Abstract:
A two-stage vaporizer includes two vaporizing stages joined by a vaporization chamber located gravitationally below the first vaporizing stage and gravitationally above the second vaporizing stage. A separator covering an outlet within the vaporization chamber allows vaporized precursor from both vaporizing stages to pass through the outlet to chemical vapor deposition system and prevents any remaining liquid precursor from passing through the outlet. The liquid precursor is premixed with carrier gas just prior to entry into the vaporizer. Additional flows of carrier gas pass through the two vaporizing stages in opposite directions to carry the vaporized precursor to the outlet.

Description:
TECHNICAL FIELD  
         [0001]    Chemical vapor deposition (CVD) systems can be used to deposit thin films on substrates by decomposing vapor precursors within low-pressure reactors. The vaporization of the precursors takes place prior to entry of the precursors into the reactors.  
         BACKGROUND  
         [0002]    Purified forms of metals, metal compounds, and other materials can be deposited in uniformly thin layers onto substrates by decomposing vaporized precursors of the materials. The depositions take place inside reactors with evacuatable environments and temperature controls. Many of the precursors take liquid form at ambient temperatures and are vaporized at higher temperatures just prior to entry into the reactors.  
           [0003]    High deposition rates for such chemical vapor deposition (CVD) processes require correspondingly high delivery rates of vaporized precursors into the reactors. Vaporization of liquid precursors can be carried out by mixing the liquid precursor with a carrier gas or by atomizing the liquid precursor in a suspended gas. Liquid flow rates into vaporizers are limited by conversion capabilities of the vaporizers to vaporize the liquid precursors. Incomplete vaporization can result in the passage of large droplets of the liquid precursor into the reactors. The entry of liquid precursor into reactors, which is referred to as “flooding”, contaminates the reactors and diminishes pumping performance. Flooding increases deposition processing time by requiring more time to evacuate the reactors.  
         SUMMARY OF INVENTION  
         [0004]    Our invention provides opportunities for vaporizing liquid precursors at high rates and for delivering the vaporized precursors to low-pressure reactors for processing, while preventing the delivery of any remaining liquid precursor to the reactors. Droplets of liquid precursor remaining after a first stage of vaporization are trapped and subject to a second stage of vaporization. More efficient vaporization enables the higher rates of vaporization to be achieved. Throughput processing rates can also be improved by avoiding passage of liquid precursor droplets into the reactors.  
           [0005]    One example of a precursor vaporizer for a chemical vapor deposition system has an inlet arrangement for admitting a liquid precursor and a carrier gas into the vaporizer. A first vaporizing stage vaporizes a portion of the liquid precursor into the carrier gas. A second vaporizing stage located gravitationally below the first vaporizing stage vaporizes another portion of the liquid precursor into the carrier gas. A vaporization chamber interconnects the first and second vaporizing stages. An outlet conveys the vaporized precursor from both vaporizing stages to a reactor of the chemical vapor deposition system. The outlet is connected to the vaporizing chamber out of liquid communication with the first vaporizing stage and extending gravitationally above the second vaporizing stage to prevent the remaining liquid precursor from reaching the reactor.  
           [0006]    The inlet arrangement preferably includes separate conduits that support flows of carrier gas through both vaporizing stages towards the vaporizing chamber. The flows of carrier gas supported by the inlet arrangement can include (a) a first flow of the carrier gas through the first vaporizing stage in a direction corresponding to a gravitationally directed seepage of the liquid precursor through the first vaporizing stage and (b) a second flow of the carrier gas through the second vaporizing stage in a direction opposed to a gravitationally directed seepage of the liquid precursor through the second vaporizing stage.  
           [0007]    A separator within the vaporization chamber can be used to allow the liquid precursor to reach the second vaporizing stage and to allow the vaporized precursor to pass through the outlet. In addition, the separator can prevent the liquid precursor from passing through the outlet, preferably by diverting the liquid precursor from the outlet. For example, the separator can be formed as a roof over the outlet with pervious under-eaves structure for admitting the vaporized precursor under the roof.  
           [0008]    The inlet arrangement also preferably includes a mixing valve that mixes the liquid precursor with the carrier gas in advance of the first vaporizing stage. The mixing valve regulates flow rates of the liquid precursor into the vaporizer. A signal from a flow meter to the mixing valve can be used to adjust the flow rates of the liquid precursor into the vaporizer.  
