Abstract:
A mesh is created from selected focus points having locations on a surface of interest of a sample being micro-imaged. The mesh and associated focus settings of the focus points are used to define adjacent focus facets forming a focus surface substantially coincident with the surface of interest of the sample. In micro-imaging the sample, a micro-imaged portion of the surface of interest is segmented for the purpose of acquiring tile images. A tile image focus location is used to extract a tile image focus setting from the focus surface. A tile image focus determination process selects a focus facet coincident with the tile image focus location and interpolates a tile image focus setting from focus settings associated with the focus points defining the focus facet. If a coincident focus facet is not found, the tile image focus setting is set to the focus setting of a nearest focus point. Dependence on autofocus is thus eliminated, providing faster imaging and better focused images.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is the first application filed for the present invention. 
     TECHNICAL FIELD 
     The invention relates to the field of micro-imaging and, in particular, to methods and apparatus for acquiring high magnification images of a tilted and/or uneven sample using a micro-imaging system having a shallow depth-of-field. 
     BACKGROUND OF THE INVENTION 
     In the field of micro-imaging a high magnification imaging instrument is used to acquire images of a sample. Magnification powers of 100× or more are used for micro-imaging semiconductor integrated circuits (IC) to extract design and layout information for the purposes of design verification, product quality assurance and reverse engineering. At such high magnifications, a parameter of the high magnification microscope known in the art as the depth-of-field becomes very important. 
     According to what is know in the art as the “Thin Lens Approximation”, while imaging an object using a theoretical optical system only an infinitesimally thin object plane in front of the optical system is in focus on an infinitesimally thin image plane, behind the optical system. The depth-of-field corresponds to the thickness of the image plane and therefore, in theory, the depth-of-field approximates zero. In practice, while imaging an object, features behind and in front of the theoretical object plane are also focused on the image plane. The depth-of-field is the thickness of the slice around the object plane that can be imaged in-focus. 
     Imaging objects at most one to two orders of magnitude smaller than constituent optical elements of an optical imaging system operating at small powers of magnification, the depth-of-field is large enough to capture an entirety of such objects in focus. However in micro-imaging sample IC&#39;s using a 100× power magnification: the optical elements are at most a few orders of magnitude larger than traces on a sample IC (˜1 mm:˜1 μm). The width of traces on the sample IC is just wide enough that interference/diffraction effects are minimal when using ultraviolet light, and the pixel size of the CCD is comparable to the trace width. This results in a dept-of-field that is about the size of the width of a trace or about the thickness of a deposition layer on an IC. 
     Semiconductor components manufactured on a silicon substrate of an IC are several deposition layers in height. Traces interconnecting components on the silicon substrate transcend deposition layers. The components and interconnecting traces form a relief on the silicon substrate. Focusing not only becomes very important, autofocusing techniques are unsuitable because a range of focus settings of the micro-imaging system will appear to provide in-focus images, each image correctly focusing on different features distributed over a range of deposition layers. 
     U.S. Pat. No. 5,647,025 entitled “AUTOMATIC FOCUSING OF BIOMEDICAL SPECIMENS APPARATUS” which issued on Jul. 8, 1997 to Frost, et al. describes an apparatus for inspecting biological specimens and a method for automatically focusing on features using morphological criteria such as brightness, contrast, size, shape, texture and context. The apparatus is adapted to extract a focus measure concurrent with performing pattern recognition. The pattern recognition is optimized for biological cell detection in a particular size range and having a particular geometry. Methods for detecting biological cell nuclei are also presented. While this invention has merit, it is not suited for micro-imaging sample IC&#39;s to extract design and layout information. The methods described by Frost only provide suitable autofocusing at 4× magnification with a field of view of 1.4 mm square. At this magnification the depth-of-field has a substantial thickness enabling reliable focusing on discrete biological cells having cell nuclei. A comparable depth-of-field is not available at 100× magnifications required for micro-imaging a sample IC. 
     Frost&#39;s methods provide a timely inspection of a slide having approximately 700 fields of view. In micro-imaging sample IC&#39;s a surface of interest is typically divided in excess of 10,000 fields of view each corresponding to a tile-image to be acquired. Autofocusing operations performed according to Frost&#39;s teachings to acquire tile images would be time consuming and therefore unsuitable. Minimizing the time taken to acquire tile images is very important as pointed out in co-pending United States patent application entitled “METHOD AND SYSTEM FOR RECALIBRATION DURING MICRO-IMAGING TO DETERMINE THERMAL DRIFT”, which was filed on Jun. 15, 2000 and assigned Ser. No. 09/594,169, the specification of which is incorporated herein by reference. 
