drawn out of the plasma, and are accelerated across the cathode dark space. The target has a lower potential than the region in which the plasma is formed. Therefore, the target attracts positive ions. Positive ions move towards the target with a high velocity. Positive ions impact the target and cause atoms to physically dislodge or sputter from the target. The sputtered atoms then propagate to a substrate where they deposit a film of sputtered target material. The plasma is replenished by electron-ion pairs formed by the collision of neutral molecules with secondary electrons generated at the target surface.  
      Magnetron sputtering systems use magnetic fields that are shaped to trap and to concentrate secondary electrons, which are produced by ion bombardment of the target surface. The plasma discharge generated by a magnetron sputtering system is located proximate to the surface of the target and has a high density of electrons. The high density of electrons causes ionization of the sputtering gas in a region that is close to the target surface.  
      One type of magnetron sputtering system is a planar magnetron sputtering system. Planar magnetron sputtering systems are similar in configuration to diode sputtering systems. However, the magnets (permanent or electromagnets) in planar magnetron sputtering systems are placed behind the cathode. The magnetic field lines generated by the magnets enter and leave the target cathode substantially normal to the cathode surface. Electrons are trapped in the electric and magnetic fields. The trapped electrons enhance the efficiency of the discharge and reduce the energy dissipated by electrons arriving at the substrate.  
      Conventional magnetron sputtering systems deposit films that have relatively low uniformity. However, the film uniformity can be increased by mechanically moving the substrate and/or the magnetron, but such systems are relatively complex and expensive to implement. Conventional magnetron sputtering systems also have relatively poor target utilization. By poor target utilization, we mean that the target material erodes in a non-uniform manner. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
      This invention is described with particularity in the detailed description. The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
       FIG. 1  illustrates a cross-sectional view of a known magnetron sputtering apparatus having a pulsed power source.  
       FIG. 2  illustrates a cross-sectional view of an embodiment of a magnetron sputtering apparatus according to the present invention.  
       FIG. 3  illustrates a cross-sectional view of the anode and the cathode assembly of the magnetron sputtering apparatus of  FIG. 2 .  
       FIG. 4  illustrates a graphical representation of the applied power of a pulse as a function of time for periodic pulses applied to the plasma in the magnetron sputtering system of  FIG. 2 .  
       FIG. 5  illustrates graphical representations of the absolute value of applied voltage, current, and power as a function of time for periodic pulses applied to the plasma in the magnetron sputtering system of  FIG. 2 .  
       FIG. 6A  through  FIG. 6D  illustrate various simulated magnetic field distributions proximate to the cathode assembly for various electron ExB drift currents according to the present invention.  
       FIG. 7  illustrates a cross-sectional view of another embodiment of a magnetron sputtering apparatus according to the present invention.  
       FIG. 8  illustrates a graphical representation of pulse power as a function of time for periodic pulses applied to the plasma in the magnetron sputtering system of  FIG. 7 .  
       FIG. 9A  through  FIG. 9C  are cross-sectional views of various embodiments of cathode assemblies according to the present invention.  
       FIG. 10  illustrates a cross-sectional view of another illustrative embodiment of a magnetron sputtering apparatus according to the present invention.  
       FIG. 11  is a cross-sectional view of another illustrative embodiment of a magnetron sputtering apparatus according to the present invention.  
       FIG. 12  is a flowchart of an illustrative process of sputter deposition according to the present invention.  
       FIG. 13  is a flowchart of an illustrative process of controlling sputtering rate according to the present invention. 
    
    
     DETAILED DESCRIPTION  
      The magnetic and electric fields in magnetron sputtering systems are concentrated in narrow regions close to the surface of the target. These narrow regions are located between the poles of the magnets used for producing the magnetic field. Most of the ionization of the sputtering gas occurs in these localized regions. The location of the ionization regions causes a non-uniform erosion or wear of the target that results in poor target utilization.  
      Increasing the power applied between the target and the anode can increase the amount of ionized gas and, therefore, increase the target utilization. However, undesirable target heating and target damage can occur. Furthermore, increasing the voltage applied between the target and the anode increases the probability of establishing an undesirable electrical discharge (an electrical arc) in the process chamber.  
      Pulsing the power applied to the plasma can be advantageous since the average discharge power can remain low while relatively large power pulses can be periodically applied. Additionally, the duration of these large voltage pulses can be preset so as to reduce the probability of establishing an electrical breakdown condition leading to an undesirable electrical discharge. However, very large power pulses can still result in undesirable electrical discharges and undesirable target heating regardless of their duration.  
       FIG. 1  illustrates a cross-sectional view of a known magnetron sputtering apparatus  100  having a pulsed power source  102 . The known magnetron sputtering apparatus  100  includes a vacuum chamber  104  where the sputtering process is performed. The vacuum chamber  104  is positioned in fluid communication with a vacuum pump  106  via a conduit  108 . The vacuum pump  106  is adapted to evacuate the vacuum chamber  104  to high vacuum. The pressure inside the vacuum chamber  104  is generally less than 100 Pa during operation. A feed gas source  109 , such as an argon gas source, is introduced into the vacuum chamber  104  through a gas inlet  110 . The gas flow is controlled by a valve  112 .  
      The magnetron sputtering apparatus  100  also includes a cathode assembly  114  having a target material  116 . The cathode assembly  114  is generally in the shape of a circular disk. The cathode assembly  114  is electrically connected to a first output  118  of the pulsed power supply  102  with an electrical transmission line  120 . The cathode assembly  114  is typically coupled to the negative potential of the pulsed power supply  102 . In order to isolate the cathode assembly  114  from the vacuum chamber  104 , an insulator  122  can be used to pass the electrical transmission line  120  through a wall of the vacuum chamber  104 . A grounded shield  124  can be positioned behind the cathode assembly  114  to protect a magnet  126  from bombarding ions. The magnet  126  shown in  FIG. 1  is generally ring-shaped having its south pole  127  on the inside of the ring and its north pole  128  on the outside of the ring. Many other magnet configurations can also be used.  
      An anode  130  is positioned in the vacuum chamber  104  proximate to the cathode assembly  114 . The anode  130  is typically coupled to ground. A second output  132  of the pulsed power supply  102  is also typically coupled to ground. A substrate  134  is positioned in the vacuum chamber  104  on a substrate support  135  to receive the sputtered target material  116 . The substrate  134  can be electrically connected to a bias voltage power supply  136  with a transmission line  138 . In order to isolate the bias voltage power supply  136  from the vacuum chamber  104 , an insulator  140  can be used to pass the electrical transmission line  138  through a wall of the vacuum chamber  104 .  
      In operation, the pulsed power supply  102  applies a voltage pulse between the cathode assembly  114  and the anode  130  that has a sufficient amplitude to ionize the argon feed gas in the vacuum chamber  104 . This typical ionization process is referred to as direct ionization or atomic ionization by electron impact and can be described as follows:
 
Ar+ e   − →→Ar + +2 e   − 
 
      where Ar represents a neutral argon atom in the feed gas and e − represents an ionizing electron generated in response to the voltage pulse applied between the cathode assembly 114 and the anode 130. The collision between the neutral argon atom and the ionizing electron results in an argon ion (Ar   + ) and two electrons.  
      The negatively biased cathode assembly  114  attracts positively charged ions with sufficient acceleration so that the ions sputter the target material  116 . A portion of the sputtered target material  116  is deposited on the substrate  134 .  
      The electrons, which cause the ionization, are generally confined by the magnetic fields produced by the magnet  126 . The magnetic confinement is strongest in a confinement region  142  where there is relatively low magnetic field intensity. The confinement region  142  is substantially in the shape of a ring that is located proximate to the target material. Generally, a higher concentration of positively charged ions in the plasma is present in the confinement region  142  than elsewhere in the chamber  104 . Consequently, the target material  116  is eroded rapidly in areas directly adjacent to the higher concentration of positively charged ions. The rapid erosion in these areas results in undesirable non-uniform erosion of the target material  116  and, thus relatively poor target utilization.  
