CMOS-Compatible Fine Pitch Al-Al Bonding

The metal-metal bonding has become more promising for fine-line hermitic sealing and electronic packaging applications. Even though aluminum has CMOS compatibility and extensive utilization in MEMS, its fine pitch bonding and bonding at low thermal budgets is still challenging. In this paper, we have demonstrated a fine pitch
$(\sim 6\ \mu \mathrm{m})$
direct Aluminum - Aluminum bonding process achievable at a low temperature
$\leq 350 {}^{\circ} \mathrm{C}$
, which is compatible with CMOS Back-End-Of-Line (BEOL) processes. Successful bonding was achieved by using optimally ultra-thin titanium as a surface passivation layer. The passivation layer not only helps in protecting the surface from oxide formation, but also helps in the modification of surface microstructure and morphology. Systematic investigation of surface oxidation and grain structure using EDS and XRD, as well as AFM analysis of surface roughness revealed that the optimal passivation layer thickness to be
$\sim$
2–4 nm. With optimized passivation layer thickness, wafer level blanket bonding has been achieved at a temperature and pressure of
$\leq 300^{\circ}\mathrm{C}$
and
$< 1\text{MPa}$
, respectively. Reliable interface bonding quality in fine-pitch structures (with
$\sim$
16 % metal pattern density) has been achieved at
$\leq 350{}^{\circ} \mathrm{C}$
bonding temperature and
$\geq 1$
MPa bonding pressure. These investigations suggest that passivation materials can help reduce the bonding temperature further to achieve advanced interconnect bonding and MEMS sealing at low thermal budgets.