Abstract:
Miniature resistive gas detectors incorporate thin films that can selectively identify specific gases when heated to certain characteristic temperatures. A solid state gas sensor module is disclosed that includes a gas sensor, a heater, and a temperature sensor, stacked over an insulating recess. The insulating recess is partially filled with a support material that provides structural integrity. The solid state gas sensor module can be integrated on top of an ASIC on a common substrate. With sufficient thermal insulation, such a gas detector can be provided as a low-power component of mobile electronic devices such as smart phones. A method of operating a multi-sensor array allows detection of relative concentrations of different gas species by either using dedicated sensors, or by thermally tuning the sensors to monitor different gas species.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure generally relates to the field of microelectronic sensors and, in particular, gas sensors integrated with circuitry on a semiconductor substrate. 
         [0003]    2. Description of the Related Art 
         [0004]    Miniature solid state gas sensors integrated with microelectronics allow construction of an “electronic nose” that can selectively detect the presence of, for example, toxic substances such as carbon monoxide (CO), or vapor associated with controlled substances such as ethanol. 
         [0005]    It is well known that thin films made of certain metal materials, for example, semiconductor metal oxides (SMOs), experience a change in resistivity when they are exposed to certain gases at certain temperatures, as described in U.S. Pat. No. 4,485,667. Typically, SMO sensors operate at temperatures between 200 and 500 C. One such example of an SMO material is tin oxide (SnO 2 ) which, when heated to 380 C-400 C and then exposed to methane gas, experiences a chemoresistive reaction (1) that produces free electrons, thereby altering the resistivity of the tin oxide film: 
         [0000]      SnO 2 +CH 4 →CO 2 +H 2 O+e −   (1)
 
         [0000]    SMO sensors based on SnO 2  or nickel oxide (NiO) are currently manufactured and sold by companies such as Hanwei Electronics Co., Ltd. of Zhengzhou in the Henan province of China, and Figaro Engineering, Inc. of Glenview, Ill. 
         [0006]    By integrating a tin oxide thin film with an integrated circuit and a heating element, it is possible to construct an electronic gas detector suitable for use in a home or industrial environment, for example, or a portable breathalizer for use by law enforcement officers. Integration of chemical sensors with microelectronics on a common semiconductor die is described in U.S. Patent Application Publications 2012/0168882 and 2012/0171713, to Cherian and LeNeel, a co-inventor of this patent application, which publications are hereby incorporated by reference in their entirety. 
         [0007]    One problem that arises in designing SMO thin film gas sensors is that the associated heaters tend to consume large amounts of electrical power to heat the SMO thin films to operating temperatures in the range of about 100 C-500 C, so the heaters tend to quickly drain the battery of the portable sensor device. 
         [0008]    Another challenge is that the SMO sensors may have difficulty distinguishing between two gases. For example, carbon monoxide (CO) sensors also tend to be sensitive to hydrogen gas (H 2 ). Thus, it is desirable to operate the sensor only within a small temperature range around the temperature at which the sensor has peak sensitivity to the particular gas of interest. Furthermore, instead of heating one sensor to different temperatures to detect different gases, it would be advantageous to dedicate specific sensors to specific gases. Alternatively, it would be advantageous to assemble multiple data points from a plurality of sensors that are all tuned to a particular gas, to increase specificity and to obtain more precise measurements. 
         [0009]    Another challenge lies in providing thermal insulation for the gas sensors so that neighboring devices are not heat-damaged by such extreme temperatures. In some environments, heating the SMO sensor to temperatures in the range of 100 C-400 C can pose a safety risk—for example, if there exists a sufficient concentration of an ambient gas for which the combustion temperature is within the range of the operating temperature of the sensor. Some existing SMO sensor products include insulating layers between the heater and the substrate. U.S. Patent Application Publication 2009/0243003, entitled “Manufacturing Method of a Gas Sensor Integrated on a Semiconductor Substrate,” assigned to the same assignee as this patent application, addresses thermal insulation by forming an insulating cavity buried in the semiconductor substrate, the cavity being filled with air. However, although air is an effective thermal insulator, a substrate having an air cavity is generally structurally unstable, and prone to collapse. Thus, the depth of such an air cavity may be structurally limited. 
