U.S. patent application number 12/857779 was filed with the patent office on 2010-12-09 for system for measuring liquid flow rates.
Invention is credited to David B. Blackford, Derek R. Oberreit, Frederick R. Quant.
Application Number | 20100312499 12/857779 |
Document ID | / |
Family ID | 39365336 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100312499 |
Kind Code |
A1 |
Blackford; David B. ; et
al. |
December 9, 2010 |
SYSTEM FOR MEASURING LIQUID FLOW RATES
Abstract
A system for monitoring non-volatile residue concentrations in
ultra pure water includes a nebulizer for generating an aerosol
composed of multiple water droplets, a heating element changing the
aerosol to a suspension of residue particles, and a condensation
particle counter to supersaturate the dried aerosol to cause
droplet growth through condensation of a liquid onto the particles.
The nebulizer incorporates a flow dividing structure that divides
exiting waste water into a series of droplets. The droplets are
counted to directly indicate a waste water flow rate and indirectly
indicate an input flow rate of water supplied to the nebulizer. The
condensation particle counter employs water as the condensing
medium, avoiding the need for undesirable chemical formulations and
enabling use of the ultra pure water itself as the condensing
medium.
Inventors: |
Blackford; David B.; (North
Oaks, MN) ; Quant; Frederick R.; (Shoreview, MN)
; Oberreit; Derek R.; (Roseville, MN) |
Correspondence
Address: |
Frederick W. Niebuhr;Haugen Law Firm PLLP
1130 TCF Tower, 121 South Eighth Street
Minneapolis
MN
55402
US
|
Family ID: |
39365336 |
Appl. No.: |
12/857779 |
Filed: |
August 17, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11935810 |
Nov 6, 2007 |
7777868 |
|
|
12857779 |
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Current U.S.
Class: |
702/45 ;
73/861.61 |
Current CPC
Class: |
G01N 15/065 20130101;
G01F 3/00 20130101; G01F 1/661 20130101 |
Class at
Publication: |
702/45 ;
73/861.61 |
International
Class: |
G06F 19/00 20060101
G06F019/00; G01F 1/42 20060101 G01F001/42 |
Claims
1. A device for generating an aerosol composed of multiple droplets
of a test liquid, including: a first conduit for receiving a test
liquid at an input flow rate; a second conduit for receiving
pressurized gas; a merger region, open to the first conduit and to
the second conduit for simultaneous reception of the test liquid
and the pressurized air, adapted to generate an aerosol composed of
multiple droplets of a first portion of the test liquid suspended
in the gas; an aerosol exit passage open to the merger region for
conducting the aerosol away from the merger region; a liquid exit
passage open to the merger region and a measuring section
downstream of the liquid exit passage, adapted to conduct an output
flow comprised of a second portion of the test liquid away from the
merger region; and a flow sensor disposed along the measuring
section and adapted to generate a sensor signal indicating an
output flow rate of the output flow.
2. The device of claim 1 wherein: the liquid exit passage
incorporates a flow dividing structure adapted to separate the
output flow into discrete test liquid segments; and the flow sensor
is disposed at a sensing location downstream of the flow dividing
structure to detect each test liquid segment as it passes the
sensing location.
3. The device of claim 2 wherein: the flow sensor is adapted to
optically detect the test liquid segments and to generate an
electrical pulse responsive to each test liquid segment passing the
sensing location.
4. The device of claim 2 wherein: the flow dividing structure
comprises a vessel disposed to collect the output flow, the vessel
having an orifice at a bottom thereof adapted to serially release
the test liquid segments in droplet form.
5. The device of claim 2 wherein: the flow dividing structure
further is adapted to separate the output flow into test liquid
segments substantially uniform in volume.
6. The device of claim 1 wherein: the liquid exit passage comprises
a first compartment downstream of the merger region for collecting
the output flow, a second compartment downstream of the first
compartment, and a wall between the first and second compartments
adapted to direct the output flow from the first compartment into
the second compartment via spillage over a top of the wall after
the first compartment is filled with the test liquid; and the
second compartment is exposed to ambient pressure, and is
substantially isolated from the merger region by the test liquid
occupying the first compartment.
7. The device of claim 6 wherein: the second compartment
incorporates structure for releasing the test liquid therefrom in
discrete test liquid segments.
8. The device of claim 1 further including: an aerosol drying stage
disposed along the aerosol exit passage for evaporating the first
portion of the test liquid, whereby the aerosol downstream of the
drying stage is characterized by residue particles suspended in the
gas; and a concentration indicating component downstream of the
drying stage and adapted to generate residue concentration
information based on received residue particles.
9. The device of claim 1 wherein: the first conduit comprises an
elongate axially directed flow restricting orifice adapted to
gradually reduce a pressure of the test liquid in the downstream
direction toward the merger region, whereby the test liquid enters
the merger region at a pressure just above atmospheric
pressure.
10. The device of claim 1 further including: a processor coupled to
receive the sensor signal and adapted to generate an indication of
the input flow rate based on the sensor signal.
11. A droplet generating and flow measuring apparatus, including:
an input conduit defining an upstream region of a liquid path for
conveying a test liquid at an input flow rate; a merger region open
to the input conduit to receive the test liquid for merger with
pressurized gas to generate an aerosol composed of multiple
droplets of a first portion of the test liquid suspended in the
gas; an aerosol exit passage open to the merger region for
conveying the aerosol away from the merger region; a liquid exit
passage defining a downstream region of the liquid path, open to
the merger region to convey a second portion of the test liquid
away from the merger region at an output flow rate; a flow dividing
structure disposed along the liquid path and adapted to separate
the test liquid into discrete test liquid segments; and a flow
sensor, disposed at a sensing location along the liquid path
downstream of the flow dividing structure to detect each of the
test liquid segments as it passes the sensing location.
