Patent Document

This application is a divisional application of, and claims priority from U.S. application Ser. No. 09/064,359 filed Feb. 27, 1998 now U.S. Pat. No. 6,179,277, the complete disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is directed to liquid vaporizer systems and, more specifically, to liquid vaporizer systems for low vapor pressure liquids. 
     Semiconductor fabrication processes may include the use of chemical vapor deposition (CVD) to deposit a thin film of material on semiconductor wafers. CVD processes are often preferred because of their ability to provide highly uniform layers of film. CVD typically comprises introducing gases into a reaction chamber in the presence of a substrate, wherein the gases react and deposit a film on the substrate surface. Some of the CVD materials originate as liquids, and are evaporated and transported in the gaseous state to the desired reaction chamber for CVD. 
     Liquid reactant sources are often used with CVD processes. For example, titanium tetrachloride (TiCl 4 ) is used as a reactant gas in CVD to deposit titanium-containing film layers onto a substrate surface. Tetrakisdimethylamidotitanium (TDMAT), a metal-organic material, also is used in CVD processes. 
     CVD using TiCl 4  and TDMAT typically involves the vaporization of liquid TiCl 4  or TDMAT and the transportation of the gaseous TiCl 4  or TDMAT to the reaction chamber using a carrier gas. Care must be exercised to fully vaporize the liquid compound. If the compound is not fully vaporized and is thus delivered to the reaction chamber as a gas/liquid mixture, liquid compounds in the reaction chamber can lead to poor uniformity of the deposited film and other undesired process results. This also can cause poor repeatability wafer to wafer because liquid droplets deposited on the walls of the process piping can later evaporate, causing fluctuations in vapor concentration in the reaction chamber. Once fully vaporized, care also must be exercised to maintain the compound in a gaseous state while it is being transported to the reaction chamber. Condensation of TiCl 4  or TDMAT can lead to undesirable deposition process variability and possibly the failure of line components. 
     The vaporization of TDMAT presents unique problems, in part due to the fact that TDMAT has a low vapor pressure. It is desirable, therefore, to provide an efficient mechanism for fully vaporizing TDMAT metal-organic compound, and for transporting the gaseous compound to the reaction chamber while avoiding the condensation of TDMAT liquid out of the gas. 
     SUMMARY OF THE INVENTION 
     The present invention provides for improved liquid vaporizer systems and methods for their use. Vaporizer systems of the present invention are likely to be particularly useful for the vaporization of TDMAT and other liquid sources having relatively low vapor pressures. The present invention uses a two-step vaporization process to assist in the full vaporization of such liquids. 
     In one embodiment, a liquid vaporizer system of the present invention includes a vaporizer unit having first and second inlets and an outlet. The vaporizer system further includes a vessel having an inlet and an outlet, whereby the vessel inlet is operably connected to the vaporizer unit outlet. The vessel contains a plurality of passages which operably connect the vessel inlet with the vessel outlet. In this manner, liquids and/or gases flowing into the vaporizer unit through either or both of its two inlets exit the vaporizer unit outlet and enter the vessel inlet. Liquids and/or gases pass through the plurality of passages and exit the vessel outlet. 
     In one aspect, the first vaporizer inlet is adapted to receive a liquid and the second vaporizer inlet is adapted to receive a carrier gas. The vaporizer unit preferably comprises a control valve for controlling the amount of liquid passing therethrough, and a vaporization valve for causing the liquid which has passed through the control valve to be at least partially vaporized and transported out the vaporizer outlet by the carrier gas. A vaporizer passageway is provided which connects the control valve with the vaporization valve to allow liquid to flow from the control valve into the vaporization valve. 
     The vaporizer unit further preferably comprises a temperature-controlled housing structure containing both the control valve and the vaporization valve. A temperature control mechanism is in communication with the housing structure for maintaining a thermostatic condition inside the housing structure. In this manner, a desired amount of heat can be transmitted to the liquids or gases passing through the control valve and vaporization valve, thereby assisting with the vaporization of the liquid. Exemplary vaporizer units of the present invention are disclosed in U.S. Pat. No. 5,440,887 and U.S. Pat. No. 5,272,880, the complete disclosures of which are hereby incorporated by reference. Both U.S. Pat. No. 5,440,887 and U.S. Pat. No. 5,272,880 are assigned to Applied Materials, Inc. of Santa Clara, Calif., the assignee of the present application. 