      Dramatically increasing the power applied to the plasma can result in more uniform erosion of the target material  116 . However, the amount of applied power necessary to achieve this increased uniformity can increase the probability of generating an electrical breakdown condition that leads to an undesirable electrical discharge between the cathode assembly  114  and the anode  130  regardless of the duration of the pulses. An undesirable electrical discharge will corrupt the sputtering process and cause contamination in the vacuum chamber  104 . Additionally, the increased power can overheat the target and cause target damage.  
       FIG. 2  illustrates a cross-sectional view of an embodiment of a magnetron sputtering apparatus  200  according to the present invention. The magnetron sputtering apparatus  200  includes a chamber  202 , such as a vacuum chamber. The chamber  202  is coupled in fluid communication to a vacuum system  204  through a vacuum control system  206 . In one embodiment, the chamber  202  is electrically coupled to ground potential. The chamber  202  is coupled by one or more gas lines  207  to a feed gas source  208 . In one embodiment, the gas lines  207  are isolated from the chamber and other components by insulators  209 . Additionally, the gas lines  207  can be isolated from the feed gas source using in-line insulating couplers (not shown). A gas flow control system  210  controls the gas flow to the chamber  202 . The gas source  208  can contain any feed gas, such as argon. In some embodiments, the feed gas includes a mixture of gases. In some embodiments, the feed gas includes a reactive gas.  
      A substrate  211  to be sputter coated is supported in the chamber  202  by a substrate support  212 . The substrate  211  can be any type of work piece such as a semiconductor wafer. In one embodiment, the substrate support  212  is electrically coupled to an output  213  of a bias voltage source  214 . An insulator  215  isolates the bias voltage source  214  from a wall of the chamber  202 . In one embodiment, the bias voltage source  214  is an alternating current (AC) power source, such as a radio frequency (RF) power source. In other embodiments (not shown), the substrate support  212  is coupled to ground potential or is electrically floating.  
      The magnetron sputtering apparatus  200  also includes a cathode assembly  216 . In one embodiment, the cathode assembly  216  includes a cathode  218  and a sputtering target  220  composed of target material. The sputtering target  220  is in contact with the cathode  218 . In one embodiment, the sputtering target  220  is positioned inside the cathode  218 . The distance from the sputtering target  220  to the substrate  211  can vary from a few centimeters to about one hundred centimeters.  
      The target material can be any material suitable for sputtering. For example, the target material can be a metallic material, polymer material, superconductive material, magnetic material including ferromagnetic material, non-magnetic material, conductive material, non-conductive material, composite material, reactive material, or a refractory material.  
      The cathode assembly  216  is coupled to an output  222  of a matching unit  224 . An insulator  226  isolates the cathode assembly  216  from a grounded wall of the chamber  202 . An input  230  of the matching unit  224  is coupled to a first output  232  of a pulsed power supply  234 . A second output  236  of the pulsed power supply  234  is coupled to an anode  238 . An insulator  240  isolates the anode  238  from a grounded wall of the chamber  202 . Another insulator  242  isolates the anode  238  from the cathode assembly  216 .  
      In one embodiment, the first output  232  of the pulsed power supply  234  is directly coupled to the cathode assembly  216  (not shown). In one embodiment, the second output  236  of the pulsed power supply  234  is coupled to ground (not shown). In this embodiment, the anode  238  is also coupled to ground (not shown).  
      In one embodiment (not shown), the first output  232  of the pulsed power supply  234  couples a negative voltage impulse to the cathode assembly  216 . In another embodiment (not shown), the first output  232  of the pulsed power supply  234  couples a positive voltage impulse to the anode  238 .  
      In one embodiment, the pulsed power supply  234  generates peak voltage levels of up to about 30,000V. Typical operating voltages are generally between about 100V and 30 kV. In one embodiment, the pulsed power supply  234  generates peak current levels of less than one ampere to about 5,000 A or more depending on the size of the magnetron sputtering system. Typical operating currents varying from less than a few amperes to more than a few thousand amperes depending on the size of the magnetron sputtering system. In one embodiment, the power pulses have a repetition rate that is below 1 kHz. In one embodiment, the pulse width of the pulses generated by the pulsed power supply  234  is substantially between about one microsecond and several seconds.  
      The anode  238  is positioned so as to form a gap  244  between the anode  238  and the cathode assembly  216  that is sufficient to allow current to flow through a region  245  between the anode  238  and the cathode assembly  216 . In one embodiment, the gap  244  is between approximately 0.3 centimeters (0.3 cm) and ten centimeters (10 cm). The volume of region  245  is determined by the area of the sputtering target  220 . The gap  244  and the total volume of region  245  are parameters in the ionization process as will be discussed with reference to  FIG. 3 .  
      An anode shield  248  is positioned adjacent to the anode  238  so as to protect the interior wall of the chamber  202  from being exposed to sputtered target material. Additionally, the anode shield  248  can function as an electric shield to electrically isolate the anode  238  from the plasma. In one embodiment, the anode shield  248  is coupled to ground potential. An insulator  250  is positioned to isolate the anode shield  248  from the anode  238 .  
      The magnetron sputtering apparatus  200  also includes a magnet assembly  252 . In one embodiment, the magnet assembly  252  is adapted to create a magnetic field  254  proximate to the cathode assembly  216 . The magnet assembly  252  can include permanent magnets  256 , or alternatively, electro-magnets (not shown). The configuration of the magnet assembly  252  can be varied depending on the desired shape and strength of the magnetic field  254 . In alternate embodiments, the magnet assembly can have either a balanced or unbalanced configuration.  
      In one embodiment, the magnet assembly  252  includes switching electro-magnets, which generate a pulsed magnetic field proximate to the cathode assembly  216 . In some embodiments, additional magnet assemblies (not shown) can be placed at various locations throughout the chamber  202  to direct different types of sputtered target materials to the substrate  212 .  
      In one embodiment, the magnetron sputtering apparatus  200  is operated by generating the magnetic field  254  proximate to the cathode assembly  216 . In the embodiment shown in  FIG. 2 , the permanent magnets  256  continuously generate the magnetic field  254 . In other embodiments, the magnetic field  254  is generated by energizing a current source (not shown) that is coupled to electro-magnets. In one embodiment, the strength of the magnetic field  254  is between about one hundred and two thousand gauss. After the magnetic field  254  is generated, the feed gas from the gas source  208  is supplied to the chamber  202  by the gas flow control system  210 . In one embodiment, the feed gas is supplied to the chamber  202  directly between the cathode assembly  216  and the anode  238 .  
      In one embodiment, the pulsed power supply  234  is a component in an ionization source that generates the weakly-ionized plasma. The pulsed power supply applies a voltage pulse between the cathode assembly  216  and the anode  238 . In one embodiment, the pulsed power supply  234  applies a negative voltage pulse to the cathode assembly  216 . The amplitude and shape of the voltage pulse are such that a weakly-ionized plasma is generated in the region  246  between the anode  238  and the cathode assembly  216 . The weakly-ionized plasma is also referred to as a pre-ionized plasma. In one embodiment, the peak plasma density of the pre-ionized plasma is between about 10 6  and 10 12  cm −3  for argon feed gas. The pressure in the chamber can vary from about 10 −3  to 10 Torr. The peak plasma density of the pre-ionized plasma depends on the properties of the specific magnetron sputtering system and is a function of the location of the measurement in the pre-ionized plasma.  
