Abstract:
A method for vaporizing a liquid for subsequent thin film deposition on a substrate. The method comprises vaporizing a liquid which is disposed within a tubular porous metal body. The porous metal body comprises a first surface defining a first carrier gas flow path and a second surface defining a second carrier gas flow path in a substantially opposite direct to the first carrier flow path. Vapor is generated from the liquid and added to a carrier gas that passes sequentially in direct contact along the first and second surfaces of the porous metal body to form a gas/vapor mixture with the carrier gas first flowing along the first surface and then along the second surface thereby providing a gas/vapor mixture for thin film deposition.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 60/740,029, filed Nov. 28, 2006, the content of which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     This invention relates to methods and apparatus for liquid precursor vaporization for thin film deposition and semiconductor device fabrication. The precursor liquid is vaporized in a precise and controlled manner to generate a high purity gas/vapor mixture that is substantially free of particulate contaminants. The gas/vapor mixture is then introduced into a chamber for film deposition and semiconductor device fabrication. 
     Thin film formation by chemical vapor deposition (CVD) is a well-known process in semiconductor device fabrication. In conventional CVD, a precursor vapor is introduced into a chamber in which one or more semiconductor wafers are held at a suitable temperature and pressure to form a thin film on the wafer surface. Insulating, conducting and semi-conducting thin films can be formed by the CVD process using suitable precursor chemicals. If the precursor is a liquid at room temperature, the liquid must be vaporized to form a vapor for film deposition. The process is often referred to as metal organic CVD, or MOCVD, if the precursor liquid is a metal organic compound. Apparatus for liquid precursor vaporization plays an important role in CVD and MOCVD applications. It must be designed properly and be capable of generating vapor with repeatability and accuracy to achieve uniform, high quality thin films for commercial device fabrication in semiconductor and related industries. 
     When the desired film thickness is small and approaches a few nanometers in overall thickness, an Atomic Layer Deposition (ALD) process can be used. In ALD two complementary vapor pairs are used. One, such as ammonia, is first chemisorbed onto the wafer surface to form a monolayer of molecules of the first vapor. A second vapor is then introduced into the chamber to react with the first chemisorbed vapor layer to form a single monolayer of the desired film. The process is repeated as many times as is necessary in order to form multiple atomic film layers with the desired overall thickness. The ALD process produces film with good step coverage and excellent conformity to the topography and underlying surface structure on the wafer. The film thickness can also be precisely controlled. For these reasons, ALD is finding increasing use in advanced semiconductor device fabrication involving small geometrical dimensions. 
     Vapor generation in a controlled manner is possible by direct liquid injection. Direct liquid injection is accomplished through a direct liquid vaporizer (DLI vaporizer), in which liquid is injected into a heated chamber for vaporization. The method is generally limited to vapor generation at rates that are higher than a few milligrams per second. When the desired vapor generation rate is low, it becomes increasingly more difficult to control the small amount of liquid that needs to be injected. Alternative methods must then be used. 
     A commonly used alternative is the bubbler. In a conventional bubbler, a carrier gas is bubbled through a heated precursor liquid to saturate the gas with vapor. The gas/vapor mixture then flows into the CVD or ALD chamber by opening and closing valves. Such prior art bubblers are shown and described in U.S. Pat. Nos. 5,288,325 and 6,579,372. 
     A prior art ALD deposition system is described by Hausmann et al. (Atomic Layer Deposition of Hafnium and Zirconium oxides using metal amide precursors), Chem. Meter. 14, 43-50-4358, 2002). An external volume is used to control the amount of vapor to be delivered to the deposition chamber. By opening and closing the on-off valves connected to the external volume, the external volume is first filled with vapor from a heated vaporization chamber and then emptied into the deposition chamber. Dielectric thin films such as metal oxides and nitrides including SiO 2 , HfO 2 , Zro 2 , WO 3 , and WN have been deposited by this method using ALD (Becker et al.,  Diffusion barrier properties of tungsten nitride films grown by atomic layer deposition from bis ( tertbutylimido )  bid ( dimethylamido ) tungsten and ammonia, applied physics letters , Vol. 82, No. 14, 7 Apr. 2003; Hausmann et al.  Surface morphology and crystallinity control in the atomic later deposition  ( ALD )  of hafnium and zirconium oxide thin films , Journal of Crystay Growth, 249, pgs. 251-261, 2003). 
