Abstract:
A gas sensor using a metal organic framework material can be fully integrated with related circuitry on a single substrate. In an on-chip application, the gas sensor can result in an area-efficient fully integrated gas sensor solution. In one aspect, a gas sensor can include a first gas sensing region including a first pair of electrodes, and a first gas sensitive material proximate to the first pair of electrodes, wherein the first gas sensitive material includes a first metal organic framework material.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/592,547 filed on Jan. 30, 2012, which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to an integrated gas sensor. 
       BACKGROUND 
       [0003]    Over the past decades, sensor microsystems developed from simple transducers to a complete microsystem can include signal conditioning, analog-to-digital conversion, intelligence, and wireless link. Integrating sensors with electronic circuitry can offer several advantages, e.g., reduced size, weight, and cost, improved signal transduction, lower power consumption, and improved immunity to external noise sources. 
         [0004]    Gas sensors find numerous industrial, medical, scientific, and military applications. One important application can include monitoring of toxic gases in oil and petrochemical industries. Another application can be building wireless gas sensor networks for environmental monitoring over large areas. Although the topic of gas sensing is relatively old, recent reviews show that the selectivity, dynamic range, and integration level of available gas sensors continue to fall short of the requirements of different applications. 
       SUMMARY 
       [0005]    A gas sensor using a metal organic framework material can be fully integrated with related circuitry on a single substrate. In an on-chip application, the gas sensor can result in an area-efficient fully integrated gas sensor solution. 
         [0006]    In one aspect, a gas sensor can include a first gas sensing region including a first pair of electrodes, and a first gas sensitive material proximate to the first pair of electrodes, wherein the first gas sensitive material includes a first metal organic framework material. 
         [0007]    The gas sensor can further include a monitor module electrically connected to the first pair of electrodes and configured to monitor at least one electronic property of the first gas sensitive material. The electronic property of the first gas sensitive material monitored by the monitor module includes one or more of impedance, resistance, or capacitance. 
         [0008]    In another aspect, a chip can include the gas sensor described herein. 
         [0009]    In another aspect, an on-chip microsystem for gas sensing can include the gas sensor described herein. 
         [0010]    In another aspect, a method of making a gas sensor can include forming a first pair of electrodes on a substrate, and forming a first gas sensitive metal organic framework material proximate to the first pair of electrodes. 
         [0011]    The method can further include forming a monitor module on the substrate to monitor at least one electronic property of the first gas sensitive metal organic framework material. The method can include forming a second pair of electrodes on the substrate. 
         [0012]    In another aspect, a gas sensor system can include a substrate, and a plurality of gas sensor described herein on the substrate. 
         [0013]    In another aspect, a method of monitoring a gas concentration includes exposing a gas sensor region to a detectable gas; monitoring at least one electronic property of a gas sensitive material including a metal organic framework material; and correlating the electronic property of the gas sensitive material with the presence of the detectable gas 
         [0014]    The first pair of electrodes can be interdigitated. Both of the first pair of electrodes and the monitor module are on a single substrate. 
         [0015]    A material is proximate to another structure of the device if it is nearby that structure in a manner that allows the device to function. In some embodiments, the proximate material is in contact with the other structure of the device. In other embodiments, the proximate material is in contact with another material which is in contact with the other structure of the device. For example, the first gas sensitive material can be on top of the first pair of electrodes. 
         [0016]    The gas sensor can further include a second gas sensing region including a second pair of electrodes, and a second gas sensitive material proximate to the second pair of electrodes. The second gas sensitive material can include a second metal organic framework material. The first and second pair of electrodes can be on the same substrate. The first and second pair of electrodes can have different compositions, different dimensions, or combinations thereof. The first and second metal organic framework materials can have different compositions, different dimensions, or combinations thereof. The gas sensor can include a plurality of gas sensing regions, such as, 2, 3, 4 or more regions in a single gas sensor. 
         [0017]    The gas sensor can include a passivation layer between the gas sensitive material and the electrodes. The gas sensor can include a filter layer proximate to the gas sensitive material for enhancing the selectivity of the gas sensor. The filter layer can include a metal organic framework. 
         [0018]    The gas sensor can include a wireless communication module to transmit a measurement result of the monitor module. The gas sensor can be configured to receive power from a power unit. The power unit can include a battery. The power unit can include an energy harvesting device to collect energy from one or more of ambient energy sources, including solar energy, thermal energy, or radio frequency energy. In certain embodiments, the energy harvesting device can be a component of a microsystem including the gas sensor, for example, on the same chip. 
         [0019]    Other aspects, embodiments, and features will be apparent from the following description, the drawings, and the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  is a diagram illustrating a gas sensor. 
           [0021]      FIGS. 2  A-C are diagrams illustrating gas sensors with different configurations. 
           [0022]      FIG. 3  is a diagram illustrating a gas sensor. 
