Abstract:
A method and system for controlling arcs in a DC sputtering system with a passive circuit is presented. The arc control system includes a sputtering chamber that houses an anode and a sputtering target formed from a target material and serving as a cathode. A DC power supply provides a DC voltage between the cathode and anode such that a cathode current flows from the anode to the cathode. A resonant network is coupled between the DC power supply and the chamber. The resonant network has sufficient Q so that in reaction to an arc, the cathode current resonates through zero, causing a positive voltage to be applied between the cathode and anode. A reverse voltage clamp is coupled across the resonant network to clamp the cathode voltage to a predetermined reverse voltage. The reverse cathode voltage inhibits subsequent arcing by positively charging insulated deposits on the sputtering target. The arc control system limits the quantity of energy that is dissipated by the arc.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. patent application Ser. No. 09/678,993, filed on Oct. 4, 2000. The disclosure of the above application is incorporated in its entirety herein by reference. 
     
    
     
         [0002]    BACKGROUND AND SUMMARY OF THE INVENTION  
           [0003]    The present invention relates generally to plasma-based thin film processing. More particularly, the invention relates to an arc control apparatus for DC sputtering systems that decreases the recovery time from an arc.  
           [0004]    With the increasing demand for optical and disk media such as CD, DVD, MD, MO, DLC films, and hard disks; the importance of the sputtering processes that are used in the manufacture of these media continues to increase. There are numerous types of sputtering systems, all of which are employed to deposit insulating or conductive coatings on devices ranging from semiconductors to drill bits. The films that are generally applied to optical and disk media are typically created with a sputtering process having poor control over the sputtering gas, i.e. a significant fraction of atmosphere and petrochemical volatilities are in the chamber at the beginning of the process.  
           [0005]    During the initial sputtering phase, atmosphere is introduced into the plasma chamber to combine with freed target material. The resulting compound, typically oxides and nitrides, may form a film on the surface of the target. This is referred to as target poisoning, and will cause arcing in DC sputtering. Arcing, though inevitable in these processes is a mixed blessing. The arc often removes the poisoning from the target, but it may also generate undesirable particles to damage the substrate or disk.  
           [0006]    Further sources of arcing include contaminants such as moisture, atmospheric gases, inclusions, and outgassing from the workpiece may also cause arcing.  
           [0007]    In addition to particulate defects, arcing may lead to a defect commonly known as “mousebites”, which is illustrated in FIG. 1. A mousebite  9  is an irregularity that generally occurs along the edges of a workpiece coating. The occurrence of mousebites is related to the configuration of the power supply and attached energy storage components. In applications such as coating CDs, the effect of mousebites can be attenuated by increasing the portion of the workpiece adjacent to the edges that is masked from coating. However, masking limits the usable area of the workpiece, thereby increasing the need for maintenance, i.e. replacement of the mask due to coating buildup.  
           [0008]    To control arcs, conventional DC sputtering systems include arc suppression systems that are either attached to or integrated into the power supply. Arc suppression systems may be divided into two groups. The first type, periodic arc control systems, cause a periodically occurring interruption or voltage reversal of the cathode voltage in an attempt to avoid arcing. The second type, arc initiated control systems, bring about an interruption only after the beginning of an arc has been detected.  
           [0009]    Periodic suppression systems universally employ at least one active switch (in shunt or series to the cathode) to interrupt the flow of current or to apply a reverse voltage to the cathode. The frequency and pulse width of the switch are normally set so that arcing is suppressed, thereby eliminating defects that result from target poisoning and its associated arcs. A disadvantage of periodically interrupting the cathode voltage is that the basic deposition rate may be reduced since the cathode voltage is not continuously applied. Another disadvantage of periodic suppression systems is the additional cost of the active devices and associated control circuits. Generally, periodic suppression systems are only employed when defect free deposition is required, such as in the manufacture of semiconductors.  
