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Claims  |
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What is claimed is:
1. A mass flow measuring assembly for performing a multiplicity of
measurements of mass flow of a gas entering the assembly at a flow rate
less than a predetermined maximum flow rate from a gas storage container
at a pressure less than one atmosphere and exiting the assembly into a
vacuum chamber, each measurement performed within not more than a
predetermined maximum response time, the assembly comprising:
a capillary tube having opposed first and second ends and a central portion
equidistant between said first and second ends, the tube having a
circumferential outer surface and a generally cylindrical bore of a
preselected diameter determining a preselected wall thickness, the tube
second end in fluid communication with the gas storage container;
a diffuser section having first and second apertures, the first aperture
attached to and in fluid communication with the tube first end, and
further having a conical, distally diverging passage therethrough
symmetric about an axis of symmetry substantially in-line with the tube
bore, said passage terminating proximally in said first aperture and
distally in said second aperture, said second aperture in fluid
communication with the vacuum chamber, said passage minimizing pressure
drop of the gas exiting said second aperture; and
first and second temperature-measuring elements, said elements in thermal
contact with the outer surface of the capillary tube at about the central
portion, said elements separated by a gap of a preselected width, said
maximum flow rate being 10 sccm and said maximum response time being 4
seconds.
2. The measuring assembly of claim 1, wherein the temperature-measuring
elements are selected from the group consisting of resistance
thermometers, thermocouples, and thermistors.
3. The measuring assembly of claim 2, wherein the resistance thermometers
are self-heated.
4. The measuring assembly of claim 2, wherein:
the bore diameter is in a range from 0.035 to 0.065 inch;
the wall thickness is in a range from 0.001 to 0.003 inch; and
the conical passage of the diffuser section diverges distally at a small
total angle.
5. The measuring assembly of claim 4, wherein the total angle is in a range
from about 15.degree. to about 23.degree..
6. A mass flow measuring assembly for performing a multiplicity of
measurements of mass flow of a gas entering the assembly at a preselected
flow rate in a range from 0.25 to 10 sccm from a gas storage container at
a pressure less than 10 Torr and exiting the assembly into a vacuum
chamber, each measurement performed within not more than 4 seconds, the
assembly comprising:
a capillary tube having opposed first and second ends and a central portion
equidistant between said first and second ends, the tube having a
circumferential outer surface and a generally cylindrical bore of a
preselected diameter, said outer surface and bore determining a
preselected wall thickness, the tube second end in fluid communication
with the gas storage container;
a first diffuser section having first and second apertures, said first
aperture attached to and in fluid communication with the tube first end,
and further having a conical, distally diverging passage therethrough
symmetric about an axis of symmetry substantially in line with the tube
bore, said passage diverging distally at a preselected small total angle
and terminating proximally in said first aperture and distally in said
second aperture, said second aperture in fluid communication with the
vacuum chamber, said passage minimizing pressure drop of the gas exiting
said second aperture;
a second diffuser section having first and second apertures, said first
aperture attached to and in fluid communication with the tube second end,
and further having a conical, distally diverging passage therethrough
symmetric about an axis of symmetry substantially in-line with the tube
bore, said passage diverging distally at said preselected total angle and
terminating proximally in said first aperture and distally in said second
aperture, said passage reducing pressure drop of the gas exiting said
first aperture and entering said tube second end, said second aperture in
fluid communication with the gas storage container; and
first and second resistance thermometers, each thermometer being formed as
a segment of an iron-nickel alloy wire of a predetermined diameter, the
segments of equal length, each segment wound in a tight coil around the
outer surface of the tube central portion, the coils of equal length and
separated by a predetermined gap.
7. The measuring assembly of claim 6, wherein:
the tube bore diameter is in a range from 0.035 to 0.065 inch;
the tube wall thickness is in a range from 0.001 to 0.003 inch;
the wire diameter is in a range from 0.0003 to 0.0006 inch;
the gap separating the coils is in a range from 0.015 to 0.025 inch;
the length of each wound coil is in a range from 0.015 to 0.030 inch; and
the total angle of divergence of each said conical, distally diverging
passage is in a range from about 15.degree. to about 23.degree..
8. The measuring assembly of claim 7, wherein the first and second diffuser
sections each are determined circumferentially by a distally tapering
outer surface, and a portion of each outer surface is circumscribed by a
generally annular compression-type seal.
9. The measuring assembly of claim 7, wherein the first and second diffuser
sections each are determined circumferentially by a generally cylindrical
outer surface having a circumferential groove within which is disposed an
O-ring.
