Abstract:
An apparatus and method for making a grayscale photo mask and a three-dimensional grayscale diffractive optical element operate as follows. A grayscale photo mask is obtained by exposing a laser direct write (LDW) glass material to laser beam radiation from a first laser beam of modulated power moved over a grid of discrete locations on the LDW material, the modulated power being in accordance with grayscale pattern data, and, while the first laser beam is moved over the discrete locations of the grid, exposing the grid to a second laser beam, the power of the second laser beam being less than the bleach threshold of the glass material, to provide each of the discrete locations with a gray scale level to provide a predetermined gray scale pattern of varying optical transmissivity on the LDW material to produce a grayscale mask. An optical element is obtained by exposing a photoresist layer on a wafer to radiation transmitted through the grayscale mask; and removing material from the photoresist layer to provide a predetermined varying thickness of the photoresist layer as determined by the gray scale patterns on the grayscale mask to produce the three-dimensional grayscale diffractive optical element. The optical element may be a diffraction grating. The power of the second laser exposed on said discrete locations may be between about 50 mW and about 5 W, and the power of the first laser exposed on the discrete locations may be between about 20 mW and about 500 mW.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention is directed to an apparatus and method for making grayscale photo masks used as a light filter to transfer an image from one device to another and optical elements made using such masks.  
         [0003]     2. Discussion of the Related Art  
         [0004]     Conventional photolithographic technology is based on binary photolithography, which basically produces a step-like binary structure on a material of interest. Grayscale technology is a type of photolithographic technology that is mainly used in the opto-electronics industry. Grayscale photolithography has been developed to make multi-level steps or ultimately a continuous structure.  
         [0005]      FIG. 16  illustrates a conventional binary photolithographic process, and  FIG. 19  shows an example of a binary diffraction grating made with a binary photo mask.  
         [0006]     As illustrated in  FIG. 16 , a type of conventional binary photo mask is made by a glass wafer with a chromium pattern on the wafer, where the transmission is binary, i.e., 0 or 1. In  FIG. 16 , step  1  involves preparation of a wafer (for example, glass) on the surface of which a micro-structure is to be formed. In step  2 , the wafer is placed on a vacuum chuck, and then, using a spin-coater, a photo resist is spin-coated onto the wafer, by dropping resist drops onto the spinning wafer. Step  3  represents preparation of the photo mask. An exposure step (step  4 ) is performed using a mask aligner or a stepper by exposing the photo resist with light of a certain wavelength through a photo mask loaded on an optical system to project light onto the photo resist on the wafer to make a desired pattern on the photo resist corresponding to the pattern formed on the photo mask in advance. A development step (step  5 ) is performed to develop the photo resist on the wafer, where the exposed portion is removed and the unexposed portion stays on the wafer. Next, an etching step (step  6 ) is performed to etch the wafer with developed photo resist using any plasma etcher or chemical etcher, where the remaining photo resist plays a role to avoid being exposed in the plasma or chemical etcher while bare parts of the wafer are etched to make a structure corresponding to the original pattern on the photo mask.  
         [0007]     An example of a photolithographic process is described in European Patent Application 0 588 781 which discloses a method for exposure of a pattern by writing with a focused laser light onto a photosensitive substrate. During the writing procedure, the substrate is moved in a first direction, and the optics are continuously moved to focus laser light on the photosensitive surface in a second direction orthogonal to the first direction. The focused laser light is spread on the photosensitive surface in the first direction to form an extended focus area, and the focused laser light is independently controlled to hit position increments spaced in the first direction across the extended focus area. Another example is described in U.S. Pat. No. 6,490,390 which discloses an optical interferometric system for fabricating fiber Bragg gratings in photosensitive optical fibers by using an acousto-optic element to generate and control the radiation pattern with a high resolution. The radiation pattern may be programmable in that different radiation patterns may be produced by controlling the acousto-optic element. The fiber is exposed to an interference pattern to write a grating pattern in the fiber.  
         [0008]     A gray scale photo mask is a mask that has a desired pattern provided on a given material described by the transmission of light and is used to project light onto a massive number of wafers having a photo resist thereon. A grayscale photo mask may be made using any number of materials using a technique to alter the transmission of the material to be multi-level or gray-level.  
