Abstract:
An antenna composed of composite material which can be screen printed is proposed for RFID transponders and other RF communication systems.

Description:
RELATED PATENTS AND APPLICATIONS  
       [0001]    Related U.S. Pat. Nos. assigned to the assignee of the present invention include: 5,866,044, which is a continuation of application Ser. No. 08/2339,609, filed Nov. 15, 1994 (now abandoned). Other U.S. Pat. Nos. include: 5,521,601; 5,528,222; 5,538,803; 5,550.547; 5,552,778; 5,554,974; 5,563,583; 5,565,847; 5,606,323; 5,635,693; 5,673,037; 5,680,106; 5,682,143; 5,729,201; 5,729,697; 5,736,929; 5,739,754; 5,767,789; 5,777,561; 5,786,626; 5,812,065; 5,821,859; 5,828,318; 5,831,532; 5,850,181; 5,874,902; 5,889,489; 5,909,176; and 5,912,632. U.S. Patent applications assigned to the assignee of the present invention include: application Ser. No. 099/119,569, filed Jul. 20, 1998, which is a division of Ser. No. 08/734,492 (now U.S. Pat. No. 5,866,044. The above identified U.S. Patents and U.S. Patent applications are hereby incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The field of the invention is the field of Radio Frequency (RF) transponders (RF Tags) which receive RF electromagnetic radiation from a base station and send information to the base station by modulating the load of an RF antenna.  
         BACKGROUND OF THE INVENTION  
         [0003]    RF Tags can be used in a multiplicity of ways for locating and identifying accompanying objects, items, animals, and people, whether these objects, items, animals, and people are stationary or mobile, and transmitting information about the state of the of the objects, items, animals, and people. It has been known since the early 60&#39;s in U.S. Pat. No. 3,098,971 by R. M. Richardson, that electronic components on a transponder could be powered by radio frequency (RF) power sent by a “base station” at a carrier frequency and received by an antenna on the tag. The signal picked up by the tag antenna induces an alternating current in the antenna which can be rectified by an RF diode and the rectified current can be used for a power supply for the electronic components. The tag antenna loading is changed by something that was to be measured, for example a microphone resistance in the cited patent. The oscillating current induced in the tag antenna from the incoming RF energy would thus be changed, and the change in the oscillating current led to a change in the RF power radiated from the tag antenna. This change in the radiated power from the tag antenna could be picked up by the base station antenna and thus the microphone would in effect broadcast power without itself having a self contained power supply. In the cited patent, the antenna current also oscillates at a harmonic of the carrier frequency because the diode current contains a doubled frequency component, and this frequency can be picked up and sorted out from the carrier frequency much more easily than if it were merely reflected. Since this type of tag carries no power supply of its own, it is called a “passive” tag to distinguish it from an active tag containing a battery. The battery supplies energy to run the active tag electronics, but not to broadcast the information from the tag antenna. An active tag also changes the loading on the tag antenna for the purpose of transmitting information to the base station.  
           [0004]    The “rebroadcast” or “reflection” of the incoming RF energy at the carrier frequency is conventionally called “back scattering”, even though the tag broadcasts the energy in a pattern determined solely by the tag antenna and most of the energy may not be directed “back” to the transmitting antenna.  
           [0005]    In the 70&#39;s, suggestions to use tags with logic and read/write memories were made. In this way, the tag could not only be used to measure some characteristic, for example the temperature of an animal in U.S. Pat. No. 4,075,632 to Baldwin et. al., but could also identify the animal. The antenna load was changed by use of a transistor. A transistor switch also changed the loading of the transponder in U.S. Pat. No. 4,786,907 by A. Koelle.  
           [0006]    Prior art tags have used electronic logic and memory circuits and receiver circuits and modulator circuits for receiving information from the base station and for sending information from the tag to the base station.  
           [0007]    The continuing march of semiconductor technology to smaller, faster, and less power hungry has allowed enormous increases of function and enormous drop of cost of such tags. Presently available research and development technology will also allow new function and different products in communications technology.  
           [0008]    A fundamental problem with RF tags is that the cost of the tag must be reduced to a level small compared to the cost of the product to which the tag is attached, which would then allow many more tags to be used and so that high volume production can cut the costs even further. The cost of the tags is the cost of the semiconductor chip, the antenna, the substrate supporting the antenna and chip, and the attachment cost. The present invention lowers the cost and increases the function of the antenna attached to the substrate.  
         OBJECTS OF THE INVENTION  
         [0009]    It is an object of the invention to produce an RF transponder comprising components which can be made at low cost. It is a further object of the invention to provide an RF transponder comprising a flexible antenna and substrate. It is a further object of the invention to provide an RF transponder which can be made by screen printing techniques. It is a further object of the invention to provide an RF transponder having enhanced environmental resistance. It is a further object of the invention to provide an RF transponder in a thin package. It is a further object of the invention to provide an RF transponder antenna which may easily be electrically connected to a semiconductor chip.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is an RF antenna made from a composite material, wherein the composite material preferably comprises electrically conducting particles in a matrix, and wherein the electrically conducting particles have such a high density that the electrical conductivity of the composite material is large enough for the antenna to receive RF signals sufficient to activate the RF tag. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a sketch of a prior art RFID tag and base station.  
         [0012]    [0012]FIG. 2 is a sketch of a plan view of an embodiment of the invention.  