     Therefore in micro-imaging a surface of interest for a sample IC, there is a need for methods and apparatus for providing focus settings to enable the acquisition of a very large number of tile-images of the surface of interest. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an interface for selecting locations for a plurality of focus points on a surface of interest of a sample IC to be micro-imaged and to select a focus setting associated with each focus point. 
     It is another object of the invention to compute a virtual focus surface that substantially mimics a surface topology found by components and interconnecting traces manufactured on sample IC silicon substrate, using a plurality of focus point locations and the associated focus settings. 
     It is yet another object of the invention to compute a tile image focus setting at a tile image focus location based on the focus surface and the plurality of focus points. 
     According to one aspect of the invention, there is provided an apparatus for micro-imaging an uneven surface of interest of a sample comprising means for selecting and storing positional coordinates of focus points associated with a sample coordinate space defined by the sample, to create a focus point list, means for determining an associated focus setting for each focus point in the focus point list, and means for generating a focus surface using the focus point list, as well as means for extracting a tile image focus setting at a tile image focus location from the focus surface, and means for positioning a micro-imaging system to acquire a tile image at the tile image focus setting. 
     The means for selecting and storing positional coordinates of focus points associated with a sample coordinate space preferably comprises a man-machine interface for sending control messages to the micro-imaging system, and receiving image data from the micro-imaging system. The means for determining a focus setting associated with each focus point further comprises means for extracting a focus measure from image data received from the micro-imaging system. The means for extracting a focus measure is prefreably an algorithm that performs pixel operations on the image data to extract, for example, a sharpness measure. The means for generating a focus surface using the focus point list comprises means for grouping focus points from the focus point list into focus point groups, the focus point groups being stored in a focus point group list. The means for grouping focus points into focus point groups comprises a focus point grouping algorithm, the positional coordinates and the associated focus setting of each focus point in a focus point group forming a focus facet, and the focus surface comprises abutting focus facets. The focus point grouping algorithm is preferably a mesh generation algorithm, for example, a triangular mesh generation algorithm. If so, each focus point group is a focus point triad. A preferred triangular mesh generation algorithm is the Delaunay triangulation algorithm, which is well known in the art. In order to ensure accurate focusing, the means for grouping focus points into focus point groups preferably further comprises a focus point group exclusion algorithm for excluding from the focus point group list a focus point group having substantially collinear focus points. 
     The means for extracting a tile image focus setting at a tile image focus location from the focus surface comprises an algorithm that selects a focus facet coincident with the tile image focus location, interpolates at the tile image focus location the focus settings associated with the focus points in the focus point group associated with the selected focus facet and sets the tile image focus setting to the interpolated focus setting. If the tile image focus location is not coincident with a focus facet, the means for extracting a tile image focus setting at a tile image focus location from the focus surface comprises an algorithm for selecting a closest focus point to the tile image focus location and setting the tile image focus setting to the focus setting associated with the closest focus point. 
     The invention also provides a method of generating a focus surface for determining a focus setting for a micro-imaging system used to capture micro-images of a surface of interest of a sample. The method comprises steps of: selecting a plurality of focus points respectively having focus point positional coordinates with respect to a sample coordinate space, to create a focus point list, at least some of the focus points being selected in close proximity to each other in the vicinity of an abrupt change in elevation of the surface of interest; determining a focus setting for each focus point; and compiling a list of focus point groups from the focus point list. The focus point list is stored after it is created. 
     Each of the focus point groups comprises three focus points that form a focus point triad. Each focus point triad defines a triangular mesh cell in the sample coordinate space. The combination of the focus point positions and the focus point settings of the focus points defining a triangular focus facet and the focus surface comprises adjacent triangular focus facets. Mesh cells are generated by a mesh generation algorithm using the plurality of focus points. The mesh generation algorithm preferably comprises a triangulation algorithm, such as the Delaunay triangulation algorithm. 
