Surface activated dielectric bonding is more and more attractive as a key technology to achieve further high-performance CMOS based devices independent on scaling. The major challenge of dielectric bonding is to decrease the process temperature in order to be compatible with CMOS processing. Although the conventional SiO₂-SiO₂ bonding has already been comprehensively investigated, there might be some limitations in terms of thermal budget. In the past, we have demonstrated low temperature bonding using PECVD-SiCN as interfacial layer, where we have obtained more than 2200 mJ/m² of adhesion energy at 250 ^oC of post annealing temperature [1].

In this work, the composition of SiCN has been tuned aiming at the identification of the key elements taking part in the bonding mechanism and to further increase the adhesion energy.

The SiCN composition was successfully controlled by tuning the CVD deposition process, as it is proven by elastic recoil detection (ERD) measurements which enable us to monitor the ratio of the different elements (C,N,H,O, Si) in the different films. A table with ERD results is reported below. Also, the film densities are determined by using the mass differences of the processed wafers. The density becomes lower with increasing carbon concentration. The density variation impacts the CMP removal rate, which is critical for actual hybrid bonding process. On the other hand, no difference on the roughness is seen for all SiCN films.

After film planarization by CMP, wafer bonding test were done on 300 mm wafers by combining a pair of the same films as interfacial layer. Prior to bonding, N2 plasmas were applied on the both surfaces. No voids were observed for all the pairs, even after post bond annealing. The highest bonding strength is obtained from the carbon rich SiCN (SiCN #2). This indicates that the higher adhesion by SiCN may be attributed to the carbon dangling bond formed by plasma activation, rather than hydrogen or nitrogen.

Further mechanisms are discussed to understand the present low-temperature bonding technique.

[1] E. Beyne et.al., “Scalable, sub 2µm Pitch, Cu/SiCN to Cu/SiCN Hybrid Wafer-to-Wafer Bonding Technology” IEEE IEDM 2017, 32.4.1 (2017)