Abstract:
A method of repairing or augmenting a metal line buried beneath at least one cover layer comprising the steps of creating a via through the cover layer to the metal line, repairing or augmenting the metal line, and filling the via. Also provided is apparatus for repairing or augmenting a metal line buried beneath at least one cover layer comprising means for creating a via through the cover layer to the metal line, means for repairing or augmenting the metal line, and means for filling the via. Also provided is a substrate having a metal line buried beneath at least one cover layer wherein the metal line has been repaired or augmented according to the process comprising the steps of creating a via through the cover layer to the metal line, repairing or augmenting the metal line, and filling the via.

Description:
FIELD OF THE INVENTION 
     This invention relates to repair of metal line and, more particularly, to the repair or augmentation of metal line covered with a dielectric material using laser ablation. 
     BACKGROUND OF THE INVENTION 
     Metal lines are commonly used for connecting circuits and personal computer boards and multichip modules, among many other applications. In some of these applications, the metal lines are covered by a dielectric material. In such devices, defects or openings in the metal line are difficult to repair because access to the metal lines is hindered by the dielectric cover layer. Presently, when defects in the metal lines of such devices are discovered, the device is considered irreparable, and it is thrown away. 
     A technique for repairing such buried metal lines is desirable. Such a technique would reduce production costs and waste of devices incorporating buried metal lines. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for repairing or augmenting a metal line buried beneath at least one cover layer by creating a via through the cover layer to the metal line, repairing or augmenting the metal line, and filling the via. 
     The invention also provides a substrate having a metal line buried beneath at least one cover layer wherein the metal line has been repaired or augmented according to the process of creating a via through the cover layer to the metal line, repairing or augmenting the metal line, and filling the via. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side sectional view of a buried metal line, with an opening therein, on a substrate. 
     FIG. 2 is a top sectional view of the substrate shown in FIG.  1 . 
     FIG. 3 is a side sectional view of the substrate depicted in FIG. 1 during repair according to an exemplary embodiment of the present invention. 
     FIG. 4 is top sectional view of the substrate shown in FIG.  3 . 
     FIG. 5 is a side sectional view of the substrate shown in FIG. 1 during repair according to an exemplary embodiment of the present invention. 
     FIG. 6 is a side plan view of laser metal deposition apparatus including an exemplary embodiment of the present invention. 
     FIG. 7 is a side plan view of an exemplary unit cell for metal deposition and bonding according to the present invention. 
     FIG. 8 is a top sectional view of the substrate shown in FIG.  5 . 
     FIG. 9 is a side sectional view of the substrate shown in FIG. 1 during repair according to an exemplary embodiment of the present invention. 
     FIG. 10 is a top sectional view of the substrate shown in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention provides a technique for repairing or augmenting an electrode or circuit, typically in the form of a metal line, when the electrode or circuit is buried beneath a layer of dielectric or other cover material. 
     FIG. 1 is an exemplary depiction of such a device having a buried metal line. The device shown includes a substrate  10  which is typically soda lime glass. Metal line  12  is deposited on substrate  10 . In the illustrated embodiment, metal line  12  comprises three separate layers,  12   a ,  12   b , and  12   c . Layers  12   a  and  12   c  are chromium, and layer  12   b  is copper. Disposed over metal line  12  and substrate  10  is a dielectric layer  11 , which is SiO 2  in this embodiment and is about 20 microns thick. Dielectric layer  11  completely covers metal line  12  in the illustrated embodiment. 
     FIG. 2, which is a top sectional view of the device shown in FIG. 1, illustrates a defect, or opening,  13  in metal line  12 . Such an opening  13  results in a break in electrical contact in the circuit which produces a defective device. Accordingly, it is desirable to repair opening  13  to reestablish contact between the separated portions of metal line  12 . Before the present invention, devices with defects such as opening  13  depicted in FIG. 2 were merely discarded as unusable. With the present invention, however, it is possible to repair defects such as opening  13 . 