       BRIEF SUMMARY 
       [0010]    A miniature resistive SMO gas sensor module as described herein includes an SMO gas sensor, a resistive heater, and a resistive temperature sensor integrated with an application specific integrated circuit (ASIC). The gas sensor, heater, and temperature sensor are stacked adjacent to an insulating cavity that is structurally supported by pillars. In one example, the pillars are made of polyimide. The resistive heater is located next to the SMO gas sensor to conserve battery power. The resistive temperature sensor is placed in close proximity to the SMO gas sensor to ensure accurate temperature measurements. Electrical tests using pulsed signals indicate that use of the disclosed heat confinement structure lowers power consumption from 900 mW to less than about 5 mW. 
         [0011]    An exemplary method of fabricating the miniature resistive SMO gas sensor module incorporates the resistive devices with an ASIC after the ASIC is formed. In one embodiment, the SMO gas sensor module, having feature sizes in the range of about 1 μm-10 μm, is constructed on top of a completed ASIC that is fabricated using 0.13 μm or 0.18 μm process technology. Such an integration scheme is made possible by restricting thermal processing of the SMO gas sensor module to temperatures lower than about 400 C. One advantage of integrating the gas sensor module closely with the ASIC is that less wiring is required between the ASIC circuitry and the gas sensor components. With less wiring, there is less signal noise, and less power is dissipated. 
         [0012]    A method of operating a gas sensor array made up of a plurality of gas sensor elements facilitates detection of relative concentrations of different gas species, either by using dedicated sensors, or by thermally tuning the sensors to monitor different gas species. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]    In the drawings, identical reference numbers identify similar elements. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. 
           [0014]      FIG. 1A  is a plot of SMO gas sensor responses to different gases according to the prior art. 
           [0015]      FIG. 1B  is a plot of SMO gas sensor response as a function of temperature, according to the prior art. 
           [0016]      FIG. 2  is a top plan view of a solid state gas sensor module according to one embodiment described herein. 
           [0017]      FIG. 3  is a magnified view of the solid state gas sensor module shown in  FIG. 2 . 
           [0018]      FIG. 4  is a composite cross-sectional view of the solid state gas sensor module shown in  FIGS. 2 and 3 . 
           [0019]      FIG. 5  is a cross-sectional view of a solid state gas sensor module integrated on top of a completed ASIC chip. 
           [0020]      FIG. 6A  is a top plan view of a heater test module as described herein. 
           [0021]      FIG. 6B  is a plot of measured heater temperatures as a function of power applied to the heater shown in  FIG. 6A . 
           [0022]      FIG. 7  is a flow diagram showing generalized steps in a method of fabricating a solid state gas sensor module as described herein. 
           [0023]      FIGS. 8A-10B  are cross-sectional views of the solid state gas sensor module at various stages during fabrication according to the method shown in  FIG. 7 . 
           [0024]      FIG. 11  is a top plan view of a circuit layout for a sensor array in which array elements are solid state gas sensor modules as described herein. 
           [0025]      FIG. 12  is an electrical schematic diagram corresponding to the multi-sensor array shown in  FIG. 11 . 
           [0026]      FIG. 13  is a flow diagram showing generalized steps in an exemplary method of operating the multi-sensor array shown in  FIGS. 11 and 12 . 
           [0027]      FIG. 14  is a plot of SMO gas sensor responses to different gas species as a function of temperature. 
           [0028]      FIGS. 15A-15C  are plots of SMO gas sensor responses to different gas species as a function of temperature when operated according to the method shown in  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various aspects of the disclosed subject matter. However, the disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and methods of semiconductor processing comprising embodiments of the subject matter disclosed herein have not been described in detail to avoid obscuring the descriptions of other aspects of the present disclosure. 
