Patent Publication Number: US-2007117383-A1

Title: Precursor material delivery system with staging volume for atomic layer deposition

Description:
RELATED APPLICATIONS  
      This application is a divisional under 35 U.S.C. § 121 and claims priority under 35 U.S.C. § 120 from U.S. application Ser. No. 10/660,365, filed Sep. 10, 2003, issued as U.S. Pat. No. 7,141,095, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 60/410,067, filed Sep. 11, 2002, and which is a continuation-in-part of U.S. patent application Ser. No. 10/400,054, filed Mar. 25, 2003, now U.S. Pat. No. 6,936,086, all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD  
      The field of the present disclosure relates to methods and devices for storing precursor materials in a thin film deposition process, such as atomic layer deposition; conditioning such precursor materials in preparation for deposition, e.g., by adjusting their temperature and/or pressure; and introducing pulses of vaporized precursor material into a reaction space of a thin film deposition system.  
     BACKGROUND  
      Atomic layer deposition (“ALD”), formerly known as atomic layer epitaxy (“ALE”), is a thin film deposition process that has been used to manufacture electroluminescent (“EL”) displays for over 20 years. See, e.g., U.S. Pat. No. 4,058,430 of Suntola et al., incorporated herein by reference. The films yielded by the ALD technique have exceptional characteristics such as being pinhole free and possessing almost perfect step coverage. Recently, ALD has been proposed for use in the semiconductor processing industry for depositing thin films on semiconductor substrates, to achieve desired step coverage and physical properties needed for next-generation integrated circuits. ALD offers several benefits over other thin film deposition methods commonly used in semiconductor processing, such as physical vapor deposition (“PVD”) (e.g., evaporation or sputtering) and chemical vapor deposition (“CVD”), as described in Atomic Layer Epitaxy (T. Suntola and M. Simpson, eds., Blackie and Son Ltd., Glasgow, 1990).  
      In contrast to CVD, in which the flows of precursors are static (i.e., flow rates are steady during processing), precursor flows in ALD processing are dynamic. There are many precursor delivery system components, such as mass flow controllers and particle filters, that can be used in CVD processing in which flow resistance and switching speed are not especially important. However, the inventors have recognized that such delivery system components have limited utility in ALD processes and equipment, due to the dynamic precursor flows and fast switching needed in ALD.  
      Successful ALD growth typically requires the sequential introduction of two or more different precursor vapors into a reaction space around a substrate. ALD is usually performed at elevated temperatures and pressures. For example, the reaction space may be heated to between 200° C. and 600° C. and pumped down to a pressure of approximately 1 Torr. In a typical ALD reactor, the reaction space is bounded by a reaction chamber sized to accommodate one or more substrates. One or more precursor material delivery systems (also known as “precursor sources”) are typically provided for feeding precursor materials into the reaction chamber.  
      After the substrates are loaded into the reaction chamber and heated to a desired processing temperature, a first precursor vapor is directed over the substrates. Some of the precursor vapor chemisorbs on the surface of the substrates to make a one monolayer thick film. For true ALD, the molecules of precursor vapor will not attach to other like molecules, and the process is therefore self-limiting. Next the reaction space is purged to remove excess of the first vapor and any volatile reaction products. Purging is typically accomplished by introduction of an inert or non-reactive purge gas into the reaction space. After purging, a second precursor vapor is introduced. Molecules of the second precursor vapor chemisorb or otherwise react with the chemisorbed first precursor molecules to form a thin film product of the first and second precursors. To complete the ALD cycle, the reaction space is again purged to remove any excess of the second vapor as well as any volatile reaction products. The steps of first precursor pulse, purge, second precursor pulse, and purge are typically repeated hundreds or thousands of times until the desired thickness of the film is achieved.  
      A key to successful ALD growth is to have the first and second precursor vapors pulsed into the reaction chamber sequentially and without overlap. An ideal set of ALD precursor pulses would be a pair of Delta functions, as illustrated in  FIG. 1 , which is a simplified timing diagram representing two cycles of a simple ALD process. With reference to  FIG. 1 , alternating pulses of a first precursor  12  and a second precursor  14  are separated by intervals  16 , which can be made small compared to the duration “d” of each of the pulses  12  and  14 . For simplicity of illustration, the pulses  12  and  14  are shown in  FIG. 1  as having equal duration, but unequal pulse durations would also be feasible.  
      As noted above,  FIG. 1  illustrates an ideal set of precursor pulses. However, in practice, imperfections in the precursor delivery system, precursor adsorption on the walls of the delivery system and reaction chamber, and fluid flow dynamics cause the concentration of precursor material in the ALD reaction space to have a leading edge slope and an exponential decay during purge.  FIG. 2  is a simplified timing diagram illustrating respective first and second pulses  22  and  24  in an ALD reactor, each with a leading edge slope  26  and exponential decay  28 . With reference to  FIG. 2 , because the actual pulses  22  and  24  are not Delta functions, they will overlap if the second precursor pulse  24  is started before the first precursor pulse  22  is completely decayed, as illustrated by overlap region  29 . If substantial amounts of both of the first and second precursor chemicals are present in the reaction space at the same time, then non-ALD growth can occur, which can generate particles, non-uniform film thickness, and other defects. To prevent the problems caused by non-ALD growth, the pulses  22  and  24  are desirably separated by a purge interval that is long enough to prevent overlap  29 .  
       FIG. 3  illustrates a purge interval  32 , between respective first and second precursor pulses  34  and  36 , that is sufficiently long to prevent overlap. For simplicity, the purge interval  32  is illustrated as having a duration similar to the duration of the pulses  34  and  36 . However, in practice, it is common for purge times in an ALD process to be 10 times longer than the pulse times, due to long exponential decays of precursor pulses caused by flow restrictions and cold spots in the flow path. For example, a ALD process including precursor vapor pulses having a duration of 50 milliseconds (ms) may require pulse intervals of 500 ms or longer to prevent overlap and achieve good film thickness uniformity. Long purge intervals increase processing time, which substantially reduces the overall efficiency of the ALD reactor. The present inventors have recognized that reducing the rise and decay times also reduces the overall time required for each ALD process cycle without causing non-ALD growth, thereby improving the throughput of the ALD reactor.  
      Conventionally, precursors have been stored and vapors delivered from glass tubes placed inside the reactor, as described in U.S. Pat. No. 4,389,973 of Suntola et al., incorporated herein by reference. The flow of each precursor vapor is controlled by so-called “inert gas valving,” which involves controlling the direction of an inert gas flowing through the tube containing the precursor chemical. Conventional inert gas valving has been employed for about 20 years for the fabrication of EL displays, including its use with certain solid precursors like ZnCl 2  and MnCl 2 . However, the present inventors have found that the particle requirements for other applications, particularly semiconductor processing, are far more stringent than those required for EL display manufacturing. Conventional precursor delivery methods and inert gas valving do not provide a barrier to prevent particles present in powdered precursors from being carried into the reaction space with the pulses of precursor vapor. Further, the conventional methods cannot accommodate certain highly reactive precursors useful for semiconductor processing, which cannot be loaded in an open tube due to their reactivity with air and/or moisture.  
      For most films grown by ALD, unwanted particles in or on the film will reduce the manufacturing yield. It is therefore important that the precursor delivery system does not emit particles. Preventing particles is especially difficult when one or more of the precursors exist in powdered form at room temperature and pressure. CVD systems commonly include a high efficiency particle filter that can block up to 99.99999% of particles smaller than 0.003 microns. However, the present inventors have found that CVD-type high efficiency particle filters are unsuitable for use in ALD processing because they are highly resistive to flow, which leads to long precursor rise and/or decay times. High efficiency particle filters also have a tendency to become blocked by coarse particles emanating from a supply of precursor material, which can cause system failures and yield losses in manufacturing. A new type of ALD-oriented particle filtering is therefore needed.  
