Patent Publication Number: US-2006012794-A1

Title: Optical sensor system and method for detection of hydrides and acid gases

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This is a continuation of the U.S. patent application Ser. No. 10/462,526 for “Optical Sensor System and Method for Detection of Hydrides and Acid Gases” filed on Jun. 16, 2003 in the name of Jose I. Amo. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an optical sensor system and method for detecting the presence of hydrides and acid gases, e.g., in semiconductor manufacturing process effluents and semiconductor manufacturing plant environments.  
      2. Description of the Related Art  
      In the manufacture of semiconductor products, hydrides and acid gases are frequently used, e.g., as etchants, source reagents, cleaning agents, reactants, chamber cleaning materials, process stream diluents, and treating agents for abatement of effluents of the semiconductor manufacturing plant.  
      In such applications, there is a need for corresponding monitoring equipment and techniques for detecting the presence of hydrides and acid gases in specific process streams, or environments of the semiconductor manufacturing facility, to ensure the absence of such gases where they pose a health or safety risk, or are required to be removed in the treatment of gaseous streams containing same.  
      An illustrative application of such type involves scrubbing of hydrides and acid gases from streams in which they are present, using dry scrubbing apparatus. The dry scrubber apparatus is typically arranged as a single- or multi-vessel installation, in which the scrubber vessel contains a dry scrubber composition such as a chemical sorbent material that irreversibly reacts with the hydrides and acid gases that are present in a gas stream flowed through the vessel.  
      Since such scrubbing consumes the active dry scrubber medium, it is desirable to monitor the gas discharged from the scrubber vessel. By such monitoring, the approach to “breakthrough” of the hydride and/or acid gas components from the vessel is sensed, and the vessel can be taken off-stream to enable change-out of the scrubber medium, whereby fresh dry scrubber material is provided for renewed on-stream processing of the hydride- and/or acid gas-containing stream.  
      The point at which the bed of dry scrubber material in the vessel becomes substantially exhausted by cumulative reaction with hydride and/or acid gas components in the gas stream is termed the “end point” of the scrubber material bed, and a corresponding sensor that is arranged to sense the end point is termed an “end point sensor.” 
      Specific scrubber systems of such type can also include hydride and/or acid gas sensors that are disposed in the bed, e.g., at an intermediate portion thereof, or at scattered multiple sensing locations in the bed of dry scrubber material, to permit the progressive exhaustion of the scrubber material to be monitored, so that the need for change-out of the scrubber medium from the vessel can be more accurately scheduled during the active service life of the dry scrubber material, particularly in instances where the hydride and/or acid gas-containing stream is highly variable in concentrations of such gas stream components.  
      Conventional scrubber sensors, e.g., end point sensors, include two main categories of devices, electrochemical-based sensors and tape-based sensors.  
      Electrochemical sensors have two potential shortcomings in use. First, the analyte used to react with the gas stream has a tendency to dry out when used in applications where the gas stream being analyzed is of low relative humidity. Such low relative humidity conditions result in erratic measurements and ultimately sensor failure. Gas streams exhausted from semiconductor manufacturing tools and flowed to scrubbers are typically dry in character, making the use of electrochemical sensors problematic. Second, conventional electrochemical sensors are sometimes cross-sensitive to hydrogen gas (H 2 ), a significant disadvantage since hydrogen is frequently used as a reducing gas in semiconductor processes and correspondingly is present as a common component in gas streams being monitored for hydrides and acid gases.  
      Tape-based sensors also have inherent limitations when used for scrubber endpoint or general environmental safety monitoring. Such sensor units are characteristically expensive in the first instance, and their cost of ownership (COO) is substantial as a result of having to continually replace color-changing tapes. Such units have movable parts that require maintenance and sometimes result in failure. For example, sensor units of such type are equipped with a pump that draws up to 1 standard liter per minute (slpm) to the sensor tape, for contact therewith so that a color change of the tape indicates the presence of the target gas species. Tape-based sensors also employ electric motors for scrolling the tape after each sampled point. These and associated moving parts have lifetime and reliability issues.  
