Abstract:
A method of fabricating a circuit lead, the method comprising: (a) depositing a slurry upon a substrate in a predetermined pattern, the substrate including a plurality of substantially uniformly patterned micropores operative to drain a fluid component of the slurry from the surface of the substrate, while maintaining conductive particles of the slurry on a surface of the substrate; and (b) drying the conductive particles to secure the conductive particles upon a surface of the substrate and provide a circuit lead. The invention also includes an electronic circuit comprising: (a) a substrate including a plurality of micropores that are substantially uniformly patterned; (b) a microchip; and (c) a circuit lead in electrical communication with the microchip and contacting the substrate, the circuit lead comprising conductive particles deposited upon the substrate by ejecting a slurry from an inkjet printer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is claims priority to, and is a continuation of, U.S. patent application Ser. No. 10/973,106, filed Oct. 25, 2004, now U.S. Pat. No. 7,055,756, the disclosure of which is hereby incorporated by reference. 

   RELATED ART 
   1. Field of the Invention 
   The present invention is directed to inkjet printing, and more specifically to inkjet printing utilizing conductive ink deposition to fabricate conductive patterns upon a filtered substrate. 
   2. Related Art 
   Radio Frequency Identification (RFID) tags have been developed and envisioned to replace the bar code as the preeminent future identification tool. Present day methods of manufacturing RFID antennae include costly plating and etching processes similar to current semiconductor device manufacturing techniques. 
   RFID is an automatic identification technology whereby digital data encoded in an RFID tag or “smart label” is captured by a reader using radio waves and, therefore, RFID does not require the tag or label to be visually apparent in order to read its stored data. An RFID system consists of a tag, which is made up of a microchip with an antenna, and a reader with an antenna. The reader sends out electromagnetic waves and the tag antenna is tuned to receive these waves and transmit stored data on the microchip to the reader. RFID tags are either “passive” (no battery) or “active” (self-powered by a battery), with a passive RFID tag drawing power from an electromagnetic field created by the reader to power the microchip&#39;s circuits. The microchip then modulates the waves and sends the waves back to the reader where the reader converts the new waves into digital data. RFID tags can be read-only (stored data can be read but not changed), read/write (stored data can be altered or re-written), or a combination, in which some data is permanently stored while other memory is left accessible for later encoding and updates. 
   Therefore, there remains a need in the art for more widespread use of RFID tags, as well as techniques, and devices produced from such techniques, that reduce the costs associated with RFID tag fabrication. In addition, there is a need in the art for increased quality control and consistency between devices and device subsets produced for RFID applications. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to inkjet printing, and more specifically to inkjet printing utilizing conductive ink deposition to fabricate conductive patterns upon a filtered substrate. The present invention includes methods, and devices manufactured using such methods, for fabricating RFID tags, and more specifically, RFID antennae. The present invention makes use of conductive inks comprising a carrier fluid and suspended conductive particles that are ejected onto a printable medium to create conductive patterns. The present invention incorporates substrates having channels adapted to draw away the carrier fluid from the surface of the substrate to leave behind the conductive particles on the surface. In this manner, a droplet of conductive ink spreads over a smaller area than using conventional substrates and allows for greater precision and density in depositing the conductive particles. Exemplary substrates for use with the present invention may be subjected to a vacuum or elevated temperature environment to dry the conductive particles and stabilize the positioning of the particles on the substrate surface. 
   It is a first aspect of the present invention to provide a method of fabricating a circuit lead, the method comprising: (a) depositing a slurry upon a substrate in a predetermined pattern, the substrate including a plurality of substantially uniformly patterned micropores operative to drain a fluid component of the slurry from the surface of the substrate, while maintaining conductive particles of the slurry on a surface of the substrate; and (b) drying the conductive particles to secure the conductive particles upon a surface of the substrate and provide a circuit lead. 
   In a more detailed embodiment of the first aspect, the plurality of micropores are generally vertically oriented. In a further detailed embodiment, the slurry deposition is carried out using an inkjet printer. In still a further detailed embodiment, the slurry deposition includes repositioning at least one of the substrate and a nozzle of the inkjet printer to deposit the conductive particles upon the substrate in the predetermined pattern. In a more detailed embodiment, the act of drying the substrate includes subjecting the substrate to an elevated temperature ambient. In a more detailed embodiment, the act of drying the substrate includes applying a vacuum to the substrate. 
