Abstract:
A method of laser milling an aperture in a workpiece for use with manufacturing ink-jet nozzles includes initially illuminating a surface of the workpiece with a laser beam at a point within an outer perimeter of a desired aperture and a distance away from the outer perimeter sufficient to substantially avoid initial ablation of the outer perimeter. The laser beam is driven substantially in the direction of the outer perimeter at a variable rate controlled to avoid deformation of the outer perimeter. Material of the workpiece is ablated in a pattern designed to substantially remove material within the outer perimeter, thereby forming the aperture.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application Serial No. 60/398,639 which was filed on Jul. 25, 2002 and is incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention generally relates to laser drilling, and particularly relates to a method for milling repeatable exit holes in ink jet nozzles.  
         BACKGROUND OF THE INVENTION  
         [0003]    Material ablation by pulsed light sources has been studied since the invention of the laser. Etching of polymers by ultraviolet (UV) excimer laser radiation in the early 1980s led to further investigations and developments in micromachining approaches using lasers—spurred by the remarkably small features that can be drilled, milled, and replicated through the use of lasers. A recent article entitled “Precise drilling with short pulsed lasers” (X. Chen and F. Tomoo, High Power Lasers in Manufacturing, Proceedings of the SPIE Vol. 3888, 2000) outlines a number of key considerations in micromachining. Other recent patents of interest include the following:  
           [0004]    U.S. Pat. No. 6,260,957, “Ink jet printhead with heater chip ink filter,” describes a silicon ink filter for a heater chip of an ink jet printhead that is formed by micromachining and laser drilling. The heater chip may contain a plurality of such filters for the plurality of nozzles of the printhead. The filter has a via constituting an ink entrance area formed by micromachining and a plurality of bores formed at the exit side of the via produced by laser drilling. Protective layers are preferably disposed over the heater chip substrate prior to micromachining and laser drilling.  
           [0005]    U.S. Pat. No. 6,089,698, “Nozzles and methods of and apparatus for forming nozzles,” describes nozzles for an ink jet printer formed by laser ablation in a nozzle plate that has previously been bonded to the body of the printer. The laser beam is caused to converge at a point in front of the nozzle plate so that a nozzle is formed which tapers toward the outlet. First and second beam masks are established in front of a focusing lens with the masks being respectively conjugate in the lens with the nozzle inlet and outlet, which are of different shape. The nozzle has a central land that controls the ink meniscus and avoids the ejected drop receiving a sideways kick from the nozzle wall.  
           [0006]    U.S. Pat. No. 6,023,041, “Method for using photoabsorptive coatings and consumable copper to control exit via redeposit as well as diameter variance,” describes a method of forming a through-via in a laminated substrate by applying a polymeric photo-absorptive layer on an exposed bottom surface of a laminated substrate. A through-via is laser drilled in the substrate from a top of the substrate through the substrate to a bottom of the substrate. The photoabsorptive layer formed on the bottom surface of the substrate is then removed.  
           [0007]    European Patent No. EP0867294, “Ink jet printhead nozzle plates,” describes a method for making an inkjet printhead nozzle plate from a composite strip containing a nozzle layer and an adhesive layer. The adhesive layer is coated with a polymeric sacrificial layer prior to laser ablating the flow features in the composite strip. A method is also provided for improving adhesion between the adhesive layer and the sacrificial layer. Once the composite strip containing the sacrificial layer is prepared, the coated composite strip is then laser abated to form flow features in the strip in order to form the nozzle plates. After forming the flow features, the sacrificial layer is removed. Individual inkjet printhead nozzle plates are separated from the composite strip by singulating the nozzle plates with a laser.  