           [0009]    The two vaporizing stages and the intermediate vaporizing chamber are preferably supported within a thermally conductive body that supports transfers of heat to the vaporization process. However, the mixing valve is preferably supported by a thermal isolator for insulating the mixing valve from the thermally conductive body. One or more heating elements positioned within the thermally conductive body heat the first and second vaporizing stages without substantially heating the mixing valve.  
           [0010]    The second vaporizing stage preferably includes a trap for capturing the liquid precursor below a level of the outlet and a porous medium within the trap to increase surface area. A carrier gas passageway provides for conducting carrier gas through the porous medium to vaporize the liquid precursor captured in the trap. Preferably, the carrier gas passageway is arranged to convey the precursor vaporized by the second vaporizing stage in a direction opposed to gravity en route to the outlet in the vaporizing chamber.  
           [0011]    During operation, the mixer preferably combines a liquid precursor with a carrier gas at a first temperature low enough to avoid significant decomposition of the liquid precursor. The first and second vaporizing stages promote vaporization of the liquid release agent at a second temperature high enough to avoid significant condensation of the vaporized precursor. The mixing is preferably carried out at ambient temperatures to prevent the mixing valve from becoming clogged with prematurely decomposed solids. The vaporizing stages, however, are preferably heated well above ambient temperatures to prevent condensation of the vaporized precursor.  
           [0012]    A precursor for a low-pressure processing system can be vaporized in accordance with our invention by a series of steps for increasing vaporization efficiency and overall processing rates. A liquid precursor and a carrier gas are admitted into a vaporizer. A portion of the liquid precursor is vaporized into the carrier gas at a first vaporizing stage. A remaining liquid portion of the precursor from the first vaporizing stage is separated from the vaporized portion of the precursor. The remaining liquid portion of the precursor is passed to a second vaporizing stage. At least a portion and preferably all of the remaining liquid portion of the precursor are vaporized at the second vaporizing stage. The vaporized precursor from both vaporizing stages is passed through an outlet located gravitationally below the first vaporizing stage and gravitationally above the second vaporizing stage.  
           [0013]    Preferably, the admission of the liquid precursor and the carrier gas includes mixing the liquid precursor with the carrier gas at a temperature low enough to avoid significant decomposition of the liquid precursor. Flow rates of the liquid precursor into the vaporizer can be regulated by a mixing valve that accepts a feedback signal from a flow meter. The mixing valve is preferably thermally isolated from the first and second vaporizing stages to conduct the mixing operation at ambient temperature.  
           [0014]    Both vaporizing stages conduct flows of the carrier gas in opposite directions. The carrier gas is conducted through the first vaporizing stage in a direction corresponding to a gravitationally directed seepage of the liquid precursor through the first vaporizing stage and the carrier gas is conducted through the second vaporizing stage in a direction opposed to a gravitationally directed seepage of the liquid precursor through the second vaporizing stage.  
           [0015]    The separation of the two states of the precursor between vaporizing stages preferably includes allowing the vaporized precursor from the first vaporizing stage to pass through the outlet and preventing the remaining liquid precursor from the first vaporizing stage from passing through the outlet. The liquid precursor remaining from the first vaporizing stage is preferably diverted from the outlet to the second vaporizing stage. The separation preferably takes place within a vaporization chamber interconnecting the two vaporizing stages. The vaporized precursor from both vaporizing stages preferably passes through the same outlet within the vaporizing chamber.  
           [0016]    The remaining liquid portion of the precursor from the first vaporizing stage is captured in a trap at the second vaporizing stage below a level of the outlet. The carrier gas is preferably conducted through a porous medium within the trap to vaporize the liquid precursor captured in the trap. The carrier gas is flowed through the trap in a direction opposed to gravity en route to the outlet. 
       
    
    
     DRAWINGS  
       [0017]    [0017]FIG. 1 is a diagram of an exemplary chemical vaporization system incorporating a vaporizer system arranged in accordance with our invention.  
         [0018]    [0018]FIG. 2 is a perspective view of the vaporizer system showing more specific components of the system including a vaporizer, a mixing valve, a shut-off valve, and a flow meter.  
         [0019]    [0019]FIG. 3 is an exploded view of the vaporizer showing the various components of its assembly.  
         [0020]    [0020]FIG. 4 is a side cross-sectional view of the vaporizer oriented as intended with respect to a vertical axis of gravity. 
