Microscopy is used in most technology processes where two-dimensional distributions, that
are digital images of the shape and appearance of integrated circuit (IC) features, reveal
important information. Optical microscopy and diffraction gratings can be used to measure
the placement of IC features with a precision of less than 1 nm (1σ) with
a new technique of separately viewing multiple levels. However, the microscope’s optical
and video camera distortion may cause significant errors in the IC feature measurement
under some circumstances. In this paper, the optical and video camera distortions of an
optical microscope used in IC feature measurement were studied by analyzing digital images
of a precision grating. MATLAB programs were used to extract the value of intensity of
each pixel in the grating image. By matching a position dependent, phase-varied ideal
sinusoidal wave to the processed grating digital image, the phase of the best-fit sine
wave was observed to vary by the equivalent of many nanometers at different locations
within the digital image of the grating. This variation was similar, but significantly
different, for different microscope objectives. It is believed to arise from optical
distortion within the microscope, and possibly also from distortion within the camera.
Impact of optic chromatic aberration and microscope stability on the measurement of IC
features was also studied. The method described here is inexpensive and easy to implement
since it does not require any sophisticated equipment or controlled environment. This
technique provides an attractive option for small companies, university labs and
instrument manufacturers.