Abstract:
A programmable apparatus for laser beam shaping including a preprogrammable mircomirror array produces a spatial energy distribution suitable for accurately and rapidly marking, machining and processing materials. The preprogrammed micromirror array redistributes the laser output beam energy to produce a desired two-dimensional machining pattern on a work piece. The energy pattern created by the preprogrammed micromirror array is changeable between successive pulses of the laser to create accurate, complex three-dimensional machined shapes in a work piece not easily achieved by conventional machining systems. A special application of this invention is laser beam shaping to produce a uniform spatial energy distribution, i.e. homogenizing the beam from a laser with non-uniform energy distribution. Continuous adjustment of beam shaping is provided to maintain beam homogenization in accordance with changes in laser beam output energy profile.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of U.S. Provisional Application Ser. No. 60/207,115 entitled “Laser Beam Shaping Device and Apparatus for Material Machining” filed on May 25, 2000 the entire contents and substance of which are hereby incorporated in total by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a device in which a preprogrammed micromirror array redistributes the laser output beam energy to produce a desired two-dimensional machining pattern on a work piece. 
     2. Description of Related Art 
     It is known that in laser machining, particular importance is attached to both the quality of the beam produced by a laser and the shaping of that beam for the desired use. The laser output beam quality and shape determine the quality, quantity and efficiency of work piece machining. In many lasers, the output energy distribution over the beam profile is nonhomogeneous and if not reshaped to produce a uniform distribution would result in uneven machining over the work piece surface. Various approaches have been used to homogenize a laser beam profile to solve this problem. 
     One of the simplest devices to produce a homogenized beam profile is described by Fan et al. in U.S. Pat. No. 4,744,615 issued on May 17, 1988. In the Fan et al. patent, a coherent laser beam having non-uniform spatial intensity distributions is transformed by a multi-mirror tunnel in which the laser energy is reflected off the multiple mirrors such that there is a uniform energy profile at the exit of the tunnel. 
     U.S. Pat. No. 5,041,862, Rossman et al. “Lens Screen”, issued on Aug. 20, 1991 discloses a two-dimensional optical lenslet array that divides the laser beam into beamlets, which are subsequently reimaged to overlap their individual beamlet profiles producing an homogenized beam profile. Similarly, reshaping and homogenization of a laser beam is conventionally performed by XMR cylindrical lens arrays like 2Z17-E0125 or monolithic lens arrays of crossed cylindrical lenses. The simplest set-up consists of an array of crossed cylindrical followed by a focusing lens. The rectangular lenslets of the array have a clear aperture of rectangular shape. Collimated light coming into the lens array will be divided by the lenslets into beamlets of a rectangular beam profile. Each beamlet is focused by the lens array and diverges after the focal plane. The convex lens refracts each beamlet so that it fills the focal of the optical axis. 
     In addition, the prior art also includes: 
     U.S. Pat. No. 5,864,430, Dickey et al entitled “Gaussian beam profile shaping apparatus, method therefor and evaluation thereof”, issued on Jan. 26, 1999 discloses a method and apparatus for mapping a Gaussian beam into a beam with uniform irradiation profile by exploiting the Fourier transform properties of lenses. 
     U.S. Pat. No. 5,925,271 to Pollack, et al. and entitled, “Laser beam shaping device and process including rotating mirror”, issued Jul. 20, 1999. discloses a system in which one stationary beam shaping mirror and at least one rotary mirror is used to achieve an elliptical beam on a surface. 
     German patent DE 19724060A1 describes an apparatus which creates a homogenized excimer laser beam by splitting the beam, inverting the profile and coaxial recombination of the inverted beam profile with the original profile. The practical implementation of this beam homogenizer is manufactured by Micro/Las GMBH. 
     The foregoing patents describe technology, which accomplish beam shaping with fixed and/or rotating optical components and are capable of changing beam profile shape by mechanically replacing or reorienting the optical components. 
     Homogenized laser beams are used for a variety of materials machining purposes including drilling, contouring surfaces, cutting, scribing, trimming and pattern depositions. Typically, Co2 lasers are used for scribing, drilling and machining. Excimer lasers are used for film ablation, flex circuits and relief cutting. YAG lasers are used for trimming resistors, capacitors, marking and cutting of metals, semiconductors and absorptive synthetics. 
     In the case of U.S. Pat. No. 5,676,866 to Baumen et al, an apparatus is disclosed that creates an array of beamlets, which are individually deflected on to a work piece to simultaneously drill holes at different points. A beam homogenizer is described, for example, in U.S. Pat. No. 5,041,862, Rossman et al. for the purpose of making certain that the array of machining beamlets all have the same intensity. 
     Laser beam machining with uniform beam profiles requires beam homogenization when the laser output beam energy profile is non-uniform. Further, changes in patterns for laser machining of a work piece is limited to changes/adjustments of optical components and or mechanical displacements of the work piece. 
     The prior art does not appear to disclose or suggest a suitable a method of laser beam homogenization and work piece shaping that has the ability to provide high speed, high resolution, and complex pattern laser beam machining of a work piece. 
