Interaction of Ni with SiGe for electrical contacts in CMOS technology
Abstract: This thesis investigates the reactive formation of Ni mono-gernanosilicide, NiSi1-uGeu, for contact metallization of future CMOS devices where Si1-xGex can be present in the gate, source and drain of a MOSFET. Although the investigation has been pursued with a strong focus on materials aspects, issues related to process integration in MOSFETs both on conventional bulk Si and ultra-thin body SOI have been taken into consideration. The thesis work has taken a balance between experimental studies and theoretical calculations.The interaction between Ni films and Si1-xGex substrates, polycrystalline (poly) as in the gate or single-crystal (sc) as in the source/drain, leads to the formation of a ternary solid solution NiSi1-uGeu with the MnP structure in a wide range of temperature from 450 to 850oC. A linear variation of the lattice parameters of the NiSi1-uGeu with u is determined. A number of key observations are made: (1) the agglomeration of NiSi1-uGeu on Si1-xGex at a lower temperature compared to that of NiSi on Si, (2) the absence of NiSi2 up to 850 oC when Ge is present, and (3) a substantial Ge out-diffusion from the NiSi1-xGex and a precipitation of Ge-richer SiGe around the NiSi1-uGeu grains. These observations are interpreted referring to the ternary phase diagram for the Ni-Si-Ge system presented in this work.Possible factors influencing the morphological stability of NiSi1-uGeu films on Si1-xGex are discussed: (1) mechanical strain in the epitaxial Si1-xGex, (2) the favorable formation of NiSi at the expense of NiGe, (3) grain growth in poly-Si1-xGex, and (4) grain grooving in NiSi1-uGeu on sc-Si1-xGex. Energetically, the former two factors have been found to play a comparable, yet major role in the morphological instability of NiSi1-uGeu. The inter-diffusion of Si and Ge in NiSi1-uGeu and Si1-xGex provides the kinetic pathway for the morphological evolution. On Si1-xGex epitaxially grown on Si(100), a strong preferential orientation of the resulting NiSi1-uGeu film is found; NiSi films formed on Si show no specific film texturing. Furthermore, layer sequence and layer thickness of Si/SiGe or SiGe/Si are found to strongly affect the film texture in the resulting NiSi1-uGeu. Epitaxy of NiSi on NiSi1-uGeu, and vice versa, occurs across the compositional boundary, which confirms Ni as the dominant diffusion species during germanosilicide formation.The presence of Ge reduces the contact resistivity for NiSi1-uGeu on p-tyep Si1-xGex, as expected. For poly-Si1-xGex doped by B to 1020cm-3, a contact resistivity of 9x10-8 Ωcm2, 5 times lower than for the corresponding NiSi/Si contact, is obtained. On n-type Si1-xGex doped by As to 1020 cm-3, the opposite is true regarding the effect of Ge and a contact resistivity of 2x10-5 Ωcm2, 20 times higher than for the corresponding NiSi/Si contact, is obtained.When formed in the source/drain regions of a MOSFET fabricated on ultra-thin body SOI, a severe lateral growth of NiSi and Ni2Si into the channel region is revealed if the initial Ni thickness is too thick and if the silicidation conditions are not carefully controlled. This leads to a Schottky contact S/D MOSFET due to the consumption of the entire source/drain. In order to realize a low source/drain resistance for MOSFETs on ultra-thin SOI, satisfying the Roadmap recommendation for the 45-nm technology node, simplified calculations have been performed and an elevated source/drain structure is clearly shown to be advantageous.
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