Abstract:
In one embodiment the present invention includes a direct-write laser lithography system. The system includes a reel-to-reel feed system 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 a moving mirror, and is intense enough to ablate the metal but not so strong as to destroy the tape substrate. The ablated metal becomes deposited to form circuit structures on a target structure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       BACKGROUND 
       [0002]    The present invention relates to flexible circuits, and in particular to methods, systems, and devices for manufacturing flexible circuits in high volumes and at low costs. 
         [0003]    Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
         [0004]    Radio frequency identification (RFID) device 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 (PCBs) 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 PCBs are made by adding traces to a bare substrate by electroplating. 
         [0006]    Three common subtractive methods are used to make PCBs. 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 HPGE 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., RFIDs on flexible printed circuits (FPC) using DuPont&#39;s KAPTON polyimide film. 
         [0011]    Thus, there is a need for improved systems and methods for electronic circuit formation. The present invention solves these and other problems by providing systems and methods for using a laser to ablate metal for deposition of circuit structures onto another medium. 
       SUMMARY 
       [0012]    Embodiments of the present invention improve systems and methods related to the formation of electronic circuits and related electronic components. 
         [0013]    A direct-write laser lithography embodiment of the present invention comprises a reel-to-reel or sheet feed system that presents the reverse side of a single-sided metal-coated media to a laser for ablation of the metal. The laser beam is swept laterally across the media by a moving mirror, and is intense enough to ablate the metal but not so strong as to destroy the media substrate. The ablated metal adheres to a target medium to form circuit structures on the target medium. 
         [0014]    According to another embodiment, a laser movement system moves the laser in relation to the metal-coated media in order to direct the laser beam without mirrors. 
         [0015]    One feature of certain embodiments of the present invention is a system that can produce RFID circuits on flexible printed circuits at a low cost per unit. 
         [0016]    Another feature of certain embodiments of the present invention is a manufacturing method for flexible printed circuits that allows for continuous production. 
         [0017]    The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a block diagram of a direct-write laser lithography system according to an embodiment of the present invention that uses a laser to ablate metal from film wound reel-to-reel or sheets fed from a sheet feeding system. 
           [0019]      FIG. 2  is a block diagram of a direct-write laser lithography system according to another embodiment of the present invention that does not use mirrors for directing the laser. 
           [0020]      FIG. 3  is a plan view diagram of a RFID device constructed with a flex circuit antenna etched by the system of  FIG. 1  or  FIG. 2 . 
           [0021]      FIG. 4  is a flowchart of a method of laser circuit deposition according to an embodiment of the present invention. 
           [0022]      FIG. 5  is a block diagram of a control system for controlling laser ablation according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Described herein are techniques for reverse side film laser circuit etching. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include obvious modifications and equivalents of the features and concepts described herein. 
         [0024]      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 metal-on-film substrate tape  104  wound on a supply reel  106  and a take-up reel  108 . 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. 
         [0025]    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 from the tape  104 . The ablated metal  120  then adheres to a target structure  122 . The laser  114  is controlled to ablate such that the ablated metal  120  forms circuit structures on the target structure  122 . 
         [0026]    It is theorized that the laser causes the metal to ablate, partially melt, partially vaporize, or partially become plasma. The partially molten or partially vaporized ablated metal  120  then projects toward the target surface  122 . Upon contact with the target surface  122 , the ablated metal  120  sticks to the target surface in a pattern that generally corresponds to the path followed by the laser  114  as it ablated the metal. In such a manner, ablation by the laser causes the ablated metal to deposit itself in circuit patterns on the target surface  122 . 
         [0027]    The target structure  122  is generally a flexible material, such that traditional circuit deposition techniques (chemical etching, chemical deposition, etc.) are unworkable or inefficient. Materials envisioned for the target structure  122  include various non-metallic surfaces such as textile, leather, wood, glass, polyvinyl chloride (PVC), organic fibers, etc. However, note that even though the motivation behind certain embodiments of the present invention is to deposit circuit structures onto flexible materials, the techniques of the various embodiments of the present invention also allow the deposition onto more traditional materials such as printed circuit boards, metal, etc. 
         [0028]    The above-described process is referred to generally as “subtractive ablative deposition”. The process is “subtractive” in that the ablation subtracts the metal from the coated sheet  104 , “ablative” in that the laser ablates the metal from the coated sheet, and involves “deposition” in that the ablated metal becomes deposited on the target structure  122 . 
         [0029]    Observe in the embodiment shown in  FIG. 1  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. The result is less laser energy is needed to get the job done. 
         [0030]    The materials used for the transparent film substrate 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 15 W diode pumped YAG laser will produce the desired results. 