         [0030]    Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” 
         [0031]    Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same aspect. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more aspects of the present disclosure. 
         [0032]    Reference throughout the specification to integrated circuits is generally intended to include integrated circuit components built on semiconducting substrates, whether or not the components are coupled together into a circuit or able to be interconnected. Throughout the specification, the term “layer” is used in its broadest sense to include a thin film, a cap, or the like and one layer may be composed of multiple sub-layers. 
         [0033]    Reference throughout the specification to conventional thin film deposition techniques for depositing silicon nitride, silicon dioxide, metals, or similar materials include such processes as chemical vapor deposition (CVD), low-pressure chemical vapor deposition (LPCVD), metal organic chemical vapor deposition (MOCVD), plasma-enhanced chemical vapor deposition (PECVD), plasma vapor deposition (PVD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), electroplating, electro-less plating, and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. For example, in some circumstances, a description that references CVD may alternatively be done using PVD, or a description that specifies electroplating may alternatively be accomplished using electro-less plating. Furthermore, reference to conventional techniques of thin film formation may include growing a film in-situ. For example, in some embodiments, controlled growth of an oxide to a desired thickness can be achieved by exposing a silicon surface to oxygen gas or to moisture in a heated chamber. 
         [0034]    Reference throughout the specification to conventional photolithography techniques, known in the art of semiconductor fabrication for patterning various thin films, includes a spin-expose-develop process sequence typically followed by an etch process. Alternatively or additionally, photoresist can also be used to pattern a hard mask (e.g., a silicon nitride hard mask), which, in turn, can be used to pattern an underlying film. 
         [0035]    Reference throughout the specification to conventional etching techniques known in the art of semiconductor fabrication for selective removal of polysilicon, silicon nitride, silicon dioxide, metals, photoresist, polyimide, or similar materials includes such processes as wet chemical etching, reactive ion (plasma) etching (RIE), washing, wet cleaning, pre-cleaning, spray cleaning, chemical-mechanical planarization (CMP) and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. In some instances, two such techniques may be interchangeable. For example, stripping photoresist may entail immersing a sample in a wet chemical bath or, alternatively, spraying wet chemicals directly onto the sample. 
         [0036]    Specific embodiments are described herein with reference to gas sensors that have been produced; however, the present disclosure and the reference to certain materials, dimensions, and the details and ordering of processing steps are exemplary and should not be limited to those shown. 
         [0037]      FIGS. 1A and 1B  show sensitivity characteristics of tin oxide (SnO 2 ) SMO gas sensors. The resistance of a thin film of tin oxide changes in the presence of various gases of interest. Such a change occurs at temperatures within the range of about 100 C-400 C, and in the presence of oxygen.  FIG. 1A  is a log-log plot  100  derived from a data sheet for existing discrete sensors manufactured by Figaro USA, Inc. of Arlington Heights, Ill.  FIG. 1A  shows normalized resistance vs concentration in parts per million of the tin oxide sensor for five different gases. For example, the bottom curve shows that the resistance of a tin oxide thin film changes dramatically with concentration of methane gas (CH 4 ), while the top two curves show that the resistance of the tin oxide thin film hardly varies with concentrations of air or ethanol. Thus, tin oxide is most sensitive to detect methane with little interference from ethanol, for example. The tin oxide sensor is also sensitive to liquid petroleum gas (LPG), commonly known as propane or iso-butane, and hydrogen gas (H 2 ), which correspond to the next two steepest curves, after methane. 
