Abstract:
The present invention relates to optimizing optical power from a laser processing system, and more specifically to a method of optimizing the optical power by employing parallel laser processing techniques to maximize fabrication quality and yield. The method in the present invention includes the steps of: determining the specification for the final product, selecting the proper combination of optical power and processing method for processing a single feature, determining the maximum number of features in pattern to be parallel processed, selecting a portion of the desired pattern that can be parallel processed, designing the DOEs, manufacturing the DOEs, incorporating the DOE into laser processing system, operating and controlling the laser processing system, and determining if more DOEs are needed to complete the laser processing.

Description:
FIELD OF THE INVENTION 
   The present invention relates to optimizing the use of optical power in laser processing systems, and more specifically to a method of optimizing the use of optical power in parallel processing systems to maximize fabrication quality and yield. 
   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 or material transformation patterning, or resistor trimming). However, most laser processing systems are very costly and inefficient. For example, single feature laser processing systems process one feature (i.e. pattern, hole or via) through ablative, additive, or transformational means at a time and are therefore incapable of efficiently operating in large volume manufacturing environments. Many manufacturers have sought means to reduce cost by increasing yield. However, increasing yield often requires higher optical power from the laser. Increasing optical power reduces the processing time each feature thus yield is increased, but this increase in optical power often has a negative effect of lowering the quality in the fabricated devices due to overexposure. Therefore there exists a need to reduce cost by increasing manufacturing yield without sacrificing manufacturing quality. Likewise, there exists a need to increase manufacturing quality without sacrificing manufacturing yield. 
   One way that manufacturers have sought to increase manufacturing yield and reduce cost is through parallel processing, where a energy beam is split in order to process more than one feature at a time. Diffractive optical elements (DOEs) are often employed in parallel laser processing systems because they are capable of providing highly efficient and highly uniform beam splitting. Unlike conventional optical components that utilize refraction and/or reflection, DOEs enable parallel processing by optically diffracting and directly controlling the optical phases of the beam. Therefore, a wide range of applications including, for example, multi-spot beam splitters or shapers, can be expected. 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. 
   A method of patterning holes in a surface of an object through the use of parallel processing with a DOE can be found in U.S. Pat. No. 6,635,849, entitled “Laser beam machine for micro-hole machining.” The &#39;849 patent details a process of beam splitting to increase manufacturing yield. To ensure that manufacturing quality is also met, the &#39;849 patent details a process where masks control the optical power and the beam diameter from over-exposing the object under process. However, masks (e.g. aperture, imaging or pinhole masks allow only a small portion of the laser&#39;s beam to be used, wasting 20% to 80% of the optical power. Thus, the highest potential yield is not realized and manufacturing cost is increased. Therefore there exists a need to more economically use the optical power supplied by laser processing systems. 
   It is an object of the invention to reduce cost by increasing manufacturing yield without sacrificing manufacturing quality. 
   It is another object of this invention to increase manufacturing quality without sacrificing manufacturing yield. 
   It is yet another object of this invention to more economically use the optical power supplied by laser processing systems. 
   SUMMARY OF THE INVENTION 
   The present invention seeks to optimize the use of laser power to balance the two key competing aspects of laser processing: process quality and manufacturing throughput. This optimization is achieved first through making reference to the specification of the end product which specification includes parameters that define quality and cost. The first step is to find the optimum laser processing parameters for a single feature that is repeated multiple times on the desired product. Based on these parameters and the available laser power, the maximum number of features that can be processed in parallel is determined. At this point it can be ascertained if the entire process can be completed with one DOE or if the process needs to be broken down into sub-processing steps with multiple DOEs being designed and fabricated for use on each part of the end product. After the DOEs are fabricated, the laser system is controlled and operated in order to fabricate the product. 
   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 laser processing system; and 
       FIG. 2  illustrates a functional block diagram method of optimizing the use of optical power in parallel processing systems to maximize fabrication quality and yield. 
   