      In one embodiment, the pulsed power supply  234  generates a low power pulse having an initial voltage of between about one hundred volts and five kilovolts with a discharge current of between about 0.1 amperes and one hundred amperes in order to generate the weakly-ionized plasma. In some embodiments the width of the pulse can be in on the order of 0.1 microseconds up to one hundred seconds. Specific parameters of the pulse are discussed herein in more detail in connection with  FIG. 4  and  FIG. 5 .  
      In one embodiment, prior to the generating of the weakly-ionized plasma, the pulsed power supply  234  generates a potential difference between the cathode assembly  216  and the anode  238  before the feed gas is supplied between the cathode assembly  216  and the anode  238 .  
      In another embodiment, a direct current (DC) power supply (not shown) is used to generate and maintain the weakly-ionized or pre-ionized plasma. In this embodiment, the DC power supply is adapted to generate a voltage that is large enough to ignite the pre-ionized plasma. In one embodiment, the DC power supply generates an initial voltage of several kilovolts with a discharge current of several hundred milliamps between the cathode assembly  216  and the anode  238  in order to generate and maintain the pre-ionized plasma. The value of the current depends on the power level generated by the power supply and is a function of the size of the magnetron. Additionally, the presence of a magnetic field in the region  245  can have a dramatic effect on the value of the applied voltage and current required to generate the weakly-ionized plasma.  
      In some embodiments, the DC power supply generates a current that is between about 1 mA and 100 A depending on the size of the magnetron and the strength of a magnetic field in the region  245 . In one embodiment, before generating the weakly-ionized plasma, the DC power supply is adapted to generate and maintain an initial voltage between the cathode assembly  216  and the anode  238  before the introduction of the feed gas.  
      The pre-ionized or weakly-ionized plasma can be generated by numerous other techniques including UV radiation techniques, X-ray techniques, electron beam techniques, ion beam techniques, or ionizing filament techniques, for example. In one embodiment, an alternating current (AC) power supply can be used. Generally, an AC power supply can require less power to generate a weakly-ionized plasma than a DC power supply.  
      Forming a weakly-ionized or pre-ionized plasma substantially eliminates the probability of establishing a breakdown condition in the chamber  202  when high-power pulses are applied between the cathode assembly  216  and the anode  238 . The probability of establishing a breakdown condition is substantially eliminated because the weakly-ionized plasma has a low-level of ionization that provides electrical conductivity through the plasma. This conductivity substantially prevents the formation of a breakdown condition, even when high power is applied to the plasma.  
      Once the weakly-ionized plasma is formed, high-power pulses are then generated between the cathode assembly  216  and the anode  238 . In one embodiment, the pulsed power supply  234  generates the high-power pulses. The desired power level of the high-power pulse depends on several factors including the desired deposition rate, the density of the pre-ionized plasma, and the size of the magnetron, for example. In one embodiment, the power level of the high-power pulse is in the range of about one kilowatt to about ten megawatts or more. This power level range corresponds to target densities that are on the order of 0.01 kilowatt per square centimeter to more than ten kilowatts per square centimeter.  
      Each of the high-power pulses are maintained for a predetermined time that, in alternate embodiments, is approximately one microsecond to ten seconds. The repetition frequency or repetition rate of the high-power pulses, in one embodiment, is in the range of between about 0.1 Hz to 1 kHz. In order to minimize undesirable target heating, the average power generated by the pulsed power supply  234  can be less than one megawatt depending on the size of the magnetron. In one embodiment, the thermal energy in the sputtering target  220  is conducted away or dissipated by liquid or gas cooling such as helium cooling (not shown).  
      The high-power pulses generate a strong electric field between the cathode assembly  216  and the anode  238 . This strong electric field is substantially located in the region  245  that is in the gap  244  between the cathode assembly  216  and the anode  238 . In one embodiment, the electric field is a pulsed electric field. In another embodiment, the electric field is a quasi-static electric field. By quasi-static electric field, we mean an electric field that has a characteristic time of electric field variation that is much greater than the collision time for electrons with neutral gas particles. Such a time of electric field variation can be on the order of ten seconds. The strength and the position of the strong electric field will be discussed in more detail with reference to  FIG. 3 .  
      The high-power pulses generate a highly-ionized or a strongly-ionized plasma from the weakly-ionized plasma. The discharge current density that is formed from this strongly-ionized plasma can be as high as about five-hundred amperes per squared centimeter or more for a pressure that is as high as about ten Torr. Since the sputtering target  220  is typically negatively biased, the positively charged ions in the strongly-ionized plasma accelerate at high velocity towards the sputtering target  220 . The accelerated ions impact the surface of the sputtering target  220 , causing the target material to be sputtered. The strongly-ionized plasma of the present invention results in a very high sputtering rate of the target material.  
      In addition, the strongly-ionized plasma tends to diffuse homogenously in the region  246 . The homogenous diffusion results in accelerated ions impacting the surface of the sputtering target  220  in a more uniform manner than with conventional magnetron sputtering. Consequently, the surface of the sputtering target  220  is eroded more evenly and, thus higher target utilization is achieved. Furthermore, since the target material is sputtered more uniformly across the surface of the sputtering target  220 , the uniformity and homogeneity of the material deposited on the substrate  211  is also increased without the necessity of rotating the substrate  211  and/or the magnet assembly  252 . The physical mechanism responsible for this homogenous diffusion is described with reference to  FIG. 6A  through  FIG. 6D .  
      In one embodiment, the high-power pulsed magnetron sputtering system  200  of the present invention generates a relatively high electron temperature plasma and a relatively high density plasma. One application for the high-power pulsed magnetron sputtering system  200  of the present invention is ionized physical vapor deposition (IPVD), which is a technique that converts neutral sputtered atoms into positive ions to enhance the sputtering process.  
       FIG. 3  illustrates a cross-sectional view of the cathode assembly  216  and the anode  238  of  FIG. 2 . In one embodiment, the strong electric field  260  is located in the region  245  between the cathode assembly  216  and the anode  238 . The strong electric field  260  facilitates a multi-step ionization process that substantially increases the rate at which the strongly-ionized plasma is formed.  
      The feed gas  264  flows between the cathode assembly  216  and the anode  238 . A pre-ionizing voltage is applied between the cathode assembly  216  and the anode  238  across the feed gas  264  which forms the weakly-ionized plasma. The weakly-ionized plasma is generally formed in the region  245  and diffuses to a region  266  as the feed gas  264  continues to flow. The electrons in the weakly-ionized plasma are substantially trapped in the region  266  by the magnetic field  254 . In one embodiment (not shown), the magnetic field  254  is generated in the region  245  to substantially trap electrons where the weakly-ionized plasma is ignited.  
      After the formation of the weakly-ionized plasma, a high-power pulse is then applied between the cathode assembly  216  and the anode  238 . This high-power pulse generates the strong electric field  260  in the region  245  between the cathode assembly  216  and the anode  238 . The strong electric field  260  results in collisions occurring between neutral atoms and ions in the weakly ionized plasma. These collisions generate numerous excited atoms in the weakly-ionized plasma. The accumulation of excited atoms in the weakly-ionized plasma alters the ionization process. Instead of direct ionization, the strongly-ionized plasma is generated by a multi-step ionization process having an efficiency that increases as the density of excited atoms in the weakly-ionized plasma increases.  
      The distance or gap  244  between the cathode assembly  216  and the anode  238  is chosen so as to maximize the rate of excitation of the atoms. The value of the electric field  260  in the region  245  depends on the voltage level applied by the pulsed power supply  234  ( FIG. 2 ) and the size of the gap  244  between the anode  238  and the cathode assembly  216 . In alternative embodiments, the strength of the electric field  260  can vary between about 10 V/cm and 10 5  V/cm depending on various parameters and operating conditions of the magnetron system. In some embodiments, the gap  244  can be between about 0.30 cm and about 10 cm depending on various parameters of the process. In one embodiment, the electric field  260  in the region  245  is rapidly applied to the pre-ionized or weakly-ionized plasma. In some embodiments, the rapidly applied electric field  260  is generated by a voltage pulse having a rise time that is between about 0.1 microsecond and ten seconds.  
