Low-Temperature Deposited SiO2 for Advanced Chiplet
Authors: Kitagawa Hayato , Onishi Koki , Fuse Junya , Uedono Akira , Iwata Tomoya , Inoue Fumihiro 
Published: February 2024

D2W hybrid bonding is a crucial technology for chiplets and 3D HBMs [1]. However, the sequential die bonding process has not been matured yet, in particular due to the compatibility of process tools with high-level particle control, e.g. die level cleaning, activation, and die handling. The current packaging tools are not designed for hybrid bonding, therefore, the control of the die surface cleanliness is still far from the requirement for assembly tools compared to the wafer level tools. In order to mitigate the issues above and lead time for D2W stacking, massive D2W bonding by using a temporary carrier wafer with populated dies (so-called collective D2W bonding) is introduced as an intermediate solution. The collective D2W bonding is a promising integration method since it can be processed in matured wafer-level tools for die-level cleaning, inspection, activation, and even bonding. Furthermore, reconstructed D2W hybrid bonding is recently proposed and developed to overcome some issues seen inon collective D2W bonding, e.g. huge gap in between die. The drawback for both collective D2W and reconstructed D2W is the propagation of misalignment for chip placement on a carrier wafer and actual hybrid bonding at Wafer-to-Wafer (W2W) step. Overall, the use of organic temporary bonding materials, which are typically polymers regardless of mechanical or laser release, has some issues/limitations for collective D2W bonding. e.g. die shift during population and W2W bonding, chemical/thermal compatibility, and difficulties for chemical mechanical polishing due to the elastic properties.
We proposed a newly developed temporary bonding method for die population on a carrier wafer with CVD dielectric film (Fig.1(a)) [2]. In this study, the debonding mechanism of low-temperature (150°C) deposited SiO2 (LT-SiO2) at the bonding interface was investigated.
According to TDS measurement, water is the major outgas below 250°C (Fig. 1(b)). It indicates that the water outgases from LT-SiO2 are much larger than that from Thermal SiO2. The surface will become the interface after bonding, and it form voids by water outgassing by post-bond annealing. Fig. 1(C) shows the results of PAS at different annealing temperatures. The S parameter of LT-SiO2 is lower than the thermally oxidized SiO2, which can be considered that atomic-level vacancies are occupied by water and other residual substances due to low-temperature deposition [3]. The larger increase of the S parameter at the temperature range above the deposition temperature (150℃) supports this hypothesis. Also, a little increase of the S parameter near the surface suggests that water re-entered the sub-surface from the atmosphere after annealing, which is correlated with the TDS result. Moreover, when the higher annealing temperature and PAS measurement were performed, the closer S parameter of LT-SiO2 is to that of the thermally oxidized SiO2 by outgassing and densification.
Furthermore, when measurements of bonding strengths after post-bond annealing (250℃) were performed, the bonding strength of LT-SiO2 (1.34[J/m2]) is lower than thermally oxidized SiO2 (4.41[J/m2]). In addition, the lower the pre-bond annealing temperature, the larger the void area and the lower the bonding strength. This can be considered that water in the LT-SiO2 that has not been fully outgassed by low-temperature pre-bond annealing is desorbed, resulting in void formation at the interface and lower bonding strength. Therefore, we can make LT-SiO2 bonding and debonding as temporary bonding by the thermal release method.
[1] F. Inoue et.al., “Protective layer for collective die to wafer hybrid bonding” 2019 International 3D Systems Integration Conference (3DIC)
[2] F. Inoue et.al., “Inorganic Temporary Direct Bonding for Collective Die to Wafer Hybrid Bonding” 2023 Electronic Components and Technology Conference (ECTC)
[3] M. Sometani, et. al., J. Appl. Phys 51 021101 (2012)

Figure 1