Abstract:
A method of capturing and removing metallic debris created on a target side of a target metal specimen undergoing laser micromachining entails providing a barrier that encompasses the immediate volume surrounding a laser cutting head output nozzle to contain the ejected debris and extracting the debris through a vacuum outlet. A preferred system implementing this approach to debris management includes a barrier in the form of a flexible fiber brush configured in the shape of a ring and positioned to trap ejected debris within a localized area surrounding a target area where the laser beam is incident on the target metal specimen. The ring brush is made of material that is robust to molten metals. An inert gas directed at a high flow rate along the target surface of the metal specimen carries ejected surface debris trapped in the ring brush toward a vacuum outlet.

Description:
RELATED APPLICATION 
     This application claims benefit of U.S. Provisional Patent Application No. 61/073,672, filed Jun. 18, 2008. 
    
    
     COPYRIGHT NOTICE 
     © 2008 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d). 
     TECHNICAL FIELD 
     The system disclosed addresses management of debris generated by laser processing and, more specifically, by laser micromachining of small-scale target specimen features. 
     BACKGROUND INFORMATION 
     Machining metals and other target specimens using a laser beam generates a significant amount of molten debris. Most of the debris is ejected from the immediate region surrounding the laser cut, as the laser beam blasts through the target specimen material. Debris from the area within the width of the laser cut, or “kerf,” may be ejected by a high-pressure jet of cutting head gas flowing along the laser beam propagation axis and out of a nozzle through which the laser beam is focused. Thus, the laser beam propagates and the cutting head gas flows along a common axis. Remaining debris particles are ejected at high velocity (several km/sec) along trajectories perpendicular to the kerf, both axially of (i.e., normal to) the target surface and parallel to the target surface of the target specimen undergoing machining. The sizes of these particles range from sub-millimeter to sub-micron, and the particle temperatures are high, typically at least several hundred degrees centigrade. Without proper debris containment during the laser micromachining process, the laser system becomes polluted with axial debris and requires daily cleaning and maintenance. In addition, surface debris may block the laser beam cutting path, reducing ablation efficiency. 
     The current state of the art of debris management in semiconductor micromachining systems is highly dependent on the application. In some applications, such as for example, semiconductor wafer scribing, processing may be restricted to the wafer backside, thereby completely avoiding the target surface and steering clear of active layers of circuitry. Other applications address debris ejected through the underside of a target material undergoing laser micromachining, while the remainder of the debris on the target surface of the material is not managed or contained. Most laser micromachining systems are designed with proper covers and shields to protect sensitive subsystem components from vapor and molten deposits, but these shields intercept and trap only a small portion of the ejected material. Although they protect the micromachining equipment, the shields do not address quality assurance of the electronic parts being processed. 
     When drilling prescribed holes, a “sandwich” technique may be used that entails covering both surfaces of the target with a protective layer, drilling through both the protective coverings and the target material, and later peeling off the coverings and surface debris together (Tuan A. Mai, “Toward Debris-free Laser Micromachining,”  Industrial Laser Solutions,  23:1, 2008). Another similar technique entails coating a surface with a benign protective layer (e.g., photoresist) that traps debris and can be dissolved after laser processing. Yet another technique entails cutting in the presence of a water spray or a water film bathing the target surface; however, the presence of liquid tends to result in mist or condensation affecting the laser optics (Sun and Longtin, “Ultrafast Laser Micromachining with a Liquid Film,”  Proc. ICALEO,  2001). 
     Brushes have been used as debris management devices in related industries, such as printed circuit board (PCB) milling that uses end mills to drill macroscopic holes in a plastic PCB backplane to enable routing of the printed circuits. Some designs incorporate vacuum exhaust, but the systems currently implementing these designs do not fully encompass the cutting area. A considerable amount of material may, therefore, escape from the debris containment system. In the PCB milling application, an external vacuum hose may be attached to the back of the circuit board to enable intermittent application of vacuum pressure to remove the board material as it is drilled out. Alternatively, a brush may surround the drill bit, or “end mill,” and associated end mill spindle, and a brush housing that supports the brush may be equipped with a vacuum port to exhaust debris generated by drilling the board material. An example of such PCB milling equipment is a Final Touch 101 depaneling router system, available from Precision PCB Products of Irvine, Calif. 
     SUMMARY OF THE DISCLOSURE 
     A method of capturing and removing debris created on a target side of a target specimen undergoing laser micromachining entails providing a barrier that encompasses the immediate volume surrounding a laser cutting head output nozzle to contain the ejected debris and extracting the debris through a vacuum outlet. A preferred system implementing this approach to debris management includes a barrier in the form of a flexible fiber brush configured in the shape of a ring and positioned to trap ejected debris within a localized area surrounding a target area where the laser beam is incident on the target specimen. The target specimen is preferably made of metal, and the ring brush is made of material that is robust to molten metals. The perimeter of the ring brush is positioned to encompass the axis of propagation of the laser beam, and the distance from the propagation axis to the ring brush perimeter is made sufficiently large to allow the molten debris to cool before it encounters the brush. An inert gas directed at a high flow rate along the target surface of the metal specimen carries ejected surface debris trapped in the ring brush toward a vacuum outlet. 
