Abstract:
A system for indicating an amount of a process liquid contained within an interior of an ampoule. A first conduit is in selective fluid communication with the interior of the ampoule, and has a first opening configured for disposal below an upper surface of the process liquid. The first conduit introduces a carrier gas into the interior of the ampoule. A second conduit is also in selective fluid communication with the interior of the ampoule, and has a second opening configured for disposal above the upper surface of the process liquid. The second conduit receives the carrier gas from the interior of the ampoule. A pressure differential sensor is disposed between and is in selective fluid communication with the first conduit and the second conduit. The pressure differential sensor senses a pressure differential between the first conduit and the second conduit. An indicator indicates the amount of the process liquid in the ampoule, based at least in part upon the pressure differential between the first conduit and the second conduit. Thus, the system as described above provides selectively continuous monitoring of the amount of the process liquid that is in the ampoule. Furthermore, the system monitors the amount in real time, and is relatively inexpensive to implement in a new chemical vapor deposition system, or to add to an existing chemical vapor deposition system.

Description:
FIELD 
     This invention relates to the field of process control. More particularly, the invention relates to monitoring the height of a liquid in a container as the fluid is used during a process step of manufacturing an integrated circuit. 
     BACKGROUND 
     In processing integrated circuits, such as semiconductor devices, titanium nitride is often deposited by metal organic chemical vapor deposition (MOCVD) to achieve highly conformal step coverage, such as in high aspect ratio vias and contacts. Typically, the deposition process employs tetrakis-dimethylamino titanium (TDMAT) as the precursor chemical. To transfer TDMAT into the deposition chamber, a carrier gas, such as helium, is bubbled through liquid phase TDMAT that is provided in a heated ampoule. The carrier gas conveys the TDMAT in the vapor phase to the deposition chamber. 
     During processing, care is typically used to ensure that the TDMAT ampoule does not run empty or below a minimum threshold amount, such as a minimum height level within the ampoule. If the TDMAT liquid in the ampoule drops below the minimum threshold amount, the desired thickness of titanium nitride may not be deposited during the MOCVD process, typically resulting in a significant reduction in process yield. 
     Several approaches to monitoring TDMAT levels have been attempted, including (1) estimating the TDMAT usage based on a processed substrate count, (2) using a pressurizing test to determine the empty volume in the ampoule, (3) using discrete level sensors in the ampoule, which sense an object floating on the liquid and detect the location of the floating object as it passes by discrete points, and (4) using sonic level sensors in the ampoule. However, each of these prior methods have significant disadvantages which limit their effectiveness and applicability. For example, the first method provides an inaccurate estimation, the second method cannot be accomplished in real-time and is too time-consuming and obtrusive, and the third and fourth methods are cost-prohibitive. 
     What is needed, therefore, is a system for real-time, continuous, unobtrusive, and inexpensive monitoring of a process liquid level in an ampoule. 
     SUMMARY 
     The above and other needs are met by a system for indicating an amount of a process liquid contained within an interior of an ampoule. A first conduit is in selective fluid communication with the interior of the ampoule, and has a first opening configured for disposal below an upper surface of the process liquid. The first conduit introduces a carrier gas into the interior of the ampoule. A second conduit is also in selective fluid communication with the interior of the ampoule, and has a second opening configured for disposal above the upper surface of the process liquid. The second conduit receives the carrier gas from the interior of the ampoule. 
     A pressure differential sensor is disposed between and is in selective fluid communication with the first conduit and the second conduit. The pressure differential sensor senses a pressure differential between the first conduit and the second conduit. An indicator indicates the amount of the process liquid in the ampoule, based at least in part upon the pressure differential between the first conduit and the second conduit. 
     Thus, the system as described above provides selectively continuous monitoring of the amount of the process liquid that is in the ampoule. Furthermore, the system monitors the amount in real time, and is relatively inexpensive to implement in a new chemical vapor deposition system, or to add to an existing chemical vapor deposition system. 
     In various preferred embodiments of the invention, the amount of the process liquid in the ampoule is expressed as a height of the process liquid, such as a height from the bottom of the ampoule to the upper surface of the process liquid, or a height from the first opening of the first conduit to the upper surface of the process liquid. The pressure differential sensor is preferably a manometer, and most preferably a U tube manometer containing manometric fluid, and having a first arm in fluid communication with the first conduit and a second arm in fluid communication with the second conduit. 
