Abstract:
A microelectronic device capable of detecting multiple gas constituents in ambient air can be used to monitor air quality. The microelectronic air quality monitor includes a plurality of temperature-sensitive gas sensors tuned to detect different gas species. Each gas sensor is tuned by programming an adjacent heater. An insulating air pocket formed below the sensor helps to maintain the sensor at a desired temperature. A temperature sensor may also be integrated with each gas sensor to provide additional feedback control. The heater, temperature sensor, and gas sensors are in the form of patternable thin films integrated on a single microchip. The device can be incorporated into computer workstations, smart phones, clothing, or other wearable accessories to function as a personal air quality monitor that is smaller, more accurate, and less expensive than existing air quality sensors.

Description:
BACKGROUND 
     Technical Field 
       [0001]    The present disclosure relates to miniature sensors for use in monitoring air quality to detect gas phase molecules such as carbon dioxide and volatile organic compounds. 
       Description of the Related Art 
       [0002]    It is believed that as many as seven million premature deaths occur annually due to air pollution [ World Health Organization Report, Mar.  25, 2014]. Air pollution includes both outdoor pollution and poor indoor air quality in enclosed spaces such as, for example, homes, factories, office buildings, and high-density apartment buildings. Indoor air pollution is considered by some experts to be a larger health hazard than outdoor air pollution. Many of the illnesses and deaths associated with air pollution are attributable to the use of solid fuels for heating and cooking in third world countries. However, industrial societies using cleaner forms of energy continue to suffer health effects from indoor pollution. In a typical day, each office worker inhales and processes about fifteen cubic meters of air, exhaling about 350 liters of carbon dioxide (CO 2 ). High levels of volatile organic compounds (VOCs) exist in many buildings constructed using engineered materials that contain glues, dyes, binding agents, adhesives, and the like. Furthermore, cleaning products, solvents, paint and other coatings, furniture, carpeting, and other chemical sources also contribute VOC pollutants. VOCs include such compounds as ethanol, toluene, benzene, formaldehyde, tetrachloroethene (TCE), and methylene chloride. 
         [0003]    As heat efficiency of buildings improves and structures have become more airtight, there is less air circulation and a reduction in the exchange of air from outside to inside. As stale air accumulates within a closed space, concentrations of carbon dioxide and VOCs may rise to harmful levels. In some cases, cardio-pulmonary function may be compromised, increasing the risk of heart attacks and strokes. With continued exposure to poor air quality, over time, cancer may be triggered by such airborne toxins. Furthermore, a subtler and more common consequence of poor air quality is that the brain becomes deprived of oxygen, and productivity is reduced. A Harvard study funded by the National Institutes of Health (NIH) shows that a typical indoor CO 2  level of about 950 ppm impairs cognitive ability, ultimately lowering worker productivity. [J. G. Allen et al., “Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments,” Environmental Health Perspectives, DOI:10.1289/ehp.1510037, Oct. 26, 2015]. Consequently, green building practices have been introduced in an attempt to limit the use of VOCs and, in some cases, to require a higher outdoor air ventilation rate to prevent accumulation of both VOCs and CO 2 . 
         [0004]    Maintaining awareness of the levels of VOCs and CO 2  present in ambient air is challenging. While some people are particularly sensitive to VOCs and will experience allergic reactions such as headaches, dizziness, and irritation of the eyes, nose, and throat in a high-VOC environment, most people cannot detect hazardous levels of pollution. Because VOCs and CO 2  are both odorless, they are generally difficult to detect, and most buildings today are not equipped with multi-species gas sensors. Some portable air quality alert devices that contain CO 2  and VOC sensors are available, e.g., AirVisual Node™, Alima™, Atmotube™, Cube Sensor™, and the like; however, such devices tend to be bulky, and each unit that is capable of monitoring a personal sphere of exposure costs hundreds of dollars. 