     The invention further provides a method of determining a tile image focus setting for micro-imaging a tile image constituent of an image-mosaic representative of a surface of interest of a sample having at least a one of a tilted and an uneven surface of interest. The method comprises steps of selecting a tile image to be acquired, the tile image having an associated tile image focus location with respect to a sample coordinate space defined by the sample; determining a focus facet coincident with the tile image focus location using a focus facet list associated with a focus surface of the sample, each focus facet in the focus facet list being defined by a focus point group, and focus points in each focus point group having respective positional coordinates with respect to the sample coordinate space, and an associated focus setting; and, interpolating the focus settings of the focus points of the focus facet coincident with the tile image focus location to determine a tile image focus setting for acquiring the tile image. The tile images have a predetermined shape and the tile image focus location is typically a geometrical center of the tile image. 
     The focus facet list is preferably parsed to exclude focus facets from the focus facet list that have substantially collinear focus points, in order to ensure that such focus facets are not used for focus setting. The focus facets are excluded from the focus facet list by computing a collinearity measure using positional coordinates of the focus points of the focus facet. The collinearity of the focus points of a focus facet is determined by, for example, computing an area of a mesh cell associated with the focus facet. The computed area is used as the collinearity measure. When a collinearity measure of the focus points is determined to be less than a predetermined threshold, the focus facet is excluded from the focus facet list. 
     For tile images having tile image focus locations outside all focus facets in the focus facet list, and tile images having tile images focus locations coincident with focus facets having substantially collinear focus points, the method preferably further comprises steps of selecting from among focus points in a focus point list, a closest focus point to the tile image focus location; and, assigning the focus setting associated with the closest focus point to the tile image focus setting. 
     The invention also provides a method of acquiring tile image constituents of an image-mosaic representative of a sample having at least a one of a tilted and an uneven surface of interest that is micro-imaged using a high magnification micro-imaging system. The method comprises steps of selecting a tile image to be acquired, the tile image having an associated tile image focus location with respect to a sample coordinate space defined by the sample; determining a focus facet coincident with the tile image focus location by parsing a focus facet list of focus facet constituents of a focus surface, each focus facet in the focus facet list being defined by a focus point group, and each focus point in the focus point group having positional coordinates with respect to the sample coordinate space, as well as an associated focus setting; interpolating the focus settings of the focus points of the focus facet coincident with the tile image focus location to determine a tile image focus setting; positioning the micro-imaging system in relation to the sample so as to acquire a tile image associated with the tile image focus location; adjusting a focusing mechanism of the micro-imaging system to the tile image focus setting; and acquiring the tile image. 
     A primary advantage of the invention is ensuring that the micro-imaging system focuses correctly on a selected sample IC, and in particular on features revealed at each deconstructive step in the process of reverse engineering the IC. A further advantage is the generation of a virtual focus surface that closely follows the relief of the surface of interest to provide tile image focus settings at tile image focus locations. This eliminates reliance on autofocus while acquiring tile images using the micro-imaging system, which is known to be unreliable for that purpose. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
     FIG. 1 is a work flow diagram showing an overview of an exemplary process by which image-mosaics representative of steps in a deconstruction of a semiconductor integrated circuit (IC) are acquired; 
     FIG. 2 is a process diagram showing an exemplary progression of steps in manufacturing a sample IC; 
     FIG. 3 is a process diagram showing an exemplary progression of steps in deconstructing a sample IC for reverse-engineering; 
     FIG. 4 shows an example of a micro-imaging system comprising a computer controlled optical stage and an imaging system such as a high magnification power microscope equipped with a digital imaging system including a Charge Coupled Device (CCD) camera; 
     FIG. 5 is a schematic diagram showing a plan view of an exemplary portion of a surface of interest of a sample IC corresponding to an exemplary deconstructive step; 
     FIG. 6 is a schematic elevational diagram showing a cross-section of the sample IC silicon substrate along the line  6 — 6  of FIG. 5; 
     FIG. 7 is a schematic diagram showing an exemplary selection of focus points having coordinate positions with respect to a sample coordinate space in accordance with the preferred embodiment of the invention; 
