Characterization of Silicon Carbon Nitride for Low Temperature Wafer-to-Wafer Direct Bonding
Authors: Nagano Fuya , Iacovo Serena , Phommahaxay Alain , Inoue Fumihiro , Sleeckx Erik , De Gendt Stefan , Beyer Gerald , Beyne Eric 
Published: December 2020

Three-dimensional circuit integration in vertical stacking attracts significant attention as a promising technique to fabricate higher performance and multifunctional chips without current 2D scaling limitation. Generally, the 3D integration using micro bumps consisted of solder joints hold the major integration scheme. However, micro bumps using solders joints confront stacking issues in the case of finer pitch (> 2µm) due to misalignment during thermal compression bonding. The wafer-to-wafer (W2W) hybrid bonding system can be then introduced as one of the state-of-the-art techniques in the 3D integration scheme. In this bonding scheme, two different bonding combinations are established simultaneously, which are Cu-to-Cu and dielectric-to-dielectric, respectively. For these bonding structures, dielectric material adjacent to Cu is required as not only an insulator but also a mechanical factor to withstand the grinding process for further 3D integration.

Silicon carbon nitride (SiCN) compound has aroused great interest for bonding dielectrics due to high thermal stability and strong bond strength for direct bonding as well as Cu diffusion barrier ability. Recently back-to-face W2W hybrid bonding using backside SiCN deposition are also challenging to DRAM applications. However, in the case of DRAMs stacking, SiCN W2W direct bonding must be processed at lower temperature (<250°C) due to memory assembly thermal budget although general W2W direct bonding including deposition step of dielectrics is performed at high temperature (>350°C). In the past, we have reported that low temperature bonding using SiCN film deposited at 370°C obtained more than 2.0 J/m2 of adhesion energy at 250°C of post annealing temperature. However, W2W direct bonding using SiCN film deposited at lower temperature (<250°C) have not been studied well.

In this study, properties of W2W direct bonding with SiCN deposited at varied temperature are evaluated. Chemical and mechanical properties of SiCN film linked to the bonding quality are fundamentally studied by using specific measurements.

The SiCN films were deposited by plasma enhanced chemical vapor deposition (PECVD) at specific temperature 200°C and 370°C. After deposition, these wafers were annealed in a 10% H2/N2 atmosphere at a temperature of 200°C. CMP with typical barrier metal slurry and pad was employed to flatten SiCN surface. Subsequently, the wafer surface was activated by using N2 plasma and rinsed in deionized water. Wafers were finally bonded at room temperature in the EVG GEMINI system. Different characterization techniques have been used to understand the layer quality of the SiCN film deposited at 200°C (which indicated as SiCN 200°C in the text) in comparison with the optimized SiCN deposited at 370°C (indicated as SiCN 370°C).

Bond energies for the different wafer pairs have been calculated by using the blade insertion method. The highest bond energy ~ 2.3 J/m2 was obtained in the case of SiCN 370°C, while the bond energy for SiCN 200°C was ~ 1.4 J/m2.

ERD observed the elemental composition in bulk for SiCN 370°C and 200°C, respectively (Table 1). It is found that SiCN 200°C is not pure SiCN as SiCN 370°C but it is characterized by the presence of oxygen into the bulk. Oxygen could come from absorbed moisture since deposition and annealing steps are including no oxygen resources.

FTIR and XPS were carried out to clarify the chemical components of SiCN 370°C and SiCN 200°C (Figure 1). FTIR spectra of SiCN 200°C is compared with the spectra of SiCN 370°C in Figure 1(a). The broad peak 3100 – 3500cm-1 is assigned to N-H and O-H components, the last one due to moisture absorption from air ambient. A peak at ~1160 cm-1 can be assigned to NH3. This assignment is confirmed through XPS measurements where NH3 peak is detectable in the N1s peak (Figure 1 (c) right) while no NH3 peaks can be detected from the as-deposited SiCN 370°C (Figure 1(b) right). Such observation is suggesting the presence of residue of deposition precursors in the SiCN 200°C. In the Si2p peak of XPS spectra of Figure 1(c), it is observed that SiCN 200°C is characterized by less Si-C bonds compared to the SiCN 370°C as-deposited film. mainly Si-O bonds can be observed in the case of SiCN 200°C after N2 plasma while main peaks didn’t change from CMP for SiCN 370°C (Figure 2). The fact that in the as-deposited SiCN 200°C we do not have Si-C bonds is pointing to a missing desired property for the material since it was demonstrated that Si and C dangling bonds are essential in the process of enhancing bond energies.

Figure 1