         [0038]      FIG. 1B  shows a log-linear plot  110  of a tin oxide sensor response to carbon monoxide gas (CO) vs. operating temperature in degrees C. It is clear that the response at about 100 C is about 100 times greater than the response at 350 C. Thus, the tin oxide sensor is highly temperature dependent in this example. In general, it is found that the strongest response of the tin oxide sensor to each target gas species occurs at a different heater temperature. The ability to detect gases accurately using an SMO sensor thus requires excellent temperature control. Consequently, to make an SMO-based gas sensor, it is important to incorporate a heater as well as an accurate temperature sensor to provide feedback to control the heater. Because the relevant temperatures are so hot, it is also desirable that the heater will confine heating to a localized region during sensing. 
         [0039]      FIG. 2  shows a top plan view of a solid state gas sensor module  130 , according to one embodiment, in which a temperature sensor and a heater are co-located with the gas sensor, and a structurally stable heat confinement structure is provided in the sensing area. The solid state gas sensor module  130  as shown in  FIG. 2  includes a sensor region  132 , heater contacts  134  at metal  2 , temperature sensor contacts  136  at metal  3 , and SMO sensor contacts  138  at metal  4 . 
         [0040]      FIG. 3  shows a magnified view of the central area of the sensor region  132  of the solid state gas sensor module  130 , according to one embodiment. The sensor region  132  as shown has an active sensing area of about 50×50 μm 2 . The sensor region  132  includes three resistive components arranged as a stack of thin films: a heater  140  measuring 90×20 μm, a temperature sensor  142  measuring 100×30 μm, and an SMO sensor  144 , measuring 110×40 μm. The SMO sensor  144  has a film thickness of about 200 nm, which is much thinner than conventional SMO sensors that use bulk films typically having thicknesses within the range of about 100 μm to 1 mm. The sensors are formed vertically adjacent to one another, at three different layers, wherein the temperature sensor  142  is positioned above the heater  140  and the SMO sensor  144  is positioned above the temperature sensor. Because the component films are very thin, the different layers are partially transparent. The heater  140  is made of a tantalum aluminum (TaAl) thin film resistor, having a sheet resistance of about 160 Ω/square. The heater  140  heats the SMO sensor  144  to a temperature within the range of about 100 C-400 C while consuming less than 50 mW of power. The temperature sensor  142  is made of an 80/20 alloy of chromium silicate (CrSi), and has a sheet resistance of about 980 kΩ/square. The SMO sensor  144  is made of undoped tin oxide (SnO 2 ), and has a sheet resistance of about 900 kΩ/square. Such a tin oxide film can be doped, for example, with palladium to sense carbon monoxide, or with platinum to sense hydrogen gas, or with silver to sense hydrogen sulfide (H 2 SO 4 ). Although process integration of CrSi resistive thin films was disclosed by the present inventors in U.S. Pat. No. 8,436,426, assigned to the same assignee as the present patent application, integrating sensors made of SnO 2 , CrSi, and TaAl into a common sensor module has not been disclosed. 
