Patent Publication Number: US-8987678-B2

Title: Encapsulation of electrodes in solid media

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 13/165,556, filed Jun. 21, 2011, and a continuation-in-part of U.S. patent application Ser. No. 13/353,032, filed Jan. 18, 2012, which is a continuation of U.S. patent application Ser. No. 13/182,925, filed Jul. 14, 2011, which is a continuation of Ser. No. 12/982,606, filed Dec. 30, 2010, which claims priority from U.S. Prov. App. 61/291,288, filed Dec. 30, 2009, all of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to inductively-coupled plasma ion sources and more specifically to the means of cooling the plasma sources while providing high voltage isolation. 
     BACKGROUND OF THE INVENTION 
     Inductively-coupled plasma (ICP) sources have advantages over other types of plasma sources when used with a focusing column to form a focused beam of charged particles, i.e., ions or electrons. The inductively-coupled plasma source is capable of providing charged particles within a narrow energy range, which allows the particles to be focused to a small spot. ICP sources, such as the one described in U.S. Pat. No. 7,241,361, which is assigned to the assignee of the present invention, include a radio frequency (RF) antenna typically wrapped around a ceramic plasma chamber. The RF antenna provides energy to maintain the gas in an ionized state within the chamber. 
     The energy of ions used for ion beam processes is typically between 5 keV and 50 keV, and most typically about 30 keV. Electron energy varies between about 500 eV to 5 keV for a scanning electron microscope system to several hundred thousand electron volts for a transmission electron microscope system. The sample in a charged particle system is typically maintained at ground potential, with the source maintained at a large electrical potential, either positive or negative, depending on the particles used to form the beam. Thus, the ion beam source is typically maintained at between 5 kV and 50 kV and the electron source is typically maintained at between 500 V and 5 kV. “High voltage” as used herein means positive or negative voltage greater than about 500 V above or below ground potential. For the safety of operating personnel, it is necessary to electrically isolate the high voltage components. The electrical isolation of the high voltage plasma creates several design problems that are difficult to solve in light of other goals for a plasma source design. 
     One design difficulty occurs because gas must be brought into the high voltage plasma chamber to replenish the gas as ions leave the plasma. The gas is typically stored at ground potential and well above atmospheric pressure. Gas pressure in a plasma chamber typically varies between about 10 −3  mbar and about 1 mbar. The electrical potential of the gas must be brought to that of the high voltage plasma and the pressure of the gas must be decreased as the gas moves from the gas source into the plasma chamber. The gas must be brought into the chamber in a way that prevents a gas phase discharge, also known as arcing, which would damage the system. 
     Another design challenge is to place the radio frequency coils that provide power to the plasma as close as possible to the plasma to efficiently transfer power. Maintaining the coils at the same high potential as the plasma, however, would typically require maintaining the power supply for the coil at the high plasma potential, which would excessively complicate the power supply design and greatly increase the cost. Inductively-coupled plasma ion sources may use a split Faraday shield to reduce capacitive coupling between the coil and the plasma. The split Faraday shield must be located between the plasma and the coils and is typically well grounded. When the grounded Faraday shield is located close to the dielectric plasma container, the large electric field caused by the rapid change in potential would likely cause a gas-phase discharge if any air or other low dielectric constant gas is trapped between the Faraday shield and the dielectric plasma chamber, which discharge could damage the source. 
     Also, the energy applied to the plasma chamber generates heat. While a compact plasma source is desirable for beam formation, the more compact and powerful the plasma source, the hotter the source would become and therefore the greater the need to efficiently dissipate the heat. The high voltage can also make cooling difficult, which can limit the density of the plasma used. These conflicting requirements make the design of an ICP source very challenging. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved plasma source and an improved charged particle system having a plasma charged particle beam source. 
     This invention provides high voltage (HV) isolation and cooling of the inductively-coupled plasma source for a charged particle beam system. In one preferred embodiment, the plasma source is surrounded by a Faraday shield substantially encapsulated in a solid dielectric media that prevents gaseous high voltage breakdown at the surface of the shield. In another embodiment, the plasma source is surrounded at least in part by a static fluid that provides HV isolation. The term “static fluid” as used herein means a fluid that is not actively pumped. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a longitudinal cross-sectional schematic view of a plasma source that uses a Faraday shield for reduced coupling and an insulating fluid for high voltage isolation and cooling. 
         FIG. 2  shows a transverse cross-sectional schematic view of the plasma source of  FIG. 1 . 