      In one embodiment, the dimensions of the gap  244  and the parameters of the applied electric field  260  are varied in order to determine the optimum condition for a relatively high rate of excitation of the atoms in the region  245 . For example, an argon atom requires an energy of about 11.55 eV to become excited. Thus, as the feed gas  264  flows through the region  245 , the weakly-ionized plasma is formed and the atoms in the weakly-ionized plasma undergo a stepwise ionization process. The excited atoms in the weakly-ionized plasma then encounter the electrons that are trapped in the region  266  by the magnetic field  254 . Since excited atoms only require about 4 eV of energy to ionize while neutral atoms require about 15.76 eV of energy to ionize, the excited atoms will ionize at a much higher rate than neutral atoms. In one embodiment, ions in the strongly-ionized plasma bombard the sputtering target  220  causing secondary electron emission from the sputtering target  220 . These secondary electrons are substantially trapped by the magnetic field  254  and interact with any neutral or excited atoms in the strongly-ionized plasma. This process further increases the density of ions in the strongly-ionized plasma as the feed gas  264  is replenished.  
      The multi-step ionization process corresponding to the rapid application of the electric field  260  can be described as follows:
 
Ar+ e   − →Ar* + e   − 
 
Ar*+ e   − →Ar + +2 e 
 
 where Ar represents a neutral argon atom in the feed gas and e −  represents an ionizing electron generated in response to a pre-ionized plasma, when sufficient voltage is applied between the cathode assembly  216  and the anode  238 . Additionally, Ar* represents an excited argon atom in the weakly-ionized plasma. The collision between the excited argon atom and the ionizing electron results in an argon ion (Ar +30 ) and two electrons. 
 
      As previously discussed, the excited argon atoms generally require less energy to become ionized than neutral argon atoms. Thus, the excited atoms tend to more rapidly ionize near the surface of the sputtering target  220  than the neutral argon atoms. As the density of the excited atoms in the plasma increases, the efficiency of the ionization process rapidly increases. The increased efficiency eventually results in an avalanche-like increase in the density of the strongly-ionized plasma. Under appropriate excitation conditions, the proportion of the energy applied to the weakly-ionized plasma which is transformed to the excited atoms is very high for a pulsed discharge in the feed gas.  
      Thus, in one embodiment of the invention, high power pulses are applied to a weakly-ionized plasma across the gap  244  to generate a strong electric field between the anode  238  and the cathode assembly  216 . This strong electric field generates excited atoms in the weakly-ionized plasma. The excited atoms are rapidly ionized by secondary electrons emitted by the sputtering target  220 . This rapid ionization results in a strongly-ionized plasma having a large ion density being formed in an area proximate to the cathode assembly  216 .  
       FIG. 4  illustrates a graphical representation  300  of the applied power of a pulse as a function of time for periodic pulses applied to the plasma in the magnetron sputtering system of  FIG. 2 . At time t 0 , the feed gas from the gas source  208  flows into the chamber  202  before the pulsed power supply  234  is activated. The time required for a sufficient quantity of gas to flow from the gas source  208  into the chamber  202  depends on several factors including the flow rate of the gas and the desired pressure in the chamber  202 .  
      In one embodiment (not shown), the pulsed power supply  234  is activated before the feed gas flows into the chamber  202 . In this embodiment, the feed gas is injected between the anode  238  and the cathode assembly  216  where it is ignited by the pulsed power supply  234  to generate the weakly-ionized plasma.  
      In one embodiment, the feed gas flows between the anode  238  and the cathode assembly  216  between time t 0  and time t 1 . At time t 1 , the pulsed power supply  234  generates a pulse  302  between the cathode assembly  216  and the anode  238  that has a power level between about 0.01 kW and 100 kW depending on the size of the magnetron. The pulse  302  is sufficient to ignite the feed gas to generate the weakly-ionized plasma.  
      In another embodiment (not shown), the pulsed power supply  234  applies a potential in the gap  244  between the cathode assembly  216  and the anode  238  before the feed gas from the gas source  208  is delivered into the chamber  202 . In this embodiment, the feed gas is ignited as it flows between the cathode assembly  216  and the anode  238 . In another embodiment, the pulsed power supply  234  generates the pulse  302  between the cathode assembly  216  and the anode  238  when the feed gas from the gas source  208  is delivered into the chamber  202 .  
      The power generated by the pulsed power supply  234  partially ionizes the gas that is located in the region  245  between the cathode assembly  216  and the anode  238 . The partially ionized gas is also referred to as a weakly-ionized plasma or a pre-ionized plasma. As described herein, the formation of weakly-ionized plasma substantially eliminates the possibility of creating a breakdown condition when high-power pulses are applied to the weakly-ionized plasma.  
      In one embodiment, the power is continuously applied for between about one microsecond and one hundred seconds to allow the pre-ionized plasma to form and be maintained at a sufficient plasma density. In one embodiment, the power from the pulsed power supply  234  is continuously applied after the weakly-ionized plasma is ignited to maintain the weakly-ionized plasma. The pulsed power supply  234  can be designed so as to generate a continuous nominal power in order to generate and sustain the weakly-ionized plasma until a high-power pulse is delivered by the pulsed power supply  234 .  
      At time t 2 , the pulsed power supply  234  delivers a high-power pulse  304  across the weakly-ionized plasma. In some embodiments, the high-power pulse  304  has a power that is in the range of between about one kilowatt to ten megawatts depending on the size of the magnetron. The high-power pulse has a leading edge  306  with a rise time that is between about 0.1 microseconds and ten seconds  
      The high-power pulse  304  has a power and a pulse width that is sufficient to transform the weakly-ionized plasma to a strongly-ionized plasma. In one embodiment, the high-power pulse  304  is applied for a time that is in the range of between about ten microseconds and ten seconds. At time t 4 , the high-power pulse  304  is terminated.  
      The power supply  224  maintains the weakly-ionized plasma after the delivery of the high-power pulse  304  by applying background power that, in one embodiment, is between about 0.01 kW and 100 kW. The background power can be a pulsed or continuously applied power that maintains the pre-ionization condition in the plasma, while the pulsed power supply  234  prepares to deliver another high-power pulse  308 .  
      At time t 5 , the pulsed power supply  234  delivers another high-power pulse  308 . The repetition rate between the high-power pulses  304 ,  308  is, in one embodiment, between about 0.1 Hz and 1 kHz. The particular size, shape, width, and frequency of the high-power pulses  304 ,  308  depend on various factors including process parameters, the design of the pulsed power supply  234 , the size of the magnetron, and the design of the sputter system. The shape and duration of the leading edge  308  and the trailing edge  310  of the high-power pulse  304  is chosen to sustain the weakly-ionized plasma while controlling the rate of ionization of the strongly-ionized plasma. In one embodiment, the particular size, shape, width, and frequency of the high-power pulse  304  is chosen to control the rate of sputtering of the target material.  
       FIG. 5  illustrates graphical representations  320 ,  322 , and  324  of the absolute value of applied voltage, current, and power, respectively, as a function of time for periodic pulses applied to the plasma in the magnetron sputtering system of  FIG. 2 . In one embodiment, at time t 0  (not shown), the feed gas from the gas source  208  flows into the chamber  202  before the pulsed power supply  234  is activated. The time required for a sufficient quantity of gas to flow from the gas source  208  into the chamber  202  depends on several factors including the flow rate of the gas and the desired pressure in the chamber  202 .  
