Patent Publication Number: US-2006000811-A1

Title: Diffractive optical element changer for versatile use in laser manufacturing

Description:
FIELD OF THE INVENTION  
      The present invention relates to a diffractive optical element (DOE) array apparatus and method of using the apparatus in laser processing. More specifically, the present invention relates to a way of interchanging DOEs for use with lasers in manufacturing for versatile tasks such as drilling holes or vias of various sizes and shapes and multiple ablation or material transformation patterns in a surface of an object  
     BACKGROUND OF THE INVENTION  
      There is an ever-increasing demand for smaller and smaller electronic devices in today&#39;s high-tech marketplace. As a result, new and innovative fabrication techniques have become a focal point of many manufacturers. Many manufacturers have turned to laser processing as a means of fabrication, (e.g. for blowing fuses, via and hole drilling, ablation patterning, resistor trimming, material transformation such as curing monomers to polymers, changing refractive index, transmissivity or reflectivity and etc.). However, laser processing systems are very costly and can be inefficient. Manufacturers have sought parallel laser processing methods to increase throughput and to reduce cost. Therefore, there exists a need to use parallel laser processing to increase throughput and to reduce cost in the fabrication of electronic devices.  
      The diffractive optical element (DOE) is one method of employing parallel laser processing for electronic device fabrication. The DOE enables parallel processing by optically diffracting and directly controlling the optical phases. Therefore, a wide range of applications including, for example, multi-spot beam splitters or shapers, can be expected as a result of this preferred benefit. The beam splitting or shaping can be used for drilling holes or vias of various sizes and shapes and multiple ablation or material transformation patterns. Compared to conventional beam splitting methods such as partial mirrors or amplitude masks, the DOE is compact and capable of generating massively parallel processing patterns. Also, unlike an amplitude mask that generates a pattern by blocking most of the incident laser beam, the DOE is very efficient because it is non-absorbing.  
      A system and method of laser drilling is detailed in U.S. Patent Application 20030102291. The &#39;291 patent application describes a method of parallel laser processing with a single DOE. However, the &#39;291 patent application does not address the process of changing the DOE for additional ablation patterns, (e.g., for patterning multi-leveled circuit boards). For example, the &#39;291 patent application and other current systems require that the DOE be changed either manually or robotically in the system. Changing the DOE using current conventional methods, even robotically, can be time-consuming and inefficient and therefore costly. Alternatively, multiple laser processing machines are used, with each machine using a single DOE for a single pattern, and work pieces transferred from machine to machine for multiple patterns to be processed. However, this alternative would be more costly. Therefore, there exists a need to streamline the DOE changing process in DOE laser processing systems for quicker fabrication of electronic devices. Also, in conventional laser processing systems that use DOEs, the hole or pattern density is limited by the density of the pattern on the DOE. With the miniaturization of electronic devices year by year, there further exists a need to pattern or drill holes or cause material transformations in an object with a greater density per square inch than one DOE can provide.  
      A method and apparatus for ablating a desired high-density pattern of vias in a surface of an object can be found in U.S. Pat. No. 6,256,121, entitled “Apparatus for ablating high-density array of vias or indentation in surface of object.” The &#39;121 patent uses an X-axis and Y-axis automatic repeat positioning mechanism for redirecting a laser beam to a desired one holographic imaging lens in an array of holographic imaging lenses to make a via in a surface of the object. The repeat positioning then moves the laser beam to a different holographic imaging lens on the array for drilling another via in a different location on the object. The holographic imaging lenses may vary in application from one to another on the array to form different shapes on the surface of the object, thus, multiple shaped vias or holes can be formed in multiple locations. However, laser processing systems that use automatic repeat positioning mechanisms do not adequately employ parallel processing techniques and are therefore inefficient. Also, the addition of automatic repeat positioning mechanisms in a laser processing system adds undo complexity and cost to the manufacturer. Therefore there exists a need for a laser processing system that drills multiple holes or vias of various sizes and shapes, and further, drills multiple ablation patterns without adding complex, inefficient and costly automatic repeat positioning mechanisms for redirecting a laser beam.  
