Abstrict 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.
Claims What is claimed is:
1. A fluid mass flow meter, particularly useful in measuring fluid
mass flow in a gas process system, said flow meter comprising: a
body including a passage extending therethrough and adapted to be
in fluid flow receiving communication with a source of process fluid;
a flow restrictor disposed in said passage; a first set of plural
fluid mass flow sensors in fluid flow communication with said passage,
each of said fluid mass flow sensors having a full scale flow measurement
range capability different from each of the other fluid mass flow
sensors for measuring fluid mass flow over a substantial range of
fluid mass flow rates for fluid flowing through said flow meter.
2. The flow meter set forth in claim 1 wherein: at least one of
said mass flow sensors is a thermal mass flow sensor.
3. The flow meter set forth in claim 2 wherein: said flow meter
includes three mass flow sensors, each of said mass flow sensors
having a full scale flow measurement range different from the other
of said mass flow sensors.
4. The flow meter set forth in claim 2 wherein: all of said mass
flow sensors are thermal mass flow sensors.
5. The flow meter set forth in claim 3 wherein: the full scale
flow measurement range of each of said mass flow sensors overlaps
a portion of the full scale flow measurement range of at least one
other mass flow sensor in said flow meter.
6. The flow meter set forth in claim 1 including: a second flow
restrictor disposed in said passage in said body downstream of the
first mentioned flow restrictor and a second set of plural mass
flow sensors in fluid flow receiving communication with conduits
connected to said body for measuring fluid mass flow through said
flow meter.
7. The flow meter set forth in claim 6 wherein: each of said plural
mass flow sensors of said second set includes a full scale fluid
flow operating range which overlaps at least a portion of the full
scale mass flow measurement range of at least one other mass flow
sensor of said second set.
8. The flow meter set forth in claim 7 wherein: at least one of
said mass flow sensors of said second set is a thermal mass flow
sensor.
9. The flow meter set forth in claim 1 wherein: the full scale
flow measurement ranges of one of said mass flow sensors varies
by a factor of at least two times the full scale flow measurement
range of another mass flow sensor of said flow meter.
10. The flow meter set forth in claim 1 including: a second body
including a passage therein and a second flow restrictor disposed
in said passage in said second body, said second body and said first
body being operable to be in fluid flow receiving communication
with said source; conduit means interconnecting said bodies and
in fluid flow communication with said mass, flow sensors; and flow
control valve means for controlling fluid flow through one or both
of said bodies.
11. The flow meter set forth in claim 10 including: flow control
valve means for controlling fluid flow through said mass flow sensors
from a selected one of said bodies.
12. The flow meter set forth in claim 10 including: flow control
valve means for selectively controlling fluid flow through at least
one of said mass flow sensors.
13. The flow meter set forth in claim 10 wherein: said first flow
restrictor and said second flow restrictor are selected from a group
consisting of a plug forming an annular flow path in said passage
in said first body or said second body and a wire mesh member.
14. A fluid mass flow meter for use in measuring mass flow of a
fluid to a process, said flow meter comprising: a body including
a passage therethrough, said body being adapted to be connected
to a source of pressure gas at one end of said passage; a first
flow restrictor disposed in said passage; a fluid mass flow sensor
in fluid flow communication when said passage for measuring fluid
mass flow through said flow meter, said mass flow sensor including
a first conduit in communication with said passage on one side of
said first flow restrictor and a second conduit in communication
with said passage on an opposite side of said first flow restrictor
and a second flow restrictor interposed in one of said conduits.
15. The flow meter set forth in claim 14 including: a third flow
restrictor disposed in said one of said conduits.
16. The flow meter set forth in claim 15 wherein: said second flow
restrictor has a greater fluid flow restriction than said third
flow restrictor.
17. The flow meter set forth in claim 16 wherein: the flow restriction
characteristics of said second flow restrictor are about twenty
times greater than the flow restriction characteristics of said
third flow restrictor.
18. The flow meter set forth in claim 16 wherein: said second flow
restrictor is disposed upstream of said mass flow sensor.
19. The flow meter set forth in claim 16 wherein: said third flow
restrictor is disposed downstream of said mass flow sensor.
20. The flow meter set forth in claim 16 including: a flow control
valve interposed said second flow restrictor and said third flow
restrictor.
21. The flow meter set forth in claim 16 wherein: said fluid mass
flow sensor comprises a differential pressure transducer.
22. A fluid mass flow meter for measuring flow of a gaseous fluid,
said flow meter comprising: a body including a passage therethrough,
said body being adapted to be connected to a source of pressure
gas at one end of said passage; a flow restrictor disposed in said
passage; a fluid mass flow sensor including conduit means connected
to said passage on opposite sides of said flow restrictor with respect
to the direction of fluid flow through said passage, said fluid
mass flow sensor including a pressure transducer for sensing one
of a differential pressure across said flow restrictor and the pressure
of fluid in said passage upstream of said flow restrictor, respectively.
