US20120263870A1 - Carbon Dioxide Gas Sensors and Method of Manufacturing and Using Same - Google Patents
Carbon Dioxide Gas Sensors and Method of Manufacturing and Using Same Download PDFInfo
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- US20120263870A1 US20120263870A1 US13/267,978 US201113267978A US2012263870A1 US 20120263870 A1 US20120263870 A1 US 20120263870A1 US 201113267978 A US201113267978 A US 201113267978A US 2012263870 A1 US2012263870 A1 US 2012263870A1
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- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 title claims description 97
- 239000001569 carbon dioxide Substances 0.000 title claims description 66
- 229910002092 carbon dioxide Inorganic materials 0.000 title claims description 66
- 238000004519 manufacturing process Methods 0.000 title description 22
- 239000002228 NASICON Substances 0.000 claims abstract description 97
- 239000000758 substrate Substances 0.000 claims abstract description 96
- 239000003792 electrolyte Substances 0.000 claims abstract description 72
- 239000007784 solid electrolyte Substances 0.000 claims abstract description 54
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims abstract description 44
- 229910052697 platinum Inorganic materials 0.000 claims abstract description 29
- 150000001875 compounds Chemical class 0.000 claims abstract 2
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- 229920002120 photoresistant polymer Polymers 0.000 claims description 126
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- 229910000029 sodium carbonate Inorganic materials 0.000 claims description 83
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- MUMZUERVLWJKNR-UHFFFAOYSA-N oxoplatinum Chemical compound [Pt]=O MUMZUERVLWJKNR-UHFFFAOYSA-N 0.000 claims 1
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Images
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/406—Cells and probes with solid electrolytes
- G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
- G01N27/4073—Composition or fabrication of the solid electrolyte
- G01N27/4074—Composition or fabrication of the solid electrolyte for detection of gases other than oxygen
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
- G01N33/0004—Gaseous mixtures, e.g. polluted air
- G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
- G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
- G01N33/0036—General constructional details of gas analysers, e.g. portable test equipment concerning the detector specially adapted to detect a particular component
- G01N33/004—CO or CO2
Definitions
- the invention is in the field of carbon dioxide gas sensors.
- the detection of CO 2 is essential for a range of applications including reduction of false fire alarms, environmental monitoring, and engine emission monitoring.
- traditional smoke detectors monitoring particles can have false fire alarm rates as high as 1 in 200 in aircraft applications.
- monitoring the change of CO and CO 2 concentrations and their ratio (CO/CO 2 ) can be used to detect the chemical signature of a fire.
- Electrochemical CO 2 sensors which use super ion conductors (such as Na Super tonic Conductor or NASICON) as the solid electrolyte, and auxiliary electrolytes (such as Na 2 CO 3 /BaCO 3 ) have great potential for in-situ fire detection and other applications.
- super ion conductors such as Na Super tonic Conductor or NASICON
- auxiliary electrolytes such as Na 2 CO 3 /BaCO 3
- Sensitivity refers to the ability of the sensor to detect the desired chemical species in the range of interest.
- Selectivity refers to the ability of the sensor to detect the species of interest in the presence of interfering gases which also can produce a sensor response.
- Response time refers to the time it takes for the sensor to provide a meaningful signal. By meaningful signal it is meant that the signal has reached, for example, 90% of the steady state signal when the chemical environment experiences a step change.
- Stability refers to the degree which the sensor baseline and response are the same over time. It is desirable to use a sensor that will accurately determine the species of interest in a given environment with a response large and rapid enough to be of use in the application and whose response does not significantly drift over its operational lifetime.
- FIG. 1A is a cross-sectional schematic illustration 100 of a prior art bulk carbon dioxide gas sensor.
- reference numeral 101 is an electrolyte known as NASICON which is an acronym or partial acronym for Na 3 Zr 2 Si 2 PO 12 and is oricntcd between a platinum (paste) 103 and a Sodium Carbonate/Barium Carbonate (Na 2 CO 3 /BaCO 3 ) layer 102 .
- a reference electrode 105 engages the platinum paste and a gold working electrode 104 resides in contact with the interface of the Sodium Carbonate and/or Barium Carbonate (Na 2 CO 3 /BaCO 3 ) 102 and the NASICON 101 .
- Sodium Carbonate and/or Barium Carbonate Na 2 CO 3 /BaCO 3
- Sodium Carbonate and/or Barium Carbonate Na 2 CO 3 /BaCO 3
- quartz glass tubes (insulators) 106 for reference gases.
- FIG. 1 is a cross-sectional schematic illustration 100 of a prior art gas sensor disclosing an Alumina substrate 107 , interdigitated Platinum metal electrodes 108 , a first solid electrolyte, NASICON 109 , between the electrodes, and Sodium Carbonate and/or Barium Carbonate (Na 2 CO 3 /BaCO 3 ) 110 covering the NASICON and the electrodes.
- NASICON Sodium Carbonate and/or Barium Carbonate
- the first solid electrolyte is selected from the group consisting of NASICON, LISICON, KSICON, and ⁇ ′′-Alumina (beta prime-prime alumina in which when prepared as an electrolyte is complexed with a mobile ion selected from the group consisting of Na + , K + , Li + , Ag + , H + , Pb 2+ , Sr 2+ or Ba 2+ ).
- Sodium Carbonate and/or Barium Carbonate Na 2 CO 3 /BaCO 3
- it is meant that either a material containing Sodium Carbonate (Na 2 CO 3 ), Barium Carbonate (BaCO 3 ), or a mixture of Sodium Carbonate and Barium Carbonate may be used.
- the width of the mask and its apertures has to be absolutely perfect and the alignment has to be absolutely perfect to achieve uniform three-point contact along the joint of the metal electrodes, NASICON and Sodium Carbonate/Barium Carbonate (Na 2 CO 3 /BaCO 3 ).
- the structure depicted in FIG. 1 is very difficult to achieve.
- Photolithographic masks are aligned by hand with the aid of an electron microscope. Any misalignment of the photolithographic mask will result in photoresist trapped between NASICON and electrode finger and therefore result in a failed sensor.
- the structure of FIG. 1 is very difficult to manufacture exactly as shown. Errors in manufacturing probably will result in a failed structure such as that depicted in FIG. 5D .
- One of the innovations of the instant invention is to realize the advantages of not having to perfectly duplicate the structure of FIG. 1 , which represents the structure obtained using standard procedures of microfabrication engineers.
- Electrodes formed by the thick film technique are not sufficiently porous. Using a non-porous electrode can lead to the formation of sodium carbonate Na 2 CO 3 which hinders the working electrode. The formation and dissociation of sodium carbonate Na 2 CO 3 at the electrodes results in slower response time.
- Photolithography is used in device fabrication processes every time a pattern is transferred to a surface. It allows ion implantation or etching of a material in selected areas on the wafer (substrate).
- Photoresist is a photosensitive organic substance which is a sticky liquid with high viscosity which is typically spun onto a wafer and then thermally hardened in an oven. Photoresist may be positive or negative. When positive photoresist is exposed to light it breaks down long-chain organic molecules into shorter chain molecules which can be dissolved by a chemical solution called a developer. When negative photoresist is exposed to light it induces cross-linking of organic molecules such that a high atomic mass is achieved by producing longer-chain molecules.
- an appropriate developer solution is then used to remove the resist that has not been exposed to light.
- the transfer of the desired patterns onto the photoresist is made using ultraviolet light exposure through a mask which is typically a quartz plate.
- Masks are used in two modes. Contact lithography involves overlaying the mask directly into contact with the photoresist and proximity photolithography involves spacing the mask a distance above the photoresist.
- the use of photolithography enables miniaturization, batch processing, and more exact duplication of a given sensor structure. Employing these techniques can fundamentally change and improve the sensors produced; a significant technical challenge is to apply these techniques for some material systems such as those used for CO2 sensor production.
- a miniaturized amperometric electrochemical (solid electrolyte) carbon dioxide (CO 2 ) sensor using a novel and robust sensor design has been developed and demonstrated.
- Semiconductor microfabrication techniques were used in the sensor fabrication and the sensor is fabricated for robust operation in a range of environments.
- the sensing area of the sensor is approximately 1.0 mm ⁇ 1.1 mm.
- the sensor is operated by applying voltage across the electrodes and measuring the resultant current flow at temperatures from 450 to 600° C. Given that air ambient CO 2 concentrations are ⁇ 0.03%, this shows a sensitivity range from below ambient to nearly two orders of magnitude above ambient.
- Sensor current output versus ln [CO 2 concentration] shows a linear relationship from 0.02% to 1% CO 2 .
- One aspect of the development of the invention was to develop miniature CO 2 sensors for a wide variety of applications.
- This miniaturized CO 2 sensor can be integrated into a sensor array with other sensors such as electronics, power, and telemetry on a postage stamp-sized package.
- the complete system (“lick and stick” technology) could be placed at a number of locations to give a full-field view of what is chemically occurring in an environment.
- NASICON Na 3 Zr 2 Si 2 PO 12
- Na 2 CO 3 /BaCO 3 for CO 2 Semiconductor microfabrication techniques are used in the sensor fabrication.
- the fabrication process involves three fabrication steps: 1) deposition of interdigitated electrodes on alumina substrates; 2) deposition of solid electrolyte NASICON (Na 3 Zr 2 Si 2 PO 12 ) between the interdigitated electrodes; and 3) deposition of auxiliary solid electrolytes Na 2 CO 3 and/or BaCO 3 (1:1.7 molar ratio) on top of the entire sensing area.
- the resulting sensing area is approximately 1.0 mm ⁇ 1.1 mm.
- the multiple interdigitated finger electrodes are in contact with the solid electrolytes and the atmosphere in multiple locations rather than in just one location as is seen with single set of electrode structures.
- this approach yields increased surface area associated with three-contact boundaries as compared to other sensors with similar dimensions.
- the same sensor structure could also be applied to develop other sensors such as NO sensors with the corresponding auxiliary electrolytes NaNO 2 or NaNO 3 .
- An amperometric circuit is used to detect CO 2 .
- the detection system includes pairs of electrodes with constant voltage, V, applied across the electrodes.
- the sensing mechanism of the amperometric CO 2 sensors can be understood based on the reactions taking place at the working and reference electrode of each pair of electrodes. The following two reactions may be considered to carry current between the electrodes:
- the reduction current is the result of the reaction taking place at the working electrode where electrons are consumed.
- the oxidation current is the result of the reaction taking place at the reference electrode where electrons are released. The following reaction can then be considered to be:
- Electrodes Platinum is used as the preferred material for the electrode.
- electrodes made from other metals such as Palladium, Silver, Iridium, Gold, Ruthenium, Rhodium, Indium, or Osmium may also be used.
- non-porous or porous electrodes may be used
- the auxiliary electrolyte (Na 2 CO 3 and/or BaCO 3 ) is deposited homogeneously on the entire sensing area of the sensor, including both the working and reference electrodes.
- the deposition of an auxiliary carbonate electrolyte improves the selectivity and sensitivity of the sensor to CO 2 gases and the flow of the desired species within the electrolyte.
- depleted concentration of sodium ions (Na + ) can be recovered by the transfer of sodium ions (Na + ) from NASICON through the three-phase boundary of the electrodes, NASICON electrolyte, and an auxiliary electrolyte layer.
- the sodium carbonate, Na 2 CO 3 , deposited at the working electrode during reacting with CO 2 can be transferred to the reference electrode through the Na 2 CO 3 /BaCO 3 auxiliary carbonate electrolyte layer if temperatures are high enough, for example, 450-600° C.
- the sensing mechanism has increased performance from the Na 2 CO 3 /BaCO 3 auxiliary carbonate electrolyte layer being distributed across both the working and the reference electrodes at high operating temperatures in the 450-600° C.
- the eutectic mixture of Na 2 CO 3 /BaCO 3 as the auxiliary carbonate electrolyte layer has a lower melting temperature enabling improved flow within the electrolyte at a reduced temperature range.
- the Na 2 CO 3 /BaCO 3 auxiliary carbonate electrolyte can act as a diffusion barrier to prevent other species from reaching the electrode/electrolyte interface and interfering with the correlation of measured current with detection of the desired chemical species.
- porous platinum electrodes can be used with an auxiliary carbonate electrolyte having an increased porosity.
