Abstract:
A backfill prevention system for a gas flow conduit, comprising a gas flow monitor circuit which measures the rate and direction of gas flow through a gas flow conduit and converts the measured data into a voltage signal. A valve control circuit operably connected to a valve or valves in the gas flow conduit receives the voltage signal from the gas flow monitor circuit and closes the valve or valves in the event that the voltage signal indicates backflow of a gas through the gas flow conduit. The valve control conduit may further be provided with a first light emitting diode (LED) which is illuminated during normal flow of the gas through the conduit, and a second LED which is illuminated in the event of gas backflow through the conduit. The system is typically used in conjunction with a mass flow controller in the conduit.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to mass flow controllers which control the flow of process gases into a process chamber in the fabrication of integrated circuits on semiconductor wafers in the chamber. More particularly, the present invention relates to a backfill prevention system which may be operated in conjunction with a mass flow controller to measure flow of gas through a gas flow conduit and close a valve or valves in the conduit as needed to prevent gas backfilling of the conduit.  
         BACKGROUND OF THE INVENTION  
         [0002]    The fabrication of various solid state devices requires the use of planar substrates, or semiconductor wafers, on which integrated circuits are fabricated. The final number, or yield, of functional integrated circuits on a wafer at the end of the IC fabrication process is of utmost importance to semiconductor manufacturers, and increasing the yield of circuits on the wafer is the main goal of semiconductor fabrication. After packaging, the circuits on the wafers are tested, wherein non-functional dies are marked using an inking process and the functional dies on the wafer are separated and sold. IC fabricators increase the yield of dies on a wafer by exploiting economies of scale. Over 1000 dies may be formed on a single wafer which measures from six to twelve inches in diameter.  
           [0003]    Various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate. Many of the various processing steps, including but not limited to etching and chemical vapor deposition (CVD), used in the semiconductor fabrication process require process fluids or chemicals for the formation of integrated circuits on the wafer substrate.  
           [0004]    About 50 different types of gases are used in as many as 450 process steps in semiconductor manufacturing. Gases used in semiconductor fabrication are generally categorized as one of two types: bulk gases, which include oxygen, nitrogen, helium and argon; and specialty gases, which include chlorine and hydrogen chloride and are the process gases used to effect the circuit-fabricating chemical reactions on the semiconductor wafer substrate. Bulk gases, which include purge gases used to flush undesirable residual gases, atmospheric gases or water vapor from a process chamber, are stored in large storage tanks outside the wafer fab manufacturing area and are distributed into the proper workstation through a bulk gas distribution (BGD) system. Specialty gases are dispensed from cylinders in a gas cylinder cabinet containing a control panel. A local gas distribution system in the process area is used to deliver the gas from the cylinder to the chamber of the process tool.  
           [0005]    The molecular quantities of the reactant gases utilized in semiconductor fabrication processes are important for proper control of the reactions. According to the ideal gas law, the number of gas molecules contained in a given volume changes in proportion to to the absolute pressure and temperature. Therefore, a given volume of gas flowing into a process chamber yields various quantities of gas molecules depending on the temperature and pressure of the gas. Accordingly, mass flow controllers (MFCs), which utilize a thermal sensor that senses the heat-transfer property of a gas to detect changes in the mass flow of the gas, are used to control the flow of gases into process chambers.  