12. The apparatus of claim 11 wherein: the flow sensor is disposed
along the downstream region of the liquid path.
13. The apparatus of claim 12 wherein: the liquid exit passage
comprises a first compartment downstream of the merger region for
collecting the second portion of the test liquid, a second
compartment downstream of the first compartment, and a wall between
the first and second compartments adapted to direct the second
portion from the first compartment into the second compartment by
spillage over a top of the wall after the first compartment is
filled with the test liquid; and the second compartment is exposed
to ambient pressure, and is substantially isolated from the merger
region by the test liquid occupying the first compartment.
14. The apparatus of claim 11 wherein: the flow sensor is adapted
to optically detect the test liquid segments and to generate an
electrical pulse responsive to each test liquid segment passing the
sensing location.
15. The apparatus of claim 11 wherein: the flow dividing structure
comprises a vessel disposed downstream of the merger region to
collect the second portion of the test liquid and having an orifice
at a bottom thereof adapted to serially release the test liquid
segments in droplet form.
16. The apparatus of claim 15 wherein: the second compartment
incorporates the flow dividing structure.
17. The apparatus of claim 11 further including: a processor
coupled to the flow sensor and adapted to generate an indication of
the input flow rate based on a frequency at which the flow sensor
detects the test liquid segments.
18. The apparatus of claim 11 wherein: the flow dividing structure
is adapted to separate the test liquid into segments substantially
uniform in volume.
19. In a fluid measurement system incorporating a nebulizer and a
processor coupled to the nebulizer to receive information from the
nebulizer, a process for determining an input flow rate of a liquid
provided to the nebulizer, including: providing a liquid to a
nebulizer at least one known input flow rate; while so providing
the liquid, using the nebulizer to generate an aerosol composed of
multiple droplets of a first portion of the liquid suspended in a
gas, while conveying a second portion of the liquid along an exit
passage of the nebulizer; measuring a flow rate of the second
portion of the liquid to determine an output flow rate
corresponding to the at least one known input flow rate; providing
the known input flow rate and the corresponding output flow rate to
a processor coupled to the nebulizer; and associating the
corresponding output flow rate with the known input flow rate
within the processor to enable the processor to generate an
indication of the known input flow rate in response to receiving an
indication of the corresponding output flow rate from the
nebulizer.
20. The process of claim 19 wherein: providing the liquid comprises
providing the liquid to the nebulizer at a plurality of different
known input flow rates; measuring a flow rate comprises measuring a
plurality of different output flow rates, each corresponding to a
different one of the known input flow rates; providing the known
input flow rate and the corresponding output flow rate comprises
storing the plurality of known input flow rates and their
corresponding output flow rates to the processor; and associating
the flow rates comprises causing the processor to generate an
indication of each one of the known input flow rates in response to
receiving an indication of its corresponding output flow rate from
the nebulizer.
21. The process of claim 19 wherein: associating the flow rates
comprises determining a function relating the input flow rate and
the corresponding output flow rate, and modifying the processor
according the function to configure the computer to generate a
plurality of different input flow rates, individually, in response
to receiving indicia of a plurality of different corresponding
output flow rates from the nebulizer.
22. A liquid flow measuring apparatus, including: structure forming
a holding region disposed along a liquid flow path for accumulating
liquid received from an upstream region of the liquid flow path; a
vessel disposed downstream of the holding region in fluid
communication with the holding region through an interface allowing
liquid to flow from the holding region into the vessel responsive
to an accumulation of liquid in the holding region exceeding a
predetermined threshold, wherein liquid occupying the holding
region substantially isolates the vessel from any pressure
fluctuations along the upstream region of the liquid flow, the
vessel having an orifice adapted to separate liquid exiting the
vessel into discrete liquid segments; and a flow sensor disposed
along the liquid path at a sensing location downstream of the
vessel to detect each of the liquid segments passing the sensing
location.
23. The apparatus of claim 22 wherein: the vessel is exposed to
ambient pressure.
24. The apparatus of claim 22 wherein: the orifice is disposed at a
bottom of the vessel to serially release the liquid segments as
droplets substantially uniform in volume.
25. The apparatus of claim 22 wherein: the vessel comprises an
upright vessel wall, and the holding region comprises a compartment
adjacent the vessel adapted to accumulate liquid to a level
coinciding with a top of the vessel wall to determine said
threshold whereby liquid exceeding the threshold enters the vessel
by spillage over the wall.
26. The apparatus of claim 22 further including: a processor
coupled to the flow sensor and adapted to indicate a flow rate of
liquid along the liquid flow path based on a frequency at which the
flow sensor detects the liquid segments, wherein the flow sensor is
adapted to optically detect the liquid segments and generate an
electrical pulse responsive to each liquid segment detected.
27. The apparatus of claim 22 further including: a housing forming
a merger region disposed along the upstream region of the liquid
path to receive the liquid for merger with pressurized gas to
generate an aerosol composed of multiple droplets of a first
portion of the liquid suspended in the gas, an aerosol exit passage
open to the merger region for conveying the aerosol away from the
merger region, and a liquid exit passage open to the merger region
to convey a second portion of the liquid from the merger region to
the holding region.
Description
[0001] This application is a divisional of U.S. Non-Provisional
patent application Ser. No. 11/935,810 entitled "System for
Measuring Non-Volatile Residue in Ultra Pure Water," filed Nov. 6,
2007.
[0002] This application claims the benefit of priority based on
Provisional Application No. 60/857,548 entitled "System for
Measuring Non-Volatile Residue in Ultra Pure Water" filed Nov. 7,
2006.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to instruments for measuring
minute concentrations of impurities in liquids, and more
particularly to systems for detecting non-volatile residue
concentrations in ultra pure water and aerosol generating
components used in such systems.