10. A system for controlling mass flow of a gas entering in at a
preselected flow rate from a gas storage container at a pressure less than
one atmosphere and exiting into a vacuum chamber, the system comprising:
a mass flow measuring assembly comprising a capillary tube having opposed
first and second ends and a central portion equidistant between said first
and second ends, the tube having a circumferential outer surface and a
generally cylindrical bore of a preselected diameter, said outer surface
and bore determining a preselected wall thickness, the tube second end in
fluid communication with the gas storage container; said measuring
assembly further comprising a first diffuser section having first and
second apertures, said first aperture attached to and in fluid
communication with the tube first end, and further having a conical,
distally diverging passage therethrough symmetric about an axis of
symmetry substantially in-line with the tube bore, said passage diverging
distally at a preselected small total angle and terminating proximally in
said first aperture and distally in said second aperture, said second
aperture in fluid communication with the vacuum chamber, said passage
minimizing pressure drop of the gas exiling said second aperture; said
measuring assembly further comprising a second diffuser section having
first and second apertures, said first aperture attached to and in fluid
communication with the tube second end, and further having a conical,
distally diverging passage therethrough symmetric about an axis of
symmetry substantially in-line with the tube bore, said passage diverging
distally at said preselected total angle and terminating proximally in
said first aperture and distally in said second aperture, said passage
reducing pressure drop of the gas exiting said first aperture and entering
said tube second end, said second aperture in fluid communication with the
gas storage container; and said measuring assembly further comprising
first and second resistance thermometers, each thermometer being formed as
a segment of an iron-nickel alloy wire of a predetermined diameter, the
segments of equal length, each segment wound in a tight coil around the
outer surface of the tube central portion, the coils of equal length and
separated by a predetermined gap.
11. The system of claim 10, wherein said first and second diffuser sections
each are determined circumferentially by a distally tapering outer
surface, and a portion of each outer surface is circumscribed by a
generally annular compression-type seal.
12. The system of claim 11, further comprising:
an elongated housing comprising an upper portion and a lower portion, the
upper portion having first and second recesses, the lower portion having a
cavity within which is disposed the capillary tube, and further having a
generally circular outlet conduit, a generally circular section, a
generally circular outlet aperture, a valve element, a wall having a bore,
a plug removably inserted into the bore, and a generally circular inlet
conduit, the outlet conduit in fluid communication with said second
aperture of the first diffuser section and substantially in-line with the
axis of the first diffuser section and the tube bore, the first diffuser
section maintained rigid by the compression-type seal between said
tapering outer surface and the outlet conduit, the outlet conduit in fluid
communication with the circular section, the circular section terminating
in the outlet aperture, the valve element seated within the aperture, the
outlet conduit and circular section bounded by the wall, the inlet conduit
in fluid communication with said second aperture of the second diffuser
section and substantially in-line with the axis of the second diffuser
section and the tube bore, the second diffuser section maintained rigid by
the compression-type seal between said tapering outer surface and the
inlet conduit, the capillary tube attached to the first and second
diffuser sections maintained rigid within said cavity; and
a proportional solenoid having an armature, and a signal conditioner having
an upper portion and a lower portion, the lower portion having first and
second pairs of electrical terminals, the solenoid mounted within the
first recess of the housing upper portion, the valve element contiguous to
the armature, the signal conditioner upper portion mounted within the
second recess of the housing upper portion, the signal conditioner lower
portion extending into said cavity, the first and second resistance
thermometers in electrical contact, respectively, with the first and
second pairs of terminals.
13. The system of claim 10, wherein said first and second diffuser sections
each are determined circumferentially by a generally cylindrical outer
surface having a circumferential groove within which is disposed an
O-ring.
14. The system or claim 13, further comprising:
an elongated housing comprising an upper portion and a lower portion, the
upper portion having first and second recesses, the lower portion having a
cavity within which is disposed the capillary tube, and further having a
generally circular outlet conduit, a generally circular section, a
generally circular outlet aperture, a valve element, a wall having a bore,
a plug removably inserted into the bore, and a generally circular inlet
conduit, the outlet conduit in fluid communication with said second
aperture of the first diffuser section and substantially in-line with the
axis of the first diffuser section and the tube bore, the first diffuser
section maintained rigid by the compressed O-ring between said cylindrical
outer surface and the outlet conduit, the outlet conduit in fluid
communication with the circular section, the circular section terminating
in the outlet aperture, the valve element seated within the aperture, the
outlet conduit and circular section bounded by the wall, the inlet conduit
in fluid communication with said second aperture of the second diffuser
section and substantially in-line with the axis of the second diffuser
section and the tube bore, the second diffuser section maintained rigid by
the compressed O-ring between said cylindrical outer surface and the inlet
conduit, the capillary tube attached to the first and second diffuser
sections maintained rigid within said cavity; and
a proportional solenoid having an armature acting as an actuator, and a
signal conditioner having an upper portion and a lower portion, the lower
portion having first and second pairs of electrical terminals, the
solenoid mounted within the first recess of the housing upper portion, the
valve element contiguous to the armature, the signal conditioner upper
portion mounted within the second recess of the housing upper portion, the
signal conditioner lower portion extending into said cavity, the first and
second resistance thermometers in electrical contact, respectively, with
the first and second pairs of terminals.
