Abstract:
A method of substantially eliminating imperfections in a laser milled workpiece, wherein the imperfections result from a laser drilling process, includes attaching a pre-milled sacrificial layer to a beam exit surface of a pre-milled workpiece, wherein the pre-milled sacrificial layer has a first laser ablation rate substantially matching a second laser ablation rate of the pre-milled workpiece. A passage is formed through the pre-milled workpiece and the pre-milled sacrificial layer by ablating workpiece and sacrificial layer material with a laser, thereby producing a laser-milled workpiece and laser-milled sacrificial layer with the imperfections substantially concentrated in the laser-milled sacrificial layer. The laser-milled sacrificial layer is removed from the workpiece, thereby substantially eliminating imperfections in the laser-milled workpiece.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/398,640, filed on Jul. 25, 2002. The disclosure of the above application is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention generally relates to laser drilling systems and methods, and particularly relates to use of a sacrificial layer in a laser drilling process.  
         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. 6,323,456, “Method of forming an ink jet printhead structure,” 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 ablated to form flow features in the strip in order to form the nozzle plates. After forming the flow features, the sacrificial layer is removed and the individual inkjet printhead nozzle plates are separated from the composite strip by singulating the nozzle plates with a laser.  
           [0005]    U.S. Pat. No. 6,228,246, “Removal of metal skin from a copper-Invar-copper laminate,” describes a method of removing a metal skin from a through-hole surface of a copper-Invar-copper (CIC) laminate without causing differential etch back of the laminate. The metal skin includes debris deposited on the through-hole surface as laser or mechanical drilling of a substrate that includes the laminate as an inner plane is forming the through hole. Removing the metal skin combines electrochemical polishing (ECP) with ultrasonic. ECP dissolves the metal skin in an acid solution, while ultrasonic agitates and circulates the acid solution to sweep the metal skin out of the through-hole. ECP is activated when a pulse power supply is turned on and generates a periodic voltage pulse from a pulse power supply whose positive terminal is coupled to the laminate and whose negative terminal is coupled to a conductive cathode. After the metal skin is removed, the laminate is differentially etched such that the copper is etched at a faster rate than the Invar. To prevent the differential etching, a copper layer is formed on a surface of the substrate with an electrical resistance R 1  between the copper layer and the positive terminal of the pulse power supply. Additionally, an electrical resistance R 2  is formed between the laminate and the positive terminal of the pulse power supply. Adjustment of R 1  and R 2  controls the relative etch rates of the copper and the Invar.  
           [0006]    U.S. Pat. No. 6,120,131, “Method of forming an inkjet printhead nozzle structure,” describes a composite structure containing a nozzle layer and an adhesive layer where the adhesive layer is coated with a polymeric sacrificial layer. The coated composite structure is laser ablated to form one or more nozzles in the structure and the sacrificial layer is then removed. The sacrificial layer is preferably a water-soluble polymer, such as polyvinyl alcohol or polyethylene oxide, which is removed by directing jets of water at the sacrificial layer until it is substantially removed from the adhesive layer.  
           [0007]    U.S. Pat. No. 5,609,746, “Printed circuit manufacture,” describes a manufacturing method of a printed circuit board where a sacrificial tin-lead layer is deposited on the surface of the board by electroplating. Holes are then formed in the board by UV laser ablation. Debris from the ablation process is adsorbed on the sacrificial layer. The sacrificial layer is then removed by means of a chemical stripping process, along with the debris.  
           [0008]    U.S. Pat. No. 4,948,941, “Method of laser drilling a substrate,” describes a method of laser drilling a substrate and includes the steps of: placing a sacrificial member over the substrate, and then laser drilling through the sacrificial member. This method produces a substantially uniform hole in the substrate.  
           [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, these lasers readily drill hole sizes in the sub-micron range. 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]    Even as micro-technologies continue to provide products with ongoing decreases in size, the need for high product quality, adherence to stringent specifications, and manufacturing consistency continues. An example of a product having such stringent specifications is appreciated in consideration of the print quality and performance of an inkjet printer; this performance is closely related to tight control of the hole geometries of the inkjet workpieces (inkjet nozzles provided in inkjet nozzle plates).  
           [0012]    Inkjet nozzle design, construction, and operation are all important factors in providing high quality inkjet print resolution. Inkjet nozzle designs, which typically include specific patterns of many ink jet holes, which in turn are also specific defined geometries, provide the templates for nozzle holes drilled in a thin foil or polymer to a particular shape. Each nozzle hole includes an input section, a shaped section and an exit hole section, and each exit hole section is preferably cut with a high degree of precision respective to the design pattern. In a particular nozzle, inconsistency in nozzle hole shape leads to inconsistent expulsion of inks among the individual holes in an inkjet nozzle, which negatively affects print resolution. Therefore, imperfections in the shape of the inkjet nozzle holes respective to the design pattern negatively impact print quality.  
