Abstract:
LED chip packaging assembly that facilitates an integrated method for mounting LED chips as a group to be pre-wired to be electrically connected to each other through a pattern of extendable metal wiring lines is provided. LED chips which are electrically connected to each other through extendable metal wiring lines, replace pick and place mounting and the wire bonding processes of the LED chips, respectively. Wafer level MEMS technology is utilized to form parallel wiring lines suspended and connected to various contact pads. Bonding wires connecting the LED chips are made into horizontally arranged extendable metal wiring lines which can be in a spring shape, and allowing for expanding and contracting of the distance between the connected LED chips. A tape is further provided to be bonded to the LED chips, and extended in size to enlarge distance between the LED chips to exceed the one or more prearranged distances.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    (a) Field of the Invention 
         [0002]    The present invention is related to a packaging and mounting construction directed to LED chips and more particularly, to an LED packaging and mounting construction using MEMS fabrication techniques for forming LED chips connected by a pattern of extendable metal wiring lines formed there in between and capable of extendably mounting onto a substrate as a group, and an integrated fabrication method thereof. 
         [0003]    (b) Description of the Prior Art 
         [0004]    LEDs are commonly used for providing illumination because they are compact in size, have a lower power consumption, have a lower operating temperature and have a longer service life, so as to be gradually replacing the conventional tungsten filament bulb and fluorescent lamp. For the fabrication of LED light strings or filaments, many LED chips need to be appropriately placed in accordance with prearranged locations onto a circuit board. Conventionally, the LED chips are individually mounted and placed onto the circuit board by means of an alignment method, such as using a pick-and-place process. Thereafter, wire bonding is performed to create electrical connections between LED chips. The LED chip mounting process can be performed using an LED chip mounter. Drawbacks of conventional methods of LED chip mounting and placement include the fact that the pick-and-place process for mounting individual LED chips is time consuming and requires expensive equipment to perform such tasks. In addition, the wire bonding step to electrically connect the LEDs is also time consuming. Hence, there is a need for improvement in the related art. 
       SUMMARY OF THE INVENTION 
       [0005]    One purpose of the present invention is to provide an LED chip packaging assembly that facilitates an efficient integrated method for mounting a plurality of LED chips as a group. The plurality of LED chips has been pre-wired to be electrically connected to each other through a pattern of extendable metal wiring lines, respectively. 
         [0006]    Another purpose of the present invention is to provide a method for mounting a plurality of LED chips as a group at the same time, while the LED chips have already been electrically connected to each other through a preexisting pattern of extendable metal wiring lines. This replaces the need of mounting individual LED chips by means of pick and place mounting process and of the wire bonding process of the LED chips, respectively. 
         [0007]    To achieve one of the purposes, a wafer level processing technique under MEMS technology is utilized to form a pattern of parallel wiring lines that are suspended as well as being connected to various contact pads of the LED chips. 
         [0008]    To achieve one of the purposes, the bonding wires connecting the LED chips are made into a plurality of extendable metal wiring lines configured in a parallel direction, which can be made in a spring shape. The extendable nature of the extendable metal wiring lines allows increasing and/or decreasing of the distance between the connected LED chips without the extendable metal wiring line being damaged or broken off during usage or mounting. 
         [0009]    To achieve one of the purposes, the use of MEMS technology allows fabrication of a large number of extendable metal wiring lines that are suspended and capable of expanding and/or contracting, at the same time on one substrate or one wafer. Thus, embodiments of the present invention thereby efficiently replace the individually wiring bonding of LED chips processes. 
         [0010]    To achieve the purpose of replacing the need for using the individual pick and place mounting method for individual LED chips, embodiments of present invention utilize LED chips that are arranged at prearranged distances from one another and can be expanded or extended out in a consistent manner, such as at a constant or variable speed, by making use of a substantially uniform stretching or expanding of a tape to which the LED chips are temporarily bonded with, so as to meet the demands for different size requirements for LED chip layout. 
         [0011]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention is illustrated by way of example and not limited by the figures of the accompanying drawings in which same references indicate similar elements. Many aspects of the disclosure can be better understood with reference to the following drawings. Moreover, in the drawings same reference numerals designate corresponding elements throughout. Wherever possible, the same reference numerals are used throughout the drawings to refer to the same or similar elements of an embodiment. 