- the sensor structure employs interdigitated electrodes which can be generally thought of as interdigitated fingers. Unique fabrication processes to miniaturize the CO 2 sensor are used.
- a unique amperometric CO 2 sensor is produced using a non-standard approach as disclosed herein and has the following attributes:
- Electrode width and spacing between electrodes is around 50 ⁇ m.
- the sensor would be very difficult to make with a shadow mask if a layer of electrolyte is deposited before the electrodes as is the case in most other attempted processes.
- Interdigitated electrodes are very important for amperometric CO 2 sensors because the current output of the electrodes is summed and bussed which results in currents much higher compared to the traditional two electrodes with the same size. As a result, better sensitivity of the sensor is achieved. In other words for a given change of input to the sensor in terms of CO 2 concentration, a larger differential change in output is observed.
- the senor has a robust structure.
- the interdigitated electrodes were deposited directly on the alumina substrate with strong adhesion, which will stand the attack of humidity and vibration. This is in contrast to the approach of depositing the electrolyte first on the substrate which has less inherent stability.
- the senor has a unique arrangement of electrodes/electrolytes.
- Solid electrolyte NASICON is deposited between interdigitated fingers and the auxiliary electrolyte Na 2 CO 3 /BaCO 3 was deposited on the whole sensing area, forming greater length of three-point boundaries (electrode, solid electrolyte NASICON, and auxiliary electrolyte Na 2 CO 3 /BaCO 3 ), which is beneficial for amperometric gas sensing.
- Interdigitated finger electrodes on a substrate were used as sensor structures before but only one or mixed sensing materials were deposited.
- the interdigitated finger electrode structure is deposited with two distinctive sensing materials forming maximum length three-point contacts.
- the sensor was tested continuously for at least three weeks at high temperatures showing its robust nature.
- the sensor structure could also be used with any other sensing system which requires two distinctive deposited materials in an electrochemical cell structure.
- the senor is very easy to batch fabricate compared to the bulk-sized sensors and consumes much less power. This is specifically due to the non-standard photolithographic approach used.
- sensors may be fabricated which have good sensitivity, selectivity, response time, and stability.
- the carbon dioxide sensor produced by the innovative technique described herein is applicable to the fire detection (including hidden fire), EVA applications, personal health monitoring, and environmental monitoring.
- the sensor and its electronics are integrated into a postage stamp sized system.
- the low cost due to the batch fabrication process and its compact size make it highly affordable and thus useable in a wide array of locations.
- a process for sensing carbon dioxide is accomplished which includes the following steps: applying a constant direct current voltage across the pair of electrodes.
- the electrodes are separated by an electrolyte material containing sodium, and the electrodes are located between a layer of alumina substrate and an electrolyte layer of auxiliary carbonate.
- Carbon dioxide is then reacted with the material containing sodium at the first three-point boundary.
- the first three point boundary is located at the joinder of one of the electrodes, the electrolyte material containing sodium, and the barium containing auxiliary electrolyte.
- the second three-point boundary is located at the joinder of the other of the electrodes, the electrolyte material containing sodium, and a barium containing auxiliary electrolyte.
- FIG. 1 is a cross-sectional schematic illustration of a prior art gas sensor.
- FIG. 1A is a cross-sectional schematic illustration of a prior art bulk gas sensor.
- FIG. 2 is a cross-sectional schematic illustration of interdigitated electrodes residing on a substrate forming part of the sensor of the present invention.
- FIG. 2A is a cross-sectional schematic view taken along the lines 2 A- 2 A of FIG. 2 .
- FIG. 2B is a cross-sectional view similar to FIG. 2A with first and second solid electrolytes over the substrate and the interdigitated electrodes.
- FIG. 3 is a cross-sectional schematic illustration of a substrate with photoresist spun onto the substrate.
- FIG. 3A is a cross-sectional schematic illustration of the substrate as illustrated in FIG. 3 with a photomask oriented thereover and ultraviolet light imidizing the unmasked portions of the photoresist.
- FIG. 3B is a cross-sectional schematic illustration of the substrate illustrated in FIG. 3A with the imidized photoresist developed and removed.
- FIG. 3C is a cross-sectional schematic illustration similar to FIG. 3B with a first layer of Titanium sputtered onto the substrate.
- FIG. 3D is an enlargement of a portion of FIG. 3C illustrating the sputter deposition of the first layer of the Titanium over the substrate and the photoresist.
- FIG. 3E is a cross-sectional schematic illustration of a second layer of Platinum deposited above the first metallization layer of Titanium and the photoresist.
- FIG. 3F is a cross-sectional schematic illustration of the substrate with two interdigitated electrodes affixed to the substrate with the photoresist removed with acetone or other suitable solvent.
- FIG. 3G is a cross-sectional schematic illustration of photoresist spun over the interdigitated Titanium/Platinum electrodes and the substrate.
- FIG. 3H is a cross-sectional schematic illustration of a mask applied to the substrate and ultra violet light imidizing the unmasked portions of the photoresist.
- FIG. 3I is a cross-sectional schematic illustration of the substrate, interdigitated electrodes and photoresist left after the imidized photoresist has been developed and removed.
- FIG. 3J is a cross-sectional schematic illustration of the substrate, interdigitated electrodes with photoresist residing on a portion thereof with a layer of a first solid electrolyte deposited thereover
- FIG. 3K is a cross-sectional schematic illustration wherein the photoresist has been removed with acetone or other suitable solvent.
- FIG. 3L is a cross-sectional schematic illustration with a second solid electrolyte deposited thereover.
- FIG. 3M is a cross-sectional schematic illustration similar to FIG. 3L wherein the electrodes form tapered surfaces at the place of joinder with the electrolytes.
- FIG. 3N is an enlargement of a portion of FIG. 3M .
- FIG. 3O is a cross-sectional schematic similar to FIG. 3J of another example of the application of a first solid applied over the substrate, interdigitated electrodes, photoresist using sputter deposition.
- FIG. 3P is a cross-sectional schematic illustration similar to FIG. 3K wherein the photoresist has been removed with acetone or other suitable solvent.
- FIG. 3Q is a cross-sectional schematic having a second solid electrolyte applied over the first solid electrolyte and the interdigitated electrodes.
- FIG. 3R is a cross-sectional schematic illustration wherein a third layer, a metal oxide layer, is applied over the second solid electrolyte.
- FIG. 4 is a schematic illustration similar to FIG. 3H with the mask slightly misaligned.
- FIG. 4A is a schematic illustration of the photoresist developed and removed with photoresist remaining over the interdigitated electrodes but not centrally located (misaligned).
- FIG. 4B is a schematic illustration similar to FIG. 4A with a first solid electrolyte deposited thereover.
- FIG. 4C is a schematic illustration with the photoresist lifted off through dissolution with acetone.
- FIG. 4D is a schematic illustration similar to FIG. 4C with a second solid electrolyte deposited over the first solid electrolyte and the interdigitated electrodes.
- FIG. 5 is a schematic illustration similar to FIG. 4 with the mask misaligned above the substrate, interdigitated electrodes, and photoresist indicating the application of ultraviolet light thereto.
- FIG. 5A is a schematic illustration similar to FIG. 5 with the imidized photoresist developed and removed leaving a gap filled with photoresist adjacent the electrodes.
- FIG. 5B is a schematic illustration with a first electrolyte over the substrate, interdigitated electrodes and photoresist.
- FIG. 5C is a schematic illustration similar to FIG. 5B with the photoresist lifted off.
- FIG. 5D is a schematic illustration similar to FIG. 5C with a second electrolyte over the first electrode and interdigitated electrodes.
- FIG. 6 is a schematic illustration of one example of process steps used to make the sensors.
- FIG. 2 is a cross-sectional schematic illustration 200 of interdigitated electrodes 204 , 210 residing on a substrate 206 forming part of the sensor of the present invention.
- Positive contact pad 201 is interconnected by lead 202 to positive bus 203 which is in turn interconnected with positive interdigitated positive electrodes (fingers) 204 .
- Negative contact pad 207 is interconnected by lead 209 to negative bus 209 which in turn is interconnected with negative interdigitated negative electrodes (fingers) 210 .
- Electrodes 204 , 210 are fixedly engaged to the Alumina substrate 206 .
- the Alumina substrate 206 is an insulator and is approximately 625 ⁇ m thick.
- reference numeral 205 indicates the gap between electrodes 204 , 210 .
- the electrode width 212 W and width of the gap between electrodes 211 are both around 30 ⁇ m.
- Contact pads 201 , 207 are interconnected by a conductor 221 to battery 222 which is nominally at 1V DC.
- Amp meter 220 measures and records current in the circuit.
- FIG. 2A is a cross-sectional schematic view 200 A taken along the lines 2 A- 2 A of FIG. 2 .
- Gap 205 and electrodes 204 , 210 are illustrated as is the negative bus 209 .
- the width 211 of the gap 205 is approximately 30 ⁇ m.
- the electrodes 204 , 210 have a width of approximately 30 ⁇ m as indicated by reference numeral 212 W.
- a thin layer of Titanium 213 is beneath Platinum electrodes 204 , 210 .
- the electrode material may comprise a thin layer of PtO x followed by a relatively thick layer of Platinum.
- FIG. 2B is a cross-sectional schematic view 200 B similar to FIG. 2A with first and second solid electrolytes 212 , 211 over the substrate 206 and interdigitated electrodes 204 , 210 .
- Reference numeral 212 is also used to indicate the contour of the first electrolyte, for example, NASICON, LISICON, or ⁇ ′′-Alumina (beta prime-prime alumina in which when prepared as an electrolyte is complexed with a mobile ion selected from the group consisting of Na + , K + , Li + , Ag + , H + , Pb 2+ , Sr 2+ , or Ba 2+ ).
- the first electrolyte may be any number of solid electrolytes known for their conductivity performance.
- electrolytes may include sodium or lithium as in the case of NASICON and LISICON, but the electrolyte is not limited to materials containing these elements and may include any number of elements including but not limited to Na, Li, K, Ag, H, Pb, Sr, or Ba.
- the second solid electrolyte 211 may include Sodium Carbonate (Na 2 CO 3 ) or mixture of Sodium Carbonate (Na 2 CO 3 ) and Barium Carbonate (BaCO 3 ).
- electrolyte materials such as Li 2 CO 3 , K 2 CO 3 , Rb 2 CO 3 , SrCO 3 , Ag 2 CO 3 , PbCO 3 and their mixtures among them or others may be used as a mixture in place of or in addition to Sodium Carbonate or mixture of Sodium Carbonate (Na 2 CO 3 ) and Barium Carbonate in a second solid electrolyte layer.
- FIG. 3 is a cross-sectional schematic illustration 300 of a substrate 301 with photoresist 302 spun onto the substrate 301 .
- FIG. 3A is a cross-sectional schematic illustration 300 A of the substrate 301 as illustrated in FIG. 3 with a photomask 399 oriented thereover and ultraviolet light 305 imidizing the unmasked portions of the photoresist.
- the photomask 399 includes apertures 304 and opaque portions 303 .
- FIG. 3B is a cross-sectional schematic illustration 300 B of the substrate 301 illustrated in FIG. 3A with the imidized photoresist developed and removed.
- the imidized portion of the photoresist is the portion which has been exposed to the ultraviolet light.
- Unimidized portions 302 A of the photoresist remain on the substrate at this step.
- FIG. 3C is a cross-sectional schematic illustration 300 C similar to FIG. 3B with a first layer of titanium 303 sputtered onto the substrate 301 and the unimidized photoresist 302 A.
- FIG. 3D is an enlargement 300 D of a portion of FIG. 3C illustrating the sputter deposition of the first layer of the Titanium 303 A over the substrate 301 and the photoresist 302 A.
- Titanium layer 303 A is approximately 50 ⁇ thick and forms a good bond to the Alumina substrate which is approximately 250-625 ⁇ m thick.
- the dimensions of the interdigitated area on the Alumina substrate is approximately 1.1 mm long, 1.0 mm wide and 250 or 625 ⁇ m thick in this embodiment of the invention.
- FIG. 3E is a cross-sectional schematic illustration 300 E of a second layer of Platinum 304 A deposited above the first metallization layer of Titanium 303 A and the unimidized photoresist 302 A.
