Room-Temperature Bonding of Wafers with Smooth Au Thin Films in Ambient Air Using a Surface-Activated Bonding Method

Abstract

Wafers with smooth Au thin films (rms surface roughness: < 0.5 nm, thickness: < 50 nm) were successfully bonded in ambient air at room temperature after an Ar radio frequency plasma activation process. The room temperature bonded glass wafers without any heat treatment showed a sufficiently high die-shear strength of 47-70 MPa. Transmission electron microscopy observations showed that direct bonding on the atomic scale was achieved. This surface-activated bonding method is expected to be a useful technique for future heterogeneous photonic integration.

Direct gold bonding for flexible integrated electronics

“…A large normal or frictional load and a pristine clean flat ductile surface in an ultrahigh vacuum environment are generally required to realize cold welding in bulk metals (16,17). SAB of metals typically requires very smooth surfaces [typical root mean square (RMS) roughness of <1 nm] and was developed primarily for flat sputtered metals on Si wafers (18)(19)(20)(21)(22)(23). In both methods, metal oxides and metal hydroxides have no free electrons.…”

Comparison of Argon and Oxygen Plasma Treatments for Ambient Room-Temperature Wafer-Scale Au–Au Bonding Using Ultrathin Au Films

“…Several Au–Au bonding techniques have been investigated to achieve low-temperature bonding, including thermocompression bonding [6,7,8,9,10,11,12], atomic diffusion bonding [4,13,14], and surface activated bonding (SAB) [15,16,17,18,19]. With SAB, the surfaces to be bonded are activated by plasma pretreatment and then bonded at low temperature (<150 °C).…”

“…Furthermore, the use of Au for the bonding layer enables bonding in ambient air because gold oxide (Au 2 O 3 ) is the only metal oxide that has a positive formation enthalpy (+19.3 kJ/mol) [20]. The use of SAB with little contact pressure at room temperature in ambient air using ultrathin (<50 nm) Au films with smooth (root mean square (RMS) <0.50 nm) surfaces was recently shown to result in reliable Au–Au bonding [17].…”

Effect of Au Film Thickness and Surface Roughness on Room-Temperature Wafer Bonding and Wafer-Scale Vacuum Sealing by Au-Au Surface Activated Bonding

“…Au-Au surface activated bonding (SAB) [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] is a promising technique for low-temperature bonding. In Au-Au SAB, the Au surfaces are activated by plasma treatment and then brought into contact at low temperature (<150 • C).…”

“…Room-temperature pressureless wafer bonding was recently achieved with Au-Au SAB using ultrathin Au films (thickness <50 nm) with small grains, and thus, smooth surfaces (RMS surface roughness: <0.5 nm) [30,34]. Furthermore, room-temperature pressureless wafer-scale hermetic sealing in both air and vacuum was achieved using Au-Au SAB with ultrathin Au films (thickness: 15 nm) [35].…”