Abstract:
A method is provided for maintaining a workpiece in a focal plane of a laser drilling system. The method includes: providing a workpiece holder that is adapted to releasably retain a workpiece on a planar surface thereof, the planar surface having a recess extending therein; positioning the workpiece onto a planar surface of a workpiece holder, such that the workpiece extends across the recess formed in the workpiece holder and an exposed surface of the workpiece aligns with a focal plane of a laser drilling system; projecting a laser beam from the laser drilling system onto the exposed surface of the workpiece, thereby forming an ablation on the exposed surface of the workpiece; and directing a flow of gas onto the exposed surface of the workpiece substantially concurrent with the step of projecting a laser beam, such that the flow of gas substantially impinges on an area of the exposed surface that extends across the recess formed in the workpiece holder, thereby maintaining the exposed surface of the workpiece in the focal plane of the laser drilling system during the laser drilling operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Serial No. 60/398,376 which was filed on Jul. 25, 2002 and is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to laser drilling, and more particularly, to a method for maintaining a workpiece in a focal plane of a laser drilling system. 
     BACKGROUND OF THE INVENTION 
     Material ablation by pulsed light sources has been studied since the invention of the laser. Reports in 1982 of polymers having been etched by ultraviolet (UV) excimer laser radiation stimulated widespread investigations of the process for micromachining. Since then, scientific and industrial research in this field has proliferated—mostly spurred by the remarkably small features that can be drilled, milled, and replicated through the use of lasers. 
     Ultrafast lasers generate intense laser pulses with durations from roughly 10 −11  seconds (10 picoseconds) to 10 −14  seconds (10 femtoseconds). Short pulse lasers generate intense laser pulses with durations from roughly 10 −10  seconds (100 picoseconds) to 10 −11  seconds  (10  picoseconds). A wide variety of potential applications for ultrafast and short pulse lasers in medicine, chemistry, and communications are being developed and implemented. These lasers are also a useful tool for milling or drilling holes in a wide range of materials. Hole sizes as small as a few microns, even sub-microns, can readily be drilled. High aspect ratio holes can be drilled in hard materials, such as cooling channels in turbine blades, nozzles in ink-jet printers, or via holes in printed circuit boards. 
     Optical parallel processing of laser-milled holes is key to increasing the throughput of, and the profitability of laser micromachining. Beamsplitting devices such as diffractive optical elements are currently used in laser micromachining to divide a single beam into multiple beams to allow for parallel processing of the workpiece (i.e., material to be drilled). 
     Currently, one way to prevent a laser drilling system&#39;s sub-beams from damaging the workpiece holder is to use a workpiece holder with a large recess behind the target area, such that the sub-beams pass through the workpiece holder after milling through the workpiece itself. Performing parallel laser drilling upon a flimsy workpiece presents a set of challenges related to keeping the flimsy workpiece surface in the focal plane. A workpiece holder with a single large recess behind the target area does not provide sufficient support to keep the flimsy workpiece in the focal plane when the foil is subject to recoil pressure due to laser ablation. 
     In order to perform precision laser drilling in a parallel process system, the workpiece surface must remain in the focal plane (where the laser beams are focused) of the laser drilling system throughout the laser drilling to enable the beams to drill workpiece geometries meeting precise specifications. However, the use of thin, flimsy workpieces (workpieces that bend and move outside the focal plane of the drilling laser beam when the workpiece is impacted with the beam(s)), which are required in some applications, such as inkjet nozzles, poses a challenge because the workpiece deforms during drilling and moves outside the focal plane of the laser system. This results in poor quality laser-drilled holes and an inability to meet required product specifications. 
     When a laser drilling system&#39;s sub-beams are incident upon a flimsy workpiece, the kickback of debris causes significant recoil force upon the workpiece, causing the workpiece to deform and move outside the laser drilling system&#39;s focal plane. If the sub-beams are out of focus when incident upon the workpiece, the result will be poor quality and misshapen holes that do not meet product specifications or obtain the desired benefits of precision laser micromachining. What is needed is a way to counteract workpiece deformation when using parallel process laser drilling on a flimsy workpiece. 
