Abstract:
A fluid mass flow controller, particularly adapted for controlling mass flow rates of toxic and reactive gases used in semiconductor device fabrication, includes a control circuit connected to pressure sensors for sensing the differential pressure across a flow restrictor in the mass flow controller for controlling a valve to control the fluid mass flow rate to a setpoint. The control circuit compares the differential pressure with the downstream pressure at a measured temperature with a data set of a gas passing through the flow controller for a range of differential pressures and downstream pressures and adjusts the flow control rate accordingly. The flow controller is mechanically uncomplicated including a two part body for supporting the pressure sensors, a remotely controllable flow control valve and the flow restrictor. The flow restrictor may comprise an orifice or nozzle but preferably comprises a sintered metal plug having a predetermined porosity for the expected materials and flow conditions to which the flow controller will be exposed. Process gases to be controlled by the flow controller are tested to provide data sets of mass flow rates at selected temperatures for a range of differential pressures across a flow restrictor and a range of downstream pressures.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to a fluid mass flow controller, particularly adapted for controlling the flow of toxic or highly reactive gases used in the fabrication of semiconductor devices and the like and including a method of operation of the controller based on data which correlates controller operation with a fluid differential pressure across a flow restrictor part of the controller and the downstream fluid pressure viewed by the controller. 
     BACKGROUND 
     Some effort has been put forth to develop precision fluid mass flow controllers, particularly flow controllers for controlling the mass flow rates of fluids, such as toxic and highly reactive gases, of the type used in the fabrication of semiconductor devices, for example. In the field of semiconductor fabrication, various gases are used in etching and vapor deposition processes, which gases are toxic to humans and are also highly reactive when exposed to ambient atmospheric conditions, for example. Mass flow controllers have been developed which measure and control the flow rate of fluids of the above-mentioned type wherein the measurements are based on thermal properties of the fluids. Other fluid mass flow controllers have been developed which are based on measuring a pressure differential across a flow restrictor or orifice. The accuracy of prior art fluid mass flow controllers of the type in question here is inadequate for many applications of flow controllers. 
     Semiconductor fabrication processes may require the discharge of very precise quantities of fluids (primarily gases) into a process chamber. For example, flow rates ranging from as high as twenty liters per minute to as low as a few tenths of one cubic centimeter per minute (CCM) may be required. Moreover, the response time and stabilization rate of flow controllers used to control reactive gases in semiconductor fabrication may require that the controller be able to react to an “on” signal and be stable at the required fluid flow rate within 0.5 to 1.0 seconds. The process itself may last anywhere from a few seconds to several hours and the shutoff response time of the fluid flow controller is usually required to be less than one second. The ability for thermal based fluid mass flow controllers to react and stabilize at such rates is difficult to achieve. 
     Another problem associated with prior art fluid mass flow controllers of the general type discussed herein pertains to the requirements to calibrate the controllers for various process fluids. Prior art fluid mass flow controllers are typically calibrated using an inert or nontoxic calibration fluid which requires the development of conversion factors or conversion data sets. Since the use of toxic or highly reactive process fluids for calibrating each controller instrument is cost prohibitive and dangerous to operating personnel, prior art mass flow controllers are typically calibrated on an inert fluid, such as nitrogen or argon, or a fluid whose properties are similar to the properties of the process fluid to be controlled by the mass flow controller. This process of using calibration fluids and conversion factors introduces errors into the operation of the mass flow controllers, is time consuming and thus expensive. The inaccuracy of prior art mass flow controllers and the expense and time required to calibrate controllers during initial setup, as well as in replacement procedures, adds substantially to the cost of many manufacturing processes, including the fabrication of semiconductor devices, to the point that certain improvements in fluid mass flow controllers have been highly desired. 
