Patent Publication Number: US-8525112-B2

Title: Variable pixel density imaging

Description:
SUMMARY 
     In some embodiments a method is provided for topographically characterizing a workpiece. The method includes steps of scanning the workpiece with a scanning probe along a first directional grid, thereby scanning a reference surface and an area of interest subportion of the reference surface, at a variable pixel density including a first pixel density outside the area of interest and a second pixel density inside the area of interest to derive a first digital file characterizing topography of the workpiece; scanning the reference surface and the area of interest with the scanning probe along a second directional grid, that is substantially orthogonal to the first directional grid, at a constant pixel density to derive a second digital file characterizing topography of the workpiece; and executing computer-readable instructions stored in memory that generate a topographical profile of the workpiece in relation to the first and second digital files. 
     In some embodiments an apparatus is provided for topographically characterizing a workpiece. The apparatus has a scanning probe that obtains topographical data about the workpiece. A processor controls the scanning probe to scan the workpiece along a first directional grid, thereby scanning a reference surface and an area of interest subportion of the reference surface, at a variable pixel density including a first pixel density outside the area of interest and a second pixel density inside the area of interest to derive a first digital file characterizing topography of the workpiece, and to scan the workpiece along a second directional grid, that is substantially orthogonal to the first directional grid, at a constant pixel density to derive a second digital file characterizing topography of the workpiece. The apparatus further includes logic that generates a topographical profile of the workpiece in relation to the first and second digital files. 
     In some embodiments a topography metrology apparatus is provided having a processor controlling a scanning probe to collect first data along a variable pixel density grid and to collect second data along a constant pixel density grid to obtain topographical data about a workpiece, and means for integrating the first and second data to generate a topographical profile for the workpiece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric depiction of an atomic force microscope probe constructed and used in accordance with embodiments of the present invention. 
         FIG. 2  is an isometric depiction of a cavity transition feature in a workpiece that is suited for measuring with atomic force microscopy (AFM) techniques of the present embodiments. 
         FIG. 3  depicts a variable pixel density directional grid of the present embodiments. 
         FIG. 4A  is a two-dimensional plot of a raw scanned image profile of a cavity transition feature. 
         FIG. 4B  is a two-dimensional plot of a tilted image profile of the cavity transition feature. 
         FIG. 4C  is a three-dimensional plot of the tilted image profile shown in  FIG. 4B . 
         FIG. 4D  is a three-dimensional plot of a corrected image profile of the cavity transition feature. 
         FIG. 5A  diagrammatically depicts a first scan along a horizontal directional grid. 
         FIG. 5B  diagrammatically depicts a second scan along a vertical directional grid. 
         FIG. 6  depicts the vertical directional grid of  FIG. 3  and a constant pixel density horizontal directional grid in accordance with the present embodiments. 
         FIG. 7  is a view similar to  FIG. 6  but depicting the horizontal directional grid having been modified to match the variable pixel density of the vertical directional grid. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view an atomic force microscope  10  positioned over a surface of structure  12  (sometimes referred to herein as “workpiece”). Atomic force microscope  10  includes probe  11  having cantilever portion  14  and tip portion  16 . Atomic force microscope  10  also includes light source  18 , position sensitive detector  20 , and processor  22 . Light source  18  emits a beam  24  that is reflected by cantilever  14  and received by position sensitive detector  20 . Processor  22  receives signals from position sensitive detector  20  and provides signals to control movement of probe  11  relative to structure  12 . 
     Structure  12  in these illustrative embodiments is the pole tip region of a magnetic recording system, including slider  26  carrying reader structure  28  and writer structure  30 . The atomic force microscopy (AFM) techniques described herein are useful for measuring and imaging feature characteristics of structure  12 , such as pole tip recession (PTR) features of reader structure  28  and writer structure  30 . It should be noted that structure  12  is shown merely for purposes of illustration, and the AFM techniques described herein are also useful for measuring and imaging nanometer and micrometer scale surface features of other structures. For example, the AFM techniques may also be used to measure feature characteristics in other magnetic recording device structures, such as a cavity transition feature as shown in  FIG. 2 . 
     Atomic force microscope  10  measures physical characteristics or properties of structure  12 , such as feature dimensions and surface finish. Probe tip  16  is positioned in very close proximity (i.e., within picometers) to the surface of structure  12  to allow measurements of structure  12  over a small area. Probe tip  16  is moved relative to structure  12  using extremely precise positioning. For example, processor  22  may control motion of probe  11  such that probe tip moves along the surface of stationary structure  12 . Alternatively, processor  22  may control a device such as a tube scanner to move structure  12  while probe  11  remains stationary. As probe tip  16  moves over the surface of structure  12 , features on the surface of structure  12  cause cantilever  14  to bend in response to the force between probe tip  16  and structure  12 . 