      The inventors have also recognized a need for improved control of unwanted precursor migration between pulses (during purging).  
      U.S. Patent Application Publication No. 2001/0042523 A1 of Kesala discloses a reactant gas source contained in a vacuum shell. Liquid or solid reactant matter is held in an ampoule having an opening covered by a high efficiency particle filter. The ampoule is enclosed within a gas-tight container that defines a gas space around the ampoule. An outlet of the gas-tight container leads from the gas space through a second high efficiency particle filter and into the reaction chamber. Pulses of reactant gases are switched by a backflow of inert gas in a line between the second high efficiency particle filter and the reaction chamber.  
      U.S. Pat. No. 6,270,839 of Onoe at el. discloses a precursor source for a CVD system that does not include a mechanism for pulsing, as required in an ALD system.  
      Thus the inventors have recognized a need for improved methods and devices for storing precursor materials in a thin film deposition process, conditioning such precursor materials in preparation for deposition, and introducing pulses of vaporized precursor material into a reaction space of a thin film deposition system.  
     SUMMARY  
      A precursor delivery system for delivering pulses of precursor material to a reaction space in a thin film deposition system includes a precursor container for holding a supply of precursor material and a flow path from the precursor container to the reaction space. In a preferred embodiment, a pulse control device is interposed between the precursor container and the reaction space for selectively permitting pulses of the precursor material to flow from the precursor container to the reaction space via the flow path. In some embodiments, a staging volume may be established downstream from the precursor container and upstream from the reaction space for receiving at least one dose of the precursor material from the precursor container. The staging volume is preferably selectively isolatable from the reaction space for releasing a series of pulses of the precursor material from the staging volume toward the reaction space. The staging volume may also be selectively isolatable from the precursor container. One or more sensors may be coupled to the staging volume for sensing a physical condition of the staging volume or the precursor material present in it, such as temperature or pressure, for monitoring system performance and/or providing feedback to an automatic controller of the precursor delivery system for closed-loop control.  
      The precursor material is preferably vaporized after loading it in the precursor container by heating the precursor material or reducing pressure inside the precursor container. A vacuum line may be coupled to the precursor container for reducing pressure inside the precursor container. The vacuum line preferably bypasses a reaction chamber of the thin film deposition system to prevent particles from being drawn through the flow path and into the reaction chamber. The vaporized precursor material (hereinafter “precursor vapor”) may be drawn into the staging volume, via a pressure differential, upon opening an optional isolation valve between the precursor container and the staging volume. A particle filter may be interposed in the flow path between the precursor container and the reaction space, and preferably between the precursor container and a staging volume, for filtering particles from the precursor vapor.  
      The particle filter may include a high conductivity particle filter for preventing the particles from passing into the reaction space without significantly restricting the flow of the pulses through the flow path. In a preferred embodiment, the high conductivity particle filter defines a filter passage having multiple turns, at least one of which passes near a trap in communication with the filter passage such that the inertia of the particles causes them to travel into the trap as the precursor material flows through said turn. The particle filter may also comprise a compound filter including one or more high efficiency filters downstream from a high conductivity particle filter. In the compound filter arrangement, the high conductivity particle filter operates to remove coarse particles before the precursor reaches the high efficiency filters, thereby protecting the high efficiency filters from clogging.  
      Methods and devices in accordance with the disclosed embodiments may be useful in atomic layer deposition (“ALD”), as well as for other pulsed thin film deposition techniques, such as pulsed chemical vapor deposition (“Pulsed-CVD”) and pulsed metal-organic chemical vapor deposition (“Pulsed-MOCVD”), for example.  
      Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Non-exhaustive embodiments are described with reference to the figures, in which like reference numerals identify like elements.  
       FIG. 1  is a simplified timing diagram representing two cycles of an idealized thin film deposition process;  
       FIG. 2  is a timing diagram illustrating overlapping first and second precursor pulses in a simplified prior art ALD process, which may cause non-ALD film growth;  
       FIG. 3  is a timing diagram illustrating a simplified prior art ALD process, in which a purge interval prevents overlapping of sequential first and second precursor pulses;  
       FIG. 4  is a schematic representation of a precursor delivery system in accordance with a preferred embodiment;  
       FIG. 5  is an isometric section view illustrating an embodiment of the precursor delivery system of  FIG. 4 ;  
       FIG. 6  is an enlarged isometric section view of a removable precursor container module for use with the precursor delivery system of  FIG. 5 , including detail of an optional high conductivity particle filter positioned within the precursor container of the precursor container module;  
       FIG. 7  is a schematic representation of the precursor delivery system of  FIGS. 4 and 5 , illustrating a flow of purge gas controlled by a diffusion barrier module of the precursor delivery system during a purging step of an ALD process cycle;  
       FIG. 8  is a schematic representation of the precursor delivery system of  FIGS. 4 and 5 , illustrating the flow of precursor vapors and purge gas during a precursor pulse step of the ALD process cycle; and  
       FIGS. 9-16  show various embodiments of a high conductivity particle filter used in the precursor delivery system of  FIGS. 4-6 , of which:  
       FIG. 9  is a cross-sectional view of one embodiment;  
       FIG. 10  is a cross-sectional view of an alternative embodiment;  
       FIG. 11  is a cross-sectional view of a second alternative embodiment;  
       FIG. 12  is a cross-sectional view of a third alternative embodiment;  
       FIG. 13A  is a plan view of a first plate for another alternative embodiment;  
       FIG. 13B  is a plan view of a second plate for use with the plate of  FIG. 13A ;  
       FIG. 13C  is a perspective view of plates of  FIGS. 13A and 13B  arranged sequentially;  
       FIG. 13D  is a cross-sectional view of a filter incorporating the plates of  FIGS. 13A and 13B  arranged sequentially as shown in  FIG. 13C ;  
       FIG. 14  is a cross-sectional view of an alternative embodiment;  
       FIG. 15  is a cross-sectional perspective view of yet another alternative embodiment; and  
       FIG. 16  is a cross-sectional view of still another alternative embodiment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Throughout the specification, reference to “one embodiment,” or “an embodiment,” or “some embodiments” means that a particular described feature, structure, or characteristic is included in at least one embodiment. Thus appearances of the phrases “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout this specification are not necessarily all referring to the same embodiment.  
      Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Various embodiments can be practiced without one or more of the specific details or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or not described in detail to avoid obscuring aspects of the embodiments.  
      As used herein, terminology referring to “communication” or “fluid communication” between components is inclusive of both a direct connection between components and an indirection connection in which such communication is effected via one or more intermediate components or pathways.  
     System Overview  
       FIG. 4  shows an schematic representation of a precursor delivery system  100  in accordance with a preferred embodiment.  FIG. 5  is an isometric cross section view illustrating an embodiment of the precursor delivery system  100  of  FIG. 4 . With reference to  FIGS. 4 and 5 , a supply of precursor material (not shown) is stored in a precursor container (PC)  102 , where it is preferably vaporized before flowing through a flow path  104  of the system  100  and into a reaction space inside a reaction chamber  110  of a thin film deposition system. Precursor material may originate as a solid, liquid, gas, or mixtures thereof, although it will most commonly be in powdered or liquid form when initially loaded into precursor container  102 . When the precursor material originates in gaseous form, it is typically unnecessary to take steps for vaporizing the precursor material, such as heating or pressure reduction in precursor container  102 . Valves V 1 , V 2 , V 3 , and V 4  are used to regulate the pressure at different stages in precursor delivery system  100  and to control the flow of precursor material, as further described below. For clarity, the details of valves V 1 , V 2 , V 3 , and V 4  are omitted and, in  FIGS. 5 and 6 , the valve stems and valve actuators of the valves are merely outlined in dashed lines. Any of a variety of diaphragm valves, fast-switching shut-off valves, or other flow shut-off mechanisms may be used. A preferred diaphragm valve is described in U.S. patent application Ser. No. 10/609,134, filed Jun. 26, 2003, incorporated herein by reference.  
