Abstract:
Systems and methods are described that monitor electron beam current and voltage. The systems and methods react to fault conditions such as arcing experienced during evaporation and deposition processes to shutdown and protect associated power supply equipment. The systems and methods may provide online beam current control to provide stable operation of e-beam guns during heating and melting modes of operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Benefit is claimed of U.S. patent application Ser. No. 61/118,812, filed Dec. 1, 2008, and entitled “Vapor Deposition Electron Beam Current Control”, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. 
     
    
     BACKGROUND 
       [0002]    The disclosure relates generally to the field of vacuum deposition. More specifically, the disclosure relates to systems and methods for controlling electron beam filament current used in electron beam physical vapor deposition for aircraft engine turbine blades and accessories. 
         [0003]    Today, vacuum deposition of thin films and coatings is evolving. This is true of their processes, equipment, applications and markets. 
         [0004]    Electron beam physical vapor deposition (PVD) is a material coating technology where a coating, such as a metal, alloy, or ceramic is melted, vaporized in a vacuum, and then deposited on a work piece. The material to be deposited is converted to a vapor by physical means. Generally, the process deposits atoms or molecules one at a time. Since the process is performed in a vacuum, it is an environmentally friendly technology, suitable as a replacement for other coating processes in many applications. The technology is capable of producing coatings for a wide range of industrial applications. 
         [0005]    PVD processes are atomistic where material vaporized from a solid or liquid source is transported as a vapor through a vacuum or low-pressure environment. When it contacts the work piece, it condenses. PVD processes are used to deposit films with thicknesses in the range of a few nanometers to thousands of nanometers, however, they can be used to form multilayer coatings, thick deposits and free-standing structures. 
         [0006]    Vacuum evaporation is a PVD process where material from a thermal vaporization source reaches the substrate without collision with gas molecules in the space between the source and substrate. The trajectory of the vaporized material is in a line-of-sight. 
         [0007]    The equipment used to generate a deposition environment is an integral part of the process. The principal parts of the deposition system are the deposition chamber, evaporation tools, fixtures (which hold the parts to be coated), and the vacuum pumping system (which removes gases and vapors from the deposition chamber). 
         [0008]    Generating a vacuum reduces the gas pressure so that vaporized atoms have a long mean-free path and do not nucleate in the vapor to form soot. The vacuum also reduces the contamination level to the point that the desired film can be deposited. Fixtures hold the substrates to be coated and provide the motion, relative to the vaporization source. This is often necessary to give a uniform deposition over a large area, a complex surface or over many substrates. The deposition chamber is sized to contain the fixtures and provide room for accessories such as shutters, deposition rate monitors, heaters, etc. Proper design, construction, operation and maintenance are necessary to obtain a reproducible product with high yield and desired product throughput. 
         [0009]    The vaporization source is typically a high-energy, electron beam (e-beam) gun that is focused and rastered over the surface of the source material. E-beams are either hot-cathode or hot-filament thermionic guns where the electrons are generated by a hot filament of a high temperature alloy such as tungsten. Beam electrons are generated by applying a constant voltage, typically 20,000 to 120,000 V to the cathode. Electron emission from the cathode is increased by bombarding it with electrons from an electrically heated filament, typically at 1000 V. 
         [0010]    An axial, e-beam gun evaporation tool is shown in  FIG. 1 . During gun operation, the cathode  101  is heated electrically by passing a current through the filament  103  until it emits electrons  105  through thermionic emission. The electrons bombard the cathode  101 , heating it and leading to its own electron emission. Once the cathode  101  has begun to emit electrons, electron bombardment from the cathode in combination with direct electric current heating increases the cathode temperature, increases its electron current emission density, and results in a larger emitting surface than would be achieved through cathode operation only. 
         [0011]    Gun systems use electromagnets located within the body of the gun for beam focusing  107  and scanning  109 . Beam scanning is an integral feature because it ensures rapid, uniform, controlled, atomistic evaporation from the largest possible target  111  surface. 
