Abstract:
A thin film/MEMS electrochemical gas sensor includes a body having first and second joined subassemblies to form an interior portion of the body, and is composed of a semiconductor material. The body includes at least one opening configured to allow air to pass into the interior portion of the body. A membrane stack is located in the interior of the body, producing an electrical signal that represents a concentration of target gas in the air at the membrane stack. Conductive contacts are configured to provide electrical connection to the membrane stack to access the electrical signal produced by the membrane stack.

Description:
BACKGROUND 
     The present invention relates to a compact gas (e.g., carbon monoxide) sensor formed at least partially of a thin film material such as silicon. 
     Carbon monoxide (CO) is an example of a gas of interest to be sensed, as it is a colorless, odorless, highly toxic gas that is dangerous to humans at fairly low concentrations. CO is a commonly generated gas during early stages of combustion or heating of various materials. Detecting CO (or other gases) can be accomplished using numerous techniques including spectroscopy, electrochemical sensing, and metal oxide semiconductor (MOS) devices. Existing electrochemical CO sensors provide high performance but are relatively bulky. 
     Miniature devices fabricated from semiconductor materials such as silicon have become known as micro-electro-mechanical systems (MEMS). MEMS processing techniques allow fabrication of very small devices with high resolution. 
     SUMMARY 
     The present invention is a thin film/MEMS electrochemical gas sensor that includes a body having first and second joined subassemblies to form an interior portion of the body. The body is composed of a semiconductor material, and includes at least one opening configured to allow air to pass into the interior portion of the body. A membrane stack is located in the interior of the body, producing an electrical signal that represents a concentration of the target gas in the air at the membrane stack. Conductive contacts are configured to provide electrical connection to the membrane stack to access the electrical signal produced by the membrane stack. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a thin film/MEMS gas sensor according to an embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a thin film/MEMS gas sensor according to another embodiment of the present invention. 
         FIG. 3  is a diagram illustrating a thin film/MEMS gas sensor according to a further embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An electrochemical gas sensor for sending a target gas (such as a toxic gas) is a type of fuel cell that, rather than being configured to produce power, is configured to produce an electrical signal (current or voltage) that is related to the amount of target gas in the atmosphere. Measurement of the electrical signal gives a measure of the concentration of the target gas analyte in the atmosphere. The gas sensor includes an ion conducting proton exchange membrane positioned between and in electrical contact with first and second electrodes. Introduction of the gas to the first electrode produces an electrochemical reaction facilitated by the presence of a catalyst, where the gas molecules are oxidized into other molecules, and protons and electrons are generated by the reaction. For example, if the gas introduced is carbon monoxide (CO), the CO molecules are oxidized into carbon dioxide (CO 2 ). The protons, which are ions of hydrogen, migrate across the proton exchange membrane to the second electrode, where they react with electrons and oxygen to form water in a reduction reaction. The electrochemical reaction generates an electrical signal which is proportional to the concentration of gas at the first electrode. More details of an exemplary electrochemical gas sensor (in particular, a CO sensor) are disclosed in U.S. Pat. No. 6,200,443. 
     In existing electrochemical gas (e.g., CO) sensors as described generally above, a reservoir containing an electrolyte (e.g., water) of several cubic centimeters is employed to hydrate the proton exchange membrane, which dictates the typical size of such sensors to be at least about 5 centimeters high and 2 centimeters in diameter. Also, in a sensor of this size, the response time of the sensor can be an issue due to the length of the diffusion path from the membrane to the electrodes. Moreover, it can be difficult in these sensors to achieve consistent alignment and contact between the membrane and the electrodes, due to the crimping techniques that are employed to press the membrane and the electrodes together. A gas (e.g., CO) sensor employing electrochemical sensing techniques that has reduced size and response time, and increased accuracy with more consistent and precise contact between the membrane and the electrodes would be an improvement to the state of the art. Various embodiments of a miniature thin film micromachined gas sensor are disclosed herein. 