      It is therefore an object of this invention to use parallel laser processing to increase throughput and to reduce cost in the fabrication of electronic devices.  
      It is another object of this invention to streamline the DOE changing process in DOE laser processing systems for quicker fabrication of electronic devices.  
      It is yet another object of this invention to pattern or drill holes or cause material transformation in an object with a greater density per square inch than one DOE can provide.  
      It is yet another object of this invention to provide a laser processing system that drills holes or vias of various sizes and shapes and multiple ablation or material transformation patterns without using costly and inefficient automatic repeat positioning mechanisms for redirecting a laser beam.  
     SUMMARY OF THE INVENTION  
      In accordance with the present invention, a DOE array apparatus includes a plurality of different interchangeable DOEs for use with lasers in manufacturing for versatile tasks such as drilling holes or vias of various sizes and shapes and multiple ablation or material transformation patterns on the surface or inside an object. A method of using the apparatus in laser processing systems includes: determining a specification for the number of patterns and/or the number of layers to be patterned, designing the appropriate number of DOEs according to the product specification, assembling the DOEs into an array to be used in a laser processing system, ablating the layer on the object through laser processing, determining whether more patterns on the layer are to be processed, determining whether more layers are to be patterned, and changing and aligning the DOE for the next laser ablation pattern to be processed.  
      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  
      The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
       FIG. 1  illustrates a subassembly of a laser processing system;  
       FIG. 2A  illustrates a perspective view of DOE linear array  140 , a preferred embodiment of a subassembly of a laser processing system;  
       FIG. 2B  illustrates a top view of DOE linear array  140 , a preferred embodiment of a subassembly of a laser processing system;  
       FIG. 2C  illustrates a side view of DOE linear array  140 , a preferred embodiment of a subassembly of a laser processing system;  
       FIG. 3A  illustrates a top view of DOE rectangular array  300 , an alternate preferred embodiment of a subassembly of a laser processing system;  
       FIG. 3B  illustrates a side view of DOE rectangular array  300 , an alternate preferred embodiment of a subassembly of a laser processing system;  
       FIG. 4A  illustrates the top view of DOE wheel array  400 , an alternate embodiment of a subassembly of a laser processing system;  
       FIG. 4B  illustrates the cross-sectional view A-A′ of DOE wheel array  400 , an alternate embodiment of a subassembly of a laser processing system; and  
       FIG. 5  illustrates a functional block diagram method of operating a subassembly of a laser processing system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      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.  
      The present invention relates to a diffractive optical element (DOE) array apparatus and method of using the apparatus in laser processing systems. More specifically, the present invention relates to a way of interchanging DOEs for use with lasers in manufacturing for versatile tasks such as drilling holes or vias of various sizes and shapes and multiple ablation or material transformation patterns in a surface of an object.  
       FIG. 1  illustrates a subassembly  100  of a laser processing system for drilling holes or vias of various sizes and shapes and multiple ablation or material transformation patterns in a surface of an object including a beam  110 , a plurality of DOEs  130   a ,  130   b ,  130   c ,  130   d ,  130   e ,  130   f ,  130   g ,  130   h  and  130   i  (note that subassembly  100  may contain any number of DOEs and is not limited to nine), a linear DOE array  140 , a DOE array holder  145 , a plurality of sub-beams  150 , a scan lens  160 , and a workpiece  170  arranged as shown.  
      A pulsed or continuous wave (CW) laser (the laser must exhibit a sufficiently small bandwidth to avoid chromatic aberrations induced by the DOE) provides sufficient pulse energy or average power to ablate or transform material in workpiece  170 . In one example, the laser may be a picosecond (ps) laser (bandwidth less than 0.1 nanometer) consisting of an oscillator and a regenerative amplifier, the oscillator output power equals 35 milliwatts (mW), the pulse width is approximately 15 ps, the regenerative amplifier output power is 1 Watt (W) at 1 killohertz (kHz) the energy per pulse is 1 millijoule (mJ), the power stability is 1.0% over 12 hours and the pointing stability is approximately 1%.  