      In the embodiment shown in  FIG. 5 , the power supply  238  generates a constant power. At time t 1 , the pulsed power supply  234  generates a voltage  326  across the anode  238  and the cathode assembly  216 . In one embodiment, the voltage  326  is approximately between one hundred and two thousand volts. The period between time t 0  and time t 1  (not shown) can be on the order of several microseconds up to several milliseconds. At time t 1 , the current  328  and the power  330  have constant value.  
      Between time t 1  and time t 2 , the voltage  326 , the current  328 , and the power  326  remain constant as the weakly-ionized plasma is generated. The voltage  332  at time t 2  is between about 100V and 2,000V. The current  334  at time t 2  is between about 0.1 A and 100 A. The power  336  delivered at time t 2  is between about 0.01 kW and 100 kW.  
      The power  336  generated by the pulsed power supply  234  partially ionizes the gas that is located between the cathode assembly  216  and the anode  238 . The partially ionized gas is also referred to as a weakly-ionized plasma or a pre-ionized plasma. As described herein, the formation of weakly-ionized plasma substantially eliminates the possibility of creating a breakdown condition when high-power pulses are applied to the weakly-ionized plasma. The suppression of this breakdown condition substantially eliminates the occurrence of undesirable arcing in the chamber  202 .  
      In one embodiment, the period between time t 1  and time t 2  is between about one microsecond and one hundred seconds to allow the pre-ionized plasma to form and be maintained at a sufficient plasma density. In one embodiment, the power  336  from the pulsed power supply  234  is continuously applied to maintain the weakly-ionized plasma. The pulsed power supply  234  can be designed so as to output a continuous nominal power into order to sustain the weakly-ionized plasma.  
      Between time t 2  and time t 3 , the pulsed power supply  234  delivers a large voltage pulse  338  across the weakly-ionized plasma. In some embodiments, the large voltage pulse  338  has a voltage that is in the range of two hundred to thirty thousand volts. In some embodiment, the period between time t 2  and time t 3  is between about 0.1 microseconds and ten seconds. The large voltage pulse  338  is applied between time t 3  and time t 4 , before the current across the plasma begins to increase. In one embodiment, the time period between time t 3  and time t 4  can be between about one microsecond and ten seconds.  
      Between time t 4  and time t 5 , the voltage  340  drops as the current  342  increases. The power  344  also increases between time t 4  and time t 5 , until a quasi-stationary state exists between the voltage  346  and the current  348 . The period between time t 4  and time t 5  can be on the order of one to one hundred microseconds.  
      In one embodiment, at time t 5 , the voltage  346  is between about one hundred and thirty thousand volts, the current  348  is between about one hundred and five thousand amperes and the power  350  is between about one kilowatt and ten megawatts. The power  350  is continuously applied to the plasma until time t 6 . In one embodiment, the period between time t 5 , and time t 6  is approximately between one microsecond and ten seconds.  
      The pulsed power supply  234  delivers a high-power pulse having a maximum power  350  and a pulse width that is sufficient to transform the weakly-ionized plasma to a strongly-ionized plasma. At time t 6 , the maximum power  350  is terminated. In one embodiment, the pulsed power supply  234  continues to supply a background power that is sufficient to maintain the plasma after time t 6 .  
      In one embodiment, the power supply  234  maintains the plasma after the delivery of the high-power pulse by continuing to apply a power  352  that can be between about 0.01 kW and 100 kW to the plasma. The continuously generated power maintains the pre-ionization condition in the plasma, while the pulsed power supply  234  prepares to deliver the next high-power pulse.  
      At time t 7 , the pulsed power supply  234  delivers the next high-power pulse (not shown). In one embodiment, the repetition rate between the high-power pulses is between about 0.1 Hz and 1 kHz. The particular size, shape, width, and frequency of the high-power pulses depend on various factors including process parameters, the design of the pulsed power supply  234 , the size of the magnetron, and the design of the sputter system.  
      In another embodiment (not shown), the power supply  234  generates a constant voltage. In this embodiment, the applied voltage  320  is continuously applied from time t 2  until time t 6 . The current  322  and the power  324  change to keep the applied voltage  320  constant. The current  322  and the power  224  rise until time t 6 , where the voltage  320  is terminated.  
       FIG. 6A  through  FIG. 6D  illustrate various simulated magnetic field distributions  400 ,  402 ,  404 , and  406  that are proximate to the cathode assembly  116  for various electron ExB drift currents in the magnetron sputtering apparatus  200  of  FIG. 2 . The simulated magnetic fields distributions  400 ,  402 ,  404 , and  406  indicate that high-power plasmas having high current density tend to diffuse homogeneously in the area  246  of the magnetron sputtering apparatus  200  of  FIG. 2 .  
      The high-power pulses between the cathode assembly  216  and the anode  238  generate secondary electrons from the cathode assembly  216  that move in a substantially circular motion proximate to the cathode assembly  216  according to crossed electric and magnetic fields. The substantially circular motion of the electrons generate an electron ExB drift current. The magnitude of the electron ExB drift current is proportional to the magnitude of the discharge current in the plasma and, in one embodiment, is approximately in the range of between about three and ten times the magnitude of the discharge current.  
      In one embodiment, the substantially circular electron ExB drift current generates a magnetic field that interacts with the magnetic field generated by the magnet assembly  252 . In one embodiment, the magnetic field generated by the electron ExB drift current has a direction that is substantially opposite to the direction of the magnetic field generated by the magnet assembly  252 . The magnitude of the magnetic field generated by the electron ExB drift current increases with increased electron ExB drift current. The homogeneous diffusion of the strongly-ionized plasma in the region  246  is caused, at least in part, by the interaction of the magnetic field generated by the magnet assembly  252  and the magnetic field generated by the electron ExB drift current.  
      In one embodiment, the electron ExB drift current defines a substantially circular shape for low current density plasma. However, as the current density of the plasma increases, the substantially circular electron ExB drift current tends to describe a more complex shape as the interaction of the magnetic field generated by the magnet assembly  252 , the electric field generated by the high-power pulse, and the magnetic field generated by the electron ExB drift current becomes more acute. For example, in one embodiment, the electron ExB drift current has a substantially cycloidal shape. Thus, the exact shape of the electron ExB drift current can be quite elaborate and depends on various factors.  
      For example,  FIG. 6A  illustrates the magnetic field lines  408  produced from the interaction of the magnetic field generated by the magnet assembly  252  and the magnetic field generated by an electron ExB drift current  410  illustrated by a substantially circularly shaped ring. The electron ExB drift current  410  is generated proximate to the cathode assembly  216 .  
      In the example shown in  FIG. 6A , the electron ExB drift current  410  is approximately one hundred amperes (100 A). In one embodiment of the invention, the electron ExB drift current  410  is between approximately three and ten times as great as the discharge current. Thus, in the example shown in  FIG. 6A , the discharge current is approximately between 10 A and 30 A.  
      The magnetic field lines  408  shown in  FIG. 6A  indicate that the magnetic field generated by the magnet assembly  252  is substantially undisturbed by the relatively small magnetic field that is generated by the relatively small electron ExB drift current  410 .  
       FIG. 6B  illustrates the magnetic field lines  412  produced from the interaction of the magnetic field generated by the magnet assembly  252  and the magnetic field generated by an electron ExB drift current  414 . The electron ExB drift current  414  is generated proximate to the cathode assembly  216 .  
      In the example shown in  FIG. 6B , the electron ExB drift current  414  is approximately 300 A. Since the electron ExB drift current  414  is typically between about three and ten times as great as the discharge current, the discharge current in this example is approximately between 30 A and 100 A.  