         [0009]      FIG. 18  shows a conventional grayscale photolithographic process to make a grayscale photo mask, using LDW (Laser Direct Write) glass. LDW glass is a kind of grayscale photo mask material made by Canyon Materials Inc., San Diego, Calif. Such materials and their use are described in U.S. Pat. No. 6,524,756, U.S. Pat. No. 6,562,523 and Korolkov et al., “Application of gray-scale LDW-glass masks for fabrication of high-efficiency DOEs,” Institute of Automation and Electrometry SB RAS, Novosibirsk, 630090, Russia, January, 1999.  
         [0010]     The base transmission of LDW glass is set at a desired optical density (OD) at a certain wavelength, normally OD=2.0, for example, which means incident light transmits only approximately 1% through the glass. The transmission of LDW glass illuminated by a laser changes in accordance with the laser power. As the laser power increases, LDW glass turns more transparent. By traversing the surface of the LDW blank while modulating the laser power in accordance with desired patterns, a grayscale photo mask is obtained whose transmission varies across the surface of the LDW glass corresponding to a desired pattern. In this way, a grayscale optical density, or alternatively grayscale transmission, can be attained. This allows a grayscale structure to be made on the photo resist on a wafer to make a grayscale optical element by final etching.  FIG. 17  shows the relation between transmittance and laser power of an LDW material.  
         [0011]     More particularly, in order to make a grayscale photo mask using LDW glass, a focused laser beam is required. The laser should have a certain power to bleach LDW glass that has a threshold energy to be bleached within a practical writing time. Also, the laser should have sufficient focusing power to write a very fine pattern, e.g., having a beam spot of 1 micron or less. An Acousto-Optic Modulator (AOM) modulates the laser beam to vary the intensity of the transmitted laser beam applied to the LDW glass surface. The laser writer for writing on the LDW glass includes a laser, an AOM, a scanning mirror, a focusing lens, a translation stage, and a precise positioning mechanism using, for example, an interferometer and a computer. An example of a laser writer is that made by Microtech (Italy) having a 150 mW He—Cd laser with a 442 nm wavelength.  
         [0012]     With respect to the writing method, to write up a pattern on the LDW blank, a raster scanning method may be used. The scanning speed depends on how fast it is desired to write the pattern, for example, a 39 cm/s scan using an objective lens of NA=0.65 (NA shows focusing power) and a gas laser of 150 mW power to make a grayscale pattern with a full range of OD=2.0.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention is directed to providing a fast and inexpensive method of using a laser writer to produce an LDW grayscale photo mask with high spatial resolution, and to use such grayscale photo mask in the fabrication of optical or opto-electronic elements.  
         [0014]     According to the present invention, there is provided a method of making a grayscale photo mask, comprising providing a LDW glass material having a body portion and a surface layer formed thereon which is responsive to heat from laser beam radiation to change the optical density of the LDW glass material; exposing the surface layer of the LDW glass material to a first laser beam moved over a plurality of locations on the surface layer of the LDW glass material while modulating power of the first laser beam in accordance with grayscale pattern data; and while the first laser beam is moved over the locations on the surface layer, exposing the LDW glass material to a second laser beam, the power of the second laser beam being less than a bleach threshold of the LDW glass material, whereby each of the locations is provided with a gray scale level to provide a predetermined gray scale pattern of varying optical transmissivity on the plurality of locations of the LDW glass material to produce a gray scale mask.  
         [0015]     According to the present invention, there is further provided a method of making a grayscale photo mask, comprising providing a LDW glass material having a body portion and a surface layer formed thereon which is responsive to heat from laser beam radiation to change the optical density of the LDW glass material; exposing the surface layer of the LDW glass material to a first laser beam moved over a plurality of locations on the surface layer of the LDW glass material while modulating power of the first laser beam in accordance with grayscale pattern data; and while the first laser beam is moved over the locations on the surface layer, exposing the LDW glass material to a second laser beam, the power of the second laser beam being less than a bleach threshold of the LDW glass material, whereby each of the locations is provided with a gray scale level to provide a predetermined gray scale pattern of varying optical transmissivity on the plurality of locations of the LDW glass material to produce a gray scale mask; providing a wafer having a photo resist layer thereon; exposing the photoresist layer to radiation transmitted through the grayscale mask; and removing material from the photoresist layer to provide a predetermined varying thickness of the photoresist layer as determined by the gray scale patterns on the gray scale mask to produce the three-dimensional grayscale diffractive optical element.  
