Thermal mass flowmeter and mass flow controller, flowmetering system and method

A flowmeter (100) or mass flow controller (101) used in a manufacturing process with toxic and reactive process fluids. A fluid flow sensor (114) senses fluid flow. A set point is established based upon predetermined temperature and pressure conditions at which the fluid will be utilized in the process. A valve drive (124) operates a fluid flow valve (126) to the resulting fluid flow rate, this being based upon the sensed flow rate and the set point. A control unit (122) controls the valve drive. The control unit accesses a calibration data set to determine the amount of fluid to be delivered by the fluid flow valve based upon the sensed flow rate and the set point. This calibration data set is created for the controller over its operational range using a calibration fluid having similar thermodynamic transport properties to a process fluid. The instrument is calibrated using the calibration fluid and the data set is produced by converting the calibration data using process fluid data stored in a data base (200). Accessing the data set stored in the instrument together with routing signals over a communication network (300) permits the instrument to precisely control process fluids without having to introduce external correction factors or other adjustments to the process.

CROSS REFERENCE TO RELATED APPLICATIONS 
Not applicable. 
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
Not applicable. 
BACKGROUND OF THE INVENTION 
This invention relates to mass rate of measurement fluid flowmeters and 
flow controllers, and more particularly, to an analog or digital flowmeter 
employed in manufacturing processes such as the manufacture of 
semiconductor chips in which highly toxic and highly reactive fluids are 
used, and because of which, such flowmeters are not calibrated using the 
fluid they will be controlling during a process. 
In the manufacture of semiconductors, integrated circuit (IC) chips and the 
like, it is necessary to use a variety of fluids (gases) which are highly 
toxic and/or reactive. A reactive fluid is a gas which is corrosive, 
flammable, or pyrophoric, among other things. Proper control of these 
fluids, for example, dichlorosilane (SiH.sub.2 Cl.sub.2), is therefore 
mandatory. The same is true in other manufacturing processes as well; 
although these will not be discussed. Process control in the critical 
process steps where these fluids are used is accomplished by monitoring 
the mass flow rate of the gas and controlling appropriate valving to 
adjust the flow to a desired rate for the process condition. Measuring 
mass flow rates is old in the art. Essentially it is done using either an 
analog measuring system, or a digital based system. Regardless of which 
technique is used, there has been, and until now, continues to be, 
substantial control problems which must be overcome in order to maintain a 
process capable of producing quality chips. 
There are a number of problems which currently effect flowmeter calibration 
and performance. While these are discussed in more detail below, these 
problems are: 
a) calibrating a flowmeter using an inert gas produces inaccuracies; 
b) calibrating the flowmeter with a gas that is dangerous for one of a 
variety of reasons and which can potentially damage the instrument, if the 
instrument is exposed to air or moisture at any time subsequent to 
calibration and before installation; and, 
c) calibrating the instrument with a gas (freon, for example) that is 
environmentally unsound and which also cause one or both of the other two 
problems noted above. 
Because of thermal transport properties in gases such as those used in 
semiconductor manufacture, for example, the accuracy of current mass flow 
controllers (whether analog or digital) cannot be guaranteed to a level 
desirable both by the instrument maker and the end user. Ideally, flow 
controllers would be tested with the actual gases they control in a 
process so as to properly calibrate their performance for actual use. 
However, process capable calibration data generally currently does not 
exist because the toxic and corrosive nature of certain of these gases 
require special facilities be used to obtain the necessary information. To 
perform an instrument calibration in a facility which may be suitable for 
use with a toxic or reactive fluid is currently very expensive. This is 
particularly so where a controller may be used with one of many such gases 
and the controller must be calibrated for use with each. Contracting out 
instrument calibration available facility is also expensive. It is not 
unusual for a calibration to cost well over a thousand dollars per 
instrument. This procedure is simply not cost efficient. Rather, current 
practice is to calibrate the instrument with an inert gas such as nitrogen 
(N.sub.2) rather than any of the gases with which the controller will be 
used. The output of the instrument is then scaled using a conversion 
factor to estimate the performance of the instrument with the process gas. 
Or, the instrument can be calibrated using a "surrogate" gas. A surrogate 
gas is one which has specific heat properties which are substantially 
close to a process gas with which the controller is used. Using a 
surrogate gas reduces the magnitude of the conversion factor required to 
adapt the instrument's performance to the process gas. 
Another problem involved with instrument calibration does not involve 
either the gases with which the controller will be used or the calibration 
facilities. Rather it involves certain calibration fluids currently used 
and the residual effects such gases may have on the instrument. For 
example, if a calibration is performed with a gas such as chlorine, unless 
subsequent purging of the instrument effectively removes all traces of the 
gas, future exposure of the instrument to moisture, as when the instrument 
is exposed to air, will result in hydrochloric acid (HCl) being formed. 
Damage to the instrument caused by the acid will ruin the instrument, 
requiring a costly replacement. 
Yet another problem is simply that some gases are expensive to use and 
calibrating a flowmeter with such gases is cost prohibitive. 