      The magnetic field lines  412  that are generated by the magnet assembly  252  are substantially undisturbed by the relatively small magnetic field generated by the relatively small electron ExB drift current  414 . However, the magnetic field lines  416  that are closest to the electron ExB drift current  414  are somewhat distorted by the magnetic field generated by the electron ExB drift current  414 . The distortion suggests that a larger electron ExB drift current should generate a stronger magnetic field that will interact more strongly with the magnetic field generated by the magnet assembly  252 .  
       FIG. 6C  illustrates the magnetic field lines  418  that are produced from the interaction of the magnetic field generated by the magnet assembly  252  and the magnetic field generated by an electron ExB drift current  420 . The electron ExB drift current  420  is generated proximate to the cathode assembly  216 .  
      In the example shown in  FIG. 6C , the electron ExB drift current  420  is approximately 1,000 A. Since the electron ExB drift current  420  is typically between about three and ten times as great as the discharge current, the discharge current in this example is approximately between 100 A and 300 A.  
      The magnetic field lines  418  that are generated by the magnet assembly  252  exhibit substantial distortion that is caused by the relatively strong magnetic field generated by the relatively large electron ExB drift current  420 . Thus, the larger electron ExB drift current  420  generates a stronger magnetic field that strongly interacts with and can begin to dominate the magnetic field generated by the magnet assembly  252 .  
      The interaction of the magnetic field generated by the magnet assembly  252  and the magnetic field generated by the electron ExB drift current  420  substantially generates magnetic field lines  422  that are somewhat more parallel to the surface of the sputtering target  220  than the magnetic field lines  408 ,  412 , and  416  in  FIG. 6A  and  FIG. 6B . The magnetic field lines  422  allow the strongly-ionized plasma to more uniformly distribute itself in the area  246 . Thus, the strongly-ionized plasma is substantially uniformly diffused in the area  246 , and consequently, the sputtering target  220  is eroded more uniformly thereby resulting in higher target utilization than can be achieved than in conventional magnetron sputtering systems.  
       FIG. 6D  illustrates the magnetic field lines  424  produced from the interaction of the magnetic field generated by the magnet assembly  252  and the magnetic field generated by an electron ExB drift current  426 . The electron ExB drift current  426  is generated proximate to the cathode assembly  216 .  
      In the example shown in  FIG. 6D , the electron ExB drift current  426  is approximately 5,000 A. The discharge current in this example is approximately between 500 A and 1,700 A.  
      The magnetic field lines  424  generated by the magnet assembly  252  are relatively distorted due to their interaction with the relatively strong magnetic field generated by the relatively large electron ExB drift current  426 . Thus, in this embodiment, the relatively large electron ExB drift current  426  generates a very strong magnetic field that is substantially stronger than the magnetic field generated by the magnet assembly  252 .  
       FIG. 7  illustrates a cross-sectional view of another embodiment of a magnetron sputtering apparatus  450  according to the present invention. The magnetron sputtering apparatus  450  includes an electrode  452  that generates a weakly-ionized or pre-ionized plasma. The electrode  452  is also referred to as a pre-ionizing filament electrode and is a component in an ionization source that generates the weakly ionized plasma.  
      In one embodiment, the electrode  452  is coupled to an output  454  of a power supply  456 . The power supply  456  can be a DC power supply or an AC power supply. An insulator  458  isolates the electrode  452  from the grounded wall of the chamber  202 . In one embodiment, the electrode  452  is substantially shaped in the form of a ring electrode. In other embodiments, the electrode  452  is substantially shaped in a linear form or any other shape that is suitable for pre-ionizing the plasma.  
      In one embodiment, a second output  460  of the power supply  456  is coupled to the cathode  218 . The insulator  226  isolates the cathode  218  from the grounded wall of the chamber  202 . In one embodiment, the power supply  456  generates an average output power that is in the range of between about 0.01 kW and 100 kW. Such an output power is sufficient to generate a suitable current between the electrode  452  and the cathode assembly  216  to pre-ionize feed gas that is located proximate to the electrode  452 .  
      In operation, the magnetron sputtering apparatus  450  functions in a similar manner to the magnetron sputtering apparatus  200  of  FIG. 2 , but with some operational differences. The magnetic field  254  is generated proximate to the cathode assembly  216 . In one embodiment, the strength of the magnetic field  254  is between about one hundred and two thousand gauss. The feed gas is supplied from the gas source  208  to the chamber  202  by the gas flow control system  210 .  
      The power supply  456  applies a suitable current between the cathode assembly  216  and the electrode  452 . The parameters of the current are chosen to establish a weakly-ionized plasma in the area  246  proximate to the electrode  452 . In one embodiment, the power supply  456  generates a voltage of between about one hundred volts and five thousand volts with a discharge current that is between about 0.1 A and 100 A depending on the size of the magnetron. An example with specific parameters will be discussed herein in more detail in connection with  FIG. 8 .  
      In one embodiment, the resulting pre-ionized plasma density is in the range between approximately 10 6  and 10 12  cm −3  for argon sputtering gas. In one embodiment, the pressure in the chamber  202  is in the range of approximately 10 −3  to 10 Torr. As previously discussed, the weakly-ionized or pre-ionized plasma reduces or substantially eliminates the possibility of establishing a breakdown condition in the chamber  202  when high-power pulses are applied to the plasma.  
      The pulsed power supply  234  then generates a high-power pulse between the cathode assembly  216  and the anode  238 . The high-power pulse generates a strongly-ionized plasma from the weakly-ionized plasma. The parameters of the high-power pulse depend on various parameters including the size of the magnetron, the desired deposition rate, and the concentration of the pre-ionized plasma necessary for depositing the target material.  
      In one embodiment, the high-power pulse between the cathode assembly  216  and the anode  238  is in the range of about one kilowatt to about ten megawatts. This corresponds to target densities on the order of several kilowatts per square centimeter. In one embodiment, the ion current density that can be generated from the strongly-ionized plasma is greater than about one ampere per squared centimeter for a pressure of approximately ten mTorr.  
      In one embodiment, the high-power pulse has a pulse width that is in the range of approximately one microsecond to several seconds. In one embodiment, the repetition rate of the high-power discharge is in the range of between about 0.1 Hz to 1 kHz. In one embodiment, in order to minimize undesirable target heating, the average power generated by the pulsed power supply is less than one megawatt depending on the size of the magnetron. In one embodiment, the thermal energy in the sputtering target  220  is conducted away or dissipated by liquid or gas cooling (not shown).  
      The gas flow control system  210  provides a feed gas flow rate that is high enough to maintain the strongly-ionized plasma. Additionally, the vacuum control system  206  controls the pressure so as to maintain the pressure inside the chamber  202  in a range that supports the strongly-ionized plasma.  
      The ions in the strongly-ionized plasma accelerate towards the sputtering target  220  at high velocity and impact the surface of the sputtering target  220 . The strongly ionized plasma causes a very high sputtering rate of the target material. Furthermore, as described herein in connection with  FIG. 6A  though  FIG. 6D , the strongly-ionized plasma generated by the sputtering systems according to the present invention tends to diffuse homogenously in the area  246  due to the interaction of generated magnetic fields. This homogenous diffusion results in a more uniform distribution of ions impacting the surface of the target material  220  compared with conventional magnetron sputtering systems, thereby resulting in relatively high target utilization and relatively uniform deposition of target material on the substrate  211 .  
       FIG. 8  illustrates a graphical representation of pulse power as a function of time for periodic pulses applied to the plasma in the magnetron sputtering system of  FIG. 7 . In one embodiment, the feed gas from the gas source  208  flows into the chamber  202  at time t 0 , before either the power supply  456  or the pulsed power supply  234  are activated.  