       FIG. 1  illustrates the bubble formation process in a conventional prior art bubbler. Liquid is placed in a metal container,  50 , which is usually heated. A carrier gas is introduced into the bubbler through inlet tube  52 . As the gas leaves the bottom of tube  52 , it forms a stream of bubbles,  54 . As each bubble rises through the liquid, the surrounding liquid pressure decreases, causing the bubble volume to expand causing the bubble to rise to the surface quickly. As the bubble bursts through the liquid surface, small droplets,  56 , are formed, which are then carried by the gas/vapor mixture through the outlet tube  58 . 
     One disadvantage of the prior art bubbler is that the precursor liquid must be placed in a heated vessel for a prolonged period. Prolonged thermal contact between the liquid and the hot vessel walls can cause the precursor liquid to thermally decompose to form undesirable by-products. Another disadvantage is that with increased gas flow, the gas bubbles would rise more quickly to the liquid surface thereby reducing the residence time of the bubbles in the liquid, thereby causing the gas to become less saturated with vapor. With the bubbler, the vapor generation rate is often unknown or uncontrolled. In addition, as the bubbles burst at the liquid surface, liquid is atomized to form droplets that are entrained by the carrier gas to deposit in the downstream components. Such components as valves, fittings, tubing connection, as well as the deposition chamber are often coated with precursor droplets that have impacted on the heated metal surface and subsequently under thermal decomposition to form a non-volatile residue coating the surface. Over time, the system components would become contaminated. The thin film deposition tool itself must then be shut down for maintenance and cleaning, resulting in the loss of productivity of the tool. 
     A porous metal wall with interstitial spaces extends from the liquid reservoir for containing liquid from the reservoir. 
     SUMMARY OF THE INVENTION 
     The present invention includes an apparatus for vaporizing a liquid for subsequent thin film deposition on a substrate. The apparatus comprises a housing with an inlet and an outlet and a liquid reservoir. A mechanism controls the liquid level in the reservoir to a substantially constant level. A gas flow passageway extends along side the porous metal wall for forming a gas/vapor mixture suitable for thin film deposition. 
     The present invention also includes an apparatus vaporizing a liquid into a carrier gas for thin film deposition in which a mechanism changes the temperature of the carrier gas as it flows into the vaporization chamber in which vapor is added to the carrier gas. 
     The present invention also includes a method for generating a gas/vapor mixture for use in thin film deposition in which liquid disposed within a porous metal body is vaporized, and adding the vapor to a carrier gas passing along side the porous metal body to form a gas/vapor mixture and providing such gas/vapor mixture for thin film deposition. 
     The present invention also includes an apparatus for vaporizing a liquid for subsequent thin film deposition in which a liquid reservoir within a housing contains the liquid. A mechanism controls the liquid level in the reservoir. A gas flow passageway extends into the reservoir allowing the gas to bubble through the liquid to form a gas/vapor mixture. Additionally, a porous metal filter is positioned at an outlet to filter the gas/vapor mixture. 
     The present invention also includes an apparatus for vaporizing a liquid for subsequent thin film deposition having at least two vaporizing chambers, each vaporizing chamber having a passage for carrier gas to flow therethrough and in which carrier gas forms a gas/vapor mixture. A thermoelectric module in thermo conductive contact with the vaporizing chambers controls the temperature of the chambers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of bubble and droplet formation in a conventional prior art bubbler. 
         FIG. 2  is a schematic view of a diffusion vaporizer of the present invention. 
         FIG. 3  is a schematic view of a system for vapor generation by diffusion and film formation. 
         FIG. 4  is a schematic view of a bubbler with level control and external liquid source. 
         FIG. 5  is a schematic view of a high capacity bubbler. 
         FIG. 6  is a schematic view of a high capacity diffusion vaporizer 
         FIG. 7  is a schematic view of a multi-channel vaporizer in a metal block. 
         FIG. 8  is a schematic view of a sectional view of metal block for a multi-channel vaporizer of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention includes a new approach to liquid vaporization for gas saturation that would overcome the disadvantages of the prior art bubbler of  FIG. 1 . A small amount of liquid is placed in the vaporizer to minimize thermal decomposition. The new vaporization apparatus is also highly stable and repeatable and capable of generating high purity gas/vapor mixtures that are substantially free of particulate contaminants. The resulting high purity gas/vapor mixture is suitable for use in semiconductor and other industries for film deposition on a substrate for semiconductor device fabrication or other applications 
     A vaporization apparatus described in this invention is generally indicated at  100  in  FIG. 2 . Like reference characters will be used to indicate like elements throughout the drawings. The vaporization apparatus  100  will also be referred to as a diffusion vaporizer. Although gas saturation occurs by vapor diffusion both in the present apparatus as well as the conventional bubbler of  FIG. 1 , the apparatus of  FIG. 2  does not involve bubble formation in a liquid to provide the surface area needed for vapor diffusion into the carrier gas. Instead, a porous metal with a wetted liquid surface is provided to generate vapor for diffusion into the interior of a carrier gas stream flowing along the porous metal. 
