Abstract:
A miniature gas analyzer capable of detecting VOC gases in ambient air as well as sensing relative humidity and ambient temperature can be used to monitor indoor air quality. The VOC gas sensor is thermally controlled and can be tuned to detect a certain gas by programming an adjacent heater. An insulating air pocket formed below the sensor helps to maintain the VOC gas sensor at a desired temperature. A local temperature sensor may be integrated with each gas sensor to provide feedback control. The heater, local temperature sensor, gas sensor(s), relative humidity sensor, and ambient temperature sensor are in the form of patternable thin films integrated on a single microchip, e.g., an ASIC. 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 indoor 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 indoor 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 indoor chemical sources also contribute VOC pollutants. VOCs include such compounds as ethanol, toluene, benzene, formaldehyde, trichloroethylene (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 indoor 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 gas analyzer implemented as a micro-sensor device detects VOCs in ambient air to monitor indoor air quality. The gas analyzer also includes temperature and humidity sensors formed on the same integrated circuit chip as the VOC sensor, providing secondary information to calibrate the VOC sensor. The VOC detector is a solid state, semiconductor-metal-oxide (SMO)-based sensor formed on a semiconductor substrate, as described in a co-pending patent application by the present inventors, entitled “Integrated Air Quality Sensor” which is attorney docket number 851663.632. A multi-species gas sensor chip that is configured to detect different gases can be incorporated into indoor 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 air quality monitoring device. Such a monitor can continuously measure an air quality index that includes VOC levels or levels of other gases detectable using a solid state SMO-based material. 
         [0006]    A multi-species micro-sensor device detects multiple gas constituents in ambient air to monitor indoor air quality. In particular, three or more gas species detectors may be 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. 
         [0007]    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 indoor 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. 
         [0008]    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 
         [0009]    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. 
           [0010]      FIG. 1  is a pictorial view of a microelectronic gas analyzer in use, according to an embodiment as described herein. 
           [0011]      FIG. 2  is a block diagram of the microelectronic gas analyzer shown in  FIG. 1 , according to an embodiment as described herein. 
           [0012]      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. 
           [0013]      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. 
           [0014]      FIG. 4  is a series of graphs of concurrent test measurements of temperature, relative humidity, pressure, and VOCs of ambient air in a test chamber. 
           [0015]      FIGS. 5A, 5B, and 5C  are circuit schematics of a VOC sensor, a relative humidity sensor, and an ambient temperature sensor, respectively, according to an embodiment as described herein. 
           [0016]      FIG. 6  is a flow diagram showing steps in a method of fabricating sensors of a miniature gas analyzer according to an embodiment as described herein. 
           [0017]      FIGS. 7-9B  are cross-sectional views of a sensors at various steps in the fabrication method shown in  FIG. 6 . 
           [0018]      FIG. 10  is a top plan view of a completed miniature relative humidity sensor, according to one embodiment as described herein. 
           [0019]      FIG. 11  is a top plan view of a completed miniature VOC sensor, according to one embodiment as described herein. 
           [0020]      FIG. 12  includes a top plan view and cross-sectional views of a relative humidity sensing capacitor and a reference capacitor in an electronic package that includes wiring and contact pads, according to one embodiment as described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    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. 
         [0022]    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.” 
         [0023]    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. 
         [0024]    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. 
         [0025]    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. 
         [0026]    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. 
         [0027]    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. 
         [0028]    Specific embodiments are described herein with reference to miniature gas analyzers 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. 
         [0029]    Turning now to the Figures,  FIG. 1  shows a workstation  100  equipped with a miniature gas analyzer  102 , according to an embodiment of the present disclosure. The workstation  100  represents an indoor 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 miniature gas analyzer  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 miniature gas analyzer  102  may be a fixed component of the workstation  100 , or the miniature gas analyzer  102  may be a mobile unit that is removably attached to the workstation  100 . In one embodiment, the miniature gas analyzer  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 miniature gas analyzer  102  is communicatively coupled to the workstation  100  only while a particular user is working at the workstation  100 . The miniature gas analyzer  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 miniature gas analyzer  102  may maintain a history of air quality data specific to a fixed location of the workstation  100 . 