           [0013]      FIGS. 1-7  illustrate a plurality of schematic cross-sectional views of an integrated fabrication method according to a first embodiment of present invention. 
           [0014]      FIGS. 8-15  illustrate a plurality of schematic cross-sectional views of an integrated fabrication method according to a second embodiment of present invention. 
           [0015]      FIGS. 16-22  illustrate a plurality of schematic cross-sectional views of an integrated fabrication method according to a third embodiment of present invention. 
           [0016]      FIGS. 23-31  illustrate a plurality of schematic cross-sectional views of an integrated fabrication method according to a fourth embodiment of present invention. 
           [0017]      FIGS. 32-38   b  illustrate a plurality of schematic cross-sectional views of an integrated fabrication method according to a fifth embodiment of present invention. 
           [0018]      FIG. 39  is a top view diagram illustrating the LED chips placed on the transparent substrate and the spring structure of the extendable metal wiring line connecting adjacent bonding terminals shown in  FIG. 37  according to the fifth embodiment of present invention. 
           [0019]      FIG. 40  shows a substrate being cut into a plurality of pieces, where each piece of the substrate includes more than one LED chip. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    In the illustrated embodiments presented herein, a p-pad and an n-pad for each LED chip can be both on the same side of the LED chip. 
         [0021]    An integrated fabrication method for mounting and electrically connecting a plurality of LED chips according to a first embodiment of present invention is described as follow: Referring to  FIG. 1 , an LED chip  5  and a plurality of metal layers  10 ,  15  are both formed on one side of an LED wafer  18 . In this embodiment, the metal layer  10  comprises titanium and the metal layer  15  comprises copper. The titanium layer  10  and the copper layer  15  are deposited by e-gun evaporation method, in which the titanium layer  10  is deposited first, and followed by the copper layer  15 . In addition, a plurality of bonding pads  20  are formed on the LED chip  5 . Referring to  FIG. 2 , a photoresist layer  25  is formed to cover the top of the metal layers (Ti/Cu)  10 ,  15 . The photoresist layer  25  can be a negative resist, and the developer used can be an organic solvent. The photoresist layer  25  is then developed and exposed by a photomask and the unexposed region of the negative photoresist is dissolved away by the solvent. The photomask (not shown) is designed to have a (solid) pattern of a plurality of extendable metal wiring lines and a plurality of contacting electrodes. After developing and exposure steps, the patterns on the photomask (not shown in the figure) is transferred onto a patterned photoresist layer  25  so that the patterned photoresist layer  25  then serves as a mask itself in later processing. The removed portions of the patterned photoresist layer  25  are referred to as openings  27 . In another embodiment, the photoresist layer  25  can be a positive resist, and the photomask can be the pattern of a plurality of extendable metal wiring lines and a plurality of contacting electrodes being of opening region (exposed region). Plasma etching can be performed while using the photomask to form the patterned photoresist layer  25  as shown in  FIG. 2 . 
         [0022]    Referring to  FIG. 3 , the patterned photoresist layer  25  is being used as a mask and a metal material  29  such as copper (Cu) is filled into the openings  27  in the etched or patterned photoresist layer  25  to form a plurality of extendable metal wiring lines  30  via a process, which can be in the shape of springs (when viewed under a top view parallel to a top surface of the LED wafer  18 ). For example, to form a plurality of contacting electrodes  35  located above the bonding pads  20 . A plating layer  37  can also be formed on top of the contacting electrodes  35  above the bonding pads  20 . The plating layer  37  can be of copper or gold material. Referring to  FIG. 4 , a first tape  45  is connected to one side of the patterned photoresist layer  25 , which is adjacent to the bonding pads  20  of the LEDs, and also adjacent to the extendable metal wiring lines  30 . In this embodiment, the first tape  45  is treated as a carrier, and the first tape  45  is a flexible, stretchable and translucent plastic material with an adhesive layer