Abstract:
In a plasma ion source having an induction coil adjacent to a reactor chamber for inductively coupling power into the plasma from a radio frequency power source and designed for negative and positive ion extraction, a method for operating the source according to the invention comprises providing radio frequency power to the induction coil with a RF amplifier operating with a variable frequency connected to a matching network mainly comprised of fixed value capacitors. In this device the impedance between the RF power source and the plasma ion source is matched by tuning the RF frequency rather than adjusting the capacitance of the matching network. An option to use a RF power source utilizing lateral diffused metal oxide semiconductor field effect transistor based amplifiers is disclosed.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/701,495, filed Sep. 14, 2012, whose contents are incorporated herein for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to plasma ion sources and more particularly to methods for providing RF power, magnetic filtering, and high voltage isolation to an inductively coupled plasma ion source 
     2. Description of the Prior Art 
     Focused ion beam (FIB) tools are used for nanometer scale precision material removal. The benefits of FIB tools include nano-scale beam placement accuracy, a combined imaging and patterning system for accurate sample registration and pattern placement, and low structural damage of the area surrounding the removed volume. However, conventional FIB systems typically have a maximum removal rate of ˜5 μm 3 /s that limits their usefulness for removing volumes with dimensions exceeding 10 μm. Conventional FIB systems are further limited by the low angular intensity of the ion source, so at large beam currents the beam size dramatically increases from spherical aberration. For high beam currents and hence high removal rates of material, a high angular intensity is required, along with high brightness and low energy spread. 
     Focused ion beams are often referred to as ‘primary’ ion beams when used on secondary ion mass spectrometer (SIMS) systems. Here the term ‘primary’ is used to differentiate it from the secondary ion beam. As with FIB tools, SIMS tools use the focused primary ion beam for precise, sputter removal of atoms and molecules from a material surface. SIMS tools are primarily used to determine the spatial distribution of chemical constituents in the near surface region of a material. Oxygen primary ion beams are beneficial to enhance the yield of positively charged secondary ions from the sample material, and hence enhance the sensitivity of the SIMS measurement. Negatively charged oxygen ion beams, not only enhance the yield of positively charged secondary ions, but also result in minimal sample charge buildup when the sample is a dielectric material. The state-of-the-art SIMS instruments employ duoplasmatron ion sources to produce negative oxygen primary ion beams. Duoplasmatrons have insufficient brightness (˜40 Am −2 sr −1 V −1 ) and lifetime to produce the spatial resolution required for many SIMS applications. Duoplasmatrons also produce ion beams with a relatively large axial energy spread (˜15 eV), which is also problematic when endeavoring to produce high spatial resolution focused primary ion beams. 
     One solution is to use an inductively coupled plasma ion source. Inductively coupled plasma ion sources typically wrap an RF antenna about a plasma chamber. Energy is transferred by inductively coupling power from the antenna into the plasma. A RF power supply with 50 Ohm output impedance utilizes an impedance matching network so that the output of the RF supply can be efficiently coupled to the plasma which has an impedance substantially lower than 50 ohms. To extract negative ions from the ion source, it is typically necessary to use a transverse magnetic field near the source aperture to modify the plasma to allow negative ions to leave the plasma and to separate unwanted electrons that are extracted with the negative ion beam. 
     Other applications for this type of plasma source include its use as the primary ion source for Ion Scattering Spectroscopy (ISS), focused and projection ion beam lithography, proton therapy, and high energy particle accelerators. 
     In all cases, a high power density is deposited into the plasma from the antenna in order to create a high density plasma, and the plasma is biased to a potential of a hundred Volts or higher with respect to ground. An ion beam is extracted from a small aperture and is accelerated through a bias voltage to produce a fine beam of energetic ions. 
     One issue in the design of plasma ion sources is how to create an optimum magnetic field near the extraction aperture while a high voltage bias is applied to the plasma chamber. A second issue is how to provide radio frequency power to the antenna when the effective impedance of the plasma is very low compared to common RF power supply output impedances. This is especially problematic because the effective impedance of the plasma varies with plasma chamber gas pressure, gas composition, and the change of state during plasma ignition. A third problem is how to introduce gas into a reactor chamber that is biased to a high voltage without having high voltage breakdown through the input gas line. 
     Accordingly, the need arises for new designs and methods that provide efficient RF power to plasma ion sources, as well as improve the magnetic filtering and high voltage stability of plasma ion sources. 
     SUMMARY OF THE INVENTION 
     The present invention provides a solution for providing RF power to inductively coupled ion sources as well as improved magnetic filtering and high voltage isolation to plasma ion sources biased to high voltage. 