     In order to access metal line  12  to effect the repair, it is necessary to form a via  21  in dielectric layer  11  as shown in FIG.  3 . Via  21  is formed by laser ablation. In the exemplary embodiment illustrated in FIG. 3, an ultraviolet laser beam  20  is directed onto dielectric layer  11  such that laser beam  20  impinges on dielectric layer  11  and ablates, or removes, ions from dielectric layer  11 . As a result, an access, or via,  21  through dielectric layer  11  is produced. Via  21  may be formed in any desired shape or dimension. It is only necessary that via  21  provide access to opening  13  in metal line  12  to repair opening  13 . Electrode  12  in the exemplary embodiment shown is approximately 60 microns wide. Via  21  is approximately 100 microns wide. 
     FIG. 4 is a top view of the device shown in FIG. 3 with via  21  is formed in it. Via  21  in the illustrated embodiment is of a dimension sufficient to fully expose opening  13  in metal line  12 . 
     Once via  21  to metal line  12  is formed, opening  13  can be repaired. FIG. 5 shows a side sectional view of the deposition of metal into opening  13  to fill it and make the repair. Such deposition may be accomplished using conventional metal deposition techniques such as chemical vapor deposition, or by a technique involving laser ablation forward metal deposition. FIG. 5 is an exemplary illustration of such laser ablation forward metal deposition. Substrate  30  is provided with metal coating  31  on a first surface thereof and is disposed adjacent via  21 . Laser beam  32 , which in the illustrated embodiment is a green laser beam, is directed onto the side of substrate  30  opposite the side having metal coating  31  on it. Substrate  30  is transparent to laser beam  32  so that laser beam  32  passes through substrate  30  and impinges on metal coating  31 . Metal ions from metal coating  31  are ablated from substrate  30  into opening  13 . Laser beam  32  may be moved or scanned across substrate  30  as necessary to fill opening  13 . Deposited metal  22  fills opening  13  and connects the separated portions of metal line  12  to reestablish electrical connection between these portions. 
     FIG. 6 illustrates a laser metal deposition apparatus suitable for depositing metal line into opening  13 . In the exemplary embodiment shown in FIG. 6, laser  41  is a harmonically doubled solid state Q-switched Nd:YLF or Nd:YAG laser, available from Continuum Inc., in Santa Clara, Calif. Laser beam  53  from laser  41  is expanded by telescope  42  into expanded beam  54 . Beam  54  shines on dichroic mirror  43  which directs beam  54  into objective lens  44 . Objective lens  44  focuses the beam to a diffraction limit spot on sample  45 , which may comprise, for example, the device shown in FIG. 5 including substrate  30  and substrate  10 . 
     In the exemplary embodiment shown in FIG. 6, illuminator  51  provides light that is deflected by mirror  49  onto dichroic mirror  43 . Illuminator  51  is used as a white light source to illuminate sample  45  so that the process and location of the focused spot can be monitored. A suitable illuminator is available from Edmund Scientific Company in Barrington, N.J. 
     Also, in the exemplary embodiment, CCD camera  50  is used to image and monitor the process location. The image is fed to computer  48  which computes subsequent process locations based on a programmed path. Any state of the art video camera is suitable for this purpose. 
     Sample  45  is supported on stage  46 . Stage  46  is equipped with X-Y motion controls  55  that are controlled by computer  48 . Suitable motion controls and computer are available from New England Affiliated Technologies in Lawrence, Mass., and comprise, for example, an XY-8080 precision stage, a PCX2 controller, and a 202M microstepping drive, with the controller interfaced to a 486 IBM PC or compatible. 
     Computer  48  also controls the power of laser  41 . By adjusting the position of stage  46  and the power of laser  41 , computer  48  enables the deposition of specific patterns on sample  45 . 
