Abstract:
Improved methods and apparatus for laser ablating polymeric materials. The method includes providing a first mark for laser ablating features in the polymeric material using a laser beam. A second mask is disposed in the laser beam for attenuating laser beam. The polymeric material is laser ablated using the first mask and the second mask in combination so that ablated features made in the polymeric material have substantially uniform feature dimensions.

Description:
FIELD OF THE DISCLOSURE  
       [0001]     The disclosure relates to improved methods of laser ablating polymeric materials, and in particular to a method of laser ablating polymeric materials to provide materials having uniform laser ablated features therein from one end of the materials to a second end of the materials.  
       BACKGROUND AND SUMMARY  
       [0002]     Excimer lasers are widely used in industry to form minuscule structures or ablated features in objects due to their high-energy output and precision. Frequently, a mask is employed in the laser ablation process so that very complex structures may be ablated in the materials. For example, excimer lasers have found a place in the manufacture of nozzle plates for micro-fluid ejection heads, e.g., ink jet printheads. When manufacturing a nozzle plate for a micro-fluid ejection head, it is necessary to form precise nozzle holes in a polymeric material. In some micro-fluid ejection heads, fluid chambers and fluid channels corresponding to the nozzle holes are also ablated in the polymeric material. The quality of the micro-fluid ejection head is affected by the precision with which the polymeric material is ablated by the excimer laser ablation system.  
         [0003]     The trend for a number of years for micro-fluid ejection devices is to increase the number of nozzle holes in a nozzle plate while decreasing the fluid droplet size. As the droplet size is decreased, the diameter of the nozzle hole is correspondingly decreased. Accordingly, a small variation in nozzle diameter from one end of a nozzle array to a center portion of the nozzle plate or to another end of the nozzle plate has a greater affect on small diameter nozzles than it does on large diameter nozzles.  
         [0004]     Nozzle diameter variations may arise because of anomalies in the manufacturing of lens and optical delivery systems used in the laser system which may result in an inconsistent energy output throughout a width and length of the laser beam. In such a system, the laser beam exhibits a characteristic energy distribution along the beam profile that may result in exit nozzle hole diameter variations from an end to a middle of a nozzle plate along a y-axis of the nozzle plate as illustrated by curve A in  FIG. 1 . A profile of the laser beam energy distribution as a function of position along the y-axis of the nozzle plate would look similar to the curve A. In fact, it is this laser energy profile that causes the diameter profile. Other factors that may be affected by laser beam energy variations include nozzle hole and ablated feature wall angles and ablated feature depth.  
         [0005]     Variations in the ablated features affect the performance of the a micro-fluid ejection head. For example, variations in fluid channel size and fluid chamber dimensions may affect fluid refill times which have a direct impact on a drop mass of fluid ejected and a velocity at which the fluid is ejected. Accordingly, there is a need for improved methods of laser ablating polymeric materials to reduce nozzle hole and flow feature dimension variations from one end of the ablated material to a second end of the ablated material.  
         [0006]     With regard to the foregoing, one embodiment of the disclosure provides improved methods and apparatus for laser ablating polymeric materials. The method includes providing a first mask for laser ablating features in the polymeric material using a laser beam. A second mask is disposed adjacent the first mask for attenuating laser beam. The polymeric material is laser ablated using the first mask and the second mask in combination so that ablated features made in the polymeric material have substantially uniform feature dimensions.  
         [0007]     In another embodiment, the disclosure provides a laser beam attenuation method for a laser ablation process. The method includes providing a first mask containing ablation features therein for ablating a polymeric material. A second mask is disposed adjacent the first mask. The second mask contains opacity gradation features therein. During the laser ablation process, the second mask is moved relative to the first mask in the laser beam to provide substantially uniform laser beam energy distribution to the first mask so that ablated features in the polymeric material are substantially uniform from a first end of the material to a second end of the material.  
