Overlay performance with advanced ATHENA™ alignment strategies

After the introduction of the ATHENA(TM) alignment sensor, advanced applications of the sensor data are becoming increasingly important to meet the tightening overlay specifications for future technology nodes. As part of the total overlay budget, the effects of different alignment strategies on overlay performance need to be investigated. Keeping in mind that such strategies are simple and easy to use, two developments are addressed in this paper: advanced alignment recipes and advanced mark designs. An alignment recipe defines which signals from the sensor are used to calculate the aligned position. By making advanced use of the available data, wafer alignment can be made more accurate and more robust to processing effects. It is shown that the new Smooth Color Dynamic alignment recipes exhibit good overlay performance on STI, Cu dual damascene and W-CMP / Al-PVD layers. Since Smooth Color Dynamic also takes away the choice of a particular color in the alignment recipe, it is the preferred alignment recipe for all product layers. The optimum design of an alignment mark depends on the process characteristics. As the process characteristics may vary over time, the optimum mark design can change accordingly. To cover a larger process range, multiple alignment mark designs are combined in a new multi-grating mark: the Versatile Scribeline Primary Mark (VSPM). By measuring. all gratings during regular production, the optimum grating of a VSPM can be selected and aligned with a Smooth Color Dynamic alignment recipe. For CMP layers a further overlay improvement can be achieved if all gratings have comparable phase depths. By combining alignment signals from different,gratings in a predictive alignment recipe, wafer-to-wafer variations due to CMP effects can be reduced.