coated on one side. A ring  46  connected to the first tape  45  is a portion of a carrier (not shown in the figure), such as a circular brace, which supports the first tape  45  during the following processes. With the support of the carrier, the soft first tape  45  can be constrained in a fixed geometry and be expanded at a steady manner. Moreover, the ring  46  can be used to keep structures to firmly dwell on the first tape  45  and constrain the first tape  45  at a same position during the following manufacturing processes. Referring to  FIG. 5 , a plurality of scribing lines  48  are formed at the sides adjacent to the LED chips  5  on another surface  47  of the LED wafer  18 . These may be used to separate and break off of the LED chips  5 . Referring to  FIG. 6 , the LED chips  5  are broken off along the scribing lines  48 , while the first tape  45  remains bonded to the LED chip  5  during the breaking-off step. After the breaking-off step, the first tape  45  can be removed. Thereafter, a second tape  55  is bonded to another surface  47  of the LED wafer  18 , and then the first tape  45  is removed. In this embodiment, a ring  56  is a portion of a carrier which supports the second tape  55 . As mentioned above, the ring  56  can be used to keep the structures firmly dwell on the second tape  57  and constrain the second tape  56  at a specific position in the following manufacturing steps. The patterned photoresist layer  25  is removed by wet stripping (solvent or acid) or plasma gas stripping. The metal layer (Ti/Cu)  10 ,  15  connected to the LED wafer  18  is selectively etched. Therefore, a portion of the copper metal layer  29  remains as a part of the spring or extendable metal wiring lines  30  and as the contacting electrodes  35 , with the un-etched titanium layer  10  bonded to the contacting electrode  35 . After the selective etching of the titanium metal layer  10  by wet etching with etching solution that can etch titanium without etching copper, a pattern of a plurality of extendable metal wiring lines  30  connecting to a plurality of LED chip  5  is formed (from the remaining left-over portion of the original copper metal layer after selective etching), in which adjacent LED chips  5  are connected to each other by means of the extendable metal wiring lines  30 , respectively. The fabrication steps that have taken place in this embodiment from  FIG. 2  to  FIG. 6  can be regarded as the MEMS fabrication steps. Referring to  FIG. 7 , the contacting electrode  35  is connected or mounted to an expanding table  38 , while the second tape  55  is being removed by applying heat or UV radiation. The expanding table  38  is a fabricated metal assembly comprising of multiple translation stages (x, y, z, radial), which has a center expansion area (not shown) for holding the LED wafer  18 . The size of the center expansion area of the expanding table  38  is dependent upon the wafer size. The expanding table  38  further includes a plurality of extending arms (not shown) configured at predetermined intervals surrounding the center expansion area for extending the sides of the LED wafer  18 . 
         [0023]    Because the LED chips  5  are already detached and broken off along the scribing lines  48 , the LED chips  5  can be further separated by extending the second tape  55  to a desired length and width for mounting the LED chips  5  onto various types of desired substrates. Meanwhile, since the extendable metal wiring lines  30  has a spring-like structure arranged in a substantially parallel direction while in a contracted state, the extended connecting LED chips  5  maintain to be electrically connected to each other in the extended state of the extendable metal wiring lines  30 . The extendable metal wiring line  30  can be functional as an electrically conductive wire in either an extended/expanding state or the contracted/shortened state. A large number of extendable metal wiring lines  30  can be fabricated to one wafer arranged and configured in a specific pattern for connecting to large number of LED chips  5 , such as, i.e. 96000 LED chips on one  6 -inch wafer, at the same time. Because of the extendable physical property of the extendable metal wiring lines  30 , the distance between each pair of connected LED chips  5  can be varied and increased to a preset width to allow for or accommodate flexibility in placement and mounting of the LED chips  5  without having to worry about the extendable metal wiring lines  30  being too short or become entangled. 
