Mechanical Strength and Interface Characteristics of Transmission Laser Bonding

A laser-based bonding technique, called transmission laser bonding (TLB), is studied for the purposes of device and wafer-level packaging. The TLB technique uses the specific characteristics of a laser to bond a transparent wafer on top of an opaque wafer. When a laser beam with a specific wavelength is passed through a transparent wafer, high-density laser energy is absorbed by the opaque wafer and melts a thin surface layer, resulting in the formation of strong chemical bonds across the two wafers. A Nd:YAG pulse laser has been used to bond a transparent glass wafer to a Si substrate. The associated bond strengths under various bonding conditions are examined by a microtensile tester to quantify the bonding quality. With a contact pressure higher than 0.5 MPa, the TLB strength can reach a stable value of 10.5 MPa, which is comparable to those obtained by other popular bonding processes currently used by the packaging industry. The wafer surface conditions are evaluated by atomic force microscopy (AFM) and profilometry, while Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) are used to study the characteristics of bonding interfaces. The AFM and profilometry results reveal that the wafer roughness and flatness required by TLB can be less stringent than those specified in the current industrial standards. The AES and XPS results are used to interpret the chemical and physical aspects of TLB formation and to provide the rationale for obtaining high-quality and high-strength TLB