Abstract:
In order to evaporate a liquid substance ( 3 ) and convey it to a user, a bubbler ( 1 ) is provided, into which a carrier gas ( 7 ) is conveyed. When a minimum level ( 4 ) has been reached, an exact quantity of the substance ( 3 ) is refilled from a reservoir tank ( 15 ). For this purpose, an intermediate tank ( 25 ) is provided, which is filled with an inert gas up to a pressure P 1 . An inert gas ( 21 ) is applied to the reservoir tank ( 15 ) at a pressure P 2 , higher than P 1 . The intermediate tank ( 25 ) is connected with the reservoir tank ( 15 ), as a result of which a specific quantity of the substance ( 3 ) flows into the intermediate tank ( 25 ) and is conveyed via a connection line ( 17 ) to the bubbler ( 1 ), with inert gas ( 32 ) first having been applied to the connection line ( 17 ) at a pressure P 3 , lower than P 2.

Description:
FIELD OF THE INVENTION 
   The invention relates to an apparatus and process for refilling a bubbler used to convey a carrier gas into a liquid substance in order to evaporate the substance and supply it to a user, according to the generic part of claims  1  and  9 . 
   BACKGROUND OF THE INVENTION 
   Such an apparatus and process is already known. The user may be, for example, a reactor for chemical vapor deposition (CVD). Such CVD reactors are used in the manufacture of integrated microcircuits and similar electronic components. To meet the increasingly rigorous requirements for such components, substances of very high purity must be used for the chemical vapor deposition process. The substances involved are usually organic metallic and semi-metallic compounds in liquid form. 
   In addition to the requirement for very high purity, it is also important to ensure precise compliance with the specified gas mixture composition in the CVD reactor, and therefore a consistent quantity of the substance conveyed to the CVD reactor. In order to prevent changes in the composition due to condensation of the evaporated substance in the line to the CVD reactor, it is necessary to place the bubbler as close as possible to the reactor. The bubbler therefore has to have a small volume, which means that it has to be refilled frequently. For this purpose, in the known apparatus the liquid substance is forced out of the reservoir tank into the bubbler with an inert gas. 
   The quantity of the substance conveyed to the CVD reactor depends on the distance traversed by the carrier gas bubbles in the liquid substance as they rise through the liquid substance. Because of the evaporation of the liquid substance in the bubbler, this distance becomes progressively shorter, and the quantity of evaporated substance in the carrier gas progressively diminishes. In the known apparatus, the resulting variations in concentration are kept as low as possible by providing level sensors for the minimum level and maximum level, respectively. 
   Level sensors are, however, inherently relatively imprecise, and are not particularly reliable. The use of two level sensors is therefore associated with significant problems. In addition, the liquid substances used are often pyrophorous, and accordingly a discharge of such a substance from the bubbler due to a malfunction of the level sensor for the maximum level would represent a major hazard. 
   SUMMARY OF THE INVENTION 
   The objective of the invention is to provide a simple refilling system enabling operation precisely and reliably in accordance with the minimum and maximum levels in the bubbler. 
   This is achieved according to the invention with the apparatus described in the claims. The claims provide advantageous embodiments of the apparatus according to the invention. 
   The claims also describes a preferred process for the refilling of the bubbler according to the invention. 
   According to the invention, a precisely defined quantity of the liquid substance is conveyed to the bubbler during the refilling operation. This precise definition is governed solely by pressure measurement. Since there are commercially available pressure sensors accurate to within the per mil range, the level in the bubbler can be precisely set according to the invention. 
   The carrier gas used for the evaporation of the liquid substance in the bubbler is an inert gas, for example a noble gas such as argon. The inert gas conveyed to the reservoir tank, intermediate tank, and connection line between the reservoir tank and bubbler via the inert gas line connected thereto can also be a noble gas such as argon. 
   The level sensor used according to the invention to detect the minimum level can, for example, be a heat sensor, a float, or an ultrasonic or radar sensor, or similar. 
   Connected to the bubbler are the connection line and the supply line to the user. An immersion tube forming part of the carrier gas line is immersed in the liquid substance in the bubbler, to take up the evaporated liquid substance with the carrier gas bubbles rising through the liquid substance. 
   In order to provide a stable flow of the carrier gas with the evaporated substance to the user from the beginning of the process, a gas discharge line with a valve is also provided on the supply line, through which the carrier gas with the evaporated substance is first conveyed before the supply line to the user is opened. 
