U.S. patent number RE35,544 [Application Number 08/671,231] was granted by the patent office on 1997-07-01 for apparatus for monitoring moisture in a gas stream.
This patent grant is currently assigned to Millipore Corporation. Invention is credited to James T. Snow.
United States Patent |
RE35,544 |
Snow |
July 1, 1997 |
Apparatus for monitoring moisture in a gas stream
Abstract
A device is provided comprising a piezoelectric material and at
least one non-crystalline .[.non-crystalline.]. metal
oxide.[.,.]..Iadd.;
.Iaddend.crystalline.[.-.]..Iadd.,.Iaddend.non-zeolite metal oxide
and mixtures thereof coating chemically reactive with trace
quantities of water in chemically reactive and inert gases. The
piezoelectric material is bonded to a conductor for delivering an
alternating electric current and to a conductor for transmitting
resonant vibration frequency of the crystal. The reactive metal
oxide coating has an effective thickness which provides a
serviceable life for the coating while not being so thick as to
prevent vibration of the piezoelectric material.
Inventors: |
Snow; James T. (Nashua,
NH) |
Assignee: |
Millipore Corporation (Bedford,
MA)
|
Family
ID: |
26960802 |
Appl.
No.: |
08/671,231 |
Filed: |
June 27, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
958384 |
Oct 8, 1992 |
5339675 |
Aug 23, 1994 |
|
Reissue of: |
281286 |
Jul 27, 1994 |
05477716 |
Dec 26, 1995 |
|
|
Current U.S.
Class: |
73/24.01; 436/40;
73/31.06 |
Current CPC
Class: |
G01N
29/036 (20130101); G01N 2291/014 (20130101); G01N
2291/02845 (20130101) |
Current International
Class: |
G01N
29/02 (20060101); G01N 29/036 (20060101); G01N
27/00 (20060101); B01J 029/00 (); G01N
031/06 () |
Field of
Search: |
;73/24.01,24.04,24.06,24.03,31.06 ;436/40 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Williams; Hezron E.
Assistant Examiner: Wiggins; J. David
Attorney, Agent or Firm: Cook; Paul J. Hubbard; J. Dana
Parent Case Text
REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of U.S. application Ser.
No. 07/958,384, filed Oct. 8, 1992, which has become U.S. Pat. No.
5,339,675 as of Aug. 23, 1994.
Claims
I claim:
1. A sensing device for measuring water content in a stream of
purified gases which comprises:
a piezoelectric material having an effective mass of a coating
.[.non-crystalline metal oxide.]. on at least one surface of said
sensing device, with said coating formed of a composition
.Iadd.selected from the group consisting of a non-crystalline metal
oxide, a crystalline, non-zeolite metal oxide and mixtures thereof,
said coating being .Iaddend.chemically reactive with water to form
a metal hydroxide product, said coating having a mass which permits
said material to vibrate in response to applied alternating
electrical current,
means for applying an alternating electrical current to said
piezoelectric material and means for measuring frequency of
vibration of said piezoelectric material.
2. The device of claim 1 wherein said material has said coating on
two opposing surfaces.
3. The device of claim 1 wherein said piezoelectric material is
quartz.
4. The device of claim 2 wherein said piezoelectric material is
quartz.
5. The device of claim 1 wherein said coating is barium oxide.
6. The device of claim 2 wherein said coating is barium oxide.
7. The device of claim 3 wherein said coating is barium oxide.
8. The device of claim 4 wherein said coating is barium oxide.
9. The device of claim 1 wherein said piezoelectric material is
poly(vinylidene fluoride).
10. The device of claim 2 wherein said piezoelectric material is
poly (vinylidene fluoride).
11. The device of claim 1 wherein said coating includes a layer of
protective material between said metal oxide coating and said
piezoelectric material comprising poly(vinylidene fluoride).
12. The device of claim .[.I.]. .Iadd.1 .Iaddend.wherein said
coating includes a layer of protective material between said metal
oxide coating and said piezoelectric material comprising
polytetrafluoroethylene.
13. The device of claim 2 wherein said coating includes a layer of
protective material between said metal oxide coating and said
piezoelectric material comprising poly(vinylidene fluoride).
