The Au/Si eutectic bonding compatibility with KOH etching for 3D devices fabrication

Abstract

KOH etching and Au/Si eutectic bonding are cost-efficient technologies for 3D device fabrication. Aimed at investigating the process compatibility of KOH etching and Au/Si bonding, KOH etching tests have been carried out for Au/bulk Si and Au/amorphous Si (a-Si) bonding wafers in this paper. For the Au/bulk Si bonding wafer, a serious underetch phenomenon occurring on the damage layer in KOH etching definitely results in packaging failure. In the microstructure analysis, it is found that the formation of the damage layer between the bonded layer and bulk Si is attributed to the destruction of crystal Si lattices in Au/bulk Si eutectic reaction. Considering the occurrence of underetch for Au/Si bonding must meet two requirements: the superfluous Si and the defective layer near the bonded layer, the Au/a-Si bonding by regulating the a-Si/Au thickness ratio is presented in this study. Only when the a-Si/Au thickness ratio is relatively low are there not underetch phenomena, of which the reason is the full reaction of the a-Si layer avoiding the formation of the damage layer for easy underetch. Obviously, the Au/a-Si bonding via choosing a moderate a-Si/Au thickness ratio (⩽1.5:1 is suggested) could be reliably compatible with KOH etching, which provides an available and low-cost approach for 3D device fabrication. More importantly, the theory of the damage layer proposed in this study can be naturally applied to relevant analyses on the eutectic reaction of other metals and single crystal materials.

Modified Au/bulk Si eutectic bonding structure with reliable compatibility of KOH etching based on two-step local oxidation of silicon

“…As illustrated in [19], the adequate Au-Si eutectic reaction would result in the formation of the continuously distributed damage layer on the interface of Au-Si alloys and the bulk Si, which has been well-reasonably considered as the root cause of underetch phenomenon in KOH etching for the Au/bulk Si bonding structure. In order to protect the formed damage layer from KOH etching, it should be insulated between the Au-Si reaction interfaces and the exposed etching zones.…”

“…Still, due to the formed damage layer resulted from the Si-based eutectic reaction, we ever discussed the underetch phenomenon for the eutectic bonding (e.g. Au/ bulk Si bonding) in KOH etching [19]. Meanwhile, the Au/a-Si bonding structure adopted with a-Si as intermediate layer is illustrated to be compatible with KOH etching under certain conditions [19].…”

“…Au/ bulk Si bonding) in KOH etching [19]. Meanwhile, the Au/a-Si bonding structure adopted with a-Si as intermediate layer is illustrated to be compatible with KOH etching under certain conditions [19]. However, the compatibility between the Au/a-Si bonding structure and KOH etching is highly depend on the thickness ratios of deposited a-Si with Au films.…”

Novel fabrication techniques for ultra-thin silicon based flexible electronics

“…Furthermore, the well-established fabrication process ensures consistent uniformity and a high yield [139][140][141][142][143]. The top-down approaches encompass various transfer methods, including silicon-on-insulator (SOI) [144][145][146][147][148][149], <111> wafer undercut etching [34,139,[150][151][152][153], metal-assisted chemical etching (MACE) [154][155][156][157], epitaxy-free methods [158][159][160][161], crack-based exfoliation [162][163][164][165][166], and bonding technology [128,167,168].…”