         [0024]    An integrated fabrication method for mounting and electrically connecting a plurality of LED chips according to a second embodiment of present invention is described as follow: as shown in  FIG. 8 , a metal seed layer  100  is formed on one side  106  of a template substrate  105 . The metal seed layer  100  comprises titanium. The metal seed layer  100  can be deposited by an e-gun evaporation. As shown in  FIG. 9 , a photoresist layer  110  is formed to cover the top of the metal seed layer  100 , and the photoresist layer  110  can be a positive resist or a negative resist and is then etched or exposed/developed to serve as a mask. As shown in  FIG. 10 , using the patterned photoresist layer  110  as the mask, the metal layer  117 , comprising copper (Cu), is filled into the openings  119  of the patterned photoresist layer  110  to form a plurality of extendable metal wiring lines  120 , which can be in the shape of springs, and also to form a plurality of contacting electrodes  125 . The extendable metal wiring lines  120  form a predefined pattern (not shown in the figure) which covers the template substrate  105 . Each extendable metal wiring line  120  is connected to a pair of adjacent contacting electrodes  125  at two ends thereof. As shown in  FIGS. 11˜12 , the patterned photoresist layer  110  is removed by a process, such as wet stripping (solvent or acid) or plasma gas stripping, and selective etching is performed to remove the metal seed layer  100  so that the extendable metal wiring line  120  electrically connecting the contacting electrodes  125  remains above the template substrate  105 . That is, a portion of the metal seed layer  100   a  located below the extendable metal wiring line  120  is substantially removed by the selective etching, while the portion of the metal seed layer  100   b  beneath the contacting electrode  125  remains. The fabrication steps that have taken place in this embodiment from  FIG. 9  to  FIG. 12  can be regarded as the MEMS fabrication steps. As shown in  FIG. 13 , a first tape  130  is connected to one side of a plurality of LED chips  5 , where each of the LED chips  5  has a pair of contact pads  135 . The pair of contact pads  135  of each LED chip  5  bonded to the first tape  130  are aligned with respect to the pair of contacting electrodes  125  (bonded to the template substrate  105 ) which was formed as shown in  FIG. 10 . As shown in  FIG. 14 , the contact pads  135  are bonded with the corresponding contacting electrodes  125  at one side and connected to the corresponding portion of the metal seed layer  100   b  at the other side. The first tape  130  is removed from the LED chips  5  to form an intermediate LED chip assembly  140 . As shown in  FIG. 15 , the intermediate LED chip assembly  140  along with the LED chip  5  are connected to a final substrate  145  to be a chip on glass (COG) or chip on board (COB) structure, while removing the template substrate  105  from the intermediate LED chip assembly  140  by procedures such as heating, providing UV light, or using organic solvent. The COG structure comprises a glass substrate or a transparent substrate as a support, and the COB structure comprises a PCB board as a support. 
         [0025]    An integrated fabrication method for electrically connecting a plurality of LED chips according to a third embodiment of present invention is described. As shown in  FIG. 16 , an LED chip  5  and a plurality of bonding pads  20 , made of material such as copper, are formed on one surface of a substrate  103 . As shown in  FIG. 17 , a metal layer  116  is deposited by a process, such as e-gun evaporation, to cover the LED chips  5  and the bonding pads  20 . As shown in  FIG. 18 , a photoresist layer  110  is formed to cover the top of the metal layer  116 . The photoresist layer  110  is then etched or patterned to serve as a mask. As shown in  FIG. 19 , the etched or patterned photoresist layer  110  is used as the mask, and a metal material such as copper (Cu) is filled into the openings of the patterned photoresist layer  110  to form a plurality of extendable metal wiring lines  170 . The extendable wiring lines  170  can be in the form of a short wire having a spring shape, as well as to form the contacting electrodes  35  located above the bonding pads  20  of the LED chips  5 , located above the substrate. As shown in  FIG. 20 , the patterned photoresist layer  110  is removed by methods such as wet stripping (solvent or acid) or plasma gas stripping and a plurality of scribing lines  48  are formed above the substrate  103  by a process comprising using deep scribing laser. As shown in  FIG. 21 , the metal layer  116  is removed by selective etching to preserve a portion of the metal layer  116  which is connected between the bonding pads  20  and the contacting electrodes  35 . The bonding pads  20  connected to the LED chip  5  are left. The metal layer left is further used as a spring structure for the extendable metal wiring lines  170  and the contacting electrodes  35 . After the selective etching of the titanium metal layer, a pattern of the extendable metal wiring lines  170  connecting the LED chips  5  is formed from the copper metal layer, in which adjacent LED chips  5  are connected to each other by the extendable metal wiring lines  170 , respectively. The metal layer located below the extendable metal wiring line  170  is substantially removed by the performed selective etching. As shown in  FIG. 22 , with the LED chips  5  being already-detached along the scribing lines  48 , the substrate  103  with separated individual LED chips  5  formed thereon are further scored or scribed and separated thereby forming scribed substrate pieces. So that, a plurality of LED chip assemblies have the pattern of the extendable metal wiring lines  170  connecting adjacent LED chips  5 . 
