Localization of Non Linear Defects in Sub 14nm BEOL Structures
Terence Kane, Michael Tenney
IBM Systems Technology Group/Microelectronics Division, Hopewell Junction, New York, USA
Abstract
For leading edge 14nm and below technologies with as many as fifteen back end of the line (BEOL) metallization levels, detection of at levelnon linear resistive/open defects in copper BEOL structures pose real challenges with contemporary FIB voltage contrast/SEM voltage contrast methods.
Conventional SEM voltage contrast or FIB voltage contrast is typically applied to detecting opens or shorts in buried nodes of these BEOL structures. Our experience has shown this technique is not successful in localizing diodic or non linear typedefects.
In our current work, even after careful delayering to the line over the BEOL metallization layer, nanoprobing of these BEOL structures [Figures 1,2, 3] shows defects exhibiting pA of current at 0.7 volts. This translates toa diodic or non linear type defects.****Sections below are optional********
Introduction
14nm and lower back end of line (BEOL) technologies can involve as many as fifteen metallization levels with ultra low dielectric constant [ULK] films at multiple metal layers.
BEOL reliability stress tests such as Time to Dielectric Breakdown [TDDB] and/or electromigration stress testing involve measuring test structures designed to be sensitive to the defect densities with these advanced technologies and can
By definition, the feature contrast between two areas for voltage contrast is given by the following:
Where I2 = incident beam keV
I1 = incident beam keV
Iref = background or reference beam keV
C = contrast
Figure 1: Layout description of BEOL monitor structure measuring 7 um X 7 um for this sub 14nm technology.
Figure 13: Dark Field TEM image of defect shown in Figure 12. Note subtle nature of voiding present[I1]
Discussion
The significance of improved localization of nonlinear BEOL defects is demonstrated by
(1) wide spread presence of defects throughout the stressed BEOL structure
(2) the process implications that multiple sites are similary affected
(3) the low keV and beam currents involved with this technique did not contribute any artifacts or damage to the BEOL structure itself
Improved localization of non linear defects has enabled the detection of subtle lithography defects, process anomalies in advanced technologies as well as the BEOL detection of defects that would not be detected otherwise.
Avoiding artifacts introduced during this localization technique as well as application of low keV beam currents and FIB introduced gallium contamination has enabled TEM elemental mapping to determine root cause of failure.
Summary
SEM two point specimen current localization combined with TEM imaging/elemental mapping over comes limitations of single probe SEM passive/active voltage contrast and gallium ion beam FIB voltage contrast localization for advance technology nodes
Acknowledgements
The authors wish to acknowledge the efforts of Malik Ali and Gouda Lian in the course of this analysis.
References
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[3]L. Elliott et al , Journal of Applied Physics, 91 (11) 9116 (2002)
[4]L. Scipioni, Rainer Reche, Karl Zeiss Application Note “Voltage Contrast Imaging in Helium Ion Microscope” pp. 1-4, June 9, 2009
[5]H.Stegmann , “Voltage Contrast in Microelectronic Engineering”, March 2013
[6]T.Kane, Yun Wang, “22nm BEOL TDDB Defect Localization and Root Cause Analysis”, ISTFA 2013, pp 212-217
[I1]Point out the voiding.