WO1998002926A1

WO1998002926A1 - Piezoelectric sensor and method

Info

Publication number: WO1998002926A1
Application number: PCT/US1997/012165
Authority: WO
Inventors: Glenn M. Tom; Cynthia A. Miller
Original assignee: Advanced Technology Materials, Inc.
Priority date: 1996-07-12
Filing date: 1997-07-11
Publication date: 1998-01-22
Also published as: WO1998002926A8

Abstract

A sensor for detection of a trace fluid component in a fluid environment, comprising: a piezoelectric crystal having a fundamental resonant frequency in response to an applied oscillating electric field; a coating on the piezoelectric crystal of a sensor material which is reactive with the trace fluid component to yield a solid interaction product; means for applying an oscillating electric field to the piezoelectric crystal which generates an output resonant frequency therefrom; means for (i) sampling the output resonant frequency of the piezoelectric crystal while the oscillating electric field is applied thereto, (ii) determining the change in resonant frequency from the fundamental resonant frequency that occurs on formation of the solid interaction product when the sensor material interacts with the trace fluid in the fluid environment, and (iii) generating an output indicative of the presence of the trace fluid component in the environment; and means for flowing fluid from the fluid environment to the coating on the piezoelectric crystal so that the trace fluid component when present reacts with the coating to form the solid interaction product; wherein the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.001 to about 1000 milleHertz/min./(part-per-million of the fluid component). The sensor may be utilized for sensing of breakthrough in dry scrubbing of gases in semiconductor manufacturing, as well as for sensing of contaminant and hazardous gas species in ambient fluid environments, for environmental monitoring applications.

Description

PIEZOELECTRIC SENSOR AND METHOD

TECHNICAL FIELD

The present invention relates to sensors for monitoring fluid components and, more particularly, to a sensor apparatus and method for monitoring of low/trace concentration fluid components.

BACKGROUND ART

In the field of environmental gas monitoring, various means have been employed and/or proposed for the detection of low or trace concentrations of impurities, e.g., hazardous gas species, in air or other ambient gases. The systems currently commercially available such as the so-called MDA monitors or Kitagawa tubes, are either costly, require significant maintenance (involving replacement of consumable elements, e.g., the frequent change of color tapes in MDA monitors), require frequent recalibration, or in some instances do not measure the impurity species properly or provide useful readouts. The MDA monitor is sensitive only down to concentration levels on the order of about 5 ppm, and readings below that level are inaccurate.

Another application in which the detection of low or trace concentrations of impurities is carried out is the conventional use of dry scrubbers, i.e., sorbent beds that reactively remove undesired components of gas streams flowed therethrough. In this application it is critically important that the approach of the bed to exhaustion of its removal capacity thereof be accurately determinable. If the exhaustion of the removal capability of the bed is not detected by operating personnel, then gas requiring treatment will pass untreated through the bed and be passed to discharge, disposal or other process steps, still containing the components desired to be removed from such treatment effluent. Such non-treatment, or inadequate treatment as the point of exhaustion is approached, may entail severe consequences. By way of example, dry scrubbers are used extensively in the semiconductor manufacturing industry, where the scrubber is employed to abate hazardous gases from the effluent from the processing plant, or specific operating components thereof. The failure to detect exhaustion of the scrubber bed thus may result in deleterious exposure of facility personnel to hazardous gases, as well as environmental contamination in the ambient surroundings of the semiconductor process facility. Additionally, incidents have been reported in which eductor devices downstream of scrubbers have experienced plugging when impurities have broken through the scrubbers without being detected.

Accordingly, it has been common practice either to require change-out of the scrubber bed, viz., replacement of the scrubber material in the bed with fresh scrubber medium, well prior to the actual exhaustion of the scrubber bed, i.e., with a substantial safety margin in respect of the operating life of the scrubber bed, or else to deploy monitors that detect actual or incipient breakthrough of the scrubbable components in the gas stream egressing the scrubber bed.

The first alternative, of change-out of the scrubber material well in advance of the exhaustion of the capacity of the scrubber bed, although effective in terms of preventing discharge of scrubbable components in the effluent gas, is inefficient in respect of the wastage of scrubber medium which could otherwise be employed to remove the scrubbable component, so that the effective capacity of the scrubber bed is not utilized. As a result, the scrubber bed must be oversized to accommodate the unused scrubber material.

The second alternative, of using monitors that detect actual or threshold breakthrough of the scrubbable components in the scrubber beds, is expensive, and involves the use of costly devices which additionally require significant maintenance (involving replacement of consumable elements, e.g., the frequent change of color tapes in so-called MDA monitors, or frequent change of cells in monitors such as those commercially available under the trademark "ENMET"), require inline recalibration not infrequently, and in some instances do not to measure the impurity species properly. In general, problems of cost, accuracy and reliability plague the existing commercially available monitors in application to scrubbing systems.

Accordingly, it would be a significant advance in the art to provide a low cost, accurate, reliable, and easily fabricated and operated sensor device for monitoring of impurity species in fluid environments, such as gas streams discharged from a scrubber bed, or ambient gas environments which are monitored for the presence of contaminants.

It is another object of the invention to provide a highly sensitive and selective detection system for determining the presence of impurity species in fluid environments.

It is a further object of the invention to provide an end point detector for sensing the breakthrough of impurity species in such operations as dry scrubbing of process gases.

Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a sensor for detection of a trace fluid component in a fluid environment. Such sensor may include:

a piezoelectric crystal having a fundamental resonant frequency in response to an applied oscillating electric field;

a coating on the piezoelectric crystal of a sensor material which is reactive with the trace fluid component to yield a solid interaction product of changed mass in relation to initial mass of the sensor material interacting with the trace fluid component to yield the solid interaction product;

means for applying an oscillating electric field to the piezoelectric crystal which generates an output resonant frequency therefrom;

means for (i) sampling the output resonant frequency of the piezoelectric crystal while the oscillating electric field is applied thereto, (ii) determining the change in resonant frequency from the fundamental resonant frequency that occurs on formation of the solid interaction product when the sensor material interacts with the trace fluid component in the fluid environment, and (iii) generating an output indicative of the presence of the trace fluid component in the environment; and

means for flowing fluid from the fluid environment to the coating on the piezoelectric crystal so that the trace fluid component when present reacts with the coating to form the solid interaction product;

wherein the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.001 to about 1000 milliHertz/min./(part-per-million of the fluid component). Preferably the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.01 to about 100 milliHertz/min./(part-per-miIlion of the fluid component); more preferably, such range is from about 0.1 to about 50 milliHertz/min./(part-per-million of the fluid component); and most preferably such range is from about 0.5 to about 10 miIliHertz/min./(part-per-million of the fluid component).

The means for flowing fluid from the fluid environment to the coating on the piezoelectric crystal may for example comprise a passage having appropriate geometry, e.g., length to diameter characteristics, and/or containing a flow limiting structure such as a frit or flow-restricting orifice, so that the flow of fluid to the coating is maintained at a level which is consistent with good sensitivity and useful sensor life.

In another embodiment of the invention, the flow limiting structure may comprise a frit or flow-restricting orifice, so that the flow of fluid to the coating is maintained at a constant rate, with the pressure on the low pressure side of the flow restriction being less than 1/2 the pressure on the upstream side of the flow restriction.

In such sensor, the piezoelectric crystal may for example comprise a piezoelectric silica crystal. The coating of sensor material usefully may comprise a chemisorbent material which is chemically reactive with the trace fluid component. Useful piezoelectric crystals include those having a fundamental resonant frequency in the range of from about 1 Megahertz to about 10 Megahertz.