14. The device of claim 2 wherein said coating includes a layer of
protective material between said-metal oxide coating and said
piezoelectric material comprising polytetrafluoroethylene.
Description
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for detecting
water impurity in a gas stream. More particularly, this invention
relates to a method and apparatus for detecting water impurity
utilizing a piezoelectric crystal coated with a metal oxide
reactive with water.
At the present time ultrapure gas streams are utilized in chemical
reactions such as in the semiconductor industry. These chemical
reactions usually are conducted in sealed containers to maintain
purity since the gases often times are toxic and are conducted
under low pressure in order to decrease the probability of unwanted
side reactions. In order to maintain the requisite gas purity, the
gas is treated, prior to entering the reaction chamber, in order to
remove impurities therefrom. It is general practice to pass the gas
through a bed of resin particles which are interactive with
impurities such as water in the gas. Over time, the capacity of the
bed of resin particles for interacting with the impurities is
depleted to a point where breakthrough of impurities from the resin
bed occurs and the impurities enter the reaction zone it is
difficult to predict when undesirable depletion of resin capacity
occurs so that in the absence of independent monitoring means,
premature or late removal of the resin is likely. Premature resin
removal results in increased resin cost while late removal results
in expensive damage to reaction product. It is additionally
desirable to have a means for continuously monitoring the gas
purity level and providing a measure of gas impurity
concentration.
It has been proposed in U.S. Pat No. 5,138,867 to provide a
detection system for sensing concentration of impurities in a gas
stream which includes a sensing device which can be hygrometric,
spectrophotometric, piezoelectric or colorimetric. The specific
piezoelectric device disclosed is a surface acoustical wave (SAW)
device. In a SAW device, an acoustical wave is passed along a
surface coating on a substrate to measure the change in mass at the
interface between the coating and the substrate. Mass change in the
coating is caused by reaction of the coating with impurities in a
gas which contacts the coating. Accordingly, a reactive polymer
coating material is described that is consumed over time and
regenerable. In the SAW device, the coating must be thin; on the
order of a wavelength of the acoustic wave or thinner in order to
permit accurate measurement of impurity concentration. While this
device is extremely sensitive to impurity concentration change,
i.e. in the picogram level, it is too sensitive for use in a device
requiring an extended service, i.e., about one year or more, since
the thickness of the coating necessary to have the capacity for
extended lifetimes quickly exceeds that which permits accurate
measurements.
The detection of water vapor using materials like silica gel and
alumina on piezoelectric materials was proposed in U.S. Pat. No.
3,385,100. These types of metal oxide coatings absorb moisture
through a physisorption, rather than chemisorption, mechanism.
Water adsorption on these types of coatings occur principally
through dipole bonds (H-bonding) with surface hydroxyls. The
approximate water vapor pressure at 25.degree. C. for absorbants
like alumina are shown in the table below (Shriver, D. F.;
Drezdzon, M. A. "The Manipulation of Air-Sensitive Compounds",
2nd.ed.; John Wiley & Sons, Inc.: New York. 1986; p 72):
______________________________________ Material Water Vapor
Pressure (torr) ______________________________________ Molecular
Sieves 1 .times. 10.sup.-3 Alumina (active) 1 .times. 10.sup.-3
Silica gel 2 .times. 10.sup.-3
______________________________________
Absorbants such as alumina, silica gel and molecular sieves exhibit
a steady increase in the water vapor pressure as more moisture is
absorbed. This continual increase is undesirable, since the
sensitivity for moisture steadily decreases as more moisture is
absorbed. In addition, due to the reversible adsorption-desorption
of moisture from these type of coatings, the coated crystal would
need to be located within a carefully temperature controlled
environment to eliminate any temperature effects on the
physisorption.
A scavenger for oxygen and water vapor impurities comprising metal
hydrides are disclosed in U.S. Pat. Nos. 4,950,419 and 4,716,181.
However, these devices are not useful for selective reaction with
water in an oxygen gas containing stream since coatings of the
device are reactive with both oxygen and water.
It would be desirable to provide a means for detecting water vapor
in a gas stream which is useful and accurate for extended times. In
addition it would be desirable to provide such a means for
detecting water vapor capable of quantifying impurity concentration
in a gas, particularly a gas stream consisting of or containing
oxygen gas.