         FIG. 3  shows charged particle beam system that uses a plasma source which uses an insulating fluid for cooling and high voltage isolation. 
         FIG. 4  shows a longitudinal half-sectional schematic view of a plasma source which uses a static fluid for high voltage isolation and active cooling elements. 
         FIG. 5  shows a longitudinal half-sectional schematic view of a plasma source that uses a Faraday shield substantially encapsulated in a solid dielectric media for improved electrical isolation and reduced RF coupling. 
         FIG. 6A  shows a cross-sectional schematic view of a plasma source with integrated heat pipe cooling. 
         FIG. 6B  shows a cross-sectional schematic view of a plasma source with integrated heat pipe cooling, displaying one heat pipe. 
         FIG. 6C  shows a top view of a plasma source with integrated heat pipe cooling, displaying an example configuration of several heat pipes distributed around the perimeter of the plasma source. 
         FIG. 6D  shows a side view of a plasma source with integrated heat pipe cooling, displaying an example heat pipe configuration. 
         FIG. 7  shows a front view of a plasma source with a plasma chamber bonded to a split Faraday shield surrounded by an encapsulant. 
         FIG. 8  shows a partial cross section of the plasma source of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Designing a plasma source typically requires many tradeoffs to meet conflicting design requirements. Embodiments of the invention can provide excellent coupling between the RF coil and the plasma, efficient cooling of the plasma chamber, excellent capacitive screening, and high voltage isolation of the plasma source all of which can produce an inductively-coupled plasma that is dense, quiescent, and at high potential. 
     In some embodiments, expelling air from regions with strong electric fields and filling those volumes with a liquid or other high dielectric constant fluid provides a system designer the opportunity to make design choices with regard to the source configuration that would be otherwise unavailable. 
     The description below describes a plasma source for a focused ion beam system, but a plasma source of the present invention can be used for an electron beam system, or other system. As used herein, a “fluid” can comprise a liquid or a gas. 
       FIG. 1  shows a stylized longitudinal cross-sectional view of a plasma source  100 . The plasma source  100  includes a dielectric plasma chamber  102  having an interior wall  104  and an exterior wall  106 . Plasma chamber  102  rests on a conductive base plate  110 . Plasma  112  is maintained within the plasma chamber  102 . Extraction optics  114  extract charged particles, ion or electrons depending on the application, from plasma  112  through an opening  116  in plasma chamber  102  and opening  118  in base plate  110 . A dielectric outer shell  120 , preferably of ceramic or plastic material that transmits radio frequency energy with minimal loss, is concentric with plasma chamber  102  and defines a space  122  between outer shell  120  and plasma chamber outer wall  106 . A split Faraday shield  134  is located in space  122  and is typically concentric with the plasma chamber  102 . A pump  124  pumps a cooling fluid  126  from a reservoir/chiller  127  to space  122  through cooling fluid inlets  128  and exit through exit  132 , cooling plasma chamber  102  by thermal transfer from outer wall  106 . 
     The split Faraday shield  134  is typically fixed to ground potential and therefore the electrical potential drops rapidly between the plasma region and the split Faraday shield, thus materials between the plasma region and the split Faraday shield must have sufficiently large dielectric strength to resist arcing. The cooling fluid can be chosen to have a sufficiently high dielectric constant compared to the material of ceramic housing  102  so that the voltage drop across the fluid is sufficiently low to prevent dielectric breakdown at the operating voltage. A liquid coolant is chosen to be free of gaseous bubbles or other impurities which could present the opportunity for field enhancement and gaseous electric discharge. The cooling fluid can also be chosen to be slightly conductive in which case the fluid volume will be substantially free of electric fields and substantially all of the voltage drop will take place in the plasma chamber  102 . The cooling fluid should also have sufficient heat capacity to prevent the plasma chamber  102  from overheating without requiring a large fluid flow that requires a large pump that would consume excessive power. The plasma chamber outer wall  106  is typically maintained at a temperature of less than about 50° C. 