     In one preferred embodiment, graduated indicia on at least one of the first arm and the second arm of the U tube manometer indicate a difference in height of the manometric fluid in the first arm and the second arm. A conversion table indicates a height of the process liquid in the ampoule based at least in part upon the difference in height of manometric fluid in the first arm and the second arm. Alternately, the graduated indicia on at least one of the first arm and the second arm of the U tube manometer directly indicate a height of the process liquid in the ampoule, based at least in part upon the difference in height of manometric fluid in the first arm and the second arm. 
     In an especially preferred embodiment, the indication of the amount of the process liquid in the ampoule is based at least in part on:          h   =         Δ                 P     -   C       ρ   ×   g         ,                          
     where 
     h is a height of the process liquid in the ampoule, 
     ΔP is the pressure differential between the first conduit and the second conduit, 
     C is a constant based at least in part on a configuration of the ampoule, 
     ρ is a density of the process liquid, and 
     g is an acceleration due to gravity. 
     According to other aspects of the invention there is provided a chemical vapor deposition system including the system for indicating an amount of a process liquid as described above, and a method for determining an amount of a process liquid in an ampoule. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
     FIG. 1 depicts a liquid height measurement system according to a preferred embodiment of the present invention, 
     FIG. 2 depicts a liquid height measurement system according to a most preferred embodiment of the present invention, 
     FIG. 3 depicts a liquid height measurement system according to an alternative embodiment of the present invention, and 
     FIG. 4 depicts a liquid height measurement system according to another alternative preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, there is depicted a process liquid delivery system  10 , such as is used in integrated circuit processing, such as may be part of a metal-organic chemical vapor deposition (MOCVD) system for depositing a layer of material on a substrate. The apparatus  10  includes an ampoule  14  containing a process liquid  16 , such as liquid tetrakis-dimethylamino titanium (TDMAT). Preferably, the amount of TDMAT within the ampoule  14  at any given time during the deposition process is determined based on the height h of the process liquid  16 . As shown in FIG. 1, the height h is in one embodiment measured from the bottom surface  14   a  of the interior of the ampoule  14  to the top surface  16   a  of the process liquid  16 . 
     In preferred embodiments of the invention, two fluid conduits provide selective fluid communication to the interior of the ampoule  14 . These include an first input conduit  17  and a second output conduit  22 . An ampoule bypass line  28  and valve V 1  are provided between the input and output conduits  17  and  22 . As depicted in FIG. 1, one end of the input conduit  17  has an opening  17   a  disposed below an upper surface  16   a  of the process liquid  16 , such as adjacent the bottom interior surface  14   a  of the ampoule  14 . The height h is in one embodiment measured from the opening  17   a  of the input conduit  17  to the top surface  16   a  of the process liquid  16 . The other end of the input conduit  17  is preferably connected to a carrier gas line  18  via a valve V 2 . The carrier gas line  18  is connected to a carrier gas source  20  via a valve V 3 . In preferred embodiments of the invention, the carrier gas is helium. 
     With the valves V 2  and V 3  open (and V 1  closed), a positive pressure P i  of carrier gas is provided in the input conduit  17 , thereby causing carrier gas to emanate from the first opening  17   a  and bubble through the process liquid  16  in the ampoule  14 . As the carrier gas bubbles through the process liquid  16 , it picks up an amount of the process liquid  16  which is transferred in the vapor phase by the carrier gas into the portion of the interior of the ampoule  14  above the upper surface  16   a  of the process liquid  16 , thereby creating a positive pressure P o  within the output conduit  22 . Due to forces such as frictional flow losses and the static head pressure of the process liquid  16 , there is a pressure differential ΔP between the input pressure P i  and the output pressure P o . The process liquid vapor and the carrier gas flow into an opening  22   a  in the output conduit  22  that is disposed above the upper surface  16   a  of the process liquid  16 . The process liquid vapor and carrier gas are transferred through the output conduit  22  and a valve V 4  into a line  24 , such as one connected to an MOCVD processing chamber. 
     As depicted in FIG. 1, the apparatus  10  preferably includes diluent gas sources  30 ,  32 , and  34  connected through valves V 5 , V 6 , V 7 , and V 8  to a diluent line  26 . In preferred embodiments, the diluent gas sources  30 ,  32 , and  34  provide a mixture of nitrogen, hydrogen, and helium diluent gases to the MOCVD processing chamber via the lines  26  and  24 . 
