Thermal stability of polycrystalline silicon electrodes on ZrO2 gate dielectrics

Abstract

Thermal stability of gate stack structures composed of ZrO2 gate dielectrics and silicon electrodes was investigated. The ZrO2 films were deposited by atomic layer deposition, while the polycrystalline silicon electrodes were deposited using different variants of chemical (CVD) and physical vapor deposition (PVD). Zirconium silicide formation was noted in all CVD-electroded samples after subsequent annealing treatments at temperatures above 750 °C, but not in the room temperature PVD-electroded samples, even after gate annealing at 1050 °C. The dependence of zirconium silicide formation on the Si deposition process was explained using thermodynamic arguments which explicitly include the effects of oxygen deficiency of the metal oxide films.

Thermodynamic considerations in the stability of binary oxides for alternative gate dielectrics in complementary metal–oxide–semiconductors

“…Without direct evidence of SiO gas formation, observations of thinning of a pre-existing SiO 2 layer underneath a high-k oxide during annealing under moderately reducing conditions may also be due by decomposition and subsequent filling of O vacancies in the high-k dielectric as shown in Fig. 3(b) [8]. This mechanism is possible if the as-deposited high-k oxide was oxygen deficient, or if reducing processing atmospheres caused the high-k oxide to loose oxygen, because most high-k oxides under consideration have more negative free energies of formation than SiO 2 .…”

“…The first approach is to include gaseous species, in particular SiO, as part of the thermodynamic analysis of interfacial reactions under reducing conditions [4][5][6][7]17,18]. A different explanation is based on nonstoichiometry of the high-k oxide [8]. Both schemes are evaluated in the following sections.…”

“…While the experimental conditions under which these layers form are relatively well understood, significant disparity exists in the literature with respect to the reactions causing the formation of interfacial layers, in particular the silicides [4][5][6][7][8][9]17,18].…”

“…Recently, a mechanism for the silicide formation involving oxygen deficiency has been proposed [8]. The free energy of the oxygen deficient film/substrate system can be lowered by oxidation, if oxygen is available, or by reaction of excess metal with Si, leaving behind a stoichiometric oxide:…”

“…However, thermodynamic stability analysis considering only reactions between two stoichiometric solids appears to be insufficient for predicting stable heterointerfaces. Interfacial phases, such as silicides, SiO 2 , and silicates, are often observed in real high-k dielectric gate stacks exposed to high temperatures [3][4][5][6][7][8][9][10][11][12][13][14], as schematically illustrated in Fig. 1.…”