     The position sensitive detector  20  measures the amount of deflection in cantilever  14 , which may be used to generate an image representation of structure  12 . In particular, light source  18  (e.g., a laser) reflects light beam  24  off of cantilever  14  to position sensitive detector  20 . Position sensitive detector  20  may include two side-by-side photodiodes such that the difference between the signals generated by the photodiodes indicates the position of light beam  24  on position sensitive detector  20 , and thus the angular deflection of cantilever  14 . Because the distance between cantilever  14  and position sensitive detector  20  is generally thousands of times the length of cantilever  14 , the motions of probe tip  16  are greatly magnified. 
       FIG. 2  is a perspective view of slider  26  including air bearing surface (ABS)  42 , transition edge  43 , cavity transition  44 , and cavity  46 . Cavity transition  44 , which may be defined using ion milling techniques, has very small features varying from nanometer to micrometer scale sizes. Measurement of the features of cavity transition  44  is important in various aspects in the development of the device, including design improvement, device model validation, and device performance enhancement. For example, in a magnetic recording device, the measurement of cavity transition  44  is important for understanding flying performance of slider  26 . The AFM techniques described herein may be used to measure the properties of cavity transition feature  44 . 
     The lateral resolution of an image produced from a scan of a structure is defined by the scan area size and the number of pixels in the image. Conventionally, atomic force microscope  10  moves relative to the structure at a constant speed, and the position of probe  11  is periodically sampled by processor  22 . The resulting image has a uniform resolution across the entire scanned region. 
     Some structures may include a region or area of interest having a target feature or characteristic of which a more detailed scan may be desired. For example, a detailed scan of cavity transition  44  of slider  40  shown in  FIG. 2  may be desirable for precise measurement of the dimensions and other characteristics of the transition profile. In order to increase the resolution in the area of interest, the number of pixels or data points in the area of interest may be increased. Because the number of pixels available for a given scan is often fixed, the resolution in the area of interest is thus increased at the expense of limited views of the areas surrounding the area of interest. However, it is also important to maintain the contextual details around the area of interest such that the relative sizes and positions of features within the area of interest are more easily determinable. 
       FIG. 3  is a schematic view of an approach for increasing the amount of data at an area of interest on a scanned structure while maintaining the contextual details of the surrounding areas. In  FIG. 3A , the scan area  50  (or “reference surface”) of a structure is shown including area of interest  52 . Probe  11  moves around scan area  50  in scan pattern  54  during image acquisition in response to control signals from processor  22 . In particular, scan pattern  54  is programmed in processor  22 , and processor  22  controls movement of probe  11  relative to the structure in the programmed scan pattern  54 . As probe  11  is moved relative along scan pattern  54  within scan area  50 , data points or pixels  56  are sampled by processor  22 . That is, processor  22  periodically communicates with position sensitive detector  20  to receive information about the position of probe tip  16  relative to position sensitive detector  20 . This information is used by processor  22  to set locations of, and hence the density of, the pixels  56  in a digital file containing topographic information used to generate an image of the structure  12 . The resulting image is representative of the structure in that it includes graphical representations of the surface features and characteristics of the structure. 
     Processor  22  samples pixels  56  in the portions of scan area  50  outside of area of interest  52  as probe  11  moves along the y-direction relative to the structure. The pixels  56  are separated by a distance d y1  in the y-direction and a distance d x1  in the x-direction in areas outside of the area of interest  52 . In one embodiment, distance d x1  and distance d y1  are equal to provide a continuous lateral resolution in the portions of scan area  50  outside the area of interest  52 . 
     In order to increase the resolution at the area of interest  52 , scan pattern  54  is programmed such that more pixels  56  are sampled in the area of interest  52  during the scan than in the portions surrounding the area of interest  52 . The programmed location of the area of interest  52  in scan pattern  54  may be determined during the scan based on known position information on the scanned structure, or based on surrounding feature characteristics sensed by probe tip  16 . When probe tip  16  is close to the area of interest  52 , processor  22  reduces the distance between adjacent scan lines in the x-direction to distance d x2  to increase the density of data points  56  in the x-direction (i.e., probe tip  16  moves a smaller distance relative to the structure between adjacent scan lines). When probe tip  16  is in the area of interest  52 , processor  22  increases the number of data points  56  sampled along each scan line (i.e., decreases the spacing between each data point  56  to d y2 ), which increases the pixel density in the area of interest  52  in the y-direction. The resolution in the y-direction may be increased by, for example, increasing the rate at which processor  22  samples the position information of cantilever  14  from position sensitive decoder  20 , by adjusting the rate at which probe  11  is moved relative to the structure, or a combination of increasing the sample rate and decreasing the relative motion between probe  11  and the structure. In the illustrative embodiment shown, the resolution of the scan in the area of interest  52  is three times that in the portions of scan area  50  surrounding the area of interest  52 . 
     Atomic force microscope  10  allows the density of pixels  56  to be adjusted during the scanning process. From a single scan, the resulting image of the structure has a higher resolution in the area of interest  52  than in the remainder of scan area  50 . This scan process not only preserves the contextual details in the areas around the area of interest  52 , but also allows for greater throughput of scans and measurements of the structure since multiple scans are not required. 
