Patent Publication Number: US-2011068904-A1

Title: Rfid tag

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
     This is a continuation application of PCT/JP2008/061668, filed on Jun. 26, 2008. 
    
    
     TECHNICAL FIELD  
     The present invention relates to an RFID (Radio_Frequency_IDentification) tag. 
     BACKGROUND ART  
     Conventionally, there are known various kinds of RFID tag which exchange information with external devices represented by a reader/writer in a non-contact manner by radio waves. As a kind of this RFID tag, there has been proposed one having such a structure that a conductor pattern and an IC chip for radio wave communication are mounted on a dielectric member in sheet form. The RFID tag generally has a dielectric member, an IC chip provided on one of the top surface and the undersurface of this dielectric member, and an antenna having a conductor pattern arranged on the dielectric member (for example, see Patent Citation 1). 
     Also, there is known an RFID tag that includes a loop antenna extending like a belt over a top surface and a undersurface of a dielectric member and having both ends connected to an IC chip (for example, see Patent Citation 2). The RFID tag having the loop antenna extending like a belt over the top surface and the undersurface of the dielectric member can improve the gain of the antenna, thereby expanding a communications distance, while suppressing the area of the entire RFID tag. 
     There is a case in which a capacitor is added to the RFID tag so that the frequency response of the loop antenna matches with a communication frequency of the RFID tag. 
       FIG. 1  is a plan view of a conventional RFID tag when viewed from an IC chip side, and  FIG. 2  is a cross-sectional view of the RFID tag illustrated in  FIG. 1  taken along a line A-A. 
     An RFID tag  90  illustrated in  FIG. 1  and  FIG. 2  includes a printed board  91  made of a dielectric material, an IC chip  93 , and an antenna pattern  92 . The antenna pattern  92  extends over the top surface and the undersurface of the printed board, thereby forming a loop antenna L 9 . Both ends of the loop antenna L 9  are connected to the IC chip  93 . From the both ends of the antenna pattern  92  which are connected to the IC chip  93 , electrode sections  94  and  95  mutually extend in the same direction, respectively. The opposed parts of the electrode sections are adjacent to each other, and a capacitor C 9  is formed. The loop antenna L 9  that can be considered as an inductor electrically and the capacitor C 9  are connected in parallel, and a resonance frequency that affects a communication property is obtained by the inductance of the loop antenna L 9  and the capacitance of the capacitor C 9 . 
     Patent Citation 1: Japanese Laid-open Patent Publication No. 2002-246829 
     Patent Citation 2: Japanese Laid-open Patent Publication No. 2006-53833 
     DISCLOSURE OF INVENTION  
     Generally, in the thickness of the printed board of a RFID tag, a deviation occurs in each of individual products in a production phase. This also applies to the RFID tag  90  illustrated in  FIG. 1  and  FIG. 2 . A deviation that occurs in a thickness t of the printed board  91  affects the area of the loop antenna L 9 . As a result, a deviation occurs in the inductance of the loop antenna L 9 , and the resonance frequency deviates from a designed value. For example, in the RFID tag illustrated in  FIG. 1  and  FIG. 2 , when the inductance of the loop antenna is assumed to be L and the capacitance of the capacitor is assumed to be C, the resonance frequency f is expressed by the following equation (1). 
         f= 1/(2π( LC ) 0.5 )  (1)
 
     As indicated by this equation (1), the resonance frequency f changes according to the inductance L. 
     Here, for example, there is known a trimming technique of correcting the produced deviation of the resonance frequency, by removing a part of the conductor pattern that forms the capacitor and thereby adjusting the capacitance of the capacitor (for example, Patent Citation 1). However, it takes enormous efforts to perform the trimming for the individual RFID tags, which lowers productivity in the mass production. 
     In view of the foregoing circumstances, plural embodiments described in the present description provide an RFID tag in which a deviation of a frequency response is suppressed without adjustment by removal or extension of a conductor pattern. 
     According to an aspect of the invention, a RFID tag includes: 
     a tabular dielectric member; 
     an antenna pattern that extends over a top surface and an undersurface of the dielectric member and forms a loop antenna having both ends existing on one surface of the top surface and the undersurface; 
     a circuit chip that is electrically connected to the antenna pattern and performs communication via the loop antenna; 
     a first electrode that is connected directly or via a conductor to one of the both ends of the loop antenna and spreads on the one surface; and 
     a second electrode that is connected via a conductor to the other with respect to the one of the both ends of the loop antenna and spreads on the other surface with respect to the one surface of the top surface and the undersurface, along the first electrode. 