   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 includes a system and method for optimizing the use of optical power during parallel processing with a laser processing system. 
     FIG. 1  illustrates a laser processing system  100 , including the elements of: a laser  110 , a computer  112 , a beam  115 , a first mirror  120 , a shutter  125 , an attenuator  130 , a second mirror  135 , a beam expander  140 , a spinning half-wave plate  155 , a DOE  165 , a plurality of sub-beams  170 , a scan lens  175 , a workpiece  180 , and a workpiece holder  185 , arranged as shown. 
   Laser  110  provides sufficient pulse energy or average power to ablate or transform material in workpiece  180 . In one example, laser  110  is a picosecond (ps) laser (bandwidth less than 0.1 nanometer (nm)) 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 kilohertz (kHz) the energy per pulse is 1 millijoule (mJ), the power stability is 1.7% over 12 hours and the pointing stability is approximately 1%. 
   Beam  115  is emitted by laser  110 . 
   First mirror  120  and second mirror  135  are conventional mirrors used to direct or steer beam  115  along a specified path. Please note that the actual number of mirrors used to steer beam  115  may vary, depending the specific layout of the optical path of the drilling system. 
   Shutter  125  is a conventional mechanical shutter, like those made by Vincent Associates (e.g., model # LS6ZMZ). The purpose of shutter  125  is to allow beam  115  to illuminate the workpiece  180  when shutter  125  is in the open state, and to prevent beam  115  from illuminating workpiece  180  when shutter  125  is in the closed state. 
   Computer  112  is a computing means, like a personal computer which minimally includes: conventional input devices (e.g., keyboard, mouse); output devices (e.g., monitor, printer, disk, etc); communication means (e.g., network card, serial ports); an operating system (e.g., Microsoft Windows, Linux); and software to convert product specifications into instructions for elements within laser processing system  100 . As shown in  FIG. 1 , computer  112  has communication links to shutter  125 , attenuator  130 , and scan lens  170 . Computer  112  is required to coordinate the movements of these elements when processing complex features (like shaped holes) in workpiece  180 . In this example, computer  112  contains software applications capable of converting product, laser, and material specifications into processing algorithms required for laser processing system  100  to produce products that meet specifications. Computer  112  may be communicating only with shutter  125  in examples where laser processing system  100  is processing simple features (like straight holes) in workpiece  180 . Computer  112  has access to lookup tables that contain historical data from various combinations of lasers, workpiece materials, and processing methods. 
   Attenuator  130  is a filter that continuously controls the energy outside laser  110 . Attenuator  130 , as shown in  FIG. 1 , includes a half-wave plate, such as those manufactured by CVI Laser (e.g., model # QWPO-1053-06-2-R10), followed by a polarizer, such as one manufactured by CVI (e.g., model # CPAS-10.0-670-1064). 
   Beam expander  140  is used in the present invention to match the spot size of beam  115  to the pupil size of scan lens  175 . The specifications of beam expander  140  are selected in coordination with the specifications of beam size of laser  110  and scan lens  175 . The laser beam size from beam expander  140  should be the same size or slightly smaller than the pupil size of scan lens  175 . One example of a beam expander is made of a pair of negative and positive lenses, with a focal length of −24.9 millimeters (mm) for the negative lens, and 143.2 mm for the positive lens. 
   Spinning half-wave plate  155  changes the polarization of beam  115  to increase the smoothness of the features in workpiece  180 . In one example where laser processing system  100  is drilling tapered holes in workpiece  180 , such a change in polarization decreases rippling on the walls of the hole. In one embodiment, spinning half-wave plate  155  is a half-wave plate, such as those made by CVI Laser (e.g., model # QWPO-1053-06-2-R10), spinning at 600 revolutions per minute (RPM) driven by an electric motor. 
   DOE  165  is a highly efficient beam shaper or beam splitter and beam array pattern generator that allows laser processing system  100  to process features in workpiece  180  either singly or in parallel. In an alternate embodiment, DOE  165  is part of a DOE changer (not shown) that contains more than one DOE and provides a fast and simple way of changing the DOE used in laser processing system  100 . 
   The pattern of shaped beam or sub-beams  170  output by DOE  165  is pre-determined by the product specifications. In one example, the DOE splits beam  115  into  152  beams in the forms of 4 rows with 38 beams in each row. In another example, the DOE shapes the beam into a rectangle. 
   Scan lens  175  is preferably an f-theta (f-θ) telecentric (scan) lens. Scan lens  175  determines the spot size of shaped beam or sub-beams  170  upon workpiece  180 . The size of the shaped beam or sub-beams  170  as they enter scan lens  175  must be less than or equal to the pupil size of scan lens  175 . Telecentricity is required to keep the incident angle between shaped beam or sub-beams  170  and workpiece  180  perpendicular, which is necessary to parallel process features in workpiece  180 . In alternate embodiments where the axes of the features do not need to be parallel to each other, a non-telecentric scan lens can be used. 
   Workpiece  180  is the target of laser processing system  100 . In one example, workpiece  180  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, laser processing system  100  can process features of a wide variety of shapes and tapers in workpiece  180 . 
   Workpiece holder  185  is used in a laser drilling system to support workpiece  180  during laser drilling. Workpiece holder  185  is round, but in practice, workpiece holders could be any of a variety of shapes, including triangles, squares, rectangles, pentagons, etc. Workpiece holder  185  is made of a hard, durable, stiff, and heat-resistant material (e.g., steel, aluminum, machinable ceramic, etc.). Workpiece holder  185  is generally attached to the stage in a laser drilling system with nuts and bolts or other similar attachment means. In one example, workpiece holder  185  is attached to a fixed stage. In another example, workpiece holder  185  is attached to a X-Y axis moveable stage. This stage may be moved during laser processing to create linear or circularly symmetric features. In the case of multiple sub-beams each of these features will have a fixed correspondence to each other depending on the geometry of the sub-beams generated by the DOE. 
   In operation, laser  110  emits beam  115  along the optical path identified in  FIG. 1  above. Beam  115  propagates along the optical path, where it is incident upon first mirror  120 . First mirror  120  redirects beam  115  along the optical path, where it is incident upon shutter  125 . Computer  112  sends a signal to shutter  125  to open the shutter and illuminate workpiece  180 . Beam  115  exits shutter  125  and propagates along the optical path to attenuator  130 . Attenuator  130  filters the energy of laser  110  in order to precisely control ablation or material transformation parameters Beam  115  exits attenuator  130  and propagates along the optical path, where it is incident upon second mirror  135 . Second mirror  135  redirects beam  115  along the optical path, where it is incident upon beam expander  140 . 
   Beam expander  140  increases the size of beam  115 . Beam  115  exits beam expander  140  and propagates along the optical path, where it is incident upon spinning half-wave plate  155 . Spinning half-wave plate  155  changes the polarization of beam  115 . Upon exiting spinning half-wave plate  155 , beam  115  propagates along the optical path, where it is incident upon DOE  165 . 
   DOE  165  splits beam  115  into a plurality of sub-beams  170 , which allow parallel processing of workpiece  180 . Sub-beams  170  exit DOE  165  and propagate along the optical path, where they are incident upon scan lens  175 . Scan lens  175  determines the spot size of sub-beams  170  upon workpiece  180 . Sub-beams  170  exit scan lens  175  and propagate along the optical path, where they are incident upon workpiece  180 . Sub-beams  170  ablate or transform workpiece  180 , which is held in position by workpiece holder  185 . In an alternate embodiment, DOE  165  shapes beam  115  in order to create a unique feature. In a further embodiment, shaped beam can be split into multiple sub-beams by a second DOE disposed at the same location as DOE  165  or DOE  165  can be a compound DOE that both shapes and splits the beam. 
     FIG. 2  illustrates a functional block diagram method  200  method of optimizing the use of optical power in laser processing system  100  to maximize fabrication quality and yield. Technicians familiar with laser processing can readily adjust the method below to include other embodiments, such as parallel processing without shaped features. 
   Method  200  includes the steps of: 
   Step  210 : Obtaining Specifications for Final Product 
   In this step, specifications for final product are analyzed and converted to a digital format. Specification details include feature shape and size, quality, materials, manufacturing cost, etc. This specification is available to computer  112 . In one example, the specification is stored on computer  112 . In another example, computer  112  accesses the specification via a communication means like a network or the Internet. In one example, the specification is stored in a computer aided design (CAD) file. In another example, the specification is stored in a database table similar to Table 1 below. 
   
