Patent Publication Number: US-2017362701-A1

Title: Central source delivery for chemical vapor deposition systems

Description:
TECHNICAL FIELD 
     The disclosure relates to chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, such as chemical vapor deposition (CVD) and metalorganic chemical vapor deposition (MOCVD). In particular, various disclosed embodiments include precursor gas supplies that facilitate continuous functionality and operation of a vapor deposition system. Gas mixture generating systems generate the binary mixtures of solid, liquid or gaseous precursors in-situ, which is mixed with the carrier gas. This remotely located system provides multiple reactors with accurately premixed to desired concentration binary mixtures. 
     BACKGROUND 
     Chemical vapor deposition (CVD) is a process that can be used to grow desired objects epitaxially. Examples of current product lines of manufacturing equipment that can be used in CVD processes include the TurboDisc®, MaxBright®, and EPIK™ family of MOCVD systems, manufactured by Veeco Instruments Inc. of Plainview, N.Y. 
     Numerous industries employ processes that require accurate delivery of gas mixtures comprising a gas of interest within the carrier gas. New processes raised substantially the requirements to the accuracy, repeatability and reproducibility of delivered gas of interest in the flowing gas mixture, where the gas of interest is typically of high purity and highly corrosive. Common examples of these processes are different types of CVD (chemical vapor deposition) processes in the semiconductor, compound semiconductor, fiber-optic, and other industries. 
     A number of process parameters are controlled, such as temperature, pressure and gas flow rate, to achieve a desired crystal growth in a CVD system. Different layers can be grown using varying materials and process parameters. For example, devices formed from compound semiconductors such as group III-V semiconductors typically are formed by growing successive layers of the compound semiconductor using metal organic chemical vapor deposition (MOCVD). In this process, the wafers are exposed to a combination of gases, typically including a metal organic compound as a source of a group III metal, and also including a source of a group V element (for example, arsenic or phosphorus) which flow over the surface of the wafer while the wafer is maintained at an elevated temperature. Generally, the metal organic compound and group V source are combined with a carrier gas which does not participate appreciably in the reaction as, for example, nitrogen or hydrogen. One example of a group III-V semiconductor is indium phosphide (InP), which can be formed by reaction of indium and phosphine or aluminum gallium arsenide (AlGa 1-x As x ), which can be formed by the reaction of aluminum, gallium, and arsine. The reaction of the compounds form a semiconductor layer on a substrate having a suitable substrate. These precursor and carrier gases can be introduced by an injector block configured to distribute the gas as evenly as possible across the growth surface. 
     In order to provide proper ratios of the precursor gases, gas source systems are used in which a carrier gas is loaded with gaseous or aerosolized precursor material. For example, a carrier gas can be sparged through a liquid precursor material. In some such systems, this can be accomplished by positioning a dip tube in the liquid precursor material, and then routing the carrier gas such as nitrogen through the liquid. As the carrier gas passes through the liquid, it picks up a quantity of the precursor material. These types of systems are called “bubblers” due to the carrier gas bubbling through the liquid precursor. Typically, each bubbler includes enough liquid precursor to operate a CVD system for several hours. Likewise, in other systems, a solid precursor material can be sublimated into a carrier gas flow in a sublimator system. 
     Conventionally, carrier gas flow through the bubbler (or through the sublimater in case of the solid sources) is measured using a mass flow controller located upstream or downstream of the bubbler (or sublimator) to control the mass transfer rate of the precursor to the reactor. This is conventionally an open loop system, and for example in conventional EPI processes for generating single wafers, it provides wafer-to-wafer thickness uniformity on the order 1-2%. This approach is inaccurate and unrepeatable for several reasons, including instability of bubbler temperature and pressure, heat of vaporization effect, etc. 
     U.S. Pat. Nos. 6,116,080, 6,192,739, 6,199,423, 6,279,379 as well as Patent Application 2014/0060153 A1, all of which are assigned to Veeco Flow Technologies, disclose technique and electroacoustic binary mixture concentration sensor Piezocon® systems that provide a substantial improvement over above open loop system. Such systems are described in R. Logue et al., Deposition Rate Control During Silicon Epitaxy, Semiconductor International, Jul. 1, 2014. As described in that publication, improvements on the conventional system achieved by using certain piezoelectric concentration sensors can increase wafer-to-wafer thickness uniformity, such that deviations are less than about 0.15-0.20%. 
     CVD systems often require precursor inputs at a defined temperature or temperature range. Deviation from these defined temperatures can cause several problems. First, output from the bubblers or sublimators contains high enough concentrations of precursor gas that, if the temperature falls sufficiently, the precursor gas may condense. Second, the output from the bubblers or sublimators should be kept below pyrolyzation temperature until it is at or near the desired surface for deposition. Third, locating a high concentration vapor source remote from the reactor leads to the possibility of a pressure drop, causing adiabatic cooling of the flowing mixture and localized condensation. For this reason, conventionally, bubblers or sublimators for producing carrier gas and vapor mixtures have been kept in very close proximity to the CVD chamber, as routing precursor gas through tubing that is maintained within this specific temperature range is energy-intensive and the consequences of failure to maintain the necessary temperature can be severe. 
     U.S. Pat. No. 5,835,678 (“Li”) describes systems employing bubblers that have employed heated delivery lines and other devices to prevent the condensation of precursors that are reluctant to form vapors. These heaters can be used to extend the distance between the bubbler and the reactor. Such heaters require monitoring and constant power. Loss of power, a faulty temperature sensor, or other problems can cause undesirable buildup or settling out of the precursor material in the lines. 
     U.S. Pat. No. 8,486,191 (“Aggarwal”) describes multiple delivery paths for gas delivery to a reaction chamber. Each path contains a different gas, and the gases react only once they reach the chamber in a common mixing path. Aggarwal noted the benefits of reducing footprint of systems within the semiconductor fabrication industry. Aggarwal also describes the benefits of forming a desired concentration of precursor gas mixture well in advance of deposition. 
     U.S. Pat. No. 4,980,204 (“Fujii”) describes a gas supply system in which a gas flow rate through each vent pipe is made to be controllable individually by a flow controlling device. This can be used to create a uniform concentration of reactants in the reaction chamber. 
     None of these references, however, solve problems in the art such as maintaining continuous (or near-continuous) flow of reactant gas, from a bubbler source that can be positioned at a large distance from the reactor, and for providing that near-continuous flow of reactant gas at a variety of desired concentrations. 
     Replacement of a bubbler or sublimator can be time-intensive. Once the precursor source is consumed, any lines containing precursor gas must be purged, because many precursor gases are pyrophoric. Then the bubbler or sublimator itself can be replaced or refilled. Before reconnecting the bubbler or sublimator, however, a vacuum typically must be pulled in the lines, again to prevent damage that could be caused by the introduction of a pyrophoric material into air-filled lines. Even once reconnected to the lines, a bubbler or sublimator can take several hours of temperature conditioning before the precursor gas is at an appropriate temperature to provide precursor gas to a CVD system. 
     Furthermore, replacement of a reactor produces insufficiently accurate or repeatable deposition results. For example, in the U.S. Pat. No. 8,997,775 and US Patent application 20150167172 A1 the authors are recommending to use their methods for the low vapor pressure solid precursors, such as TrimethylIndium and Cyclopentadienil Magnesium. For reasonably chosen operating conditions for the GaN process of 17° C. and 900 torr, which prevent the condensation of the binary mixtures flowing through the concentration sensor, such as mentioned above Piezocon®, the accuracy and repeatability with Nitrogen as a carrier gas are shown in the table below. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                 Vapor 
                 Concen- 
                 Relative 
                 Relative 
               