      Accordingly, it would be a substantial advance in the art to be able to monitor gas, e.g., a gaseous stream or a gaseous environment, for the presence of hydrides and acid gases, with a monitoring system and methodology that avoids the above-discussed deficiencies of conventional hydride and acid gas sensing technologies.  
     SUMMARY OF THE INVENTION  
      The present invention relates to an optical sensor system and method for detecting the presence of hydrides and acid gases, e.g., in semiconductor manufacturing process effluents and semiconductor manufacturing plant environments.  
      In one aspect, the invention relates to a gas sensor for detection of hydride and/or acid gas species, said gas sensor comprising: 
      (a) a sensor body;     (b) a sensing element mounted on the sensor body and arranged for contact with gas susceptible to presence of said hydride and/or acid gas species therein, said sensing element changing color in contact with gas containing said hydride and/or acid gas species;     (c) a source light optical fiber mounted in the sensor body to conduct source light to the sensing element;     (d) a detector optical fiber mounted in the sensor body to receive reflected source light from the sensing element;     (e) a light source coupled to the source light optical fiber for supplying source light thereto; and     (f) a detector coupled to the detector optical fiber and arranged to receive reflected source light from the detector optical fiber and to responsively produce an output indicative of presence or absence of hydride and/or acid gas species in gas contacted with said sensing element.    

      Another aspect of the invention relates to a gas sensor for detection of hydride and/or acid gas species, comprising a color changing material that changes color in exposure to hydride and/or acid gas species, means for impinging radiation on the color changing material for reflection therefrom, and means for receiving reflected radiation from the color changing material and responsively generating an output upon a change of color of the color changing material indicative of a presence of hydride and/or acid gas species in gas contacting the color changing material.  
      Yet another aspect of the invention relates to a method of detecting hydride and/or acid gas species in a gaseous medium latently susceptible to presence of same, said method comprising contacting the gaseous medium with a color changing material that changes color in exposure to the hydride and/or acid gas species, while impinging on the color changing material an incident radiation that is reflected from said color changing material, and monitoring the reflected radiation from the color changing material to detect a change of the reflected radiation indicative of a change of color of the color changing material due to presence of hydride and/or acid gas species in said gaseous medium, thereby detecting the hydride and/or acid gas species in the gaseous medium.  
      A still further aspect of the invention relates to a effluent treatment system arranged to receive a gaseous effluent from a process generating same, said effluent treatment system comprising an effluent treatment unit constructed and arranged to remove hydride and/or acid gas species from the gaseous effluent to produce a treated gaseous effluent having reduced content of hydride and/or acid gas species, and at least one gas sensor as described hereinabove, arranged for detection of hydride and/or acid gas species in the gaseous effluent.  
      As used herein, the term “hydride” means a compound of hydrogen with another, more electropositive element or group. Examples include boranes, arsine, phosphine, silane, etc. The term “acid gas” as used herein means a gas that in aqueous solution becomes a proton donor. Examples of acid gas species frequently encountered in semiconductor manufacturing effluents include F 2 , HF, HCl, chlorine, ClF 3 , SiF 4 , Br 2 , HBr, HI, BF 3 , BCl 3 , and H n SiCl (n-4)  (where n=1 to 3, inclusive).  
      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  
       FIG. 1  is a schematic cross-sectional view of a gas-sensing probe, in accordance with one embodiment of the invention.  
       FIG. 2  is a schematic cross-sectional elevation view of an upflow scrubber unit featuring a gas-sensing probe according to another aspect of the invention.  
       FIG. 3  is a schematic representation of a dry scrubbing unit for treatment of effluent gas from a semiconductor manufacturing tool, equipped with a gas-sensing assembly according to another aspect of the invention.  
       FIG. 4  is a graph depicting the response of a fiber optics sensor according to one embodiment of the present invention versus a commercially available TLD-1 Toxic Gas Detection System (Zellweger Analytics, Inc., Lincolnshire, Ill., USA) measuring the breakthrough of arsine gas from a dry scrubber. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF  
      The present invention is based on the discovery that optical sensing of colorimetric gas-indicating elements can be carried out in a manner that overcomes the deficiencies of prior approaches to hydride and acid gas monitoring. The gas monitoring system of the present invention can be readily fabricated and efficiently operated. It has no moving parts, and is not susceptible to variable humidity conditions, hydrogen cross-reactivity or hydrogen cross-sensitivity.  