In the sensor of the invention, the means for (i) sampling the output resonant frequency of the piezoelectric crystal while the oscillating electric field is applied thereto, (ii) determining the change in resonant frequency from the fundamental resonant frequency that occurs on formation of the solid interaction product when the sensor material interacts with the trace fluid component in the fluid environment, and (iii) generating an output indicative of the presence of the trace fluid component in the environment, may comprise means such as a circuit including therein a cascaded array of frequency counters.

The sensor may be constructed and arranged so that the output indicative of the presence of the trace fluid component in the environment, comprises a calculated concentration of said trace fluid component in said environment.

In one embodiment of the invention, the sensor further comprises a flow control means for controllably flowing a selected flow rate of fluid from the fluid environment into contact with the sensor material on the piezoelectric crystal, and the aforementioned means for performing functions (i), (ii) and (iii), comprise computational means for deterrnining the calculated concentration of the trace fluid component in the fluid environment, in accordance with the algorithm:

dF/dt = δ • [C, ] • Q

wherein:

dF/dt is the time-variant differential rate of change of frequency from the fundamental resonant frequency of the piezoelectric crystal coated with the sensor material as sampled by the means for performing functions (i), (ii) and (iii);

δ is a proportionality constant;

[C, ] is the concentration of the trace fluid component; and

Q is the volumetric flow rate of the fluid of the fluid environment.

The sensor in another embodiment further comprises a flow passage accommodating flow therethrough of fluid of the fluid environment, and having a diffusional flow restrictor in the passage, arranged in relation to the sensor material to permit substantially only diffusional flow from the flow passage through the diffusional flow restrictor to the sensor material. Such diffusional flow restrictor additionally is constructed and arranged to prevent particulate solids in the fluid environment from contacting the sensor material.

The sensor in still another embodiment may include a flow passage accommodating flow therethrough of fluid from the fluid environment, and having a flow restrictor in the passage, arranged in relation to the sensor material to restrict flow of fluid from the environment to the sensor material. Such diffusional flow restrictor additionally is constructed and arranged to prevent particulate solids in the fluid environment from contacting the sensor material.

The sensor may further comprise means for removing substantially all sensor material-interactive components other than the selected trace fluid component from the fluid, before the fluid contacts the sensor material. Such removing means may advantageously comprise a chemisorbent medium having sorptive affinity for sensor material-interactive components other than the selected trace fluid component.

The sensor material in one aspect may comprise a thin film metal, such as copper, zinc, silver, aluminum, chromium, or the like.

In another aspect, the invention relates to a fluid scrubbing assembly for processing of impurity-containing fluid, including:

a scrubber vessel containing a dry scrubber composition having sorptive affinity for impurity in the impurity-containing fluid;

means for introducing impurity-containing fluid to the scrubber vessel for contacting with the dry scrubber composition therein to remove impurity from the impurity-containing fluid, and yield treated fluid; means for discharging treated fluid from the scrubber vessel;

a sensor for detection of impurity in the treated fluid, such sensor comprising:

(I) a piezoelectric crystal having a fundamental resonant frequency in response to an applied oscillating electric field;

(II) a coating on the piezoelectric crystal of a sensor material which is reactive with the impurity to yield a solid interaction product of changed mass in relation to mass of the sensor material interacting with the impurity to yield the solid interaction product;

(III) means for applying an oscillating electric field to the piezoelectric crystal which generates an output resonant frequency therefrom;

(IV) means for (i) sampling the output resonant frequency of the piezoelectric crystal while the oscillating electric field is applied thereto, (ii) determining the change in resonant frequency from the fundamental resonant frequency upon formation of the solid interaction product when the sensor material interacts with impurity in the treated fluid, and (iii) generating an output indicative of the presence of the impurity in the treated fluid; and

means for flowing at least a portion of the treated fluid to the sensor for determining, by the output indicative of the presence of impurity, when breakthrough of impurity has occurred in the dry scrubber composition in said vessel. A further aspect of the invention relates to a process for monitoring a fluid stream for determining presence of a selected component therein, such process comprising:

providing a sensor for detection of the selected component in the fluid stream, such sensor comprising:

(A) a piezoelectric crystal having a fundamental resonant frequency in response to an applied oscillating electric field;

(B) a coating on the piezoelectric crystal of a sensor material which is reactive with the selected component to yield a solid interaction product of changed mass in relation to initial mass of the sensor material interacting with the selected component to yield the solid interaction product;

applying an oscillating electric field to the piezoelectric crystal which generates an output resonant frequency therefrom;

sampling the output resonant frequency of the piezoelectric crystal while the oscillating electric field is applied thereto;

determining the change in resonant frequency from the fundamental resonant frequency upon formation of the solid interaction product when the sensor material interacts with the selected component in the fluid stream; and

generating an output indicative of the presence of the selected component in the fluid stream.

In such process, the step of generating the output indicative of the presence of the selected component in the fluid stream, comprises determimng via a programmed computer a calculated concentration of the selected component in the fluid stream. The process may further comprise flowing fluid from the fluid stream at a constant flow rate to the coated piezoelectric crystal.

The process may further comprise controllably flowing at least a portion of the fluid stream at a selected flow rate in contact with the sensor material on the piezoelectric crystal, and deteirnining the calculated concentration of the selected component in the fluid stream, in accordance with the algorithm:

dF/dt = δ • [C, ] • Q

wherein:

dF/dt is the time-variant differential rate of change of frequency from the fundamental resonant frequency of the piezoelectric crystal coated with the sensor material as sampled;

δ is a proportionality constant;

[C, ] is the concentration of the selected component in the fluid stream; and

Q is the volumetric flow rate of the fluid stream.

Such determination of the concentration of the selected component may be accompanied by the sensing and determination of concentration of water vapor content in the environment, with adjustment or correction of the concentration of the selected component to compensate for such presence of water vapor. The water vapor sensing may be effected by hygrometric sensing of the water vapor, and such hygrometric sensing may likewise be based on a piezoelectric sensor comprising a coating interactive with water vapor. In the process of the invention, the selected component may for example comprise a halide gas. By way of further example, the selected component may comprise a gas such as boron trichloride, boron trifluoride, hydrogen chloride, chlorine, fluorine, hydrogen fluoride, etc.

The process of the invention may be carried out with the sensor being constructed and arranged to be contacted by only a restricted part or portion of a main gas flow stream in a process system, so that the cumulative concentration of the impurity species reactive with the coating material on the piezoelectric crystal does not rapidly consume the coating and deplete the capacity of the sensor to detect the impurity species over a useful lifetime of operation.

In another aspect, the coated crystal is arranged in the sensor apparatus in relation to the fluid flow stream so that the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.001 to about 1000 milliHertz/min./(part-per-million of the fluid component), preferably in the range of from about 0.01 to about 100 milliHertz/min./(part-per-million of the fluid component), more preferably in the range of from about 0.1 to about 50 milliHertz/min./(part-per-million of the fluid component), and most preferably in the range of from about 0.5 to about 10 milIiHertz/min./(part-per-million of the fluid component). Such arrangement may for example entail the sampling by the coated piezoelectric crystal of a slip-stream or side-stream of a main flow of process fluid, or the restricted access of the main flow of fluid to the coated piezoelectric crystal.