               
                   
                 Pressure 
                 tration at 
                 Piezocon 
                 Piezocon 
               
               
                   
                 at 17° C., 
                 900 torr, 
                 Accuracy, 
                 Repeatability, 
               
               
                 Precursor 
                 torr 
                 ppm 
                 % 
                 % 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 TrimethylIndium 
                 0.96 
                 1067 
                 14 
                 1.9 
               
               
                 Cyclopentadienil 
                 0.02 
                 23 
                 535 
                 71 
               
               
                 Magnesium 
               
               
                   
               
            
           
         
       
     
     According to ELMOS, TrimethylIndium vapor pressure can be approximately computed as 
     
       
         
           
             
               P 
               v 
             
             = 
             
               10 
               
                 ( 
                 
                   9.735 
                   - 
                   
                     2830 
                     T 
                   
                 
                 ) 
               
             
           
         
       
     
     where T=290.15° K is the mixture condensation temperature in ° K (17° C.). After the substitution it can be determined that Pv=0.96 torr.
         Similarly we can approximately compute the vapor pressure of Cyclopentadienil Magnesium using the formula       

     
       
         
           
             
               P 
               v 
             
             = 
             
               10 
               
                 ( 
                 
                   25.14 
                   - 
                   
                     2.18 
                     × 
                     
                       log 
                        
                       
                         ( 
                         T 
                         ) 
                       
                     
                   
                   - 
                   
                     4198 
                     T 
                   
                 
                 ) 
               
             
           
         
       
     
     Substituting T=290.15° K we can compute that the vapor pressure at condensation temperature is Pv=0.02 torr. 
     The expected molar concentration is calculated as 
     
       
         
           
             MC 
             = 
             
               
                 P 
                 v 
               
               P 
             
           
         
       
     