         [0041]      FIG. 4  shows a composite cross-sectional view of the solid state gas sensor module  130  taken along the cut lines A-A′ shown in  FIG. 2 , together with contacts  134 ,  136 , and  138  shown in the same cut plane as the sensor region  132 , for convenience. The sensor region  132  includes the three stacked sensors  140 ,  142 , and  144 , separated by insulating layers  146 . The insulating layers  146  can be made of silicon nitride, for example. The temperature sensor contacts  136  are shown to the far right of the sensor region  132 . An SMO sensor contact  138  is shown to the right of and adjacent to the sensor region  132 , and the heater contact  134  is shown to the left of the sensor region  132 . The solid state gas sensor module  130  is formed on a substrate  148  made of, for example, silicon or glass. A silicon dioxide layer  149  separates the substrate  148  from the insulating layers  146 . The solid state gas sensor module  130  further includes a thermal insulation structure  150  formed in the substrate  148 . Silicon, polyimide, and air were evaluated as materials for use in the thermal insulation structure  150 , using simulations of integrated circuit structures. Air, having the lowest thermal conductivity of the three materials, at 0.001 W/mK, is preferred to occupy as much of the thermal insulation structure  150  as possible. Polyimide also has a low thermal conductivity, of about 0.15 W/mK. Silicon, on the other hand, has a thermal conductivity 1000 times greater than polyimide, at 150 W/mK. Based on the simulations, the thermal insulation structure  150  was designed to include an air gap  152  and support pillars  154  made of polyimide that structurally support the solid state gas sensor module  130 . The thermal insulation structure  150  confines heat within the solid state gas sensor module  130 , thus allowing a high temperature to be maintained in the sensor region  132 , with low power input. With reference to  FIG. 5 , in one embodiment, the substrate  148  contains control circuitry; for example, an application specific integrated circuit (ASIC)  156 . The ASIC  156  includes memory and one or more microprocessors programmed to operate the solid state gas sensor module  130 . The ASIC  156  is communicatively coupled to the heater  140 , the temperature sensor  142 , and the SMO sensor  144 , thus allowing the control circuitry within the ASIC  156  to exchange signals with the three thin film components of the solid state gas sensor module  130 . For example, the ASIC  156  can receive data signals from the temperature sensor  142  and the SMO sensor  144  and, in response, transmit control signals to the heater  140 . ASICs and sensors integrated into a common chip are well known in the art, as disclosed in U.S. Patent Publication No. 2012/0171713, U.S. Patent Publication No. 2012/0168882, and U.S. Pat. No. 8,644,053. 
         [0042]    In some embodiments, one or more components of the solid state gas sensor module  130  can be integrated directly into a top layer of the ASIC  156  to create an SMO/ASIC digital sensor module  157 . It is possible to integrate the solid state gas sensor module  130  after the ASIC is formed in the substrate  148  if the processing steps used to form the solid state gas sensor module  130  do not exceed process temperatures above about  400  C. The thermal insulation structure  150  helps to prevent heat generated in the sensor region  132  of the solid state gas sensor module  130  from affecting control circuitry in the substrate  148 . In addition, a thick oxide dielectric layer, e.g.,  5  microns or more, can be used to separate the ASIC  156  from the solid state gas sensor module  130 . One advantage of integrating the sensor with the ASIC is that direct connections between the solid state gas sensor module  130  and the ASIC  156  avoid power losses that would otherwise occur in wiring used to couple the two devices. The SMO/ASIC digital sensor module  157  has a surface area in the range of about 1 mm 2 -4 mm 2 , of which the sensing area is in the range of about 50×50 μm 2  to about 100×100 μm 2 . Typically, the ASIC uses a more advanced technology with smaller device dimensions than that of the solid state gas sensor module  130 . For example, the ASIC may be designed at the 0.18 μm or 0.13 μm technology node, while the sensor structures are in the range of 1 μm-10 μm. The ASIC  156  can be protected by a cap  158  that has an opening to expose the sensing area. 
         [0043]    Heat confinement tests were conducted using one embodiment of the heater  140  adjacent to the thermal insulation structure  150 , and temperature sensors  142   a  and  142   b  configured as shown in  FIG. 6A . The heater  140  was electrically energized at various power levels via the heater contacts  134 . In response, the temperature sensor  142   a,  located adjacent to the heater  140 , and the temperature sensor  142   b,  located a short distance away from the heater  140 , near the perimeter of the sensor region  132 , were probed via the temperature sensor contacts  136   a  and  136   b,  respectively.  FIG. 6B  shows both sets of temperature data, wherein the curve  160   a  represents data from the temperature sensor  142   a  next to the heater  140 , and the curve  160   b  represents data from the remote temperature sensor  142   b.  The curve  160   a  shows that temperatures 250 C-400 C within the operating range  159  of the solid state gas sensor module  130  are successfully maintained by supplying only 20 mW-50 mW of power to the heater  140 , thus demonstrating effective heat confinement. Meanwhile, the flat curve  160   b  confirms that the temperature stays substantially constant at the border of the sensor region  132 , thus demonstrating that thermal dissipation to neighboring devices from effects of the heater  140  is very low. 
