In performing SEM Critical Dimension (CD) measurements on photomasks in dense line and space arrays it is often difficult to distinguish between whether a feature is a line or space. This is a result of tone shifts that occur affecting contrast on target images. The inability to reliably differentiate lines and spaces leads to the inclusion of fliers, or inaccurate measurements into automated measurement results. In an effort to overcome this phenomenon a new algorithm has been developed to increase the robustness of the CD SEM measurements to insure reliable data acquisition. This new algorithm takes into account apparent tone reversals on a variety of today's photomask material types. This paper will detail the various elements of the new algorithm and show before and after test results of improved recognition performance.
INTRODUCTIONThe analysis of intensity profiles through the use of differential profiles can lead to more robust detection of dense line or space features in dense line and space arrays. New software incorporated into the Vistec LWM9000 SEM and successor models has eliminated confusion on edge selection regardless of reflectivity or tone. A second differential profile is calculated from the first differential profile of the SEM intensity profile. This second differential profile yields information that is unique to a line or a space that is independent of reflectivity or apparent tone. This information is then used to insure proper feature alignment so the correct target critical dimension (CD) can be measured. Local alignment of dense features usually occurs at 20kX and measurement of the target feature is performed at a higher magnification such as 50kX or 100kX. An in depth explanation will be given on the alignment sequence and how the second differential profile is used to distinguish between a line or a space.
ALIGNMENT AND MEASUREMENT SEQUENCEThe alignment procedure for measuring a photomask begins with a coarse three point global alignment utilizing an optical microscope image and template image, see figures 1-a and 1-b. After the coarse optical alignment is completed a fine alignment is done on the same three points using a SEM image and template, see figures 2-a and 2-b.
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