     The diffusion vaporizer  100  includes a tubular metal housing  110 , which is connected to a metal piece  120  with an inlet  122  and an outlet  124  for the carrier gas to enter and the gas/vapor mixture to exit. A porous metal piece  126  is positioned inside the metal piece  120 . A porous metal tube  130  is welded to the bottom of the metal piece  120 . All these metal pieces (including the housing  110 , metal piece  120 , porous metal piece  126  and porous metal tube  130 ) are in close thermal contact with one another and are at substantially the same temperature. 
     An electric heater  112  is clamped around the housing  110  and is in close thermal contact with the housing. When electrical power is applied to the heater  112 , the entire vaporization apparatus  100  including the tubular housing and all components inside become heated to substantially the same temperature. In some applications, when a volatile liquid is to be vaporized, it may be necessary to cool the vaporizer to a temperature below the ambient room temperature. In which case, a thermoelectric cooler (not shown) can be used in place of the electric heater  112  to cool the vaporizer to the desired lower temperature for liquid vaporization and gas saturation. 
     The tubular metal housing  110  contains liquid  140  to be vaporized. The liquid is in contact with porous metal tube  130  near the base of the apparatus  100 . By virtue of capillary surface tension, the liquid is drawn automatically into the interstitial pore space of the porous metal that forms the wall of tube  130 , rising above the free liquid surface  141 , and filling the wall of the porous metal tube  130  with liquid. Even though the liquid is in contact only with a small part of the porous metal tube near its bottom, the entire porous metal tube, including the section above the liquid surface becomes saturated and wet with liquid. As a result, liquid vaporizes from the entire wetted porous metal surface to generate vapor for gas saturation and subsequent film deposition. 
     The carrier gas enters the diffusion vaporizer at near ambient temperature through inlet  122 . The gas then flows through the interstitial pore space of porous metal piece  126 , which is distinct and separate from the porous metal tube  130 . Unlike porous metal tube  130 , porous metal piece  126  is dry and not saturated with liquid. As the dry gas flows through the dry interstitial pore space of porous metal piece  126 , the gas is quickly heated by the hot porous metal to substantially the same temperature as the porous metal. The heated gas stream flowing out of the porous metal piece  126  then flows downward within the interior space of the porous metal tube  130  for vapor saturation. 
     As the gas flows along the inside surface of porous metal tube  130 , vapor generated at the porous metal tube surface diffuses into the interior of the flowing gas stream causing the gas to become partially saturated with vapor. Vapor diffusion will continue as the gas stream continues its journey downward inside the porous metal tube  130 . As a result, the vapor concentration and partial pressure of the gas will increase. 
     Near the lower end of the porous metal tube  130 , there are several vertical slots  132  cut into the wall of the porous metal tube  130 . Upon reaching these slots, the gas flows through the slots into the surrounding annular space  143  between the tubular metal housing  110  and the porous metal tube  130  and begins to ascend through the annular space. As the gas flows in the upward direction through the annular space, vapor generated at the outside surface of the wetted porous metal tube  130  diffuses into the gas stream causing the concentration and the partial pressure of the vapor in the gas to continue to rise. Vapor diffusion into the gas will cease when the partial pressure of the vapor in the gas becomes equal to the saturation vapor pressure of the liquid at the temperature of the vaporizer. The gas then becomes saturated with vapor and exits the vaporizer through outlet  124  as a fully saturated gas stream. 
     In contrast to the conventional bubbler of  FIG. 1 , the diffusion vaporizer  100  described in this invention operates in a quiescent manner. There is no bubbling of gas through a liquid in order for the gas to become saturated with vapor. Vapor saturation occurs by virtue of vapor diffusion in a flowing gas stream in contact with a wetted porous metal surface with interstitial pore space filled and saturated with the precursor liquid. There is no bubbling of the liquid and no droplets are formed to cause the downstream components to become contaminated as in the conventional bubbler of  FIG. 1 . 
