Abstract:
A vaporization system for thin film formation and for introducing vapors into a deposition chamber for depositing films onto a semi-conductor surface has a vaporization chamber that is selectively provided with at least two different, separate, precursor liquids carried in a gas stream that may be a single carrier gas, or a selected one of a plurality of carrier gasses. When the liquids being introduced are likely to be subject to thermal decomposition from contact with high temperature surfaces, an atomizer is used at the inlet of the vaporization chamber to provide an aerosol to the vaporization chamber from one or more individual sources of liquid combined with one or more individual carrier gasses for simultaneous or sequential introduction into the vaporization chamber. The vaporization chamber may be designed to insure complete vaporization by incorporating a recirculating gas flow through heated passageways before the vaporized gas/vapor mixture exits the vaporization chamber.

Description:
[0001]     The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 60/534,286, filed Jan. 5, 2004, the content of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention provides a liquid precursor chemical vaporization system in which more than one liquid precursor chemical can be vaporized in one vaporizer, simultaneously or in sequence, and where one or more carrier gasses can be used with one or more cursor chemicals, again, simultaneously or in sequence. The capability for providing a plurality of precursor materials is incorporated into a single vaporizer, resulting in savings in costs and materials, as well as improvement in film quality and wafer throughput in a chemical vapor deposition (CVD) chamber.  
         [0003]     When liquid precursor chemicals are used for semiconductor device fabrication, it is generally necessary to vaporize the liquid, which vapor is then introduced into a process chamber containing a semiconductor wafer to form a thin film on the wafer surface. The most commonly used method for thin film formation on a wafer is by chemical vapor deposition (CVD). The CVD process is often used in combination with plasma, which is then referred to as a plasma-enhanced CVD, or a PECVD process. The chamber in which the deposition takes place can be near atmospheric in pressure, i.e. around 760 Torr, or at a reduced pressure, i.e. in a vacuum. The chamber pressure can be in the range of 1 Torr to 760 Torr, in the range above 760 Torr, in the millitorr range, or even in a high vacuum below 1 milliTorr in pressure.  
         [0004]     Since many leading-edge liquid precursor chemicals used for semiconductor device fabrication are formulated specially to provide certain specific desirable film properties, they are often quite fragile, and prone to thermal decomposition when heated. A method to avoid thermal decomposition is to atomize the liquid precursor chemical by using a compressed gas to form an aerosol containing precursor droplets, which aerosol is then passed over a heated metal surface to transfer heat to the gas and vaporize the droplets. This avoids, or greatly reduces, direct metal-to-liquid contact, and the consequent thermal decomposition that may take place due to liquid contacting the high-temperature, hot metal surface.  
         [0005]     The term aerosol is used herein to refer to atomized liquid droplets or solid particles suspended in a gas. When the desired precursor chemical is a solid, it can be dissolved in a liquid solvent and atomized to form solution droplets. Upon heating of the gas, and the evaporation of the solvent from the solution droplets, the remaining residue of solid particles is suspended in the gas before the solid itself is vaporized, and the gas with the suspended solid, essentially dry particles is also referred to as an aerosol. Thus an aerosol is a gas having either suspended liquid droplets or solid particles. The size of the suspended droplets or solid particles is usually between 1 to 10 μm in diameter, but a gas containing suspended droplets or solid particles as large as 100 μm, or as small as 0.002 μm can still be referred to as an aerosol. Droplets or particles below 0.002 μm are usually referred to as molecular clusters, but for the purpose of this invention a gas carrying any particulate matter below 100 μm in suspension referred to as an aerosol.  
         [0006]     A previous invention disclosed in U.S. Pat. No. 6,409,839, which is incorporated by reference, included a system shown in  FIG. 1  and a vaporization chamber shown in  FIG. 2 . Briefly, the system of  FIG. 1  provides a carrier gas from a source  12  introduced with a reagent liquid,  14 , into an aerosol generator,  16 , to form an aerosol, which is then introduced along a line or passageway  18  into a heated vaporization chamber  24  to form a gas/vapor mixture. This mixture of gas and vapor is then filtered through a heated filter  25 , to remove particulate contaminants that may remain in the gas phase and thus mixed in with the gas/vapor mixture. The gas/vapor mixture is then introduced into a CVD chamber  26  for film deposition. A controller  28  is used for controlling the flow of heated gas/vapor mixture into the CVD chamber  26  from the heated gas filter  25  through a heated flow restriction  29  and a heated control valve  30 . Sensors and flow controllers are used in the system, as detailed in U.S. Pat. No. 6,409,839.  
