On the feasibility of die-to-wafer inorganic dielectric bonding

A feasibility study of die-to-wafer (D2W) bonding of inorganic dielectric layers is conducted on common industrial tools in a typical cleanroom environment. With the help of an additional cleaning step after dicing of the top wafers or using stealth dicing process, 100% bonding success rate from wafers with chemical mechanical polished (CMP-ed) SiO
₂
has been achieved. Plasma treatment of the top dies has no clear impact on the bonding success rate. However, die shear test shows a significant bonding strength improvement from 10.7±2.6 MPa of samples without pre-bonding plasma treatment to 24.8±9.1 MPa with the treatment. The failure mechanism in the shear test is identified to be fractures in bulk Si, confirming the high bonding strength. The study highlights particle control as the major challenge and current bottleneck in D2W inorganic dielectric bonding. Even in the best experimental split which yields 100% bonding success rate, scanning acoustic microscopy (SAM) and infrared (IR) microscopy show voids in every stack, which are clearly caused by entrapped particles. Cross-sectioning and inspection of the bonding interface by focused ion beam (FIB) show well bonded interfaces between SiO
₂
layers in the no-void (dark) areas as revealed under SAM, and a narrow gap in the order of tens of nm in the voiding (bright) areas.