           [0006]    A typical conventional gas delivery system in a semiconductor fab facility is generally indicated by reference numeral  10  in FIG. 1 and includes a gas manifold  12  connected to a process chamber  40  of a process tool (not shown) in the facility. The gas manifold  12  may be contained in a valve manifold box (VMB, not shown) and includes a BCl 3  gas delivery conduit  14  for conducting BCl 3  to the process chamber  40 , a Cl 2  gas delivery conduit  15  for conducting Cl 2  to the process chamber  40 , an N 2 S gas delivery conduit  16  for conducting N 2 S to the process chamber  40 , a CH 3 F gas delivery conduit  17  for conducting CH 3 F to the process chamber  40 , and a CF 4  gas delivery conduit  18  for conducting CF 4  to the process chamber  40 . The BCl 3  and the Cl 2  are each delivered to the process chamber  40  typically at a pressure of about 15 psi, whereas the N 2 S, the CH 3 F and the CF 4  are delivered to the process chamber  40  typically at a pressure of about 35 psi. Each of the gas flow lines  14 - 18  is typically fitted with a manual valve  20  for manually opening and closing the corresponding gas flow line; a regulator  24  for controlling the gas pressure in the gas flow line; a filter  26  for filtering particles from the flowing gas; a mass flow controller (MFC)  30  for controlling the flow rate of each gas in the corresponding gas delivery conduit; and an upstream valve  28  and a downstream valve  32  on respective sides of the mass flow controller  30 . The gas delivery conduits are connected to a common manifold conduit  34 , from which an outlet conduit  36  conducts the gases into the process chamber  40 . A final valve  38  is provided in the outlet conduit  36 . The lower-pressure gas delivery conduits  14  and  15  may each be fitted with a V-block valve  22  which prevents backflow of gas through the respective gas delivery conduits.  
           [0007]    One of the problems associated with the conventional gas delivery system  10  is that the final valve  38  frequently becomes blocked or clogged during use and is therefore incapable of opening to establish fluid communication between the manifold conduit  34  and the process chamber  40 . Consequently, residual gas from the higher-pressure gas delivery conduits  16 - 18 , such as the N 2 S, the CH 3 F or the CF 4 , respectively, remains in the manifold conduit  34  after flow of these gases to the process chamber  40 . Accordingly, upon subsequent flow of the lower-pressure BCl 3  to the process chamber  40  through the gas delivery conduit  14 , the upstream valve  28  and downstream valve  32  are each opened and the higher-pressure CH 3 F or CF 4  backflows from the manifold conduit  34  and through the downstream valve  32 , the mass flow controller  30  and the upstream valve  28 , respectively, of the BCl 3  gas delivery conduit  14 . This gas backfill causes contamination of the BCl 3  gas delivery conduit  14  with the CH 3 F or the CF 4  gas, thereby potentially adversely affecting processes carried out in process tools connected to the valve manifold box in which the gas manifold  12  is contained. Additionally, clearing of the CH 3 F or CF 4  gas from the BCl 3  gas delivery conduit  14  results in unnecessary downtime in the semiconductor processing sequence.  
           [0008]    Accordingly, an object of the present invention is to provide a system which prevents undesired backfilling of a gas flow conduit with a gas.  
           [0009]    Another object of the present invention is to provide a backfill prevention system which prevents gas contamination of a gas flow conduit.  
           [0010]    Still another object of the present invention is to provide a backfill prevention system which is capable of closing a valve or valves in a gas flow conduit to prevent backfilling of the conduit with an undesired gas.  
           [0011]    Yet another object of the present invention is to provide a backfill prevention system which prevents undesired gas contamination of a process tool for semiconductors.  
           [0012]    A still further object of the present invention is to provide a backfill prevention system which eliminates downtime associated with clearing gas from a gas flow conduit in a semiconductor fab facility.  
           [0013]    Yet another object of the present invention is to provide a backfill prevention system which is capable of a variety of industrial applications.  
           [0014]    Another object of the present invention is to provide a backflow prevention system which utilizes a negative voltage signal that corresponds to reverse flow of gas in a gas flow conduit to close valves in the gas flow conduit and prevent gas backfill or contamination of the conduit.  
           [0015]    Yet another object of the present invention is to provide a backfill prevention system which may be utilized with a mass flow controller to sense backfilling of gas in a gas flow conduit and close valves in the conduit to prevent further backfilling of the gas in the conduit.  
