Optical overlay metrology is thought to face the challenges in precision improvement with edge-determination based
algorithm because the design rule is gradually decreasing in advanced semiconductor process. We develop a novel
algorithm for determining the overlay error of grating structures with an optical bright-field imaging tool. By evaluating
the intensity variation of the different acquired images through the analysis of the optical images obtained at different
defocus positions, the analysis curves of the focus measure versus the overlay error experimentally demonstrate the
nanometer sensitivity with the overlay of the grating structure. When plenty of images of different target pattern captured
with image sensor at different amounts of defocus position, the image intensity variation in each image can be calculated
by focus criteria. In our application, the gradient energy is the best metric of the image within the grating area because it
discriminates the main and side lobe local maximum more sharply that is a key phenomenon in our algorithm
application. Empirical models were developed to fit the experiment results of image intensity variation versus overlay
error. The experimental data show that the variations of the focus measure has nano-scale sensitivity to the overlay of the
grating structure, so that it can be used to determine the overlay error by implementing the algorithm. Thus, the
through-focus method has potential application in overlay metrology for the process control in future semiconductor
We present a new algorithm for determining nano-scale feature dimensions of grating structures with a bright-field imaging tool. The algorithm is based on the intensity and focus quality of images obtained with varying amounts of defocus. Analysis of the intensity of optical images obtained at various focal positions demonstrates nanometer sensitivity with grating structures. An empirical quadratic model was developed to fit the experimental results of image intensity versus critical dimension.
Optical interference leads to errors in the determination of the location of lines and in feature dimension measurements. Multi-peaked focus plots were observed from the metrology tools when the target includes sub-resolution lines. In this paper we present a new algorithm for determining nano-scale feature dimensions of grating structures with a bright-field metrology tool. The algorithm is based on the intensity of images obtained with varying amounts of defocus. By evaluating the variations of the different captured images through analysis of the optical images intensity obtained at various off-focus positions, the through-focus curves experimentally demonstrate nanometer sensitivity with grating structure. An empirical quadratic model was developed to fit the experimental results of image intensity deviation versus critical dimension. Our model and experimental data both shows that the grating structure with critical dimension at half pitch has maximum focus measure. A quadratic symmetry distribution data were shown when the critical dimension increase or decrease with the same dimensional intervals. The results demonstrate that the sub-wavelength feature dimensions can be evaluated using regular optical microscopes with exceptional resolution by implementing this algorithm.
Current optical overlay measurement tools utilize visible light and operate with optical resolution of approximately 0.5-1.0 μm. Such tools cannot resolve the targets based on the design rule features. Hence, reliable theoretical model-based measurements and enhanced algorithms are required to address this problem. The test targets, with features similar to those specified by the design rule, were fabricated by a high precision E-beam writer. An optical bright field overlay metrology tool was applied to acquire the optical images of the test targets. The best focus position of test target is selected using an auto focus algorithm. The focus offset is specified relative to the best focus position and the optical image data is measured with a full field-of-view CCD array. The through focus image data are analyzed to obtain the relationship between the intensity profile and the structural parameters of the test targets. These structural parameters are also verified with the CD-SEM. This work experimentally analyzes the through-focus behavior of the test targets. These targets are based on grating patterns, and while they provide more information than traditional targets, they are more sensitive to the focus position. The through focus image formed at the image plane depends on the relative focus position between the target and the optical system. An appropriate design for the optical configuration and target geometry produces a unique image at each focus position, for a specific physical feature.
We report the results of a study of grating-based target designs for overlay measurement. While improvements to measurement precision may be expected from targets with more pattern information, the main interest in these targets is the hope that they will provide more accurate measurements.
In order to test the accuracy of these targets, we have compared data taken using them with data obtained from conventional bar-in-bar targets. In general, good agreement is seen. There are instances, however, where the grating targets all produce a significantly different measurement to the bar-in-bar target. Although this isn’t proof of improvements in measurement accuracy with these newer designs, it does suggest that thorough investigation is warranted.
Meeting the stricter overlay measurement error requirements of next-generation lithography is a challenge to conventional optical metrology solution associated with bright-field microscopy. A modified thin film model was developed to simulate the optical image intensity profile from novel overlay targets with design rule features. The image is calculated based on diffraction theory, which is simpler than the rigorous application of Maxwell’s equations in three dimensions. The model is matched to the image by adding the contributions from all of the patterned regions in the target, and multiplying by a complex reflectance transfer matrix, which embodies all of the material characteristics. The overlay error in the target and the optical configuration parameters are modified to find the best fit between the image and the model. Although this method makes several assumptions about the formation of an image, very close agreement between the model and the image is obtained.