     Accordingly, several desiderata have been identified for fluid mass flow controllers, particularly of the type used in manufacturing processes as described above. Such desiderata include controller accuracy within a few percent of controller setpoint (at least one percent is desired), operation at elevated or below “normal” temperatures and various positions or attitudes (i.e., right side up, sideways, or upside down), without loss of accuracy, such as experienced by thermal based mass flow controllers, accurate measurement and control over a wide range of flow rates, fast response time from turn-on to achieving stable flow conditions, economy of manufacture and uncomplicated modular mechanical structure to facilitate servicing the flow controller and to facilitate changing the flow controller out of the fluid flow distribution system for the manufacturing process. Other features desired in fluid mass flow controllers include no requirement to calibrate each complete controller instrument at the time of manufacture or recalibrate the instrument after servicing, the provision of a reliable easily interchanged flow restrictor or orifice part, ease of verification of the operability and accuracy of the flow controller after servicing or changeout of a flow restrictor, the ability to accurately control flow rates for a wide variety of toxic and/or reactive fluids, particularly the hundreds of fluids in gaseous form which are used in semiconductor fabrication processes, and ease of changing the controller working data for flow rates for different gases or fluids in liquid form. It is to these ends that the present invention has been developed. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved fluid mass flow controller and method of operation. In particular, an improved mass flow controller and method of operation are provided for use in connection with controlling the flow of gaseous fluids used in the manufacture of semiconductor devices and the like. 
     In accordance with one aspect of the present invention, a fluid mass flow controller is provided which utilizes measurements of differential pressure across a flow restrictor and the pressure downstream of the flow restrictor to provide a more accurate reading of the actual mass flow of a particular fluid at a particular temperature. Such measurements may be carried out using only two pressure sensors or transducers and over a wide range of temperatures of the fluid being measured. 
     The present invention further provides an improved fluid mass flow controller, particularly adapted for controlling the mass flow rate of toxic and reactive gases, including those used in the fabrication of semiconductor devices wherein the flow controller includes rapid response time to stabilize at a desired setpoint flow rate and is accurate within setpoint conditions to less than one percent error. The controller is also operable to measure mass flow rates over a wide range of such flow rates, on the order of a ratio of maximum to minimum flow rates as great as 100 to 1. The mass flow controller does not require calibration with a process fluid or with a calibration fluid and thus no conversion factors are required in the flow measurement process. 
     The present invention also provides an improved fluid mass flow controller which operates by measurement of fluid differential pressures across and the fluid pressure downstream of a flow restrictor and which utilizes data for the mass flow of selected fluids within a range of differential pressures and downstream pressures to which the controller will be exposed and in which the controller will be operated. The mass flow controller and flowmeter of the invention is also operable over a wide range of inlet pressures from above atmospheric pressures to vacuum conditions experienced with so-called safe delivery systems for toxic or reactive gases. Still further, the invention includes a fluid mass flow controller which is operably associated with a control system including a suitable processor circuit, such as a digital signal processor, a nonvolatile memory for storing the aforementioned data and which may receive additional sets of data when desired. 
     The present invention further provides a fluid mass flow control apparatus which is of mechanically uncomplicated construction, is modular in form and is particularly adapted for rapid changeout of a replaceable flow restrictor, one or more pressure transducers and a single flow control valve associated with the controller. 
     The present invention still further provides a flow restrictor for which data of flow versus differential pressure and downstream pressure are available as data sets for a multiplicity of fluids, particularly adapted for use with a pressure based fluid mass flow controller or flowmeter in accordance with the invention, but is adaptable for other applications and is adapted for use with toxic and reactive gases, in particular. 
     Still further, the present invention contemplates a method for measuring and/or controlling the mass flow rate of a fluid by measuring differential pressures across a flow restrictor and the fluid pressure downstream of the restrictor, and particularly, but not limited to operating conditions wherein the downstream pressure is below atmospheric pressure. The invention further contemplates a method of operation of a fluid mass flow controller which does not require calibration of the controller with calibration fluids but utilizes predetermined data sets for a flow restrictor part of the controller for various types of process fluids, including those which may be toxic or highly reactive. 
     The invention also contemplates a fluid mass flow controller including a microcontroller or processor device adapted to receive signals from two pressure sensors, a temperature sensor and command signal inputs while providing a suitable analog output signal for a control valve associated with the fluid mass flow controller. Still further, the microcontroller is operable to support RS485 communication and various network communications for receiving data from a remote site and for supporting and inputting data to a serial EEPROM. Accordingly, the invention contemplates a method of operation of a fluid mass flow controller wherein data sets characterizing a flow restrictor for different fluids may be obtained remotely via a network for rapid change in operation of the controller on various types of fluids. 