[0004] Certain industries, most notably semiconductor fabrication,
involve extremely high standards of cleanliness and purity. A
semiconductor component may require washing with ultra pure water
after each processing stage, to remove chemicals used in that
stage. More generally, ultra pure water may be used to clean
fixtures and other tools used to handle semiconductor wafers and
other components. Any non-volatile residue present in the ultra
pure water can remain on the surface of the component after the
water has evaporated, possibly causing defects in the resulting
semiconductor device. Thus, there is a need to monitor the ultra
pure water used in such processes, to insure that the concentration
of non-volatile residue remains at or below acceptable levels.
[0005] Systems have been developed and employed successfully to
continuously monitor the quality of ultra pure water. U.S. Pat. No.
5,098,657 (Blackford et al.) discloses a system in which ultra pure
water is provided at a constant flow rate to a nebulizer where the
water is formed into droplets. The droplets are dried to provide
non-volatile residue particles. The particles can be detected
electrostatically or provided to a condensation particle counter
(CPC, also known as a condensation nucleus counter) where the
particles are "grown" into larger droplets and sensed optically.
Droplets are grown with a fluid having a relatively low vapor mass
diffusivity, e.g. butyl alcohol.
[0006] While such systems have enjoyed success, they also are
subject to difficulties that limit their utility. One of these
concerns is measuring the flow rate of the ultra pure water to the
nebulizer. Such measurement is critical, because any change in the
flow rate, e.g. due to a blockage in the water delivery system,
seriously disrupts measurement of the residue concentration. The
conventional approach is to position a rotometer just upstream of
the nebulizer. Rotometers are not particularly well suited for
measuring the extremely low flow rates involved, typically about 1
milliliter per minute. They are expensive, in part due to the need
for ultra-clean materials such as Teflon to minimize residue
contamination that would adversely affect concentration
readings.
[0007] Another problem is the accumulation of waste water in the
nebulizer. Conventional devices have employed sponges to absorb
waste water, but this only postpones the eventual need to remove
the waste water.
[0008] Another difficulty concerns the sapphire orifice plate
typically used to control the flow rate of water into the
nebulizer. A forty micron diameter orifice through the plate limits
the flow and admits water into the nebulizer. The pressure drop
across the orifice plate is sufficient to cause gasses dissolved in
the water to accumulate on the back side of the orifice plate and
form bubbles. Downstream of the nebulizer, the bubbles eventually
break free and tend to disrupt residue concentration
measurements.
[0009] As to the condensation particle counter, a concern relates
to the use of butyl alcohol or similar fluids with low vapor mass
diffusivity for growing the residue particles into droplets. Such
liquids tend to be flammable, toxic, and produce noxious odors that
frequently require vapor exhaust systems to be located near the
measuring device. Frequently the liquids are subject to health and
environmental regulations that restrict their use in indoor
environments. In addition, the liquids require equipment for
supplying, collecting and draining the liquid involved.
[0010] Another persistent problem is the relatively long time
elapsed between a change in the concentration of non-volatile
residue in the ultra pure water, and the detection of the change.
This raises the risk that contaminated water may be used in several
process stages before the condition is realized.
[0011] Accordingly, the present invention has several aspects
directed to one or more of the following objects: [0012] to provide
an aerosol generating device, e.g. a nebulizer, with a reliable
means for measuring an input flow rate of the liquid supplied to
the device without contacting or otherwise interfering with the
liquid used to generate the aerosol droplets; [0013] to provide an
aerosol generating device particularly well-suited for digital
measurement of the flow rate of the liquid into and through the
device; [0014] to provide a system and process for measuring
concentrations of non-volatile residue in test liquids, adapted to
facilitate the use of the test liquid as the condensation medium
for droplet growth onto previously dried particles for optical
detection; and [0015] to provide a non-volatile residue measuring
system with improved response times for alerting users to changes
in non-volatile concentrations, liquid flow rates and other key
parameters.
SUMMARY OF THE INVENTION
[0016] To achieve these and other objects, there is provided a
device for generating an aerosol composed of multiple droplets of a
test liquid. The device includes a first conduit for receiving a
test liquid at an input flow rate, and a second conduit for
receiving pressurized gas. A merger region, open to the first
conduit and to the second conduit for simultaneous reception of the
test liquid and the pressurized air, is adapted to generate an
aerosol composed of multiple droplets of a first portion of the
test liquid suspended in the gas. An aerosol exit passage is open
to the merger region for conducting the aerosol away from the
merger region. A liquid exit passage, open to the merger region, is
adapted to conduct an output flow comprised of a second portion of
the test liquid away from the merger region. A flow sensor is
disposed along the liquid exit passage and adapted to generate a
sensor signal indicating an output flow rate of the output
flow.
[0017] According to this aspect of the invention, the flow rate of
ultra pure water into a nebulizer can be determined by measuring
the rate at which the nebulizer outputs waste liquid. Measurement
occurs at a point downstream of the nebulizer entrance, and is
accomplished without contact or other interaction with the
nebulizer droplets used to measure residue concentration. Thus, the
flow rate measuring component has no impact on residue
concentration readings. This approach does not take into account
the entire flow of the ultra pure water or other test liquid,
although nearly 95 percent of the incoming water becomes waste
water. The specific percentage of incoming water that becomes waste
water can vary from one nebulizer to another, yet the percentage
within any given nebulizer is constant. Consequently, the system
can be calibrated to determine the incoming flow rate based on the
waste water output flow rate.
[0018] Preferably, the first conduit comprises an elongate axially
directed flow restricting opening or bore adapted to gradually
reduce a pressure of the test liquid in the downstream direction
toward the merger region, so that the test liquid enters the merger
region at a pressure just above atmospheric pressure. For example,
the first conduit can take the form of an extended length of
microbore tubing with a restricted (e.g. 500 microns in diameter)
axial passage.