The result of all of this is that process engineers responsible for 
controlling a manufacturing process and for using mass flow controllers, 
have devised various techniques to insure the accuracy of the instruments 
they employ. Each mass flow controller is delivered to its end user with a 
complete set of calibration data, this data being based upon the inert gas 
with which the calibration was performed. This data is expressed, for 
example, as a curve of flow versus set point, and the curve covers the 
entire operating range of the instrument. The process engineer, using his 
knowledge of the process and the behavior of the gas used in the process, 
is able to adapt the calibration curve for the inert gas to the actual 
process gas using his prior experience. He may employ a "black book" or 
the like containing conversion factors he will use to interpret instrument 
readings for the process gas and meter fluid flow accordingly. This 
"tweaking" however, comes at a price. Certain processes, such as the 
manufacture of semiconductor devices, require very precise process 
controls. If inaccuracies in instrumentation occur, useless product 
results. It will be understood, for example, that a conversion factor 
typically is accurate only at a single point, and the further readings are 
away from that point, the greater the divergence from a "true" value and 
the converted reading. Trial and error experimentation to determine what 
the adjustment factors for a particular instrument should be can cause 
delays and also result in lost production, increased down times, increased 
product costs, etc. Alleviation of problems concerning instrument 
calibration can have an immediate beneficial impact on many industries. 
Another area of concern is the error that arises because of the 
communications involved in signal handling and processing. All 
controllers, whether analog or digital controllers, use analog signals at 
one point or another throughout the processing and control functions 
performed by a controller. A control system may include a central control 
computer which commands analog input/output (I/0) cards of a process 
controller. The I/0 card converts digital signals from the computer to 
analog set point signals, and analog flow information signals to digital 
signals supplied to the computer. The system includes the following 
sources of potential signal error: wire and connector losses, noise 
pick-up, and analog-to-digital and digital-to-analog conversion errors. 
Use of completely digital communications between a central computer and 
mass flow controllers will eliminate various system errors. 
BRIEF SUMMARY OF THE INVENTION 
Among the several objects of the present invention may be noted the 
provision of improved flowmeters and mass flow controllers having 
significantly greater accuracy than either existing digital or analog 
flowmeters and mass flow controllers. The improvement in digital mass flow 
controller accuracy, for example, is partially the result of improved 
signal processing techniques, and partially a result of improved digital 
communications within the controller. Further, flowmeters and flow 
controllers can now be individually customized for the process gases with 
which they are used. 
An important object of the present invention is the improvement in 
measurement accuracy which results from the flowmeter's or flow 
controller's calibration for a customer's process gas or gases. The 
calibration process now eliminates the need for "tweaking" by the user's 
technical personnel and the "cut and try" techniques previously used by 
such personnel to accommodate a calibrated flowmeter or flow controller to 
the particular process. The attendant costs and wastes arising from these 
techniques are now also eliminated, and process development time is 
shortened since these steps need no longer be performed. 
Another important object of the invention is the capability of the improved 
flowmeter or flow controller to be used in a variety of processes in which 
highly toxic, highly corrosive, or expensive gases, or some combination 
thereof are normally used. Even though flowmeter or flow controller 
calibration is performed on "safe" gases, the calibration is now such that 
the thermodynamic transport properties of such gases are taken into 
account as part of the calibration process. 
A further object of the invention is the provision of an improved flowmeter 
or flow controller in which either is independently calibrated for a 
number of gases with which they are used and the calibration information 
for each gas is stored within the instrument and is readily accessible by 
a user. The personnel using the controller now no longer need to maintain 
separate "little black books" containing relevant information necessary to 
adjust the instrument's operation, depending upon the gas currently being 
used in a process. 
An additional object of the invention is the creation and usage of a 
database which contains information relating performance of a flowmeter or 
flow controller with a gas used in a process as well as that of the 
instrument with a calibration gas or gases. The database enables the 
instrument to be readily used with process gases over the entire operating 
range of the instrument; that is, the instrument is readily used with any 
of the number of gases for which the instrument is calibrated, and for the 
entire range of flow rates of these gases in a particular process. 
A further object of the invention is to provide a flowmeter or mass flow 
controller having the capability to remotely zero the flow sensor used 
with the instrument. Other instrument capabilities include a digitally 
adjustable setpoint and ramprate, and temperature monitoring for 
indicating the temperature outside the instrument's flow rate sensor. 
Also, direct indications can be provided of a sensor's raw output signals 
and a valve drive signal from the instrument so clogging or restriction of 
the sensor can be detected. Where a number of instruments are used in a 
process, the instruments can be interconnected so, for example, their 
setpoints can be simultaneously adjusted. 
It is also a provision of the improved processor of the flowmeter or flow 
controller to have sufficient data storage capability so all relevant 
information relating to a calibration is stored in the instrument and is 
readily accessible by the user. This enables a relationship between data 
collected for a process gas and representative instrument calibration 
curves using a calibration gas (N.sub.2 for example) to be derived. From 
their relationship a calibration curve for the process gas can be 
determined and stored in the instrument, or in an external database 
accessible by the instrument so this process gas calibration curve can be 
used during the process. 