     Those skilled in the art will further appreciate the above-mentioned advantages and superior 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 DRAWINGS 
     FIG. 1 is a somewhat schematic view of the fluid mass flow controller of the present invention; 
     FIG. 2 is a longitudinal, generally central section view of the controller shown in FIG.  1  and showing some features of fluid flow circuitry normally associated therewith; 
     FIG. 3 is a detail section view of a flow restrictor used in the flow controller of FIGS. 1 and 2 and in accordance with the present invention; 
     FIG. 4 is a diagram showing the mass flow rate of a gaseous fluid as a function of differential pressure across a flow restrictor and the downstream pressure, all in relatively low pressure ranges of about zero torr to about 2,000 torr; 
     FIG. 5 is a longitudinal section view of another embodiment of the flow restrictor and an associated support fitting in accordance with the invention; 
     FIG. 6 is a diagram similar to FIG. 4 showing the characteristics of another type of flow restrictor as a function of differential pressure across and pressure downstream of the flow restrictor; and 
     FIGS. 7A and 7B are flow charts of certain steps carried out in the operation of the fluid mass flow controller shown in FIGS. 1 and 2. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the description which follows, like elements are marked through the specification and drawings with the same reference numerals, respectively. The drawings may not necessarily be to scale and certain features may be shown in generalized or schematic form in the interest of clarity and conciseness. 
     Referring to FIGS. 1 and 2, an improved fluid mass flow controller in accordance with the invention is illustrated and generally designated by the numeral  20 . The mass flow controller  20  includes a two-part modular body  22  comprising generally rectangular block shaped body parts  24  and  26  which may be suitably joined to each other by conventional mechanical fasteners  28  at cooperating planar faces  24   a  and  26   a , respectively. The body parts  24  and  26  are provided with suitable fluid conductor connector portions  25  and  27  to provide for connecting the fluid mass flow controller to conduits for a system for supplying, in particular, toxic or reactive fluids in gaseous form for use in semiconductor fabrication, for example. 
     By way of example, as shown in FIG. 2, the mass flow controller  20  may be interposed in a fabrication system including a source pressure vessel  28  for pressure fluid such as tungsten hexafluoride, chlorine, or sulfur hexafluoride, for example. Source pressure vessel  28  is connected to the flow controller  20  via a suitable conduit  30  and a purge conduit  32  is also connected to conduit  30  and to a source of purge gas, not shown, for purging the flow controller to a suitable receiver or scrubber  34 , when needed. During operation of the flow controller  20 , however, a precise flow of fluid is controlled for entry into a semiconductor fabrication chamber or vessel  36  via conduit  33 . Chamber  36  is typically maintained at a substantially reduced pressure by way of one or more vacuum pumps  37 , for example. The system in which the flow controller  20  is interposed, as shown in FIG. 2, is shown by way of example in simplified form to illustrate one preferred application of the flow controller. 
     Referring primarily to FIG. 2, the body part  24  supports an electrically controlled flow control valve  40  which is removably mounted on a face  24   b  of body part  24  by conventional mechanical fasteners, not shown. Valve  40  includes an electrically actuated closure member  41  operable to throttle flow of fluid from an internal passage  42  of body part  24  to a second internal passage  44  of body part  24 . Valve  40  also includes an actuator  43  for the closure member  41 . Actuator  43  is preferably of a type using a solenoid or piezoelectric material for rapid response and fineness of control of closure member  41 . A first pressure transducer  46  is also removably mounted on body part  24  and is in communication with a passage  47  in body part  24  which is in communication with passage  44 . A second pressure transducer  48  is removably mounted on body part  26  and is in communication with a passage  49  which opens into a longitudinal passage  50  in body part  26 , which passage is also connected to conduit  33  leading to the fabrication chamber  36 . Pressure transducers  46  and  48  may be of a type commercially available from Honeywell Data Instruments Division, for example. Control valve  40  and pressure transducers  46  and  48  may be disposed within a removable cover  51 , FIG. 1, for the flow controller  20 . 
     Referring also to FIG. 3, the body part  24  includes a cylindrical counterbore  54  formed therein and concentric with the passage  44  for receiving a flow restrictor  56 . Flow restrictor  56  is supported in a tubular sleeve  58  which may be mounted in a suitable tubular adapter  60  supported in the counterbore  54  between seal rings  62 . Accordingly, the flow restrictor  56  may be easily removed from the body  22  by separating the body parts  24  and  26 , removing the flow restrictor together with its support sleeve  58  and replacing the flow restrictor with a suitable replacement restrictor of the same flow characteristics or a selected other flow characteristic. The flow restrictor  56  preferably comprises a sintered metal cylindrical plug shaped member having a predetermined porosity for allowing fluid to flow therethrough by providing restriction to flow sufficient to create a differential pressure thereacross which may be sensed by the pressure transducers  46  and  48 . Flow restrictor  56  may, for example, be fabricated of stainless steel or nickel particles suitably compressed and sintered to provide the desired porosity and flow restriction characteristic. Flow restrictor  56  is advantageously disposed in flow controller  20  downstream of control valve  40 . 