         SUMMARY OF THE INVENTION  
         [0016]    In accordance with these and other objects and advantages, the present invention comprises a backfill prevention system for a gas flow conduit, comprising a gas flow monitor circuit which measures the rate and direction of gas flow through a gas flow conduit and converts the measured data into a voltage signal. A valve control circuit operably connected to a valve or valves in the gas flow conduit receives the voltage signal from the gas flow monitor circuit and closes the valve or valves in the event that the voltage signal indicates backflow of a gas through the gas flow conduit. The valve control conduit may further be provided with a first light emitting diode (LED) which is illuminated during normal flow of the gas through the conduit, and a second LED which is illuminated in the event of gas backflow through the conduit. The system is typically used in conjunction with a mass flow controller in the conduit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0018]    [0018]FIG. 1 is a schematic view of a typical conventional gas delivery system for delivering gases to a process chamber in a semiconductor fab facility;  
         [0019]    [0019]FIG. 2 is a schematic view of a gas delivery system in implementation of the backfill prevention system of the present invention;  
         [0020]    [0020]FIG. 3 is a schematic wiring diagram illustrating a typical gas flow monitor circuit in implementation of the present invention; and  
         [0021]    [0021]FIG. 4 is a schematic wiring diagram illustrating a typical valve control circuit in implementation of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The present invention has particularly beneficial utility in gas delivery conduits of gas delivery systems used in the distribution of gases to process chambers in a semiconductor fabrication facility. However, the invention is not so limimted in application, and while references may be made to such gas delivery systems used in the semiconductor fabrication industry, the invention is more generally suitable for gas flow conduits used in a variety of industrial and mechanical applications.  
         [0023]    An illustrative embodiment of the backfill prevention system of the present invention is generally indicated by reference numeral  1  in FIG. 2 and is typically used as part of a gas delivery system  50  for delivering various gases to a process chamber  80  of a process tool (not shown) in a semiconductor manufacturing facility. The gas delivery system  50  may include a gas manifold  52  connected to the process chamber  80  in the facility. The gas manifold  52  may be contained in a valve manifold box (VMB, not shown) and typically includes a BCl 3  gas delivery conduit  54  for conducting BCl 3  to the process chamber  80 , a Cl 2  gas delivery conduit  55  for conducting Cl 2  to the process chamber  80 , an N 2 S gas delivery conduit  56  for conducting N 2 S to the process chamber  80 , a CH 3 F gas delivery conduit  57  for conducting CH 3 F to the process chamber  80 , and a CF 4  gas delivery conduit  58  for conducting CF 4 to the process chamber  80 . It will be recognized and understood that various other gases in addition to or other than those mentioned above may be delivered to the process chamber  80  through the gas delivery conduits. The BCl 3  and the Cl 2  are each delivered to the process chamber  80  typically at a pressure of about 15 psi, whereas the N 2 S, the CH 3 F and the CF 4  are delivered to the process chamber  80  typically at a pressure of about 35 psi. Each of the gas flow lines  54 - 58  is typically fitted with a manual valve  60  for manually opening and closing the corresponding gas flow line; a regulator  64  for controlling the gas pressure in the gas flow line; a filter  66  for filtering particles from the flowing gas; and an upstream valve  68  and a downstream valve  72 . The upstream valve  68  and the downstream valve  72  are typically electric solenoid valves. A mass flow controller (MFC)  70  for controlling the flow rate of each gas in the corresponding gas delivery conduit  55 - 58 . A MFC  2 , which may be modified according to the present invention, is provided in the BCl 3  gas delivery conduit  54  for controlling the flow rate of the BCl 3  therethrough. The gas delivery conduits  54 - 58  are connected to a common manifold conduit  74 , from which an outlet conduit  76  conducts the gases into the process chamber  80 . A final valve  78  is provided in the outlet conduit  76 . The lower-pressure BCl 3  gas delivery conduit  54  and Cl 2  gas delivery conduit  55  may each be fitted with a V-block valve  62  which prevents backflow of gas through the respective gas delivery conduits  54  and  55 .  
