Abstract:
Fluid mass flow meters, particularly for measuring a wide range of relatively low flow rates of gas used in semiconductor fabrication processes include a body adapted to be interposed in a purge gas line leading to or from a mass flow controller or in a process gas line with the mass flow controller. The flow meter body includes a flow restrictor interposed in a passage and plural mass flow sensors which sense overlapping full scale fluid mass flow ranges across the flow restrictor to increase the overall range of fluid mass flow rates sensed by the meter. The flow meter body may include series or parallel arranged flow restrictors, a second set of mass flow sensors, and valving to cause a set of mass flow sensors to sense fluid mass flow rates across one or both of the flow restrictors. Embodiments of the flow meter include a pressure transducer mass flow sensor and conduits arranged with additional flow restrictors therein to selectively vary the full scale measurement range of the mass flow sensor.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the priority of U.S. Provisional Patent Application 60/226,806, filed Aug. 22, 2000. 
     
    
     
       BACKGROUND  
         [0002]    Many applications of fluid mass flow devices, including fluid mass flow meters and calibration tools require a relatively large range of flow measurement capability at relatively low overall flow rates. For example, in the control of flow of gases used in the fabrication of semiconductor devices, the accuracy of the mass flow controllers must be verified repeatedly over a wide range of relatively low flow rates of gas, since the quantities of such gases directly affect the chemical and physical properties of the semiconductor devices being fabricated. Accordingly, substantially continuous or very frequent monitoring of fluid mass flow controllers is advantageous to avoid delivering gas flows to semiconductor fabrication processes at incorrect flow rates.  
           [0003]    A significant number of gases used in semiconductor fabrication processes are corrosive, pyrophoric or poisonous, or a combination of all such characteristics. The gas delivery apparatus may have multiple gas lines or conduits, each containing a mass flow controller connected to a process vessel. A source of an inert gas, such as nitrogen, is typically provided for purging the flow conduits and controllers for the various gases from time to time, to change the gas being controlled or to allow replacement or repair of the fluid mass flow controllers associated with the fabrication system or process.  
           [0004]    Due to the criticality of maintaining accuracy of gas flow rates used in semiconductor manufacturing, in particular, it is desirable to provide calibration devices, such as so-called rate of rise systems or mass flow meters to monitor the flow rates being controlled by mass flow controllers. Typically, in prior art arrangements, calibration devices or flow meters have been placed in series with each mass flow controller device, thereby complicating the overall system. Moreover, due to the wide range of full scale flow rates that fluid mass flow controllers are required to accommodate, the use of a single conventional mass flow meter as a reference for all mass flow controllers has required that the mass flow meter operate over a wider dynamic range than it is capable of maintaining for the required accuracy of flow measurements. The needed one percent of reading flow accuracy specification for most semiconductor fabrication processes is unattainable by conventional mass flow meters over the full scale operating range required.  
           [0005]    The inherent design of commercially available mass flow controller sensors contains an error component that is proportional to the full scale flow of a device. For example, a 1000 sccm (standard cubic centimeters per minute) controller that has a 0.5 percent full scale accuracy is not capable of accurate measurement at a flow rate of 50 sccm wherein the accuracy becomes 10 percent of the 50 sccm reading. However, by providing multiple full scale ranges in a device wherein parallel sensors are provided which reach full scale excitation at markedly different pressure drops across a common laminar flow element or flow restrictor and by providing one sensor to overlap the range of another, a wide dynamic range is provided and which is one improvement in accordance with the present invention.  
           [0006]    Moreover, a so called pneumatic lag error occurs when gas flowing through a mass flow meter causes a pressure loss or so called pressure drop. The magnitude of this error as a percent of full scale flow of the meter is directly proportional to the magnitude of the pressure drop and the gas accumulation volume between the mass flow meter and the mass flow controller. At moderate flow rates this error is small and short lived. However, at low flow rates the error can be significant. For example, measuring flow rates as low as 10 sccm, using conventional commercially available flow meters, such as MOLBLOC brand gas flow calibration systems available from DH Instruments, Inc., which experience differential pressures as high as 7.0 psi, may take as much as fifteen minutes to complete. However, by utilizing a sensor which has a very small pressure drop (0.001 psi) the magnitude of the pneumatic lag may be reduced substantially.  