- FIG. 3F is a cross-sectional schematic illustration 300 F of the Alumina substrate 301 with two interdigitated electrodes 304 A/ 303 A affixed to the substrate with the unimidized photoresist 302 A removed with acetone or some other suitable solvent.
- FIG. 3G is a cross-sectional schematic illustration 300 G of photoresist 355 spun over the interdigitated Titanium/Platinum electrodes 304 A/ 303 A and the Alumina substrate 301 .
- FIG. 3H is a cross-sectional schematic illustration 300 H of a photomask 399 A spaced apart and in proximity to the substrate 301 with interdigitated electrodes 304 A/ 303 A thereon and ultra violet light 308 passing through apertures 307 imidizing the unmasked (exposed) portions of the photoresist.
- Reference numeral 309 represents the width of opaque portion 306 of photomask 399 A. This width is specifically designed to be less than the width of 303 A/ 304 A.
- FIG. 3I is a cross-sectional schematic illustration 300 I of the substrate, interdigitated electrodes 303 A/ 304 A and unimidized photoresist 355 A left after the imidized photoresist has been developed and removed.
- FIG. 3J is a cross-sectional schematic illustration 300 J of the substrate 301 , interdigitated electrodes 304 A/ 303 A with unimidized photoresist residing on a portion thereof with a layer of first solid electrolyte 310 , for example, NASICON, deposited thereover.
- Reference numeral 312 indicates the portion where the NASICON is raised slightly as its deposition by E-beam evaporation follows the contour of the substrate 301 , the electrodes 303 A/ 304 A and the unimidized photoresist 355 A.
- NASICON 310 is applied at a thickness approximately equal to the thickness of the electrodes 303 A/ 304 A.
- E-beam deposition is used here as an example of very controlled, exact deposition of component layers providing nearly vertical deposition geometries. Actual applications may vary.
- one of the electrodes 303 A/ 304 A is the working electrode and the other electrode is the reference electrode.
- FIG. 3K is a cross-sectional schematic illustration 300 K wherein the unimidized photoresist 355 A has been removed with acetone, or some other suitable solvent leaving a contoured surface of NASICON and Platinum electrodes exposed.
- FIG. 3L is a cross-sectional schematic illustration 300 L with a second solid electrolyte 311 deposited over the NASICON 310 and the electrodes 303 A/ 304 A.
- the second solid electrolyte may be Sodium Carbonate (Na 2 CO 3 ), or a combination of Sodium Carbonate (Na 2 CO 3 ) and Barium Carbonate (BaCO 3 ) thereof in addition to other solid electrolytes and combinations which may include Li 2 CO 3 , K 2 CO 3 , Rb 2 CO 3 , SrCO 3 , Ag 2 CO 3 , and PbCO 3 .
- the second electrolyte layer with Na 2 CO 3 and BaCO 3 mixture performs a barrier function in that it keeps the sensor less vulnerable to humidity.
- each electrode joins the NASICON and the Sodium Carbonate or mixture of Sodium Carbonate and Barium Carbonate along a line where reduction and oxidation takes place. Current flow takes place through the NASICON.
- Reference numeral 369 represents inboard lines of three point contact of the electrodes 303 A/ 404 A, NASICON 312 , and second electrolyte Sodium Carbonate 311 .
- Reference numeral 369 A represents outboard lines of three point contact of the electrodes 303 A/ 404 A, NASICON 312 , and second electrolyte Sodium Carbonate 311 .
- the outboard lines 369 A may not exist. Such is the case when the NASICON is applied by sputtering as set forth in FIG. 3 “O” to FIG. 3R , inclusive.
- FIG. 3M is a cross-sectional schematic illustration 300 M similar to FIG. 3L wherein the NASICON 310 , 312 includes tapered surfaces 313 at the joinder of the Platinum electrodes and the Sodium Carbonate/Barium Carbonate (Na 2 CO 3 /BaCO 3 ) layer 311 .
- the tapered surfaces of NASICON are very thin which in effect creates several lines of multiple three point contacts which facilitates the reduction and oxidation processes set forth below.
- FIG. 3N is an enlargement 300 N of a portion of FIG. 3M and provides a better view of the tapered surface 313 , the electrode 304 A, first electrolyte 312 and secondary electrolyte 311 .
- the tapered surface 313 plays an important role in that it provides a better amperometric surface as the NASICON layer in the tapered surface 313 is very thin resulting in multiple lines where three (3) point contacts between the electrode, NASICON, and the auxiliary Sodium Carbonate/Barium Carbonate (Na 2 CO 3 /BaCO 3 ) electrolyte layer exist.
- tapered surfaces 313 are created as a result of heat treatment of the NASICON film at temperature as high as 850° C.
- the detection system depicted in FIGS. 2-2B and 3 - 3 R includes pairs of electrodes with constant voltage, V, applied across the multiple interdigitated electrodes.
- the sensing mechanism of the amperometric CO 2 sensors can be understood based on the reactions taking place at the working and reference electrode of each pair of electrodes.
- the following two electrode reactions may be considered:
- Reduction occurs as the result of the reaction taking place at the working electrode where electrons are consumed.
- Oxidation occurs as the result of the reaction taking place at the reference electrode where electrons are released.
- Electrodes Platinum is used as the preferred material for the electrode.
- electrodes made from other metals such as Palladium, Silver, Iridium, Gold, Ruthenium, Rhodium, Indium, or Osmium may also be used.
- non-porous or porous electrodes may be used
- the auxiliary electrolyte (Na 2 CO 3 and/or BaCO 3 and/or Li 2 CO 3 , K 2 CO 3 , Rb 2 CO 3 , SrCO 3 , Ag 2 CO 3 , PbCO 3 ) is deposited homogeneously on the entire sensing area of the sensor, including both the working and reference electrodes.
- the deposition of an auxiliary carbonate electrolyte improves flow of the desired species within the electrolyte.
- depleted concentration of sodium ions Na +
- the sodium carbonate, Na 2 CO 3 deposited at the working electrode can be transferred to the reference electrode through the Na 2 CO 3 auxiliary carbonate electrolyte if temperatures are high enough, for example, 450-600° C.
- the sensing mechanism has increased performance from the Na 2 CO 3 /BaCO 3 auxiliary carbonate electrolyte layer being distributed across both the working and the reference electrodes at high operating temperatures in the 450-600° C.
- the eutectic mixture of Na 2 CO 3 /BaCO 3 as the auxiliary carbonate electrolyte layer has a lower melting temperature enabling improved flow within the electrolyte at a reduced temperature range.
- the Na 2 CO 3 /BaCO 3 auxiliary carbonate electrolyte can act as a diffusion barrier to prevent other species from reaching the electrode/electrolyte interface and interfering with the correlation of measured current with detection of the desired chemical species.
- FIG. 3 “O” is a cross-sectional schematic 300 “O” similar to FIG. 3J and is another example of the application of a first solid electrolyte 310 A applied over the substrate 301 , interdigitated electrodes 303 A/ 304 A and unimidized photoresist 305 A using sputter deposition.
- Sputter deposition of NASICON 310 A results in a surface 320 which is contoured and does not follow the underlying components.
- Sputter deposition is used here as an example of a less exact, more diffuse deposition of component layers providing more graded deposition geometries. Actual applications may vary.
- FIG. 3 “O” is a cross-sectional schematic 300 “O” similar to FIG. 3J and is another example of the application of a first solid electrolyte 310 A applied over the substrate 301 , interdigitated electrodes 303 A/ 304 A and unimidized photoresist 305 A using sputter deposition.
- the NASICON was applied in a manner which results in the NASICON applied so as to more closely follow the contour of the underlying structure.
- Reference numeral 325 is used to indicate the NASICON above the unimidized photoresist 305 A.
- FIG. 3P is a cross-sectional schematic illustration 300 P similar to FIG. 3K wherein the unimidized photoresist 305 A has been removed with acetone or other suitable solvent leaving NASICON 310 A behind with a contoured surface 320 . Next, the auxiliary Sodium Carbonate/Barium Carbonate (Na 2 CO 3 /BaCO 3 ) electrolyte composition is applied.
- FIG. 3Q is a cross-sectional schematic 300 Q having a second solid electrolyte 311 applied over the first solid electrolyte 310 A and the interdigitated electrodes 303 A/ 304 A.
- FIG. 3R is a cross-sectional schematic illustration 300 R wherein a solid metal oxide 330 , SnO 2 , CuO, In2O3, and TiO 2 and/or a combination thereof, is applied over the second solid electrolyte 311 . It is preferred that these solid metal oxides be composed of nanoparticles. Use of this third layer of metal oxide provides enhanced performance of the sensor. This third layer of metal oxide is applied by drop deposition of SnO 2 sol gel on top of the Na 2 CO 3 /BaCO 3 and heat treat the sensor in the instant invention. It can also be deposited using e-beam evaporation or sputtering using a shadow mask which is the same as that for Na 2 CO 3 /BaCO 3 deposition. The third layer of metal oxide improves the sensor signal greatly and also enables the carbon dioxide sensor to function a temperature range as low as 200° C.
- FIG. 4 is a schematic illustration 400 similar to FIG. 3H with the photomask 499 A slightly misaligned.
- FIGS. 4-4D are illustrative of the fault tolerance of the instant invention. It is this fault tolerance which enables successful production of the device.
- the opaque portion 404 of the mask 499 A is indicated with reference numeral 404 and the ultra violet light 405 is indicated with reference numeral 405 . Still referring to FIG. 4 , it can be seen as is discussed elsewhere herein that if the opaque portions 404 of the photomask 499 A are the same width as the underlying electrodes 402 / 406 and they are misaligned, then a faulty sensor will result.
- the opaque portions of the mask have a width less than 30 ⁇ m and preferably in the range of 15-20 ⁇ m. Misalignment of the photomask 499 A (serpentine) results in decentralized unimidized photoresist 403 A. However, because the opaque portions of the mask have a width significantly smaller than the width of the electrodes perfect alignment is not necessary.
- the NASICON lip shown by reference numeral 412 and the NASICON indicated by reference numeral 313 are locations where the NASICON may be thin and in effect leads to more reaction sites close to three boundary contacts. Also, this speeds up the manufacturing process because the technician does not have to be perfect in alignment.
- reference numeral 406 is the thin layer (50 ⁇ ) of Titanium as previously described in connection with reference numeral 303 A in FIGS. 3-3R .
- Reference numeral 402 is the relatively thicker layer (4000 ⁇ ) of Platinum as previously described in connection with reference numeral 304 A in FIGS. 3-3R .
- FIG. 4A is a schematic illustration 400 A of the photoresist developed and removed with unimidized photoresist 403 A remaining over the interdigitated electrodes but not centrally located (misaligned).
- FIG. 4B is a schematic illustration similar 400 B to FIG. 4A with a first solid electrolyte such as NASICON or LISICON 410 deposited thereover by e-beam evaporation.
- FIG. 4C is a schematic illustration 400 C with the photoresist lifted off through dissolution with acetone or other suitable solvent. Raised portions of the NASICON or LISICON 410 are viewed well in FIG. 4C .
- FIG. 4D is a schematic illustration 400 D similar to FIG.
- FIG. 4C with a second solid electrolyte 411 (Barium Carbonate and/or Sodium Carbonate) deposited over the first solid electrolyte and the interdigitated electrodes.
- FIG. 4D illustrates the potential problem with misalignment discussed elsewhere herein particularly in describing FIG. 1 which can not be produced because of the stack-up of manufacturing tolerances.
- FIG. 5 is a schematic illustration 500 similar to FIG. 4 with the photomask 599 A significantly misaligned above the substrate 501 , interdigitated electrodes 503 / 504 , and photoresist 555 indicating the application of ultraviolet 508 light thereto.
- Reference numeral 503 is the Titanium layer of the electrode and reference numeral 504 is the Platinum layer of the electrode as described and similarly proportioned to the other examples given herein.
- Opaque portions of the photomask 599 A are indicated by reference numeral 506 and apertures in the mask are denoted by reference numeral 507 . Misalignment should not occur when the opaque portions 506 of the photomask 599 A are substantially smaller than the width of the electrodes as described herein. However, the illustration of FIG.
- the opaque portion 506 of the mask protects the underlying photoresist and prevents ultraviolet light from reaching the photoresist resulting in a portion of the photoresist being unimidized 555 A.
