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 + , Pb 2+ , Sr 2+  or Ba 2+ ). 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 + , Pb 2+ , Sr 2+  or Ba 2+  ions or combinations thereof.

Description:
ORIGIN OF THE INVENTION 
       [0001]    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. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention is in the field of carbon dioxide gas sensors. 
       BACKGROUND OF THE INVENTION 
       [0003]    The detection of CO 2  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 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. In recent years, there has been a significant effort to develop bulk and miniaturized electrochemical CO 2  sensors. Compared to bulk material and thick film solid electrolyte CO 2  sensors, miniaturized sensors fabricated by microfabrication techniques generally have the advantages of small size, light weight, low power consumption, and batch fabrication. 
         [0004]    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. 
         [0005]    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. 
         [0006]      FIG. 1A  is a cross-sectional schematic illustration  100  of a prior art bulk carbon dioxide gas sensor. Referring to  FIG. 1A , 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 . By Sodium Carbonate and/or Barium Carbonate (Na 2 CO 3 /BaCO 3 ), it is meant that either Sodium Carbonate (Na 2 CO 3 ) or Barium Carbonate (BaCO 3 ), or their mixtures may be used. The sensor is supported by quartz glass tubes (insulators)  106  for reference gases. 
         [0007]      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. 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 + , Pb 2+ , Sr 2+  or Ba 2+ ). By 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. 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 in  FIG. 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 (Na 2 CO 3 /BaCO 3 ). Statistically, given manufacturing tolerances 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. Simply put, 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. 
         [0008]    Previously, most solid electrolyte CO 2  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 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. 
         [0009]    Most often (in the prior art) two sensing materials were used in a solid electrolyte CO 2  sensor structure. In the effort to miniaturize a CO 2  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. 
         [0010]    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. 
       SUMMARY OF THE INVENTION 
       [0011]    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] (natural logarithm of the carbon dioxide concentration) shows a linear relationship from 0.02% to 1% CO 2 . This linear relationship allows for easy sensor calibration. Linear responses were achieved for CO 2  concentrations from 1% to 4% and to the logarithm of the CO 2  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 CO 2  sensor has the advantage of being simple to batch fabricate, small in size, low in power consumption, easy to use, and fast response time. 
         [0012]    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. 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. 
         [0013]    The development of miniature electrochemical sensors based on solid electrolytes NASICON (Na 3 Zr 2 Si 2 PO 12 ) and Na 2 CO 3 /BaCO 3  for CO 2  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 (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. 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 NaNO 2  or NaNO 3 . 
         [0014]    An amperometric circuit is used to detect CO 2 . The detection system includes pairs of electrodes with constant voltage, V, applied across the electrodes. 
         [0015]    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: 
         [0000]      Working Electrode 2Na + +CO 2 +½O 2 +2 e   − →Na 2 CO 3  
 
         [0000]      Reference Electrode Na 2 O→2Na + +½O 2 +2 e   − 
 
         [0000]    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:
 