           [0008]    U.S. Pat. No. 5,548,894, “Ink jet head having ink-jet holes partially formed by laser-cutting, and method of manufacturing the same,” describes a method of manufacturing an ink jet head including an ink-chamber member having ink chambers, and a nozzle plate secured to a front end face of the ink-chamber member and which has ink-jet holes communicating with the respective ink chambers, wherein a blank for the nozzle plate is formed by injection molding, such that blind holes are formed in one of opposite surfaces of the blank and such that each blind hole has a varying-area portion whose cross sectional area decreases in a direction from the above-indicated one of opposite surfaces of the blank toward the other surface, and the blank is subjected to laser-cutting to prepare the nozzle plate having orifice holes which cooperate with the blind holes to form the ink-jet holes. The size of each blind hole at an open end thereof is preferably smaller than the size of the ink chamber at an end thereof at which the ink chamber communicates with the ink-jet hole.  
           [0009]    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). Along with a wide variety of potential applications for ultrafast and short pulse lasers in medicine, chemistry, and communications, short pulse lasers are also useful in milling or drilling holes in a wide range of materials. In this regard, hole sizes in the sub-micron range are readily drilled by these lasers. High aspect ratio holes are also drilled in hard materials; applications in this regard include cooling channels in turbine blades, nozzles in ink-jet printers, and via holes in printed circuit boards.  
           [0010]    Creation of a repeatable hole shape that meets stringent specifications is frequently critical in quality control for manufacturing applications. Laser systems are flexible in meeting such specifications in milling because appropriate programming can easily engineer custom-designed two-dimensional (2D) and three-dimensional (3D) structures and translate such designs into numerical control of the laser in real-time. However, as the required feature size for these structures decreases, mass production of quality micromachined products becomes more difficult to conduct in a rapid, cost-effective manner that consistently meets product specifications.  
           [0011]    Key factors in inkjet printer quality derive from inkjet nozzle design, construction techniques, and operation. Nozzle design defines a need for a number of holes to be milled in the aforementioned materials. Each nozzle hole includes a shaped section and an exit hole. The exit hole is critical in controlling ink ejection from an inkjet printer nozzle. Inconsistent expulsion of ink leads to poor print quality; therefore, imperfections in the exit hole negatively impact print quality.  
           [0012]    Manufacturers of inkjet printers require that inkjet nozzle holes meet specific workpiece geometry. The measurements (e.g., input diameter, exit diameter, depth of exit hole, and taper angle) of the hole and shape of the hole (e.g., tapered with cylindrical exit hole) are critical to the product quality and the operation of the end application. In addition, inkjet nozzle manufacturing must provide manufacturing methods of laser tool operation, material controls, and inspection to achieve repeatable size and shape to ensure consistency among mass-produced nozzles.  
           [0013]    Although laser drilling inkjet nozzles provides numerous advantages and benefits over other drilling methods, defects in the final product remain a problem. Current laser drilling systems, such as those using picosecond lasers, still create defects, such as burrs and notches, in the finished product. These defects are particularly detrimental in the exit hole because the size and smoothness aspects of the exit hole are critical to acceptable inkjet nozzle performance. Burrs or notches cause restrictions in the high velocity expulsion of inks and cause variability in the position and amount of ink per dot; burrs and noteches therefore diminish print quality. Many current laser drilling techniques utilizing short pulse, low energy lasers use traditional trepanning (e.g. cutting a circular pattern to remove a core, leaving a hole) to create the exit hole. This trepanning method causes an unpredictable notch or burr to be formed in the otherwise cylindrical exit hole. As previously noted, this notch or burr is undesirable because of its negative impact on print quality. What is needed is a way to minimize these defects in laser drilling inkjet nozzles to enhance quality and consistency in manufactured inkjet nozzle. The present invention provides a solution to this need.  
         SUMMARY OF THE INVENTION  
         [0014]    According to the present invention, a method of laser milling an aperture in a workpiece for use with manufacturing ink-jet nozzles includes initially illuminating a surface of the workpiece with a laser beam at a point within an outer perimeter of a desired aperture and a distance away from the outer perimeter sufficient to substantially avoid initial ablation of the outer perimeter. The laser beam is driven substantially in the direction of the outer perimeter at a variable rate controlled to avoid deformation of the outer perimeter. Material of the workpiece is ablated in a pattern designed to substantially remove material within the outer perimeter, thereby forming the aperture.  
