Abstract:
A method of adjusting depth of focus in a laser milling system includes generating a laser beam having a focal plane, positioning a workpiece in the focal plane, wherein a surface of the workpiece is exposed to the laser beam at a point intersecting the focal plane, and adjusting a position of at least one of the workpiece and the focal plane, thereby maintaining a constant ablation rate on the exposed surface of the workpiece throughout the drilling process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application Serial No. 60/398,469 which was filed on Jul. 25, 2002 and is incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention generally relates to material ablation with pulsed light sources, and particularly relates to focus depth control in a laser milling system.  
         BACKGROUND OF THE INVENTION  
         [0003]    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.  
           [0004]    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.  
           [0005]    Ultrafast and short pulse laser systems can be designed to accommodate drilling and milling of thin materials (50 to 100 microns); however, there is a need in the field of laser micromachining to drill or mill a wider range of material thicknesses. Ablating thicker materials presents certain technological challenges, including maintaining control of the laser through the entire thickness of the material and providing a finished product that meets both customer specifications and established quality standards.  
           [0006]    Several problems continue to exist in the field of material ablation with pulsed light sources, and one such problem relates to controlling the quality specifications of a laser-drilled final product. Recent advancements in the field of laser drilling have been effective in enhancing the quality (i.e., shape, contour, and repeatability) of finished products according to customer demands. However, a growing number of customers require an increased geometric complexity in their product designs. Using conventional industry techniques, it is difficult to meet the increasing quality market needs. What is needed is a way to control the quality specifications of a laser-drilled final product.  
           [0007]    Another problem that persists in the field of material ablation with pulsed light sources relates to controlling laser drilling on a thick workpiece. In typical laser-drilling applications that work with thin materials, ablation need only be controlled through the range of a thin workpiece (such as 50 to 100 microns). However, as material thickness increases, ablation must be controlled through a wider range of thickness and through an increased number of ablation layers. As drilling is performed on thicker surfaces, maintaining laser parameters becomes increasingly more difficult. Therefore, greater control over the laser parameters that control ablation rates and hole shape geometry throughout the process becomes necessary. What is needed is a way to control laser drilling on a thick workpiece.  
           [0008]    A further problem that continues to exist in the field of material ablation with pulsed light sources relates to maintaining a constant ablation rate on the surface of a thick workpiece throughout the drilling process. During ablation of a layer in laser drilling, a material void is created at the contact point on the workpiece surface, i.e., the point at which the laser spot size intersects with the material surface. Once a void in the material is created, the expected contact point is no longer the same in that location on the workpiece due to the removal of material. Because of the variations in laser beam intensity and spot size at the contact point, the ablation rate changes. What is needed is a way to maintain a constant ablation rate on the surface of a thick workpiece throughout the drilling process.  
           [0009]    A still further problem that continues to exist in the field of material ablation with pulsed light sources relates to maintaining constant laser beam intensity and constant spot size on the surface of a thick workpiece throughout the drilling process. Laser beam spot size is measured at the point where the focused laser beam and the surface of the workpiece material intersect. At a known laser beam intensity and known spot size, the ablation rate can be calculated and thus predicted in order to meet customer specifications. In a thick workpiece, however, as layers of material are ablated away the workpiece surface is no longer in the focal plane of the laser drilling system. This means that the laser beam intensity and spot size are no longer the same as originally intended (at the point of ablation) and, thus, ablation control is decreased. A constant spot size is required to prevent hole geometry distortion and inconsistent geometrical shaping due to lack of ablation control. What is needed is a way to maintain constant laser beam intensity and constant spot size on the surface of a thick workpiece throughout the drilling process.  
         SUMMARY OF THE INVENTION  
         [0010]    According to the present invention, a method of adjusting depth of focus in a laser milling system includes generating a laser beam having a focal plane, positioning a workpiece in the focal plane, wherein a surface of the workpiece is exposed to the laser beam at a point intersecting the focal plane, and adjusting a position of at least one of the workpiece and the focal plane, thereby maintaining a constant ablation rate on the exposed surface of the workpiece throughout the drilling process.  
           [0011]    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  
       [0012]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0013]    [0013]FIG. 1 is a perspective view of a spot size alignment within a laser drilling system;  
         [0014]    [0014]FIG. 2 is a simplified schematic of a laser drilling system;  
         [0015]    FIGS.  3 A-C are magnified views of depth of focus adjustments made using a moveable stage; and  
         [0016]    [0016]FIG. 4 is a flow chart diagram depicting a method of adjusting depth of focus in a laser drilling system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    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.  
