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research-article

Far Back End of Line Aluminum Stress Reduction Methods for 2D/2.5D Fine Pitch Assemblies

[+] Author and Article Information
Krishna Tunga

IBM Corporation, Hopewell Junction, NY
krishna.tunga@gmail.com

Thomas A. Wassick

IBM Corporation, Hopewell Junction, NY
wassickt@us.ibm.com

Maryse Cournoyer

IBM Corporation, Hopewell Junction, NYIBM Corporation, Bromont, Canada
mcournoy@ca.ibm.com

1Corresponding author.

ASME doi:10.1115/1.4036368 History: Received October 02, 2016; Revised March 28, 2017

Abstract

Fine pitch interconnects when used with 2D/2.5D packaging technology offers enormous potential towards decreasing signal latency and by making it possible to package increased electrical functionality within a given area. However, fine pitch interconnects present their own set of challenges not seen in packages with coarse pitch interconnects. Increased level of stresses within the Far Back End of Line (FBEOL) layers of the chip is the primary concern. Seven different types of 2D and 2.5D test vehicles with fine pitch and coarse pitch interconnects were built and tested for mechanical integrity by subjecting them to accelerated thermal cycling between -55ºC and 125ºC. Finite-element based mechanical modeling was done to determine the stress level within the FBEOL layers of these test vehicles. For all the tested assemblies, experimental data and modeling results showed a strong correlation between reduced pitch and increased level of stresses and increased incidence of failures within the FBEOL region. These failures were observed exclusively at the passivation layer and aluminum pad interface. Experimental data in conjunction with mechanical modeling was used to determine a safe level of stress at the aluminum to passivation layer interface. Global and local design changes were explored to determine their effect on the stresses at this interface. Several guidelines have been provided to reduce these stresses for a 2D/2.5D package assembly with fine pitch interconnects. Finally, a reliable low stress configuration, that takes into account all the design changes, has been proposed that is expected to be robust with very low risk of failure within the FBEOL region.

Copyright (c) 2017 by ASME
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