     It is appreciated that a preferred objective of the process liquid vapor delivery system as described herein is to ensure that a known and sufficient amount of the process liquid vapor is conducted to the processing chamber by the carrier gas. For example, if the carrier gas is at all times substantially completely saturated with the process liquid vapor at a reasonably known temperature and pressure, then the amount of the process liquid  16  delivered to the processing chamber can be determined based on the flow rate of the carrier gas and the length of time for which it flows. This in turn can be empirically related to a thickness of a deposited layer within the processing chamber. 
     However, if a variable changes in the system, such as if the carrier gas is not substantially completely saturated with the process liquid vapor, then the empirical relation to the thickness of the deposited layer is no longer valid. One of the variables that can effect whether the carrier gas is substantially completely saturated with the process liquid vapor is the height of the process liquid  16  above the outlet  17   a  of the inlet conduit  17 . This height effects parameters such as the residence time of the carrier gas in the process liquid  16 , which in large measure tends to determine how saturated the carrier gas will be with the process liquid vapor. 
     The height of the upper surface  16   a  of the process liquid  16  in the ampoule  14  may be accounted for in a variety of ways. For example, the height may be directly determined as a distance value from the upper surface  16   a  of the process liquid  16  to either the bottom  14   a  of the ampoule  14  or to the outlet  17   a  of the inlet conduit  17 . It is appreciated that the distance from the outlet  17   a  of the inlet conduit  17  is the distance that most directly has an effect on the saturation of the carrier gas. However, with a knowledge of the distance of the outlet  17   a  of the inlet conduit  17  from the bottom  14   a  of the ampoule  14 , the length of travel of the carrier gas through the process liquid  16  can be determined. 
     Further, the degree of saturation of the carrier gas can be empirically related to other properties associated with the system  10  as well. For example, with a knowledge of the volume, height, diameter, and other such physical properties of the ampoule  14 , the weight of the process liquid  16  within the ampoule  14  can be empirically related to the degree of saturation of the carrier gas. Thus, for example, when the weight of the ampoule  14  drops below a certain previously calculated value, it is an indication that there is not a sufficient amount of the process liquid  16  above the outlet  17   a  of the inlet conduit  17  for the carrier gas to substantially completely saturate with the process liquid  16 , and that the deposition properties within the processing chamber will therefore start to change. 
     Therefore, although the height of the upper surface  16   a  of the process liquid  16  above the outlet  17   a  of the inlet conduit  17  is the parameter this is preferably ultimately of interest in the present invention, it is appreciated that this height is related to an amount of the process liquid  16  within the ampoule  14 , and that this amount of the process liquid  16  may be empirically related to a large number of other measurable parameters. Therefore, although the specific examples below are described in direct relation to the height of the process liquid  16 , it is further appreciated that the discussion could also be phrased in terms of other parameters related to the process liquid  16 , which parameters relate, either directly or empirically, to the height of the process liquid  16 . 
     To ensure that a sufficient amount of the process liquid vapor is transferred to the MOCVD processing chamber, the amount of the process liquid  16 , such as measured by the height h, is preferably maintained above a minimum threshold level. As described in more detail hereinafter, the invention provides for determining the height h of the process liquid  16  based upon the pressure differential ΔP between the input pressure P i  in the input conduit  17  and the output pressure P o  in the output conduit  22 . 
     To determine the pressure differential ΔP, the invention of FIG. 1 includes a pressure differential sensor  35 . To indicate the height h of the process liquid  16  based upon the pressure differential ΔP, the invention of FIG. 1 includes an indicator  37 . Various embodiments of the pressure differential sensor  35  and the indicator  37  are discussed hereinafter. 
     In a preferred embodiment of the invention, the pressure differential sensor  35  comprises a U-tube manometer  36  having a first arm  36   a  coupled to the first conduit  17  and a second arm  36   b  coupled to the second conduit  22 , as depicted in FIG.  2 . Within the U-tube manometer  36  is manometric fluid  40 , preferably having a mass density greater than that of the carrier gas, and which is immiscible in the carrier gas. If the mass density ρ man  of the manometric fluid  40  is much greater than the mass density ρ c  of the carrier gas (ρ man &gt;&gt;ρ c ), the pressure differential ΔP may be expressed as: 
     
       
         Δ P=P   i   −P   o =( h   i   −h   o )×ρ man   ×g,   (1) 
       
     
     where, as shown in FIG. 2, h i −h o  is the difference in the levels of the manometric fluid  40  in the two arms  36   a  and  36   b  of the U-tube manometer  36 , and g is acceleration due to gravity. 