      In another embodiment, prior to the formation of the weakly-ionized plasma, the power supply  456  and/or the pulsed power supply  234  are activated at time t 0  before the gas enters the chamber  202 . In this embodiment, the feed gas is injected between the electrode  452  and the cathode assembly  216  where it is ignited by the power supply  456  to generate the weakly-ionized plasma.  
      The time required for a sufficient quantity of gas to flow into the chamber  202  depends on several factors including the flow rate of the gas and the desired operating pressure in the chamber  202 . At time t 1 , the power supply  456  generates a power  502  that is in the range of between about 0.01 kW to about 100 kW between the electrode  452  and the cathode assembly  216 . The power  502  causes the gas proximate to the electrode  452  to become partially ionized, thereby generating a weakly-ionized plasma or a pre-ionized plasma.  
      At time t 2 , the pulsed power supply  234  delivers a high-power pulse  504  to the weakly-ionized plasma that is on the order of less than one kilowatt to about ten megawatts depending on the size of the magnetron. The high-power pulse  504  is sufficient to transform the weakly-ionized plasma to a strongly-ionized plasma. The high-power pulse has a leading edge  506  having a rise time that is between about 0.1 microseconds and ten seconds.  
      In one embodiment, the pulse width of the high-power pulse  504  is in the range of between about one microsecond and ten seconds. The high-power pulse  504  is terminated at time t 4 . Even after the delivery of the high-power pulse  504 , the power  502  from the power supply  456  is continuously applied to sustain the pre-ionized plasma, while the pulsed power supply  234  prepares to deliver another high-power pulse  508 . In another embodiment (not shown), the power supply  456  is an AC power supply and delivers suitable power pulses to ignite and sustain the weakly-ionized plasma.  
      At time t 5 , the pulsed power supply  234  delivers another high-power pulse  508 . In one embodiment, the repetition rate of the high-power pulses can be between about 0.1 Hz and 1 kHz. The particular size, shape, width, and frequency of the high-power pulse depend on the process parameters and on the design of the pulsed power supply  234  and the sputter system. The shape and duration of the leading edge  506  and the trailing edge  510  of the high-power pulse  504  is chosen to control the rate of ionization of the strongly-ionized plasma. In one embodiment, the particular size, shape, width, and frequency of the high power pulse  504  is chosen to control the rate of sputtering of the target material.  
       FIG. 9A  through  FIG. 9C  are cross-sectional views of various embodiments of cathode assemblies  216 ′,  216 ″, and  216 ″′ according to the present invention.  FIG. 9A  through  FIG. 9C  illustrate one side (the right side with reference to  FIG. 7 ) of each cathode assembly. The left side of each cathode assembly is generally symmetrical to the illustrated right side.  FIG. 9A  through  FIG. 9C  illustrate various configurations of the electrode  452  and the cathode assemblies  216 ′,  216 ″, and  216 ′″. These various configurations can affect the parameters of the electric field generated between the electrode  452  and each of the cathode assemblies  216 ′,  216 ″, and  216 ′″. The parameters of the electric field can influence the ignition of the pre-ionized plasma as well as the pre-ionization process generally. In one embodiment, these various embodiments create the necessary conditions for breakdown of the feed gas and ignition of the weakly-ionized plasma in the region between the anode  238  and each respective cathode assembly  216 ′,  216 ″, and  216 ′″.  
       FIG. 9A  illustrates one side of the cathode assembly  216 ′. In this embodiment, a sputtering target  220 ′ is substantially positioned in contact with a cathode  218 ′. The sputtering target  220 ′ extends past the bend  520  of the ring-shaped electrode  452 . In this embodiment, the electric field lines (not shown) from the electric field generated between the cathode assembly  216 ′ and the ring-shaped electrode  452  are substantially perpendicular to the cathode assembly  216 ′ along the circumference of the ring-shaped electrode  452 . This embodiment can increase the efficiency of the pre-ionization process. Furthermore, since the cathode  218 ′ is never directly exposed to the plasma, ions from the plasma do not bombard the cathode  218 ′ and therefore, any contamination that could otherwise be generated by the cathode material is substantially reduced.  
       FIG. 9B  illustrates one side of the cathode assembly  216 ″. In this embodiment, a sputtering target  220 ″ is substantially positioned in contact with a cathode  218 ″. The sputtering target  220 ″ extends to the point  524  on the cathode assembly  216 ″. In this embodiment, the electric field lines (not shown) generated between the cathode assembly  216 ″ and the electrode  452  are substantially perpendicular to the cathode assembly  216 ″ at the point  528  on the cathode  218 ″. The electric field in the gap  530  between the electrode  452  and the cathode  218 ″ is adapted to ignite the plasma from the feed gas flowing through the gap  530 . Depending on various parameters such as where the magnetic field is generated relative to the sputtering target  220 ″ and the pressure in the area proximate to the cathode assembly  216 ″, this embodiment can increase the efficiency of the pre-ionization process.  
       FIG. 9C  illustrates one side of the cathode assembly  216 ′″. In this embodiment, a sputtering target  220 ′″ is substantially positioned in contact with a cathode  218 ′″. The sputtering target  220 ′″ extends to position  532  on the cathode assembly  216 ′″. In this embodiment, the electric field lines (not shown) generated between the cathode assembly  216 ′″ and the electrode  452  are substantially perpendicular to the cathode assembly  216 ′″ at the position  538 . The electric field in the gap  540  between the electrode  452  and the cathode  218 ′″is adapted to ignite the plasma from the feed gas flowing through the gap  540 . Depending on various parameters such as where the magnetic field is generated relative to the sputtering target  220 ′″ and the pressure in the area proximate to the cathode assembly  216 ′″, this embodiment can increase the efficiency of the pre-ionization process.  
       FIG. 10  is a cross-sectional view of another embodiment of a magnetron sputtering apparatus  450 ′ according to the present invention. This embodiment is similar to the magnetron sputtering apparatus  450  of  FIG. 7 . However, in this embodiment, the electrode  452 ′, which is a component of the ionization source, substantially surrounds the cathode assembly  216 . The position of the electrode  452 ′ relative to the cathode assembly  216  is chosen to achieve particular electrical conditions in the gap  244  between the anode  238  and the cathode assembly  216 . For example, in this embodiment, since the pre-ionizing electrode  452 ′ is not physically located in the region  245 ′ between the anode  238  and the cathode assembly  216 , it does not interfere with the strong electric field that results when a high-power pulse is applied between the anode  238  and the cathode assembly  216 . Additionally, the location of the pre-ionizing electrode  452 ′ results in a more uniformly distributed weakly-ionized plasma in the region  246 ′.  
      The power supply  456  applies a substantially constant voltage between the cathode assembly  216  and the electrode  452 ′. The substantially constant voltage generates a weakly-ionized or pre-ionized plasma proximate to the electrode  452 ′ and the cathode assembly  216 . The pre-ionized plasma substantially eliminates the possibility of establishing a breakdown condition in the chamber  202  when high-power pulses are applied to the plasma. In one embodiment, the power supply  456  is a DC power supply that generates a DC voltage that is in the range of between about one hundred volts and several kilovolts with a discharge current that is in the range of between about 0.1 A and 10 A. In another embodiment, the power supply  456  is an AC power supply that generates voltage pulses between the cathode assembly  216  and the electrode  452 ′.  
      Since the electrode  452 ′ substantially surrounds the cathode assembly  216 , a distance  462  between the electrode  452 ′ and the cathode  218  can be varied by changing the diameter of the electrode  452 ′. For example, the distance  462  can be varied from about 0.1 cm to about 10 cm. The distance  462  is optimized to generate sustainable weakly-ionized plasma in the region  246 ′. The vertical position of the electrode  452 ′ relative to the cathode assembly  216  can also be varied.  
