A Rugate notch filter is fabricated by atomic layer deposition. By regulating the thickness ratio of TiO2 and Al2O3 in a nanoscale layer, the refractive index is tailored between the refractive indices of the two materials. With the method of equivalent refractive index, the continuously variable refractive index of the designed Rugate filter is dispersed into several discrete ones, so that it can be realized by the refractive index tailoring. To coincide with the thickness, the nanoscale layer is iteratively deposited in the equivalent layer. The experimental reflectance matches the designed one well, and the average reflectance is 86.7% (510-590 nm).
The mechanism of coating induced polarization state variation is analysed by the
Jones matrix. Pauli spin matrices are used to establish the relationship between
coating induced polarization state variation and polarization aberrations.
To reduce coating induced polarization aberrations, we propose that
δ = 0 and
Ts = Tp
at arbitrary incident angle should be appended as two additional optimization goals of
optical coating design when the requirements of transmittance are met. Two typical
anti-reflection (AR) coatings are designed and the polarization state variation induced by
them is simulated. The MTF (modulation transfer function) calculated by polarization ray
tracing is applied to evaluate the polarization aberrations of the practical lithography
objective system with the two AR coatings. All the obtained results show that the
coating induced polarization aberrations can be reduced by optimizing the angle
dependent properties of the optical coating without additional optical elements.
The incident angles of the optical systems with high numerical apertures, such as lithography or microscope, are larger than those of normal ones. For these systems, polarized illumination is widely adopted. The coatings on their surfaces will make s and p polarization components of oblique incident light experience diverse amplitudes and phase modulations, and induce extra polarization aberrations. We apply the vectorial diffraction theory to assess the effects of coating-induced polarization aberrations on the focusing properties of these systems. By applying the generalized Debye integral, the relationship between the parameters of coating and electric field vector near the focal spot is established. Considering x linearly polarized light as the incident light field, we evaluate the potential influence of the coatings on the intensity and the full width at half maxium of focal spots. In the further discussion, we compare the results of different coatings when the various optimization goals are set, and certify that the phase difference caused by coating has more effect on focusing property. Based on this, the additional constraint conditions of coating design are proposed to suppress such disturbance, i.e. to properly increase relative weight of phase constraint conditions. With this proposed constraint conditions, we design and optimize an anti-reflection coating with low polarization aberrations. By applying this designing, the central intensity of focal spot can be enhanced by 12.5%, and the light energy utilization will be improved effectively.
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