     One way to counteract the workpiece deformation is to reduce the atmospheric pressure in front of the workpiece. A reduction in atmospheric pressure exerts a force upon the workpiece that moves it toward the area of reduced atmospheric pressure. A sufficient reduction in atmospheric pressure in front of the workpiece counteracts the deformation of the workpiece caused by the recoil force. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method is provided for maintaining a workpiece in a focal plane of a laser drilling system. The method includes: providing a workpiece holder that is adapted to releasably retain a workpiece on a planar surface thereof, the planar surface having a recess extending therein; positioning the workpiece onto a planar surface of a workpiece holder, such that the workpiece extends across the recess formed in the workpiece holder and an exposed surface of the workpiece aligns with a focal plane of a laser drilling system; projecting a laser beam from the laser drilling system onto the exposed surface of the workpiece, thereby forming an ablation on the exposed surface of the workpiece; and directing a flow of gas onto the exposed surface of the workpiece, substantially concurrent with the step of projecting a laser beam, such that the flow of gas substantially impinges on an area of the exposed surface that extends across the recess formed in the workpiece holder, thereby maintaining the exposed surface of the workpiece in the focal plane of the laser drilling system during the laser drilling operation. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a top view of a conventional workpiece holder; 
     FIG. 1B is a top view of the conventional workpiece holder supporting a workpiece thereon; 
     FIG. 1C is a side view of the conventional workpiece holder illustrating the affect of a series of laser sib-beams incident on a surface of the workpiece; 
     FIG. 2 is a fragmentary side view of an exemplary laser drilling system which employs a gas delivery subsystem in accordance with the present invention; 
     FIG. 3 is a flowchart illustrating a method of using the gas delivery subsystem in accordance with the present invention; and 
     FIG. 4 is a perspective view illustrating the primary components of an ink-jet printer; and 
     FIG. 5 is a cross-sectional schematic view of an exemplary ink-jet head. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A shows a top view of a conventional workpiece holder  100 , including a recess  105 , a groove  110 , a vacuum source  112 , an external connection  115   a , an internal connection  115   b , an opening  120 , a first face  150 , and a second face  155 . The first face  150  is the planar surface defined between the recess  105  and the groove  110 ; whereas the second face  155  is the planar surface defined between the groove and the outer periphery of the workpiece holder  100 . 
     FIG. 1B is a top view of the conventional workpiece holder  100  supporting a workpiece  130  thereon. The workpiece  130  is fastened to workpiece holder  100 , such that the workpiece  130  extends across the recess  105  formed in the workpiece holder  100 . In a laser drilling system, the workpiece holder  100  is used to support the workpiece  130  during laser drilling. The drilling pattern  160  is the pattern of holes to be drilled by laser drilling system (not shown). An exemplary drilling pattern  160  is illustrated on the exposed surface of the workpiece  130 . 
     Workpiece holder  100  is round, but could be formed in a variety of shapes, including triangles, squares, rectangles, pentagons, etc. Workpiece holder  100  is made of a hard, durable, stiff, and heat-resistant material (e.g., steel, aluminum, machinable ceramic, etc.). Workpiece holder  100  is generally attached to the stage in a laser drilling system with nuts and bolts or other similar attachment, means. In one embodiment, the workpiece holder  100  is attached to a fixed stage. In another embodiment, the workpiece holder  100  is attached to a moveable stage. 
     Recess  105  is an opening allowing the laser system sub-beams to propagate through workpiece holder  100  without impacting and damaging workpiece holder  100 . It is readily understood that the recess  105  is larger than the drilling pattern  160  formed in the workpiece  130 . 
     Groove  110  is a grooved area around workpiece holder  100 . In a preferred embodiment, the groove  110  is rectangular in shape with corners at 90-degree angles; however, groove  110  is not limited to this shape. For instance, the groove  110  may have a circular shape. The groove  110  is dimensioned such that the workpiece  130  covers the recess  105  and the groove  110 . 
     Opening  120  is a hole that provides an opening for vacuum source  112  to remove air from groove  110  under workpiece  130 . In an exemplary embodiment, opening  120  is a round hole; however, opening  120  is not limited to this shape. Opening  120  connects with internal connection  115   b  through workpiece holder  100  and to external connection  115   a , thereby allowing air to be drawn through opening  120  by vacuum source  112 . 
     Vacuum source  112  may be implemented as a conventional vacuum pump such as those commercially available from Varian and GAST Mfg Corp. Vacuum source  112  draws air through opening  120 , internal connection  115   b , and external connection  115   a  from groove  110  underneath the workpiece, thereby effectively fastening it to workpiece holder  100 . 
     External connection  115   a  is a connection between vacuum source  112  and workpiece holder  100 . In one embodiment, the external connection  115   a  is a flexible hose connected between the vacuum source  112  and the workpiece holder  100 . The internal connection  115   b  is formed as a through hole between the internal opening  120  into the groove  110  and an opening along the external surface of the workpiece holder  100 . External connection  115   a  and internal connection  115   b  are used to remove air from groove  110  as described above. 