       FIG. 1  is a diagram illustrating thin film gas sensor  10  according to an embodiment of the present invention. The description of sensor  10  that follows describes a particular embodiment in which sensor  10  is a CO sensor, although embodiments for detecting other target gases (such as toxic gases) will be similar in many respects. Sensor  10  includes first thin film subassembly  12  and second thin film subassembly  14 . First thin film subassembly  12  is made up of semiconductor wafer  16 , formed of silicon or a similar material, dielectric layer  17 , and a membrane stack that includes top electrode membrane  18 , proton exchange membrane  20 , and bottom electrode membrane  22 . Dielectric layer  17  may be composed of a material such as silicon dioxide (SiO 2 ), a silicon nitride (Si x N y ) dielectric, or others. Top electrode membrane  18  and bottom electrode membrane  22  are semi-permeable electrically conductive membranes, such as carbon membranes. Proton exchange membrane  20  may be a proton conductive membrane of a perfluorosulfate ionomer, for example, a NAFION® membrane supplied by DuPont, with composite catalytic electrodes. The composite catalytic electrodes include an electrode material coated with a catalyst coating that facilitates electrochemical reaction, reducing the energy required for the reaction to occur. For example, in a CO sensor, the catalyst coating is a platinum alloy. Conductive via  24  extends through semiconductor wafer  16  to provide electrical contact between top electrode membrane  18  and contact pad  25 , and conductive via  26  extends through semiconductor wafer  16  and dielectric layer  17  to provide electrical contact between bottom electrode membrane  22  and contact pad  27 . Contact pads  25  and  27  may be composed of a conductive metal such as aluminum, copper, nickel, gold, or others. 
     Second thin film subassembly  14  is made up of semiconductor wafer  30 , a filter film realized in the exemplary embodiment shown in  FIG. 1  as activated carbon film  32 , and hygroscopic material  34 . Semiconductor wafer  30  may be formed with the geometry shown by high resolution MEMS processing techniques such as reactive ion etching, chemical anisotropic etching, or others, and is formed to include gas diffusion openings  36  and  38 . Activated carbon film  32 , stretches across semiconductor wafer  30  to allow gas to diffuse through it from openings  36  and  38  to the interior of the sensor, while preventing liquid from passing through. Hygroscopic material  34  is located in the interior of the sensor, and may, for example, include materials such as zeolites, alumina, other highly porous materials with affinity to water, polysulfonates, and/or dessicants such a DRIERITE® dessicant supplied by W.A. Hammond Drierite Co., to harvest (adsorb) water from the air and provide electrolyte to hydrate proton exchange membrane  20 . First thin film subassembly  12  and second thin film subassembly  14  are attached together at outer portions thereof via a known method such as fusion bonding or another wafer bonding technique. 
     In operation, gas passes through openings  36  and  38  and diffuses through activated carbon film  32 . CO in the gas interacts with the electrode stack formed by bottom electrode membrane  22 , proton exchange membrane  20  and top electrode membrane  18  to produce an electrochemical reaction. The electrochemical reaction generates an electrical signal between bottom electrode membrane  22  and top electrode membrane  18  that is proportional to the concentration of CO at bottom electrode membrane  22 . This electrical signal is detectable at contact pads  25  and  27  by virtue of their electrical connection to top electrode membrane  18  and bottom electrode membrane  22  by conductive vias  24  and  26 , respectively. The electrical signal at contact pads  25  and  27  is processed and monitored by appropriate circuitry in a manner generally known in the art to indicate the level of CO present. 
       FIG. 2  is a diagram illustrating thin film gas sensor  40  according to another embodiment of the present invention. The description of sensor  40  that follows describes a particular embodiment in which sensor  40  is a CO sensor, although embodiments for detecting other target gases (such as toxic gases) will be similar in many respects. Sensor  40  includes first thin film subassembly  42  and second thin film subassembly  44 . First thin film subassembly  42  is made up of semiconductor wafer  46 , formed of silicon or a similar material, dielectric layer  47 , and a membrane stack that includes top electrode membrane  48 , proton exchange membrane  50 , and bottom electrode membrane  52 . Dielectric layer  47  may be composed of a material such as silicon dioxide (SiO 2 ), a silicon nitride (Si x N y ) dielectric, or other polymeric dielectrics. Top electrode membrane  48  and bottom electrode membrane  52  are semi-permeable electrically conductive membranes, such as carbon membranes. Proton exchange membrane  50  may be a proton conductive membrane of a perfluorosulfate ionomer, for example, a NAFION® membrane supplied by DuPont, with composite catalytic electrodes. The composite catalytic electrodes include an electrode material coated with a catalyst coating that facilitates electrochemical reaction, reducing the energy required for the reaction to occur. For example, in a CO sensor, the catalyst coating is a platinum alloy. Conductive via  54  extends through semiconductor wafer  46  to provide electrical contact between top electrode membrane  48  and contact pad  55 , and conductive via  56  extends through semiconductor wafer  46  and dielectric layer  47  to provide electrical contact between bottom electrode membrane  52  and contact pad  57 . Contact pads  55  and  57  may be composed of a conductive metal such as aluminum, copper, nickel, gold, or others. 