     SUMMARY OF THE INVENTION 
     Briefly described, the invention combines the function of laser beam homogenization with shaping of the beam using a preprogrammed micromirror array device to produce a specified spatial energy distribution that can rapidly and accurately machine a work piece. 
     A preprogrammed thin film micromirror array (TMA) has individually addressable and moveable mirrors capable of redistributing the laser output beam energy to produce a desired two-dimensional machining pattern. Simple and complex predetermined energy patterns can be created and rapidly changed. Different patterns can be generated on successive laser energy pulses, both in energy distribution and geometric location, to create accurate, complex three-dimensional machining of a work piece, not easily achieved by conventional machining techniques. An electronic tracking system is included to precisely align the laser energy patterns with work piece features/indices. 
     A special application of the invention is laser beam homogenization, i.e. laser beam shaping, to produce a uniform spatial energy distribution from a laser with non-uniform energy distribution. The disclosed invention provides automatic, continuous adjustment of beam shaping to maintain homogenization in accordance with changes in laser beam output energy profile not available from fixed optical systems. 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic diagram of the apparatus to flexibly produce predetermined laser beam energy profiles. 
     FIG. 1B is a schematic diagram of the Schlieren optical system illustrating the beam profile that results from various micromirror tilt angels. 
     FIG. 2A illustrates the operation of a single micromirror. 
     FIG. 2B is a graph of micromirror tilt angle versus the applied voltage to the piezo electric actuator. 
     FIGS. 3A and 3B illustrate the relationship between mirror angle, laser intensity and material ablation rate. 
     FIGS. 4A and 4B illustrate the fact that no energy is transferred when the micromirror angle is zero. 
     FIGS. 5A and 5B illustrate the fact that increasing energy is transferred with increasing micromirror angle. 
     FIG. 6 is a schematic diagram of the laser Schlieren optical beam homogenizer system. 
     FIG. 7 illustrates the various laser beam energy patterns the apparatus in FIG. 1 can produce. 
     FIGS. 8A through 8F illustrate prior art beam shaping techniques. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     During the course of this description like numbers will be used to identify like elements according to the different views that illustrate the invention. 
     The apparatus for laser machining  10  according to the preferred embodiment of the invention illustrated in FIG. 1 includes a thin-film micromirror array  12 , an interface board  14 , a computer  16 , an input  17 , imaging optics  18 , beam splitter  20 , beam spatial profile measurement  22 , feature location detector &amp; illuminator  24 , a laser  26  and work piece  28 . Acceptable thin-film micromirror arrays  12  are made by Daewoo Electronics Co. Ltd. Korea under the trademark “Thin-film Micromirror Array” (TMA) and other MEMS device manufacturers. 
     The TMA, thin-film micromirror array  12 , is an array of 1024×768 mirrors, each mirror measuring 49 microns×49 microns, with a panel size of 2.54 inches diagonal. The TMA is a particular kind of reflective spatial modulator. The TMA pixel is a monolithically integrated MEMS device fabricated over a simple PMOS switch. Each pixel consists of a mirror and an actuator as shown in FIG.  2 A. Each aluminum mirror  30  has high reflectivity (better than 90%) and excellent flatness for high optical efficiency (fill factor of 94%), and the actuator  32  has linear and fast response times (less than 25 microseconds), as well as mechanical and electrical reliability (tested with no failures to 0.5×1011 cycles). 
     The TMA uses thin film piezoelectric actuators  32  in the form of micro-cantilevers. As shown in FIG. 2, a mirror  30  is connected to the cantilevers  34  themselves through a support post  36 . The cantilevers  34  themselves are anchored to the underlying substrate. A cantilever consists of the supporting layer  38 , bottom electrode  40 , piezoelectric layer  42  and top electrode  44 . When an electric field is applied between the two electrodes  40  and  44 , the piezoelectric  42  layer shrinks in the horizontal direction and expands in the vertical direction. Since the neutral plane of the cantilever  34  shifts toward the bottom of the electrode due to thickness of the supporting layer  38 , the mechanical strain of the piezoelectric layer causes vertical deflection delta  46 , of the cantilever  34  and a tilt of angle alpha  48  of the mirror  30  on top of it. The mirrors in FIG. 1A are shown at various tilt angles in accordance with the computer  16 , preselected energy distribution patterns. 
     FIG. 2A shows that the tilt angle  48  of the mirror is linearly proportional to the applied voltage within the 0 to 10 volt operating range. FIG. 3 shows the relative intensity versus mirror tilt angle alpha  48 , illustrating the change in laser energy applied to a work piece as the individual micromirrors are tilted to different angles. When the mirror is not tilted, the laser energy is substantially blocked from the imaging optics  18 . As the mirror angle increases the amount of energy directed toward the work piece is increased, reaching a maximum at maximum tilt angle alpha  48 . The laser energy applied to the work piece removes material each time the laser fires when the intensity level is above threshold. FIG. 3 shows typical corneal tissue ablation rates versus intensity levels in millijoules per square centimeter for a commercial UV excimer laser operating in the 193 nanometer wavelength region. The amount of energy that passes through to the work piece is linearly proportional to the tilt angle of each mirror as illustrated in FIGS. 4A and 4B &amp;  5 A and  5 B. 
