Mo/Si-multilayers for EUV applications prepared by Pulsed Laser Deposition (PLD)

• Published in: Microelectronic engineering 2001-09, Vol.57, p.9-15
• Main Authors: Braun, St, Dietsch, R, Haidl, M, Holz, Th, Mai, H, Müllender, S, Scholz, R
• Format: Article
• Language: English
• Subjects: Applied sciences; Electronics; Exact sciences and technology; Microelectronic fabrication (materials and surfaces technology); Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices
• ISSN: 0167-9317; 1873-5568
• DOI: 10.1016/S0167-9317(01)00425-7

In the past, the successful application of PLD for X-ray multilayer synthesis has already been demonstrated for C-spacer systems. Recently, the method has also been tested for Mo/Si layer stacks. A UHV-coating machine has been used to prepare X-ray mirrors on 4 in. substrates. The ablation of both Mo and Si targets was carried out by Nd:YAG laser irradiation using the third harmonic ( λ=355 nm) with a pulse energy E p=275 mJ and a pulse width τ=4…6 ns. Multilayers of 10…50 periods have been synthesized. Soft X-ray measurements in the EUV-range at near normal incidence show reflectivities R s of typically 60%. From HRTEM, a high stack regularity and minimum interface roughness can be deduced. In contrast to conventional technologies (coating by sputtering or e-beam evaporation) the formation of a MoSi x -interface layer happens only when depositing Mo on Si. Extremely sharp interface transitions from one individual layer to the other are observed and the total period is represented by a three-layer system. From TEM results, a structure model for PLD-prepared Mo/Si-multilayers has been deduced. The optical parameters of the layers were adapted by reflectivity curve fitting, so that the results measured in the EUV-range can be explained. Using this model, predictions of the ratio of the number of atoms N Si/ N Mo for the total stack were made and are in good agreement with results of RBS measurements. The use of the multilayers as X-ray optics requires an excellent homogeneity of the layer thickness across the entire mirror. It can be shown that the PLD technique is able to realize film uniformities with a standard deviation of the period thickness of less than 0.5%. This was confirmed by Cu–Kα-reflectometry and by near normal incidence measurements in the EUV range on 4 in. samples.