         [0044]    In one embodiment, power consumption of the heater  140  can be further reduced by applying a pulsed square wave signal to energize the heater  140 . For example, if the heater  140  is pulsed on for 60 seconds out of every 600 seconds, or 10% of the time, the temperature will be at the target, e.g., 300 C, at least at the end of the 60-second on-period. If the SMO sensor  144  is limited to a sensing period of, for example, the last 20 seconds of the 60-second on-period, substantially similar results can be obtained using only 10% of the power that would be needed to maintain the heater 140 at 300 C continuously. 
         [0045]      FIG. 7  presents a flow chart showing an exemplary sequence of steps in a generalized method  170  for fabricating the solid state gas sensor module  130  as shown in  FIG. 4 . Cross-sectional views are shown in  FIGS. 8A-10B  to illustrate intermediate stages during a fabrication process having only 12 mask layers, whereas existing multi-sensor products typically have used 35-40 mask layers. Three of the 12 mask layers are metal interconnect layers made of aluminum (Al) or, alternatively, aluminum copper (AlCu). 
         [0046]    At  172 , formation of the thermal insulation structure  150  is begun by first forming a recess in the substrate  148 . To form the recess, a  1  pm silicon dioxide layer  149  is deposited onto the substrate  148 , which is made of glass in the embodiment described herein. The silicon dioxide layer  149  is patterned for use as a hard mask to define the recess using conventional photolithography and etching methods well known to those skilled in the art. 
         [0047]    At  174 , the recess is over-filled with a bulk polyimide  182  to a total polyimide thickness of about 4 μm. A polyimide layer formed on top of the silicon dioxide layer  149  outside the recess area is then removed by patterning and etching the polyimide using standard processing techniques. The bulk polyimide  182  within the recess is then covered with about 500 nm of a first SiN layer  146   a  which provides electrical insulation with a high thermal conductivity.  FIG. 8A  shows the solid state gas sensor module  130  after step  174  is completed. 
         [0048]    At  176 , the thin film resistive heater  140  is formed. First, an 870 nm TaAl metal layer (M 1 ) is sputter-deposited onto the first SiN layer  146   a,  followed by a second metal layer M 2  made of 500 nm of AlCu. The AlCu and TaAl film stack is then patterned together using a standard resist mask or a hard mask ( FIG. 8B ). Etching can be done using either a standard wet etch or reactive ion etch process. Next, all of the aluminum structures are selectively removed except one which will provide the heater contact  134  to the resistive heater  140 . Such a selective removal can be accomplished using a mask-less slope etch process that entails use of an aluminum etchant such as a solution of phosphoric acid and nitric acid. Under-cutting of AICu and achieving a sloped profile can be done by combining a photoresist treatment with a wet etch treatment in which the ratio of phosphoric and nitric acids in the aluminum etchant is adjusted, and a surfactant is added. A sloped profile helps to manage power dissipation in the heater structure which includes a low-resistance metal (Al) adjacent to a high-resistance metal (TaAl). Next, a second dielectric SiN layer  146   b  is blanket deposited, followed by a via etch process to form an opening in the SiN to the heater contact  134 . Then, a third 500 nm thick layer of metal, M 3 , is blanket deposited over the second SiN layer  146   b  and patterned using standard techniques to form the temperature sensor contacts  136 .  FIG. 9A  shows the solid state gas sensor module  130  after step  176  is completed. 