     
    
     DETAILED DESCRIPTION  
       [0021]    An exemplary chemical vapor deposition (CVD) system  10  useful for depositing thin layers of metal or other materials on substrates such as single-crystal substrates is depicted in FIG. 1. Thermochemical vapor-phase reactions necessary for forming the thin layers take place with a reactor  12  in the form of an evacuatable processing chamber. A handling system  14  moves substrates  16  (e.g., wafers) into and out of the reactor  12 . An exhaust pump  18  evacuates gas from the reactor  12  for supporting low-pressure processing within the reactor  12 . A waste treatment system  20  (e.g., an abatement module) safely manages the -exhaust including byproducts of the reactions. A power supply  22  is regulated for temperature control and other powered functions of the reactor  12 .  
         [0022]    A supply  24  of liquid precursor containing constituents of the intended film and a supply  26  of a carrier gas are mixed together by a mixing valve  28  and delivered into a vaporizer  30 . The carrier gas supply  26  is also connected directly to the vaporizer  30 . Within the vaporizer, the liquid-phase precursor is converted into a vapor-phase precursor at an elevated temperature. The vaporized precursor is dispersed into the reactor  12  through a delivery manifold (e.g., an injector plate)  32  that functions as a diffuser. At a further elevated temperature within the reactor  12 , the film constituents of the vaporized precursor deposit onto the substrate  16  according to a process of disproportionation.  
         [0023]    A variety of liquid precursors can be used containing constituents including metal agents incorporated into metallorganic complexes for transportation in the vapor phase. Examples include PEMAT: Pentakis(ethylmethylamino)Tantalum, CUPRA SELECT: Hfac(Cu)TMVS, and Cobalt Tricarbonyl nitroso. An inert gas such as helium is preferably used as the carrier gas; but a variety of gases including argon, nitrogen, hydrogen, and oxygen can also be used.  
         [0024]    A perspective exterior view of the mixing valve  28  and vaporizer  30  in FIG. 2 shows more specific components involved with vaporization. Just in advance of the mixing valve  28 , the liquid precursor passes through both a flow meter  36  and a shut-off valve  38  along a liquid supply line  40 . The mixing valve  28 , which can be piezoelectrically actuated, receives a feedback signal from the flow meter  36  to control flow rates through the mixing valve  28 . A separate gas supply line  42  conducts the carrier gas to the mixing valve  28 . All of the liquid and gas regulating components including the mixing valve  28 , the flow meter  36 , and the shut-off valve  38  can be of conventional design for managing liquid flow rates of 0 through 5 cubic centimeters/min (ccm) and gas flow rates of 0 through 700 standard cubic centimeters per minute (sccm). Suitable components are available from Porter Instrument Company, Inc. of Hatfield, Pa.  
         [0025]    A delivery tube  44  conducts the pre-mixed liquid precursor and carrier gas into the vaporizer  30 . Additional gas lines  46  and  48  conduct preheated carrier gas directly to two different locations within the vaporizer  30 . Four sets of electrical lines  52  supply power to heating elements within the vaporizer  30 . The heating elements, though not shown, are preferably  50  watt cartridge heaters; but a variety of other heating elements could also be used. Other interior structures of the vaporizer  30  can be seen in the exploded view of FIG. 3 and the cross-sectional view of FIG. 4.  
         [0026]    The delivery tube  44  passes with wide clearance through a top flange  54  but is engaged with or itself terminates with a thermal isolator that limits transfers of heat from the vaporizer  30  to the delivery tube  44  and mixing valve  28 . Thin walls  56  (see FIG. 4) of the flange  54  inhibit the conduction of heat to the delivery tube  44  from a vaporizer body  60  and a two-part surrounding block  62 , which are both made of heat-conducting materials. The top flange  54 , the vaporizer body  60 , and other structural fittings that come into contact with the precursor are preferably made of stainless steel or other materials that are chemically inert to the precursor and support the use of metal seals. The two-part surrounding block  62 , which is intended for supporting the heating elements, can be made of aluminum or other less expensive thermal conductors.  
         [0027]    A copper gasket  64  seals the top flange  54  to a top of the vaporizer body  60 , which itself has the form of a flanged-end pipe. However, a central hole in the gasket  64  permits (a) the premixed liquid precursor and carrier gas, which enter the top flange  54  through the delivery tube  44 , and (b) additional preheated carrier gas, which enters the top flange  54  through the gas line  46 , to both enter the vaporizer body  60 . The heating elements (not shown) within the two-part surrounding block  62  provide for elevating the temperature of the vaporizer body  60  to support vaporization of the liquid precursor. A resistance temperature detector  58  monitors the temperature of the vaporizer body  60  to provide feedback control to the heating elements. Side insulating panels  66 , together with top insulating panels  68  and bottom insulating panels  70 , trap heat within the two-part surrounding block  62  to support more even heating of the vaporizer body  60 .  