     The fluid preferably comprises a liquid, such as water or Fluorinert™ FC-40, an electrically insulating, stable fluorocarbon-based fluid sold commercially by 3M Company, St. Paul, Minn. Other electrically insulating fluids, such as mineral oil, may be used. Water, such as distilled water or tap water, may be used. An insulating gas, such as sulfur hexafluoride, may also be used. A cooling pump typically pumps the cooling liquid at a rate of between 10 gal/hour and 50 gal/hour from reservoir/chiller  127 . Fluid  126  returns from exit  132  to chiller/reservoir  127  via a return conduit  133 . Alternately, the cooling fluid can be static liquid that is not mechanically pumped, allowing for significant power savings. Water at room temperature has a dielectric constant of about 80, whereas the ceramic material of the plasma chamber has a dielectric constant of about 9, which results in most of the voltage drop occurring in the ceramic. A preferred insulating fluid has a dielectric constant preferably greater than that of the dielectric material of which the plasma chamber is made. In some embodiments, the insulating fluid has a dielectric constant greater than 5, more preferably greater than 10, even more preferably greater than 20, and most preferably greater than or equal to about 40. 
     In a typical embodiment, reservoir/chiller  127  cools the cooling fluid to about 20° C. before the fluid is recirculated by pump  124 . The cooling fluid partly surrounds the plasma chamber and the coolant flows longitudinally along the plasma chamber from bottom to top. For clarity,  FIG. 1  shows cooling fluid entering space  122  on two sides at the bottom of plasma chamber  102  and exiting space  122  one on side at the top of chamber  102 . Skilled persons will understand that suitable inlets, outlets, and baffles may be used to ensure an even fluid flow around all sides of the plasma chamber  102 . 
     A Faraday shield  134  passes the radio frequency energy from RF coils  136  to energize the plasma while reducing the capacitive coupling between radio frequency coils  136  and plasma  112 . In some embodiments, the Faraday shield  134  is protected from corrosion and physical damage by being substantially encapsulated in a solid dielectric media, such as ceramic, glass, resin, or polymer, to eliminate unwanted fluid in contact with the Faraday shield and to eliminate high voltage discharge. RF coils  136  may be hollow and cooled by flow of a fluid coolant through the internal passages  137  in the coils. The plasma chamber coolant system may also pump fluid coolant through the coils, or the coils can have an independent cooling system. 
     Faraday shield  134  can be positioned such that cooling fluid  126  flows on both sides of the shield  134 . In some embodiments, the shield can be positioned against the plasma chamber outer wall  106  or onto the inside wall of shell  120 . For example, the shield can comprise a metallic layer painted or otherwise deposited on outer plasma chamber wall  106  or inside shell wall  120 . Faraday shield  134  is electrically grounded. In one embodiment, shield  134  comprises a slotted metal cylinder that is grounded by trapping a tab  138  of the Faraday shield between a portion of outer shell  120  and base plate  110 , thereby ensuring a solid ground contact. 
     The gas from which the plasma is produced must be brought from ground potential to the plasma potential along the path between the tank  150  and the plasma. In a preferred embodiment, most of the voltage change occurs where the gas pressure is relatively high and resistant to arcing. 
     Gas is provided to plasma chamber  102  from a gas source, such as a tank  150 . Tank  150  is typically maintained at ground potential and contains the gas at a high pressure. A regulator  152  reduces the pressure of the gas leaving the tank entering a conduit  154 . An optional adjustable valve  156  further reduces the pressure in the gas line or closes the conduit completely when the source is not in use. A flow restrictor, such as a capillary  158 , further reduces the gas pressure before the gas reaches plasma chamber  106 . Restrictor  158  provides a desired gas conductance between the gas line and the interior of plasma chamber  102 . Restrictor  158  is preferably in electrical contact with plasma  112  and so is at the plasma potential. In other embodiments, the flow restriction can have an electrical bias applied from a voltage source other than the plasma. An insulating shield  160  surrounds capillary  158  and a grounded metallic collar  162  at the end of insulating shield  160  ensures that the electrical potential of the gas is zero at that position. Thus, the entire electrical potential change from ground to the plasma voltage occurs within insulating shield  160  in which the gas is at a relatively high pressure and therefore resistant to arcing. 
     In one example embodiment without a valve  156 , regulator  152  reduces the pressure of the gas leaving the supply tank from 150 psig to 5 psig. The gas pressure remains at 5 psig until the gas reaches capillary  158 , and which point the gas pressure drops to the plasma chamber pressure of, for example, 0.1 Ton. Insulating shield  160  preferably has sufficient length to keep the field sufficiently low to prevent a damaging discharge. Insulating shield  160  is typically about at least about 5 mm long, and more typically between about 30 mm and 60 mm. For example, if the plasma is maintained at 30 kV, the electric field within a 10 mm shield is about 3 kV/mm, which is sufficiently low to prevent a sustained discharged in most applications. Skilled persons will understand that the local electric field will be a function of the geometry and that initial low current discharges may occur to reach a static charge equilibrium within insulating shield  160 . In some embodiments, valve  156  may reduce the gas pressure further before the gas reaches the final restrictor before the plasma. Instead of a capillary, the flow restrictor could be a valve, such as a leak valve. Any type of gas source could be used. For example, the gas source may comprise a liquid or solid material that is heated to produce gas at a sufficient rate to supply the plasma. The different output pressures of the different gas sources may require different components to reduce the pressure to that required in the plasma chamber. 