   It is a second aspect of the present invention to provide a method of fabricating a circuit lead, the method comprising: (a) depositing a slurry upon a filtration medium in a predetermined arrangement, the filtration medium including a plurality of micropores in a substantially uniform arrangement operative to filter solid conductive components of the slurry from a fluid component of the slurry; (b) drying the solid conductive components to secure a substantial portion of the solid conductive components upon a surface of the filtration medium and provide a circuit lead; and (c) mounting a microchip in electrical communication with the circuit lead. 
   In a more detailed embodiment of the second aspect, the plurality of micropores are generally vertically oriented. In a further detailed embodiment, the slurry deposition is carried out using an inkjet printer. In still a further detailed embodiment, the slurry deposition includes repositioning at least one of the filtration medium and a nozzle of the inkjet printer to deposit the solid conductive components upon the filtration medium in the predetermined arrangement. In a more detailed embodiment, the act of drying the filtration medium includes subjecting the filtration medium to an elevated temperature ambient. In a more detailed embodiment, the act of drying the filtration medium includes applying a vacuum to the filtration medium. 
   It is a third aspect of the present invention to provide an electronic circuit comprising: (a) a substrate including a plurality of micropores that are substantially uniformly patterned; (b) a microchip; and (c) a circuit lead in electrical communication with the microchip and contacting the substrate, the circuit lead comprising conductive particles deposited upon the substrate by ejecting a slurry from an inkjet printer. 
   In a more detailed embodiment of the third aspect, at least one of a median width of the micropores and a median length of the micropores is less than at least one of a median width of the conductive particles and a median length of the conductive particles. In yet another more detailed embodiment, a summation of a volume of each micropore of the substrate is greater than the volume of a carrier fluid of the slurry. In a further detailed embodiment, the micropores extend substantially through an entire thickness of the substrate. In still a further detailed embodiment, the substrate comprises a coating applied to a base substrate, where the base substrate includes at least one of paper, a polymer, a composite, and a semiconductor. 
   It is a fourth aspect of the present invention to provide an electronic circuit comprising: (a) a substrate including a plurality of micropores in a substantially uniform arrangement; and (b) an circuit lead formed on the substrate for in electrical communication with a microchip, the circuit lead comprising conductive particles deposited upon the substrate by ejecting a slurry from an inkjet printer. 
   In a more detailed embodiment of the fourth aspect, at least one of a median width of the micropores and a median depth of the micropores is less than at least one of a median width of the conductive particles and a median length of the conductive particles. In yet another more detailed embodiment, a summation of an available volume of the micropores of the substrate is greater than a volume of a carrier fluid, comprising the slurry, deposited on the substrate. In a further detailed embodiment, the micropores extend substantially through an entire thickness of the substrate. In still a further detailed embodiment, the substrate comprises a coating applied to a base substrate, where the base substrate includes at least one of paper, a polymer, a composite, and a semiconductor. 
   It is a fifth aspect of the present invention to provide a method of accurately and precisely depositing conductive particles of a conductive ink from an inkjet printer, the method comprising: (a) orienting a substrate with respect to an inkjet printer, the substrate including a plurality of generally aligned interstices adapted to filter a conductive ink; (b) depositing the conductive ink upon the substrate in a predetermined pattern using the inkjet printer, where the conductive ink comprises conductive particles and a carrier fluid; and (c) drying the conductive particles to mount the conductive particles upon a surface of the substrate and provide a conductive lead. 
   In a more detailed embodiment of the fifth aspect, the act of drying the substrate includes subjecting the substrate to an elevated temperature ambient. In yet another more detailed embodiment, the act of drying the substrate includes applying a vacuum to the substrate. In a further detailed embodiment, the conductive ink deposited on the substrate is chemically compatible with the substrate. In still a further detailed embodiment, the conductive ink comprises a slurry of solid conductive particles and the carrier fluid. In a more detailed embodiment, the act of depositing the conductive ink includes depositing the conductive ink using more than one nozzle of a printhead associated with the inkjet printer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a first exemplary embodiment in accordance with the present invention; and 
       FIG. 2  is an overhead view of an exemplary printer configuration that may be used to fabricate the antenna of the first exemplary embodiment as shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   The exemplary embodiments of the present invention are described and illustrated below to encompass methods, and devices produced in accordance with such methods, for depositing conductive materials upon a filtered substrate. Of course, it will be apparent to those of ordinary skill in the art that the preferred embodiments discussed below are exemplary in nature and may be reconfigured without departing from the scope and spirit of the present invention. In addition, those of ordinary skill will readily comprehend various devices that may be fabricated in accordance with the methods discussed herein and, therefore, the disclosure is not limited to the exemplary embodiments discussed herein, as these embodiments are for purposes of illustrating the invention only. However, for clarity and precision, the exemplary embodiments as discussed below may include optional steps or features that one of ordinary skill will recognize as not being a requisite to fall within the scope of the present invention. As used herein, micropore refers to any orderly channel within a substrate adapted to enable a fluid to flow therein and having an opening inhibiting throughput of particles having a mean diameter of 20-100 nanometers. 
