Abstract:
A method of creating a milled structure in a fixed material using a moving laser beam is disclosed, where a picosecond laser provides short pulses of light energy to produce required exposure steps, where a variable rate of laser beam movement conducts the milling upon the material, where the laser beam tool path directs the milling process to produce a milled hole of high quality and repeatability, and where the knowledge of how to measure these 3 quantities is returned as feedback into the laser system. The present invention is further embodied as a spiral milled tool path structured to achieve the customer specified tapered hole shape. The constant arc speed tool path is required to produce tapered holes to customer specification.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/334,746, filed on Nov. 30, 2001. The disclosure of the above application is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention generally relates to material ablation with pulsed light sources and particularly relates to laser drilling and laser milling. 
   BACKGROUND OF THE INVENTION 
   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. 
   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 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. 
   The ability to drill holes as small as microns in diameter is a basic requirement in many high-tech manufacturing industries. The combination of high resolution, accuracy, speed, and flexibility has allowed laser processing to gain acceptance in many industries, including the manufacture of integrated circuits, hard disks, printing devices, displays, interconnects, and telecommunication devices. 
   There exist multiple methods for laser machining; however, when fine features are to be drilled, tolerances are smaller for the finished product in laser micromachining. In this case, the process used must provide consistent, predictable, and repeatable results to satisfy the end application. Computer control via algorithms and software in laser micromachining provides the opportunity for fine control of hole geometry and the consistency required for a profitable, mass-production manufacturing facility. This opportunity should not be squandered, as many problems continue to exist related to micromachining. 
   One problem that persists in the field relates to avoiding manufacturing off-specification products with micromachining. This problem is persistent because, in micromachining, the tolerance for error is low and consistency is critical from product to product. For example, inkjet nozzle holes must be manufactured consistently to provide equal ink ejection from each hole when used. When a process is not consistent or repeatable, the manufacturing line produces off-specification products that result in wasted time and energy, mandatory rework, and reduced throughput. This in turn reduces profitability of a manufacturing facility. What is needed is a way to avoid manufacturing off-specification products with micromachining. Another persistent problem related to micromachining involves production of consistent, repeatable results in milling. As noted above, consistency and repeatability are important factors in producing technically acceptable, high quality micro-machined products. However, current methods of milling are not designed to ensure that the required hole geometry is consistent from item to item in the manufacturing line. What is needed is a way to produce consistent, repeatable results in milling. 
   A further persistent problem relating to micromachining involves providing guidelines for creating tool path geometry; in recent history, milling techniques that produce predictable and repeatable hole geometries have proven difficult to achieve. Trial and error methods have been used to manufacture desired hole geometries: parameters are iteratively changed to reach the desired shape. A typical procedure is to step through the desired tool path radius linearly over time; however, this technique introduces uneven pitches in the spiral path, which causes variations in the radial overlap. The uneven ablation that results is undesirable. An algorithmic approach proves mildly successful, in that a desired shape is produced using a constant angular velocity and tool pitch. However, this process does not compensate for the spacing of exposure steps generated near the center of the hole as shown in FIG.  1 . What is needed is a way to provide guidelines for creating tool path geometry. 
   A still further persistent problem relating to micromachining involves providing a laser drilling system tool path allowing for constant material removal. Current requirements for milling require total material ablation across the workpiece target area. Past techniques include such methods as excimer laser ablation and a constant angular velocity approach, shown in FIG.  1 . However, these techniques do not provide the flat surface required by customer specifications. What is needed is a way to provide a laser drilling system tool path allowing for constant material removal. A still further persistent problem relating to micromachining involves maintaining constant exposure of a laser source on a workpiece when the tool path is changing. In a constant pulse laser system, the laser is pulsed at a fixed repetition rate; therefore, the uniform ablation is translated into a required constant propagation speed of the laser strike point onto the workpiece. When using a semi-circular motion, such as spiraling, the linear speed of the strike point should be constant throughout the laser milling process to maintain constant ablation. What is needed is a way to maintain constant exposure of a laser source on a workpiece when the tool path is changing. 
   SUMMARY OF THE INVENTION 
   In a first aspect, the present invention is a method of creating a milled structure in a fixed material using a moving laser beam, where a picosecond laser provides short pulses of light energy to produce required exposure steps, where a variable rate of laser beam movement conducts the milling upon the material, where the laser beam tool path directs the milling process to produce a milled hole of high quality and repeatability, and where the knowledge of how to measure these 3 quantities is returned as feedback into the laser system. 