- FIG. 5A is a schematic illustration 500 A similar to FIG. 5 with the imidizcd photoresist developed and removed leaving a gap filled with unimidized photoresist 555 A appearing just to the left of the electrodes 503 / 504 .
- This photoresist 555 A which lies next to the electrodes 503 / 504 will interfere with the proper function of the electrodes as it prevents the joinder of the electrodes, NASICON, and the Carbonate layer as illustrated in FIG. 5D . It also blocks the movement of Na ion in NASICON between reference electrode and working electrode, which is also a critical factor for sensor to work or function.
- FIG. 5B is a schematic illustration 500 B with a first electrolyte NASICON deposited by e-beam deposition over the substrate, interdigitated electrodes, and photoresist. It will be noticed that the NASICON 510 does not abut the electrodes 503 / 504 on the left hand side of FIG. 5B .
- FIG. 5C is a schematic illustration 500 C similar to FIG. 5B with the unimidized photoresist 555 A lifted off with acetone. NASICON 510 includes a raised portion 512 .
- FIG. 5D is a schematic illustration 500 D similar to FIG. 5C with a second electrolyte 511 over the first electrolyte and the interdigitated electrodes 503 / 504 .
- the electrodes are interdigitated and may involve 8-10 pairs of electrodes in order to sum enough current to provide the desired sensitivity.
- Currents ranging from nano to micro amps are generated by the application of 1.0 Volts or higher dc across the sensor electrode bus as illustrated schematically in FIG. 2 .
- the fabrication of carbon dioxide sensors includes three steps: 1) Deposition of platinum interdigitated finger electrodes on Alumina substrates; 2) Deposition of solid electrolyte called NASICON (Na 3 Zr 2 Si 2 PO 12 ) or LISICON (Li 3 Zr 2 Si 2 PO 12 ) between the finger electrodes; and 3) Deposition of auxiliary electrolytes sodium carbonate and/or barium carbonate (Na 2 CO 3 /BaCO 3 , 1:1.7 in molar ratio for the combination) on the upper surfaces of the electrodes.
- NASICON Na 3 Zr 2 Si 2 PO 12
- LISICON Li 3 Zr 2 Si 2 PO 12
- the Platinum interdigitated finger electrodes were deposited as follows: Alumina substrates (250 ⁇ m or 625 ⁇ m in thickness) were patterned with photoresist and an interdigitated finger electrode photomask. A 50 ⁇ layer of Titanium and a 4000 ⁇ layer of Platinum were deposited on the Alumina substrate by sputter deposition. After development and removal, the substrates were then patterned again to cover the top of interdigitated finger electrodes with photoresist.
- NASICON solid electrolyte between the finger electrodes and the Na 2 CO 3 /BaCO 3 was performed as follows.
- the solid electrolyte NASICON was deposited by e-beam evaporation or sputtering.
- a liftoff process which uses acetone to remove unimidized photoresist was conducted to remove NASICON on the upper surfaces of the electrodes resulting in the NASICON mainly staying between the interdigitated finger electrodes and exposing most of the electrode surface.
- the substrate was heated in an oven at 850° C. for 2 hours.
- Na 2 CO 3 /BaCO 3 (1:1.7 in molar ratio) was then deposited on the upper surfaces of the electrodes and the NASICON surface by sputtering using a shadow mask.
- the use of shadow mask in this step is to prevent the Na ion in deposited NASION being washed away by photolithograph process, which is not obvious and not a typical practice of standard microfabrication process.
- the substrates were heated in an oven at 686° C. for 10 minutes and 710° C. for 20 minutes. Different concentrations of carbon dioxide gases were tested by the sensors at temperatures ranging from 450-600° C. The sensor was tested by applying a voltage to the electrodes and measuring the resulting current. A linear response to carbon dioxide concentrations between 1% to 4% was achieved. Linear responses of the natural logarithmic of carbon dioxide concentrations between 0.02% to 1% was achieved.
- the resulting miniature CO 2 sensor can be integrated into a sensor array with other sensors and electronics, power, and telemetry on a stamp sized package.
- the complete system (“lick and stick” technology) can be placed at a number of locations including some hidden areas to give a full-field understanding of what is occurring in an environment.
- the same sensor structure could also be applied to develop NO x or SO x with the corresponding auxiliary electrolytes NaNO 2 and NaNO 3 , or Na 2 CO 3 and Na 2 SO 4 .
- FIG. 6 is a schematic illustration 600 of one example of process steps used to make the sensors. The process steps are described below and have been described hereinabove.
- an Alumina substrate is coated with photoresist 302 .
- a photomask 399 is then applied selectively 602 imidizing ultra violet light using an interdigitated finger electrode photomask and developing and removing the imidized photoresist.
- sputtering 603 a 50 ⁇ layer of Titanium 303 A onto the Alumina 301 substrate and unimidized photoresist 302 A is performed.
- the sputtering of the Titanium is followed by sputtering 604 a 4000 ⁇ layer of Platinum onto the Titanium.
- the unimidized photoresist 302 A is lifted off 605 with acetone or other solvent to remove the unimidized photoresist 302 A as well as the Titanium 303 A and Platinum 304 A thereover forming electrodes on the Alumina substrate.
- Another layer of photoresist is then applied 606 to the Alumina substrate 301 and electrodes 303 A/ 304 A.
- the photoresist is selectively imidized 607 by applying imidizing ultraviolet light 308 using an interdigitated finger electrode photomask 399 A and then developing and removing the imidized photoresist.
- Electron beam evaporation or sputtering 608 of NASICON over the Alumina substrate, the electrodes and the unimidized photoresist follows.
- the step 620 of depositing a metal oxide may be accomplished by drop deposition of metal oxide sol gel or by sputtering/e-beam deposition using a shadow mask
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Abstract
A gas sensor includes a substrate and a pair of interdigitated metal electrodes selected from the group consisting of Pt, Pd, Au, Ir, Ag, Ru, Rh, In, and Os. The electrodes each include an upper surface. A first solid electrolyte resides between the interdigitated electrodes and partially engages the upper surfaces of the electrodes. The first solid electrolyte is selected from the group consisting of NASICON, LISICON, KSICON, and β″-Alumina (beta prime-prime alumina in which when prepared as an electrolyte is complexed with a mobile ion selected from the group consisting of Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+ or Ba2+). A second electrolyte partially engages the upper surfaces of the electrodes and engages the first solid electrolyte in at least one point. The second electrolyte is selected from the group of compounds consisting of Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+ or Ba2+ ions or combinations thereof.
Description
- The invention described herein was made by employees and by employees of a contractor of the United States Government, and may be manufactured and used by the government for government purposes without the payment of any royalties therein and therefor.
- The invention is in the field of carbon dioxide gas sensors.
- The detection of CO2 is essential for a range of applications including reduction of false fire alarms, environmental monitoring, and engine emission monitoring. For example, traditional smoke detectors monitoring particles can have false fire alarm rates as high as 1 in 200 in aircraft applications. Alternatively, monitoring the change of CO and CO2 concentrations and their ratio (CO/CO2) can be used to detect the chemical signature of a fire. Electrochemical CO2 sensors which use super ion conductors (such as Na Super tonic Conductor or NASICON) as the solid electrolyte, and auxiliary electrolytes (such as Na2CO3/BaCO3) have great potential for in-situ fire detection and other applications. In recent years, there has been a significant effort to develop bulk and miniaturized electrochemical CO2 sensors. Compared to bulk material and thick film solid electrolyte CO2 sensors, miniaturized sensors fabricated by microfabrication techniques generally have the advantages of small size, light weight, low power consumption, and batch fabrication.
- Four factors are typically cited as relevant in determining whether a chemical sensor can meet the needs of an application, namely, sensitivity, selectivity, response time and stability. Sensitivity refers to the ability of the sensor to detect the desired chemical species in the range of interest. Selectivity refers to the ability of the sensor to detect the species of interest in the presence of interfering gases which also can produce a sensor response. Response time refers to the time it takes for the sensor to provide a meaningful signal. By meaningful signal it is meant that the signal has reached, for example, 90% of the steady state signal when the chemical environment experiences a step change. Stability refers to the degree which the sensor baseline and response are the same over time. It is desirable to use a sensor that will accurately determine the species of interest in a given environment with a response large and rapid enough to be of use in the application and whose response does not significantly drift over its operational lifetime.
- Current bulk or thick film solid electrolyte carbon dioxide sensors have the disadvantages of being large in size, high in power consumption, difficult in batch fabrication, and high in cost. The carbon dioxide sensor design described herein has the advantage of being simple to batch fabricate, small in size, low in power consumption, easy to use, and fast responding.
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FIG. 1A is a cross-sectionalschematic illustration 100 of a prior art bulk carbon dioxide gas sensor. Referring toFIG. 1A ,reference numeral 101 is an electrolyte known as NASICON which is an acronym or partial acronym for Na3Zr2Si2PO12 and is oricntcd between a platinum (paste) 103 and a Sodium Carbonate/Barium Carbonate (Na2CO3/BaCO3)layer 102. Areference electrode 105 engages the platinum paste and agold working electrode 104 resides in contact with the interface of the Sodium Carbonate and/or Barium Carbonate (Na2CO3/BaCO3) 102 and the NASICON 101. By Sodium Carbonate and/or Barium Carbonate (Na2CO3/BaCO3), it is meant that either Sodium Carbonate (Na2CO3) or Barium Carbonate (BaCO3), or their mixtures may be used. The sensor is supported by quartz glass tubes (insulators) 106 for reference gases. -
FIG. 1 is a cross-sectionalschematic illustration 100 of a prior art gas sensor disclosing anAlumina substrate 107, interdigitatedPlatinum metal electrodes 108, a first solid electrolyte, NASICON 109, between the electrodes, and Sodium Carbonate and/or Barium Carbonate (Na2CO3/BaCO3) 110 covering the NASICON and the electrodes. The first solid electrolyte is selected from the group consisting of NASICON, LISICON, KSICON, and β″-Alumina (beta prime-prime alumina in which when prepared as an electrolyte is complexed with a mobile ion selected from the group consisting of Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+ or Ba2+). By Sodium Carbonate and/or Barium Carbonate (Na2CO3/BaCO3), it is meant that either a material containing Sodium Carbonate (Na2CO3), Barium Carbonate (BaCO3), or a mixture of Sodium Carbonate and Barium Carbonate may be used. An important feature of electrochemical cells of this type are the three-contact boundaries seen in 100. It is the intersection of 108, 109, and 110. These contacts significantly determine the effectiveness of the sensor and their number and surface area should be maximized. The inventors of the instant patent application disclosed this structure in a conference in Lisbon, Portugal in 2004 and this structure was illustrated or described in an FAA website thereafter. This structure is a schematic and not ideally achievable for a number of reasons. First, to obtain the structure exactly as illustrated inFIG. 1 a perfectly sized and aligned mask is necessary. In other words the width of the mask and its apertures has to be absolutely perfect and the alignment has to be absolutely perfect to achieve uniform three-point contact along the joint of the metal electrodes, NASICON and Sodium Carbonate/Barium Carbonate (Na2CO3/BaCO3). Statistically, given manufacturing tolerances the structure depicted inFIG. 1 is very difficult to achieve. Photolithographic masks are aligned by hand with the aid of an electron microscope. Any misalignment of the photolithographic mask will result in photoresist trapped between NASICON and electrode finger and therefore result in a failed sensor. Simply put, the structure ofFIG. 1 is very difficult to manufacture exactly as shown. Errors in manufacturing probably will result in a failed structure such as that depicted inFIG. 5D . One of the innovations of the instant invention is to realize the advantages of not having to perfectly duplicate the structure ofFIG. 1 , which represents the structure obtained using standard procedures of microfabrication engineers. - Previously, most solid electrolyte CO2 sensors developed were bulk sized or thick film based as illustrated in
FIG. 1A , which involves complicated fabrication process of hot press or screen printing. The power consumption of these sensors is very high and batch fabrication is very difficult. Porous electrodes are typical: Electrodes formed by the thick film technique are not sufficiently porous. Using a non-porous electrode can lead to the formation of sodium carbonate Na2CO3 which hinders the working electrode. The formation and dissociation of sodium carbonate Na2CO3 at the electrodes results in slower response time. - Most often (in the prior art) two sensing materials were used in a solid electrolyte CO2 sensor structure. In the effort to miniaturize a CO2 sensor, the standard approach was to first deposit one sensing electrolyte on the substrate, the electrodes were then deposited on top of the electrolyte, and finally the auxiliary electrolyte was deposited on the electrodes. Humidity, liquid chemical processing, and/or physical vibration tends to erode or loosen the electrolyte underneath the electrodes. This structure limited the application of standard microprocessing techniques one might employ such as photolithography. These properties limited the miniaturization of the sensor using this structure, because the electrodes could only be deposited by a shadow mask, which usually produces electrodes with less integrity when the feature is very small. That is one reason few stable and functional miniaturized sensors of this type exist.