           [0010]    Arc initiated control is employed for the manufacture of devices in which lower cost and reduced deposition times are the primary requirements. Traditionally, an arc initiated control system senses the initiation of an arc, and in response disables the power supply driving the cathode. Generally, these control systems permit a significant quantity of energy to be dissipated in the arc before disabling the power applied to the cathode. The quantity of energy dissipated in the arc is also dependent on the type of power supply to which the arc initiated control system is attached. Since arc control systems do not completely prevent arcing like periodic suppression systems, but instead act in response to the detection of an arc, particulate defects to varying degrees will occur. In addition, with some combinations of power supplies and arc initiated control systems, mousebite defects may also occur. In addition, deposition times may also be adversely affected, since while an arc occurs there is no deposition on the substrate.  
           [0011]    While the prior art can be used to provide a DC sputtering system, it has not proven capable of providing low cost arc control that does not adversely affect the deposition time or the quality of the coating. Accordingly, it is desirable to provide a low defect arc control system that does not compromise the deposition rate of the process. In addition, eliminating defects from the workpiece is desirable. Also, it is desirable to reduce deposition defects caused by target poisoning.  
           [0012]    The present arc control system and method provide a system for controlling an arc. The arc control system includes a sputtering chamber that houses an anode and a sputtering target formed from a target material and serving as a cathode. A DC power supply provides a DC cathode voltage such that a cathode current flows from the anode to the cathode. A resonant network is coupled between the DC power supply and the chamber. The resonant network has sufficient Q so that in reaction to the occurrence of an arc, the cathode current resonates to a negative value as is well known in the art. A reverse voltage clamp is coupled across the resonant network to clamp the cathode voltage to a predetermined reverse voltage value, and allow the negative portion of the resonant waveform to drive the cathode and reverse charge the target surface. Thus reverse current of the network is allowed to flow unimpeded to the cathode. This reverse current then charges the cathode to a clamped positive voltage.  
           [0013]    For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 illustrates an exemplary workpiece having mousebites;  
         [0015]    [0015]FIG. 2 is a block diagram of a sputtering system constructed in accordance with the teachings of the invention;  
         [0016]    [0016]FIG. 3 is a schematic diagram of a presently preferred embodiment of the invention;  
         [0017]    [0017]FIG. 3A is a schematic diagram of a voltage clamp;  
         [0018]    [0018]FIG. 3B is a schematic diagram of another voltage clamp;  
         [0019]    [0019]FIG. 4A illustrates a sputtering system during a voltage reversal;  
         [0020]    [0020]FIG. 4B illustrates a conventional sputtering system during a voltage reversal;  
         [0021]    [0021]FIG. 4C illustrates a sputtering system in accordance with the principles of the present invention during a voltage reversal; and  
         [0022]    [0022]FIG. 5 is a signal diagram showing the cathode current and voltage waveforms associated with an arc. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    Referring to FIG. 2, a DC sputtering system  10  according to the present invention is shown. In the presently preferred embodiment of the invention, the DC sputtering system  10  uses a DC sputtering process to deposit a coating on a workpiece  16 . Although, the workpiece in the present embodiment is an optical disk storage media such as CDs and DVDs, it is within the scope of the invention to coat other items such as drill bits, glass panels, toys, cutting tools, optical equipment for any substrate, or a sputtered thin film. This process is unusual in that there typically is about 50 percent atmosphere in the sputtering chamber at the beginning of the deposition process. The oxygen and nitrogen of the air may convert some aluminum deposition to a AI2O3 and AIN deposition for the first part of the run. During the early portion of this process, arcing may occur due to outgassing, atmosphere contamination, etc.  
         [0024]    The sputtering system  10  includes a sputtering chamber  12  that provides a controlled environment for the deposition process. A vacuum pump  11  is typically used to maintain the sputtering chamber  12  at a controlled pressure. The workpiece  16  may be a CD, DVD, cutting blade, or other item to be coated. A sputtering target  18 , configured as a cathode, serves as a source of material for the coating. In the presently preferred embodiment the target  18  is formed from aluminum, although other suitable materials and alloys such as Gold, Si, Ta, B, and Ti may be used. Another piece of conductive material within the sputtering chamber  12  serves as an anode  20 . The cathode  18  and anode  20  are coupled to a DC power supply  24  which supplies electrical energy for inducing a plasma within the sputtering chamber  12 . In the presently preferred embodiment the atmospheric gas introduced at the start of the process is a contaminant. A controlled amount of a sputtering gas, for providing anions that flow within the plasma, is also supplied to the sputtering chamber  12 . Typically argon or another noble gas is used as the sputtering gas.  