         [0012]      FIG. 2  is a diagram of an exemplary process. The work piece  201  is maintained at a specific temperature to ensure good adhesion of the evaporated material. Two  203 ,  205  of six e-beam guns are directed at trays containing crushed ceramic or graphite positioned adjacent to the work piece  201  for indirect heating. Up to four guns  207 ,  209 ,  211 ,  213  are used for evaporation.  FIG. 2  also shows the e-beam directing system  221  and workpiece motion controller  223 . 
         [0013]    Specifying and controlling the maximum e-beam gun power for high or low vacuum evaporation systems is problematic because of the uncertainty involved in determining the magnitude of the various e-beam energy losses between generation of the beam and generation of the vapor. The amount of e-beam energy actually available for material evaporation depends upon the energy losses such as those experienced inside of a gun due to some fraction of the beam impinging on various portions of the gun, in the gas and vapor cloud  227  due to electron scattering collisions, from the evaporant material surface as a result of electron backscattering, through conduction into the crucible containing the melt material, from the radiating molten evaporant surface, and through convection caused by the gas jet blowing across the evaporant surface. With the above described energy losses and operating environment, problems related to electrical discharges manifest themselves from high voltage in the presence of high vapor. The vapor may become ionized resulting in interaction with the hot filament. An exemplary threshold for coaters is about 16 kV accelerating voltage: below this voltage the beams do not penetrate the vapor very well. At operational voltages, arcing can occur. Arcing induces rapid, out of control filament current. Arcs caused by ionized vapors occur between the coater enclosure and the high-voltage circuits. Most often arcs occur in the guns and HV input circuits. 
         [0014]    Pre-arc or non-arc interactions can occur between electron beam guns. This is evidenced by filament current fluctuations that may occur spontaneously or when the beam current is intentionally altered on any one or more guns. Thermal process control may be lost during these events, with attendant substrate temperature excursions that may adversely affect the applied coating characteristics (microstructure). 
         [0015]    It is a challenge to ensure consistent production while offering protection for the e-beam gun support systems. It is therefore desirable to develop a system and method that protects gun subsystems in the event of an arc within a gun or within the processing environment. 
       SUMMARY 
       [0016]    Although there are various systems and methods that control filament and beam current for electron beam guns used in vapor deposition, such systems and methods are not completely satisfactory. It would be desirable to have systems and methods that monitor electron beam current and voltage, and react to fault conditions such as arcing experienced during evaporation and deposition processes to shutdown and protect associated power supply equipment. 
         [0017]    Although arc down protection and arc recovery systems may be elements of multi-electron beam gun coating systems, and these functions may be provided as a byproduct of the present method and apparatus, the present disclosure can provide enhanced thermal stability that is manifested into the process via electron beam gun filament current control. Thermal stability of the parts in the process is dependent upon input process parameters such as time, temperature, pressure, injected oxygen relationships. These parameters influence the characteristics that determine coating durability. Thus, attempts to control these parameters to improve stability are constrained by the desired coating characteristics. 
         [0018]    The disclosure provides online beam current control to provide stable operation of e-beam guns during heating and melting modes of vapor deposition operation. Beam current supplied to each gun used in a process may be monitored. The measured current may be used in a closed-loop feedback control to indirectly adjust beam current by adjusting filament current. Power supply protection may be provided for one or more e-beam guns having a shared, common beam high voltage power supply. 
         [0019]    One aspect of the disclosure provides methods for controlling a power supply for an electron beam. Methods according to this aspect may start with defining a high voltage level setpoint for a high voltage beam power supply output, defining a beam current setpoint, defining a filament current setpoint for a filament power supply output, monitoring the high voltage beam power supply output voltage, monitoring current supplied to a cathode of the electron beam, and determining whether a fault condition has occurred based on the high voltage beam power supply output voltage and filament current. 