     According to this basic mode, the inductance of the loop antenna increases with increase in the thickness of the dielectric member, but a capacitance is formed between the first electrode and the second electrode provided via the dielectric member, and the capacitance of this capacitance also increases with the increase in the thickness of the dielectric member. Thus, a deviation of a resonance frequency due to a deviation of the inductance resulting from the thickness of the dielectric member acts in a direction suppressing a deviation of the capacitance. Therefore, the deviation of the frequency response is suppressed without adjustment by removal or extension of the conductor pattern. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view of a conventional RFID tag when viewed from an IC chip side. 
         FIG. 2  is a cross-sectional view of the RFID tag illustrated in  FIG. 1  taken along a line A-A. 
         FIG. 3  is a plan view of an RFID tag according to a specific first embodiment when viewed from a IC chip side. 
         FIG. 4  is a cross-sectional view of the RFID tag illustrated in  FIG. 3  taken along a line B-B. 
         FIG. 5  is a plan view of an RFID tag according to a specific second embodiment when viewed from an IC chip side. 
         FIG. 6  is a cross-sectional view of the RFID tag illustrated in  FIG. 5  taken along a line C-C. 
         FIG. 7  is a cross-sectional view of an RFID tag according to a specific third embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
     Specific embodiments of the RFID tag described for the basic mode will be described below with reference to the drawings. 
       FIG. 3  is a plan view of an RFID tag according to a specific first embodiment when viewed from a IC chip side, and  FIG. 4  is a cross-sectional view of the RFID tag illustrated in  FIG. 3  taken along a line B-B. 
     An RFID tag  10  illustrated in  FIG. 3  includes a tabular dielectric member  11 , an antenna pattern  12 , an IC chip  13  connected to the antenna pattern  12 , a first electrode  14  and a second electrode  15  spreading on the top surface and the undersurface, respectively, of the tabular dielectric member  11 . 
     The RFID tag  10  is an electronic device that exchanges information with a not-illustrated reader/writer in a non-contact manner, and receives energy of an electromagnetic field given by the reader/writer as electric energy, and the IC chip  13  is driven by the electric energy. The antenna pattern  12  functions as an antenna for communication, and the IC chip  13  performs radio communication through the antenna pattern  12 . Here, the IC chip  13  is equivalent to an example of the circuit chip in the above-described basic mode. 
     Incidentally, among persons skilled in the art in the technical field of the present application, there is a case in which the “RFID tag” used in the description of the present application is called a “RFID tag inlay”, by regarding the “RFID tag” as an internal member (inlay) for the “RFID tag”. Alternatively, there is a case in which this “RFID tag” is called a “wireless IC tag”. Further, a non-contact IC card also is included in this “RFID tag”. 
     Subsequently, each part of the RFID tag  10  will be described. 
     The dielectric member  11  is formed by a dielectric material. The dielectric material is, for example, a resin, and is a synthetic resin represented by polyethylene terephthalate (PET) and epoxy based resin. 
     The antenna pattern  12 , the first electrode  14  and the second electrode  15  are formed as a conductor pattern formed on the surface of the dielectric member  11 . The antenna pattern  12  extends like a belt over the top surface and the undersurface of the dielectric member  11  and forms a loop antenna L 1 . Here, of the top surface and the undersurface of the dielectric member  11 , one surface where both ends  12   a  and  12   b  of the loop antenna L 1  are provided is referred to as the top surface, and the other surface is referred to as the undersurface. 
     The IC chip  13  is disposed on the top surface of the dielectric member  11 , and is connected to the both ends  12   a  and  12   b  of the loop antenna L 1  by solder or the like, thereby being electrically connected to the loop antenna L 1 . The IC chip  13  wirelessly communicates with the reader/writer not illustrated, through the loop antenna L 1 . 
     The first electrode  14  spreads like a plane on the top surface of the dielectric member  11 , and is connected to the one end  12   a  of the loop antenna L 1  directly. To be more specific, the first electrode  14  continuously extends from the antenna pattern  12 , and is disposed to protrude in a direction crossing a direction in which the antenna pattern  12  extends. 
