Patent Publication Number: US-6907897-B2

Title: Diaphragm valve for high-temperature precursor supply in atomic layer deposition

Description:
TECHNICAL FIELD 
   The present invention relates to a diaphragm valve that is particularly useful in high-temperature thin film deposition systems and equipment. 
   BACKGROUND OF THE INVENTION 
   Atomic Layer Deposition (“ALD”), also known as Atomic Layer Epitaxy (“ALE”), is a method of depositing thin films onto a substrate that involves sequential and alternating self-saturating surface reactions. The ALD process is described in U.S. Pat. No. 4,058,430 of Suntola et al., which is incorporated herein by reference. ALD offers several benefits over other thin film deposition methods, such as Physical Vapor Deposition (“PVD”) (e.g., evaporation or sputtering) and Chemical Vapor Deposition (“CVD”), which are well known to those skilled in the art, as described in  Atomic Layer Epitaxy  (T. Suntola and M. Simpson, eds., Blackie and Son Ltd., Glasgow, 1990). ALD methods have been proposed for use in depositing thin films on semiconductor wafer substrates, to achieve desired step coverage and physical properties needed for next-generation integrated circuits. 
   Successful ALD growth requires the sequential introduction of two or more precursor vapors into a reaction space around the substrate surface. Typically, ALD is performed at elevated temperatures and reduced pressures. For example, the reaction space may be heated to between 150° C. and 600° C., and operated at a pressure of between 0.1 mbar and 50 mbar. Even at such high temperatures and low operating pressures, pulses of precursor vapors are not delta functions, meaning they have a substantial rise and decay time. Sequential pulses of precursor vapors will overlap if the second pulse is started before the first pulse is completely decayed, i.e., before excess first precursor vapor is substantially eliminated from the reaction space. If substantial amounts of the different precursor vapors are present in the reaction space at the same time, then non-ALD growth can occur, which can generate particles or non-uniform film thickness. To prevent this problem, the pulses of precursor vapor are separated by a purge interval during which the reaction space is purged of excess amounts of the first precursor vapor. During the purge interval, the reaction chamber is purged by flushing the reaction chamber with an inert gas, application of a vacuum, pumping, suction, or some combination thereof. 
   The ALD reaction space is typically bounded by a reaction chamber, which is fed by one or more precursor material delivery systems (also called “precursor sources”). The size of the reaction space is affected by the dimensions of the reaction chamber needed to accommodate the substrate. Some reaction chambers are large enough to fit multiple substrates for batch processing. However, the increased volume of the reaction space in a batch processing system may require increased precursor pulse durations and purge intervals. 
   To prevent overlap of the precursor pulses and to form thin films of relatively uniform thickness, an ALD process may require purge intervals that are ten times longer than the duration of precursor vapor pulses. For example, a thin film deposition process may include thousands of precursor vapor pulses of 50 ms duration alternating with purge intervals of 500 ms duration. Long purge intervals increase processing time, which can substantially reduce the overall efficiency of an ALD reactor. The present inventors have recognized that reducing the rise and decay times also reduces the overall time required for precursor pulse and purge without causing non-ALD growth, thereby improving the throughput of the ALD reactor. 
   A precursor material delivery system may typically include one or more diaphragm valves positioned in a flow path of the system, for preparing and dispensing one or more precursor vapors. The precursor vapors are pulsed into the reaction chamber by opening and closing the appropriate diaphragm valves in the precursor delivery system. Diaphragm valves can also be used for controlling the flow of inert gases and other materials into and out of the ALD reactor. Known diaphragm valves commonly have an actuator for opening and closing a flexible diaphragm against a valve seat. When the diaphragm is in the open position, the precursor vapor is allowed to pass through a valve passage and enter the reaction chamber. When closed, the diaphragm blocks the valve passage and prevents the precursor vapor from entering the reaction chamber. Because ALD processing can require many thousands of cycles of precursor pulse and purge for forming a film on a single workpiece, valves used in an ALD system should have very high durability and be able to perform millions of cycles without failure. 
   Hydraulic and pneumatic actuators typically include dynamic seals that can fail under the high temperatures and large number of cycles required for delivery of precursor gases and purge gases in an ALD system. 
   Solenoid type actuators are desirable because they typically have a faster response time than pneumatic and hydraulic actuators, and are capable of a large number of open-close cycles. However, solenoid actuators generate heat when electric current is applied and, like hydraulic and pneumatic valves, solenoid actuated valves can fail when exposed to the high temperatures required for maintaining some precursor materials in vapor form. Heat can degrade the insulation around the solenoid windings, resulting in electrical shorting between windings and failure of the solenoid coil. It can also melt a plastic bobbin around which the solenoid coil is wound. The present inventors have recognized that active cooling of the actuator to avoid heat-related failure tends to also draw heat from the diaphragm, valve seat, and walls of the valve passage, which can cause the precursor material to condense or solidify in the valve passage. Condensation and buildup of precursor material on the diaphragm and valve seat can cause the valve to leak or clog, leading to undesirable non-ALD growth and particles in the reaction chamber. 
   For successful ALD processing, precursor gases are typically delivered to the reaction chamber at temperatures in excess of 100° C. and often between 200° C. and 300° C., particularly the varieties of precursor materials used for forming thin films on semiconductor substrates. With a conventional diaphragm valve, a significant amount of heat is conducted from the flow path through the valve, where it dissipates to the surrounding environment. Heat dissipation through the valve can result in cooling of the flow path and the associated condensation problems discussed above. To avoid condensation, the flow path may be heated, as described, for instance, in U.S. Provisional Patent Application No. 60/410,067 filed Sep. 11, 2002, tilted “Precursor Material Delivery System for Atomic Layer Deposition,” which is owned by the assignee of the present invention and incorporated herein by reference. However, heating the flow path may fend to contribute to overheating of the actuators in conventional diaphragm valves. The present inventors have recognized a need for an improved diaphragm valve in which the valve passage, diaphragm, and valve seat can be kept hot enough to prevent the precursor vapor from condensing (typically in the range of 130° C. to 260° C. or hotter), without overheating the valve actuator. 