A further object of the invention is to provide an instrument having stored 
data sets for various system operating pressures. The controller of the 
instrument is responsive to a pressure sensor reading or pressure input 
information from a process control to interpolate between data sets where 
the sensed pressure is intermediate the pressure values for which the data 
sets were produced. 
Yet another object of the invention is the establishment of a system of 
fluid flowmeters or flow controllers each of which independently functions 
within some part of a manufacturing process. The system includes a 
communications network by which each flowmeter or flow controller can 
separately, quickly access a database containing relevant information for 
use by the instrument. This enables each instrument to have the 
information readily available by which the instrument can readily and 
precisely monitor and/or control the fluid flow portion of the process 
with which it is associated. 
Finally, it is a particular object of the invention to provide a flowmeter 
and mass flow controller which can be calibrated quickly, efficiently, and 
at a reasonable cost, yet provide the necessary precision required when 
used in a manufacturing process. Further, it is also an object to reduce 
the complexity of the monitoring and control system in which the 
instrument is used. This is achieved by an improved communications system 
that minimizes wiring. In so doing, the overall reliability of the 
monitoring and control system is significantly enhanced, resulting in 
substantial savings in process costs for the manufacture of semiconductor 
devices, for example. 
In accordance with the invention, generally stated, a flowmeter or mass 
flow controller is used in a manufacturing process such as for the 
manufacture of semiconductor chips. The flowmeter or mass flow controller 
meters, or meters and controls the flow of one of a variety of fluids used 
in the process, and a number of meters and/or controllers may be used with 
the same or different fluids. The process fluid is used in the process 
under a variety of temperature and pressure conditions. And, the fluids 
may be toxic, corrosive, or otherwise reactive. The mass flow meter 
comprises a fluid flow sensor for sensing fluid flow through a passage by 
which the fluid is directed to a portion of the process where it is used. 
The flow meter provides an output signal to the user that accurately 
represents the flow passing through the instrument at a given time. To do 
this, the instrument includes a processor which accesses stored 
calibration information derived for one or more process fluids the mass 
flowmeter measures and covers the operating range of the instrument. The 
signal from the flow sensor is processed by the instrument's processor 
using the calibration curve, temperature and pressure information to give 
an accurate indication of the flow rate. 
The mass flow controller comprises the same sensing and signal processing 
elements as the mass flowmeter with the addition of a valve drive that 
operates a fluid flow valve to control the mass flow rate of fluid into 
the process and a control unit. A set point is established by an external 
input supplied by the user to establish a desired flow rate for a process 
fluid. The control unit of the instrument operates the valve drive. To do 
this, the control unit includes a processor which accesses stored 
calibration information derived for one or more process fluids the mass 
flow controller controls and which covers the operating range of the 
instrument. From this calibration curve, the fluid flow rate for the 
process fluid to be delivered by the valve is determined. The calibration 
information stored in the instrument is derived from calibration data for 
a calibration fluid which is not the process fluid whose flow is now being 
controlled, but which has similar thermodynamic transport properties. The 
calibration information is stored in a data base and the instrument's 
calibration is established for a particular process fluid by adapting the 
instrument's calibration curve for a calibration gas at certain set point 
conditions over the operating range of the instrument using the process 
fluid data stored in the data base. As a consequence, even though the 
instrument is calibrated with an inert gas, for example, the instrument 
can now accurately meter mass flow of a process fluid it is monitoring 
without external involvement. This, even though the process fluid is a 
toxic, reactive fluid. A system of process control employing multiple mass 
flowmeters and/or mass flow controllers in which set point information is 
supplied to each instrument, and a method of calibrating a flowmeter or 
mass flow controller with an inert fluid and adapting the resulting 
calibration curve so the instrument can be used for toxic, reactive 
process fluids are also disclosed. Other objects and features will be in 
part apparent and in part pointed out hereinafter.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings, FIG. 1 represents a prior art analog mass flow 
controller (MFC) 10. In an analog MFC, the functional components of the 
controller are implemented using resistors, potentiometers, capacitors, 
amplifiers, etc. In this device, a flow rate sensor 12 is a thermal sensor 
which, as is well-known in the art, converts the flow rate of a gas into 
an electrical voltage signal. In a flow controller manufactured by the 
assignee of the present application, the amplitude of this signal is a 
function of the thermal gradient (temperature difference) between an 
upstream and downstream monitoring location, and hence measured flow rate. 
A thermistor 14 is connected in series with windings (not shown) of the 
sensor to provide compensation for shifts in the sensor calibration 
resulting from temperature effects on a measurement. Use of the thermistor 
typically provides a linear or first order compensation. The full scale 
output voltage of sensor 12 is on the order of 50 mVDC. 
The sensor output is provided to a gain and linearization module 16 in 
which the analog output signal from the sensor is amplified, linearized, 
and then supplied to a junction point 18. Module 16 employs feedback to 
produce a linear output to the summing point and controller, filtering to 
eliminate noise effects on the output signal, and adjustable components 
(potentiometers) for controller calibration. The output signal from module 
16 is, for example, variable from 0-5 VDC, and a setpoint input to the 
controller also varies between 0-5 VDC. These signals are summed at 
junction point 18 and their difference is provided to a controller 20 
which uses the difference value to determine the position of a fluid flow 
control valve V. The valve position is controlled by a valve drive 22 to 
which outputs from controller module 20 are provided. The controller 
module takes into account factors such as the established operating 
setpoint, and overshoot, undershoot, and steady-state operating conditions 
to determine the valve V position. 