      A staging volume (VOL)  114  is defined by the walls of a volume module  116 , which is interposed in flow path  104  downstream from precursor container  102  and upstream from reaction chamber  110 . Staging volume  114  is selectively isolatable from precursor container  102  and reaction chamber  110  for receiving and holding at least one dose of precursor material from which one or more pulses of precursor vapor may be released through flow path  104  and into reaction chamber  110 . A particle filter module (PF 2 )  120  prevents particles from being transported into staging volume  114  when it receives a dose of the precursor from precursor container  102 . Particle filter module  120  may include a high conductivity particle filter, as described below; a high efficiency filter; or a compound filter including a high conductivity particle filter followed by a series of one or more high efficiency filters. In the embodiment of  FIGS. 4 and 5 , particle filter module  120  includes a series of four filters  122   a ,  122   b ,  122   c , and  122   d  arranged in flow path  104 , of which the first filter  122   a  is a high conductivity filter and the second, third, and fourth filters  122   b ,  122   c ,  122   d  are high efficiency filters. The high conductivity filter of the first filter  122   a  may preferably be of the type described below with reference to  FIGS. 9-11 , including a spiraling filter passage flanked by a series of inertial traps. In some embodiments, filters  122   a ,  122   b ,  122   c , and  122   d  comprise filters of progressively increasing efficiency to prevent clogging. For example, the first filter  122   a  may be the coarsest filter and the last filter may be the finest. In other embodiments (not shown), particle filter module  120  may include a greater or fewer number of filters in series.  
      A fast switching pulse control device  124  includes a pulse valve (V 4 )  126  for controlling the timing and duration of pulses of precursor material released into reaction chamber  110 . Pulse valve  126  may include a diaphragm valve for selectively interrupting flow path  104 . A suitable diaphragm valve is described in U.S. patent application Ser. No. 10/609,134, filed Jun. 26, 2003, which is incorporated herein by reference. In alternative embodiments (not shown), pulse control device  124  includes other devices for releasing pulses of precursor vapor, such as an inert gas valve. Pulse control device  124  preferably includes a diffusion barrier (DB)  130  positioned in the precursor flow path between pulse valve  126  and reaction chamber  110 . One purpose of diffusion barrier  130  is to purge flow path  104  and reaction chamber  110  between pulses by injecting an inert gas such as nitrogen (N 2 ) into flow path  104  at a location upstream from reaction chamber  110  and downstream from pulse valve  126 . Diffusion barrier  130  also operates so that any precursor material that might leak through pulse valve  126 , when closed, is prevented from diffusing into reaction chamber  110  and reacting with a different precursor material being delivered into reaction chamber  110  by a second precursor delivery system (not shown).  
      In one embodiment, a high conductivity particle filter (HCPF)  140  is the last component in flow path  104  of precursor delivery system  100  before reaction chamber  110  to prevent particles from reaching reaction chamber  110 . High conductivity particle filters may also be placed at other locations along flow path  104 , to prevent particles from clogging or fouling valves, high efficiency particle filters, and other components of precursor delivery system  100 .  
      An isolation valve (V 2 )  146  is positioned downstream from particle filter module  120  and upstream from volume module  116  for selectively isolating staging volume  114  from precursor container  102 . Isolation valve  146  may include a diaphragm valve for selectively interrupting flow path  104  between staging volume  114  and precursor container  102 . A suitable diaphragm valve is described in U.S. patent application Ser. No. 10/609,134. A pump  152  is coupled to precursor container  102  and can draw a vacuum within precursor container  102  independently of vacuum drawn at other locations in precursor delivery system  100  or the thin film deposition reactor. A vacuum path  156  between precursor container  102  and pump  152  is selectively opened and closed by a vacuum valve (V 1 )  158 . Vacuum valve  158  may include a diaphragm valve for selectively interrupting vacuum path  156 , such as the diaphragm valve described in U.S. patent application Ser. No. 10/609,134. Pump  152  may be the same pumps used to reduce the pressure in the reaction chamber  110  or independent pumps, but vacuum path  156  preferably bypasses reaction chamber  110  so that any particles drawn into vacuum path  156  from precursor chamber  102  will not pass through reaction chamber  110 . By bypassing reaction chamber  110 , vacuum path  156  also facilitates the use of less expensive precursor chemicals because it allows high vapor pressure contaminants and byproducts, such as water, to be removed from the supply of precursor material without contaminating the reaction space  110 . Isolation valve  146  preferably prevents the portion of flow path  104  located downstream from isolation valve  146  from the effects of pumps  152 . Vacuum valve  158  and isolation valve  146  may cooperate so that normally no more than one of them is open at a time. A particle filter (PF 1 )  160  ( FIG. 4 ) may be interposed between precursor container  102  and vacuum valve  158  to prevent particles from clogging or fouling vacuum valve  158 . Particle filter  160  preferably comprises a high conductivity particle filter of the type described below with reference to  FIGS. 9-16 . Alternatively, particle filter  160  may include a high efficiency particle filter.  
      A boost valve (V 3 )  164  connects a source of inert boost gas  166  to staging volume  114  at a location downstream from isolation valve  146 . The diffusion barrier  130  includes a control valve (V 5 )  168  and a network of flow channels with flow restrictors R 1 , R 2 , and R 3 . Diffusion barrier  130  includes an input channel  174  from a purge gas source  176  supplying an inert gas, and an output channel  178  to a diffusion barrier pump  182 . Purge gas source  176  and diffusion barrier pump  182  may be shared or combined with the source of boost gas  166  and the vacuum pump  152 , respectively.  
       FIG. 6  is an enlarged sectional perspective view of a removable precursor container module  190  of the system  100  of  FIG. 5 , including precursor container  102 , vacuum valve  158 , and particle filter module  120 . A valve  194  (not shown in  FIGS. 4 and 5 ) is included at the end of module  190  opposite from vacuum valve  158  to allow precursor container  102  to be sealed before it is disconnected from precursor delivery system  100 , to thereby prevent leakage of precursor material into the workspace when changing precursor container module  190 . When precursor container module  190  is removed, isolation valve  146  ( FIGS. 4 and 5 ) is shut to prevent leakage of precursor from staging volume  114  or loss of pressure in reaction chamber  110 .  
      A high conductivity particle filter  196  (not shown in  FIGS. 4 and 5 ) is optionally disposed within precursor chamber  102  above a reservoir  198  where a supply of precursor material is held (not shown). Filter  196  may take the place of or be in addition to particle filter  160  ( FIG. 4 ). Also, if high conductivity particle filter  196  is used, then first filter  122   a  of particle filter module  120  may optionally be a high efficiency filter or may be omitted. In yet another embodiment (not shown), filter module  120  may be entirely replaced by high conductivity particle filter  196 .  
      Filter  196  includes a series of stacked plates  202  and non-aligned apertures  204  creating a labyrinth filter passage within the precursor container  102  that passes by a series of inertial traps for filtering particles without significantly impeding the flow of precursor vapor. The filter passage extends between an inlet  203  communicating with flow path  104  at an upstream part of filter  196 , and an outlet  205  communicating with flow path  104  at a downstream part of filter  196 . (In the embodiment of  FIG. 6 , the flow path  104  begins at the reservoir  198  and follows the labyrinth filter passage through the filter  196  to outlet  205  and beyond.) Spacer pins (not shown, see  FIG. 13 ) hold plates  202  in spaced relation and keep plates  202  held together as a unit that is easily replaceable and removable. The entire unit preferably rests on a lip  208  circumscribing precursor container  102 , but could also hang from a removable lid  210  of precursor container  102  or stand on legs (not shown) extending to the bottom  212  of precursor container  102 . Plates  202  may be closely fit to the walls of precursor container  102 , thereby eliminating the need for a resilient seal or bushing therebetween. By preventing the transmission of particles, filter  196  protects high efficiency particle filters  122   a - d  from clogging and increases their life. A high conductivity filter design similar to filter  196  is described below in greater detail with reference to  FIGS. 13A, 13B ,  13 C, and  13 D. Various other embodiments of high conductivity particle filters useful in precursor container module  190  are described below with reference to  FIGS. 9-16 .  
      A pair of monitoring ports  206  and  207  are optionally provided for facilitating fluid measurements immediately upstream and immediately downstream of particle filter  120 . Optional pressure sensors may be inserted in ports  206  and  207  for measuring a pressure drop across particle filter  120  to determine conductance of the filter and to provide a signal or alarm when particle filter  120  is clogged and requires cleaning or replacement. Pressure sensors may be coupled to controller  250  ( FIG. 4 ) for monitoring and control purposes. Alternatively, ports may be plugged if not used.  
     Precursor Loading and Stabilization  
      A precursor such as ZrCl 4  is loaded into the precursor container  102  under possibly inert conditions such as a nitrogen-filled glove box. Lid  210  is sealed with an o-ring  214  and valves  158 ,  194  are leak tight. Precursor container module  190  is connected to downstream components of precursor delivery system  100  via bolts extending through mounting holes  216  and flow path  104  is sealed by an o-ring fitted in an annular groove  218  at the connecting end of precursor container module  190 . The pressure in precursor container  102  is reduced by opening vacuum valve  158  to the bypass path  156  (vacuum path). The precursor material is also heated to operating temperature by a heater  222  positioned adjacent precursor container module  190 , which can lead to an increase in pressure within precursor container  102  due to the expulsion of crystalline or adsorbed water and reaction by products with the precursor material. Pressure buildup in precursor container  102  may be periodically released by again opening vacuum valve  158 . Once the precursor material has reached operating temperature, the precursor source is stabilized and ready for use. A thermocouple (not shown) may be used to monitor the temperature of the precursor material and a pressure transducer (not shown) may be used to monitor the pressure in precursor container  102 , for closed loop control of heater  222  and vacuum valve  158 , via an automatic controller  250  ( FIG. 4 ).  
      The various components of the precursor delivery system  100  are preferably formed in or supported on one or more thermally conductive blocks  230  or other solid bodies, preferably made of a heat-resistant thermally conductive material, such as aluminum, stainless steel, titanium, or another suitable metal. Precursor delivery system  100  may be made modular through the use of more than one block  230  removably joined and sealed, thereby facilitating equipment modifications, repair, and replacement. Blocks  230  together form an elongate thermally conductive body extending from the precursor container  102  to the reaction chamber  110 . In an alternative embodiment, precursor delivery system  100  may be formed in a single block of solid thermally conductive material, to eliminate the possibility of leakage at seams between the blocks  230  of  FIG. 4 . Because blocks  230  are each formed of a solid block of thermally conductive material, they provide around and along flow path  104  a thermal pathway having low thermal resistance, which allows a positive temperature gradient to be maintained along the flow path  104  (the temperature preferably increases toward the reaction space) via heater  222  and a second heater  234  ( FIG. 5 ). A positive temperature gradient ensures that precursor vapor flowing through flow path  104  will not encounter cold spots downstream from precursor container  102  and condense in flow path  104 . In particular, heaters  222  and  234  ensure that the temperature of staging volume  114  is maintained at a higher temperature than precursor container  102 , so that precursor material will not condense when released from precursor container  102  into staging volume  114 .  
     Operation and ALD Processing  
      Referring to the schematic of  FIG. 4 , the following is a preferred sequence of steps for sending pulses of precursor vapor to the reaction chamber  110 :  
      1) Isolation valve  146  is opened to allow precursor vapor to flow into the staging volume  114  for filling of staging volume  114 .  
      2) A control circuit  250  monitors the pressure in staging volume  114  via a pressure sensor  238 .  
      3) When the pressure reaches a preset or target level, the control circuit  250  signals the isolation valve  146  to close.  
      4) The control circuit  250  then signals the inert gas boost valve  164  to open.  
      5) A control circuit  250  monitors the pressure in staging volume  114  via the pressure sensor  238 .  
      6) When the pressure reaches a preset operating level, the control circuit  250  signals the inert gas boost valve  164  to close.  
      7) The diffusion barrier control valve  168  is opened to change the direction of the diffusion barrier flow as shown in  FIG. 8 .  
      8) The precursor pulse valve  126  is opened, which allows a pulse of the precursor vapor to flow from staging volume  114  towards reaction chamber  110 .  
      9) The precursor vapor travels through high conductivity particle filter (HCPF)  140  where it must make several high speed changes of direction. This action separates any remaining particles from the precursor vapor due to the higher inertia of the particles.  
      10) The precursor vapor enters the reaction chamber  110  where a single monolayer chemisorbs on the surface of the substrate.  
      11) Pulse valve  126  is then closed and the concentration precursor material present in reaction chamber  110  decays through the action of inert purge gas supplied by diffusion barrier  130  or otherwise (i.e., the reaction chamber is purged of the precursor material). Diffusion barrier control valve  168  is closed to change the direction of the diffusion barrier flow as shown in  FIG. 7 , to prevent any precursor material leaking through pulse valve  126  from reaching reaction chamber  110 .  
      12) After a suitable purge time has elapsed, a pulse of second precursor material is released into reaction chamber  110  via a second precursor delivery system (not shown) acting in a similar manner as described above.  
      13) The pulsing sequence of the two (or more) precursors is repeated until the desired thickness of the film is reached.  
      At the end of the process the integrity of the seals in valves  126 ,  146 , and  164  may be checked by filling staging volume  114  to a predetermined pressure and using pressure sensor  238  to check for a pressure change in staging volume  114  over time. If the pressure decreases then it is assumed that pulse valve  126  is leaking; if the pressure increases it is assumed that either boost valve  164  or isolation valve  146  is leaking. Either situation suggests maintenance is needed.  
      The level of the precursor may be checked by filling staging volume  114  to a predetermined pressure with inert gas (via boost valve  164 ), then opening isolation valve  145 . The pressure measured by pressure sensor  238  will decrease as the inert gas expands into the precursor container in the open space above the supply of precursor material. The ideal gas equation PV=nRT can be used to calculate the volume additional volume filled by the expanding gas, which represents the open space above the supply of precursor material in the bottom of precursor container  102 . The volume can then be compared to baseline information on the known volume of an empty precursor container  102 , to determine the amount of precursor material remaining in precursor container  102 .  