       FIG. 2  shows a transverse cross-sectional view of plasma source  100  in  FIG. 1 .  FIG. 2  shows that the outer wall  106  of plasma chamber  102  is corrugated, that is, it is composed of a series of ridges  202  and valleys  204 . The Faraday shield  134  is positioned against the ridges  202 , defining passages  206  for the cooling fluid to flow between the valleys  204  and the shield  134 . In the embodiment shown in  FIG. 2 , the Faraday shield  134  comprises a metal sleeve that slips over plasma chamber outer wall  106 . A portion of the metal sleeve is then bent outward at the bottom to form grounding tab  138  ( FIG. 1 ), which is trapped between plasma chamber  102  and ground plate  110 . Cooling fluid  126  flows through the space  122  which is bounded by the plasma chamber outer wall  106  and the shell  120 . The Faraday shield is “split”, that is, there are longitudinal slots in the shield, which allow for inductive coupling between the RF antenna and the plasma  112 . In an alternative embodiment, the outer wall  106  may be smooth and the Faraday shield formed with corrugations. Alternatively, neither the wall  106  nor the faraday shield may be corrugated. 
       FIG. 3  shows a charged particle beam that uses the plasma source of  FIG. 1 . At the top of the ion column, an inductively-coupled plasma (ICP) ion source  302  is mounted, comprising an electromagnetic enclosure  304 , a source chamber  306 , and an induction coil  308 , which includes one or more windings of a conductive material. In the embodiment shown in  FIG. 3 , a coolant reservoir and chiller  390  provides coolant to a pump  391 , which provides coolant by conduit  392  to a coolant region around source chamber  306 . The coolant then flows back to coolant reservoir and chiller  390  through a return conduit  393 . In an alternative embodiment, the coolant region around the source chamber  306  contains a static liquid for high voltage isolation. In such embodiments. reservoir/chiller  390  and coolant pump  391  can be eliminated or can be used to circulate a cooling fluid that does not enter a high voltage region. In yet another embodiment, liquid in the plasma source  302  is cooled by one or more heat pipes, as described in more detail below. 
     An RF power supply  340  is connected to a match box  341  by an RF coaxial cable  342 . The match box  341  is connected to the induction coil  308  by coil leg extentions  343 . The induction coil  308  is mounted coaxially with the source chamber  306 . To reduce capacitive coupling between the induction coil  308  and the plasma generated within the source chamber  306 , a split Faraday shield (not shown) may optionally be mounted coaxially with the source chamber  306  and inside the induction coil  308 . When a split Faraday shield is used in the ICP ion source  302 , the high voltage (typically several hundred to a few thousand volts) across the induction coil  308  will have minimal effect on the energies of the ions extracted from the bottom of the ICP ion source  302  into the ion column. This will result in smaller beam energy spreads, reducing the chromatic aberration in the focused charged particle beam at or near the substrate surface. 
     The presence of a plasma within the source chamber  306  may be detected using the light emitted by the plasma and collected by the source-facing end of optic fiber  344 , and transmitted through optic fiber  344  to a plasma light detection unit  345 . An electrical signal generated by the plasma light detection unit  345  is conducted through cable  346  to a programmable logic controller (PLC)  347 . The plasma on/off signal generated by the plasma light detection unit  345  then passes from the PLC  347  through cable or data bus  348  to the plasma source controller  351  executing plasma source control software. Signals from the plasma source controller  351  may then pass through cable or data bus  352  to the focused ion beam (FIB) system controller  353 . The FIB system controller  353  may communicate via the Internet  354  to a remote server  355 . These details of the interconnections of the various components of the FIB system control are for exemplary purposes only. Other control configurations are possible as is familiar to those skilled in the art. 