           [0013]    Although laser drilling of 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 induce burr and notch defects in the finished product. These defects are particularly detrimental in the exit hole because the size and smoothness specifications 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, causing poor print quality. Most 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. This notch or burr is undesirable because of the negative impact it has on print quality. Insofar as the industry has a preference to use stainless steel as the best nozzle plate (workpiece) material in inkjet nozzles, there are also certain machining challenges in eliminating burrs and notches respective to the hardness properties of stainless steel alloys.  
           [0014]    What is needed is a way to minimize defects in stainless steel laser drilling inkjet nozzles and thereby to enhance quality and consistency in manufactured inkjet nozzles. The present invention provides a solution to this need.  
         SUMMARY OF THE INVENTION  
         [0015]    According to the present invention, a method of substantially eliminating imperfections in a laser milled workpiece, wherein the imperfections result from a laser drilling process, includes attaching a pre-milled sacrificial layer to a beam exit surface of a pre-milled workpiece, wherein the pre-milled sacrificial layer has a first laser ablation rate substantially matching a second laser ablation rate of the pre-milled workpiece. A passage is formed through the pre-milled workpiece and the pre-milled sacrificial layer by ablating workpiece and sacrificial layer material with a laser, thereby producing a laser-milled workpiece and laser-milled sacrificial layer with the imperfections substantially concentrated in the laser-milled sacrificial layer. The laser-milled sacrificial layer is removed from the workpiece, thereby substantially eliminating imperfections in the laser-milled workpiece.  
           [0016]    A number of advantages are provided with the invention. Elimination of notches or aberrations, which are normally formed in the high volume laser drilling manufacturing process, is one benefit. The method also provides flexibility in the choice and thicknesses of sacrificial layers. Since it uses low cost processing and low cost materials, the invention is cost effective. When copper is the sacrificial layer, the copper also functions in capturing debris (as described, for instance, in background patent U.S. Pat. No. 5,609,746). Since aberrations and notches are effectively eliminated, higher power lasers are deployed to further speed the drilling process. Finally, removal of the sacrificial layer (especially in the case of copper) is, in one alternative, delayed until the drilled nozzle plate is delivered for final integration with its inkjet cartridge, providing a basis for a cleaner inkjet head.  
           [0017]    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  
       [0018]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0019]    [0019]FIG. 1 presents a schematic of a laser drilling system;  
         [0020]    [0020]FIG. 2 (FIGS. 2A through 2E) illustrates a method of using a sacrificial layer to make holes using a laser drilling system;  
         [0021]    [0021]FIG. 3 provides a perspective view showing major constituent components of an ink-jet printer; and  
         [0022]    [0022]FIG. 4 provides a schematic cross-sectional view of an ink-jet head. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    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.  
         [0024]    In overview, one embodiment of the present invention provides a method of eliminating aberrations and notches in an inkjet workpiece by (1) providing a workpiece of stainless steel, (2) depostiting a polymer layer on the workpiece, (3) defining a metal layer on the polymer layer, (4) defining holes in the workpiece, the polymer layer and the metal layer where aberrations and notches are randomly created in the metal layer and (5) removing the metal layer and hence also removing all the random aberrations and notches. This is very advantages where the workpiece is an inkjet nozzle and where the shaped holes each have exactly the same shape.  
         [0025]    The present invention provides a method of manufacturing an inkjet nozzle structure that produces controlled and repeatable nozzle shapes without random aberrations or notches normally caused in high volume manufacturing by the lack of control of the laser ablation drilling process. These aberrations or notches are eliminated by using a sacrificial layer where the aberrations or notches are created (instead of within the final structure). However, the shape of the exit holes is controlled since the random aberrations or notches that are normally created by the laser drilling process in the workpiece are instead created in the sacrificial layer only, and are subsequently removed when the sacrificial layer is removed. This process creates a final article of manufacture structure that prevents the laser drilling defects from impacting the quality of the final exit hole.  
         [0026]    Turning now to specific details in the preferred embodiments, FIG. 1 shows a simplified schematic of a laser drilling system  100 , including a laser  105 , a beam  107 , a shutter  110 , an attenuator  115 , a beam expander  120 , a spinning half-wave plate  125 , a first mirror  108 , a second mirror  117 , a third mirror  121 , a fourth mirror  122 , a piezo electric transducer (PZT) scan mirror  130 , a diffractive optical element (DOE)  135 , a plurality of sub-beams  137 , a scan lens  140 , a microfilter  145 , an image transfer lens  150 , and a workpiece  155 , arranged as shown. All elements of laser drilling system  100  are conventional in laser micromachining.  