         [0020]    Another aspect involves deriving a beam current value based on the filament current and the high voltage beam power supply voltage. 
         [0021]    Yet another aspect of the disclosure is an electron beam power supply system. Systems according to this aspect of the disclosure comprise a high voltage beam power supply having an output coupled to a cathode of the electron gun, a filament power supply having an output coupled to a filament of the electron gun, and a processor monitoring voltage applied to the cathode and current applied to the filament, the processor outputting control signals to the high voltage beam power supply and the filament power supply wherein the control signals are responsive to the cathode voltage and the filament current and indicate fault conditions experienced by the electron beam. 
         [0022]    Another aspect of the system involves a high voltage output level setpoint for predetermining a high voltage level, a filament power supply current setpoint for predetermining a filament current level, and a beam current setpoint for predetermining a beam current level wherein the beam current depends on the high voltage level and the filament current level. 
         [0023]    Other objects and advantages of the methods and systems will become apparent to those skilled in the art after reading the detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is an exemplary electron beam gun. 
           [0025]      FIG. 2  is an exemplary cross-sectional view of a vapor deposition chamber. 
           [0026]      FIG. 3  is an exemplary schematic of a control system. 
           [0027]      FIG. 4  is an exemplary plot showing beam current and filament current at various values of high accelerating voltage. 
           [0028]      FIG. 5  is a block diagram of an exemplary method according to the invention. 
           [0029]      FIG. 6  is an exemplary plot showing system operation during a fault condition. 
           [0030]      FIG. 7  is an exemplary schematic of the beam current regulator. 
           [0031]      FIG. 8  is an exemplary plot of output signal beam current regulator  317  for filament current control. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Embodiments are described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Further, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “mounted,” “connected,” and “coupled” are used broadly and encompass both direct and indirect mounting, connecting, and coupling. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
         [0033]    The disclosure is not limited to any particular software language described or implied in the figures. A variety of alternative software languages may be used for implementation. Some components and items are illustrated and described as if they were hardware elements, as is common practice within the art. However, various components in the method and system may be implemented in software or hardware. 
         [0034]    Embodiments provide systems and methods for controlling filament current for electron beam guns used in vapor deposition. 
         [0035]      FIG. 3  is a schematic of a deposition control system  301 . The exemplary system uses six e-beam guns. However, other numbers of guns may be employed. The subscript  x  identifies components and signal paths that are associated with one gun ( x =1, 2, 3, . . . 6). 
         [0036]    The control system  301  controls the beam current (I_BEAM) supplied to all guns  303   x  (I_BEAM x ) used in a process chamber  305  for PVD. The amount of beam current (I_BEAM x ) supplied to each gun  303   x  depends on the magnitude of the accelerating voltage and filament current (I_FILAMENT x ). 
         [0037]    During vapor deposition, the chamber  305  vacuum, the properties of the material  307  being evaporated, electrical arcing, and other phenomena which occur inside the chamber  305  during processing all may affect gun  303   x  filament current (I_FILAMENT x ). The simultaneous operation of several guns  303   x  sourced from a common high voltage (HV) power supply affect gun  303   x  stability. 
         [0038]    The common HV beam power supply system  308  includes an HV closed loop regulator  309 , an SCR controller  311 , an HV power supply  313 , and an HV rectifier  315 . The HV rectifier outputs an HVDC (direct current) to the gun cathodes (I_BEAM). 
         [0039]    The system  301  uses a thyristor regulator  317   x  in the filament current path for each gun  303   x  in conjunction with closed-loop feedback as part of the overall control. Each gun  303   x  has a dedicated filament regulator system  321   x  that includes a beam current regulator  317   x , an SCR controller  323   x , a filament supply  325   x , and a filament rectifier  327   x . 