15. A system for measuring mass flow of a gas entering in at a preselected
flow rate from a gas storage container at a pressure less than one
atmosphere and exiting into a vacuum chamber, the system comprising:
a mass flow measuring assembly comprising a capillary tube having opposed
first and second ends and a central portion equidistant between said first
and second ends, the tube having a circumferential outer surface and a
generally cylindrical bore of a preselected diameter, said outer surface
and bore determining a preselected wall thickness, the tube second end in
fluid communication with the gas storage container; said measuring
assembly further comprising a first diffuser section having first and
second apertures, said first aperture attached to and in fluid
communication with the tube first end, and further having a conical,
distally diverging passage therethrough symmetric about an axis of
symmetry substantially in-line with the tube bore, said passage diverging
distally at a preselected small total angle and terminating proximally in
said first aperture and distally in said second aperture, said second
aperture in fluid communication with the vacuum chamber, said passage
minimizing pressure drop of the gas exiting said second aperture; said
measuring assembly further comprising a second diffuser section having
first and second apertures, said first aperture attached to and in fluid
communication with the tube second end, and further having a conical,
distally diverging passage therethrough symmetric about an axis of
symmetry substantially in-line with the tube bore, said passage diverging
distally at said preselected total angle and terminating proximally in
said first aperture and distally in said second aperture, said passage
reducing pressure drop of the gas exiting said first aperture and entering
said tube end, said second aperture in fluid communication with the gas
storage container; and said measuring assembly further comprising first
and second resistance thermometers, each thermometer being formed as a
segment of an iron-nickel alloy wire of a predetermined diameter, the
segments of equal length, each segment wound in a tight coil around the
outer surface of the tube central portion, the coils of equal length and
separated by a predetermined gap.
16. The system of claim 15, wherein said first and second diffuser sections
each are determined circumferentially by a distally tapering outer
surface, and a portion of each outer surface is circumscribed by a
generally annular compression-type seal.
17. The system of claim 16, further comprising:
an elongated housing comprising an upper portion and a lower portion, the
upper portion having a recess, the lower portion leaving a cavity within
which is disposed the capillary tube, and further having a generally
circular outlet conduit and a generally circular inlet conduit, the outlet
conduit in fluid communication with said second aperture of the first
diffuser section and substantially in-line with the axis of the first
diffuser section and the tube bore, the first diffuser section maintained
rigid by the compression-type seal between said tapering outer surface and
the outlet conduit, the inlet conduit in fluid communication with said
second aperture of the second diffuser section and substantially in-line
with the axis of the second diffuser section and the tube bore, the second
diffuser section maintained rigid by the compression-type seal between
said tapering outer surface and the inlet conduit, the capillary tube
attached to the first and second diffuser sections maintained rigid within
said cavity; and
a signal conditioner having an upper portion and a lower portion, the lower
portion having first and second pairs of electrical terminals, the upper
portion mounted within the recess of the housing upper portion, the signal
conditioner lower portion extending into said cavity, the first and second
resistance thermometers in electrical contact, respectively, with the
first and second pairs of terminals.
18. The system of claim 15, wherein said first and second diffuser sections
each are determined circumferentially by a generally cylindrical outer
surface having a circumferential groove within which is disposed an
O-ring.
19. The system of claim 18, further comprising:
an elongated housing comprising an upper portion and a lower portion, the
upper portion having a recess, the lower portion having a cavity within
which is disposed the capillary tube, and further having a generally
circular outlet conduit and a generally circular inlet conduit, the outlet
conduit in fluid communication with said second aperture of the first
diffuser section and substantially in-line with the axis of the first
diffuser section and the tube bore, the first diffuser section maintained
rigid by the compressed O-ring between said cylindrical outer surface and
the outlet conduit, the inlet conduit in fluid communication with said
second aperture of the second diffuser section and substantially in-line
with the axis of the second diffuser section and the tube bore, the second
diffuser section maintained rigid by the compressed O-ring between said
cylindrical outer surface and the inlet conduit, the capillary tube
attached to the first and second diffuser sections maintained rigid within
said cavity; and
a signal conditioner having an upper portion and a lower portion, the lower
portion having first and second pairs of electrical terminals, the upper
portion mounted within the recess of the housing upper portion, the signal
conditioner lower portion extending into said cavity, the first and second
resistance thermometer in electrical contact, respectively, with the first
and second pairs of terminals. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to instrumentation for measuring and
controlling the flow of fluids, and more particularly to an assembly for
measuring the mass flow of gases which can be incorporated into a flow
controller or flowmeter.