         [0016]     According to the present invention, there is also provided an apparatus for making a grayscale photo mask, the apparatus comprising: a first laser that emits a laser beam; a modulator that modulates the power of the laser beam in accordance with grayscale pattern data to produce a first laser beam of modulated power; a drive section that moves the first laser beam of modulated power over a plurality of locations on a surface layer of an LDW glass material to change the optical density of the LDW glass material at the plurality of locations; and a second laser that, while the first laser beam is moved over the plurality of locations on the surface layer of the LDW glass material, exposes the LDW glass material to a second laser beam, the power of the second laser beam being less than a bleach threshold of the LDW glass material, whereby each of the plurality of locations is provided with a gray scale level to provide a predetermined gray scale pattern of varying optical transmissivity on the LDW glass material to produce a gray scale mask.  
         [0017]     According to the present invention, there is further provided an apparatus for making a three-dimensional grayscale diffractive optical element, comprising: (a) an apparatus for making a grayscale photo mask comprising: a first laser that emits a laser beam; a modulator that modulates the power of the laser beam in accordance with grayscale pattern data to produce a first laser beam of modulated power; a drive section that moves the first laser beam of modulated power over a plurality of locations on a surface layer of an LDW glass material to change the optical density of the LDW glass material at the plurality of locations; and a second laser that, while the first laser beam is moved over the plurality of locations on the surface layer of the LDW glass material, exposes the LDW glass material to a second laser beam, the power of the second laser beam being less than a bleach threshold of the LDW glass material, whereby each of the plurality of locations is provided with a gray scale level to provide a predetermined gray scale pattern of varying optical transmissivity on the LDW glass material to produce a gray scale mask; (b) an exposing section that exposes the photoresist layer to radiation transmitted through the grayscale mask; and (c) a removing section that removes material from the photoresist layer to provide a predetermined varying thickness of the photoresist layer as determined by the gray scale patterns on the gray scale mask to produce the three-dimensional grayscale diffractive optical element.  
         [0018]     The LDW glass material may have another surface layer on the body portion and the second laser beam may be exposed on the another surface layer of the LDW glass material. The another surface layer may be a back side of the LDW material relative to the surface layer which is exposed to the first laser beam. The second laser beam may expose a wider area of the LDW glass material than the first laser beam. The second laser beam may expose the LDW glass material to the second laser beam for a period of time prior to exposure of the LDW material to the first laser beam.  
         [0019]     The optical element may be a diffraction grating. The power of the second laser exposed on the discrete locations may be between about 50 mW and about 5 W, and the power of the first laser exposed on the discrete locations may be between about 20 mW and about 500 mW. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, described in brief below.  
         [0021]      FIG. 1  shows a preferred embodiment of the apparatus of this invention for making a grayscale element.  
         [0022]      FIGS. 2, 3  and  4  illustrate a method of using the apparatus of  FIG. 1 .  
         [0023]      FIGS. 5A, 5B  and  5 C show examples of grayscale patterns on LDW glass that may be formed according to the present invention.  
         [0024]      FIGS. 6A and 6B  show examples of grayscale blazed gratings that may be made according to the present invention.  
         [0025]      FIG. 7  shows an example of an optical pickup that may employ a grating made according to the present invention.  
         [0026]      FIG. 8  shows an example of a grating unit that may employ a grating made according to the present invention.  
         [0027]      FIG. 9  shows an example of a grating pattern that may be formed according to the present invention.  
         [0028]      FIGS. 10A and 10B  a Fresnel lens that may be made according to the present invention.  
         [0029]      FIG. 11  shows the contour of a 4 level fan-out grating that may be made according to the present invention.  
         [0030]      FIG. 12  shows another embodiment of the present invention using illumination from the back side.  
         [0031]      FIG. 13  shows the vacuum chuck of the  FIG. 12  embodiment with opening in the extended support.  
         [0032]      FIG. 14  shows a grayscale photo mask layout with fiducials.  
         [0033]      FIG. 15  shows another embodiment of the present invention using a fiber-coupled diode laser.  
         [0034]      FIG. 16  shows a conventional binary photolithographic process.  
         [0035]      FIG. 17  shows the relation between transmittance and laser power.  
         [0036]      FIG. 18  shows a conventional grayscale photolithographic process.  
         [0037]      FIG. 19  shows an example of a binary diffraction grating made with a binary photo mask. 