       FIG. 1  is a schematic cross-sectional view of an optical gas-sensing probe  10 , in accordance with one embodiment of the invention. The probe  10  includes a tubular probe body  12  defining a hollow tubular interior passage  14 , and having a lower end  16  that is fabricated with threaded surface  18  for matable engagement with complementary threading  20  of the retaining cap  22  of the probe.  
      Directionally, the lower end  16  of the tubular probe body may be termed the “distal” end, with the upper open end of the tubular probe body correspondingly being termed the “proximal” end. The length of the tubular probe body, as measured along its longitudinal axis, may for example be on the order of from about 2 to about 100 cm in a specific embodiment, although it will be recognized that the size, shape and orientation of the tubular probe body as well as the gas sensor probe itself, may be widely varied in the general practice of the present invention, as regards specific applications and embodiments of the invention.  
      The retaining cap  22  is fabricated with an opening in its distal face  40 . In this opening is disposed a protective mesh element  38 . The function of the protective mesh element  38  is to permit ingress of gas from the bulk volume of gas being monitored for hydride(s) and/or acid gas(es), while occluding any deleterious particulates from entering the active sensing region of the probe. Alternatively, a gas-selective membrane can be substituted for or used in addition to the mesh in order to enhance the selectivity for the gas being analyzed. The bulk volume of gas may be a gaseous environment, e.g., in a room, or chamber, in which the gas sensing probe is disposed. Alternatively, the bulk volume of gas may be a flowing stream of gas, with which the gas sensing probe is contacted.  
      The active sensing region of the probe  10  comprises a distal portion  26  of the hollow tubular interior passage  14  that is forward of the fiber optics holder  24  and rearward of the optical window  34 . A colorimetric (color-changing) element  36  is disposed on or in front of the optical window, and may comprise a sheet or web or other suitable article formed of a material that in the presence of the target gas species undergoes a colorimetric change. Examples of such calorimetric element  36  as useful for hydride gas detection include, without limitation: silver nitrate, copper hydroxide, copper carbonate, copper sulfate, mercury chloride, gold (Au +3 ) complexes, and commercially available color-indicating dosimeters such as Gas Monitoring Dosimeter Type-1-Hydrides (GMD Systems, Inc., Pittsburgh, Pa.) and SAFEAIR® Arsine (K&amp;M Environmental, Virginia Beach, Va.). For acid gas detection, a calorimetric matrix element containing iron oxide or acid/base indicators (such as methyl orange, bromocresol green, methyl red, bromocresol purple, bromothymol blue, and phenol red) can advantageously be employed.  
      The fiber optics holder  24  can comprise a cylindrical plug or disk element having a through-hole passage  46  therein, in which a source light optical fiber  28  is disposed with its end terminating at or in proximity to the distal face of the fiber optics holder. The fiber optics holder  24  also has a second through-hole passage  48  therein, in which a detector optical fiber  30  is disposed. The fiber optics holder  24  can be constructed as shown with a frontal cavity  32  in its distal face, with the detector optical fiber terminating at an end that is positioned at the floor of such frontal cavity or otherwise within the cavity.  
      Although the invention is illustratively shown and described with reference to embodiments including separate optical fibers for transmittal of source radiation and for transmittal of reflected radiation, it will be recognized that optical waveguide structures and elements of widely varying type may be employed in the general practice of the invention.  
      For example, the optical waveguide structure may comprise an optical fiber bundle in which source radiation and reflected radiation are transmitted by respective component fibers of the bundle, with all source radiation transmitting fibers being “pigtailed” to one side and coupled to a radiation source at the proximal end of the optical fiber array, and with all reflected radiation transmitting fibers being correspondingly pigtailed to another side and coupled to detector means at the proximal end of the optical fiber array.  
