Improved method for optical linewidth measurements

PTB offers an optical linewidth (CD) measurement service for micro and sub-micro structures. This well-established service makes use of a metrological UV-microscope to measure cross-section light intensity profiles. They are analyzed with a threshold method, where the lines’ edge positions are derived from the expected light intensity level (threshold) at the lines’ edge position in a simulated profile. The method is simple and easily applicable. However, it does not exploit the whole available information since only a small part of the measured profile data is analyzed. In addition it assumes that the focusing is perfect. Both issues are limiting the achievable measurement uncertainty. With a new method we overcome these limitations. Now the full profile data of all measurement scans recorded with varying focus are analyzed. This is accomplished by a parallel least-squares fitting routine with free focus position parameter for each scan and with common geometry parameters for all scans. Due to the rigorous image calculation the computational costs are quite high, but the statistical noise and a systematical error influence can be reduced so that a lower uncertainty can be offered.