         [0026]    An integrated fabrication method for electrically connecting a plurality of LED chips to a substrate according to a fourth embodiment of present invention is described. As shown in  FIG. 23 , a metal seed layer  100  is formed on one side of a transparent substrate  300 . The metal seed layer  100  can be deposited by e-gun evaporation. As shown in  FIG. 24 , a photoresist layer  110  is formed to cover the top of the metal seed layer  100  and the photoresist layer  110  is then etched or patterned to serve as a mask. As shown in  FIG. 25 , using the patterned photoresist layer  110  as the mask, metal material, such as copper is filled into the openings  119  of the patterned photoresist layer  110  to form a plurality extendable metal wiring lines  120 . The plurality extendable metal wiring lines  120  can be in the shape of springs. Metal material  117 , comprising copper, is filled into the openings  119  to form a plurality of contacting electrodes  125 . The extendable metal wiring lines  120  form a predefined pattern covering the transparent substrate  300 . Each extendable metal wiring line  120  is connected to a pair of adjacent contacting electrodes  125  at both ends thereof, respectively. As shown in  FIG. 26 , the photoresist layer  110  is removed by methods comprising wet stripping (solvent or acid) or plasma gas stripping, so that, the metal which fills in the openings to form the extendable metal wiring lines  120  and the contacting electrodes  125  remain. Each extendable metal wiring line  120  is connected to a pair of adjacent contacting electrodes  125 . In addition, a pattern of the extendable metal wiring lines  120  connecting the contacting electrodes  125  is formed on the transparent substrate  300 . As shown in  FIG. 27 , selective etching is performed to remove the metal seed layer  100  so that the extendable metal wiring line  120  connecting the contacting electrodes  125  remain above the transparent substrate  300 . The metal seed layer  100  located below the extendable metal wiring line  120  is substantially removed by the selective etching. The fabrication steps that have taken place in this embodiment from  FIG. 24  to  FIG. 27  can be regarded as the MEMS fabrication steps. As shown in  FIG. 28 , a first tape  130  is bonded to one side of a plurality of LED chips  5 , where the LED chips  5  each has a pair of contact pads  135 . The contact pads  135  of each LED chip  5  bonded to the first tape  130  are aligned with respect to the contacting electrodes  125  (bonded to the transparent substrate) formed as shown in  FIG. 25 . As shown in  FIG. 29 , the contact pads  135  are bonded with the corresponding contacting electrodes  125 . The first tape  130  is removed from the LED chips  5  to form an intermediate LED chip assembly  140 . As shown in  FIG. 30 , the transparent substrate  300  is cut and broken off. As shown in  FIG. 31 , the LED chips  5  connected by the extendable metal wiring lines  120  are both movable and/or expandable in a vertical direction (along y-axis), and in a horizontal direction (along x-axis) thereby allowing convenient usage for a construction comprising the individual LED chips  5  and a connected individual pieces of the transparent substrate  300  connected to the flexible substrate (not shown). 
         [0027]    An integrated fabrication method for electrically connecting a plurality of LED chips according to a fifth embodiment of present invention is described here. As shown in  FIG. 32 , a metal seed layer  100  is formed on one side of the transparent substrate  300 . The metal seed layer  100  can be deposited by e-gun evaporation. As shown in  FIG. 33 , a photoresist layer  110  is formed to cover the metal seed layer  100 . The photoresist layer  110  is then etched to serve as a mask. As shown in  FIG. 34 , using the etched photoresist layer  110  as the mask, metal material, comprising copper, is filled into the openings  119  of the etched photoresist layer  110  to form a plurality of extendable metal wiring lines  120  and a plurality of bonding terminals  310 . The extendable metal wiring lines  120  can be in the shape of springs. The extendable metal wiring lines  120  form a predefined pattern covering an area such as that of the transparent substrate  300 . Each extendable metal wiring line  120  is connected to a pair of adjacent bonding terminals  310  at both ends thereof, respectively. As shown in  FIG. 35 , the etched photoresist layer  110  is removed by process comprising wet stripping with solvent or acid applied and plasma gas stripping, but the metal forming the extendable metal wiring lines  120 , and the bonding terminals  310  remain. Each extendable metal wiring line  120  is connected to a pair of adjacent bonding terminals  310 . In addition, a pattern of the extendable metal wiring lines  120  connecting many of the bonding terminals  310  is formed on the transparent substrate  300 . As shown in  FIG. 36 , selective etching is performed to remove the metal seed layer  100 , so that the extendable metal wiring line  120  connecting the bonding terminals  310  remains above the transparent substrate  300 . The metal seed layer  100  below the extendable metal wiring line  120  is substantially removed by the selective etching. The fabrication steps that have taken place in this embodiment from  FIG. 33  to  FIG. 36  can be regarded as the MEMS fabrication steps. As shown in  FIG. 37 , a plurality of LED chips  5  are formed on a first tape  130  through the bonding pads  20  of the LED chips  5 . Then the first tape  130  and LED chips  5  formed thereon are connected to the transparent substrate  300  by procedures such as heating, providing UV light, or using organic solvent. Wherein, the bonding terminals  310  and the extendable metal wiring lines  120  can be optionally attached to the first tape  130 . 