     Deposition of a metal line onto a substrate is illustrated in further detail in FIG. 7. A first substrate (corresponding to substrate  30  in FIG.  5  and thus illustrated with the same reference numeral), which is glass in the exemplary embodiment, is disposed in the path of focused laser beam  32 . Glass substrate  30  has metal coating  31  disposed on the side of glass substrate  30  furthest from objective lens  44 . Coating  31  may be deposited on substrate  30  by standard sputtering deposition or metal plating. Beam  32  passes through glass substrate  30  and impinges on metal coating  31  from the back side; that is, at the surface interface of metal coating  31  and glass substrate  30 . The contact of laser  32  with metal coating  31  results in ablation of metal coating  31 . During ablation, metal ions  62  accelerate away from metal coating  31 . 
     A second substrate (corresponding to substrate  10  in FIG.  5  and thus illustrated with the same reference numeral), which is also glass in the exemplary embodiment, is disposed adjacent the side of first glass substrate  30  having metal coating  31  thereon. As metal ions  62  accelerate from glass substrate  30  as a result of the ablation caused by focused laser beam  32 , metal ions  62  contact second glass substrate  10 . 
     An electric field is applied across first glass substrate  30  and second glass substrate  10  using power supply  52 . Power supply  52  is used to create the electric field in this embodiment. Power supply  52  has a positive electrode  60  attached to the initiated metallic line to insure a permanent chemical seal as discussed below. A negative electrode  61  is connected to second glass substrate  10 . In the exemplary embodiment of the invention, the voltages applied across the electrodes are at least 300 volts. 
     The electric field drives positively charged metal ions  62  toward second glass substrate  10 . The transfer of metal ions  62  from metal film  31  to surface  63  is due to the electrostatic force and laser ablation-generated acoustic shock waves. 
     The electric field applied across first glass substrate  30  and second glass substrate  10  also assists the bonding of metal ions  62  to second glass substrate  10 . Because of the contact of the negative electrode with second glass substrate  10 , the positive ions such as sodium ions in glass substrate  10  migrate away from surface  63  toward the negative electrode. This leaves behind negative ions such as oxygen in the glass substrate  10 . These negative ions electrostatically bond with the positive metal ions that contact surface  63 . A permanent chemical seal due to a thin metal oxide layer is formed after the electric field is removed. Conducting metal lines can thus be formed on surface  63  of second substrate  10  from metal ions  62 . 
     A hot plate  67  may be used to augment the migration of positive ions within second glass substrate  10  to the negative electrode and thus enhance the bonding of metal ions  62  to surface  63  of second glass substrate  10 . The heat increases the diffusion and allows for greater mobility of the ions in the glass. 
     Alternatively, electrode  61  may be connected directly to the metal line that is being deposited or repaired on substrate  10  (such as metal line  12 ). This helps channel ions  62  directly to the metal line. 
     FIG. 8 shows a top sectional view of the exemplary device after deposit  22  is formed in opening  13 . Via  21  should now be filled to complete repair of the device. 
     FIG. 9 illustrates this filling. Dielectric, or frit, filling  70  is formed in via  21  over repaired metal line  12 . Frit  70  is typically glass powder. The powder may be dispensed by depositing it with a dispenser into via  21 , or by screen printing over metal line  12 , according to methods known to those skilled in the art. Frit  70  is then laser annealed to melt the frit and solidify it. The laser annealing can be accomplished using a high power laser diode emitting a beam that is focused onto frit  70 . A laser diode or YAG laser may be focused on the frit to melt it and fuse it with dielectric layer  11 . The laser should heat the frit to approximately 600° C. to accomplish the fusion. By way of illustration only, if a one watt ultraviolet laser is used and focused to a 50 micron waist, the laser need only be focused on the frit for about 3 to 5 seconds to accomplish the fusion. 
     FIG. 10 is a top view of the device after deposition of frit  70  filling via  21  over metal line  12 . The completed device is now ready for reuse in any application for which it was originally suited. 
     Repair of metal line according to the present invention provides an efficient, economical technique for preservation of electrode and circuit devices. Rather than having to discard the devices and manufacture entirely new ones, the same devices can be simply repaired and reused. This results in considerable savings in cost and materials. 
     Although this invention has been described with reference to particular embodiments, it is not intended to be limited thereto. Rather, the scope of the invention is intended to be interpreted according to this scope of the claims.