Developments in realistic design for aperiodic Mo/Si multilayer mirrors

Developments in realistic design for aperiodic Mo/Si multilayer mirrors A.L. Aquila, F. Salmassi, F. Dollar, Y. Liu, and E.M. Gullikson Center for X-Ray Optics, Lawrence Berkeley National Lab 2-400, Berkeley, CA 94720 Abstract: Aperiodic multilayers have been designed for various applications, using numeric algorithms and analytical solutions, for many years with varying levels of success. This work developed a more realistic model for simulating aperiodic Mo/Si multilayers to be used in these algorithms by including the formation of MoSi 2 . Using a genetic computer code we were able to optimize a 45° multilayer for a large bandpass reflection multilayer that gave good agreement with the model. I. Introduction Aperiodic multilayer structures have been developed [1,2] for various applications that require larger wavelength/energy bandpasses than can be achieved with periodic structures. These structures have been used with great success under the title of “supermirrors” in both neutron optics, and grazing incidence hard x-ray optics. However, the use of aperiodic multilayers in large angle soft x-ray/extreme ultraviolet (EUV) optics has been hampered due to lack of agreement in the optimizing simulations compared to the deposited thin film optics. Often the measured reflectivity can differ significantly from the design goal. While these differences could result from inaccuracies in the deposition process there is also the predictable error caused by the intermixing at the multilayer interfaces. The motivation of this work is to demonstrate improvements in the simulation of aperiodic multilayers to allow for there use in applications such as bandpass filters, and integrated reflectivity mirrors. The Mo/Si multilayer system is probably the best studied system due to its importance as a high reflectance coating for EUV lithography and astronomy. This paper demonstrates for the Mo/Si system that when the formation of MoSi 2 is properly included in the design of an aperiodic multilayer structure, it is possible to fabricate a multilayer with a performance very close to the design. The silicide simulation process is discussed in Section 2. Section 3 discusses the computational optimization process and section 4 describes the sample preparation and compares the designed and measured reflectivity. II. Silicide It is well known that the intertermixing of the Mo and Si plays an important role in determining the structure of a Mo/Si multilayer. The multilayer not