Grid Batch Measurement Pitfall

Grid Batch Measurement Pitfall

Description

• New measurment of test record encountered a difficulty regarding the global scan depth settings.
• Remember we had to apply a relatively large global depth to accommodate a wide range of depth gap due to the disc surface warp.
• However,
  it is discovered that such a setting causes scan saturation for quite a
  lot of FOVs, i.e., acquired depth data were distorted. The exact reason
  is not fully understood yet, although a few evidence was obtained
  through repeated scanning with different parameters, such as scan depth
  and intensity:
  • When this depth is large (e.g., 300um) and the
    turret tilt is not optimized for the FOVs, in which the groove depth
    range is located around the deepest portion of our scan depth range,
    the resulting depth image tend to get saturated.
  • Given a FOV
    that's naturally tilted relative to the turret. Let Scan #1 be a
    reference scan (normal result) with its depth range adapted to the
    target FOV's without tilt correction, Scan #2 a scan with long depth
    range regarding the target groove without tilt correction, and Scan #3
    with same depth range but with tilt correction; all three scans were
    performed on the same FOV. A quick test reveals that the distortion of
    depth data of #2 could be so large that the depth level of measured
    groove top edges of #2 could be as low as that of the groove bottom of
    #1. This is an obvious distortion. However, #3 generates the similar
    result with #1.
    
  • By chaning the intensity to a lower value but without tilt correction didn't remove the saturation.
  • Scan #1: http://kakyolab.xicp.net/0.report/26.saturation/saturation_1.tif
  • Scan #2: http://kakyolab.xicp.net/0.report/26.saturation/saturation_2.tif
    
  • Scan #3: http://kakyolab.xicp.net/0.report/26.saturation/saturation_3.tif
    

Plan

• Because
  tilt setting is static during batch measurement and it is empirically
  safe to assume depth consistency between surface areas that are close
  to each other, the ways to avoid saturation would probably one of the
  two approaches
  • Finely sub-divide the target region into smaller ones, and perform individual grid measurements on them:
    • Pros: this is the easiest way in terms of development
    • Cons: the surface situation is unpredictable; therefore the sub-division can hardly be designed systematically.
  • Dynamically
    monitoring the scanning intensity window, do a fancy image recognition
    to warn the scanning process about the absence of interference patterns
    (moving stripes), then automate the GUI program of Vision to restart
    the scanning.
  • Pros: the ideal way for mass production.
  • Cons: requires significant development effort, invoving image recognition and window process management.
    
• For now, I would go with the subdivision for the current data.