Fraunhofer Institute for Integrated Systems and Device Technology IISB
The Computational Lithography and Optics Group warmly congratulates Varun Jadhav on successfully defending his Master’s thesis titled “Analytical Modeling of Light Diffraction from Periodic Structures on EUV Mask using Modal Methods" on July 8, 2026.
In his thesis, Varun investigated the optical behavior of periodic absorber structures on extreme ultraviolet (EUV) masks, where pronounced waveguiding effects arise due to absorber thicknesses exceeding the illumination wavelength and the refractive index contrast between absorber and vacuum openings. While rigorous electromagnetic solvers such as Rigorous Coupled-Wave Analysis (RCWA) can accurately reproduce these effects, they often offer limited direct insight into the physical propagation mechanisms inside the absorber structures.
To address this, Varun developed an analytical waveguide model for periodic reflective EUV mask structures based on a Bloch-Floquet formulation. His approach extends classical modal methods to lossy absorber materials by solving a complex transcendental eigenvalue problem, enabling the computation of Bloch modes with complex effective refractive indices. The electromagnetic field inside the absorber is then reconstructed as a superposition of these modes, while reflected diffraction orders are determined using a Fourier-based formulation together with coupling to the reflective multilayer.
Validation against RCWA simulations in Dr.LiTHO showed excellent agreement for both near-field intensity distributions and reflected diffraction efficiencies, with deviations below 1.1 percentage points. Through systematic parameter studies involving absorber thickness, incidence angle, and complex absorber refractive index, Varun demonstrated that the model accurately captures the dominant physical trends governing reflective EUV mask behavior.
A key outcome of the work is the insight that optical propagation inside the absorber is governed by only a small number of weakly attenuated Bloch modes, while the reflected diffraction efficiencies are determined by the interplay of modal confinement, attenuation, phase matching, and diffraction outcoupling. Overall, the proposed analytical framework provides a physically interpretable and efficient approach for understanding waveguide-mediated diffraction effects in reflective EUV masks.
We sincerely congratulate Varun on this excellent achievement and thank him for his dedication and valuable contribution to our research. We wish him all the best for his future academic and professional journey!