Abstract:
In the previously known methods for tuning the antenna resonant circuit, in the case of passive transponders, their range in a high-frequency carrier field is generally limited to a few cm as the supply voltage for the integrated circuit gained from the carrier field falls very quickly with increasing distance.  
     According to the new method, the range of passive transponders in a high-frequency electromagnetic carrier field is substantially increased, in that the real and imaginary parts connected to the contact surface of the antenna contact are minimized and the energy absorbed from the carrier field by means of a serial resonant circuit is used to the greatest possible extent for generating the supply voltage for the integrated circuit of the transponder.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method for tuning an antenna resonant circuit of a passive transponder according to the preamble of patent claim 1.  
           [0003]    2. Description of the Related Technology  
           [0004]    Passive transponders are used in the field of contactless communication for identification (RFID). For this purpose, the carrier wave transmitted by the base station is modulated by the transponder. As passive transponders do not have their own power supply, the energy for the integrated circuit of the passive transponder has to be drawn from the carrier wave by absorption modulation. In the currently used passive 125 kHz systems, this is achieved in the electromagnetic near-field region by an inductive coupling. The energetic range achieved in this way lies in the range from a few cm to around 0.5 m and is dependent upon the prevailing national HF regulations. With the further increasing security requirements placed upon the identification, ever higher data transmission rates are required in order to maintain short identification times. This can only be achieved with high carrier frequencies in the UHF (868 MHz) or microwave (2.45 GHz) ranges. Dipole antennas are usually used for these frequencies. The objective of the development in this field is to achieve long ranges with passive systems and acceptable transmit powers of the base station.  
           [0005]    From HF technology it is known, as described for example in RFID Handbook 2nd edition 2000, p.121, that the length of the dipole antenna has to be matched to the wavelength of the sender in order to draw energy from the field of the sender. A complete absorption of the received electromagnetic wave is achieved by matching the impedance to the wave impedance defined by the design of the antenna. If the transponder antenna is located at a fairly long distance from the sender, the antenna voltage resulting therefrom is too low to generate a supply voltage for the transponder by a simple rectification. Because of the HF regulations in the European countries, the ranges of existing UHF transponders lie below one meter.  
         SUMMARY OF THE INVENTION  
         [0006]    The object of the present invention is to provide a method for tuning an antenna resonant circuit of a passive transponder by means of which the range of a passive transponder is increased in the case of a high-frequency electromagnetic carrier field. The second object is to provide a semiconductor arrangement for implementing the method.  
           [0007]    The first-named object of the invention is solved by a method of the type mentioned at the beginning, with the features of patent claim 1. The second-named object of the invention is solved by the features described in patent claim 6. Favorable embodiments are the objects of subclaims.  
           [0008]    According to this, the essence of the invention consists of increasing the range of a passive transponder by making the operational energy capacitively absorbed from a high-frequency electrical field by means of a serial resonant circuit exclusively available to an input part of the transponder for generating a supply voltage for the integrated circuit. The parasitic capacitive and resistive parts, which form a current path between the contact surface (BP) and the reference potential, are reduced in order to increase the quality of the antenna resonant circuit, in which the input part of the transponder, which has a semiconductor body with an integrated circuit with a signal part with reference potential connector, and which has an antenna arrangement arranged on a substrate, and the antenna arrangement is linked to the integrated circuit via a contact surface connected on the semiconductor body, and in which the antenna forms a serial resonant circuit with the input capacitance of the integrated circuit.  
           [0009]    The advantage of the new method is that the high voltage values generated from the very low antenna voltage by means of the serial resonant circuit are not absorbed by parasitic, capacitive and resistive parts connected to the contact surface. The power absorbed by the antenna is thus available to the input part of the integrated circuit for generating the supply voltage. As the serial resonant circuit only has a very low inductance, reducing the capacitive part (imaginary part) of the contact surface enables the quality of the signal part to be raised. In the case of passive RFID systems, the range is significantly increased by the improved efficiency of the receiver part of the transponder.  
           [0010]    In a development of the method, it is advantageous to divide the current path between the contact surface and the reference potential at at least one point into a first current path and a second current path by means of a series connection and a parallel connection of capacitive and resistive parts, so that a parallel connection of low capacitance of low quality, the value of which preferably lies in the range of a few fF (10e-18 Farad), and a parallel connection of a low capacitance of high quality, the value of which preferably lies in the range of a few fF, is switched in series to a first capacitance of high quality.  
           [0011]    In another development of the method, the low capacitances are produced by means of diodes, as the pn junctions needed for the manufacture of diodes can be easily realized in the manufacturing process of the integrated circuit. Furthermore, spatially bounded pn junctions can be manufactured with a low capacitance value.  