         [0030]    In one embodiment, a statistical summary  106  presented on the display  104  includes a humidity reading, a temperature reading, a VOC gas 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. 
         [0031]      FIG. 2  shows components of the miniature gas analyzer  102 , according to an embodiment of the present disclosure. The miniature gas analyzer  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 miniature gas analyzer  102  may support wired or wireless data communication. The micro-sensor array  124  may be implemented as an application-specific integrated circuit (ASIC) chip. A conventional analog-to-digital converter (ADC) may also be implemented on board the 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). 
         [0032]    The micro-sensor array  124  includes one or more gas sensors, which may include a VOC gas sensor or a plurality of VOC gas sensors, as well as other environmental sensors such as, for example, a pressure sensor, a humidity sensor, a temperature sensor, a flow sensor, and the like. The environmental sensors that sense ambient humidity and temperature may be used to calibrate readings of one or more of the gas sensors according to calibration instructions stored in the electronic memory  122  and executed by the microprocessor  120 . 
         [0033]    The environmental sensors may be implemented as described in related patent documents by the same inventor as the present patent application, including U.S. Pat. No. 9,176,089, entitled “Integrated Multi-sensor Module,” and U.S. Patent Publication No. 2014/0294046, entitled “Microelectronic Environmental Sensing Module,” both of which are herein incorporated by reference in their entireties. Alternatively, the environmental sensors may be implemented as described herein, or with some features described in the related patent documents and other features as described herein. 
         [0034]    The gas sensor portion of 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 gas sensor portion of the micro-sensor array  124  may be implemented as described in the co-pending patent application entitled, “Integrated Air Quality Sensor,” [attorney docket number 851663.632], which is assigned to the same entity as the present patent application, and is summarized herein and incorporated by reference in its entirety, which implementation has some features that differ from those of Shankar. Alternatively, the gas sensor portion of the micro-sensor array  124  may be implemented so as to combine certain features of Shankar&#39;s gas sensors with certain other features of LeNeel&#39;s gas sensors. In one embodiment, the entire air quality monitor  102  is on a single substrate  222 . 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. 
         [0035]      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-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. 
         [0036]      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. 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 VOC 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 . 
         [0037]      FIG. 4  is a series of output plots  146  showing exemplary real-time trends of air quality data detected by the miniature gas analyzer  102 . The upper time trend labeled “ADC count” shows the output of an A-to-D converter associated with VOC gas sensor measurements as detected by the micro-sensor array  124 . At about 2360 seconds, the VOC sensor registers the presence of a VOC gas as indicated by the rising ADC count value  148 . Meanwhile, output values indicating concurrent measurements of the other environmental sensors, pressure, humidity, and temperature, remain constant, for example, at 1020 mT, 51%, and 21.5 C, respectively, while the VOC sensor reacts. The concurrent measurements can be used by the microprocessor  120  to calibrate the VOC gas sensors. 
         [0038]      FIG. 5A  shows a circuit schematic of the micro-sensor array  124 , according to an embodiment of the present disclosure. A single element  150  of the micro-sensor array  124  includes a local 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  120  according to programmed instructions, so as to tune the gas sensor  156  to be sensitive to a particular gas. The local temperature sensor  152  can be used as a feedback control device for automatically adjusting the resistive heater  154 . Power is delivered to the resistive heater  154  via a heater signal line that is driven at a voltage V h  and carries a current I h . The gas sensor  156  includes the thin film  142  made of the thin film gas sensing material  140  shown in  FIG. 3B . A temperature of the gas sensor  156  is determined by the voltage V h  and a resistance R H  of an associated resistive heater  154 . The element  150  of the micro-sensor array  124  can be operated within a selected temperature range by selecting a particular gas sensing material  140  and then controlling the resistance R H  of the resistive heater  154  to tune the thin film  142  to the desired sensitivity. For example, the element  150  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 to detect methane, or within a temperature range of 300 C-350 C to detect carbon monoxide. In one embodiment, the local 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 of the gas sensor  156  is done in a confined manner as explained below. 