           [0007]    Another problem associated with fluid mass flow calibration devices or meters is related to changes in either the electronic characteristics, the fluid system of the device or the heat transfer system of the device, any of which will result in a calibration shift. However, sensor and electronic drift on one instrument set may be detected by comparing its flow data to data from an instrument set whose flow range is directly above and/or below the instrument set in question. Still further, errors in mass flow control due to clogging of the flow passages by unwanted material can be detected by using flow restrictors or laminar flow elements which have markedly different hydraulic diameters thereby exhibiting different propensities to clogging. Moreover, such errors can also be detected by comparing data of one instrument set with another and knowing the relative hydraulic diameters of the laminar flow elements of each instrument set. The problems associated with prior art mass flow control calibration and measurement described above have been overcome by the present invention.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides an improved fluid mass flow meter, particularly adapted for measuring a wide range of fluid mass flow rates in processes including, in particular, processes requiring precise gaseous mass flow rates in semiconductor fabrication, for example.  
           [0009]    In accordance with one aspect of the present invention an improved fluid mass flow meter is provided which is preferably disposed in a supply conduit for an inert gas used to purge process gases from multiple mass flow controllers flowing gases into the chambers of a semiconductor process apparatus. The improved mass flow meter can thus be valved in series with each individual mass flow controller and used as a reference to detect a calibration shift in a mass flow controller when operating on the inert gas. Such operation can be indicative of a calibration shift on any of the process gases which might be controlled by the mass flow controller during a working process. At least certain embodiments of the invention are also operable to be placed in line with the mass flow controller(s) for measuring the process gases directly.  
           [0010]    In accordance with another aspect of the present invention. A fluid mass flow meter is provided which is operable to route the same gas flow through different flow measuring devices. In one embodiment of the invention a mass flow meter is provided which is operable to serially flow fluid through two flow restrictors. Moreover, the mass flow meter includes duplicate sets of mass flow sensors arranged in parallel across each flow restrictor.  
           [0011]    In accordance with another embodiment of the invention a mass flow meter is provided wherein fluid flow is directed through a first flow restrictor and then subsequently through a second flow restrictor and wherein a single set of parallel arranged mass flow sensors is operable to sense flow through each restrictor currently receiving the flow.  
           [0012]    In accordance with another aspect of the invention a fluid mass flow meter is provided which is arranged such that mass flow sensors are provided with individual operating ranges which overlap, but which ranges are markedly different and increase from a relatively low value to a relatively high value to allow an expanded measurement range. The invention also provides a mass flow meter wherein a flow restrictor or laminar flow element and associated mass flow sensors generate markedly different pressure drops when flowing the same quantity of fluid. The flow restrictors of the different mass flow sensors are sized such that the magnitude of the pressure drop resulting from a flow through the sensor that produces a full scale output signal is markedly different.  
           [0013]    In accordance with still a further aspect of the invention, fluid mass flow meters are provided wherein the degree of overlap between the flow ranges of the flow sensors is sufficient to allow multiple measurements to be taken concurrently. Comparisons of the concurrent readings may be used to generate an alarm signal should one of the independent sensors provide signals which deviate from another sensor. By providing an arrangement wherein two laminar flow elements or flow restrictors and three different sensors are used in the mass flow meter, six different operating ranges are provided resulting in a very wide range of full scale flow measurement capability.  
           [0014]    Still further, the present invention provides a method wherein calibration verification for fluid mass flow controllers installed in semiconductor fabrication process apparatus may be provided. However, the wide dynamic range mass flow meter of the invention may be used in other applications.  
