Patent Publication Number: US-2011057540-A1

Title: Saw transponder for sensing pressure

Description:
TECHNICAL FIELD 
     The present invention relates to a surface acoustic wave (SAW) transponder for sensing pressure, and more particularly, to a SAW transponder for sensing pressure, in which a pressure sensor is connected to a SAW transponder receiving an applied radio frequency (RF) signal to generate a surface acoustic wave (SAW) and which can detect a change of a pressure through an amplitude change of a SAW. 
     BACKGROUND ART 
       FIG. 1  is a schematic view of a structure of a conventional surface acoustic wave (SAW) transponder using a surface acoustic wave (SAW). 
     The SAW transponder has a structure in which a plurality of inter digital transducer (IDT) metal electrodes are arranged on a piezoelectric substrate  1 , which is formed of a material such as LiNbO 3  having piezoelectricity, wherein the IDT metal electrodes are arranged along a propagating direction of SAW. 
     When an external transmit/receive device (not shown) wirelessly transmits an interrogation pulse signal, which is a radio frequency (RF) signal, the pulse signal is applied to a transmit/receive IDT  2  through an antenna  3  of the SAW transponder. When the pulse signal having a high frequency is incident to the transmit/receive IDT  2 , the SAW is generated by the piezoelectric substrate  1 . 
     The SAW generated from the transmit/receive IDT  2  proceeds to a detect IDT  4 . A part of the SAW continually proceeds in a proceeding direction, and simultaneously a reflective wave having an opposite direction to the proceeding direction are generated. The generated reflective wave is applied to the transmit/receive IDT  2 . The transmit/receive IDT  2  converts the applied reflective wave to a pulse signal (wireless response signal) which is an electric signal, and then wirelessly transmits the reflective wave to the external transmit/receive device through the antenna  3 . At this time, when a pressure sensor  5  of which impedance is varied according to the variance of the pressure is connected to the detect IDT  4 , the amplitude of the reflective wave is changed according to the variance of the impedance of the pressure sensor  5 . The external transmit/receive device can detect a pressure of a place on which the pressure sensor  5  is equipped by analyzing the shape of the reflective wave. 
     The SAW transponder for sensing pressure does not require an additional power supply. The SAW transponder for sensing pressure can supply a power through a RF signal applied to the transmit/receive IDT  2 , and can wirelessly transmit a detected pressure signal to the external transmit/receive device. Due to such advantages, the SAW transponder for sensing pressure is used in a tire pressure monitoring system (TPMS) detecting a pressure of the inside of a tire of a vehicle. 
     To use the SAW transponder for sensing pressure in the TPMS, there is a need for a pressure sensor that can sensitively and quantitatively detect the change of an external to pressure. In addition, there is a need for a SAW transponder for sensing pressure in which an impedance of the pressure sensor is linearly dependent to the change of the external pressure and a pressure signal can be easily analyzed by an external device. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a structure of a conventional surface acoustic wave (SAW) transponder using a surface acoustic wave (SAW); 
         FIG. 2  is a schematic view illustrating a structure of a SAW transponder for sensing pressure according to an embodiment of the present invention; 
         FIG. 3  is a cross-sectional view of a capacitive pressure sensor of the SAW transponder for sensing pressure taken along a line of  FIG. 2 ; 
         FIG. 4  is a view illustrating a lower electrode of the capacitive pressure sensor of the SAW transponder for sensing pressure of  FIG. 2 ; 
         FIGS. 5 through 8  are views illustrating a method of manufacturing the SAW transponder for sensing pressure of  FIG. 2 ; 
         FIGS. 9 and 10  are views illustrating a pressure change of the SAW transponder for sensing pressure of  FIG. 2 ; and 
         FIG. 11  is a view illustrating an RF pulse waveform of a state illustrated in  FIG. 3 ; and 
         FIGS. 12 and 13  are views illustrating RF pulse waveforms of states that are respectively illustrated in  FIGS. 9 and 10 . 
     