In another aspect, the invention relates to a fluid scrubbing process for treating impurity-containing fluid, comprising:

contacting impurity-containing fluid with a dry scrubber composition to remove impurity from the impurity-containing fluid, and yield treated fluid; detecting impurity in the treated fluid, by the steps comprising:

providing:

(I) a piezoelectric crystal having a fundamental resonant frequency in response to an applied oscillating electric field; and

(II) a coating on the piezoelectric crystal of a sensor material which is reactive with the impurity to yield a solid interaction product of changed, e.g., increased or decreased, mass in relation to initial mass of the sensor material interacting with the impurity to yield the solid interaction product;

determining the change in resonant frequency from the fundamental resonant frequency incident to the formation of the solid interaction product when the sensor material interacts with impurity in the treated fluid;

generating an output indicative of the presence of the impurity in the treated fluid; and

flowing at least a portion of the treated fluid to the sensor for determining, by the output indicative of the presence of impurity, when breakthrough of impurity has occurred in the dry scrubber composition. Other aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a flow restricting orifice such as may be usefully employed in the broad practice of the invention, to limit the flow of impurity-containing gas to a piezoelectric crystal sensor of the invention.

Figure 2 is a schematic view of a dry scrubber system featuring a piezoelectric crystal sensor assembly according to one embodiment of the invention.

Figure 3 is a perspective schematic view of a fluid monitor assembly according to one embodiment of the invention.

Figure 4 is a sensor assembly, according to another embodiment of the invention.

Figure 5 is a sensor assembly, according to still another embodiment of the invention.

Figure 6 is a graph of frequency response as a function of time, for a Zn electrode piezoelectric crystal sensor, in exposure to HCI at 5 ppm concentration and 2500 ppm water.

Figure 7 is a graph of frequency response as a function of time, for a Ag piezoelectric crystal electrode sensor, in response to HCI at 5 ppm concentration and 2500 ppm water.

Figure 8 is a graph of frequency as a function of water concentration, in ppm, showing the frequency response of a zinc electrode piezoelectric crystal sensor, determined with BCI3. Figure 9 is a graph showing zinc electrode coated piezoelectric crystal sensor frequency response as a function of flow in standard cubic centimeters of gas per minute (seem), with varied flow at 5 ppm HCI and 2500 ppm water.

Figure 10 is a graph of dHz/dt, the rate of frequency change, as a function of chloride concentration, for a Zn electrode sensor device, at constant flow, temperature and pressure, at 50 seem and 2500 ppm water conditions.

Figure 11 is a graph of frequency as a function of time, showing the response characteristics of a sensor representative of the present invention, and the response characteristics of an MDA sensor, with boron trichloride (BCI3).

Figure 12 is a graph of the rate of change of frequency per unit of trace impurity, as a function of the flow rate of the fluid stream containing such impurity, for a Zn electrode piezoelectric sensor according to one embodiment of the invention, in which a 5/16 inch flow restricting orifice is employed to restrict the flow of the impurity (HCl)-containing fluid to the sensor.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

The disclosures of the following applications are hereby incorporated herein by reference in their entirety: United States patent application no. 08/678,572 filed July 12, 1996; United States patent application no. 08/679,258 filed July 12, 1996; and all applications from which they claim priority, or from which priority is claimed.

The present invention utilizes piezoelectric crystals coated with electrode sensor materials such as thin metal film coatings of Cu, Zn, Ag, Al, Cr, etc., to provide highly sensitive detectors for halide and other gases, when the gases contact and react with the electrode sensor material under operating conditions.

In the sensor of the invention, the piezoelectric crystal coated with the electrode sensor material is subjected to an input frequency, such as by means of an appropriately constructed and arranged oscillator circuit coupled in operative relationship to the piezoelectric crystal. The output frequency of the piezoelectric crystal coated with the electrode sensor material then is monitored and the change of the frequency in relation to the natural harmonic frequency of the coated crystal is determined, e.g., by a cascaded counter assembly.

By this arrangement, the contacting of a halide gas with the coating material on the crystal will cause reaction to yield a metal halide reaction product of different mass than the initial mass of the metal on the crystal. As a result of such mass change, the frequency response characteristics of the coated crystal will change, and this frequency change thus will reflect the presence of the halide component in the gas contacted with the coating film on the piezoelectric crystal.

Accordingly, in the practice of the invention involving sensing of halide gaseous components, the frequency of an oscillator in the piezoelectric crystal circuit thus may be readily monitored to detect halogenation of the electrode, involving chemical reactions such as the following:

Zn + 2HC1 → ZnCl₂ + H₂

Zn + F₂ -» ZnF₂

It is readily feasible in the practice of the invention to tailor the reactivity of the coating material on the piezoelectric crystal, by choice of different materials, to obtain the appropriate desired sensitivity to different trace gases. For example, set out below are several illustrative thermodynamic equilibrium constants, for the reaction of HCI with different electrode (piezoelectric crystal coating) materials:

2HCl(g) + 2Ag — 2 AgCl + H₂(g) Keq=10⁶ 2HCl(g) + 2Cu → CuCl + H₂(g) Keq= 1O¹⁷ 2HCl(g) + Zn → ZnC12 + H₂ (g) Keq= 10³⁵

From this list one would predict that of these three piezoelectric crystal coating materials, Zn would be the most sensitive to HCI, and Ag would be the least. In like manner, a desired sensitivity coating material can readily be selected, for various other and specific gas components of interest, in a given sensing or monitoring application of the present invention.

In the broad scope of practice of the present invention, the piezoelectric crystal features a coating of a material which is interactive with the gas species of interest, to yield an interaction product which alters the frequency response of the piezoelectric crystal, so that the presence of the gas species is readily detectable in the gas contacted with the coated crystal. Thus, the coating material may suitably comprise a material which is irreversibly chemically reactive with the gas species of interest, to produce a reaction product which is of a different mass than the original coating material, being either greater or smaller in magnitude in relation to the virgin coating on the crystal.

In the use of coating materials which are consumed in contact of the gas species of interest therewith, the sensor coating can be consumed in scrubbing applications by breakthrough of impurities from the scrubber bed, so that the concentration of the impurities is suddenly increased from a zero or near-zero concentration to a high concentration. The coating can also be consumed in such scrubbing applications by a continuing low level leakage in the scrubber system. If the lifetime of the scrubber bed is sufficiently long, the sensor could be used up before the breakthrough of the scrubber bed occurs. An end point sensor product, therefore, needs to be designed to alert the user if there is a "gas alarm" condition indicative of breakthrough of the gas species from the scrubber bed, or a "system fault" condition indicative of a continuing leak of the gas species of interest which has consumed a significant portion of the coating material. The sensor system may therefore be constructed and arranged so that a gas alarm is tripped if a sufficient rate of change of the frequency over time has occurred. The sensor system may also be constructed and arranged so that system fault will occur if the life of the coated crystal has been terminated, or if the capacity of the sensor coating has been substantially exhausted before a sufficiently large rise in frequency has been obtained.

This gas alarm and system fault distinction can be accommodated by measuring the differential frequency rate of change, dF/dt (F = frequency), and the change in frequency from the start of the life of the sensor. If a large dF/dt is measured, such measurement indicates the occurrence of breakthrough of impurity from the scrubber bed. If no large dF/dt is measured but the sensor response has damped out from weight gain (of reaction product incident to leakage impurity reacting with the coating material slowly over a period of time), such response damping indicates that the sensor has been consumed without impurity breakthrough of the scrubber bed. This use of two "trip points," indicative of impurity breakthrough as well as significant leakage consumption of the coating material, is unique in the design and operation of scrubber systems, and achieves a substantial advance in the art.