         [0007]      FIG. 2  shows a detailed design of the vaporization chamber  24 . The aerosol from generator  16  is introduced into the vaporization chamber through line or passageway  18 , and through a multitude of heated passage ways  34  in a block  36  provided with a heater  38  to heat the carrier gas and vaporize the droplets in the aerosol to form the desired heated gas/vapor mixture in an outlet chamber  40 .  
         [0008]     The heated gas/vapor mixture is then filtered through the heated porous metal or ceramic filter,  25 , and then through the heated flow restriction  29  in the form of a coiled capillary tube before it is introduced into the CVD chamber  26  ( FIG. 1 ) through the heated control valve  30 . Again, details of construction are included in U.S. Pat. No. 6,409,839. The prior art device described above and similar devices are designed to vaporize a single liquid precursor chemical with the help of a single carrier gas. In applications where more than one liquid precursor chemical is needed, it is necessary to use a plurality of such vaporizers, one for each precursor chemical application. This leads to unnecessary duplication of physical components, cumbersome installation, difficulty in operation, and high cost.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides a liquid precursor chemical vaporization system having a vaporization chamber in which more than one liquid precursor chemicals can be vaporized simultaneously or in sequence. One or more carrier gases can be used with one or more of the precursor chemicals, again simultaneously or in sequence. The capability is incorporated into a single vaporizer with the selection and sequencing of the precursor and carrier gases can be controlled as desired, this leads to simplification of semiconductor wafer fabrication equipment design, installation, and operation, as well as cost reduction. The invention thus gives rise to savings in materials, cost of construction, installation, maintenance, and more importantly, improvement in film quality, and wafer throughput. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a block diagram of a prior art arrangement for deposition of a thin film on a semiconductor;  
         [0011]      FIG. 2  is a schematic cross sectional view of an aerosol vaporization chamber used in the system of  FIG. 1 ;  
         [0012]      FIG. 3  is a block diagram of a vaporization system made according to a first form of the present invention;  
         [0013]      FIG. 3A  is a schematic representation of the system of  FIG. 3  showing the atomizer in greater detail;  
         [0014]      FIG. 4  is a block diagram of a vaporization system made according to a second form of the present invention;  
         [0015]      FIG. 5  is a schematic block diagram of a further modified form of the present invention;  
         [0016]      FIG. 6  is a schematic block diagram of a system similar to  FIG. 5  with a modified atomizer;  
         [0017]      FIG. 7  is a sectional view of a two stage vaporization chamber useful with the present invention; and  
         [0018]      FIG. 8  is a schematic top view of the chamber of  FIG. 7  with parts broken away. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]      FIG. 3  is a schematic diagram of a multi-liquid precursor chemical vaporization system  50  in one its simplest forms. It includes a single heated vaporization chamber  52  connected to a liquid input source comprising two or more supply sources of different precursors of liquids  54 A,  54 B, and  54 C, each comprising a liquid precursor chemical of the desired chemical nature. Typical liquid precursor chemicals are tetraethyoxisilane (TEOS), and tetraeylborate (TEB), which can be used to make borosilicate glass (BSG) thin films of a low dielectric constant. These chemicals are relatively stable and can be vaporized by direct liquid to metal contact in some applications. The liquid precursor chemicals are usually supplied through individual and separate liquid-flow controllers  56 A,  56 B, and  56 C each being conventionally equipped with a flow sensor to sense the rate of liquid flow, and a flow control valve that can be adjusted or varied to provide the desired liquid flow rate to the vaporizer  52 . Also included in the lines from the flow controllers to the vaporizer  52  are positive shut-off valve  58 A,  58 B, and  58 C that can be turned off to stop the liquid flow from the liquid source to the vaporizer  52  when the system is idle or shut down for maintenance or repair, or when only a selected one or two liquid precursors are needed. The output connection of the shut-off valves  58 A- 58 C are connected to one or more vaporizer inlet tubes  60  through nozzles  61 .  
