Abstract:
A direct-write laser lithography system comprises a reel-to-reel feed system in a vacuum chamber that presents the clear film-side of a single-sided metal-clad tape to a laser for direct patterning of the metal. The laser beam is swept laterally across the tape by rotating mirrors, and is intense enough to ablate the metal but not so strong as to destroy the tape substrate. In one instance, two specialized lasers are used, one set to ablate large field areas, and the other tuned to scribe fine features and lines. The ablated metal blows off in a downward direction and is collected for recycling.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to flexible circuits, and in particular to methods, systems, and devices for manufacturing flexible circuits in very high volumes and at very low costs. 
         [0003]    2. Description of the Prior Art 
         [0004]    Radio frequency identification device (RFID) technology is proliferating everywhere and into everything. Right now, a worldwide effort is stepping into high gear to replace the familiar universal product code (UPC) barcodes on products with RFID tags. The ink and labels used to print UPC barcodes is very inexpensive, and the costs of RFID chips and printed circuit antennas are under a lot of pressure to match them. Large, expensive items, of course, are not price sensitive to the cost of a typical RFID tag. But mass produced commodity items need tags that cost only a few cents. 
         [0005]    The majority of printed circuit boards (PCB&#39;s) are made by depositing a layer of copper cladding over the entire substrate, then subtracting away the unwanted copper by chemical etching, leaving only the desired copper traces. Some PCB&#39;s are made by adding traces to a bare substrate by electroplating. 
         [0006]    Three common subtractive methods are used to make PCB&#39;s. Etch-resistant inks can be screened on the cladding to protect the copper foils that are to remain after etching. Photoengraving uses a photomask to protect the copper foils, and chemical etching removes the unwanted copper from the substrate. Laser-printed transparencies are typically employed for phototools, and direct laser imaging techniques are being used to replace phototools for high-resolution requirements. PCB Milling uses a 2-3 axis mechanical milling system to mill away copper foil from the substrate. A PCB milling machine operates like a plotter, receiving commands from files generated in PCB design software and stored in HPGL or Gerber file format. 
         [0007]    Additive processes, such as the semi-additive process starts with an unpatterned board and a thin layer of copper. A reverse mask is then applied. Additional copper is plated onto the board in the unmasked areas. Tin-lead and other surface platings are then applied. The mask is stripped away, and a brief etching step removes the now-exposed thin original copper laminate from the board, isolating the individual traces. 
         [0008]    The additive process is commonly used for multi-layer boards because it favors making plating-through holes (vias) in the circuit board. 
         [0009]    Circuit etching methods that use chemicals, coatings, and acids are slow, expensive, and not environmentally friendly. Mechanical etching has been growing rapidly in recent years. mechanical milling involves the use of a precise numerically controlled multi-axis machine tool and a special milling cutter to remove a narrow strip of copper from the boundary of each pad and trace. 
         [0010]    Conventional laser etching of circuit traces is from the side with the metal to be etched. The metal, smoke, and debris goes flying directly in the path of the laser beam trying to do its work. The laser and its optics need frequent cleaning in order to maintain etching efficiency. But lasers can be a very fast, environmentally safe way to mass produce printed circuits, e.g., RFID&#39;s on flexible printed circuits (FPC) using DuPont&#39;s KAPTON polyimide film. 
       SUMMARY OF THE INVENTION 
       [0011]    Briefly, a direct-write laser lithography embodiment of the present invention comprises a reel-to-reel or sheet feed system in a vacuum chamber that presents the reverse side of a single-sided metal-coated media to a laser for direct patterning of the metal. The laser beam is swept laterally across the media by rotating mirrors, and is intense enough to ablate the metal but not so strong as to destroy the media substrate. In one instance, two specialized lasers are used, one set to ablate large field areas, and the other tuned to scribe fine features and lines. The ablated metal blows off in a downward direction and is collected for recycling. 
         [0012]    An advantage of the present invention is that a system is provided that can produce RFID circuits on flexible printed circuits at exceedingly low cost per unit. 
         [0013]    Another advantage of the present invention is manufacturing method is provided that produces very little waste and that readily recycles the metals ablated from the tapes. 
         [0014]    A further advantage of the present invention is manufacturing method for flexible printed circuits is provided for continuous production. 
         [0015]    These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures. 
     