     Gas is provided to the source chamber  306  by inlet gas line  320  which leads to inlet restrictor  328 , which leads to the interior of the source chamber  306 . Restrictor  328  is maintained at an electrical potential closer to the potential of the plasma in chamber  306  than to the potential of the gas source  310  and regulator  332  so that the voltage drop occurs primarily across gas of higher pressure. Insulating shield  329  insulates the gas line upstream of restrictor  328  and is terminated with a grounded collar  331 . 
     A gas supply system  310  for the ICP source comprises a gas supply  330 , a high purity gas regulator  332 , and a needle (regulating) valve  334 . The gas supply  330  may comprise a standard gas bottle with one or more stages of flow regulation, as would be the case for helium, oxygen, xenon or argon feed gases, for example. Alternatively, for gases derived from compounds which are solid or liquid at room temperature, gas supply  330  may comprise a heated reservoir. Other types of gas supplies  330  are also possible. The particular choice of gas supply  330  configuration is a function of the type of gas to be supplied to the ICP source. Gas from supply  330  passes through high purity gas regulator  332 , which may comprise one or more stages of purification and pressure reduction. The purified gas emerging from high purity gas regulator  332  passes through an optional needle valve  334 . Gas emerging from optional needle valve  334  passes through a hose  336  to an optional second needle valve  338 , mounted in close proximity to the ICP source. Gases emerging from needle valve  338  pass through inlet gas line  320 , which connects through restriction  328  to the top of the source chamber  306 . 
     At the bottom of the ICP source  302 , a source electrode  357  serves as part of the ion beam extraction optics, working in conjunction with the extractor electrode  358  and the condenser  359 . A plasma igniter  360  is connected to a source electrode (not shown), enabling the starting of the plasma in the source enclosure  306 . Other known means of igniting the plasma can also be used. Details of the operation of the ICP source are provided in U.S. Pat. No. 7,241,361, issued Jul. 10, 2007, incorporated by reference herein. The source electrode  357  is biased through the igniter  360  to a high voltage by beam voltage power supply (PS)  361 . The voltage on the source electrode  357  determines potential of the plasma and therefore the energy of the charged particles reaching the substrate surface in the case of singly-ionized atomic or molecular ion species or electrons. Doubly-ionized ion species will have twice the kinetic energy. The extractor electrode  358  is biased by extractor power supply  363 , while the condenser  359  is biased by condenser power supply  362 . The combined operation of the source electrode  357 , the extractor  358 , and the condenser  359  serves to extract and focus ions emerging from the ICP source  302  into a beam which passes to the beam acceptance aperture  364 . The beam acceptance aperture  364  is mechanically positioned within the ion column by the beam acceptance aperture actuator  365 , under control of the FIB system controller  353 . Typical voltage settings may be roughly +30 kV for power supply  361 , roughly 15 kV for power supply  362  and roughly 15 kV for power supply  363 . 
     The ion column illustrated in  FIG. 3  shows two electrostatic einzel lenses  366  and  367 , used to form a highly demagnified (roughly 1/125×) image of the virtual source in the ICP source  302  at or near the surface of substrate  368 , mounted on stage  369  controlled by a sample stage controller  337 . The first einzel lens,  366 , referred to as “lens 1” or “L1,” is located directly below the beam acceptance aperture  364  and comprises three electrodes with the first and third electrodes typically being grounded (at 0 V), while the voltage of the center electrode  370  is controlled by lens 1 (L1) power supply (PS)  371 . The lens 1 power supply  371  is controlled by the FIB system controller  353 . 
     Between the first einzel lens  366  and the second einzel lens  367  in the ion column, a beam defining aperture assembly  372  is mounted, comprising one or more beam defining apertures (three apertures are shown in  FIG. 1 ). Typically, the beam defining aperture assembly  372  would comprise a number of circular apertures with differing diameter openings, where any one of which could be positioned on the optical axis to enable control of the beam current and half-angle at the substrate surface. Alternatively, two or more of the apertures in the beam defining aperture assembly  372  may be the same, thereby providing redundancy to enable the time between aperture maintenance cycles to be extended. By controlling the beam half-angle, together with corresponding adjustments of the lenses, the beam current and diameter of the focused ion beam at or near the substrate surface may be selected, based on the spatial resolution requirements of the milling or imaging operations to be performed. The particular aperture to be used (and thus the beam half-angle at the substrate) is determined by mechanical positioning of the desired aperture in the beam defining aperture assembly  372  on the optical axis of the column by means of the beam defining aperture (BDA) actuator  373 , controlled by the FIB system controller  950 . 