     FIG. 8 is a schematic elevational diagram showing positional relationships of focus points shown in FIG. 7 in a cross-section through the sample IC silicon substrate taken along line  8 — 8  of FIG. 7; 
     FIG. 9 is a schematic perspective diagram showing a schematic component manufactured on a silicon substrate and relative positions of focusing planes of images acquired in extracting focus measures in a coarse focus search; 
     FIG. 10 is a graph showing an example of a variation of a sharpness measure in a progression of focus settings of images acquired in performing a coarse focus search; 
     FIG. 11 is a schematic perspective diagram showing a schematic component manufactured on a silicon substrate and relative positions of focusing planes of images acquired in extracting focus measures in a fine focus search; 
     FIG. 12 is a graph showing an example of a variation of a sharpness measure in a progression of focus settings of images acquired in performing a fine focus search; 
     FIG. 13 is a schematic diagram showing a superposition of a computed triangular mesh over the portion of a surface of interest in accordance with an exemplary embodiment of the invention; 
     FIG. 14 is a schematic perspective diagram showing focus points defining a focus facet according to a preferred embodiment of the invention; 
     FIG. 15 which appears on sheet seven of the drawings, is a schematic elevational diagram showing a cross-section of a focus surface and its positional relationship to the relief of the surface of interest of the sample IC; 
     FIG. 16 is a schematic diagram showing positional relationships between the triangular mesh and tile images of the surface of interest of the sample IC to be acquired in accordance with a preferred embodiment of the invention; 
     FIG. 17 is a schematic diagram showing a tile image acquisition sequence prescribed by an exemplary tiling algorithm in accordance with a preferred embodiment of the invention; 
     FIG. 18 is another schematic diagram showing a tile image acquisition sequence prescribed by another exemplary tiling algorithm in accordance with another embodiment of the invention; 
     FIG. 19 is a schematic diagram showing an exemplary tile image acquisition sequence used in acquiring specified tile images in accordance with yet another embodiment of the invention; 
     FIGS. 20A and 20B are flow diagrams showing a process by which focus settings are provided at tile image focus locations given a focus surface, in accordance with a preferred embodiment of the invention. 
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a work-flow diagram showing an overview of an exemplary prior art process by which image-mosaics representative of steps in the deconstruction of a semi-conductor integrated circuit (IC) are acquired. IC&#39;s  110  are fabricated on a wafer  112 . The wafer  112  comprises a mono-crystalline silicon substrate which is a natural insulator. Doping the silicon substrate with other chemical elements can change the properties of the silicon, including making the silicon substrate a semi-conductor or a conductor. Such substrate processing is performed as part of a manufacturing process  114  of chip  116 . In packaging a chip  116 , a die  120  is cut in a step  118  from the wafer  112  and is encapsulated in step  122  to form the chip  116 . 
     The manufacture of integrated circuits typically involves a verification process  124  by which wafers  112 , cut dies  120 , or portions thereof are inspected using a micro-imaging system  126  to extract design and layout information for design validation or purposes of competitive analysis. 
     For the purposes of product quality assurance or competitive analysis, for example, a process of reverse-engineering  128  is performed on the chip  116 . A first step in the reverse-engineering process  128  is a decapsulation  130  of the chip  116  to remove the die  120 . Die  120  is inspected using the micro-imaging system  126  to extract design and layout information. The micro-imaging system  126  may include high magnification optical microscopes, scanning electron microscopes, field emission electron microscopes, or the like. Design and layout extraction from a die  120  or portion thereof involves a process of deconstruction  132  by which layers formed during the manufacturing process  114  are removed step-by-step. 
     High magnification tile images  134  of the sample die  120  are acquired between each deconstructive step  132  under the control of a computer workstation  136 . The computer workstation  136  controls the micro-imaging system  126  using control signals  138 . The computer workstation  136  receives tile image data  140  from the micro-imaging system  126  and saves the tile image data  140  to memory, typically high capacity storage  142 , such as a hard disk. Generally, the tile image data  140  is transmitted to the high capacity storage  142  and stored in a compressed format minimizing data transfer requirements between the computer workstation  136  and the high capacity storage  142  and, minimizing data storage requirements at the high capacity storage  142 . 
     The stored tile images  134  are assembled into image-mosaics  144 , each image-mosaic  144  representing a surface of interest of the die  120  at a deconstructive step  132 . During acquisition of the tile images  134  of the die  120 , a sample coordinate space  146  is defined. The sample coordinate space  146  is used to align the tile images  134  and the image-mosaics  144 . 
