A 3D structured CMOS image sensor is investigated wherein two layers (photodiode array layer and signal processing circuit layer) are stacked using a low-temperature Au/SiO₂ hybrid bonding technique [1]. We previously reduced the pitch of Au electrodes from 40 μm to 6 μm and confirmed that the Au electrodes could be electrically interconnected via the bonding technique [2]. To further enhance its performance, higher integration by multilayer stacking is essential. Therefore, we developed a multilayering technique by direct bonding and manufactured a three-layered daisy-chain test device to examine the numerous interconnections.

The device is fabricated by the process described in Fig. 1 using the following steps: (a) an aluminum wiring layer is formed in a SiO₂ dielectric layer on a Si substrate; (b) first via holes are formed on the surface of the SiO₂ layer and are embedded with electroplated Au; (c) an Au/SiO₂ bonding surface is formed by chemical–mechanical polishing (CMP), and the wafer is diced into the top-, middle-, and base-layer chips; (d) the bonding surfaces of the top- and middle-layer chips are activated by O₂ plasma; (e) the top- and middle-layer chips are directly bonded by applying a force of 20,000 N for 60 min at 330 °C; (f) the Si substrate of the middle-layer chip is removed by grinding and XeF₂ vapor-phase etching; (g) second via holes are formed on the exposed backside of SiO₂ of the middle-layer chip and are embedded with an electroplated Au; (h) an Au/SiO₂ bonding surface is formed on the backside of the middle-layer chip by CMP; (i) the bonding surfaces of the middle- (backside) and base-layer chips are activated by O₂ plasma; and (j) the bonded (top- and middle-layer) chip and the base-layer chip are directly bonded by applying a force of 20,000 N for 60 min at 330 °C. Figure 2 shows a cross-sectional scanning electron microscopy (SEM) image of the three-layered daisy-chain test device. No voids were observed at the two bonded interfaces. Details of the three-layered stacking process and experimental results will be discussed in the presentation.

1. M. Goto et al., IEEE Trans. Electron Devices, vol. 62, no. 11, pp. 3530–3535, (2015).

2. Y. Honda et al., ECS Trans., vol. 75, no. 9, pp. 103–106, (2016).