Calibration of analog device 10 is performed by determining and adjusting 
the flow of a calibration fluid at three points within the metering range 
of the instrument. These points reflect 0%, 50%, and 100% of the 
instrument's scale range. Based upon the instrument's performance, the 
potentiometers within module 16 are adjusted so the resulting calibration 
curve is essentially as represented by the dashed line in FIG. 2. That is, 
they are adjusted to control the instrument's zero, span, and linearity. 
As can be seen in the Figure, the ideal curve is a straight line (the 
solid line) extending between the 0,0 and 100,100 co-ordinates on the 
curve. However, the calibration curve may have a positive or negative 
offset at the respective ends of the curve; i.e., at the higher and lower 
flow rates. It will be understood that the dashed line representation in 
the Figure is exaggerated for purposes of understanding the performance 
capabilities of the instrument. The actual worst case error of a 
calibrated instrument is on the order of .+-.1% full scale. 
Referring to FIG. 3, a prior art digital mass rate flow controller 30 
includes a flow sensor 32 and a temperature sensor 34. Here, an analog 
output signal from each sensor is separately provided to an 
analog-to-digital converter (ADC) 36, 38 respectively. ADC 36 is, for 
example, a twenty-four bit converter, as is ADC 38. The digital output 
from each converter is applied as a separate mathematical input to a 
microprocessor 40. Microprocessor 40 incorporates three elements. First is 
a microcontroller 42, second is a 64K by 8 erasable programmable 
read-only-memory or EPROM 44, and third is a 4K by 8 EEPROM 46. Operating 
software for running controller 30 is stored in EPROM 44, and product 
information and calibration tables are stored in EEPROM 46. The software 
implemented in the microprocessor performs the linearization and filtering 
functions performed in module 16 of the analog controller 10, as well as 
the controller 20 functions of the analog instrument. In addition, the 
microprocessor has enhanced performance capabilities in these areas as 
well as the capability to provide performance outputs to the user on a 
timely basis. 
The control output from the microprocessor is a digital signal supplied to 
a digital-to-analog converter (DAC) 48 which produces an analog signal for 
valve drive 22 to open and close valve V. 
For purposes of this application, it will be understood that the 
distinction between an analog and a digital flowmeter is that in an analog 
unit, the basic signal conditioning and control functions are performed 
using an operational amplifier (op-amp). In a digital unit, a 
microprocessor performs these functions. It will further be understood 
that in a flowmetering system, a digital flowmeter, for example, may be 
used with an analog communications system. Other variations are also 
possible depending upon the user's system in which a flowmeter or flow 
controller is installed. 
Calibration of a digital flow controller differs significantly from the 
calibration of an analog flow controller. Now using variable digital 
values, a full scale flow rate having an accuracy on the order of .+-.2% 
is produced. Next, the flow controller is operated at a number of 
different set points (ten, for example) over the operating range of the 
instrument. Performance data is accumulated for each set point. An 
equation is now generated using the resulting text data. The equation 
represents the calibration curve for the instrument over the entire 
operating range of the instrument. Using the equation, a table of 
calibration points (twenty-five, for example) is created and stored in 
memory 46 of the controller. A plot of the flow rate vs. set point curve 
is illustrated in FIG. 4. The values displayed on the curve of FIG. 4 are 
corrected using temperature information from sensor 34. The combination of 
the information from the curve, and the temperature correction, result in 
a worst case flow rate error on the order of .+-.0.2% full scale. Memory 
46 of controller 30 is capable of storing multiple calibration curves so 
the controller can be separately calibrated for multiple gases and 
multiple flow rates. 
As previously mentioned, it has heretofore been impractical to always 
calibrate a controller with the process gas with which the controller is 
used. Rather, an inert calibration gas, or a surrogate gas having similar 
fluid thermodynamic properties to the process gas have been used for 
calibration. The subsequent user of the controller then applies a 
conversion factor between measured flow rate data, and the calibration 
curve data, to generate a desired flow rate value for a particular set 
point. This conversion factor is based upon the relative thermodynamic 
properties of the calibration fluid and a process fluid with which the 
controller is used. As noted, users of the flow controller have separately 
determined a conversion factor for use with a particular process fluid 
under given set point conditions. This leads to process inefficiencies, as 
well as errors in performance. 
Referring now to FIG. 5, a flowmeter of the present invention is indicated 
generally 100 and a flow controller 101. The flowmeter or flow controller 
can be used individually; or, as shown in FIG. 5, in a system having a 
plurality of other flowmeters and flow controllers indicated MFC2. . . 
MFCn. When used in a system, respective flowrneters and flow controllers 
are in communication with a process control 102 is used to monitor the 
process and to establish set point conditions for each instrument. In FIG. 