         [0013]    [0013]FIG. 3 is a sketch of an elevation view of an embodiment of the invention.  
         [0014]    [0014]FIG. 4 is a sketch of an elevation view of an embodiment of the invention.  
         [0015]    [0015]FIG. 5 is a sketch of an elevation view of an embodiment of the invention.  
         [0016]    [0016]FIG. 6 is a sketch of an elevation view of an embodiment of the invention.  
         [0017]    [0017]FIG. 7 is a sketch of an elevation view of an embodiment of the invention.  
         [0018]    [0018]FIG. 8 is a sketch of a plan view of an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    [0019]FIG. 1 shows a prior art RF identification (RFID) system where a base station  10  controlled by a computer  12  sends RF energy to an antenna  14  which broadcasts the RF energy  16  to an RF tag  18  through an antenna  19 .  
         [0020]    [0020]FIG. 2 shows a plan view of the apparatus of the invention. Electrical circuitry  24  is connected to an antenna  19 , which is made of a material  20  which is a composite material, preferably a composite of a large number of small electrically conducting particles in a matrix. The composite material is connected to electrical circuitry  24  at points  22 . Composite material  20  is most preferably silver powder mixed in a thermoplastic matrix. Composite materials  20  matrix is preferably an elastomeric polymer and/or a thermoplastic material. The antenna  19  is preferably made by screening a paste of metal powder, polymer material, and solvent through a screen on to a substrate (shown later). While the paste is still wet, the electrical circuitry  24  is preferably mechanically and electrically bonded to the material  20  of the invention by contacting electrical contacts of the electrical circuitry  24  with the wet paste, and then driving off the solvent and/or curing the polymer matrix material. Surprisingly, material  20  of the invention may have sufficiently low electrical resistance that the apparatus of FIG. 2 works as an RFID tag with only slightly reduced read distance from an RFID tag with an antenna made from conventional copper. Conventional wisdom in the antenna art dictates that the antenna material have as low a resistivity as possible. The material  30  has preferably 30 by volume electrically conducting powder mixed in with the matrix material. With the most preferred material of the invention, the dried metal powder matrix material has a resistivity only 10 times that of solid copper which equals 1.7 micro-ohm-cm. It is important that the resistivity be as low as possible, preferably in the range from about 10 micro-ohm-cm to about 40 micro-ohm-cm. It is most preferably for the material to have a resistivity less than 20 micro-ohm-cm. Other conducting particle composite materials are preferably copper particles, copper particles covered with a silver film, and copper particles covered with a gold film. Other preferred composite materials  20  are composites of conducting polymers and elastomeric polymers.  
         [0021]    With the material  20  of the invention, the steps of forming an antenna by stamping or etching solid metal electrical conductors, then joining the metal to the semiconductor chip are combined into the steps of screening paste and pressing the electrical circuit connections into the wet paste. This leads to an enormous decrease in the cost of the tags.  
         [0022]    While FIGS.  1 - 8  show antennas as dipole antennas, all antennas as known in the art of RF communication are anticipated by the inventors, including but not limited to patch antennas, spiral antennas, horn antennas, reflector antennas, folded dipole antennas, and polarizing antennas.  
         [0023]    [0023]FIG. 3 shows an elevation view of the apparatus of the invention. Material  20  is shown electrically contacting an electrically conducting material  30  which is electrical contact with electrical circuitry  24 . Electrical circuitry  24  is most preferably a single semiconductor chip. Electrically conducting material  30  may be a pad on the chip, a bump on the chip or another means as known in the art to make contact to a chip. Material  20  is most preferably screen printed on to a substrate (shown later). The formed antenna  19  may be stripped off the screen printed substrate after curing, or may most preferably be left on the substrate.  
         [0024]    [0024]FIG. 4 shows the material  20  of the invention directly contacting electrical contacts of the a chip  40  so that the chip  40  is both mechanically connected to and electrically connected to the material  20  of the antenna  19 .  
         [0025]    [0025]FIG. 5 shows the material  20  of the invention connected to an electrically conducting pad  50  which is wire bonded with wire  54  to chip  40  through a conducting pad or bump  52  on the chip  40 .  
         [0026]    [0026]FIG. 6 shows the antenna of the invention supported by a substrate  60 , and also shows that the chip  40  is encapsulated with encapsulant  62 . The substrate  60  and the encapsulant  62  protect the chip from the environment. The substrate  60  is formed from epoxy-glass printed circuit board or from a polymer such as polyimide, polyethylene terphalate (PET), mylar, polyester, pollycarbonate, or other substrates as are known in the art. In particular, the substrate  60  and the material  20  of the antenna may be elastic so that the system is flexible.  
         [0027]    [0027]FIG. 7 shows the material of the invention and the chip protected by the environment by a layer  72  which is sealed to substrate  60  and which covers material  20  and chip  40  completely. Material for layer  72  is advantageously EVA (Ethylene vinyl acetate), which adheres well with PET and is used extensively in the food industry.  
         [0028]    [0028]FIG. 8 shows a plan view of the invention, where battery  80  is attached to chip  40  by electrical connections  82 . Electrical connections  82  are preferably made of the same material  20  as antenna  19 , and are bonded at the same time that chip  40  and material  20  are bonded together. A suitable battery is a thin film battery comprising MnO 2 , Lithium foil, and a polymer electrolyte (PAN) with a thickness of 14 mils. The apparatus of the invention may also comprise a passive RFID tag.  
         [0029]    Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.