      Referring again to the embodiment illustratively depicted in  FIG. 1 , the source light optical fiber  28  and the detector optical fiber  30  extend readwardly from the fiber optics holder  24  to a source/detector module  50 . The source/detector module  50  is arranged to generate radiation of a specific character, e.g., a coherent light of a specific wavelength/frequency character. The generated radiation is supplied by the source/detector module  50  to the source light optical fiber  28 , for transmittal along the fiber and discharge from the end face of the fiber in the distal portion  26  of the hollow tubular interior passage  14 , as radiation  42 .  
      The radiation  42  passes from the distal face of the fiber  28  and through the radiation-transmissive window  34  to the colorimetric element  36 , from which it is reflected in the distal portion  26  of the hollow tubular interior passage  14  as reflected radiation  44 . The reflected radiation  44  impinges on the distal end face of the detector optical fiber  30  in cavity  32 , and is transmitted along the detector optical fiber to the source/detector module  50 . The source/detector module  50  receives the reflected radiation from the detector fiber  30 . In response to the character of the reflected radiation conducted to module  50  from the detector optical fiber  30 , the module  50  generates an output indicative of the presence of the target gas (hydride and/or acid gas) species in the gas contacted with the color changing material  36 , when the target gas species is present and produces a color change of the color changing material  36 .  
      For such purpose, the module  50  may contain a detector, such as a photodiode, photoresistor, or other photosensitive or optoelectronic detector component, which is coupled to the proximal end of the detector optical fiber  30 , and is operative to produce an output indicative of the presence of the target gas (hydride and/or acid gas) species in the gas contacted with the color-changing material  36 .  
      For example, the source/detector module  50  can be arranged to submit the return radiation signal from the detector optical fiber  30  to signal processing operations in comparator circuitry in the source/detector module  50 , against a signal based on the source light that is transmitted by the source light optical fiber  28  to the color-changing material  30 .  
      It will be appreciated that the source/detector module  50  can be arranged in any of a wide variety of arrangements to provide a source light for supply to the source light optical fiber  28 , and to receive the reflected light from the detector optical fiber  24 , with appropriate circuitry, computational means (programmable general purpose computers, microprocessors, programmable logic controllers, registers, etc.), and the like, to generate an output indicative of the presence of the target gas species in the gas environment being monitored by the optical gas sensor. Such output may be indicative of mere presence of the target gas species in the monitored gas, or may be constituted to provide a quantitative indication of the concentration of the target gas species when present in the monitored gas contacted with the color-changing material of the sensor.  
      In one embodiment, the module  50  includes comparator circuitry that generates an output based on the source light and return reflected light characteristics. If the calorimetric element  36  in exposure to gas being monitored for the presence of the target gas species does not change color, thereby indicating the absence of the target gas species in the monitored gas environment, the comparator circuitry in the source/detector module  50  responsively outputs a null signal corresponding to such absence of the target gas species.  
      If, however, the colorimetric element  36  in exposure to gas being monitored for the presence of the target gas species changes color, thereby indicating the absence of the target gas species in the monitored gas environment, the comparator circuitry in the source/detector module  50  receives an altered reflected radiation signal from the detector optical fiber. The comparator circuitry then responsively outputs a positive signal corresponding to the presence of the target gas species. Such positive signal then can be passed to output, recording or alarm means indicative of such presence of the target gas species.  
      The radiation-transmissive window  34  may be formed of any suitable material of construction appropriate to the nature of the radiation that is being employed for the monitoring of gas for the presence or absence of the target gas species. For example, when the radiation is visible light, the radiation-transmissive window  34  can be formed of light-transmissive materials such as glass, quartz, fused silica, sapphire, etc. If the radiation is IR or UV radiation, correspondingly transmissive materials can be used, e.g., zinc selenide, zinc sulfide, sapphire, CaF 2 , MgF 2 , and other materials through which the radiation of the specific spectral character can pass for impingement on and reflection from the colorimetric element  34 .  