         [0025]    For this particular application of the invention, wherein an optical disk is coated, a pair of shields are included in the chamber  12  for masking the outer and inner edges of the optical disk that is used as the workpiece  16 . The shields provide well defined outer and inner radii for the optical disk. An outer shield  14  is positioned between the target material  18  and the optical disk  16  to prevent the deposition of coating material on the outer edge. An inner shield  17  is likewise positioned to prevent coating of the inner edge of the optical disk.  
         [0026]    The DC power supply  24  provides electrical energy for the sputtering process. The power supply  24  converts unregulated AC line power to regulated DC power suitable for powering the sputtering system  10 . In the presently preferred embodiment of the invention, the DC power supply  24  is a switch mode power supply, however, the scope of the invention is not limited by the type of power supply. For example, other types of power supplies such as SCR and diode-transformer power supplies may be used. The positive  26  and negative  28  outputs of the DC power supply  24  are coupled to the anode  20  and cathode  18 , respectively. The power supply  24  provides the required voltage/current to the sputtering chamber  12 . As will be recognized by those skilled in the art, the nominal voltage is to be suitable for the target material and sputtering operation to be performed. Therefore, the scope of the invention includes sputtering processes employing a wide range of voltages. A resonant network  30  and voltage clamp  32  are connected between the DC power supply  24  and the cathode  18  and anode  20 . The resonant network  30  stores electrical energy that drives a voltage reversal at the cathode  18  and anode  20  during an arc event. Although, the resonant network  30  is illustrated as being separate from the power supply  24 , it is within the scope of the invention to integrate the resonant network  30  into the power supply  24 . Indeed, as will be subsequently explained, the performance of the resonant network  30  is correlated to the output filter of power supply  24 . The voltage clamp  32  limits the amplitude of the voltage reversal that is applied across the cathode  18  and anode  20  during an arc and may additionally be configured to clamp excess normal voltages to protect the power supply  24  and the workpieces  16 .  
         [0027]    Referring to FIG. 3, a partial electrical schematic of the presently preferred embodiment of the sputtering system  10  is illustrated. The DC power supply  24  includes, in addition to other elements, an output filter  33  that comprises an output inductor  34  and an output capacitor  36 . Those skilled in the art will recognize that there are multiple output filter configurations that will operate with the claimed invention. For example, multiple stage filters, damped output filters, and high impedance output filters are all within the scope of the invention.  
         [0028]    The resonant network  30  is configured to store energy during normal system  10  operation. The stored energy is subsequently released to the sputtering chamber  12  during an arc occurrence to enhance the system response. A resonant capacitor  38  is connected across the positive and negative outputs  26  and  28  of the DC power supply  24 . In the presently preferred embodiment, a 0.1 μF polypropylene film capacitor is used for the resonant capacitor  38 , although many values may be used dependent on the output design of the power supply  24  and magnetron cathode  18 . A resonant inductor  40  is connected from the resonant capacitor  38  to voltage clamp  32 . The resonant inductor  40  stores energy to be used in response to an arc, and also limits the rate of increase in output current during an arc. In the presently preferred embodiment, the resonant inductor  40  is 12 μH, though other values may be used.  