   U.S. Pat. No. 5,326,078 of Kimura, U.S. Pat. No. 6,116,267 of Suzuki et al., and U.S. Pat. No. 6,508,453 of Mamyo describe known diaphragm valves for controlling the flow of high temperature gases for semiconductor manufacturing. 
   The present inventors have recognized that a need remains for a valve in which the diaphragm is kept at a temperature sufficient to prevent condensation of ALD precursor materials while not exceeding the temperature limits of the actuator. The inventors have also recognized a need for a durable valve that transitions from an open position to a closed position more quickly than prior art valves. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a diaphragm valve may include a heating body that thermally contacts a valve body of the valve and extends proximal to an outer side of the diaphragm opposite the valve passage. The heating body forms a thermally conductive pathway between the valve body and the diaphragm that facilitates maintaining an operating temperature at the diaphragm. When the valve body is heated, the heat is conducted toward the diaphragm by the heating body. Such a construction is useful, for example, in an ALD system for preventing high-temperature precursor gases from condensing or freezing in the valve passage. 
   In a preferred embodiment, a plunger extends through a central opening in the heating body to operably couple a valve actuator to the diaphragm. In some embodiments, a thermally resistive member such as a thin section, a hollow part, or an insulating material, for example, may be interposed between the valve passage and the actuator, for attenuating heat transfer between the valve passage and the actuator. 
   Additional aspects and advantages of the invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric section view of a precursor delivery system including several diaphragm valves; 
       FIG. 2  is a cross section elevation view of one of the diaphragm valves of  FIG. 1 , with a diaphragm of the diaphragm valve shown in a closed position; 
       FIG. 3  is a cross section view of the diaphragm valve of  FIG. 2 , taken along line  3 — 3  of  FIG. 2 ; 
       FIG. 4  is an enlarged cross section view detailing the region of a valve passage, valve seat, and diaphragm of the diaphragm valve of  FIG. 2 , with the diaphragm shown transitioned to an open position; and 
       FIG. 5  is an enlarged cross section view detailing the seating region of a diaphragm valve including an alternative valve seat having a seating ridge suitable for use with a plastic diaphragm. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  is an isometric section view of a precursor material delivery system  100  of an ALD reactor  102 , which comprises an exemplary environment of use for valves  104   a - 104   e , in accordance with a first preferred embodiment. With reference to  FIG. 1 , a supply of precursor material is stored in a precursor container  106 , where it is heated and vaporized before flowing through a flow path  110  of the precursor material delivery system  100  (generally from left to right in  FIG. 1 ) and into a reaction chamber  112 . ALD reactor  102  will typically have two or more precursor material delivery systems  100  connected to reaction chamber  112 . Precursor material delivery system  100  includes electric heaters  116  and  118  for heating precursor materials in the flow path  110 . Valves  104   a - 104   e  are used to control the flow of precursor material and regulate pressure of the precursor vapor at different stages in precursor material delivery system  100 . 
   Precursor material delivery system  100  preferably includes removable modules  120  having bodies  122  machined from solid blocks of thermally conductive material, such as aluminum, titanium, or stainless steel. Modules  120  have various different functions, such as storage, vaporization, valving, filtering, and pulsing of precursor materials, and purging with inert gases. Modules  120  preferably all have a heavy construction that promotes diffusive conduction of heat from heaters  116  and  118  to promote a smooth temperature gradient along the length of precursor material delivery system  100 , increasing in temperature toward reaction chamber  112 . The downstream heater  118  may operate at a temperature slightly higher than the upstream heater  116  to facilitate the temperature gradient. In an alternative embodiment (not shown), a greater number of heating zones may be employed. A positive temperature gradient is important for preventing undesirable condensation or freezing of precursor gases in flow path  110  at any point downstream from precursor container  106 . The magnitude of the temperature gradient is not typically important, so long as the temperature and pressure conditions within flow path  110  are sufficient to prevent condensation or freezing of precursor vapors. To maintain vaporization, heaters  116  and  118  may typically be operated at temperatures in the range of approximately 50° C. and 300° C. 
   A volume module  124  is provided downstream from the precursor container  106  for preparing a dose of gas-phase precursor material. A particle filter module  128  prevents particles from being transported from precursor container  106  into volume module  124 . Valve  104   d  is a diaphragm valve used to control the timing and duration of pulses of precursor vapor introduced into the reaction chamber  112  by precursor material delivery system  100 . A diffusion barrier module  140  includes valve  104   e  for controlling the direction of an inert gas flow in a barrier section  144  of flow path  110  located between diaphragm valve  104   d  and reaction chamber  112 . 
     FIG. 2  is a cross-section elevation view of a diaphragm valve  200  in accordance with a preferred embodiment, which is exemplary of valves  104   a - 104   e  (FIG.  1 ). With reference to  FIG. 2 , diaphragm valve  200  includes a valve body  210  that defines a valve passage  214  through which a medium can flow when diaphragm valve  200  is open. Valve passage  214  includes an inlet  216  and an outlet  218 , which are selectively interruptible by a flexible diaphragm  220 , which blocks valve passage  214  when flexed to a closed position, as shown in FIG.  2 . Valve body  210  is preferably integrally formed with the body  122  of one of the modules  120  of precursor material delivery system  100  (FIG.  1 ). Forming valve passage  214  in module body  122  facilitates connection of inlet  216  and outlet  218  to adjoining portions of flow path  110  in adjoining modules  120  of precursor material delivery system  100 . Alternatively, valve body  210  may comprise a separate structure, which, when used in an ALD precursor material delivery system  100 , may be coupled to the module body  122 . Valve body  210  is preferably formed from a solid billet of material having good thermal conductivity. However, valve body  210  may, alternatively, be formed of multiple parts or by means other than machining from a solid billet, such as by molding or casting, for example. Suitable valve body materials for use in ALD system  102  include aluminum, titanium, and stainless steel. Other materials such as copper, brass, other metals, and molded materials such as high temperature plastics and molded metals may also be suitable for use in valve body  210  depending on the environment in which diaphragm valve  200  is to be used. 