         [0031]    According to other embodiments, the tape  104  is radiused so the substrate  110  is under compression and the metal cladding  112  is under tension where they encounter the laser beam  118 . Such mechanical stresses and the force of gravity may assist with ablation and not require all the separation energy come from the laser and its heating effects. According to further embodiments, heating, or pre-heating tape  104  may also be used to assist to get the materials up to the points where the metal will ablate more readily and with less violence. According to other embodiments, the tape  104  may be cooled prior to ablation, for example, using liquid nitrogen. Cooling may make a metal such as copper more brittle so that it ablates more easily. The choice of heating, cooling or neither may depend upon the specific material. 
         [0032]    The tape  104  may also be referred to as a coated tape. In general, the term “coated” includes both “laminated”, which refers to an adhesive material between the substrate  110  and the metal cladding  112 , as well as “sputtered”, which refers to a chromium material between the substrate  110  and the metal cladding  112 . These materials help the substrate  110  and the metal cladding  112  to adhere together. 
         [0033]    Although a reel-to-reel tape system is shown in the embodiment of  FIG. 1 , note that other embodiments may instead use a sheet feeder system, or other structure for presenting the tape  104  for ablation. The choice of reel-to-reel tape system, sheet feeder system, or other structure will depend upon various design factors, including the form factor of the coated tape  104 . 
         [0034]    The mirror  116  may be implemented in various ways. According to one embodiment, the mirror  116  is a swinging mirror that may be tilted on one or more axes, for example, the x-axis or the y-axis. The mirror  116  may be part of a galvo head device. According to another embodiment, the mirror  116  may be a rotating mirror, for example, a many-sided prism type structure that is rotated to direct the laser beam. 
         [0035]      FIG. 2  represents a reverse-side laser ablatement system embodiment of the present invention, which is referred to herein by the general reference numeral  200 . System  200  comprises a laser  202 , such as a YAG laser that can operate a relatively high power levels, for example, 15 W. It operates in an atmosphere  204  selected with a view toward improving laser operation and reducing the cost of operating the whole of system  204 . For example, some applications will be able to do best with an atmosphere  204  of either normal air, reduced pressure, vacuum, or dry, or inert atmospheres like nitrogen or argon. A beam  118  of laser light travels through atmosphere  204  and enters the “back side” of a coated tape  104  comprising a dielectric substrate  110  and a metal cladding  112 . An optional intermediate layer may be used between the dielectric substrate  110  and the metal cladding  112 . If used, the intermediate layer may comprise UV absorption materials, in the case of a UV laser  202 , or other wavelength selective energy absorbing materials coordinated with the selection of laser  202 . A sheet feeder system  230  moves the coated sheet  104 . 
         [0036]    It is a feature of the embodiment shown in  FIG. 2  that the material that comprises dielectric substrate  110  be substantially transparent to the laser light beam  118  so that a transitioning beam will be able to deposit a maximum of energy into the metal ablatement area (and to an intermediate heating area if the optional intermediate layer is present). It is desirable that the material of dielectric substrate  110  survive the exposure to laser beam  118  with substantially no damage or heating. It can do that if such material is effective at transmitting the light wavelengths used by laser  202 . So the choice of laser can affect the choice of materials for dielectric substrate  110 , and vice versa. 
         [0037]    If the intermediate layer is present, such intermediate heating area is used to overpressure the ablatement area and stress it to assist in ablating metal  120 . If the intermediate layer is not used, then the transitioning beam reaches metal ablatement area directly and melts and vaporizes metal to produce ablating metal  120  according to patterns written by a patterning control block  222 . 
         [0038]    In general, metal cladding  112  will comprise material conductive to electricity, and dielectric substrate  110  will comprise electrically insulative materials so that patterning control  222  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. 
         [0039]    Laser  202 , and in particular beam  118 , is positioned in coordination with patterning control  222  by means such as pen-plotter mechanisms, x-y stages, micro-mirrors, a galvo head device, etc. according to design tradeoffs in various embodiments. The patterning control  222  in combination with the sheet feeder system  230  work together so that the laser beam  118  ablates the metal from the coated sheet  104  at the desired location. Additional lasers can be included to improve job throughput, or they can be specialized to do wide area or fine feature ablations. Such lasers can use different wavelengths and laser types to assist in such specialization and job sharing. According to another embodiment, to improve throughput, a beam splitter may split a beam from a single laser into multiple beams that are directed by multiple galvo head devices. 
         [0040]    The use of a pen-plotter type positioning mechanism for laser  202  permits the propagation distance that beam  118  has to travel through atmosphere  204  to be reduced as compared to certain embodiments that interpose a mirror between the laser and the substrate  110 . Such then would permit atmosphere  204  to be ordinary air, whereas a longer travel distance could necessitate the use of vacuum in certain embodiments. 
         [0041]    The coated sheet  104  may be implemented in various form factors, and the components of the system  200  may be varied in accordance with the form factor of the coated sheet  104 . Conversely, the form factor of the coated sheet  104  may be varied in accordance with the components of the system  200 . For example, a reel-to-reel tape system (similar to that shown in  FIG. 1 ) may be implemented in the system  200 , in which case the coated sheet  104  may be a coated tape. As another example, the metal layer  112  may have a thickness such that coated sheet  104  may be in sheet form, in which case a sheet feeder may be implemented in the system  200 . 
         [0042]    Various materials for substrate  110  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 
                 15 
                 12 
                 8 
                 20 
                 20 
                 70 
               