     FIG. 2 is a process diagram showing an exemplary prior art progression of steps followed during the manufacture of an IC. The diagram shows a progression of cross-sections through a silicon substrate, representing exemplary steps in manufacturing a component such as a junction. In step  152  of the progression, the silicon substrate is doped using diffusion and/or ion implantation techniques to change its characteristics and, in particular, to form P-wells, well known in the art. In step  154 , another implantation is performed to form N-type sources and drains. A gate oxide layer is deposited between the sources and drains, and a field oxide is deposited in other areas of the chip in step  156 . A polysilicon gate layer is deposited in step  158 , and in steps  160  and  162  the deposition of oxide layers is effected. Metal layers for providing connectivity between the gates, sources and drains are deposited in step  164 . Step  166  illustrates the deposition of a passivation layer, typically used to protect the IC from physical damage and/or contamination with dust particles before it is encapsulated in step  122  (FIG.  1 ). 
     FIG. 3 is a process diagram showing an exemplary prior art progression of deconstructive steps used to reverse-engineer a sample IC. Step  170  illustrates a cross-section through a silicon substrate of a die  120  after decapsulation in step  130  (FIG.  1 ). Steps  172 ,  174 ,  176 ,  178 ,  180  and  182  illustrates a progressive removal of the deposited material layers, such as the passivation layer, metalization layers, polysilicon layers, base contact layers, the field oxide layer, etc. This results in an exposed silicon substrate (step  182 ) including the well structures manufactured during steps  152  and  154  (FIG.  2 ). In order to reveal the well structure, the back surface of the die  120  may also be deconstructed. Steps  184  and  186  show the progressive deconstruction of the back surface of the die  120  to expose the P- and N-wells. In extracting design and layout information both surfaces of the die  120  may be micro-imaged, and therefore both represent surfaces of interest. 
     FIGS. 2 and 3 also show components spanning deposition layers, the surface of interest having a relief and traces that follow the relief. 
     FIG. 4 shows a prior art example of a micro-imaging system used to acquire tile images of a sample IC. The micro-imaging system  126  typically includes an optical stage schematically illustrated in the diagram at  200 . The optical stage  200  provides positioning of the die  120  with respect to the high magnification microscope  210 . The optical stage  200  has a vertical axis of displacement  202  and two horizontal axes of displacement  204  and  206 . Collectively the axes of displacement  204  and  206  provide motion of the die  120  in a field of view of the microscope  210 . Axis of displacement  202  provides positioning of the die  120  in a direction perpendicular to the field of view of the microscope  210  and therefore enables focusing of high magnification optics of the microscope  210  onto a surface of the die  120 . 
     Acquisition of tile images  134  is facilitated by a digital imaging system such as a Charge Coupled Device (CCD) camera  230  having pixelated light sensing elements. The optical stage  200  and CCD camera  230  receive control signals  138  from a computer workstation  136  to position the die  120  and move it into focus under the microscope  210 . Other control signals  138  effect the capture of tile images  134  which are transferred as digital data  140  to the computer workstation  136 . 
     FIG. 5 is a schematic diagram showing a plan view of an exemplary portion of a surface of interest of a sample IC corresponding to a deconstructive step in reverse-engineering the sample IC. A representation of the field of view of the micro-imaging system  126  (FIG. 4) is presented to an analyst via the computer workstation  136  (FIG. 1) in a view. The analyst cannot see all features in focus (sharpness)due to the shallow depth-of-field of the micro-imaging system  126 , and also due to the fact that the components project above the silicon substrate (not shown) of the sample IC die  120 . An analyst can, however, discern components such as capacitor components  200  due to their relative size and shape, and perhaps also due to their relationship to other components on the silicon substrate. The analyst can likewise discern abrupt changes in elevation of the surface of interest such as shown at  202  by observing abrupt changes in sharpness of the focus in the field of view. 
     FIG. 6 is a schematic elevational diagram showing a cross-section of the sample IC silicon substrate taken along line  6 — 6  of FIG.  5 . As is apparent, the deconstruction of a sample IC generally does not yield flat surfaces. Components manufactured on the silicon substrate of the die  120  project above the surface of the silicon substrate. By gradually changing the focus setting of the micro-imaging system  126  while observing an image of the surface displayed on the computer workstation  136  as the image is refreshed, the analyst can also observe sloped portions of the relief such as shown at  204  and relatively flat portions such as shown at  206 . 