         [0020]     Also connected to the vaporizer  52  through an inlet tube  60  is a gas source comprising one or more carrier gas supply sources  62 A,  62 B, and  62 C each providing gas to an input tube or tubes  60  through a gas flow controller  64 A,  64 B, or  64 C. The gas flow controllers are conventional units that control the rate of gas flow, from the source in relation to a desired set-point value. Each gas flow line also is equipped with a shut-off valve  66 A,  66 B, or  66 C for positively shutting off the respective gas supply the unit. The inlet tubes introduce the liquid under pressure provided at the supply source for example, through nozzles  61  into a gas from one of the gas supply sources to carry liquid drops into the chamber  52 . The nozzles are designed to break the liquid into drops that can be carried along with the gas flow.  
         [0021]     The vaporization chamber  52  is also equipped with a temperature sensor  68 , and a heater  70  to maintain the chamber interior space at a desired temperature.  
         [0022]     The controller  72  can be an analog controller or a micro-processor based digital controller. The controller is connected to receive signals from the temperature sensor  68  along line  74 , and will control the heater  70  along line  76  to heat the vaporization chamber to the desired temperature as needed for vaporization. The temperature is adjustable according to the need of the specific liquid precursor that is to be vaporized, and this can be set with any desired type of set point control  80  that would provides an input signal to the controller  72 .  
         [0023]     The liquid flow controllers and the gas flow controllers shown, generally include an internal flow sensor and an adjustable valve. The signal from each flow sensor produces an output which can be used as an input to an electronic controller to control the flow rate to the desired set-point value. The electronic controller is usually internal to the flow controller. Alternatively, the flow sensor output can be connected to an external controller  72 . Controller  72  then in turn is connected to adjust an internal valve in each of the liquid or gas flow controllers, to provide the proper flow rate. The controller  72  is also connected to control the shut off valves  66 A- 66 C and  58 A- 58 C.  
         [0024]     The controller  72  can be any desired type of electronic controller. It can be digital, or analog. The controller will sense feedback signals and control outputs for adjusting the flow in the individual valves for the liquid or the gas. For some simple applications, the control can be done manually. In which case, an equivalent manual control can be provided so that an operator will make the necessary manual adjustment to provide the proper flow rates and the temperatures to the vaporizer.  
         [0025]     An output opening of the vaporization chamber  52  is connected with a suitable line or passageway  82  to a process chamber  84  that can be used for processing semi-conductor wafers, or the like. The process chamber  84  is a chemical vapor deposition (CVD) chamber and has a heater  85 , and a temperature sensor  86 , both of which can be connected to the computer controller  72  for controlling the temperature in the process chamber  84 . A vacuum source  88  is also connected to the process chamber for providing the desired internal conditions for appropriate processing of semi conductor wafers.  
         [0026]     It should be noted that in  FIG. 3 , the precursor liquid may be fed to the vaporization chamber and vaporized in a conventional manner by direct contact with the hot metal surface of a heated plate  90 , without first being atomized to form droplets. Atomization would not be necessary if the liquid precursor is sufficiently stable at the vaporization temperature so it will not thermally decompose. Direct-contact vaporization, rather than atomization-and-vaporization would be an alternative method for multi-liquid precursor vaporization, when stable precursors are used.  
         [0027]     In the embodiment of  FIG. 3 , there is no attempt to atomize the liquid by a high velocity gas. The liquid should be relatively volatile and easy to vaporize. With easily vaporized liquids the arrangement of  FIG. 3A  can be used with the atomizer using liquid and gas inlets shown in  FIG. 3 . The liquid and gas aerosol from the atomizer  60  can be introduced into a simple heated vaporization chamber  53 A of a housing  53  through a tube  53 B. Tube  53 B discharges the aerosol near the bottom of the chamber  53 A. As the gas and liquid come in contact with the heated metal surface  53 C, the gas is heated, and the liquid is vaporized. The gas vapor mixture then flows upwardly and exits from the side to the CVD chamber. The heated chamber  53 A does not need to be in form of a cylindrical volume. Various geometrical arrangements can be made to insure that the gas and liquid can be heated properly, and that would be obvious to those skilled in the art of heat exchanger design.  