         [0008]     An advantage of the methods described herein can include the ability to compensate for laser beam energy variations that affect ablated features in an ablated substrate such as a polymeric material used for a nozzle plate. Laser beam energy compensation may be achieved regardless of whether variations in the laser beam energy are caused by poorly aligned optics, aging optics or interactions between the laser beam and byproducts or plumes emanating from the ablated material during the ablation process.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     Further advantages of the embodiments will become apparent by reference to the detailed description of exemplary embodiments when considered in conjunction with the drawings, wherein like reference characters designate like or similar elements throughout the several drawings as follows:  
         [0010]      FIG. 1  is a two-dimensional graph of nozzle diameter versus y-axis position of a nozzle on a nozzle plate made by a prior art process;  
         [0011]      FIG. 2  is a schematic view of a laser ablation system for ablating substrates according to a prior art process;  
         [0012]      FIG. 3  is a cross-sectional view, not to scale, of a portion of a micro-fluid ejection head;  
         [0013]      FIG. 4  is a perspective view, not to scale, of a fluid cartridge containing a micro-fluid ejection head made according to the disclosure;  
         [0014]      FIG. 5  is a perspective view, not to scale, of a device for activating micro-fluid ejection heads on fluid cartridges according to  FIG. 4 ;  
         [0015]      FIG. 6  is a partial plan view, not to scale, of a portion of a nozzle plate containing nozzle holes and a portion of a mask used to make nozzle holes in the nozzle plate;  
         [0016]      FIG. 7  is a schematic view of a laser ablation system for ablating substrates according to an embodiment of the disclosure;  
         [0017]      FIG. 8  is a perspective view, not to scale, of a first and second mask for laser ablation systems according to the disclosure;  
         [0018]      FIG. 9  is a two-dimensional graph of an opacity curve versus nozzle plate y-axis for a laser ablation masking process according to a first embodiment of the disclosure;  
         [0019]      FIG. 10  is a two-dimensional graph of opacity curves versus nozzle plate y-axis for a laser ablation masking process according to a second embodiment of the disclosure;  
         [0020]      FIG. 11  is a two-dimensional graph of opacity curves versus nozzle plate y-axis for a laser ablation masking process according to a third embodiment of the disclosure;  
         [0021]      FIG. 12  is a schematic view of a laser ablation system for ablating substrates according to another embodiment of the disclosure;  
         [0022]      FIG. 13  is two-dimensional graph of flow feature depth versus nozzle plate y-axis for a prior art laser ablation process;  
         [0023]      FIG. 14  is two-dimensional graph of flow feature depth versus nozzle plate y-axis for a laser ablation masking process according to a fourth embodiment of the disclosure; and  
         [0024]      FIG. 15  is two-dimensional graph of flow feature depth versus nozzle plate y-axis for a laser ablation masking process according to a fifth embodiment of the disclosure. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0025]     Laser ablation of materials for micro-miniature devices such as micro-fluid ejection heads is an efficient process for forming multiple ablated features in a substrate. A conventional laser ablation system  10  is illustrated schematically in  FIG. 2 . According to the system  10 , an excimer laser  12  generates a coherent light beam that travels down a telescope section  14 . Within the telescope section  14  are two lenses (not shown) which change the shape and focus of the coherent light beam. The coherent light beam is then expanded into individual light beams and then recombined by a homogenizer  16 .  
         [0026]     The homogenized coherent light beam  18  is then further focused by a condenser lens  20  and field lens  22  and is directed upon and through a mask  24 . The mask  24  is made of a transparent material such as quartz and typically coated on one side with a light reflecting material such as chrome or a dielectric layer to provide transparent and opaque areas for forming laser ablated features in a substrate  26 .  
         [0027]     A portion  28  of the coherent light beam  18  emitted by laser  12  passes through the transparent portion of the mask  24  while the opaque portions of the mask  24  reflect other portions of the coherent light beam  18 . The portion  28  of the coherent light beam  28  passing through the mask  24  is further reduced by a factor of five times by a reduction lens  30  to provide a reduced beam  32 .  