           [0023]      FIGS. 4  A-B are diagrams illustrating the capacitance to voltage conversion circuits. 
           [0024]      FIG. 5  is a diagram illustrating a gas sensor. 
           [0025]      FIG. 6  is a diagram illustrating a gas sensor. 
           [0026]      FIG. 7  is a diagram illustrating a gas sensor. 
           [0027]      FIG. 8  is a diagram illustrating a gas sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Integrated gas sensors can be fabricated with standard circuitry, including complementary metal oxide circuitry (CMOS). CMOS gas sensors are described, for example, in W. Gardner, P. K. Guha, F. Udrea &amp; J. A. Covington, “CMOS interfacing for integrated gas sensors: A review,” IEEE Sensors Journal, vol. 10, no. 12, pages 1833-1848, 2010, which is incorporated by reference in its entirety. Metal organic framework materials (MOFs) are crystalline materials composed of both inorganic and organic components in a porous networked structure. Metal organic framework materials exhibit exceptionally high specific surface area, in addition to tunable pore size and functionality, which make them attractive in many applications, including gas storage, gas separation, catalysis, drug delivery, light-emitting devices, and sensing. See, for example, Scott T. Meek, “Metal-Organic Frameworks: A Rapidly Growing Class of Versatile Nanoporous Materials,” Advanced Materials. 2011; 23(2):249-267; Mark D. Allendorf, Adam Schwartzberg, Vitalie Stavila &amp; A. Alec Talin, “A Roadmap to Implementing Metal-Organic Frameworks in Electronic Devices: Challenges and Critical Directions.” Chemistry—A European Journal, vol. 17, no. 41, pages 11372-11388, 2011; and O. Shekhah, J. Liu, R. A. Fischer and Ch. Wöll, “MOF thin films: existing and future applications” Chem. Soc. Rev., 2011, 40, 1081-1106, each of which is incorporated by reference in its entirety. An integrated sensor using metal organic framework material capitalizes on the unique properties of MOFs that allow them to be integrated in gas sensor microsystems. The sensor microsystem can be implemented in a lab-on-chip (LoC) for environmental monitoring and assessment of manufacturing plant conditions, air quality, and air pollution. Examples of LoC structures can be found, for example, in Ebrahim Ghafar-Zadeh &amp; Mohamad Sawan, “CMOS capacitive sensors for lab-on-chip applications: A multidisciplinary approach” Springer, 2010, which is incorporated by reference in its entirety. 
         [0029]    The metal organic framework material can passivate an active layer of a gas sensor. Alternatively, the metal organic framework material can be selected to change in capacitance or resistance upon exposure to a gas to be detected. The properties of the metal organic framework material can be adjusted by design of the material. Due to the presence of inorganic and organic molecules in the metal organic framework material, the pore size and the chemical behavior of the metal organic framework material can be tuned to satisfy specific requirements, which make them more versatile than other porous inorganic materials Metal organic framework materials can serve as transducers by changing electronic, optical, or structural properties upon exposure to an analyte. Examples of transduction mechanisms include change in dielectric constant, change in luminescence signal, structural swelling, and mass change. These types of changes can be detected by various methodologies, including by monitoring capacitance or resistance of the material. The versatility of metal oxide framework materials due the ability to tune pore size and chemical functionality, in addition to their exceptionally high surface area, make these materials attractive for gas sensing applications. Gases for detection can include, but are not limited to, ammonia, carbon monoxide, carbon dioxide, volatile organic compounds (VOCs), water vapor, and light hydrocarbons. 
         [0030]    A schematic of an integrated gas sensor system is shown in  FIG. 1 . Gas sensor  100  includes two metal electrodes  20  and gas sensitive layer  30 . Various electrode structures, such as interdigitated electrode structures, can be used. Electrodes can be built using the top metal layers of standard CMOS process with CMOS circuitry implemented on substrate  10 . 
         [0031]    Interface circuits to measure the change in resistance, capacitance or impedance (real and imaginary components) can be implemented on chip. The integrated microsystem can also include radiofrequency transceiver for wireless connectivity. Examples of sensors with wireless capability are described, for example, in Radislav A. Potyrailo, Cheryl Surman, Nandini Nagraj &amp; Andrew Burns, “Materials and Transducers Toward Selective Wireless Gas Sensing,” Chemical Reviews, vol. 111, no. 11, pages 7315-7354, 2011, which is incorporated by reference in its entirety. Control logic and memory can be used to store and process readings. The sensor or system can be powered by a battery or use energy harvesting techniques to collect energy from ambient energy sources, such as, for example, solar energy, thermal energy, or radiofrequency energy, or combinations thereof. 