      Linear DOE array  140  holds a plurality of DOEs  130   a ,  130   b ,  130   c ,  130   d ,  130   e ,  130   f ,  130   g ,  130   h  and  130   i . A DOE is an optical element that acts as a beam splitter or shaper to allow a laser processing system to drill parallel holes or vias of various sizes and shapes and multiple ablation or material transformation patterns on a material on workpiece  170 .  
      DOE array holder  145  is holds the DOE array  140  and is used to index the array through subassembly  100  in linear steps, one DOE per step.  
      Beam  110  is a laser beam, for example, from a ps laser. Sub-beams  150  are formed by beam  110  being transmitted through DOE  130   a.    
      Scan lens  160  is an f-theta telecentric (scan) lens. Scan lens  160  determines the spot size of sub-beams  150  upon workpiece  170 . The beam size that enters scan lens  160  must be less than or equal to the pupil size of scan lens  160 . Telecentricity is required to keep the incident angle between sub-beams  140  and workpiece  170  perpendicular, which is necessary to drill parallel holes in workpiece  170 . In an alternate embodiment, a non-telecentric lens is used to drill angled holes, if parallel holes in the work piece are not required.  
      Workpiece  170  is the target for subassembly  100 . In one example, workpiece  170  is a stainless steel inkjet nozzle foil; however, the present invention may be generalized to a variety of workpiece materials, such as polymers, semiconductor metals, or ceramics. In alternate embodiments, subassembly  100  can drill holes or cause material transformations of a wide variety of shapes and tapers in workpiece  170 .  
      A brief description of the operation of subassembly  100  is provided below. In alternate embodiments, changes in the elements of subassembly  100  may be required. The present invention is not limited to the current selection and arrangement of elements in subassembly  100 .  
      In operation, beam  110  is emitted from a laser source; for example, a ps laser (not shown) propagates along the optical path identified in  FIG. 1 . DOE  130   a  splits beam  110  into sub-beams  150 . Sub-beams  150  exit DOE  130   a  and propagate along the optical path, where they are incident upon scan lens  160 . Scan lens  160  determines the spot size of sub-beams  150  upon workpiece  170 . Sub-beams  150  exit scan lens  160  and propagate along the optical path, where they are focused onto workpiece  170 . Sub-beams  150  ablate or transform workpiece  170 . Alternate holes, vias or patterns may be ablated or transformed with a different DOE  130   b, c, d, e, f, g, h , or  i  on the linear DOE array  140 .  
       FIGS. 2A, 2B  and  2 C illustrate DOE linear array  140  including DOE base  210  holding a plurality of DOEs  130   a ,  130   b ,  130   c ,  130   d ,  130   e ,  130   f ,  130   g ,  130   h  and  130   i  and DOE array holder  145  holding DOE array  140 .  
       FIG. 2A  illustrates a perspective view of DOE linear array  140 .  FIG. 2B  illustrates a top view of DOE linear array  140 .  FIG. 2C  illustrates a side view of DOE linear array  140 .  
      DOE linear array  140  is one of the preferred array embodiments of subassembly  100 . There may be any number of DOEs  130  held on DOE base  210  according to the product specifications. In one example the DOE base  210  is an aluminum (Al) plate with openings. The openings are counter-bored so the clear aperture is smaller than the size of the DOE. The DOEs are attached to the DOE base  210  by mechanical means or by an adhesive. The DOE array holder  145  holds DOE base  210  and is used to index the array through subassembly  100  in linear steps, one DOE per step. DOE  130   a  then  130   b  then  130   c  and so forth are sequenced through subassembly  100  one at a time in the X axis direction by indexing or mechanical stage mechanism as is in common use in optical assemblies. DOEs are often an etching on a glass substrate or alternatively could be a spun coat resist that has been made using a laser beam writer. DOEs  130   a ,  130   b ,  130   c ,  130   d ,  130   e ,  130   f ,  130   g ,  130   h  and  130   i  may have separate etchings for ablation or material transformation. However, in some cases it may be preferable to duplicate DOEs in different locations on the subassembly in order to reduce changeover time when swapping patterns.  
