Abstract:
An efficient method of and a system for performing topography measurement facilitates increasing laser machining throughput. Topography measurements at multiple points on a target specimen or continuous real time measurement and monitoring of the target specimen surface topography and target specimen thickness can be performed during a laser machining process. Measurement of the thickness of the target specimen to be laser machined would permit fine tuning of laser energy delivered and result in higher quality target material removal.

Description:
COPYRIGHT NOTICE 
     © 2005 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR § 1.71(d). 
     TECHNICAL FIELD 
     The present invention relates to laser machining target materials and, in particular, to a system and method for automatic measurement of one or both of the topography and thickness of a layer of a sequentially laminated target from which material is removed to form vias of repeatable quality at increased yield. 
     BACKGROUND INFORMATION 
     Lasers are used to drill vias in and remove material from electronic materials products. The epoxy or resin often used in dielectric layers of electrical circuit boards is among the types of material typically removed by such a laser. For a machining laser beam to reliably and consistently remove a layer of material, it is desirable that one or both of the depth of focus and image plane of the beam fall within the depth of the layer of material undergoing removal. Variations in either the thickness or the topography of the layer undergoing removal, or in the topography of other target layers, may alter the relative location of the layer with respect to one or both of the depth of focus and image plane of the beam and thereby result in inconsistently drilled and poor quality vias. 
     The use of a machining laser beam and beam positioning system to drill vias in a sequentially laminated target is well known in the art. Such sequentially laminated targets typically include conductive layers and dielectric layers and are used as circuit boards in electronic circuit applications. 
     There are four main quality metrics that characterize a via. They include the taper of the via, the roundness of the via, the smoothness of the wall, and the cleanliness of the bottom surface. When the depth of focus of the machining laser beam is outside of the layer from which material is to be removed, vias will be drilled with nonuniform diameters. The via diameter may change 10%-20% if the layer thickness varies more than the depth of focus of the machining laser beam. When the thickness of the layer requiring material removal is small, excess power imparted by the machining laser beam can result in an over-drilled via, which exhibits one or both of poor wall quality and out-of-tolerance via size. If the layer of material is thick, insufficient power can result in incomplete via formation. Via quality is thus dependent on accurate perception of the surface height and thickness of the layer from which material is to be removed. 
     The state of the art for measuring the topography of a sequentially laminated target entails either touching the surface of the target with a probe and measuring its displacement or focusing a camera on a portion of the surface. Lowering and raising a probe or focusing a camera consumes an industrially significant amount of time, which elapses before the actual material removal process. Because of the time associated with current methods of measurement, the height of each target is measured only at a single location. While it permits adjustment of the depth of focus of the machining beam based on variations from target to target, a single measurement does not account for variations in topography of a single target. 
     Although the thickness of a single layer of material may vary by only 6 microns, the height of a target surface can vary by more than 60 microns. Since some layers requiring material removal can be as thin as 25 microns, the variation in surface height of the target is more than sufficient to cause the depth of focus to fall outside of the target layer and thereby reduce the quality of any via drilled in that layer. As technology continues to demand miniaturization, vias will likely continue to shrink in diameter, depth, or both, and, therefore, be formed by lasers of shorter (e.g., UV) wavelengths. At smaller dimensions, increased quality and repeatability are even more vital to the proper functioning of vias. 
     Variations in the thickness of the layer from which material is to be removed may also reduce the quality of vias formed in the layer. When the thickness of the layer is unknown, excessive or insufficient amounts of energy may be applied by the machining laser beam during material removal, leading to either damage to the underlying conductive layer or an incompletely drilled via. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention use in a machining laser beam system a tracking device to measure in real time variations in one or both of the surface height and layer thickness of regions of a sequentially laminated target preparatory to laser machining material from them. The tracking device provides signals that correlate to distance changes it detects. Examples of a suitable tracking device include a laser triangulation, a capacitance or an eddy current probe, or a confocal device. Higher quality vias can be achieved by altering one or both of the relative position of the target and machining laser beam and the energy of the machining laser beam in response to measurements taken by the tracking device. The tracking device can be used in association with a machining laser beam of any shape under conditions in which the depth of focus can fall outside a specified operational tolerance. 