   In a second aspect, the present invention is a spiral milled tool path structured to achieve the customer specified tapered hole shape. The constant arc speed tool path is required to produce tapered holes to customer specification. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. For example, while the present invention is described herein with reference to modifying angular speed as a function of radius for a spiral tool path that is round in shape, it should be understood that a constant arc speed can be obtained differently for different applications requiring a spiral that is not round in shape. Thus, a rate of traversal of the laser beam with respect to the surface of the workpiece is more generally modified as a function of distance from at least one fixed axis (for example, an oval has two relevant axes). 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  is a block diagram of a constant angular velocity tool path; 
       FIG. 2  is a block diagram depicting a simplified schematic of a laser drilling system; 
       FIG. 3A  is a block diagram of constant arc speed tool path corresponding to a round, inward spiral; 
       FIG. 3B  is a block diagram of a constant arc speed tool path corresponding to a round, outward spiral; 
       FIG. 4  is a flowchart diagram depicting a method of laser milling; 
       FIG. 5  is a perspective view showing major constituent components of an ink-jet printer; and 
       FIG. 6  is a schematic cross-sectional view of an ink-jet head. 
   

   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 is a method of milling using a constant tool path algorithm (or alternatively, “milling algorithm”) that can be used to produce holes in a consistent, repeatable process. Further, the process can be used to parallel-process a plurality of milled holes simultaneously. 
   As noted above, an algorithmic approach proves mildly successful, in that a desired shape is produced using a constant angular velocity and tool pitch. As also noted above, this process does not compensate for the spacing of exposure steps generated near the center of the hole. 
   Referring to  FIG. 1 , a constant angular velocity tool path (tool path)  100  includes an initial voltage at the outer contour (V max )  110 , a plurality of laser exposure steps  120 , and the spacing of tool pitch  130 . Using this approach, a large number of exposure steps  120  are generated near the target center, which result in excessive ablation in this area. In the present example, approximately 10,000 laser exposure steps  120  are used to create the spiraling shape of tool path  100 . 
   Referring to  FIG. 2 , a simplified schematic of a laser drilling system  200 , includes a laser  205 , a beam  207 , 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 diffractive optical element (DOE)  235 , a plurality of sub-beams  237 , a scan lens  240 , a microfilter  245 , an image transfer lens  250 , and a workpiece  255 , 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. 
   A brief description of the elements and 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 . 
   In operation, picosecond laser  205  emits beam  207  along the optical path identified in FIG.  2 . Beam  207  propagates along the optical path, where it is incident upon first mirror  208 . First mirror  208  redirects beam  207  along the optical path, where it is incident upon shutter  210 . Shutter  210  opens and closes to selectively illuminate the work piece material. Beam  207  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  207  exits attenuator  215  and propagates along the optical path, where it is incident upon second mirror  217 . Second mirror  217  redirects beam  207  along the optical path, where it is incident upon beam expander  220 . 
   Beam expander  220  increases the size of beam  207  to serve two purposes. First, it increases the beam size for the correct functioning of the DOE  235  as beam splitter. For the DOE  235  to function correctly, the beam size incident upon DOE  235  needs to be big enough to cover several periods of DOE  235 . Second, it increases the beam size to match the scan lens pupil size. Beam  207  exits beam expander  220  and propagates along the optical path, where it is incident upon third mirror  221 . Third mirror  221  redirects beam  207  along the optical path, where it is incident upon fourth mirror  222 . Fourth mirror  222  redirects beam  207  along the optical path, where it is incident upon spinning half-wave plate  225 . Spinning half-wave plate  225  changes the polarization of beam  207 . Upon exiting spinning half-wave plate  225 , beam  207  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 milling algorithm (not shown) to drill the holes in workpiece  255 . PZT scan mirror  230  redirects beam  207  along the optical path, where it is incident upon DOE  235 . 
   DOE  235  splits beam  207  into a plurality of sub-beams  237 , which allow parallel drilling of workpiece  255 . Sub-beams  237  exit DOE  235  and propagate along the optical path, where they are incident upon scan lens  240 . Scan lens  240  determines the spot size of sub-beams  237  upon workpiece  255 . Sub-beams  237  exit scan lens  240  and propagate along the optical path, where they are incident upon microfilter  245 . Microfilter  245  equalizes the intensities of sub-beams  237 . Sub-beams  237  exit microfilter  245  and propagate along the optical path, where they are incident upon image transfer lens  250 . Image transfer lens  250  re-images the focal spots of sub-beams  237  onto workpiece  255 . Sub-beams  237  ablate workpiece  255  in a pattern according to the pre-defined milling algorithm. 