         [0018]    The present invention provides a system for and a method of continuously optimizing the depth of focus of a laser drilling system using a moveable workpiece stage to provide consistent, controlled ablation throughout the milling process.  
         [0019]    Referring to FIG. 1, a perspective view of a spot size alignment  100  within a laser drilling system includes a workpiece  112 , a beam  107 , a depth of focus region  110 , and an optical axis  150 . Depth of focus region  110  is a region along optical axis  150  of beam  107  in which the spot size of beam  107  maintains a relatively constant radius that can be quantitatively determined.  
         [0020]    Adjustments to the optical path of beam  107  or workpiece  112  along optical axis  150  of beam  107  may adjust depth of focus region  110  to an optimal point. The spot size of beam  107  is calculated to center the spot size of beam  107  within depth of focus region  110 , thereby aligning beam  107  to the designated surface contact point on workpiece  112 .  
         [0021]    [0021]FIG. 2 shows a simplified schematic of a laser drilling system  200 , including a laser  205 , a shutter  210 , an attenuator  215 , a beam expander  220 , a spinning half-wave plate  225 , a first mirror  208 , a second mirror  217 , a third mirror  221 , a fourth mirror  222 , a piezo electric transducer (PZT) scan mirror  230 , a scan lens  240 , and a moveable stage  257 , arranged as shown. Although the present invention uses a picosecond laser system, the present invention may be generalized for use with other laser systems, such as excimer, CO 2 , and copper vapor laser systems.  
         [0022]    Although all elements of laser drilling system  200  are conventional, a brief description of the operation of laser drilling system  200  is provided below. In alternate embodiments, changes in the elements of laser drilling system  200  may be required. The present invention is not limited to the current selection and arrangement of elements in laser drilling system  200 .  
         [0023]    In operation, picosecond laser  205  emits beam  107  along the optical path identified in FIG. 2 above. Beam  107  propagates along the optical path, where it is incident upon first mirror  208 . First mirror  208  redirects beam  107  along the optical path, where it is incident upon shutter  210 . Shutter  210  opens and closes to selectively illuminate the workpiece material. Beam  107  exits shutter  210  and propagates along the optical path to attenuator  215 . Attenuator  215  filters the energy of picosecond laser  205  in order to precisely control ablation parameters. Beam  107  exits attenuator  215  and propagates along the optical path, where it is incident upon second mirror  217 . Second mirror  217  redirects beam  107  along the optical path, where it is incident upon beam expander  220 .  
         [0024]    Beam expander  220  increases the size of beam  107  to match the pupil size of scan lens  240 . Beam  107  exits beam expander  220  and propagates along the optical path, where it is incident upon third mirror  221 . Third mirror  221  redirects beam  107  along the optical path, where it is incident upon fourth mirror  222 . Fourth mirror  222  redirects beam  107  along the optical path, where it is incident upon spinning half-wave plate  225 . Spinning half-wave plate  225  changes the polarization of beam  107 . Upon exiting spinning half-wave plate  225 , beam  107  propagates along the optical path, where it is incident upon PZT scan mirror  230 . PZT scan mirror  230  moves in a pre-defined pattern using a drilling algorithm (not shown) to drill the holes in workpiece  112 . PZT scan mirror  230  redirects beam  107  along the optical path, where it is incident upon scan lens  240 . Scan lens  240  determines the spot size of beam  107  upon workpiece  112 . Beam  107  exits scan lens  240  and propagates along the optical path, where it is incident upon workpiece  112 . Beam  107  ablates workpiece  112  in a pattern according to the pre-defined drilling algorithm. Moveable stage  257  is adjusted on a vertical axis to maintain accurate depth of focus and spot size of beam  107  on the contact surface of workpiece  112  during ablation. It is readily understood that to maintain accurate depth of focus, the scan lens  240  may also be adjustable along the vertical axis, such that the workpiece  112  remains in a fixed position.  
         [0025]    FIGS.  3 A-C show magnified views of depth of focus adjustments  300  made using moveable stage  257 , and include a focal point  310  and an ablated region  312 . Additional elements of laser drilling system  200  are shown in FIGS.  3 A-C for the purpose of clarity.  
         [0026]    As shown in FIG. 3A, scan lens  240  directs beam  107  to focal point  310  on the surface of workpiece  112  for material ablation. FIG. 3B illustrates how ablated region  312  is formed by beam  107 , thereby causing focal point  310  to become displaced a distance equal to the depth of the ablated layer. The feed distance and direction of moveable stage  257  is identified in the present invention as ΔZ.  