         [0040]    The beam current regulator  317   x  takes into account operational features of the controller (e.g., ENERPRO Corporation SCR series FCOG 6100 (three phase firing board) or FCRO 2100 (single phase firing/regulator board) controllers). The level of FCRO 2100 firing board input command is from 0 up to +5 VDC; at the same time delay angle varies from 180 to 0 degrees. Other devices with the appropriate specifications can be used. 
         [0041]    The system controls beam current (I_BEAM) in dependence on individual gun filament current (I_FILAMENT x ). Individual beam current (I_BEAM x ) feedback (V_FB_I_BEAM_GUN x ) provides an independent, on-line control for each gun  303   x  in use and provides stable operation for different processing modes. Presetting filament current (I_FILAMENT x ) decreases the value of beam current deviation if the system terminates on a fault or HV arc which may occur in the chamber during processing. 
         [0042]    A processor  329  accepts feedback from the common HV output  339  (HV_FB), individual gun  303   x  beam current (V_FB_I_BEAM_GUN x ), and based on a control logic, outputs in response to the feedback signals that control the common HV power supply system  308  and each gun filament regulator system  321   x . An HV voltage divider  343  provides a low voltage representation of the HV output  339  as feedback (HV_FB). 
         [0043]    Each gun  303   x  comprises a cathode  331   x , an accelerating electrode  333   x  and a deflection coil  335   x . DC filament current  319   x  (I_FILAMENT x ) heats the cathode  331   x  liberating electrons from its surface through thermionic emission. 
         [0044]    The cathode  331   x  is coupled to a negative output of the HV rectifier  315 . The corresponding positive output of the HV rectifier  315  is coupled to a chamber  305  ground terminal and an accelerating electrode  333   x  ground terminal for each gun  303   x . When high voltage is applied to each cathode  331   x , electrons are emitted and accelerate through the accelerating electrode  333   x  forming an electron beam  337   x . 
         [0045]    The deflection coil  335   x  rasters the electron beam  337   x  over an object  307  for evaporation, or tray (not shown) for heating within in the chamber  305 . A discussion of raster control is beyond the scope of this disclosure. 
         [0046]    Process parameters such as heating, and evaporant temperature and rate depend upon the cathode and filament currents. Two plots of HV beam current versus filament current corresponding to two different HV setpoints are shown in  FIG. 6 . The Y axis shows the reading of controllable value in arbitrary units. X axis—time. Unit of measurement is defined by the name of controllable value. Various controllable values are shown on one Y axis for better understanding of their dependency in time: HV level  920 ; HV threshold comparator detector level  922 ; arc comparator detector current level  924 ; beam current  926 ; filament current  928 . 
         [0047]    The HV feedback (HV_FB) is coupled to the HV regulator  309  and to an HV level comparator  345  input that is part of the processor  329 . Beam current sensors  347   x  produce beam current values (V_FB_I_BEAM_GUN x ) corresponding to each gun  303   x . 
         [0048]    An arc detector  349  which is part of the processor  329  logic monitors the HV level. The system may enable beam current regulators of live guns as soon as HV level reaches desired value (e.g., 90% of operating voltage HV). An arc detector  349  is used to detect over current conditions which indicate shorting or HV arcing. Beam current (V_FB_I_BEAM_GUN x ) is also used as feedback for the beam current regulator  317   x . The system compensates for electron beam  337   x  perturbations that occur by monitoring the variation of each gun&#39;s beam current and voltage. 
         [0049]    The power source for the SCR controller  311  and HV supply  313  is three phase alternating current (ac). Single phase ac is supplied to the SCR controller  323   x  and the filament supply  325   x . 
         [0050]    The processor  329  analyzes the beam (I_BEAM) (HV_FB) and current (V_FB_I_BEAM_GUN x ) feedback. Processor  329  logic analyzes whether an over current condition exists. The processor  329  controls the common HV power supply system  308  and each gun filament regulator system  321   x . 