SUMMARY OF THE INVENTION
The present invention provides a coated piezoelectric apparatus for
detecting the presence of water impurity in a wide variety of
gases. A piezoelectric crystal is coated on one or more opposing
surfaces with .[.a.]. .Iadd.at least one .Iaddend.non-crystalline
metal oxide.Iadd.; crystal1iline non-zeolite metal oxide and
mixtures thereof coating .Iaddend.which reacts with water to form a
metal hydroxide. The metal oxide can be deposited on the
piezoelectric crystal by a suitable thin film deposition process.
Alternatively the metal oxide coating can be formed in situ by
first coating the crystal with a metal which is reactive with
oxygen followed by exposure to an oxygen-containing gas to convert
the metal coating to the desired metal oxide. The mass of reactive
metal oxide coating applied is large enough so that there is enough
material to continue to react with water for the service lifetime
that is desired, while not being so heavy as to prevent or
substantially reduce vibration of the piezoelectric crystal due to
its mass or the mass of the reaction product. The crystal is
subjected to an alternating electric field and the resonant
vibration caused by the electric field is detected. The reaction of
water with the reactive metal oxide coating will form a metal
hydroxide with an accompanying change of mass. The mass change will
cause a change in reasonant frequency, i.e., a mass increase causes
a resonant frequency decrease due to a damping effect which is
measured. The resonant frequency measured can be correlated to
water impurity concentration in the incoming gas by means of a
standard curve. The piezoelectric crystal can be coated with a
protective polymer such as poly(vinylidene fluoride) (PVDF) or
polytetrafluoroethylene to protect the piezoelectric from corrosive
gases such as hydrogen fluoride. The reactive coating is coated on
the protective polymer. The combined mass of the two coatings must
meet the mass criteria set forth above.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view of a coated piezoelectric crystal device of
this invention.
FIG. 2 is a top view of an apparatus of this invention.
FIG. 3 is a schematic view illustrating the use of the apparatus of
this invention.
FIG. 4 is a graph showing the maximum weight of material that can
be deposited on a crystal and remain within the crystal linear
range.
FIG. 5 shows the effect of pressurized air contact with the coated
piezoelectric crystal device of this invention.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The coated piezoelectric crystals of this invention comprise a
piezoelectric substrate such as natural quartz, lithium niobate,
lead metaniobate, lead zirconate titanate, poly(vinylidene
difluoride) (PVDF), Rochelle salts, tourmaline, ethylenediamine
tartrate, dipotassium tartrate, ammonium dihydrogen phosphate or
the like, preferably quartz (and PVDF in hydrogen fluoride gas
streams). The coating is applied to one or both of the largest
surfaces of the substrate.
The behavior of the piezoelectric crystals can be explained using
the Sauerbrey equation:
wherein .DELTA. f is the observed frequency change, f.sub.o is the
fundamental frequency of the crystal, m is the mass change at the
electrode surface, N is the frequency constant for the crystal, A
is the surface area of the deposit, and .rho. is the density of the
piezoelectric crystal. As evidenced by the equation, the observed
frequency change is linearly dependent on the mass change but
varies to the square of the fundamental frequency. Thus, a 2.0-MHz
crystal will be four times more sensitive than a 1.0 MHz crystal.
Good correlation has been found between the observed and calculated
frequency change when the crystal linear range is assumed to be 1%
of f.sub.o as shown in FIG. 4. Outside this range, this linear
relationship between frequency and mass change will deteriorate and
above a critical mass, the crystal will cease to oscillate. Thus,
as shown in FIG. 4, the maximum amount of material that the crystal
can accommodate and still vibrate within the linear response range
will vary according to the frequency of the piezoelectric crystals
utilized and is generally between about 0.1 and 3.5 milligrams.