     Referring again to FIG. 1, the flow controller  20  is adapted to be operated by a control circuit or system including a microcontroller characterized as a digital signal processor  70  which is operably connected to a non-volatile memory, such as an EEPROM  72 , a power supply  74  and a suitable valve driver circuit  76 . The microcontroller  70  is operably connected to the valve  40  for effecting movement of the closure member  41  by way of the driver  76 . The microcontroller  70  is also operably connected to the pressure transducers  46  and  48  and to a temperature sensor  78  which may be located to sense the temperature of fluid flowing through the controller  20  at a predetermined location. The microcontroller  70  is also operably connected to a suitable interface  80  for receiving command signals, data sets and programming changes from various sources. 
     The microcontroller  70  is preferably a TMS320 LF2407 fixed point microcontroller available from Texas Instruments Incorporated. The pressure sensors  46  and  48  operate in a plus/minus 0.5 volt range with fourteen to sixteen bit resolution as analog inputs to the microcontroller  70  which carries its own A/D and D/A converters. Other analog inputs will be for the temperature sensor  78  and a zero to five volt set point command signal input with twelve bit resolution. The microcontroller  70  also provides analog output signals for controlling operation of the valve  40  via the driver  76 . Communication with the microcontroller  70  may be via an RS485 4-wire communication link and/or a CAN (Controller Area Network). The microcontroller  70  is also capable of supporting a JTAG interface for emulation and debug and a powerup bootloader function for programming. The memory  72  is preferably a serial EEPROM of at least four thousand bytes. 
     The microcontroller  70  requires a closed loop control function to be executed at a rate of about one hundred times per second between the inputs for the pressure sensors  46  and  48  and the output signal for controlling the valve  40 . Communication through interface  80  is carried out while the control loop is functioning although new data transfer or transfer to the memory  72  may be supplied when control loop updates are not being maintained. 
     An important aspect of the present invention resides in the discovery that, in a normal operating range of the mass flow controller  20 , the fluid flow rate is a function not only of the differential pressure across the flow restrictor  56  but also the absolute downstream pressure corresponding substantially to the pressure in the fabrication chamber  36 . FIG. 4, for example, shows a typical characteristic of flow in standard cubic centimeters per minute (SCCM) as a function of the differential pressure (torr) across the flow restrictor  56  and also as a function of the downstream pressure (torr) in the passage  50 , conduit  33  and fabrication chamber  36 . The diagram of FIG. 4 indicates that the flow characteristics of a fluid flowing across a restrictor, in the pressure ranges indicated in the diagram, may be in accordance with a three-dimensional surface indicated by numeral  90 . The flow characteristic or surface  90  is for a particular temperature. In the diagram of FIG. 4, the mass flow characteristic  90  for the fluid tested was conducted at 25° C. As indicated in FIG. 4, measurements taken at lower temperatures would provide flow characteristics indicated by the surfaces  92  and  94 , for example. The flow characteristic indicated by surface  92  is for a temperature lower than the temperature for the flow characteristic indicated by surface  90  and the flow characteristic which is determined by the surface  94  is at a temperature lower than the measurements taken for developing the flow characteristic surface  92 . 
     It will also be noted from viewing FIG. 4 that a mass flow rate across a flow restrictor, particularly for the pressure ranges indicated in the diagram, varies with the downstream pressure. For example, if the downstream pressure is approximately 0.0 torr and the pressure differential across the flow restrictor is approximately 1575.0 torr, the flow rate for the particular restrictor tested is about 280 SCCM. However, if the downstream pressure is 760.0 torr (standard atmospheric pressure), the flow rate for the same pressure differential across the flow restrictor is approximately 500 SCCM. Accordingly, the behavior of fluids flowing across a flow restrictor, particularly in gaseous form, is dependent not only on temperature and differential pressure but also the pressure downstream of the flow restrictor. 