       BRIEF SUMMARY 
       [0005]    A multi-species micro-sensor device detects multiple gas constituents in ambient air to monitor air quality. In particular, three or more gas species detectors are formed on a single integrated circuit chip, e.g., an application-specific integrated circuit (ASIC) that includes a volatile organic compound (VOC) sensor and a CO 2  sensor. The ASIC may also include other types of environmental sensors, as well as a processor and a memory. Such a miniature multi-species sensor chip can be seamlessly and invisibly integrated into many different products. For example, a multi-species gas sensor chip can be incorporated into fixtures, such as desktop computers or displays, to monitor an individual&#39;s work environment. In addition, an integrated sensor chip can be incorporated into mobile devices such as laptop computers, smart phones, clothing, watches, and other accessories to function as a personal monitoring device for air quality. Such an integrated multi-species gas sensor can continuously monitor an air quality index that includes levels of various gas species along with humidity, temperature, and the like. 
         [0006]    An integrated multi-species gas micro-sensor is smaller, more accurate, and less expensive than existing air quality sensors. The multi-species gas micro-sensor includes a VOC sensor in the form of a conformal thin film less than 0.2 micron thick. The multi-species gas micro-sensor also includes a heater having a low temperature coefficient of resistance. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    In the drawings, identical reference numbers identify similar elements or acts unless the context indicates otherwise. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. 
           [0008]      FIG. 1  is a pictorial view of a microelectronic air quality monitor in use, according to an embodiment as described herein. 
           [0009]      FIG. 2  is a block diagram of the microelectronic air quality monitor shown in  FIG. 1 , according to an embodiment as described herein. 
           [0010]      FIG. 3A  is a pictorial view of a thick sensor material in powder form that is structured to sustain a bulk chemical reaction, according to the prior art. 
           [0011]      FIG. 3B  is a pictorial view of a sensor material in the form of a thin film that is structured to sustain a surface chemical reaction, according to an embodiment as described herein. 
           [0012]      FIG. 4  is a circuit schematic of a multi-species gas sensor array including sensors for detecting three different gas species according to an embodiment as described herein. 
           [0013]      FIG. 5  is a data table listing temperatures at which various gas species can be detected using different sensor materials, according to an embodiment as described herein. 
           [0014]      FIG. 6  is a flow diagram showing steps in a method of fabricating a multi-species gas micro-sensor according to an embodiment as described herein. 
           [0015]      FIGS. 7-10  are cross-sectional views of a multi-species gas sensor following steps of the fabrication method shown in  FIG. 6 . 
           [0016]      FIG. 11  is a top plan view of a VOC sensor, according to one embodiment as described herein. 
           [0017]      FIG. 12  is a top plan view of a chip layout corresponding to the sensor array shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    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 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. 
         [0019]    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.” 
         [0020]    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 an 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. 
         [0021]    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. 
         [0022]    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. 
         [0023]    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. 
         [0024]    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. 
         [0025]    Specific embodiments are described herein with reference to air quality 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. 
         [0026]    Turning now to the Figures,  FIG. 1  shows a workstation  100  equipped with an air quality monitor  102 , according to an embodiment of the present disclosure. The workstation  100  represents a fixture such as a desktop computer, a laptop computer, a kiosk, a wall-mounted display, or the like. The workstation  100  includes a display  104  that presents air quality data in the form of a statistical summary  106  and a trend chart  108 . The air quality data is sensed locally by the air quality monitor  102  and is then analyzed by electronic components for presentation on the display  104 . The electronic components that process and analyze the air quality data may be located within the workstation  100 , or at a remote location communicatively coupled to the workstation  100  by a wired or wireless connection, e.g., a network connection. The air quality monitor  102  may be a fixed component of the workstation  100 , or the air quality monitor  102  may be a mobile unit that is removably attached to the workstation  100 . In one embodiment, the air quality monitor  102  may be part of a smart phone, a tablet computer, a laptop computer, a watch, a pendant, an article of clothing, or another type of mobile unit associated with a user of the workstation  100 , wherein the air quality monitor  102  is communicatively coupled to the workstation  100  only while a particular user is working at the workstation  100 . The air quality monitor  102  may maintain a user history of locations and associated air quality data to monitor the user&#39;s exposure to certain air pollutants. Alternatively, the air quality monitor  102  may maintain a history of air quality data specific to a fixed location of the workstation  100 . 
