Diffraction-based Overlay Measurement Research Articles

Bottom grating asymmetry-induced inaccuracy in diffraction-based overlay measurement

Background: Overlay (OV) is a very important indicator for characterizing the pattern position accuracy between two layers in an integrated circuit in the manufacturing industry. The self-aligned double-patterning (SADP) process, which is limited to the resolution of deep ultraviolet lithography, is being extensively used for the volume manufacturing of integrated circuits. However, compared to the planar process, the SADP process poses new challenges for OV control.Aim: We investigate the bottom grating asymmetry of a microdiffraction-based overlay (μDBO) target in the SADP process and its effect on OV measurement.Approach: The bottom grating of the OV metrology target is analyzed through scanning electron microscopy and transmission electron microscopy. The imbalance of the −1st- and +1st-order diffraction light intensities of the asymmetrical bottom OV target is characterized. Finally, the effects of light polarization and wavelength on the OV accuracy are simulated.Results: Asymmetric behavior of bottom OV grating is identified. OV dependency on the measurement-light polarization is determined. The simulation results indicate that wavelength and polarization affect the OV accuracy simultaneously.Conclusions: We studied the bottom grating asymmetry-induced inaccuracy in DBO measurement and can assist in understanding the mechanism of measurement light-interaction with the OV grating to enhance the DBO accuracy to a new level.

Selection of DBO measurement wavelength for bottom mark asymmetry based on FDTD method

A physical model for simulating overlay metrology employing diffraction based overlay (DBO) principles is built. It can help to optimize the metrology wavelength selection in DBO. Simulation result of DBO metrology with a model based on the finite-difference time-domain (FDTD) method is presented. A common case (bottom mark asymmetry) in which error signals are always induced in DBO measurement due to the process imperfection were discussed. The overlay sensitivity of the DBO measurement across the visible illumination spectrum has been performed and compared. After adjusting the model parameters compatible with the actual measurement conditions, the metrology wavelengths which provide the accuracy and robustness of DBO measurement can be optimized.

Artificial Neural Network for Diffraction Based Overlay Measurement

Diffraction-based overlay (DBO) accuracy is critical to the intelligent nanolithography process control for producing advanced semiconductor fabrication nodes. Optical gratings located on various layers are commonly used as the targets for the detection of the overlay displacement offset in DBO measurement. The asymmetry in intensity between the 1st and -1st beams diffracted by the targets is used for the prediction of grating displacement offset. This paper describes the effect of grating targets with sidewall angles (SWAs) on asymmetry in intensity and proposes an artificial neural network (ANN) method for enhancing the accuracy of grating displacement offset prediction. Grating targets with a 1:3 line-to-pitch ratio and SWA profiles varying from 86° to 90° were employed in this paper. The asymmetry in the intensity of the designed targets was computed for incident beams with transverse-electric and transverse-magnetic polarization at visible wavelength. An ANN feed-forward model was developed for the displacement offset prediction. The ANN, the conventional linear model, and the regression models were evaluated using diffraction data calculated by a numerical electromagnetic solver. The mean square error and the mean of the residual indicated that using the ANN model and incident beams at wavelengths of 600, 650, and 750 nm is substantially more effective for prediction than the conventional linear model is.