         [0027]    DOE  135  is a highly efficient beamsplitter and beam array pattern generator so that laser-drilling system  100  drills parallel holes in workpiece  155 . The pattern of sub-beams  137  output by DOE  135  is predetermined by the specifications of the holes to be drilled in workpiece  155 . In an alternate contemplated embodiment pursuant to anticipated improvements in beam quality of excimer lasers, an excimer laser with a kinoform is used in place of DOE  135 . In one example, DOE  135  splits the single incident laser beam from laser  105  into  152  beams in the forms of  4  rows with  38  beams in each row. (See Holmér and Hård&#39;s 1995 paper “Laser-machining experiment with an excimer laser and a kinoform” in Applied Optics which is hereby incorporated herein by reference).  
         [0028]    Scan lens  140  determines the spot size of sub-beams  137  upon workpiece  155 . The beam size that enters scan lens  140  must be less than or equal to the pupil size of scan lens  140 . Telecentricity is required to keep the incident angle between sub-beams  137  and workpiece  155  essentially perpendicular, which is necessary to drill parallel holes in workpiece  155 . Scan lens  140  is preferably an f-theta telecentric (scan) lens. In alternate embodiments where the axes of the holes do not need to be parallel to each other, a non-telecentric scan lens is used.  
         [0029]    Microfilter  145  equalizes the uniformity of sub-beams  137  emitted from laser  105  and through DOE  135 . Microfilter  145  consists of dielectric coatings on a glass substrate, and is designed and fabricated according to the intensity patterns of the sub-beams of DOE  135 . In one embodiment, microfilter  145  provides two transmission values, 100% and 98%, in a pattern of  152  individual filters of 4 rows with 38 filters in each row (correspondent to DOE  135  as discussed above). In this embodiment, each of the individual filters is circular in shape with a diameter of 250 microns.  
         [0030]    Image transfer lens  150  maintains image quality, spot size, and telecentricity, while preventing blowback of ablated particles from workpiece  155  onto microfilter  145  by distancing workpiece  155  an additional focal length away from microfilter  145 . In this regard, ablated particles present a hazard to microfilter  145  respective to the proximity between microfilter  145  and workpiece  155 . In one embodiment, the image transfer lens consists of two telecentric scan lenses, identical to scan lens  140 , placed back-to-back, with the pupil planes of the two scan lenses coinciding in the middle.  
         [0031]    Workpiece  155  is the target for picosecond laser drilling system  100 . In this example, workpiece  155  is a stainless steel inkjet nozzle foil; however, the present invention is, in alternative embodiments, generalized to a variety of workpiece materials, such as polymers, semiconductor metals, or ceramics. In alternate embodiments, picosecond laser drilling system  100  drills holes of a wide variety of shapes and tapers in workpiece  155 .  
         [0032]    In operation, laser  105  emits beam  107  along the optical path shown in FIG. 1 above. 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 to be 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 .  
         [0033]    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 PZT scan mirror  130 . PZT scan mirror  130  moves in a pre-defined pattern using a drilling algorithm in execution by a real-time control computer (not shown but which should be apparent) to drill the holes in workpiece  155 . PZT scan mirror  130  redirects beam  107  along the optical path, where it is incident upon DOE  135 . DOE  135  splits beam  107  into a plurality of sub-beams  137 , which allow parallel drilling of workpiece  155 . Sub-beams  137  exit DOE  135  and propagate along the optical path, where they are incident upon scan lens  140 . Scan lens  140  determines the spot size of sub-beams  137  upon workpiece  155 . Sub-beams  137  exit scan lens  140  with the correct spot size and propagate along the optical path, where they are incident upon microfilter  145 . Microfilter  145  equalizes the uniformity of sub-beams  137 . Sub-beams  137  exit microfilter  145  and propagate along the optical path, where they are incident upon image transfer lens  150 . Image transfer lens  150  maintains the properties of sub-beams  137  and focuses sub-beams  137  onto workpiece  155 . Sub-beams  137  ablate workpiece  155  in a pattern according to the pre-defined drilling algorithm.  
         [0034]    Turning now to a closer consideration of details in the invention, FIG. 2, including FIGS. 2A through 2E, illustrates a method of using a sacrificial layer to make holes using a laser drilling system.  
         [0035]    In FIG. 2A, a workpiece  210  (commensurate with the more generalized workpiece  155  of FIG. 1) is provided as the basis of structure  200 . Workpiece  210  consists of a stainless steel substrate, which will be used to form an inkjet nozzle. Stainless steels are optimal materials for an inkjet nozzle since they are flexible, durable, and resistive to degradation from the ink environment used in the printer system.  
         [0036]    In FIG. 2B, a polymer layer  220  is applied to completely coat one side of workpiece  210 . Polymer layer  220  is a hydrophobic material and its purpose is to improve the ink ejection from the inkjet printer. This polymer is typically a 20 to 100 micron thick film of polyimide which is formed by any of a number of deposition processes, including but not limited to (1) spin application and cure, (2) atmospheric deposition of a polymeric film and cure, or (3) roll and press lamination of an adhesive and a polymer film, such as in U.S. Pat. No. 6,120,131.  