         [0024]    The backfill prevention system  1  includes a thermal-sensing gas flow monitor circuit  3  (FIG. 3) which may be a part of the mass flow controller  2  (FIG. 2) of the BCl 3  gas delivery conduit  54 . Alternatively, the gas flow monitor circuit  3  may be separate from the mass flow controller  2 . The gas flow monitor circuit  3  includes a switch  5 , a voltage source  6 , an upstream thermal induction coil  8 , and a downstream thermal induction coil  9  which are coiled around an upstream portion  59  and a downstream portion  61 , respectively, of the BCl 3  gas delivery conduit  54  of the gas delivery system  50 . The upstream thermal induction coil  8  and the downstream thermal induction coil  9  use the heat-transfer property of the gas flowing through the gas delivery conduit  54  to measure the mass flow rate of the gas in the gas delivery conduit  54 , in the same manner as conventional mass flow controllers. The gas flow monitor circuit  3  further includes a first resistor (R 1 ) and a second resistor (R 2 ) in series. A primary voltage signal lead  42  leads from the circuit  3  between the first resistor R 1  and the second resistor R 2 . A secondary voltage signal lead  43  leads from the upstream thermal induction coil  8  and the downstream thermal induction coil  9 . Accordingly, when the switch  5  is closed, current having a voltage V 1  flows along a current path  4  through the first resistor R 1  and the second resistor R 2 . Some of the current flows through the primary voltage signal lead  42 , and some of the current flows along a current path  7  through the upstream induction coil  8 , the downstream induction coil  9  and the secondary voltage signal lead  43 . V 0  defines the voltage potential between the primary voltage signal lead  42  and the secondary voltage signal lead  43 . Depending on the rate of flow of BCl 3  gas through the BCl 3  gas delivery conduit  54  at the upstream portion  59  and the downstream portion  61  of the BCl 3  gas delivery conduit  54 , V 0  has various values. The relationship between V 0  and these values is expressed by the formula Vo=Vl*[(Rd-Ru)/(Rd+Ru)], where Ru=the flow rate of BCl 3  flowing through the upstream portion  59  of the BCl 3  gas delivery conduit  54  and Rd=the flow rate of BCl 3  flowing through the downstream portion  61  of the BCl 3  gas delivery conduit  54 . Accordingly, when the rate of BCl 3  flow through the BCl 3  gas delivery conduit  54  is constant, Rd=Ru and V 0 =0. When the BCl 3  flow rate increases, Rd&gt;Ru and V 0  increases to a positive value. When the BCl 3  flow rate decreases, Rd&lt;Ru and V 0 &lt;0. In operation of the system  1  as hereinafter further described, this negative voltage condition for V 0  occurs when a higher-pressure gas such as the N 2 S, CH 3 F or CF 4  begins to backfill the BCl 3  gas delivery conduit  54  in the direction opposite the normal flow of BCl 3 , indicated by the arrows in FIG. 3. Accordingly, Rd initially falls below Ru and V 0  assumes a negative value.  
         [0025]    The primary voltage signal lead  42  and the secondary voltage signal lead  43  are connected to a valve control circuit  82 , shown in FIG. 4, through voltage signal wiring  44 . The valve control circuit  82  typically includes a manual switch  84 , a relay  85 , a typically yellow LED  86 , a typically green LED  87 , a first resistor R 1  (2.2 KΩ ½ w), a second resistor R 2  (50 KΩ ½ w), a third resistor R 3  (1 KΩ ½ w), a fourth resistor R 4  (33 KΩ ½ w), and a fifth resistor R 5  (220Ω ½ w). The relay  85  is connected to an upstream valve switch  69  of the upstream valve  68  and to a downstream valve switch  73  of the downstream valve  72  through relay wiring  90 . Under circumstances in which no backfilling of N 2 S, CH 3 F or CF 4  gas occurs in the BCl 3  gas delivery conduit  54 , current flows along a primary current path  83  and illuminates the yellow LED  86  to indicate normal conditions in the BCl 3  gas delivery conduit  54 . Under circumstances in which N 2 S, CH 3 F or CF 4  begins to backfill the BCl 3  gas delivery conduit  54  in the direction opposite the normal flow of BCl 3 , indicated by the arrows in FIG. 3, current flows along the secondary current path  89  and illuminates the green LED  87 . Simultaneously, the relay  85  switches current flow to the upstream valve switch  69  and downsream valve switch  73 , causing these switches to close the normally-open upstream valve  68  and the downstream valve  72 , respectively, of the BCl 3  gas delivery conduit  54  and thereby halting further backfilling of N 2 S, CH 3 F or CF 4  in the BCl 3  gas delivery conduit  54 .  