     Second thin film subassembly  44  includes semiconductor wafer  60 , a filter film realized in the exemplary embodiment shown in  FIG. 2  as activated carbon film  62 , and hygroscopic material  64 . Semiconductor wafer  60  may be formed with the geometry shown by high resolution MEMS processing techniques such as reactive ion etching, chemical anisotropic etching, or others, and is formed to include gas diffusion openings  66  and  68 . Activated carbon film  62  stretches across semiconductor wafer  60  to allow gas to diffuse through it from openings  66  and  68  to the interior of the sensor, while preventing liquid from passing through. Microheater  70  is integrated into activated carbon film  62 , with electrical connection being provided by conductive via  72  through semiconductor wafer  60  to contact pad  73 , and by conductive via  74  through semiconductor wafer  60  to contact pad  75 . Contact pads  73  and  75  may be composed of a conductive metal such as aluminum, copper, nickel, gold, or others. Microheater  70  may be made using metallic traces, doped silicon, or doped polysilicon in exemplary embodiments. Hygroscopic material  64  is located in the interior of the sensor, and may, for example, include materials such as zeolites, alumina, other highly porous materials with affinity to water, polysulfonates, and/or dessicants such a DRIERITE® dessicant supplied by W.A. Hammond Drierite Co., to harvest (adsorb) water from the air and provide electrolyte to hydrate proton exchange membrane  50 . First thin film subassembly  42  and second thin film subassembly  44  are attached together at outer portions thereof via a known method such as fusion bonding or another wafer bonding technique. 
     In operation, gas passes through openings  66  and  68  and diffuses through activated carbon film  62 . Microheater  70  is controlled for operation to maintain the temperature of the CO sensor to be near the temperature of the gas diffusing into the sensor. 
     CO in the gas interacts with the electrode stack formed by bottom electrode membrane  52 , proton exchange membrane  50  and top electrode membrane  48  to produce an electrochemical reaction. The electrochemical reaction generates an electrical signal between bottom electrode membrane  52  and top electrode membrane  48  that is proportional to the concentration of CO at bottom electrode membrane  52 . This electrical signal is detectable at contact pads  55  and  57  by virtue of their electrical connection to top electrode membrane  48  and bottom electrode membrane  52  by conductive vias  54  and  56 , respectively. The electrical signal at contact pads  55  and  57  is processed and monitored by appropriate circuitry in a manner generally known in the art to indicate the level of CO present. 
       FIG. 3  is a diagram illustrating thin film gas sensor  80  according to a further embodiment of the present invention. The description of sensor  80  that follows describes a particular embodiment in which sensor  80  is a CO sensor, although embodiments for detecting other target gases (such as toxic gases) will be similar in many respects. Sensor  80  includes first thin film subassembly  82  and second thin film subassembly  84 . First thin film subassembly  82  is made up of semiconductor wafer  85 , formed of silicon or a similar material, a filter film realized in the exemplary embodiment shown in  FIG. 3  as activated carbon film  86 , dielectric layer  87 , and a membrane stack that includes top electrode membrane  88 , proton exchange membrane  90 , and bottom electrode membrane  92 . Dielectric layer  87  may be composed of a material such as silicon dioxide (SiO 2 ), a silicon nitride (Si x N y ) dielectric, or others. Top electrode membrane  88  and bottom electrode membrane  92  are semi-permeable electrically conductive membranes, such as carbon membranes. Proton exchange membrane  90  may be a proton conductive membrane of a perfluorosulfate ionomer, for example, a NAFION® membrane supplied by DuPont, with composite catalytic electrodes. The composite catalytic electrodes include an electrode material coated with a catalyst coating that facilitates electrochemical reaction, reducing the energy required for the reaction to occur. For example, in a CO sensor, the catalyst coating is a platinum alloy. Conductive via  94  extends through semiconductor wafer  85  to provide electrical contact between top electrode membrane  88  and contact pad  95 , and conductive via  96  extends through semiconductor wafer  85  and dielectric layer  87  to provide electrical contact between bottom electrode membrane  92  and contact pad  97 . Contact pads  95  and  97  may be composed of a conductive metal such as aluminum, copper, nickel, gold, or others. A gas diffusion path is formed to the membrane stack by gas diffusion opening  98 , which allows gas to diffuse through activated carbon film  86  to top electrode membrane  88 . 