           [0015]    A number of advantages are provided with the invention. Print quality in inkjet printers is generally improved even as quality and consistency in manufacturing inkjet nozzles is also improved. Defects in laser drilling inkjet nozzle holes are minimized. The elimination of notching provides for repeatable exit holes in inkjet nozzles. The method and system of the present invention also add little cost to the manufacturing of inkjet nozzles insofar as the method can be implemented using currently-deployed systems and additional tools to the conventional components of a laser drilling system are not required in many cases.  
           [0016]    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  
       [0017]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0018]    [0018]FIG. 1 presents a schematic of a laser drilling system;  
         [0019]    [0019]FIG. 2 shows cross-section detail in an exemplary workpiece geometry drilled using laser drilling system;  
         [0020]    [0020]FIG. 3 shows an exit hole finished using traditional trepanning techniques;  
         [0021]    [0021]FIG. 4 illustrates a prior art method of finishing exit holes in inkjet nozzles using traditional trepanning techniques;  
         [0022]    [0022]FIG. 5 provides a derived image from a photo of an inkjet nozzle hole with a notching problem;  
         [0023]    [0023]FIG. 6 shows an exit hole finished with punch-through and spiral technique;  
         [0024]    [0024]FIG. 7 illustrates a method of finishing exit holes in inkjet nozzles using a punch-through and spiral technique;  
         [0025]    [0025]FIG. 8 shows an exit hole finished using punch-through and slow circling technique;  
         [0026]    [0026]FIG. 9 illustrates a method of finishing exit holes in inkjet nozzles using a punch-through and slow circling technique;  
         [0027]    [0027]FIG. 10 shows an exit hole finished using a two-pass trepanning technique;  
         [0028]    [0028]FIG. 11 illustrates a method of finishing exit holes in inkjet nozzles using a two-pass trepanning technique;  
         [0029]    [0029]FIG. 12 provides a perspective view showing major constituent components of an ink-jet printer; and  
         [0030]    [0030]FIG. 13 a schematic cross-sectional view of an ink-jet head. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0032]    In overview, one embodiment of the present invention provides a method of creating repeatable exit holes meeting required shape specifications with a laser drilling system by punching through the workpiece at a radius A which is less than the radius B of the exit hole, maneuvering the laser to the exit hole diameter, and circling around the exit hole diameter. In another embodiment, an overview of the present invention provides removal of an initial mass X from the hole, where mass X is less than the total mass Y to be ultimately removed from the hole, and then removing the remaining mass Y-X. In implementing all cutting, the required specifications contemplate an essential conformance between the actual excision edge to the ideal perimeter (which is pursued by the laser in cutting that excision edge) to within a predetermined threshold value (or threshold function if the tolerance modifies along the perimeter) so that acceptable quality in performance of the exit holes or other holes is fully realized.  
         [0033]    The two embodiments reflect resolution of two distinct theories surrounding the cause of a “tuna can effect”—that is, a single burr or notch in the exit hole. One such theory is that the notch or burr in the exit hole that is caused at the point where the laser beam first “punches through” the material at the exit hole perimeter. The punch-through in the first embodiment overviewed resolves this insofar as the punch-through is controlled to occur at a point that is not located on the perimeter (e.g. at a point less than the finished diameter of the exit hole), so that the notch is eliminated. The second theory is that burrs or notches are generated when the laser beam drills the exit hole, progressively removes material from the section of the exit hole area, and the mass of the material being ablated eventually causes an uneven break or notch in the exit hole. This second embodiment overviewed resolves this insofar as a portion of material is removed from the exit hole prior to the final finishing step, so that when the exit hole is finished at final diameter, the amount of mass has been reduced, thus minimizing the size of the notch or eliminating it altogether. In both embodiments, a determination of the material ablation rate from the workpiece when incised by the cutting beam is used to define the rate of material removal at any moment; a punch hole location needs to be defined within the portion of material to be removed at a distance from the perimeter such that the material ablation rate and the spot size of the laser minimize distortion of the excision edge to less than a threshold value (specified tolerance of imperfection in the excision edge) when the cutting beam cuts a pilot hole in the workpiece; a laser beam path needs to be defined which provides sufficient distance from the perimeter such that the material ablation rate and the spot size minimize distortion of the excision edge to less than the predetermined threshold value when the cutting beam progressively incises the workpiece; and a beam progression rate function needs to be effectively determined for moving the cutting beam along the beam path as a function of the position of the beam respective to the perimeter such that the material ablation rate and the spot size minimize distortion of the excision edge to less than the predetermined threshold value as the cutting beam progressively incises the workpiece.  