           [0012]    Furthermore, it is advantageous to create the first capacitance, which needs a high quality, by implanting a conductive layer in an insulating layer lying under the contact surface. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0013]    The method according to the invention is described in the following by means of the embodiments in conjunction with the drawings. They show:  
         [0014]    [0014]FIG. 1 an equivalent HF circuit diagram of an input part of a transponder, and  
         [0015]    [0015]FIG. 2 a  a cross-section through a contact surface for an antenna connector within an integrated semiconductor arrangement according to the previous state-of-the-art, and  
         [0016]    [0016]FIG. 2 b  the equivalent HF circuit diagram of the contact surface of FIG. 2 a , and  
         [0017]    [0017]FIG. 3 a  a cross-section through a contact surface for an antenna connector in which the real and imaginary parts linked to the contact surface are suppressed by means of a Schottky diode arrangement, and  
         [0018]    [0018]FIG. 3 b  a cross-section through a contact surface in which, in contrast to FIG. 3 a , the Schottky diode arrangement is replaced by a conventional diode arrangement, and  
         [0019]    [0019]FIG. 3 c  the equivalent HF circuit diagram for the cross-section shown in FIG. 3 a.   
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    The equivalent HF circuit diagram shown in FIG. 1 describes the structure of a receiver part of a passive transponder TR for drawing energy by absorption from a high-frequency electromagnetic carrier field of a base station (not shown). The transponder TR consists of an antenna AN and an integrated circuit IS which are both arranged in a common enclosure (not shown). The antenna AN is linked to the integrated circuit IS by means of a contact surface BP and a bonding wire, whereby the contact surface BP is linked to a signal part ST within the integrated circuit IS. The antenna AN lies in series to a parallel connection consisting of the contact surface BP and the signal part ST. The task of the signal part ST, which contains, for example, a voltage doubling circuit, is to generate a supply voltage for the integrated circuit IS by means of the energy absorbed by the antenna AN.  
         [0021]    The circuit arrangement of the equivalent circuit diagram is explained in more detail in the following. In the equivalent circuit diagram, the antenna AN consists of an alternating voltage generator GA, which is connected to the reference potential, and a series resistor RA. Furthermore, the series resistor RA is linked to a coil L 1  which is connected in series with the parallel connection of the contact surface BP and the signal part ST. The contact surface BP consists of a series connection comprising a capacitor CP and a resistor RP linked to the reference potential. The signal part ST can be represented by a series connection of a capacitor CS and a load resistor RS connected to the reference potential.  
         [0022]    The principle of operation of the equivalent circuit diagram is explained in the following. The alternating voltage generator GA represents the energy absorbed by the antenna AN from the carrier field of the base station (not shown), which in the form of an alternating voltage excites the series resonant circuit, which is formed by the resistor RA, the coil L 1 , and the capacitors CP and CS, and the resistors RP and RS. The loss arising in the antenna AN during the excitation is described by the resistor RA, the phase shift between current and voltage in the antenna by the coil L 1 . In a description of the alternating voltage, the loss is represented as a real part and the phase shift as an imaginary part. Furthermore, the contact surface BP has an imaginary part, represented by the capacitor CP, and a real part, represented by the resistor RP. The signal part ST also has an imaginary part, represented by the capacitor CS, and a real part, represented by the resistor RS. In order to generate the greatest possible voltage from the energy absorbed from the electromagnetic field, the resonant frequency of the series resonant circuit is tuned to the carrier frequency at either 868 MHz or 2.45 GHz. In accordance with the resonance condition determined by the Thomson equation, the total capacitance of the two capacitors CP and CS and the inductance of the coil L 1  must be adequate for the resonance condition. The excitation voltage of the alternating voltage generator GA, which lies in the range of a few μV, is increased excessively by the series resonant circuit in proportion to the quality of the series resonant circuit. In order to achieve a large overvoltage, the attenuation, that is the sum of the real parts, must be low and the ratio of the imaginary parts of inductance and capacitance must be high in order to make the whole of the energy drawn from the carrier field available for generating a supply voltage to the signal part at a high voltage level. As the inductance value of the coil L 1  is low, the capacitance values of the capacitors CP and CS must have very low values in the femto range in order to tune the series resonant circuit to the resonant frequency of the carrier frequency, that is the capacitance CP (imaginary part) linked to the contact surface BP must be reduced. Moreover, the real part linked to the contact surface BP must be very small in order to achieve a high quality in the series resonant circuit and to make the absorbed energy available to the signal part ST, represented by the resistor RS, for generating the supply voltage for the integrated circuit IS.  
