IWLPC (Wafer-Level Packaging) Conference Proceedings

Ultra-Thin Gold Passivation as a Viable Alternative for Achieving Low Temperature Low Pressure Cu-Cu Thermo-Compression Bonding

Authors: Satish Bonam*, Asisa Kumar Panigrahi, Shikhar Jain, Siva Rama Krishna Vanjari, Shiv Govind Singh
Company: Indian Institute of Technology Hyderabad
Date Published: 10/18/2016   Conference: IWLPC (Wafer-Level Packaging)

Abstract: The viability of effective surface passivation using an optimally engineered ultra-thin Gold (Au) layer for low temperature, low pressure Cu-Cu wafer on wafer (WoW) bonding is demonstrated in this work. The gold layer not only performs the dual role of protecting the Copper (Cu) surface from oxidation and reducing the effective surface roughness, but also allows the Cu to diffuse across layers effectively. Oxidation of Cu is the major bottleneck in carrying out low temperature, low pressure Cu-Cu bonding. Gold is chosen as passivation layer as it is a noble metal that is immune to environmental changes. The key challenge addressed in this endeavor is to identify the right thickness that passivates Cu and at the same time decreases the surface roughness of Cu. Minimal surface roughness is essential for a good quality bonding.

In this endeavor, the gold passivation layer thickness was optimized by performing AFM studies for varying thickness of gold to study the effect on surface roughness. Simultaneously Cu-Cu wafer bonding experiments were carried out to study the bond strength. All the samples were prepared on full 4 inch p-type silicon (100) wafers coated having 30 nm Titanium (Ti), 225 nm of Copper (Cu) and the required thickness of Au sputtered deposited sequentially. Ti layer acts as an adhesive layer between Si and Cu. At an optimized value of 3 nm Au passivation, a good bonding interface was obtained at a pressure of 0.33 MPa and 150°C. The RMS roughness obtained of thin Au coated Cu sample was observed to be 0.88 nm which is half of the roughness obtained for pure Cu sputtered sample without Au passivation (~2.2nm). Higher thickness of Au did not yield in a good bonding under the same conditions even though the roughness is on a lower side, as Au tends to become diffusive barrier at higher thickness. Cleaving samples into small pieces did not cause any disintegration suggesting a good quality bonding. SEM analysis suggested the formation of a homogenous merged film with no bonding interface. Quantitative bond strength analysis resulted in an excellent strength of 200 MPa.

Key Words: 

3D IC, Bonding, Gold, Passivation

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