     The flow characteristics indicated in FIG. 4 at various temperatures, differential pressures across the flow restrictor and downstream pressures are for a sintered metal type flow restrictor, such as the flow restrictor  56 . Alternatively, viewing FIG. 6, a similar flow characteristic is indicated for a sharp edged circular orifice at 25° C. and is indicated by numeral  95 . The specific flow characteristics shown in FIGS. 4 and 6 are for nitrogen gas although other gases are indicated to behave in accordance with the general flow characteristics shown in FIGS. 4 and 6 for the types of flow restrictors described herein. 
     Accordingly, a flow characteristic in accordance with the diagrams of FIGS. 4 and 6 may be developed for particular types of flow restrictors used in connection with a mass flow controller, such as the controller  20 , and for various fluids in liquid and gaseous form, including the process gases or vapors used in semiconductor fabrication. 
     Data points representing the three-dimensional flow characteristics, such as the surfaces  90 ,  92  and  94  in FIG. 4, may be developed in various ways and entered into the memory  72  of the flow controller  20 . The flow controller microcontroller  70 , when operated in a set point mode can be programmed to command operation of the valve  40  to adjust the flow through the flow controller  20  to approach the setpoint by sensing the pressure differential across the flow restrictor by the pressure transducers  46  and  48  to determine the actual flow rate, repeatedly, until the flow rate is essentially that programmed into the microcontroller  70  as the setpoint or pursuant to instructions input to the microcontroller. The data points representing the surfaces  90 ,  92   94  for a particular gas may be obtained using conventional flow measuring equipment. 
     A rate of change mass flow measuring apparatus may also be used to obtain the data points. Moreover, such a flow measuring apparatus may be used to verify the operation of a flow controller, such as the flow controller  20  within its design specification, and such apparatus may also be used to verify whether or not a particular flow restrictor is within its design specification. Once a design specification has been established for a flow restrictor and a flow controller of the types described herein, the performance of each may be verified by a rate of change mass flow measuring apparatus or other mass flow measuring apparatus or devices and use of an inert gas so that toxic and highly reactive gases are not required to be used during verification tests on the complete flow controller or on a flow restrictor, respectively. For example, a selected number of data points may be verified at flow rates of 50, 100, 500 and 3,000 SCCM at 30 psig inlet pressure, with exhaust pressure being atmospheric, for a flow controller, such as the controller  20 , or for a flow restrictor, such as flow restrictor  56 . Data points representing the design specification of the flow controller  20  may also be entered into the memory  72  to verify the operability of the flow controller when tested with the aforementioned rate of change flow measuring apparatus. A suitable rate of change or so-called rate of rise mass flow measuring apparatus is commercially available. 
     Moreover, the fluid mass flow controller  20  may also be connected via its interface  80  with a network adapted to be connected to a source of data for any fluid which has been tested in conjunction with a controller of the same type as the flow controller  20 . In this way, any gas to be controlled by the flow controller  20  may have its flow characteristics entered into the memory  72  by merely querying a database stored in a suitable processor. For example, a vendor of the flow controller  20  may have selected data sets stored on a suitable processor and memory associated therewith for a wide variety of gases, each data set corresponding substantially to the type of data sets that would provide the flow characteristics shown in FIGS. 4 and 6 for any one type of flow restrictor, respectively. An authorized customer using a flow controller, such as the flow controller  20 , and desiring to begin using the controller with a particular gas would merely make an inquiry to the vendor source and download the needed data set directly to the microcontroller  70  and its memory  72  via a network such as the Internet, for example. 
     Operation of the microcontroller  70  is generally in accordance with the flow diagrams of FIGS. 7A and 7B and will now be described in further detail. The microcontroller or processor  70  is operable to execute closed loop control and communication functions. Closed loop control is preferably executed at a rate of 100 times per second and requires execution of lookup tables or polynomial calculations. All code may be written in “C”. The functions of the microcontroller or processor  70  are summarized in the flow diagram of FIG.  7 A. Step  100  in FIG. 7A indicates a 10 millisecond interrupt to drive the key functions of the processor  70 . In step  102 , the processor obtains  64  samples of downstream pressure XD 1  and averages the samples. In step  104 , the processor  70  obtains  64  samples of the upstream pressure XD 2  and averages the samples. Step  106  is an averaging of  32  samples of an analog output signal for control of the valve  40  identified by the software tag CV 1 SN. Step  108  indicates operation of the processor  70  in the signal mode to obtain 32 samples of a zero to five volt setpoint command signal input in step  110 , and a 32 sample zero to five volt analog output signal in step  112 . Step  114  indicates when analog inputs are shorted to ground. Step  116  indicates the processor obtaining 32 samples of the signal from temperature sensor  78 , indicated as TE 1 , and averaging such samples. Step  118  provides for converting the signal inputs to English units of pressure, flow and temperature. Step  120  in FIG. 7A is the execution of a calculation of flow routine using, for example, the surfaces  90 ,  92  and  94  of FIG.  4 . New processor proceeds to the control mode at step  122 . 