         [0023]    The cross-section through a contact surface within an integrated semiconductor arrangement as represented in FIG. 2 a  shows the previous state-of-the-art. The identifier DA denotes a mirror axis, that is the representation shown is symmetrical to the axis DA. The function of the contact surface is to make a bonding surface available to connect the antenna AN to the integrated circuit IS.  
         [0024]    The structure of the cross-sectional view is explained in the following in conjunction with a simplified consideration of the alternating voltage. An insulating layer ISO, which insulates the contact surface ALB 1  from an underlying, low-doped zone D 1  of a first conductivity type, is formed under the contact surface ALB 1  on a semiconductor body made of silicon, which lies in an uppermost metal level. A low doped zone D 2  of the second conductivity type, which is in general described as substrate doping, adjoining zone D 1  denotes the reference potential and is connected to a contact ALK by means of a highly doped zone D 2 P, of the second conductivity type. In order to ensure adequate process reliability in the manufacture of the integrated circuit IS, a large minimum distance a 1 , lying in the range of some 10 μm, must be maintained between the contact surface ALB 1  and the contact ALK, and this determines the length of the zone D 2  between the zone D 1  and the zone D 2 P. In a simplified consideration of the alternating voltage, the layer sequence consisting of the contact surface and the zone D 1  corresponds to a capacitor with a capacitance value CP, in which the low doped zone D 1  of the opposite conductivity type to that of the substrate doping has the effect that the capacitive contribution CP of the contact surfaces is not very great. A resistor RP lies in series with the capacitor CP and has a high value as a result of the long length of the zone D 2  in combination with the low doping.  
         [0025]    The alternating voltage of the layer structure shown in FIG. 2 a  is discussed in detail in the following by means of the equivalent HF circuit diagram shown in FIG. 2 b . The contact surface ALB 1  and the zone D 1  form a capacitor with the capacitance C 1 . Connected in series to this latter there is a diode DD 1  D 2 , which is formed by the boundary surface of the zones D 1 /D 2 , and a resistor RS 1 , which results from the length of the zone D 2  between the diode DD 1 D 2  and the contact ALK. In the equivalent HF circuit diagram, the diode DD 1 D 2  is shown as a parallel connection consisting of a resistor XRC 2  and a capacitor C 2 , and a resistor RSC 2  connected in series to them. The total capacitance CP is determined by the sum of the capacitance values C 1  and C 2 , the series resistor RP is given by the sum of the resistors RSC 2  and RS 1 . Despite the low doping of zone D 1  and the series connection of the capacitances C 1  and C 2 , the capacitance value CP is very high and lies in the range of 40 fF because the zone D 1  has a large and the diode DD 1 D 2  a very large boundary surface. As a result of the large boundary surface, the resistor XRC 2  and the resistor RSC 2  are small despite the low doping of the zone D 1 , that is the capacitance has a large loss contribution. On the other hand, the resistor RS 1  and thus the resistor RP are very large as a result of the great length and low doping of zone D 2 . As the contribution of the real part is thus large, a substantial part of the energy absorbed by the antenna is lost through the contact surface. Moreover, the large capacitance value gives rise to a high imaginary part and, in conjunction with the low inductance value, makes it difficult for the resonant frequency of the series resonant circuit to be tuned to the carrier frequency. The energetic range of the passive transponder cannot be increased with acceptable expenditure without a drastic reduction of the real and imaginary parts linked to the contact structure.  
         [0026]    The layer structure represented in FIG. 3 a  shows an arrangement for realizing the method according to the invention with which, within a conventional manufacturing process of an integrated circuit, the real and imaginary parts connected to the contact surface are very much reduced by an advantageous layer structure, so that the physical parameters of the series resonant circuit determine the contributions of the signal part ST.  
         [0027]    As explained in connection with FIG. 2, the axis DA is mirror-symmetrical. The structure is explained in the following, in which, building upon the explanations provided in conjunction with FIG. 2, only the differences are described. Underneath the contact surface ALB 1 , a second conductive layer ALB 2 , the size of which is matched to the contact surface ALB 1 , is introduced within the insulating layer in order to achieve good screening. The layer ALB 2  is connected laterally to the zone D 1  in order to thus create a Schottky diode DALB 2 D 1 . The zone D 1  completely surrounds the zone D 2 P, which represents a connector doping for the contact ALK. In contrast to the distance al in FIG. 2 a , the distance a 2  between the contact ALK and the conductive layer ALB 2  is significantly less and lies in the range of about 1 μm. The total length of the zone D 1  is determined by the width of the connection of the layer ALB 2  and a distance a 2 , which is given by the distance between the layer ALB 2  and the reference potential connection ALK. For the description of the current flow in an equivalent HF circuit diagram, a differentiation is made between a first current path  1  and a second current path  2  which are switched parallel to one another.  