   Whereas the intermediate tank is preferably located close to the central reservoir tank, the bubbler is generally located close to the user, with the reservoir tank and intermediate tank usually being placed lower than the bubbler and the user. In a building, the connection line can be in the form of a riser, often several meters long, from the reservoir tank and intermediate tank to the bubbler. 
   Where several bubblers are to be filled using a central reservoir tank via the intermediate tank, the section of the connection line from the immersion tube in the reservoir tank to the inert gas line forms the main section of the connection line, from which individual lines in the form of risers lead to the respective bubblers. Between the inert gas line and each of the bubblers, there is then a valve in each individual line. The pressure regulator in the inert gas line can then be used to set pressures individually in the risers to the bubblers, which may, for example, be located on different floors. 
   In the apparatus according to the invention, when the minimum level has been detected by the minimum level sensor, the level sensor actuates the control device, which then performs the following steps by actuation of the appropriate valves:
     (1) The valve in the carrier gas line and the valve in the supply line are closed, and the bubbler is evacuated, e.g. via the gas discharge line or inert gas line connected to the connection line, to the vapor pressure of the liquid substance  3 . In the empty intermediate tank, a predefined pressure (P 1 ) is set via the inert gas line connected to the tank.   (2) A predefined pressure (P 2 ) is set in the reservoir tank via the inert gas line connected to the reservoir tank, that pressure being greater than the pressure (P 1 ) set in step (1) in the intermediate tank.   (3) The reservoir tank is connected with the intermediate tank in order to fill a quantity of liquid substance into the intermediate tank corresponding to the pressure difference (ΔP) between the pressure (P 2 ) in the reservoir tank and the pressure (P 1 ) in the intermediate tank×the volume of the intermediate tank according to Boyle/Mariott&#39;s law, P×V=const.   (4) The connection between the reservoir tank and the intermediate tank is severed. This means there is now a precisely defined quantity of the liquid substance in the intermediate tank.   (5) Via the inert gas line connected to the connection line, a pressure (P 3 ) is set in the section of the connection line between the junction line of the intermediate tank and the bubbler, and in the bubbler, that pressure being lower than the pressure reached in the intermediate tank in step (3).   (6) The intermediate tank and the bubbler are connected with one another in order to fill a quantity of the liquid substance into the bubbler corresponding to the pressure difference between the pressure (P 2 ) in the intermediate tank and the pressure (P 3 ) in the bubbler and the section of the connection line between the inert gas line and the bubbler minus the hydrostatic pressure from the difference in height between the intermediate tank and the bubbler. This means that the precisely predefined quantity in the intermediate tank has been transferred from the intermediate tank to the bubbler.   

   In order to empty the connection line, it is advantageous if the following steps are then also carried out:
     (7) The reservoir tank is evacuated, and   (8) The liquid substance in the connection line is sucked back into the evacuated reservoir tank.   

   Following the filling of the bubbler, carrier gas is conveyed into the liquid substance in the bubbler, such that the carrier with the evaporated liquid substance contained therein is first discharged via the gas discharge line until such time as a stable flow of the carrier gas with the evaporated substance has been reached, at which time the supply line to the user is opened. 
   The set-up according to the invention has the effect of significantly increasing safety. For example, the level sensor for the maximum level is replaced by pressure controllers or sensors, which are significantly more reliable and precise and also less expensive. Whereas level sensors are single-point-of-failure devices, according to the invention redundancy is provided, since in each of steps 1 to 6 there are two pressure sensors involved. In addition, the connection line is filled only for the duration of the refilling of the bubbler, and is otherwise product-free, i.e. for the majority of the time. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is now explained in more detail on the basis of examples, with reference to the drawing.  FIGS. 1 to 6  illustrate an embodiment of the apparatus in diagrammatic form according to the invention, with the individual steps for refilling the bubbler. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   According to  FIG. 1 , a bubbler  1  has a tank  2  which is filled with a liquid substance  3 , and which in  FIG. 1  has a minimum level  4 , detected by a level sensor  5 . 
   Immersed in the liquid substance  3  is an immersion tube  6 , through which a carrier gas is conveyed into the liquid substance  3  by means of a carrier gas line  7  with a valve  8 , for the evaporation of the liquid substance  3 . 