   Referring to  FIG. 3A , a constant arc speed tool path  300 A and includes an initial outer contour exposure voltage (V max )  310 , a plurality of exposure steps  320  having constant arc speed and spacing, and the spacing of tool pitch  330 . In operation, the desired tool path  300 A, in the present example, consists of many revolutions separated by a tool pitch  330 , which can be constant or variable depending on the desired final shape. Utilizing this constant arc speed tool path provides a way to avoid manufacturing off-specification products and a way to produce repeatable results in milling. V max    310  determines the outer radius of the spiral in tool path  300 A. Each revolution, as shown, has many discrete exposure steps  320 , which are specified by the software algorithm described in step  430  of method  400  below. In reference to  FIGS. 2 and 3 , as laser  205  pulses at a fixed repetition rate, the uniform ablation is translated into a constant propagation speed of PZT scan mirror  230  to direct the laser strike point onto exposure steps  320  of workpiece  255 . 
   The constant arc speed tool path depicted in  FIG. 3A  provides for a flat surface in workpiece  255  being ablated. Maintaining this flat surface in workpiece  255  provides a laser drilling system tool path allowing for constant material removal. This constant arc speed tool path also provides a way to provide a laser drilling system tool path allowing for constant material removal. 
   During the manufacturing process employing the present invention, milling is also performed outward as the second half of the milling process. When the laser milling reaches the end of the inward spiral at t=T, the laser strike point is directed moving in an outward spiral tool path  300 B as shown in FIG.  3 B. After the laser strike point reaches the maximum radius for the next layer of milling at t=T′, the next inward spiral begins. 
   Referring to  FIG. 4 , a method  400  of laser milling includes several steps. At step  410 , an operator or technician provides a control system (not shown), such as a computer, that is capable of running an algorithm via a software program. The control system is electronically connected to PZT scan mirror  230  to provide operational control signals for implementation of the algorithm. At step  420 , the 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. The operator or technician determines the voltage, V max    310 , by considering entrance hole diameter, laser spot size, and voltage response of PZT scan mirror  230 . The operator or technician also uses the spot size of laser  205  to determine the minimum allowable tool pitch  330  of tool path  300 A. For example, if the spot size is 10 microns, tool pitch  330  should be a maximum of 10 microns to prevent under-ablated ridges from forming along outer walls of the radial contours. A pitch size around two microns works well with the 10-micron laser spot. A typical 40-volt of bias on the PZT scan mirror  130  deflects beam  107  by about 45 microns on workpiece  255 . At step  430 , the operator or technician launches software code (not shown), which resides in the control system identified in step  410  above to calculate the radius and angular speed over the period of laser drilling, T, to manufacture the desired hole geometry. For example, the following formula, Formula (A), describes the radius “r” along tool path  300 A at any given time “t” during the laser drilling: 
               r   ⁡     (   t   )       =         r   0   2     -       (         r   0   2     -     r   min   2       T     )     ⁢   t                 (   A   )             
 
   Similarly, the following formula, Formula (B), describes the angular velocity “ω” along tool path  300 A at any given time “t” during the laser drilling to achieve constant arc speed: 
               ω   ⁡     (   t   )       =       ω   0     ⁢       r   0       r   ⁡     (   t   )                   (   B   )             
 
   Also, when the laser milling reaches the end of the inward spiral tool path  300 A at t=T, the laser strike point is directed moving in an outward spiral tool path  300 B determined by the following equations during T≦t≦T′. Formula (C), describes the radius “r” along tool path  300 A at any given time “t” during the laser drilling: 
                 r   ′     ⁡     (   t   )       =         2   ⁢     r   min   2       -     r   0   2     +       (         r   0   2     -     r   min   2       T     )     ⁢   t                 (   C   )             
 
   Similarly, the following formula, Formula (D), describes the angular velocity “ω” along tool path  300 B at any given time “t” during the laser drilling to achieve constant arc speed: 
                 ω   ′     ⁡     (   t   )       =       ω   0     ⁢       r   0         r   ′     ⁡     (   t   )                   (   D   )             
 
   These four formulas are used to formulate the tool paths for drilling conical shapes, which resides in algorithmic form in the software on the control system. This step provides guidelines to create tool path geometry. 
   At step  440 , the control system transmits the results of the algorithm executed in step  430  to a tool path controller (not shown), such as a microprocessor, to initiate execution of the tool path and commence laser drilling. At step  450 , the controller identified in step  440  transmits voltages to PZT scan mirror  230  over time that correspond to the digital output of the algorithm executed in step  430 . The voltages are applied to PZT scan mirror  230  to translate its position in accordance with the calculated tool path and desired hole geometry in workpiece  255 . At step  460 , laser drilling system  200  mills workpiece  255  per the tool path algorithm, in a pattern illustrated in  FIG. 3  above. In the present invention, laser milling is performed using a layer-by-layer spiraling algorithm (“tool path”), thus, forming a tapered hole by decreasing V max    310  for successive spirals. 