         [0028]      FIG. 4  is a schematic diagram as a second system  92  showing the use of an atomizer  94  for multi-liquid precursor chemical vaporization. This is the preferred form for multi-precursor chemical vaporization. Typical precursor chemicals include tetraethyoxisilane (TEOS), tetraeylborate (TEB), tetramethycyclotetrasiloxane (TOMACTS), triethyloxyphosphine and oxide (TEPO), which can be used in suitable combination in a CVD deposition chamber to deposit phosphosilicate glass (PSG), borosilicate glass (BSG), or boro-phosphosiicate glass (BPSG) thin films of a low dielectric constant (low k dielectrics). The atomizer  94  is shown schematically and has an input line  96  connected to two (or more) gas sources  98 A and  98 B that are pressurized sources. Each gas source  98 A,  98 B is connected to the input line  96  through a gas flow controller  100 A,  100 B and a positive shut-off valve,  102 A and  102 B, respectively. The plurality of gas sources allows more than one carrier gas to be used with the atomizer  94 . The atomizer  94  includes an orifice plate  104 , typically with a small diameter orifice opening so that the pressure drop across the orifice can be greater than the critical pressure drop needed to produce a gas flow at sonic velocities for fine droplet atomization.  
         [0029]     Downstream of the orifice plate  104  is a gas flow passageway  106  in the atomizer that has two or more liquid input tubes  108 A or  108 B connected thereto. Each tube  108 A,  108 B is connected to a separate supply source  110 A or  110 B of a liquid precursor chemical through a liquid flow controller  112 A or  112 B and a positive shut-off valve  114 A or  114 B. When the precursor liquid from one of the sources  110 A or  110 B is flowing (under pressure from the supply source) into the gas flow passageway  106 , it is injected by nozzles and atomized by the high velocity gas jet flowing through the same passageway  106  from orifice plate  104  thereby forming small liquid droplets. The gas and liquid droplet mixture i.e. the aerosol, then flows out of the gas flow passageway into the heated vaporization chamber  116 . The liquid pressures, nozzles, sizes and gas flow requirements for atomization are well known.  
         [0030]     The flow controllers for the liquid precursor and the carrier gases are conventional and include flow sensors and adjustable valves connected to an electronic controller, which can be internal to the flow controller, or located outside as shown in  FIG. 3 .  
         [0031]     The vaporization chamber  116  is usually electrically heated. The heater  118  provides energy needed to heat a block  120  on the internal cavity  121  of the vaporization chamber  116  to the desired temperature so as to provide the energy needed to heat the carrier gas and vaporize the liquid droplets in the aerosol formed at the atomizer  94 . A temperature probe or sensor  122  is provided to sense the temperature. A controller, such as controller  72  similar to that shown in  FIG. 3  is used to control the heater to keep the vaporizer block  120  at the set-point value.  
         [0032]     The block  120  is provided with a multitude of parallel passageways  124  through which the aerosol can flow and be heated by heat transfer, first to the gas and then to the droplets for vaporization. The parallel passageways  124  reduce the gas velocity through each passageway to allow more time for the gas to be heated and the droplets to vaporize. By this means the gas can be heated more efficiently in a small volume so that the vaporizer can be made more compact for a given rate of gas and liquid flow.  
         [0033]     The atomizer  94  shown is especially convenient when two or more liquid precursor chemicals are needed for use with one or more carrier gases. If the same carrier gas can be used with all the liquid precursors, only one carrier gas supply needs to be provided and one gas flow controller and one gas shut-off valve need to be installed.  
         [0034]     In operation, when the vaporizer  116  has reached the desired operating temperature, the carrier gas from one or more supply  98 A or  98 B will be turned on. This can be accomplished by a control signal sent from a computer (similar to controller  72  shown in  FIG. 3 ) to open the shut-off valve  102 A or  102 B (or both) and a signal to the gas flow controller or controllers will provide gas flow at the desired value. The same controller such as controller  72  shown in  FIG. 3  can then send a signal to open the desired liquid shut-off valve  114 A or  114 B and set the liquid flow rate to the desired set-point value with the liquid flow controller  112 A or  112 B.  
         [0035]     These adjustments can occur simultaneously, or in sequence. For instance, it may be desirable to turn on the gas flow first, and allow a brief time of delay to allow the gas flow to be stabilized before turning on the liquid flow to form a gas/liquid mixture. The gas/liquid mixture or aerosol containing liquid droplets is passed through headed passageway  124  to vaporize the liquid droplets, and then the hot carrier gas vaporized precursors is passed through a heated filter  126  and through an output line or passageway  128  (which may also be heated) for introduction into a process chamber  130  for film deposition.  
