High precision large area PLD of X-ray optical multilayers

Abstract

To realize high quality X-ray optical multilayer stacks on large areas a double-beam PLD source was integrated into a commercial MBE system. Optimization of ablation conditions and film growth regime, resp., for various kinds of homogeneous thin films and multilayer systems has been realized by a reproducible variation of pulse energy and repetition rate of each of the two Nd:YAG-lasers. In addition, the lasers can be independently controlled by a predetermined pulse delay. Thus, plasma parameters of two plumes generated from locally separated origins can be influenced by the pulse delay of the Nd:YAG-lasers, too. The influence of laser parameters and pulse delay on thin film growth is investigated by the deposition of Ni/C layer stacks. Optimum irradiation conditions are zero delay and moderate pulse energies. Multilayer interface roughnesses on the order of σ R≈0.1 nm are deduced from high resolution electron microscopy (HREM)-micrographs. The interface roughness increases with higher pulse energy. For changing the pulse delay from τ=0 ns to τ=2.5 ms, a destruction of the layer stack is observed. Laterally graded Ni/C multilayers showing X-ray optical activity were synthesized with these optimized deposition parameters in the period thickness range from 3 to 5 nm. Average values of thickness gradients typically Δ t/Δ x≈2×10 −8 for 4″ substrate length in maximum and period thickness variations on the order of σ t≈0.1 nm are confirmed by grazing incidence X-ray reflectometry and HREM.