         [0027]    In one embodiment, the statistical summary  106  presented on the display  104  includes a humidity reading, a temperature reading, a volatile organic compound concentration reading, a location, a time stamp, and an overall office air quality index. The statistical summary  106  is exemplary and may include more or fewer data items than are shown in  FIG. 1 . One or more of the data items may be displayed as a time series graph on the trend chart  108  that occupies a portion of the display  104  so that a user of the workstation  100  can be informed of local air quality in real time. The trend chart  108  may display time trends of individual data items in succession, on a rotating basis. Alternatively, a plurality of time trends may be displayed simultaneously on the trend chart  108 . The trend chart  108  may be configurable by the user or by a system administrator. 
         [0028]      FIG. 2  shows components of the air quality monitor  102 , according to an embodiment of the present disclosure. The air quality monitor  102  is a microelectronic device that includes at least a microprocessor  120 , an electronic memory  122 , and a micro-sensor array  124 . The microprocessor  120  is communicatively coupled to the electronic memory  122  and the micro-sensor array  124 . The electronic memory  122  is configured to store instructions for execution by the microprocessor  120  and to store data received from the micro-sensor array  124 . The micro-sensor array  124  may also be coupled directly to the electronic memory  122 . Any one of the communication paths among components of the air quality monitor  102  may support wired or wireless data communication. The micro-sensor array  124  may be an application-specific integrated circuit (ASIC) chip. A portion or all of the electronic memory  122  may be implemented on board the ASIC chip. Furthermore, all components of the air quality monitor may be co-integrated as a system-on-chip (SOC). 
         [0029]    The micro-sensor array  124  may be implemented as described in a related patent document entitled, “Integrated SMO Gas Sensor Module,” [U.S. patent application Ser. No. 14/334,572 to Shankar et al., published as U.S. Patent Publication No. 2016/0018356, hereinafter “Shankar”], which is assigned to the same entity as the present patent application, and is herein incorporated by reference in its entirety. Alternatively, the micro-sensor array  124  may be implemented as described herein, which implementation has some features that differ from those of Shankar. Alternatively, the micro-sensor array  124  may be implemented so as to combine certain features of Shankar&#39;s gas sensor with certain other features of the gas sensor as described herein. In one embodiment, the entire air quality monitor  102  is on a single substrate  222  (see  FIG. 7 ). In other embodiments, the micro-sensor array  124  is on its own silicon substrate and the microprocessor  120  and the electronic memory  122  are together on a single silicon substrate. 
         [0030]      FIGS. 3A, 3B  contrast the prior art with the present invention for providing an air quality sensor.  FIG. 3A  shows a bulk sensor material  130 , known in the art. The bulk sensor material  130  is in the form of a powder that is structured to sustain a chemical reaction with ambient air. The bulk sensor material  130  is made up of particles  132  that may include multi-crystalline grains of a reactive material. Ambient gas can flow through bulk sensor material, for example, along a circuitous path  134 , which facilitates contact between the ambient gas molecules and surfaces of the particles  132 . The bulk sensor material  130  may be, for example, tin oxide (SnO 2 ) having a thickness in the range of about 5 μm to 20 μm. The bulk sensor material  130  is typically sintered at a temperature of 600 C. The bulk sensor material  130  is a known system and will therefore not be further described. It is large and bulky, and does not fit on a silicon substrate. 
