Improved Thermal Boundary Conductance in Annealed Direct Wafer Bonded Si-Ge

Abstract

The evolution of structural and thermal properties of wafer bonded Si|Ge with annealing is investigated in this study. Theoretical work suggests that the thermal boundary conductance between amorphous Si and Ge provide improved thermal properties as compared to a crystalline Si to Ge interface, and wafer bonded systems provides a fabrication method to experimentally test this theory [1]. EVG® ComBond® equipment was used for bonding under high vacuum (~10-8 mtorr) at room temperature to remove unwanted native surface oxides [2-4]. The bombardment of the surfaces prior to bonding produces an amorphous region at the bonded interface that has been seen in many other bonded systems [5,6]. In the as-bonded sample, high resolution scanning transmission electron microscopy revealed a ~1.2 nm amorphous region at the bonded interface. Subsequent annealing was done in an effort to recrystallize the amorphous interface. Previous work has shown that recrystallization between Si|Si wafer bonded samples occurred when annealed at 450 °C for 12 hours [3]. After annealing at 600 °C for 2 hours, the amorphous interface was observed to decrease to ~0.9 nm. Complete recrystallization is observed when the sample was annealed at 600 °C for 48 hours.Preliminary thermal results show that the thermal boundary conductance (TBC) of the as bonded sample is 47 ± 5 MW/(m2K). The TBC for the sample annealed at 600 °C for 48 hours is 94 ± 5 MW/(m2K), an improvement by a factor of two compared to the as-bonded interface. These results demonstrate that the TBC can be improved through annealing of the interface and that improvements due to an amorphous-amorphous interface do not dominate the thermal boundary conductance in this system. We consider that the improvements are due to recrystallization of the interface and/or to increased interdiffusion, which has been observed to increase TBC in epitaxially grown Si-Ge interfaces [7]. K. Gordiz, et al., J. Appl. Phys., 121(2), p.025102 (2017)V. Dragoi, et al., ECS Trans., 86(5), 23 (2018)C. Flötgen, et al., ECS Trans., 64(5), 103 (2014)M.E. Liao, et al., ECS Trans., 86(5), 55 (2018)Y. Xu, et al., Ceramics International, 45, 6552 (2019)F. Mu, et al., Appl. Surf. Sci., 416, 1007 (2017)Z. Cheng, et al., Nature communications, 12(1), p.6901 (2021) Figure 1