         [0027]    [0027]FIG. 3C illustrates the adjustment made (shown as ΔZ) to refocus beam  107  on the contact surface of workpiece  112 . The distance that moveable stage  257  moves (ΔZ) is predetermined based on the calculated ablation rate.  
         [0028]    In operation, initial depth of focus settings are determined for focal point  310  of beam  107  on the contact surface of workpiece  112 . As ablation region  312  is formed, a control unit (not shown) mechanically adjusts the feed distance ΔZ of moveable stage  257  to realign focal point  310  of beam  107  to the depth of the next layer to be ablated. This technique provides a continuous ablation of material within workpiece  112  while providing constant laser intensity to focal point  310  within selected depth of focus region  110 .  
         [0029]    [0029]FIG. 4 shows a method  400  of adjusting the depth of focus in laser drilling system  200 , the steps of which are now described with reference to FIGS.  1 - 3 .  
         [0030]    At step  410 , a desired hole geometry is determined. In this step, an operator or technician uses customer-specified information, such as CAD files, and technical notes to determine the desired hole geometry, including taper angle, exit hole diameter, and entrance hole diameter. In the present invention, the desired shape and dimensions are circular holes. However, this invention is not limited by a designated shape and may also be applied to the drilling of a general pattern geometry, such as lines, curves, squares, etc.  
         [0031]    At step  420  a laser ablation rate is experimentally determined. In this step, an operator or technician determines the ablation rate (or material removal rate) with a given set of laser drilling parameters. Governing parameters of ablation rate include repetition rate, spot size, and laser power. Prior to milling workpiece  112 , a number of trial milling runs are performed to measure and make adjustments to laser drilling system  200  to affect the ablation rate.  
         [0032]    At step  430 , a finished depth of the workpiece hole is set. In this step, an operator or technician determines the final desired hole depth of workpiece  112  using the material thickness of workpiece  112  and the desired hole shape as derived from step  410 .  
         [0033]    At step  440 , a feed rate is determined. In this step, an operator, technician, or automated tool uses calculations derived from steps  410 ,  420 , and  430  to determine the feed rate of moveable stage  257 . The feed rate is equal to the determined ablation rate expressed in ablated depth per unit time for any particular drilling application. For example, if the ablation rate is determined to be 1 μm/sec for a particular material and size and shape for a given set of laser parameters, the feed rate will be 1 μm/sec.  
         [0034]    At step  450 , data is input to a control unit. In this step, an operator, technician, or automated tool inputs the data derived from steps  420 ,  430 , and  440  into a control unit to enable adjustment of moveable stage  257 .  
         [0035]    At step  460 , a workpiece is ablated. In this step, laser drilling system  200  adjusts depth of focus parameters to the surface of workpiece  112 . Laser drilling system  200  then performs a milling algorithm to ablate material within workpiece  112 . Ablation of workpiece  112  within entire ablated region  312  at pre-determined ablation depths, are based on the ablation rate as determined in step  420 .  
         [0036]    At step  470 , a determination is made regarding whether a desired hole depth has been achieved. In this decision step, the milling algorithm determines whether the desired milled shape has been achieved. The milled shape is considered to be achieved when the milling algorithm has completed the pre-set number of ablation layers and moveable stage  257  has achieved a predetermined setpoint for the desired hole depth as determined in step  430 . If yes, method  400  ends; if no, method  400  returns to step  420 .  
         [0037]    The present invention has several advantages. A first advantage of the present invention is that it provides a way to control the quality specifications of a laser-drilled final product. A second advantage of the present invention is that it provides a way to control laser drilling on a thick workpiece. A third advantage of the present invention is that it provides a way to maintain a constant ablation rate on the surface of a thick workpiece throughout the drilling process. A fourth advantage of the present invention is that it provides a way to maintain constant laser beam intensity and constant spot size on the surface of a thick workpiece throughout the drilling process. A fifth advantage of the present invention is that it provides a way to make adjustments to the depth of focus using only one moveable element. A sixth advantage of the present invention is that it allows simplified patterns to be made without taking into account material thickness.  
         [0038]    The present invention also has some disadvantages. One disadvantage of the present invention is that if the feed rate of the moveable stage is too slow, then it may be difficult for the invention to maintain a consistent geometric hole shape. However, further advances in tooling hardware, software, and microcontrolling devices in the field of micromachining may soon alleviate this area of concern.  
         [0039]    There also exists another way to solve the same problem(s) addressed by the invention. One other way to solve the same problem is to adjust the depth of focus of the laser drilling system by manually adjusting individual laser system elements (such as the scan lens). However, this technique is time consuming and cumbersome, and could introduce optical alignment problems in the laser drilling system.  
         [0040]    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.