         [0036]     In the event that the process application calls for the introduction of a mixture of two or more precursor vapors in a carrier gas, the carrier gas flow can be turned on along with both (or the desired number) of the precursor liquid flows. The aerosol from atomizer  94  would thus comprise droplets of two or more precursor liquids suspended and mixed in with the same carrier gas. Upon heating of the gas and vaporization of the precursor liquid droplets as the aerosol passed along passageway  124 , the gas/vapor mixture then contains the vapor from the two or more precursor chemicals. This gas mixture can then be delivered to the process chamber  130  for thin film deposition. The high velocity atomizer gas will insure that the droplets are uniformly mixed with the carrier gas and that the gas/vapor mixture will also have a uniform composition both spatially and in time.  
         [0037]     If the specific application calls for the delivery of a precursor vapor with its own specific carrier gas, the flows of the specific gas and the specific precursor liquid can be turned on and controlled to provide the proper carrier gas and liquid flow to generate the desired droplet aerosol at the desired rate, and upon heating and vaporization in the vaporization chamber, the proper carrier gas/precursor vapor mixture can thus be generated. This can be followed by a second step where a second set of carrier gas/precursor liquid combination is used to generate a second combination of carrier gas/liquid precursor aerosol, and a second carrier gas and a second vapor mixture following vaporization.  
         [0038]     As will be clear to those knowledgeable in semiconductor device fabrication the atomizer and vaporizer arrangement described above will provide a great deal of flexibility for the semiconductor device fabrication plant, also known as the “device fab”, or simply as the “fab”. One carrier gas can be used with two or more liquid precursor supply systems to generate a mixture of gas with two or more precursor vapors that can be introduced into the process chamber to generate a thin film comprising multiple components of chemical species provided by different liquid precursor chemicals. It can also be used to generate different layers of material in sequence by the suitable choice of carrier gas and liquid precursor to achieve the desired film property.  
         [0039]     The system of  FIG. 4  can be used to generate a thin film, and “dope” the film with the desired chemical species simultaneously or in sequence to generate film with unusual properties or qualities. This approach to liquid precursor chemical vaporization, and the vaporizer constitutes a significant advance, not just in vaporization technology, but in semiconductor device fabrication technology as well. The use of a heated filter  126  in the vaporizer  116  will insure that the gas/vapor mixture leaving the vaporizer  116  is nearly free of particulate contaminants to insure high film quality that is necessary for high product quality and device yield.  
         [0040]     The operating pressure of the atomizer, i.e. the absolute pressure upstream of the orifice  104  is typically twice the absolute pressure downstream, so that there is sonic flow through the orifice. For example, if the downstream pressure is 1 atmosphere, or 760 Torr, the upstream pressure would be typically around 2 atmosphere, or around 1500 Torr or higher in absolute pressure. Since the atomizer outlet is connected to the vaporization chamber, the pressure downstream of the atomizer orifice should be similar to the pressure in the chamber. In some cases, the vaporization chamber may need to be operated at a lower pressure, and the pressure upstream of the orifice would also have to be lower. For instance, if the chamber pressure is, say 100 Torr, then the pressure upstream of the orifice should be around 200 to 300 Torr. The pressures are related to insure sonic flow at the orifice.  
         [0041]      FIG. 5  shows a modified deposition system  92 A that has a modified form of the input atomizer to the same vaporization chamber  116  as shown in  FIG. 4  for multi-liquid precursor liquid vaporization. As shown in  FIG. 5 , the same arrangement as  FIG. 4  is disclosed except that separate atomizers  142 A and  142 B are built into the same atomizer head  140 . Only two atomizer passageways are shown for clarity.  
         [0042]     Each atomizer  142 A and  142 B is provided with one source of gas  98 A,  98 B, controlled by a separate gas flow controller  100 A,  100 B and a separate positive shut-off valve  102 A,  102 B, respectively. Each atomizer has an orifice plate  144 A or  144 B, respectively. Similarly, the liquid supply tube  108 A or  108 B opens into a separate chamber  146 A and  146 B forming output chambers, or passageway of the atomizers  142 A and  142 B. Each atomizer is thus provided with one source of liquid precursor from source  110 A or  110 B, one liquid flow controller  112 A or  112 B, and a positive shut-off valve  114 A,  114 B.  
         [0043]     Additional atomizer passageways and orifice plates of substantially the same design can be incorporated into the same atomizer head. The number of atomizer passageways that can be incorporated into a single atomizer head for droplet precursor chemical vaporization is limited only by space requirements, and by the number of precursor liquids that need to be vaporized in a single piece of equipment in a specific installation.  