     In one embodiment implemented with a laser triangulation device, a tracking light beam reflects off of the target and is received by a laser beam position sensor. The reflected light received is then processed to provide information about the topography and thickness of the layer from which material is to be removed. Based on the information received, the machining laser beam system adjusts the position of the image column of the beam waist of a pulsed machining laser beam relative to the target along the axial distance between the objective lens and the target. The machining laser beam system also adjusts the number of laser pulses used in the material removal process. For via formation, the number of pulses corresponds to the amount of machining laser beam energy applied to form a via. For other machining applications using either or both of topography and thickness information, the number of pulses corresponds to the amount of machining laser beam energy applied to, for example, dice a semiconductor wafer, sever a semiconductor memory link, or trim resistive or other target material. Other embodiments use a capacitance or an eddy current probe or a confocal device as a tracking device to adjust the axial distance in similar manner. 
     Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art machining laser beam system and beam positioning system configured to laser machine a target specimen. 
         FIG. 2  is a fragmentary cross-sectional view of a two-layer sequentially laminated target as it appears after a via has been formed by laser machining. 
         FIG. 3  shows the objective lens and depth of focus of the machining laser beam in relation to the target. 
         FIGS. 4A ,  4 B, and  4 C depict respective front, plan, and side elevation views of an embodiment of a machining laser and laser positioning system that has been fitted with two tracking lasers and associated position sensing devices. 
         FIG. 5  is a diagram showing a tracking laser beam, laser beam position sensor, and relevant surfaces of a sequentially laminated target. 
         FIG. 6  is a diagram showing an alternative embodiment of the system components of  FIG. 5 . 
         FIG. 7  shows a capacitance or an eddy current probe operating as a tracking device. 
         FIG. 8  shows a confocal probe operating as a tracking device. 
         FIG. 9  shows a capacitance probe or an eddy current probe of  FIG. 7  cooperating with a laser triangulation device of  FIG. 6  to measure the thickness of an opaque target layer. 
         FIGS. 10A and 10B  depict the positional relationships among the tracking beam, beam position sensor, and sequentially laminated target of  FIG. 5  for two regions of incidence of the tracking beam on the top surface of the laminated target. 
         FIG. 11  is a graph showing the relationships between laser beams having different energy profiles and distances from the objective lens to a target surface. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of this invention are implemented with a machining laser beam that performs via drilling and other electronic circuit material removal processes. The machining laser beam is typically produced by a pulsed laser source in optical association with an objective lens, which focuses the laser into a beam suitable for drilling a target specimen mounted on a support structure. Typical targets include sequentially laminated boards often used as printed circuit boards in the electronics industry. 
     With reference to  FIG. 1 , a machining laser beam system  10  includes a laser  12  from which propagates a pulsed output beam  14  along a beam path  18 . Laser output  14  may be manipulated by a variety of well-known optical devices including beam expander lens components  16  that are positioned along beam path  18  before being directed by a series of beam directing components  20  of a beam positioning system  22 . Laser output beam  14  propagates through an objective lens  26 , such as a focusing or telecentric scan lens, for incidence as a machining laser beam  28  on a sequentially laminated target  30  that is secured to a target specimen mount  32 . 
     Beam positioning system  22  is used to alter the relative position of machining beam  28  and target  30  and may move one or both of machining beam  28  and mount  32 . Beam positioning system  22  operates to move machining beam  28  relative to target  30  in the X-, Y-, and Z-axis directions, in which the Z axis is defined along the machining beam axis and is substantially orthogonal to the surface of target  30 . The axial distance measured from the point at which machining beam  28  exits objective lens  26  and strikes the surface of target  30  is thus altered by movement of target  30  or objective lens  26  along the Z axis. 