         [0031]      FIG. 3B  shows a thin film gas sensing material  140 , suitable for use in the micro-sensor array  124 , according to an embodiment of the present disclosure that is an improvement over the sensor of  FIG. 3A . The thin film gas sensing material  140  has a structure that supports surface conduction of ambient gas along a substantially straight path  144 , and a surface reaction between the ambient gas and a dense, multi-crystalline thin film  142  that is made of a thin film gas sensing material  140 . In one example, the thin film  142  is a tin oxide (SnO 2 ) film of thickness 100 nm, about 100 times thinner than the bulk sensor material  130 . Other gas sensing materials that can be used as the thin film  142  include zinc oxide (ZnO 2 ) and indium oxide (In 2 O 3 ). The thin film  142  may be formed by sputter deposition, followed by sintering at a low temperature of 400 C. The resulting thin film  142  is so dense that it is classified as a ceramic as opposed to a powder. Part or all of the thin film  142  may then be capped with a thin coating of platinum (Pt). The sensitivity of thin film gas sensing materials  140  to various gases that may be present in ambient air is known to change as a function of temperature. The platinum coating may assist in transferring heat to the thin film  142 . 
         [0032]      FIG. 4  shows a circuit schematic of the micro-sensor array  124 , according to an embodiment of the present disclosure. Elements of the micro-sensor array  124 , (three shown:  150   a, b, c ) include a temperature sensor  152 , a resistive heater  154 , and a gas sensor  156  that are formed together on a common substrate. The resistive heater  154  is electronically controlled by the microprocessor according to programmed instructions, so as to tune the gas sensor  156  to be sensitive to a particular gas. The temperature sensor  152  can be used as a feedback control device for automatically adjusting the resistive heater  154 . Power is delivered to the resistive heaters  154  via a heater signal line  158  that is driven at a voltage V h . The gas sensor  156  includes the thin film  142  in the form of the thin film gas sensing material  140  shown in  FIG. 3B . The temperature of each gas sensor  156  is determined by the voltage V h  and a resistance R H  of an associated resistive heater  154 . Each element of the micro-sensor array  124  can be operated within a different temperature range when the resistances R H  have different values. This can be accomplished by using different sensing materials in the different elements of the micro-sensor array  124 . For example, a first element  150   a  of the micro-sensor array  124  may include tin oxide (SnO 2 ) as an active sensing material and may be operated within a temperature range of 400 C-500 C, while a second element  150   b  of the micro-sensor array  124  may include ZnO 2  as an active sensing material and may be operated in a temperature range of 300 C-350 C. In one embodiment, each temperature sensor  152  is configured as a Wheatstone bridge that includes three fixed resistors R 1 , R 2 , and R 3 . To control dissipation of heat and power consumption, heating is done in a confined manner as explained below. 
         [0033]      FIG. 5  shows a table  300  that lists which gases can be detected by VOC sensors based on material and operating temperature, according to an embodiment of the present disclosure. For example, when a VOC sensor made of SnO 2  is heated to an operating temperature of 100 C, it is capable of detecting hydrogen gas. But when the SnO 2  sensor is heated to an operating temperature of 300 C, it will detect carbon monoxide (CO), and at 400 C, it will detect methane. When a VOC sensor made of ZnO 2  is heated to 300 C, it detects nitrogen oxide (NO 2 ). When a VOC sensor made of InO 2  is heated to 300 C, it will detect sulphur dioxide (SO 2 ). Other sensor materials such as the various oxide compounds listed in the first column of the table  300  can be substituted for, or used in addition to, SnO 2 , ZnO 2 , and InO 2  in the VOC sensors. Accordingly, each one of the VOC sensors in the micro-sensor array  124  can be tuned to sense a selected gas by controlling the associated heater that is disposed adjacent to the sensor. It is advantageous to construct the VOC sensor array to ensure that the heat is confined to the local region of the VOC sensor and that it remains at the desired temperature over time, and to sustain the accuracy of the sensor elements. 
         [0034]    In one embodiment, the same physical material is heated to different temperatures at different times to sense different gases. In one example, at a first time, the SnO 2  layer is heated to about 200 C to detect butane and propane. At a later time, the very same material is heated to about 300 C to detect CO. The local temperature sensor adjacent to the material provides a feedback signal to ensure that the SnO 2  material is at the desired temperature for sensing the selected gas. 