     The vessel passages preferably have a cumulative surface area that is at least two times greater than, and more preferably, at least ten times greater than an internal surface area of an identical size vessel without the passages. The vessel preferably comprises at least five passages, and more preferably, at least 10 passages connecting the vessel inlet and outlet. 
     In one aspect, the vessel is generally cylindrical in shape. The vessel further preferably comprises a thermally conductive material. In one aspect, the vessel comprises stainless steel and, in another aspect, the vessel comprises aluminum. In addition, other thermally conductive materials, such as other metals or metal alloys, may be used. 
     The liquid vaporizer system further preferably comprises a heat source in communication with the vessel. In one aspect, the vessel heat sources comprises a blanket heater wrapped at least part way around the vessel. In this manner, an outer surface of the vessel can be heated to a desired temperature by the vessel heat source. Use of a thermally conductive material for the vessel facilitates the transmission of heat throughout the vessel structure, and to liquid and/or gas passing through the vessel to assist with vaporization. 
     In another aspect, the vaporizer system further comprises a gas passageway operably attached to the vessel outlet and adapted to be attached to a reaction chamber. In this manner, gases exiting the vessel outlet may be transported to a desired reaction chamber for CVD or other processes. The gas passageway is preferably an insulated, heated passageway to help maintain gases exiting the vessel outlet in a gaseous state. 
     In another aspect, the liquid vaporizer system further includes a preheater operably connected to at least one of the vaporizer inlets. Such a preheater is preferably used for preheating a carrier gas before the carrier gas enters the vaporizer unit. In this manner, the heated carrier gas facilitates the vaporization of a desired liquid, such as liquid TDMAT. 
     The present invention further provides a method for vaporizing liquid/gas mixtures. The method includes the step of flowing a liquid and a carrier gas into the vaporizer unit wherein the liquid is at least partially vaporized to create a liquid/gas mixture. The liquid/gas mixture is passed through a heated vessel wherein the liquid/gas mixture is further vaporized into a product gas. 
     In one aspect of the method, the liquid/gas mixture is passed through a plurality of generally parallel passages contained within the vessel. The passages preferably have a cumulative surface area that is at least two times greater than an internal surface area of an identical size vessel without the passages. The liquid preferably has a vapor pressure that is less than about 82 Torr at 100 degrees Celsius. In one aspect, the liquid comprises TDMAT, however, other liquids may be used within the scope of the invention. In another aspect, the carrier gas comprises helium, however, other inert gases, such as argon, nitrogen, or a combination of inert gases, may be used. In still another aspect, the liquid/gas mixture comprises at least some liquid TDMAT. The product gas preferably comprises gaseous TDMAT. 
     In one embodiment, the method further comprises the step of heating the vaporizer unit to a desired temperature, preferably, between about 50 degrees Celsius and about 200 degrees Celsius. In another aspect, the method further comprises the step of heating the carrier gas to a desired temperature, preferably between about 50 degrees Celsius and about 200 degrees Celsius, before the gas enters the vaporizer unit. 
     The method further preferably comprises the step of heating an outer surface of the vessel to a desired temperature. In one aspect, the vessel outer surface is heated to at least about 80 degrees Celsius. In another aspect, the vessel outer surface preferably is heated to between about 50 degrees Celsius and about 200 degrees Celsius. In this manner, the heated vessel facilitates further vaporization of any remaining liquid. 
     In still another aspect, the method further includes the step of passing the product gas through an insulated, heated passageway to prevent the condensation of liquid from the product gas. An outer surface of the heated passageway is preferably heated to between about 50 degrees Celsius and about 200 degrees Celsius. 
     In one embodiment of the present invention, an apparatus for vaporizing liquid/gas mixtures passing therethrough includes a vessel comprising a thermally conductive material, and having an inlet and an outlet. The inlet is operably connected to the outlet by a plurality of passages contained within the vessel. A heat source is in communication with the vessel for heating an outer surface of the vessel to a desired temperature. The passages have a cumulative surface area that is at least two times greater than an internal surface area of an identical size vessel without such passages. The vessel preferably comprises stainless steel and/or aluminum, however, other thermally conductive materials may be used within the scope of the present invention. 