23. The flow meter set forth in claim 22 including: a temperature
sensor for sensing the temperature of fluid flowing through said
passage.
24. The flow meter set forth in claim 22 wherein: said pressure
transducer comprises a differential pressure transducer connected
to said conduit means for measuring a differential pressure across
said flow restrictor and said mass flow sensor includes an absolute
pressure reference device for sensing the absolute pressure in said
passage downstream of said flow restrictor.
25. The flow meter set forth in claim 24 including: flow restriction
means disposed in said conduit means and providing a pressure divider
to modify the pressure differential seen by said pressure transducer.
26. The flow meter set forth in claim 25 wherein: said flow restriction
means includes a first flow restriction disposed in said conduit
means between said passage and a branch conduit connected to said
conduit means and said pressure transducer and a second flow restriction
disposed in said conduit means between said first branch conduit
and a second branch conduit connected to said conduit means and
said pressure transducer.
27. The flow meter set forth in claim 26 including: shutoff valve
means disposed in said conduit means between said first flow restriction
and said second flow restriction.
28. The flow meter set forth in claim 22 wherein: said fluid mass
flow sensor includes a pressure transducer for measuring the absolute
pressure of fluid in said passage upstream of said flow restrictor
and said fluid mass flow sensor includes an absolute pressure reference
device for measuring the absolute pressure of fluid in said passage
downstream of said flow restrictor.
29. The flow meter set forth in claim 28 including: flow restriction
means disposed in said conduit means and providing a pressure divider
to modify the pressure seen by said pressure transducer.
30. The flow meter set forth in claim 29 wherein: said flow restriction
means includes a first flow restriction disposed in said conduit
means between said passage and a branch conduit connected to said
conduit means and said pressure transducer and a second flow restriction
disposed in said conduit means between said first branch conduit
and a second branch conduit connected to said pressure reference
device.
31. The flow meter set forth in claim 30 including: shutoff valve
means disposed in said conduit means between said first flow restriction
and said second flow restriction.
32. A fluid mass flow meter, particularly useful in measuring fluid
mass flow in a gas process system, said flow meter comprising: a
first body including a first passage extending therethrough and
operable to be in fluid flow receiving communication with a source
of process fluid; a second body including a second passage extending
therethrough and adapted to be in fluid flow receiving communication
with said source of process fluid; first and second flow restrictors
disposed in said first and second passages, respectively; plural
fluid mass flow sensors operable to be in fluid flow communication
with said first and second passages, said mass flow sensors having
predetermined full scale flow measurement ranges, respectively,
for measuring fluid mass flow over a substantial range of fluid
flow rates through said flow meter, said fluid mass flow sensors
each being operably connected to a first conduit operable to be
in fluid flow communication with said passages upstream of said
first and second flow restrictors and said fluid mass flow sensors
being operably connected to a second conduit operable to be in fluid
flow communication with said passages at a point downstream of said
first and second flow restrictors.
33. The flow meter set forth in claim 32 including: a flow control
valve operable to direct fluid flow from said source to one or both
of said first and second passages.
34. The flow meter set forth in claim 32 including: a flow control
valve operably associated with said flow meter for shutting off
fluid flow through at least one of said fluid mass flow sensors
while permitting fluid flow through at least another of said fluid
mass flow sensors.
35. The flow meter set forth in claim 32 including: a third flow
restrictor disposed in one of said conduits between said plural
mass flow sensors and one of said first and second passages.
36. The flow meter set forth in claim 32 wherein: at least one
of said fluid mass flow sensors is a thermal fluid mass flow sensor.
37. The flow meter set forth in claim 36 wherein: each of said
fluid mass flow sensors is a thermal mass flow sensor.
Description CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority of U.S. Provisional
Patent Application 60/226806 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] 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] FIG. 2 is a schematic diagram of another preferred embodiment
of a thermal sensor based mass flow meter in accordance with the
invention;
[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] FIG. 4 is a schematic diagram of still another thermal sensor
based fluid mass flow meter in accordance with the invention;
[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] 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] 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 12a and
a second section 12b. Conduit section 12a 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 12b 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 12b 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 16a 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. 5660207 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. 5660207 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 54a 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 12a and
12b through branch conduits 12c and 12d, respectively, as illustrated.
A remotely controllable valve 72 is disposed in conduit 12a between
inlet port 70 and branch conduit 12c and a remotely controllable
valve 74 is disposed in conduit 12c between conduit 12a 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 76a.
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 60b. The flow meter 60b 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 60b,
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 60b 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 60c. The flow meter 60c is similar in some respects
to the flow meters 60 and 60b 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 78a and 80a.
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.'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 12a and
12b 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 12a is connected to inlet port 106
and conduit section 12b 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 118a to a suitable absolute pressure
reference device 122. Transducer 120 is also connected to conduit
116 by branch conduit 116a. 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 10a. The flow meter 100a 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 100a includes
a pressure transducer 120a having an absolute pressure reference
chamber 121 formed therein. In this way the flow meter 100a 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 120a.
[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.
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