     
       
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Sample of specification data 
             
           
        
         
             
                 
               Feature 
                 
               Melt 
                 
               # of 
               Pattern of 
             
             
               Material_name 
               shape? 
               Absorption? 
               Temp? 
               Size? 
               Features? 
               features? 
             
             
                 
             
           
        
         
             
               SteelFoil1 
               Cone 
               1.88 × 10 5  cm −1   
               1535° C.  
               20 μm 
               500 
               Regular 
             
             
               AlFoil2 
               Polygon1 
               1.21 × 10 6  cm −1   
               660° C. 
               40 μm 
               200 
               Linear 
             
             
               PolymerFilm1 
               Cylinder 
               2.08 × 10 6  cm −1   
               110° C. 
               80 μm 
               2000 
               Random 
             
             
               . . . 
                 
               . . . 
               . . . 
             
             
                 
             
           
        
       
     
   
   Method  200  proceeds to step  220 . 
   Step  220 : Selecting Combination of Optical Power, Processing Method, and Material for Single Feature 
   In this step, computer  112  selects the best combination of optical power and processing method to meet product specifications from step  210  for a single feature. Examples of possible lasers include: CW, millisecond, microsecond, nanosecond, picosecond, and femtosecond. Examples of possible laser processing methods include: percussion, trepanning (flycutting), milling, zoom processing and material transformation in the absence of ablation (e.g. conversion of monomer to polymer, refractive index or transmissivity changes). Computer  112  accesses lookup tables and based on historical results stored in database (not shown) computer  112  selects the best combination of laser and processing method based on historical data that shows results of the various processing methods when used with the material that was selected in step  210 . In one example, computer  112  accesses a database (not shown) with product specification and results data for the available lasers and processing methods. 
   