         [0028]     As is appreciated by a person of ordinary skill in the art the amount of reduction by the reduction lens  30  may vary depending on the size of features desired and the quality of the lenses  30  that are available. The reduced coherent light beam  32  is used to ablate the substrate  26  and form structures and features of a desired size and shape in the substrate  26 . During the ablation process, the substrate  26  is typically supported on a platen  34 .  
         [0029]     Laser ablation as described above may be used to form a plurality of features in the substrate  26 . An example of the use of laser ablation is for the provision of a nozzle plate  36  for a micro-fluid ejection head  40 . A portion of a micro-fluid ejection head  40  is illustrated in  FIG. 3 . The micro-fluid ejection head  40  includes a semiconductor substrate  42  containing a plurality of layers  44  providing a fluid ejection actuator  46 . In the embodiment illustrate in  FIG. 3 , the ejection actuator  46  is a heater resistor. However, the disclosure is not intended to be limited to any particular ejection actuator or to micro-fluid ejection heads.  
         [0030]     The nozzle plate  36  is attached to the substrate  42  and layers  44  to provide the micro-fluid ejection head  40 . As shown, the nozzle plate  36  has formed therein, as by laser ablation, nozzle holes  48 , a fluid chamber  50 , and a fluid flow channel  52 . The fluid chamber  50  and fluid flow channel  52  are collectively referred to herein as “flow features.” Fluid flowing through the fluid flow channel  52  to the fluid chamber  50  is heated by the ejection actuator  46  to provide a vapor bubble that forces fluid through the nozzle hole  48  and onto a fluid receptive medium. In another embodiment, a nozzle plate may contain nozzle holes only, and a separate thick film layer, attached to the substrate  42  provides the fluid chambers  50  and fluid flow channels  52 .  
         [0031]     In one embodiment, the micro-fluid ejection head  40  may be attached to a fluid cartridge  54  as shown in  FIG. 4 . The fluid cartridge  54  may include a fluid reservoir body  56  and a micro-fluid ejection head portion  58  for supply of fluid from the body  56  to the ejection head  40 . As shown, the nozzle plate  36  for the micro-fluid ejection head  40  includes a plurality of nozzle holes  48  in one or more substantially linear arrays of nozzle holes  48 .  
         [0032]     Activation of the ejection actuators  46  on the micro-fluid ejection head  40  is controlled by a ejection control device. In the case of a micro-fluid ejection head  40  for ejecting ink, an ink jet printer  60  ( FIG. 5 ) may provide a suitable control device. Electrical contact between the ejection head  40  and printer  60  is provided by a tape automated bonding (TAB) circuit or flexible circuit  62  containing contact pads  64  for electrical connection to the control device. The contact pads  64  on the flexible circuit  62  are in electrical communication with the ejection head  40  as by conductive traces  66 .  
         [0033]     In the conventional laser ablation system  10  illustrated in  FIG. 2 , there are typically variations in ablated features from a first end of the nozzle plate  36  to a second end of the nozzle plate  36  along a length of the nozzle plate  36 . A portion of a prior art nozzle plate  70  having a first end portion  72 , a second end portion  74 , and middle portion  76  is illustrated in  FIG. 6 . A corresponding mask  78  for laser ablating features in the nozzle plate  70  is also illustrated in  FIG. 6 . The mask contains transparent openings  80 ,  82 , and  84  for forming nozzle holes  86 ,  88 , and  90  respectively in the nozzle plate  70 . Due to variations in reduced coherent beam  32  ( FIG. 2 ) described above from the first end  72  to the second end  74  of the nozzle plate  70 , nozzle holes  86  and  88  have larger diameters than nozzle holes  90  in a center portion  76  of the nozzle plate  70 .  
         [0034]     Typically laser ablation system  10  produces nozzle diameter variations of ±1 micron from the largest nozzles  86  and  88  to the smallest nozzle  90  as shown by curve A ( FIG. 1 ). Without desiring to be bound by theory, it is believed that laser beam energy is lower for the nozzles  90  and higher for the nozzles  86  and  88  thereby increasing the wall angle of the nozzles  86  and  88 , resulting in a larger exit diameter for nozzles  86  and  88 .  