     and converted to parts per million (ppm) by multiplying by 10 6 . Similar to other measuring devices, the performance of Piezocon® has some limits and this is especially affected at low concentration. In the Piezocon® manual is shown a way of computation of the expected accuracy and repeatability of the concentration measurement. Computed for both above precursors relative to measured concentration accuracy and repeatability with Nitrogen as a carrier gas are shown the table above. These parameters are computed only for the concentration sensor and do not include performance of all other components of the control system, such as proportional valves, pressure sensor, etc. As can be seen in the table above, estimated performance cannot be considered acceptable for the contemporary MOCVD or CVD processes and could be improved. 
     Furthermore, conventional processes require wide dynamic range of the precursor delivery. For example, process 0791 for the Propel HVM reactor requires TrimethylAluminum delivery in the range from 0.711 mg/min to 50.82 g/min, or about 70 times. Conventional tools include up to six independent reactors, therefore the required range for 6 reactors can vary up to 420 times. Existing delivery systems “on-demand” are designed as synchronous systems, meaning they are unable to satisfy required dynamic range because the contemporary controlling components, such as mass flow controllers, proportional valves, and other standard components, have acceptable accuracy and repeatability in the range of about 5-10 times their lowest setting. For the required wide dynamic range of the precursor delivery system, this accuracy range is insufficient. 
     Some systems attempt to overcome this shortcoming using either dilution or double-dilution architectures. Employing these approaches leads to a wasting of expensive precursors by directing substantial amounts of the binary mixture to the scrubber during the run. 
     In addition, typically for the best performance during the deposition it is required to have sharp interfaces for each precursor. However, in the above example for the  791  process the dynamic range is such that at 20° C. and 900 torr it will require the mass flow controller set points from 10.9 sccm to 779 sccm. At a mass flow rate of 10.9 sccm, the flow velocity will be on the order of 15 mm/s and at these flow rates reaching sharp interfaces is difficult. For reasonably sharp interfaces, the flow rate typically must be at least several hundred sccm. 
     Whenever in one of the bubblers/sublimaters the remaining amount of precursor becomes low, the reactor is stopped for the replacement of this bubbler/sublimater. Typically bubbler replacement is a time-consuming process of the reactor&#39;s downtime because it includes the following steps: multiple cycles of vacuum/purge of the bubbler&#39;s legs after closing the bubbler&#39;s manual valves for avoiding chemical reaction between the precursor and the water vapor in the air, removing the old bubbler and replacing it with the new one, repeating multiple cycles of vacuum/purge of the bubbler&#39;s legs, leak testing, stabilizing the bubbler&#39;s temperature at its operating condition and finally carefully opening the bubbler&#39;s manual valves preventing bubbler&#39;s splashing, which sometimes occurs when the bubbler&#39;s headspace pressure is above line pressure. 
     Recovery time after changing a bubbler or sublimator depends on the flow rate through the bubbler/sublimater, bubbler&#39;s or sublimater&#39;s headspace, flow velocity and the length of tubing, or other factors. Changing one precursor gas source or reactor affects not only the newly connected source or reactor but also previously running reactor due to the cross-talk. Conventionally, the only way for implementing a synchronous precursor delivery system for multiple reactors without negatively affecting the process has been to synchronize all the reactors, and purge out the mixture at the required flow rate until it reaches a desired concentration. As a rule, the reactors are not synchronized, however. Therefore, in unsynchronized systems, large quantities of time could be lost during bubbler or sublimator replacement. 
     When a carrier gas is flowing through a small bubbler, typically at the volumetric flow rates of over 5 LPM the carrier gas is picking up not only the vaporized precursor but also small micro droplets of liquid. They also undergo secondary vaporization inside the heated to higher temperature downstream lines, which creates unstable concentration of the precursor negatively affecting the process. If we feed the same source to multiple reactors, this problem will be substantially amplified. 
     SUMMARY 
     Systems and methods are described herein that facilitate use of a CVD system continuously. The systems and methods shown herein include multiple precursor gas sources, and structures for independently connecting or disconnecting those sources for replacement. Use of multiple sources reduces the downtime associated with disconnecting and replacing a precursor gas source, which often requires several hours during which lines are vented, the precursor gas source is disconnected, a new precursor gas source is attached and heated to a desired operating temperature, and then the lines are re-purged before being provided with the output from the new precursor gas source. According to some embodiments, these replacement steps can be accomplished for one precursor gas source while another continues to provide precursor gas to the CVD system, resulting in an elimination of downtime related to changing out the precursor gas source. 
     By providing user inputs for diluting the outputs of these multiple precursor gas sources, mixtures of precursor gas in carrier gas can be generated that have sufficiently low concentrations to be routed to a remote CVD system even at relatively low temperatures. Therefore, in embodiments many precursor gas sources, located remotely from the CVD chamber, can be independently operated and replaced as needed without interrupting a supply of precursor gas to the CVD chamber. This prevents cluttering on the top of the tool, and generally makes use, repair, and maintenance of the tool less cumbersome. 
     According to one embodiment, a system for providing precursor gas includes a user interface comprising a plurality of carrier gas inputs, a primary precursor gas source configured to receive a carrier gas from one of the plurality of carrier gas inputs and produce a primary precursor gas mixture, an auxiliary precursor gas source configured to receive a carrier gas from one of the plurality of carrier gas inputs and produce an auxiliary precursor gas mixture, and an output configured to receive a precursor gas mixture by combining at least a portion of the primary precursor gas mixture, at least a portion of the auxiliary precursor gas mixture, and a carrier gas from at least one of the plurality of carrier gas inputs. 
     According to another embodiment, a method for continuous operation of a chemical vapor deposition system includes providing a carrier gas at a first user input and routing it to the inlet of a primary precursor gas source to generate a precursor gas mixture at an outlet of the primary precursor gas source, providing a carrier gas at a second user input and routing it to the inlet of an auxiliary precursor gas source to generate a precursor gas mixture at an outlet of the auxiliary precursor gas source, combining the precursor gas mixture of the primary precursor gas source and the precursor gas mixture of the auxiliary gas source to form a combined precursor gas mixture, mixing at least a portion of the combined precursor gas mixture with a carrier gas from a third user input to form a diluted precursor gas mixture that has a sufficiently low concentration that the precursor gas is fully soluble in the carrier gas above a temperature, and routing the diluted precursor gas mixture, at or above the temperature, to a remote chemical vapor deposition tool. 
     The above summary of the invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The detailed description and claims that follow more particularly exemplify these embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  is a flow diagram depicting two precursor gas sources configured to provide continuous supply of a precursor gas for chemical vapor deposition, according to an embodiment; 
         FIG. 2  is a diagram depicting a system of valves and lines that permit for selective removal or replacement of a precursor gas source during continuous operation of a reactor, according to an embodiment; 
         FIG. 3  is a diagram of a system for mixing and accumulating the output of the system of valves and lines of the embodiments shown in  FIG. 2 . 
     