As an illustrative embodiment of the invention, the frequency response curve of a sensor system for sensing of chloride gases with a zinc electrode coating on the piezoelectric coating can be described using the equation below. This equation contains a chloride gas concentration term, a gas flow rate term, and water, pressure and temperature terms (the water term reflecting the presence of relative humidity moisture in the gas being sensed). Lifetime of the piezoelectric crystal is not included in this equation because for the breakthrough or endpoint sensor used for scrubbing applications, coating lifetime can be treated as linear and is therefore taken into account with a numerical constant, α, as a coefficient in the equation. In other applications such as environmental monitor sensors, the sensor coating lifetime will be reflected as a separate term in the algorithm. The equation for the scrubber breaktlvrough sensor is:

dF/dt = α[HCl][Flow Rate][7.3 - 7.4*10-^{( 00056)[H} ₂ ⁰⁾]b dP/dt + g dT/dt + noise terms wherein:

F= frequency (Hz); α= general coefficient for chloride gas concentration, flow rate and water concentration; b= pressure coefficient which is small relative to the first term in the equation; and g= temperature coefficient which is small relative to the first term in the equation.

For the use of the sensor in scrubber applications, the water concentration variable, the pressure and the temperature noise terms are added to the constant, α. To avoid complications a new constant for the simplified equation will be represented as k. The simplified equation for the end point application then becomes:

dF/dt = k[HCl][Flow Rate]

wherein: k = proportionality constant which includes the water concentration, temperature coefficient, pressure coefficient and lifetime terms.

With a zinc coated sensor, k - 40 Hz(min.)^"'(ppm) (1pm)"¹ at a water concentration of about 2500 ppm. This sensitivity is sufficiently high so that breakthrough of impurity in the scrubber bed can be detected within a few minutes of breakthrough transition first occurring. Due to the first order dependence on flow in the above equation, it is important to have a constant known flow through the sensor housing in which the sensor element comprising the coated piezoelectric crystal is disposed for contacting with the gas egressing the scrubber bed.

Another issue which is important in the scrubber bed applications of the invention is keeping particulates away from the sensor element, in order to avoid false alarms due to additional loading of the particulates on the crystal.

To maintain such a constant flow and to avoid contamination of the sensor element with particulates, a frit or a flow restrictor may be deployed in the gas flow passage, e.g., conduit, through which the gas being sampled is flowed. Such flow restriction means may be employed to force the flow to be purely or substantially diffusional in character, and it will act as a particle filter at the same time. An example of such a flow restrictor device 10 is shown in Figure 1, interposed between conduit 12, whose end 14 is joined to the sensor housing (not shown) and conduit 16, whose end 18 is joined to the manifold of the scrubber bed assembly (also not shown). The flow restrictor may in a specific embodiment comprise a 1/4" "TEFLON" plug in a F25 tee which has a single 5/16"- 18 tapped hole in it to allow diffusion of the gas to the sensor. The single hole will provide enough medium for gas to diffuse through without clogging.

If there are many particulates in the gas stream then in place of such a single hole flow restrictor, a porous frit may alternatively be utilized.

In some instances, the gas being monitored for the presence of a specific halide may contain other halide species, or more generally, the coating material used in the sensor may be chemically reactive with a number of species in the gas. In such instances, it may be necessary to provide ancillary treatment of the gas to remove the species thereof which are not of interest in the monitoring or detection process.

For example, if the sensor is not selective for chloride gas of a specific type, but rather responds similarly to all three chloride gases in a gas containing BCI3, HCI and Cl₂ which is undergoing scrubbing treatment, then it may be desirable to install a guard column or other extraneous chloride gas removal means, upstream of the sensor receiving the gas being monitored.

Thus, a reactive chemical removal agent for use in a guard column can be selected by examination of standard electrode potentials. For example, in the case of a two- component gas mixture (HCI and Cl₂) where the sensor is intended to selectively detect HCI but not Cl₂, electrode potential analysis shows that Fe(II) may be usefully employed in a guard column to obtain this selectivity. A positive net electrode potential (E°) yields a favorable reaction, and a negative E° yields an unfavorable reaction, in respect of the following reactions:

Fe(II) + Cl₂ → Fe(III) + 2 Cl- E°= 0.589V (favorable)

FeCl₂ + HCI → FeCl₃ + 1/2 H₂ E° = -0.771V (unfavorable) Cu or Cu(I) would also be sufficient for this purpose, as shown by the following reactions:

Cu + Cl₂ -* Cu (I) + 2C1- E° = 0.84V (favorable)

Cu(I) + Cl₂ → Cu(II) + 2C1- E°=l .207V (favorable)

Cu + HCI → CuCl₂ + H₂ E° = -0.52V (unfavorable)

CuCl + HCI — CuCl₂ + H₂ E° = -0.153V (unfavorable)

Pb or Ca would not be suitable candidate materials for such purpose because they both react favorably with HCI and Cl₂.

To determine the proper species of removal agent for the guard column one must examine the standard electrode potentials of the components. If the addition of the electrode potentials for the two components is positive then the reaction is favorable, and if the addition is negative then the reaction will not occur readily. There are many possible choices for materials which will selectively react with the gas component to be "masked" from exposure to the sensor. Care must be exercised in this determination to pick a reactive component which reacts only with the gas species to be masked, and not the gas species to be sensed by the piezoelectric sensor.

Modification of the sensor coatings to provide oxidizing characteristics may be utilized as a suitable technique to provide sensitivity to hydride gases. For example, oxidation of a Cu, Cr, or Ag electrode coating to the corresponding oxide salt may be carried out for such purpose. Such oxides react with the hydrides to form nonvolatile salts (and hydrogen/water). There is a net gain in weight (relative to the starting sensor coating material) when such reaction occurs. Mass-sensitive piezoelectric sensors can thus be used to readily and economically detect the occurrence of such reaction: 3CuO + 2AsH₃ → Cu₃As₂ + 3H₂O

As in the case of the chloride reactions, it is possible within the broad scope of the invention to readily tailor the reactivity of the sensor material and guard bed reactive material for a specific end use application of the present invention.

As described, the sensor device of the present invention may be readily fabricated and deployed to provide accurate and reliable sensing of impurity species of interest in gas scrubbing applications of the type wherein a solid scavenger or chemisorbent material having removal capability for the impurity is contacted with the gas to remove the impurity therefrom, and wherein the sensor is utilized to determine the presence of breakthrough and/or leakage of the impurity from the bed or beds in the scrubbing system.

The gas sensor of the present invention also has utility for environmental monitoring applications in which the coated piezoelectric crystal is provided to sense the presence of undesired components in a fluid environment such as air or other ambient gases.

Figure 2 is a schematic view of a dry scrubber system featuring a piezoelectric crystal sensor assembly according to one embodiment of the invention. This process system comprises a scrubber vessel 20 containing a quantity of a dry scrubber material 22 as a bed or mass in the vessel. The dry scrubber vessel 20 is arranged in receiving relationship to the process facility 24, which discharges a waste gas in line 26. The waste gas stream containing the impurity to be scrubbed from the gas enters the dry scrubber vessel in line 26 for scrubbing therein to deplete the gas of scrubbed component.

The dry scrubber vessel 20 has a vertically upstanding discharge conduit 28 disposed in the interior volume of the scrubber vessel, with its lower end open to receive scrubbed gas for flow upward in the conduit 28 and discharge therefrom at the open upper end of the conduit in the direction indicated by arrow M. The open upper end of the conduit 28 terminating exteriorly of the scrubber vessel 20 may be arranged to discharge the scrubbed gas to an exhaust means of the process facility, or otherwise such conduit at its open upper end may be joined to other flow passage means or apparatus for further treatment and/or disposition of the scrubbed gas.

The dry scrubber vessel 20 also has disposed therein and terminating exteriorly thereof a sampling conduit 30 receiving scrubbed gas at its open lower end for flow upwardly therein. Exterior of the scrubber vessel 20, a guard bed 32 is provided in conduit 30 for removing from the scrubbed gas stream any extraneous impurities which may react with the reactive coating of the piezoelectric sensor 36, and thereby adversely affect the sensor's accuracy for the impurity species of interest.