         [0035]     In the case where flow channels, fluid chambers, and nozzles are ablated in the nozzle plate material  70 , the flow channels and fluid chambers adjacent the end portions  72  and  74  of the nozzle plate  70  are ablated more deeply than the flow channels and fluid chambers in the middle portion  76  of the nozzle plate  70 . Such added depth adjacent the end portions  72  and  74  may cause the nozzles  86  and  88  to be larger than the nozzles  90  even if all of the nozzles  86 ,  88 , and  90  were ablated with substantially the same wall angles.  
         [0036]     For the sake of simplicity so far in our discussion, the mask  78  and the nozzle plate  70  contain only a single row of nozzle holes. However, in a typical micro-fluid ejection head nozzle plate  70  at least two, and often more, rows of nozzle holes exist. Where a plurality of rows of nozzle holes are ablated simultaneously, consideration must be given to variations in diameter sizes of nozzle holes from one row to another. As would be appreciated by a person of ordinary skill in the art, similar variations can be anticipated along the width of a rectangular coherent light beam as well as along the length of the beam as discussed above. The method for correcting such variations in energy output discussed in the embodiments of the disclosure may be employed for any number of rows of nozzle holes and flow features formed in a nozzle plate or other substrate.  
         [0037]     One method used to correct the foregoing problem is described, for example, in U.S. Pat. No. 6,089,959 to Komplin, the disclosure of which is incorporated by reference. In the method described in the &#39;959 patent, a modified mask  24  is constructed having adjusted feature dimensions to compensate for laser beam energy variations from one end portion  72  to a second end portion  74  of the nozzle plate  70 . Actual nozzle plate  70  feature dimensions are used to determine how the mask is to be modified to compensate for laser beam energy variations.  
         [0038]     While the foregoing method is effective to compensate for a particular laser beam energy profile, other laser beam energy profiles may exist which require numerous variations of mask  24 . It has been observed that the energy profile may change so often that it may not be feasible to provide all of the masks  24  required to provide uniform feature ablation during a manufacturing process.  
         [0039]     Hence, an adjustable method of laser beam energy compensation is provided by embodiments of the disclosure. Referring to  FIG. 7 , a modified laser ablation system  100  includes an adjustable gray scale mask  102  including mask portions  102 A and  102 B. The adjustable gray scale mask  102  is disposed between the condenser lens  20  and the reduction lens  30  and may preferably be disposed adjacent the primary mask  24  for attenuation of the laser beam energy. In one embodiment, illustrated in  FIG. 8 , the mask portions  102 A and  102 B include gray scale patterns  104 , which may consist of chrome squares smaller in size than the resolution of the laser system  100  imaging optics. However, any shape of gray scale patterns  104  may be sufficient to attenuate the laser beam energy.  
         [0040]     As shown in  FIG. 8 , the number or size of the gray scale patterns  104  increases toward a distal ends  106 A,  106 B of the mask  102  thereby providing increased opacity of the mask  102  toward the distal ends thereof. Curve B in  FIG. 9  provides a plot of mask  102  opacity as a function of a nozzle plate y-axis which may provide ablated features for the nozzle plate  36  where the ablated features, for example nozzles holes  48 , may otherwise have a nozzle diameter profile as shown in  FIG. 1  in the absence of mask  102 .  
         [0041]     The gray scale patterns  104  of mask  102  may be effective to reflect a portion of the incoming laser beam  108 , thus reducing the laser beam energy at distal ends of the mask  24  along a y-axis of the mask  24 . Mask  102  may thus be effective to equalize the laser energy profile across the length and width of the beam  108  thereby equalizing the ablated features made in the nozzle plate  36 .  
         [0042]     During a laser ablation process, mask portions  102 A and  102 B are adjustable in the y direction by motorized or micrometer mount for example. Moving mask portions  102 A and  102 B toward or away from each other may be used to modify the energy profile of the beam  108  by modifying the opacity profile of the mask  102 . One or both of the mask portions  102 A and/or  102 B may be moved to compensate for any particular laser beam energy profile. The mask portions  102 A and  102 B may be moved to a set position for the energy profile of the laser beam  108 , or may be continuously movable during the laser ablation process.  
