Low temperature Cu-Cu thermocompression bonding assisted by electrochemical desorption of a self-assembled monolayer
This paper reports low temperature Cu-Cu thermocompression bonding with the help of self-assembled monolayer (SAM) desorption. SAM layer protects copper from oxidation. It should be desorbed just before bonding. The desorption was carried out using cyclic voltammetry in aqueous potassium hydroxide (KOH) solution. Contact angle measurements carried out before and after desorption indicates successful desorption of SAM. The bonding was carried out at 150 °C and has yielded in excellent bond strength of 520 N.
Application of self assembly monolayer (SAM) in lowering the process temperature during Cu-Cu diffusion bonding of 3D IC
In this research work, we investigate the feasibility of lowering the bonding temperature by refining the copper surface with the application of a self assembly monolayer (SAM). First, the stability of SAM when it is subjected to different exposure environments is measured quantitatively by the change in the contact angle. The different environments includes: 1) exposure in clean room ambient; and 2) high temperature annealing. The contact angle is reduced for these two cases suggesting that SAM degrades or desorbs over a period of time and during high temperature annealing. Next, the formation of SAM is further analyzed qualitatively by X-ray photoelectron spectroscopy (XPS) to verify its adsorption and degradation properties. The result shows that SAM is able to slow down the oxidation process on copper surface. Bonding experiments are then performed in order to confirm the effectiveness of SAM in achieving a low temperature bonding. The formation of SAM has been shown useful in protecting copper surface from oxide formation; and at moderate temperature annealing (≪300 °C), it readily desorbs prior to bonding. Surface passivation with SAM can therefore help to lower the bonding temperature by provide a clean and a larger contact area for successful bonding.
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.