      As used in such context, the term “colorimetric” is intended to be broadly construed as inclusive of changes that involve moderation or alteration of the incident radiation signal in a manner that is different when the element  34  has been exposed to the target gas species, in relation to the response of the element to incident radiation in the absence of exposure to the target gas species. Thus, the element  34  can be responsive to the presence of the target gas species by chemical reaction that alters the radiation absorbance or transmissivity of the material of the element, by adsorptive behavior (the element having a sorptive affinity for the target gas species, and the adsorbent-bearing element having different radiation-response characteristics than the element which has not been exposed to the target gas species).  
      The radiation-generating and radiation receiving elements in the source/detector module  50  can be of any suitable type. By way of example, the radiation generated by the module can, as mentioned, be in the visible, UV, or IR spectral regime, and can be generated by a radiation source such as a light-emitting diode, laser, or other source of the desired spectral character. The corresponding detector in the source/detector module  50  can likewise be of any suitable type, e.g., a phototransistor or a photoresistor. The radiation source can be arranged to emit the generated radiation to a splitter, with a first part of the radiation being sent to the optical fiber  28  and a second part of the generated radiation being passed to an optoelectronic transducer or other means to generate a reference signal, e.g., for supply to comparator circuitry in the module  50 .  
      The source/detector module  50  can be arranged with any other suitable signal processing circuitry, computational elements (gate elements, registers, microprocessors, etc.), and response output indicative of the presence of the target gas species in the gas environment being monitored. Such signal processing circuitry and other elements can be chip-based, as a unitary integrated circuit module, or may be constructed and arranged in any suitable manner, using any suitable means, for providing the capability of outputting a response when the probe  10  is exposed to the target gas species, and the colorimetric element  34  undergoes a responsive change that alters the radiation transmittance/reflectance/absorbance characteristics of the color change element and produces an output indicative of the presence of the target gas species in the monitored gaseous medium.  
      The radiation-transmissive window  34  is preferably utilized to isolate the internal elements of the probe from the gas being monitored, so that the gas does not flow into the interior passage of the tubular member  12  and contaminate the probe and associated electronics. In some instances, the window can be eliminated from the probe assembly, e.g., where the target gas species is not toxic or hazardous, or where other control means are provided (e.g., a hydride or acid gas getter material is disposed in the interior passage of the tubular member  12 ) that render the window superfluous.  
      The window when employed can also be coated with an antireflective coating to prevent reflection of the radiation and enhance transmissivity of the window to the impinged radiation and radiation reflected from the calorimetric element.  
      In a preferred embodiment, the colorimetric element is capable of changing color when exposed to low concentrations (e.g., concentrations&lt;1000 ppm, more preferably &lt;100 ppm, and most preferably &lt;10 ppm) of hydride gases (e.g., arsine, phosphine, silane, etc.) or acid gas species (e.g., hydrogen fluoride, hydrogen chloride, boron trifluoride, etc.). The calorimetric element can be formed of any suitable material, such as for example, impregnated paper, color-indicating polymer, granules of active (color-changing) materials, or a thin film directly coated on the optics window.  
      The protective mesh element  38  likewise can be of any suitable type, e.g., a screen, mesh, frit, sintered matrix, fibrous (woven or non-woven) web, or the like, which prevents particulate solids from impacting or interfering with the color change layer (calorimetric element), yet simultaneously allowing diffusion of gas to the calorimetric element to take place.  
      In operation, as the color of the colorimetric element changes in exposure of the element to hydride or acid gas, the reflected light changes in intensity, and is transferred through the detector optical fiber to the detector in the source/detector module  50 . The change of the detected light can be processed as above described using comparator or other processing circuitry, and converted to a concentration reading, whose magnitude is determined by the extent of the change of the detected light. Alternatively, on exceeding a predetermined threshold, the change of detected light can be employed to generate a corresponding signal that incursion of the target gas species into the monitored gas environment has occurred. In another alternate embodiment, the detected light signal derivative with respect to time can be used to distinguish between a slow drift in response versus the fast light-to-dark transition that is characteristic of hazardous gas exposure.  
