Impact of morphological defects on the electrical breakdown of ultra thin atomic layer deposition processed Al2O3 layers
Introduction
Oxide based thin film transistors have intensively been discussed concerning different aspects, such as defect generation, degradation and breakdown mechanisms [1], [2]. Nevertheless, focusing on breakdown effects, thin oxide layers have not completely been understood yet. Particularly, the breakdown behavior at thin film thicknesses below 50 nm still lacks a viable physical theoretical background. As one of the first groups, Lin et al. observed an unexpected increase in the disruptive strength Ebd = Ubd/d for Al2O3 at layer thicknesses less than 6 nm, where Ubd means the breakdown voltage and d the thickness of the dielectric [3]. A step towards the understanding of the breakdown properties at low oxide thicknesses has been made by Blonkowski [4]. By assuming a filamentary growth of percolation paths through the dielectric induced by an external electric field at the value of the disruptive strength he presented the first analytical and comprehensive approach for predicting the measured breakdown electric fields. The modeling depends on the layer thickness, bulk filament characteristic times and the band gap among some other parameters [4]. The reason for the formation of filamentary percolation paths is mainly attributed to defects inside the dielectric structure in terms of irregularities of the stoichiometry inside the dielectric. These defects are deduced to result from broken atomic bonds or oxygen deficiency, structural failures such as local melting points after voltage stress and nanocrystallites resulting from imperfect amorphous layer growth. In fact, these defects are stress driven or process dependent, respectively. In the following, we investigate the impact of existing morphological defects in ultra thin oxide layers like pin holes and spatially varying material densities and its relationship to the voltage breakdown properties by examining thin films of Al2O3 prepared by ALD.
Section snippets
Experimental details
In order to measure the morphological defect density of the dielectric layers, Cu electroplating has been performed as previously reported by Zhang et al. [5]. According to [6], tunneling currents at low thicknesses can occur, but do not play a significant role, since the resulting Cu bumps should be small and homogeneously shaped, which is in contrast to our observations. The dielectric layer was deposited onto an Al electrode deposited on an indium tin oxide (ITO) precoated glass substrate.
Results and discussion
First, the density of morphological defects in Al2O3 prepared by ALD has been investigated. Atomic layer deposited monolayers can exhibit different forms of imperfections under real circumstances leading to an inhomogeneous coverage of the substrate and resulting in the growth of pin holes. This is attributed to the surface relief effect at the first few ALD cycles, which has been simulated numerically by Neizvestny et al. [8]. The surface relief effect leads to remaining pin holes inside the
Conclusion
In this contribution, we report on the continuous increase of the disruptive strength towards lower thicknesses of ultra thin Al2O3 layers prepared by ALD with a concomitant increase of the morphological defect density. The morphological defects detected by Cu electroplating act as pin holes and conducting paths in the dielectric but have no disadvantageous influence on the disruptive strength as shown in the analytical model of Blonkowski [4]. Accordingly, the dielectric breakdown is mainly
Acknowledgments
The authors would like to thank the German Federal Ministry of Education and Research for their financial support, Dr.-Ing. Michael Kröger, Dipl.-Phys. Daniela Donhauser and Dipl.-Phys. Diana Nanova from the InnovationLab Heidelberg for the SEM pictures of the Cu structures and Justyna Rodziewicz for the preparation of the substrates.
References (16)
- et al.
Thin Solid Films
(2009) - et al.
Surf. Coat. Technol.
(2011) - et al.
Comput. Mater. Sci.
(2006) - et al.
Thin Solid Films
(2002) Solid State Electron.
(1997)- et al.
J. Appl. Phys.
(2005) - et al.
IEEE Trans. Device Mater. Reliab.
(March 2005) - et al.
Appl. Phys. Lett.
(2005)
Cited by (11)
Bayesian machine learning for efficient minimization of defects in ALD passivation layers
2021, ACS Applied Materials and InterfacesFailure modes of protection layers produced by atomic layer deposition of amorphous TiO<inf>2</inf>on GaAs anodes
2020, Energy and Environmental ScienceGate-first process compatible, high-quality in situ SiN<inf>x</inf> for surface passivation and gate dielectrics in AlGaN/GaN MISHEMTs
2019, Journal of Physics D: Applied Physics