Abstract:
A new method and apparatus for moving an excimer laser beam relative to a workpiece to control the wall profile of laser machined features, such as holes and grooves. An excimer laser beam is displaced relative to a workpiece in a substantially circular motion and the substantially circular motion is further displaced relative to the workpiece to correspond to a desired shape.

Description:
FIELD OF INVENTION 
     The present invention relates generally to a method and apparatus for controlling the wall profile of laser machined features, such as holes and grooves. More specifically, it relates to a system for moving a laser beam relative to a workpiece to machine the walls of the machined features therein to achieve a desired profile. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of certain products, features, such as holes and grooves, are produced in workpieces by ablating the workpiece material with a laser beam, e.g., Gugger U.S. Pat. No. 3,576,965. When a laser, such as an excimer laser, ablates a hole or groove, it exhibits a small “natural” taper. A taper of 5-9 degrees half angle is typical for laser machined holes in many plastics such as polyamide and polyester. This natural taper varies slowly with laser beam energy density, wavelength and numerical aperture. However, these parameters are difficult to control. Moreover, where a larger taper angle is required, it is necessary to use a different approach. 
     One approach involves controlling (a) the angle of the laser beam striking the workpiece relative to the axis of a hole being machined and (b) the distance between the intersection of the beam with the surface of the workpiece and the axis of the hole. Corfe et al. U.S. Pat. No. 5,043,553. This approach is, however, difficult to implement because of its inherent complexity and, for an excimer laser, because of the difficulty in focusing the laser across the image field. 
     Other approaches attempt to control wall taper angle by controlling the fluence used in a laser beam or by refocusing a projection lens. These approaches have only limited success. See Sheets U.S. Pat. No. 4,940,881. 
     Finally, another approach involves displacing a laser beam relative to a workpiece. One way to do this involves interposing a thin refractive disc in the laser beam and rotating the refractive disc not about its axis but instead about the optical axis or an axis parallel to it, a so-called “wobble plate.” In a preferred embodiment, the refractive disc has parallel faces and is inclined at a small angle to a reference plane perpendicular to the optical axis. In another embodiment, the faces of the refractive element are at a small angle to each other. Sheets U.S. Pat. No. 4,940,881. In the preferred embodiment, the taper angle of the wall is dependent on the angle at which the refractive element is inclined. It has, however, proven time consuming to change that angle in practice, and the required apparatus for rotating the refractive disc is relatively large and complicated. 
     It is an object of the present invention to move a laser beam relative to a workpiece to machine the taper angle of the walls of laser machined features therein, such as holes and grooves,at specified angles, which angles may be readily and remotely changed. It is also an object of the present invention to move a laser beam relative to a workpiece to machine the walls of the machined features therein to achieve other desired profiles. 
     SUMMARY OF THE INVENTION 
     A new method and apparatus for moving an excimer laser beam relative to a workpiece to control the wall profile of laser machined features, such as holes and grooves. In a preferred embodiment, an excimer laser beam is displaced relative to a workpiece in a substantially circular motion and the substantially circular motion is further displaced relative to the workpiece to correspond to a desired shape. 
     The foregoing and other objects, features and advantages of the current invention will be apparent from the following detailed description of preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     IN THE DRAWINGS 
     FIG. 1 is a schematic of a preferred embodiment of the present invention. 
     FIG. 2 illustrates different side profiles of the walls of a machined hole. 
     FIG. 3 illustrates a preferred distribution of laser pulses for machining a hole with a wall taper angle resulting from a relative circular motion of a laser beam and a workpiece. 
     FIG. 4 is an elevated view of a groove machined with a preferred embodiment. 
     FIG. 5 illustrates a distribution of laser pulses for machining a groove with a tapered wall angle resulting from a reciprocal motion of a laser beam and a workpiece. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Excimer lasers are used to micromachine workpieces of various materials. Each pulse of an excimer laser removes only a very thin layer of material of the order of 0.1-0.2 microns. It is possible therefore to sculpt a desired cross section shape of the features to be micromachined into the material by displacing the laser beam relative to the workpiece and controlling the number of pulses applied in a given location. For example, if the center of a hole to be drilled in the workpiece is exposed to more pulses then the outer edge of the hole, the profile of the resulting hole will be tapered. 