- Photolithography is used in device fabrication processes every time a pattern is transferred to a surface. It allows ion implantation or etching of a material in selected areas on the wafer (substrate). Photoresist is a photosensitive organic substance which is a sticky liquid with high viscosity which is typically spun onto a wafer and then thermally hardened in an oven. Photoresist may be positive or negative. When positive photoresist is exposed to light it breaks down long-chain organic molecules into shorter chain molecules which can be dissolved by a chemical solution called a developer. When negative photoresist is exposed to light it induces cross-linking of organic molecules such that a high atomic mass is achieved by producing longer-chain molecules. In the example of longer chain molecules, an appropriate developer solution is then used to remove the resist that has not been exposed to light. The transfer of the desired patterns onto the photoresist is made using ultraviolet light exposure through a mask which is typically a quartz plate. Masks are used in two modes. Contact lithography involves overlaying the mask directly into contact with the photoresist and proximity photolithography involves spacing the mask a distance above the photoresist. The use of photolithography enables miniaturization, batch processing, and more exact duplication of a given sensor structure. Employing these techniques can fundamentally change and improve the sensors produced; a significant technical challenge is to apply these techniques for some material systems such as those used for CO2 sensor production.
- A miniaturized amperometric electrochemical (solid electrolyte) carbon dioxide (CO2) sensor using a novel and robust sensor design has been developed and demonstrated. Semiconductor microfabrication techniques were used in the sensor fabrication and the sensor is fabricated for robust operation in a range of environments. The sensing area of the sensor is approximately 1.0 mm×1.1 mm. The sensor is operated by applying voltage across the electrodes and measuring the resultant current flow at temperatures from 450 to 600° C. Given that air ambient CO2 concentrations are ˜0.03%, this shows a sensitivity range from below ambient to nearly two orders of magnitude above ambient. Sensor current output versus ln [CO2 concentration] (natural logarithm of the carbon dioxide concentration) shows a linear relationship from 0.02% to 1% CO2. This linear relationship allows for easy sensor calibration. Linear responses were achieved for CO2 concentrations from 1% to 4% and to the logarithm of the CO2 concentrations from 0.02% to 1%. These sensing measurement results, but not the method of sensor fabrication, were disclosed in the April 2004 American Ceramic Society presentation and at the Fire Prevention Conference in Lisbon November 2004. This CO2 sensor has the advantage of being simple to batch fabricate, small in size, low in power consumption, easy to use, and fast response time.
- One aspect of the development of the invention was to develop miniature CO2 sensors for a wide variety of applications. This miniaturized CO2 sensor can be integrated into a sensor array with other sensors such as electronics, power, and telemetry on a postage stamp-sized package. Like a postage stamp, the complete system (“lick and stick” technology) could be placed at a number of locations to give a full-field view of what is chemically occurring in an environment.
- The development of miniature electrochemical sensors based on solid electrolytes NASICON (Na3Zr2Si2PO12) and Na2CO3/BaCO3 for CO2 is an important aspect of the instant invention. Semiconductor microfabrication techniques are used in the sensor fabrication. The fabrication process involves three fabrication steps: 1) deposition of interdigitated electrodes on alumina substrates; 2) deposition of solid electrolyte NASICON (Na3Zr2Si2PO12) between the interdigitated electrodes; and 3) deposition of auxiliary solid electrolytes Na2CO3 and/or BaCO3 (1:1.7 molar ratio) on top of the entire sensing area. The resulting sensing area is approximately 1.0 mm×1.1 mm. The multiple interdigitated finger electrodes are in contact with the solid electrolytes and the atmosphere in multiple locations rather than in just one location as is seen with single set of electrode structures. Thus, this approach yields increased surface area associated with three-contact boundaries as compared to other sensors with similar dimensions. The same sensor structure could also be applied to develop other sensors such as NO sensors with the corresponding auxiliary electrolytes NaNO2 or NaNO3.
- An amperometric circuit is used to detect CO2. The detection system includes pairs of electrodes with constant voltage, V, applied across the electrodes.
- The sensing mechanism of the amperometric CO2 sensors can be understood based on the reactions taking place at the working and reference electrode of each pair of electrodes. The following two reactions may be considered to carry current between the electrodes:
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Working Electrode 2Na++CO2+½O2+2e −→Na2CO3 -
Reference Electrode Na2O→2Na++½O2+2e − - The reduction current is the result of the reaction taking place at the working electrode where electrons are consumed. The oxidation current is the result of the reaction taking place at the reference electrode where electrons are released.
The following reaction can then be considered to be: -
Overall Reaction Na2O+CO2→Na2CO3 - Platinum is used as the preferred material for the electrode. However, electrodes made from other metals such as Palladium, Silver, Iridium, Gold, Ruthenium, Rhodium, Indium, or Osmium may also be used. In addition, non-porous or porous electrodes may be used
- The auxiliary electrolyte (Na2CO3 and/or BaCO3) is deposited homogeneously on the entire sensing area of the sensor, including both the working and reference electrodes. The deposition of an auxiliary carbonate electrolyte improves the selectivity and sensitivity of the sensor to CO2 gases and the flow of the desired species within the electrolyte. At the working electrode, depleted concentration of sodium ions (Na+) can be recovered by the transfer of sodium ions (Na+) from NASICON through the three-phase boundary of the electrodes, NASICON electrolyte, and an auxiliary electrolyte layer. The sodium carbonate, Na2CO3, deposited at the working electrode during reacting with CO2 can be transferred to the reference electrode through the Na2CO3/BaCO3 auxiliary carbonate electrolyte layer if temperatures are high enough, for example, 450-600° C.
- These mechanisms allow the sensor to measure CO2 but recover back to its initial state. The sensing mechanism has increased performance from the Na2CO3/BaCO3 auxiliary carbonate electrolyte layer being distributed across both the working and the reference electrodes at high operating temperatures in the 450-600° C. The eutectic mixture of Na2CO3/BaCO3 as the auxiliary carbonate electrolyte layer has a lower melting temperature enabling improved flow within the electrolyte at a reduced temperature range. The Na2CO3/BaCO3 auxiliary carbonate electrolyte can act as a diffusion barrier to prevent other species from reaching the electrode/electrolyte interface and interfering with the correlation of measured current with detection of the desired chemical species.
- In order to facilitate a faster response time, porous platinum electrodes can be used with an auxiliary carbonate electrolyte having an increased porosity. The sensor structure employs interdigitated electrodes which can be generally thought of as interdigitated fingers. Unique fabrication processes to miniaturize the CO2 sensor are used.
- A unique amperometric CO2 sensor is produced using a non-standard approach as disclosed herein and has the following attributes:
- First is the miniature size of the sensor with interdigitated electrodes. The fabrication of electrodes with photolithography enables the sensor to have a small sensor sizes with a sensing area of approximately 1.0 mm×1.1 mm (electrode width and spacing between electrodes is around 50 μm). Further miniaturization is possible and the size can be varied to control sensor properties. The sensor would be very difficult to make with a shadow mask if a layer of electrolyte is deposited before the electrodes as is the case in most other attempted processes. Interdigitated electrodes are very important for amperometric CO2 sensors because the current output of the electrodes is summed and bussed which results in currents much higher compared to the traditional two electrodes with the same size. As a result, better sensitivity of the sensor is achieved. In other words for a given change of input to the sensor in terms of CO2 concentration, a larger differential change in output is observed.
- Secondly, the sensor has a robust structure. The interdigitated electrodes were deposited directly on the alumina substrate with strong adhesion, which will stand the attack of humidity and vibration. This is in contrast to the approach of depositing the electrolyte first on the substrate which has less inherent stability.
- Thirdly, the sensor has a unique arrangement of electrodes/electrolytes. Solid electrolyte NASICON is deposited between interdigitated fingers and the auxiliary electrolyte Na2CO3/BaCO3 was deposited on the whole sensing area, forming greater length of three-point boundaries (electrode, solid electrolyte NASICON, and auxiliary electrolyte Na2CO3/BaCO3), which is beneficial for amperometric gas sensing. Interdigitated finger electrodes on a substrate were used as sensor structures before but only one or mixed sensing materials were deposited. The interdigitated finger electrode structure is deposited with two distinctive sensing materials forming maximum length three-point contacts. The sensor was tested continuously for at least three weeks at high temperatures showing its robust nature. The sensor structure could also be used with any other sensing system which requires two distinctive deposited materials in an electrochemical cell structure.
- Finally, the sensor is very easy to batch fabricate compared to the bulk-sized sensors and consumes much less power. This is specifically due to the non-standard photolithographic approach used.
- Using the process disclosed herein, sensors may be fabricated which have good sensitivity, selectivity, response time, and stability.
- The carbon dioxide sensor produced by the innovative technique described herein is applicable to the fire detection (including hidden fire), EVA applications, personal health monitoring, and environmental monitoring. The sensor and its electronics are integrated into a postage stamp sized system. The low cost due to the batch fabrication process and its compact size make it highly affordable and thus useable in a wide array of locations.
- A process for sensing carbon dioxide is accomplished which includes the following steps: applying a constant direct current voltage across the pair of electrodes. The electrodes are separated by an electrolyte material containing sodium, and the electrodes are located between a layer of alumina substrate and an electrolyte layer of auxiliary carbonate.
- Carbon dioxide is then reacted with the material containing sodium at the first three-point boundary. The first three point boundary is located at the joinder of one of the electrodes, the electrolyte material containing sodium, and the barium containing auxiliary electrolyte.
- An oxide of sodium is then reacted at a second three-point boundary. The second three-point boundary is located at the joinder of the other of the electrodes, the electrolyte material containing sodium, and a barium containing auxiliary electrolyte.
- Finally, the resulting current is measured and the change in current is correlated to the concentration of carbon dioxide.
