The Method Of Diffraction-based Overlay Metrology

As semiconductor process nodes continue to shrink,it is becoming more and more important to measure overlay accurately and precisely.In order to detect the alignment quality of different lithography layers,the overlay target is usually processed in the scribe line of the Si wafer,and then it can reflect the overlay error in the device by simply measuring the target.Nowadays,the Diffraction-Based Overlay(DBO)method has become the mainstream due to the its inherited advantages of non-contact,non-destructive,and high efficiency.Among theDBO methods,the eDBO(empirical DBO)method is the most popular method,and it can be realized based on the empirical linear relationship between overlay and the diffraction information.Compared with the traditional diffraction information,Mueller matrix contains 15 Mueller matrix elements,and it can identify the direction of the overlay error,which is typically useful for the overlay measurement.For these reasons,how to measure overlay error using the Mueller matrix is very significant.The sensitivity of eDBO method is defined,which is used for analysis on the overlay target,and the best elements are selected based on the simulation results.Then,we analysed the causes of the measured error for the eDBO method,and we defined the evaluation criterion.On this basis,the empirical laws of the measurement performance,which are decided by the target parameters or the measurement configuration,are discussed by the simulation.The overlay target design is an important issues in the overlay metrology,and we innovatively proposed to use the multi-objective optimization to improve all the criterion of overlay measurement simultaneously,which can supply the different optimal Pareto parameters of overlay target and measured configuration,to overcome all the situations in the semiconductor process.This thesis is mainly about the method of overlay metrology,and there are three main parts,including the elements selection for overlay measurement,the evaluation method for the overlay measured error,and the multi-objective optimization for overlay target design,which lay a solid foundation for the overlay metrology.