     Beneath the beam defining aperture assembly  372 , the second einzel lens  367 , referred to as “lens 2” or “L2,” is shown. The first and third electrodes are typically grounded (0 V), while the voltage of the center electrode  374  is controlled by lens 2 (L2) power supply (PS)  375 . The lens 2 power supply  375  is controlled by the FIB system controller  353 . A column/chamber isolation valve  376  is positioned somewhere between the source  302  and the sample chamber  378 . Isolation valve  376  enables the vacuum in the ion column vacuum chamber  377  to be maintained at high levels, even if the vacuum level in the sample chamber  378  is adversely affected by sample outgassing, during sample introduction and removal, or for some other reason. A source/chamber turbopump  379  is configured to pump the sample chamber  378  through a pumping line  380 . Turbopump  379  also pumps the ion column enclosure  377  through pumping line  381 . 
     The details of the FIB system illustrated in  FIG. 3  are for exemplary purposes only—many other FIB system configurations are capable of implementing a multiple mode embodiment of the present invention for milling and imaging. For example, the ion column illustrated in  FIG. 3  shows two electrostatic einzel lenses. The ion column may alternatively be implemented using a single electrostatic einzel lens, or more than two electrostatic lenses. Other embodiments might include magnetic lenses or combinations of two or more electrostatic or magnetic quadrupoles in strong-focusing configurations. For the purposes of this embodiment of the present invention, it is preferred that the ion column forms a highly demagnified image of the virtual source (in the ICP source  302 ) at or near the surface of the substrate  368 . Details of these possible demagnification methods are familiar to those skilled in the art. 
       FIG. 4  shows a half-sectional view of another embodiment of a plasma source  400  that includes a dielectric plasma chamber  402  having an inner wall  404  and an outer wall  406 .  FIG. 4  shows a static fluid  408  positioned in a cavity  410  between the antenna coils  436  and the outer wall  406  of the plasma chamber and a shell  416 . Static fluid may comprise a liquid, such as Fluorinert, oil, or distilled water, or gas, such as sulfur hexafluoride. A split Faraday shield  412  is also positioned between shell  416  and outer wall  406 . Faraday shield  412  can be positioned against shell  416  as shown, against outer wall  406 , or away from both walls and immersed in the static fluid  408 . When positioned between a grounded split Faraday shield  412  and the outer wall  406 , fluid  408  provides part of the high voltage isolation of the plasma chamber. Static fluid  408  is preferably not circulated outside of the source  400  by an external pump, although static fluid  408  may move internally by convection within. One or more optional cooling devices  414  assist in cooling the plasma chamber  402 . Cooling devices  414  may comprise cooling loops that encircle the plasma chamber and through which a fluid circulates. Because cooling devices  414  are positioned outside of the Faraday shield, which is at ground potential, these devices do not perform any voltage isolation and hence any type of cooling fluid may be used in cooling devices  414 . Alternatively, cooling devices  414  may comprise one or more thermoelectric coolers, such as Peltier effect coolers. The RF coils  436  may be hollow and cooled by flow of a coolant through the internal passages  437  in the coils. 
       FIG. 5  shows a half-sectional view of a plasma source  500  of another embodiment of the invention.  FIG. 5  shows the Faraday shield  512  substantially encapsulated in a solid dielectric media  516 , which is positioned between the RF coils  536  and the plasma chamber outer wall  506 . Solid dielectric media  516  can comprise, for example, a ceramic material such as alumina or quartz, a resin, or an epoxy encapsulant such as Stycast W-19 or Stycast 2762, sold commercially by Emerson &amp; Cumming Specialty Polymers, Billerica, Mass. An optional gap between the dielectric media  516  and outer wall  506  defines a fluid cavity  510 , which can be filled with a fluid, such as Fluorinert, distilled water, oil (for example mineral oil), or sulfur hexafluoride. Non-encapsulated portions  538  of the Faraday shield  512  are available to form grounding connections. In some embodiments, fluid  508  is pumped through the fluid cavity  510  and then through a cooler, using a system similar to the system shown in  FIG. 1 . In other embodiments, fluid  508  is not pumped outside the source and remains within fluid cavity  510 . The RF coils  536  may be hollow and cooled by flow of a coolant through the internal passages  537  in the coils. 