         [0028]    Typically, the temperature of the vaporizer body  60  is raised with respect to an ambient temperature (approximately  24  degrees centigrade, ° C.) of the delivery tube  44  to between 55° C. and 65° C. for supporting and maintaining vaporization of the precursor. However, the preferred vaporization temperature can be varied in accordance with the vaporization characteristics of particular precursors.  
         [0029]    A first vaporizing stage  72  within the vaporizer body  60  contains a porous frit  74  having a large surface area formed by voids and passages to facilitate vaporization of the liquid precursor. The frit  74  is made of an inert material, such as sintered nickel, to avoid chemically reacting with the precursor. Suitable frits are available from Mott Industrial. Passages through the frit  74  are sized large enough to allow the liquid precursor to seep through the frit  74  without pooling. The number and size of the passages and the overall dimensions (e.g., diameter and thickness) of the frit  74  are set to maximize surface area for vaporization while limiting pressure drops accompanying passage of vaporized precursor through the frit  74 . The total pressure drop through the vaporizer is preferably less than 20 Torr.  
         [0030]    The vaporized precursor transported by the flow of the preheated carrier gas through the frit  74  and any remaining liquid precursor moved in the same direction through the frit  74  by the force of gravity enter a vaporization chamber  76  within the vaporizer body  60 . Within the vaporization chamber  76  is an opening  78  of an outlet tube  80  that extends through a bottom of the vaporizer body  60 . The outlet tube  80  passes through a bottom flange  82  that is sealed to the bottom of the vaporizer body  60  through a copper gasket  84 . An extension of the outlet tube  80  connects the vaporizer  30  to the delivery manifold  32  of the reactor  12 .  
         [0031]    A splash cone  86  forms a roof over the outlet opening  78  to prevent any of the remaining liquid precursor seeping through the frit  74  of the first vaporizing stage  72  from passing through the opening  78  of the outlet tube  80 . However, the splash cone  86  is elevated on posts  88  above the outlet opening  78  to provide gaps  90  under eaves of the roof structure of the splash cone  86  for admitting the vaporized portion of the liquid precursor through the outlet opening  78 .  
         [0032]    The remaining liquid precursor that is diverted from the outlet opening  78  by the splash cone  86  descends through the vaporization chamber  76  to a second vaporizing stage  92  within the vaporizer body  60 . Another frit  94 , which is preferably more porous that the frit  74  but occupies more volume, exposes the remaining liquid precursor to a substantially increased surface area. The gas line  48  directs a flow of the preheated carrier gas opposed to the seepage direction of the remaining liquid precursor through the frit  94  for returning the remaining precursor in a vaporized form to the vaporization chamber  76 . The flow of vaporized precursor from the second vaporizing stage  92  is combined with the flow of vaporized precursor from the first vaporizing stage  72  through the outlet opening  78  for delivery to the reactor  12 .  
         [0033]    The more porous frit  94  can be made of a less dense material such as aluminum foam and has an annular shape surrounding the outlet tube  80 . A suitable media for the frit  94  is available from Energy Research and Generation, Inc. under the trade name DUOCELL.  
         [0034]    The second vaporizing stage  92  occupies a trap  96  within the vaporizer body  60  for capturing the -remaining liquid precursor below a level of the outlet opening  78 . The trap  96  within the vaporizer body  60  is enclosed by the bottom flange  82  surrounding the outlet tube  80 , which passes without interruption through the trap  96 . The frit  94  fills the trap  96  to support vaporization of the liquid precursor captured within the trap  96 . Any of the liquid precursor reaching the second vaporizing stage  92  remains captured within the trap  96  until transformed into a vapor state and transported by the preheated carrier gas into the vaporization chamber  76 .  
         [0035]    The vaporization process can be initiated by preheating the vaporizer body  30  and initiating flows from the precursor and carrier gas supplies  24  and  26 . The mixing valve  28  combines the liquid precursor with the carrier gas at a first temperature low enough to avoid significant decomposition of the liquid precursor. Ambient temperature is usually adequate for this purpose. The premixed liquid precursor and carrier gas are kept at ambient temperature until the mixture is admitted into the vaporizer  30 , which is preferably heated well above ambient temperatures (e.g., 55° C. to 65° C.) to promote vaporization and to avoid any subsequent condensation of the vaporized precursor. The temperature of the vaporizer  30 , however, is kept well below the temperature required for decomposition of the precursor in the reactor  12 .  