5, flowmeter 100, which is shown to be a digital flowmeter, is connected 
to a fluid flow control portion of a process 104. The portion of the 
process with which flowmeter 100 is associated includes a fluid flow 
passage 106, an inlet 108 to the passage, an outlet 110 from the passage, 
and a bypass 112 through which a portion of the process fluid flows. In 
common parlance, bypass 112 is also referred to as a restrictor, flow 
shunt, or flow splitter. Fluid flow through bypass 112 is monitored by a 
flow sensor 114 of the flowmeter, and by a temperature sensor 116. A 
pressure sensor 117 may also be used by the instrument. The fluid flow 
information gathered by sensor 114 is an analog signal output to an A/D 
converter 118. Similarly, the output of temperature sensor 116 (or 
pressure sensor 117) is an analog output which is provided as an input to 
an A/D converter 120. The digital signal outputs of the A/D converters are 
supplied to a microprocessor 122 of the flowmeter. Stored within a memory 
portion of the microprocessor are a series of data sets representing 
calibration curves developed for the instrument using data developed 
specifically for the process fluids with which the flowmeter or flow 
controller is used and for specific fluid pressure and fluid flow 
conditions. The microprocessor, utilizing the data set or fluid 
calibration curve for established set point conditions for the process, 
and the process fluid flow data, is now able to generate a fluid flow 
signal by which accurate flow rates are achieved. The result is the 
production of a control signal for a valve drive 124 by which the valve 
drive can open or close a flow control valve 126 and precisely control 
process fluid flow through the passage. The control signal from 
microprocessor 122 is a digital signal supplied to a D/A converter 128 to 
produce an analog signal used by valve drive 124. 
Referring to FIGS. 6 and 7, the flow charts of the Figures. set forth how a 
determination is made as whether or not a flowmeter 100 or mass flow 
controller 101 is to be used with a process fluid for which flow control 
data exists, whether or not flow data for a particular process fluid is 
already stored in a data base; and, if not, how flow control data for the 
process fluid is developed, stored in the data base, and used to create a 
data set stored in a memory portion of microprocessor 122 of the 
instrument. When an order for a flow controller is received as indicated 
at step S1, the order typically includes a set of operating criteria in 
which the instrument will be used to control flow rate of a process fluid. 
This criteria includes the process fluids with which the controller will 
be used, as well as the flow range, and temperature and pressure 
conditions. A determination is therefore first made as to whether flow 
control information for the fluid or fluids and the range of operating 
conditions are currently in the data base. This is step S2 in FIG. 6. If 
so, the next determination is whether the flowmeter or mass flow 
controller will be an analog or digital instrument. This occurs at step 
S3. 
If the instrument is an analog instrument, then the instrument is 
constructed and a calibration is performed on the instrument using 
nitrogen gas, for example. This calibration is then matched to a companion 
curve generated from the stored flow data for the process fluid. This is 
step S4. At step S5, a quality control check is performed to verify that 
the companion curve does match. If there is verification, then the 
instrument is shipped as indicated at step S6. 
If the instrument is to be a digital flowmeter or mass flow controller, 
then at step S7, the instrument is constructed and calibrated. Again, 
nitrogen gas is the calibration fluid. Now, a scaling or conversion factor 
is used to determine full scale flow of nitrogen and a conversion factor 
equation is developed based on the calibration results. Generation of the 
scaling factor is discussed hereafter. The conversion factor equation is 
stored in the microprocessor memory of the instrument. At step S8, the 
equation is used to produce a calibration for the instrument for the 
process gas with which the controller is used, and the given set of 
operating conditions. Next a quality control check is made of the 
instrument. This is step S9. If successful, the digital mass flow 
controller is shipped. 
Returning to step S2, if there is currently no information in the data base 
for a particular process fluid or set of operating conditions for a 
process fluid, then we proceed to step S10. At step S10, it is determined 
if there is any flow data for a particular process fluid; and if so, what 
are the "bounding" conditions for the data. That is, what are the 
temperature and pressure conditions for which flow data was obtained, and 
how closely do these bounding conditions approximate those under for which 
the instrument will be used with the fluid. If there is no relevant 
information, then data base information will be developed at step S11 and 
as discussed with reference to FIG. 7. If there is bounding information 
for the process gas as indicated at step S12, a conversion factor is 
developed by which a companion curve can be generated for use in the 
instrument's calibration. This is the conversion factor used at step S8 in 
the calibration of a digital mass flow controller. 
Referring to FIG. 7, step S11 involves generation of process fluid 
information for inclusion in a data base. The data is accumulated for a 
variety of process fluids for a range of operating conditions. At step 
S13, the threshold question to be asked is what data to collect. There are 
two sets of such data as indicated by steps S14 and S15. Step S14 is the 
information requested by the customer for the process gases and sets of 
conditions under which the gas will be used. Step S14 includes the 
calibration information normally generated by the instrument manufacturer. 
That is, the manufacturer will have a standard calibration procedure (or 
procedures) which is normally performed on each instrument. From the 
information gathered at steps S14 and S15, an overall calibration plan for 
the instrument is defined at step S16. This information now includes all 
of the process fluids with which the instrument will possibly be used, 
full scale flow values, and the range of temperature and pressure 
conditions for the various process fluids. 