     For a specific vaporizer operating at a specific temperature, there is a certain maximum gas flow rate at which the gas will be fully saturated with vapor before its exit from the vaporizer. If the gas flow rate is below this maximum, the gas will be saturated with vapor. If the gas flow rate is higher than the maximum, the gas will be partially saturated and will emerge from the vaporizer as a partially saturated gas stream. The maximum gas flow rate for 100% vapor saturation depends on the vaporizer design, such as the diameter and the length of the porous metal tube  130 , the insider diameter of metal housing  110 , the operating temperature and pressure of the vaporizer, and the thermo-physical properties of the liquid and vapor as well as those of the gas. 
     Although only a small amount of liquid  140  is contained in the vaporizer, this liquid can be replenished in the manner shown in  FIG. 3 . An external reservoir  150  containing a larger volume of the liquid is connected to the vaporizer through tube  152 . Connected to the reservoir  150  is an external liquid source  162  containing liquid under pressure. This external pressurized liquid source  162  is connected to reservoir  150  through tube  164  and solenoid valve  160 . Mounted on the reservoir  150  are two level sensors,  154  and  156 , with sensing surfaces placed inside reservoir  150 . The valve  160  will open when the liquid level inside the reservoir  150  drops to the sensing level of the sensor  156 . When the valve  160  is open, liquid will then flow from the external pressurized liquid source through the connecting tubing  166  and  164  to the reservoir  150 . When the liquid level rises to the sensing level of sensor  154 , the valve  160  will close. Since vaporizer  100  and the reservoir  150  are connected through the tube  152 , the liquid level inside the vaporizer  100  will also rise and fall as the liquid level in the reservoir  150  rises and falls. By this means, the liquid level inside the vaporizer  100  is controlled by the two level sensors to within their respective sensing levels. To insure that the gas pressure inside the vaporizer and that inside the reservoir  150  are the same, a small capillary tube  158  is connected as shown to equalize the gas pressure between the vaporizer  100  and reservoir  150  to allow the liquid level in the vaporizer and in the sensor reservoir to be substantially the same. 
     To supply carrier gas at the desired flow rate into the vaporizer  100 , a gas flow controller  170  is provided. The gas flow controller  170  receives pressurized gas from the pressurized gas source  172  and delivers the gas at the required flow rate to the vaporizer  100  through its inlet  122 . To control the temperature of the vaporizer to a specific set point value, a temperature sensor  114  is placed in close thermal contact with the vaporizer  100 . For automatic control of the vaporizer, an electronic controller  180  is provided. Controller  180  has a multitude of input signal lines, shown generally at  182  and a multitude of output lines shown generally at  184 , to provide the required control signal to various parts of the system that need to be controlled. The signal from the temperature sensor, the level sensors, and flow sensor in the flow controller are all connected to the electronic controller via its input signal lines. The appropriate control signals are then applied to the heater, the solenoid valve  160  and the flow controller to allow the temperature, the liquid level and the carrier gas flow rate to be controlled to their respective set point values. 
     The carrier gas and the gas/vapor mixture formed by using the present invention is characterized by being substantially particle free. By particle free is meant that the gas is not an aerosol that contains particulate matter carried by the gas or suspended within the gas. Such particulate matter in the carrier gas may cause condensation or droplet formation of the vaporized liquid which would not be useful for thin film deposition. Small particles can also deposit on the substrate causing defects to form in the film, resulting in impure films or films of inferior quality that is undesirable for thin film deposition for semiconductor device fabrication. 
     The gas/vapor mixture generated by the apparatus and method described above can be introduced into the deposition chamber  190  containing the substrate  192  on which thin film is to be deposited as shown in  FIG. 3 . As the gas/vapor mixture enters the chamber, it encounters the substrate, which is held at a suitable temperature for film deposition. As a result, a thin film is deposited on the wafer. The gas/vapor mixture then exits the chamber to an external vacuum pump (also not shown). 
       FIG. 4  shows a new improved bubbler. The bubbler, shown generally at  200 , includes an inlet  212  for the carrier gas to enter the bubbler and an outlet  224  for the gas/vapor mixture to exit the bubbler. Upon entering the inlet through  212 , the carrier gas flows down tube  214 , then out of tube outlet  216  to form a stream of bubbles  218  in the surrounding liquid. The bubbles then rise through the liquid. Upon reaching the liquid surface  226 , the bubbles burst to form droplets. Dissolved impurities in the liquid appear as residue particles  220 , when the droplets vaporize in the carrier gas. These particles are suspended in the carrier gas/vapor mixture. When the mixture flows through the porous metal filter  222 , the particles are removed, producing a pure gas/vapor mixture that is substantially free of particulate contaminants as it leaves the bubbler through exit  224 . 