         [0000]      Overall Reaction Na 2 O+CO 2 →Na 2 CO 3  
 
         [0016]    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 
         [0017]    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. 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, 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. 
         [0018]    These mechanisms allow the sensor to measure CO 2  but recover back to its initial state. 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. 
         [0019]    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 CO 2  sensor are used. 
         [0020]    A unique amperometric CO 2  sensor is produced using a non-standard approach as disclosed herein and has the following attributes: 
         [0021]    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 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. 
         [0022]    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. 
         [0023]    Thirdly, the sensor 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. 
         [0024]    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. 
         [0025]    Using the process disclosed herein, sensors may be fabricated which have good sensitivity, selectivity, response time, and stability. 
         [0026]    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. 
         [0027]    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. 
         [0028]    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. 
         [0029]    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. 
         [0030]    Finally, the resulting current is measured and the change in current is correlated to the concentration of carbon dioxide. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a cross-sectional schematic illustration of a prior art gas sensor. 
           [0032]      FIG. 1A  is a cross-sectional schematic illustration of a prior art bulk gas sensor. 
           [0033]      FIG. 2  is a cross-sectional schematic illustration of interdigitated electrodes residing on a substrate forming part of the sensor of the present invention. 
           [0034]      FIG. 2A  is a cross-sectional schematic view taken along the lines  2 A- 2 A of  FIG. 2 . 
           [0035]      FIG. 2B  is a cross-sectional view similar to  FIG. 2A  with first and second solid electrolytes over the substrate and the interdigitated electrodes. 
           [0036]      FIG. 3  is a cross-sectional schematic illustration of a substrate with photoresist spun onto the substrate. 
           [0037]      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. 
           [0038]      FIG. 3B  is a cross-sectional schematic illustration of the substrate illustrated in  FIG. 3A  with the imidized photoresist developed and removed. 
           [0039]      FIG. 3C  is a cross-sectional schematic illustration similar to  FIG. 3B  with a first layer of Titanium sputtered onto the substrate. 
           [0040]      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. 
           [0041]      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. 
           [0042]      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. 
           [0043]      FIG. 3G  is a cross-sectional schematic illustration of photoresist spun over the interdigitated Titanium/Platinum electrodes and the substrate. 
           [0044]      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. 
           [0045]      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. 
           [0046]      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 
           [0047]      FIG. 3K  is a cross-sectional schematic illustration wherein the photoresist has been removed with acetone or other suitable solvent. 
           [0048]      FIG. 3L  is a cross-sectional schematic illustration with a second solid electrolyte deposited thereover. 
           [0049]      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. 
           [0050]      FIG. 3N  is an enlargement of a portion of  FIG. 3M . 
           [0051]      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. 
           [0052]      FIG. 3P  is a cross-sectional schematic illustration similar to  FIG. 3K  wherein the photoresist has been removed with acetone or other suitable solvent. 
           [0053]      FIG. 3Q  is a cross-sectional schematic having a second solid electrolyte applied over the first solid electrolyte and the interdigitated electrodes. 
           [0054]      FIG. 3R  is a cross-sectional schematic illustration wherein a third layer, a metal oxide layer, is applied over the second solid electrolyte. 
           [0055]      FIG. 4  is a schematic illustration similar to  FIG. 3H  with the mask slightly misaligned. 
           [0056]      FIG. 4A  is a schematic illustration of the photoresist developed and removed with photoresist remaining over the interdigitated electrodes but not centrally located (misaligned). 
           [0057]      FIG. 4B  is a schematic illustration similar to  FIG. 4A  with a first solid electrolyte deposited thereover. 
           [0058]      FIG. 4C  is a schematic illustration with the photoresist lifted off through dissolution with acetone. 
           [0059]      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. 
           [0060]      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. 
           [0061]      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. 
           [0062]      FIG. 5B  is a schematic illustration with a first electrolyte over the substrate, interdigitated electrodes and photoresist. 
           [0063]      FIG. 5C  is a schematic illustration similar to  FIG. 5B  with the photoresist lifted off. 
           [0064]      FIG. 5D  is a schematic illustration similar to  FIG. 5C  with a second electrolyte over the first electrode and interdigitated electrodes. 
           [0065]      FIG. 6  is a schematic illustration of one example of process steps used to make the sensors. 
       
    
    
       [0066]    The drawings will be better understood when reference is made to the Description of the Invention and Claims which follow hereinbelow. 
       DESCRIPTION OF THE INVENTION 
       [0067]      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 μm thick. 
         [0068]    Still referring to  FIG. 2 , 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 μm. Sec  FIG. 2A . 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. 
         [0069]      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 . In one example illustrated herein, the width  211  of the gap  205  is approximately 30 μm. The electrodes  204 ,  210  have a width of approximately 30 μm as indicated by reference numeral  212 W. A thin layer of Titanium  213  is beneath Platinum electrodes  204 ,  210 . Alternatively, the electrode material may comprise a thin layer of PtO x  followed by a relatively thick layer of Platinum. 
         [0070]      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 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 + , Pb 2+ , Sr 2+ , or Ba 2+ ). 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 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 ). Other 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. 
         [0071]      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 . 
         [0072]      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. 
         [0073]      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. 
         [0074]      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 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-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. 
         [0075]      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. 
         [0076]      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 . Next,  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. Once imidization of the photoresist  355  is complete the imidized portions of the photoresist are subjected to developer and removed leaving the structure in  FIG. 3I .  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. 
         [0077]    Next,  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. In the examples set forth herein (drawing  FIGS. 3-3R ) one of the electrodes  303 A/ 304 A is the working electrode and the other electrode is the reference electrode. As indicated in connection with  FIGS. 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. 
         [0078]      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. 
         [0079]      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. Further, it selectively reacts with Carbon Dioxide. at the three point contacts NASICON  310 , the electrode  303 A/ 304 A, 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 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 . Depending on the process used for applying the NASICON, 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. 
         [0080]      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 . It is believed that 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. 
         [0081]    Still referring to  FIG. 3M  it is believed that the tapered surfaces  313  are created as a result of heat treatment of the NASICON film at temperature as high as 850° C. 
         [0082]    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. 
         [0083]    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: 
         [0000]      Working Electrode 2Na + +CO 2 +½O 2 +2 e   − →Na 2 CO 3  
 
         [0000]      Reference Electrode Na 2 O→2Na + +½O 2 +2 e   − 
 
         [0084]    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. 
         [0085]    The following overall reaction can then be considered to be: 
         [0000]      Overall Reaction Na 2 O+CO 2 →Na 2 CO 3  
 