   The evaporated liquid substance  3  is conveyed via a supply line  9  to a user, not shown, e.g. a CVD reactor. The supply line  9  connected to the gas chamber  11  of the bubbler  1  has a valve  12 . Between the gas chamber  11  and the valve  12 , a gas discharge line  13  with a valve  14  is connected to the supply line  9 . 
   The liquid substance  3  for refilling the bubbler  1  is stored in a reservoir tank  15 . Immersed in the liquid substance  3  in the reservoir tank  15  is an immersion tube  16 , which is connected by a connection line  17  with the gas chamber  11  of the bubbler  1 , with a filling valve  18  being fitted on the end of the connection line  17  facing the bubbler  1 . 
   Connected to the gas chamber  19  of the reservoir tank  15  is an inert gas line  21  with a valve  22 . The reservoir tank  15  can also be evacuated via the inert gas line  21 . 
   On the end part of the connection line  17  facing the reservoir tank  15 , an intermediate tank  25  is connected to the connection line  17  via a junction line  23  with a valve  24 , with said intermediate tank having a significantly smaller volume than that of the reservoir tank  15 . The junction line  23  is connected at the base  26  of the intermediate tank  25 . An inert gas line  28  with a valve  29  is connected to the upper part of the intermediate tank  25 . 
   Between the junction line  23  and the immersion tube  16 , a further valve  31  is provided in the connection line  17 . In addition, between the junction line  23  and the filling valve  18 , an inert gas line  32  with valve  33  is connected to the connection line  17 , again on the end part of the connection line  17  facing the reservoir tank  5 . Another valve  34  is provided in the connection line  17  between the junction line  23  and the inert gas line  32 . 
   Pressure controllers  35 ,  36 , and  37 , respectively, are provided in the inert gas line connected to the gas chamber  19  of the reservoir tank  15 , in the inert gas line  28  connected to the intermediate tank  25 , and in the inert gas line  32  connected to the connection line  17 . 
   Through the connection of the junction line  23  to the base  26  of the intermediate tank  25 , the liquid substance is fully evacuated from the intermediate tank  25  if a sufficiently high inert gas pressure is applied to the intermediate tank  25 , as shown in  FIGS. 3 and 4  and described in greater detail below. 
   Just as a vacuum can be connected to the inert gas line  21  connected to the gas chamber  19  of the reservoir tank  15 , a similar vacuum connection can be made to the inert gas line  32  connected to the connection line  17 . A vacuum can also be applied to the gas discharge line  13 , in order to evacuate the connection line and the tank  2  of the bubbler  1 . 
   The section of the connection line  17  between the inert gas line  32  and the filling valve  18  is in the form of the riser  17 ′. In a situation where several bubblers  1  for several users are to be filled from the reservoir tank  15  with the intermediate tank  25 , the section of the connection line  17  from the immersion tube  16  to the inert gas line  32  forms the main section, from which individual lines  17 ′,  17 ″, and  17 ′″ then lead to the respective bubbler  1  in each case, as shown diagrammatically in  FIG. 1 . Lines  17 ′,  17 ″, and  17 ′″ each have valves  38 ′,  38 ″, and  38 ′″ between the inert gas line  32  and the respective bubbler  1 , located in the end parts of individual lines  17 ′,  17 ″, and  17 ′″, facing the inert gas line  32 . 
   The following example is provided, in combination with  FIGS. 1 to 6 , as further explanation of the invention; evacuated chambers in the tanks are shown in white, gas-filled chambers in gray, and chambers filled with liquid in a dark shade. It should also be noted that this example describes an apparatus with only one riser  17 ′, i.e. without valve  38 ′. 
   The tank  2  has a volume of e.g. 1,000 cm 3 . When the level of the liquid substance  3  in the bubbler  1  according to  FIG. 1  falls to the minimum level  4  of e.g. 500 cm 3 , the level sensor  5  actuates the control device C, which, in order to refill the bubbler  1  with the liquid substance  3 , actuates valves  8 ,  12 ,  14 ,  18 ,  22 ,  24 ,  29 ,  31 ,  33 , and  34  as follows (connections not shown for clarity): 
   1 st  Step ( FIG. 1 ) 
   Valves  8 ,  12 ,  22 ,  24 ,  29 ,  31 ,  33 , and  34  are closed and valves  14  and  18  opened, in order to evacuate the tank  2  of bubbler  1  via the gas discharge line  13  and the section  17 ′ of connection line  17  between the inert gas line  32  and the tank  2 . The intermediate tank  25 , which has a volume of e.g. 1,000 cm 3 , is empty, i.e. it does not contain any liquid substance  3 . Valve  29  is opened, and the intermediate tank  25  fills with inert gas until the pressure controller  36  displays a predefined pressure P 1 , of e.g. 200,000 Pa, in the intermediate tank  25 . Valve  29  is then closed. 