     The disclosed system contains ejected surface debris and thereby enables automatic capture and disposal of the surface debris and the axial debris produced by the laser micromachining of the target specimen. The flexible fiber brush material sustains temperatures of up to at least several hundred degrees and does not impart damage on contact with the target surface. Standard laser-based via drilling equipment may be retrofitted with, or re-designed to accommodate, the components necessary to provide the surface gas flow, debris containment, and vacuum exhaust. 
     Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a laser cutting head and debris management components that include a ring-shaped flexible fiber brush skirt positioned to capture debris generated by laser micromachining a target specimen. 
         FIG. 2  is a fragmentary isometric view of the laser cutting head and debris management components including the flexible fiber ring brush of  FIG. 1 . 
         FIG. 3  is a replica of  FIG. 1  but is annotated to indicate flow paths of cutting gas and ejection paths of debris generated by laser micromachining of the target specimen. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIGS. 1-3  show a laser cutting head  90  of a laser micromachining system. Laser cutting head  90  includes laser micromachining-generated debris management (i.e., containment and removal) components  100  associated with a laser focusing lens assembly  102  and a laser cutting head alignment assembly  104  that are adjoined as a unitary structure. Lens assembly  102  includes light beam focusing optical components  106  (shown in  FIGS. 1 and 3  collectively as a single lens component), and cutting head alignment assembly  104  includes at its bottom end a nozzle and purge gas mount  108  to which a laser beam and gas flow output nozzle  110  is affixed. A debris removal collar  112  supports cutting head alignment assembly  104  and nozzle and purge gas mount  108 . Optical components  106  of lens assembly  102  are positioned safely behind a protective debris window  114  set and sealed by an O-ring  116  in a recess  118  in the top end of nozzle and purge gas mount  108 , where it is adjoined with cutting head alignment assembly  104 . 
     A laser beam  120  emitted by a laser source (not shown) propagates along a beam propagation and cutting head gas flow common axis  122  through lens assembly  102  and output nozzle  110  of cutting head alignment assembly  104 . Laser beam  120  is focused by lens assembly  102  and directed by cutting head alignment assembly  104  for incidence on a target surface  124  of a target specimen  126  that is secured to a chuck  128 . Cutting head alignment assembly  104  is configured for lateral positioning of common axis  122  by a three-point adjustment relative to debris removal collar  112 . A purge gas inlet  130  admits into a conically shaped gas pressure chamber  132  of nozzle and purge gas mount  108  high pressure inert cutting head gas as laser beam  120  propagates through gas pressure chamber  132 . Laser beam  120  propagates and high pressure cutting head gas flows through output nozzle  110  to, respectively, cut material from target specimen  126  and eject debris material from a kerf formed in the region of material cut from target specimen  126 . 
     The following description is presented with reference to use of an infrared (IR) laser beam  120  in the formation of through-holes in target specimen  126  of metal material. Other suitable target materials include polyvinyl alcohol-coated metal; glass; ceramics; and any number of composite materials, including KEVLAR and carbon fiber. 
     Debris ejected from the kerf may be categorized as axial debris  138  and surface debris  140  having trajectories that are substantially perpendicular and substantially parallel, respectively, to target surface  124  of metal specimen  126 . Two challenges associated with capturing such ejected debris include a wide range of different topographies (i.e, hills, valleys, and canyons) of target surface  124  that can trap surface debris  140  and the high temperature of molten metal debris ejected. A change in topography over target surface  124  can be, for example, a five mm stair step presented by a clamp  142  holding target specimen  126  in place against chuck  128 . 
     A preferred embodiment of debris management components  100  includes a flexible fiber brush skirt  148  in the shape of a ring functioning as a barrier that captures micromachining debris by encompassing as much space as possible within an internal volume  150  surrounding output nozzle  110 . In this preferred embodiment, ring brush  148  is configured as an annulus centered around common axis  122  and having a mean radius  152  at target surface  124 . In general, however, ring brush  148  may be configured in the shape of an oval or a straight-sided polygon. Ring brush  148  downwardly depends from and, for ease of replacement, is releasably mounted to a barrier or brush mounting plate  154 . 
     Ring brush  148  appears in cross section in  FIGS. 1 and 3  as two trapezoids equidistantly positioned from common axis  122  at output nozzle  110  and having bristles in which clamp  142  is partly enmeshed. Ring brush  148 , which traps ejected surface debris  140  at near molten temperatures, is made of flexible fiber material to ensure it does not scratch or damage components undergoing micromachining. Ring brush  148  is therefore preferably fashioned from a fiber having a very high melting temperature, such as polytetrafluoroethylene (PTFE) “Teflon®” material, which melts at 680 F, or carbon fiber, which melts at 1500 F. The flexing of ring brush  148  as it contacts different target surface topographies causes debris caught in the brush bristles to flake off of them, thereby exhibiting a self-cleaning property. Ring brush  148  has mean radius  152  of a value setting an effective perimeter distance and an internal volume boundary that allow ejected surface debris  140  to cool to a temperature below the melting temperature of ring brush  148  before its bristles trap ejected surface debris  140 . 