     These and other features and advantages of the invention will appear from the following description in which the preferred embodiment has been set forth in detail in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic depiction of a liquid vaporizer system according to the present invention; 
     FIG. 2 is a front sectional view of a vaporizer unit according to the present invention; 
     FIG. 3 is a sectional view of the vaporizer unit of FIG. 2 taken along line  3 — 3  in FIG. 2; 
     FIG. 4 is a schematic sectional view of the vaporizer unit of FIGS. 2 and 3 showing its temperature control mechanism; 
     FIG. 5 is an overall view of a vessel that is part of a liquid vaporizer system according to the present invention; 
     FIGS. 6 a, b  and  c  depict cross-sectional views of an inlet end cap, a middle portion, and an outlet end cap of the vessel depicted in FIG. 5, respectively; 
     FIGS. 7 a-c  depict top planar views of the inlet end cap, the middle portion, and the outlet end cap of the vessel depicted in FIG. 5, respectively; 
     FIG. 8 is a partial cutaway view of the vessel depicted in FIG. 5 showing a blanket heater surrounding the vessel; 
     FIG. 9 is an overall view of an insulated, gas passageway as part of a liquid vaporizer system according to the present invention; and 
     FIGS. 10 a  and  b  depict an overall view and a cross-sectional view, respectively, of a preheater as part of a liquid vaporizer system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A liquid vaporizer system  10  according to the present invention will now be described. FIG. 1 depicts a schematic of liquid vaporizer system  10 . Vaporizer system  10  includes a carrier gas line  12  through which a carrier gas such as helium, nitrogen, or the like is transported. For the vaporization of liquid TDMAT, the carrier gas is preferably a combination of inert gases, such as helium and nitrogen. Thus, the term carrier gas shall be understood to refer to an inert gas or a combination of inert gases. The desired carrier gas flows through a mass flow controller  11 , which operates to control the amount of carrier gas flowing therethrough. The carrier gas is preheated in a preheater  14  and continues through carrier gas line  12  into a vaporizer unit  16 . 
     A desired liquid, such as liquid TDMAT, flows through a liquid source line  13  and into and through a liquid flow meter  15 . Liquid flow meter  15  operates to measure the amount of liquid flowing therethrough. The desired liquid enters a liquid line  18  and is delivered to vaporizer unit  16 . Vaporizer unit  16 , whose details will be described in conjunction with latter figures, at least partially vaporizes the TDMAT. 
     A liquid/gas mixture of TDMAT and the carrier gas exits vaporizer unit  16  through a liquid/gas mixture line  20  and enters a vessel  22  in which further vaporization of the TDMAT occurs as described in conjunction with latter figures. A product gas comprising gaseous TDMAT exits vessel  22  and proceeds through a product gas line  24  to a reaction chamber  26 . Chemical vapor deposition or other desired processes involving gaseous TDMAT can then occur in reaction chamber  26 . 
     Carrier gas line  12 , liquid source line  13  and liquid line  18  may comprise stainless steel tubing or the like. Similarly, liquid gas mixture line  20  and product gas line  24  may comprise stainless steel tubing or the like. In one embodiment, lines  12 ,  13 ,  20  and  24  comprise ¼inch diameter tubing, and line  18  comprises ⅛inch diameter tubing. 
     The present invention further anticipates operably attaching preheater  14  to vaporizer unit  16  without the need to have carrier gas line  12  therebetween. Such a connection may be accomplished by fittings and joints, such as face sealed tubing or the like. Likewise, vaporizer unit  16  and vessel  22  may be operably attached to one another without the need to use liquid/gas mixture line  20 . 
