Fluorine-enhanced low-temperature wafer bonding of native-oxide covered Si wafers

Abstract

The bonding energy of bonded native-oxide-covered silicon wafers treated in the ${\mathrm{HNO}}_3∕{\mathrm{H}}_2\mathrm{O}∕\mathrm{HF}$ or the ${\mathrm{HNO}}_3∕\mathrm{HF}$ solution prior to room-temperature contact is significantly higher than bonded standard RCA1 cleaned wafer pairs after low-temperature annealing. The bonding energy reaches over $2000\mathrm{mJ}∕{\mathrm{m}}^2$ after annealing at 100 °C. The very slight etching and fluorine in the chemically grown oxide are believed to be the main contributors to the enhanced bonding energy. Transmission-electron-microscopic images have shown that the chemically formed native oxide at bonding interface is embedded with many flake-like cavities. The cavities can absorb the by-products of the interfacial reactions that result in covalent bond formation at low temperatures allowing the strong bond to be retained.