   2 nd  Step ( FIG. 2 ) 
   Valve  22  is opened, and inert gas is conveyed via the inert gas line  21  to the evacuated gas chamber  19  in the reservoir tank  15  until the pressure controller  35  displays a predefined pressure P 2 , of e.g. 400,000 Pa, in the reservoir tank  15 . 
   3 rd  Step ( FIG. 2 ) 
   Valves  24  and  31  are opened, in order to connect the reservoir tank  15  with the intermediate tank  25 . Liquid substance  3  then flows from the reservoir tank  15  into the intermediate tank  25 , until the pressure in the gas chamber  20  of the intermediate tank is P 2 , i.e. 400,000 Pa. In accordance with the equation P×V=const., at that time the intermediate tank  25  is filled with 500 cm 3  of the liquid substance  3 . 
   4 th  Step ( FIG. 2 ) 
   Valves  24  and  31  are closed, thereby severing the connection between the reservoir tank  15  and the intermediate tank  25 . The pressure in the gas chamber  19  of the reservoir tank  15  is reduced via valve  22 , and valve  22  is then closed. This concludes the phase of the transfer of exactly 500 ml of the liquid substance  3  from the reservoir tank  15  into the intermediate tank  25 . 
   5 th  Step ( FIG. 3 ) 
   Valves  18  and  33  are opened in order to fill the section  17 ′ of the connection line  17  between the inert gas line  32  and the bubbler  1 , and tank  2  of bubbler  1 , with inert gas, until the pressure controller  37  displays a predefined pressure P 3  of e.g. 50,000 Pa. Valve  33  is then closed. 
   6 th  Step ( FIG. 4 ) 
   The volume of the gas chamber  20  in the intermediate tank  25  is 500 cm 3 , at a pressure of 400,000 Pa. 
   The volume of the gas chamber in the riser section  17 ′ of the connection line  17  between the inert gas line  32  and the bubbler  1  is 250 cm 3 , and the volume of the gas chamber  11  of the tank  2  of the bubbler  1  is 500 cm 3 , i.e. the total volume of the riser  17 ′ and the gas chamber  11  is 750 cm 3 , at a pressure of 50,000 Pa. 
   Valves  24  and  34  are then opened. This means that 500 cm 3  of liquid substance  3  is transferred into the riser  17 ′ and the bubbler  1 , 250 cm 3  into the riser  17 ′, and 250 cm 3  into the bubbler  1 . The maximum level  38  in the bubbler  1  is then reached. Pressure equilibrium is reached if the emptied intermediate tank  25  with a gas chamber  20  of 1,000 cm 3  is 200,000 Pa, and the pressure in the gas chamber  11  of tank  2  is 200,000 Pa minus the hydrostatic pressure of the riser  17 ′, which is dependent on the height of the riser  17 ′ and the density of the liquid substance  3 ; the hydrostatic pressure with trimethyl aluminum as the liquid substance  3  and riser  17 ′ height of 7 m, for example, is 50,000 Pa. 
   7 th  Step ( FIG. 5 ) 
   Valve  22  is opened and the reservoir tank  15  is evacuated via the inert gas line  21  to the vapor pressure of the liquid substance  3 , which for trimethyl aluminum, for example, is 1.036 Pa. The valve  22  is then closed. 
   8 th  Step ( FIG. 6 ) 
   Valves  31  and  34  are opened, as a result of which the liquid substance contained in the riser  17 ′ (250 cm 3 ) is sucked back into the reservoir tank  15 . Valves  31  and  34  are then closed. 
   Whereas steps 1 to 4 transfer the liquid substance  3  from the reservoir tank  15  into the intermediate tank  25 , and steps 5 and 6 transfer the liquid substance  3  from the intermediate tank  25  into the bubbler  1 , the purpose of steps 7 and 8 is to empty the connection line  17  after completion of the refilling process. 
   Thus, with the apparatus according to the invention, there are no measurements to be captured. Nor is there any process signal to be obeyed for the termination of the refilling cycle. 
   Furthermore, with the apparatus according to the invention, the duration of a refilling cycle is short, e.g. in the above example the cycle duration is only approx. 1 minute.