Abstract:
A humidity sensor element for a humidity sensing device includes a rigid, p-doped silicon substrate, a non-porous terminal on one side of the substrate, a porous terminal on a second side of the substrate, and a layer of polyphenylsulfone between the porous terminal and the substrate. The sensor element displays improved linear response with humidity changes and very low hysteresis.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to humidity sensor elements for use in humidity-sensing devices, and more particularly to humidity sensor elements containing sulfone polymers.  
           [0003]    2. The Prior Art  
           [0004]    Moisture-sensing devices which detect humidity levels by measuring the electrical capacitance of moisture-sensing elements containing sulfones are known. For example, U.S. Pat. No. 5,069,069 discloses moisture-sensing devices containing moisture-sensing films whose active layer is composed of either polyethersulfone or polysulfone. Although such known moisture-sensing devices work reasonably well, there is always a need to find alternative moisture-sensing polymers for use in moisture-sensing elements that will have more favorable physical and chemical characteristics, such as a more linear response with change in bulk dielectric constant with relative humidity, and thus provide a better functioning moisture-sensing device. I have discovered such an element.  
         SUMMARY OF INVENTION  
         [0005]    According to my invention, a moisture or humidity sensor element for use in a moisture or humidity-sensing device includes polyphenylsulfone as its sensor polymer. Due to the advantageous properties of polyphenylsulfone such as solvent resistance and hydrolytic stability, the sensor element will display almost linear changes in dielectric constant with moisture level variation, and devices using the sensor element will display very low hysteresis and a low time constant (about 20 seconds at room temperature with hysteresis at 1% RH).  
           [0006]    The inventive sensor element includes a rigid conductive substrate, a non-porous terminal layer on one side of the substrate, a porous or permeable terminal layer on a second side of the substrate, and a layer of polyphenylsulfone between the substrate and the porous terminal layer. The rigid, conductive substrate is advantageously made of p-doped silicon and the non-porous terminal layer is advantageously made of dual layers of gold and chromium. The porous terminal layer is made of gold, dual layers of gold and chromium, or a composite layer of lampblack and a binder which is permeable to water vapor and at least partially miscible in polyphenylsulfone.  
           [0007]    For enhanced physical characteristics of the sensor element, including adhesion of the layers and integrity of the polyphenylsulfone layer, the doped silicon substrate is covered on both sides with a layer of silicon oxide. Other polymeric adhesion layers can be advantageously located between the polyphenylsulfone layer and the silicon oxide layer, and between the polyphenylsulfone layer and the porous terminal layer.  
           [0008]    The invention will be better understood by reference to the attached drawings, taken with the following discussion. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]    In the drawings,  
         [0010]    [0010]FIG. 1 shows, an enlarged scale, a cross section of a humidity sensor element in accordance with a first preferred embodiment of the invention,  
         [0011]    [0011]FIG. 2 is a cross section similar to FIG. 1 of a humidity sensor element in accordance with a second preferred embodiment,  
         [0012]    [0012]FIG. 3 is a cross section similar to FIG. 1 of a humidity sensor element in accordance with a third preferred embodiment,  
         [0013]    [0013]FIG. 4 is a schematic illustration of first humidity sensor apparatus which utilizes the humidity sensor elements of FIG. 1 or  2 ,  
         [0014]    [0014]FIG. 5 is a cross section of a humidity sensor element as shown in FIG. 2 when modified to work in the FIG. 4 apparatus,  
         [0015]    [0015]FIG. 6 is a schematic illustration of a second humidity sensor apparatus which utilizes the humidity sensor elements of FIGS. 1, 2 or  3 , and  
         [0016]    [0016]FIG. 7 is a schematic illustration of a third humidity sensor apparatus which utilizes the humidity sensor elements of FIGS. 1, 2 or  3 . 
     
    
     DETAILED DESCRIPTION  
       [0017]    [0017]FIGS. 1, 2 and  3  show cross sections through three preferred humidity sensor elements of the present invention. In the following discussion the terms “above” and “below” will be used based on relative vertical positionings as shown in FIGS. 1, 2 and  3 .  
         [0018]    Referring first to FIG. 1, the sensor element  20  is seen to include a rigid conductive substrate  1  made of doped silicon, a humidity sensor layer  2  of polyphenylsulfone located above the substrate, a terminal layer  3  of porous gold located above the sensor layer  2 , and a non-porous terminal layer  4  of a chromiumgold composite located below the substrate  1 . The terminal layers  3  and  4  are intended for connection to the leads of a humidity testing circuit. The pores in the gold terminal layer  3  are sufficiently large to allow passage of water vapor molecules therethrough for contact with the sensor layer  2 . This is achieved by vapor deposition of the gold under controlled conditions. The polyphenylsulfone layer is between about 1 and 3 microns in thickness.  