     FIG. 1 is a schematic of a preferred embodiment of the present invention. An excimer laser  10  projects a laser beam  12  on to and through a mask  14 , which contains a pattern of holes or openings. The mask  14  may, in another embodiment, be mounted on a motorized rotary or linear translation stage so that the pattern to be machined may be selected from among a series of patterns in the mask. 
     The laser beam continues along optical path  16  on to and through a first flat transparent refractive plate  18 . The first refractive plate  18  is disposed perpendicularly to the optical path  16 . The first refractive plate  18  is preferably formed of a material minimizing absorption of ultraviolet light from the laser beam  12 , preferably having a thickness in the range of 0.5-6.0 mm. The first refractive plate  18  is mounted on a first scanner  20  that can rotate the first refractive plate  18  about a first axis  22  perpendicular to the optical path  16 . In a preferred embodiment, the scanner is a galvanometer. 
     Rotating the first refractive plate  18  about the first axis  22  causes the laser beam to be displaced in a linear direction perpendicular to the first axis  22 . 
     The laser beam continues along optical path  16  onto and through a second flat transparent refractive plate  24 . The second refractive plate  24  is disposed perpendicularly to the optical path  16  and is made of similar material and a similar shape as the first refractive plate  18 . The second refractive plate  24  is mounted on a second scanner  26  that can rotate the second refractive plate  24  about a second axis  28  perpendicular to the optical path  16  and at an angle, preferably perpendicular to the first axis  22 . In a preferred embodiment, the second scanner is also a galvanometer. Rotating the second refractive plate  24  about the second axis  28  causes the laser beam to be displaced in a linear direction perpendicular to the second axis  28 . 
     The laser beam continues along optical path  16  onto and through a projection lens  30  that sharply focuses the mask image on a workpiece  32 . The laser beam is deflected by the rotation of the refractive plates. For small scan angles, the deflection on the workpiece is given by:        Δ   =     M   ·       (     n   -   1     )     n     ·   d   ·     θ   i                              
     where M is the demagnification of the projection lens, n and d are the index of refraction and thickness respectively of the refractive plate and θ, is the scan angle. 
     The workpiece  32  is mounted on a worktable  34 . In a preferred embodiment, the work table comprises two motorized stages for linear movement in two substantially orthogonal directions. The work table  34  is connected to a computer  40 . The computer  40  can control the linear motion of the work table  34  and workpiece  32  in two directions, which directions are perpendicular to the optical axis, and at an angle, preferably perpendicular, to each other. 
     The first scanner  20  is connected to a first driver  50 , and the second scanner  26  is connected to a second driver  56 . In a preferred embodiment, the drivers are analog servo amplifiers for driving the scanners. The first driver  50  and second driver  56  are connected to a controller  58 . The controller  58  takes digital input from a computer, for example a scan pattern, and converts it to analog voltage output to activate the first driver  50  and the second driver  56 . The controller is, in turn, connected to a computer  40 . Thus, scan patterns can be entered in the computer  40 . The computer  40  can cause the displacement of the laser beam  12  in relation to the workpiece  32  in two linear directions, which are at an angle to each other and orthogonal to the optical axis  16 . The computer can cause the displacement by either one of two techniques or by a combination of the two techniques. The first technique involves the displacement of the laser beam  12  through the use of the first scanner  18  and the second scanner  24  as described in detail above. The second technique involves the displacement of the workpiece  32  on the work table  34 , also as described in detail above. 
     As an example of the use of a preferred embodiment of the present invention, as is shown in FIG. 2, one may desire to drill a hole through a workpiece  32  with an entrance diameter D i  ( 60 ), and an exit diameter of D o  ( 62 ), thereby causing the taper angle of the walls of the hole A ( 64 ) to be fixed. Referring to FIG. 3, in order to drill such a hole, the laser beam is imaged on the workpiece into a disc of diameter D x  ( 70 ), which image is slightly smaller than the entrance hole diameter D i  ( 60 ) of the hole that is to be machined but larger than the exit hole diameter D o  ( 62 ) of the hole. A relative circular motion is induced between the laser beam and the workpiece such that the center of the laser beam traces a circle of diameter D R  ( 72 ). The relative circular motion is induced by displacing the laser beam according to the present invention or displacing the workpiece according to the present invention. 
     FIG. 3 illustrates how the laser pulses are distributed on the workpiece with this relative circular motion. The center area is exposed by many more pulses than the outer region. This results in a differential exposure rate along the radius of the hole, resulting in the desired wall taper shown in FIG.  2 . The wall taper angle is controlled by controlling the diameter of the laser beam image D x  ( 70 ) and the diameter of the circular motion D r  ( 72 ) of the center of the laser beam. In first approximation, we have: 
     
