Ellipsometry in the EUV regime

As semiconducting devices grow smaller in scale, measuring their critical dimensions becomes more challenging. Knowledge of these dimensions is crucial, as several of these parameters control the quality of the transistors being manufactured. A popular non-destructive measurement technique is ellipsometry, where polarized light is diffracted off of the grating and the diffraction intensities are fit against a simulated model. Visible light, however, is unable to decorrelate parameters for the smallest and most complex devices, requiring the development of ellipsometers that operate at shorter wavelengths. We conduct a sensitivity analysis of a new ellipsometer designed and being built by NIST. This ellipsometer uses extreme ultraviolet (EUV) radiation from NIST’s SURF-III synchrotron. Its distinguishing feature is the use of two rotating four-mirror devices to control polarization. The rotating compensators allow the ellipsometer to probe the entire Mueller matrix in the EUV. With a combination of Monte Carlo simulations and least squares fitting, we demonstrate the superior sensitivity of an ellipsometer employing both EUV radiation and polarization control. We simulate ellipsometry experiments with several industrially relevant geometries, and observe the parametric decorrelation of important parameters with shorter wavelength. We also observe the benefits of employing phase control for certain geometries, like stacked thin films. Numerical and analytical justification is offered for this improved sensitivity.