         [0019]    A first adhesion layer  5  of silicon oxide is located below the substrate  1  and between the substrate and the terminal layer  4  to adhere the terminal layer to the substrate. A second adhesion layer  6  of silicon oxide is located above the substrate  1  and below a layer  7  formed of an aminosilane or amino polyamic acid, which layer  7  also acts as an anchoring layer between the sensor layer and the substrate. The silicon oxide layers  5  and  6  can be thermally grown on the substrate  1 . The layer  7  can be formed by coating layer  6  with either aminopropylsilane followed by a 1-3% solution of polyamic acid to form polyimide (Dupont Series P12000 or P12600). The polyimides form covalent bonds with the substrate  1  via the aminopropylsilane layer and are miscible in polyphenylsulfone.  
         [0020]    Located between the sensor layer  2  and the terminal layer  3  is an anchoring layer  8  of mercaptopropylsilane. This layer is needed because the gold terminal layer  3  does not attach well to the polymeric sensor layer  2 . A further layer  9  of mercaptopropylsilane is located above the terminal layer  3  to help attach an electrical lead to the terminal layer, as well as an applied water vapor-permeable barrier coating  10 . This barrier coating can be in the form of polysulfone applied from a butyrophenone-acetone solution.  
         [0021]    The sensor element  20   a  of FIG. 2 includes a rigid conductive substrate  1   a  of doped-silicon and layers  2   a ,  3   a ,  4   a ,  5   a ,  6   a ,  7   a  and  10   a  similar to layers  2 - 7  and  10  in the FIG. 1 embodiment; however, the porous terminal layer  3   a  is made of dual layers of chromium and gold (chromium deposition at 3-5 a/sec followed by gold at 3-4 a/sec), and no anchoring layer similar to layer  8  in FIG. 1 is included. The chromium-gold composite layer  3   a  is between about 400 and 700 angstoms in thickness.  
         [0022]    The sensor element  20   b  of FIG. 3 includes a rigid conductive substrate  1   c  of doped silicon and layers  2   b ,  3   b ,  4   b ,  5   b ,  6   b ,  7   b  and  10   b  similar to layers  2 - 7  and  10  in the FIG. 1 embodiment; however, the water vapor-permeable terminal layer  3   b  is made of a composite of lampblack and a polymer binder. This layer can be formed from a mixture of lampblack and a binder of polyethersulfone, a soluble aramid (such as the condensation product of bis (4-aminophenyl) ether and isophthaloyl chloride), or a soluble polyimide (such as a condensation product of 3,3′, 4,4′ benzophenone tetracarboxylic diahydride and 5(6)-amnio-1-(4-aminophenyl)-1,3,3′ trimethylindane in a suitable solvent such as dimethylsulfoxide, butryrophenone, tetrahydrofuran, 1-4 dioxane, acetophenone, cyclohexanone, m-cresol or butyrolactone. No attachment layer similar to layer  9  is needed. The terminal layer  3   b  is between about 5 and 25 microns in thickness.  
         [0023]    [0023]FIGS. 4, 6 and  7  depict embodiments of humidity sensor devices in which the inventive humidity sensor elements of this invention can be used. In FIG. 4 a conventional T05 can  30  with connectors  31  and  32  includes a sensor element  35  according to the present invention (either sensor element  30  or  30   a ) located on a flat mounting plate  33 , a lead wire  34  from connector  32  being attached (bonded) to upper terminal layer of the sensor element, while the lower terminal layer is connected to the connector  31  by a layer of conductive epoxy (not shown). For best results, when the humidity sensor element of FIG. 1 is used in the humidity sensor device of FIG. 4, it is modified to include a gold via  30  for attachment of a lead (not shown) to the element, the gold via  30  extending from above the barrier coating  10   c  through the polymeric anchoring layer  9   c , and through the sensor layer  2   c  (see FIG. 5) so as to provide an adequate and durable connection of the lead to the element  20   c . The via can cover between 1 and 5% of the surface area of the terminal  3   c  and be provided by vapor deposition of gold (5,000-10,000 angstroms).  
         [0024]    In FIG. 6 the sensor apparatus  40  includes an alumina substrate  41 , conductive traces  42  and  43  (for connection to a lead frame), and a conductive mounting plate  44  on which a sensor element of the present invention (either sensor element  30 ,  30   a  or  30   b ) is positioned. A portion  43   a  of trace  43  extends through the alumina substrate  41  to attach to the lower terminal of the sensor element, while a conductive stitch  46  that extends around an insulating stitch  47  connects the upper terminal of the sensor element to the conductive trace  43 .  
         [0025]    In FIG. 7 the sensor apparatus  50  includes a mold  51  having hinges  52 ,  53  and a porous roof  51   a , lead frames  54  and  55 , and a conductive mounting plate  56  on which a sensor element  58  of the present invention (either sensor element  30 ,  30   a  or  30   b ) is positioned. The lead frame  55  is electrically connected to the mounting plate  56 , which in turn is electrically connected to the lower terminal layer of the sensor element. A contact spring  57  is positioned between the sensor element  58  and the lead frame  54  to electrically connect the upper terminal layer thereof with the lead frame  54 .  
         [0026]    Although various preferred embodiments of the invention have been shown and described, modifications can be made therein and still fall within the scope of the appended claims. For example, the barrier layer can be excluded from the inventive sensor element when used in certain sensor apparatus, e.g., the sensor apparatus of FIG. 4.