    
    
     While embodiments are amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     According to embodiments, systems include at least a primary and an auxiliary precursor gas source. In embodiments, the precursor gas source can be either a bubbler or sublimator, though in the description provided before “bubbler” is used to refer to either of these, or any other precursor gas source, for convenience. One of ordinary skill in the art would understand that these precursor gas sources depend on a desired precursor gas, and are often interchangeable. 
     According to embodiments, systems include multiple bubblers that can be operated independently, and tubing or piping systems that can be disconnected from one or more of the bubblers without disrupting supply of the precursor gas. In this way, the need for downtime to change a bubbler is reduced or obviated. The tubing or piping systems can also be connected to additional inputs such that a sufficiently low precursor gas concentration within the carrier gas is created, and precursor gas can be routed from a location remote from the reactor chamber. 
     As described in more detail below, a precursor generation source, precursor gas conditioning, and precursor gas delivery subsystems can be provided to continuously deliver precursor gas mixture to a reactor housing or tool used in CVD systems. Because precursor gas mixture is generated and accumulated that has a relatively low concentration, it is not necessary to position the bubbler or other precursor gas source directly on the reactor chamber or tool itself. The ability to position precursor gas sources further from the reactor housing facilitates a smaller tool foot-print, and therefore the tighter cleanliness requirements associated with some semiconductor applications can be more easily met. Re-layout of the tool for serviceability can also be accomplished much more easily without the precursor gas source arranged on the tool itself. In embodiments, the system can facilitate scaling, or addition of more precursor to reactors. By enabling accumulation of multiple concentrations of precursor gas in carrier gas, reduced venting of precursor gas mixes is accomplished, and an improvement in run-to-run and tool-to-tool matching due to controlled and stable delivery of flux is possible. 
       FIG. 1  depicts an embodiment of a system  110  for providing a precursor gas at a desired concentration for chemical vapor deposition. System  110  includes carrier gas source  112 , which can be any carrier gas used to deliver precursor gas. For example, in embodiments carrier gas source  112  can be a pressurized source of hydrogen, nitrogen, or argon gas, in embodiments. In alternative embodiments various other inert or noble gases can be used as a carrier gas. 
     Carrier gas source  112  provides carrier gas to a user interface  114 . Depending on the process there are different carrier gases and most commonly used in CVD processes are Nitrogen, Hydrogen, Argon, Helium, or others. User interface  114  is an interface that can be used either manually or automatically to adjust the amount of carrier gas that is delivered via each of a series of lines. For example, in the embodiment shown in  FIG. 1 , there are four lines (first input line  116 , second input line  118 , third input line  120 , and fourth input line  122 ). These four input lines can receive more or less of the carrier gas, depending on the settings at user interface  114 . In embodiments, such settings can be modified based upon feedback from sensors within the rest of system  110 , as will be described in more detail below. Furthermore, various alternative embodiments may include a different number of input lines, depending on the desired number of precursor gas concentration(s) and bubblers, as described in more detail below. 
     First, second, third, and fourth input lines ( 116 ,  118 ,  120 , and  122 , respectively) pass through heat exchanger  124 , in the embodiment shown in  FIG. 1 . Heat exchanger  124  can be used to ensure that the carrier gas flowing through the first through fourth input lines ( 116 ,  118 ,  120 , and  122 , respectively) is at a desired input temperature. In alternative embodiments, some or all of the first through fourth input lines ( 116 ,  118 ,  120 , and  122 , respectively) may not be routed through heat exchanger  124 . Additionally or alternatively, one or more of the first through fourth input lines ( 116 ,  118 ,  120 , and  122 , respectively) can be routed through individual heat exchangers (not shown), such that each line can be controlled and set to a desired input temperature. In such embodiments, the temperature associated with each of the first through fourth input lines ( 116 ,  118 ,  120 , and  122 , respectively) can be independently set and monitored. In embodiments, controlled flow rate for each of the first through fourth input lines ( 116 ,  118 ,  120 , and  122 , respectively) can be set, whether using a single heat exchanger  124  or multiple heat exchangers, using user interface  114 . User interface  114  can include regulators, flowmeters, shutoff valves, or other devices that can modify the throughput of the carrier gas at each of the input lines  116 - 122 . 
     Carrier gas in the first through fourth input lines ( 116 ,  118 ,  120 , and  122 , respectively) that has passed through heat exchanger  124  can be used to generate precursor gas mixtures having a desired concentration and a desired temperature. The addition of precursor gas to the carrier gas is accomplished using a system of bubblers and piping. 
     In the embodiment shown in  FIG. 1 , first input line  116  is routed to primary bubbler  126 . Primary bubbler  126  is an apparatus configured to create a mixture of carrier gas and precursor gas. In one embodiment, primary bubbler  126  comprises a quantity of liquid precursor material. The precursor material is a material that can be used in chemical vapor deposition. In embodiments, the precursor material can be pyrolyzable such that, when heated, epitaxial growth of a desired material can occur on a substrate. 
     Primary bubbler  126  can receive a carrier gas supply, which can be applied to a pressure regulator at its inlet in embodiment. Depending on the intended CVD process, there are different carrier gases used. Most commonly used in CVD processes are Nitrogen, Hydrogen, Argon, and Helium. In some MOCVD processes, either Nitrogen or Hydrogen is used at a pressure of about 15-30 psig. A mass flow controller (MFC) (not shown) can also be applied to the carrier gas inlet to primary bubbler  126 . This source MFC ensures the flow with the vaporized/sublimated precursor is at a desired rate. In embodiments, a second MFC, called a dilution MFC, is supplied with the carrier gas only and directed to the outlets without mixing with the precursor material. In order to avoid condensation in the MFCs, they are heated, for example with heat exchangers. In one embodiment, input MFCs can be heated up to the temperature at least 5° C. higher than the temperature of the bubbler. 
     After picking up precursor molecules at the vapor pressure of the precursor material, the high concentration mixture can be directed to a concentration sensor (not shown) at the outlet of the primary bubbler  126 . The concentration sensor can also be heated to prevent condensation. In one embodiment, the sublimator temperature is 55° C., its pressure controlled by a back pressure regulator is 1150 torr, and the temperature of the MFCs and Piezoelectric concentration sensor is 60° C.-65° C., then the performance of the concentration measurement will be higher than conventional systems. The table below shows accuracy and repeatability of the concentration measurement in the TrimethylIndium/Nitrogen and Cyclopentadienil Magnesium/Nitrogen binary mixtures at the sublimater temperature of 55° C. and its pressure of 1150 torr. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                 Vapor 
                 Concen- 
                 Relative 
                 Relative 
               