     
    
     DETAILED DESCRIPTION  
       [0038]      FIG. 1  shows a preferred embodiment of the apparatus of this invention for making a grayscale photo mask on an LDW blank which is a plate having a body portion and a surface layer formed thereon which is responsive to heat to change the optical density of the LDW glass material. This apparatus includes a high power main (first) laser  1  (which may be gas or semiconductor) with power of, e.g., 20 mW to 500 mW; an acousto-optic modulator (AOM)  3 ; a shutter  23 ; a polarization beam splitter  25 ; a quarter wave plate  27 ; a scanning mirror  5 ; an objective lens  7 ; an interferometer  9  (including a laser and a detector); a translation stage  11  (including all axes); a computer  13 ; an auxiliary (second) laser  17 ; and a reflection mirror  21 .  FIG. 1  shows an LDW glass plate  15  (which may be of any desired size, e.g., 5 or 6 inches square), held onto the translation stage  11  by a vacuum chuck  19 , for being processed to form the grayscale photo mask.  
         [0039]     The method of using this apparatus is as follows, with reference to  FIGS. 1, 2 ,  3  and  4 . Grayscale data for creating the mask is prepared and stored in computer  13 . Patterns are written on LDW glass plate  15  with a focused laser beam by modulating the laser beam from main laser  1  using AOM  3  based on the data stored in computer  13  and raster scanning in one direction while the translation stage  11  moves in the other direction (see  FIG. 3 ). Interferometer  9  corrects the writing position in real time to achieve precise alignment.  
         [0040]     As shown in  FIG. 4 , the exposure step involves exposing photo resist  43  on a wafer  45  in a stepper (1× optical system) or mask aligner  41  (5× or 10× optical system) through an LDW grayscale photo mask  47  made by the method described above. Then, the exposed photo resist on the wafer is developed in a developer  49 , and thereafter, the wafer together with the developed photo resist on the wafer is etched in etcher  51  using any etching method such as plasma etching, for example, RIE (Reactive Ion Etching) and wet etching using chemicals.  
         [0041]     Referring again to  FIG. 1 , auxiliary laser  17  performs an auxiliary heating operation of the LDW glass  15  with an amount of heat energy that does not exceed the bleach threshold of the LDW glass  15 . The use of the auxiliary laser enables reduction of the power of main laser  1 , which avoids certain problems caused by use of a very high power main laser. Also, another disadvantage of a very high power laser  1  is that the optics can be easily damaged and the LDW glass can incur surface deformation due to melting. Also, the higher the laser power, the greater the expense. The main laser beam  1  should be tightly focused down to the surface of LDW glass plate  15  using objective lens  7  to write a fine resolution pattern onto LDW plate  15 . Another advantage of the use of auxiliary laser  17  is that the scan speed of the main laser  1  can be increased.  
         [0042]     The mechanism of turning color in LDW glass upon illumination of a focused laser beam is the result of heat effect. There is a bleach threshold which is the threshold of the temperature where LDW glass starts to change its color. While illuminating the auxiliary laser on the LDW glass to get its temperature close to the threshold temperature, the main laser is applied to exceed the threshold temperature. Both lasers should be used simultaneously. Otherwise once the auxiliary laser is turned off, the LDW glass could be cooled down very quickly. It is noted that, with respect to heating with a laser beam, there is a decay time of heat dissipation, which is a function of heat capacitance. Glass has a rather long decay time compared to metal. Thus, the auxiliary laser may preheat the glass from a point in time a few ms or a few tens of ms before application of the main laser beam. In other words, the auxiliary beam may be applied, not only during the writing operation of the main beam, but also for a short time prior to beginning of the writing operation with the main beam. Of course, the auxiliary beam covers a wider area than the beam spot of the main beam and thus heats some areas of the LDW glass surface prior to application of the write beam to such areas; thus, the auxiliary laser beam is applied to such areas before the main laser begins to write. The auxiliary beam may be applied a short time before the write operation of the main beam even begins.  
         [0043]     Because the LDW glass plate  15  is written by the effect of heat, the main laser  1  can be any type of laser, such as solid state, semiconductor and gas, and its wavelength can be any wavelength provided the laser beam is focused and has a heat effect. The main laser  1  should be a continuous laser that is modulated by AOM  3 . The main laser  1  can be, for example, an He—Cd gas laser with 150 mW power, such as model number KR1801C, made from KIMMON Electric Co., Ltd., Japan. Alternatively, the first laser  1  can be directly driven by a laser driver (not shown) in accordance with grayscale pattern data to produce a first laser beam of modulated power, in which case the AOM  3  can be eliminated.  
