Elsevier

Thin Solid Films

Volume 534, 1 May 2013, Pages 172-176
Thin Solid Films

Impact of morphological defects on the electrical breakdown of ultra thin atomic layer deposition processed Al2O3 layers

https://doi.org/10.1016/j.tsf.2013.02.076Get rights and content

Highlights

  • Defects in atomic layer deposited Al2O3 films detected by Cu electroplating

  • Defect density rises as layer thickness decreases.

  • Relative permittivity decreases at lower layer thicknesses.

  • Despite of increasing defect density, breakdown electric field increases.

  • Defect density not correlated to those defects supporting electrical breakdown

Abstract

We report on the continuous increase of the breakdown electric field, also known as disruptive strength, of an ultra thin layer based on Al2O3 prepared by low temperature atomic layer deposition (ALD) by reducing its thickness down to 3 nm. By measuring the disruptive strength for lower thicknesses, we demonstrate that these observations are in agreement with recent reports. Furthermore we detected an increase within the disruptive strength towards lower thicknesses together with a rise of the pin hole density. The pin holes, originating from an inhomogeneous growth of the dielectric and referred to as morphological defects and current conducting paths, are detected by Cu electroplating and result in a lower permittivity of the dielectric. As a conclusion, the dielectric breakdown of thin, low temperature ALD processed Al2O3 layers does not seem to be negatively influenced by the increase of the pin hole density. Thus, the increase of the disruptive strength is either due to the morphological defects in the form of pin holes or a material phenomenon that is not affected by its nanoporous structure.

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.

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