         [0051]    The processor  329  outputs on/off control commands to the common HV power supply system  308  SCR controller  311  (HV_ON/OFF) and HV regulator  309  (REG_HV_ON/OFF), and each gun filament regulator control system  321   x  beam current regulator  317   x  (REG_I_ON/OFF x ) and SCR controller  323   x  (FILAMENT_ON/OFF x ) thereby turning on the common power supply  308  and gun filament regulator systems  321   x  to energize or de-energize a gun. 
         [0052]    A method of operation is shown in  FIG. 5 . After applying power to the system (step  505 ), a user adjusts the HV level setpoint  341 , beam setpoint  353 , and filament setpoint  351  values (step  510 ). 
         [0053]    The processor  329  reviews a list of permissives regarding possible electrical fault conditions for a determination of system availability (step  515 ). If no fault conditions are found (step  520 ), the gun system is ready for operation (step  525 ). The invention turns on the HV regulator  309  (REG_HV_ON) and SCR controller  311  (HV_ON) (step  530 ) and gradually ramps-up (RAMPING_HV) the HV power supply output  339  via the regulator  309  to the HV level setpoint  341  (step  535 ). 
         [0054]    The HV level setpoint  341  is used to define a desired HV level. The beam current regulator  317   x  (REG_I_ON x ) and SCR controller  323   x  (FILAMENT_I_ON x ) are then turned on (step  540 ). 
         [0055]    As soon as the predetermined HV level is reached, the beam current regulator  317   x  output ramps-up (RAMPING_I_BEAM x ) to its setpoint  351  (step  545 ). The system is in operation (step  550 ). 
         [0056]    If a fault condition occurs (step  555 ), the arc detector  349  turns the processor  329  control outputs for the SCR regulator  311  (HV_OFF), HV regulator  309  (REG_HV_OFF), beam current regulator  317   x  (REG_I_OFFx) to OFF (steps  560 ,  565 ), thereby de-energizing beam  308  power supplies and shutting down the evaporating tools for a finite period of time. If one gun arcs, all guns may be shutdown. The desired finite period of time depends upon the properties of evaporating materials and may vary within the range of 0.2-5 seconds. The duration of such time is adjusted through the adjustment of the duration of disabling pulse (t pause  940  after arc  942  in  FIG. 6 ) that is generated by arc detector  349  as soon as arc is detected. 
         [0057]      FIG. 6  is a plot of an arc transient and the system response over time. Beam current lags behind filament current because of cathode thermal lag. Lines of filament current and beam current are shown in arbitrary units since beam current value depends upon the gun design, cathode material, working value of accelerating voltage. Filament set point  351  value depends upon these factors as well. Line  924  of ARC Level shows the thresholds of arc current detector  349  response for each gun. Beam current increase up to the preset value of arc level causes arc current detector  349  to generate control pulse (t pause after ARC) that disables high-voltage source and beam current regulators of all guns. Lines beam current  926  and filament current  928  show beam current and gun filament values. As soon as arc current detector  349  detects arcing it disables high-voltage source and beam current regulators of all guns. At the same time beam current rapidly drops down to 0 and filament current decreases to the preset value Fil Preset (see  FIG. 4 ). Available filament current keeps hot state of cathode and protects cathode from abrupt variation of its length at beam current enabling\disabling. As soon as HV level reaches 90% of operating voltage HV threshold detector  345  allows enabling of beam current regulators. Such control logic provides beam setting (hit) to the initial position. Filament current increases at the preset ramping and restores the working value of beam current. 
         [0058]    When the processor  329  logic determines that the fault has ended and the event is over (step  515 ) and operation can resume (step  520 ), the HV regulator  309  and SCR controller  311  are reset and turned on ((REG_HV_ON), (HV ON)) (step  525 ). At the same time, a power supply ramping signal (RAMPING_HV) is output from the processor  329  that ramps-up the regulator  309  output to the HV setpoint  341  (step  530 ). 