       FIGS. 3A and 3B  illustrate DOE rectangular array  300  including DOE base  310  holding a plurality of DOEs  130   a ,  130   b ,  130   c ,  130   d ,  130   e ,  130   f ,  130   g ,  130   h  and  130   i  and DOE array holder  345  holding DOE base  310 . DOE square array  300  is an alternate embodiment of subassembly  100  to replace DOE linear array  140 . DOE rectangular array  300  has equal preference as an embodiment to that of DOE rectangular array  140 . However, the two dimensional rectangular array  300  will require different translation than the one dimensional DOE linear array  140  when the DOEs are sequenced through subassembly  100 .  
       FIG. 3A  illustrates a top view of DOE rectangular array  300 .  FIG. 3B  illustrates a side view of DOE rectangular array  300 .  
      There may be any number of DOEs  130  held on DOE base  310  according to the product specifications. In one example the DOE base  310  is an aluminum (Al) plate with openings. The openings are counter-bored so the clear aperture is smaller than the size of the DOE. The DOEs are attached to the DOE base  310  by mechanical means or by an adhesive. The DOE array holder  345  holds DOE base  310  and is used to index the array through subassembly  100  in steps, one DOE per step. DOE  130   a  then  130   b  then  130   c  and so forth are sequenced through subassembly  100  one at a time in the X and Y axis directions by indexing or mechanical stage mechanism as is in common use in optical assemblies. DOEs are often an etching on a glass substrate or alternatively could be a spun coat resist that has been made using a laser beam writer. DOEs  130   a ,  130   b ,  130   c ,  130   d ,  130   e ,  130   f ,  130   g ,  130   h  and  130   i  all have separate etchings for ablation or material transformation.  
       FIG. 4A  and  FIG. 4B  illustrate DOE wheel array  400  including DOE base  410  holding a plurality of DOEs  130   a ,  130   b ,  130   c ,  130   d ,  130   e ,  130   f ,  130   g ,  130   h  and  130   i . DOE wheel array  300  is an alternate embodiment of subassembly  100  to replace DOE rectangular array  140 . There may be any number of DOEs  130  held on DOE base  410  according to the product specifications. In one example the DOE base  410  is an aluminum (Al) plate with openings. The openings are counter-bored so the clear aperture is smaller than the size of the DOE. The DOEs are attached to the DOE base  410  by mechanical means or by an adhesive. The DOE array holder  445  holds DOE base  410  and is used to index the array through subassembly  100  in steps, one DOE per step. DOE  130   a  then  130   b  then  130   c  and so forth are sequenced through subassembly  100  one at a time in theta (θ) rotational axis direction by indexing or mechanical stage mechanism as is in common use in optical assemblies. DOEs are often an etching on a glass substrate or alternatively could be a spun coat resist that has been made using a laser beam writer. DOEs  130   a ,  130   b ,  130   c ,  130   d ,  130   e ,  130   f ,  130   g ,  130   h  and  130   i  all have separate etchings for ablation or material transformation.  
       FIG. 4A  illustrates the top view of DOE wheel array  400 .  FIG. 4B  illustrates the cross-sectional view A-A′ of DOE wheel array  400 .  