         [0049]    At  178 , the thin film resistive temperature sensor  142  is formed. A 200-nm film of CrSi is sputter-deposited over the M 3  layer, followed by 50 nm of a third SiN layer,  146   c.  In an alternative embodiment, 100 nm of platinum (Pt) is substituted for the CrSi film to create the temperature sensor  142 . Next, the third SiN layer  146   c  is patterned, followed by a pad etch process to form an opening to M 3 . ( FIG. 9B ). Then, a fourth 500 nm thick layer of metal, M 4 , is blanket deposited over the third SiN layer  146   c,  and M 4  is patterned using, for example, a slope etch process to form the SMO sensor contacts  138 .  FIG. 10A  shows the solid state gas sensor module  130  after step  178  is completed. 
         [0050]    At  180 , the thin film resistive SMO sensor  144  is formed. First, the SMO film is sputter-deposited over the third SiN layer  146   c  and the SMO film is patterned using standard processing techniques, followed by a pad etch process to form an opening to M 4 . Finally, the polyimide support pillars  154  are formed by etching through the SiN layers  146   a,    146   b,  and  146   c  on either side of the sensor region  132  and then removing large portions of the bulk polyimide  182  using an isotropic oxygen plasma etch. Techniques suitable for pillar formation are further described in U.S. patent application Ser. No. 13/907,708 by the same inventors as the present patent application.  FIG. 10B  shows the solid state gas sensor module  130  after step  180  is completed. 
         [0051]      FIG. 11  shows a set of three solid state gas sensor modules,  130   a,    130   b,  and  130   c,  arranged to form a multi-sensor array  190 . The small area of the gas sensor module design as described above is particularly suitable for use in such an array. One way to operate the multi-sensor array  190  is to evaluate the same gas sample simultaneously at three different temperatures to achieve a more accurate result for the species concentration. For example, the solid state gas sensor module  130   a  can be set to the target temperature, and the solid state gas sensor modules  130   b,    130   c  can be set to ±50 C from the target temperature, respectively. By increasing accuracy of the measurement, noise is reduced, and specificity is improved. Another way to operate the multi-sensor array  190  is to set each one of the solid state gas sensor modules to a different temperature target to detect different gas species having different thermal fingerprints. For example, the solid state gas sensor module  130   a  can be set at 100 C to detect ethanol while the solid state gas sensor module  130   b  is set at 300 C to detect CO and the gas sensor module  130   c  is set at 400 C to detect methane. 
         [0052]      FIG. 12  shows an electrical schematic diagram of the multi-sensor array  190 , configured as a current divider circuit to which a common voltage V h  is applied. In  FIG. 12 , V cc  represents a supply voltage, V h  is a heater input voltage, V t  is an output voltage from the temperature sensor, and V o  is an output voltage from the SMO sensor. The solid state gas sensor module  130   a  is configured with a heater  140   a  having a large heater resistance R ha , so that less current will be delivered to the solid state gas sensor module  130   a  according to I=V h /R ha . Consequently, the power dissipated in the heater  140   a,  given by P=I 2 R ha  will be small, producing a relatively low temperature of 100 C to detect ethanol, for example. Likewise, the solid state gas sensor module  130   c  is configured with a heater  140   c  having a small heater resistance R hc , so that more current will be delivered to the sensor module  130   c  and the corresponding power dissipated in the heater  140   c  will be large, to produce a temperature of 500 C, to detect methane, for example. The solid state gas sensor module  130   b  is configured with a heater  140   b  having a medium heater resistance R hb , so as to produce a temperature of 300 C, to detect carbon monoxide, for example. 
         [0053]    Either one of the two operating schemes described above can be carried out using a 1×3 multi-sensor array  190 , which occupies a surface area of about 1 mm 2 . Alternatively, a 3×3 sensor array allows for redundant measurements at each of three different temperatures, effectively producing more accurate data for each of three different gas species. However, a 3×3 sensor array requires more power and takes up more chip real estate. A 3×3 sensor array like the ones described herein occupies a surface area of about 3 mm 2 . Another way to operate the multi-sensor array  190  is to perform a rough measurement to determine the dominant gas species in a sample, and then to tune all of the gas sensors to measure the dominant species. Generalized steps in such a thermo-cycling method  200  are outlined in  FIG. 13  as follows: 
         [0054]    At  202 , SMO sensors  144   a,    144   b,  and  144   c  are heated to different prescribed temperatures to target three different gas species thought to be present in the ambient environment. 