2. Description of the Related Art
The measurement and control of the mass flow of gases is important in many
industries. During the manufacture of semiconductor chips, for example,
many of the processes require precise reaction of two or more gases under
carefully controlled conditions. Since chemical reactions occur at the
molecular level, control of mass flow is the most direct way to regulate
the absolute and relative quantities of gaseous reactants.
There have been developed in the art a variety of instruments for measuring
the mass flow rate of gases from less than one standard cubic
centimeter(s) per minute (sccm) to more than 500,000 sccm. The prevalent
design of such instruments requires that the flowing gas be divided into
at least two portions. In a typical instrument, a relatively small portion
of the total flow is routed through a sensor assembly where the mass flow
is measured, while the remainder, i.e., almost all of the flow, is routed
through a flow splitter assembly disposed generally parallel to the sensor
assembly. The sensor assembly includes a capillary tube around which are
wound two resistance thermometers as identical as possible in electrical
and mechanical characteristics. Each thermometer is wound in a tight coil
in thermal communication with the outer surface of the tube. The
thermometers form two legs of an electronic bridge; the other two legs are
usually fixed resistors. When a voltage is applied across the bridge,
current flows through each thermometer, causing it to self-heat. When
there is no flow of gas through the capillary tube, the thermometers heat
up identically. As gas begins to flow through the tube, the gas first
cools the upstream thermometer and then the downstream thermometer which
is cooled less because the gas is now slightly warmer due to heating by
the upstream thermometer. The resultant temperature differential is a
function of both the mass flow rate and the properties of the particular
gas. As disclosed in U.S. Pat. No. 1,193,488, C. C. Thomas more than
eighty years ago utilized separate sensor and flow splitter assemblies and
resistance thermometers in designing gas flowmeters. In U.S. Pat. No.
1,222,492, he disclosed a flowmeter which measured the rate of gas mass
flow by imparting heat to raise the gas temperature while automatically
regulating the imparting of heat so as to keep constant either the rate at
which heat is imparted or the temperature rise produced.
Typically, a signal conditioning circuit is used to compensate for
nonlinearities in sensor response, account for changes in operating
parameters such as resistance values, and convert analog output signals
into digital format. Variations of the basic sensor assembly design
include: interposing a heater coil between the two thermometer coils and
reducing the bridge current so that the thermometers do not self-heat;
substituting thermocouples or thermistors for the resistance thermometers;
or arranging the signal conditioning circuit so that it maintains each
thermometer at the same temperature, using the difference in wattage to
the coils to determine the mass flow rate.
In some semiconductor processes such as ion implantation, the process gases
must be stored and handled with great care and not be wasted, because they
are toxic, highly reactive and expensive. Examples are arsine, phosphine
and boron trifluoride. An ion implanter chamber operates at a very low
pressure approaching a hard vacuum, drawing small portions of gas at low
flow rates, typically 0.25 to 10 sccm, out of a storage container through
a mass flow controller. Should an accident occur breaking the vacuum, gas
stored conventionally in a pressurized container would be released into
the environment, with expensive if not hazardous consequences. As
disclosed in U.S. Pat. Nos. 5,518,528, 5,704,965 and 5,707,424, a gas
storage system has been developed by Advanced Technology Materials, Inc.
(ATMI) of Danbury, Conn. wherein gases are stored at slightly less than
atmospheric pressure in special containers filled with porous resin beads
which adsorb gas molecules on their collectively large total surface area.
In the event of a vacuum break there will be minimal escape of gas. The
amount of gas in a container varies non-linearly with pressure; the bottle
is 100% full at atmospheric pressure (760 Torr), but may still be half
full at 75 Torr. Since the gas is expensive and since it is also expensive
to shut down and change the container, it is desirable to be able to
operate the system at the lowest possible container pressure to avoid
wasting gas and having frequent shutdowns. In order to withdraw
essentially all of the stored gas, it is important that the mass flow
controller have a very low pressure drop at the rated flow of the
implanter. With existing controllers, the combined pressure drop through
the sensor and flow splitter assemblies at a nominal flow rate of 5 sccm
and an exit pressure of zero Torr (common operating parameters) results in
a residual gas pressure in the container of 50 to 80 Torr. Because of the
mechanism whereby the gas desorbs from the porous beads, at such pressures
only about 60 percent of the gas can be extracted, resulting in a
substantial economic penalty. Pressure drop in the flow controller is due
to the small diameter of the sensor tubing, resistance of the flow
splitter, multiple right angle turns traversed by gas entering and exiting
the sensor assembly, and losses at the entrance and exit of the sensor
assembly. Thus, there is a need for a mass flow controller which has
suffici | | |