     It should be noted that scan pattern  54  is merely illustrative, and other scan patterns may be used for imaging a structure having different characteristics. For example, if a structure includes multiple areas of interest, the scan pattern may be programmed to increase the sampling rate or reduce the scan speed at the multiple areas of interest to increase the resolution in those areas. In addition, scan pattern  54  may include multiple levels of resolution within the same scan area  50 . 
     When a structure (such as slider  26 ) is scanned by atomic force microscope  10 , it is held in position on a linear stage or other positioning device, such as in a tray, by a fixture, or with adhesive. However, due to positioning errors, the structure may not be precisely aligned with atomic force microscope  10  with respect to the contours of the programmed scan pattern. For example, the structure may be skewed in one direction relative to the scan pattern, or the structure may be tilted relative to the plane of the scan pattern. The image resulting from the misaligned scan thus may not represent the true profile of the structure, making an accurate measurement of the dimensions of the scanned structure and features of the structure difficult. In addition, even if the positioning of the structure relative to atomic force microscope  10  is perfect, variations in the components of atomic force microscope  10  (e.g., due to environmental conditions) may result in a misaligned image. 
     For example, in a scan of slider  26  ( FIG. 2 ), the dimensions and characteristics of cavity transition  44  may be measured. Measurement of cavity transition  44  is important for understanding flying performance of slider  26 , as well as for device design improvement, model validation, and performance enhancement. Cavity transition  44  may be measured relative to another feature on slider  26 , such as ABS  42 , to allow for analysis of the shape of cavity edge  43  and cavity transition  44 . However, if the image of ABS  42  is skewed or tilted due to mispositioning of slider  26  or due to performance of atomic microscope  10 , characteristics of cavity transition  44  may be difficult to measure. 
     For example, at micrometer-level scan lengths, bowing can occur in traditional tube scanner atomic force microscopes, which may also produce curvature in the resulting image. The amplitude and shape of the bowing vary between atomic force microscopes, and may change with aging, temperature, and humidity. Positional offsets between scans of the same surface in the same scan area may also vary the curvature in the corresponding image. To avoid the contribution of bowing to measurement error, a reference scan may be taken on a flat surface with the same scan settings (e.g., scan size and offsets) of a regular scan. The reference scan is subsequently subtracted from the regular scan to obtain an image without curvature due to the bowing effect. However, any real curvature in the surface of the reference scan will be added to the measurement results. In addition, scan defects, irregular scan lines, and particle contamination in the reference scan may add error to the measurement results. 
       FIGS. 4A-4D  show another approach to correcting the skew (or “drift”) and tilt in an image based on a scan of slider  26  including cavity transition feature  44 . To minimize alignment artifacts caused by variations in the components of atomic force microscope  10  and environmental conditions, the scan was performed at a 0.2 Hz sampling rate with probe tip  16  having a radius of less than about 30 nm proximate to slider  26 . Each step described herein with regard to correcting the drift and tilt in the image may be performed by processor  22  or by a microprocessor based system external to atomic force microscope  10 . 
       FIG. 4A  is a two-dimensional plot of a raw scanned image profile based on a scan of slider  26  including cavity transition  44  under the above conditions. The contour lines shown in the plot in  FIG. 4A  represent height changes in slider  26  relative to the z-axis. For context, transition edge  43  and cavity transition  44  are labeled in the image. As is shown, the image representative of slider  26  drifts relative to the y-axis. In addition, the height change relative to the z-axis should be more pronounced at cavity transition  44 , and ABS  42  should include no contour lines because it should be co-planar with the x-axis and the y-axis. However, because few contour lines are shown at cavity transition  44  and several contour lines are shown around ABS  42 , the image is also tilted relative to the desired orientation (i.e., with ABS  42  parallel with the xy-plane). The correction of the tilt in the image will be described with regard to  FIGS. 4C and 4D . 
     The drift in the two-dimensional view of slider  26  may be corrected by choosing a feature in the scan and re-orienting the image based on that feature. For example, a derivative map of the image (i.e., a plot of the derivative at every location in the image) shown in  FIG. 4A  may be generated to determine the locations of transition features (e.g., transition edge  43 ) in slider  26 . The transition feature may then be used as an alignment index for aligning the image within the two-dimensional view.  FIG. 4B  shows the two-dimensional image of slider  26  including cavity transition  44  after aligning the image relative to the x-axis and y-axis based on transition edge  43  to correct the image for drift. 
       FIG. 4C  is a three-dimensional plot of the tilted image profile of slider  26 . As can be seen, while transition edge  43  is aligned with the y-axis, ABS  42  is tilted relative to the xy-plane. To facilitate measurement of cavity transition  44 , the image of slider  26  may be rotated relative to the xy-plane. While this rotation may be performed in Cartesian coordinates, the rotation is simplified by first converting each data point in the image of slider  26  to spherical coordinates. Thus, for each data point having coordinates (x, y, z), the corresponding spherical coordinates (R, θ, φ) are given by 
             R   =         x   2     +     y   2     +     z   2                     θ   =     arctan   ⁡     (     y   x     )                   φ   =     arctan   (     z         x   2     +     y   2           )           
where R is distance from the origin to the data point, θ is the angle from the xz-plane to the point, and φ is the angle from the xy-plane to the point.
 
