| Copper thin films have been widely used in the fields of microelectronics and micromachine due to their excellent electrical conductivity, great electromigration resistance and good plasticity and ductility. However, the modification of Cu thin films is inevitable owing to their poor adhesion, low strength and hardness. Because of the complementarities of refractory metals and copper, copper-refractory metals composite films often exhibit special structure and excellent comprehensive properties. Therefore, three immiscible systems of Cu-W, Cu-Mo and Cu-Nb prepared by magnetron sputtering and orthogonal array optimization technology were considered in this paper. The Cu-W, Cu-Mo and Cu-Nb composite thin films with wide composition and the Cu/W nanomultilayers were prepared by combined targets co-sputtering and elemental targets alternate deposition, respectively. The effects of W, Mo and Nb on the structural, electrical' and mechanical properties and thermal stabilities of Cu composite thin films were analyzed via the structure and performance tests. The correlations between structures, properties and modulation parameters of Cu/W nanomultilayers were also investigated. These results not only provide experimental basis and important support for the application of copper-refractory metal films, but also have important guidance to the design and development of novel structure-functional thin films.In this study, the uniform process parameters were optimized via reasonable evaluating indicators, which are target power density PD of 8 W/cm2, the sputtering pressure P of 2 Pa and the target-substrate distance DTS of 140 mm. These parameters provide a guarantee for the reaearch on the content effects. The thin film samples were prepared by using the originally designed and manufactured mosaic and overlay structure targets. The composition of thin films is controllable effectively by the simply adjustment of target area fraction. Results presented here provide a new method for the preparation of Cu-based composite thin films.By using the unbalanced magnetron sputtering technology, the Cu composite thin films with metastable and amorphous structure were prepared. It is obvious that the structures of Cu thin composite films affected by the content of W, Mo and Nb:(1) With the low content of W, Mo and Nb((2.12?16.19)at.%W, (2.19?15.12)at.%Mo and (1.16? 23.43)at.%Nb content), the composite thin films exhibit metastable quasi-miscibility homogeneous structure containing the fcc Cu(W), Cu(Mo) and Cu(Nb) metastable quasi solid solution. (2) With the middle content of W, Mo and Nb((24.65 ?38.2)at.%W, (20.26?35.15)at.%Mo and (35.32?48.1)at.%Nb content), Cu-W and Cu-Mo films are metastable quasi-miscibility high-homogeneous structure containing (fcc Cu(W)+bcc W(Cu)) and (fcc Cu(Mo)+bcc Mo(Cu)) metastable quasi solid solution, whereas Cu-Nb film is a mixture of Nb amorphous and Cu crystalline. (3) With the high content of W, Mo and Nb((53.14?55.92)at.% W, (58.6?74.91)at.%Mo and (53.36?60.4)at.%Nb content), the Cu-W and Cu-Mo films also are metastable quasi-miscibility structure containing bcc W(Cu) and bcc Mo(Cu) metastable quasi solid solution, while Cu-Nb film is the complete amorphous structure.The appropriate addition of W, Mo and Nb has a significant grain refinement effect and can improve the surface smoothness of Cu composite thin films:with the increase of the W, Mo and Nb content, the grain size gradually reduces to less than 4 nm and the value of RMS is low to .93 nm. These results can be contributed to the low diffusion rate of W, Mo and Nb, and the nuclei of uniform dispersion which prevents the diffusion and migration of Cu atoms effectively. The Cu composite thin films containing W, Mo and Nb have the intrinsic stress of 102 MPa, which is in the state of elastic compressive stress.The electrical and mechanical properties of Cu composite films were significantly affected by the content of W, Mo and Nb:with the increase of the content of W, Mo and Nb, the resistivity p, yield strength 00.2, elastic modulus E and hardness H of Cu composite thin films increased gradually, whereas the critical strain of crack initiation ?c and the friction coefficient?k decreased gradually. The values of ?p??0.2 and ?c for the thin films with high W and Mo content decrease, while the values of ? and ?k for the Cu-Nb films with the middle and high content of Nb significantly increase. The changes of electrical and mechanical properties are closely related to the structural changes. Generally, the resistivity of Cu-Nb film is the highest and that of Cu-W film is the lowest, and the elastic modulus and hardness of Cu-Nb film are the lowest, suggesting the influence of the component element nature. Cu-2.12at.%W film has the lowest resistivity (11.4 ??·cm), Cu-35.15at.%Mo film has the highest yield strength (1.302 GPa) and the lowest friction coefficient (0.089), Cu-1.16at.%Nb film possesses the largest critical strain of crack initiation (1.95%), and Cu-74.91at.%Mo film exhibits the highest elastic modulus (168.85GPa) and hardness (9.19 GPa). The addition of W, Mo and Nb can affect the bonding force of film-substrate for Cu composite thin films significantly that the bonding force increases firstly and then decrease with the increase of content. Cu-38.2at.%W film has the strongest bonding force (31.07 N).The structure of Cu composite thin films was changed in different degrees by heat treatment. In general, the recovery and initial recrystallization occur during annealing at the temperature ranging from 200?400?. When the critical annealing temperature is 650?, the phase-separation and crystallization transition occur in the Cu composite thin films with middle and high W, Mo and Nb content, respectively, which two-phase mixing structure of Cu-rich+W, Mo and Nb-rich forms. When the annealing temperature is 650?, the phase separation is thorough and the two-phase structure is complete with the increase of annealing time. During the annealing, the strain of Cu composite thin films is relaxed and the intrinsic compressive stress is weakened. The evaluated thermal stability indicates that the temperature limit of the structure stability for the Cu composite thin films is the range of 400?650?. The morphology of annealed Cu composite thin films is obviously changed, and the surface roughness was significantly increased due to the convex Cu aggregate particles. Some aggregate particles have the hexagonal or quasi-symmetric octagon due to the lowest energy cuboctahedral structure. Annealing can change the mechanical and electrical properties of thin films that the values of E and H usually decrease and the resistivity monotonically decreases with the increase of annealing temperature or time. But for Cu-Nb films, the values of E and H increase after crystallization during annealing. After annealing at 650? for 5 h, the values of E and H for Cu-74.91at.%Mo films are still the highest (162.5 and 7.24 GPa, respectively), and that of resistivity for Cu-2.12at.% W films is still the lowest (5.47 ??·cm).The Cu and W layers in Cu/W nanomultilayers show a nanocrystalline structure. The Cu{111} and W{110} textures decrease or disappear with the increase of ? The interplanar spacing of W(110) decreases with the decrease of ? or with the increase of ?. The mixed-transition layer exists in the interface of Cu/W layers. When the thickness of layers increases, the grain size and the surface smoothness of Cu layer increases significantly, whereas the grain size and surface smoothness of W layer do not change obviously. The modulation parameter ? and ? significantly affect the mechanical and electrical properties of multilayers. Generally, when ? and ? increase, the yield strength, elastic modulus, hardness and resistivity of multilayers decrease, while the critical strain of crack initiation ?c increases with the decrease of ? or with the increase of ?. The change of modulation parameters can result in the change of layer thickness, the amount of interface between layers and microstructure of sublayers (especially Cu layer). The dislocation motion and the electron scattering status in intralayer and interlayer, and the stress intensity factor of W layer also change. Therefore, the performances of multilayers change regularly. |
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