The guard bed may therefore contain a chemisorbent scavenger for the extraneous impurity species, so that the sample gas stream in conduit 39 is passed to the piezoelectric sensor depleted in such extraneous fluid component(s). Intermediate the guard bed 32 and the piezoelectric sensor 36 is an optional flow restriction, which may for example be of the type illustratively shown in Figure 1 , for the purpose of maintaining the flow rate of the sample gas passed to the piezoelectric sensor at a level consistent with good operating life characteristics of the sensor.

As an alternative flow restricting feature, the diameter of the conduit 30 may be significantly less than the diameter of conduit 28, so that the side stream in conduit 30 is correspondingly only a portion of the flow discharged from the vessel 20 in conduit 28.

As a still further alternative flow restricting feature, the conduit 30 may with the conduit 48 downstream of the guard bed 32, form a main flow passage for discharge of scrubbed gas from the scrubber vessel (in lieu of, or in addition to, the conduit 28), and the conduit 39 may be provided with appropriate dimensions to attenuate the flow of gas to the piezoelectric sensor 36. For example, the conduit 39 may have a diameter which is smaller than the diameter of conduits 30 and 48, or alternatively, the conduit 39 may simply by virtue of its length from the junction with conduit 48 to the sensor 36 serve to diminish the flux of the sampled scrubbed gas to an appropriate level.

Thus, the main flow of scrubbed gas from the scrubber vessel may be substantial, e.g., 40 liters per minute or more, and such gas flow would if directly contacted with the sensor coating rapidly deplete the coating even at low trace levels of the impurity, due to the cumulative large volume which would be experienced by the coating.

Accordingly, it is desired in the practice of the present invention to restrict the flux of the sampled gas stream to the sensor such that the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.001 to about 1000 milliHertz/min./(part-per-million of the fluid component), preferably in the range of from about 0.01 to about 100 milliHertz/min./(part-per- million of the fluid component), more preferably in the range of from about 0.1 to about 50 milliHertz/min./(part-per-million of the fluid component), and most preferably in the range of from about 0.5 to about 10 milliHertz/min./(part-per- million of the fluid component). Such arrangement may as previously described entail the sampling by the coated piezoelectric crystal of a slip-stream or side-stream of a main flow of process fluid, or the restricted access of the main flow of fluid to the coated piezoelectric crystal.

The sampled gas stream after contact with the coating on the piezoelectric sensor is discharged from the sensor 36 in line 44, from which the sampled gas may be recycled to the main gas stream, or otherwise disposed of in the process facility. As shown in Figure 2, the piezoelectric sensor 36 comprising coated crystal 38 is operatively coupled, e.g., by signal transmission lines 40 and 41 to electronics module 42.

The electronics module includes suitable output means, e.g., comprising a liquid crystal display (not shown), which may numerically display a concentration value or other information for the impurity gas being monitored. Alternatively, the output means may provide a colorimetric display, e.g., with red indicating a hazardous or dangerously high concentration of the gas component of interest, yellow indicating a tolerable but high concentration of the gas component, and green indicating that the gas component concentration is within acceptable concentration limits. As still other alternatives, the output means may comprise a audible alarm, other visual display (e.g., a flashing light), or any other suitable output means.

The electronics module 42 is constructed and arranged for (i) sampling the output resonant frequency of the piezoelectric crystal while the oscillating electric field is applied thereto, (ii) determining the change in resonant frequency from the fundamental resonant frequency that occurs on formation of the solid interaction product when the sensor material interacts with the trace fluid component in the sampled fluid stream, and (iii) generating an output indicative of the presence of the trace fluid component in the fluid stream, with the coated piezoelectric crystal exhibiting a frequency response rate to the trace fluid component in the range of from about 0.001 to about 1000 milliHertz/min./(part-per-million of the fluid component).

The sensor device of the present invention may be readily fabricated and deployed to provide accurate and reliable sensing of impurity species of interest in a wide variety of fluid environments, e.g., air or other ambient gases.

Point of use environmental sensors according to the present invention may utilize a simple pump arrangement to draw ambient gases across the face of the sensor, with the sensed change in frequency being used to determine the concentration of the impurity gas species in the ambient environment. The output of such environmental sensor can be time averaged or instantaneous in character.

Sampling of gas from the environment being monitored may be effected by flowing the gas sampled from the environment through a tube at a suitably low flow rate, with the sensor being disposed in the tube and mounted for sensing of the gas. Such an arrangement does not require additional dilution flow and can operate in a low flow regime.

An environmental monitoring assembly according to one embodiment of the invention is shown in Figure 3. This environmental monitoring assembly 100 comprises a housing 102 containing therein a microelectronics module comprising motherboard 104 and ancillary board 106 which are operatively interconnected to the gas sensor element 1 12. The gas sensor element comprises a piezoelectric crystal having coated thereon a thin film of a sensor material with which the specific gas of interest is reactive to yield a metal-containing reaction product, as well as to the moisture sensor element 110 comprising a piezoelectric crystal. The moisture sensor element 110 may comprise an uncoated piezoelectric crystal, or it may have coated thereon a thin film of a sensor material 11 1 with which the water in the sampled gas is reactive to yield a metal hydride and/or metal oxide reaction product.

Alternatively, another type of moisture sensor element or assembly could be used, such as capacitive hygrometers, resistive hygrometers, etc.

The microelectronics module is in turn operatively connected to the output means comprising liquid crystal display 108 which may numerically display a concentration value for the gas species being monitored. Alternatively or additionally, the output means may provide a colorimetric display, e.g., with red indicating a hazardous or dangerously high concentration of the gas component of interest, yellow indicating a tolerable but high concentration of the gas component, and green indicating that the gas component concentration is within acceptable concentration limits. As still other alternatives, the output means may comprise a audible alarm, other visual display (e.g., a flashing light), a tactile indicating means such as a vibratory oscillator when the monitoring unit is worn on the person of a user thereof, or any other suitable output means.

The factors which control the response of the environmental monitor of Figure 3 are flow rate (FR), the concentration of water, the monitored gas species concentration, and the life of the piezoelectric crystal. The response is not linear over the life of the crystal.

In the environmental monitor of Figure 3, the sample gas is pulled across the two sensor elements 110 and 112 by an eductor 126. Gas is supplied to the eductor by eductor gas inlet 116, which may be suitably coupled to a source of "driver" gas, such as a container of compressed air. The eductor features suction tube 120 for drawing air, admitted into the housing by inlet fitting 114, across the sensor elements to the gas outlet 122. A frit (not shown) is provided in the inlet fitting 1 14 to the sensor cavity, to prevent particulate contamination. There is a second flow limiting orifice or frit (also not shown) in the inlet line to the eductor. This flow restriction provides constant flow through the sensor chamber. This constant flow condition can be maintained if the pressure on the low pressure side of the flow restriction is less than 1/2 the pressure on the upstream side of the flow restriction.

The concentration of water is measured by the piezoelectric crystal sensor 1 10. Alternatively, other moisture sensor means may be employed as discussed hereinabove.

Figure 4 shows an exploded view of a sensor assembly according to another embodiment of the invention, comprising the sensor element 150 and the housing 160. The sensor element 150 comprises the piezoelectric crystal 154 which is coated with a suitable material interacting with the fluid component of interest to yield an interaction product of differing mass characteristic than the original coating material. The coated crystal is mounted on the plug member 152, with the respective leads of the piezoelectric crystal 154 protruding exteriorly of the plug member when the plug member is engaged with the housing 160 with the coated crystal extending into the cavity 162.