           [0015]    Although preferred embodiments of the invention are described herein those skilled in the art will further appreciate the above noted advantages and features of the invention together with other important aspects thereof upon reading the detailed description which follows in conjunction with the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a schematic diagram of one preferred embodiment of a primarily thermal sensor based fluid mass flow meter in accordance with the invention;  
         [0017]    [0017]FIG. 2 is a schematic diagram of another preferred embodiment of a thermal sensor based mass flow meter in accordance with the invention;  
         [0018]    [0018]FIG. 3 is a schematic diagram of another preferred embodiment of a thermal sensor based fluid mass flow meter in accordance with the invention;  
         [0019]    [0019]FIG. 4 is a schematic diagram of still another thermal sensor based fluid mass flow meter in accordance with the invention;  
         [0020]    [0020]FIG. 5 is a schematic diagram of a preferred embodiment of a pressure sensor based fluid mass flow meter in accordance with the invention;  
         [0021]    [0021]FIG. 6 is a schematic diagram of another preferred embodiment of a pressure sensor based fluid mass flow meter in accordance with the invention; and  
         [0022]    [0022]FIG. 7 is a table of selected design features and exemplary full scale flow rates for certain embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    In the description which follows like elements are marked throughout the specification and drawing with the same reference numerals, respectively. The drawing FIGURES are generalized schematic diagrams in the interest of clarity and conciseness.  
         [0024]    Referring to FIG. 1, there is illustrated a fluid mass flow meter in accordance with the invention and generally designated by the numeral  10 . The fluid mass flow meter  10  is adapted to be interposed in a gas flow conduit  12  having a first section  12   a  and a second section  12   b . Conduit section  12   a  is operable to be connected to a source of inert gas, not shown, such as nitrogen, for purging the flow conduits and mass flow controllers of a semiconductor fabrication process system. Discharge conduit  12   b  is operable to be connected to respective ones of the aforementioned mass flow controllers, not shown. Flow meter  10  includes a body  14  including a somewhat divergent flow passage  16  in communication with an inlet port  17  and with a substantially constant diameter continuing flow passage  18 . Passage  18  is connected to conduit  12   b  at a discharge port  19 . The flow meter  10  includes a first flow restrictor  20  disposed in passage  16 . Flow restrictor  20  is characterized as a solid plug element supported in passage  16  in such a way as to provide a substantially annular flow passage  16   a  disposed about the outer circumference of the plug type flow restrictor  20  and delimited by the wall of passage  16 . A second flow restrictor  22  is disposed in passage  18  and preferably comprises a generally conical shaped wire mesh element as shown schematically in FIG. 1 and throughout other figures of the drawings. Flow restrictors  20  and  22  may also be referred to herein as laminar flow elements (LFE) . Flow restrictors used with flow meters in accordance with the invention may not require to have an entirely linear performance characteristic over the entire range of their operation. However, flow restrictors which are characterized as laminar flow elements are generally preferred for use with the flow meters of the present invention. Various configurations of flow restrictors, some of which may be characterized as LFEs, may be used with the present invention including, for example, porous sintered metal plugs or plugs with multiple parallel conduits or flow passages formed therein. Other forms of flow restrictors or LFEs may also be used with the flow meters of the invention.  
         [0025]    Mass flow meter  10  includes a first mass flow sensor  24  interposed in a conduit  26  connected to conduits  28  or  30  which are in communication with the passage  16  on opposite sides of the flow restrictor or LFE  20 . A second mass flow sensor  32  is arranged in parallel with mass flow sensor  24  and includes a conduit  34  in flow communication with the conduits  28  and  30 . Mass flow sensors  24  and  32  are arranged in parallel. Mass flow sensors  24  and  32  are of the thermal type and may be similar to the type described in my U.S. Pat. No. 5,660,207, issued Aug. 26, 1997. Also, the mass flow sensors  24  and  32  may be of a type manufactured by the Millipore Corp. as one of their FC 2900 Series sensors. Mass flow sensor  24  may have a conduit inner diameter of 0.010 inches, for example, for conduit section  26  and which generates a pressure drop of 3.0 inches of water (0.1 psi) when operating at a full scale condition on nitrogen gas at so-called typical room temperature and pressure. Mass flow sensor  32  may also be of the type described in my U.S. Pat. No. 5,660,207 or one of a type manufactured by Millipore Corp. as their model FC 490 series and includes a conduit section  34  having an inner diameter of 0.022 inches and operable to generate a pressure drop of 0.1 inches water (0.003 psi) when operating at full scale on nitrogen gas at typical room temperature and pressure. An additional flow restrictor may be placed in series with the mass flow sensor  32  to achieve a targeted 0.3 inches of water flow resistance.  