   In the preferred embodiment, inlet  216  and outlet  218  extend in a generally axial direction relative to diaphragm valve  200 . However, in alternative embodiments (not shown), valve passage  214  may include a straight-through passage extending transversely to diaphragm valve  200 . Still further alternatives may include a weir formed in the valve body between the inlet and outlet. Many other means and structures may be used for defining valve passage  214  to handle the flow of a medium such as a fluid (liquid and/or gas) or slurry. 
   Inlet  216  and outlet  218  extend into a cylindrical blind bore  226  bordered by a rim  228  against which diaphragm  220  is secured. Bore  226  is deep enough to accommodate a valve seat  230  against which diaphragm  220  is pressed when transitioned to the closed position. Bore  226  is also sized to allow the medium to flow through valve passage  214  between inlet  216  and outlet  218  when diaphragm  220  is transitioned to the open position (FIG.  4 ). Thus, bore  226  forms side and bottom boundaries of a central chamber  232  ( FIG. 4 ) of valve passage  214 . In a preferred embodiment, diaphragm  220  is a flexible disc-shaped member having a central section that is pressed against or pulled away from valve seat  230  selectively in response to an applied actuation force. Diaphragm  220  includes a first side  234  positioned proximal to valve passage  214  and forming an upper boundary of central chamber  232 . A second side  236  of diaphragm  220  opposite first side  234  is engaged by a means for applying actuation force, such as an actuator  240 . 
   Diaphragm valve  200  is shown oriented with actuator  240  extending vertically from diaphragm  220  and valve passage  214 . However, diaphragm valve  200  could also be oriented with actuator  240  extending to the side of, below, or at an incline relative to diaphragm  220  and valve passage  214 . Furthermore, diaphragm  220 , valve passage  214 , and other parts of diaphragm valve  200  may be oriented in many different ways. For example, in an alternative valve body including a weir, the valve passage may be oriented at an angle to promote drainage across the weir when the valve is in the open position, as is common in prior art diaphragm valves. Thus, the designations of top, bottom, upper, lower, side, front, back, and other similar designations are used as a matter of convenience to describe the preferred embodiment, oriented as it is shown in the drawing figures, and should not be construed as limiting the scope of the invention. 
   Diaphragm  220  is preferably formed of a flexible plastic or elastomeric material. In some ALD systems, diaphragm  220  is preferably formed of a thin, molded disc of a plastic material such as polytetrafluoroethylene (“PTFE”), which may be of the type sold by E. I. du Pont de Nemours &amp; Company, Wilmington, Del., USA, under the TEFLON® trademark. PTFE is a preferred diaphragm material for use in a precursor delivery system that delivers aluminum chloride (AlCl 3 ) to the reaction chamber  112 . While PTFE is desirable for its purity, inertness, chemical resistance, heat resistance, and toughness, other plastic materials, such as polyvinylidene fluoride (“PVDF”), for example, may also be suitable for use in diaphragm  220 . In ALD systems used in semiconductor manufacturing, diaphragm  220  may preferably be formed of an elastomer material, such as VITON® brand fluoroelastomer (FKM) made by DuPont Dow Elastomers LLC, Wilmington, Del., USA. Other suitable elastomeric materials for diaphragm  220  include ethylene propylene diene monomer (“EPDM”); silicone rubber; nitrile rubber; chloroprene rubber (neoprene); natural rubber; and perfluorinated elastomers (FFKM), such as KALREZ® made by DuPont Dow Elastomers LLC, CHEMRAZ® made by Greene, Tweede &amp; Co., Medical &amp; Biotechnology Group, Hatfield, Pa., USA, and SIMRIZ® sold by Freudenberg-NOK, Plymouth, Mich., USA. In some ways, elastomers are less desirable than plastics due to the inferior high-temperature resistance of elastomers and the tendency of fillers in some elastomers to contaminate precursor materials flowing through valve passage  214 . However, elastomers such as VITON, EPDM, and others have good chemical resistance, good purity, and excellent sealing capabilities, making them preferred diaphragm materials for use with many of the ALD precursors used in semiconductor processing. Alternatively, diaphragm  220  may be formed of metal, especially when the temperature of the medium will exceed 260° C., having the potential to degrade elastomer materials. However, metal diaphragms are more vulnerable to fatigue-related failure and breakage than plastic and elastomeric diaphragms. Diaphragm  220  is preferably formed of a solid disc of material, but may also include structures that are not disc shaped, composite structures, and any other flexible shapes and structures that can be transitioned between open and closed positions. Thus, the term “diaphragm” is to be construed broadly to include any member that both borders valve passage  214  when open and can be moved or flexed to a closed position, thereby blocking valve passage  214 . 
   To help prevent corrosion and/or buildup of precursor materials in flow path  110 , the valve passage  214 , diaphragm  220 , and valve seat  230  may be coated with a passivation layer. The passivation layer may comprise an oxide, such as Al 2 O 3 , ZrO 2 , HfO 2 , TiO 2 , Ta 2 O 5 , SnO 2 , or Nb 2 O 5 ; a nitride, such as AlN, ZrN, HfN, TiN, TaN, NbN, or BN; a carbide, such as TiC, TaC, ZrC, or HfC; or mixtures thereof. However, other passivation materials and coatings may be used. Passivation is particularly important when using halide-based precursors, to prevent exchange reactions between the halide-based precursors and the metal typically used in valve body  210  and valve seat  230 . The specific composition of the passivation layer is selected for compatibility with the type of precursor or other medium with which diaphragm valve  200  is used. Other considerations, such as thermal properties, electrical properties, durability, and malleability, for example, may also be important factors in the selection of the material used for passivation. 
   Actuator  240  is operably coupled to diaphragm  220  for applying an actuation force for transitioning diaphragm  220  from the open position to the closed position. In an alternative embodiment, actuator  240  transitions diaphragm  220  from the closed position to the open position, or in both directions. However, the preferred diaphragm valve  200  for use in precursor material delivery system  100  is of a normally closed configuration. Actuator  240  preferably includes a solenoid  246  that can be energized by application of an electric current to drive a plunger  250  that transmits force to diaphragm  220 . Solenoid  246  is the preferred actuator for diaphragm valve  200  due to its speed and generally low maintenance requirements. Alternatively, actuator  240  may include a different means for actuating diaphragm  220 , such as a pneumatic or hydraulic cylinder, for example. Other devices and methods of actuating diaphragm  220 , such as piezoelectric devices, for example, may also be used. 
   Plunger  250  of actuator  240  includes a first end section  256  engaged by solenoid  246  and a second end section  258  coupled to diaphragm  220 . Plunger  250  may be coupled to diaphragm  220  in many ways. For example, diaphragm  220  may include a head  262  or ball end that extends from second side  236  of diaphragm  220  and snaps into lateral openings  266  ( FIG. 3 ) in second end section  258  of plunger  250 . This snap-fit connection between head  262  and plunger  250  allows actuator  240  to pull the central section of diaphragm  220  away from valve seat  230 . It may also allow diaphragm  220  to be conveniently removed for repair or replacement without completely disassembling actuator  240 , plunger  250 , and other components of diaphragm valve  200 . 
   Actuator  240  includes a stop  276  secured to solenoid  246  at its distal end  278  and extending into the center of solenoid  246  to limit outward travel of plunger  250 . Stop  276  is preferably formed of a magnetic material (i.e., a material having a high permeance) to reduce the reluctance in the magnetic circuit of solenoid  246 . More specifically, stop  276  reduces the high-reluctance air gap between distal end  278  of solenoid  246  and plunger  250 , thereby reducing the overall reluctance in the magnetic circuit and intensifying the magnetomotive force exerted on plunger  250  by solenoid  246 , when energized. The magnetomotive actuation force is further increased as plunger  250  moves closer to stop  276 , i.e., when the low permeance gap between plunger  250  and stop  275  is reduced. In other embodiments, stop  276  is made of a nonmagnetic material or omitted entirely. A spring  280 , preferably interposed between stop  276  and plunger  250 , biases plunger  250  and diaphragm  220  toward the closed position wherein first side  234  of diaphragm  220  is pressed against valve seat  230  to block valve passage  214 . Spring  280  is preferably seated in a counterbore in first end section  256  of plunger  250 , but alternative embodiments may involve placement of spring  280  in another location or use of other means for biasing plunger  250  relative to valve seat  230 . For example, in a normally open embodiment (not shown) plunger  250  is biased away from valve seat  230 , and plunger  250  is driven toward valve seat  230  when actuator  240  is activated. In yet other embodiments, spring  280  may be omitted, in which case diaphragm  220  may be driven in both the opening and closing directions by actuator  240 . In yet another embodiment, spring  280  is omitted and diaphragm  220  has a domed shape that is inherently resilient, providing an integral return spring force. Skilled persons will appreciate that many other means and devices may be employed for effecting return of diaphragm  220  to its normal position. 
   Preferably, diaphragm  220  is secured to valve body  210  by a heating body  290  to form a substantially hermetic seal along a perimeter of diaphragm  220  where it is clamped against rim  228  by heating body  290 . Heating body  290  includes a proximal end  294  that is relieved to define a space  296  adjacent second side  236  of diaphragm  220 . Space  296  provides clearance for diaphragm  220  when diaphragm  220  is moved to the open position ( FIG. 4 ) and is substantially enclosed, although a small amount of clearance is provided around plunger  250  to allow plunger  250  to move freely in response to activation of actuator  240 . For valves used in ALD systems, the clearance around plunger  250 , the space  296 , and any other passages in fluid communication with space  296  are preferably sealed to prevent leakage beyond valve  200  in the event that precursor or other medium escapes around the perimeter of diaphragm  220  or in the event that diaphragm  220  ruptures. However, it may not be necessary to hermetically seal space  296 , particularly when diaphragm valve  200  is used in applications other than ALD systems. In the preferred embodiment, enclosed space  296  is defined, at least in part, by proximal end  294  of heating body  290 . However, in alternative embodiments (not shown), space  296  is defined by one or more other components of diaphragm valve  200 , such as, for example, the valve body, the actuator housing, a valve stem, or another structural member extending proximal to second side  236  of diaphragm  220 . 