         [0036]    The admission of the premixed liquid precursor and carrier gas into the vaporizer  30  is accompanied by the admission of additional carrier gas, which is preheated to promote immediate vaporization of the liquid precursor. Some of the liquid precursor may actually be vaporized even prior to reaching the porous frit  74  associated with the first vaporizing stage  72 . However, the increased surface area provided by the frit  74  combined with the flow of preheated carrier gas through the frit  74  vaporizes a more significant portion of the liquid precursor.  
         [0037]    The vaporized portion of the liquid precursor is transported by the carrier gas through the frit  74  in the same direction as the gravitationally directed seepage of the remaining portion of the liquid precursor through the frit  74 . Both portions exit the frit  74  into the vaporization chamber  76  connecting the first and second vaporizing stages  72  and  92 .  
         [0038]    Within the vaporization chamber  76 , the liquid portion of the precursor is separated from the vaporized portion of the precursor. The liquid portion descends into the second vaporizing stage  92 , and the vaporized portion escapes through an outlet opening  78  for delivery to the reactor  12 . The splash cone  86  positioned over the outlet opening  78  prevents the liquid portion of the precursor from entering the outlet opening  78 . Any liquid precursor that would otherwise drip into the outlet opening  78  is diverted from the opening by the roof-like structure of the splash cone  86 . However, gaps  90  formed by posts  88  that support the splash cone  86  above the outlet opening  78  admit the vaporized portion of the liquid precursor into the outlet pipe  80  through passages under the eaves of the roof-like splash cone  86 .  
         [0039]    The remaining liquid portion reaching the second vaporizing stage  92  continues to descend by gravitationally directed seepage through the frit  94 . Although more porous than the frit  74 , the frit  94  occupies substantially more volume to avoid becoming saturated by any temporary accumulations of the liquid precursor within the trap-like structure of the second vaporizing stage  92 . The preheated carrier gas from the gas line  48  enters the trap  96  of the second vaporizing stage  94  near the bottom of the frit  94  and flows towards the vaporization chamber  76  in a direction opposed to the gravitationally directed seepage of the liquid precursor. The remaining precursor vaporized by the conditions of the second vaporizing stage  92  is transported by the oppositely directed carrier gas into the vaporization chamber  76  and combined with the precursor vaporized by the first vaporizing stage for escape through the common outlet opening  78  en route to the reactor  12 .  
         [0040]    The two vaporizing stages together with the premixing of the liquid precursor and carrier gas can increase vaporization efficiency and deposition rates. Overall processing time can be reduced by avoiding the passage of liquid precursor into the reactor  12 .  
         [0041]    Vaporization processing includes three main controls for regulating the concentration of precursor delivered to the reactor  12 . These include the flow rate of the precursor, the flow rate of the carrier gas, and the temperature of the vaporizer body  30 . Increased flow rates of the precursor can support higher deposition rates of the film constituents of the precursor within the reactor  12 . For example, precursor flow rates of 1.5 ccm of a metallorganic compound of copper (CUPRA SELECT) together with carrier gas (helium) flow rates of 120 sccm can support copper deposition rates of around 1700 Angstroms per minute (A/min). The vaporizer  30  is expected to support precursor flow rates of 2.5 ccm or more without clogging.  
         [0042]    Between deposition operations, the carrier gas can be left flowing through the vaporizer  30  to purge any fluids left within the vaporizer  30 . The flow of liquid precursor is stopped by the shut-off valve  38 . However, the flow of carrier gas can be maintained to purge the mixing valve  28  and delivery line  44 . Both porous frits  74  and  94  can be replaced or cleaned on regular intervals.  
         [0043]    The invention is expected to be especially useful for metallorganic chemical vapor deposition (MOCVD) operations used for such purposes as flat panel display manufacturing or thin film head production.  
         [0044]    Although the invention has been illustrated with respect to a single embodiment, the invention can be practiced with a variety of other components and component configurations to achieve similar benefits. More than one of our new vaporizers can be used for supplying the same reactor with either multiple precursors or an increased amount of a single precursor along parallel delivery paths.