   In fabricating a Radio Frequency Identification (RFID) antenna utilizing an inkjet printer, a number of considerations may be involved in achieving the desired conductivity, uniformity, and repeatability that include, without limitation, the type conductive ink deposited, any post-deposition treatment of the antenna, the software and printing mode utilized to deposit the ink, the printer and type of printhead used in depositing the ink, and the media upon which the ink is deposited. While the present invention, as discussed herein, may appear to be directed to the latter consideration, it is to be understood that the latter consideration is simple one piece of the present invention. 
   During a conductive ink printing process, at least two mechanisms are involved in forming the conductive pattern: (1) deposition and subsequent connection of the individual conductive particles to form a conductive path; and (2) absorption of a liquid carrier phase that may include constituents such as, without limitation, water, a surfactant, and a humectant. To achieve the first mechanism most efficiently, a relatively smooth and solid substrate surface is favorable to reduce gaps between conductive particles from elevational changes and discontinuities in the surface. To achieve the second mechanism, however, a surface having a non-uniform topography is advantageous to provide hollows or cavities for the separation of the carrier fluid from the conductive particles, which hasten drying of the conductive particles stabilizing the relative position of the conductive particles on the substrate surface. Therefore, at first glance, it might appear that these interests are competing and cannot be concurrently accommodated. Nevertheless, the present invention is operative to accommodate each of these interests without placing both in direct competition with one another. 
   Referencing  FIG. 1 , a first exemplary embodiment comprises an RFID tag  10  that includes a substrate  12  having a microchip  14  positioned thereon. Those of ordinary skill are familiar with the plethora of techniques for mounting a microchip  14  to a substrate  12  and, therefore, an exhaustive explanation has been omitted for purposes of brevity. The microchip  14  includes at least one contact pad  16  in electrical communication with an inkjet deposited antenna  18  that comprises a plurality of conductive particles  20  deposited in a predetermined pattern. 
   An exemplary substrate  12  in accordance with the present invention includes a generally smooth surface  22  with a plurality of openings  24  within the surface of the substrate  12  leading to micropores  26  within the substrate. The substrate  12  is comprised of materials that are not especially chemically reactive with the deposited components in contact therewith and those of ordinary skill will understand the utility of the substrate  12  being reactively dormant during deposition of the conductive ink and later processing. The substrate  12  may be comprised of aligned particles that are spaced apart to create micropores  26  therebetween. Exemplary materials for use as the substrate  12  include, without limitation, ceramic particles. In this exemplary substrate  12 , the micropores  26  are vertical and linearly oriented, however, it is to be understood that angled micropores and non-linear micropores may be operative with the present invention and these operative orientations likewise fall within the scope of the present invention. It is to be understood that the uniformity of the substrate particles is of increasing importance to fabricate micropores  26  that are repeatable and generally aligned. In particular, the dimensions of the micropores  26  are such that the mean particle size of the conductive particles  20  deposited thereover will not completely plug the micropores. Commercially available substrates that may be used with the present invention include media from Pictorico (www.pictorico.com). 
   Referring to  FIGS. 1 and 2 , an inkjet printer  28  may be utilized to deposit the conductive particles  20  of the RFID antenna  18 . The inkjet printer  28  includes an ink reservoir  30  in fluid communication with a plurality of nozzles  32  of a printhead  34  and controls  36  operative to position the nozzles  32  over particular locations of the substrate  12 . A bitmap may be communicated to the printer  28  and translated by the controls  36  to create a printing sequence where the nozzles  32  of the printhead  34  will be fired to eject ink onto the substrate  12 . In this exemplary embodiment, the ink comprises a slurry that includes conductive particles  20  and a carrier fluid  38  (See  FIG. 1 ) that may include a surfactant and a humectant. Those of ordinary skill are familiar with the various commercially available conductive inks that may be used in accordance with the present invention. Such inks include, without limitation, silver collide inks and are available from Nippon Paint Company (www.nipponpaint.co.jp). 