     An exemplary beam positioning system  22  is described in detail in U.S. Pat. No. 5,751,585 of Cutler et al. and may include ABBE error correction described in U.S. Pat. No. 6,430,465 of Cutler, both of which patents are assigned to the assignee of this patent application. Beam positioning system  22  preferably employs a translation stage positioner that controls at least two platforms or stages  40  and  42  and supports beam directing components  20  to target and focus machining beam  28  to a desired laser target position  46 . In a preferred embodiment, the translation stage positioner is a split-axis system in which a Y stage  40 , typically moved by linear motors, supports and moves target  30  along rails  48 , an X stage  42  supports and moves a fast positioner  50  and objective lens  26  along rails  52 , the Z-axis dimension between X and Y stages  40  and  42  is adjustable, and beam directing components  20  align beam path  18  through any turns between laser  12  and a fast steering mirror  54 . A typical translation stage positioner is capable of a velocity of 500 mm/sec and an acceleration of 1.5 G. For convenience, the combination of fast positioner  50  and one or more translation stages  40  and/or  42  may be referred to as a primary or integrated positioning system. An example of a preferred laser system that contains many of the above-described positioning system components is a Model 5320 laser system or others in its series manufactured by Electro Scientific Industries, Inc., the assignee of this patent application. Skilled persons will appreciate, however, that a system with a single X-Y stage for target specimen positioning and one or both of a fixed beam position and stationary galvanometer may alternatively be employed. 
     A laser system controller  56  preferably synchronizes the firing of laser  12  to the motion of stages  40  and  42  and fast positioner  50  in a manner well known to skilled persons. Skilled persons will appreciate that laser system controller  56  may include integrated or independent control subsystems to control and/or provide power to any or all of these laser components and that such subsystems may be remotely located with respect to laser system controller  56 . 
     The parameters of machining beam  28  are selected to facilitate substantially clean, sequential drilling, i.e., via formation, in a wide variety of metallic, dielectric, and other target materials that may exhibit different optical absorption, material removal threshold, or other characteristics in response to UV, visible, or other appropriate wavelengths of light. 
       FIG. 2  depicts a cross-sectional view of a sequentially laminated target  30  of the type typically processed by machining laser beam system  10  of  FIG. 1 . For convenience, target  30  is depicted as having only two layers  60  and  62 . Layer  62  supports layer  60 , from which material is removed to form a via  64 . For vias to be consistently and properly drilled, the depth of focus of machining beam  28  is established within layer  60 . Thus, the depth of focus is set between a surface  66  of sequentially laminated target  30  and a surface  68  of layer  62  that is beneath layer  60 . Layer  60  has a thickness  70 , which is the distance between surfaces  66  and  68 . 
     Layer  62  may contain, for example, standard metals such as aluminum, copper, gold, molybdenum, nickel, palladium, platinum, silver, titanium, tungsten, metal nitrides, or combinations thereof. A conventional metal layer  62  varies in thickness, typically between 9 μm-36 μm, but may be thinner or as thick as 72 μm. Sequential conductive layers in a single target  30  are typically made of the same material. 
     Dielectric matrix or layer  60  may contain, for example, a standard organic dielectric material such as benzocyclobutane, bismaleimide triazine, cardboard, cyanate esters, epoxies, phenolics, polyimides, polytetrafluorethylene, various polymer alloys, or combinations thereof. Conventional organic dielectric layers vary considerably in thickness, but are typically much thicker than metal layers such as layer  62 . An exemplary thickness range for organic dielectric layers  60  is about 30 μm-400 μm. 
     Via diameters preferably range from 25 μm-300 μm, but laser system  10  may produce vias that have diameters as small as about 5 μm-25 μm or greater than 1 mm. Because the preferred material removal spot size of machining beam  28  is preferably about 25 μm-75 μm, vias larger than 25 μm may be produced by trepanning, concentric circle processing, or spiral processing. 
     Thus, with reference to  FIG. 2 , the quality of via  64  relates directly to the smoothness of via walls  72 , the cleanliness of a via bottom  74 , and the top and bottom diameters (i.e., taper) of the via. 
       FIG. 3  shows machining beam  28  in relation to target  30  during via drilling. An image column  76  of a focused beam waist  78  of machining beam  28  corresponds to its depth of focus and identifies a region within which the focus of machining beam  28  is sufficiently tight to consistently drill vias of a specified diameter. An image plane  80  identifies the optimum plane of focus for machining beam  28 . To obtain repeatable and high-quality vias, image column  76  extends throughout layer  60 . If the distance from objective lens  26  to target surface  66  and/or conductive surface  68  changes, image column  76  may fall either above or below layer  60 , resulting in a lower quality via. 