     Turning now to FIG. 2, a preferred vaporizer unit  16  will be described. As shown in FIG. 2, vaporizer unit  16  comprises a liquid inlet  50  for receiving a desired liquid such as TDMAT. Liquid line  18  is operably connected to liquid inlet  50  to allow the transportation of liquid TDMAT to vaporizer unit  16 . A control valve  52  operates to control the amount of liquid TDMAT passing therethrough. Liquid TDMAT proceeds through a vaporizer passageway  56  and into a vaporization valve  54 . As best seen in FIG. 3, vaporizer unit  16  further includes a carrier gas inlet  60  as shown here operably attached to carrier gas line  12 . As the carrier gas enters vaporizer unit  16 , it proceeds into and through vaporization valve  54 . The carrier gas transports at least partially vaporized TDMAT through vaporization valve  54  and a liquid/gas TDMAT mixture exits a vaporizer unit outlet  62 . 
     FIGS. 2 and 3 depict a temperature-controlled housing structure  58  enclosing control valve  52  and vaporization valve  54 . As seen in FIG. 4, a temperature control mechanism or heater controller  64  is operably attached to housing structure  58  to maintain a thermostatic condition inside housing structure  58 . By controlling the temperature within housing structure  58 , preferably at an elevated temperature between about 50 degrees Celsius and about 200 degrees Celsius, and more preferably, between about 70 degrees Celsius and about 90 degrees Celsius, TDMAT can be at least partially vaporized. Alternatively, TDMAT is at least partially vaporized by its contact with the carrier gas. 
     The preceding discussion in conjunction with FIGS. 2-4 describes a preferred embodiment of vaporizer unit  16  according to the present invention. A more detailed description is provided in U.S. Pat. Nos. 5,440,887 and 5,272,880, the complete disclosures of which have been previously incorporated by reference. 
     The preferred vaporizer unit  16  has experienced considerable success in vaporizing tetraethyl orthosilicate (TEOS, having a vapor pressure of about 82 Torr at 100 degrees celsius), and other materials. However, due in part to the fact that TDMAT has a low vapor pressure (about 9.1 Torr at 100 degrees Celsius), vaporizer unit  16  typically only partially vaporizes TDMAT. As previously described, the presence of partially vaporized TDMAT can provide undesirable characteristics in chemical vapor deposition processes. 
     In an attempt to fully vaporize TDMAT, consideration was given to heating liquid/gas mixture line  20  to a temperature sufficient to complete the vaporization of TDMAT. However, for mixture line  20  comprising a ¼inch diameter tubing, this required heating the outer surface of such tubing to at least 115 degrees Celsius. At such a temperature, TDMAT breaks down into undesirable by-products which negatively affect the chemical vapor deposition process conducted in the reaction chamber. In addition, gas at this temperature may cause cracking of fittings, injection valves, and other line components. Therefore, at least part of the present invention is the realization that to provide full vaporization of TDMAT without increasing liquid/gas mixture line  20  temperature, and hence TDMAT temperature, to an undesirable level, TDMAT should be exposed to a larger heating surface at a lower temperature. 
     Referring now to FIG. 5, a preferred vessel  22  of liquid vaporizer system  10  will now be described. Vessel  22  comprises inlet and outlet end caps  74  operably attached to a middle portion  76 . The combination of end caps  74  and middle portion  76  results in vessel  22  having a cylindrical shape. Liquid/gas mixture line  20  is operably attached to vessel  22  in a manner which allows a liquid/gaseous TDMAT mixture to enter vessel  22 . Product gas line  24  in turn is operably attached to vessel  22  to allow a product gas to exit vessel  22 . Lines  20  and  24  can be operably attached to vessel  22  by a number of methods, including by the use of face sealed tubing or the like. As previously noted, vessel  22  also may be operably attached to vaporizer unit  16  without the need to use mixture line  20  therebetween. 
     Vessel  22  preferably comprises a thermally conductive material, and more preferably, stainless steel and/or aluminum. The choice of such a metal provides desirable thermal conductive properties to allow an external heat source to heat vessel  22  to a desired temperature. Vessel  22  may also comprise other thermally conductive materials, including other metals or metal alloys. Vessel  22  preferably comprises a disk-shaped or cylindrical vessel  22 , although other shaped vessels are possible within the scope of the present invention. In one embodiment, vessel  22  preferably has a diameter that is between about one inch and about three inches, and a height that is between about one half inch and about two inches. Such a size and shape facilitates the incorporation of vessel  22  into exemplary mainframe units currently commercially available, including the Precision 5000®, the Centura® 5200, and the Endura® 5500 systems from Applied Materials, Inc. of Santa Clara, Calif. 