     In accordance with the preferred embodiment of the invention, a plurality of focus points  230 ,  232  and  234  are selected on a surface of an IC. Each focus point has a respective positional x-y coordinate with respect to a sample coordinate space defined by the sample IC die  120 . An associated focus setting is determined for each of the focus points  230 ,  232  and  234  to enable the calculation of focus settings at other locations on the surface intermediate the respective focus points. 
     FIG. 7 is a schematic diagram showing an exemplary selection of focus points  230 ,  232  and  234  having coordinate positions with respect to a sample coordinate space defined by the sample IC die  120 . FIG. 8 is a schematic elevational diagram showing positional relationships of focus points shown in FIG. 7 in a cross-section through the sample IC silicon substrate taken along line  8 — 8 . 
     Using the computer workstation  136  (FIG. 1) the analyst selects focus points  230 ,  232  and  234  by pointing at displayed features of the sample IC die  120  displayed in the view as the computer workstation  136  is operated in a focus point selection mode. In accordance with a preferred embodiment of the invention, focus points such as  232  are selected in close proximity to an abrupt change in elevation  202  of the 3-dimensional relief on the surface of the sample IC die  120 . 
     FIG. 9 is a schematic perspective diagram illustrating focus setting determination at a focus point located on a common component  240  manufactured on a silicon substrate. Relative positions of focus planes  250  of images acquired in a process of acquiring focus measures  250  during a coarse focus search. 
     FIG. 10 is a graph illustrating a variation of a sharpness measure  262  associated with a progression of focus settings of images acquired during the coarse focus search shown in FIG.  9 . 
     FIG. 11 is a schematic perspective diagram illustrating component  240  with respect to relative positions of focus planes  250  of images acquired in a process of acquiring focus measures  260  during a fine focus search. 
     FIG. 12 is a graph showing an example of a variation of a sharpness measure  266  for a progression of focus setting images acquired during the fine focus search shown in FIG.  11 . 
     In accordance with a preferred embodiment of the invention, a focus setting determination is performed by selecting a focus setting used during the acquisition of a best-focused image from a series of images acquired during a monotonically varying progression of focus settings. The sample IC die  120  (FIG. 4) is positioned by moving the optical stage  200  along the vertical axis  202  (FIG. 4) at focus settings below and above a it current focus setting. The coarse focus search is first performed by moving the optical stage  200  in coarse increments. An image  250  in a series is captured at each focus increment, and displayed on the computer workstation  136 . A focus measure  260  is derived from each image in the series. Preferably, the focus measure  260  is an image sharpness measure, but other focus measures can also be used. Various algorithms for calculating the sharpness of an image are known to persons skilled in the art. The focus measure  260  is used to select the best focused image in the series. The variation in the focus measure may be monotonically varying or peaked. 
     If the variation in the focus measure is monotonic (not shown) then the best focus setting is the focus setting associated with the image having the highest focus measure (unique sharpness). However, another focus search is preferably run to determine if a higher focus measure exists in order to locate a peak in the variation of the focus measure. 
     If the variation  262  of the image sharpness measure is peaked, as shown in FIG. 10, then a fine focus search is preformed after the coarse focus search. In accordance with the invention the optical stage  200  is moved in fine increments for the fine focus search. Fine increments around the highest image sharpness measure  264  of the peak corresponding to the coarse focus search are used to find an image having the highest image sharpness measure  268 . Focus searching is performed using coarse and fine focus adjustment increments in accordance with the preferred embodiment of the invention. However, other methods may be used. It should also be understood that the invention is not limited to mechanical focus adjustment systems. Persons skilled in the art will understand that focus adjustment may also be achieved by changing lens properties, as is the case in focused ion beam imaging, or the like. 
     In accordance with the invention, the selected focus points such as  230 ,  232  and  234  are used as input to a mesh generation algorithm. Mesh generation algorithms extract point groupings to define mesh cells. A well-known mesh generation algorithm is the Delaunay triangulation algorithm. The Delaunay triangulation algorithm generates point groupings having three points, which are referred to as a “point triad”. Each point triad defines vertices of a triangular mesh cell. An exemplary triangular mesh cell specified by a focus point triad is shown in FIG. 13 at  270 . The juxtaposed mesh cells form a “triangular mesh” generated by the Delaunay triangulation algorithm. 