         [0034]    Turning now to specific details in the embodiments, FIG. 1 shows a simplified schematic of a laser drilling system  100 , including a laser  105 , a beam  107 , a first mirror  108 , a second mirror  117 , a third mirror  121 , a fourth mirror  122 , a shutter  110 , an attenuator  115 , a beam expander  120 , a spinning half-wave plate  125 , a scan mirror  130 , a scan lens  140 , and a workpiece  155 , arranged as shown. In one embodiment, laser  105  is a picosecond laser system.  
         [0035]    In operation, laser  105  emits beam  107  along the optical path between Laser  105  and Workpiece  155 . Beam  107  propagates along the optical path, where it is incident upon first mirror  108 . First mirror  108  redirects beam  107  along the optical path, where it is incident upon shutter  110 . Shutter  110  opens and closes to selectively illuminate the workpiece material. Beam  107  exits shutter  110  and propagates along the optical path to attenuator  115 . Attenuator  115  filters the energy of laser  105  in order to precisely control ablation parameters. Beam  107  exits attenuator  115  and propagates along the optical path, where it is incident upon second mirror  117 . Second mirror  117  redirects beam  107  along the optical path, where it is incident upon beam expander  120 .  
         [0036]    Beam expander  120  increases the size of beam  107  to match the pupil size of scan lens  140 . Beam  107  exits beam expander  120  and propagates along the optical path, where it is incident upon third mirror  121 . Third mirror  121  redirects beam  107  along the optical path, where it is incident upon fourth mirror  122 . Fourth mirror  122  redirects beam  107  along the optical path, where it is incident upon spinning half-wave plate  125 . Spinning half-wave plate  125  changes the polarization of beam  107 . Upon exiting spinning half-wave plate  125 , beam  107  propagates along the optical path, where it is incident upon scan mirror  130 .  
         [0037]    Scan mirror  130  moves in a pre-defined pattern using a milling algorithm in real-time execution by a control computer (not shown but which should be apparent) to drill the holes in workpiece  155 . Scan mirror  130  redirects beam  107  along the optical path, where it is incident upon scan lens  140 . Scan lens  140  determines the spot size of beam  107  upon workpiece  155 . Beam  107  exits scan lens  140  and propagates along the optical path, where it is incident upon workpiece  155 . Beam  107  ablates workpiece  155  in a pattern according to the pre-defined milling algorithm. The milling algorithm is defined and communicated to laser drilling system  100  with a computer (not shown). The computer sends signals to shutter  110  and scan mirror  130  according to the parameters specified in the milling algorithm.  
         [0038]    [0038]FIG. 2 shows workpiece geometry  200  as a cross-section, including workpiece  155 , an outer diameter  260 , an exit hole diameter  280 , and an exit hole depth  290 .  
         [0039]    Workpiece geometry  200  is a cross-section of a cone-shaped hole in an inkjet nozzle, which can be drilled using laser drilling system  100 . However, workpiece geometry  200  is provided as one embodiment and the present invention is not limited to use with this shape.  
         [0040]    Specific parameters for workpiece geometry  200 , including outer diameter  260 , exit hole diameter  280 , and exit hole depth  290 , are measurements specified according to requirements of the inkjet cartridge manufacturer.  
         [0041]    Turning now to a consideration of the prior art, FIGS. 3 and 4 illustrate a prior art method of finishing exit holes in inkjet nozzles using traditional trepanning techniques. FIG. 3 is an illustration of an exit hole  300 , created with the trepanning technique described in FIG. 4 and including a laser beam path  310  and a perimeter  320 . Perimeter  320  is defined by the radius as measured from the centerpoint of exit hole  300 .  