         [0026]    Mass flow meter  10  includes a third fluid mass flow sensor  38  interposed in a conduit  40  in communication with the passage  16  across the flow restrictor or LFE  20 , as indicated schematically in FIG. 1. Mass flow sensor  38  may be one of several types. One preferred type is a micromachined flow sensor available from Honeywell Inc., Freeport, Ill. as their model AWM42150VH. This sensor is rated at a full scale flow of 25 sccm which, beyond that point, significant non-linearity characteristics start to result from measuring mass flow. Another type of sensor which may be used is commercially available from Yamatake Corporation, Tokyo, Japan.  
         [0027]    Still further, the mass flow meter  10  includes a second set of flow sensors  24  and  32  interposed in conduits  42  and  44 , respectively, in communication with conduits  46  and  48  and in parallel flow arrangement. Sensors  24  and  32  of the second set are in fluid flow communication with passages  16 ,  18  across the flow restrictor or LFE  22 , as shown by the schematic diagram of FIG. 1. A mass flow sensor  38  is interposed in a conduit  50  in communication with passages  16 ,  18  across the flow restrictor  22 , as indicated in FIG. 1. Output signals from all of the mass flow sensors of the flow meter  10  may be carried to a suitable recording device  54  which may be connected to a digital processor or CPU  54   a  for processing and managing the recorded data from the sensors of the apparatus  10 , FIG. 1, as indicated, for appropriate handling and recording. Flow sensor  38  provides the lowest flow restriction, on the order of 0.01 to 0.03 inches of water (0.0003 to 0.001 psi) and, as such, act as the primary references used for measuring lower flows. The flow restriction for the sensors  38  may be accomplished with the 0.060 inch internal diameter thermal sensor or the above identified sensor available from Honeywell Inc.  
         [0028]    Referring now to FIG. 2, a first alternate embodiment of a flow meter in accordance with the invention is illustrated and generally designated by the numeral  60 . The mass flow meter  60  is adapted to be interposed in conduit  12  in the same manner as the flow meter  10 , as illustrated. Mass flow meter  60  includes bodies  62  and  64  having respective flow passages  66  and  68  formed therein and corresponding somewhat to the passages  16  and  18  of the embodiment of FIG. 1, respectively. Bodies  62  and  64  may be integrally joined. An LFE or flow restrictor  20  is interposed in passage  66  which is in communication with an inlet port  67  and a discharge port  69 . Flow restrictor or LFE  22  is disposed in passage  68  which is in communication with an inlet port  70  and a discharge port  71 . Flow meter body  62  is in fluid flow communication with conduits  12   a  and  12   b  through branch conduits  12   c  and  12   d , respectively, as illustrated. A remotely controllable valve  72  is disposed in conduit  12   a  between inlet port  70  and branch conduit  12   c  and a remotely controllable valve  74  is disposed in conduit  12   c  between conduit  12   a  and inlet port  67 , as illustrated. Valves  72  and  74  may be operated by a suitable data recorder and controller  76  operably associated with a CPU  76   a . Valves  72  and  74  are operated in conjunction with each other to direct fluid flow from the aforementioned source to flow meter bodies  62  or  64 , as required for operation of the flow meter in accordance with the invention.  