               
                   
                 Pressure 
                 tration at 
                 Piezocon 
                 Piezocon 
               
               
                   
                 at 55° C., 
                 1150 torr, 
                 Accuracy, 
                 Repeatability, 
               
               
                 Precursor 
                 torr 
                 ppm 
                 % 
                 % 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 TrimethylIndium 
                 12.9 
                 11220 
                 1.36 
                 0.18 
               
               
                 Cyclopentadienil- 
                 0.52 
                 452 
                 27.2 
                 3.6 
               
               
                 Magnesium 
               
               
                   
               
            
           
         
       
     
     Comparing accuracy and repeatability between these results and those of the conventional system described previously, concentration measurement can be improved roughly 10 times for the Trimethyllndium/Nitrogen mixture and about 20 times for the Cyclopentadienil Magnesium/Nitrogen mixture. Overall repeatability of the delivery system after the dilution can be estimated as 
       δ=√{square root over (δ Piezo   2 +δ MFC1   2 +δ MFC2   2 )}
 
     Primary bubbler  126  contains such precursor material in the liquid state and a mechanism for bubbling or sparging the carrier gas through the liquid precursor. Bubbling the carrier gas through the liquid precursor causes the carrier gas to collect some of the precursor material as vapor and/or liquid aerosol. This mixture of carrier gas, vapor, and/or liquid aerosol, referred to hereinafter as the precursor gas mixture, can be used for deposition in a CVD system. In one embodiment, primary bubbler  126  comprises a tank of liquid precursor material and a dip tube through which carrier gas from first input line  116  can be routed. 
     Primary bubbler  126  can be heated to a desired temperature such that the vapor pressure of the liquid precursor is known. Furthermore, primary bubbler  126  is sealed against ingress from ambient air, because primary bubbler  126  often contains pyrophoric materials. As such, when primary bubbler  126  is empty or must be replaced for any other reason, it may take significant time to safely remove and replace it. 
     Likewise, auxiliary bubbler  128  is configured to provide the precursor gas. Auxiliary bubbler  128  is similar to primary bubbler  126 , but auxiliary bubbler  128  receives carrier gas input from fourth input line  122 . 
     Primary bubbler  126  and auxiliary bubbler  128  provide precursor gas outputs via primary bubbler outlet line  130  and auxiliary bubbler outlet line  132 , respectively. Primary bubbler outlet line  130  splits into two lines: low concentration primary bubbler outlet line  130 L and high concentration primary bubbler outlet line  130 H. Likewise, auxiliary bubbler outlet line  132  splits into two lines: low concentration auxiliary bubbler outlet line  132 L and high concentration auxiliary bubbler outlet line  132 H. 
     Low concentration output  134  receives carrier gas from second input line  118 , low concentration primary bubbler outlet  130 L, and low concentration auxiliary bubbler outlet  132 L. Low concentration output  134  can include a mixer, in embodiments, to combine the outputs from these lines. Additionally or alternatively, in some embodiments low concentration output  134  can include an accumulator tank or hose. 
     Low concentration output  134  provides low concentrations of precursor gas in carrier gas for CVD processes. The concentration of precursor gas provided by low concentration output  134  is often significantly lower than the concentration of precursor gas provided at primary bubbler outlet line  130  or auxiliary bubbler outlet line  132 . In order to generate the desired low concentration of precursor gas in carrier gas, second input line  118  can provide relatively large quantities of carrier gas to dilute the mixture provided by low concentration primary bubbler outlet  130 L and low concentration auxiliary bubbler outlet  132 L. 
     In the embodiment shown in  FIG. 1 , low concentration output  134  can be provided even if one of the bubblers ( 126 ,  128 ) is not providing any output. For example, if primary bubbler  126  is not providing any output to low concentration primary bubbler outlet line  130 L, low concentration output  134  can nonetheless create a desired low concentration mixture by combining the gas from second carrier gas input line  118  and low concentration auxiliary bubbler outlet line  132 L. Likewise, if auxiliary bubbler  128  is not providing any output to low concentration auxiliary bubbler outlet line  132 L, low concentration output  134  can nonetheless create a desired low concentration mixture by combining the gas from second carrier gas input line  118  and low concentration primary bubbler outlet line  130 L. Therefore, so long as one of the two bubblers ( 126 ,  128 ) is installed at any given time, low concentration output  134  can generate a desired concentration of precursor gas in carrier gas by adjusting the quantity of carrier gas provided by second carrier gas input line  118  at user interface  114 . 
     High concentration output  136  provides relatively higher concentrations of precursor gas than those provided by low concentration output  134 . The concentration of precursor gas within the carrier gas is still lower than the output of primary bubbler  126  and auxiliary bubbler  128 . To generate the desired concentration of precursor gas in carrier gas, high concentration output  136  receives carrier gas from third input line  120 , high concentration primary bubbler outlet  130 H, and high concentration auxiliary bubbler outlet  132 H. High concentration output  136  can include a mixer, in embodiments, to combine the outputs from these lines. Additionally or alternatively, in some embodiments high concentration output  136  can include an accumulator tank or hose. 