[0019] The microbore tubing, like the previously used orifice
plate, controls the flow rate of the water into the nebulizer, with
the added benefit of forming a gradual reduction in water pressure,
to just above atmospheric pressure at the nebulizer entrance. With
the severe pressure drop at the entrance avoided, gasses dissolved
in the ultra pure water tend to remain in solution rather than form
gas bubbles. Consequently, downstream disruptions in residue
measurements due to gas bubbles are minimized or completely
avoided.
[0020] Another aspect of the present invention is a droplet
generating and flow measuring apparatus. The apparatus includes an
input conduit defining an upstream region of a liquid path for
conveying a test liquid at an input flow rate. A merger region is
open to the input conduit to receive the test liquid for merger
with pressurized gas, to generate an aerosol composed of multiple
droplets of a first portion of the test liquid suspended in the
gas. An aerosol exit passage is open to the merger region for
conveying the aerosol away from the merger region. A liquid exit
passage, defining a downstream region of the liquid path, is open
to the merger region to convey a second portion of the test liquid
away from the merger region at an output flow rate. A flow dividing
structure is disposed along the liquid path and adapted to separate
the test liquid into discrete test liquid segments substantially
uniform in volume. A flow sensor is disposed at a sensing location
along the liquid path downstream of the flow dividing structure, to
detect each of the test liquid segments as it passes the sensing
location.
[0021] The preferred flow dividing structure is a vessel disposed
downstream of the merger region to collect the second portion of
the test liquid. The vessel has an orifice at its bottom adapted to
serially release the test liquid segments in droplet form. For
example, the aerosol generator can be provided with a weir and
standpipe arrangement. As water collects in a trap at the bottom of
the aerosol generator, it rises above the weir and overflows into
the standpipe. A nozzle at the bottom of the standpipe forms the
orifice, which is sized to prevent the water or other test liquid
from continuously or rapidly draining. Instead, due to surface
tension effects, liquid is prevented from leaving the standpipe
until the collected liquid reaches a threshold, whereupon the
liquid has sufficient weight to overcome surface tension and exits
the standpipe as a droplet. If the pressure in the standpipe
remains substantially constant, the size of the droplets likewise
is constant, and the waste water flow rate is determined by the
frequency at which the droplets leave the collection volume.
Individual droplets are counted by optical components, to measure
waste liquid flow rate and, through calibration, to determine the
flow rate of liquid into the nebulizer.
[0022] The division of the waste liquid flow into individually
counted droplets is particularly well suited for generating digital
data. Signals generated when counting the droplets can be provided
directly to a digital processor, with no need for an
analog-to-digital converter. An additional benefit of this approach
is that the droplets exit the nebulizer rather than accumulating
within the nebulizer. At the same time, water remains in the trap,
weir and standpipe after each droplet leaves the standpipe. The
remaining water acts as a seal to prevent outside air from entering
the nebulizer. Outside air entry must be prevented, since airborne
particles otherwise would mingle with the residue agglomerate
particles leaving the nebulizer and cause erroneous residue
concentration readings.
[0023] Another aspect of the invention is an instrument for
measuring non-volatile residue in a test liquid. The instrument
includes a conduit arrangement comprising an entrance conduit for
conveying a test liquid downstream, and first and second downstream
conduits fluid coupled to the entrance conduit for respectively
conveying first and second portions of the test liquid. An aerosol
generating stage is fluid coupled to the first downstream conduit
to receive the first portion of the test liquid, and is adapted to
generate an aerosol comprised of multiple test liquid droplets
suspended in a gas. An aerosol drying stage is provided downstream
of the aerosol generating stage for evaporating the test liquid as
the aerosol is conveyed therealong, whereby the aerosol downstream
of the drying stage consists essentially of non-volatile residue
particles suspended in the gas. A droplet forming stage, downstream
of the drying stage, includes a holding component adapted to
receive a condensing medium in liquid form and release the
condensing medium in vapor form as the aerosol is conveyed
therealong, to supersaturate the aerosol and cause droplet growth
through condensation of said medium onto the residue particles. A
droplet detector is disposed at a sensing location downstream of
the droplet forming stage and adapted to detect the droplets
resulting from the condensation as they pass the sensing location.
The droplet forming stage is coupled to the second downstream
conduit to receive the second portion of the test liquid, and to
provide the second portion of the test liquid to the holding
component as the condensing medium.
[0024] In systems used to monitor ultra pure water, this entails
the use of water as the working medium in the condensation particle
counter. Using water avoids the health and environmental concerns
associated with butyl alcohol and other perfluorinated
hydrocarbons. This eliminates the need to supply, store and recover
such fluids, and to separate such fluids from the ultra pure water.
Further, since the ultra pure water being monitored serves as the
working medium in the condensation particle counter, the working
medium can be supplied and replenished through a direct connection
of the CPC to the water supply.
[0025] When water is used as the working medium, the aerosol stream
is saturated with water vapor and proceeds to a condensing region
surrounded by wetted walls that are heated to provide a temperature
higher than that of the saturated aerosol stream. Maximum
supersaturation occurs at the center of the aerosol flow, because
the mass diffusivity of water exceeds the thermal diffusivity of
air.
[0026] One of the advantages of using water as the working fluid in
a CPC is a substantially higher threshold at which spontaneous
nucleation (also called homogeneous nucleation) can occur compared
with the previously available butyl alcohol based CPC. An improved
coincidence correction algorithm in the water-based CPC also
contributes to a considerably higher permitted particle/droplet
throughput rate. Further, a much shorter drying column and aerosol
path from the nebulizer to the CPC considerably reduce diffusion
losses and the time elapsed from generating the aerosol to sensing
droplets to generate residue concentration information. As a result
of these advantages, the concentration information from the CPC is
available virtually in real time, and can encompass concentrations
ranging from a single part per trillion to 60 parts per billion in
the single count mode. If desired, a photometric mode can be
employed to increase the upper limit to more than 500 parts per
billion.