     To provide a constant degree of vapor saturation in the carrier gas, the liquid level in the bubbler  200  is controlled. A sensor reservoir  230  contains a level sensor  232  that detects the liquid level in the reservoir by contact or non-contact level sensing techniques. The sensor reservoir  230  is connected to the bubbler through tubes  234  and  236  to equalize the gas and liquid pressure in the bubbler with those in the sensor reservoir  230 . The liquid level  238  in the sensor reservoir  230  is thus the same as the liquid level of liquid  226  in the bubbler. Alternatively, the sensor itself can be incorporated into the bubbler, thereby eliminating the need for a separate reservoir for level sensing. Upon detecting a liquid level below a desired set-point, the sensor provides an electrical output to actuate a valve  240  to allow liquid in an external liquid reservoir  250  to flow through the valve  240  and the tube  234  into the bubbler. When the liquid level reaches the desired set-point, the valve  240  is closed. 
     The external liquid reservoir  250  contains a supply of liquid sufficient to operate the bubbler for a period of time for example a day, a week or a month. The liquid in the supply reservoir is pressurized by compressed gas applied to inlet  252 . The gas pressure is regulated by a pressure regulator (not shown) to a constant but adjustable value. The liquid in the reservoir  250  can thus be set to a specific constant pressure, the pressure being sufficient to cause the liquid to flow from the reservoir through the valve  240  and the tube  234  into the bubbler  200  when the valve  240  is open. 
     It is common practice in the semiconductor industry to use the liquid reservoir described in U.S. Pat. No. 5,288,325 or as illustrated in  FIG. 1  as a bubbler. As the liquid is consumed due to vaporization, the level within the prior art bubbler falls. When the level has fallen to a low level, the reservoir (bubbler) must be removed from the system and refilled. Alternatively, a new reservoir (bubbler) containing a fresh supply of liquid can be used to replace the one that has been exhausted. Refilling and replacing the prior art bubbler requires the system to be shut down leading to the loss of production during the shut-down period. The bubbler and the liquid supply scheme shown in  FIG. 4  allows the bubbler  200  to continue to operate for vapor saturation and semiconductor thin film deposition, while the external liquid supply reservoir  250  is being replenished or replaced. 
     In addition, since the liquid level in the bubbler  200  is controlled to a constant or near constant level, the degree of vapor saturation in the carrier gas is also precisely controlled, leading to improved rate of film formation and more uniform film thickness being produced by the film deposition process. In a conventional reservoir bubbler shown in  FIG. 1 , the liquid level continues to fall during operation, leading to variation in vapor saturation in the carrier gas, especially at high carrier gas flow rates, leading to variable film deposition rate, as well as non-uniform film thickness. 
     The present invention also includes a bubbler generally indicated at  200  in  FIG. 4 . The bubbler  200  is typically made of stainless steel. The bubbler is typically slender in shape and small approximately ⅓ the size of the reservoir bubbler shown in  FIG. 1 . The small size of bubbler  200  and its low mass would make the bubbler easier to heat or cool quickly to the desired operating temperature. The amount of electrical power needed for heating or cooling is also reduced when compared to the conventional reservoir bubbler. The small size, quick thermal response, and low power consumption leads to cost and size savings that can be considerable since the production system used for film deposition in which the bubbler  200  is used is usually placed in a clean room, which are quite expensive to build, when the system size is large. 
     To vaporize liquid of a low volatility, the bubbler  200  usually must be heated to a high enough temperature to generate sufficient vapor pressure for film deposition. An electrical band heater  228  is clamped tightly around the bubbler as shown in  FIG. 4  to provide the desired heating power to the bubbler. A thermocouple temperature sensor is usually included to provide temperature feed back for bubbler temperature control. Both the temperature sensor and feed back controller are common industrial devices and are not shown in  FIG. 4 . 
     When the liquid to be vaporized is volatile, the bubbler may need to be controlled to a temperature that is near the ambient temperature of the room in which the bubbler  200  is located. In some cases, the bubbler  200  must be cooled to below room temperature. When heating and cooling are needed a thermoelectric module  242  can be used. The thermoelectric module is also referred to as a Peltier module. The thermoelectric module  242  provides both cooling and heating by simply reversing the polarity of the DC voltage applied to the module. 