         [0028]    The layer structure shown in FIG. 3 b  presents an alternative embodiment, in which the Schottky diode DALB 2 D 1  shown in FIG. 3 a  is replaced by a standard model of a diode DD 2 PD 1 , by an additional zone D 2 P being arranged underneath the contact area of the metallic conductive layer ALB 2 . In place of the Schottky diode, a conventional diode is formed by the boundary surfaces of the zones D 2 P and the zone D 1 . This is advantageous if the metalization process within the manufacturing process does not allow the formation of a metal semiconductor junction, but good HF characteristics are nevertheless required. As a result of the small boundary surface of the doping zones D 2 P and D 1 , the diode DD 2 PD 1  has a low capacitance value. Moreover, the length of the zone D 1  remains substantially unchanged. In the HF description shown in  3   c , the equivalent circuit diagram of the diode DD 2 PD 1  corresponds to the Schottky diode DALB 2 D 1  in FIG. 3 a.    
         [0029]    The equivalent HF circuit diagram shown in FIG. 3 c  describes the layer structure shown in FIG. 3 a . The contact surface ALB 1  and the layer ALB 2  together with the layer ISO form the capacitance C 1 A. In series to this, lies the parallel connection consisting of the current path  1  and the current path  2 . The current path  1  consists of the Schottky diode DALB 2 D 1 . In the equivalent circuit diagram, the Schottky diode DALB 2 D 1  comprises a capacitor C 3 A, which lies parallel to a resistor XRC 3 A, and a resistor RS 3 A lying in series to the parallel connection. A low capacitance value with a high quality is achieved by the metal semiconductor contact. A capacitance C 2 A, which is formed by the layer ALB 2  and the zone D 2  together with the zone ISO, lies in the current path  2  in series with a resistor RSA, which is determined by the layer resistance of the zone D 2 , and in series with the diode DD 2 D 1  which results from the boundary surface of the zones D 1 /D 2 . In the equivalent circuit diagram, the diode DD 2 D 1  comprises a capacitor C 4 A, which is switched parallel to a resistor XRC 4 A, and a resistor RSA 4  lying in series to the parallel connection. Both current paths pass through the layer D 1  which has a negligible resistance because of the small lateral extent. The diode DD 1 D 2 P then lies in series. In the equivalent circuit diagram, the diode DD 1 D 2 P comprising a capacitance C 5 A, which lies parallel to a resistor XRC 5 A, in which a resistor RS 5 A lies in series to the parallel connection, and the capacitance C 5 A has a low dissipative resistance and a high quality as a result of the small boundary surface and the high doping of the zone D 2 P. Moreover, the contact ALK is represented by a resistor RS, the resistance of which is very low as a result of the metallic conductive layer.  
         [0030]    The principle of operation is explained in the following. In order to suppress the imaginary part of the contact surface, comprising the contribution of the capacitors, small capacitances C 3 A and C 5 A with high quality are switched in series with the large capacitance C 1 A in the current path  1 . In particular, the capacitance C 3 A, which is derived from the equivalent circuit diagram of the Schottky diode, has a very low capacitance value in the range of a few fF. Furthermore, the real part, that is the losses through the series resistor, is also suppressed to the greatest possible extent in current path  1  because both the contributions of the metallic conductive layers ALB 2  and ALK are negligible, and also the series resistance comprising the contribution of the zone D 1  is also very low because of the very short distance a 2 . Furthermore, the series resistor RSA lying in current path  2  is able to be made high by low doping in comparison to the series resistance of the metallic conductive layer ALB 2  lying in current path  1 . However, as path  2  lies parallel to path  1 , the relatively large real part of the current path  2  is suppressed. Furthermore, as a result of the low doping of the zones D 1  and D 2 , the capacitance of the capacitor C 4 A is low despite the relatively large boundary surface, and at the same time it has a very poor quality in comparison to the capacitance C 3 A resulting from the Schottky diode. The contribution to the imaginary part is thus suppressed by current path  2 . The HF current flowing through current path  2  is thus negligible. In total, the small real and imaginary parts of the components lying in current path  1  thus determine the contribution of the contact surface to the real and imaginary parts of the series resonant circuit. As the contributions of the contact surface are extremely small, they may be ignored in comparison to the contributions given by the signal part ST of the integrated circuit IS.  
         [0031]    As no changes to the arrangement of the individual components arise from the layer structure shown in FIG. 3 b , the explanations given in connection with the drawing documentation of FIG. 3 c  also apply to the layer structure shown in FIG. 3 b.