[0027] Another aspect of the invention is a process for determining
an input flow rate of a liquid, provided to a nebulizer in a fluid
measurement system incorporating the nebulizer and a processor
coupled to the nebulizer to receive information from the nebulizer.
The process includes:
[0028] (a) providing a liquid to a nebulizer at least one known
input flow rate;
[0029] (b) while so providing the liquid, using the nebulizer to
generate an aerosol composed of multiple droplets of a first
portion of the liquid suspended in a gas, while conveying a second
portion of the liquid along an exit passage of the nebulizer;
[0030] (c) measuring a flow rate of the second portion of the
liquid to determine an output flow rate corresponding to the at
least one known input flow rate;
[0031] (d) providing the known input flow rate and the
corresponding output flow rate to a processor coupled to the
nebulizer; and
[0032] (e) associating the corresponding output flow rate with the
known input flow rate within the processor to enable the processor
to generate an indication of the known input flow rate in response
to receiving an indication of the corresponding output flow rate
from the nebulizer.
[0033] The input and output rates may be associated through a
calibration process in which several different input rates lead to
the measurement of several different output flow rates
corresponding individually to the input rates. The rates can be
stored to the processor as a look-up table whereby the processor,
upon receiving an indication of a measured output flow rate, is
caused to generate the corresponding input flow rate.
[0034] Alternatively, the input and output flow rates may be
associated through a function, e.g. a direct linear function based
on a determination that for the nebulizer involved, the output flow
rate is a certain percent of the input flow rate.
[0035] Yet another aspect of the invention is a process for
measuring non-volatile residue concentrations in a monitoring
system including an aerosol forming stage and a droplet growth
stage downstream of the aerosol forming stage. The process
includes:
[0036] (a) providing a test liquid flow to a non-volatile residue
monitoring system;
[0037] (b) using a first portion of the test liquid flow to
generate an aerosol composed of multiple droplets of the test
liquid suspended in a gas;
[0038] (c) drying the aerosol to evaporate the test liquid and
thereby provide a dried aerosol consisting essentially of multiple
non-volatile residue particles suspended in the gas;
[0039] (d) using a second portion of the test liquid flow to
supersaturate the dried aerosol and cause droplet growth through
condensation of the test liquid onto the non-volatile residue
particles; and
[0040] (e) following the droplet growth, detecting the droplets
formed by said condensation to determine a concentration of the
non-volatile residue in the test liquid.
[0041] Thus, non-volatile residue measuring systems configured
according to the present invention generate more reliable
concentration information in virtually real time and over a wider
range of residue concentrations. The critical flow rate of ultra
pure water to the nebulizer is monitored without contacting the
ultra pure water used to provide the nebulizer output, and in a
manner particularly well suited to generating digital flow rate and
concentration data.
IN THE DRAWINGS
[0042] For a further understanding of the foregoing features and
advantages, reference is made to the following detailed description
and to the drawings, in which:
[0043] FIG. 1 is a diagrammatic view of a non-volatile residue
measuring system configured according to the present invention;
[0044] FIG. 2 is a more detailed schematic view of part of the
system;
[0045] FIGS. 3-5 are sectional views illustrating a nebulizer of
the system;
[0046] FIGS. 6-9 schematically illustrate the operation of a flow
measurement component of the nebulizer; and
[0047] FIG. 10 is a schematic view of a condensation particle
counter of the system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0048] Turning to the drawings, FIG. 1 is a diagram of a system 16
for monitoring the concentration of non-volatile residue in ultra
pure water. The water may be used in a semiconductor fabrication
process stage or other application requiring high purity. System 16
continuously monitors the water to insure an acceptably low residue
concentration as the water is supplied to the process stage.
[0049] As seen in the figure, ultra pure water from a water supply
18 and gas from a compressed air or nitrogen source 20 are supplied
to a nebulizer 22 to generate an aerosol including droplets of the
water suspended in the air or nitrogen. The aerosol is provided to
a condensation particle counter (CPC) 24. Most of the water
provided to nebulizer 22, typically close to 95 percent, is not
used to form droplets, but instead is provided to a flow sensor 26
used to monitor the flow rate of water into the nebulizer through
direct measurement of the waste water flow. The waste water is
drained from the nebulizer after flow sensing, as indicated at
28.
[0050] The aerosol output of nebulizer 22 is dried to reduce the
aerosol to suspended residue particles provided to particle counter
24. As the aerosol travels through the particle counter, it is
first saturated, and then channeled through a condensation or
supersaturation region in which the residue particles act as nuclei
for water condensation. Thus, the residue particles "grow" into
considerably larger droplets that are optically detected and
counted to generate non-volatile residue concentration information.
The concentration information is provided to a microprocessor 30.
The microprocessor provides the information to a video display
terminal 32 to generate a continuously updated record of
non-volatile residue concentration in the ultra pure water. The
microprocessor also receives water flow rate information from flow
sensor 26.
[0051] Particle counter 24 includes an exit 33 through which the
aerosol is drawn out of the CPC after the droplets are counted.
Water used in the CPC saturation and condensation regions is
provided from water supply 18 via a line 34.
[0052] FIG. 2 illustrates system 16 in more detail. Ultra pure
water from supply 18 is received through a sapphire flow-limiting
orifice 36 and delivered to a water pressure regulator 38.
Compressed air source 20 provides air under pressure to an air
pressure control regulator 40. The air pressure control regulator
controls air pressure to the water pressure regulator 38. An air
pressure of 30 psi produces a water pressure of 30 psi. A pressure
transducer 41 is operably coupled to water pressure regulator 38 to
insure maintenance of the desired pressure. The water pressure
regulator automatically shuts off the flow of water to downstream
system components, which eliminates the potential for water leaks
that might occur if the water continued to flow after a loss of the
air pressure.