   The printing sequence is operative to acknowledge the position of the microchip  14  and contact pad  16  on the substrate  12 . More specifically, the printing sequence will call for the deposition of the ink, including the conductive particles  20 , onto the substrate  12  to form the antenna  18  in electrical communication with the contact pad  16 . Those of ordinary skill are familiar with the plethora of antenna designs and orientations available for use with RFID tags  10  that may be carried out using an inkjet printer  28  in accordance with the present invention, each of which concurrently fall within the scope of the present invention. 
   Referencing  FIGS. 1 and 2 , ink is ejected from the nozzles  32  and deposited onto the substrate  12  and the openings  24  therein allow the carrier fluid  38  to be drawn into the micropores  26  and away from the surface  22 . This separation of carrier fluid  38  and conductive particles  20  results in less carrier fluid  38  in contact with the conductive particles  20  above the surface  22 , providing fewer avenues of movement for the particles  20  in contact with carrier fluid  38  above the substrate surface  22 . The concentration of conductive particles  20  resulting from less carrier fluid  38  on the surface  22  of the substrate  12  allows for more precise and accurate placement of the conductive particles  20 , which may result in better connections between the deposited conductive particles  20 . This can be sharply contrasted with prior art systems where the droplet spread out over the substrate before being considerably absorbed, creating a greater footprint. After the ink is deposited, the conductive particles  20  on the surface  22  of the substrate  12  are dried to stabilize the orientation of the particles on the surface. 
   Drying of the conductive particles  20  on the substrate surface  22  may be carried out using numerous techniques. A first exemplary technique includes subjecting the substrate  12  and deposited particles  20  to ambient conditions and allowing the fluid components  38  of the ink to vaporize and effectively dry the conductive particles  20 . A second exemplary technique includes subjecting the substrate  12  and deposited particles  20  to a vacuum to draw off the fluid components  38  of the ink and effectively dry the conductive particles  20 . A third exemplary technique includes subjecting the substrate  12  and deposited particles  20  to an elevated temperature environment to vaporize the fluid  38  and effectively dry the conductive particles  20 . A fourth exemplary technique includes subjecting the substrate  12  and deposited particles  20  to a vacuum in a heated environment to vaporize the fluid  38  and effectively dry the conductive particles  20 . Those of ordinary skill are familiar with other techniques for drying a deposited material on a substrate, and such techniques concurrently fall within the scope of the present invention. 
   It is also within the scope and spirit of the present invention to fabricate antennae  18  on the substrate  12  without a microchip  14  and/or a contact pad  16  being mounted to the substrate. In this manner, the antenna  18  is prefabricated and may be later mounted to the contact pad  16  of the microchip  14  to comprise the RFID tag  10 . 
   It is also within the scope of the present invention to mount the substrate  12  to another base substrate (not shown) comprising a polymer, a paper base, a semiconductor, or a composite, where the substrate  12  may be applied to the base substrate prior to or subsequent to deposition of the conductive particles  20 . 
   It is also within the scope of the present invention that the median width of the micropores  26  and a median depth of the micropores  26  are less than at least one of a median width of the conductive particles  20  and a median length of the conductive particles  20 . It should also be understood that the thickness of the substrate  12  may be such that the micropores  26  extend from a top surface through to a bottom surface, thereby extending the entire thickness of the substrate  12 . It is to be understood that the micropores  26  need not extend substantially the entire thickness of the substrate  12  in order to fall within the scope of the present invention. Still further, the volume available within the micropores  26  of the substrate  12  is preferably greater than the volume of the carrier fluid  38  deposited on the substrate  12 , however, substrates having micropore  26  volumes less than the eventual volume of carrier fluid  38  deposited thereon do not necessarily fall outside of the scope of the present invention. 
   Following from the above description and invention summaries, it should be apparent to those of ordinary skill in the art that, while the methods and apparatuses herein described constitute exemplary embodiments of the present invention, the invention contained herein is not limited to this precise embodiment and that changes may be made to such embodiments without departing from the scope of the invention as defined by the claims. Additionally, it is to be understood that the invention is defined by the claims and it is not intended that any limitations or elements describing the exemplary embodiments set forth herein are to be incorporated into the interpretation of any claim element unless such limitation or element is explicitly stated. Likewise, it is to be understood that it is not necessary to meet any or all of the identified advantages or objects of the invention disclosed herein in order to fall within the scope of any claims, since the invention is defined by the claims and since inherent and/or unforeseen advantages of the present invention may exist even though they may not have been explicitly discussed herein.