       FIG. 4A  is a front view of a machining laser beam system  90  to which are mounted similar tracking laser beam sources  92  and  94  emitting respective tracking laser beams  96  and  98  that propagate along separate beam paths near the path of machining beam  28 .  FIGS. 4B and 4C  show respective top and side views of laser system  90  of  FIG. 4A . Laser system  90  is of similar construction to that of laser system  10 , except for the addition of tracking laser beam sources  92  and  94  and associated ancillary devices operating to process information carried by or derived from tracking beams  96  and  98 . Components common to laser systems  10  and  90  are identified by the same reference numerals. 
     Beam positioning system  22  causes relative translational movement of machining beam  28  along surface  66  of target  30  and relative axial movement between objective lens  26  and target  30 . Tracking beams  96  and  98  are offset by fixed, known distances from and are positioned relative to machining beam  28  so that different ones of tracking beams  96  and  98  lead machining beam  28  as it traverses the machining path in the opposite directions along the X axis. Although not shown, tracking laser beams  96  and  98  may also be configured to lead machining laser beam  28  as it travels along the Y axis. 
     Alternatively, if the topography of surface  66  or thickness  70  of layer  60  varies only slightly over the offset distance between machining beam  28  and a selected one of tracking beams  96  and  98 , the selected tracking beam may be employed to measure the topography of surface  66  and thickness  70  of layer  60  near laser target position  46 . The selected tracking beam need not lead machining beam  28 , and measurements can be taken once objective lens  26  is located above laser target position  46 . This is so because the selected tracking beam is sufficiently near laser target position  46  such that the measurements taken approximate the parameters relating to laser target position  46 . 
       FIG. 5  shows the positional relationship of tracking beam  96  and an associated beam position sensor  100  for measuring surface topography and layer thickness of target  30 . Tracking beam source  92  emits tracking laser beam  96 , which is incident on and a portion of which reflects off of surface  66  of target  30  to form a first reflected light beam  102 . First reflected light beam  102  is then received by beam position sensor  100 . The characteristics of first reflected light beam  102 , as measured by beam position sensor  100 , are processed to determine an axial distance  104  measured between a known location relative to tracking laser beam  96  and surface  66  at the instantaneous location of incidence of tracking laser beam  96  on surface  66 . In the embodiments described and as shown in  FIG. 5 , the known location is the exit surface of objective lens  26 . Axial distance  104  corresponds to the height of surface  66  at the measurement location. Determining axial distance  104  continually as tracking beam  96  moves along surface  66  provides a surface typography measurement of target  30 . This enables laser system controller  56  to cause corresponding movement of objective lens  28  along beam path  18  to substantially maintain the position of image column  76  on surface  66  or within layer  60 . 
       FIG. 5  further shows the function of measuring ablation layer thickness  68 . If layer  60  is at least partly transparent and the angle of incidence θ is within a specified range, a portion of tracking laser beam  96  propagates through layer  60  and reflects off surface  68  of electrically conductive layer  62  beneath to form a second reflected light beam  106 . This is true, for example, of a printed circuit board in which an epoxy top layer  60  is supported on a copper layer  62 . Second reflected light beam  106  is separated from first reflected light beam  104  by a displacement distance  108 , which corresponds to the thickness of layer  60 . Laser position sensor  100  is operable to measure and provide an output signal corresponding to displacement distance  108 . Electrical signals generated by incidence of first and second reflected light beams  102  and  106  on position sensor  100  provide information about displacement distance  108 , which corresponds to thickness  70  of layer  60 . Laser system controller  56  can process the outputs of beam position sensor  100  to determine the actual thickness  70  of layer  60  and thereafter adjust accordingly the number of pulses of laser energy applied to layer  60  to form via  64  to compensate for variation in thickness  70 . Skilled persons will appreciate that measuring one or both of target surface topography and target layer thickness entails repetitive sampling of beam position sensor  100  by laser system controller  56  as the tracking beam travels with machining beam  28  along its beam path. 
       FIG. 6  shows an alternative configuration to the system of  FIG. 5 , in which tracking laser source  92  and beam position sensor  100  are housed in the same compartment. One such commercially available laser triangulation device is a Model opto NCDT 1400, manufactured by Micro Epsilon, Koenigbacher Strasse 15 d-94496, Ortenburg, Germany. 