     
       
             
           
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Sample of laser characteristics data accessed by computer 112 
             
           
        
         
             
                 
               Wave- 
               Pulse 
                 
                 
               Repetition 
             
             
               Laser_name 
               length 
               Energy 
               Pulse_width 
               Spot_size 
               Rate 
             
             
                 
             
             
               Picosecond1 
               1053 nm 
                1 mJ 
               20 ps 
               10 μm 
               1 kHz 
             
             
               CW 
                248 nm 
               n/a 
               n/a 
               10 μm 
               continuous 
             
             
               Picosecond2 
               1064 nm 
               10 mJ 
               40 ps 
               10 μm 
               2 kHz 
             
             
               . . . 
                 
               . . . 
               . . . 
               . . . 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               Illustrative laser processing data accessed by computer 112 
             
           
        
         
             
                 
                 
                 
               Ablation 
                 
             
             
               Laser Processing 
               Hole Shape 
               Ablation rate? 
               Threshold 
               Material? 
             
             
                 
             
             
               Flycutting-Algorithm-F1 
               Straight 
               1 mm 2  × 1 μm/1 mJ 
                 1 J/cm 2   
               W 
             
             
                 
                 
               pulse 
             
             
               Zoom Processing - 
               Cone 
               1 mm 2  × 1 μm/0.1 mJ 
               0.2 J/cm 2   
               Al 
             
             
               Algorithm-ZP1 
                 
               pulse 
             
             
               Percussion 
               tapered 
               1 mm 2  × 1 μm/0.5 mJ 
               0.5 J/cm 2   
               Fe 
             
             
                 
                 
               pulse 
             
             
               . . . 
             
             
                 
             
           
        
       
     
   
   Method  200  proceeds to step  230 . 
   Step  230 : Determining the Maximum Number of Features in Pattern to be Parallel Processed 
   In this step, the available optical power is divided by the optimum required optical power for a single feature as defined above. Laser  110  has a pre-defined amount of optical power. This amount of optical power is divided by the optimum amount of optical power for a single feature defined in step  220  and combined with a coefficient that estimates loss of optical power from DOE  165  to determine how many features can be processed in parallel. 
   Method  200  proceeds to step  240 . 
   Step  240 : Selecting Portion of Desired Pattern that can be Parallel Processed 
   In this step, computer  112  (or operator) selects the portion of the pattern to be processed. In one example where the product specification pattern contains 100 features, and 50 features have been identified in step  230  as the maximum number of features to be parallel processed, a portion of 50 features (representing ½ of all features of the final product) would be selected in this step. Method  200  proceeds to step  250 . 
   Step  250 : Developing Algorithm for Laser Processing to Specification 
   In this step, an algorithm is developed that combines the characteristics of the laser, laser processing method, and materials to meet the product specification. This algorithm will be used by computer  112  to direct how sub-beams  170  ablate workpiece  180 . The algorithm is used by computer  112  to control shutter  125 , attenuator  130 , and scan lens  175  and produce the desired shape in workpiece  180 . Method  200  proceeds to step  260 . 
   Step  260 : Designing and Manufacturing DOEs 
   In this step, DOE  165  is designed split beam  115  into sub-beams  170 . DOE  165  is designed to split beam  115  into a number of sub-beams equal to the number of features to be parallel processed, as determined in step  240 . In the example mentioned in step  240 , where the number of features is 100 and 50 features is the maximum number of features to be parallel processed, two DOEs are likely be needed. Any number of DOEs may be designed to accommodate the number of features desired from step  240 . After DOE  165  is designed it can be manufactured to match specifications. In one example, an optics vendor, like MEMS Optical (Huntsville Ala.) manufactures DOE  165 . Method  200  proceeds to step  270 . 
   Step  270 : Operating and Controlling Laser Processing System 
   In this step, workpiece  180  is in place, computer  112  sends a signal to open shutter  125  and laser processing begins. Scan lens  175  is adjusted by computer  112  to process workpiece  180  according to the combination of optical power, processing method, and materials selected in step  220 . In one example where the laser processing method selected is “zoom processing,” computer  112  adjusts the radius of the annulus and dwell time of sub-beams  170  to meet specifications defined in step  210 . In one example where laser processing system  100  is using zoom processing to drill shaped holes, dwell time correlates to the amount of material abated from workpiece  180 . Computer  112  also adjusts attenuator  130  to adjust the optical power of beam  115 . All adjustments by computer  112  are done according to algorithm defined in step  250 . If more than one DOE  165  is needed to complete the product specified, DOE  165  is replaced with another DOE that has a different sub-beam pattern, creating another set of features on workpiece  180 . 
   Because information is available to build the required database lookup tables, and to compare and select the best system and operating parameters for DOE-supported laser processing in a given situation, manufacturing quality and manufacturing yield are maximized while optimally and economically using the optical power supplied by a laser processing system through efficient parallel processing. 
   After step  270 , product manufacturing is complete and method  200  ends. 
   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.