     FIGS. 6 a - 6   c  show cross-sectional views of inlet and outlet end caps  74  and middle portion  76 , taken through the middle thereof. As seen in FIGS. 6 a  and  6   c , each end cap  74  has an outer cap rim  75  having a height that is greater than the height of the remainder of end cap  74 . As a result, when end caps  74  are operably attached to middle portion  76 , a small gap is created between an inner surface  77  of end caps  74  and the upper and lower surfaces of middle portion  76 . Inlet end cap  74 , to which liquid/gas mixture line  20  is operably attached, includes a vessel inlet  70  for receiving the liquid/gas mixture. Likewise, outlet end cap  74  is operably attached to product gas line  24  and has a vessel outlet  72  to allow a product gas to exit vessel  22 . End caps  74  may be affixed to middle portion  76  by welding, adhesive or the like. Further, end caps  74  and middle portion  76  may be formed as a single piece of thermally conductive material. 
     Middle portion  76  comprises an outer surface  80 , a solid core  82  and a plurality of passages  78 . Middle portion  76  may comprise a solid piece of thermally conductive material such as stainless steel, with passages  78  bored or drilled out therefrom. Alternatively, middle portion  76  may be formed or molded with passages  78 , for example, by extruding a honeycomb-shaped structure to a desired length. As best seen in FIGS. 7 a - 7   c , which depict top planar views of inlet and outlet end caps  74  and middle portion  76 , a liquid/gas mixture entering vessel  22  enters vessel inlet  70 . The liquid/gas mixture is unable to proceed directly through vessel.  22  due to solid core  82 . Instead, liquid/gas mixture proceeds through plurality of passages  78  and exits vessel  22  through vessel outlet  72 . 
     By providing a plurality of passages  78  through which the liquid/gas TDMAT mixture proceeds, the liquid/gas mixture is exposed to a surface area that is larger than an internal surface area of vessel  22  if vessel  22  were a hollow vessel without such passages  78 . Vessel  22  with a plurality of passages  78 , therefore, exposes the liquid/gas TDMAT mixture to a larger heated surface area and any liquid TDMAT remaining in the liquid/gas mixture can be further vaporized. As a result, vessel outlet  72  allows a product gas to exit vessel  22  which preferably comprises fully vaporized TDMAT. As depicted in FIG. 7 b , one preferred embodiment has passages  78  having circular cross-sections, however, the shape of passages  78  can be varied within the scope of the invention. Further, the number of passages  78  may be larger or smaller than the number of passages  78  depicted in FIG.  7 B. 
     In one preferred embodiment, vessel  22  is cylindrical in shape and has a diameter of two inches and a height equal to one inch. Two hundred and forty-six (246) passages  78  are provided, each having a diameter of 0.081 inch. The passages  78  allow liquid/gas mixtures passing therethrough to be exposed to a surface area that is over 100 times greater than the surface area such mixtures would be exposed to in a one inch long, ¼inch outer diameter (0.18 inch inner diameter) tubing. 
     As seen in FIG. 8, a vessel heat source  84  preferably is operably attached to the outer surface of vessel  22 . In this manner, vessel  22  outer surface preferably is heated to between about 50 degrees Celsius and about 200 degrees Celsius to assist with the vaporization of liquids. FIG. 8 depicts vessel heat source  84  as a blanket heater, however, other heat sources may be used. As previously noted, passing a liquid/gas mixture of TDMAT through the one quarter (¼) inch diameter tubing required that the outer surface of such tubing be heated to at least 115 degrees Celsius in order to fully vaporize the remaining liquid TDMAT. Use of vessel  22  having a larger surface area exposed to the liquid TDMAT allows vessel  22  to be heated to a lower temperature than the one quarter inch diameter tubing. For vaporization of TDMAT in the present invention, the outer surface of vessel  22  preferably is heated to between about 80 degrees Celsius and about 100 degrees Celsius, and more preferably, to about 95 degrees Celsius. By reducing vessel  22  temperature to about 95 degrees Celsius, TDMAT is no longer broken down into undesirable byproducts. As a result, a more homogeneous gaseous TDMAT is produced, thereby resulting in more favorable chemical vapor deposition processes. In addition, the lower temperature reduces the likelihood of cracking or other damage to line components. 
