Microstructure And Poroperties Of Sputtered Copper And Copper Alloy Thin Films

As the development of the manufacture techniques for integrated circuits, Cu has replaced Al as the interconnect material owing to its low resistivity and high electromigration resistance. However, microstructure control is becoming increasingly important as interconnect line-widths decrease below 0.5μm, because performance and reliability become more critically affected by specific microstructure than by average bulk properties. Much attention has been paid to the method of alloying, which could both control the microstructure and improve the properties of thin films. The basic researches of Cu alloy films play an important role in predicting the performance of Cu interconnects, optimizing the damascene technology and enhancing the reliability of Si-ICs.In this study, Cu(Cr) and Cu(Zr) films were deposited by magnetron sputtering by using the mosaic structure target with optimizing parameters. Effects of the Cr and Zr dopants on the microstructure and properties of Cu films were investigated. The research results could provide a guideline not only for the expanded application of Cu(Cr) and Cu(Zr) alloy films, but also for the design and the developmentof Cu alloy films with high strength and electrical conductivity.The Cu1-xMx(M=Cr, Zr) alloy targets with the mosaic structure were originally designed and manufactured. Cu1-xMx(M=Cr, Zr, x<0.05, at.%) alloy films with different thickness and composition were deposited on Si(100) substrates by using our mosaic structure target. The composition of alloy films with high purity is in control. Results presented here provide a new method for magnetron sputtering Cu film contain insoluble substances.Effects of the sputtering pressure P, the distance between the target and the substrate D, the alloying dopant and the annealing temperature on the texture and morphology of films were investigated. The (111) texture, as the increase of D and P, increases first and decreases subsequently. When the P is 0.5 Pa and the D is 200 mm, the films have the strongest (111) texture and the fine morphology. The texture of sputtered films is determined by both the surface energy and the strain energy of the system. In a certain range of temperature, Cr or Zr dopant increases the (111) texture of Cu films, which is related with the refinement of grains in alloy films and the increase of internal stress of alloy films.Evident grain growth in Cu(2.6 at.%Cr) alloy films occurs at the temperature above 450℃. After annealing at 450℃, Cu(Cr) alloy films exhibit a bimodal grain size distribution with fine grains. Annealing at higher temperatures results in an abnormal grain growth. No evident Cr precipitates are found in Cu(Cr) alloy films after annealing. The bimodal grain size distribution is considered to be ascribed to the growth of favorably oriented grains.Cross-section TEM images show that the annealed Cu films exhibit a'bamboo'grain structure, and grain boundary grooves have formed at the film/substrate interface. By contrast, a typical columnar structure can be seen in Cu(Cr) alloy films, which is in good agreement with the structure zone model. Cr dopant inhibits the grain growth and the grooving process in annealed Cu films. Moreover, an extremely high density of {111} twin boundary is found in Cu(Cr) alloy films, in which the estimated twin spacing and twin lamella are about 6 nm. The twin boundary density in Cu(Cr) films is about 18.6×108 m2/m3, which is 20 times as much as that of annealed Cu films. The formation of nano-twin is related with the internal stress and the preferential <111>-orientated grain growth. As a result, the hardness and young's modulus of alloy films increase markedly, without degrading the conductivity much after annealing.After annealing at 500℃, Cu films agglomerate on Si(100) substrates because of the significant grain growth. However, Cr dopant refines the grains, inhibits the agglomeration and enhances the thermal stability of Cu films on Si(100) substrates. In contrast, Zr dopant accelerates the diffusion and reaction between the Cu film and the Si(100) substrate and degrades the thermal stability of Cu films, which are related with the'purifying effect'of Zr. As a result, the final resistivity of Cu(Cr) films is approached to that of Cu films, while the resistivity of Cu(Zr) films increases sharply after annealing.On the contrary, for the Cu(Zr) films on SiO2 substrates, Zr segregates at the film/substrate interface, thus, an effective Zr-rich layer at the interface is formed, which inhibits the diffusion between the film and the substrate. As a result, the thermal stability and adhesion between the film and the substrate are improved. The formation of the interfacial barrier layer is depended on the alloying element and its content in alloy films.Cr dopant improves the electromigration resistance remarkably. The electromigration lifetimes of Cu(Cr) lines are 10-100 times longer than those of Cu lines. The improvement of electromigration resistance of Cu(Cr) alloy lines is related with the segregation of Cr on the film surface, the increase of {111} twin boundaries density and the Cu(111) texture of alloy lines.