         [0055]    At  204 , the multi-sensor array  190  is exposed to a gas sample of the ambient environment. 
         [0056]    At  206 , electrical signals are generated by the SMO sensors  144   a,    144   b,  and  144   c  in response to changes in thin film resistivity of the common SMO material. 
         [0057]    At  208 , the electrical signals from the three different SMO sensors are compared to determine which of the three gas species is the dominant gas species in the gas sample. 
         [0058]    At  210 , the heaters  140   a,    140   b,  and  140   c  are all tuned to the same temperature associated with the dominant gas species. 
         [0059]    At  212 , all three SMO sensors  144   a,    144   b,  and  144   c  are used to measure the dominant gas species so that statistics can be calculated to determine with greater accuracy the concentration of the dominant gas species. 
         [0060]    With reference to  FIG. 14 , in steps  202 - 206 , depending on the exact temperatures of the heaters, the common SMO material used in the SMO sensors may respond to several different gas species a, b, c, d, and e at the same time, i.e., the species differentiation may not be distinct. For example, if the operating temperatures of the different heaters differ by 100 C, the SMO closest to a heater set at 300 C, e.g., SMO  144   b,  may be influenced somewhat by gases b and d that are detectable at 200 C and 400 C respectively, but not influenced significantly by gases a and e that are detectable at 100 C and 500 C, respectively. As the operating temperatures are brought closer together, the species differentiation becomes worse. This is because statistical distributions associated with a particular SMO material response at different temperatures are not mutually exclusive, but instead have some amount of overlap, as illustrated in  FIG. 14 . Consequently, for better differentiation of gas species, it may be desirable to operate adjacent SMO sensors  144   a  and  144   c  at 100 C to detect gas species a, and at 500 C to detect gas species e, and to forego the measurements of species b and d at 200 C and 400 C, respectively. Alternatively, different SMO materials could be used to improve the differentiation of gas species within a narrower temperature range. Since, in the present semiconductor implementation, it is not feasible to incorporate multiple SMO materials into the multi-sensor array  190 , it is therefore desirable to operate with temperatures for adjacent array elements being spaced apart by no less than 50 C. 
         [0061]    On the other hand, while entirely different SMO materials may not be feasible, implanting different portions of the SMO film with different dopants is a practical alternative that allows operating at a common temperature while still achieving a high degree of gas species differentiation. For example, in a 3×3 sensor array, one row of sensors may be fabricated by doping the SnO 2  film with platinum to detect H 2 SO 4 , while a second row of SMO sensors is doped with palladium to detect CO, and the third row of SMO sensors can be doped with gold. The doping can be accomplished during an in-situ plasma vapor deposition (PVD) process as the SnO 2  is being formed, or by implant doping after the SnO 2 film is deposited. With doped SMO sensor films, the ability to differentiate gas species improves as illustrated in  FIGS. 15A ,  15 B, and  15 C. For example, in  FIG. 15A , the SMO sensor response to methane at 400 C is significantly greater than the response to CO at 325 C or ethanol at 150 C; similarly, in  FIG. 15B , the SMO sensor response to CO at 325 C is significantly greater than the response to methane at 400 C or ethanol at 150 C; and in  FIG. 15C , the SMO sensor response to ethanol at 150 C is significantly greater than the response to CO at 325 C or methane at 400 C. 
         [0062]    The structure, size, and thermal properties of the solid state gas sensor modules  130  as described herein thus permit flexibility in operation of the sensor array  190 . 
         [0063]    The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
         [0064]    It will be appreciated that, although specific embodiments of the present disclosure are described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, the present disclosure is not limited except as by the appended claims. 
         [0065]    These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.