[0053] The ultra pure water proceeds through a porous (20-60
micron) sintered steel filter 42 designed to remove coarse material
such as plastic fragments, which otherwise might block the flow at
downstream components. A 3-way tee 44, downstream of filter 42,
incorporates a control orifice having a diameter of 430 micrometers
which restricts the water flow to the tee to about 100 milliliters
per minute at 30 psi water pressure. Tee 44 divides the ultra pure
water flow into a sample flow of about 1 milliliter per minute, and
a waste water flow of about 99 milliliters per minute.
[0054] A capillary 46 guides the sample flow from tee 44 to an
entrance 48 of nebulizer 22. Capillary 46, preferably a section of
polytetrafluoroethylene (PTFE) or perfluoroalkoxy (PFA) tubing with
a 500 micron axial bore, controls the flow rate of water into the
nebulizer, and produces a pressure gradient that reduces the water
pressure to just above atmospheric pressure at nebulizer entrance
48.
[0055] The water pressure along capillary 46 is reduced gradually
and continually over the capillary length in the downstream
direction. Thus, for a given capillary inlet water pressure, a
longer capillary provides a liquid flow at a lower pressure at the
capillary exit (i.e. the nebulizer entrance). In system 16, the
preferred length of the capillary is about 9 inches (23 cm). The
optimum capillary length can depend on a variety of factors,
including water pressure at tee 44, pressure of air entering
nebulizer 22, and the exact diameter of the capillary
microbore.
[0056] In a general sense, capillary 46 provides a flow-controlling
conduit to the nebulizer in which the liquid pressure upstream of
the nebulizer is reduced, at a rate of at most about one pound per
square inch, over each inch of capillary length. More preferably,
each inch of capillary length entails a pressure reduction in the
range of 0.5-1.0 psi. For example, capillary 46 may reduce the
pressure from about 5-10 psi at a point just beyond filter 44 to
about 1 psi at the nebulizer entrance. In the preferred approach,
the capillary has a uniform diameter axial bore over its complete
length, and the desired rate of pressure reduction is achieved over
the full capillary length.
[0057] With the gradual decrease in water pressure along capillary
46, the tendency for gasses dissolved in the water to form bubbles
is substantially eliminated. This avoids a problem in previous
residue monitoring systems, in which gas bubbles forming at the
nebulizer entrance and then passing through the nebulizer would
momentarily disrupt downstream residue measurement. To further
reduce this problem, tee 44 is advantageously oriented to direct
the sample stream downward while directing the waste water stream
upward, whereby any bubbles present in the water tend to rise with
the waste stream.
[0058] Air from source 20 is provided at about fifty psi to a
pressure regulator 50. Downstream, the air passes through a high
efficiency particle air (HEPA) filter 52, and then is supplied to a
nebulizer air entrance 54 at a pressure of 15 psi and a flow rate
of 0.55 liters per minute through a conduit 56. Further, air is
provided to a point downstream of nebulizer 22 through a conduit
58. Conduit 58 includes a control orifice 60 for limiting the air
flow to a rate of about 2 liters per minute.
[0059] FIGS. 3, 4 and 5 illustrate nebulizer 22 in greater detail.
Nebulizer 22 includes a housing section 62 forming a merger region
64 where incoming water and pressurized air mingle to form multiple
droplets. The flow of pressurized air into the nebulizer at 54
creates a slightly negative pressure in region 64 that draws water
into the nebulizer from capillary 46. The droplets leave nebulizer
22 as an aerosol, traveling upwardly through a passage 66. The
proportion of residue relative to water varies with the purity of
the water, but is substantially constant over different droplet
sizes. Nebulizer 22 generates droplets of different sizes, but
there is sufficient consistency in the aggregate such that the
droplet count, or more accurately the count of residue particles
corresponding to the droplets, is a reliable indicator of
non-volatile residue concentration in the water.
[0060] A thermoelectric device 68 functions as a heat sink to
maintain a stable temperature of about 25.degree. C. in the
nebulizer merger region. This promotes more uniform droplet sizes.
As the aerosol leaves the nebulizer via aerosol passage 66, a
heating element 70 along passage 66 evaporates the water to
transform the aerosol into a suspension of residue particles.
[0061] A housing section 72 below housing section 62 forms a waste
water receiving compartment 74, a downstream holding compartment
76, and a fluid passage 78 located between and coupling
compartments 74 and 76. Water not forming the aerosol, i.e. a major
portion (e.g. 95 percent) of the water entering nebulizer 22,
descends directly into compartment 74, where the water accumulates
as indicated at 82.
[0062] A water retention vessel 84 includes an upright cylindrical
standpipe 86 centered within holding compartment 76, and a
truncated conical wall 88 that converges downwardly at about a 60
degree angle to an exit orifice 90.
[0063] An upper region of holding compartment 76 is open to
atmospheric pressure through a passage 92. A cylindrical interior
wall 94 of the holding compartment cooperates with standpipe 86 to
form an annular water holding region 96. A plug 98 is removable
from housing section 72 to drain the holding region. The hole for
plug 9g is also used for machining
[0064] Waste water at first accumulates in receiving compartment
74, then proceeds through passage 78 to holding region 96. The
pressurized air flowing into the nebulizer creates a positive
pressure in the receiving compartment. As a result, water in
holding region 96 is pushed upwardly until the water level reaches
the top of standpipe 86. Further addition of water in receiving
compartment 74 causes the water to spill over standpipe 86 into
vessel 84. The standpipe thus functions as a weir, with water
pushed upwardly and over the top of the weir due to the positive
pressure in the receiving compartment.