     
       IN THE DRAWINGS 
         [0016]      FIG. 1  is a functional block diagram of direct-write laser lithography system embodiment of the present invention that uses a single laser to ablate metal from film wound reel-to-reel or sheets feed from a sheet feeding system; 
           [0017]      FIG. 2  is a functional block diagram of dual-direct-write laser lithography system embodiment of the present invention that uses a one laser to ablate wide fields of metal, and another set to scribe very fine lines and features; 
           [0018]      FIG. 3  is a plan view diagram of a typical RFID device constructed with a flex circuit antenna etched in the system of  FIG. 1  or  2 ; and 
           [0019]      FIG. 4  is a cross-sectional view diagram of a reverse-side laser ablatement system embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]      FIG. 1  represents a direct-write laser lithography system embodiment of the present invention, and is referred to herein by the general reference numeral  100 . System  100  is used to manufacture flexible printed circuits (FPC), and comprises a vacuum chamber  102  in which are disposed a metal-on-film substrate tape  104  wound on a supply reel  106  and a take-up reel  108 . A vacuum of about 115 torr (0.15 atm) is used. The tape  104  has a transparent film substrate  110  and a thin-film metal cladding  112 . The transparent film substrate  110  may comprise polyimide, PEN, polyester, polycarbonate, etc. The thin-film metal cladding  112  may include copper (CU), aluminum (AL), platinum (PT), etc. 
         [0021]    A laser  114  is used to ablate off the metal from the backside of tape  104  as it translates from supply reel  106  to take-up reel  108 . A mirror  116  moves a laser beam  118  to various lateral points across the tape  104 . Once laser beam  118  is positioned properly, a pulse of energy is generated enough to ablate metal  120  away. These are patterned to create electrical circuits, e.g., RFID antennas. A metal collection and recycle  122  captures the ablated metal  120  and recycles it. 
         [0022]    It is important to observe that the ablated metal  120  does not fly or plume into the path of laser beam  118  because the ablation is on the opposite side to the laser. Further, laser beam  118  is unimpeded by normal atmosphere because the whole is enclosed in vacuum chamber  102 . The result is less laser energy is needed to get the job done. If the laser energy is too high, the ablated material will convert to plasma and the vapors can coat components inside the chamber. The goal is to carry away pieces for recycling, so it is better if the ablation dislodges or tears away solid chunks of metal. So breaking the adhesive bond between the metal and the substrate of the tape is an important step in the ablation. 
         [0023]    The materials used for the transparent film substrate  110  and the wavelength of laser beam  118  are chosen such that the energy absorbed by the substrate will be minimal and be able to pass the laser energy through to concentrate on ablating the metal  120 . This could be assisted by placing an energy absorbing material between the transparent film substrate  110  and a thin-film metal cladding  112 . The choice of type and power level of laser  114  will be empirically derived, but initial indications are that a 1.5W carbon-dioxide laser will produce the desired results. 
         [0024]    There is a balance between what kinds of laser beams  118  will be good for wide area ablating of metal, and what kind will provide clean sharp features. An alternative embodiment of the present invention uses two lasers, one for wide area ablating of metal, and the other set to write clean sharp features. 
         [0025]      FIG. 2  shows a dual direct-write laser lithography system embodiment of the present invention, and is referred to herein by the general reference numeral  200 . System  200  is used to manufacture flexible printed circuits (FPC), and comprises a vacuum chamber  202  in which are disposed a metal-on-film substrate tape  204  wound on a supply reel  206  and a take-up reel  208 . The tape  204  has a transparent film substrate  210  and a thin-film metal cladding  212 . The transparent film substrate  210  may comprise polyimide, PEN, polyester, polycarbonate, etc. The thin-film metal cladding  212  may include copper (CU), aluminum (AL), platinum (PT), etc. 
         [0026]    A coarse laser  214  is used to ablate off wide fields of metal from the backside of tape  204  as it translates from supply reel  206  to take-up reel  208 . A first mirror  216  moves a coarse laser beam  218  to various lateral points across the tape  204 . Once coarse laser beam  218  is positioned properly, a pulse of energy is generated to ablate field metal  220  away. Such ablated metal takes heat away and is caught and recycled by metal collection and recycle  222 . 
         [0027]    A fine laser  244  is used to ablate off fine lines of metal from the backside of tape  204 , after it finishes with coarse laser beam  218 . A second mirror  226  moves a fine laser beam  228  to various lateral points across the tape  204 . Once fine laser beam  218  is positioned properly, e.g., within 50-micrometers, a pulse of energy is generated to ablate precise lines and spots of metal  230  away. 
         [0028]    Gravity is used to assist the falling away and collection of ablated metals  220  and  230 . It may also be useful to radius tape  204  so the substrate  210  is under compression and the metal cladding  212  is under tension where they encounter laser beams  118  and/or  228 . Such mechanical stresses and the force of gravity can assist with ablation and not require all the separation energy come from the laser and its heating effects. Heating, or pre-heating tape  204  would also be helpful to get the materials up to the points where the metal will ablate more readily and with less violence. 
         [0029]    Having to balance between what kinds of laser beams would be good for wide area ablating of metal, and what kind would provide clean sharp features is avoided in the system  200  of  FIG. 2  by using the two different specialized lasers  214  and  224 . 
         [0030]    Various materials for substrate  110  and  210  can be used, the best depending on several variables. A typical substrate tape is  460  mm wide. Table I summarizes the properties of several popular materials. (As reported by LPKF Laser &amp; Electronics AG.) 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                 KAPTON 
                 APICAL 
                 UPILEX 
                 KALADEX 
                 MYLAR 
                 MAKROFOL 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Tg (° C.) 
                 385 
                 &gt;500 
                 &gt;500 
                 122 
                 80 
                 153 
               