         [0044]     With the aid of auxiliary laser  17 , the power of the main laser  1  can range from much lower power, e.g., minimum of about 20 mW up to optionally about 500 mW (but preferably no greater than 150 mW) depending upon the system, whereas, without an auxiliary laser  17 , there is a requirement of high power of the single writing laser of around 500 mW. Regarding the power range of 20 mW to 500 mW for the main laser  1 , the minimum 20 mW is valid with low scan speed such as 1 cm/s and with the existence of the auxiliary laser  17 , while the higher power is limited by 500 mW because otherwise, the optics may be easily damaged and LDW glass may incur deformation due to melting at such higher powers. The main laser beam should be tightly focused onto the surface of the LDW glass using the objective lens to write a finely resolved pattern.  
         [0045]     The auxiliary laser  17  can be any type of laser and can be either a continuous laser or a pulsed laser as long as it provides a heat effect. The laser beam from the auxiliary laser  17  can be collimated or focused using a lens. This auxiliary beam is delivered on the portion of LDW plate  15  where a pattern is being written while making a spot with a spot size  29 , as shown in  FIG. 2 , that covers a wide area, e.g., (strip width)×(track width), written and applied before and while the main laser beam scans one strip. The power of the auxiliary laser  17  can take any level, provided the bleaching power is not exceeded, depending on how much the laser is focused. When the beam of the auxilliary laser is focused tightly, the power can be low. Where the beam is loosely focused or collimated, the power should be large. The laser beam of auxiliary laser  17  should be low enough that it does not cause a phase change on the surface of LDW plate  15  but high enough that LDW plate  15  is locally heated enough to enable the above-discussed significant reduction of the power of the main laser  1 .  
         [0046]     With respect to the writing method, to write up a pattern on the LDW blank, a raster scanning method may be used, in which the main laser beam is raster-scanned in one direction (for example in the x-direction) while the LDW glass moves in the y-direction to scan one strip of the pattern. After finishing the first strip, the LDW glass moves to the starting point of the next strip. In this way, all of the strips are written.  FIG. 3  illustrates a raster scan method with a scan speed of 78 cm/s (for a first lens) and 39 cm/s (for a second lens), and a strip width of 200 microns (for the first lens) and 100 microns (for the second lens).  
         [0047]     The following table shows some exemplary combinations of parameters and obtained results with He—Cd main laser wavelength of 442 nm and auxiliary laser diode with wavelength of 780 nm for pattern size of 5 mm by 5 mm.  
                                                                                                               Example #                1   2   3   4   5   6                        Set   Main Laser power (mW)   150   50   50   50   50   50       parameters   Aux. Laser power (mW)   0   50   100   100   100   100           Scan speed (cm/s)   40   40   120   120   120   120           Track separation (um)   0.1   0.1   0.1   0.2   0.2   0.2           Lens NA   0.65   0.65   0.65   0.65   0.65   0.4           LDW background OD   2.0   2.0   2.0   2.0   1.0   1.0       Obtained   Feature size on LDW   0.5   0.5   0.5   0.5   0.5   1.0       result   (μm)           Gray level   64   64   64   32   16   8           Process time (min)   60   60   20   10   5   5                  
 
         [0048]     Products made by the method and apparatus of the present invention are grayscale optical elements, for example, a grating used in an optical pickup in an optical drive such as a CD, a DVD and a Blu-ray (high definition) drive. The grating plays an important role of splitting the laser beam carrying information from an optical disk into a sensor to process the data. A conventional grating used in an optical pickup is a binary type that has a number of periodic rectangular grooves. A problem of binary gratings is low diffraction efficiency, i.e., the grating throws away about 70% of the incident beam. For a CD and a DVD drive, there is a demand to decrease the laser power due to a heat problem that comes from high laser power. Thus, if the diffraction efficiency goes up, a lower power laser can be used, which may relax the problem. For a Blu-ray drive, there is a shortage of laser power due to low reflectivity of optics that causes power loss for this particular wavelength. Thus, there is a need to increase the diffraction efficiency for efficient use of the laser power. One method to improve the diffraction efficiency is to make a so-called blazed grating, which is roof-shaped rather than rectangular groove shape. This blazed grating is made only by grayscale technology.  FIGS. 6A and 6B  show examples of binary and blazed gratings that may be made according to the present invention.  FIG. 7  shows an example of an optical pickup that may employ a grayscale grating produced according to the present invention.  FIG. 8  shows an example of a grating unit that may employ a grating produced according to the present invention.  FIG. 9  shows an example of a grating pattern that may be formed according to the present invention.  