         [0059]    A filament on signal (FILAMENT_ON) is output from the processor  329  to SCR controller  323   x  and beam regulator  317   x  in addition to a beam current regulator  317   x  on signal (REG_I_ON x ) (step  535 ). 
         [0060]    Processor  329  outputs the beam current regulator ramping signal (RAMPING_I_BEAM x ) when the beam voltage (HV_FB) reaches 80%-90% of the HV setpoint  341  (step  540 ). The system returns to operation (step  545 ). 
         [0061]      FIG. 7  is the beam current regulator  317   x  for each filament regulator system  321   x . The filament current setpoint  351  allows for the smooth ramp-up of filament current as soon as the SCR controller  323   x  is turned on (FILAMENT_I_ON x ). 
         [0062]    The beam current setpoint is set via potentiometer  353  or from another analog device. A comparator  701  compares a desired beam current setpoint  353  against actual beam current feedback (V_FB_I_BEAM_GUNx) and outputs a difference, or error signal. The error signal is amplified by a PI (proportional-integral) error amplifier  703 . An analog adder  705  adds the filament current setpoint  351  to the amplifier  703  output. 
         [0063]    A limiter  707  limits the output of the adder  705  output preventing saturation using the beam current setpoint  353  which is non-linear amplified  709  and added with the filament setpoint  351  which modifies the limit  707 .  FIG. 8  shows the output signal current regulator  317  for filament current control. 
         [0064]    A control device  711  (e.g., an electronic switch) outputs a signal in response to the beam regulator signal (REG_I_ON/OFF x ) that modifies the characteristics of the error amplifier  703  during arcing. As soon as arc occurs control device  711  disables the error amplifier  703 . In this case output signal of regulator  317  corresponds to I Fil preset (see  FIG. 4 ) 
         [0065]    Filament current presetting  713  sets I filament level via the regulator  317   x  when gun&#39;s filament is ON. Such filament level is adjusted for each gun individually from filament set point  351 . An adjustment procedure for finding set point  351  is: 1) preset HV set point  341  at operating level HV; 2) preset set point I beam  353  to approximately 0 mA; 3) increase slowly the filament set point  351  until beam current is about 5 mA. The resultant value of filament set point  351  is fixed. 
         [0066]    The disclosure facilitates operating mode of regulator  317   x  and cathode  331   x  of a gun at beam current stabilization. The use of closed loop beam current feedback in regulator  317   x  which control of prefabricated SCR controller  323  of filament current of each EB gun provides stable operation of the system under various modes of evaporation and deposition. Points I_FIL_PRESET A and B in  FIG. 4  can be considered as initial value of cathode filament at operating accelerating voltage 25 kV (point A) or 18 kV (point B). 
         [0067]    Points A, B show approximate values of filament current at 25 kV and 18 kV accelerating voltage when beam current starts showing up. 
         [0068]    The control is carried out in the following manner. See  FIG. 3B ,  FIG. 4 . Filament set point  351  sets initial current value of cathode filament (I_FIL_PRESET), that depends upon the preset operating voltage of HV supply (see points A, B  FIG. 4 ). Beam set point  353  sets signal level for Beam current close loop regulator  317  that transmits the command to the input of SCR CONTROLLER  323  that provides control over silicon-controlled rectifiers of Filament Supply  325 . From the output of Filament Supply  325  filament, gate voltage is supplied to filament current rectifier  327  that provides control over DC filament current  319  of cathode  331  of gun  303 . Signal of beam current-sensing device of the gun  303  is used as the signal of negative feedback in beam current close loop regulator  317 . Beam current value deviation from the preset value causes regulator  317  to generate filament control signal that compensates for beam current variation. 
         [0069]    Although the disclosure herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present disclosure. For example, the principles may be implemented in the retrofit or other reengineering of existing systems used for existing purposes. Physical and operational details of such existing systems and purposes (e.g., component configurations, particular voltages, particular currents, and the like) will influence or dictate details of any particular implementation. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.