Next, the manufacturer builds a number of instruments as indicated at step 
S17. By building a minimum number of instruments, statistical validity of 
the instruments' calibration can be determined. As indicated at step S18, 
the instruments are then calibrated. Part of this procedure includes 
ranging the full scale output for a calibration gas equivalent of the 
actual (process) gas for given sets of conditions. That is, obtaining data 
for the defined range of conditions using a gas having thermodynamic 
transport properties which closely match those of the process fluid. After 
testing is complete, the instruments are transported (step S19) to a 
calibration installation which has facilities to develop the flow data for 
the process fluids with which the instruments are used. 
At the test facility, and as indicated at step S20, the instruments are 
separated into analog and digital groupings. For an analog instrument, an 
actual gas calibration is performed using a process fluid at each of a set 
of pressure and temperature conditions (i.e., P1-T1, P2-T2, . . . Pn-Tn). 
This is step S21. At step S22, a calibration check procedure is performed 
using a calibration gas at the same pressure conditions as the process gas 
and the calibration gas calibration data is compared against that obtained 
for the process fluids. The instruments are then returned to the 
manufacturer (step S23) where calibration tests are made at the various 
pressure conditions using the calibration gas (step S24). The resulting 
calibration data is now checked (step S25) and if the results correlate 
with those from the testing facility, the flow data for the process fluid 
for the given sets of conditions are entered into a data base 200 
established for this purpose. If the results do not correlate, then the 
process set out in steps S20-S24 is repeated. 
For a digital flowmeter, as indicated at step S26, data for the actual gas 
is collected at specific set point (temperature and pressure) conditions. 
If additional testing is desired, in order to obtain bounding conditions 
for performance predictions, then further actual gas testing is performed 
for additional conditions (step S27). If no additional testing is done, 
then a calibration check (similar to that performed at step S22 for analog 
controllers) is performed (step S29). Thereafter, the instruments are 
returned to the manufacturer (step S30) for the manufacturer to perform a 
calibration check at his facility (step S31). Again, if the calibrations 
check out, the data is incorporated in data base 200. If not, steps 
S25-S31 are repeated. 
It will be understood that now, unlike with previous instruments and 
calibration systems, one or more data sets can be created for each 
flowmeter or mass flow controller, not only for each process fluid with 
which the instrument is used, but for the range of flow conditions which 
will be experienced in carrying out the process with which the fluid is 
used. These data sets are represented by stored calibration curves. Now, 
when a set point is established, the control means of the instrument can 
access the appropriate data set to provide the appropriate flow control 
signal to the valve means for sensed temperature and/or pressure 
conditions. This capability eliminates the need for external manipulation 
of process temperature and pressure data, to provide flow control inputs 
into the process. Further, once the data base 200 is established, it can 
be updated, amended, etc. as additional process fluid information is 
collected. This not only improves the quality of instrument calibration, 
but reduces the time and cost involved in performing a calibration. 
Referring now to FIGS. 8A and 8B, there is presented a simplified 
calibration/linearization method for a digital flowmeter or mass flow 
controller. 
TABLE 1 
______________________________________ 
1 2 
Set Measur- 3 4 
point and 
ed Curve Desired 
5 6 
Raw Flow, fit N2 Flow, Desired Signal 
Signal in Engr. 
Signal N2 Flow N2 N2 as gas 
Units, N2 
______________________________________ 
0.000 0 0 0 0.000 0 
0.625 160 125 0.488 125 
1.250 320 320 250 0.977 250 
1.875 470 375 1.496 375 
2.500 600 600 500 2.083 500 
3.125 700 625 2.790 625 
3.750 800 800 750 3.516 750 
4.375 900 875 4.253 875 
5.000 1000 1000 1000 5.000 1000 
______________________________________ 
Using the data from the above table 1, the curve shown in FIG. 8A is 
plotted for flow volume in standard cubic centimeters per minute as the 
abscissa and a set point and raw signal value as the ordinate. The range 
for the set point is from 0.0 to 5.0, and five points are plotted to 
generate the curve. The set point values are listed in column 1 of the 
chart, the five plotted points in column 2. Once the curve has been 
created, the curve fit flow values listed in column 3 are taken directly 
from the plot. 
FIG. 8B illustrates a calibration curve for a digital flowmeter or flow 
controller in which the ordinate is the same as in FIG. 8A. Now, the 
abscissa is for a desired signal and represents a modified set point 
value. These value are derived from the measured flow data for a set point 
as follows: 
A desired flow is listed in column 4 of the chart. The desired signal 
representing this flow is equal to the desired flow value of column 4, 
divided by the curve fit flow value of column 3, and with the result of 
the division multiplied by the raw signal value of column 1. That is, 
EQU Desired signal=(desired flow/curve fit flow)*raw signal 
As an example of how the curve of FIG. 8B is generated, for a raw signal 
value of 0.625 (point X in FIG. 8B), the desired flow value in column 4 is 
125, and the curve fit flow value in column 3 is 160. Using the above 
equation, the desired signal value is 
EQU desired signal=(125/160)*0.625=0.488 
which is the value entered in column 5. In column 6, the desired signal is 
expressed in engineering units. The values in this column are arrived at 
by multiplying the desired signal value in column 5 by a gas scaling 
factor. The values calculated for columns 5 and 6 now represent stored 
calibration data. 