    
    
     EXPLANATION OF REFERENCE NUMERALS DESIGNATING THE MAJOR ELEMENTS OF THE DRAWINGS 
       
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 100; SAW transponder for sensing 
                 10; transmit/receive IDT 
               
               
                   
                 pressure 
               
               
                   
                 20; detect IDT 
                 30; capacitive pressure sensor 
               
               
                   
                 40; antenna 
                 50; detect IDT 
               
               
                   
                 6; piezoelectric substrate 
                 31; substrate 
               
               
                   
                 32; lower electrode 
                 33; dielectric layer 
               
               
                   
                 34; upper electrode 
                 341; supporting portion 
               
               
                   
                 342; electrode portion 
               
               
                   
                   
               
            
           
         
       
     
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
     The present invention provides a surface acoustic wave (SAW) transponder for sensing pressure, that can effectively detect an external pressure since the electric capacity varies according to the change of an external pressure, wherein the variations of the external pressure and the electric capacity have a linear relationship with each other. 
     Technical Solution 
     According to an aspect of the present invention, there is provided a surface acoustic wave (SAW) transponder for sensing pressure including a transmit/receive inter digital transducer (IDT) receiving an applied signal to generate a surface acoustic wave (SAW) and receiving a SAW to generate a radio frequency (RF) signal, a detect IDT reflecting the SAW transmitted from the transmit/receive IDT to return the SAW to the transmit/receive IDT, and a capacitive pressure sensor which is electrically connected to the detect IDT to modulate an amplitude of the SAW reflected by the detect IDT, wherein electric capacity of the capacitive pressure of the capacitive pressure sensor is changed according to a surrounding pressure, wherein the capacitive pressure sensor includes a substrate, a conductive lower electrode formed on the substrate to cover a predetermined area of the substrate, a dielectric layer formed on the substrate to cover the lower electrode, and an upper electrode comprising a ring-type supporting portion disposed on the substrate, and an electrode portion that is supported by the supporting portion, that hermetically seals a space above the lower electrode, which is surrounded by the supporting portion, and that is formed to contact the dielectric layer while being elastically deformed according to an increase of a pressure applied from an upper part, wherein electric capacity of the upper electrode and the lower electrode is changed according to an area of the electrode portion, which contacts the dielectric layer. 
     ADVANTAGEOUS EFFECTS 
     According to the present invention, a structure of a surface acoustic wave (SAW) transponder for sensing pressure is improved, and thus the change of an external pressure can be effectively detected. 
     In addition, since the variations of the external pressure and the pressure sensor have a linear relationship with each other, the change of the external can be easily and quantitatively detected, and an external transmit/receive device can easily analyze a pressure signal. 
     BEST MODE 
     Preferred embodiments of the present invention will now be described with reference to the attached drawings. 
       FIG. 2  is a schematic view illustrating a structure of a surface acoustic wave (SAW) transponder for sensing pressure  100  according to an embodiment of the present invention.  FIG. 3  is a cross-sectional view of a capacitive pressure sensor  30  of the SAW transponder  100  for sensing pressure taken along a line III-III of  FIG. 2 .  FIG. 4  is a view illustrating a lower electrode of the capacitive pressure sensor  30  of the SAW transponder for sensing pressure  100  of  FIG. 2 . 
     Referring to  FIGS. 1 through 3 , the SAW transponder for sensing pressure  100  includes a transmit/receive inter digital transducer (IDT)  10 , a detect IDT  20 , a standard IDT  50 , a capacitive pressure sensor  30  and an antenna  40 . 
     The transmit/receive IDT  10 , the detect IDT  20  and the standard IDT  50  are each an IDT formed on a piezoelectric substrate  6  having piezoelectricity, such as LiNbO 3 . The IDT applies an electric signal or a radio frequency (RF) signal to a plurality of metal electrodes parallel to one another and generates a surface acoustic wave (SAW). IDTs are well known to one of ordinary skill in the field in which SAW is used, and thus its detailed descriptions will be omitted. 
     Since the transmit/receive IDT  10  is connected to the antenna  40 , RF signals received from the outside through the antenna  40  are converted into SAW, or SAW applied from the standard IDT  50  or the detect IDT  20  is converted into RF signals so that converted signals may be transmitted to the outside through the antenna  40 . 