     The height h of the process liquid  16  in the ampoule  14  is related to the pressure differential ΔP according to:                h   =         Δ                 P     -   C       ρ   ×   g         ,           (   2   )                                
     where ρ is the mass density of the process liquid and C is a constant based on the configuration of the system, and which preferably accounts for factors such as flow friction. 
     Combining equations (1) and (2), the height h of the process liquid  16  in the ampoule  14  may be determined according to:              h   =             (       h   i     -     h   o       )     ×     ρ   man       -     C   g       ρ     .             (   3   )                                
     As depicted in FIG. 2, the two arms  36   a  and  36   b  of the U-tube manometer  36  are preferably transparent to allow observation of the manometric fluid  40 , and each arm  36   a  and  36   b  includes graduated indicia  42  which provide a reference for determining the difference in height (h i −h o ) in each arm. The fluid height indicator  37  of this embodiment is preferably in the form of a conversion table  38 . The conversion table  38 , which may be in the form of a printed or a computerized look-up table, relates several values of the height difference (h i −h o ) to corresponding values of the height h, according to the relationship of equation (3). Using the conversion table  38 , the operator may determine the height h of the process liquid  16  in the ampoule  14  based on the observed height difference (h i −h o ). 
     However, and as introduced above, it may not be essential to directly determine the height of the process liquid  16 , as the degree of saturation of the carrier gas can be empirically related to other parameters. For example, the degree of saturation of the carrier gas can be related directly to a difference of the levels of the manometric fluid  40  within the two arms  36   a  and  36   b  of the manometer  36 . In this embodiment, it may be determined that when the difference between the levels of the manometric fluid  40  drops down to a given minimum value, it is an indication that there is not a sufficient height of the process liquid  16  above the outlet  17   a  of the inlet tube  17  for the carrier gas to be sufficiently saturated with the process liquid  16 , and that the deposition conditions within the processing chamber will start to change, and that therefore more process liquid  16  should be added to the ampoule  14  before another deposition process is commenced. 
     One skilled in the art will appreciate that the U-tube manometer  36  may be any one of various known designs, such as a tilted-arm manometer, or a manometer in which one arm has a much larger cross-sectional area than the other arm. 
     In an alternative embodiment of the invention, the pressure differential sensor  35  comprises a digital manometer  39 , as depicted in FIG.  3 . The digital manometer  39 , such as one of the HHP-2000 series manometers manufactured by Omega Engineering, Inc., includes a first input  39   a  coupled to the first conduit  17  and a second input  39   b  coupled to the second conduit  22 . The digital manometer  39  preferably senses the pressure differential ΔP and provides a digital readout of the pressure differential ΔP, such as on an integrated display screen. Based on the digital readout, the operator may determine the height h of the process liquid  16  in the ampoule  14  based on the relationship of equation (2). Alternately, the pressure differential ΔP sensed by the digital manometer  39  may be downloaded to a processor  44  which calculates the height h based on equation (2). The calculated value of the height h may then be displayed on a display device  46 , such as a computer monitor. Preferably, if the height h drops below a minimum threshold value, the processor  44  generates an alarm indication which is visually and audibly presented to the operator. In this embodiment, the processor  44  and the display device  46  together comprise the fluid height indicator  37 . 
     In yet another alternative embodiment of the invention, the pressure differential sensor  35  comprises a pair of pressure sensors  41   a  and  41   b,  as depicted in FIG.  4 . The pressure sensors  41   a  and  41   b  generate pressure signals which are conditioned by an instrumentation interface  48  and provided to a processor  44 . Based on the pressure signals, the processor  44  determines tee pressure differential ΔP, and calculates the height h based on equation (2). As described in the previous embodiment, the calculated value of the height h may be displayed on a display device  46 , such as a computer monitor. With this embodiment, care must be taken to properly calibrate the pressure sensors  41   a  and  41   b  based on a reference pressure value before determining the pressure differential ΔP. In this embodiment, the instrumentation interface  48 , the processor  44 , and the display device  46  together comprise the fluid height indicator  37 . 
     As mentioned above, it is appreciated that these various devices need not calculate the height of the process liquid  16 . For example, the value of the pressure differential ΔP may be directly used, and when the value drops to a predetermined minimum, then the process liquid  16  within the ampoule  14  is replenished. 
     It is further appreciated that the various embodiments of the invention as described herein may be used to determine the height of practically any liquid through which a gas is being bubbled. Further, the invention is not limited to determining the height of a liquid. The invention is also applicable to determining the height of a high-density gas through which a lower-density gas is being bubbled. 
     The foregoing description of preferred embodiments for this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as is suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.