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FIG. 1 is a cross-sectional schematic illustration of a prior art gas sensor. -
FIG. 1A is a cross-sectional schematic illustration of a prior art bulk gas sensor. -
FIG. 2 is a cross-sectional schematic illustration of interdigitated electrodes residing on a substrate forming part of the sensor of the present invention. -
FIG. 2A is a cross-sectional schematic view taken along thelines 2A-2A ofFIG. 2 . -
FIG. 2B is a cross-sectional view similar toFIG. 2A with first and second solid electrolytes over the substrate and the interdigitated electrodes. -
FIG. 3 is a cross-sectional schematic illustration of a substrate with photoresist spun onto the substrate. -
FIG. 3A is a cross-sectional schematic illustration of the substrate as illustrated inFIG. 3 with a photomask oriented thereover and ultraviolet light imidizing the unmasked portions of the photoresist. -
FIG. 3B is a cross-sectional schematic illustration of the substrate illustrated inFIG. 3A with the imidized photoresist developed and removed. -
FIG. 3C is a cross-sectional schematic illustration similar toFIG. 3B with a first layer of Titanium sputtered onto the substrate. -
FIG. 3D is an enlargement of a portion ofFIG. 3C illustrating the sputter deposition of the first layer of the Titanium over the substrate and the photoresist. -
FIG. 3E is a cross-sectional schematic illustration of a second layer of Platinum deposited above the first metallization layer of Titanium and the photoresist. -
FIG. 3F is a cross-sectional schematic illustration of the substrate with two interdigitated electrodes affixed to the substrate with the photoresist removed with acetone or other suitable solvent. -
FIG. 3G is a cross-sectional schematic illustration of photoresist spun over the interdigitated Titanium/Platinum electrodes and the substrate. -
FIG. 3H is a cross-sectional schematic illustration of a mask applied to the substrate and ultra violet light imidizing the unmasked portions of the photoresist. -
FIG. 3I is a cross-sectional schematic illustration of the substrate, interdigitated electrodes and photoresist left after the imidized photoresist has been developed and removed. -
FIG. 3J is a cross-sectional schematic illustration of the substrate, interdigitated electrodes with photoresist residing on a portion thereof with a layer of a first solid electrolyte deposited thereover -
FIG. 3K is a cross-sectional schematic illustration wherein the photoresist has been removed with acetone or other suitable solvent. -
FIG. 3L is a cross-sectional schematic illustration with a second solid electrolyte deposited thereover. -
FIG. 3M is a cross-sectional schematic illustration similar toFIG. 3L wherein the electrodes form tapered surfaces at the place of joinder with the electrolytes. -
FIG. 3N is an enlargement of a portion ofFIG. 3M . -
FIG. 3O is a cross-sectional schematic similar toFIG. 3J of another example of the application of a first solid applied over the substrate, interdigitated electrodes, photoresist using sputter deposition. -
FIG. 3P is a cross-sectional schematic illustration similar toFIG. 3K wherein the photoresist has been removed with acetone or other suitable solvent. -
FIG. 3Q is a cross-sectional schematic having a second solid electrolyte applied over the first solid electrolyte and the interdigitated electrodes. -
FIG. 3R is a cross-sectional schematic illustration wherein a third layer, a metal oxide layer, is applied over the second solid electrolyte. -
FIG. 4 is a schematic illustration similar toFIG. 3H with the mask slightly misaligned. -
FIG. 4A is a schematic illustration of the photoresist developed and removed with photoresist remaining over the interdigitated electrodes but not centrally located (misaligned). -
FIG. 4B is a schematic illustration similar toFIG. 4A with a first solid electrolyte deposited thereover. -
FIG. 4C is a schematic illustration with the photoresist lifted off through dissolution with acetone. -
FIG. 4D is a schematic illustration similar toFIG. 4C with a second solid electrolyte deposited over the first solid electrolyte and the interdigitated electrodes. -
FIG. 5 is a schematic illustration similar toFIG. 4 with the mask misaligned above the substrate, interdigitated electrodes, and photoresist indicating the application of ultraviolet light thereto. -
FIG. 5A is a schematic illustration similar toFIG. 5 with the imidized photoresist developed and removed leaving a gap filled with photoresist adjacent the electrodes. -
FIG. 5B is a schematic illustration with a first electrolyte over the substrate, interdigitated electrodes and photoresist. -
FIG. 5C is a schematic illustration similar toFIG. 5B with the photoresist lifted off. -
FIG. 5D is a schematic illustration similar toFIG. 5C with a second electrolyte over the first electrode and interdigitated electrodes. -
FIG. 6 is a schematic illustration of one example of process steps used to make the sensors. - The drawings will be better understood when reference is made to the Description of the Invention and Claims which follow hereinbelow.
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FIG. 2 is a cross-sectionalschematic illustration 200 ofinterdigitated electrodes substrate 206 forming part of the sensor of the present invention.Positive contact pad 201 is interconnected bylead 202 topositive bus 203 which is in turn interconnected with positive interdigitated positive electrodes (fingers) 204.Negative contact pad 207 is interconnected bylead 209 tonegative bus 209 which in turn is interconnected with negative interdigitated negative electrodes (fingers) 210.Electrodes Alumina substrate 206. TheAlumina substrate 206 is an insulator and is approximately 625 μm thick. - Still referring to
FIG. 2 ,reference numeral 205 indicates the gap betweenelectrodes electrode width 212W and width of the gap betweenelectrodes 211 are both around 30 μm. SecFIG. 2A . Contactpads conductor 221 tobattery 222 which is nominally at 1V DC.Amp meter 220 measures and records current in the circuit. -
FIG. 2A is a cross-sectionalschematic view 200A taken along thelines 2A-2A ofFIG. 2 .Gap 205 andelectrodes negative bus 209. In one example illustrated herein, thewidth 211 of thegap 205 is approximately 30 μm. Theelectrodes reference numeral 212W. A thin layer ofTitanium 213 is beneathPlatinum electrodes -
FIG. 2B is a cross-sectionalschematic view 200B similar toFIG. 2A with first and secondsolid electrolytes substrate 206 andinterdigitated electrodes Reference numeral 212 is also used to indicate the contour of the first electrolyte, for example, NASICON, LISICON, or β″-Alumina (beta prime-prime alumina in which when prepared as an electrolyte is complexed with a mobile ion selected from the group consisting of Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+, or Ba2+). The first electrolyte may be any number of solid electrolytes known for their conductivity performance. These electrolytes may include sodium or lithium as in the case of NASICON and LISICON, but the electrolyte is not limited to materials containing these elements and may include any number of elements including but not limited to Na, Li, K, Ag, H, Pb, Sr, or Ba. The secondsolid electrolyte 211 may include Sodium Carbonate (Na2CO3) or mixture of Sodium Carbonate (Na2CO3) and Barium Carbonate (BaCO3). Other electrolyte materials such as Li2CO3, K2CO3, Rb2CO3, SrCO3, Ag2CO3, PbCO3 and their mixtures among them or others may be used as a mixture in place of or in addition to Sodium Carbonate or mixture of Sodium Carbonate (Na2CO3) and Barium Carbonate in a second solid electrolyte layer. -
FIG. 3 is a cross-sectionalschematic illustration 300 of asubstrate 301 withphotoresist 302 spun onto thesubstrate 301.FIG. 3A is a cross-sectionalschematic illustration 300A of thesubstrate 301 as illustrated inFIG. 3 with aphotomask 399 oriented thereover andultraviolet light 305 imidizing the unmasked portions of the photoresist. Thephotomask 399 includesapertures 304 andopaque portions 303. -
FIG. 3B is a cross-sectionalschematic illustration 300B of thesubstrate 301 illustrated inFIG. 3A with the imidized photoresist developed and removed. The imidized portion of the photoresist is the portion which has been exposed to the ultraviolet light.Unimidized portions 302A of the photoresist remain on the substrate at this step. -
FIG. 3C is a cross-sectionalschematic illustration 300C similar toFIG. 3B with a first layer oftitanium 303 sputtered onto thesubstrate 301 and theunimidized photoresist 302A. -
FIG. 3D is anenlargement 300D of a portion ofFIG. 3C illustrating the sputter deposition of the first layer of theTitanium 303A over thesubstrate 301 and thephotoresist 302A.Titanium layer 303A is approximately 50 Å thick and forms a good bond to the Alumina substrate which is approximately 250-625 μm thick. Overall, the dimensions of the interdigitated area on the Alumina substrate is approximately 1.1 mm long, 1.0 mm wide and 250 or 625 μm thick in this embodiment of the invention.FIG. 3E is a cross-sectionalschematic illustration 300E of a second layer ofPlatinum 304A deposited above the first metallization layer ofTitanium 303A and theunimidized photoresist 302A. -
FIG. 3F is a cross-sectionalschematic illustration 300F of theAlumina substrate 301 with twointerdigitated electrodes 304A/303A affixed to the substrate with theunimidized photoresist 302A removed with acetone or some other suitable solvent. -
FIG. 3G is a cross-sectionalschematic illustration 300G ofphotoresist 355 spun over the interdigitated Titanium/Platinum electrodes 304A/303A and theAlumina substrate 301. Next,FIG. 3H is a cross-sectionalschematic illustration 300 H of aphotomask 399A spaced apart and in proximity to thesubstrate 301 withinterdigitated electrodes 304A/303A thereon andultra violet light 308 passing throughapertures 307 imidizing the unmasked (exposed) portions of the photoresist.Reference numeral 309 represents the width ofopaque portion 306 ofphotomask 399A. This width is specifically designed to be less than the width of 303A/304A. Once imidization of thephotoresist 355 is complete the imidized portions of the photoresist are subjected to developer and removed leaving the structure inFIG. 3I .FIG. 3I is a cross-sectionalschematic illustration 300I of the substrate, interdigitatedelectrodes 303A/304A andunimidized photoresist 355A left after the imidized photoresist has been developed and removed. - Next,
FIG. 3J is a cross-sectionalschematic illustration 300J of thesubstrate 301, interdigitatedelectrodes 304A/303A with unimidized photoresist residing on a portion thereof with a layer of firstsolid electrolyte 310, for example, NASICON, deposited thereover.Reference numeral 312 indicates the portion where the NASICON is raised slightly as its deposition by E-beam evaporation follows the contour of thesubstrate 301, theelectrodes 303A/304A and theunimidized photoresist 355A.NASICON 310 is applied at a thickness approximately equal to the thickness of theelectrodes 303A/304A. E-beam deposition is used here as an example of very controlled, exact deposition of component layers providing nearly vertical deposition geometries. Actual applications may vary. In the examples set forth herein (drawingFIGS. 3-3R ) one of theelectrodes 303A/304A is the working electrode and the other electrode is the reference electrode. As indicated in connection withFIGS. 2-2B above, there may be 8 to 10 pairs of working and reference electrodes which combine in an interdigitated fashion to generate enough current to produce sufficient sensitivity of the sensor. Other numbers of pairs may be used. The number of electrode pairs used in a sensor depends upon the application. -
FIG. 3K is a cross-sectionalschematic illustration 300K wherein theunimidized photoresist 355A has been removed with acetone, or some other suitable solvent leaving a contoured surface of NASICON and Platinum electrodes exposed. -
FIG. 3L is a cross-sectionalschematic illustration 300L with a secondsolid electrolyte 311 deposited over theNASICON 310 and theelectrodes 303A/304A. The second solid electrolyte may be Sodium Carbonate (Na2CO3), or a combination of Sodium Carbonate (Na2CO3) and Barium Carbonate (BaCO3) thereof in addition to other solid electrolytes and combinations which may include Li2CO3, K2CO3, Rb2CO3, SrCO3, Ag2CO3, and PbCO3. The second electrolyte layer with Na2CO3 and BaCO3 mixture performs a barrier function in that it keeps the sensor less vulnerable to humidity. Further, it selectively reacts with Carbon Dioxide. at the three point contacts NASICON 310, theelectrode 303A/304A, and the Sodium Carbonate or mixture of Sodium Carbonate and Barium Carbonate. As described elsewhere herein, each electrode joins the NASICON and the Sodium Carbonate or mixture of Sodium Carbonate and Barium Carbonate along a line where reduction and oxidation takes place. Current flow takes place through the NASICON.Reference numeral 369 represents inboard lines of three point contact of theelectrodes 303A/404A,NASICON 312, and secondelectrolyte Sodium Carbonate 311.Reference numeral 369A represents outboard lines of three point contact of theelectrodes 303A/404A,NASICON 312, and secondelectrolyte Sodium Carbonate 311. Depending on the process used for applying the NASICON, theoutboard lines 369A may not exist. Such is the case when the NASICON is applied by sputtering as set forth in FIG. 3“O” toFIG. 3R , inclusive. -
FIG. 3M is a cross-sectionalschematic illustration 300M similar toFIG. 3L wherein theNASICON surfaces 313 at the joinder of the Platinum electrodes and the Sodium Carbonate/Barium Carbonate (Na2CO3/BaCO3)layer 311. The tapered surfaces of NASICON are very thin which in effect creates several lines of multiple three point contacts which facilitates the reduction and oxidation processes set forth below.FIG. 3N is anenlargement 300N of a portion ofFIG. 3M and provides a better view of the taperedsurface 313, theelectrode 304A,first electrolyte 312 andsecondary electrolyte 311. It is believed that thetapered surface 313 plays an important role in that it provides a better amperometric surface as the NASICON layer in the taperedsurface 313 is very thin resulting in multiple lines where three (3) point contacts between the electrode, NASICON, and the auxiliary Sodium Carbonate/Barium Carbonate (Na2CO3/BaCO3) electrolyte layer exist. - Still referring to
FIG. 3M it is believed that thetapered surfaces 313 are created as a result of heat treatment of the NASICON film at temperature as high as 850° C. - The detection system depicted in
FIGS. 2-2B and 3-3R includes pairs of electrodes with constant voltage, V, applied across the multiple interdigitated electrodes. - The sensing mechanism of the amperometric CO2 sensors can be understood based on the reactions taking place at the working and reference electrode of each pair of electrodes. The following two electrode reactions may be considered:
-
Working Electrode 2Na++CO2+½O2+2e −→Na2CO3 -
Reference Electrode Na2O→2Na++½O2+2e − - Reduction occurs as the result of the reaction taking place at the working electrode where electrons are consumed. Oxidation occurs as the result of the reaction taking place at the reference electrode where electrons are released.