     In a plasma ion source having an induction coil adjacent to a reactor chamber for inductively coupling power into the plasma from a radio frequency power source, a method for operating the source according to the invention comprises providing radio frequency power to the induction coil with a RF amplifier operating with a variable frequency connected to a matching network mainly comprised of fixed value capacitors. In this device the impedance between the RF power source and the plasma ion source is matched by tuning the RF frequency rather than adjusting the capacitance of the matching network. An option to use a RF power source utilizing lateral diffused metal oxide semiconductor field effect transistor based amplifiers is disclosed. 
     A magnetic circuit design for coupling magnetic flux into a plasma reactor chamber that is biased to a high voltage is also disclosed. The magnetic circuit comprises a source of magnetic flux located at ground potential combined with a high permeability magnetic circuit that channels magnetic flux through a high voltage gap to provide magnetic flux into a plasma reactor chamber and an exit aperture that is biased to high voltage. This design has a portion of a magnetic circuit in contact with a plasma reactor chamber that is biased to high voltage, where the circuit can transfer magnetic flux from a second ground based portioned of a magnetic circuit while electrically isolating the two portions of the magnetic circuit. 
     A gas feed insulator design that improves the high voltage standoff by reducing the electric field along the insulator path is further disclosed. This design results in a short path within the insulator in the direction of the electric field to minimize avalanche breakdown within the insulator. 
     Finally, the patent describes an ion source that allows for negative ion extraction (or positive ions). The negative ion extraction is facilitated by the transverse magnetic field, that&#39;s created by the magnetic circuit. Optimal negative ion generation is achieved with an RF frequency that&#39;s greater than 40 MHz. Impedance matching at 40 MHz or above, is made significantly easier by using fixed capacitors and a variable RF frequency to achieve the matching. Variable frequency impedance tuning also allows the impedance match circuit to be more compact, lower complexity and power efficient. Variable frequency tuning eliminates the need for mechanically adjustable capacitors that require servo or stepper motors to drive them, thus minimizing the complexity of the impedance matching device. 
     The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention that proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing a side elevation of a plasma ion source implemented according to teachings of the present invention. 
         FIG. 2  is a schematic view showing a side elevation of a preferred magnetic circuit design used in the plasma ion source of  FIG. 1 . 
         FIG. 3  is a schematic view showing a side elevation of a preferred gas inlet insulator used in the plasma ion source of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an inductively coupled plasma source that includes a ceramic tube  10 , enclosing the plasma chamber  14 , welded between a first upstream metal flange  34  and a second downstream vacuum flange  13 . Windings of an RF antenna  16  are wrapped around, but electrically isolated (e.g. slightly spaced) from, the cylindrical outer walls of ceramic tube  10 . As will be further appreciated through the description below, RF antenna  16  is further electrically isolated from the cylindrical outer walls of the ceramic tube  10  by filling of the space between them with a dielectric fluid. A split Faraday shield  18  surrounds the cylindrical outer walls of ceramic tube  10  and is interposed between the RF antenna  16  and the ceramic tube  10 . Shield  18  includes a plurality of slits through the wall of the shield  18  facing the ceramic tube  10 , with a space between the shield  18  and tube  10  being filled with the dielectric fluid. 
     A gas inlet  20  communicates with an upstream opening A in the plasma chamber  14  to supply material that is later formed into a plasma. Plasma chamber  14  is referenced to high voltage while the surrounding RF shielding—e.g. implemented as Faraday shield  18 —and RF antenna  16  making plural windings about the ceramic tube  10 , are referenced to ground. A source of radio frequency power  45 , and an impedance matching network comprised of multiple capacitors  49  are electrically connected to the RF antenna  16 . The impedance matching network  49  uses fixed value high voltage capacitors in a radio frequency circuit and in some instances trimmer capacitors may be used to make small adjustment to the capacitance of portions of this circuit. 
     The source of radio frequency power  45  can operate over a range of frequencies in the High Frequency and Very High Frequency bands. Preferably, radio frequency power should operate above about 40 Mhz when powering a plasma source that contains a magnetic circuit, as this has been found to provide efficiency advantages. 
     A source electrode of the plasma chamber has an exit aperture B, opposite the side adjacent the gas inlet  20 , and is in further communication with an extraction electrode  11  and focusing optics located downstream (not shown). Ions extracted from the plasma are then focused into a beam and directed downstream within a ceramic vacuum break  22 . A magnetic circuit consisting of an electromagnetic or permanent magnet source of a magnetic flux  50 , outer magnetic poles  52  based at ground potential, and inner magnetic poles  54  biased at the reactor chamber voltage operate to produce a magnetic field near the exit aperture B. This magnetic field can be set to a relatively high strength to modify the plasma potential to allow for negative ion extraction and separate electrons from the extracted ion beam, or set to a relatively low strength to just limit electron loss to the source electrode and enhance positive ion extraction without significantly modifying the plasma potential. The plasma tube  10  is attached to the vacuum flange  13  through a non-magnetic tube  56  brazed to the plasma tube  10 . 