         [0029]    Flow meter  60  includes mass flow sensors  24 ,  32  and  38  interposed in conduits  82 ,  84  and  86 , respectively, in communication with conduits  78  and  80 . Conduits  78  and  80 , as shown, extend between and are in fluid flow communication with passages  66  and  68  of the flow meter  60 . Conduits  82 ,  84  and  86  extend between conduit  78  and  80 , as illustrated, and incorporate the mass flow sensors  24 ,  32  and  38  therein, respectively. Remotely controllable shutoff valves  88  and  90  are operably connected to data recorder and controller  76  and are interposed in conduit  78 , as illustrated. Shut-off valve  88  is disposed between passage  68  and mass flow sensors  24 ,  32  and  313  while shut-off valve  90  is disposed between passage  66  and the aforementioned mass flow sensors.  
         [0030]    The mass flow meters  10  and  60 , shown in FIGS. 1 and 2, are operable to be valved in series with each mass flow controller, not shown, to be used as a reference to detect a calibration shift in the associated mass flow controller while operating on an inert gas, such as nitrogen, which would be indicative of a calibration shift also to be experienced by the same mass flow controller when operating on a process gas. The desired accuracy over the entire dynamic measurement range of a mass flow controller is assured by the use of redundant sets of mass flow sensors and associated flow restrictors or LFEs as shown for the mass flow meter of FIG. 1 or a set of mass flow sensors may be alternately associated with a particular flow restrictor or LFE, as for the flow meter  60  of FIG. 2.  
         [0031]    Referring now to FIG. 3, still another embodiment of a thermal sensor based flow meter is illustrated and generally designated by the numeral  60   b . The flow meter  60   b  utilizes a substantial number of components of the flow meter  60  except for elimination of the remotely controllable valves in conduit  78  which interconnects the bodies  62  and  64 . Remotely controllable valve  88  is shown moved to a position disposed in conduit  86  between conduit  80  and mass flow sensor  38 . Alternatively, a flow restrictor or LFE  91  is shown interposed in conduit  78  at the approximate former location of valve  88 . Still further, in the arrangement of the mass flow meter  60   b , valve  74  has been eliminated. Valve  72  may be controlled to shut off flow through the body  64  at relatively low flow conditions and remotely controllable valve  88  is operable to close to shut off flow through the sensor  38  to avoid subjecting the sensor  38  to flow conditions at relatively high differential pressures across that sensor. Accordingly, a substantially wide range of fluid flows through the flow meter  60   b  may be accurately recorded thanks to the arrangement of the bodies  62  and  64 , the flow restrictors or LFEs  20  and  22  and the sensors  24 ,  32  and  38 , together with the control elements  72  and  88 . Of course, all of the flow meter embodiments described herein are pre-calibrated so that the mass flows being sensed by the respective sensors can be correlated with the total flow through the meter for whatever flow paths are available for such flow to pass through the respective meters.  
         [0032]    Referring now to FIG. 4, still another embodiment of a thermal sensor based flow meter is illustrated and generally designated by the numeral  60   c . The flow meter  60   c  is similar in some respects to the flow meters  60  and  60   b  but enjoys a different arrangement of the bodies  62  and  64  and the sensors  24 ,  32  and  38 . For operations at relatively high flow rates, all flow is directed through body  62  and passage  66  as well as only flow sensor  24  by actuating valves  72  and  88  to shut off flow through body  64  as well as through flow sensors  32  and  38 . This operating mode is carried out primarily due to the non-linearity of sensor  38  at higher flow rates. As shown in FIG. 4, the sensors  24 ,  32  and  38  are arranged in their respective conduits  82 ,  84  and  86  which interconnect conduits  78   a  and  80   a . Valve  88  is interposed sensors  24  and  32  to shut off flow to the sensors  32  and  38  at the aforementioned high flow conditions. Under such conditions valve  72  is also closed.  
         [0033]    Other non-thermal based sensors may be capable of use with the flow meters of the invention. Differential pressure transducers, such as Honeywell Inc.&#39;s model PPT1C, could be used with appropriately different flow restrictions therein, or accuracy and stability may be obtained also using a Model 698AA13TRA sensor available from MKS, Andover, Mass. or by using a piezo-electric based pressure transducer or transducers. However, the last mentioned type of mass flow sensor may present a significant cost disadvantage.  