[0065] So long as the levels of water in receiving compartment 74
and holding region 96 are above passage 78, the passage functions
as a trap to isolate the interior of vessel 84 from the nebulizer
interior, in the sense that the water prevents the direct passage
of air or any other gas from one of these regions to the other.
[0066] This has several beneficial effects. First, it tends to
isolate the vessel interior from any pressure fluctuations in the
merger region or elsewhere within the nebulizer. Consequently, the
pressure inside vessel 84 is determined by ambient pressure outside
of nebulizer 22. The pressure is essentially constant, virtually
unaffected by pressure fluctuations inside the nebulizer.
[0067] Another benefit is that while the interior of vessel 84 is
exposed to ambient air for pressure control, water in holding
region 96, passage 78 and receiving compartment 74 prevents ambient
air from reaching the nebulizer interior.
[0068] Nebulizer 22 includes a fluid flow measuring section 100
disposed below housing section 72 and forming an open region below
holding compartment 76. Water leaving vessel 84 through exit
orifice 90 descends through the open region to a basin 102, from
which the waste water is removed from the nebulizer.
[0069] A light emitting diode 104 and a detector 106 are mounted
within housing section 100 along the open region. The housing
section further includes a reflective surface 108 exposed to the
open region and positioned with respect to the diode and sensor
such that water descending through the open region passes between
the diode and the reflective surface. In the preferred version,
this arrangement affords convenient access to the diode and
detector. As an alternative, a diode and detector can be placed on
opposite sides of the droplet region.
[0070] A feature of nebulizer 22 is that waste water is drained
through exit orifice 90, eliminating the need to periodically
extract waste water from the nebulizer. Further, the waste water is
released incrementally, in a manner especially conducive to digital
measurement of fluid flow.
[0071] The manner of waste water flow measurement is perhaps best
understood from FIGS. 6-9, which schematically show conical wall 88
of vessel 84, diode 104, detector 106 and reflective surface 108.
Exit orifice 90 has a diameter (e.g. about 1 mm) selected for
temporary retention of water within vessel 84. More particularly,
as seen in FIG. 6, water is present in the vessel up to a lower
threshold 110. The water is retained by surface tension forces,
which at this point overcome the tendency of the water to pass
through exit orifice 90 due to gravity.
[0072] In FIG. 7, water has accumulated to a level above the lower
threshold. Water is suspended below the exit orifice, in an early
stage of droplet formation.
[0073] In FIG. 8, water has accumulated to the point of reaching an
upper threshold 112. At this point, the weight of the water is
sufficient to overcome surface tension effects. A droplet 114 is
formed and breaks free from the water remaining in the holding
component (FIG. 9). When the droplet breaks free, the water level
descends to or near to the lower threshold, and the process
repeats.
[0074] Water overflowing standpipe 86 tends to enter vessel 84 in
bursts, intermittently overcoming surface tension to break the
meniscus at the top of the standpipe. To modulate these bursts and
ensure a more uniform flow of droplets through orifice 90, a piece
of felt or natural sponge is inserted into the standpipe.
[0075] It has been found that if the positive pressure in vessel 84
is essentially constant (i.e. subject only to changes in
atmospheric pressure), the droplets leaving exit orifice 90 are
substantially identical in size. As a result, the speed or
frequency of droplet formation depends on the rate at which waste
water is supplied to the vessel. Thus, the droplet frequency
provides a direct measurement of the waste water flow rate.
[0076] The dimensions of machined parts and the proportion of
incoming water to waste water can vary from one nebulizer to
another. Accordingly, nebulizer 22 is calibrated so that the
droplet frequency directly indicates the flow rate of ultra pure
water into the nebulizer.
[0077] Calibration involves supplying water or another liquid to
nebulizer 22 at a known constant input flow rate while operating
the nebulizer to generate an aerosol. In this manner a portion of
the incoming liquid is used to faun the aerosol droplets, while a
second portion or remainder of the liquid descends to receiving
compartment 74, eventually to exit the nebulizer in the form of
droplets 114.
[0078] As each droplet 114 descends between diode 104 and
reflective surface 108, it momentarily alters the light received by
detector 106. In response, the detector provides an electrical
pulse or signal to processor 30 via a line 116 (FIG. 1). As noted
above, a substantially constant pressure in vessel 84 results in a
substantially uniform size in droplets 114. The pulses are counted,
and the frequency indicates the waste water flow rate, i.e. an
output flow rate corresponding to the known input flow rate.
[0079] Calibration entails modifying processor 30 to individually
associate a plurality of different output flow rates with a
plurality of input flow rates. This can be accomplished by
measuring multiple output rates in conjunction with supplying water
to the nebulizer at multiple different known input flow rates, to
create a look-up table in the processor. Alternatively, based on
measuring a single output flow rate for a known input flow rate, or
measuring several different output flow rates in conjunction with
supplying water at several known flow rates, the user generates a
function relating the input and output flow rates. The function may
be a direct linear function, e.g. that the output rate is a given
fraction or percent of the input flow rate. Or, the function may be
more complex. In either event, processor 30 is modified to
incorporate the function, and to use the function to generate an
indication of the input flow rate responsive to receiving an
indication of the output flow rate from the nebulizer.
[0080] To this end, and with reference to FIG. 1, processor 30 has
a counter register 35 for accumulating a count of pulses from
detector 106, and a conversion register 37 in the form of a
function or look-up table for determining the input flow rates.
Thus, processor 30, based on stored calibration data, generates the
fluid flow rate based on the droplet count.
[0081] The droplet generation frequency provides an accurate flow
rate measurement, despite any fluctuation in the positive pressure
in receiving compartment 74. Vessel wall 86 tends to dampen any
changes in flow rate due to differences in the positive pressure,
smoothing the flow out of exit orifice 90. If desired, the droplet
frequency can be used as feedback to adjust the flow rate.