       FIG. 4 . demonstrates the time-dependent response of a fiber-optics calorimetric system according to one embodiment of the present invention during a scrubber breakthrough experiment. Both the fiber optics probe constructed in accordance with the present invention and a TLD-1 Toxic Gas Detection System (commercially available from Zellweger Analytics, Inc., Lincolnshire, Ill., USA) were used to measure the exhaust gas stream discharged from a dry scrubber. The scrubber was challenged with a 2% arsine/H 2  gas mixture at a linear velocity of 2 cm/second. The fiber optic color-indicating element consisted of filter paper impregnated with copper sulfate. When arsine gas began to break through the scrubber bed (515 seconds into the test), the signal in the fiber optics device showed a marked increase coinciding with the TLD-1 Toxic Gas Detection System response.  FIG. 4  depicts the direct voltage response in addition to the voltage gradient (derivative) of the signal. In this case, either curve could be used to detect gas breakthrough, although the derivative provides a more differentiated pattern. Inspection of the color-indicating element after completion of the test corroborated that the paper had changed from white to black.  
       FIG. 2  is a schematic cross-sectional elevation view of an upflow scrubber unit  60  featuring a gas-sensing probe  88  according to another aspect of the invention. The upflow scrubber unit  60  shown in  FIG. 2  includes a vessel  62 , which may be in the form of a canister, cylindrical container, or other housing, within which is disposed a bed of finely divided dry scrubber material (“Dry Media”)  74 .  
      The bed may be of any suitable dry scrubber material, such as metal oxide scrubber medium, or other suitable scrubber medium having abatement capability for the hydrides and acid gases in the effluent stream passed therethrough. Examples of scrubbing media suitable for the abatement of hydride gases include GV-5 (Matheson, Montgomeryville, Pa., USA), and NPT 170B (Sud-Chemie, Louisville, Ky., USA). Materials useful to scrub acid gases include 451D and S450 (ATMI, Inc., Danbury, Conn., USA). The bed of dry scrubber medium preferably comprises a chemisorbent material that irreversibly reacts with the hydrides and acid gases in the effluent stream to effect removal of such gases from the effluent stream.  
      The dry scrubber medium  74  can be in any suitable discontinuous form, such as in the form of pellets, spherical particles, monolithic forms, rings, or other regular or irregular shapes, and of suitable size to accommodate the operating characteristics of the unit. Such operating characteristics may be based on allowable pressure drop across the bed, associated blower/compressor requirements for effecting flow of the effluent stream, superficial velocity constraints required to avoid fluidization of bed material, etc. The operating characteristics and the corresponding scrubber design, bed sizing and vessel geometry are readily determinable within the skill of the art, for the provision of a scrubber unit of appropriate character for a specific effluent stream containing hydride and/or acid gas species.  
      The bed of dry scrubber medium  74  may be reposed in the vessel  62  on a support  80 . The support can comprise a screen, mesh, grid or other suitable support structure accommodating passage of the effluent gas stream therethrough, for flow of the effluent gas through the bed of dry scrubber medium  74 .  
      The bed of dry scrubber medium  74  is located in the vessel  62 , with a lower plenum  68  located below the bed and an upper head space  64  located above the bed.  
      The vessel  62  is equipped with an effluent gas stream inlet  70  which can comprise a conduit having an open upper end  72 , extending downwardly through the bed of dry scrubber medium  74 , and terminating at an open lower end  78 . By such arrangement, the effluent stream can be introduced into the open end  72  of the inlet conduit, in the direction indicated by arrow “A,” for flow through the interior tubular passage  76  of the conduit to the open lower end  78  thereof, with discharge of the effluent gas into the lower plenum  68 . From the lower plenum, the effluent stream is flowed upwardly through the bed of dry scrubber medium  74 , in the direction indicated by arrows “B.” 
      The effluent stream in passing upwardly through the bed of dry scrubber medium  74  is contacted with the scrubber medium particles in the bed, thereby effecting removal of hydrides and acid gases from the effluent stream. The hydride- and acid gas-reduced effluent stream then passes from the upper end of the bed of dry scrubber medium  74  into the upper head space  64  and is discharged from vessel  62  in outlet  82  through open discharge end  84  thereof. The discharged gas, reduced in content of hydrides and acid gases, flows from the outlet in the direction indicated by arrow “C” and may be subjected to further treatment, recycled to an upstream process unit, or released to the atmosphere, as appropriate.  