         [0044]     The advantage of the vaporizer design in  FIG. 5  compared to that shown in  FIGS. 3 and 4  is that each liquid flow in  FIG. 5  is confined in its own liquid flow passageway in the atomizer and into an inlet to the vaporization chamber. The different liquids will thus not mix, or come into contact with each other before they are atomized to form a separate droplet aerosol for vaporization. In some situations, mixing of precursor liquid chemicals may be undesirable and needs to be avoided. The disadvantage of the apparatus based on the design in  FIG. 5  is that separate atomizing gas flows are used to atomize different liquids, so the mass concentration of the droplet aerosol in the carrier gas is lower than the designs shown in  FIGS. 3 and 4 . In the latter cases, the same atomizing gas is used to atomize two or more liquid precursor droplets. The mass concentration of the droplet aerosol in the carrier gas can thus be lower with the multiple atomizers of  FIG. 5 .  
         [0045]     In some applications, the precursor liquid may have a high molecular weight that may be in excess of 300 or more. Some of these precursor liquids may also have a high viscosity, making it difficult to atomize to form droplets. Since the viscosity of most, if not all, substances decreases with increasing temperature, the liquid may be heated to a higher temperature for ease of atomization.  
         [0046]     In  FIG. 6 , a system  92 B includes a heater  150  in the supply line for the precursor liquid from source  110 B, which for illustrative purposes, is assumed to have a high viscosity. Heaters can be used in the supply lines of all precursor liquids, if desired. By heating the precursor liquid to a suitably high temperature, the liquid viscosity can be reduced, thereby making it easy to atomize. However, it is important that the heating is not excessive to cause thermal decomposition.  
         [0047]     A modified atomizer head  152  has two atomizers  154 A and  154 B that can be applied to a wider range of liquids, including those that cannot be easily atomized at room temperature. The atomizer head  152  is provided with a mounting flange  156  that is insulated from the vaporization chamber  116  with a layer of insulation  158  to prevent the atomizer head  152  and the liquid precursor in contact with the head from being over-heated by the high temperature used for droplet vaporization in the vaporization chamber  116 . If the insulation layer  158  is inadequate to keep the atomizer head  152  sufficiently cool, a stream of cooling gas from a source  160  can be directed through passageways  162  in the atomizer head  152  to keep the atomizer head at a moderate to low temperature. In some instances, it may be necessary to use a liquid coolant from a source to keep the atomizer head temperature in a reasonable operating range.  
         [0048]     As explained earlier,  FIG. 2  is a schematic diagram of the vaporizer shown in the previous invention disclosed in U.S. Pat. No. 6,409,839. This prior art vaporizer is further improved in the current invention.  
         [0049]      FIG. 7  is a schematic diagram of a modified form of the invention including an improved vaporizer  180 . An atomizer  182  is used to form a droplet aerosol containing the desired precursor liquid chemicals. One gas source  184  and one liquid source  186  are shown for clarity, but it is understood that the atomizer  182  can be designed to include more than one gas source and more than one liquid source. The atomizer includes an orifice  188  through which the gas discharges as shown previously and has an outlet  189 . The output flow from the atomizer  182  would thus contain one or more than one liquid source chemicals in droplet form. As this aerosol leaves the atomizer  182  it is in the form of a high velocity aerosol jet represented by arrow  190 . As this jet of gas containing suspended droplets enters the heated vaporization chamber  192 , it is heated in a suitable manner with a heater represented schematically at  194 . The aerosol mixes with the heated gas to form a heated aerosol jet. Small droplets will evaporate quickly in this heated gas mixture, larger droplets will evaporate more slowly and may contain some partially vaporized droplets. The result is a heated gas mixture containing vapor and some partially vaporized droplets. This heated gas mixture, due to the momentum conservation principle in gas flow, will continue to travel at a relatively high velocity for a considerable distance. This high velocity heated aerosol stream is then directed at a mixing orifice  196  and into a cylindrical passageway  198  formed in a heated metal block  199  downstream of the orifice  196 . The orifice  196  is substantially the same size as, or can be smaller than the passageway  198 .  
