High precision large area PLD of X-ray optical multilayers

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 NirC layer stacks. Optimum irradiation conditions are zero delay and moderate pulse energies. Multilayer interface roughnesses on the order of sRf0.1 nm are deduced from high resolution electron microscopy HREM.-micrographs. The interface roughness increases with higher pulse energy. For changing the pulse delay from ts0 ns to ts2.5 ms, a destruction of the layer stack is observed. Laterally graded NirC 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 DtrD xf2=10y8 for 4Y substrate length in maximum and period thickness variations on the order of stf0.1 nm are confirmed by grazing incidence X-ray reflectometry and HREM. q1998 Elsevier Science B.V.