     The detect IDT  20  is formed on the piezoelectric substrate  6 , which is on a path along which SAW generated by the transmit/receive IDT  10  is transmitted, and are spaced from the transmit/receive IDT  10  by a predetermined distance. Accordingly, the detect IDT  20  reflects the SAW transmitted from the transmit/receive IDT  10  to return the SAW to the transmit/receive IDT  10 . The detect IDT  20  includes a plurality of detect IDT elements  21 ,  22 ,  23 ,  24  and  25 . The detect IDT elements  21 ,  22 ,  23 ,  24  and  25  are arranged so that intervals d 1 , d 2 , d 3  and d 4  between adjacent ones of the detect IDT elements  21 ,  22 ,  23 ,  24  and  25  may be the same according to the amplitude of the SAW in order to reduce loss of the SAW. The detect IDT  20  includes five detect IDT elements  21 ,  22 ,  23 ,  24  and  25 , as illustrated in  FIG. 2 . Since the detect IDT  20  is electrically connected to the capacitive pressure sensor  30 , the detect IDT  120  modulates the is amplitude of the SAW to reflect the SAW according to electric capacity of the capacitive pressure sensor  30 . 
     Since the standard IDT  50  is also formed on the piezoelectric substrate  6 , and is disposed between the transmit/receive IDT  10  and the detect IDT  20 , the standard IDT  50  reflects a part of the SAW generated from the transmit/receive IDT  10  to return the part of the SAW to the transmit/receive IDT  10 , and transmits the other parts of the SAW to the detect IDT  20 . Since an external impedance such as the capacitive pressure sensor  30  is not connected to the standard IDT  50 , the SAW reflected by the standard IDT  50  is a standard for determining the amplitude change of SAW reflected by the detect IDT  200 . 
     The capacitive pressure sensor  30  includes a substrate  31 , a lower electrode  32 , a dielectric layer  33  and an upper electrode  34 . 
     The substrate  31  is formed of an insulating material, that is, pyrex glass. 
     The lower electrode  32  is configured in a structure in which a conductive metal (e.g., aluminum or copper) covers a predetermined area of an upper surface of the substrate  10 . The lower electrode  32  includes a plurality of lower electrode elements  321 ,  322 ,  323 ,  324  and  325  spaced apart one another. As illustrated in  FIG. 4 , the lower electrode  32  includes five lower electrode elements  321 ,  322 ,  323 ,  324  and  325 . As illustrated in  FIG. 2 , the lower electrode elements  321 ,  322 ,  323 ,  324  and  325  are respectively and electrically connected to the detect IDT elements  21 ,  22 ,  23 ,  24  and  25 . 
     The dielectric layer  33  is formed on the substrate  31  to cover the lower electrode  32 , and is formed of a material having dielectricity. In particular, the dielectric layer  33  may be formed of SiO 2  material that is usually used in a micro electro mechanical system (MEMS) technology and is easily deposited. 
     The upper electrode  34  includes a supporting portion  341  and an electrode portion  342 . 
     The supporting portion  341  is formed on an upper surface of the substrate  31 , and is formed to have a square ring shape. 
     The electrode portion  342  is supported by the supporting portion  341 , and the upper space  37  of the lower electrode  32  surrounded by the supporting portion  341  is hermetically sealed. Thus, the empty upper space  37  is formed between the dielectric layer  33  and the electrode portion  342 . The electrode portion  342  is formed of a conductive material. While the electrode portion  342  is elastically deformed according to the increase of a pressure applied on the upper surface of the electrode portion  342 , the electrode portion  342  is configured to contact the dielectric layer  33 . The electrode portion  342  of the upper electrode  34  is formed of silicon (Si) to which boron (B) ion or phosphorus (P) ion is ion-implanted. The silicon (Si), which is a nonconducting substance, becomes conductive by ion-implantation of the boron (B) ion or the phosphorus (P) ion. 
     As a pressure applied to an upper surface of the electrode portion  342  increases, the electrode portion  342  is elastically deformed, and thus an area of the electrode portion  342 , which contacts the dielectric layer  33 , is increased. As the pressure applied to the upper surface of the electrode portion  342  decreases, the electrode portion  342  is elastically restored. Thus, when the area of the electrode portion  342 , which contacts the dielectric layer  33 , is decreased and the pressure becomes less and less, the electrode portion  342  stops contacting the dielectric layer  33 . 
     As illustrated in  FIG. 2 , the supporting portion  341  of the upper electrode  34  is electrically connected to the detect IDT elements  21 ,  22 ,  23 ,  24  and  25 . 
     Hereinafter, a method of manufacturing the capacitive pressure sensor  30  will be described in detail. 
     The SAW transponder for sensing pressure  100  is manufactured using a common MEMS technology. 
     First, the lower electrode  32  having a predetermined area is deposited on the substrate  31  formed of pyrex glass. 
     SiO 2  is deposited on the lower electrode  32  deposited on the substrate  31  to cover the lower electrode  32 , and thus the dielectric layer  33  is formed to obtain a structure of  FIG. 5 . 
     As illustrated in  FIG. 6 , by etching an additional silicon (Si) substrate  50  in order to form the upper electrode  34 , a concave shape for forming the upper space  37 , which is hermetically sealed, is formed over the dielectric layer  33 . 