         [0050]     As the heated aerosol stream flows through the mixing orifice  196 , carrying with it the entrained heated gas from the vaporization chamber, a negative pressure is created in the upper part  192 A of the vaporization chamber  192 . This negative pressure sets up a continuous re-circulating gas flow as depicted by the arrows  200  showing the direction of the re-circulating gas flow. As this re-circulating gas flows upward through the individual small cylindrical passageways  202  arranged on annular lines  203  concentric with and surrounding the central large tubular passageway  198 , (see  FIG. 8 ) the gas flow in each passageway  202  is at a relatively low value and flowing at a relatively low gas low velocity. Any unvaporized droplets in the re-circulating gas flow entering each cylindrical passageway from the bottom of the heated metal block  199  will thus spend considerable amount of time in the small cylindrical passageway. As each gas stream emerges from the exit on the top of the block  199  and into the upper part  192 A of the chamber  192 , droplets contained in the gas stream when the gas stream first entered the chamber  192  from the atomizer would be completely evaporated. By choosing the proper mixing orifice size, and the distance between the orifice  196  and the atomizer outlet  189 , a high volume re-circulating gas flow can be established. Re-circulating gas flow as high as ten times or more of the gas flow at the atomizer outlet  189  can be easily maintained this way, without the use of a pump, or other devices involving moving mechanical parts.  
         [0051]     The re-circulating gas-flow vaporizer  180  described above forms a Stage one  205  of a two stage and improved vaporization system depicted in  FIG. 7 . Stage two  208  of the vaporization system comprises a cylindrical metal block  210  in a lower portion  192 B of chamber  192  having numerous cylindrical flow passageways  212  in the cylindrical metal block  210 , similar to those in Stage one. The heated gas flow at the outlet  198 A of the central tubular passageway  198  is above an imperforate surface of a center plug  211  of the block  210 . Gas flow at the outlet  198 A that is not re-circulated is directed through the passageways  212  of the Stage two vaporizer block  210  to fully vaporize the droplets before the gas stream passes through a heated filter  214  located downstream of block  210 . Any un-vaporized droplets flowing through the filter  214  will cause the filter to become clogged. The second vaporization stage  208  will insure that this does not happen and that the droplets are completely vaporized and no droplets would remain even at a comparatively low vaporization temperature, to minimize the possibility of material decomposition. The gas/vapor mixture flows from an outlet  216  and can then enter a CVD chamber (as shown in  FIG. 4 ) located downstream, either directly, if the CVD chamber can be operated at the same pressure as the vaporizer, or through a flow restriction, which can be in the form of an orifice, a length of small diameter capillary tube, etc. before the gas/vapor mixture is introduced into the CVD chamber operating at a lower pressure than the vaporizer. The flow from the inlet  189  to the outlet  216  of the chamber  192  follows a flow direction or flow path axis in the vaporization chamber.  
       Applications  
       [0052]     There are wide varieties of applications where the improved vaporizer system and the multi-liquid precursor vaporization system described in this specification can be used. Particularly important are insulating thin films of a low or a high dielectric-constant, also referred to as low-k or high-k dielectrics. These films are used as insulating layers in semiconductor device fabrication on a silicon wafer. Simple silicon dioxide (SiO 2 ) thin films of a low dielectric constant can be made using a single precursor chemical such as Tetraethyloxisilane (TEOS) or Tetramethyclotetrasiloxane (TOMACTS). Tatalum pentoxide (Ta 2 O 5 ) thin films of a high dielectric constant can also be made using a single precursor chemicals such as tantalum tetraeoxydimethyaminoethoxide (TAT-DMAE). Silicon nitride (Si 3 N 4 ) thin films, also of a high dielectric constant, can be made by the LPCVD process using the precursor chemical Bis(terbutylamino) silane (BTBAS). Thin glass films constaining the elements silicon, boron, and/or phosphorous can be made by a CVD process using suitable precursor chemicals containing these elements. Common precursor chemicals include Tetraethyloxisilane (TEOS), Tetraethylborate (TEB), and Triethyloxyphosphine oxide (TEPO), which can be used in a suitable combination to make thin films of boro-silicate glass (BSG), phosphor-silicate glass (PSG), or boro-phospho silicate glass (BPSG). Other liquid precursor chemicals are constantly being developed. Some will require the vaporization of one single precursor chemical. Others will require two or more precursor liquid chemicals to be vaporized. To have a method and device that can be used to vaporize multiple liquid precursors in the same apparatus will lead to saving in cost of the equipment, and provide a degree of control that is hither-to-fore impossible for semiconductor thin film deposition.  
         [0053]     Instead of having holes through the metal blocks shown, the heat conductive metal blocks can be made of a heat conductive porous material. The porous material will form passageways for allowing heat to transfer to the gas/vapor mixture flowing therethrough.  
         [0054]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.