   To relieve pressure behind diaphragm  220 , space  296  is preferably vented. Venting of enclosed space  296  may provide one or more benefits. For example, venting can reduce or prevent resistance to the movement of diaphragm  220  that would otherwise be caused by compression or expansion of gases trapped in space  296 . When the medium flowing through valve passage  214  has a lowered operating pressure, as is the case in an ALD precursor material supply, suction may be applied in conjunction with venting to reduce a pressure differential acting on diaphragm  220 . Suction can also be applied to generate a vacuum of the same pressure as the medium in valve passage  214 , thereby equalizing the pressures on respective first and second sides  234  and  236  of diaphragm  220 . In some embodiments, suction can be applied to venting to achieve a pressure in space  296  that is slightly less than the medium in valve passage  214 , to thereby assist actuator  240  in opening diaphragm  220 . Thus, in the preferred embodiment, the venting may advantageously reduce the force necessary to actuate diaphragm  220  and move it to the open position, and may also reduce the spring force necessary to return diaphragm  220  to the closed position. Similar force reductions are possible in an alternative normally open configuration, in which case the direction of actuation and spring forces would be reversed. By reducing forces needed to transition diaphragm  220  between the open and closed positions, venting may also extend the life of diaphragm  220  and prevent solenoid burnout. Extending the life of valves  104   a-e  in ALD precursor material delivery system  100  can significantly decrease downtime and improve yields in ALD reactor  102 . Applying suction to space  296  has the further benefit of improving safety, in that any gas that leaks around or through diaphragm  220  is pumped away. This feature is of particular benefit when using toxic precursor materials, which might otherwise leak into human workspaces. Applying a vacuum to space  296  also reduces the density of gas in valve space  296 , which restricts a convective pathway from diaphragm  220  to actuator  240 . 
   Venting is preferably accomplished by a venting passage, an embodiment of which is described below with reference to  FIGS. 2 and 3 .  FIG. 3  is a cross section view of diaphragm valve  200  taken along lines  3 — 3  of FIG.  2 . With reference to  FIGS. 2 and 3 , the venting passage includes a first vent passage section  302  extending through heating body  290  and communicating with space  296 ; a second vent passage section  306  passing through valve body  210 ; and an annular connecting passage  310  that extends around a mid-section of heating body  290  to link together the first and second vent passage sections  302  and  306 . In other embodiments (not shown), the venting passage follows a different path, through one or more other parts of diaphragm valve  200 . The formation of at least a portion of the venting passage in valve body  210  and, particularly, in body  122  of module  120 , provides a convenient means for connecting a pump  316  or other source of suction to second vent passage section  306 . More specifically, connection of pump  316  to the venting passage may include connecting the pump  316  to a manifold (not shown) that serves one or more of the diaphragm valves  104   a - 104   e  and, possibly, other modules  120  of precursor material delivery system  100  where suction is needed. Pump  316  is operable to draw a vacuum in space  296  relative to the pressure outside of valve body  210  (typically atmospheric pressure). 
   Resilient seals  328  and  382  are provided to prevent leakage around heating body  290  and to allow a vacuum to be achieved in space  296  behind diaphragm  220 . As used herein, the term “vacuum” is used loosely to describe a fluid pressure that is lowered from its atmospheric or otherwise normal pressure. The suction generated by pump  316  preferably reduces the pressure in space  296  to a pressure that is the same as or close to the fluid pressure of the medium flowing through valve passage  214 , thereby equalizing or nearly equalizing a differential force on diaphragm  220 . In an alternative embodiment for use with a high-pressure medium, the pressure in space  296  is increased by application of a positive fluid pressure instead of suction. However, changing the fluid pressure in space  296  is optional and, therefore, pump  316  may be omitted in some embodiments. In a preferred ALD precursor material delivery system  100 , pump  316  or another means for generating suction is operable to reduce the pressure in space  296  to between approximately 0.1 mbar and approximately 20 mbar, which is comparable to the operating pressure of precursor vapors in flow path  110  and valve passage  214 . 
   Diaphragm valve  200  preferably includes features that enhance reliability when valve  200  is used to control the flow of a high temperature medium, such as an ALD precursor vapor used for depositing a thin film on a semiconductor wafer substrate, for example. The improved thermal design of diaphragm valve  200  may provide advantages over conventional diaphragm valves, in which solenoid actuators (or other types of actuators) are vulnerable to heat-related failure. For example, one conventional solenoid-actuated diaphragm valve is rated for operating temperatures of up to 140° C. When the operating temperature exceeds 140° C., the solenoid can overheat and melt a plastic bobbin supporting the solenoid coil and/or melt insulation around the coil windings, thereby causing blockage of the plunger, short circuiting of the coil, and other modes of failure. In non-solenoid valves, high operating temperatures can cause failure due to permanent deformation of structural components, melting or deformation of resilient seal materials in the actuator, and other causes. 
   Heat conducted to actuator  240  can also cool diaphragm  220  or valve body  210  enough to cause the medium to condense or freeze within valve passage  214  or on surfaces bordering valve passage  214 . Condensation of the medium is particularly troublesome in an ALD precursor material delivery system  100 , because particles or condensation can cause blockage in the delivery system  100  or may propagate into the reaction chamber  112 , causing flaws in the films being formed. Condensation on the surfaces of diaphragm  220  and/or valve seat  230  can also cause leakage of precursor past valve  200 , when closed, which can cause non-ALD growth in reaction chamber  112 . 
   The operating temperature will depend on the vapor pressure of the particular precursor medium, but will typically be in the range of 130° C. to 220° C. To prevent condensation or freezing of the precursor gases as they travel along flow path  110 , the precursor is gradually heated with a positive temperature gradient toward reaction chamber  112 . In the preferred embodiment, the heat is provided by heaters  116  and  118  in two zones along precursor material delivery system  100 , although a different number of zones and heaters may be used in an alternative embodiment (not shown). Heat may be provided by means other than electric heaters, but will generally result in the conduction of heat into valve body  210 . To evenly and smoothly distribute heat along flow path  110 , valve body  210  and bodies  122  of other modules  120  are preferably formed of a thermally conductive material such as aluminum, titanium, or stainless steel. 
