Impact of moisture on the reliability of 3-D & OIO technologies

Since more than 50 years, corrosion of the metal lines in integrated-circuits, resulting from the presence of atmospheric moisture near the metal, is a serious reliability challenge [1, 2]. Traditionally, corrosion issues have been mitigated by the utilization of thick passivation stacks, consisting in an association of silicon nitride and silicon oxide, and preventing moisture from reaching the metal lines [3].

Nevertheless, in this new microelectronic era, where complex products are built by combining different chips, leveraging on new interconnect modules such as optical IO, and polymer based RDL technologies, more and more applications are incompatible with the classical passivation stacks. For example, at imec, silicon photonics chips require opening of the passivation layers to connect the optical fibers [4], resulting in the direct exposure of the underneath BEOL layers to atmospheric moisture leading to severe reliability concerns such as metal line corrosion [3] or waveguide mode shifting due to refractive index increase [5]. In the realm of 3D technologies, the introduction of novel materials such as polymers, molds or unprotected SiCN bonding layers for e.g., raises concerns of new moisture induced reliability issues. To guarantee reliable 3D technologies and products, the impact of moisture on the reliability of advanced packaging technologies must be assessed.

The aim of this PhD is to understand the risk posed by moisture uptake and its impact on the reliability performance of OIO and 3D interconnects. The degradation mechanisms induced by moisture uptake should be identified and understood to propose possible mitigation strategies. In more details, the following achievements are expected:

• Comparison and selection of the most appropriate characterization methodologies to monitor moisture diffusion in dielectrics.
• Characterization of the moisture diffusion kinetics in various materials (eg; SiN, SiCN, Polymers...);
• Identification of the possible failure modes and their relative mechanisms for the different dielectric and polymer materials used in OIO and 3D technologies;
• Assessing the impact of moisture on dielectrics and polymers electrical and optical performances;
• Assessing the impact on the electrical and optical performances of waveguides and copper lines when surrounded by moisture exposed dielectric materials. 

 To reach these goals, the following activities are foreseen:

• Deep understanding of the OIO & 3D technologies and their applications;
• Building moisture diffusion models for the different dielectric & polymer materials used in OIO& 3D technologies
• Electrical & optical characterization of moisture exposed dielectric & polymer materials.
• Statistical data analysis of reliability data extracted from electrical characterization;
• Building of analytical/computer models for dielectric & polymer lifetime predictions;
• Interaction with OIO and 3-D process integration engineers to provide them feedback on the reliability characterization results.
• Proposition of various mitigation strategies to build reliable products. 

Who you are:

• You have a material sciences background and enjoy experimental work.
• Prior experience with moisture diffusion in dielectric/polymer materials would be beneficial.
• Deep knowledge of characterization methodologies such as ellipsometry or FTIR would be an asset.
• Prior exposure to ellipsometry and FTIR spectrum analysis methodology is an advantage. 

[1] Frankenthal, R. P. & Becker, W. H., Corrosion Failure Mechanisms for Gold Metallizations in Electronic Circuits, Journal of The Electrochemical Society, Vol. 126, No. 10, 1979.

[2] Leppänen, J. et al., Aluminium corrosion in power semiconductor devices, Microelectronics Reliability, Vol. 137, 2022.

[3] Comizzoli et al., Corrosion of Aluminum IC Metallization with Defective Surface Passivation Layer, International Reliability Physics Symposium, 1980.

[4] He, J. et al., V-Groove assisted passive assembly of single-mode fibers to ultra-broadband polarization-insensitive edge couplers for silicon photonics, European Conference on Optical Communication (ECOC 2019).

[5] Y. Zhang, M. Cui, Refractive Index Sensor Based on the Symmetric MIM Waveguide Structure, J. Electron. Mater. 48 (2) (2019)