Suitable metal oxide coatings are those which react with water,
permit detection of water at concentrations of 1 part per billion
(ppb) and do not add contaminant to purified gas. The
non-crystalline metal oxide.Iadd.; crystalline, non-zeolite metal
oxide; and mixtures thereof .Iaddend.coatings most useful in the
present invention are those which react with water to form the
metal hydroxide which then reacts further with water to form
hydrates of the hydroxide. Metal oxides which merely adsorb or
absorb water are not useful in the present invention. The metal
.Iadd.oxide .Iaddend.should be reactive with water at a temperature
between about 10.degree. and 60.degree. C., preferably between
about 20.degree. and 35.degree. C. to form a metal hydroxide
product having a melting point of at least about 60.degree. C.
Representative suitable metal oxides include the oxides of Groups
IA and IIA elements, i.e. lithium, magnesium, potassium, strontium,
barium, sodium, calcium or the like. It is preferred to utilize the
metal oxides formed of a metal having a low molecular weight such
as lithium or magnesium since the mass change as a result of
reaction with water is larger and more easily detectable as
compared to coatings containing higher molecular weight metals. The
chemical reaction of the desired metal oxides of this invention
differs significantly from the physical interaction of the metal
oxides e.g. alumina, silica gel and molecular sieves with moisture.
The water vapor absorption isotherms for the desired metal oxides
have plateau regions corresponding to the different phases and do
not continuously increase as observed with silica gel and alumina.
Therefore, the reactivity and sensitivity toward moisture is
constant in these regions. This extends the capability of these
coatings for accurate low-level moisture detection and simplifies
the interpretation of moisture calibration curves. In addition, the
chemical reaction of moisture with the desired metal oxides
eliminates the need for careful temperature control that is
required with the other metal oxide coatings that have
adsorption-desorption equilibria. One of the desired coatings,
barium oxide, additionally has a lower water vapor pressure of
7.times.10.sup.-4 torr compared to alumina and related metal
oxides. In addition, analysis of reaction kinetic data showed
similar reaction probabilities for the reactions of water with BaO
and Ba metal, a known getter for moisture (Verhoeven, J.; Van
Doveren, H. Applications of Surface Science 1980, 6, 225)
The detector of this invention is useful in monitoring water vapor
impurity concentration in oxygen containing gases such as oxygen or
nitrous oxide; inert gases, such as helium, argon, nitrogen;
silicon containing gases such as silane, dichlorosilane,
trichlorosilane; dopants such as arsine, phosphine, diborane;
etchants such as halocarbon 14, halocarbon 16, halocarbon 218,
sulfur hexafluoride, chlorine, hydrogen bromide or reactants such
as hydrogen chloride, hydrogen fluoride or ammonia. When monitoring
an oxygen-containing gas, a metal coating is first converted to a
metal oxide by the gas stream which then is reactive with water
vapor to produce the effects discussed above. The mass change
corresponding to this conversion must not exceed the maximum mass
specified for the crystal discussed above. Alternatively, the metal
oxide may be deposited on the piezoelectric crystal by a suitable
thin film deposition technique.
Referring to FIG. 1, the composite structure of this invention
includes a piezoelectric crystal which is coated with the metal
oxide coating materials of this invention set forth above to form
coatings 12 and 14. A conductive lead wire 16 such as a copper wire
is bonded to crystal 10 such as with solder 18 in order to input
alternating electrical energy into crystal 10.
Referring to FIG. 2, an apparatus of this invention is shown
including the piezoelectric crystal coating 12 and connected to
lead wire 16. A second lead wire 22 is bonded to piezoelectric
crystal 10 which transmits vibration of the coated crystal to an
electronic circuit such as described below. The electronic circuit
monitors the frequency of vibration of the energized crystal. The
coatings 12 and 14 are applied by an acceptable thin film
deposition technique, e.g., sputtering. The film can be applied as
the metal oxide directly or as the metal which then can be
converted to the metal oxide by reaction with oxygen. For the
deposition of a metal conductor e.g. magnesium, DC sputtering is a
possible mode. For other metals or metal oxides, e.g. barium,
barium oxide, RF is the recommended sputtering technique. In a
process of utilizing an RF planar magnetron sputtering system for
the deposition of barium, e.g., Model CrC-100 Planar Magnetron
Sputtering System with optional 200 watt RF power supply
manufactured by Plasma Sciences, Inc., a potential is applied to a
barium target. In the sputtering process, the barium target will
become negatively sell-biased creating an enrichment of ions in
front of the target. The ions strike the target and sputtering is
obtained. The crystal is positioned on a pedestal located at a
distance of one to three inches from the bottom of the sputtering
head. The crystal is masked so deposition occurs only on the
desired areas of the piezoelectric electrodes. Typical sputtering
operations with this piece of equipment are performed at a pressure
range of 2 to 10 mtorr and RF power of 50 to 150 W with argon as
the sputtering gas. For the sputter deposition of metal oxides,
reactive RF sputtering may be employed with 1-5% oxygen in argon as
the sputtering gas.