         [0037]    In FIG. 2C, a metal layer  230 , such as copper, is applied to completely coat polymer layer  220 , and provide a new beam exit surface of workpiece  210 . Metal layer  230  is selected to have similar properties to workpiece  210  such that it ablates similarly using laser drilling system  100 . Metal layer  230  is deposited by any of (1) electroless plating of copper on a seed layer of sputtered copper, (2) evaporation, (3) sputtering, or (4) chemical vapor deposition. Typically, copper is deposited to a total thickness of 20-100 microns. Alternative metal materials that can be deposited include aluminum, aluminum alloys, nickel, nickel alloys, and the like. The material is chosen to match as closely as possible the laser ablation properties of workpiece  210  in terms of its ablation rate and thermal dispersion rate as well in consideration of its selective etch properties from stainless steel. In this regard, metal layer  230  must be a substance having (1) a laser ablation rate sufficiently comparable to the workpiece  210  material ablation rate such that aberrations formed from the cutting beam are formed essentially in metal layer  230 , (2) a thermal dispersion rate sufficiently comparable to the workpiece  210  material thermal dispersion rate such that aberrations formed from the cutting beam are formed essentially in metal layer  230 , and (3) a selective etch property to the etchable material respective to the material of the workpiece  210  and an etching substance selected for use in etching metal layer  230  from the workpiece  210 .  
         [0038]    In FIG. 2D, holes in-group  251  and in-group  252  are drilled into structure  200  using laser drilling system  100  of FIG. 1. Holes in-group  251  and in-group  252  are drilled according to pre-determined size and geometry specifications, and are drilled by ablating workpiece  210 , polymer layer  220  and metal layer  230 . As shown, aberrations or notches  253  are created in holes in-group  251 , because of the variability of laser ablation parameters. Aberrations or notches  253  are created randomly in holes that are ablated, and always occur near the exit region. In FIG. 2, aberrations or notches  253  are shown in the metal layer  230 . Metal layer  230  is of sufficient thickness that any random aberrations or notches  253  are always created in metal layer  230  and not in workpiece  210 .  
         [0039]    In FIG. 2E, metal layer  230  is removed via a selective wet etch, which removes metal layer  230  but does not affect either polymer layer  220  or workpiece  210 . Copper is removed using either a wet etch step, such as a combination of ammonium persulfate/NH 4 OH, or a combination of Fe(NO 3 )/HCl (see “Metallography, Principles and Practice” by George Vander Voort); or a plasma etch (reactive ion etch such as BCl 3  and Cl). However, this etch does not etch the polymer or stainless steel. As can be seen, by removing metal layer  230 , aberrations or notches  253  in metal layer  230  are also removed. Thus, the final inkjet nozzle holes in-group  251  and  252  are produced without these random aberrations or notches  253  and thus provide a controlled shape for inkjet use.  
         [0040]    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. 3 and 4.  
         [0041]    As shown in FIG. 3, an ink-jet printer  340  has an ink-jet head  341  capable of recording on a recording medium  342  via a pressure generator. Ink droplets emitted from ink-jet head  341  are deposited on the recording medium  342 , such as a sheet of copy paper, so that recording is performed on the recording medium  342 . The ink-jet head  341  is mounted on a carriage  344  capable of reciprocating movement along a carriage shaft  343 . More specifically, the ink-jet head  341  is structured such that it reciprocates in a primary scanning direction X in parallel with the carriage shaft  343 . The recording medium  342  is timely conveyed by rollers  345  in a secondary scanning direction Y. The ink-jet head  341  and the recording medium  342  are relatively moved by the rollers  345 .  
         [0042]    Turning now to FIG. 4, further details in in-jet head  341  are shown. Pressure generator  404  is preferably a piezoelectric system, a thermal system, and/or equivalent system. In this embodiment, the pressure generator  404  corresponds to a piezoelectric system which comprises an upper electrode  401 , a piezoelectric element  402 , and an under electrode  403 . A nozzle plate  414  (an instance of workpiece  155 ) comprises a nozzle substrate  412  and a water repellent layer  413 . The nozzle substrate  412  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  412  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  409 , a pressure chamber  405 , an ink passage  411 , a nozzle  410 . Ink droplets  420  are ejected from nozzle  410  as pressure generator  404  pushes on pressure chamber element  406 .  
         [0043]    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).  
         [0044]    From the foregoing it will be understood that the present invention provides a provides a system and method for cutting a workpiece with a laser cutting tool with a high degree of precision in the quality of the conformance of the dimensions of the removed portion to the dimensions of the design used in the cutting operation 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.