     FIG. 1C shows a side view of workpiece holder  100 , including recess  105 , groove  110 , workpiece  130 , first face  150 , and second face  155 . Of particular interest, several sub-beams  145  are shown incident upon the surface of the workpiece  130 . The sub-beams may be emitted from a beamsplitter (not shown) and are used to perform parallel process laser drilling of the drilling pattern  160  in the targeted workpiece  130 . Sub-beams  145  are focused at a focal plane  135 . 
     However; due to the flimsy nature of the workpiece, the surface of the workpiece  130  is shown not aligned with the focal plane  135  of the laser drilling system. In one exemplary embodiment, the workpiece  130  may be further defined as a stainless steel inkjet nozzle foil. The result of drilling operation deforms the workpiece  130  such that is does not meet product specifications (e.g., hole size, hole shape, taper angle). The deformation of workpiece  130  is the problem solved by the present invention. 
     In operation, vacuum source  112  is turned on to hold workpiece  130  against workpiece holder  100  by removing air from groove  110 , through opening  120 , internal connection  115   b , and external connection  115   a , creating a reduced atmospheric pressure in groove  110  such that the ambient atmospheric pressure fastens workpiece  130  to workpiece holder  100 . Sub-beams  145  propagate from a beamsplitter (not shown) in a laser drilling system (not shown), are incident upon workpiece  130 , and are maneuvered to drill the desired workpiece geometry in workpiece  130 . The recoil pressure caused by debris kickback during ablation by sub-beams  145  causes workpiece  130  to deform and moves the targeted pattern area of workpiece  130  out of focal plane  135 . 
     In accordance with the present invention, the laser drilling system further includes a gas delivery subsystem  200  as shown in FIG.  2 . The gas delivery subsystem  200  is comprised of a gas delivery means  250 , including a nozzle  260 . The gas delivery subsystem  200  is generally operable to direct a flow of gas onto the exposed surface of the workpiece  130 . 
     Gas delivery means  250  may be implement as an air pump (e.g., an air compressor) that delivers gas flow  265  from a nozzle  260  therein. The gas delivery means  250  may contain a regulator that controls the flow and force of the gas, as well as an air filtration system to ensure that the gas is clean (e.g., free of dust, oil and excessive moisture) when incident upon workpiece  130 . The nozzle  260  is used to direct the gas flow  265  upon workpiece  130  at an angle θ. In one embodiment, the nozzle  260  is the AIR KNIFE nozzle manufactured by Exair. 
     Angle θ is the angle between gas flow  265  and workpiece  130 . Angle θ is possibly between 1 and 50 degrees, and is preferably 10 degrees. Angle θ is important to gas delivery subsystem  200  to counteract ablation pressure and remove debris, but angle θ is also selected so that it does not contribute to workpiece deformation. If angle θ is too large, it contributes to workpiece deformation. 
     Gas flow  265  is a flow of gas used to perform two important functions in the gas delivery subsystem  200 . Examples of gasses used to create gas flow  265  include (but are not limited to) air, nitrogen, and argon. The first function of gas flow  265  is to create a reduced atmospheric pressure in front of the target area of workpiece  130  that exerts a force upon workpiece  130  to counteract the recoil pressure upon workpiece  130 . The second function of gas flow  265  is to remove debris from the surface of workpiece  130  during drilling. Debris removal further contributes to the ability of laser micromachining to create a product that meets specification. When incident upon workpiece  130 , the gas flow  265  has a range of speed of 2-132 m/s, optimally 15 m/s, and a range of flow of 0.3-4.1 cubic feet per minute (CFM), optimally 0.98 CFM, thereby creating a reduction in atmospheric pressure in the range of 2.7 to 56,000 Pascal, optimally 536 Pascal. In addition, the gas flow  265  has a humidity range of 10-1000 parts per million (ppm) and a particulate size range of 0.01-0.1 micrometer. In one example, gas flow  265  is comprised of an air flow. In another example, gas flow  265  is comprised of nitrogen, or other inert gas. 
     In operation, workpiece  130  is removably attached to workpiece holder  100  via vacuum source  112 , as previously discussed. Gas delivery means  250  delivers gas through the nozzle  260  to the surface of workpiece.  130  at angle θ, thereby creating a reduced atmospheric pressure in front of the target area of workpiece  130 . The force of sub-beams  145  upon workpiece  130  is countered by the reduced atmospheric pressure, such that the workpiece  130  remains in the focal plane  135  throughout drilling. 
     Gas delivery system  200  solves the problems left unresolved in the prior art and keeps the surface area of flimsy workpiece  130  in focal plane  135  of sub-beams  145  of a laser drilling system by creating a reduced atmospheric pressure in front of the pattern target area of workpiece  130  that counteracts the recoil pressure upon workpiece  130 . 
