Measuring overlay between two layers of semiconductor devices is a crucial step during electronic chip fabrication. We present dark-field digital holographic microscopy that addresses various overlay metrology challenges that are encountered in the semiconductor industry. We present measurement results that show that the point-spread function of our microscope depends on the position in the field-of-view. We will show that this novel observation can be explained by a combination of the finite bandwidth of the light source and a wavelength-dependent focal length of the imaging lens. Moreover, we will also present additional experimental data that supports our theoretical understanding. Finally, we will propose solutions that reduce this effect to acceptable levels.

A dark-field Digital Holographic Microscope with a single lens for imaging is a potential candidate for future overlay metrology on semiconductor wafers. Aberrations caused by this single lens are computationally corrected allowing high-resolution imaging over a large wavelength range. However, the spatially-coherent imaging conditions in our microscope introduce coherent imaging artifacts that can limit the metrology performance. We present computational apodization of the optical field in the exit pupil of the lens as a potentially effective solution to mitigate these coherent imaging effects. A comparison of experimental data and simulations is presented that demonstrates the importance of this apodization in metrology applications. Moreover, our data also shows that exploiting the full potential of DHM requires an imaging lens with low optical scattering levels.

Precise, accurate, and fast characterization of overlay between two layers of semiconductors during chip fabrication is an important monitoring and feedback step. Overlay, which is to be measured within the accuracy of a nanometer, is susceptible to many small imperfections in the measurement system. A digital holographic microscope measures the complex field of overlay targets using simple optics followed by computational algorithms for overlay metrology. We improved the precision of overlay measurement by correcting the inhomogeneities and asymmetries caused by the illumination spots, presenting a robust method resulting from simple calibration steps.