       FIG. 2  shows a detailed view of the magnetic circuit. All elements are retained within a can  24  that surrounds the whole assembly and is held at ground potential. The assembly in this way forms spaces (e.g. spaces  26   a ,  26   b ,  26   c , and  26   d ) between the can  24  and the outer surface of the plasma chamber  14  and ceramic vacuum break  22  (space  26   a ), between the antenna  16  and the Faraday shield  18  (space  26   b ), between the ceramic plasma tube  10  and the Faraday shield  18  (space  26   c ), and particularly between the outer magnetic poles  54  and the plasma tube  10  (space  26   d ). All spaces are in fluid communication with one another, and form a fluid circuit, so that a fluid introduced through inlet  28  can be pumped through all spaces and exit outlet  30 . 
     Operation of the RF plasma source results in significant power being deposited into the plasma and the antenna, which would create thermal issues and failure within the device if efficient heat dissipation were not implemented. Air cooling may be used, but such is typically inefficient within a small space such as that defined by the plasma ion source. Water cooling is also a possibility for cooling, but it has poor dielectric properties and its reactivity with other materials can create problems. Furthermore, maintaining different parts of the assembly at different voltages also creates operational issues absent adequate insulation between the parts. 
     The invention uses a dielectric fluid as a coolant. The dielectric fluid has been found to exhibit efficient coolant properties when used in the environment of the plasma ion source. Furthermore, a dielectric fluid provides high voltage isolation between the plasma chamber (at high voltage) and the other parts at ground potential including the outer magnetic poles. The dielectric fluid also can provide high voltage isolation between the antenna and the Faraday shield as well as between the impedance matching network capacitors and the surrounding ground planes. A cooling circuit is created within the device so that the dielectric fluid circulates between the plasma chamber  14  and the can  24  throughout the device, and preferably through spaces  26   a ,  26   b ,  26   d , and  26   c . Use of the dielectric coolant around the inductively coupled plasma ion source has been found to keep the plasma chamber and antenna operating at a stable temperature (near room temperature and reduces the gaps required for high voltage stability. More specifically, use of the dielectric fluid both electrically insulates the plasma chamber, so that it can be biased to 30 kV and up, and efficiently transfers heat away from the plasma chamber. The advantages of this approach are:
         1. The entire outer surface of the plasma chamber, the impedance matching network, the magnetic flux source, and the antenna can have their thermal energy efficiently transferred away from the ion source.   2. The split Faraday shield can be held at ground potential while maintaining a minimal gap between the shield and the plasma chamber. Consequently, the antenna can be in close proximity to the plasma chamber so that power coupling is more efficient.   3. The outer magnetic poles can be held at ground potential while maintaining a minimal gap between the outer magnetic pole and the plasma chamber. Consequently, the outer magnetic pole can be in close proximity to the plasma chamber and the inner magnetic poles so that magnetic flux is coupled efficiently between the inner and outer magnetic poles.   4. The antenna, and the matchbox and RF amplifier used to drive the antenna, are ground referenced (i.e. are not biased at the same potential as the plasma).   5. The dielectric fluid is chemically inert and has a very low power dissipation factor (loss tangent) at RF frequencies. This is unlike water that is susceptible to large variations in its loss tangent depending on its purity.       

     In a preferred embodiment, the dielectric coolant is a fluorinated fluid, and the pumping flow is between a heat exchanger (or chiller) and the plasma source. The coolant preferably has a low relative permittivity (dielectric constant), in the range of between 1 and 3, and a high dielectric strength (e.g. greater than 10 kV/mm). The preferable flow rate through the cooling circuit is between about 0.5 and 1.5 gallons per minute, with a most preferred rate of approximately 1 gallon/minute. 