     Turning now to FIG. 9 a gas passageway as part of the liquid vaporizer system  10  will be described. FIG. 9 depicts product gas line  24  surrounded by an electric resistance heater  90 . Electric resistance heater  90  may comprise a blanket heater, although other heat sources may be used. Heater  90  is used to heat product gas line  24  to a desired temperature and to insulate line  24  in order to prevent the condensation of liquids, such as liquid TDMAT, on the internal surface of product gas line  24 . Product gas line  24  preferably is heated to between about 50 degrees Celsius and about 200 degrees Celsius. As previously noted, the presence of liquid TDMAT is undesirable in chemical deposition processes and, therefore, the use of a heated insulated, product gas line  24  is desirable. 
     As previously discussed in conjunction with FIG. 1, carrier gas line  12  may preferably pass through a preheater  14 . As shown in FIGS. 10 a  and  10   b , preheater  14  can comprise heating elements  15  operably attached to the outer surface of carrier gas line  12 . In this manner, the outer surface of carrier gas line  12  is heated, preferably to between about 50 degrees Celsius and about 200 degrees Celsius. As a result, carrier gas passing through carrier gas line  12  is at least partially heated. Alternatively, vessel  22  may be used as preheater  14 . In this manner, carrier gas line  12  delivers a carrier gas to vessel  22 , which heats the carrier gas passing therethrough as previously described. Carrier gas then flows into vaporizer unit  16  and a second vessel  22 , which operate as previously described to vaporize a desired liquid, such as TDMAT. 
     Referring to FIG. 1, a method of vaporizing liquid/gas mixtures will now be described. A carrier gas, such as helium and/or nitrogen, is introduced into vaporizer unit  16 , preferably after passing through mass flow controller  11  and preheater  14 . A liquid, preferably liquid TDMAT, is introduced into vaporizer unit  16 , preferably after passing through liquid flow meter  15 . The liquid is at least partially vaporized in vaporizer unit  16  to create a liquid/gas mixture. The liquid/gas mixture is then passed through heated vessel  22 , wherein the liquid/gas mixture is further vaporized into a product gas. Such further vaporization in heated vessel  22  is accomplished by passing the liquid/gas mixture through a plurality of passages as previously described. 
     In one preferred embodiment, the liquid comprises TDMAT and the carrier gas comprises a combination of helium and nitrogen. Preheater  14  is heated to a desired temperature that preferably is between about 90 degrees Celsius and about 120 degrees Celsius, which in turn heats the carrier gas passing through carrier gas line  12 . Vaporizer unit  16  is heated to a desired temperature that preferably is between about 70 degrees Celsius and about 90 degrees Celsius, and the carrier gas and liquid TDMAT pass therethrough. A liquid/gas TDMAT mixture exits vaporizer unit  16  and passes through vessel  22 . Vessel  22 &#39;s outer surface is heated to a desired temperature that preferably is between about 80 degrees Celsius and about 100 degrees Celsius, and more preferably, is about 95 degrees Celsius. The product gas exiting vessel  22  is transported to chamber  26  through heated, insulated passageway  24 . Passageway  24  is heated to a desired temperature that preferably is between about 80 degrees Celsius and about 110 degrees Celsius. As a result, liquid TDMAT is fully vaporized by passing through vaporizing unit  16  and vessel  22  at the preferred temperatures, and is transported in heated passageway  24  to chamber  26  in the gaseous state. 
     The invention has now been described in detail. However, it will be appreciated that certain changes and modifications may be made. For example, the present invention may be used with other liquids having a relatively low vapor pressure, such as triethylphosphate (TEPO) or liquids marketed under the trade name Cupra Select by Schumacher (a unit of Air Products and Chemicals, Inc.), located in Carlsbad, Calif. It is further understood that the liquid vaporizer systems and methods of the present invention can facilitate vaporization of other liquids having vapor pressures higher than that of TDMAT, such as liquid TEOS or TiCl 4 . Therefore, the scope and content of this invention are not limited by the foregoing description. Rather, the scope and content are to be defined by the following claims.

Technology Category: c