      The upflow scrubber unit  60  is equipped with a gas-sensing probe  88  that can be constructed as shown and described with reference to  FIG. 1  hereof. The probe  88  is fixtured in a mounting block  86  through which a tubular body containing the optical fibers extends to a distal end where the optical fibers are coupled to the sensing head of the probe. The mounting block  86  also has a temperature sensor  90  fixtured therein. The temperature sensor  90  may be arranged to sense temperature at various points along the length of the bed (in the vertical direction, as shown in the orientation of  FIG. 2 ). For example, the temperature sensor may comprise thermocouples that are spaced apart at regular or otherwise predetermined intervals along the length of the bed, to measure temperature change incident to exothermic (or endothermic) abatement reactions taking place when the effluent stream is contacted with the dry scrubber medium in the bed.  
      The optical fiber leads (source light optical fiber and detector optical fiber) extending upwardly from the mounting block  86  can, as shown in  FIG. 1 , be coupled to a suitable control module, comprising a light source and detection capability. The temperature sensor  90  likewise may have associated leads or signal transmission lines (not shown in  FIG. 2 ) that are coupled with such control module to provide further monitoring or adjustment capability.  
      For example, the temperature sensor may be arranged to monitor temperature at discrete locations along the height of the bed, so that in the case of exothermic abatement reactions in the bed, any hot spots are detected and corrective action taken to avoid excessive heating problems, e.g., a control signal indicative of the excessive heating may be transmitted to a flow control valve in the effluent stream supply line, so that the flow rate of effluent gas to the scrubber unit is reduced, to reduce bed temperature in the scrubber to an appropriate level. Other monitoring and control arrangements can be implemented using a temperature sensor of the type shown in  FIG. 2 , to enhance operability and efficiency of the scrubber unit.  
      In operation, the gas-sensing probe  88  senses the presence of hydride and/or acid gas species at the lower end (sensing head) of the probe, to determine when such contaminant species have “broken through” at the sensor location in the bed. For example, the probe may be disposed with its sensing head in the exit end portion of the bed, at a point that is 90% of the length of the bed, as measured from its lower face (or alternatively stated, the sensing head of the probe may be positioned at a 10% depth into the bed as measured from its upper exit face), whereby 90% breakthrough of acid gas and/or hydride components can be sensed, indicating that only 10% of bed capacity remains.  
      In this manner, the probe is appropriately arranged to sense the approach of bed exhaustion (consumption of the dry scrubber medium), and the corresponding need to change-out the dry scrubber medium, e.g., by replacing the canister unit containing the spent dry scrubber medium, and/or switching of the effluent stream to a second vessel containing fresh scrubber medium.  
      It will be recognized that the optical sensor probe  88  may have a sensing head that is disposed at any suitable height within the bed, as appropriate to the monitoring of the scrubbing operation and the correlative action of determining the progress of the scrubber unit toward exhaustion of the dry scrubber medium, so that change-out of the scrubber medium or canister comprising same is done in a timely fashion.  
      Further, multiple probes may be employed, each having an associated sensing head disposed at a different position in the scrubber bed, along the direction of gas flow therethrough, to provide a fully monitored capability for the scrubbing and change-out operations.  
       FIG. 3  is a schematic representation of a dry scrubbing system for treatment  100  of effluent gas from a semiconductor manufacturing tool, in which the system includes a gas-sensing assembly according to another aspect of the invention.  
      The dry scrubbing system  100  of  FIG. 3  comprises a dry scrubbing unit  60  of the type illustrated and described with reference to  FIG. 2  hereof. The scrubber unit  60  features a first gas sensor  88  of the type shown in  FIG. 1 , arranged for determination of 90% breakthrough of the contaminant species of the effluent stream, as previously described in connection with  FIG. 2 .  