     In some embodiments, dielectric media  516  can be positioned against outer wall  506  without an intervening fluid. To avoid an air gap in such embodiments, dielectric media  516  should fit tightly against outer wall  506 . Air gaps can also be avoided by providing a flowable material to fill displace any air between outer wall  506  and dielectric media  506 . The flowable media can be, for example, a high dielectric constant grease or gel. The flowable material can remain liquid or may solidify after positioning the dielectric media relative to the plasma chamber. In some embodiments, the dielectric media can comprise a flowable medium that hardens or remains liquid. For example, a flowable, hardenable material may be applied to outer wall  506  and/or to Faraday shield  512  before Faraday shield  512  is slipped over outer wall  506 , so that the Faraday shield is positioned around outer wall  506 , with the flowable medium filling any gap between Faraday shield  512  and the outer wall  506 . The flowable medium may also coat the Faraday shield on the side opposite to outer wall  506 , thereby preventing contact between any cooling fluid and the Faraday shield. In some embodiments, the Faraday shield can be molded into the wall of plasma chamber  502 . 
       FIG. 6A  through  FIG. 6D  show multiple views of another embodiment of a plasma source  600  with integrated heat pipe cooling. The plasma source  600  includes a dielectric plasma chamber  604  having an interior wall  628  and an exterior wall  626 .  FIG. 6A  shows a cross-sectional schematic view of the embodiment of plasma source  600  which incorporates a preferred heat pipe cooling device. See cross-section cut line B-B of  FIG. 6C . A “heat pipe” is a heat-transfer device that utilizes phase transition to efficiently transfer heat. In this embodiment, a static fluid coolant  602  surrounds the plasma chamber  604  having one or more heat pipes  606  integrated into the upper portion of the coolant jacket  608 . The coolant  602  is evaporated by heat from the plasma chamber  604  creating coolant vapor  610  which rises toward the cooling fins  612 . Heat from the coolant vapor  610  is dissipated through the cooling fins  612  and transferred into the surrounding air causing the coolant vapor to cool. As the coolant vapor cools, it condenses and flows back into coolant jacket  608 . Alternately, the liquid inside of the heat pipe  606  may be separate from the liquid in the cooling jacket  608 , having the evaporation-condensation cycle self-contained within the heat pipe. 
     Preferably, multiple heat pipes are integrated into the upper portion of the coolant jacket  608  to provide increased heat dissipation capability. The coolant  602  is positioned in the coolant jacket  608  which is positioned between the antenna coils  620  and the outer wall  626  of the plasma chamber, preferably positioned between a split Faraday shield  624  and the outer wall  626 . Alternately, the split Faraday shield may be substantially encapsulated in a solid dielectric media. When positioned between a grounded split Faraday shield  624  and the outer wall  626 , liquid  602  provides part of the high voltage isolation of the plasma chamber. Static liquid coolant  602  is not circulated outside of the source  600  by an external pump, although static liquid coolant  602  may move internally by convection and also by the gravity flow of the condensing coolant. In some embodiments, liquid coolant carries heat away from wall  626  by convection and without a phase change, with the hot liquid rising, being cooled, for example, by cooling fin  612 , and flowing back into cooling jacket  608 . Coolant can flow in contact with outer wall  626 , or it can flow in cooling channels outside of outer wall  626 . The RF coils  620  may be hollow and cooled by flow of a coolant through the internal passages  622  in the coils. Gas enters the plasma chamber  604  through gas inlet  614 , and charged particles are pulled from the plasma chamber  604  by an extractor electrode  632 . 
       FIG. 6B  shows a cross-sectional view through cut line A-A of the plasma source  600  in  FIG. 6A , illustrating one heat pipe. The source end of the heat pipe  606  is connected to the coolant jacket  608  which surrounds the plasma chamber  604 . Static liquid coolant  602  occupies the coolant jacket space. The opposite end of the heat pipe  606  is connected to the cooling fin mount  618 . Liquid coolant is referred to as static because it is not actively pumped, although it will be understood that the liquid may move due to thermal gradients in the liquid. 
       FIG. 6C  shows a top view of the plasma source  600  with an example configuration of integrated heat pipes. In this embodiment, eight heat pipes  606  are integrated radially to the plasma source and located near the top of the plasma source. Each heat pipe  606  has a cooling fin mount  618  attached to the outward end of the heat pipe. One or more cooling fins  612  are connected to each cooling fin mount  618 . 
       FIG. 6D  shows a side view of the plasma source  600  in  FIG. 6C  with one of the eight heat pipes, heat pipe  634 , cut away to further illustrate the cooling fin and heat pipe arrangement. For clarity, only the foreground heat pipes are depicted in this diagram. Heat pipes  606  are integrated radially to the plasma source and located in the upper portion of the plasma source. Each heat pipe  606  has a cooling fin mount  618  with one or more cooling fins  612  attached. 