The housing 160 features an opening 164 by which a gas can be flowed into the cavity 162 containing the sensor element 150. Although not shown in the front perspective view of Figure 4, the housing 160 has another opening therein, opposite opening 164 and in register with such opening, for discharge from the housing of the gas flowed past the coated piezoelectric crystal.

The leads 156 and 158 of the sensor element may be coupled in circuit relationship to suitable electronics means shown schematically as electronics module 166 in Figure 3, by which the presence and concentration of the gas impurity species can be detected. The electronics module 166 is coupled to the sensor element leads 156 and 158 by wires 163 and 165, respectively.

Electronics module 166 provides the functions of (i) sampling the output resonant frequency of the piezoelectric crystal while the oscillating electric field is applied thereto, (ii) deteirnining the change in resonant frequency from the fundamental resonant frequency incident to the formation of the solid interaction product when the sensor material interacts with the trace fluid component in the fluid being monitored, and (iii) generating an output indicative of the presence of the trace fluid component in such fluid.

In a specific embodiment of the sensor assembly shown in Figure 4, the housing 160 may comprise an aluminum housing which has the cavity 162 machined into it for insertion of the sensor element, as well as two feedthrough (1/4" NPT) openings (opening 162 and the opposite opening not shown in Figure 4) for the gas to flow through the sensor. In the body of this housing is the flow restricting orifice. This 1/4" aluminum housing fits directly on the scrubber vessel, or the housing of an environmental fluid monitor, and the front end driver electronics are plugged directly onto the legs (leads 156 and 158) of the sensor assembly. The resulting assembly may be coupled to a sensor tube of the scrubber vessel, or otherwise joined in flow sensing communication with the scrubber vessel or scrubber bed therein. The resulting assembly may be coupled to a sensor tube of a scrubber vessel or otherwise joined in flow sensing communication with a scrubber vessel, scrubber bed, or environmental monitor.

Figure 5 is an exploded perspective view of another sensor assembly according to the present invention, comprising the sensor element 180 and the receiving fitting 190. The sensor element 180 comprises the piezoelectric crystal 182 which is coated with a suitable material interacting with the fluid component of interest to yield an interaction product of differing mass characteristic than the original coating material. The coated crystal is mounted on the plug member 184, with the respective leads 186 and 188 of the piezoelectric crystal protruding exteriorly of the plug member when the plug member is engaged with the receiving fitting 190 with the coated crystal extending into the cavity 192.

In a specific embodiment, the receiving fitting comprises a KF25 blank which will fit into a KF25 tee having a flow restricting orifice in the same leg as the sensor. The electronics associated with the sensor element plug directly into the legs of the sensor unit (leads 186 and 188).

It will be appreciated that the sensor device of the invention may assume a wide variety of conformations and arrangements in the broad practice of the invention, consistent with the specific end use of the sensor device, and the nature and extent of the output function thereof.

Figure 6 is a graph of frequency as a function of time, showing the frequency response of a Zn electrode piezoelectric crystal sensor according to an illustrative embodiment of the invention, in exposure to HCI. The slope of the line in this plot determines the frequency change expected over time at 5 ppm HCI, 2500 ppm water, and 50 seem HCI. This number is 6.9 Hz/min. To put this number in perspective, if sampling were carried out for 10 min. the expected frequency change would be «70Hz, and the signal to noise (S/N) ratio is 35. Such frequency change is easily detected with the system of the present invention.

Figure 7 is a graph of frequency as a function of time, showing the frequency response of a silver (Ag) electrode piezoelectric crystal sensor according to an illustrative embodiment of the invention, in exposure to HCI. The frequency response over this interval, 0.025 Hz/min., is much smaller than that obtained with the Zn electrode (see Figure 6), and corresponds to a 0.2 Hz change at a ten minute sampling period. In general, the Ag electrode sensor is much less sensitive than the zinc electrode, as predicted hereinabove.

Figure 8 is a graph of frequency as a function of water concentration, in ppm, showing the frequency response of a zinc electrode piezoelectric crystal sensor according to an illustrative embodiment of the invention. The effect of water on the zinc sensor coating was determined by examining the frequency change over time with variation in water concentration, and with the water dependence curve determined with BCI3. This figure shows that water is a catalyst in the corrosion reaction and that it accelerates the reaction to a point. At a maximum water concentration the rate of the reaction is constant. The water concentration at which the rate is constant is approximately 3000 ppm.

In relation to the previously discussed algorithm for frequency change with time, the analysis can be simplified for applications such as end point monitoring of scrubber beds (to determine breakthrough) by assuming that the water term is constant and therefore can be dropped from the equation. The pressure and temperature expressions can also be removed because they are very small relative in magnitude to the flow and chloride concentration values. In other applications where accuracy is more important these terms are retained in the equation.

The first order flow rate dependence of the endpoint sensor of the invention in application to HCI sensing was determined by the variation of flow at constant water, temperature, pressure and HCI concentration. The resulting data are reflected in the graph of Figure 9, showing zinc electrode coated piezoelectric crystal sensor frequency response as a function of flow in standard cubic centimeters of gas per minute (seem), with varied flow at 5 ppm HCI and 2500 ppm water. From this data it is seen that as the flow doubles the frequency response rate of change, dHz/dt, doubles as well, indicative of a first order dependence relationship.

Figure 10 is a graph of dHz/dt, the rate of frequency change, as a function of chloride concentration, for a Zn electrode sensor device representative of the scrubber sensor of the present invention. Figure 10 shows this data (dHz/dt vs. HCI concentration) at constant flow, temperature and pressure, at 50 seem at 2500 ppm water conditions.

Figure 11 is a graph of frequency as a function of time, showing the response characteristics of a sensor representative of the present invention, and the response characteristics of an MDA sensor, with boron trichloride (BCI3). The data show that the sensor of the present invention has similar response times as the MDA sensor.

The response of the piezoelectric crystal sensor of the invention is proportional to the flow rate. In a high flow regime (e.g., in the range of 5 to 100 1pm) a flow restricting orifice may be present to extend the life of the crystal and to control its response. Figure 12 is a graph of the frequency response (change of frequency per unit time per ppm of HCI), as a function of flow rate of gas in liters per minute (lpm). This graph shows the frequency response of a Zn electrode piezoelectric crystal sensor to HCI at different flow rates when utilizing a flow restricting orifice with an inside diameter of 5/16 inch. As shown by this graph, the flow restriction afforded by the orifice is sufficient to accommodate a 40 liter per minute flow of HCl-containing gas, restricting the flux at the Zn coating on the piezoelectric crystal so that the Δfrequency/minute/ppm of HCI is in the range of 4.8 to 6.4, thereby providing excellent dynamic frequency response characteristics consistent with superior operating life of the sensor.

Various of the foregoing graphs and examples reflect flow rates of significant magnitude, above the magnitude of rates which may be employed in most environmental fluid monitoring applications. Nonetheless, these examples, variously reflecting the use of flow restriction means to attenuate the flux of the reactive fluid species on the piezoelectric crystal coating, are useful in illustrating the operational characteristics of coated piezoelectric crystals which are usefully employed in environmental fluid monitor apparatus and processes within the broad scope of the instant invention.

The foregoing data and examples show that the piezoelectric crystal sensor of the invention provides an effective and simple means and method for determining the presence of a dilute or trace component in a gas. The invention contemplates the provision on a piezoelectric crystal substrate of a reactive coating which possesses high sensitivity and selectivity for a wide variety of gas species, e.g., chlorides, fluorides, hydrides, etc. in correspondingly diverse fluid environments.