         [0034]    Referring now to FIG. 5, another embodiment of a mass flow meter in accordance with the invention is illustrated and generally designated by the numeral  100 . The flow meter  100  is also adapted to be disposed in a conduit  12  between conduit sections  12   a  and  12   b  and includes a body  102  having a diverging flow passage  104  formed therein and in communication with an inlet port  106  and a discharge  108 . Conduit section  12   a  is connected to inlet port  106  and conduit section  12   b  is connected to discharge port  108 . A flow restrictor or LFE  110  is suitably disposed in passage  104  between lateral branch ports  112  and  114 . Ports  112  and  114  are connected to conduits  116  and  118  which are in communication with a differential pressure type transducer  120  having a wide dynamic range, and suitably connected via a conduit section  118   a  to a suitable absolute pressure reference device  122 . Transducer  120  is also connected to conduit  116  by branch conduit  116   a . A suitable temperature sensor  124  is supported on body  102  for measuring the temperature of fluid flowing through passage  104 , as indicated. Differential pressure transducer  120  may be of a type commercially available, such as a model  600  series, manufactured by MKS of Andover, Mass. Output signals from the transducer  120  are communicated to a data recorder and controller  76  which is also operable to operate a flow control valve  128  which may be connected to conduits  116  and  118  by a branch conduit  130 , as shown. Conduit  130  also includes a suitable flow restrictor or LFE  132  disposed therein. A third LFE or flow restrictor  134  may be disposed in conduit  116 , as shown in the schematic diagram of FIG. 5, upstream of transducer  120 .  
         [0035]    If the dynamic measurement range of the pressure transducer  120  is desired to be relatively low, flow restrictors  134  and  132  together with flow control valve  128  may be arranged as indicated in FIG. 5. Flow restrictor  134  is adapted to provide a markedly higher flow resistance than the flow resistance of restrictor  132 , on the order of about twenty times greater, for example. By positioning the flow restrictor or LFE  134  upstream of the pressure transducer  120  and positioning the flow restrictor or LFE  132  as indicated in FIG. 3, a pressure divider is provided to shift the pressure differential seen by the transducer  120  when the valve  128  is open. When valve  128  is closed the pressure divider effect disappears.  
         [0036]    Referring now to FIG. 6, another embodiment of a pressure sensor based flow meter is illustrated and generally designated by the numeral  10   a . The flow meter  100   a  utilizes the body  102 , the annular plug type flow restrictor  110  and all of the other elements indicated in FIG. 6 which correspond to the same elements of FIG. 5 and the flow meter  100 . However, the flow meter  100   a  includes a pressure transducer  120   a  having an absolute pressure reference chamber  121  formed therein. In this way the flow meter  100   a  may be interposed in conduits handling corrosive or otherwise hazardous gases since such gases will not act on both sides of the sensor or its diaphragm for the transducer  120   a.    
         [0037]    [0037]FIG. 7 is a table of certain design characteristics for the flow meter embodiments of FIGS. 1, 2 and  3 . The parameters “CHAR DIM” refer to the effective bore or hydraulic diameters of the respective LFEs and sensors. The terms SEN_LB, SEN_BB and SEN_HW refer to the respective sensors  24 ,  32  and  38 , as indicated in FIG. 7. The term FS refers to full scale flow in SCCM and the term dP@FS refers to the differential pressure across the element indicated in inches of water at full scale flow.  
         [0038]    The construction and operation of the embodiments of the invention shown and described is believed to be within the purview of one skilled in the art based on the foregoing description read in conjunction with the drawings. Conventional materials and fabrication methods used for flow meters and flow controllers for gases used in semiconductor fabrication may be used to construct the flow meters described herein. Although preferred embodiments of the invention have been described in detail herein those skilled in the art will recognize that various substitutions and modifications may be made without departing from the scope and spirit of the appended claims.