      Turning again to  FIG. 4 , the precursor material is loaded into precursor container  102  at a pressure of about 1 bar. Many precursors cause a large pressure increase in the container upon first heating, which can lead to large amounts of particles in the reaction chamber  110  if the pressure increase is released via the flow path  104  toward the reaction chamber  110 . To eliminate this source of particles, vacuum valve  158  and vacuum path (bypass path)  156  allows excess pressure and agitated particles to be directed to pumps  152  away from reaction chamber  110 . To prevent vacuum valve  158  from becoming plugged, a particle filter  160  may be provided. Particle filter  160  inhibits particles from powdered precursors (and droplets, in the case of liquid precursors) from being carried into staging volume  114 . Particle filter  160  may include a high efficiency particle filter having low flow conductivity, or a high conductivity particle filter  196  ( FIG. 6 ), or a combination of both kinds of filters. Because filter  160  is not located between pulse valve  126  and reaction chamber  110 , a high flow resistance of filter  160  will not increase the rise or decay time of precursor pulses. If filter  160  has a high resistance (low conductivity), it can also act as a flow dampener to reduce turbulence in precursor container  102  during pump down of precursor container  102  (i.e., when reducing pressure in precursor container  102  via vacuum path  156  and pumps  152 ). Reducing turbulence in precursor container  102  further reduces the incidence of particle transmission into flow path  104  and reaction chamber  110 .  
      For improved process control it is advantageous to control the dose of precursor in each pulse. A control system  250  (“controller” in  FIG. 4 ) typically includes a computer and drive electronics (for solenoid valves) or a pneumatic control system (for pneumatically driven valves) for driving the valves  126 ,  146 ,  158 ,  164 ,  168  between their open and closed states. Control system  250  coordinates the operation of the valves to direct the precursor material to the reaction chamber, as described above, and, via diffusion barrier  130 , to prevent any leaked precursor from reaching the reaction chamber  110 . To improve the repeatability of the dose of precursor material included in each pulse released by pulse valve  126 , pressure sensor  238  may be used to provide closed-loop feedback to control system  250 . In one embodiment, a sensor may be coupled to the staging volume for sensing a physical condition of the staging volume or the precursor material present in it, such as temperature or pressure, for monitoring system performance and/or providing feedback to an automatic controller of the precursor delivery system for closed-loop control.  
      Another potential advantage of pressure sensor  238  is the ability to do diagnostic tests on valves periodically or before each process run, to monitor valve leakage or failure. Over time all of the valves that come in contact with the precursor vapors will begin to leak, which can lead to non-uniform films and non-ALD growth. By setting the pressure of staging volume  114  to a certain level and determining the rate of change of the pressure it may be possible to determine if a valve is leaking significantly. It may also be possible to determine if the precursor supply has been exhausted, as described above.  
      It may also be possible to increase the pressure of staging volume  114  before each pulse, by injecting N 2  or another inert gas via boost valve  164 . Pressure increase provided by an inert gas boost enhances the injection of precursor material vapor into the reaction chamber  110 , which is especially important for low vapor pressure precursors. Alternatively, an inert gas boost may be utilized to increase the pressure in staging volume  114  before the precursor vapor is released into it from precursor container  102 , so that the pressure difference between the precursor container  102  and the staging volume  114  is reduced. A reduced pressure differential between precursor container  102  and staging volume  114  during filling of staging volume  114  may help prevent turbulence upon opening of isolation valve  146 , which can otherwise cause particles to be stirred and transmitted into staging volume  114 , especially when using high vapor pressure powder precursors.  
      Pulse valve  126  allows precursor vapor to travel from staging volume  114  toward reaction chamber  110 . Pulse valve  126  is the principal barrier preventing the precursor material from entering the reaction chamber  110  at undesired times. However, diaphragm valves of the type used for pulse valve  126  will eventually begin to leak after prolonged exposure to most precursor vapors. To prevent leaked precursor vapor from reaching the reaction chamber  110  and causing CVD growth, the so-called “diffusion barrier” concept is employed. In this manifestation of the “diffusion barrier” concept, any vapor leaked from pulse valve  126  will preferentially be carried towards the “P 2  out” outlet channel  178  by a backflow of inert gas, as shown in  FIG. 7 , in which pumps  182  draw the leaked vapor away from the system  100 , bypassing reaction chamber  110 . As shown in  FIG. 8 , when a pulse of precursor vapor is released, the diffusion barrier control valve  168  is opened to release a forward flow of inert gas (such as N 2 ), to prevent the precursor vapor from being drawn into pump  182  and bypassing the reaction chamber  110 . The gas flows during the pulse time are illustrated in  FIG. 8 . The ability to switch between a backflow of inert gas in flow path  104 , as shown in  FIG. 7 , and a forward flow of inert gas, as shown in  FIG. 8 , merely through opening and closing of a single control valve  168 , is achieved by adjusting flow restrictors R 1 , R 2 , and R 3  to properly balance the pressures at each intersection in diffusion barrier  130 .  
      During the purge cycle ( FIG. 7 ), pulse valve  126  is closed to stop the flow of precursor material from the staging volume  114 , and any remaining precursor gases are purged by a flow of inert gas from inlet channel  174  (“IG in”) though the diffusion barrier  130  (as illustrated in  FIG. 7  by black arrows). The inert gas also flows through the high conductivity particle filter  140  and the reaction chamber  110  to purge residual precursor material. The backflow of inert gas is pumped through pumps  182  in diffusion barrier  130  upstream from the inlet channel  174 . This backflow of inert gas carries out of the system through pump  182  any precursor chemical that may leak through the pulse valve  126  (as indicated by the white arrows).  
      One potential benefit of the diffusion barrier  130  is that the flow of inert gas may tend to push the flow of the precursor material during the purge stage, reducing the delay between the time when the precursor material is released from staging volume  114  and when it enters the reaction chamber  110 . An even greater boost is provided by inert gas boost module  164 ,  166 , coupled to the flow path upstream from staging volume  114 . When boost valve  164  is opened, inert gas is pumped into the staging volume  114  via inlet IG 1 . If staging volume  114  holds only a single dose of precursor material, boost valve  164  may be opened when pulse valve  126  is opened, to push precursor material from the staging volume  114  into the reaction chamber  110  more quickly. When the pulse valve  126  is closed, the boost valve  164  may also be closed to maintain the lowered working pressure of staging volume  114 . Isolation valve  146  may then be opened to recharge staging volume  114  with another dose of the precursor material.  
      Preferably, staging volume  114  holds more than one dose of precursor material, in which case the use of an inert gas boost during pulsing may be unnecessary. Further, when the staging volume  114  holds much more than a single pulse, a smaller pressure differential need be applied between staging volume  114  and reaction chamber  110  in order to release sufficient precursor vapor for thin film deposition processing. A smaller pressure differential further reduces turbulence and the transmission of particles from staging volume  114  through flow path  104 . For example, staging volume  114  should desirably hold enough precursor vapor so that upon release of a single pulse of precursor vapor (and without inert gas boost or other inflow into staging volume  114 ), the pressure inside staging volume  114  decreases less than 50% of its pre-pulse pressure. More preferably the pressure decreases no more than 30% of its pre-pulse pressure.  
      A high conductivity particle filter  140  is preferably the last element in the flow path  104  before reaction chamber  110 . It is important that this filter is highly conductive, as any flow resistance will lead to a lengthening of the precursor decay. To separate particles from the precursor vapor without adding significant resistance in the flow path  104 , the high conductivity particle filter  140  includes a labyrinth filter passage that requires the precursor vapor to make many fast changes of direction. The inertia of particles carried by the precursor vapor causes them to be trapped in dead ends of the labyrinth (i.e., “inertial traps”) while the precursor vapor can continue to flow through the high conductivity particle filter  140 . As described below with reference to  FIGS. 9-16 , the labyrinth and inertial traps may be formed of a variety of different structures and may have a variety of different shapes and orientations.  
     High Conductivity Particle Filters  
      Referring to  FIG. 9 , a cross-sectional view of one embodiment of a high conductivity particle filter  410  is shown. The filter  410  provides a primarily two-dimensional flow that captures unwanted particles. In order to separate particles from a vapor stream, the higher inertia of particles is used to separate the particles.  