     As previously described with respect to low concentration output  134 , high concentration output  136  can maintain a desired concentration output even when one of the bubblers ( 126 ,  128 ) is not providing any output. This can be accomplished for either output ( 134  or  136 ) by manually or automatically adjusting the quantity of carrier gas provided by second input line  118  or third input line  120 , respectively. 
     System  110  therefore is capable of providing both high concentration and low concentration precursor gas mixtures, even when primary bubbler  126  or auxiliary bubbler  128  is removed from service. For example, if primary bubbler  126  is removed to be refilled or replaced, the desired precursor gas concentrations can still be provided by auxiliary bubbler  128  until such time as primary bubbler  126  is brought back online, and vice versa. This reduces or eliminates downtime associated with replacing bubblers in conventional systems. 
       FIG. 2  is a more detailed view of one embodiment of piping system  210  within cutout  2  of  FIG. 1 . In particular,  FIG. 2  depicts a series of valves V 1 -V 12  that can be used with In alternative embodiments, various other piping systems  210  could be employed that would facilitate interchangeable delivery of precursor gas from primary bubbler  126  and auxiliary bubbler  128  of  FIG. 1 . 
     Piping system  210  of  FIG. 2  includes several components similar to those previously depicted with respect to  FIG. 1 . Parts in  FIG. 2  that are similar to those previously depicted in  FIG. 1  have similar reference numerals, iterated by a factor of 100. For example, piping system  210  includes first input line  216 , second input line  218 , third input line  220 , and fourth input line  222 . First input line  216  is coupled to primary bubbler  226 , and fourth input line  222  is coupled to auxiliary bubbler  228 . Primary bubbler  226  outputs a concentrated mixture of precursor gas in carrier gas at primary bubbler outlet line  230 , and auxiliary bubbler provides a concentrated mixture of precursor gas in carrier gas at auxiliary bubbler outlet line  232 . Primary bubbler outlet line  230  splits into low concentration primary bubbler outlet line  230 L and high concentration primary bubbler outlet line  230 H. Auxiliary bubbler outlet line  232  splits into low concentration auxiliary bubbler outlet line  232 L and high concentration auxiliary bubbler outlet line  232 H. 
     In addition to those components shown in  FIG. 2  that are similar to those previously described with respect to  FIG. 1 ,  FIG. 2  shows several structural features that facilitate the delivery of precursor gas from either or both of the bubblers  226  and  228 . For example, in the embodiment shown in  FIG. 2 , valves V 1 -V 18  are arranged to facilitate removal or replacement of one bubbler while high and low concentration outputs  236  and  234  are still provided. 
     Valves V 1 -V 4  control the input to primary bubbler  226 . Valve V 1  is positioned along first input line  216 . Second valve V 2  is positioned at primary bubbler  226 . A line towards a vacuum is connected to first input line  216  between first valve V 1  and second valve V 2 , controlled by valve V 3 . The combination of valves V 1 -V 3  permit for the line to be used to provide carrier gas to bubbler  226  (with valves V 1  and V 2  open but valve V 3  closed), purged (with valves V 1  and V 2  closed but valve V 3  open). Valve V 4  can be opened or closed to operate a bypass line. By closing valve V 1  or valves V 2  and V 3 , and opening valve V 4 , carrier gas can bypass primary bubbler  226  altogether and be routed directly to be combined with the contents of the output lines  230  and  232 . 
     Similar structures are provided for control of the input to auxiliary bubbler  228 . Fifth valve V 5  is positioned along fourth input line  222 . Sixth valve V 6  is positioned at auxiliary bubbler  228 . A line towards a vacuum is connected to fourth input line  222  between fifth valve V 5  and sixth valve V 6 , controlled by seventh valve V 7 . A bypass line to the output lines  230  and  232  is operated by valve V 8 . Valves V 5 -V 8  can be controlled as previously described with respect to valves V 1 -V 4 , respectively, but to control the lines coupled to the input of auxiliary bubbler  228 , rather than the lines coupled to the input of primary bubbler  226 . 
     The outputs of primary bubbler  226  and auxiliary bubbler  228  are similarly controlled by a series of valves. While the inputs (i.e., the lines coupled to first carrier gas input  216  and fourth carrier gas inlet  222 ) are typically provided with inert gas, the outputs of the bubblers  226  and  228  can contain precursor material, which can be pyrophoric, toxic, or hazardous in some other way, depending upon the precursor used for any particular chemical vapor deposition process. 