         [0042]    For purposes of clarity, the distance between laser beam path  310  and perimeter  320  may be as large as 20 μm. The actual distance between laser beam path  310  and perimeter  320  is set according to the spot size of laser beam  107  and the ablation rate (amount of material removed by laser beam  107  in a specified time) to meet the pre-determined workpiece geometry.  
         [0043]    [0043]FIG. 4 illustrates a method  400  of finishing exit holes in inkjet nozzles using traditional trepanning techniques, including the following steps:  
         [0044]    In Step  410 , reducing lateral speed of laser beam, the lateral speed (a scalar component of the angular velocity) of laser beam  107  is reduced from the speed used in milling the shaped hole by 50 to 90 percent to a slower speed used to finish cylindrical exit hole  300 . Slower speeds tend to provide a better defined circular hole shape. In one embodiment, the nozzle hole and cylindrical exit hole  300  is shaped according to workpiece geometry  200 , as shown in FIG. 2.  
         [0045]    In Step  420 , trepanning at exit hole perimeter, laser beam  107  is circled repeatedly at perimeter  320  until sufficient material is ablated to cause separation of the material inside laser beam path  310 . This results in an exit hole  300  with a radius that meets the pre-determined workpiece geometry.  
         [0046]    [0046]FIG. 5 provides a derived image from a photo of an inkjet nozzle hole with a notching problem to show the actual results of drilling inkjet nozzle holes and the notches according the above-described prior art methodology. Note the notch  501  in the exit hole  502 .  
         [0047]    Turning now to a description of embodiments according to the present invention, key improvements are embedded in the milling algorithms that define how shapes are drilled in workpiece  155 .  
         [0048]    As noted in the overview at the beginning of the discussion of the preferred embodiments, the present invention provides considerations in creating repeatable exit holes meeting required shape specifications with a laser drilling system. These considerations are based on two distinct theories surrounding the cause of a “tuna can effect”, that is, a single burr or notch in the exit holes upon finishing.  
         [0049]    The first consideration eliminates the notch or burr in the exit hole that is caused at the point where laser beam  107  first “punches through” (when beam  107  reaches the full depth of, and initially pierces workpiece  155 ) the material at the exit hole perimeter. The punch-through is controlled to occur at a point that is not located on the perimeter (e.g. at a point less than the finished diameter of the exit hole), so that the notch is eliminated.  
         [0050]    The second consideration removes burrs or notches that are generated when laser beam  107  drills the exit hole, progressively removes material from the section of the exit hole area, and the mass of the material being ablated eventually causes an uneven break or notch in the exit hole. This second method involves removing a portion of material from the exit hole prior to the final finishing step, so that when the exit hole is finished at final diameter, the amount of mass has been reduced, thus minimizing the size of the notch or eliminating it altogether.  
         [0051]    In further detail, FIGS. 6 and 7 illustrate a method of finishing exit holes in inkjet nozzles using a punch-through and spiral technique. FIG. 6 shows an exit hole  600 , created with the punch-through and spiral technique described in FIG. 7 and including a starting point  610 , a laser beam path  620 , and a perimeter  630 .  
         [0052]    For purposes of clarity, the distance between laser beam path  620  and perimeter  630  may be as large as 20 μm. The actual distance between laser beam path  620  and perimeter  630  is set according to the spot size of laser beam  107  and the ablation rate (amount of material removed by laser beam  107  in a specified time) to meet the pre-determined workpiece geometry.  
         [0053]    [0053]FIG. 7 illustrates a method  700  of finishing exit holes in inkjet nozzles using a punch-through and spiral technique.  
         [0054]    In Step  710 , Punching through workpiece at center of exit hole, laser beam  107  is focused at the center of exit hole  600  until it punches through workpiece  155 .  