     Second thin film subassembly  84  includes semiconductor wafers  100  and  101 . Semiconductor wafer  100  may be formed with the geometry shown by high resolution MEMS processing techniques such as reactive ion etching, chemical anisotropic etching, or others. Semiconductor wafer  101  is formed to be complementary to semiconductor wafer  100 , so that the two wafers may be attached (such as by fusion bonding or another known wafer bonding technique) to form reservoir  102 . Reservoir  102  may have a depth between 10 and 500 micrometers (μm) in an exemplary embodiment. Semiconductor wafer  100  also includes hydration opening  104 , which allows electrolyte (e.g., water) from reservoir  102  to evaporatively hydrate proton exchange membrane  90 . First thin film subassembly  82  and second thin film subassembly  84  are attached together at outer portions thereof via a known method such as fusion bonding or another wafer bonding technique. 
     In operation, gas passes through openings  98  and diffuses through activated carbon film  86 . CO in the gas interacts with the electrode stack formed by top electrode membrane  88 , proton exchange membrane  90  and bottom electrode membrane  92  to produce an electrochemical reaction. The electrochemical reaction generates an electrical signal between top electrode membrane  88  and bottom electrode membrane  92  that is proportional to the concentration of CO at top electrode membrane  88 . This electrical signal is detectable at contact pads  95  and  97  by virtue of their electrical connection to top electrode membrane  88  and bottom electrode membrane  92  by conductive vias  94  and  96 , respectively. The electrical signal at contact pads  95  and  97  is processed and monitored by appropriate circuitry in a manner generally known in the art to indicate the level of CO present. 
     Various embodiments of thin film CO sensors are described above and shown in  FIG. 1-3 . The bodies of these sensors are composed of a semiconductor material, such as silicon or other materials, that can be processed by high resolution batch processing MEMS techniques. The sensors therefore have significantly smaller sizes than sensors constructed according to the current state of the art. For example, the thicknesses of the semiconductor wafers may be between about 25-550 micrometers (μm), the thicknesses of the top and bottom semi-permeable electrically conductive membranes may be between about 100 nanometers (nm) and 500 μm, the thickness of the proton exchange membrane may be between about 10 nm and 140 μm, the dielectric layer may be between about 1 nm and 640 μm, the activated carbon film may be between about 1-300 μm, the hygroscopic material may have sizes between about 10 nm and 10 μm, and the microheater (in the embodiment of  FIG. 2 ) may have a thickness between about 100 nm and 5 μm. The contact pads may have thickness between about 100 nm and 5 μm. Thus, the total thickness of a sensor as disclosed herein may be less than about 3 millimeters (mm), and in many embodiments less than about 1 mm or even about 0.1 mm in thickness. This is a significant reduction in size in comparison to CO sensors constructed according to the state of the art, which are typically about 10 centimeters in thickness. 
     The CO sensor disclosed herein also may exhibit a significantly improved response time compared to sensors constructed according to the state of the art. The response time of a CO sensor is related to the length of the diffusion path for gas to travel through the sensor and the membrane stack of the sensor. The gas diffusion path in the CO sensor disclosed herein is significantly shorter than in sensors constructed according to the state of the art, resulting in a corresponding improvement in the response time of the sensor. 
     The CO sensor disclosed herein is also able to be constructed in a more structurally sound and consistent manner than many sensors of the prior art. Contact and alignment between the proton exchange membrane and the top and bottom electrode membranes in the membrane stack is consistently achieved by the thin film deposition of those layers (such as by a spin coating process, for example). This was not always the case in prior sensors, which pressed the electrodes and membrane together by crimping of the outer container. 
     The present invention has been described herein by illustrations of several embodiments of a CO sensor. It should be understood that the principles of the present invention are also applicable to a number of target gas sensors, such as sensors for detecting toxic gases such as propane, methane, ammonia, or others. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.