     Although it has a higher melting temperature than that of PTFE material, carbon fiber is less preferred because it is less resilient and is, therefore, slow to return to its nominal shape when encountering changes in target surface topography. A drape made of suitable material would provide adequate performance if target surface  124  is flat. 
     An alternative ring brush  148  exhibiting longer lifetime is constructed with hybrid bristles in the form of concentric ring members that include an outer ring of PTFE bristles and an inner ring of carbon fiber. The outer ring of PTFE bristles has better resilience and memory, but it melts when laser cutting head  90  is in continuous production use. The inner ring of carbon fiber does not melt; therefore, the outer PTFE bristles function as a support mechanism for the inner carbon fiber bristles, which protect the PTFE bristles from the molten debris. 
     A vacuum outlet port  156  of a fluid passageway  158  formed in debris removal collar  112  is in fluid communication with internal volume  150  to enable continuous evacuation by a remote vacuum pump (not shown) of surface debris  140  generated by the cutting operation of laser beam  110  and contained within internal volume  150  by ring brush  148 . 
       FIG. 2  is a three-dimensional rendering of laser cutting head  90 , as it appears when viewed upwardly from metal specimen  126 . In the preferred embodiment shown, debris management components  100  include brush mounting plate  154  to which ring brush  148  is mounted and vacuum outlet port  156  fitted into fluid passageway  158  formed in debris removal collar  112 . Debris management components  100  are retrofitted to a standard laser-based printed circuit board via drilling system, such as a Model 5500 system manufactured by Electro Scientific Industries, Inc., the assignee of this patent application. The Model 5500 system is retrofitted with laser heads emitting IR laser beams to cut metal specimen  126 . Ring brush  148  is shown mounted to brush mounting plate  154  fixed on the bottom surface of nozzle and purge gas mount  108 , and vacuum outlet port  156  is shown emerging from debris removal collar  112 . Ring brush  148  has a perimeter  200 , which need not be completely closed but is preferably substantially continuous, with points along perimeter  200  being located sufficiently far away from nozzle  110  to allow ejected particles to cool before making contact with ring brush  148 . A small gap  206  in ring brush  148  provides to a gas conduit or hose  208  ( FIGS. 1 and 3 ) access to purge gas inlet  130 . Hose  206  and the bristles of ring brush  148  that contact the outer surface of hose  208  extending through gap  206  cooperate to provide a substantially closed barrier in that it prevents escape of surface debris  140  from internal volume  150 .  FIG. 2  also shows, included within laser cutting head  90 , components of a vision alignment subsystem  210  to which are mounted several sets of numerous LEDs  215  (only eight of which shown for simplicity) used to illuminate the micromachining operation. 
       FIG. 3  is a replica of  FIG. 1  but is annotated to indicate, using arrows, paths of cutting head gas flow, exhaust gas flow, and surface gas flow through various cavities within laser cutting head  90 . A jet of cutting head inert gas  298  introduced into gas inlet  130  is confined within conically shaped gas pressure chamber  132 , providing a substantially vertical downwardly directed cutting head gas flow  300  that issues from output nozzle  110 . Cutting head gas flow  300  issuing from output nozzle  110  includes a portion of gas escaping into internal volume  150  and a portion of gas flowing through the kerf formed in target specimen  126  during micromachining. The portion of cutting head gas escaping into internal volume  150  is extracted through fluid passageway  158  and out of outlet port  156  by the remote vacuum pump, thus forming along target surface  124  a generally horizontal surface gas flow  305  that encounters in its path surface debris  140 . Specifying the rate of surface gas flow  305  to be equal to or greater than the rate of cutting head gas flow  300  directs primarily toward vacuum outlet port  156  the path of ejected surface debris  140  confined within internal volume  150 . A preferred rate of surface gas flow  305  is about 1.25 times the rate of cutting head gas flow  300  because it facilitates connection of hose  208  of workable diameter to purge gas inlet  130 . When it reaches vacuum outlet port  156 , surface gas flow  305  includes a mixture of inert cutting head gas and ambient air from the space within internal volume  150 . In addition to confining ejected debris, ring brush  148  confines surface gas flow  305  and intensifies the action of negative pressure within internal volume  150 , thereby increasing vacuum efficiency. Thus, encompassing internal volume  150  by ring brush  148  and extracting about 1.25 times the cutting head gas flow  300  enables automatic capture and disposal of the axial debris  138  and ejected surface debris  140  generated by the laser micromachining process. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.