       FIG. 11  illustrates a cross-sectional view of another illustrative embodiment of a magnetron sputtering apparatus  450 ″ according to the present invention. The magnetron sputtering apparatus  450 ″ is similar to the magnetron sputtering apparatus  450  of  FIG. 7 . The electrode  452 ″ is a component of an ionization source. However, the electrode  452 ″ is coupled to a first power supply  464  and also to an additional second power supply  466 . The position of the electrode  452 ″ relative to the cathode assembly  216  is chosen to achieve particular sputtering characteristics.  
      A first output  468  of the first power supply  464  is coupled through the insulator  458  to a first side  470  of the electrode  452 ″. A second output  472  of the first power supply  464  is coupled to a second side  474  of the electrode  452 ″ through an insulator  476 . The first power supply  464  is adapted to generate a current through the electrode  452 ″. The current essentially generates heat in the electrode  452 ″. The heated electrode  452 ″ emits electrons in the area  245 ″.  
      In one embodiment, the first power supply  464  is a DC power supply and applies a substantially constant current through the electrode  452 ″. In another embodiment, the first power supply  464  is an AC power supply.  
      A first output  478  of the second power supply  466  is coupled to the anode  238  through an insulator  480 . A second output  482  of the second power supply  466  is coupled to the second side  474  of the electrode. The second power supply  466  is adapted to apply a voltage between the electrode  452 ″ and the anode  238 . The second power supply  466  can be an AC power supply or a DC power supply. In one embodiment, the second power supply  466  generates a voltage in the range of about one hundred volts and several kilovolts with a discharge current that is in the range of between about 0.1 A and 10 A.  
      In one embodiment, the second power supply  466  applies a substantially constant voltage that generates a weakly-ionized or pre-ionized plasma proximate to the electrode  452 ″ and the cathode assembly  216 . The pre-ionized plasma substantially eliminates the possibility of establishing a breakdown condition in the chamber  202  when high-power pulses are applied to the plasma.  
      The pulsed power supply  234  then generates a high-power pulse between the cathode assembly  216  and the anode  238 . The high-power pulse generates a strongly-ionized plasma from the weakly-ionized plasma. The parameters of the high-power pulse depend on various parameters including the size of the magnetron, the desired deposition rate, and the concentration of the pre-ionized plasma necessary for depositing the target material, for example.  
       FIG. 12  is a flowchart  600  of an illustrative process of sputter deposition according to the present invention. The process is initiated (step  602 ) by activating various systems in the magnetron sputtering apparatus  200  of  FIG. 2 . For example, the chamber  202  is initially pumped down to a specific pressure (step  604 ). Next, the pressure in the chamber  202  is checked (step  606 ). In one embodiment, feed gas is then pumped into the chamber (step  608 ). The gas pressure is then checked (step  610 ). If the gas pressure is correct, the chamber pressure is then re-checked (step  612 ). If the chamber pressure is correct, an appropriate magnetic field is generated proximate to the feed gas (step  614 ). In one embodiment, the magnet assembly  252  of  FIG. 2  includes at least one permanent magnet, where magnetic field is generated constantly, even before the process is initiated. In another embodiment, a magnetic assembly (not shown) includes at least one electromagnet, where the magnetic field is generated only when the electromagnet is operating.  
      When the magnetic field is appropriate (step  616 ), the feed gas is ionized to generate a weakly-ionized plasma (step  618 ). In one embodiment, the weakly-ionized plasma can be generated by creating a relatively low current discharge in the gap  244  between the cathode assembly  216  and the anode  238  of  FIG. 2 . In another embodiment, the weakly-ionized plasma can be generated by creating a relatively low current discharge between the electrode  452  and the cathode assembly  216  of  FIG. 7 . In yet another embodiment, the electrode  452 ″ is heated to emit electrons proximate to the cathode assembly  216  of  FIG. 11 . In the embodiment of  FIG. 11 , a relatively low current discharge is created between the anode  238  and the electrode  452 ″.  
      In the embodiment shown in  FIG. 2 , the weakly-ionized plasma is generated by applying a potential across the gap  244  between the cathode assembly  216  and the anode  238  before the introduction of the feed gas. In the embodiment shown in  FIG. 7 , the weakly-ionized plasma is generated by applying a potential difference between the electrode  452  and the cathode assembly  216  before the introduction of the feed gas to generate the weakly-ionized plasma.  
      When the gas is weakly ionized (step  620 ), a strongly-ionized plasma is generated from the weakly-ionized plasma (step  622 ). In one embodiment, the strongly-ionized plasma is generated by applying a high-power pulse in the gap  244  between the cathode assembly  216  and the anode  238 . As previously discussed, the high-power pulse results in a strong electric field being generated in the gap  244  between the anode  238  and the cathode assembly  216 . The strong electric field results in a stepwise ionization process. In one embodiment, the strongly-ionized plasma is substantially homogeneous in the area  246  of  FIG. 2 . This homogeneity results in substantially uniform erosion of the sputtering target  220  and, therefore, relatively high sputtering target utilization.  
      The cathode assembly  216  attracts ions from the strongly-ionized substantially uniform plasma because the cathode assembly  216  is negatively biased relative to the anode  238 . This causes the ions to bombard the cathode assembly  216  causing sputtering of the target material.  
      In one embodiment, the sputter deposition is monitored (step  628 ) by known monitoring techniques. Once the sputter deposition is completed (step  630 ), the sputter process is ended (step  632 ).  
       FIG. 13  is a flowchart  650  of an illustrative process of controlling the sputter rate according to the present invention. The process is initiated (step  602 ) by activating various systems in the magnetron sputtering apparatus  200  of  FIG. 2 . For example, the chamber  202  is initially pumped down to a specific pressure (step  604 ). Next, the pressure in the chamber  202  is evaluated (step  606 ). In one embodiment, feed gas is then pumped into the chamber (step  608 ). The gas pressure is evaluated (step  610 ). If the gas pressure is correct, the pressure in the chamber  202  is again evaluated (step  612 ). If the pressure in the chamber  202  is correct, an appropriate magnetic field is generated proximate to the feed gas (step  614 ).  
      Assuming that the magnetic field is appropriate (step  616 ), the feed gas is ionized to generate a weakly-ionized plasma (step  618 ). In one embodiment, the weakly-ionized plasma can be generated by creating a relatively low current discharge between the cathode assembly  216  and the anode  238  of  FIG. 2 .  
      After the weakly-ionized plasma is generated (step  620 ), a strongly-ionized plasma is generated from the weakly-ionized plasma (step  622 ). In one embodiment, the strongly-ionized plasma is generated by applying a high-power pulse in the gap  244  between the cathode assembly  216  and the anode  238 . In one embodiment, the strongly-ionized plasma is substantially homogeneous in the area  246  of  FIG. 2 . This homogeneity results in more uniform erosion of the sputtering target  220 .  
      The cathode assembly  216  attracts ions from the strongly-ionized substantially uniform plasma because the the cathode assembly  216  is negatively biased relative to the anode  238 . This causes the ions to bombard the cathode assembly  216  causing sputtering of the target material.  
      In one embodiment, the sputter rate is monitored (step  652 ) by known monitoring techniques. If the sputter rate is not sufficient (step  654 ), the power delivered to the plasma is increased (step  656 ). Increasing the magnitude of the high-power pulse applied in the gap  244  between the cathode assembly  216  and the anode  238  increases the power delivered to the plasma. The sputter rate is again evaluated (step  652 ). This process continues (step  658 ) until the sputter rate is sufficient (step  654 ). Once the sputter deposition is completed (step  660 ), the sputter process is ended (step  662 ).  
      Equivalents  
      While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined herein.