INDUSTRIAL APPLICABILITY

The invention is a sensor for detecting and monitoring low/trace concentration fluid components in a fluid stream or environment. The sensor is a lower cost alternative to current monitoring techniques and, yet, provides improved accuracy, reliability, and operability. The sensor includes a piezoelectric crystal featuring a material coating which is reactive with the gas species of interest to yield an interaction product which alters the frequency response of the piezoelectric crystal. The invention has utility as an monitor for the detection of low or trace concentrations of impurities, hazardous chemicals, or other undesirable components in a fluid environment, such as ambient air. The invention also has utility as an end point detector in treating effluent in the semiconductor manufacturing industry. The invention may also have utility, inter alia, as a monitor for particulate scrubber technology in an industrial environment.

While the invention has been described herein with reference to specific aspects, features, and embodiments, it will be appreciated that other variations, modifications, and embodiments are possible, and all such variations, modifications, and embodiments therefore are to be regarded as being within the spirit and scope of the invention.

Claims

THE CLAIMSWHAT IS CLAIMED IS:

1. A sensor for detection of a trace fluid component in a fluid environment, comprising:

means for flowing fluid from the fluid environment to the coating on the piezoelectric crystal so that the trace fluid component when present reacts with the coating to form the solid interaction product; wherein the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.001 to about 1000 milliHertz/min./(part-per-million of the fluid component).

2. A sensor according to claim 1 , wherein the piezoelectric crystal comprises a piezoelectric silica crystal.

3. A sensor according to claim 1, wherein the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.01 to about 100 rnilliHertz/min./(part-per-million of the fluid component).

4. A sensor according to claim 1, wherein the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.5 to about 10 milliHertz/min./(part-per-million of the fluid component).

5. A sensor according to claim 1, wherein the piezoelectric crystal has a fundamental resonant frequency in the range of from 1 Megahertz to 10 Megahertz.

6. A sensor according to claim 1, wherein the means for (i) sampling the output resonant frequency of the piezoelectric crystal while the oscillating electric field is applied thereto, (ii) determining the change in resonant frequency from the fundamental resonant frequency that occurs on formation of the solid interaction product when the sensor material interacts with the trace fluid component in the fluid environment, and (iii) generating an output indicative of the presence of the trace fluid component in the environment, comprises a circuit including therein a cascaded array of frequency counters.

7. A sensor according to claim 1 , wherein said output indicative of the presence of the trace fluid component in said environment, comprises a calculated concentration of said trace fluid component in said environment.

8. A sensor according to claim 7, further comprising a flow control means for controllably flowing a selected flow rate of fluid of said fluid environment in contact with the sensor material on said piezoelectric crystal, and wherein the means for performing functions (i), (ii) and (iii), comprise computational means for determining said calculated concentration of said trace fluid component in the fluid environment, in accordance with the algorithm:

dF/dt = δ ^« [C, ] ^» Q

wherein:

dF/dt is the time-variant differential rate of change of frequency from the fundamental resonant frequency of the piezoelectric crystal coated with the sensor material as sampled by said means for performing functions (i), (ii) and (iii);

δ is a proportionality constant;

[C, ] is the concentration of the trace fluid component; and

Q is the volumetric flow rate of the fluid of the fluid environment,

9. A sensor according to claim 1, further comprising a flow passage accommodating flow therethrough of fluid of the fluid environment, and having a diffusional flow restrictor in the passage, arranged in relation to the sensor material to permit substantially only diffusional flow from the flow passage through the diffusional flow restrictor to the sensor material, said diffusional flow restrictor additionally being constructed and arranged to prevent particulate solids in the fluid environment from contacting the sensor material.

10. A sensor according to claim 1, further comprising means for removing from the fluid before its contacting with the sensor material substantially all sensor material-interactive components other than said trace fluid component.

11. A sensor according to claim 10, wherein the sensor material-interactive components removing means comprises a chemisorbent medium having sorptive affinity for said sensor material-interactive components other than said trace fluid component.

12. A sensor according to claim 1, wherein the sensor material comprises a thin film metal.

13. A sensor according to claim 1, wherein the thin film metal is selected from the group consisting of copper, zinc, silver, aluminum and chromium.

14. A sensor according to claim 1, wherein said means for carrying out functions (i), (ii) and (iii), are constructed and arranged to provide (A) an output gas alarm condition indicative of breakthrough of the trace fluid component, and (B) an output system fault condition indicative of a continuing leak of the trace fluid component when the trace fluid component has consumed a significant portion of the coating material.

15. A sensor according to claim 1, constructed and arranged to measure the differential frequency rate of change, dF/dt, and the change in frequency from the start of the life of the sensor, whereby if a large dF/dt is measured, such measurement indicates the occurrence of brealcthrough of impurity from the scrubber bed, and if no large dF/dt is measured but sensor response has damped out from weight gain incident to leakage trace impurity reacting with the coating material slowly over a period of time, such response damping indicates that the sensor has been consumed without trace impurity breakthrough.

16. A fluid scrubbing assembly for processing of impurity -containing fluid, comprising:

a scrubber vessel containing a dry scrubber composition having sorptive affinity for impurity in said impurity-containing fluid;

means for introducing impurity-containing fluid to the scrubber vessel for contacting with the dry scrubber composition therein to remove impurity from the impurity-containing fluid, and yield treated fluid;

means for discharging treated fluid from the scrubber vessel;

a sensor for detection of impurity in the treated fluid, said sensor comprising:

(II) a coating on the piezoelectric crystal of a sensor material which is reactive with the impurity to yield a solid interaction product of increased mass in relation to mass of the sensor material interacting with the impurity to yield said solid interaction product;

(IV) means for (i) sampling the output resonant frequency of the piezoelectric crystal while said oscillating electric field is applied thereto, (ii) determining the change in resonant frequency from the fundamental resonant frequency upon formation of said solid interaction product when the sensor material interacts with impurity in said treated fluid, and (iii) generating an output indicative of the presence of the impurity in said treated fluid; and

means for flowing at least a portion of the treated fluid to the sensor for determining, by said output indicative of the presence of impurity, when breal tlirough of impurity has occurred in the dry scrubber composition in said vessel.

17. An environmental gas monitor for detection of a trace fluid component in a fluid environment, comprising:

means for (i) sampling the output resonant frequency of the piezoelectric crystal while the oscillating electric field is applied thereto, (ii) determining the change in resonant frequency from the fundamental resonant frequency that occurs on formation of the solid interaction product when the sensor material interacts with the trace fluid component in the fluid environment, and (iii) generating an output indicative of the presence of the trace fluid component in the environment; means for flowing fluid from the fluid environment at a constant flow rate to the coating on the piezoelectric crystal so that the trace fluid component when present reacts with the coating to form the solid interaction product;

wherein the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.001 to about 1000 milliHertz/min./(part-per-million of the fluid component).

18. An environmental gas monitor according to claim 17, wherein the piezoelectric crystal comprises a piezoelectric silica crystal.

19. An environmental gas monitor according to claim 17, wherein the coating of sensor material comprises a chemisorbent material which is chemically reactive with the trace fluid component.

20. An environmental gas monitor according to claim 17, wherein the piezoelectric crystal has a fundamental resonant frequency in the range of from 1 Megahertz to 10 Megahertz.

21. An environmental gas monitor according to claim 17, wherein the means for (i) sampling the output resonant frequency of the piezoelectric crystal while said oscillating electric field is applied thereto, (ii) deterniining the change in resonant frequency from the fundamental resonant frequency incident to the formation of said solid interaction product when the sensor material interacts with said trace fluid component in said fluid environment, and (iii) generating an output indicative of the presence of the trace fluid component in said environment, comprises a circuit including therein a cascaded array of frequency counters.

22. An environmental gas monitor according to claim 17, wherein said output indicative of the presence of the trace fluid component in said environment, comprises a calculated concentration of said trace fluid component in said environment.