         [0035]      FIG. 6  is a flow diagram showing a sequence of steps in a method  200  for fabricating the sensor array shown in  FIG. 4 , according to an embodiment of the present disclosure. All of the steps in the method  200  can be carried out at temperatures at or below 400 C. With reference to  FIGS. 7-10 , the multi-species gas sensors  156 , suitable for detecting VOCs, are formed adjacent to resistive heaters  154 . 
         [0036]    At  202 , a thick oxide  224  is formed on a substrate  222  using, for example, a conventional thermal growth process. The substrate  222  may be, for example, a silicon substrate or a glass substrate having a thickness in the range of about 500 μm to 600 μm. The thick oxide  224  has a thickness in the range of about 3 μm to 10 μm. 
         [0037]    At  204 , cavities about 2 μm deep are formed in the thick oxide  224  by patterning the thick oxide  224 , using conventional photolithography and etching techniques. For example, the thick oxide  224  may be patterned using a photoresist and etched using a wet chemical etchant such as hydrofluoric acid (HF). The cavities may have sloped sides. 
         [0038]    At  206 , the cavities are filled with a 4-μm thick layer  228  of polyimide to form polyimide wells  226  as shown in  FIG. 7 . The polyimide material can be, for example, a material such as HD8220 available from Fujifilm corporation of Tokyo, Japan. The polyimide wells  226  can be cured at a temperature of 325 C for one hour to reduce the thickness to 3 μm, wherein about 2 μm of the polyimide layer is below the surface of the thick oxide  224  and about 1 μm of the polyimide layer is above the surface of the thick oxide  224 . Next, a 300-nm thick silicon nitride capping layer  228  (e.g., Si 3 N 4 ) is formed on top of the polyimide wells  226  using a conventional method of conformal thin film deposition. 
         [0039]    At  208 , the resistive heaters  154  are formed as 150-nm thick heating elements  230  made of tantalum aluminum (TaAl), according to one embodiment of the present disclosure. TaAl features a low thermal coefficient (TCR) that results in a stable resistance. A first metal layer is formed on top of the heating elements  230  and patterned to form contacts  232  to the heating elements  230 . The contacts  232  can be made of any metal suitable for use as integrated circuit interconnects such as, for example, aluminum copper (AlCu) having a thickness of about 500 nm. The contacts  232  may be etched so as to have sloped sides. The contacts  232  and the heating elements  230  are covered with a first conformal interlayer dielectric (ILD)  234 , e.g., another 300-nm thick layer of Si 3 N 4 . Vias are then formed in the conformal ILD  234  and filled with a second metal layer  236  made of AlCu having a thickness of 500 nm. 
         [0040]    At  210 , temperature sensing elements  238  are formed by patterning a high-TCR thin film that is conformally deposited over the second metal layer  236 . The temperature sensing elements  238  can be made of, for example, platinum (Pt) having a thickness of about 20 nm. A second conformal ILD  240  is then deposited over the temperature sensing elements  238 . The second conformal ILD  240  can be 30 nm of Si 3 N 4 . In some embodiments, the temperature sensing elements  238  are optional and may be omitted, depending on a desired level of calibration and accuracy. 
         [0041]    At  212 , multi-species gas sensors are formed adjacent to the resistive heaters  154 . A first VOC sensor  242  is formed as a patterned tin oxide (SnO 2 ) film having a thickness in the range of about 30 nm to 100 nm. The first VOC sensor  242  is formed over selected ones of the polyimide wells  226 . A first VOC sensor cap  244  is formed as a 50-nm thick SiO 2  film that is conformally deposited over the first VOC sensor  242 . 