      The dry scrubbing unit  60  is disposed in a gas cabinet  102  containing, in addition to the dry scrubber unit  60 , a control module  104  including light source elements  112 , detector elements  114 , and electronics boards  116 , as mounted in a box  118  of the module. The cabinet may be of a type having access doors, instrument panels associated with the control module  104  and vents or other air circulation structures whereby clean dry air can be flowed through the enclosed interior volume of the cabinet.  
      In the  FIG. 3  system, a second gas sensor of the type shown in  FIG. 1  is provided as scrubber exhaust sensor  130 , to monitor the treated gas discharged from the dry scrubbing unit  60  for the presence of contaminant species (hydride and/or acid gas species) therein.  
      The  FIG. 3  system also includes a third gas sensor of the type shown in  FIG. 1 , provided as a cabinet air sensor  106 , to monitor the interior environment gas of the gas cabinet  102  for the presence of the contaminant hydride and/or acid gas species in such environment.  
      The  FIG. 3  system is arranged with the first gas sensor  88  coupled via source light optical fiber  120  and detector optical fiber  122  to the light source  112  and light detector  114 , respectively, with such light source and light detector in turn being operatively coupled with electronics board  116  in box  118 .  
      In like manner, scrubber exhaust sensor  130  is coupled by source light optical fiber  124  and detector optical fiber  126  with the light source and light detector components, which in turn are coupled with the associated electronic board  116  in box  118 . Similarly, cabinet air sensor  106  is coupled via source light optical fiber  108  and detector optical fiber  110  with respective light source and detector components, which in turn are coupled with the associated electronics board  116  in box  118 .  
      The electronics boards  116  are each coupled with respective light emitting diode (LED) light source elements  112  and light detector elements  114 , and such electronics boards comprise appropriate electronic circuitry and components for actuating the light source and detection elements, for sensing of the respective environments in which the individual sensors  88 ,  130 , and  106  are disposed.  
      The  FIG. 3  system thus provides three-point sensing for comprehensive monitoring of the cabinet sub-environments for the presence of hydride and/or acid gas species of interest. The individual sensors in combination with associated electronics in the control module can be arranged to provide outputs of varied type, including (i) audible alarms indicative of hydride and/or acid gas breakthrough, (ii) visual output gauges, (iii) menu-driven displays on computer monitors operatively linked to the control module, (iv) printed reports of the monitoring operation and sensing events, etc., as will be readily understood and implementable by those of ordinary skill in the art, without undue experimentation, based on the disclosure herein.  
      It will be correspondingly apparent that the gas sensors of the present invention may be widely varied in structure and arrangement, with respect to specific applications and implementations thereof  
      The advantages of the optical gas sensors of the present invention include the following: 
          (2) low cost compared to available alternative detectors;     (3) insensitivity of the detector to relative humidity conditions and changes of relative humidity;     (4) ability to eliminate cross-sensitivities of the sensing element by simple selection of specific color-change material for the sensor;     (5) ability to non-extractively sample a gaseous environment, thereby enhancing safety, with no requirement of returning sampled gas to the main gas stream of effluent;     (6) low cost of ownership, involving one-time use of the colorimetric sensing element and replacement thereof only when it colorimetrically changes in detecting the presence of the target hydride and/or acid gas species, i.e., no continuously replaced consumables are required;     (7) capability of ready reuse of the optical gas sensor, by simple change of colorimetric target gas species indicator material subsequent to a detection event;     (8) absence of moving parts, which makes the optical gas sensor less susceptible to failure and life-time and maintenance issues;     (9) high efficiency in light transfer by fiber optics; and     ( 10 ) inherent safety, involving light as the only energy delivered to the sensing element.        

      While the invention has been described herein in reference to specific aspects, features and illustrative embodiments of the invention, it will be appreciated that the utility of the invention is not thus limited, but rather extends to and encompasses numerous other aspects, features and embodiments, as will readily suggest themselves to those of ordinary skill in the art, based on the disclosure herein. Accordingly, the claims hereafter set forth are intended to be correspondingly broadly construed, as including all such aspects, features and embodiments, within their spirit and scope.