      The filter  410  includes a filter passage (hereinafter “flow path”)  412  that may be formed in a block  414 . The flow path  412  is configured as a continuous spiral in communication with an input  416  and an output  418 . The arrows indicate the direction of vapor flow through the flow path  412 . The output  418  may be oriented perpendicular to the flow path  412 . In application, the flow path  412  provides one-way directional flow of a vapor stream from the input  416  to the output  418 . Configured as shown, the flow path  412  is a plane curve that moves around the fixed point of the output  418  while constantly approaching the output  418 .  
      The flow path  412  is in communication with a plurality of tangential particle reservoirs or traps  420 . As vapor travels through the flow path  412  the particles have greater inertia than the vapor. As the vapor travels through the curve, the inertia of particles does not allow the particles to follow and the particles are captured in the traps  420  while the vapor continues. Several traps  420  disposed along the flow path  412  provide a highly efficient filter  410  that does not constrain flow. The exact number of traps  420  may vary and depends, in part, on system design limitations.  
      The filter  410  may be formed from a block  414  of heat-resistant material, such as metal. The material may be aluminum, silicon, titanium, copper, stainless steel or other high thermal conductivity material. In manufacturing, the flow path  412  may be drilled or otherwise machined from the block  414 . A lid may then be placed on the block  414  to seal the flow path  412 . The filter  410  may be interchangeable in a modular system to facilitate equipment modifications, repair, and replacement. After forming the filter  410 , the filter  410  (including the walls of the flow path  412 ) may be coated with Al 2 O 3  or other chemically resistant material to protect the filter  410  from corrosive vapors and/or abrasive particles.  
      Referring to  FIG. 10 , another embodiment of a high conductivity particle filter  422  is shown that also relies on a primarily two-dimensional flow path  424 . The filter  422  may be formed in a manner similar to the previous embodiment. The flow path  424  is in communication with an input  426  and an output  428 . The flow path  424  is configured as a spiral, but not a curved spiral as in  FIG. 9 . As defined herein, the term spiral refers to a path that moves around and approaches a fixed point, such as an output. Thus, the spiral need not be continuously curving, but does move around and approaches a fixed point.  
      The movement around the fixed point may be achieved through angled turns  430 . The angled turns  430  of  FIG. 10  are approximately 45 degrees relative to the flow path  424 . Of course, turns having other angles may also be used. The flow path  424  includes two angled turns  430  in order to negotiate a 90-degree turn in the block  414 . One of skill in the art will appreciate that the configuration of the spiral flow path  424  may vary and the embodiments shown herein are for exemplary purposes only. For example, the block  414  may not have a rectangular cross section in which case, the flow path  424  may be adjusted accordingly. As such, the angles and the number of turns may be varied as required.  
      A trap  432  is disposed before an angled turn  430  such that the trap  432  continues along the direction of the flow path  424  before the angled turn  430 . As the vapor stream approaches the turn, the inertia of the particles is greater than that of the vapor. As the vapor stream passes through an angled turn  430 , particles continue along the former path of the flow path  424  and into a trap  432 . The filter  422  includes several traps  432  to provide high filtering efficiency. The traps  432  do not limit the flow of a vapor stream, which allows for high conductivity.  
      The turns need not all have the same angle in order to accommodate the flow path. For example, in the embodiment shown in  FIG. 10 , two 90-degree angles are used for the first and last turns  434  in the flow path  424 . Based on design considerations, the angles of the turns  430 ,  434  may vary. Furthermore, not every turn  430 ,  434  needs to have a corresponding trap  432 . Nevertheless, in order to maximize efficiency it is desirable to include a greater number of traps.  
      Referring to  FIG. 11 , a cross-sectional view of another embodiment of a high conductivity particle filter  436  is shown. As in the foregoing embodiments, the filter  436  may be formed from a block  414  with a flow path  438  machined within. The flow path  438  is similar to the embodiments of  FIGS. 9 and 10  in that it spirals around and approaches an output  440 . The flow path  438  is also in communication with an input  442  for introducing a vapor stream into the filter  436 .  
      The spiral flow path  438  is comprised entirely of 90-degree angled turns  444 . In alternative implementations, the angle of the turns  444  may vary. A trap  446  is disposed prior to an angled turn  444  such that the trap  446  continues along the direction of the flow path  438  before the angled turn  444 . As the vapor stream passes through an angled turn  444 , particles continue along the former path of the flow path  438  and into a trap  446 . Traps  446  may be placed prior to each turn  444  to maximize the efficiency of the filter  436 .  
      The flow paths shown in  FIGS. 9-11  may be altered into various configurations and still provide a spiral that approaches a central point. A flow path may include a combination of features heretofore described. For example, a flow path may include 45-degree angled turns, 90-degree angle turns and turns of other angles. A flow path may also include a combination of curves and angled turns. In an alternative embodiment, the input and the output may be reversed such that the flow path originates at a center point and moves around the center point as it approaches the output. In such an embodiment, the traps are disposed in an alternative configuration to capture particles. Thus, high conductivity particle filters are not necessarily limited to the embodiments shown, which are for exemplary purposes only.  
      Referring to  FIG. 12 , a cross-sectional view of another embodiment of a high conductivity particle filter  450  is shown. The filter  450  may be formed from a block of heat resistant material as in previous embodiments. The filter  450  includes a housing  451  that surrounds elements of the filter  450 , such as a flow path  452 . The flow path  452  is in communication with an input  454  and an output  456  and includes a series of 180-degree turns  457  to separate particles from a vapor stream.  
      The filter  450  includes a series of baffles aligned to define paths and traps. The filter  450  includes a major baffle  460  that defines a path  462  for a vapor stream. The housing  451  provides an opposing side and also defines the path  462 . A minor baffle  464 , that is substantially in the same plane as a corresponding major baffle  460 , defines a trap  458  to capture particles. The housing  451  also defines the trap  458 . The trap  458  continues in the same direction as the path  462 . The turns  457  require abrupt directional changes and particle inertia will cause particles to enter traps. As in previous filters, the trap  458  is a dead end to capture and retain particles. As the names indicate, the major baffle  460  has a greater length than the minor baffle  464 . Accordingly, the path  462  is longer than a corresponding trap  458 .  
      An aperture  466  separates the major and minor baffles and is nonaligned with a subsequent adjacent aperture. The aperture  466  may also be nonaligned with the input  454  and output  456 . The aperture  466  provides the only exit for a vapor stream from the path  462  to a subsequent path. The aperture  466  may be referred to as providing the only flow path exit from the path  462 . The flow path is defined as passing from the input  454  to the output  456  in the direction indicated by the arrows. Thus, the vapor stream must pass through the aperture  466  and be subject to a 180-degree angled turn  457 .  
      As the vapor stream enters the filter  450 , the vapor stream enters the path  462 . The input  454  may be disposed perpendicular to the major baffle  460 . The vapor stream continues along the path  462 , toward the trap  458 , until encountering the aperture  466 . Since the vapor has less inertia than the particles, the path of the vapor will tend to bend and travel through the aperture  466 . The particles, due to their greater inertia, will tend to continue on their direction and enter the trap  458 .  
      A second major baffle  470  is disposed parallel to the first minor baffle  464 , and together the first minor baffle  464  and the second major baffle  470  defines a pocket  472  that serves as a secondary trap to capture particles ejected from the flow path  462  after the flow path has passed through the aperture  466 .  
