| Summary: | Wafer level bonding process is a key fabrication step in many integration systems including microelectromechanical (MEMs) and nanoelectromechanical (NEMs). Often harsh bonding conditions used during process, result in large thermomechanical stresses built-up that leads to undesired degradation in device performance.
Our recent study revealed the capability of self-assembled monolayers (SAMs) in reducing the bonding temperature needed to bond copper surfaces from 300°C to 60°C. In this study, room temperature copper bonding is demonstrated successfully with the help of the organic monolayers. Further investigation is made to evaluate the influence of chain length of these monolayers to alleviate bonding temperature. We found that all alkanethiols coated copper (CnH2n+1SH, at n = 6, 11, 18) showed superior bond strength (>25MPa) as compared to that of the uncoated copper (<23MPa) at bonding temperatures from 25°C to 80°C. We also discover that bond strength of copper bonding increases with alkanethiols chain length (C18>C11>C6). Alternatively, a contrasting behavior was demonstrated in gold joints where bonding strength decreases with alkanethiols chain length (C6>C11>C18). This is attributed to 1) Quality of surface passivation by SAMs (C6, C11 and C18), and 2) Mechanical displacement affected by modulus (E) of metals. These points are further elaborated in the discussion of this report.
Since it is crucial for bonding surface to be clean in order to achieve superior bonding strength, the enhancement exhibited is attributed to an effective surface passivation by the organic layer. This ultrathin layer, which behaves as a milder layer as compared to the bulk oxide layer is believe to be much easier displaced for bond formation. In short, we postulate that good bonding strength is achieved through excellent surface passivation through SAMs and ease of mechanical displacement of SAMs.
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