   Heating body  290  is positioned in thermal contact with valve body  210  and extends proximal to second side  236  of diaphragm  220  to thereby form a thermally conductive pathway between valve body  210  and diaphragm  220 . The thermally conductive pathway facilitates maintenance of an operating temperature at diaphragm  220  sufficient to prevent condensation in valve passage  214 . Heating body  290  is interposed between diaphragm  220  and actuator  240  and includes a central opening  322  in alignment with diaphragm  220  and actuator  240 , and through which plunger  250  extends for coupling actuator  240  to diaphragm  220 . Second end section  258  of plunger  250  is preferably formed of a thermally conductive material and sized to closely but slidably fit within central opening  322 , so that heat is readily transmitted from heating body  290  to diaphragm  220  through plunger  250 . A core  326  of heating body  290  extends into a counterbore in valve body  210  above rim  228  and is shaped to define the annular connecting passage  310  (FIG.  3 ). A seal  328 , such as an O-ring, is positioned around core  326  to form a hermetic seal between heating body  290  and valve body  210  at an axially distal location relative to annular connecting passage  310 . A flange  332  of heating body  290  extends radially outward from core  326  adjacent an outer surface  334  of valve body  210 . Flange  332  contacts outer surface  334  along a relatively large area, thereby improving heat conduction from valve body  210  to heating body  290 . Flange  332  also provides a structure suitable for securing heating body  290  to valve body  210 , for example, with one or more screws or other fasteners  392  (FIGS.  2 - 3 ). Flange  332  also compresses seal  328  against valve body  210  when secured by fasteners  392 . Heating body  290  is preferably comprised of a material having a high thermal conductivity, such as aluminum, stainless steel, titanium, copper, or other metals, for example. 
   When diaphragm  220  is in the closed position in contact with valve seat  230 , heat is conducted to diaphragm  220  via valve seat  230 . Conduction from valve seat  230  helps replace in diaphragm  220  the heat lost by dissipation through plunger  250  and actuator  240  into the surrounding environment. Valve seat  230  is, accordingly, formed of a material having a relatively high thermal conductivity, such as aluminum, titanium, or another metal, for example. When used with a diaphragm made of an elastomeric material, valve seat  230  preferably includes a substantially flat annular seating surface  342  extending radially from inlet  216 . Seating surface  342  provides increased contact area between diaphragm  220  and valve seat  230  when diaphragm  220  is closed. The increased contact area reduces the contact resistance (thermal) between valve seat  230  and diaphragm  220 . Preferably valve seat  230  contacts a substantial portion of first side  234  of diaphragm  220  to promote heat transfer from valve seat  230  to diaphragm  220  along a thermally effective contact area opposite where plunger  250  contacts second side  236  of diaphragm. In the preferred embodiment, the area of contact between seating surface  342  and diaphragm  220  is comparable to the contact area between plunger  250  and diaphragm  220 . When diaphragm  220  is closed, valve seat  230  may contact between approximately 5% and 100% of the portion of first side  234  of diaphragm  220  exposed to central chamber  232 . More preferably, valve seat  230  may contact between approximately 12% and 50% of the exposed area of first side  234  of diaphragm  220 , when diaphragm  220  is closed. 
   Seating surface  342  may also be polished or otherwise made smooth to further reduce contact resistance and to reduce leakage of medium between valve seat  230  and diaphragm  220  when diaphragm  220  is in the closed position. A passivation layer over first side  234  of diaphragm  220  can further enhance conduction of heat from valve seat  230  to diaphragm  220 . For example, a passivation layer on first side  234  may comprise a layer of aluminum oxide (Al 2 O 3 ) or another metallic coating having a thickness of between approximately 10 nm and approximately 100 nm. 
     FIG. 5  is a cross sectional view of an alternative embodiment diaphragm valve  500  including a plastic diaphragm  520 . With reference to  FIG. 5 , plastic diaphragm  520  is preferably formed of PTFE or another resilient, high-purity, chemically inert material, such as PVDF, for example. Because plastic diaphragm  520  does not seal as easily as elastomeric diaphragms, diaphragm valve  500  includes a modified valve seat  530  having a ring-shaped seating ridge  522  that extends upwardly from seating surface  542  toward diaphragm  520 . Seating ridge  522  is sufficiently prominent and sized to permanently deform first side  534  of diaphragm  520  when diaphragm  520  is pressed against valve seat  530 . Seating ridge  522  surrounds inlet  516  and is preferably located immediately adjacent inlet  516  to reduce the amount of spring force necessary to cause permanent deformation of diaphragm  520 . However, in other embodiments (not shown) seating ridge  522  may be located outwardly of inlet  516  or in another location. Seating ridge  522  may be flat-topped, as shown in  FIG. 5 , or may have another shape, such as a knife edge. However, seating ridge  522  differs from knife-edge valve seats of the prior art in that seating ridge  522  is short enough to allow the first side  534  of diaphragm  520  to be pressed against the surrounding seating surface  542  after seating ridge  522  has formed a ring-shaped hit channel  544  in first side  534 . In comparison, because particles in some environments can lodge on flat surfaces and interfere with closure of the diaphragm, prior art diaphragm valves prevent leaks by using sharp valve seats that are tall enough to prevent areal contact between the diaphragm and flat surfaces around the sharp seating edge. In the context of an ALD precursor material delivery system, sealing interference is best prevented by improved heat transfer between the valve seat  530  and diaphragm  520 , to prevent particle formation due to cooling of the diaphragm. 
   Seating ridge  522  is preferably between approximately 0.5 mm and 1.5 mm in height above seating surface  542  to provide the desired permanent deformation of hit channel  544 , while allowing areal contact between first side  534  of diaphragm  520  and seating surface  542  of valve seat  530  after hit channel  544  has been formed. The initial formation of hit channel  544  in first side  534  of diaphragm  520  may require a break-in period in which valve  500  is cycled prior to use. To provide the increased contact area between valve seat  530  and diaphragm  520  that promotes heat transfer, annular seating surface  542  may be sized and shaped similarly to that of the seating surface  342  of the embodiment of  FIGS. 2-4 , described above. For example, the area of contact between seating surface  542  and diaphragm  520  may be comparable to the contact area between a second end section  558  of plunger  550  and a second side  536  of diaphragm  520 . When diaphragm  520  is closed, valve seat  530  may contact between approximately 5% and 100% of the portion of first side  534  of diaphragm  520  exposed to central chamber  532  and, more preferably, between approximately 12% and 50% of the exposed area. 