     The second electrode  15  spreads on the undersurface of the dielectric member  11  along the first electrode  14 , and is connected, via a conductor, to the other end  12   b,  with respect to the one end  12   a  to which the first electrode  14  is connected, of the both ends  12   a  and  12   b  of the loop antenna L 1 . To be more specific, the second electrode  15  is connected to the other end  12   b  via a penetrating conductor  16  formed to penetrate the dielectric member  11 . The end  12   b  of the loop antenna L 1  disposed on the top surface of the dielectric member  11  and the second electrode  15  disposed on the undersurface of the dielectric member  11  are connected to each other over a short distance via the penetrating conductor  16 . Therefore, as compared to a case in which a conductor goes by way of an edge of the dielectric member  11 , parasitic resistance and parasitic inductance decrease. 
     The first electrode  14  and the second electrode  15  are disposed to face each other, between which the dielectric member  11  is interposed, and a capacitor C 1  is formed by the first electrode  14  and the second electrode  15 . The capacitor C 1  and the loop antenna L 1  are connected in parallel with respect to the IC chip  13 . The frequency response of the loop antenna L 1  is determined according to the characteristics of the loop antenna L 1  and the capacitor C 1 . When the inductance of the loop antenna L 1  is assumed to be L and the capacitance of the capacitor C 1  is assumed to be C, a resonance frequency f is expressed by the following equation (2). 
         f= 1/(2π( LC ) 0.5 )  (2)
 
     Here, when a dielectric constant of the dielectric member  11  is assumed to be ε, each of the area of the first electrode  14  and the second electrode  15  is assumed to be S, and the distance between the first electrode  14  and the second electrode  15 , namely the thickness of the dielectric member  11 , is assumed to be t, the capacitance C of the capacitor C 1  is expressed by the following equation (3). 
         C=εr×S/t   (3)
 
     As indicated by the above equation (3), when the thickness t of the dielectric member  11  increases, the capacitance C decreases. 
     When the RFID tag  10  is produced, a deviation occurs in the thickness t of the dielectric member  11  in each of the individual products. When the thickness t of the dielectric member  11 , for example, increases, the loop length of the loop antenna L 1  becomes long, and the inductance L increases. However, when the thickness t of the dielectric member  11  increases, the capacitance C of the capacitor C 1  decreases, as indicated by the above equation (3). In other words, when the thickness t of the dielectric member  11  increases, the inductance L increases, and the capacitance C decreases. As a result, a change in the resonance frequency f caused by the increase in the inductance L is suppressed by the decrease in the capacitance C. Thus, the deviation of the resonance frequency f resulting from the deviation occurring in the thickness t of the dielectric member  11  is suppressed. 
     Next, a specific second embodiment of the RFID tag will be described. In the following description of the second embodiment, the same elements as those of the embodiment described so far are provided with the same reference characters, and a feature different from the above-described embodiment will be described. 
       FIG. 5  is a plan view of an RFID tag according to the specific second embodiment when viewed from an IC chip side, and  FIG. 6  is a cross-sectional view of the RFID tag illustrated in  FIG. 5  taken along a line C-C. 
     An RFID tag  20  illustrated in  FIG. 5  and  FIG. 6  includes, in addition to a first electrode  24  spreading on the top surface of the dielectric member  11  and a second electrode  25  spreading on the undersurface, a third electrode  28  and a fourth electrode  29  provided inside the dielectric member  11 . 
     The third electrode  28  spreads between the first electrode  24  and the second electrode  25 , and is connected to the one end  12   a  of the loop antenna L 1  through a conductor  27 . Further, the fourth electrode  29  spreads between the first electrode  24  and the third electrode  28 , and is connected to the other end  12   b  of the loop antenna L 1  through the conductor  16 . The conductor  27  that connects the one end  12   a  of the loop antenna L 1  to the third electrode  28  is a penetrating conductor that penetrates the dielectric member  11 . The conductor  16  that connects the other end  12   b  of the loop antenna L 1  to the second electrode  25  and the third electrode  28  also is a penetrating conductor that penetrates the dielectric member  11 . A capacitor C 2  is formed by these first electrode  24 , second electrode  25 , third electrode  28  and fourth electrode  29 . To be more specific, a first capacitor C 2   a  is formed by the first electrode  24  and the fourth electrode  29  next to the first electrode  24 , a second capacitor C 2   b  is formed by the fourth electrode  29  and the third electrode  28  next to the fourth electrode  29 , and a third capacitor C 2   c  is formed by the third electrode  28  and the second electrode  25  next to the third electrode  28 . The capacitor C 2  is formed by a composite of the first capacitor C 2   a,  the second capacitor C 2   b,  and the third capacitor C 2   c.    
