Ultra-Thin Gold Passivation as a Viable Alternative for Achieving Low Temperature Low Pressure Cu-Cu Thermo-Compression BondingAuthors: 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)
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.
3D IC, Bonding, Gold, Passivation
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