Abstract:
A method for laser machining through micro-holes having desired geometric cross-section requirements in a thin, substantially homogenous material.

Description:
DESCRIPTION OF RELATED APPLICATION 
       [0001]    This application claims priority of Provisional Patent Application Ser. No. 60/915,770 filed May 3, 2007, which is incorporated by reference herein in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention generally relates to the machining or drilling of micro-holes in micro-thin materials. 
       BACKGROUND 
       [0003]    In recent times, technological investment and advancement has been made in laser micro-machining systems to drill micro-holes (referred to as micro-vias) in thin layer materials to establish pathways between printed circuit board layers. In these processes, the hole drilling process is typically followed by a plating process of the micro-vias to establish electrical connection or conductivity between the layers. In this type of drilling, it is typical that the micro-via cross sectional geometry, i.e. the via top and bottom hole diameters and the steepness or taper of the via sidewall, were not specifically required or could vary so long as the electrical performance of the plated vias was not affected. For example, it is typical that the electrical performance of the plated vias are primarily determined by their top and bottom diameters and an important consideration was the roughness of the via wall as relatively large protrusions into the via cavity may interfere with the plating process. Respecting the via geometry, the typical specification requirements for such applications were simply biased toward steeper and smoother via walls. 
       BRIEF SUMMARY 
       [0004]    As the above drilling and machining processes were not intended to and/or are not applicable or useful where the specific geometry of the tapered through hole is required, for example where specific entry and exit hole diameters are required or desired, there exists a need for the establishment of parameters for such machining of through holes and a particular process to drill such holes with required or specified geometries. The ability to drill micro through-holes with specified entry and exit opening sizes, which thereby establish the taper or angle of the sidewall, is particularly useful where the through holes are used for the transmission of light through the thin material. By drilling micro through-holes with controlled sizes of the entry and exit openings in light transmitting applications, the amount of light transmission as well as the divergent light characteristics of the output beam through the exit opening can be controlled and optimized for the particular commercial application. For example, where a micro through-hole sidewall is too steep (approaching a 90 degree angle from a surface on the exit opening side of the material up to the nearest sidewall), there is generally high light transmission through the hole, but relatively poor light divergence requiring a user to be looking directly at the exit hole to see the transmitted light. To the contrary, if the micro-through-hole sidewall is not steep enough (relatively substantially less than a 90 degree angle from a surface on the exit opening side of the material up to the nearest sidewall), this results in greatly decreased light transmissibility through the micro through-hole when the hole is illuminated from the side with the larger hole opening (typically, the entry side). 
         [0005]    The inventive method for laser machining micro-holes having specific cross-sectional geometries through a material is disclosed. The inventive process is particularly useful, although not limited to, single-layer materials with substantially homogeneous composition. An exemplary application is where the thin material and micro holes are used for the transmission of light. 
         [0006]    The method includes establishing the drilling parameters including laser power and laser pulse repetition frequency values that are required to remove material in the material to be machined. Other parameters include at least one tooling trajectory or path which the laser follows in removing material to form the micro-hole as well as the tooling speed rate at which the material is removed along the tooling trajectory. 
         [0007]    In one example, the tooling trajectory includes a three-phase trajectory path including a first trajectory to define the first micro-hole opening, a second trajectory to define the tapered inner wall of the hole and a third trajectory to define a second hole opening opposite the first hole opening. 
         [0008]    In another example, the first laser trajectory includes at least one concentric revolution about a longitudinal axis of the micro-hole. The second trajectory includes an open path of concentric revolutions of decreasing radial length about the longitudinal axis. The third trajectory includes at least one concentric revolution about the longitudinal axis. 
         [0009]    Other applications of the present invention will become apparent to those skilled in the art when the following description is read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein: 
           [0011]      FIG. 1  is an example of a three-phase tooling trajectory to drill an exemplary micro-hole in a thin material; 
           [0012]      FIG. 2  is a graph showing cumulative fluence distribution as a function of position on a work surface resulting from the phase  1  trajectory in  FIG. 1 ; 
           [0013]      FIG. 3  is a graph illustrating cumulative fluence distribution as a function of position on a work surface resulting from the phase  2  trajectory in  FIG. 1 ; 
           [0014]      FIG. 4  is a graph showing the cumulative fluence distribution as a function of position on a work surface resulting from the phase  3  trajectory as shown in  FIG. 1 ; 
           [0015]      FIG. 5  is a compilation graph showing the cumulative fluence distributions as a function of position on a work surface resulting from the individual trajectories shown in  FIGS. 2-4 ; and 
           [0016]      FIG. 6  is an exemplary SEM picture of a micro-hole resulting from the exemplary three phase tooling trajectory shown in  FIGS. 1-5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to  FIGS. 1-6 , a method for drilling or machining micro-holes all of the way through a thin substrate or material, referred to as a through hole, is disclosed. It has generally been determined through experimentation described below that several process parameters are useful and should be considered in drilling through micro-holes to obtain a required or specified hole geometry. 