     FIG. 3 illustrates an exemplary method  300  for reducing atmospheric pressure proximate to the target area of the workpiece using the gas delivery subsystem  200 . The method generally includes the steps of: placing the workpiece on the workpiece holder; fastening the workpiece to the workpiece holder; turning on purge gas; drilling a pattern into the workpiece; turning off purge gas; and unfastening and removing the workpiece from the workpiece holder. 
     First, the workpiece  130  is placed on workpiece holder  100  at step  310 . For instance, an automated machine may obtain the workpiece  130  to be drilled and places it upon the workpiece holder  100  in a mass-manufacturing environment. In another instance, a system operator places the workpiece  130  upon workpiece holder  100  by hand. 
     Next, the workpiece  130  is fastened to workpiece holder  100  at step  320 , such that it is stationary during laser drilling. In one example, workpiece  130  is fastened by turning on vacuum source  112  to remove air from groove  110 , sealing workpiece  130  against first and second faces  150 ,  155  of the workpiece holder  100 . In another example, workpiece  130  is fastened to workpiece holder  100  with an adhesive. 
     At step  330 , the gas delivery means  250  is turned on and gas flow  265  is incident upon workpiece  130 . Gas flow  265  performs the functions of: (1) creating a zone of reduced atmospheric pressure in front of workpiece  130  to counteract the recoil pressure exerted upon workpiece  130  by sub-beams  145 ; and (2) removing drilling debris from the pattern target area of workpiece  130 . Creating the zone of reduced atmospheric pressure is critical in solving the problem of keeping a flimsy workpiece in the focal plane of a parallel process laser drilling system. 
     A drilling pattern is then drilled at step  340  into the exposed surface of the workpiece  130 . In this step, the desired pattern is drilled by maneuvering sub-beams  145  upon workpiece  130 . In one example, pre-defined milling algorithms (and, if required, correction algorithms) are stored in a computer (not shown) and communicated to elements of the laser drilling system (not shown). 
     Upon completion of the laser drilling operation, the gas delivery means  250  is turned off at step  350 , such that gas flow  265  is no longer incident upon workpiece  130 . 
     Finally, the workpiece  130  is unfastened from the workpiece holder  100  at step  360  and then removed from the workpiece holder  100  at step  370 . In one example, the vacuum source  112  is turned off, breaking the air seal between the workpiece  130  and the workpiece holder  100 , thereby allowing removal of the workpiece  130 . In another example, the adhesive seal between workpiece  130  and workpiece holder  100  is broken to allow removal of workpiece  130 . 
     If necessary, a subsequent workpiece  130  can be placed upon workpiece holder  100 . If so, processing returns to step  310  of the method; otherwise processing is complete. 
     A laser drilling system in accordance with the present invention may be used to construct a nozzle plate of an ink-jet head as further described below. Referring to FIG. 4, an ink-jet printer  1140  includes an ink-jet head  1141  capable of recording on a recording medium  1142  via a pressure generator. The ink-jet head  1141  is mounted on a carriage  1144  capable of reciprocating movement along a carriage shaft  1143 . 
     In operation, ink droplets emitted from the ink-jet head  1141  are deposited on the recording medium  1142 , such as a sheet of copy paper. The ink-jet head  1141  is structured such that it can reciprocate in a primary scanning direction X in parallel with the carriage shaft  1143 ; whereas the recording medium  1142  is timely conveyed by rollers  1145  in a secondary scanning direction Y. 
     FIG. 5 further illustrates the construction of an exemplary inkjet head  1141 . The ink-jet head is primarily comprised of a pressure generator  1104  and a nozzle plate  1114 . In this embodiment, the pressure generator  1104  is a piezoelectric system having an upper electrode  1101 , a piezoelectric element  1102 , and a lower electrode  1103 . Although a piezoelectric system is presently preferred, it is envisioned that other types of systems (e.g., a thermal-based system) may also be employed by the ink-jet head  1141 . 
     The nozzle plate  1114  is further comprised of a nozzle substrate  1112  and a water repellent layer  1113 . The nozzle substrate  1112  may be constructed from a metal or resin material; whereas the water repellant layer  1113  is made of fluororesin or silicone resin material. In this exemplary embodiment, the nozzle substrate  1112  is made of stainless steel having a thickness of 50 um and the water repellent layer  1113  is made of a fluororesin having a thickness of 0.1 um. 
     The ink-jet head  1141  further includes an ink supplying passage  1109 , a pressure chamber  1105 , and an ink passage  1111  disposed between the pressure generator  1104  and the nozzle plate  1114 . In operation, ink droplets  1120  are ejected from the nozzle  110 . The nozzle  1110  is preferably formed without flash and foreign matter (e.g., carbon, etc.) in the nozzle plate. In addition, the accuracy of the nozzle outlet diameter is 20 um±1.5 um. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.