         [0032]    The metal organic framework materials can be used as the main functional element (gas sensitive layer  30 ) or as an auxiliary element. If used as the main functional element, the metal organic framework material is deposited or grown onto sensing electrodes  20  and the electric properties of the material is monitored upon exposure to the gas analyte. The electrical property being monitored can be the impedance (real and imaginary components), resistance, or capacitance of the sensing layer. 
         [0033]    Most metal organic framework materials are dielectrics. In this case, the capacitance will be the property being monitored. The change in capacitance can be due to a change in dielectric constant or swelling of the gas sensitive layer upon exposure to a specific gas. 
         [0034]    The deposition of metal organic framework materials as the gas sensitive material on the electrodes and the configuration of the gas sensor can take many forms as shown in  FIG. 2 . The bond pad etch can be used to remove any passivation layer and deposit the gas sensitive material  30  directly on top of metal electrodes  20  (A of  FIG. 2 ). Interfacial layer  40  can be grown or deposited between electrodes  20  and gas sensitive material  30  as a passivation layer or adhesion layer (B of  FIG. 2 ). In this case only the capacitance of gas sensitive material  30  can be monitored. Electrodes  20  can also have multilayer structure to increase the surface area, hence the total capacitance and the percentage of electric fields lines passing through gas sensitive material  30  (C of  FIG. 2 ). 
         [0035]    To be used as an auxiliary element of the gas sensor, metal organic framework material can act as a filter to enhance the selectivity of the gas sensor. As shown in  FIG. 3 , metal organic framework filter film  50  can be deposited on top of gas sensitive material  30 , which can be another metal organic framework material or a different material. 
         [0036]    For measuring the small variations of resistance, a Wheatstone bridge can be used to convert the change in resistance into voltage. For capacitance to voltage conversion, continuous time or discrete time circuits can be used. Examples of continuous time circuit (A) and discrete time circuits (B) are shown in  FIG. 4 . Using fully differential circuits can yield improved performance. The voltage signal can be further digitized by on-chip analog-to-digital conversion circuitry. 
         [0037]    Using differential signal arrangement can yield improvement in performance. As shown in  FIG. 5 , reference electrodes  21  can be used in addition to sensing electrodes  20  in order to provide the differential change in impedance (real and imaginary components), capacitance or resistance. Reference electrodes  21  can be covered with passivation only, or can be covered with the same layers of sensing electrodes  20  but the packaging is arranged such that only sensing electrodes  20  are exposed to the gas analyte. Examples of using different signal arrangement are described, for example, in C. Hagleitner, D. Lange, A. Hierlemann, O. Brand &amp; H. Baltes, “CMOS single-chip gas detection system comprising capacitive, calorimetric and mass-sensitive microsensors,” Solid-State Circuits, IEEE Journal of, vol. 37, no. 12, pages 1867-1878, 2002, which is incorporated by reference in its entirety. Electronic circuit  60  can be fabricated on the same substrate. 
         [0038]    To sense different gases, several dies (each with a different metal organic framework material) can be integrated in a single package. Alternatively, different metal organic framework materials can be grown on the same die where each one is selective to a different gas, as shown in  FIG. 6 . Multiple metal organic framework materials can be also used to sense the same gas, but the units can have different physical parameters to have different dynamic range, sensitivity, etc., as shown in  FIG. 7 , and then sensor fusion techniques are used to produce an overall enhanced response. For example, metal organic framework materials can be different and have difference size or thickness. 
         [0039]    In order to compensate for the effect of humidity, in addition to the gas sensitive material, another material can be used to sense humidity. A temperature sensor can be also integrated in order to compensate for the effect of temperature on the sensor response. 
         [0040]    There are a few available methods to form metal organic framework materials on electrodes. Conditions are selected to achieve adhesion and stability for the gas sensor application. Metal organic framework materials can be formed from solvothermal mother solutions. In this approach, the substrate with sensing electrodes can be simply immersed into the solution during metal organic framework formation. Another method can be microwave-induced thermal deposition. To make a gas sensor with multiple layers of materials, liquid phase epitaxy (LPE) can be used. The substrate with sensing electrodes can be immersed into solutions of the reaction partners in a sequential, stepwise fashion to synthesize crystalline metal organic framework materials as thin films. 
         [0041]    The sensor microsystem can include an integrated power unit and/or a power connecting interface. The sensor can be powered by a battery as an external or integrated component. The sensor can be self-powered using energy harvesting. As shown in  FIG. 8 , power unit  70  can be implemented on the same chip/substrate to supply power to electronic circuit  60 . Power unit  70  can include a battery and/or an energy harvesting device to collect energy from one or more of ambient energy sources, including solar energy, thermal energy, or radio frequency energy. 
         [0042]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, although two capacitance to voltage conversion circuits are shown, the gas sensor can include any other suitable capacitance to voltage conversion circuit that can provide satisfied performance. 
         [0043]    Other embodiments are within the scope of the following claims.