         [0043]     The mask  102  shown in  FIG. 8  is only slightly greater in width than the laser beam  108 , e.g., about 6.5 millimeters wider than the laser beam  108 . A standard square mask  102  of approximately 10 to 15 centimeters may be used. Accordingly, with a 10 to 15 centimeter square mask  102 , several gray scale patterns, varying in opacity, may also be placed on the mask  102  in the x-axis direction ( FIG. 8 ). Such a mask  102  may allow handling ablated features having different feature dimension profiles. In terms of the mask  102 , the following terms are defined:  
         [0044]     a) “inboard mask edge” is defined as the edge  110 A or  110 B of the mask portion  102 A or  102 B closest to the central x-axis of the beam  108 .  
         [0045]     b) “outboard mask edge” is defined as the distal edge  106 A or  106 B of the mask portion  102 A or  102 B that is furthest from the central x-axis of the beam  108 .  
         [0046]     c) “opacity gradient” is defined as the change in opacity from the inboard edge  110 A or  110 B to the outboard edge  106 A or  106 B of the mask portion  102 A or  102 B. Accordingly, a positive opacity gradient refers to a mask  102  having a zero opacity at the inboard edge  110 A or  110 B, and some opacity greater than zero at the outboard edge  106 A or  106 B. A negative opacity gradient may have a zero opacity gradient at the outboard edge  106 A or  106 B and some opacity greater than zero at the inboard edge  110 A or  110 B. A linear opacity gradient may have similar opacity moving from the inboard edge  110 A or  110 B to the outboard edge  106 A or  106 B.  
         [0047]     In order to handle ablated features having dimension profiles of different magnitudes, several opacity gradients may be provided on each mask portion  102 A or  102 B. For the purposes of illustration only, the ablated feature profiles are nozzle hole  48  exit diameters in the nozzle plate  36 . In other embodiments, the ablated feature formed in the nozzle plate  36  may be flow features such as fluid chambers  50  and flow channels  52 .  
         [0048]     In the alternate embodiments, mask  102  may include mask portions  102 A or  102 B having a positive opacity gradient, a negative opacity gradient, and a linear opacity gradient. Such mask portions  102 A or  102 B may be movable in both the y-axis direction and in the x-axis direction by either micrometer or motorized mount in order to select the desired opacity gradient.  
         [0049]     For a mask  102  having a dimension of 10 to 15 centimeters square, the number of opacity gradients that may be provided on the mask  102  is limited. Accordingly, it may be desirable to change the opacity curve B ( FIG. 9 ) without moving the mask  102  to a different opacity gradient position. One way to do achieve uniform nozzle hole  48  diameters in a nozzle plate  36  is illustrated graphically in  FIG. 10  wherein a mask  102  having a single opacity gradient is used. Before ablating the nozzle holes  48  in the nozzle plate  36 , the mask portions  102 A and  102 B may be moved apart so that the gray scale patterns  104  in the mask portions  102 A and  102 B are beyond outer limits of the laser beam  108  in the y-axis direction. Such a mask position provides an opacity profile that is illustrated graphically in  FIG. 10  as curve C. Opacity profile C has a relatively wide flat area  112 .  
         [0050]     In an alternative process, the inboard edges  110 A and  110 B may be moved closer together to provide an opacity profile illustrated graphically in  FIG. 10  as curve D for ablating nozzle holes  48  in the nozzle plate  36 . Such an opacity profile (curve D) has a relatively narrow flat area  114 .  
