A study on Al texture, hillock formation and electromigration in Al-0.5wt%Cu/TiN(/Ti)/SiO2/Si interconnect structure

Autor: Lih-Ping Wang, 王麗萍
Rok vydání: 1999
Druh dokumentu: 學位論文 ; thesis
Popis: 88
This work presents an experimental study of a Al-0.5wt%Cu/TiN/Ti interconnect structure and a Cu/TiN/TiSi2/Si contact structure relating to their applications in IC metallization technology. For the Al-0.5wt%Cu/TiN/Ti interconnect structure and the Al texture, hillock formation and electromigration were studied under the influence of the insertion of Ti interlayer or/and Ar pretreatment sputtering. For the Cu/TiN/TiSi2/Si contact structure, the Cu-Si interdiffusion was also investigated to understand the void formation and Cu3Si precipitation. Hillock formation and electromigration are two important concerns in reliability of Al-base interconnects. It has been recognized that Al(111) texture strongly affects the electromigration performance and also influences the hillock formation. It has been proven that higher Al(111) texture is preferred to improve the electromigration resistance of Al-based interconnect. The insertion of Ti interlayer and Ar pretreatment sputtering, carried out on the SiO2 surface prior to Ti deposition, was previously reported to increase the Al(111) texture, thus to enhance the electromigration resistance. The effect of Ti interlayer on hillock formation and Al(111) texture of Al-Cu films was characterized by FESEM, AFM XTEM and XRD in Al-Cu/TiN/Ti interconnect structure. Two kinds of hillocks were found on the Al-Cu films for both multilayers with and without Ti after 400°C annealing for 30 minutes in N2 ambient. One of them is the round hillock having the same morphology for both multilayers with and without Ti. It has the size of 0.2-0.7 mm in height and 1-3 mm in diameter. It is a newly formed Al crystallite lying over more than 200 original columnar grains. The dominant mechanism responsible for the formation of round hillocks is considered as the grain boundary diffusion. On the other hand, the width of the other kind of hillock is much smaller than that of the round hillock and its morphology is different for multilayers with and without Ti. For the multilayer without Ti, it is the wedge-like hillock with an asymmetric wedge shape in side view. This hillock consists of more than 200 original columnar grains which extrude upwards to form an inclined surface with an off-surface angle of 0.6-3.5°. Some of them are formed besides the round hillock. All the wedge-like hillocks were found oriented toward the same direction along one of two in-plane Si directions with asymmetric distribution of Al(111) texture. The oriented wedge-like hillock is a new type of hillock. The most likely mechanism responsible for the formation of wedge-like hillocks is considered to be the dislocation creep rather than the grain boundary diffusion. In contrast, the irregular flat hillock was observed with the round hillock on the multilayer with Ti. The size of the irregular flat hillock is comparable with the wedge-like hillock on the multilayer without Ti, but without the asymmetric shape as the latter. The difference in hillock morphology on the multilayers with and without Ti is attributed to the difference in the distribution of Al(111) texture. The pretreatment sputtering of Ar on the SiO2 surface prior to Ti deposition is to increase the electromigration resistance of 1mm Al-Cu/TiN/Ti interconnects while greatly degrades the Al(111) texture. Chemical analysis shows a fairly large amount of oxygen and nitrogen segregated near the Al-Cu/TiN interface. T his phenomenon is attributed to the enhancement of electromigration resistance and degradation of Al(111) texture. The study of the Cu-Si interdiffusion in Cu/TiN/TiSi2/Si contact structure will be submitted as a special topic. The interests were aroused from the importance of the use of Cu film in the current metallization technology due to its lower resistivity and superior resistance to electromigration than Al film. It has been studied [Chang et al., J. Appl. Phys. 75. 4874 (1994).] that the TiN/TiSi2 bilayer can act effectively as a diffusion barrier up to 475 °C between the Cu film and Si substrate. The TiN/TiSi2 bilayer was formed by annealing of Ti/Si in rapidly thermal nitridation in either one-step (850°C/30 min./N2) or two-step (650°C/30 min./N2; TiN etched and 850°C-/30 min./N2) processes. The interdiffusion between Cu and Si through TiN/TiSi2 barrier leads to the formation of voids near the Cu/TiN interface and h''''-Cu3Si under TiSi2 in Si substrate after annealing above 450°C and 500°C, respectively, for 30 minutes. The shape of the Cu3Si precipitate was observed to be inverse-pyramidal with its oblique plane parallel to the Si{111} planes and the edges of square base parallel to two in-plane Si[011] directions. The high oxygen content in the TiN film is attributed to the cause in the oxidation of both Cu3Si and the surrounding Si region for the sample prepared by two-step process after annealing at 500 - 600°C.
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