         [0055]    In Step  720 , Spiraling outwards to exit hole perimeter, laser beam  107  is gradually spiraled along laser beam path  620  from starting point  610  at the center point, to exit hole perimeter  630  at a rate of 0.1 sec/revolution to 1 sec/revolution. As laser beam  107  spirals, workpiece  155  material is ablated, causing separation of the material inside laser beam path  620 , and exit hole  600  grows until laser beam  107  reaches perimeter  630 .  
         [0056]    In Step  730 , circling at exit hole perimeter, laser beam  107  is circled at perimeter  630  at a rate of 0.1 sec/revolution to 1 sec/revolution to finish exit hole  600  with a round perimeter  630 .  
         [0057]    As the specifics of the instantaneous progression rate of beam  107  respective to the location of beam  107  to each position on the locus of points defining perimeter  630  are resolved for progression around perimeter  630 , Steps  720  and  730  proceed according to a beam progression rate function for moving the cutting beam  107  along the beam path as a function of the position of beam  107  respective to perimeter  630  such that the material ablation rate and the spot size minimize distortion of the edge of exit hole  600  to less than a predetermined threshold value as cutting beam  107  progressively incises the nozzle plate body. Such a function is first determined empirically for a particular design of workpiece  115 , and then the function is either expressed in the control program of a computer controlling beam  107  or taught to a technician operating beam  107 . Other embodiments either embed the function in electrical control circuitry or in the design of a mechanical cam.  
         [0058]    Method  700  eliminates burrs or notches in the final exit hole  600  caused by laser beam  107  punch-through because the damaging punch-through occurs at a location inside and away from perimeter  630 .  
         [0059]    In a further embodiment, FIGS. 8 and 9 illustrate a method of finishing exit holes in inkjet nozzles using a punch-through and slow circling technique. FIG. 8 shows an exit hole  800 , created with the punch-through and slow circling technique described in FIG. 9, including a starting point  810 , a laser beam path  820 , and a perimeter  830 .  
         [0060]    For purposes of clarity, the distance between laser beam path  820  and perimeter  830  may be as large as 20 μm. The actual distance between laser beam path  820  and perimeter  830  is set according to the spot size of laser beam  107  and the ablation rate (amount of material removed by laser beam  107  in a specified time or as a function of time in the cutting cycle) to meet the predetermined workpiece geometry.  
         [0061]    [0061]FIG. 9 illustrates a method  900  of finishing exit holes in inkjet nozzles using a punch-through and slow circling technique.  
         [0062]    In Step  910 , Punching through workpiece inside exit hole perimeter, laser beam  107  focuses on starting point  810 , which is located at a point inside perimeter  830  of exit hole  800 , and punches through workpiece  155 .  
         [0063]    In Step  920 , Maneuvering laser beam to point on exit hole perimeter, laser beam  107  is maneuvered from starting point  810  to a point on perimeter  830 .  
         [0064]    In Step  930 , Circling laser at exit hole perimeter at reduced speed, laser beam  107  is circled slowly at perimeter  830  at a rate of 0.1 sec/revolution to 1 sec/revolution until sufficient material is ablated to cause separation of the material inside laser beam path  820 . This results in an exit hole  800  with a radius that meets pre-determined workpiece geometry within a predefined tolerance defined by a threshold value. Laser beam path  820  continues until laser beam  107  has completed 360 degrees at exit hole perimeter  830 .  
         [0065]    Method  900  eliminates burrs or notches in the final exit hole  800  caused by laser beam  107  punch-through because the damaging punch-through occurs at a location inside and away from perimeter  830 .  
         [0066]    Illustrating yet another embodiment, FIGS. 10 and 11 illustrate a method of finishing exit holes in inkjet nozzles using a two-pass trepanning technique. FIG. 10 shows an exit hole  1000 , created with the two-pass trepanning technique described in FIG. 11, including a laser beam path  1010 , a laser beam path  1020 , and a perimeter  1030 .  
         [0067]    [0067]FIG. 11 illustrates a method  1100  of finishing exit holes in inkjet nozzles using a two-pass trepanning technique, including the following steps:  
         [0068]    In Step  1110 , Trepanning at perimeter less than exit hole perimeter, laser beam  107  is circled repeatedly at laser beam path  1010  at a rate of 0.1 sec/revolution to 1 sec/revolution until sufficient material is ablated to cause separation of the material inside laser beam path  1010 .  