         [0029]    The voltage clamp  32  limits the amplitude of the voltage applied to the sputtering chamber  12 . The voltage clamp  32  includes a reverse voltage clamping device  44  connected from cathode  18  to anode  20 . The reverse voltage clamping device  44  limits the reverse voltage applied to the sputtering chamber  12  to a level that aids in extinguishing the arc but low enough to prevent back sputtering or mousebites. In the presently preferred embodiment of the invention, a forward voltage clamping device  42  is connected in series with the reverse voltage clamping device  44 , however the scope of the invention encompasses connecting a reverse voltage clamping device  44  in parallel with a forward clamping device  42  (see FIG. 3B, voltage clamp  32 B) as well as a reverse voltage clamping device  44  with no forward clamping device (see FIG. 3A, voltage clamp  32 A). In the presently preferred embodiment the reverse voltage clamping device  44  is a unidirectional zener diode (transorb) in series with the forward voltage clamping device  42 . Two less preferred implementations are illustrated in FIGS. 3A and 3B as voltage clamps  32 A and  32 B. In voltage clamp  32 A, the reverse voltage clamping device  44  is a series of unidirectional zener diodes in series with a reverse biased diode. In voltage clamp  32 B, the reverse voltage clamping device  44 , is a bi-directional zener diode that is in series with a string of unidirectional zener diodes.  
         [0030]    The forward voltage clamping device  42  protects the sputtering system  10  from excessive forward output voltages. The selection and implementation of forward voltage clamping devices is well-known to those skilled in the art. In presently preferred embodiment a series of unidirectional zener diodes are used as the forward voltage clamping device.  
         [0031]    Actual deposition of the coating requires the ignition of a plasma within the sputtering chamber  12 . The plasma is created by applying a voltage between the anode  20  and the cathode  18  that is sufficiently high to cause ionization of at least a portion of the sputtering gas. The intense electric field associated with the applied voltage strips electrons from the gas atoms, creating anions and electrons that flow within the plasma. The anions are accelerated by the steady-state electric field into the target  18  with sufficient kinetic energy to cause the anions to knock atoms from the target  18 . Some of the freed target atoms combine with atmosphere that is present within the sputtering chamber  12  at the beginning of the process. The remaining freed target atoms that are uncombined also disperse throughout the sputtering chamber  12  coating the exposed surfaces. Throughout the process arcing occurs intermittently due to target poisoning, outgassing from the workpiece  16 , contaminants, and flakes amongst other causes.  
         [0032]    Referring to FIGS. 3 and 5, the operation of the sputtering system  10  during an arc will be described. When an arc occurs, the impedance from cathode  18  to anode  20  dramatically decreases, causing a rapid rise in cathode current  48 . The increase in cathode current  48  is supplied from energy stored in the output capacitor  36  and resonant capacitor  38 . As the transient current continues to flow, energy stored in the output capacitor  36  and resonant capacitor  38  transfers into the resonant inductor  40 . The cathode current  48  reaches a peak value when the voltage across capacitors  36  and  38  resonates to  0 . As those skilled in the art will realize, the peak value of the cathode current  48  is related to the ratio of the energy stored in resonant inductor  40  and capacitors  36  and  38 . This ratio is commonly referred to as the Q of the filter. As the cathode current  48  begins to decrease, energy starts to transfer out of resonant inductor  40  back into capacitors  36  and  38 , reverse charging the capacitors. The voltage applied from cathode  18  to anode  20 , cathode voltage  50 , reverses as charge continues to flow into capacitors  36  and  38 .  
         [0033]    The amplitude of the reverse voltage applied to the cathode  18  continues to increase until the reverse voltage clamping device  42  becomes active. One prior technique allows this voltage to reverse to a nearly equal value of the normal cathode voltage, (+500 volts to −500 volts). Another prior technique clamps the negative voltage at or about zero volts, allowing no reverse current to flow to the cathode. In the presently preferred embodiment, the reverse voltage clamping device  42  is preferably selected to clamp the cathode  18  to a voltage less than 200V. The present invention recognizes that mousebites are caused by substantial reverse power flow through the anode  20 . Therefore, for mousebites to occur, there must be substantial reverse voltage in addition to reverse current applied to the cathode  18 . Mousebites are caused by back sputtering from the workpiece when a sufficient reverse voltage and current are applied. In the present embodiment, back sputtering begins to occur at reverse voltage levels of 200 volts or greater. The mousebites occur at the boundaries of the workpiece because these are the locations of the highest electric field intensity, and therefore the first locations to breakdown the reactive gas and allow reverse current flow. It is within the scope of the invention to select a clamping voltage for the clamping device  42  that is sufficient to extinguish arcing but is less than the back sputtering voltage that will cause mousebites.  