         [0039]      FIG. 5B  shows a schematic of a relative humidity sensor  160  configured as a parallel combination of a variable capacitor C rh  and a reference capacitor C 0 .  FIG. 5C  shows a schematic of a temperature sensor  162  that includes four resistors, R 6 , R 7 , R 8 , R 9  in a Wheatstone bridge arrangement. The temperature sensor  162  will measure a temperature of the ambient environment, in contrast to the local temperature sensor  152  that measures an internal heater temperature for adjusting operation of the VOC gas sensor. 
         [0040]      FIG. 6  is a flow diagram showing a sequence of steps in a method  200  of fabricating a VOC gas sensor and environmental sensors in the sensor array  124  of the miniature gas analyzer  102 , according to an embodiment of the present disclosure. All of the steps in the method  200  following the initial thermal oxide growth can be carried out at temperatures at or below 400 C. Some of the processing steps form dual-purpose films that are patterned in the VOC sensor area so as to perform a first function, and are patterned differently in the humidity sensor area, for example, so as to perform a second function. 
         [0041]    With reference to  FIGS. 7-9B , the gas sensor  156 , suitable for detecting VOCs, is formed adjacent to the resistive heater  154 , the local temperature sensor  152 , the relative humidity sensor  160 , and the ambient temperature sensor  162 , as follows: 
         [0042]    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-600 μm. The thick oxide  224  has a thickness in the range of about 3 μm-10 μm, as shown in  FIG. 7 . 
         [0043]    At  204 , a cavity 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 cavity may have sloped sides, as shown in  FIG. 7 . 
         [0044]    At  206 , the cavity is filled with a 4-μm thick first layer of polyimide to form a polyimide well  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 well  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 well  226  using a conventional method of conformal thin film deposition. 
         [0045]    At  208 , the resistive heater  154  and a bottom plate of the capacitive relative humidity sensor  160  are both formed from a common 150 nm thick metal layer made of tantalum aluminum (TaAl), according to one embodiment of the present disclosure as shown in  FIG. 8A . TaAl features a low thermal coefficient (TCR) that results in a stable resistance. The TaAl metal layer is therefore a multi-use film—a first portion of the TaAl layer serves as a heating element  230  of the resistive heater  154 , while a second portion of the TaAl layer serves as a bottom plate  231  of the capacitor C rh , and a third portion of the TaAl layer serves as a bottom plate  233  of the reference capacitor C o . 
         [0046]    At  210 , A first metal layer is then formed on top of the TaAl layer and patterned to form contacts  232  to the heating elements  230  and to the bottom plates  231 ,  233 , as shown in  FIG. 8A . 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 have sloped sides. 
         [0047]    At  212 , a second polyimide layer is formed and patterned so as to create active polyimide structures  235  among the metal contacts  232 . The active polyimide structures  235  will serve as dielectrics of the parallel plate capacitors C rh  and C o . The active polyimide structures  235  may have thicknesses in the range of about 6.0 μm-8.0 μm and may be made of a commercially available polyimide material that is sensitive to humidity. The contacts  232 , the TaAl layer, and the active polyimide structures  235  then are covered with a first conformal interlayer dielectric (ILD)  234 , e.g., another 300 nm thick layer of Si 3 N 4 . Vias  236  are then etched through the conformal dielectric layer  234  and filled with a second metal layer made of AlCu having a thickness of 500 nm, as shown in  FIG. 8B . 
         [0048]    At  214 , temperature sensing elements  238  are formed by conformally depositing and patterning a high-TCR thin film over the second metal layer. In the vicinity of the VOC sensor, the patterned high-TCR film functions as a temperature sensor, while in the vicinity of the relative humidity sensor  160 , the patterned high-TCR film forms a metal cap over the vias  236  and contacts  232  as shown in  FIG. 8B . The temperature sensing elements  238  can be made of, for example, platinum (Pt) or chromium silicide (CrSi 2 ) having a thickness of about 20 nm. A second conformal ILD  240  is then deposited over the temperature sensing elements  238  as shown in  FIGS. 9A and 9B . The second conformal ILD  240  can be, for example, 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. 
