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.
Temporary passivation of Cu for low temperature (&#60; 300°C) 3D wafer stacking
Self-assembled monolayer (SAM) of alkane-thiol is applied to provide temporary passivation on Cu surface for protection against oxidation. The thermal stability of SAM is improved by storage at low temperature and in inert ambient. Surface analysis shows that SAM is effective in retarding surface oxidation and the degree of protection is higher for SAM with longer chain length. Wafer-to-wafer bonding result shows shear strength enhancement in the Cu-Cu bond when the appropriate chain length is used. Longer chain is less susceptible to thermal desorption hence no improvement in the bond strength is obtained.
A study of kinetics of adsorption of alkanethiols on gold using electrochemical impedance spectroscopy
The growth kinetics of self assembled monolayer of alkanethiols of different chain lengths and concentrations on an evaporated gold surface on mica has been studied in situ and in real time using electrochemical impedance spectroscopy. The interfacial capacitance obtained from the measurement of the imaginary component of impedance is used to calculate the surface coverage as a function of time. From this data, rate constants of adsorption of alkanethiols have been measured in 0.1 M LiClO4 in ethanol solution. The results show that the adsorption follows Langmuir kinetics at higher thiol concentration of 20 μM and partial diffusion controlled kinetics at 5 μM and lower. The adsorption at a concentration of 1 mM is so rapid that ∼40% of the full monolayer is covered within the first 500 ms.