Electromagnetic modeling of darkfield defect detection in patterned wafers

A model for simulating the physics of dark-field defect metrology in 45 nanometer design rule (DR) patterned wafers is presented. The model is used to determine the feasibility of applying dark field scattering methodology to detect defects of concern in these wafers. Scenarios representative of two key steps in the manufacturing process of patterned wafers were investigated. The first is the step representing the Metal 1 layer process and the second is the step representing a polysilicon line array.;These scenarios were derived from an intentional defect array produced by SEMATECH. These were imported into a finite difference time domain simulation space where the illuminating fields were injected using a teleportation window thus creating a finite well-defined illuminated region and isolating the scattered field for analysis. The scattering environments compared included ideal defect-free scenarios, defect-free scenarios with realistic line edge roughness and scenarios with both defects and realistic line edge roughness.;The scattered field from these scenarios was propagated to the far field hemisphere establishing both the defect signatures and the characteristic background scattering of realistic 45nm DR patterned wafer scenarios. Holographic (amplitude and phase) subtraction of the scattering pattern of a realistic defect-free pattern from a pattern containing a defect was performed to determine the theoretical signal to noise level expected.;Seven measures of detectability are proposed based on these signatures and these are summarized in a single Figure of Merit (FoM) that characterizes the likelihood of detection of a given defect. Based on the FoM the detectability of each defect type versus size has been established. It is concluded that using optical darkfield methods with shallow angle illumination of the pattern, holographic subtraction and a low pass Fourier Optics filter would enable detection of defects as small as 25nm across at the Metal 1 level and 10nm across in a poly line scenario. This conclusion is validated by using a blind numerical experiment where several scenarios with and without defect were generated and examined according to the proposed detection approach.