     The outputs of primary bubbler  226  are controlled by valves V 9 -V 11 . Ninth valve V 9  is provided at primary bubbler  226 , and can be used to prevent egress of precursor material therefrom. Ninth valve V 9  is similar to second valve V 2 , in that it is a part of primary bubbler  226 . With ninth valve V 9  open, the precursor gas mixture can flow to primary bubbler outlet line  230 . In order to facilitate purging of primary bubbler outlet line  230 , a vacuum line is coupled to primary bubbler outlet line  230  via tenth valve V 10 . As shown in  FIG. 2 , tenth valve V 10  and third valve V 3  connect first input line  216  and primary bubbler outlet line  230 , respectively, to the same vacuum line. In alternative embodiments, separate vacuum lines could be used. Eleventh valve V 11  is positioned along primary bubbler outlet line  230 . Eleventh valve V 11  can be opened when precursor gas mixture is provided by primary bubbler  226 , and closed otherwise. When eleventh valve V 11  is closed, a vacuum can be pulled on primary bubbler outlet line  230  via tenth valve V 10 , but the vacuum is fluidically separated from the low concentration output  234  and the high concentration output  236 . 
     The outputs of auxiliary bubbler  228  are controlled by valves V 12 -V 14 , in a similar fashion to the controls previously described with respect to valves V 9 -V 11 . By selectively opening and closing valves V 12 -V 14 , precursor gas mixture can be provided by auxiliary bubbler  228 , or vacuum can be applied to the auxiliary bubbler output line  232 . Such vacuum can be used to facilitate removal or replacement of primary bubbler  226  and auxiliary bubbler  228  without exposing a pressurized line of hazardous precursor gas mixture to ambient atmosphere. 
     The embodiment shown in  FIG. 2  includes a flowmeter  244 . Flowmeter  244  can be, for example, a piezoelectric flow meter as described in U.S. Pat. No. 6,279,379 (filed Nov. 19, 1999), configured to determine the concentration of precursor gases within a flowing precursor gas mixture using time-of-flight measurements. The signal output by flowmeter  244  can include information relating to the flow rate and/or the concentration of precursor gases within the precursor mixture. Utilizing this information, a user can either manually or automatically control second input line  218  and third input line  220  in order to create high and low concentration outputs  236  and  234 , as shown. 
     Fifteenth valve V 15  and sixteenth valve V 16  route precursor gas mixture towards the low concentration output  236 . As shown in  FIG. 2 , the combined output of primary bubbler output line  230  and auxiliary bubbler output line  232  is present at the input to fifteenth valve V 15 . Fifteenth valve V 15  can be variably adjustable to permit a desired quantity of precursor gas mixture through. Sixteenth valve V 16  is a back pressure regulator valve that does not allow the output to reach a pressure that is above a desired threshold. 
     In embodiments, a concentration measurement device, such a piezoelectric concentration sensor, can be used to determine the mass flow of the dilution carrier at fifteenth valve V 15  gas to insure that the concentration exiting the central source delivery system is accurate. In various embodiments, other temperature or concentration sensors can be positioned throughout the system to ensure that those aspects of the system are well-controlled. 
     The precursor gas mixture that passes through both variable fifteenth valve V 15  and back pressure regulating sixteenth valve V 16  can be augmented with additional carrier gas from third input line  220 . Often, the precursor gas mixture provided by primary bubbler output line  230  and/or auxiliary bubbler output line  232  has a higher concentration of precursor gas than needed for deposition. Furthermore, excessive concentration of precursor gas in the lines can cause settling out or condensation, as described above. By routing in additional carrier gas from third input line  220  to dilute the precursor gas in the low concentration output  236  line, such unwanted phenomena can be avoided. Likewise, as described above with respect to  FIG. 1 , additional components such as a mixer (not shown) can be employed in the low concentration output  236  line in order to prevent settling out, stratification, or condensation, and/or convert liquid aerosol precursor to vapor precursor. In embodiments, the precursor gas mixture at low concentration output  236  can be sufficiently mixed and low-concentration that even at relatively low temperatures, such as 70° C., condensation will not occur. 
     Likewise, seventeenth valve V 17  and eighteenth valve V 18  route precursor gas mixture towards the low concentration output  234 . Seventeenth valve V 17  can be variably adjustable to permit a desired quantity of precursor gas mixture through. Eighteenth valve V 18  is a back pressure regulator valve that does not allow the output to reach a pressure that is above a desired threshold. The precursor gas mixture that passes through both variable seventeenth valve V 17  and back pressure regulating eighteenth valve V 18  can be augmented with additional carrier gas from second input line  218 . 
     In embodiments, more carrier gas is routed from second input line  218  than from third input line  220 . More precursor gas mixture can also be routed through variable fifteenth valve V 15  than variable sixteenth valve V 16 , in embodiments. Accordingly, the ratio of precursor gas to carrier gas can be higher in the low concentration output  236  than in the low concentration output  234 . 
     As shown in  FIG. 2 , even in the absence of any output from primary bubbler  226 , precursor gas mixture can be provided by auxiliary gas mixture  228 . By closing valves V 1 , V 2 , V 9 , and V 11 , and by opening valves V 3  and V 10 , the input and output lines to primary bubbler  226  can be purged even while auxiliary bubbler continues to provide precursor gas mixture at flowmeter  244 . By then closing valves V 3  and V 10 , and venting the input and output lines, primary bubbler  226  can be removed and replaced. The input and output lines to primary bubbler  226  can be vacuum-purged again, and then valves V 1  and V 2  reopened to provide carrier gas to the new or refilled primary bubbler  226 , and valves V 9  and V 11  reopened to allow egress of precursor gas mixture to flowmeter  244 . 