     For lasers operating at 308 or 206 nanometers (produced by commercially available lasers) polymetric materials are ablated with similar energy densities e.g. around 200 millijoules per square centimeter per pulse. Material removal proceeds at about 0.1 micron per pulse for polyimide. Silicon dioxide and silicon nitride require energy densities of about 600 millijoules per square centimeter for similar material removal rates. 
     For operation with an ultraviolet laser  26 , micromirrors  12 , imaging optical components  18 , are constructed of known efficient UV materials and coatings to minimize losses. The thin-film micromirror array  26 , package window also uses materials to minimize losses. 
     The present invention provides pre-selection and control of laser beam homogenization and shaping. As shown in FIG. 1, the computer  16  is provided a preselected profile by input  17 , connected to the micromirror array  12  through an interface board  14 , to modify the output beam energy profile of the laser  26 . The micromirrors  12  are continuously moveable for reflecting varying amounts of energy from the energy source, element by element as shown in FIGS. 4A,  4 B,  5 Aand  5 B. The computer  16  is programmed by the input  17  to move the mirrors to partially reflective positions to spatially vary the laser beam energy profile. 
     In addition, by examining features or indices on the work piece  28 , using the feature location detector and illuminator  24 , information about the position and orientation of the work piece is fed into computer  16 . Similarly the beam spatial profile measurement  22  provides the computer  16 , with the position and orientation of the beam profile. Changes in position/orientation of the work piece are examined by computer  16  and compensated for by adjusting the micromirror array to reposition/reorient the beam energy profile relative to the work piece. This active electronic tracking system can compensate for a variety of mechanical, vibration and “run out” errors. In the case of laser eye surgery, this would be an electronic eye tracking system. 
     To homogenize the spatial intensity distribution of the laser beam, a beam spatial profile measurement is made  22 , fed to computer  16 , and compared with the desired spatial profile. The computer feeds the micromirrors  12  through an interface board  14  to set each mirror to an angle that changes the amount of energy at each pixel in accordance with the desired beam profile. To homogenize the beam, the desired profile is uniform across the beam and the mirrors are so set. An important feature of this invention is the ability of computer  16  to monitor the laser beam profile  22  and to adjust micromirror array  12  in accordance with the desired profile as selected by the input  17 . Adjustments can be rapidly made on a pulse-by-pulse basis. 
     The optical system is based on Schlieren optics to control the energy intensity through continuous changes of the reflection angle of each mirror as shown in FIG.  6 . In order to modulate the laser beam intensity of the individual mirror pixels, a projection stop  4 , is used as a light valve. The laser energy reflected on the mirror surface forms an intensity image on the projection stop plane  52  and this image moves along the horizontal axis as the reflection angle changes. When the mirror does not tilt, all the laser beam energy passes through the aperture of the projection stop  54  and is re-imaged as the output beam  56 . When the mirror is deflected to its maximum position, the laser energy for that pixel is blocked from the output by the projection stop. The amount of energy that passes through the projection stop is linearly proportional to the tilt angle alpha  48  of each mirror FIG. 1A The creation of a homogenized laser beam profile is a special case of the more general beam-forming capabilities of this invention. With a two-dimensional micro mirror array with continuously adjustable mirror positions, all beam energy profiles from a simple Gaussian shape to complex energy profiles are possible e.g. annular rings and multiple Gaussian beamlets with different peak energy intensities FIG.  7 . 
     The simplest beam-shaping technique in common use is a simple aperture, where a variable iris is placed in the beam path to select the central portion of the beam energy. When an aperture is used in concert with imaging, more complex beam profiles can be achieved as illustrated in FIGS. 8A-8F. 
     To illustrate the invention, the graphs in FIGS. 3A,  3   b ,  4 A and  4 B, and  5 A and  5 B illustrate how beam shaping can be applied to the ablation of material. The invention, however, has broader implications for application to optical data storage, optical switching networks, nonlinear optical systems, photochemistry, micro machining, as well as materials processing. Consider the use of this invention for photolithography and material removal. One approach is to create a continuous distribution of energy at the surface of a work piece to achieve a photolithographic image or material removal with a predetermined profile. These predetermined patterns may have arbitrary shapes including non-contiguous pattern arrays, and time dependant spatial energy distribution in arbitrary patterns. Another feature is the ability to provide an electronic tracking system that maintains the geometric integrity between the desired spatial energy profile and the work piece using features as an index. It is understood that this invention maybe also be applied to various tissue modification/removal applications e.g. photorefractive keratotomy. 
     While the invention has been described with reference to the preferred embodiment thereof, it will be appreciated by those of ordinary skill in the art that modifications can be made to the structure and elements of the invention without departing from the spirit and scope of the invention as a whole.