   Valve seat  530  may include a polished surface finish and/or passivation similar to the surface treatments described above in connection with valve seat  230  ( FIGS. 2-4 ) of the type used with elastomeric diaphragm  220 . Because the plastic material used in diaphragm  520  of  FIG. 5  is stiffer than elastomeric materials of the diaphragm  220  of  FIGS. 2-4 , flexibility may be improved in diaphragm  520  by reducing the thickness of diaphragm  520  or, preferably, by forming an annular thin region  552  between a head  562  of diaphragm  520  and where diaphragm is mounted against a rim  528  of valve body  510 . Diaphragm  520  and valve seat  530  are preferably rotationally secured to prevent relative rotation that can cause leakage due to misalignment between seating ridge  522  and hit channel  544 . Preventing relative rotation between diaphragm  520  and valve seat  530  also facilitates the formation on first side  534  of a micro-roughness that mates against corresponding micro-roughness of seating surface  542 , to thereby promote a hermetic seal. 
   Referring again to  FIG. 2 , a thermally resistive member is preferably interposed between valve passage  214  and actuator  240  to restrict or throttle the transfer of heat from valve passage  214  (i.e., from heating body  210  and/or diaphragm  220 ) to actuator  240 . The thermally resistive member may comprise one or more structures for attenuating heat transfer between valve passage  214  and actuator  240 , or between valve body  210  and actuator  240 , or between heating body  290  and actuator  240 , or between actuator  240  and one or more other parts of diaphragm valve  200 . 
   One kind of thermally resistive member comprises a section of reduced cross sectional area between valve passage  214  and/or valve body  210  and actuator  240 . For example, plunger  250  may include a hollow region  348  between respective first and second end sections  256  and  258 . Hollow region  348  and the surrounding thin cylindrical wall of plunger  250  attenuate heat transfer between diaphragm  220  and actuator  240 . As described above, attenuation of heat transfer prevents heat-related failure of solenoid  246  and cooling of diaphragm  220 , which can otherwise result in condensation of the medium in valve passage  214 . 
   To further inhibit heat transfer through plunger  250 , plunger  250  may have a composite construction, wherein first end section  256  is formed of a magnetic material, second end section  258  is formed of a thermally conductive material (for conducting heat from heating body  290  to diaphragm  220 ), and an insulating central section  352  between respective first and second end sections  256  and  258 . Central section  352  may be formed of a material having a substantially lower thermal conductivity than second end section  258  or may have a structure resulting in lower thermal conductivity than second end section  258 . 
   Another kind of thermally resistive member includes a valve stem  360  supporting actuator  240  over and apart from heating body  290  and valve body  210 . Valve stem  360  may include a section of reduced cross sectional area  364  for attenuating heat transfer between heating body  290  and actuator  240 . To increase contact resistance, valve stem  360  preferably contacts heating body  290  and/or valve body  210  along only a very small area, if at all. For example, valve stem  360  may be supported on a small step  368  of a central boss  372  of heating body  290 . An insulating pedestal  376  formed of a thermally resistive material such as plastic or ceramic may extend around or be positioned around a perimeter of flange  332  of heating body  290  to separate valve stem  360  from heating body  290 . Insulating pedestal  376  may comprise a ring of insulating material, or may, alternatively, comprise a set of posts extending from flange  332  about its perimeter. 
   An elastomeric or plastic seal  382  is positioned around boss  372  and between heating body  290  and valve stem  360 . Seal  382  prevents leakage of gases between heating body  290  and valve stem  360 . An annular dead air space  386  may be formed between valve stem  360  and heating body  290  and between seal  382  and insulating pedestal  376 . Dead air space  386  further insulates valve stem  360  from heating body  290 . Actuator  240  may be secured to valve stem  360  by press fitting of solenoid  246  onto valve stem  360 , by adhesives, or by other means. Valve stem  360  and heating body  290  are attached to valve body  210  by one or more screws  392  extending through holes in the radial portion of valve stem  360  and the flange  332  of heating body  290 . Screws  392  are threaded into valve body  210  and thermally insulated from valve stem  360  by insulating washers  396  positioned under the heads of screws  392 . Insulating washers  396  may be made of a plastic material such as PTFE, for example. 
   A thermally insulating slide bushing  402  is interposed between plunger  250  and actuator  240 . Slide bushing  402  is preferably formed of a thermally insulating plastic material such as PTFE that also has a low coefficient of sliding friction against the inner surface of valve stem  360  within which first end section  256  of plunger  250  rides. Slide bushing  402  advantageously may inhibit heat transfer between plunger  250  and actuator  240 , reduce frictional resistance to movement of plunger  250 , and reduce wear and particle generation that can foul the movement of plunger  250  within actuator  240 . 
   A blocking member  410  is interposed between plunger  250  and stop  276 . Blocking member  410  is preferably comprised of a durable plastic material that cushions the impact of plunger  250  against stop  276  when solenoid  246  is energized. Cushioning of the impact can prevent cracking of stop  276  and/or plunger  250 , thereby preventing the formation of particles that can foul the movement of plunger  250  within actuator  240 . A suitable plastic material for blocking member  410  is PTFE. Blocking member  410  may also have thermal insulating properties to attenuate heat transfer between plunger  250  and stop  276  when plunger  250  is in the fully open position, in contact with blocking member  410 . 