         [0051]     In yet another alternative process, nozzle holes  48  in the nozzle plate  36  may be ablated for a portion of a depth of the nozzle holes  48  with the mask portions  102 A and  102 B at the position that provides the opacity profile of curve C. During ablation of the remaining depth of the nozzle holes  48  through the nozzle plate  36 , the mask portions  102 A and  102 B may be moved together to the position that provides the opacity profile of curve D thereby providing a hybrid opacity profile illustrated graphically as curve E. The hybrid opacity profile (curve E) provides a unique opacity profile that cannot be readily obtained using a stationary mask technique for curves C and D. The resulting opacity profile curve E is an average of the opacity profiles of curve C and curve D. Switching from opacity curve C to opacity curve D at times other than half way through the ablation process may be effective to provide other opacity profile curves. It will be appreciated that the foregoing embodiment may also be applied to flow features such as fluid chambers  50  and flow channels  52  ( FIG. 3 ) ablated in the nozzle plate  36 .  
         [0052]     Another method for adjusting the opacity curve for a laser ablation system using a limited number of mask positions is illustrated and described with reference to  FIG. 11 . In the embodiment illustrated in  FIG. 11 , mask  102  contains multiple gray scale patterns  104  providing multiple opacity gradients. In this embodiment, a first mask position has a standard opacity gradient illustrated by opacity curve F in  FIG. 11 . A second mask position has a different opacity gradient illustrated by opacity curve G in  FIG. 11 . During an ablation process for forming nozzle holes  48  through a thickness of the nozzle plate  36 , the mask portions  102 A and  102 B are placed in first mask position for ablating a portion of the depth of the nozzle holes through the nozzle plate  36 . The mask is then moved in the x-axis direction to the second mask position to complete forming the nozzle holes  48  through the thickness of the nozzle plate  36 . The resulting opacity gradient curve H is an average of the gradient curves F and G for the first and second positions in the x-axis direction of the mask portions  102 A and  102 B. Accordingly, using a mask  102  containing the first and second opacity gradients may be used for providing a variety of average opacity gradients depending on the depth of ablation used for each of the mask positions. A 10 to 15 centimeter square mask  102  may contain up to ten to 12 different gray scale patterns  104  providing different opacity gradients. Accordingly, the foregoing embodiments may include the combination of more than two different gray scale patterns  104  during the ablation process.  
         [0053]     Yet another embodiment of the disclosure will now be described with reference to  FIGS. 12-14 . Instead of mask portions  102 A and  102   b , opaque objects  116 A and  116 B are placed in the path of the laser beam  108  ( FIG. 12 ) during later stages of the ablation process. As described above, flow features such as fluid chambers  50  and fluid channels  52  have an ablation profile for the depth of the feature ablated in the nozzle plate  36  that is similar to the ablation profile for nozzle holes illustrated by curve A in  FIG. 1  along the y-axis of the nozzle plate  36 . The flow feature depth profile for a prior art ablation process is illustrated in  FIG. 13  by curve I. Accordingly, end portions of the nozzle plate  36  may have a depth that ranges from about 1-2 μm greater than center portions of the nozzle plate  36  for an overall ablated depth of 17 microns.  
         [0054]     During the laser ablation process for ablating flow features in a polyimide nozzle plate  36 , each laser pulse may ablate the material to a depth of about 0.2 μm, thereby requiring about ten ablation pulses to ablate a depth of 2 μm. In order to equalize the ablation depth of the nozzle plate from one end to the other and avoid the depth variation illustrated in  FIG. 13 , opaque objects  116 A and  116 B may moved into the outer edges of the beam  108  when there are about ten ablation pulses remaining. The opaque objects  116 A and  116 B are effective to limit ablation of the nozzle plate material where the depth of the features would otherwise be about 2 μm greater than the depth of the features in the center portions of the nozzle plate  36  as shown by curve I ( FIG. 13 ).  
         [0055]     With nine ablation pulses remaining for ablating the flow features, the opaque objects  116 A and  116 B are moved toward each other in the outer edges of the beam  108  to limit ablation of the nozzle plate where the depth of the features would otherwise be about 1.8 μm greater than the depth of the features in the center portions of the nozzle plate  36  as shown by curve I ( FIG. 13 ). The foregoing procedure of moving the opaque objects in the outer edges of the laser beam  108  are continued until only one ablation pulse remains. At that point, the opaque objects  116 A and  116 B are close enough to each other to block the beam  108  where the depth of the features would otherwise be more than about 0.2 μm greater than the depth of the features in the center portions of the nozzle plate  36  as shown by curve I ( FIG. 13 ).  