   As previously discussed, the present invention is not limited to the spiral shape; in alternate embodiments, other tool path algorithms keeping uniform exposure for varied shapes can be used. Also, V max  can be decreased in various ways between successive layers to achieve a desired contour in a finished workpiece. The vertical cross section containing the axis of the hole determines how the V max (i) is progressed where i is the number of steps for reducing the V max . A linear function of V max (i+1)=V max (i)−ΔV max  results in a constant taper with fixed taper angle. Another function of V max (i+1)=V max(i) −(ΔV max *i) makes the taper angle less and less steep as radius is reduced. On the other hand, V max (i+1)=V max (i)−(ΔV max /i) makes the taper angle progressively steeper. In general, the Vmax(i) needs to be determined by the cross section (or shape) specification. 
   At step  470 , the tool path algorithm identified in step  430  determines whether the desired hole geometry has been achieved. The hole geometry has been achieved when the tool path algorithm has completed the pre-calculated number of necessary spiral ablations. Conventional measuring techniques such as use of confocal microscopy and optical profilometry can also be used to determine if the desired hole geometry has been reached. If yes, method  400  ends; if no, method  400  returns to step  450 . 
   A nozzle plate of an ink-jet head may be constructed with the laser drilling system of the present invention as further detailed below. 
   As shown in  FIG. 5 , an ink-jet printer  500  has an ink-jet head  502  capable of recording on a recording medium  504  via a pressure generator. Ink droplets emitted from the ink-jet head  502  are deposited on the recording medium  504 , such as a sheet of copy paper, so that recording can be performed on the recording medium  504 . The ink-jet head  502  is mounted on a carriage  506  capable of reciprocating movement along a carriage shaft  508 . More specifically, the ink-jet head  502  is structured such that it can reciprocate in a primary scanning direction X in parallel with the carriage shaft  508 . The recording medium  504  is timely conveyed by rollers  510  in a secondary scanning direction Y. The ink-jet head  502  and the recording medium  504  are relatively moved by the rollers  510 . 
   Referring to  FIG. 6 , a pressure generator  600  is preferably a piezoelectric system, a thermal system, and/or equivalent system. In this embodiment, the pressure generator  600  corresponds to a piezoelectric system which comprises an upper electrode  602 , a piezoelectric element  604 , and an under electrode  606 . A nozzle plate  608  comprises a nozzle substrate  610  and a water repellent layer  612 . The nozzle substrate  610  is made of metal, resin, and/or equivalent material. The water repellant layer  612  is made, for example, of fluororesin or silicone resin. In this embodiment, the nozzle substrate  610  is made of stainless steel and has a thickness of 50 um, and the water repellent layer  612  is made of a fluororesin and has a thickness of 0.1 um. The ink-jet ink is filled in an ink supplying passage  614 , a pressure chamber  616 , an ink passage  618 , and a nozzle  620 . Ink droplets are ejected from the nozzle  620  as the pressure generator  600  pushes the pressure chamber element  620 . 
   As a result of the present invention, very good nozzles are formed without flash and foreign matter (carbon etc) in the nozzle plate. Further, the accuracy of the nozzle outlet diameter is 20 um±1.5 um. 
   The present invention has several advantages. A first advantage of the present invention is that it avoids manufacturing off-specification products with micromachining. A second advantage of the present invention is that it provides a way to produce consistent, repeatable results in milling. A third advantage of the present invention is that it provides a system and guidelines for creating tool path geometry. A fourth advantage of the present invention is that it maintains constant exposure of a laser source on a workpiece without active laser power control. A fifth advantage of the present invention is that it provides constant material removal. A sixth advantage of the present invention is that the spiraling milling effect provides a continuous, consistent, and seamless laser ablation of a workpiece. A seventh advantage of the present invention is that the spiraling milling provides a way to machine micro features with cylindrical symmetry using laser ablation. An eighth advantage of the present invention is that it provides uniform material removal with predictable ablation rate so that an arbitrary profile may be established. 
   The present invention also has some disadvantages. One disadvantage of the present invention is that it is time intensive. However, any milling operation will require a similar amount of time to perform and thus is not a significant concern. A second disadvantage of the present invention is that it provides an increase in operational speed at the expense of control. However, the alternative closed loop system that provides additional control is too slow for cost effective mass manufacturing environment. 
   Another way to solve the same problem is to fire the laser at a faster rate when the hole radius is at the outer exposure steps. However, this approach requires additional process control that is difficult to synchronize and manage in the laser system. 
   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.