     FIG. 7B illustrates how the calculation of flow routine is carried out using sets of so-called three dimensional maps, such as the surfaces  90 ,  92  and  94 , for example, for respective operating temperatures and whereby the flow is calculated as a function of the variables of differential pressure across the flow restrictor  56 , the downstream pressure in the flow passage  50  and the temperature sensed by the sensor  78 . A set of flow runs over a range of downstream pressures and flow rates is obtained for the flow restrictor  56 . This data set is fitted to an array of three dimensional curves. The so-called map can be thought of as flow on the z axis mapped to differential pressure, XD 2 −XD 1 , on the x axis and discharge or downstream pressure, XD 1  on the y axis. 
     The best-fit process generates curves at various values of y. Typically curves of x versus z might be generated for XD 1  being equal to 1, 50, 100, 300, 500 and 700 torr, for example. Then the process is repeated at another operating temperature. The calibration data is then mapped from floating point numbers to the fixed point quantities that are used in the processor. These tables are download to the processor and are called during the flow calculations. The get calibration data of step  124 , FIG. 7B, is carried out by obtaining the calibration maps or surfaces at the nearest temperature above and below the temperature sensed by sensor  78 . At steps  126  and  128 , flow is calculated by interpolating the differential pressure XD 2 −XD 1  for two curves in the calibration data (CAL DATA). Flow at the current calibration temperature is calculated by interpolating between calibration flow data points. At steps  130  and  132  flow at the current CAL DATA temperature is calculated by interpolating between Flow( 0 ) and Flow(i) by the value of XD 1  and the y axis values for Flow( 0 ) and Flow( 1 ). Flow is calculated by interpolating between the Flow@Temp( 0 ) and Flow@Temp( 1 ) by the value of TE 1  and the temperatures for the two CAL DATA sets selected. 
     Referring briefly to FIG. 5, as previously mentioned the flow restrictor  56  may be adapted for operation in conjunction with other flow controllers and related devices. The flow restrictor  56  may, for example, be removably mounted in a conventional fitting, such as a face seal union fitting  110 . The fitting  110  includes a longitudinal through passage  112  which is counterbored at one end to provide a bore  114  for receiving the cylindrical plug flow restrictor  56  and its tubular support sleeve  58 . The sleeve  58  may be a light press fit in the bore  114 . By way of example, a flow restrictor for use in conjunction with the flow controller  20  may be characterized as a cylindrical plug having a diameter of approximately 0.18 inches and a length of approximately 0.18 inches and may be formed of porous sintered stainless steel, nickel or Hastelloy C-22. The solid steel sleeve  58  may be formed of 316L stainless steel. It is contemplated that the manufacturing tolerances of the flow restrictor  56  may be such as to require only verification of the performance characteristics of the restrictor by verifying the mass flow rates of, for example, 50, 100, 500 and 3,000 SCCM at a pressure upstream of the restrictor of 30 psig with exhaust to atmosphere. 
     Accordingly, no calibration or calibration conversion factors are necessary for the flow restrictor  56  or the flow controller  20 . When once placed in use, the flow controller  20  and/or the flow restrictor  56  may be verified as to its operability by flowing predetermined quantities of an inert gas through these devices using the aforementioned rate of change flow measuring apparatus or a similar apparatus to verify performance. The flow restrictor and/or the flow controller may then be placed in or returned to service with assurance that the respective devices will perform in accordance with a flow characteristic, such as that indicated in FIG. 4, for example. 
     The construction and operation of the mass flow controller  20  and the flow restrictor  56 , as well as the method of operation of the flow controller as set forth hereinabove, is believed to be readily understandable to those of ordinary skill in the art. Moreover, the flow controller  20  functions as a flowmeter and may be used as a flowmeter as well as for controlling fluid flow rate to a setpoint condition. 
     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 to the invention without departing from the scope and spirit of the appended claims.