         [0028]    According to the fifth embodiment, either the wire bonding structure  180  as shown in  FIG. 38   a  or the metallization structure  190  as shown in  FIG. 38   b  can be adopted to form an electrical path connecting to external power supply or other LED chip  5 . As shown in  FIG. 38   a , the transparent substrate  300  is cut and broken up into a plurality of individual transparent substrate pieces  300   a  wherein each piece  300   a  holds one LED chip  5 . An electrical connection is made using wire bonding from the bonding terminal  310  to one of the bonding pads  20  of the LED chip  5 . Meanwhile, the bonding terminals  310  and the extendable metal wiring line  120  formed in the steps depicted in  FIG. 34  and  FIG. 35  are electrically connected each other. As shown in  FIG. 38   b , the transparent substrate  300  is cut and broken up into a plurality of individual transparent substrate pieces  300   b  wherein each piece  300   b  holds/comprises one LED chip  5 . Using lithography and metallization technology, an electrical connection is made from the bonding terminal  310  to one of the bonding pad  20  of the LED chip  5 . Meanwhile, the bonding terminals  310  and the extendable metal wiring line  120  formed in the steps depicted in  FIG. 34  and  FIG. 35  are electrically connected each other. As shown in  FIG. 39 , a top view diagram illustrating the placement of the LED chips  5  on the transparent substrate  300  and the spring structure of the extendable metal wiring line  120  connecting adjacent bonding terminals  310  shown in  FIG. 37  according to the fifth embodiment of present invention. 
         [0029]    In the illustrated embodiments, including first to fifth embodiments, the thickness t (as shown for example in  FIG. 3  of the first embodiment) of the extendable metal wiring lines can be between 0 microns to 100 microns. In another embodiment, the thickness t is less than 70 microns. 
         [0030]    As illustrated in some of the above embodiments, the tape supporting the LED chips can be either at a farther distance away from or at a close proximity/adjacent to the extendable metal wiring line. The tape used in the above embodiments can be expanded or extended in one direction or more directions. In all of the illustrated embodiments, the extendable metal wiring line can be suspended. In the embodiments, the tape is made by a thin film layer (not shown) coated by an adhesive layer (not shown). The adhesive layer is made of acrylic based composition. The thin film layer is made of a PVC composition. In addition, the extendable metal wiring line can be made of other metals, such as Ni, Ag, or Au. 
         [0031]    In other alternative embodiments, the pads for the LED chip can be formed on opposite surfaces of the LED chip or on the same surface. Thus, the p-pad and the n-pad can also be on different sides of the LED chip. 
         [0032]    In one embodiment, a substrate  50  holds a plurality of LED chips  5 , in which the substrate  50  can be cut into a plurality of substrate pieces  51 . Each piece  51  of the substrate  50  includes more than one LED chips  5  as shown in  FIG. 40 . The width of the extendable metal wiring lines in the embodiments of present invention can be &lt;70 um, such as 5 um, 10 um, 27 um, 57 um or 62 um. The length of the extendable metal wiring lines can be 255 μm when contracted, or 1200 μm when extended. Meanwhile, the contracted length of the extendable metal wiring line can be quantified as A, and the extended length of the extendable metal wiring line can be estimated to be 3.5 A to 4.0 A. The ratio t/2R of the radius of curvature (R) and the thickness (t) of the extendable metal wiring line cannot exceed the yield strain of the material of the extendable metal wiring line. 
         [0033]    It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the embodiments or sacrificing all of its material advantages.