               
                 CTE (ppm/° C.) 
                 15 
                 12 
                 8 
                 20 
                 20 
                 70 
               
               
                 tensile 
                 24 
                 15–24 
                 35 
                 32 
                 28–32 
                 20–25 
               
               
                 strength 
               
               
                 Kpsi 
               
               
                 water 
                 2.9 
                 2.2 
                 1.2 
                 &lt;1 
                 &lt;1 
                 0.35 
               
               
                 absorp. 
               
               
                 (%/wt.) 
               
               
                 dielectric 
                 7 
                 9.4 
                 6.8 
                 3.4 
                 3.5 
                 2.8 
               
               
                 strength 
               
               
                   
               
             
          
         
       
     
         [0031]    KAPTON, APICAL, and UPILEX are brand names of various forms of polyimide, KALADEX is a polyethylene naphthalate (PEN), MYLAR is a polyester, and MAKROFOL and LEXAN are polycarbonates. 
         [0032]    The choice of metal for cladding  112  and  212  depends on several tradeoffs. In general, the thinner the metal, the easier is the laser ablation. Thinner materials will have higher sheet resistances, as measured in Ohms per square. A balance between these must be made. Copper is a good choice for circuit wiring, but the copper material absorbs and dissipates heat very efficiently, and that counters the spot heating effects the laser is trying to obtain for ablation. Aluminum is better in this regard, but gold and platinum may have to be used if the application is in a corrosive environment. The metals&#39; reflectivity, absorptivity, and thermal conductivity are key parameters in the choice of metal to use. LPKF Laser &amp; Electronics AG reported on three of these metals, as in Table II. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                   
                   
                 thermal 
                   
               
               
                   
                   
                 reflectivity 
                 conductivity 
                 Absorptivity 
               
               
                   
                 metal 
                 248 nm 
                 (W/cm2K−1) 
                 248 nm 
               
               
                   
                   
               
             
             
               
                   
                 copper 
                 0.366 
                 3.98 
                 0.62 
               
               
                   
                 gold 
                 0.319 
                 3.15 
                 0.66 
               
               
                   
                 aluminum 
                 0.924 
                 2.37 
               
               
                   
                   
               
             
          
         
       
