In-line focus monitor structure and method using top-down SEM

A test structure in a photoresist layer on a substrate for monitoring the best focus setting in a lithographic process, said structure comprising:

• a large rectangular end, and
• several extensions of equal length and equal width attached to one side of said large rectangular end and extending in a parallel array wherein the spacing between the extensions is equal to the minimum feature size in the device pattern to be monitored and is equal to the width of said extensions.

A method of monitoring the best focus condition for a lithographic process, said method comprising:

• lithographically transferring a test structure from a reticle into a photoresist layer on a substrate, said test structure comprising at least one large rectangular end and several rectangular shapes of equal length and equal width attached to one side of the large rectangular end and extending in a parallel array wherein the spacing between the rectangular shapes is equal to the minimum feature size in the pattern to be monitored and is equal to the width of said shapes,
• measuring the edge width of at least one convex and one concave section of the test structure with a CD-SEM, said structure having been generated at various locations in the photoresist film using a range of focus settings in an exposure tool, and
• determining the intersection of a line representing the edge widths from concave sections and a line representing the edge widths from convex sections in a plot of edge width as a function of exposure focus settings used to print the pattern.