      A vapor stream passing through the aperture  466  enters a second path  468  that is defined by the second major baffle  470  and the first major baffle  460 . The second major baffle  470  is disposed to create a 180-degree turn  457  for the vapor stream. The second major baffle  470  is separated from a second minor baffle  474  by a second aperture  476 . The second minor baffle  474  is substantially in the same plane as the second major baffle  470  and defines a second trap  478 . The second trap  478  continues in the same direction as the second path  468  to capture particles. The second major baffle  470  is longer than the second minor baffle  474  as the second path  468  is longer than the second trap  478 .  
      The second aperture  476  provides the only exit for a vapor stream passing from the first path  462  to the second path  468 . The second aperture  476  is nonaligned with the aperture  466  or a subsequent downstream aperture.  
      Additional major and minor baffles with separating apertures may be similarly disposed to create a series of 180-degree turns  457  and corresponding traps. Some particles, especially smaller particles, may be able to follow the vapor through one or more apertures without being captured in a trap. Further, while the traps are designed to retain particles, it remains possible for particles collected in a trap to be drawn back into the vapor stream. Accordingly, multiple stages of filtering are used to increase the overall effectiveness of the filter  450 .  
      To increase the chances that a particle will be captured, the velocity of the stream should be as high as possible at the turn  457 . The inertia differences that separate particles from the vapor are a function of the velocity of the flow and, in particular, the velocity of the particles. Accordingly, the path leading up to a trap should be as long as space allows, which will allow sufficient room in which to accelerate the particles to a substantial linear velocity before reaching the turn adjacent the trap.  
      The output  456  may be disposed perpendicular to a final major baffle  477  and is in communication with a final path  479 . The number of baffles and turns may vary based on design considerations, but allows for high conductivity while maintaining the efficiency of the filter  450 .  
      The surface of each trap  458 ,  478  and pocket  472  may be modified to help retain particles in the traps and pockets. For example, one or more of the trap and pocket surfaces may be roughened or have an adhesive coating applied, to cause particles to adhere to the surfaces. The entire flow path may include a rough surface or an adhesive coating as well. In this implementation, particles traveling through the flow path would be collected and retained by the flow path surface.  
      Referring to  FIG. 13A , a plan view of a plate  480  for use in another embodiment of a high conductivity particle filter is shown. The plate  480  may be formed of a heat resistant material and in any number of shapes including a circle, oval, ellipse, rectangle and the like. The plate  480  includes an aperture  482  that provides an exit for a vapor stream passing through a filter. The aperture  482  may be aligned off-center so as to be nonaligned with a filter input and output. The aperture  482  is not disposed on the perimeter or edge of the plate  480 , rather the aperture  482  is disposed at an intermediate location on the surface area of the plate  480 . As such, the surface area of the plate  480  surrounds the aperture  482 , and the aperture does not contact a perimeter of the plate  480 . The plate  480  serves as a retaining wall to capture and retain particles.  
      Referring to  FIG. 13B , a plan view of a second plate  484  is shown for use in series with the plate  480  of  FIG. 13A . The second plate  484  may be formed of a similar shape and size as the first plate  480 . The second plate  484  also includes a second aperture  486  that provides an exit for vapor stream passing through a filter. As with the first plate  480 , the aperture  486  is disposed at an intermediate location on the surface area of the plate  480 . The second plate  484  may, in fact, be identical to the first plate  480 . However, when disposed adjacent to the first plate  480 , the second plate  484  may be rotated 180 degrees such that the second aperture  486  is nonaligned with the first aperture  482 .  
      Referring to  FIG. 13C , a perspective view of a series of plates  480 ,  484  is shown. The plates  480 ,  484  are aligned as they may be disposed in a high conductivity particle filter. The number of plates  480 ,  484  may vary based on design considerations and desired filtering efficiency. Each plate  480 ,  484  is spaced apart from one another to form a chamber therebetween. The plates  480 ,  484  are disposed such that the apertures  482 ,  486  are nonaligned with sequential apertures. For good conductivity, the spacing between the plates is preferably the same as the average diameter of the aperture, which is preferably the same as the average diameter of the input and output.  
      Referring to  FIG. 13D , a cross-sectional view of a high conductivity particle filter  488  is shown which includes plates  480 ,  484  within a housing  490 . The housing  490  couples to each plate  480 ,  484  and fixes the plates  480 ,  484  in spaced-apart relation. The housing  490  may be cylindrical or other shape, and has sealed first and second ends  492 ,  494  to define an interior  496 . The housing  490  and the plates  480 ,  484  define multiple sequential chambers  498  within the interior  496 . The housing  490  is secured to each plate  480 ,  484  so that the corresponding aperture  482 ,  486  provides the only exit from one chamber  498  to an adjacent chamber.  
      An input  500  provides passage through the first end  492  and is in communication with a first chamber  502 . Similarly, an output  504  provides passage through the second end  494  and is in communication with a final chamber  506 . The input  500  and output  504  may be disposed perpendicular to the surface area of the plates  482 ,  484 . The input  500  and output  504  may be nonaligned with the sequential apertures  482 ,  486 .  
      The filter  488  may be characterized as providing a three-dimensional flow path, as vapor movement is not primarily confined to two dimensions. A vapor stream must pass through the provided aperture to exit each chamber and undergoes a series of turns. Sequential apertures  482 ,  486  are preferably distanced from each other as much as possible to lengthen the flow path and increase the velocity of the vapor stream. As the vapor stream passes through the apertures  482 ,  486 , the particles, having a greater inertia, will continue along their former path and collect in traps of the chambers  498  adjacent the apertures. A series of plates  480 ,  484  and chambers  498  provide a highly efficient filter without unnecessary flow resistance. The interior surfaces of the chambers  498  may be modified to encourage particle adhesion. For example, the interior surfaces of a chamber  498  may be roughened or coated with an adhesive to retain particles.  
      In one embodiment (not shown), the plates  480 ,  484  may be spaced progressively closer to one another along a flow path to sequentially decrease the volumes of the chambers. Accordingly, the first chamber  502  would have a greater volume than the second chamber  508 , the subsequent chamber would have a volume less than the second chamber  508 , and so forth. The final chamber  506  may be configured with the smallest volume of all the previous chambers. Progressively decreasing the chamber volumes gradually decreases the cross-section of the flow path through the filter  488  and increases the velocity of a vapor stream. An increased vapor stream velocity increases the likelihood of smaller particles being retained in a trap  498 . Apertures  482 ,  486  may also have sequentially decreasing diameters to decrease the cross-section of the flow path.  
      Referring to  FIG. 14 , a cross-section of another embodiment of a high conductivity particle filter  510  is shown. The particle filter  510  includes a housing  512  with sealed first and second ends  514 ,  516 , which define an interior  518 . The filter  510  includes an input  520  and an output  521 , which allows passage through the first and second ends  514 ,  516  respectively.  
      The filter  510  includes tubes  522  that are disposed parallel to one another. Each tube  522  has sealed first and second ends  524 ,  526  and a first (input) aperture  528  and a second (output) aperture  530  disposed along the length of the tube  522 . The apertures  528 ,  530  allow for a flow path  536  through the tube  522  and define first and second traps  532 ,  534  within each tube  522 . The traps  532 ,  534  extend from corresponding apertures  528 ,  530  to the respective second and first sealed ends  526  and  524 . As such, each trap  532 ,  534  is a “dead end” in which particles are captured and retained in a manner similar to previously described embodiments.  
      Each tube  522  includes a path  537  which may be generally defined as the length of the tube  522  from the first aperture  528  to the second aperture  530 . Vapor exiting the path  537  must turn through the output aperture  530  and particles, having a higher inertia than the vapor, continue in the same direction and enter a trap  534 .  