         [0034]    The present invention also recognizes that providing a reverse voltage less than the sputtering voltage during an arc may reduce the recovery time of the sputtering system  10 . The reverse voltage applied to the cathode  18  aids in extinguishing the arc. In the presently preferred embodiment, the sputtering system  10  recovers from an arc within about 6μ secs. In addition, less energy is dissipated in the arc since a portion of the energy from the energy storage components is dissipated within the reverse voltage clamping device  42 . The quantity of particulate defects generated by the arc is reduced since less energy is dissipated within the arc and the duration of the arc is shortened. Also, applying a reverse voltage to the cathode  18  reduces the tendency for subsequent arcing during reactive deposition processes by reducing the charge stored by the parasitic capacitance formed from the target material and insulator compounds.  
         [0035]    After the arc is extinguished, the cathode current continues to resonate. As the cathode current  48  passes through zero amps, the cathode voltage  50  reestablishes the sputtering voltage and power.  
         [0036]    Referring to FIGS. 4A, 4B, and  4 C, the state of the sputtering system  10  during a voltage reversal is contrasted with conventional sputtering systems. FIG. 4B illustrates the operation of a conventional sputtering system during a voltage reversal. FIG. 4C illustrates the operation of the presently preferred embodiment of the sputtering system  10  during a voltage reversal. During normal operation, prior to an arc occurring, target material sputtered off from the cathode  18  forms a coating  19  on the workpiece  16 . As the coating  19  is formed, a low-level negative charge accumulates within the coating  19  as well as along the surface of the coating  19 . When a voltage reversal occurs, the voltage on the cathode  18  swings positive relative to the anode  20 . Positive anions that previously were attracted to the cathode  18  are increasingly repelled as the reverse voltage increases in magnitude.  
         [0037]    In conventional sputtering systems (see FIG. 4B), the positive anions instead of flowing towards the cathode  18 , increasingly flow towards the negatively charged workpiece  16 . As the anions flow towards the workpiece  16 , they are attracted towards the outer and inner edges of the coating  19  due to the high electric fields present in those regions. When the anions strike the coating  19 , momentum transfer occurs causing portions of the coating  19  to be back sputtered, with the majority of the sputtering occurring along the edges. Mousebites  9  are exhibited as the back sputtering along the edges of the coating  19  continues. The back sputtered coating  19  which comprises mainly aluminum with a smaller proportion ofAI2O3, is deposited on the inner surfaces of the sputtering chamber  12  including the shields  14  and  17 , and the cathode  18 .  
         [0038]    In the presently preferred embodiment of the sputtering system  10 , the magnitude of the voltage reversal is clamped at a sufficiently low voltage to prevent the flow of positive anions towards the negatively charged workpiece  16 . Since anions do not strike the coating  19 , the mousebites  9  that plagued conventional sputtering systems are prevented from occurring in the presently preferred embodiment.  
         [0039]    The arc control system of the present invention minimizes the time to recover from an arc, thereby increasing the proportion of process time during which deposition of material occurs. Additionally, the system prevents the formation of mousebites during an arc.  
         [0040]    Also, the arc control system decreases the arc energy that is dissipated in the chamber, thereby reducing defects in the workpiece. In addition, the period of time before subsequent arcs occur is lengthened, again increasing the process time during which deposition of material occurs.  
         [0041]    Further, the arc control system is designed with a comparatively small number of passive components.  
         [0042]    Thus it will be appreciated from the above that as a result of the present invention, an arc control method for DC sputtering systems is provided by which the principal objectives, among others, are completely fulfilled. It will be equally apparent and is contemplated that modification and/or changes may be made in the illustrated embodiment without departure from the invention. Accordingly, it is expressly intended that the foregoing description and accompanying drawings are illustrative of preferred embodiments only, not limiting, and that the true spirit and scope of the present invention will be determined by reference to the appended claims and their legal equivalent.