         [0042]    A second VOC sensor  246  is formed at a different location on the same substrate  222 , as shown in  FIG. 8A . In one embodiment, the second VOC sensor  246  is a patterned zinc oxide (ZnO 2 ) film having a thickness of about 100 nm. When the ZnO 2  film is being deposited and patterned, the area of the first sensor  242  is covered with the appropriate mask to protect it while sensor layer  246  is being formed. By selectively patterning the ZnO 2  film, the second VOC sensor  242  can be formed over different ones of the polyimide wells  226  than the SnO 2  film that is used for the first VOC sensor  242 . In this way, a single process flow can be used to fabricate different types of VOC sensors, each sensor being paired with a heater and a temperature sensor. A second VOC sensor cap  248  is formed as a 50-nm thick SiO 2  film that is conformally deposited over the second VOC sensor  246 , as shown in  FIG. 8B . 
         [0043]    A third VOC sensor  250  is formed as a patterned indium oxide (In 3 O 3 ) film having a thickness of about 150 nm, as shown in  FIG. 8B . The third VOC sensor  250  is formed over selected ones of the polyimide wells  226  at different locations on the same substrate  222 . The first and second VOC sensor films are masked in the patterning process for the third sensor  250 . 
         [0044]    The layers  242 ,  246  and  250  are specific examples of the thin film  142  shown and described in  FIG. 3B . Other materials besides the specific ones shown in  FIGS. 7-8B  may be used to sense different gases as explained with respect to  FIG. 5 . 
         [0045]    With reference to  FIG. 9 , vias are formed in the second conformal ILD  240  and are filled with a third metal layer  252  made of AlCu having a thickness of about 500 nm. 
         [0046]    At  214 , a passivation layer  254  is formed over the third metal layer  252  and the VOC sensors, as shown in  FIG. 10 . The passivation layer  254  may be made of SiN. The passivation layer  254  is patterned to expose the VOC sensors, and to provide a signal path via the various metal layers to access the temperature sensing elements  238  and the contact  232  to the heating element  230 . Each one of the first, second, and third VOC sensors has an exposed active sensing area of about 200 μm 2 ×100 μm 2 . 
         [0047]    At  216 , some of the polyimide material is removed from the polyimide wells  226 . Openings  256  are formed by etching through the VOC sensor layers and ILD layers to expose the polyimide wells  226 . A second film removal step is then performed to remove polyimide material from the polyimide wells  226 , leaving air pockets  260  underneath the heating elements  230 . The air pockets  260  have widths  262 . The widths  262  of the air pockets are desirably much larger than the openings  256 , so that air is effectively trapped within the air pockets  260  while being maintained at an atmospheric pressure of the ambient air. A curing step can then be performed at 400 C for two hours at atmospheric pressure to shrink and harden polyimide material remaining in the polyimide wells  226 , thereby solidifying the walls of the air pockets  260 . The air pockets  260  provide thermal insulation to trap heat produced by the heating elements  230  so that the heat is spatially confined within a local vicinity of the adjacent VOC sensor and is not transmitted to other VOC sensors in the micro-sensor array  124 . 
         [0048]      FIG. 11  shows a top plan view of an exemplary temperature sensor  152  and an exemplary resistive heater  154 , according to an embodiment of the present disclosure. The resistive heater  154  can be designed as a metal mesh heating element  230  in which the openings  256  lead to the air pockets  260  located below the heating element  230 . The contact  232  provides electrical power to the heating element  230 . The temperature sensor  152  is disposed in a layer above the heating element  230 , and extends to a position directly below the VOC sensor. 
         [0049]      FIG. 12  shows a physical layout in silicon of the entire circuit of the micro-sensor array  124  on the substrate  222 , according to an embodiment of the present disclosure. The view in  FIG. 12  shows three elements, on a single integrated circuit, of the micro-sensor array  124  that correspond to the three elements shown schematically in  FIG. 4 . Contact pads  180  provide electrical connections to the micro-sensor array elements  150   a, b, c , for access by the microprocessor  120  and the electronic memory  122 . Electrical signal paths  182  are also indicated in  FIG. 12 . Resistors R 1 , R 2 , R 3 , and so on, are shown as serpentines. A total footprint of the exemplary three-element array shown in  FIG. 12  is 2.6 mm×0.9 mm=2.34 mm 2 . 
         [0050]    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 entireties. 
         [0051]    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. The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
         [0052]    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.