     As a dotted line of  FIG. 6 , boron (B) ion or phosphorus (P) is ion-implanted, and thereby, permeates a predetermined thickness into the Si substrate  35 , and as a result the Si substrate  35  becomes conductive. 
     The Si substrate  35  is reversed as illustrated in  FIG. 6 . The Si substrate  35  is attached onto the substrate  31  formed of pyrex glass as illustrated in  FIG. 5  using an anodic bonding method to obtain a structure of  FIG. 7 . Using such method, the empty upper space  37  is formed between the dielectric layer  33  and the electrode portion  342  of the upper electrode  34 . Meanwhile, prior to attaching the Si substrate  35  onto the substrate  31  formed of pyrex glass, a portion of each of the lower electrode elements  321 ,  322 ,  323 ,  324  and  325  of the lower electrode  32 , which contacts the substrate  31  formed of pyrex glass, is treated to be insulated. Thus, short circuiting is prevented between the upper electrode  34  and each of the lower electrode elements  321 ,  322 ,  323 ,  324  and  325 . 
     Lapping is performed onto the Si substrate  35  of the structure of  FIG. 7 . The Si substrate  35  is planed to a predetermined degree as illustrated in  FIG. 8  to be etched. Then, only the upper electrode  34 , into which boron (B) ion or phosphorus (P) ion permeates, remains. As a result, the capacitive pressure sensor  30 , which is used in the SAW transponder for sensing pressure  100  of  FIG. 4 , is completed. 
     Hereinafter, the function of the SAW transponder for sensing pressure  100  will be described. 
     First, when an external transmit/receive device (not shown) transmits interrogation pulse signals, which are high frequency pulse signals or RF signals, to the SAW transponder for sensing pressure  100  by wireless, the interrogation pulse signals are applied to the transmit/receive IDT  10  through the antenna  40  of the SAW transponder for sensing pressure  100 . 
     When the high frequency pulse signals are incident to the transmit/receive IDT  10 , the piezoelectric substrate  6  generates the SAW. 
     When the SAW reach the standard IDT  50 , a part of the SAW is reflected to be returned to the transmit/receive IDT  10 , and the other parts of the SAW are transmitted to the detect IDT  20 . 
     The SAW reaching the detect IDT  20  via the standard IDT  50  are sequentially reflected by the detect IDT elements  21 ,  22 ,  23 ,  24  and  25  to be returned to the transmit/receive IDT  10 . 
     At this time, the detect IDT  20  modulates the amplitude of the SAW to reflect the SAW according to electric capacity of the capacitive pressure sensor  30  connected to the detect IDT  20 . 
     The SAW, which are reflected by the standard IDT  50  and the detect IDT  20  and reach the transmit/receive IDT  10 , is converted into RF signals by the transmit/receive IDT  10  to be transmitted to the external transmit/receive device via the antenna  40 . Since the RF signals include RF signals of which amplitudes are modulated by the capacitive pressure sensor  30 , the RF signals are analyzed by the external transmit/receive device, and thus a pressure of a place, where the capacitive pressure sensor  30  is equipped, can is be recognized. In the SAW transponder for sensing pressure  100  including the standard IDT  50 , since the standard IDT  50  is not connected to an external impedance such as the capacitive pressure sensor  30 , the amplitudes of the SAW is not changed. Accordingly, by comparing pulse signals reflected by the standard IDT  50  with pulse signals reflected by the detect IDT  20 , the change of the pressure can be effectively recognized. 
     When surrounding pressure is not large, the capacitive pressure sensor  30  is in a state such that the electrode portion  342  of the upper electrode  34  and the dielectric layer  33  may not contact each other. At this time, since the electrode portion  342  of the upper electrode  34  and the dielectric layer  33  are separated from each other, and the empty upper space  37 , which is an air layer, is formed between the electrode portion  342  of the upper electrode  34  and the dielectric layer  33 , the capacitance between the upper electrode  34  and the lower electrode  32  has a relatively small quantity. 
     In such state, when the interrogation pulse signals are transmitted to the transmit/receive IDT  10 , the pulse signals reflected by the standard IDT  50  and the detect IDT elements  21 ,  22 ,  23 ,  24  and  25  are not largely different in terms of amplitudes of the pulse signals, as illustrated in  FIG. 11 . 
     When the pressure of the space, where the capacitive pressure sensor  30  is equipped, increases, the electrode portion  342  of the upper electrode  34  is elastically deformed due to a pressure applied to the upper surface of the electrode portion  342 , and a portion of the electrode portion  342  contacts the dielectric layer  33 , as illustrated in  FIG. 9 . Since the dielectric layer  33  has relatively higher dielectricity than the air, the capacitance is increased between the upper electrode  34  and the lower electrode element  323  that is located on a central part of the lower electrode  32 . Accordingly, impedance is changed between the lower electrode element  323  that is located on a central part of the lower electrode  32  and the upper electrode  34 , and thus pulse signals can be observed as illustrated in  FIG. 12 . As illustrated in  FIG. 12 , each amplitude of the pulse signals reflected by the detect IDT element  23  connected to the lower electrode element  323  that is located on a central part of the lower electrode  32  is smaller than that of pulse signals reflected by other detect IDT elements  21 ,  22 ,  24  and  25 . 