23. An environmental gas monitor according to claim 22, further comprising a flow control means for controllably flowing a selected flow rate of fluid of said fluid environment in contact with the sensor material on said piezoelectric crystal, and wherein the means for performing functions (i), (ii) and (iii), comprise computational means for determining said calculated concentration of said trace fluid component in the fluid environment, in accordance with the algorithm:

wherein:

d is a proportionality constant;

[Cj ] is the concentration of the trace fluid component; and

Q is the volumetric flow rate of the fluid of the fluid environment.

24. An environmental gas monitor according to claim 17, further comprising an eductor including an eductor suction tube for drawing fluid from the fluid environment along a flow path for contacting thereof with the sensor material, and gas inlet and gas outlet passages for applying suction to the eductor suction tube.

25. An environmental gas monitor according to claim 17, wherein the sensor material comprises a thin film metal.

26. A sensor according to claim 25, wherein the thin film metal is selected from the group consisting of copper, zinc, silver, aluminum and chromium.

27. An environmental gas monitor according to claim 17, further comprising a second sensor for sensing the presence of water vapor in the fluid environment.

28. An environmental gas monitor according to claim 27, further comprising means for determining concentration of trace fluid component in the fluid environment, and for utilizing the sensing of the presence of water vapor in the fluid environment to adjust concentration determined for the trace fluid component, for water vapor in the fluid environment.

29. An environmental gas monitor according to claim 17, wherein the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.1 to about 50 milliHertz/min./(part-per-million of the fluid component).

30. An environmental gas monitor according to claim 17, wherein the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.5 to about 10 milliHertz/min./(part-per-million of the fluid component).

31. An environmental gas monitor according to claim 17, wherein the means for flowing fluid from the fluid environment at a constant flow rate to the coating on the piezoelectric crystal comprise a flow passage containing a flow limiting structure therein, characterized by a pressure on a low pressure side of the flow limiting structure being less than 1/2 the pressure on an upstream side of the flow limiting structure.

32. An environmental gas monitor according to claim 17, wherein the flow limiting structure is constructed and arranged to prevent particulate solids in the fluid environment from contacting the sensor material.

33. An environmental monitoring assembly comprising a housing containing therein (i) a gas sensor assembly including (A) a piezoelectric crystal having coated thereon a thin film of a sensor material with which a specific environmental gas species is reactive to yield a metal-containing reaction product, and (B) a moisture sensor, (ii) an electronics module operatively connected to the gas sensor assembly, and arranged to determine the presence of the specific environmental gas species as sensed by the gas sensing assembly, (iii) output means operatively coupled with the electronics module and arranged for displaying an output indicative of the presence of the specific environmental gas species as sensed by the gas sensing assembly, and (iv) means for drawing fluid from the fluid environment in contact with the gas sensing assembly at a constant flow rate so that the specific environmental gas species when present reacts with the sensor material to form the metal-containing reaction product; wherein the coated piezoelectric crystal exhibits a frequency response rate to the trace fluid component in the range of from about 0.001 to about 1000 milliHertz/min./(part-per-million of the fluid component).

34. An environmental monitoring assembly according to claim 33, wherein the means for drawing fluid from the fluid environment in contact with the gas sensing assembly at a constant flow rate comprise an eductor.

35. A process for monitoring a fluid stream for determining presence of a selected component therein, said process comprising:

providing a sensor for detection of the selected component in the fluid stream, said sensor comprising: (A) a piezoelectric crystal having a fundamental resonant frequency in response to an applied oscillating electric field;

(B) a coating on the piezoelectric crystal of a sensor material which is reactive with the selected component to yield a solid interaction product of increased mass in relation to mass of the sensor material interacting with the selected component to yield said solid interaction product;

sampling the output resonant frequency of the piezoelectric crystal while said oscillating electric field is applied thereto;

determining the change in resonant frequency from the fundamental resonant frequency incident to the formation of said solid interaction product when the sensor material interacts with said selected component in said fluid stream; and

generating an output indicative of the presence of the selected component in said fluid stream;

wherein the coated piezoelectric crystal exhibits a frequency response rate to the selected component in the range of from about 0.001 to about 1000 milliHertz/min./(part-per-million of the fluid component).

36. A process according to claim 35, wherein the piezoelectric crystal comprises a piezoelectric silica crystal.

37. A process according to claim 35, wherein the coated piezoelectric crystal exliibits a frequency response rate to the selected component in the range of from about 0.01 to about 100 milliHertz/min./(part-per-million of the fluid component).

38. A process according to claim 35, wherein the piezoelectric crystal has a fundamental resonant frequency in the range of from 1 Megahertz to 10 Megahertz.

39. A process according to claim 35, wherein the step of generating said output indicative of the presence of the selected component in said fluid stream, comprises determining via a programmed computer a calculated concentration of said selected component in said fluid stream.

40. A process according to claim 39, further comprising controllably flowing at least a portion of the fluid stream at a selected flow rate in contact with the sensor material on said piezoelectric crystal, and determining said calculated concentration of said selected component in said fluid stream, in accordance with the algorithm:

dF/dt = δ • [Q ] • Q

wherein:

δ is a proportionality constant;

[C, ] is the concentration of the selected component in said fluid stream; and

Q is the volumetric flow rate of the fluid stream.

41. A process according to claim 35, further comprising restricting the flow of the fluid stream to permit only diffusional flow of fluid to the sensor material, and preventing particulate solids in the fluid stream from contacting the sensor material.

42. A process according to claim 35, further comprising removing from the fluid before its contacting with the sensor material substantially all sensor material- interactive components other than said selected component.

43. A process according to claim 42, wherein the sensor material-interactive components are removed by contact of the fluid stream with a chemisorbent medium having sorptive affinity for said sensor material-interactive components other than said selected component.

44. A process according to claim 35, wherein the sensor material comprises a thin film metal.

45. A process according to claim 44, wherein the thin film metal is selected from the group consisting of copper, zinc, silver, aluminum and chromium.

46. A process according to claim 35, wherein the selected component is a halide gas.

47. A process according to claim 35, wherein the selected component is selected from the group consisting of boron trichloride, boron trifluoride, hydrogen chloride, chlorine, fluorine, and hydrogen fluoride.

48. A fluid scrubbing process for treating impurity-containing fluid, comprising:

contacting impurity-containing fluid with a dry scrubber composition to remove impurity from the impurity -containing fluid, and yield treated fluid; detecting impurity in the treated fluid, by the steps comprising:

providing:

determining the change in resonant frequency from the fundamental resonant frequency incident to the formation of said solid interaction product when the sensor material interacts with impurity in said treated fluid;

generating an output indicative of the presence of the impurity in said treated fluid; and

flowing at least a portion of the treated fluid to the sensor for determining, by said output indicative of the presence of impurity, when breakthrough of impurity has occuπed in the dry scrubber composition.

49. A method for detection of a trace fluid component in a fluid environment, comprising:

providing a piezoelectric crystal having a fundamental resonant frequency in response to an applied oscillating electric field, with a coating on the piezoelectric crystal of a sensor material which is reactive with the trace fluid component to yield a solid interaction product of changed mass in relation to initial mass of the sensor material interacting with the trace fluid component to yield the solid interaction product;

determining the change in resonant frequency from the fundamental resonant frequency that occurs on formation of the solid interaction product when the sensor material interacts with the trace fluid component in the fluid environment;

generating an output indicative of the presence of the trace fluid component in the environment; and

flowing fluid from the fluid environment at a constant flow rate to the coating on the piezoelectric crystal so that the trace fluid component when present reacts with the coating to form the solid interaction product;