     As shown in FIG. 13, a triangular mesh  272  substantially covers the surface of interest. A corner  274  of the surface of interest is not covered by the triangular mesh  272 . The degree of coverage of the surface of interest by the triangular mesh is dependent on the position of focus points  230 ,  232  and  234  selected. The focus points are preferably selected so that mesh cells having a small area, such as shown at  276 , are generated by the Delaunay triangulation algorithm in areas where there are abrupt changes in elevation on the surface of interest. The selection of closely spaced focus points  232  in the vicinity of the abrupt, changes  202  in elevation yields small area mesh cells  276  in those areas. 
     FIG. 14 is a schematic perspective diagram illustrating a focus facet associated with a mesh cell. The focus settings associated with each focus point  230 ,  232  and  234  in each focus point triad and the positional coordinates of each focus point  230 ,  232  and  234  in each focus point triad define a focus facet  280  associated with the mesh cell  270 . Focus facets  280  are juxtaposed and form a “focus surface” (see FIG. 15) in a hybrid three-dimensional coordinate space  284  defined by the x-y coordinates of the respective focus points on the sample coordinate space and respective focus settings associated with each focus point. Each mesh cell represents a projection of the focus facet onto the surface of interest of the die  120 . 
     FIG. 15 is a schematic elevational diagram showing a cross-section of an exemplary focus surface in accordance with a preferred embodiment of the invention and its relationship to the relief of the surface of interest of the sample IC. The cross-section shown in FIG. 15, taken along line  15 — 15  of FIG. 13, shows that the focus surface closely conforms to the relief on the sample IC die  120 , if the focus points are well selected. The dashed line represents a locus of focus points. Sections  288  representative of a focus facet associated with mesh cells  276  is steeply inclined due to the abrupt change in elevation of the relief. 
     FIG. 16 is a schematic diagram showing positional relationships between the mesh cells shown in FIG.  13  and tile images of the surface of interest of the sample IC to be acquired. The tiling pattern for the surface of interest of the sample IC die  120  can be determined by a tiling algorithm, or specified apriori. Tiling algorithms typically output a sequential list of tiles which can be used to effect the order in which tile images of the surface of interest are acquired. 
     FIGS. 17 and 18 are schematic diagrams showing exemplary tile image acquisition sequences output by tiling algorithms. As will be understood by those skilled in the art, images of the entire surface of interest are not always required. Consequently, other tile image acquisition patterns may be followed, such as shown in FIG.  19 . It should also be understood that a tiling algorithm is not required to output a tile list having tiles of the same size or the same geometry. FIG. 16 shows the portion of the surface of interest tiled with quadrilateral tiles such as rectangular tiles  290  and square tiles  292 . Each tile image  134  having a geometry specified by tiles  290  and  292  has an associated tile image focus location  294 . Typically, the tile image focus location  294  is located on a geometrical center of the corresponding tile but may be positioned elsewhere. 
     In accordance with the preferred embodiment of the invention focus points are selected to generate a triangular mesh  272  that covers the surface of interest in such a way that each tile image focus location is coincident with at least one mesh cell. In practice features enabling focusing may not be available on certain portions of the surface of interest, such as would be the case with a portion of the substrate having no components or traces. FIG. 16 also shows a tile  296  having a tile image focus location  294  outside of the triangular mesh  272 . If this occurs, a tile image can only be acquired by using the focus setting of the nearest focus point, focus point  234  in this example. 
     FIGS. 20A and 20B are flow diagrams showing a process by which focus settings are determined for tile image focus locations, given a focus surface derived in accordance with a preferred embodiment of the invention. A process for acquiring tile images for an image-mosaic is begun in step  300 . A list of tile images to be acquired, typically specifying a tile image acquisition sequence, is retrieved in step  302 , and a focus facet list is retrieved in step  304 . The image-mosaic acquisition process proceeds by selecting from the tile image list a first tile image to be acquired in step  306 . For each selected tile image, the image-mosaic acquisition process determines the tile image focus location  294  (FIG. 16) in step  308 . As noted above, the tile image focus location is generally the geometrical center of the tile image to be acquired. The focus facet list is searched for a focus facet coincident with the tile image focus location  294 . 