With respect to the curve shown in FIG. 8B, once all of the desired signal 
values have been calculated, the calibration is linear fit between 
adjacent points. The table of data points for this curve are stored in the 
memory portion of the microprocessor for the instrument. Now, when a set 
point is established, the desired signal representing measured flow of the 
process fluid for that set point can be found in a look up table in the 
memory. Thus, as illustrated in FIG. 8B, for a set point of 4.000, the 
desired signal can be readily established. Here, it is approximately 
3.850. 
Referring to the table 2 set out below and the FIGS. 9A and 9B, a 
calibration performed on a digital flowmeter first comprises performing 
the steps involved in collecting the measured flow data set out in column 
2 of the table, using nitrogen gas, for the set point conditions listed in 
column 1 of the table. As in the previously described calibration, a curve 
fit is made using the measured flow data. The resulting fitted curve is 
indicated C1 in FIG. 9A. Next, similar data is gathered for a process gas, 
the actual gas (AG), with which the digital flow meter would be used, and 
a calculated AG curve is generated as indicated by curve C2 in FIG. 9A. 
TABLE 2 
__________________________________________________________________________ 
5 
1 2 3 4 Theoretical 
6 7 8 
Set point and 
Measured 
Curve fit 
Data set 
Actual Flow 
Desired Flow 
Desired Signal 
Signal in Engr. 
Raw Signal 
Flow, N2 
N2 Flow 
C.F. eqn 
AG AG AG as gas 
Units, AG 
__________________________________________________________________________ 
0.000 0 0 1.000 
0.000 0 0.000 0 
0.625 160 1.000 
160.000 
100 0.391 100 
1.250 310 310 0.938 
290.625 
200 0.860 200 
1.875 455 0.872 
396.915 
300 1.417 300 
2.500 575 575 0.850 
488.750 
400 2.046 400 
3.125 690 0.843 
581.571 
500 2.687 500 
3.750 790 790 0.838 
661.625 
600 3.401 600 
4.375 895 0.822 
735.889 
700 4.162 700 
5.000 1000 1000 0.800 
800.000 
800 5.000 800 
__________________________________________________________________________ 
From these two curves, a ratio of values between the respective data values 
used in generating the curves can be created. The respective ratios for 
each set point are tabulated in column 4 above. Using this information, 
theoretical actual flow values for the actual gas can be calculated using 
the equation: 
EQU theoretical actual flow=curve fit equation value (column 4)*curve fit N2 
flow (column 3) 
Using this equation, the values listed in column 5 are tabulated. 
Referring to table 3, and FIGS. 10A and 10B, the data set equation 
correction factors tabulated in column 4 of table 2, are arrived at as 
follows. 
TABLE 3 
______________________________________ 
B C D E F 
A Average Average Average 
Average 
Ratio of 
Set point and 
Measured Measured Curve Fit 
Curve Fit 
Average 
Raw Signal 
Flow, AG Flow, N2 Ag flow 
N2 Flow 
Curve Fits 
______________________________________ 
0.000 0 0 0 0 
0.625 160 160 1.000 
1.250 300 320 300 320 0.938 
1.875 410 470 0.872 
2.500 500 600 510 600 0.850 
3.125 590 700 0.843 
3.750 680 800 670 800 0.838 
4.375 740 900 0.822 
5.000 800 1000 800 1000 0.800 
______________________________________ 
In FIG. 10A, curve X1 is a plot of the measured flow data for the actual 
gas, and curve X2 the measured flow data for the N2 gas. These plots are 
measured on the average measured flow values listed in columns B and C of 
table 3. The values respectively listed in columns D and E are the flow 
values for the set point values of column A, as taken from curves X1 and 
X2. The ratio values listed in column F of the table are arrived at by 
dividing the value for actual gas flow listed in column D by the N2 flow 
value in column E. Thus for the set point value 0.125, the actual gas flow 
value 300 divided by the corresponding value 320 for N2 gas yields a ratio 
of 0.938. A curve X3 shown in FIG. 10B is a plot of the calculated ratios 
shown in column F. In accordance with the teachings of the invention, the 
actual gas and nitrogen or calibration gas values listed in table 3, and 
the calculated ratio values, are stored in the data base now used for mass 
flow controller calibration. 
In column 6 of table 2, desired flow values for the actual gas are listed. 
For each desired flow level, a corresponding desired signal level can be 
determined from the equation: 
EQU desired signal=(desired flow/curve fit flow)*(raw signal) 
Thus for example, for a desired actual gas flow of 100, the desired signal 
is calculated as 
EQU desired flow=(100(column 6 value)/160(column 2 value))*0.625(column 1 value 
) 
The resultant value is entered in column 7. After these values are 
produced, the curve C3 shown in FIG. 9B is generated. Also, and as listed 
in column 8 of the table, the signal in engineering units for the actual 
gas can be created by multiplying the desired signal values of column 7 by 
a gas scaling factor. 