         [0069]    In Step  1120 , Trepanning at exit hole perimeter, laser beam  107  is circled repeatedly at perimeter  1030  at a rate of 0.1 sec/revolution to 1 sec/revolution until sufficient material is ablated to cause separation of the material inside laser beam path  1020 . This results in an exit hole  1000  with a radius that meets pre-determined workpiece geometry.  
         [0070]    Method  1100  eliminates burrs or notches in final exit hole  1000  caused by the mass of exit hole  1000  material creating a notch or burr in exit hole  1000 , because a portion of material from exit hole  1000  is removed prior to the final finishing step, such that when exit hole  1000  is finished at its final diameter, the amount of mass is reduced, thus minimizing the size of the notch or eliminating it altogether.  
         [0071]    The above approaches are further augmented in another embodiment by securing an etchable material layer to the beam exit surface of the nozzle plate body, controlling the continuous incision to cut through said nozzle plate body to the interface between the nozzle plate body and the etachable material, and etching away the etchable material after the cutting beam has fully traversed the beam path (see the discussions of U.S. Pat. No. 6,023,041 and EP0867294 as referenced in the Background discussion of this Specification).  
         [0072]    A nozzle plate of an ink-jet head may be constructed with the laser drilling system of the present invention as further detailed in FIGS. 12 and 13.  
         [0073]    As shown in FIG. 12, an ink-jet printer  1240  has an ink-jet head  1241  capable of recording on a recording medium  1242  via a pressure generator. Ink droplets emitted from ink-jet head  1241  are deposited on the recording medium  1242 , such as a sheet of copy paper, so that recording can be performed on the recording medium  1242 . The ink-jet head  1241  is mounted on a carriage  1244  capable of reciprocating movement along a carriage shaft  1243 . More specifically, the ink-jet head  1241  is structured such that it can reciprocate in a primary scanning direction X in parallel with the carriage shaft  1243 .The recording medium  1242  is timely conveyed by rollers  1245  in a secondary scanning direction Y. The ink-jet head  1241  and the recording medium  1242  are relatively moved by the rollers  1245 .  
         [0074]    Turning now to FIG. 13, further details in in-jet head  1241  are shown. Pressure generator  1304  is preferably a piezoelectric system, a thermal system, and/or equivalent system. In this embodiment, the pressure generator  1304  corresponds to a piezoelectric system which comprises an upper electrode  1301 , a piezoelectric element  1302 , and an under electrode  1303 . A nozzle plate  1314  (an instance of workpiece  155 ) comprises a nozzle substrate  1312  and a water repellent layer  1313 . The nozzle substrate  1312  is made of metal, resin and/or equivalent material. The water repellant layer is made of fluororesin or silicone resin. In this embodiment, the nozzle substrate  1312  is made of stainless steel and has a thickness of 50 um, and the water repellent layer is made of a fluororesin and has a thickness of 0.1 um. The ink-jet ink is filled in an ink supplying passage  1309 , a pressure chamber  1305 , an ink passage  1311 , a nozzle  1310 . Ink droplets  1320  are ejected from nozzle  1310  as pressure generator  1304  pushes on pressure chamber element  1306 .  
         [0075]    As a result of the present invention, very good nozzles are formed without flash and foreign matter (carbon etc) in the nozzle plate. Further, the accuracy of the nozzle outlet diameter is 20 um±1.5 um (a preferred predefined acceptable threshold value for tolerance between the perimeter and the excision edge of the 20 um diameter nozzle outlet).  
         [0076]    From the foregoing it will be understood that the present invention provides a provides a system and method for removing a portion from a workpiece with a laser cutting tool with special value in using a laser to mill exit holes in inkjet nozzles. While the invention has been described in its presently preferred form, it will be understood that the invention is capable of certain modification without departing from the spirit of the invention as set forth in the appended claims.  
         [0077]    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.