         [0086]    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 
         [0087]    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. 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, 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. 
         [0088]    These mechanisms allow the sensor to measure CO 2  but recover back to its initial state. 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. 
         [0089]      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. In  FIG. 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 the unimidized photoresist  305 A. 
         [0090]      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. 
         [0091]      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. 
         [0092]      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. 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 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. 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 of  FIG. 4 , reference numeral  406  is the thin layer (50 Å) 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 Å) of Platinum as previously described in connection with reference numeral  304 A in  FIGS. 3-3R . 
         [0093]      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. 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. 
         [0094]      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. 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 in  FIGS. 3-3R  and  4 - 4 D, 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. 
         [0095]      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. 
         [0096]      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 . 
         [0097]    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 . 
         [0098]    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. 
         [0099]    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. 
         [0100]    Deposition of the 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. 
         [0101]    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. 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 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 . 
         [0102]      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. 
         [0103]    First, 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. Next, sputtering  603 , a 50 Å 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 Å layer of Platinum onto the Titanium. 
         [0104]    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. Lifting off  609  the unimidized photoresist and NASICON thereover with acetone or other solvent is then performed so as to enable the deposition of secondary 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 
       REFERENCE NUMERALS 
       [0000]    
       
           100 —schematic of prior art device 
           100 A—schematic of prior art device 
           101 —NASICON 
           102 ,  110 —Sodium Carbonate/Barium Carbonate (Na 2 CO 3 /BaCO 3 ) 
           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 
           200 A—schematic view of section  2 A- 2 A 
           200 B—view of section  2 A- 2 A 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 
           212 W—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 
           300 A—schematic view of mask over substrate with photoresist spun thereover 
           300 B—schematic view of substrate with imidized photoresist developed and removed 
           300 C—schematic view of substrate and unimidized photoresist with a thin layer of Titanium thereover 
           300 D—enlargement of a portion of  FIG. 3C   
           300 E—schematic view of second metal layer of Platinum applied over the first metal layer of Titanium 
           300 E—schematic view of interdigitated electrodes and substrate after liftoff of photoresist 
           300 G—schematic view of photoresist spun over the interdigitated electrodes and substrate 
           300 H—schematic view of mask placed over photoresist 
           300 I—schematic view of substrate, electrodes and unimidized photoresist after the imidized photoresist has been developed and removed 
           300 J—schematic view of NASICON deposited by c-beam evaporation over the substrate, electrodes, and photoresist 
           300 K—schematic view with the photoresist lifted off 
           300 L—schematic view similar to  FIG. 3K  with a second electrolyte deposited over the NASICON and electrodes 
           300 M—schematic view of another example of the invention wherein multiple three point contacts occur between the NASICON, the electrodes and the second electrolyte 
           300 N—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 
           300 P—schematic view similar to  FIG. 3K  wherein the unimidized photoresist has been lifted off 
           300 Q—schematic view a second electrolyte sputtered, using a shadow mask, over the NASICON and electrodes 
           300 R—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 
           302 A,  305 A,  355 A,  403 A,  555 A—unimidized photoresist 
           305 ,  308 ,  405 ,  508 —UV light 
           303 ,  306 ,  404 ,  506 —opaques portions of photomask 
           303 A,  406 ,  503 —thin first Titanium metal layer 
           304 ,  307 ,  507 —aperture in mask 
           304 A,  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 
           310 A—NASICON, first solid electrolyte, sputter deposited 
           311 ,  411 ,  511 —second solid electrolyte Sodium Carbonate/Barium Carbonate (Na 2 CO 3 /BaCO 3 ) 
           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, SnO 2 , CuO, and TiO 2 . 
           369 —inboard three point electrical contact 
           369 A—outboard three point electrical contact 
           399 ,  399 A,  499 A,  599 A—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 
           400 A—schematic view similar to  FIG. 3I  with the imidized photoresist developed and removed 
           400 B—schematic view similar to  FIG. 3J  with NASICON deposited by e-beam evaporation over the substrate, electrodes and unimidized photoresist 
           400 C—schematic view similar to  FIG. 3K  with the unimidized Photoresist lifted off 
           400 D—schematic view similar to  FIG. 3L  wherein a second 
           500 —schematic view similar to  FIG. 4  with the mask misaligned 
           500 A—schematic view similar to  FIG. 4A  with the unimidized photoresist extending beyond the electrodes 
           500 B—schematic view similar to  FIG. 4B  with NASICON deposited over the substrate, unimidized photoresist and electrodes. 
           500 C—schematic view similar to  FIG. 4C  with the unimidized photoresist developed and removed. 
           500 D—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 
       
     
         [0198]    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.