               
                 (ppm/° C.) 
               
               
                 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 
                 ? 
                 9.4 
                 6.8 
                 3.4 
                 3.5 
                 2.8 
               
               
                 strength 
               
               
                   
               
             
          
         
       
     
         [0043]    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. 
         [0044]    The choice of metal for cladding  112  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 is to be made in each embodiment. 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 
               
               
                   
                   
               
               
                   
                   
                 reflectivity 
                 thermal conductivity 
                 absorptivity 
               
               
                   
                 metal 
                 248 nm 
                 (W/(cm 2  ° K) 
                 248 nm 
               
               
                   
                   
               
             
             
               
                   
                 copper 
                 0.366 
                 3.98 
                 0.62 
               
               
                   
                 gold 
                 0.319 
                 3.15 
                 0.66 
               
               
                   
                 aluminum 
                 0.924 
                 2.37 
               
               
                   
                   
               
             
          
         
       
     
         [0045]    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 15 W diode pumped YAG laser. 
         [0046]    In addition, the choice of metal will also depend upon the particular target material  122  selected. For example, a flexible material with a fine weave such as TYVEK brand material could involve a relatively thin layer of metal  112  on the sheet  104 . It is theorized that the smaller weave allows less metal to be deposited yet still form a working circuit structure. As another example, a flexible material with a coarse weave such as cotton fibers could involve a relatively thick layer of metal  112  on the sheet  104 . It is theorized that the larger weave has more space between the layers of the weave, requiring more metal to be deposited in order to form a working circuit structure. 
         [0047]    Furthermore, the properties of the metal (such as the thickness, reflectivity, conductivity and absorptivity) will influence the attributes of the laser (such as the power level and wavelength). 
         [0048]    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. Excimer lasers operate in the ultraviolet (UV), below 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., greater than 500 mW continuous, or 10 J/cm 2  pulsed. 
         [0049]    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 (CO 2 ) laser has the longest wavelength at 10600 mm. 
         [0050]      FIG. 3  represents an RFID device  300  with an antenna on a substrate manufactured with system  100  or system  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 . More specifically, the film substrate  302  was used as the target structure  122 . The dimensions of the RFID device  300  may vary as desired, for example, between 1 and 4 inches in length. 
         [0051]    The RFID device  300  is one example of an electrical circuit that may be formed according to embodiments of the present invention. Embodiments of the present invention may also be used to form other electrical circuits and electronic devices. As another example, embodiments of the present invention may be used to form thermal circuits such as flexible heaters. 
         [0052]      FIG. 4  is a flowchart of a method  400  of laser circuit etching according to an embodiment of the present invention. The method  400  may be implemented by various embodiments of the present invention, such as the embodiment shown in  FIG. 1 , the embodiment shown in  FIG. 2 , etc., and variations thereof. 