     By disposing the third electrode  28  and the fourth electrode  29  inside the dielectric member  11 , a large-capacitance capacitor can be formed by increasing the capacitance of the capacitor, even when the thickness of the dielectric member  11  is large. Incidentally, the RFID tag  20  can be produced by, for example, a production method similar to that of a well-known multilayer wiring board. 
     When the RFID tag  20  is produced, there are two cases, namely, a case in which the deviation occurring in the thickness of the dielectric member  11  affects any of a distance between the first electrode  24  and the fourth electrode  29 , a distance between the fourth electrode  29  and the third electrode  28 , and a distance between the third electrode  28  and the second electrode  25 , and a case in which the deviation occurring in the thickness of the dielectric member  11  affects these three distances almost equally. However, in either case, when, for example, the thickness of the dielectric member  11  increases, the capacitance of the capacitor C 2  formed by the composition of the first capacitor C 2   a,  the second capacitor C 2   b  and the third capacitor C 2   c  decreases. As a result, even when there is a change in the thickness t of the dielectric member  11 , a change in the resonance frequency f caused by an increase of the inductance L in the loop antenna L 1  is suppressed by a decrease of the capacitance C in the capacitor C 2 . Therefore, the deviation of the resonance frequency f due to the deviation occurring in the thickness t of the dielectric member  11  is suppressed. 
     In the specific embodiment described above, a structure in which two electrodes are disposed inside the dielectric member has been described, but next, there will be described a third embodiment in which the number of electrodes provided inside the dielectric member is increased. 
       FIG. 7  is a cross-sectional view of an RFID tag according to the specific third embodiment. A structure appearing in a plan view of a RFID tag  30  of the third embodiment is the same as that of the RFID tag  20  of the second embodiment illustrated in  FIG. 5  and thus, the plan view of the RFID tag  30  is omitted. Further, in the following description of the third embodiment, the same elements as those of the above-described embodiments are provided with the same reference characters, and a feature different from the above-described embodiments will be described. 
     The RFID tag  30  illustrated in  FIG. 7  is different from the RFID tag  20  described with reference to  FIG. 6  in that the RFID tag  30  further includes, inside the dielectric member  11 , a fifth electrode  31  spreading between the fourth electrode  29  and the first electrode  24  and a sixth electrode  32  spreading between the fifth electrode  31  and the first electrode  24 . The fifth electrode  31  is connected, together with the first electrode  24  and the third electrode  28 , to the one end  12   a  of the loop antenna L 1 . The sixth electrode  32  is connected, together with the fourth electrode  29  and the second electrode  25 , to the other end  12   b  of the loop antenna L 1 . A capacitor C 3  is formed by the first electrode  24 , the sixth electrode  32 , the fifth electrode  31 , the fourth electrode  29 , the third electrode  28 , and the second electrode  25 . The RFID tag  30  can form the capacitor C 3  of a larger capacitance and readily supports a wide range of resonance frequencies. 
     In the RFID tag  30  illustrated in  FIG. 7 , like the RFID tag  10  of the first embodiment and the RFID tag  20  of the second embodiment described above, the inductance of the loop antenna L 1  and the capacitance of the capacitor C 3  are tied to the thickness of the dielectric member  11 . For example, when the thickness of the dielectric member  11  increases, the capacitance of the capacitor C 3  decreases, while the inductance of the loop antenna L 1  increases, on a principle similar to that of the RFID tag  20  of the second embodiment illustrated in  FIG. 6 . Therefore, according to the RFID tag  30 , when there is a change in the thickness of the dielectric member  11 , a change of the resonance frequency f caused by an increase of the inductance L in the loop antenna L 1  (cf.  FIG. 5 ) is suppressed by a decrease of the capacitance C in the capacitor C 3 . Thus, a deviation of the resonance frequency f resulting from a deviation occurring in the thickness of the dielectric member  11  is suppressed. 
     Incidentally, in the above description of each of the specific embodiments, the antenna pattern extending like a belt is taken as an example of the antenna pattern in the basic mode described in “Disclosure of Invention”, but this antenna pattern may be linear, other than being shaped like a belt. 
     Further, in the above description of each of the specific embodiments, the first electrode  14  directly connected to one of the both ends of the loop antenna has been taken as an example of the first electrode in the basic mode described in “Disclosure of Invention”, but this first electrode may be connected to the loop antenna via a conductor pattern. 
     Furthermore, in the above description of each of the specific embodiments, the one connected via the penetrating conductor is taken as an example of the second electrode, but this second electrode may be, for example, one connected via a conductor pattern extending along an edge of the dielectric member, other than a penetrating conductor section. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described To be more specific, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.