     The image of slider  26  may be rotated relative to the xy-plane to level ABS  42  by offsetting the angle φ by a correction angle α based on the slope of the tilted ABS  42 , 
             α   =     arctan   ⁡     (       ⅆ   y       ⅆ   x       )             
where dy/dx is the slope of tilted ABS  42  relative to the xy-plane. To rotate the image of slider  26 , each data point may be offset by correction angle α and converted from spherical coordinates back to Cartesian coordinates. Thus, for each data point having coordinates (R, θ, φ−α), the corresponding Cartesian coordinates (x, y, z) are given by
 
 x=R  cos(φ−α)cos(θ)
 
 y=R  cos(φ−α)sin(θ)
 
 z=R  sin(φ−α)
 
     A plot of the three-dimensional image after rotation, which reflects the true profile of the slider  26  and cavity transition  44 , is shown in  FIG. 4D . 
     During an AFM scan, the processor  22  moves the probe tip  16  along the scanned surface faster in one direction than in the other direction. For example, as depicted in the embodiments of  FIG. 3  above, probe tip  16  samples the reference surface  50  along a first directional grid running vertically, or in other words along the y-axis direction. Thus, for each scan line in the first directional grid, a length of a group of pixels  56  is sampled in the y-direction (the “fast scan” direction), while a width of only a single pixel is sampled in the x-direction (the “slow scan” direction”). As discussed, the first directional grid encompasses both the reference surface  50  and the area of interest  52  subportion of the reference surface  50 . Thus, the first directional grid ultimately performs a scan of a variable pixel density, because it includes a first pixel density d x1 , d y1  outside the area of interest  50  and a second pixel density d x2 , d y2  inside the area of interest  52  to derive a first digital file characterizing topography of the workpiece. 
     In the fast scan direction, it takes a relatively shorter time to move the probe tip  16  from one end of the reference surface  50  to the other end, in comparison to the time it takes to move in the slow scan direction from one end of the reference surface  50  to the other end. Without correcting for the differences in the times, process variations such as environmental vibrations, machine drifting, and airflow along probe  11  can cause curvature artifacts in the image in the slow scan direction. 
       FIGS. 5A and 5B  diagrammatically depict a paired scanning scheme that can be used to correct for such image curvature artifacts in an AFM scan. For discussion sake, the scans depicted in  FIGS. 5A and 5B  encompass the reference surface  50 , including the area of interest  52 , and are orthogonally directed with respect to each other. No particular scan pattern itself is depicted in these FIGS., but rather they generally diagrammatically depict a horizontal directional scanning grid ( FIG. 5A ) and a vertical directional scanning grid ( FIG. 5B ). 
     In  FIG. 5A , the scanned reference surface  50  can be represented by a matrix of pixels a xy  (x, y=1˜n, where n is the periphery dimension of the reference surface  50 ). For simplicity, the reference surface  50  shown is square, but the curvature artifact correction method described is also applicable to scan areas having other dimensions and characteristics. For this horizontal directional grid, the fast scan direction is the x-direction and the slow scan direction is the y-direction. The fast scan direction has an average profile ā x  and the slow scan direction has an average profile ā y , where 
     