         [0049]    At  216 , a VOC gas sensor  242  is formed over a polyimide well  226 , adjacent to a resistive heater  154 , as shown in  FIG. 9A . The VOC gas sensor  242  may be made of tin oxide (SnO 2 ) having a thickness in the range of about 30 nm-100 nm. Alternatively, the VOC gas sensor  242  may be made of zinc oxide (ZnO 2 ) having a thickness of about 100 nm, or indium oxide (In 3 O 3 ) having a thickness of about 150 nm. Vias  252  are formed in the second conformal ILD  240  and are filled with a third metal layer made of TiW and AlCu having a thickness of about 500 nm. The third metal layer may overlap a portion of the VOC gas sensor film that is outside the vicinity of the polyimide well  226 , as shown in  FIG. 9A . Where the third metal layer is situated over an active polyimide structure  234 , the third metal layer is patterned to form a metal mesh top capacitor plate  255 . 
         [0050]    At  218 , a passivation layer  254  is formed over the metal mesh top capacitor plates  255  and the VOC sensor  242 , as shown in  FIGS. 9A, 9B . The passivation layer  254  may be made of SiN. The passivation layer  254  is patterned to expose the VOC sensor  242 , and to provide a signal path via the various metal layers to access the temperature sensor  238  and the contact  232  to the heating element  230 . The VOC sensor  242  has an exposed active sensing area of about 200 μm 2 ×100 μm 2 . Openings  237  are etched through the passivation layer  254 , through holes in the metal mesh top capacitor plate  255 , and through the various ILD layers below the metal mesh top plate of C rh  to expose the active polyimide to ambient air. Additional lateral removal of a plurality of polyimide plugs having widths  239  increases the surface area of polyimide that contacts the ambient air. Polyimide removal may utilize a dry etch or a wet etch chemistry suitable for removing photoresist. When the humidity of the ambient air changes, the dielectric constant, κ, of the active polyimide structure will be affected, and will cause variation in the capacitance of C rh  relative to the reference capacitor C o . A completed relative humidity sensor  160  is shown in cross-section in  FIG. 9B . 
         [0051]    Meanwhile, during the same processing step, polyimide material is removed from the polyimide wells  226 . Openings  256  are formed by etching through the VOC sensor  242  and the ILD layer stack to expose the polyimide wells  226 . The additional lateral etching step removes 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 . 
         [0052]      FIG. 10  shows a top plan view of the relative humidity sensor  160 , in which the metal mesh top plate  255  of C rh  is on the left and the metal mesh top plate of the reference capacitor C o  is on the right. Each parallel plate capacitor in the relative humidity sensor  160  has a surface area of about 200×300 microns. The top plate and bottom plate overlap area is about 11.2E-8 m 2 . The dielectric constant of the active polyimide material is about κ=3. The capacitance of the relative humidity sensor therefore can be estimated as: 
         [0000]    
       
      
       C=∈A/d  
      
     
         [0000]      =(3∈ o   F/m )(11.2 E -8 m 2 /(7.0 E -6 m)
 
         [0000]      =0.425 pF. 
         [0053]      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. 
         [0054]      FIG. 12  shows a top plan view of the relative humidity sensor  160 , along with cross-sectional scanning electron micrographs (SEMs) of the metal mesh top plates  255  of C rh  (left) and the reference capacitor C o  (right). The openings  237  exposing the active polyimide structures  235  to ambient air are evident in the left hand image, while no such openings are evident in the right hand image. 
         [0055]    By fabricating the temperature sensor  152 , the resistive heater  154 , one or more VOC sensors  156 , the relative humidity sensor  160 , and the ambient temperature sensor  162  using the same processing steps as outlined above, it is possible to co-integrate all five sensor functions on the same die, creating a full gas analyzer. 
         [0056]    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 hersein by reference, in their entireties. 
         [0057]    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. 
         [0058]    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.