         [0018]    The first parameters to be considered are with the particular laser used to drill the holes, the power of the laser and the pulse repetition frequency of the laser to drill or machine the micro-holes. As best seen in  FIG. 6 , through experimentation to drill a through micro-hole  1  in an aluminum alloy material  3  having a thickness  15  of 400 micrometers (μm) having a 90 μm diameter entry opening  5  and a 30 μm diameter exit opening  7 , a 532 nanometer (nm) DPSS Nd:YAG laser is suitable. The successful experiment in drilling the above-described hole  1  was carried out with 4.5 watt (W) of average laser power on the above-described material  3  work surface delivered at 10 Kilohertz (Khz). The above successful machining process as described above was carried out at a laser focal plane with an optical topology that yielded a 28 μm focused Gaussian beam spot on the work surface  12 . It is understood that other lasers, laser power and pulse repetition frequencies deviating from the above experiment, known by those skilled in the art, may be used without deviating from the present invention. It is further understood that different materials and thicknesses may be used. 
         [0019]    The next parameter to be considered in the present machining method is the tooling trajectory or path that the laser will follow in drilling the micro-hole  1 . As best seen in  FIGS. 1-5 , it has been determined through experimentation that a three-phase tooling trajectory  20  for drilling through micro-holes is particularly useful in drilling where specified or particular hole geometries are required. The preferred three-phase tooling trajectory includes a first trajectory (or phase  1 )  30 , a second trajectory (or phase  2 )  40  and a third trajectory (or phase  3 )  50 . 
         [0020]    Referring to  FIGS. 1 ,  2  and  6 , a preferred first tooling trajectory  30  of the preferred three-phase tooling trajectory  20  is illustrated. It has been determined through experimentation that a first trajectory  30  across a first surface  12  of material  3  is useful to define the top (entry) diameter or first opening  5  of the micro-hole  1 . In the preferred first tooling trajectory  30 , a closed path circular-shaped trajectory is suitable to define the first opening  5  in the first material surface  12  as best seen in  FIG. 6 . As best seen in  FIG. 1 , a circular-shaped trajectory  30  consisting of at least one complete revolution around longitudinal axis  25  is used. In a preferred example, a plurality of complete circular revolutions  30  about longitudinal axis  25  was sufficient to establish the first opening  5  while also removing some bulk material  3 . 
         [0021]    Through experimentation to drill a 90 μm diameter first opening  5  and a 30 μm diameter exit second opening  7  in 400 μm thick aluminum, using a 28 μm focused Gaussian beam spot, the first trajectory  30  consisted of five circular revolutions around longitudinal axis  25  with a 60 μm diameter at a cutting speed of 73 millimeters per second (mm/sec). 
         [0022]    In the second phase or trajectory  40  of the preferred three-phase tooling trajectory  20 , an open path  40  consisting of concentric circles decreasing in radial distance or length from axis  25  beginning at a point along first trajectory  30  and ending at a point along third trajectory  50  was suitable to drill the required via  1  having inner wall  10 . 
         [0023]    Through experimentation to drill the above-described hole geometry, second trajectory  40  included five open spiral revolutions about longitudinal axis  25  at a radial pitch or radial reduction of 10 millimeters per revolution (mm/rev) starting at the first trajectory 60 μm outer drilling diameter and ending at the 10 μm diameter (third trajectory  50 )  7  at a cutting speed of 73 mm/sec. was suitable. 
         [0024]    In the preferred third tooling phase or trajectory  50 , a closed trajectory path of at least one concentric revolution  50  about the longitudinal axis  25  is used. In a preferred example, numerous closed revolutions in a circular-shaped path about longitudinal axis  25  was used. 
         [0025]    Through experimentation to drill the above described hole  1 , the third trajectory  50  included  5  complete circular-shaped revolutions about longitudinal axis  25  with a 10 μm diameter  7  at a cutting speed of 73 mm/sec. 
         [0026]    Although the disclosed experiment for drilling the above-described hole  1  in material  3  using three separate trajectories in the specific manner described, it is understood that variation in the parameters and trajectories known by those skilled in the art may be used without deviating from the present invention. 
         [0027]    It is further understood that although the preferred three-phase tooling trajectory  20  including first  30 , second  40  and third  50  trajectories are disclosed as separate, interrupted trajectories, the first  30 , second  40  and third  50  trajectories can be seamless or carried out in a continuous uninterrupted process. It is further understood that additional or a fewer amount of tooling phases or trajectories may be used depending on the material  3  and other parameters discussed herein as known by those skilled in the art. It is further understood that the experimental hole  1  described above with the particular geometry shown in  FIG. 6 , is exemplary only and other size and shape holes with associated first opening  5 , second opening  7  and tapered inner wall  10  in material  3  may be made through the inventive process without deviating from the present invention. 
         [0028]    The fourth process parameter to be considered is the tooling speed or rate at which the laser moves along material  3  to remove material  3 . Through experimentation, it was determined that under the experimental parameters described above, 73 mm/sec. was a suitable tooling speed to produce a satisfactory via with the desired geometry. It is understood that other tooling speeds known by those skilled in the art may be used without deviating from the invention. 
         [0029]    The above-described embodiments have been described in order to allow easy understanding of the invention and do not limit the invention. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structure as is permitted under the law.