- The following overall reaction can then be considered to be:
-
Overall Reaction Na2O+CO2→Na2CO3 - Platinum is used as the preferred material for the electrode. However, electrodes made from other metals such as Palladium, Silver, Iridium, Gold, Ruthenium, Rhodium, Indium, or Osmium may also be used. In addition, non-porous or porous electrodes may be used
- The auxiliary electrolyte (Na2CO3 and/or BaCO3 and/or Li2CO3, K2CO3, Rb2CO3, SrCO3, Ag2CO3, PbCO3) is deposited homogeneously on the entire sensing area of the sensor, including both the working and reference electrodes. The deposition of an auxiliary carbonate electrolyte improves flow of the desired species within the electrolyte. At the working electrode, depleted concentration of sodium ions (Na+) can be recovered by the transfer of sodium ions (Na+) from NASICON through the three-phase boundary of the electrodes, NASICON electrolyte, and an auxiliary electrolyte layer. The sodium carbonate, Na2CO3, deposited at the working electrode can be transferred to the reference electrode through the Na2CO3 auxiliary carbonate electrolyte if temperatures are high enough, for example, 450-600° C.
- These mechanisms allow the sensor to measure CO2 but recover back to its initial state. The sensing mechanism has increased performance from the Na2CO3/BaCO3 auxiliary carbonate electrolyte layer being distributed across both the working and the reference electrodes at high operating temperatures in the 450-600° C. The eutectic mixture of Na2CO3/BaCO3 as the auxiliary carbonate electrolyte layer has a lower melting temperature enabling improved flow within the electrolyte at a reduced temperature range. The Na2CO3/BaCO3 auxiliary carbonate electrolyte can act as a diffusion barrier to prevent other species from reaching the electrode/electrolyte interface and interfering with the correlation of measured current with detection of the desired chemical species.
-
FIG. 3 “O” is a cross-sectional schematic 300 “O” similar toFIG. 3J and is another example of the application of a firstsolid electrolyte 310A applied over thesubstrate 301, interdigitatedelectrodes 303A/304A andunimidized photoresist 305A using sputter deposition. Sputter deposition ofNASICON 310A results in asurface 320 which is contoured and does not follow the underlying components. Sputter deposition is used here as an example of a less exact, more diffuse deposition of component layers providing more graded deposition geometries. Actual applications may vary. InFIG. 3J , the NASICON was applied in a manner which results in the NASICON applied so as to more closely follow the contour of the underlying structure.Reference numeral 325 is used to indicate the NASICON above theunimidized photoresist 305A. -
FIG. 3P is a cross-sectionalschematic illustration 300P similar toFIG. 3K wherein theunimidized photoresist 305A has been removed with acetone or other suitable solvent leavingNASICON 310A behind with acontoured surface 320. Next, the auxiliary Sodium Carbonate/Barium Carbonate (Na2CO3/BaCO3) electrolyte composition is applied.FIG. 3Q is a cross-sectional schematic 300Q having a secondsolid electrolyte 311 applied over the firstsolid electrolyte 310A and theinterdigitated electrodes 303A/304A. -
FIG. 3R is a cross-sectionalschematic illustration 300R wherein asolid metal oxide 330, SnO2, CuO, In2O3, and TiO2 and/or a combination thereof, is applied over the secondsolid electrolyte 311. It is preferred that these solid metal oxides be composed of nanoparticles. Use of this third layer of metal oxide provides enhanced performance of the sensor. This third layer of metal oxide is applied by drop deposition of SnO2 sol gel on top of the Na2CO3/BaCO3 and heat treat the sensor in the instant invention. It can also be deposited using e-beam evaporation or sputtering using a shadow mask which is the same as that for Na2CO3/BaCO3 deposition. The third layer of metal oxide improves the sensor signal greatly and also enables the carbon dioxide sensor to function a temperature range as low as 200° C. -
FIG. 4 is aschematic illustration 400 similar toFIG. 3H with thephotomask 499A slightly misaligned.FIGS. 4-4D are illustrative of the fault tolerance of the instant invention. It is this fault tolerance which enables successful production of the device. Theopaque portion 404 of themask 499A is indicated withreference numeral 404 and theultra violet light 405 is indicated withreference numeral 405. Still referring toFIG. 4 , it can be seen as is discussed elsewhere herein that if theopaque portions 404 of thephotomask 499A are the same width as theunderlying electrodes 402/406 and they are misaligned, then a faulty sensor will result. For this reason it is necessary that the opaque portions of the mask have a width less than 30 μm and preferably in the range of 15-20 μm. Misalignment of thephotomask 499A (serpentine) results in decentralizedunimidized photoresist 403A. However, because the opaque portions of the mask have a width significantly smaller than the width of the electrodes perfect alignment is not necessary. The NASICON lip shown byreference numeral 412 and the NASICON indicated byreference numeral 313 are locations where the NASICON may be thin and in effect leads to more reaction sites close to three boundary contacts. Also, this speeds up the manufacturing process because the technician does not have to be perfect in alignment. This is in contrast to standard industry practice which emphasizes increasing tight alignment and deposition procedures; the approach here is to allow and in fact take advantage of diffuse deposition and inexact alignments to improve the sensor response. In the example ofFIG. 4 ,reference numeral 406 is the thin layer (50 Å) of Titanium as previously described in connection withreference numeral 303A inFIGS. 3-3R .Reference numeral 402 is the relatively thicker layer (4000 Å) of Platinum as previously described in connection withreference numeral 304A inFIGS. 3-3R . -
FIG. 4A is aschematic illustration 400A of the photoresist developed and removed withunimidized photoresist 403A remaining over the interdigitated electrodes but not centrally located (misaligned).FIG. 4B is a schematic illustration similar 400B toFIG. 4A with a first solid electrolyte such as NASICON or LISICON 410 deposited thereover by e-beam evaporation.FIG. 4C is aschematic illustration 400C with the photoresist lifted off through dissolution with acetone or other suitable solvent. Raised portions of the NASICON orLISICON 410 are viewed well inFIG. 4C .FIG. 4D is aschematic illustration 400D similar toFIG. 4C with a second solid electrolyte 411 (Barium Carbonate and/or Sodium Carbonate) deposited over the first solid electrolyte and the interdigitated electrodes.FIG. 4D illustrates the potential problem with misalignment discussed elsewhere herein particularly in describingFIG. 1 which can not be produced because of the stack-up of manufacturing tolerances. -
FIG. 5 is aschematic illustration 500 similar toFIG. 4 with thephotomask 599A significantly misaligned above thesubstrate 501, interdigitatedelectrodes 503/504, andphotoresist 555 indicating the application ofultraviolet 508 light thereto.Reference numeral 503 is the Titanium layer of the electrode andreference numeral 504 is the Platinum layer of the electrode as described and similarly proportioned to the other examples given herein. Opaque portions of thephotomask 599A are indicated byreference numeral 506 and apertures in the mask are denoted byreference numeral 507. Misalignment should not occur when theopaque portions 506 of thephotomask 599A are substantially smaller than the width of the electrodes as described herein. However, the illustration ofFIG. 5 is being made to demonstrate that a problem is more likely to occur when the mask width equals the width of the electrodes as is the standard industry practice and direction. As the width of the opaque portion of the photomask increases or approximates the width of the electrode, the probability of misalignment increases. As was the case of the examples illustrated inFIGS. 3-3R and 4-4D, theopaque portion 506 of the mask protects the underlying photoresist and prevents ultraviolet light from reaching the photoresist resulting in a portion of the photoresist being unimidized 555A. -
FIG. 5A is aschematic illustration 500A similar toFIG. 5 with the imidizcd photoresist developed and removed leaving a gap filled withunimidized photoresist 555A appearing just to the left of theelectrodes 503/504. Thisphotoresist 555A which lies next to theelectrodes 503/504 will interfere with the proper function of the electrodes as it prevents the joinder of the electrodes, NASICON, and the Carbonate layer as illustrated inFIG. 5D . It also blocks the movement of Na ion in NASICON between reference electrode and working electrode, which is also a critical factor for sensor to work or function. -
FIG. 5B is aschematic illustration 500B with a first electrolyte NASICON deposited by e-beam deposition over the substrate, interdigitated electrodes, and photoresist. It will be noticed that theNASICON 510 does not abut theelectrodes 503/504 on the left hand side ofFIG. 5B .FIG. 5C is aschematic illustration 500C similar toFIG. 5B with theunimidized photoresist 555A lifted off with acetone.NASICON 510 includes a raisedportion 512.FIG. 5D is aschematic illustration 500D similar toFIG. 5C with asecond electrolyte 511 over the first electrolyte and theinterdigitated electrodes 503/504. - In describing the success or failure of the carbon dioxide sensor the electrodes are interdigitated and may involve 8-10 pairs of electrodes in order to sum enough current to provide the desired sensitivity. Currents ranging from nano to micro amps are generated by the application of 1.0 Volts or higher dc across the sensor electrode bus as illustrated schematically in
FIG. 2 . - The fabrication of carbon dioxide sensors includes three steps: 1) Deposition of platinum interdigitated finger electrodes on Alumina substrates; 2) Deposition of solid electrolyte called NASICON (Na3Zr2Si2PO12) or LISICON (Li3Zr2Si2PO12) between the finger electrodes; and 3) Deposition of auxiliary electrolytes sodium carbonate and/or barium carbonate (Na2CO3/BaCO3, 1:1.7 in molar ratio for the combination) on the upper surfaces of the electrodes.
- The Platinum interdigitated finger electrodes were deposited as follows: Alumina substrates (250 μm or 625 μm in thickness) were patterned with photoresist and an interdigitated finger electrode photomask. A 50 Å layer of Titanium and a 4000 Å layer of Platinum were deposited on the Alumina substrate by sputter deposition. After development and removal, the substrates were then patterned again to cover the top of interdigitated finger electrodes with photoresist.
- Deposition of the NASICON solid electrolyte between the finger electrodes and the Na2CO3/BaCO3 was performed as follows. The solid electrolyte NASICON was deposited by e-beam evaporation or sputtering. A liftoff process which uses acetone to remove unimidized photoresist was conducted to remove NASICON on the upper surfaces of the electrodes resulting in the NASICON mainly staying between the interdigitated finger electrodes and exposing most of the electrode surface. The substrate was heated in an oven at 850° C. for 2 hours. Na2CO3/BaCO3 (1:1.7 in molar ratio) was then deposited on the upper surfaces of the electrodes and the NASICON surface by sputtering using a shadow mask. The use of shadow mask in this step is to prevent the Na ion in deposited NASION being washed away by photolithograph process, which is not obvious and not a typical practice of standard microfabrication process. The substrates were heated in an oven at 686° C. for 10 minutes and 710° C. for 20 minutes. Different concentrations of carbon dioxide gases were tested by the sensors at temperatures ranging from 450-600° C. The sensor was tested by applying a voltage to the electrodes and measuring the resulting current. A linear response to carbon dioxide concentrations between 1% to 4% was achieved. Linear responses of the natural logarithmic of carbon dioxide concentrations between 0.02% to 1% was achieved.
- The resulting miniature CO2 sensor can be integrated into a sensor array with other sensors and electronics, power, and telemetry on a stamp sized package. Like a postage stamp, the complete system (“lick and stick” technology) can be placed at a number of locations including some hidden areas to give a full-field understanding of what is occurring in an environment. The same sensor structure could also be applied to develop NOx or SOx with the corresponding auxiliary electrolytes NaNO2 and NaNO3, or Na2CO3 and Na2SO4.