     The first focus facet in the focus facet list is selected in step  310 . The image-mosaic acquisition process determines the focus point triad associated with the selected focus facet (step  312 ). A preliminary check is performed in step  314  to determine whether the x-y coordinates of the focus points in the focus point triad are collinear. Well known mathematical manipulations that provide a measure of collinearity may be used for this purpose. The area of the mesh cell defined by the focus point triad can be computed as a simple measure of collinearity. A mesh cell having a zero area is defined by collinear focus points. In accordance with a preferred embodiment of the invention, collinearity is ascertained using a collinearity threshold. A mesh cell having an area that is less than or equal to a predefined threshold is assumed to have collinear focus points. If a focus facet is determined to be associated with a mesh cell having collinear focus points, that focus facet is excluded from the focus facet list in step  316 . Focus facets associated with mesh cells  276  having a small surface area are typically adjacent abrupt changes  202  in the elevation of the relief on the surface of interest of the sample IC die  120 . The collinearity threshold can be selected to exclude a majority of small area mesh cells, such as  276 . As described above with respect to FIG. 15, focus facets associated with mesh cells  276  having a small surface area are generally steeply inclined. Therefore, deriving focus settings from such focus facets may result in out-of-focus tile images, because of the shallow depth-of-field of the imaging equipment. 
     If the x-y coordinates of the focus points in the focus point triad associated with the selected focus facet are not collinear, as determined in step  314 , the image-mosaic acquisition process determines whether the tile image focus location is located within the mesh cell associated with the selected focus facet (step  318 ). Mathematical methods for determining whether the tile image focus location is located within the mesh cell are well known in the art. On determining that the tile image focus location is coincident with the mesh cell associated with the selected focus facet, a tile image focus setting  298  (FIG. 14) is set in step  320  to a value interpolated with respect to the tile image focus location. The x-y positional coordinates and associated focus settings of the focus points in the focus point triad associated with the selected focus facet are used as inputs to the interpolation calculation. 
     The optical stage  200  is positioned in accordance with the specified x-y coordinates of the tile image focus location in step  322 . The field-of-view of the micro-imaging system  126  is brought into focus in step  324  using the tile image focus setting and a tile image  134  is acquired in step  326 . If the end of the tile image sequence list has not been reached, as ascertained in step  328 , the image-mosaic acquisition process selects a next tile image to be acquired (step  330 ) and resumes from step  308 . If it is determined that the end of the tile image sequence list has been reached in step  328 , the image-mosaic acquisition process ends in step  332 . 
     If the tile image focus location is not found to be coincident with the mesh cell associated with the selected focus facet (step  318 ), or if the selected focus facet was discarded from the focus facet list (in step  316 ), and, the end of the focus facet list has not been reached (step  334 ), the image-mosaic acquisition process selects a next focus facet from the focus facet list (step  336 ), and resumes from step  312 . If the end of the focus facet list is reached in step  334 , the tile image focus location is either outside the triangular mesh  272 , or the image focus location is coincident with a focus facet having focus points that are determined to be co-linear, as described above. In order to determine a tile image focus setting, the image-mosaic acquisition process therefore searches for a closest focus point to the tile image focus location. The associated focus setting of the closest focus point is then used as the tile image focus setting. 
     A process of searching for the closest focus point is described with reference to FIG.  20 B. In step  350 , variable (D) is set to an arbitrary value greater than any dimension of the IC die  120  to ensure that all focus points are considered. A default value such as a largest expressible value for the variable D may be used. A pointer referencing a closest focus point is set to NULL in step  352 . The image-mosaic acquisition process locates the focus point list in step  354  and a first focus point is selected from the list in step  356 . The current distance (CD) between the tile image focus location and the selected focus point is determined in step  358 . 
     If CD is less than D, that is the currently calculated distance is less than the distance between the tile image focus location and the closest focus point found so far, which is ascertained in step  360 , D is set to CD in step  362  and the nearest focus point pointer is set in step  364  to reference the selected focus point. The image-mosaic acquisition process parses the entire focus point list by selecting a next focus point in the focus point list (steps  366  and  368 ). On reaching the end of the focus point list in step  366  the process sets the tile image focus setting to the focus setting associated with the focus point indicated by the nearest focus point pointer. Thereafter, the optical stage is positioned in step  322 , the focus is set in step  324  and the image is acquired in step  326 . 
     The invention therefore provides a method and apparatus for focusing a micro-imaging system on a tilted or uneven surface for acquiring tile images of the surface quickly and efficiently. After an initial acquisition of focus points, the image capture process proceeds automatically without operator intervention. 
     The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.