     Again, when the pressure of the space, where the capacitive pressure sensor  30  is equipped, increases, the elastic deformation of the electrode portion  342  of the upper electrode  34  is increased. Then, the portion of the electrode portion  342 , which contacts the dielectric layer  33 , is increased as illustrated in  FIG. 10 . Due to the reason described is above, the capacitance is increased between the upper electrode  34  and each of the lower electrode elements  322 ,  323  and  324  that are the central three of the lower electrode  32 . In this case, the amplitudes of pulse signals reflected by the detect IDT elements  22 ,  23  and  24  connected to the lower electrode elements  322 ,  323  and  324  that are the central three of the lower electrode  32  are smaller than of the amplitudes of pulse signals reflected by other detect IDT elements  21  and  25 , as illustrated in  FIG. 13 . 
     Meanwhile, when the pressure of the space, where the capacitive pressure sensor  30  is equipped, decreases, the electrode portion  342  of the upper electrode  34  is elastically restored. Simultaneously, the portion of the electrode portion  342 , which contacts the dielectric layer  33 , is decreased, and thus, the amplitudes of the pulse signals transmitted from the transmit/receive IDT  10  are changed. 
     Since the SAW transponder for sensing pressure  100  wirelessly supplies power, an additional power supply is not required. In addition, since the SAW transponder for sensing pressure  100  wirelessly transmits and receives pulse signals, the SAW transponder for sensing pressure  100  can be effectively equipped even on a place such as the inner part of a tire, where it is difficult to equip a wired pressure sensor. 
     In the case where the lower electrode  32  includes the lower electrode elements  321 ,  322 ,  323 ,  324  and  325 , since the pulse signals reflected by the detect IDT elements  21 ,  22 ,  23 ,  24  and  25 , which are respectively connected to the lower electrode elements  321 ,  322 ,  323 ,  324 , and  325 , are separately converted, the quantitative change of an exterior pressure can be easily digitalized to be detected from the pulse signals illustrated in  FIGS. 11 through 13 . 
     In addition, compared with the case where a capacitive pressure sensor is used, in which a pressure is measured using the change of a distance between two electrodes including a dielectric layer therebetween, the SAW transponder for sensing pressure  100  can have improved sensitivity with respect to pressure due to a large variation of capacitance since the SAW transponder for sensing pressure  100  measures a pressure using a touch mode capacitive pressure sensor  30  in which a change of an area of the electrode portion  342  of the upper electrode  34 , which contacts the dielectric layer  33 , is employed. 
     MODE OF THE INVENTION 
     Although the embodiment of the present invention has been described, the surface acoustic wave (SAW) transponder for sensing pressure  100  is not limited thereto. 
     For example, although the number (i.e. five) of the detect IDT elements  21 ,  22 ,  23 ,  24  and  25  is described to be the same of that of the lower electrode elements  321 ,  322 ,  323 ,  324  and  325 , the numbers of the detect IDT elements and the lower electrode elements may be variously changed. In addition, each connection between the detect IDT elements and the lower electrode elements is not limited to a one-to-one combination. 
     That is, the detect IDT elements may be connected in various combinations with the lower electrode elements. 
     The lower electrode may include one lower electrode having a predetermined area rather than a plurality of lower electrode elements. In this case, according to an area of an electrode portion of the upper electrode, which contacts the dielectric layer, the capacitance between the upper electrode and the lower electrode is changed, and accordingly the variation of the amplitude of the pulse signal reflected by the detect IDT is increased in proportion to the change of the capacitance. Accordingly, in an external transmit/receive device, the variation of the amplitude of the pulse signal is analyzed and the pressure is recognized. In this case, since the capacitance of the pressure sensor is increased in proportion to the increase of the area of the electrode portion of the upper electrode, which contacts the dielectric layer, the changes of the pressure and the capacitance have linear relationship with each other. Accordingly, the pressure can be easily converted from the capacitance. 
     In addition, the SAW transponder for sensing pressure may be configured in a structure having no standard IDT. When the SAW transponder for sensing pressure does not include the standard IDT, the pulse signals reflected by the standard IDT and the detect IDT are not compared with each other. At this time, the pressure change is analyzed according to the amplitude change of the pulse signals reflected by the detect IDT, and the pressure change can be detected.