Study On Formation Mechanism And Properties Of Self-Assembled Cu-Cr Multilayers

The self-assembled metallic multilayers are prepared by magnetron sputtering with the dual target co-deposition.Due to the special size effect and interface structure,the film not only has higher hardness but also has well toughness.There are broad application prospects in the field of microelectronics.At home and abroad,the current research of self-assembled metallic nano-multilayers mainly focused on the influence of coating process parameters on the microstructure and mechanical properties of the film.It had not deeply investigated the formation mechanism of selfassembled multilayers.This paper mainly explored the formation mechanism of CuCr self-assembled multilayers which was based on the thermodynamic and kinetic calculations.In addition,it verified the theoretical calculations with experimental results and explored the relationship between the mechanical properties and the microstructure of self-assembled multilayers.According to the thermodynamic calculations,it obtained the Gibbs free energy of solid solution and amorphous phase of Cu-Cr system,as well as the corresponding interface energy of different modulation periods of multilayered structure.Finally,the Gibbs free energy diagram of Cu-Cr system under equilibrium state was drawn.The calculation results showed that the corresponding interface energy of multilayered structure was higher with the smaller modulation period.It calculated the energy of deposited atoms under the actual coating process parameters.Besides,it calculated the distance of uphill diffusion of Cu atoms from the perspective of kinetics.It mainly simulated the relationship among the energy of deposited atoms,the corresponding interface energy of different modulation periods and the thermal diffusion distance of Cu-Cr atoms when the Cu-48 at.%Cr film spontaneously formed nano-multilayers.The study found that the Cu-Cr system could spontaneously form the nano-multilayered structure just when the energy of deposited atoms was greater than the corresponding interface energy of different modulation periods.And the energy of deposited atoms should not be too high or the modulation period should not be too small.The Cu-Cr atoms tended to be mixed when the energy of deposited atoms was greater than the diffusion energy of Cr atoms or the uphill diffusion distance of Cu atoms was greater than the thickness of the monolayer.It also destroyed the spontaneous formation of the multilayered structure.When the coating power and other process parameters were the same,by analyzing the microstructure of Cu-Cr films with different process parameters,it could be seen that the film gradually formed alternating stacked layered structures as the working pressure dropped below 1 Pa.When the working pressure and other process parameters were the same,the film gradually formed alternating stacked layered structures as the rotations of the sample stage decreased below 8 rpm.The experimental results further verified the thermodynamic calculation.The experimental results also showed that the lower working pressure,larger coating power and smaller target base distance could increase the energy of the deposited atoms,thus improved the ability of the film to spontaneously form multilayered structures.According to the test of mechanical properties,the nano-indentation hardness of the Cu-Cr film increased from 4.2 ± 0.1 GPa to 9.3 ± 0.2 GPa as the film structure transited from the mixed state to the multilayered structure(?=7 nm).The main reason for the increase in hardness was the pinning effect of much interfaces on dislocations.In addition,the increase of hardness was also related to the crystallinity and grain orientation of the film.When the working pressure was 0.2 Pa,the Cu-Cr film not only had higher hardness but also had well toughness through the analysis of the indentation morphology.