      The tubes  522  are in communication with one another to provide a sinuous flow path that includes a series of paths  537  and turns. Traps  532 ,  534  are disposed adjacent each aperture  528 ,  530  to capture particles unable to negotiate a turn. The number of tubes  522  used for a flow path may vary based on system design constraints and desired efficiency of the filter  510 .  
      The first and second apertures  528 ,  530  provide communication between the tubes  522  in the filter  510  as shown in  FIG. 14 . Thus, whether an aperture may be characterized as an input or output is relative to the tube since an output for one tube is an input for an adjacent tube.  
      The last tube in the flow path is defined herein as the output tube  538  and is in communication with or passes through the output  521 . The output tube  538  may have an open end  540  to provide an exit for the vapor stream as shown in  FIG. 14 . Alternatively, the output tube  538  may have one or more output apertures.  
      In the embodiment shown in  FIG. 14 , the filter  510  provides split paths  536   a  and  536   b . After passing through the input  520  into the interior  518 , the vapor stream is bifurcated into the two flow paths  536   a  and  536   b . Each flow path passes through a series of parallel tubes  522  configured with paths  537  and apertures  528 ,  530 . The flow paths  536   a  and  536   b  merge when reaching the output tube  538  before exiting the filter  510 . One of skill in the art will appreciate that the tubes  522  may be arranged in series to provide a single flow path, or two or more flow paths.  
      The filter  510  may further include one or more preliminary traps  542  adjacent the input  520 . The preliminary traps  542  may be formed by the extending the walls of the tubes  522  beyond their sealed first ends  524 . The preliminary traps  542  may be disposed such that incoming vapor stream must turn and pass over the traps  542  before entering into the tubes  522 . As in previous embodiments, the preliminary traps  542  and the previously discussed first and second traps  532 ,  534  may have their interior surface roughened or coated with an adhesive to retain particles. The entire interior surface of the tubes  522  and the output tube  538  may include a rough surface or an adhesive coating to capture and retain particles.  
      A method of increasing velocity is to decrease the cross section of paths  537 . Thus, the tubes  522  may be configured with progressively decreasing cross sectional areas in the direction of a flow path. Decreasing the cross sectional area of a flow path increases the velocity of a fluid as it travels along the flow path.  
       FIG. 15  is a perspective cross-section view of an alternative embodiment of a high conductivity particle filter  546  similar to the filter  510  of  FIG. 14 . With reference to  FIG. 15 , the filter  546  is formed of concentric tubes  548  having progressively smaller diameters as the flow path traverses from a first tube  550  to subsequent tubes  552 ,  554 ,  556 , and  558 . The decreasing diameters of the tubes  550 ,  552 ,  554 ,  556 , and  558  form progressively smaller cross-sectional flow areas as the flow path (or paths) proceeds to the output tube  538 . Apertures  530  may also be configured with incrementally decreasing diameters along a defined flow path.  
      The vapor stream proceeds from tube  550  to  552  to  554  to  556  to  558  and, since the cross section is decreasing, the vapor stream velocity is increasing, thereby increasing the inertia of any particles in the vapor. The decreasing diameters and increasing particle inertia encourage separation of the increasingly smaller particles from the vapor stream as the flow proceeds to the outlet  540 .  
      Referring to  FIG. 16  another alternative embodiment of a filter  560  having high conductivity is shown. The traps  562  include an orifice  564  that is in communication with a pump or a bypass line (not shown). An orifice  564  may be effectively implemented with traps of previously discussed embodiments.  
      An orifice  564  may have a cross-section that is approximately 1 to 5 percent as large as the cross-sectional area of the vapor flow channel  566 . The orifices  564  communicating with a pump improve the ability of the filter  560  to capture and retain particles from a vapor stream  572 . The orifices  564  also provide a means for cleaning the traps in-situ, without disassembling the filter  560 , to thereby prevent the traps from becoming filled with particles that might otherwise be drawn back into the vapor stream  572 . The resistance of the orifices  564  should be high enough so that the majority (e.g., preferably more than 90 percent) of the vapor stream  572  flowing through the filter  560  does not go through an orifice  564 , but rather continues to the exit of the filter  560 .  
      To direct the particles toward an orifice  564 , a trap  568  may have sidewalls that are tapered toward the orifice  564  in a funnel configuration. In this implementation, particles traveling through the orifice  564  are directed away from the trap  568  down a separate path  570 . The particles are permanently removed from the vapor stream  572 . Some traps  568  may have tapering configurations while other traps  562  do not. Furthermore, some traps  562  may have orifices  564  while others do not.  
      The high conductivity particle filters described herein provide a flow path with turns and traps to capture particles. The number of turns and traps ensure filter efficiency. The turns preferably involve abrupt high-speed changes of direction, which separates particles from vapor due to higher inertia. The filter&#39;s high conductivity offers little flow resistance, thereby speeding up precursor vapor pulse decay. Faster switching times for precursor vapor are possible due to the decreased resistance. Although the filter is described for use in a precursor vapor delivery system, the filter may also be used in a pumping line, a reaction chamber, and other applications.  
      Depending upon the location of the filter, the preferred dimensions and operating conditions will vary. When the filter is in a precursor delivery system of an ALD system or other thin film deposition system, it may typically operate at a temperature in the range of 120° C. to 250° C. and at a pressure in the range of 1 to 10 Torr with flows less than 1 standard liter per minute (slm). If the filter is located near a reaction chamber, it may typically operate at a temperature in the range of 200° C. to 500° C. and at a pressure of 0.5 to 5 Torr at flows in the range of 1 to 10 slm. If the filter is located in the pumping line, it may operate near room temperature at pressures in the range of 0.1 to 10 Torr and at flows in the range of 1 to 10 slm.  
     Passivation  
      In embodiments of the precursor delivery system  100 , the interior surfaces of the flow path  104  (including valves and high conductivity particle filters) exposed to the vapor stream are preferably coated or passivated to prevent chemical reactions. Otherwise, the precursor vapor stream may react with the surface of the material of which the filter is made. Reactions affect the concentration of a vapor stream and destabilize precursor delivery system  100 . The coating or passivation may include, for example, oxides such as Al 2 O 3 , ZrO 2 , HfO 2 , TiO 2 , Ta 2 O 5 , and Nb 2 O 5 ; nitrides such as AlN, ZrN, HfN, TiN, TaN, and NbN; or carbides such as AlC, ZrC, HfC, TiC, TaC, and NbC; and mixtures thereof.  
     Uses  
      Precursor material delivery systems  100  in accordance with the embodiments described herein are preferred for precursors that are solids at temperatures they are vaporized. Examples of such precursors include metal halides, metal β-diketonates, and organometal compounds. In particular, such systems are preferred for hafnium tetrachloride (HfCl 4 ), zirconium tetrachloride (ZrCl 4 ), aluminum trichloride (AlCl 3 ), tantalum pentachloride (TaCl 5 ), niobium pentachloride (NbCl 5 ), molybdenum pentachloride (MoCl 5 ), tungsten hexachloride (WCl 6 ), platinum (II) acetylacetonate (Pt(acac) 2 ), and tris(cyclopentadienyl)scandium (Sc(Cp) 3 ), among others. As noted above, precursor material delivery systems in accordance with the disclosed embodiments may be adapted for use in various types of thin film deposition systems, including ALD systems, CVD systems, MOCVD systems, PVD systems and others, especially when it is desirable or necessary to deliver pulses of precursor vapor to the reaction chamber in such systems. Furthermore, precursor material delivery systems in accordance with various embodiments may accept precursors originating in any of a number of different forms, including solid, liquid, gas, fluid, slurry, powder, and mixtures thereof.  
      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 without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.