       FIG. 3  depicts a system  300  for delivery of precursor gas material from a low concentration output  334  and a high concentration output  336  to a CVD chamber  342 . In order to prevent stratification and increase homogeneity, a static mixer  346 L is provided to mix gas from the low concentration output  334  and likewise a static mixer  346 H is provided to mix gas from the high concentration output  336 . The mixed gas is then provided to an accumulator tank ( 348 L and  348 H, respectively). 
     Gas mixture held in each of the accumulator tanks  348 L,  348 H can be vented via nineteenth valve V 19  and twentieth valve V 20 , respectively. Such venting can be used when the pressure within the accumulator tanks  348 L and  348 H becomes too high, or when the CVD process is complete, for example. Alternatively, gas within accumulator tanks  348 L,  348 H can be provided to CVD chamber  342  via twenty-first valve V 21  or twenty-second valve V 22 , respectively. 
     In embodiments, each of the valves V 19 -V 22  can be variable valves, such that a desired flowrate can be established to each corresponding outlet. Furthermore, flowmeters  350 L and  350 H can be used to determine the flow rate and/or the precursor gas concentration from each accumulator tank  348 L and  348 H, respectively. Information sensed by flowmeters  350 L and  350 H can therefore be used to modify the setting of each of the valves V 19 -V 21 . 
     Combining the elements of  FIGS. 1-3 , a system is provided in which a precursor generation source, precursor gas conditioning, and precursor gas delivery can be accomplished continuously. Downtime associated with removal or replacement of a bubbler or other precursor gas source can be obviated. Furthermore, because precursor gas mixture is generated that has a relatively low concentration, it is not necessary to position the bubbler or other precursor gas source directly on the reactor chamber or tool itself because the precursor gas mixture is not so prone to condensation or stratification. The ability to position precursor gas sources further from the reactor housing itself facilitates a smaller tool foot-print, and therefore tighter cleanliness requirements for semiconductor applications can be more easily met, as well as facilitating re-layout of the tool for serviceability. In embodiments, the system can facilitate scaling, or addition of more precursor to reactors. By enabling accumulation of multiple concentrations of precursor gas in carrier gas, reduced venting of precursor gas mixes is accomplished, and an improvement in run-to-run and tool-to-tool matching due to controlled and stable delivery of flux is possible. 
     In embodiments, relatively more or fewer precursor gas sources and concentration outputs can be provided. In one alternative embodiment, a third bubbler can be provided, which is also coupled to a carrier gas inlet. The outlet of the third bubbler can be comingled with the outputs of the other two precursor gas sources prior to splitting into low and high concentration output lines. Alternatively or additionally, a third concentration output line can be generated, with a separate carrier gas input line to facilitate mixing to a desired concentration, and an associated mixer, accumulator, and flowmeter can be provided for the third output line. Those of skill in the art will recognize that, with any number of precursor gas sources greater than 1, and with any number of output lines equal to or greater than 1, the systems described explicitly herein can be modified to accomplish the benefits described above. 
     Various embodiments of systems, devices and methods have been described herein. These embodiments are given only by way of example and are not intended to limit the scope of the invention. It should be appreciated, moreover, that the various features of the embodiments that have been described may be combined in various ways to produce numerous additional embodiments. Moreover, while various materials, dimensions, shapes, configurations and locations, etc. have been described for use with disclosed embodiments, others besides those disclosed may be utilized without exceeding the scope of the invention. 
     Persons of ordinary skill in the relevant arts will recognize that the invention may comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features of the invention may be combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, the invention can comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art. Moreover, elements described with respect to one embodiment can be implemented in other embodiments even when not described in such embodiments unless otherwise noted. Although a dependent claim may refer in the claims to a specific combination with one or more other claims, other embodiments can also include a combination of the dependent claim with the subject matter of each other dependent claim or a combination of one or more features with other dependent or independent claims. Such combinations are proposed herein unless it is stated that a specific combination is not intended. Furthermore, it is intended also to include features of a claim in any other independent claim even if this claim is not directly made dependent to the independent claim. 
     Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims included in the documents are incorporated by reference herein. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein. 
     For purposes of interpreting the claims for the present invention, it is expressly intended that the provisions of Section 112(f) of 35 U.S.C. are not to be invoked unless the specific terms “means for” or “step for” are recited in a claim.