         [0053]    In step  402 , a coated sheet is provided. As discussed above, the coated sheet comprises a dielectric substrate layer and a metal foil layer. The coated sheet may be in various form factors, such as in tape form or in sheet form. The specific form factor of the coated sheet may depend upon the specific embodiment of the laser etching device. The form factor of the coated sheet may also depend upon the properties of the metal layer. For example, a tape form factor may be suitable for a thinner metal layer, and a sheet form factor may be suitable for a thicker metal layer. Finally, as discussed above, the properties of the metal may depend upon the specific target material  122  selected. 
         [0054]    In step  404 , the target material is provided. As discussed above, the target material may be a flexible material that may be unsuitable for the formation of circuit structures according to traditional circuit formation techniques. 
         [0055]    In step  406 , subtractive ablation is performed. As discussed above, the laser ablates metal in a defined pattern, and the ablated metal conforms to the pattern as it becomes deposited to the target material. In this manner, circuit structures are formed on the target material. 
         [0056]      FIG. 5  is a block diagram of a control system  500  for controlling laser ablation according to an embodiment of the present invention. The control system  500  includes a master control block  502 , beam control block  504 , position control X block  508 , and position control Y block  510 . The control system  500  generally controls the operation of the laser etching system according to the various embodiments of the present invention. The control system  500  may be implemented in hardware, software, or a combination of hardware and software. 
         [0057]    The master control block  502  generally coordinates the other components of the control system  500 . The master control block may store a program or other set of instructions for performing a specific set of ablations, and may then instruct the other components of the control system in accordance with the program or other instructions. 
         [0058]    The beam control block  504  controls the operation of a laser in an embodiment of the present invention (for example, laser  114  in  FIG. 1 ) via control signals. The control signals may indicate the activation of the laser, the power of the laser, or other controllable attributes of the laser in accordance with the specifics of the ablation desired. 
         [0059]    The position control X block  508  controls, via control signals, the relative position between the laser and the coated sheet in an embodiment of the present invention. For example, in the laser etching system  100  of  FIG. 1 , the position control X block  508  controls the movement of the coated film  104  from one reel to another. The movement may be from the reel  108  to the reel  106 , or vice versa. As another example, in the laser etching system  200  of  FIG. 2 , the position control X block instructs the patterning control  222 , for example, to move the laser  202  along an x-axis, along a y-axis, or in a combination of x-axis and y-axis movement. 
         [0060]    The position control Y block  510  controls, via control signals, other aspects of the relative position between the laser and the coated sheet not otherwise controlled by the position control X block  508  in an embodiment of the present invention. For example, in the laser etching system  100  of  FIG. 1 , the position control Y block  510  controls the mirror  116 . In such manner, the movement of the coated film  104  and the mirror  116  can be coordinated so that the laser beam  118  ablates at the desired location on the coated film  104 . 
         [0061]    According to another embodiment, the position control Y block  510  controls, via control signals, the relative position between the metal sheet and the target material. 
         [0062]    As discussed above, the systems and methods according to various embodiments of the present invention are suitable for flexible circuit manufacturing techniques. Flexible circuits may be used in many different applications, including RFID antennas, RFID tag circuitry, membrane switches, flexible heaters and printed circuits, data compact disks, and data video disks. 
         [0063]    The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents will be evident to those skilled in the art and may be employed without departing from the spirit and scope of the invention as defined by the claims. The terms and expressions that have been employed here are used to describe the various embodiments and examples. These terms and expressions are not to be construed as excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the appended claims.