Journal
MICROMACHINES
Volume 7, Issue 12, Pages -Publisher
MDPI
DOI: 10.3390/mi7120234
Keywords
wafer bonding; thermo-compression bonding; pre-treatment; Cu-Cu bonding; 3D integration
Categories
Funding
- Creation of Innovation Centers for Advanced Interdisciplinary Research Areas Program from the Japanese Ministry of Education, Culture, Sports, Science and Technology
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To increase the yield of the wafer-level Cu-Cu thermo-compression bonding method, certain surface pre-treatment methods for Cu are studied which can be exposed to the atmosphere before bonding. To inhibit re-oxidation under atmospheric conditions, the reduced pure Cu surface is treated by H-2/Ar plasma, NH3 plasma and thiol solution, respectively, and is covered by Cu hydride, Cu nitride and a self-assembled monolayer (SAM) accordingly. A pair of the treated wafers is then bonded by the thermo-compression bonding method, and evaluated by the tensile test. Results show that the bond strengths of the wafers treated by NH3 plasma and SAM are not sufficient due to the remaining surface protection layers such as Cu nitride and SAMs resulting from the pre-treatment. In contrast, the H-2/Ar plasma-treated wafer showed the same strength as the one with formic acid vapor treatment, even when exposed to the atmosphere for 30 min. In the thermal desorption spectroscopy (TDS) measurement of the H-2/Ar plasma-treated Cu sample, the total number of the detected H-2 was 3.1 times more than the citric acid-treated one. Results of the TDS measurement indicate that the modified Cu surface is terminated by chemisorbed hydrogen atoms, which leads to high bonding strength.
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