      It should be understood by those familiar with DOEs as beam splitters that DOEs are translationally invariant and not rotationally invariant. These characteristics of DOEs indicate that the arrangements in  FIG. 2  or  FIG. 3  (DOE linear array  140  or DOE rectangular array  300 ) are preferable to DOE wheel array  400 . This is in direct contradiction to the teaching of U.S. Pat. No. 6,452,132 “Laser Hole Boring Apparatus”. In the embodiment shown in the 132 patent, the problem of the DOE wheel is exacerbated by the use of circular mounting holes and no measures are advised for dealing with this problem. The rectangular array  140  or square array  150  embodiments tolerate DOEs that are slightly misaligned with each other on subassembly  100  without causing any manufacturing defects when subassembly  100  is employed. But if DOE wheel array  400  is used as an alternate embodiment then due diligence must be adhered to, because DOEs do not allow for any misalignment rotationally.  
       FIG. 5  illustrates a functional block diagram method  500  of operating subassembly  100  for drilling holes or vias of various sizes and shapes and multiple ablation or material transformation patterns in a surface of an object and includes the steps of:  
      Step  510 : Determining product&#39;s number of patterns/layers specification  
      In this step, the number of patterns and the number of layers to pattern according to the product specifications is determined. Also, the sequence of patterns to be laid on the workpiece or sequence of layers to be processed is determined in this step. Method  500  proceeds to step  520 .  
      Step  520 : Designing and manufacturing multiple DOEs to match product specification  
      In this step, DOEs are designed to match product specifications defined in step  510 . For example, designing and manufacturing is done using known methods of DOE fabrication. Each new pattern requires a separate DOE to be designed and manufactured. Method  500  proceeds to step  530 .  
      Step  530 : Assembling DOE array  
      In this step, the DOEs are assembled into an array of DOEs, for example, DOE linear array  140 , DOE rectangular array  300 , or DOE wheel array  400 . The support for each of DOE linear array  140 ,. DOE rectangular array  300 , or DOE wheel array  400  may be made of a glass substrate, or structured as a mechanical mount that allows DOEs to be accurately placed on the substrate. Each DOE is indexed by its position in the DOE array. While not required, it is desirable to have the DOE array index correspond to the pattern or layer sequence determined in Step  510 . Method  500  proceeds to step  540 .  
      Step  540 : Processing pattern  
      In this step, the pattern on workpiece  170  is created, for example with a milling algorithm (not shown) employed by a laser processing system, which includes subassembly  100 .  
      For example, greater hole density on one layer may be achieved by patterning with DOE  130   a  and then DOE  130   b  (after steps  560  and  570 ) and so forth if desired. Thus, a streamlined, non-complex, parallel laser processing system able to drill denser holes than conventional means is achieved.  
      Method  500  proceeds to step  550 .  
      Step  550 : More patterns on layer? 
      In this decision step, it is determined if the layer just ablated or transformed in step  540  needs additional patterns to be processed. If yes, then method  500  proceeds to step  570 . If no, then method  500  proceeds to step  560 .  
      Step  560 : Next layer? 
      In this decision step, it is determined if there are more layers to be patterned on workpiece  170 . If yes, then method  500  proceeds to step  570 . If no, then method  500  ends.  
      Step  570 : Changing and aligning DOE  
      In this step, the next DOE  130  is sequenced for use on subassembly  100  from the DOE linear array  140  or optionally DOE rectangular array  300  or optionally DOE wheel array  400 . For example, DOE  130   b  would be used after DOE  130   a . A simple changing and aligning mechanism is employed with stops, for example, a gas-driven actuator with fixed index points. Due diligence must be adhered to such that no rotational misalignment occurs in between each DOE change and alignment since DOEs are translationally invariant and not rotationally invariant. DOE linear array  140  and DOE square array  300  are the preferred embodiments, but DOE wheel array  400  may be used as alternate embodiments in subassembly  100 .  
      The ability to change DOE  130  from one pattern to another streamlines the laser drilling process and allows for increased speed and throughput in a manufacturing process.  
      Method  500  proceeds to step  540  for further processing.  
      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.