FIB damage of Cu and possible consequences for miniaturized mechanical tests

https://doi.org/10.1016/j.msea.2007.01.046Get rights and content

Abstract

Cu specimens were exposed to Ga+ ion bombardment for varying conditions of ion energy, ion dose, and incident angle in a focussed ion beam workstation. Conventional transmission electron microscopy investigations were employed to analyze the Ga+ ion induced damage. The extent of visible damage was minimized by reducing the ion energy and furthermore by using grazing incident ions. Concentration depth profiles of the implanted Ga were measured by Auger electron spectroscopy. Concentrations of up to 20 at.% Ga were found several nanometers below the surface. Ga contents of more than 2 at.% were detected within a depth of up to ∼50 nm. Mechanical consequences in terms of possible hardening mechanisms are discussed, taking into account the experimental findings along with Monte Carlo simulations. A non-negligible influence of the ion damage is predicted for submicron-sized samples.

Introduction

From the 1980s onwards the focussed ion beam (FIB) microscope has been mainly used in the semiconductor industry as a tool for device imaging, modification and mask repair. Several years ago, the FIB technique has found a broader variety of applications in material science [1]. This increased use is due to several key factors: (i) site-specific preparation of thin foils for transmission electron microscopy (TEM) investigations, (ii) a strong channelling contrast allowing discrimination of different grain orientations, and (iii) secondary ion mass spectroscopy (SIMS) and/or energy dispersive X-ray (EDX) spectroscopy attached to the FIB workstation for local chemical analyses. Recently, the FIB became popular as a tool for machining miniaturized samples [2], [3], [4], [5], [6], [7] to investigate the influence of sample dimensions on mechanical properties. As the surface to volume ratio is large for submicron-sized test structures, any surface modifications by ion bombardment and implantation may critically alter the mechanical properties.

So far, detailed studies on FIB induced Ga+ ion damage were mainly performed on semiconductor materials, especially Si [8], [9], [10], [11], [12], [13], [14], [15]. TEM investigations revealed the width of the amorphous surface layer introduced by ion milling to be in the order of several tens of nanometers, depending mainly on the kinetic energy and incidence angle of the used ions, and on the milling geometry [8], [9], [10], [11], [12], [13]. Comparison of these results to Monte Carlo simulations, mainly using the SRIM code [11], [13], [14], [15], showed good agreement.

Few data on other materials is available [16], [17], [18], [19], and the applied methods and parameters differ remarkably, preventing a conclusive comparison. Therefore, we decided to investigate the Ga+ ion damage of Cu, a material we frequently investigate with respect to mechanical size effects [5], [7]. In addition to TEM investigations of the Ga damaged microstructure, Auger electron spectroscopy (AES) measurements were carried out for reliable information on the depth profile of the Ga concentration. The results are compared to SRIM calculations. Finally, the impact on mechanical data obtained by FIB-made mechanical test structures is discussed.

Section snippets

TEM investigations

For the TEM investigation of the Ga+ ion damage a 150 nm thick polycrystalline Cu film with an average grain size of approximately 300 nm deposited on a ∼100 nm thick amorphous SiNX membrane was used. The Cu film was intentionally damaged with Ga+ ions using a dual-beam workstation (LEO XB1540) consisting of a high-resolution field-emission scanning electron microscope (SEM) and an integrated scanning Ga+ FIB column. The kinetic energy of the primary Ga+ ions can be varied between ∼2 keV and 30 keV.

Results

In this section, we first describe the TEM results of the Cu samples exposed to different milling conditions under perpendicular and grazing ion impact. Subsequently, the AES measurements of the implanted Ga concentrations are presented.

Discussion

In order to reduce the damage introduced by the Ga+ ion bombardment, several possibilities exist. For example: (i) use of a protective layer, (ii) low incidence angle of the impinging Ga+ ions, (iii) low ion energies, and (iv) optimized milling geometries in order to avoid redeposition. A further possibility to reduce the thickness of the damaged layer would be to increase the atomic mass of the impacting ions, as was demonstrated by Jamison et al. [13] for Ga+ and In+ ions on Si. However, most

Conclusions

TEM and AES investigations were performed to elucidate the damage experienced by Cu exposed to Ga+ ion bombardment. The influences of ion energy, ion dose and incidence angle were examined. Conventional TEM studies reveal the necessity to reduce the Ga damage by reducing the ion energy, milling current, and incidence angle, if pre-existing defects are to be analyzed. AES measurements were conducted to quantify the concentration and penetration depth of the Ga+ ions. Ga concentrations as large

Acknowledgements

The authors thank M. Pečar for support with the AES analysis, J. Thomas for assistance with the EDX-TEM investigations, and Prof. Dr. R. Pippan for valuable discussions. Financial support within research activities of the Materials Center Leoben under the frame of the Austrian Kplus Competence Center Programme is gratefully acknowledged.

References (40)

  • M.W. Phaneuf

    Micron

    (1999)
  • D.M. Dimiduk et al.

    Acta Mater.

    (2005)
  • J.R. Greer et al.

    Acta Mater.

    (2005)
  • C. Motz et al.

    Acta Mater.

    (2005)
  • Z. Wang et al.

    Appl. Surf. Sci.

    (2005)
  • J.P. McCaffrey et al.

    Ultramicroscopy

    (2001)
  • S. Rubanov et al.

    Mater. Lett.

    (2003)
  • D.J. Larson et al.

    Ultramicroscopy

    (1999)
  • C.R. Hutchinson et al.

    Ultramicroscopy

    (2003)
  • J. Yu et al.

    Mater. Lett.

    (2006)
  • J.M. Cairney et al.

    Micron

    (2003)
  • T.J. Balk et al.

    Acta Mater.

    (2003)
  • T. Hebesberger et al.

    Acta Mater.

    (2005)
  • B.N. Singh et al.

    J. Nucl. Mater.

    (1993)
  • D. Kiener et al.

    Acta Mater.

    (2006)
  • M.J. Caturla et al.

    J. Nucl. Mater.

    (2000)
  • B. von Blanckenhagen et al.

    Acta Mater.

    (2004)
  • R.L. Fleischer

    Acta Mater.

    (1963)
  • D.V. Kudashov et al.

    Mater. Sci. Eng. A

    (2004)
  • E. Pereiro-Lopez et al.

    Acta Mater.

    (2006)
  • Cited by (406)

    View all citing articles on Scopus
    View full text