       
         
           Di=Dx+Dr 
         
       
     
     
       
         
           Do=Dx−Dr 
         
       
     
     The total exposure time must be properly adjusted. The inner area of the hole is exposed longer than the outer area of the hole. The laser exposure is stopped when the inner area of the hole is drilled through the desired depth. This depth is the thickness of the material for through holes, but it can be smaller than the thickness of the material for a blind hole. For a through hole, the exposure time must be controlled tightly such that the inner region just drills through the thickness of the material. If the exposure is increased much beyond this point, the taper angle becomes steeper and the exit hole diameter Do increases. 
     The present invention may also be used to achieve more complex side-wall profiles. Displacement of the laser beam by the first and second scanners and displacement of the workpiece by the worktable are fully programmable. Also, the mask pattern to be used may be changed to achieve more complex side-wall profiles. For example, the “funnel” shape profile of the hole in FIG. 2b can be achieved by first using the preferred embodiment as described above to produce the wall angle taper of the wider part of the funnel to a given depth t ( 63 ). The diameter D r  ( 72 ), as shown in FIG. 3, of the circle traced by the center of the laser beam is then decreased, preferably to zero, to produce the parallel wall profile of the narrower part of the funnel. Alternatively, a smaller diameter pattern in the mask can be chosen. The funnel profile is of interest for many microfluidic applications. 
     The side wall profiles of other features can also be machined with a similar approach. Referring to FIG. 4, one can control the taper angle of a laser machined groove using the present invention. To produce the groove  78  shown in FIG. 4, with top width W t  ( 80 ) and bottom width W b  ( 82 ), the laser beam is imaged on the workpiece into a disc of diameter D x  ( 90 ), the diameter of which image is slightly smaller than the top width W t  ( 80 ) that is to be machined but larger than the bottom width W b  ( 82 ). A relative circular motion is induced between the laser beam and the workpiece such that the center of the laser beam traces a circle of diameter D r  ( 92 ). The relative circular motion is induced by displacing the laser beam according to the present invention. The circular motion of the laser beam is then moved linearly along the direction of the groove ( 78 ) to be machined. 
     This results in the groove shown in FIG. 4 with the wall taper angle controlled by controlling the diameter of the laser beam image D x  ( 90 ) and the diameter of the circular motion D r  of the center of the laser beam such that: 
     
       
         
           W 
           i 
           =Dx+Dr 
         
       
     
     
       
         
           W 
           o 
           =Dx−Dr 
         
       
     
     An example of the use of another preferred embodiment of the present invention is shown in FIG.  5 . To produce the groove  98  shown in FIG. 5, with top width W t  ( 100 ) and bottom width W b  ( 102 ) the laser beam is imaged on the workpiece into a disc of diameter D x  ( 110 ), the diameter of which is slightly smaller than the top width W t ( 100 ). A relative linear motion is induced between the laser beam and the workpiece, according to the present invention, such that the laser beam traces a linear path  120  transverse to the direction of the groove  98  to be machined. After the laser beam has traversed the top W t ( 100 ) of the groove it is displaced relative to the workpiece, according to the present invention, to trace a linear path along the direction of the groove  98 . After the displacement along the direction of the groove, another linear motion is induced between the laser beam and the workpiece such that the laser beam traces a linear path  122  transverse to the direction of the groove. The total exposure time must be properly adjusted at various points on the traces transverse to the groove to achieve the desired depth and wall taper angle for the groove. 
     In the preferred embodiments discussed above, with (i) two refractive plates mounted on galvanometric scanners or (ii) a movable worktable was used to produce relative displacement of the laser beam and the workpiece in two directions perpendicular to the optical path. Other techniques may be used to produce this relative displacement. For example, two mirrors mounted on galvanometric scanners or on gimbal mounts activated by linear activators such as piezoelectric activators may be used. Alternatively, the mask can be moved by a translation stage to move the mask pattern. 
     The present invention has been particularly shown and described above with reference to various professional embodiments, implementations and applications. The invention is not limited, however, to the embodiments, implementations or applications described above, and modification thereto may be made within the scope of the invention.