       
         
           
             
               
                 
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     Similarly, in  FIG. 5B , the reference surface  50  can be represented by a matrix of pixels b xy  (x, y=1˜n, where n is the periphery dimension of the reference surface  50 ). In this vertical directional grid, the fast scan direction is the y-direction and the slow scan direction is the x-direction. The fast scan direction has an average profile  b   y  and the slow scan direction has an average profile  b   x : 
     
       
         
           
             
               
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     The average profiles along the fast scan direction in the two scans, ā x  and  b   y  represent the true profile of the structure, while the average profiles along the slow scan direction, ā y  and  b   x , are a combination of the true profile of the structure and curvature artifacts due to tilting and drifting of atomic force microscope  10  and other environmental effects along the slow scan direction. Where a uniform pixel density is used in scanning for both of the orthogonal directional grids, a flattened profile that corrects for drift, c xy , is defined by: 
     
       
         
           
             
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     However, in scans employing a variable pixel density the high pixel density area will be dominant in the curve fitting results of this derivation due to the high pixel count per unit area, skewing the results for purposes of correcting for drift. The present embodiments resolve that difficulty without sacrificing the benefits of variable pixel density scanning where both a limited high resolution and a large field of view are needed simultaneously. 
     Continuing from  FIG. 3 , which again depicts the processor executing computer-readable instructions to scan along the first directional grid, in this case a vertical grid like the diagrammatic depiction of  FIG. 5B , with a variable pixel density to obtain a higher resolution in the area of interest  52  than otherwise in the reference surface  50 . The pixel pattern for the first directional grid is depicted in  FIG. 3  by the pattern of empty circles. The metrology data for generating a topographical profile is obtained from data collected in that first directional grid. 
       FIG. 6  depicts the manner in which the processor  22  then scans the reference surface  50  and the area of interest  52  with the scanning probe along the second directional grid, in this case a horizontal grid, that is substantially orthogonal to the first directional grid. The second directional grid is only used for flattening the first grid to correct for drift. Accordingly, a constant pixel density that is less than the density of the area of interest  52  can be used to improve processing throughput while achieving acceptable flattening results. In these illustrative embodiments the constant pixel density of the second directional grid is matched to the constant pixel density of the first directional grid outside the area of interest  52 . The pixel pattern for the second directional grid is depicted in  FIG. 6  by the circles of the first directional grid now having X&#39;s in them. 
     The processor  22  then continues to execute computer-readable instructions stored in memory to modify the second directional grid by interpolating pixels between those in the constant pixel density so that each scan line matches the variable line density of the first directional grid.  FIG. 7  depicts the interpolated pixels by the circles of the first directional grid now being solidly filled in. The processor  22  can then generate a topographical profile of the workpiece that is corrected for drift in terms of the digital files obtained from scanning along the first and second directional grids according to this derivation: 
     
       
         
           
             
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     Generally, embodiments of the present invention contemplate a topography metrology apparatus and associated methodology of using a processor controlling a scanning probe to collect first data along a variable pixel density grid and to collect second data along a constant pixel density grid to obtain topographical data about a workpiece. Further, means for integrating the first and second data are employed to generate a topographical profile for the workpiece. For purposes of this description and meaning of the appended claims, the term “means for integrating” specifically encompasses a meaning that includes the disclosed structure and equivalents thereof. “Means for integrating” does not include previously attempted solutions at variance with the disclosed structure such as those that collect data from two constant pixel density grids or two variable pixel density grids. 
     In summary, a topographic profile of a structure is generated using atomic force microscopy. The structure is scanned such that an area of interest of the structure is scanned at a higher resolution than portions of the structure outside of the area of interest. An image of the structure is then generated based on the scan. The resulting image of the structure has a higher resolution in the area of interest than in the remainder of scan area. This scan process preserves the contextual details in the areas around the area of interest. The structure is scanned along a variable pixel density directional grid, and to correct for drift along the slow axis the structure is also scanned along an orthogonal constant pixel density directional grid. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 
     It is to be understood that even though numerous characteristics and advantages of various embodiments of the invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts and values for the described variables, within the principles of the present embodiments to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.