Gate dielectrics breakdown (BD) is one of the most challenging areas in the field of semiconductor device reliability. Despite massive research efforts conducted for over 50 years by industry and academia, a consensus has not yet been reached on the microscopic nature of the degradation process leading to BD, and none of the models proposed over the years provide a comprehensive and consistent description of all the experimental observations. Furthermore, time-dependent dielectric breakdown (TDDB) testing is typically done under field- and/or temperature-accelerated conditions. Reliable models are thus required to interpret the results and extrapolate them to real operating conditions. Having an accurate, physics-based breakdown model is thus vital for a correct prediction of device lifetime (or, equivalently, of the maximum operating voltage for 10 years lifetime). We have developed a microscopic breakdown (BD) model in which chemical bonds in amorphous oxide films are weakened by carrier injection and trapping into pre-existing structural defects (precursors) and by the electric field. The model goes much beyond the existing ones by consistently explaining the role of both current and temperature, along with the role of the electric field [1-3].

The model stems from the results of extensive computational studies, carried out on Archer2 and Young, highlighting the critical role of electron and hole trapping at pre-existing structural features in amorphous structures (precursors) of gate dielectrics, such as SiO₂, HfO₂, Al₂O₃ [1,2].  The developed breakdown model comprises three key material-dependent processes, shown in Fig. 1 for the case of the SiO₂: (i) the presence of precursor electron trapping sites, such as oxygen vacancies (V_O) and specific structural features in amorphous oxides, like wide O-Si-O bond in SiO₂ or elongated bonds in HfO₂ [4], capable of trapping two electrons; (ii) the breakage of the adjacent bond, weakened by electron trapping at the precursor site, electric field and temperature [Fig. 1 (steps 1-3), and Fig. 3]; and (iii) an effective process leading to the formation of new precursors that provides the self-sustainability to the whole mechanism (steps 4-5 in Fig. 1).

Figure 1 Schematic representation of the two-electron injection-driven microscopic processes at the bases of the degradation and breakdown. 1) Precursor (T) in SiO2. 2) Two electrons occupy precursor. 3) Field- and temperature-activated oxygen vacancy (V) generation. 4) Two electrons occupy a vacancy. 5) Generation of a new precursor. 6) Field- and temperature-activated generation of a second vacancy (after two electrons are trapped into the precursor).

The processes outlined in Fig. 1 have been investigated in amorphous SiO₂ by combining classical molecular dynamics (MD) calculations of amorphous structures with Density Functional Theory (DFT) calculations of precursor trap properties as well as energy barriers and field acceleration factors for the trap generation processes [1]. Similar mechanisms have been demonstrated also in both crystalline and amorphous HfO₂ [2] and are being investigated in other materials (e.g. Al₂O₃, TiO₂, Ga₂O₃), suggesting their possible universality [3]. DFT calculations [4] suggest the wide O-Si-O bonds and the elongated Hf-O bonds as the fundamental V_O precursors in SiO₂ and HfO₂, respectively. DFT calculations [5] demonstrate that an applied electric field only weakly affects barriers for the creation of oxygen vacancy-interstitial defect pairs and diffusion of interstitial O ions, which remain high (typically 6-8 eV). However, injection of extra electrons from electrodes significantly lowers barriers for defect creation, which are further reduced by the field to around 1-2 eV. Thus, bias application facilitates the injection of electrons into the oxide; these extra electrons reduce energy barriers for the creation of O vacancies, and these barriers as well as those for O ion diffusion are further lowered by the field. Further electron trapping by V_O facilitates creation of new V_Os nearby both in SiO₂ and HfO₂ [see for example Fig. 1 (steps 5 and 6)] [6]. This so-called “energetic correlation” effect, where pre-existing O vacancies locally increase the generation rate of additional vacancies, accelerates the oxide degradation process [2]. The dynamics of such processes is, however, material dependent [3-5]: in SiO₂ the structure distortions caused by doubly occupied V_O can generate a new wide O-Si-O bonds, which in turn could generate a new V_O. However, in HfO₂ new V_O can be directly generated from a doubly occupied V_O, acting as a V_O precursor itself. The described carrier-assisted processes provide an explanation of the V_O formation, despite the strength of the involved bonds.

However, to what extent the structure and properties of oxygen vacancies in amorphous solids are similar to those in the crystalline phase are still debated. The validity of this analogy and the experimental and theoretical evidence of the effects of oxygen deficiency in amorphous oxide films are critically discussed in ref. [7]. To test the validity of existing defect models, ab initio molecular dynamics on Archer2 was used with a non-local density functional to model the structure and electronic properties of oxygen-deficient amorphous alumina. Unlike some previous studies, the formation of deep defect states in the bandgap caused by the oxygen deficiency is found. Apart from atomistic structures analogous to crystal vacancies, the formation of more stable defect states characterized by the bond formation between under-coordinated metal ions is shown.

Outcomes

Dielectric breakdown of materials is a complex and multiscale problem at the interface between materials science, physics, solid-state electrochemistry and electronic engineering. This new class of atomistic models based on extensive computational modelling using HPC facilities reconciles many breakdown theories within a more universal picture and provide fundamental insights on the evolution of dielectric degradation with an unprecedented level of detail. Extensive simulations also elucidate the structure of point defects in amorphous oxides.

Collaborators

Andrea Padovani, Luca Larcher, Paolo La Torraca, University of Modena and Reggio Emilia, Italy and Applied Materials, Italy. David Gao, Nanolayers Research Computing Ltd.

Publications

1. A. Padovani, D. Z. Gao, A. L. Shluger, L. Larcher, “A microscopic mechanisms of dielectric breakdown in SiO₂ films: an insight from multi-scale modeling,” J. Appl. Phys., vol. 121, p. 155101, 2017. DOI: https://doi.org/10.1063/1.4979915 .
2. J. Strand, L. Larcher, A. Padovani, P. La Torraca, and A. L. Shluger, “Dielectric breakdown in HfO₂ dielectrics: using a multiscale modelling to identify the critical physical process involved in oxide degradation,” J. Appl. Phys., vol. 131, p. 234501, 2022. DOI: https://doi.org/10.1063/5.0083189.
3. A. Padovani, P. La Torraca, J. Strand, L. Larcher and A. L. Shluger, Dielectric breakdown of oxide films in electronic devices. Nat Rev Mater 9, 607–627 (2024). https://doi.org/10.1038/s41578-024-00702-0
4. 4. J. Strand, M. Kaviani,D. Gao, Al-M. El-Sayed, V. V. Afanas’ev, and A. L. Shluger, Intrinsic charge trapping in amorphous oxide films: status and challenges, J. Phys.: Condens. Matter vol. 30, p. 233001, 2018. DOI 10.1088/1361-648X/aac005
5. J.Strand, J. Cottom, L. Larcher, A. L. Shluger, Effect of electric field on defect generation and migration in HfO2, Phys. Rev. B, vol. 102, p. 014106, 2020. DOI: 10.1103/PhysRevB.102.014106
6. D. Z. Gao, J. Strand, M. S. Munde, and A. L. Shluger, “Mechanisms of oxygen vacancy aggregation in SiO₂ and HfO₂”, Frontiers in Physics, vol. 7, article 43, March 2019. DOI: https://doi.org/10.3389/fphy.2019.00043 .
7. J. Strand, A. L. Shluger, On the Structure of Oxygen Deficient Amorphous Oxide Films. Adv. Sci., vol. 11, 2306243, 2024. https://doi.org/10.1002/advs.202306243

Contact for Further Information

Alex Shluger (a.shluger@ucl.ac.uk), University College London