       FIG. 7  shows a first alternative tracking device  109   a  for use in system  10 , in which a capacitance probe or an eddy current probe is used to track the surface topography of conductive layer  62  of target  30  having an optically transparent dielectric layer  60 . Examples of a commercially available capacitance probe and a commercially available eddy current probe are, respectively, a Model capa NCDT 620 and a Model eddy NCDT 3300, both of which are manufactured by Micro Epsilon, Koenigbacher Strasse 15 d-94496, Ortenburg, Germany. 
       FIG. 8  shows a first alternative tracking device  109   b  for use in system  10 , in which a confocal device composed of a light tracking source and a feedback device is housed in the same compartment. Such a device is implemented in accordance with confocal optical principles and uses precisely located lenses to change polychromatic light into multiple monochromatic light sources that are focused at precise distances. The reflections from the monochromatic light sources are then used to determine the distance of the surface or surfaces measured.  FIG. 8  shows confocal tracking device  109   b  tracking surfaces  66  and  68  simultaneously. One such commercially available confocal device is a Model opto NCDT 2400, manufactured by Micro Epsilon, Koenigbacher Strasse 15 d-94496, Ortenburg, Germany. 
       FIG. 9  shows the capacitance probe or eddy current probe  109   a  of  FIG. 7  and laser triangulation device of  FIG. 6  arranged in combination to monitor the thickness of dielectric layer  60  if it is not transparent to laser light. The wavelength of a beam emitted by a laser triangulation device is typically about 650 nm. Dielectric target materials suitable for laser ablation are manufactured by Ajinomoto Fine-Techno Co., Inc., Japan, and are included in the Ajinomoto Buildup Film product family. Three examples include ABF SH9K (transparent), ABF GX3 (transparent), and ABF GX13 (opaque version). 
     The methods of measurement of the target surface topography and thickness  70  of layer  60  are described more fully with reference to  FIGS. 10A and 10B . Two operational modes corresponding to measurement of surface topography of target  30  and measurement of layer thickness  70  may be performed separately or concurrently, depending on user needs and limitations of the hardware chosen. Both operational modes entail adjusting the position of image column  76  of focused beam waist  78  relative to target  30 . 
     In a first operational mode, tracking beam  96  is incident on surface  66  of target  30  at an angle θ with respect to a normal to surface  66 . A portion of tracking beam  96  is reflected from surface  66  at a location A as first reflected light beam  102 , which is received by beam position sensor  100 . Beam position sensor  100  comprises a two-dimensional array of nominally identical sensor elements  110 . For sake of clarity in describing the operational modes,  FIGS. 10A and 10B  show a one-dimensional or linear array position sensor  100  (i.e., one row of a two-dimensional array) that produces an output indicative of the sensor element  110  receiving first reflected light beam  102 . A two-dimensional array position sensor  100  produces an output indicative also of the array quadrant of the sensor element  110  that receives first reflected light beam  102 . 
     The angle θ and the fixed distance between a reference point  112  of emission from the exit surface of objective lens  26  and beam position sensor array  100  can be used, in conjunction with simple geometry, to determine for each location of incidence the distance from reference point  112  of emission to surfaces  66  and  68 . For instance, if the distance from reference point  112  of emission to a first sensor element  110   1 , of beam position sensor  100  is designated D, the unit length of each sensor element  110  is L, and n is the position number of the sensor element  110  on which first reflected light beam  102  is received (counting from the sensor element  110  nearest to tracking beam  96 ), a distance from reference point  112  of emission to surface  66  or  68  could be determined by the expression
 
(D+n[L])/(2[tan θ]).
 
 FIG. 10A  shows tracking beam  96  striking target  30  at a location A, where the surface height and layer thickness  70  correspond to the expected values for those variables. First reflected light beam  102  is received by a fourth sensor element  110   4  of the beam position sensor  100 , corresponding to a distance  114   A  from reference point  112  of emission to target surface  66  at location A.  FIG. 10B  shows tracking beam  96  striking target  30  at a location B, where the surface height and layer thickness  70  are both decreased. Although location B is farther away from reference point  112  of emission of tracking beam  96  than is location A, laser system controller  56  responds to the output of beam position sensor  100  to cause beam positioning system  22  to move objective lens  26  downward along the Z axis so that first reflected light beam  102  travels the same distance before striking beam position sensor  100 . Because beam position sensor  100  moves in concert with objective lens  26 , first reflected light beam  102  is received by fourth sensor element  110   4  corresponding to distance  114   A  from reference point  112  of emission to target surface  66  at location B, where the surface height of target  30  is reduced relative to that at location A. The bent solid lines extending through layer  60  to beam position sensor  100  in  FIGS. 10A and 10B  represent the actual beam path produced by application Snell&#39;s law of refraction to light propagating through different media. (The degree of beam path bending shown is exaggerated to more clearly demonstrate the effect of refraction.)