   When formed of a nonmagnetic material, such as PTFE or another plastic, blocking member  410  introduces a magnetic discontinuity between stop  276  and plunger  250  that can reduce a “release time” after removal of electric current from solenoid  246  before spring  280  will begin to move plunger  250  away from stop  276 . The magnetic discontinuity introduced by blocking member  410 , in effect, reduces the magnetic field at the extreme distal end of first end section  256  of plunger  250  by providing a nonmagnetic separation between the magnetically conductive stop  276  and the magnetically conductive first end section  256  of plunger  250 . By way of further explanation, the attractive magnetic force on plunger  250  that is generated by solenoid  246  is not immediately removed when electric current to solenoid  246  is cut off. Rather, a certain amount of time must pass before the magnetic force decays below a threshold at which spring  280  can begin to move plunger  250  away from stop  276 . Blocking member  410  reduces release time by reducing the holding force between solenoid  246  and plunger  250 . Reducing the release time results in quicker switching from the on state to the off state, making it possible to shorten the total open time of diaphragm valve  200 . 
   Prior art diaphragm valves, such as the ones described in U.S. Pat. No. 5,326,078 of Kimura and U.S. Pat. No. 6,116,267 of Suzuki et al., for example, include a valve seat having a sharp seating surface for deforming the diaphragm or increasing localized pressure on the diaphragm when it is pressed against the valve seat. As described above, diaphragm  220  may be comprised of an elastomeric material such as VITON® or EPDM, for example. When exposed to certain heated precursors and chemicals, such as ZrCl 2 , for example, elastomer materials can become brittle, making them vulnerable to cracking and shearing against a sharp valve seat. In the preferred embodiment, seating surface  342  of valve seat  230  is characterized by an absence of sharp features, which may help prevent scoring and eventual shearing or cracking of diaphragm  220 . Seating surface  342  is preferably larger than 5 mm 2  and more preferably larger than 25 mm 2 . While it is desirable to size valve seat  230  large enough to prevent diaphragm  220  from shearing, the shape and size of valve seat  230  may, nevertheless, be selected so that the biasing force from spring  280  will cause slight surface deformation of first side  234  of diaphragm  220 . Surface deformation causes first side  234  to better conform to seating surface  342 , thereby reducing leakage of medium that can otherwise result from micro-roughness of first side  234  and/or seating surface  342 . Surface deformation may include elastic deformation, or plastic deformation, or both. A smooth or polished surface finish of seating surface  342  may further improve the ability of diaphragm  220  to provide a leak-tight seal when pressed against seating surface  342 . 
   As described above with reference to  FIG. 5 , when the diaphragm is comprised of a plastic material such as PTFE or PVDF, a predominantly flat seating surface may be less desirable. As compared to diaphragms formed of elastomeric material, a plastic diaphragm  520  ( FIG. 5 ) has a greater hardness and is, thus, more difficult to seal against the valve seat. With reference to  FIG. 5 , valve seat  530  for use with a plastic diaphragm  520  preferably includes a seating ridge  522  extending from the seating surface  542  around inlet  516 . The seating ridge  522  causes plastic deformation of first side  534  of diaphragm  520  when it is pressed against valve seat  530  during a break-in period. Diaphragm  520  can be pre-cycled to help break it in before commencing use of diaphragm valve  500 . Plastic deformation occurring during pre-cycling or a break-in period imparts a ring-shaped hit channel  544  to diaphragm  520  that tightly mates against seating ridge  522  to prevent leakage. A similar sharp edge valve seat may also be desirable to increase localized sealing pressure when diaphragm  520  is made of metal, although it may be unnecessary or undesirable to plastically deform a metal diaphragm. 
   To ensure a tight seal, the valve seat  230 ,  530  and diaphragm  220 ,  520  are secured to the respective valve body  210 ,  510  and plunger  250 ,  550  to thereby prevent relative rotation. Preventing relative rotation between the valve seat and diaphragm ensures that the same location on diaphragm  220 ,  520  contacts valve seat  230 ,  530  in the same place every time the diaphragm valve  200 ,  500  is closed. 
     FIG. 4  is an enlarged cross section view detailing valve passage  214 , valve seat  230 , and diaphragm  220 , with the diaphragm  220  shown transitioned to the open position. With reference to  FIG. 4 , annular seating surface  342  of valve seat  230  is sufficiently large so that first side  234  of diaphragm  220 , when flexed to its slightly convex closed position, will not contact an outer peripheral edge  418  of seating surface  342 . Seating surface  342  may also be curved along outer peripheral edge  418 , as shown in  FIG. 4 , or may be slightly crowned (not shown), to further prevent scoring or shearing of diaphragm  220 . Valve seat  230  is generally pedestal-shaped and includes a threaded neck  422  extending opposite seating surface  342 . Valve seat  230  is screwed into valve body  210  to achieve good thermal contact. Preferably, valve seat  230  is threaded into an inlet portion of valve passage  214 . However, in an alternative embodiment (not shown), inlet  216  and outlet  218  are reversed so that valve seat  230  is threaded into an outlet passage formed in valve body  210 . Valve seat  230  may be coated with a passivation layer in a manner similar to diaphragm  220  and valve passage  214  (as described above) to prevent corrosion of valve seat  230  or buildup of precursor materials on seating surface  342  or inside inlet  216 . A seat O-ring  430  is interposed between an upper pedestal portion of valve seat  230  and a lower surface of blind bore  226  of valve body  210  to provide a leak-tight seal between valve seat  230  and valve body  210 . A spacer ring  440  or shim is interposed between upper pedestal portion of valve seat  230  and a floor of blind bore  226  of valve body  210  for establishing an axial position of valve seat  230  relative to valve body  210 . Spacer ring  440  prevents overcompression of O-ring  430  and establishes an axial position of seating surface  342  relative to valve body  210  and diaphragm  220 . Precise axial positioning of seating surface  342  allows for improved control of the seating pressure of diaphragm  220  against seating surface  342 , thereby enhancing leak-tightness without applying excessive force that might cause scoring on first side  234  of diaphragm  220 . 
   It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.