         [0056]     A resulting flow feature depth profile provided by the foregoing process is illustrated graphically in  FIG. 14 . In  FIG. 14 , curve J represents the ablation depth profile of the features obtained by moving an opaque object into the outer edges of the beam  18  during later stages of the laser ablation process. According, the method provides a more uniform flow feature depth across a flow feature array in the y-axis direction of the nozzle plate  36 . It will be appreciated that different depth profiles may require different movement of the opaque objects  116 A and  116 B to achieve a profile generally as shown by curve J ( FIG. 14 ).  
         [0057]     The foregoing process using opaque objects  116 A and  116 B may also be used to provide uniform nozzle hole exit diameters in the nozzle plate  36  material along a nozzle hole array in the y-axis of the nozzle plate. Again, using  FIG. 13  for reference as an illustration of the depth profile for nozzle holes made by a prior art laser ablation process, the opaque objects  116 A and  116 B are moved into the outer edges of the beam  108  when there are about 10 laser ablation pulses remaining so that nozzle holes toward the outer edges of the nozzle plate  36  along the y-axis are exposed to fewer laser pulses. Reducing the number of pulses for the outer portions of the nozzle plate is effective to reduce the wall angles of such nozzle holes thereby reducing the exit hole diameters so that the exit hole diameters are substantially the same as the exit hole diameters in center portions of the nozzle plate  36 . Otherwise, outer edges of the laser beam  108 , having more energy than inner portions of the beam  108  would ablate the nozzle holes to a greater depth toward the outer edges of the nozzle plate  36  thereby increasing the wall angle of the nozzle holes.  
         [0058]     In a further embodiment, nozzle holes  48  having uniform exit diameters along the nozzle plate  36  y-axis from one end of the nozzle plate  36  to the other end of the nozzle plate  36  may be made without providing flow features such as fluid chambers  50  and flow channels  52  having a uniform depth profile. For example, the opaque objects  116  ( FIG. 12 ) may be moved into the outer edges of the beam  108  with 20 pulses left. Hence, the ablation depth of the flow features toward the end portions of the nozzle plate  36  will be 2 μm less than the depth of the flow features in the center portion of the nozzle plate  36 . Subsequently, with 18 pulses left the opaque objects  116  may be moved toward each other to a portion of the nozzle plate  36  where the depth of the flow features would have otherwise been 1.8 μm greater than the depth of the flow features in the center portion of the nozzle plate. The foregoing procedure is repeated until only two ablation pulses are left for ablating the flow features so that the opaque objects  116  are positioned where the ablation depth of the flow features would have otherwise been 0.2 μm greater than the depth of the flow features in the center portion of the nozzle plate  36 . Accordingly, the depth profile for the flow features would be similar to K in  FIG. 15 .  
         [0059]     After ablating the flow features, the nozzles  48  may be ablated through the remaining thickness of the nozzle plate  36  using conventional techniques such as a nozzle hole mask  78  ( FIG. 6 ). The decreased ablation depth of the flow features toward the end portions of the nozzle plate  36  increases the remaining thickness through which the nozzles  48  are ablated in the end portions of the nozzle plate as compared to the center portions of the nozzle plate. Thus, the exit diameter of the nozzle holes  48  in the end portions of the nozzle plate would be reduced, thereby avoiding the non-uniform nozzle hole exit diameters described with respect to  FIG. 6 .  
         [0060]     In a further modification, a uniform flow feature depth may be provided as shown in  FIG. 14 . Uniform nozzle exit diameters may then be provided by use of the gray scale mask  102  as described above.  
         [0061]     It is contemplated, and will be apparent to those skilled in the art from the preceding description and the accompanying drawings, that modifications and changes may be made in the embodiments of the disclosure. Accordingly, it is expressly intended that the foregoing description and the accompanying drawings are illustrative of exemplary embodiments only, not limiting thereto, and that the true spirit and scope of the present disclosure be determined by reference to the appended claims.