       FIG. 7  shows a front view of a portion of a plasma source  700  including a dielectric structure  702  having an interior cavity (not visible) for containing a plasma. That is, the dielectric structure  702  forms the walls of the plasma chamber. A split Faraday shield  708  is positioned such that the shield is in contact with, and preferably intimately bonded to, dielectric structure  702 . Preferably, Faraday shield  708  is configured such that there are substantially no voids (that is, empty spaces that could be filled by air, cooling fluid, or any other fluid) between the shield and the dielectric structure  702 . The absence of voids around the dielectric structure  702  ensures that no high voltage arc discharge can occur. Substantially no voids means that any voids that are present are sufficiently small to prevent damaging arcing. 
     The shield  708  can be for example, similar to the shield described in U.S. patent application Ser. No. 13/353,032, which is assigned to the assignee of the present invention. An encapsulant  710  is applied to surround split Faraday shield  708 . For illustration,  FIG. 7  shows the Faraday shield  708  visible through a translucent encapsulant  710 . Split Faraday shield  708  has gaps  722  to reduce eddy currents that drain energy from the RF coils. The encapsulant  710  preferably contacts and adheres to the regions of the dielectric structure  702  in the gaps  722  and, in some embodiments, in other regions that are not covered by shield  708 . There are preferably substantially no voids between the encapsulant and the dielectric structure in regions where the encapsulant contacts the dielectric structure. In other words, there are preferably no voids between the encapsulant and the dielectric structure regardless of whether potions of the shield are between the encapsulant and the dielectric structure. A portion of the split-Faraday shield  708 , such as regions  712 , extends from beneath the encapsulant  710  in order to provide a means of electrical connection to the shield. The extensions may also be, for example, in the form of tabs. Instead of regions of the Faraday shield extending from the encapsulant, an electrical contact could be made through a gap in the encapsulant or a conductor could extend out of the encapsulant from the Faraday shield. 
     The encapsulant  710  is preferably a thin, leak-proof, dielectric material that adheres to and protects the covered portion of the shield  708  and of the dielectric structure  702 . The encapsulant is not electrically conductive so that no eddy currents can be supported within the media. Encapsulant  710  should be thin to increase heat conductance from structure  702 . The thickness of a preferred encapsulant is less than 10 mm, more preferably less than 5 mm, and even more preferably less than 3 mm. Encapsulant  710  is preferably highly thermally conductive in order to present the lowest possible thermal barrier so the plasma chamber may be efficiently cooled by different cooling methods, such as the ones described in previous embodiments. Furthermore, the encapsulant  710  is preferably non-porous so that there is no fluid contact with the split-Faraday shield where it is surrounded by the encapsulant  710 . Suitable materials for the encapsulant  710  may include, but are not limited to epoxy, enamel, and glass frit. 
     Using a split-Faraday shield  708  in contact with dielectric structure  702  and configured so that there is no air between the Faraday shield and the dielectric structure, as well as using an encapsulant that protects the shield and allows no air between the encapsulant and the Faraday shield, prevents air and cooling liquid from coming in to the high DC voltage region between the plasma chamber and the Faraday shield, thereby simplifying the design of the system. 
       FIG. 8  shows a partial sectional view of the embodiment shown in  FIG. 7 . The dielectric structure  702  defines an inner wall  804  and an outer wall  806  of a plasma chamber  808 . A plasma is maintained within plasma chamber  808 . Split Faraday shield  708  is positioned such that it is in contact with outer wall  806 . Encapsulant  710  is applied to a portion of outer wall  806  such that the encapsulant is well adhered to the Faraday shield  708  and to the portions of the outer wall  806  that remain exposed by the slits in the split Faraday shield, thereby excluding any air or cooling fluid between the Faraday shield and the outer wall  806 . Regions  712  of the split Faraday shield extend from beneath the encapsulant  710 , either above the encapsulant, below the encapsulant, or both above and below the encapsulant. Cooling methods, such as the ones described by previous embodiments, may be supplied in the space  832  between the encapsulant  710  and a dielectric outer shell  834 , which is preferably made of ceramic or plastic material that transmits radio frequency energy with minimal loss. The RF coils  812  may be hollow and cooled by flow of a coolant through the internal passages in the coils. 
     Materials and structures described in one embodiment or described as part of the prior art may be used in other embodiments. Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments described herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.