Bonding of SiO2 and SiO2 at Room Temperature Using Si Ultrathin Film
Authors: Utsumi Jun , Ide Kensuke , Ichiyanagi Yuko 
Published: February 2020

In three-dimensional integration technology, the bonding of metal electrode and insulator hybrid interface is very important technique. The hybrid interface serves as both an electrical connection and　mechanical bond. However, the bonding of these hybrid interfaces is a challenging issue. Since the conventional bonding process requires high-temperature annealing, there exist various problems such as thermal damage, low throughput, and low alignment accuracy. As the surface activated bonding (SAB) is a bonding method carried out at room or low temperatures [1], the bonding method is expected to solve these problems. By the SAB method, direct bonding metal materials such as Cu or Al is easy. However, it is very difficult to directly bond insulator materials such as SiO2 or SiN [2], because these surfaces are rendered inactive immediately after being activated. We have reported on the bonding technique at room temperature using only Si ultrathin films for insulator materials, and we have shown that high bonding strength is achieved [3]. The surface of electrode is also covered with Si film by this bonding method. Thus, it is very important to reveal the influence of this Si film thickness on the SiO2/SiO2 bonding. In this report, we have investigated the relationship between the SiO2/SiO2bonding strength and the thickness of Si ultrathin film.
Figure 1 shows a schematic illustration of the bonding apparatus (Mitsubishi Heavy Industries, Ltd., MWB-08AX) used in this experiment. Surface activation is carried out by an Ar fast atom beam (FAB). In the normal SAB process, the upper and lower wafers are irradiated with the Ar-FAB at the same time. The bonding procedure which we propose for hybrid bonding is as follows. The one of bonding wafers is held by the electric static chuck (ESC), and Si blanket wafer is placed on the lower side as the sputtering target. First, only the lower wafer is irradiated with FAB1 (first irradiation) after activating electrode surface of the upper wafer. The blanket Si wafer is then exchanged with the other bonding wafer. Only the upper wafer is irradiated with FAB2 (second irradiation) after activating electrode surface of the lower wafer, and the Si film surface on the upper wafer is also etched by FAB2. A Si thin film is then deposited on the lower wafer surface. The upper wafer and the lower wafer are optically aligned and then bonded with high load.
Figure 2 shows the relationship between the surface energy and the Si ultrathin film thickness for the bonding of Si blanket wafers (8 in) with a thermal oxide. Applied voltage and current of Ar beam source operated in these experiments were about 1.8 kV and 100 mA, respectively. The background vacuum pressure is about 2×10-6Pa. The surface energy of wafers was about 1 J/m2 for thicker than about 3 nm. A cross-sectional TEM image of SiO2/SiO2bonding is shown in Fig. 3. The bonded wafers were treated with a FAB1 irradiation time of 5 min and a FAB2 irradiation time of 1 min. No micro voids were observed, and Si intermediate layer, of which the thickness is about 3nm, was seen at the bonding interface.
In conclusion, we showed that high bonding strength was achieved in SiO2/SiO2 bonding with the Si film thickness of about 3 nm by this bonding method.

Reference:
[1] H. Takagi, K. Kikuchi, R. Maeda, T. R. Chung, and T. Suga, Appl. Phys. Lett. 68, 2222 (1996).
[2] H. Takagi, R. Maeda, T. R. Chung, and T. Suga, Sensors and Actuators, A 70, 164 (1998).
[3] J. Utsumi, K. Ide, and Y. Ichiyanagi, Jpn. J. Appl. Phys., 55, 026503 (2016).

Figure 1