     When heating and cooling are needed, a conductive metal piece is substituted for the heater  228 . The conductive piece of metal is tightly clamped around the bubbler  200  to provide thermal conductive contact. The metal piece is usually made of aluminum because of the low density of the aluminum, its low thermal mass, and its high thermal conductivity. The metal piece is then thermally coupled to the thermoelectric module  242  as shown to provide the needed heating and/or cooling to control the bubbler  200  at near room temperature or below room temperature. 
       FIG. 5  shows a vaporizer  300  having a single bubbler  302  using two small diameter metal tubes  364  and  366  for bubble generation and carrier gas saturation. The carrier gas, upon entering the inlet  312 , flows to a small chamber  362  to which the two small diameter tubes  364  and  366  are attached. The gas then flows through both tubes and forms two streams of bubbles in the surrounding liquid  340 . The gas/vapor mixture from both tubes then flows through the porous metal filter  322  and exits the vaporizer  300  through outlet  320 . The overall capacity of the bubbler  302  for vapor saturation in a carrier gas can thus be doubled. In principle, as many tubes as needed can be used to increase the carrier gas flow capacity of the bubbler  302  to a high value. For simplicity, such a multi-tube bubbler having more than two tubes is not shown. Only two tubes are shown in the design of the bubbler  302  in  FIG. 8 . 
     The same approach described above to increase the capacity of the bubbler can also be used to increase the capacity of the Diffusion Vaporizer  100  shown in  FIG. 2 . 
       FIG. 6  shows a Multi-Tube Diffusion Vaporizer  400  in which only two porous metal diffusion tubes  430  and  431  are shown for clarity. The carrier gas enters the Multi-Tube Diffusion Vaporizer  400  through inlet  422 . The carrier gas enters chamber  423 , which is provided with a porous metal piece  426 . As the gas flows through the porous metal piece  426 , it is heated to nearly the same temperature as the surrounding metal. The heated gas flow then enters the two porous-metal tubes  430  and  431 . The porous metal tubes are partly immersed in liquid  440  near the bottom of the vaporizer  400  and the tubes  430 ,  437  are saturated with liquid in their interstitial pore spaces. As the carrier gas flows along the inside and outside surfaces of the porous metal tubes, the carrier gas becomes saturated with vapor by gas-phase vapor diffusion into the gas. The carrier gas then becomes saturated with the precursor vapor before exiting the Multi-Tube Diffusion Vaporizer through outlet  424 . Even though only two porous metal diffusion tubes are shown, more than two tubes can be used to further increase the capacity of the diffusion vaporizer. 
     A dual-channel vaporizer is shown generally at  500  in  FIG. 7 . The dual-channel vaporizer  500  includes a metal block  520  in which two vaporizers  510  and  512  are positioned. The vaporizers can be either the porous metal diffusion type described with respect to  FIGS. 2 and 3 , or be the bubbler type in operating principle as described with reference to  FIGS. 4 and 5 . A sectional view A-A through the metal block  520  is shown in  FIG. 8 . The metal block is usually made of a light weight metal such as aluminum. There are two cylindrical cavities,  530  and  532 , in the metal block. The block itself is split into two halves so that the metal can be tightly clamped around the cylindrical shaped vaporizers  510  and  512  to provide good thermal contact between the vaporizers  510  and  512  and metal block  520 . A thermoelectric module  540 , i.e. a Peltier module, in close thermal contact with the metal block  520  provides heating or cooling as may be needed to control the temperature of the metal block  520 , hence that of the vaporizers  530  and  532 , to a desired value. For temperatures near room temperature or below room temperature, the thermoelectric heating and cooling module  540  must be used in order to achieve precision temperature control. The designs illustrated in  FIGS. 7 and 8  make it possible to use a single thermoelectric module  540  to provide precision temperature control for two separate independent vaporizers both operating at the same or substantially the same temperature. 
     If the vaporizer is to be controlled to a temperature that is sufficiently high compared to the ambient room temperature, a simple electric heater will suffice. The approach shown in this invention, as illustrated in  FIGS. 7 and 8  can also be used for heating, in which case  540  will become a simple electric heater, rather than a thermoelectric heating and cooling module. 
     The above approach to using a single thermoelectric module, or heater, as the case may be, to control two separate vaporizers to the same temperature can also be used to control a multitude of vaporizers, i.e. more than just two vaporizers, to the same temperature. For instance, to control four vaporizers with the same thermoelectric module, it is necessary only to provide four cylindrical cavities in metal block  520  to accommodate the four cylindrical shaped vaporizers. Again, the vaporizers can be the diffusion type, the bubbler type, or a combination of both. The possibilities of such combinations will become clear to those who have read and understood this invention and will not be further described. 
     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.