-
FIG. 6 is aschematic illustration 600 of one example of process steps used to make the sensors. The process steps are described below and have been described hereinabove. - First, an Alumina substrate is coated with
photoresist 302. Aphotomask 399 is then applied selectively 602 imidizing ultra violet light using an interdigitated finger electrode photomask and developing and removing the imidized photoresist. Next, sputtering 603, a 50 Å layer ofTitanium 303A onto theAlumina 301 substrate andunimidized photoresist 302A is performed. The sputtering of the Titanium is followed by sputtering 604 a 4000 Å layer of Platinum onto the Titanium. - The
unimidized photoresist 302A is lifted off 605 with acetone or other solvent to remove theunimidized photoresist 302A as well as theTitanium 303A andPlatinum 304A thereover forming electrodes on the Alumina substrate. Another layer of photoresist is then applied 606 to theAlumina substrate 301 andelectrodes 303A/304A. The photoresist is selectively imidized 607 by applyingimidizing ultraviolet light 308 using an interdigitatedfinger electrode photomask 399A and then developing and removing the imidized photoresist. Electron beam evaporation or sputtering 608 of NASICON over the Alumina substrate, the electrodes and the unimidized photoresist follows. Lifting off 609 the unimidized photoresist and NASICON thereover with acetone or other solvent is then performed so as to enable the deposition ofsecondary electrolyte 610 using a shadow mask over the NASICON and the electrodes. The step 620 of depositing a metal oxide may be accomplished by drop deposition of metal oxide sol gel or by sputtering/e-beam deposition using a shadow mask -
- 100—schematic of prior art device
- 100A—schematic of prior art device
- 101—NASICON
- 102, 110—Sodium Carbonate/Barium Carbonate (Na2CO3/BaCO3)
- 103—Platinum paste
- 104—sensing electrode
- 105—reference electrode
- 106—quartz glass tube
- 107—Alumina
- 108—interdigitated Platinum metal electrodes
- 109—NASICON
- 200—schematic of interdigitated metal electrodes
- 200A—schematic view of
section 2A-2A - 200B—view of
section 2A-2A with NASICON and Barium Carbonate/Sodium Carbonate thereover - 201—contact pad
- 202—lead
- 203—positive bus
- 204—interdigitated positive metal electrodes
- 205—gap between electrodes
- 206, 301—substrate (insulator)
- 207—contact pad
- 208—lead
- 209—negative bus
- 210—interdigitated negative metal electrodes
- 211—width of gap between electrodes
- 212—contour of NASICON after liftoff of photoresist
- 212W—width of electrode
- 213—thin layer of Titanium metal
- 220—amp meter
- 221—conductor
- 222—battery or electrical potential
- 300—schematic view of substrate with photoresist spun thereovcr
- 300A—schematic view of mask over substrate with photoresist spun thereover
- 300B—schematic view of substrate with imidized photoresist developed and removed
- 300C—schematic view of substrate and unimidized photoresist with a thin layer of Titanium thereover
- 300D—enlargement of a portion of
FIG. 3C - 300E—schematic view of second metal layer of Platinum applied over the first metal layer of Titanium
- 300E—schematic view of interdigitated electrodes and substrate after liftoff of photoresist
- 300G—schematic view of photoresist spun over the interdigitated electrodes and substrate
- 300H—schematic view of mask placed over photoresist
- 300I—schematic view of substrate, electrodes and unimidized photoresist after the imidized photoresist has been developed and removed
- 300J—schematic view of NASICON deposited by c-beam evaporation over the substrate, electrodes, and photoresist
- 300K—schematic view with the photoresist lifted off
- 300L—schematic view similar to
FIG. 3K with a second electrolyte deposited over the NASICON and electrodes - 300M—schematic view of another example of the invention wherein multiple three point contacts occur between the NASICON, the electrodes and the second electrolyte
- 300N—schematic view of an enlargement of a portion of
FIG. 3M - 300 “O”—schematic view similar to
FIG. 3J wherein NASICON is sputtered over the substrate, electrode and the photoresist - 300P—schematic view similar to
FIG. 3K wherein the unimidized photoresist has been lifted off - 300Q—schematic view a second electrolyte sputtered, using a shadow mask, over the NASICON and electrodes
- 300R—schematic view of a third electrolyte sputtered, using a shadow mask, over the second electrolyte
- 301, 401, 501—Alumina, substrate (non-conductive)
- 302, 355, 403, 555—photoresist
- 302A, 305A, 355A, 403A, 555A—unimidized photoresist
- 305, 308, 405, 508—UV light
- 303, 306, 404, 506—opaques portions of photomask
- 303A, 406, 503—thin first Titanium metal layer
- 304, 307, 507—aperture in mask
- 304A, 402, 504—second Platinum metal layer
- 305, 308—ultraviolet light
- 309—width of opaque portion of mask
- 310, 410, 510—NASICON, first solid electrolyte, e-beam deposited
- 310A—NASICON, first solid electrolyte, sputter deposited
- 311, 411, 511—second solid electrolyte Sodium Carbonate/Barium Carbonate (Na2CO3/BaCO3)
- 312, 412, 512—raised portion of NASICON
- 313—extended three-point contact
- 320—contour of sputter deposited NASICON
- 325—sputter deposited NASICON
- 330—layer of metal oxide, SnO2, CuO, and TiO2.
- 369—inboard three point electrical contact
- 369A—outboard three point electrical contact
- 399, 399A, 499A, 599A—photomask
- 400—schematic view similar to
FIG. 3H with the mask slightly misaligned although still over the electrodes electrolyte is sputter deposited over the NASICON and electrodes - 400A—schematic view similar to
FIG. 3I with the imidized photoresist developed and removed - 400B—schematic view similar to
FIG. 3J with NASICON deposited by e-beam evaporation over the substrate, electrodes and unimidized photoresist - 400C—schematic view similar to
FIG. 3K with the unimidized Photoresist lifted off - 400D—schematic view similar to
FIG. 3L wherein a second - 500—schematic view similar to
FIG. 4 with the mask misaligned - 500A—schematic view similar to
FIG. 4A with the unimidized photoresist extending beyond the electrodes - 500B—schematic view similar to
FIG. 4B with NASICON deposited over the substrate, unimidized photoresist and electrodes. - 500C—schematic view similar to
FIG. 4C with the unimidized photoresist developed and removed. - 500D—schematic view similar to
FIG. 4D with a second solid electrolyte deposited over the NASICON, unimidized photoresist and metal electrodes - 600—one example of process steps used to fabricate the sensor
- 601—applying photoresist on Alumina substrate
- 602—applying, selectively, imidizing ultra violet light using an interdigitated finger electrode photomask, developing and removing the imidized photoresist
- 603—sputtering a 50 Å layer of Titanium onto the Alumina substrate and unimidized photoresist
- 604—sputtering a 4000 Å layer of Platinum onto the Titanium
- 605—lifting off with acetone or other solvent the unimidized photoresist, Titanium and Platinum thereover forming electrodes on the Alumina substrate
- 606—applying photoresist to the Alumina substrate and electrodes
- 607—applying, selectively, imidizing ultraviolet light using an interdigitated finger electrode photomask, developing and removing the imidized photoresist
- 608—electron beam sputtering of NASICON over the Alumina substrate, the electrodes and the unimidized photoresist
- 609—lifting off with acetone or other solvent the unimidized photoresist and NASICON thereover
- 610—depositing secondary electrolyte using a shadow mask over the NASICON and the electrodes
- 620—depositing metal oxide using metal oxide sol gel or by sputtering /e-beam deposition
- The invention has been set forth by way of example. Those skilled in the art will recognize that changes may be made to the invention without departing from the spirit and the scope of the claims which follow herein below.
Claims (15)
1-23. (canceled)
24. A process for fabricating a carbon dioxide sensor including the following steps:
depositing platinum interdigitated finger electrodes on an Alumina substrate, said electrodes having an upper surface;
depositing solid electrolyte NASICON between said finger electrodes and on a portion of said upper surface of said interdigitated finger electrodes; and,
depositing an auxiliary electrolyte material comprising by a compound selected from the group consisting of Sodium Carbonate, Barium Carbonate, and mixtures thereof on top of the electrodes.
25. A process for fabricating a carbon dioxide sensor as claimed in claim 24 wherein said step of depositing interdigitated finger electrodes includes the following steps:
patterning said alumina substrate with photoresist and an interdigitated finger electrode photomask;
developing and removing imidized photoresist;
depositing a layer of 50 A of material selected from the group consisting of Platinum Oxide and Titanium on said Alumina substrate;
depositing a layer of 4000 A of Platinum on said Alumina substrate;
patterning said alumina substrates again to cover said upper surface of said interdigitated finger electrodes and said Alumina substrate with photoresist; and,
developing and removing imidized photoresist.
26. A process for fabricating a carbon dioxide sensor as claimed in claim 25 wherein said step of depositing solid electrolyte NASICON between said finger electrodes and on a portion of said upper surface of said interdigitated finger electrodes includes the following steps:
depositing solid NASICON over said Alumina substrate, said interdigitated finger electrodes and the unimidized photoresist;
removing said unimidized photoresist with acetone to remove NASICON from said upper surface of said interdigitated finger electrodes exposing said upper surfaces of said interdigitated electrodes; and,
heating said substrate in an oven at a temperature range of 850° C.
27. A process for fabricating carbon dioxide sensor as claimed in claim 26 wherein the step of the depositing an auxiliary electrolyte material comprising sodium carbonate and barium carbonate on top of the electrodes includes the steps of:
sputtering solid auxiliary electrolyte layer between said interdigitated finger electrodes and on said NASICON using a shadow mask; and,
heating said substrates in an oven at approximately 686° C. for 10 minutes and at approximately 710° C. for 20 minutes.
28. A process for fabricating a carbon dioxide sensor as claimed in claim 26 wherein said step of depositing solid NASICON over said Alumina substrate, said interdigitated finger electrodes and the unimidized photoresist said NASICON solid electrolyte is deposited by e-beam evaporation.
29. A process for fabricating a carbon dioxide sensor as claimed in claim 26 wherein said step of depositing solid NASICON over said Alumina substrate, said interdigitated finger electrodes and the unimidized photoresist NASICON solid electrolyte is deposited by sputtering.
30. A process for fabricating a carbon dioxide sensor as claimed in claim 24 wherein said step of depositing solid electrolyte NASICON between said finger electrodes and on a portion of said upper surface of said interdigitated finger electrodes is performed by e-beam evaporation.
31. A process for fabricating a carbon dioxide sensor as claimed in claim 24 wherein said step of depositing solid electrolyte NASICON between said finger electrodes and on a portion of said upper surface of said interdigitated finger electrodes is performed by sputtering.
32. (canceled)
33. (canceled)
34. A process for fabricating a carbon dioxide sensor including the following steps:
applying photoresist on Alumina substrate;
applying, selectively, imidizing ultraviolet light using an interdigitated finger electrode photomask, developing, and removing the immidized photoresist;
sputtering a 50 A layer of Titanium to the Alumina substrate and unimidized photoresist;
sputtering a 4000 A layer of Platinum to the Titanium;
lifting off with solvent the unimidized photoresist, titanium, and platinum thereover forming electrodes on the alumina substrate;
applying photoresist to the Alumina substrate and electrodes;
applying, selectively, imidizing ultraviolet, light using an interdigitated finger electrode photomask, developing, and removing the imidized photoresist;
depositing NASICON over the Alumina substrate, the electrodes, and the unimidized photoresist;
lifting off with solvent the unimidized photoresist and NASICON thereover; and
depositing secondary electrolyte using a shadow mask over the NASICON and the electrodes.
35. A process for fabricating a carbon dioxide sensor as claimed in claim 34 wherein said step of depositing NASICON over the Alumina substrate, the electrodes, and the unimidized photoresist, is performed using electron beam evaporation.
36. A process for fabricating a carbon dioxide sensor as claimed in claim 34 wherein said step of depositing NASICON over the Alumina substrate, the electrodes, and the unimidized photoresist is performed using sputter deposition.
37. A process for fabricating a carbon dioxide sensor gas sensor as set forth in claim 24 using microfabrication processing with fault tolerant processing and maximization of three boundary contacts.
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US20170343503A1 (en) * | 2014-12-22 | 2017-11-30 | Robert Bosch Gmbh | Sensor for Measuring the Carbon Dioxide Concentration in a Gas Mixture, and Method for Manufacture Thereof |
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US8702962B1 (en) * | 2007-05-25 | 2014-04-22 | The United States Of America As Represented By The Administrator Of National Aeronautics And Space Administration | Carbon dioxide gas sensors and method of manufacturing and using same |
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