The oscillator design is based on a CMOS Pierce Oscillator. The
basic physics internal to the crystal is that of a sound wave
propagating through the crystal. The initial electrical input at
one voltage difference causes an expansion of the crystal. The
initial voltage is produced by one end of the inverter gate due to
its 180.degree. phase difference from the other end of the
inverter. The piezoelectric crystal behaves electrically like a
high Q(quality) LC network, or physically like a mass spring. As
the mass per unit length of the spring changes, so will the
frequency of the springs oscillations. By changing the mass on the
surface of the crystal frequency changes of the crystal oscillator
can be detected.
Two CMOS Pierce Oscillators are used in the electronics circuit.
One oscillator is used as a reference, the other as a mass sensor.
The two signals are fed into a flip-flop that gives the difference
frequency of the two crystals. This serves two purposes; one to
null Out any temperature effects, the other to give a smaller
frequency value, e.g., 1,000,000 Hz reference signal, and 999,000
Hz sensor signal, will produce a 1,000 Hz difference frequency. The
difference frequency is applied to a microcontroller counter input
and frequency cycles summed for a given time period. Knowing the
number of total cycles and the time period, frequency in Hz can be
calculated. A ten second sample time gives 1/10 Hz resolution of
the difference frequency.
Referring to FIG. 3, the sensor apparatus 8 of this invention is
positioned on housing 30 and includes a gas inlet 32 and a gas
outlet 34. Gas to be purified is introduced through inlet 36,
through screen 38 and into resin bed 40. Optional resin bed 40
functions to scavenge water vapor from the incoming gas. Purified
gas is passed out screen 42, through final filter 44 and through
outlet 46 to a zone for chemical reaction (not shown). Alternate
device configurations are possible, including a device in which the
coated crystal is mounted directly within the flowing gas
stream.
The following example illustrates the present invention and is not
intended to limit the same.
EXAMPLE 1
A stainless steel housing consisting of all stainless steel
components, metal-to-metal face seal and metal gasket seal
connections and inlet and outlet diaphragm valves to seal the
housing from outside atmosphere when required was constructed to
permit testing of coated piezoelectric quartz crystals. A quartz
crystal with fundamental frequency of 5.050 MHz was RF sputter
coated in an argon glove box with barium metal at 55 W, 5 mTorr
pressure for 7 minutes each side to deposit a total of 166 .mu.g of
barium metal and decreased the frequency of the crystal to 5.014
MHz. The crystal, previously mounted on a HC-51/U type holder with
glass feedthroughs and holder welded onto metal header, was
inserted into the stainless steel housing and sealed with metal
gasket seal. The housing was removed from the glove box and
connected to a high purity test stand. Connected to the upstream
diaphragm valve of the housing was a Waferpure.RTM. Mini XL gas
purifier (Model No. WPMV200SC) to remove water vapor from the test
gas. A flow of purified nitrogen from a cryogenic source at 1.0
slpm was started through the housing containing the barium-coated
crystal. After ca. 20 hours, the gas was switched to a 2% oxygen in
argon mixture to accelerate the conversion of the barium coating to
barium oxide. The gas flow was periodically stopped to pressurize
the housing with the oxygen gas mixture, especially during
overnight periods. After ca. 143 hours, the gas was switched to air
(Grade 0.1,[H.sub.2 O]<3 ppm) to convert any remaining
accessible barium to barium oxide. The purifier was removed after
162 hours to challenge the formed barium oxide coated sensor to the
unpurified air. As shown in FIG. 5, exposure of the sensor to
moisture resulted in a large and immediate frequency decrease
corresponding to conversion of the sensor coating to barium
hydroxide.
* * * * *