         [0049]     Other products that may be made by the method and apparatus of the present invention include any optics having curved surfaces such as aspheric lenses, aspheric mirrors and any diffractive optics such as fan-out DOEs, Fresnel lenses or Fresnel mirrors. These optics are used in cameras, displays and optical drives.  FIGS. 10A and 10B  show a Fresnel lens that may be made according to the present invention.  FIG. 11  shows the contour of a 4 level fan-out grating that may be made according to the present invention.  
         [0050]     Another embodiment of the present invention is shown in  FIG. 12 . In this embodiment, as an alternative to illuminating an auxiliary laser beam onto the LDW plate  15 , it is possible to place an auxiliary laser  17 A under LDW plate  15  on the vacuum chuck  19 A, in which case the auxiliary laser beam strikes LDW plate  15  from its back side. As LDW plate  15  moves with the translation stage  11 , the vacuum chuck  19 A may have an opening (window)  121  to expose the effective patterning area  121 .  FIGS. 12 and 13  are a side view and a top view of the system, respectively. An auxiliary laser  17 A is set in the space underneath the vacuum chuck  19 A with a fold mirror  21 A and an objective lens  123  both held by a holder (not shown). The auxiliary laser beam is delivered from the auxiliary laser  17 A and passes through shutter  125  to be reflected by fold mirror  21 A and converges through objective lens  123  to make a spot on the back side of LDW plate  15 . The auxiliary laser beam strikes LDW plate  15  from the back side while the beam from main laser  1  illuminates the front side of LDW plate  15 . LDW plate  15  is put on the vacuum chuck  19 A so that the silver layer faces upward and the main laser beam comes from the top down to the silver layer. The auxiliary laser beam comes from the back side through the base glass material, which has a certain thickness, e.g., 2.3 mm thickness for 5 inch plates or 6.35 mm thickness for 6 inch plates, to reach the silver layer. Therefore, the auxiliary laser beam may be focused by objective lens  123  so that the focal point is within the thickness of the silver layer.  
         [0051]     For stepper use, an LDW grayscale photo mask may have an effective area for patterning with a certain size, e.g., 94.3 mm×75.4 mm, spanned from the center of the mask.  FIG. 14  shows a mask design with three fiducials  127  and an effective patterning area  129 . The fiducials  127  are for alignment of the mask itself against the optical axis of the stepper. The area outside the effective patterning area  129  is not used. The vacuum chuck  19 A necessarily has an opening  121  to allow the laser beam to go through. The shape of the opening  121  may be equal to or larger than the effective patterning area  129  of LDW plate  15 .  
         [0052]      FIG. 13  is an example of the vacuum chuck  19 A. The translation stage can not sit below the vacuum chuck  19 A. So the vacuum chuck  19 A may have an extended support arm  131  being secured on the top part of the translation stage  11  at one edge while the other edge holds LDW plate  15 . To avoid vibration and bend of the support arm  131 , the support arm  131  may be made with sufficient thickness to provide good rigidity. The vacuum chuck  19 A also may have a ring-shaped groove  133  fabricated outside the perimeter of the effective patterning area  129  to suck LDW plate  15 , as shown in  FIG. 13 .  
         [0053]     The auxiliary laser  17 A can thus sweep in the sense of relative motion inside all effective patterning area  121  from the back side. A mechanical or optical shutter  125  for the auxiliary laser  17 A may be used to synchronize with the main laser  1 .  
         [0054]      FIG. 15  illustrates yet another embodiment in which the auxiliary laser is a diode laser  17 B coupled to an optical fiber  153  with about 100 to about 200 microns core diameter, for example. A spacer  155  makes a space for the beam path from the fiber-coupled diode laser  17 B. The laser beam is delivered through a fiber  153  attached directly to the exit pupil of the diode laser  17 B. An objective lens  151  may be attached at the other edge to converge the laser beam. A fold mirror  21 A is placed next to the objective lens  151  to reflect the laser beam up to the back side of LDW plate  15 . A sleeve  157  is sustains in the space holding the fiber  153  and the fold mirror  21 A so that the vacuum chuck  19 B can slide together with the translation stage  11  without touching the sleeve  157 . The sleeve  157  may be a hollow cylinder, which has an opening on the top portion for the laser beam to go through. The laser beam is focused to make a spot on the silver layer of LDW plate  15 . The spacer  155  can alternatively be made by four pillars. A suitable fiber-coupled diode laser is available for example from Coherent, Inc. with a part name Fiber-Coupled single emitter, 810 nm and 2.4 W, part number 1006430.  
         [0055]     Although the preferred embodiments of the present invention have been described and disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the scope and spirit of the invention as set forth in the accompanying claims.