For flowmeter 100 or flow controller 101 of FIG. 5, the microprocessor 122 
has stored therein data sets of process fluid calibration information 
which effectively comprises a series of curves C3. These curves are for 
all the process gases with which the controller is used and allows the 
instrument to provide accurate flow control for each of the process gases 
for the entire range of set point conditions with which may be encountered 
by the instrument as part of the process. Each of the other digital mass 
flow controllers MFC2-MFCn is similarly calibrated for the process fluids 
with which they are used. The process control 102 to which each of the 
mass flow controllers is connected, provides updated set point and other 
relevant information to each of the units. The process control can poll 
each separate instrument to obtain status and other pertinent information 
used to control the process. 
An important advantage of such an instrument calibrated in accordance with 
the method of the invention, is that the flow control curve C3 developed 
by each controller for each process fluid, eliminates the need for 
"tweaking", or otherwise having to refine flow process information 
developed by a flow controller to a fluid flow rate for a process fluid. 
As previously mentioned, it is not only desirable to calibrate digital 
flowmeters and mass flow controllers using the method of the invention, 
but analog instruments as well. Accordingly, table 4 includes calibration 
gas data and process fluid or actual gas data. 
TABLE 4 
______________________________________ 
2 3 4 5 
1 Average Average Average 
Average 
Set point and 
Measured Linearized Measured 
Curve Fit 
Raw Signal 
Flow, AG Flow, AG Flow N2 
N2 Flow 
______________________________________ 
0.000 0 0 0 0 
0.625 140 
1.250 300 220 270 270 
1.875 375 
2.500 510 400 480 480 
3.125 590 
3.750 680 580 720 720 
4.375 850 
5.000 800 800 1000 1000 
______________________________________ 
The data in column 2 of table 4 represents average measured flow data for 
the actual process gas. After linearization, the data is plotted as shown 
by curve C4 in FIG. 11, and listed in column 3 of the table. After data 
has been similarly acquired for the calibration gas, the process steps 
previously described with respect to digital instrument calibrations are 
performed. The result is the curve C5 in FIG. 11 and the data points 
listed in column 5 of the table. 
The digital flow meters MFC1-MFCn shown in FIG. 5, could be analog flow 
meters with the same process control capability being realizable. Again, 
the flow controllers could be connected in a communications system with a 
process control whereby the process control is able to provide set point 
and other relevant information to each controller and receive current 
process fluid flow information in return. 
A further advantage of the invention is the establishment of an improved, 
digital communications system 300 for routing information to and from the 
process control and individual instruments. This digital system eliminates 
signal errors resulting from noise and other effects. Elimination of such 
errors increases the precision with which the process is controlled 
thereby increasing the quality of the product produced by the process. 
What has been described is an improved flowmeter or mass flow controller 
having significantly greater accuracy than conventional digital or analog 
units. The instrument has both improved signal processing and digital 
communications capabilities, and can be specifically calibrated for the 
manufacturing process in which it will be used. It is a particular 
advantage of the method of the invention to quickly and efficiently 
calibrate digital and analog flowmeters and mass flow controllers, and to 
do so at a reasonable cost while providing a high precision instrument 
such as is needed in certain manufacturing processes. Additionally, the 
complexity of signaling and controlling a process is reduced because of 
the improved system's communications. Overall, monitoring and control 
capabilities are increased which produces savings in process costs for the 
manufacture of articles such as semiconductor devices. Calibration is 
based upon a particular customer's process gas or gases and eliminates the 
"tweaking" and "cut and try" techniques now used to accommodate an 
instrument to a particular application. Although calibration is done using 
"safe" gases, instead of the highly toxic and highly reactive gases with 
which an instrument is actually used, the thermodynamic transport 
properties of such gases are readily taken into account during 
calibration. Representative units are independently calibrated for each of 
a number of gases with which it is used, with the calibration information 
of each gas stored within a memory of a flowmeter or flow controller, the 
instrument having sufficient data storage capability so all relevant 
instrument and calibration data is stored in the instrument and is readily 
accessible by the user. To facilitate instrument calibrations, a database 
is created containing information relating to a unit's operation with a 
gas as well to that of the calibration gases. The database enables 
calibration accuracy to be consistent over the unit's entire operating 
range, regardless of which gas with which the instrument is used, and the 
entire range of gas flow rates. The improved instrument also has a remote 
capability, a digitally adjustable setpoint and ramprate, and temperature 
monitoring for indicating the temperature outside the instrument's flow 
ate sensor. A direct indication is also provided of the Mw sensor signal 
and valve drive signal to detect sensor clogging or restriction. Multiple 
flow controllers can be interconnected into a flowmetering system for 
facilitating process control wherein each flowmeter is able to access the 
database to obtain information pertinent to just that flowmeter to enable 
each flowmeter to separately regulate fluid flow in respective areas of 
the process. 
In view of the foregoing, it will be seen that the several objects of the 
invention are achieved and other advantageous results are obtained. 
As various changes could be made in the above constructions without 
departing from the scope of the invention, it is intended that all matter 
contained in the above description or shown in the accompanying drawings 
shall be interpreted as illustrative and not in a limiting sense.