     
         [0033]    Early proof-of-concept tests were made with different thicknesses of metal on a polyethylene terephthalate (PET) substrate, and at different reel-to-reel tape speeds, e.g., 0.2 μm Cu at 2.5 m/s, 0.5 μm Cu at 2.5 m/s, 0.2 μm Al at 3.0 m/s, and 0.5 μm Al at 3.0 m/s. The laser was a 1.5W CO2 laser. 
         [0034]    Many kinds of lasing mediums are used for lasers, and the mediums determine the wavelength of the coherent light produced. The right one to use here depends on the films, metals, and processing speeds decided. Eximer lasers operate in the ultraviolet (UV), &lt;425 nm. The Argon:Fluorine (Ar:F) laser operates at 193 nm, and Krypton:Fluoride (Kr:F) at 248 nm. The nitrogen UV laser emits light at 337 nm. The Argon laser is a continuous wave (CW) gas laser that emits a blue-green light at 488 and 514 nm. The potassium-titanyl-phosphate (KTP) crystal laser operates in green, around 520 nm. Pulsed dye lasers are yellow and about 577-585 nm. The ruby laser is red and about 694 nm. The synthetic chrysoberyl “alexandrite” laser operates in the deep red at about 755 nm. The diode laser operates in the near infrared at about 800-900 nm. The right laser to use in embodiments of the present invention will probably be the hazardous Class-IV types, e.g., &gt;500 mW continuous, or 10 J/cm2 pulsed. 
         [0035]    YAG lasers are infrared types that use yttrium-aluminum-garnet crystal rods as the lasing medium. Rare earth dopings, such as neodymium (Nd), erbium (Er) or holmium (Ho), are responsible for the different properties of each laser. The Nd:YAG laser operates at about 1064 nm, the Ho:YAG laser operates at about 2070 nm, and the “erbium” Er:YAG laser operates at just about 2940 nm. YAG lasers may be operated in continuous, pulsed, or Q-Switched modes. The carbon-dioxide (CO2) laser has the longest wavelength at 10600 nm. 
         [0036]      FIG. 3  represents an RFID device  300  with an antenna on a substrate manufactured with system  100  or  200 . The RFID device  300  comprises a film substrate  302  on which has been laser-patterned a folded dipole antenna. A RFID chip  304  is attached to a bond area  306 , and these are connected to left and right antenna elements  308  and  310 . 
         [0037]      FIG. 4  represents a reverse-side laser ablatement system embodiment of the present invention, which is referred to herein by the general reference numeral  400 . System  400  comprises a laser  402 , such as a CO 2  laser that can operate a relatively high power levels. For example, 1.5W. It operates in an atmosphere  404  selected to optimize laser operation and cost of operating the whole of system  404 . For example, some applications will be able to do best with an atmosphere  404  of either normal air, reduced pressure, vacuum, or dry, or inert atmospheres like nitrogen or argon. A beam  406  of laser light travels through atmosphere  404  and enters the “back side” of a laminate comprising a dielectric substrate  408 , an optional intermediate layer  410 , and a metal cladding  412 . If used, the intermediate layer  410  may comprise UV absorption materials, in the case of a UV laser  402 , or other wavelength selective energy absorbing materials coordinated with the selection of laser  402 . 
         [0038]    It is important that the material which comprises dielectric substrate  408  be substantially transparent to the laser light beam  406  so that a transitioning beam  414  will be able to deposit a maximum of energy in an intermediate heating area  416  (if present) and metal ablatement area  418 . The material of dielectric substrate  408  must survive the exposure to laser beam  406  with substantial no damage or heating. It can do that if such material is effective at transmitting the light wavelengths used by laser  402 . So the choice of laser can affect the choice of materials for dielectric substrate  408 , and vice versa. 
         [0039]    Such heating area  416  is used to overpressure ablatement area  418  and stress it to assist in ablating metal  420 . If intermediate layer  410  is not used, then transitioning beam  414  reaches metal ablatement area  418  directly and melts and vaporizes metal to produce ablating metal  420  according to patterns written by a patterning control  422 . The metal cladding  412  may be pre=patterned to reduce the amount of metal that must be ablated on-line in final patterning, e.g., into RFID antenna circuits and other electronics boards. 
         [0040]    In general, metal cladding  412  will comprise material conductive to electricity, and dielectric substrate  408  will comprise electrically insulative materials so that patterning control  422  can produce rigid or flexible printed circuits. Typical metals are copper, aluminum, gold, silver, platinum, etc. Typical insulators are polyimide, polycarbonate, silicon dioxide, alumina, glass, diamond, etc., in tapes, boards, films, and dice. 
         [0041]    Laser  402 , and in particular beam  406 , is positioned in coordination with patterning control  422  by conventional means, e.g., pen-plotter mechanisms, x-y stages, micro-mirrors, etc. Additional lasers can be included to improve job throughput, or they can be specialized to do wide area or fine feature ablatements. Such lasers can use different wavelengths and laser types to assist in such specialization and job sharing. 
         [0042]    The use of a pen-plotter type positioning mechanism for laser  402  would permit the propagation distance that beam  406  has to travel through atmosphere  404  to be kept to an absolute minimum. Such then would permit atmosphere  404  to be ordinary air, whereas a longer travel distance would necessitate the use of vacuum, as in  FIGS. 1 and 2 . 
         [0043]    Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the “true” spirit and scope of the invention.