       FIG. 3  is a side elevation view in section of a plasma ion source constructed according to a preferred embodiment of the invention. Like elements from  FIG. 1  are labeled but not discussed further here. Moving downstream from gas source  20 , an insulator conducts gas from the gas source to a leak valve  32  while providing electrical insulation between the gas source and the leak valve. The leave valve  32  supplies a low pressure (e.g. approximately 10-100 mTorr) gas into the plasma chamber  10 . The leak valve  32  cuts the pressure of gas in the line from about 1-100 psi supplied to the gas inlet  20  and present in the insulator  36  to about 10-100 mTorr at the flange  34  and the reactor chamber  14 . As will be appreciated from a description of  FIG. 3 , high voltage applied to downstream flange  13  contributes to the igniting of plasma within ceramic chamber  14 . This conducts potential upstream along the plasma so that ceramic chamber  14 , leak valve  32  and flange  34  are electrically coupled together at the same (high voltage) potential. 
     An insulator  36  is interposed between the gas source connected to the gas inlet  20  and the downstream plasma source to electrically isolate elements electrically connected to the biased plasma chamber  14  to the gas source at ground potential. The insulator consists of a narrow tube or capillary that is hermetically sealed to the gas inlet on one end and the leak valve  32  at the other end. The insulator has a spiral or serpentine path such that the electric field produced between the components biased to high voltage and those at ground potential is largely perpendicular to the path of the tube with a low electric field component that is along the path of the tube. The insulator length along the spiral or serpentine path is much longer than the voltage drop length so there is a low average electric field along the insulator path. The path along the electric field is kept physically short by the narrow tube diameter so avalanche breakdown is suppressed. The gas pressure within the insulator is between about 1-100 psi and the high pressure in combination with the short avalanche breakdown path in the insulator prevents the gas from breaking down when holding off the bias voltage. Accordingly, everything on top of the insulator  36  in  FIG. 3  is maintained at ground while that below the insulator is kept at the high voltage applied. 
     In one aspect of the invention, the plasma tube  10  is electrically isolated from ground by a combination of mechanical insulating support structures and the dielectric fluid. The insulating support structures are configured to position the plasma tube  10  containing the inner magnetic poles  54  within the assembly so that it does not touch can  24 , shield  18 , RF antenna  16 , outer magnetic poles  52 , or other elements held at a different potential. Examples of such insulated support structures include ceramic vacuum break  22  and insulator  36  ( FIG. 2 ). An external high voltage (between 0 and 50 kV) is applied to the plasma tube and the current design uses the dielectric fluid to isolate the plasma tube at applied voltage from the Faraday shield  18 , antenna  16 , and other structures in the source at ground potential. This design is unique in that each of the plasma tube, the antenna, and the impedance matching network is electrically isolated and cooled by the dielectric fluid. 
     Another aspect of the design is that the plasma tube is substantially or completely immersed into the dielectric fluid so that the fluid can effectively transfer heat away from the plasma tube, antenna, and impedance matching network. In wafer processing configurations, the whole plasma chamber may not be immersed in fluid but typically an antenna is immersed in fluid and a dielectric window near the antenna is partially immersed in fluid. However, most of the chamber containing the plasma is not immersed in fluid. 
     In the present design, ions are removed from the plasma and then the ions interact with a substrate outside of the plasma. The external voltage is applied to control how energetic the ions are when they interact with the substrate. The ions are typically focused and deflected after they have been extracted from the plasma. The present design includes a means for extracting the ions and accelerating the ions to the applied voltage. 
     Also described is a method for operating a plasma ion source having induction coils adjacent to outer walls of a reactor chamber for inductively coupling power into the plasma from a radio frequency power source. The method comprises actively biasing the reactor chamber to a high voltage and pumping a dielectric fluid into contact with the induction coils and a substantial portion of the outer walls of the reactor chamber wherein both the reactor chamber and induction coils are electrically isolated and cooled by the dielectric fluid. The step of actively biasing the reactor chamber to a high voltage preferably includes biasing the reactor chamber to a voltage above 10 kV. 
     The dielectric fluid can be pumped in a circuit through a plurality of spaces in fluid communication with one another. The plurality of spaces include a space adjacent the induction coils, a space adjacent outer walls of the reactor chamber, and a space adjacent a vacuum break downstream of the reaction chamber. The method further preferably includes enclosing the induction coils with a split Faraday shield and enclosing the Faraday shield, reaction chamber, and vacuum break in a can to retain the dielectric fluid. The Faraday shield is preferably maintained at a ground potential. The circuit through which dielectric fluid is pumped includes a space between the can and the vacuum break and the can and plasma chamber. 
     The method can further include maintaining different elements of the plasma ion source at different voltages and providing a high voltage isolation between the antenna and Faraday shield. Furthermore, the step of pumping the dielectric fluid includes pumping the fluid through a circuit into and out of the plasma ion source at a rate of between about 0.5 and 1.5 gallons per minute and more preferably approximately 1 gallon/minute. 
     Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications and variation coming within the spirit and scope of the following claims.