         [0026]    Referring again to FIGS.  2 - 4 , in application of the backfill prevention system  1 , the various gases flow individually into the process chamber  80  through the respective gas delivery conduits  54 - 58  according to the processing requirements in the process chamber  80 . The BCl 3  and Cl 2  each flows at a pressure of typically about 15 psi, whereas the N 2 S, the CH 3 F and the CF 4  each flows at a higher pressure of typically about 35 psi. The upstream valve  68  and the downstream valve  72  of each gas delivery conduit  54 - 58  remains closed when the corresponding gas is not being distributed to the process chamber  80 . For example, as the BCl 3  flows through the BCl 3  gas delivery conduit  54  to the process chamber  80 , both the downstream valve  68  and upstream valve  72  of the BCl 3  gas delivery conduit  54  is open while the upstream valve  68  and the downstream valve  72  of each of the remaining gas delivery conduits  55 - 58  are closed to prevent those respective gases from flowing into the BCl 3  gas delivery conduit  54 . However, in the event that the higher-pressure N 2 S, CH 3 F or CF 4  is introduced into the process chamber  80  prior to introducing the BCl 3  into the process chamber  80 , residual higher-pressure N 2 S, CH 3 F or CF 4  remains in the manifold conduit  74  and backflows through the downstream valve  72  and upstream valve  68  of the BCl 3  gas delivery conduit  54 . Under normal circumstances, in which the BCl 3  flows normally through the gas delivery conduit  54  in the direction indicated by the arrows in FIG. 3, Ru=Rd and V 0 =0. Accordingly, current flows along the primary current path  83  of the valve control circuit  82  shown in FIG. 4 and illuminates the yellow LED  86  to indicate normal flow of BCl 3  through the BCl 3  gas delivery conduit  54 . In the event that the flow rate of the BCl 3  in the BCl 3  gas delivery conduit  54  increases, Ru&lt;Rd and V 0 &gt;0, and current likewise flows along the primary current path  83  of the valve control circuit  82  and illuminates the yellow LED  86  to indicate normal flow of BCl 3  through the BCl 3  gas delivery conduit  54  .  
         [0027]    In the event that residual N 2 S, CH 3 F or CF 4  from the manifold conduit  74  begins to backfill the BCl 3  gas delivery conduit  54  in the direction opposite the normal flow of BCl 3 , Rd initially falls below Ru, due to the reverse-flowing residual N 2 S, CH 3 F or CF 4  in the BCl 3  gas delivery conduit  54 . Consequently, V 0  assumes a negative value. This negative V 0  value is transmitted to the valve control circuit  82  through the voltage signal wiring  44 , and current flows through the secondary current path  89  instead of through the primary current path  83 . Accordingly, the green LED  87  is illuminated, and this indicates a backfill condition in the BCl 3  gas delivery conduit  54 . Furthermore, the relay  85  initiates current flow through the switch current path  90 , which current actuates the upstream valve switch  69  of the upstream valve  68  and the downsream valve switch  73  of the downstream valve  72  to close the upstream valve  68  and the downstream valve  72  and thus, prevent further backflow of the N 2 S, CH 3 F or CF 4  into the BCl 3  gas delivery conduit  54 .  
         [0028]    While the backfill prevention system  1  is heretofore described in conjunction with the BCl 3  gas delivery conduit  54 , it is understood that the backfill prevention system  1  of the present invention may be used in conjunction with the upstream valve  68  and downstream valve  72  of the Cl 2  gas delivery conduit  55 , instead of or in addition to the BCl 3  gas delivery conduit  54 , to prevent backfilling of the Cl 2  gas delivery conduit  55  with the N 2 S, CH 3 F or CF 4 .  
         [0029]    While the preferred embodiments of the invention have been described above, it will be recognized and understood that modifications can be made in the invention and the appended claims are intended to cover all such modifications which fall within the spirit and scope of the invention.  
         [0030]    Having described our invention with the particularity set forth above, we claim: