Abstract:
A temperature sensor including a capacitive circuit including an input terminal for the application of an input voltage, an output terminal for the reading of an output voltage of the circuit, and a reference potential terminal, a voltage circuit for applying a predetermined voltage to the input terminal of the circuit and a circuit for reading the voltage at the output terminal of the capacitive circuit and converting the read voltage into a temperature measurement. According to the invention, the capacitive circuit includes a fir capacitor, connected between the input and output terminals, and having a capacitance decreasing according to temperature; and a second capacitor, connected between the input terminal and the terminal at the reference potential, and having a capacitance increasing along with temperature.

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of temperature sensors, and more particularly capacitive temperature sensors. 
     BACKGROUND OF THE INVENTION 
     As known per se, the capacitance of a capacitor varies according to temperature. By measuring this capacitance or any other variable directly associated therewith, and knowing the relationship between the temperature and the capacitance value, it is thus possible to measure the capacitor temperature. This principle is the base of so-called “capacitive” temperature sensors. The use of a single capacitor however does not provide a sufficient capacitance variation range to obtain an accurate measurement of the temperature over a wide range. 
     SUMMARY OF THE INVENTION 
     The present invention aims at providing a capacitive sensor of simple design, having a high sensitivity over a wide temperature range. 
     For this purpose, the present invention aims at a temperature sensor comprising:
         a capacitive circuit comprising an input terminal for the application of an input voltage, an output terminal for the reading of an output voltage of the circuit, and a reference potential terminal;   a voltage circuit for applying a predetermined voltage to the input terminal of the circuit; and   a circuit for reading the voltage at the output terminal of the circuit and converting the read voltage into a temperature measurement.       

     According to the invention, the capacitive circuit comprises:
         a first capacitor, connected between the input and output terminals, and having a capacitance decreasing according to temperature; and   a second capacitor, connected between the input terminal and the terminal at the reference potential, and having a capacitance increasing along with temperature.       

     In other words, the output voltage of the capacitive circuit thus follows relation: 
                 V   out     ⁡     (   T   )       =             C   1     ⁡     (   T   )       +       C   2     ⁡     (   T   )             C   1     ⁡     (   T   )         ×     V   in             
where:
         V in  is the voltage between the input terminal and the reference terminal of the capacitive circuit;   V out  is the voltage between the output terminal and the reference terminal of the capacitive circuit;   T is the temperature of the capacitive circuit;   C 1  is the capacitance of the first capacitor; and   C 2  is the capacitance of the second capacitor.       

     Thus, by using such an assembly, a very high sensitivity is obtained over a wide temperature range, particularly due to variations of capacitance C 2  according to temperature. Starting from the expression of V out , particularly for low and medium frequencies, when the temperature increases, the value of capacitance C 2  increases while, in the mean time, the value of capacitance C 1  decreases. Thus, the value of V out  at the sensor output increases. When the temperature decreases, the value of capacitance C 2  decreases while, in the mean time, the value of capacitance C 1  increases, which implies a decrease in the value of V out  at the output. The strong variation of the value of V out  at the sensor output accompanying the variation of the temperature, for a constant V in , thus implies a high sensitivity. Such an assembly behaves as an amplifying assembly and enables to have a maximum response at the capacitive circuit output. 
     According to an embodiment of the invention, the capacitive circuit comprises an insulating substrate and a stack formed on the substrate, successively comprising:
         a first conductive electrode formed on the substrate;   a layer of a first dielectric;   a second conductive electrode;   a layer of a second dielectric; and   a third conductive electrode,
 
the first capacitor being formed by the first dielectric layer sandwiched between the first electrode and the second electrode, and the second capacitor being formed by the second dielectric layer sandwiched between the second electrode and the third electrode
       

     More specifically:
         a first, a second, and a third different conductive tracks, each comprising a conductive connection area, are formed on the insulating substrate, the first track being connected to the first electrode;   the first dielectric layer is deposited on at least the first electrode and a portion of the second track;   the second metal electrode is deposited on the first dielectric layer at least vertically above the first electrode and a portion of the second track, the second electrode being connected to the second track by means of a metal connection crossing the first dielectric layer;   the second dielectric layer is deposited on at least the second electrode and vertically above a portion of the third track; and   the third metal electrode is deposited on the second dielectric layer at least vertically above the second electrode and a portion of the third track, the third electrode being connected to the third track by means of a metal connection crossing the first and second dielectric layers.       

     A compact system is thus obtained, since there only exist three electrodes for two capacitors and the same are stacked on each other. This circuit may have a wide range of capacitor surface areas, particular large surface area, and may be formed by means of simple manufacturing techniques. 
     According to an embodiment, the dielectric of the first capacitor is a fluorinated polymer having a dielectric constant smaller than or equal to 2, for example, a dielectric constant equal to 2 (∈ r =2). 
     Such a fluorinated polymer is characterized by a low adherence to the second metal electrode deposited thereon, for example, by silk screening of a silver-based ink. Such a characteristic low adherence enables to create microgaps between this fluorinated dielectric polymer and the second conductive electrode. Such microgaps will generate a variable electric capacitance which will be taken into account in the total specific electric capacitance of the fluorinated polymer. This can enable to have a very good sensitivity to temperature and to capacitive touch effects. 
     Particularly, the first dielectric layer has a thickness in the range from 0.1 micrometer to 2 micrometers. 
     Advantageously, the sensor comprises a circuit for reading the capacitance of the first capacitor and for converting the read capacitance into a temperature measurement detected on the first capacitor. A capacitive pressure sensor is thus obtained with no additional capacitive component. 
     According to an embodiment, the dielectric of the second capacitor is a ferroelectric polymer or copolymer, particularly polyvinylidene fluoride-chlorotrifluoroethylene, or a polyvinylidene fluoride, or a copolymer of one and/or the other thereof. Such a polymer has very large capacitance variations according to temperature. More particularly, the second dielectric layer has a thickness in the range from 1 micrometer to 5 micrometers. 
     Advantageously, the first dielectric is a piezoelectric ferroelectric polymer or copolymer, and the sensor comprises a circuit for reading the voltage at the input terminal and for converting the measurement of a pressure applied to the electrodes of the second capacitor into a read voltage. A capacitive touch sensor is thus obtained with no additional capacitive component. 
     According to an embodiment, each of the first, second, and third electrodes has a thickness in the range from 30 nanometers to 15 micrometers. 
     According to an embodiment, the substrate is a flexible plastic substrate, particularly made of PEN (polyethylene naphthalate) or of PET (polyethylene terephthalate), which enables to form a flexible capacitive circuit and to increase the sensitivity of the pressure sensor and of the touch sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood on reading of the following description provided as an example only in relation with the accompanying drawings, where the same reference numerals designate the same or similar elements, among which: 
         FIG. 1  is a simplified view of a pressure sensor according to the invention; 
         FIG. 2  is a simplified cross-section view of a stack forming the capacitors of the capacitive circuit of  FIG. 1 ; 
         FIGS. 3 to 8  are simplified top views of a method of manufacturing a capacitive circuit comprising the stack of  FIG. 2 ; and 
         FIGS. 9 and 10  are plots of the capacitance of the first and of the second capacitor of the capacitive circuit respectively according to frequency and for different temperatures. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a temperature sensor  10  according to the invention comprises a capacitive circuit  12  having two input terminals  14 ,  16  and two output terminal  18 ,  20 , and having its voltage transfer function from a voltage V in  applied between input terminals  14 ,  16  to a resulting output voltage V out  between output terminals  18 ,  20  varying in known fashion according to the temperature of circuit  12 . 
     More particularly, input terminal  16  and output terminal  20  are reference terminals and are taken to a same potential, for example, a ground potential, and capacitive circuit  12  comprises a first capacitor  22 , connected between input terminal  14  and output terminal  18 , and a second capacitor  24 , connected between input terminals  14 ,  16 . 
     The transfer function between voltage V out  and voltage V in  is thus provided by relation: 
                       V   out     ⁡     (   T   )       =             C   1     ⁡     (   T   )       +       C   2     ⁡     (   T   )             C   1     ⁡     (   T   )         ×     V   in               (   1   )               
where:
         T is the temperature of capacitive circuit  12 ;   C 1  is the capacitance of first capacitor  22 ; and   C 2  is the capacitance of second capacitor  24 .       

     First capacitor  22  has a capacitance which decreases according to temperature T. More particularly, first capacitor  22  comprises a dielectric interposed between two conductive electrodes, for example, metallic or organometallic, and made of a fluorinated polymer having a low dielectric constant smaller than or equal to 2 (∈ r =2), for example, a dielectric constant equal to 2. 
     Second capacitor  24  has a capacitance which increases along with temperature T. More specifically, second capacitor  24  comprises a dielectric interposed between two conductive electrodes, for example, metallic or organometallic, and made of a ferroelectric polymer or copolymer. 
     The expression of V out  in relation (1) can be rewritten, for a constant voltage V in , according to relation:
 
 V   out ( T )= S×T+V   0   (2)
 
where:
         S is the detector sensitivity expressed in V/° C.; and   V 0  is a threshold voltage.       

     Sensitivity S is thus equal to 
     
       
         
           
             S 
             = 
             
               
                 
                   ∂ 
                   
                     V 
                     out 
                   
                 
                 
                   ∂ 
                   T 
                 
               
               ⁢ 
               
                 ❘ 
                 
                   
                     V 
                     in 
                   
                   = 
                   
                     constant 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     e 
                   
                 
               
             
           
         
       
     
     Temperature sensor  10  further comprises a voltage source  26  for applying a predetermined voltage V in  between input terminals  14 ,  16 , as well as a read circuit  28  connected to output terminals  18 ,  20  for reading the voltage V out  and analyzing the measurement thereof. 
     More particularly, read circuit  28  comprises means for sampling the measured voltage and means for polling a table of output voltages measured according to the detected temperature, said table being previously determined and stored in circuit  28 . 
     The dielectric of second capacitor  24  is a ferroelectric material which also has piezoelectric properties after the dipoles have been oriented in the ferroelectric material. This material is advantageously made of polyvinylidene fluoride-chlorotrifluoroethylene (also called “PVDF-CTFE”), a polyvinylidene fluoride, or the copolymer of one and/or the other thereof. To have the piezoelectric properties (that is, the material generates electric charges when a force is exerted thereon), it is necessary to orient the dipoles by applying a strong D.C. electric field of a value in the order of 2 V/μm at 50-60° C. for several hours. Sensor  10  comprises a circuit  30  connected between terminals  14 ,  16  and capable of determining a pressure exerted on second capacitor  22  according to the voltage V in  measured when current source  26  is disconnected from input terminals  14 ,  16 . 
     Circuit  30  for example comprises means for sampling the measured voltage and means for polling a table of pressures according to the measured pressure, said table being previously determined and stored in circuit  30 . 
     Optionally, temperature sensor  10  also comprises a circuit  32  connected to first capacitor  22  to determine the temperature applied thereto. For example, circuit  32  comprises means for measuring the capacitance of capacitor  22  and for determining the detected temperature according to the measured capacitance, particularly by polling a table of pressures according to the capacitance previously determined and stored in circuit  32 . 
     An embodiment of capacitors  22  and  24  is illustrated in the simplified cross-section view of  FIG. 2 . This drawing shows a stack formed on an insulating substrate  40 . Substrate  40  is advantageously flexible and made of plastic, for example, PEN or PET. The stack comprises:
         a first conductive electrode  42 , for example, metallic or organometallic, formed on substrate  40 ;   a dielectric layer  44  used to form first capacitor  22 ;   a second conductive electrode  46 , for example, metallic or organometallic;   a dielectric layer  48  used to form second capacitor  24 ;   a third conductive electrode  50 , for example, metallic or organometallic; and   an insulating encapsulation layer  52 , advantageously flexible and made of an insulator or of a dielectric having a low dielectric constant, for example, the same material as that forming dielectric layer  44 .       

     First capacitor  22  is thus formed of dielectric layer  44  sandwiched between first electrode  42  and second electrode  46 . Second capacitor  24  is formed of dielectric layer  48  sandwiched between second electrode  46  and third electrode  50 . 
     Three terminals schematically illustrated at references  54 ,  56 ,  58  are respectively connected to the three electrodes  42 ,  46 , and  50  to measure the different voltages. 
     Referring to simplified top views  3  to  8 , a method of manufacturing a capacitive circuit comprising a stack such as described hereabove will now be described. 
     The method starts with the full-plate deposition of a metal layer on substrate  40 , by a thickness in the range from 30 nm to 300 nm, for example, an Au, Cu, or Ag layer. The deposition for example is a physical vapor deposition (“PVD”). 
     The method carries on with the removal of portions of the metal layer to form first electrode  42  on substrate  40 , as well as three different metal tracks  60 ,  62 ,  64 , each ending in a connection area  66 ,  68 ,  70 . First track  60  is formed in continuation of electrode  42  ( FIG. 3 ). These elements are for example formed by means of a photolithography or a laser etching, laser etching allowing a low-cost production. 
     A first dielectric layer  72 , advantageously a layer of fluorinated polymer having a low dielectric constant, of a thickness in the range from 0.1 micrometer to 2 micrometers, is then deposited on the assembly except on connection areas  66 ,  68 ,  70  and ends  74 ,  76  of the second and third tracks  62 ,  64  ( FIG. 4 ). For example, layer  72  is formed by silk-screening. The assembly is then annealed. 
     A metal layer  78 , having a thickness in the range from 5 micrometers to 15 micrometers, for example, an Ag layer, is then deposited, for example, by silk screening, to form second electrode  46  vertically above first electrode  42 , as well as a metal track  80  prolonging electrode  46 . Metal track  80  is deposited above end  74  of second track  62  to form an electric contact with this end ( FIG. 5 ). The assembly is then annealed. 
     The method then carries on with the deposition of a second dielectric layer  82 , advantageously a PVDF-CTFE layer, having a thickness in the range from 1 micrometer to 5 micrometers, except on connection areas  66 ,  68 ,  70  and end  76  of third track  64  ( FIG. 6 ). This deposition is for example performed by silk screening by means of an ink based on PVDF-CTFE. Usually, PVDF-CTFE, which is a semi-crystalline polymer, is produced in the form of a powder. By dissolving the PVDF-CTFE powder in a solution comprising a first solvent, N-M-methyl-2-pyrrolidone (known as “NMP”), and a second solvent, propyl methoxyacetate (known as “PMA”) with a minority amount of acetone, an ink having a viscosity compatible with silk-screening deposition and spin-coating techniques is obtained. Once the PVDF-CTFE layer has been deposited, the assembly is annealed. 
     A metal layer  84 , having a thickness in the range from 5 micrometers to 15 micrometers, for example, an Ag layer, is then deposited, for example, by silk screening, to form third electrode  50  vertically above first and second electrodes  42 ,  46 , as well as a metal track  86  prolonging electrode  50 . Metal track  86  is deposited above end  76  of third track  64  to form an electric contact with this end ( FIG. 7 ). The assembly is then annealed. 
     Finally, an encapsulation layer is deposited on the assembly, except on connection areas  66 ,  68 ,  70 , advantageously a flexible dielectric insulating layer, for example, the same material as that forming dielectric layer  44 . A metallization of the vias above connection areas  66 ,  68 ,  70  is then achieved to obtain connection terminals  54 ,  56 ,  58  ( FIG. 8 ). 
     A very compact and flexible capacitive circuit enabling to measure both temperature and pressure is thus obtained. The techniques used are further conventional and enable to obtain, if need be, planar capacitors having very large surface areas. 
       FIG. 9  is a plot of several capacitance curves of first capacitor  42 ,  44 ,  46  according to the frequency of the voltage applied between terminals  56 ,  58 , and for different temperatures. The facing surface area of the electrodes is in this example equal to 2 mm 2 . As can be observed, the capacitance value decreases as temperature increases. 
       FIG. 10  is a plot of several capacitance curves of second capacitor  46 ,  48 ,  50  according to the frequency of the voltage applied between terminals  54 ,  56 , and for different temperatures. The facing surface area of the electrodes is in this example equal to 1.77 mm 2 . As can be observed, the capacitance value increases as temperature increases. 
     As can be seen in  FIGS. 8 and 9 , there appears that by integrating both capacitors in the capacitive circuit according to the above-described connection diagram, a temperature sensor of high sensitivity covering a temperature range at least from 20° C. to 90° C. at low and medium frequency is obtained. Voltage V in  may be adaptable to the sensor and in the case illustrated by the curves, its amplitude is in the range from 0.5 volt to a few volts.