[0082] Further, in spite of fluctuations in the positive pressure
in compartment 74 or the flow rate into the nebulizer, the water
level in compartment 74 prevents ambient air from entering the
nebulizer. Water in holding region 96 prevents ambient air from
reaching the nebulizer interior via passage 92. Ambient air is kept
out of the nebulizer to insure that it cannot mingle with the
aerosol, to prevent particles suspended in the air from affecting
residue concentration measurement.
[0083] As the aerosol stream proceeds upwardly in passage 66, it is
heated by heating element 70 to a temperature of 120 degrees to
evaporate the ultra pure water, thus transforming the aerosol into
a particle suspension rather than a droplet suspension. A
thermistor 111 (FIG. 4) monitors the temperature of the air steam
as it exits passage 66. The thermistor is used in a control loop to
keep the temperature at 120 degrees C. A thermal switch 113 is used
to switch off the heater in the event of overheating. Filtered
dilution air flowing at a rate of about 2.0 liters per minute
enters through a port 115 to lower the dew point. The dried,
diluted residue aerosol at a flow rate of about 2.5 liters per
minute exits through a fitting 117 and is delivered to the droplet
growth stage, condensation particle counter 24.
[0084] Returning to FIG. 2, the aerosol path from nebulizer 22 to
particle counter 24 is about 18 centimeters in length, as compared
to the approximately 80 centimeter path in conventional systems.
The shorter aerosol path considerably reduces particle loss due to
diffusion. The shorter water pathway coupled with the shorter
aerosol pathway also considerably reduces the system response time,
to about 90 seconds as compared to nearly ten minutes in
conventional systems. Thus, unacceptable concentrations of
impurities in the water are determined earlier, and damage can be
minimized through more immediate corrective action.
[0085] FIG. 10 illustrates condensation particle counter 24 in more
detail. The CPC includes a droplet growth column 118 including a
substantially rigid cylindrical outer wall 120 and a porous
cylindrical inner liner or wick 122. Wick 122, formed of a ceramic
material, is adapted to receive and hold water, and thereby provide
water vapor to an internal passage 124 surrounded by the wick. If
desired, wick 122 can be mounted removably to facilitate inspection
and convenient replacement. A lower, saturation region 126 of
passage 124 is maintained at a near ambient temperature, e.g. at
20.degree. C. A heating element 128 is used to maintain an upper,
droplet growth region 130 of the chamber at an elevated
temperature, e.g. 60.degree. C. As aerosol from nebulizer 22
proceeds upwardly through passage 124, it becomes saturated along
region 126. As the aerosol travels through region 130, it becomes
supersaturated with water vapor. All particles in the aerosol
having at least a threshold size of 5 nm become nucleation sites
for droplet growth due to water condensation.
[0086] As the particles proceed upwardly through growth region 130,
two counteracting phenomena are at work. First, due to the elevated
temperature the wetted wick generates increased water vapor, which
travels radially inward away from the wick toward the center of
passage 124. Second, the heated walls tend to transfer heat to the
droplet growth region. However, because of the relatively high mass
diffusivity of water vapor, the water vapor reaches the center of
passage 124 more quickly then the heat. Consequently the particles
and their immediately adjacent air, even while being warmed, remain
sufficiently cool for supersaturation and the resulting
condensation and droplet growth.
[0087] A laser diode 132 and light sensor 134 are disposed above
droplet growth column 118 on opposite sides of the aerosol stream.
Each droplet alters or interrupts light transmission to the sensor
to generate an electrical pulse. The pulses are provided to
processor 30, and the pulse count yields the non-volatile residue
concentration.
[0088] With reference to FIG. 2 as well as FIG. 10, a pump 136
draws the aerosol out of CPC 24 and provides it to a waste outlet
138.
[0089] Given the use of water rather than butyl alcohol as the
condensation medium, there is no need for equipment designed to
supply, circulate and collect the medium and maintain that medium
separately from the ultra pure water. Further, the water being
monitored can be used as the CPC condensation medium. To this end,
as seen in FIG. 2, water from tee 44 not provided to the nebulizer
is fluid coupled over a line segment 140 through a backflush valve
142, a line segment 144, a solenoid valve 146 and a line segment
148 to the CPC. In FIG. 1 this is represented by line 34. As seen
in FIG. 10, CPC 24 includes a reservoir 150 fluid coupled to the
water supply through solenoid valve 146. The solenoid valve
normally is closed. When a level sensor 152 in the reservoir senses
that the water level in the reservoir has receded below a
predetermined threshold, it opens valve 146 to replenish the water
supply in the reservoir. Reservoir 150 can be provided with a
fitting for draining excess water, as indicated at 154.
[0090] Gravity and capillary forces move water from reservoir 150
to wick 122, to insure that the wick remains wetted to provide
water vapor along the saturation and growth sections.
[0091] Returning to FIG. 2, backflush valve 142 can be closed, and
ultra pure water provided in a reverse direction along line 140
from a backflush connection 158. The flow proceeds in the reverse
direction through valve 44, filter 42, regulator 38 and inlet
orifice 36, to maintain system efficiency by dislodging any
blockage that might occur along these components. Compressed air
from source 20 not provided to the nebulizer is directed through a
conduit 160 and a venturi regulator 162 to a venturi flow guide
164. The air, along with waste water from line 140 and nebulizer
22, is channeled by the venturi guide to waste outlet 138.
[0092] Thus, in accordance with the present invention, a system for
monitoring non-volatile residue concentrations in ultra pure water
generates more reliable information virtually in real time, to
facilitate more effective management of processes that depend on
water purity. The flow rate of the ultra pure water is measured
accurately without contacting water used to generate the sample
measurement aerosol, in a manner well suited for generating digital
flow measurement data.
* * * * *