 
     When tracking laser beam  96  is incident on surface  66  at locations A and B, beam position sensor  100  produces output signals corresponding to the respective sensor elements  110   4  and  110   7  on which second reflected light beam  106  is incident. Laser system controller  56  responds to the output signals to cause beam positioning system  22  to move objective lens  26  along beam path  18  to axial distance  104  that maintains the position of image column  76  on surface  66 . 
     In a second operational mode, laser system  90  is operable to automatically adjust the amount of energy applied by machining beam  28  based on the measured thickness  70  of layer  60  near the location to be drilled. As in the first operational mode, tracking beam  96  is produced at an angle θ and first reflected light beam  102  is received by beam position sensor  100 . In the second operational mode, however, a portion of tracking beam  96  propagates through target surface  66  and reflects off of surface  68  of second layer  62 , which is typically a metallic conductor. This second reflected light beam  106  is also received by the beam position sensor  100 . Beam position sensor  100  receives two portions of tracking laser beam  96  as reflected by surfaces  66  and  68  of the sequentially laminated target  30 . 
       FIG. 10A  shows fourth sensor element  110   4  and tenth sensor element  110   10  receiving first and second reflected light beams  102  and  106 , respectively. Distance  114   B  is the reference datum for surface  68  of conductive layer  62  relative to reference point  112 .  FIG. 10B  shows that, when tracking beam  96  moves over a location where thickness  70  of layer  60  of target  30  is diminished, displacement distance  108  between the first and second reflected light beams  102  and  106  is also diminished and fourth sensor element  110   4  and seventh sensor element  110   7 , receive reflected light beams  102  and  106 , respectively. The greater thickness  70  of layer  60  in  FIG. 10A  results in a span of six sensor elements  110  (between sensor elements  110   4  and  110   10 ), whereas the reduced thickness  70  in  FIG. 10B  results in a span of only three sensor elements  110  (between sensor elements  110   4  and  110   7 ). The span between sensor elements  110  is, therefore, directly proportional to thickness  70  of layer  60 , so the output signal from the beam position sensor  100  may be processed to identify thickness  70  of layer  60  and adjust by a corresponding amount the machining beam energy applied to remove the appropriate amount of material from target  30  for the thickness measured. 
     Monitoring thickness  70  of dielectric layer  60  can be accomplished by filtering first reflected light beam  102  in sensor element  110   10  and filtering reflected light beam  106  in sensor element  110   4 . If the double reflection implementation described above does not operate properly, it is possible to use two beam position sensors  100 , one of which monitoring dielectric surface  66  and the other of which monitoring conductive surface  68 . Each of sensors  100  has a filter such that one filter on a first beam position sensor  100  eliminates the dielectric reflected light beam  102  and the other filter on a second beam position sensor  100  eliminates conductive layer reflected light beam  108 . Light polarizing filters would be suitable for use as the filters in detecting the difference in distance  70  between layers  66  and  68 . 
       FIG. 11  presents a graph  120  that illustrates the characteristics of a Gaussian profiled machining beam  122  and an image profiled machining beam  124  with respect to the diameter of the laser spot versus the axial distance between the target and the objective lens. Gaussian profiled machining beam  122  and image profiled machining beam  124  represent alternative shapes of energy profiles of machining laser beam  28 . The theoretical image plane is the mathematical ideal distance between objective lens  26  and target surface  66 . Obtaining this distance and maintaining it throughout the process is greatly desired. Deviation from the theoretical image plane is tolerable, but, as the spot size of machining laser beam  28  shrinks, the amount of allowable deviation from image plane  80  becomes so small that the variations on the topography of target  30  begins to vary the via size to the extent that drilled vias may become unacceptable from a quality standpoint. This is the reason why the equipment is designed to follow the topography of the target. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.