A Study of SiCN Wafer-to-Wafer Bonding and Impact of Wafer Warpage

Wafer to wafer bonding studies were carried out on blanket and patterned highly warped wafers using SiCN as bonding dielectric material to gain a deeper understanding of SiCN-to-SiCN direct bonding and to assess the impact of wafer shape on bonding overlay results. Regarding the fundamental understanding of SiCN bonding, characterization data show that the SiCN-to-SiCN interface is oxidized, suggesting strongly bonded due to the presence of covalent bonds, already at room temperature, in contrast to what has been hypothesized for $\text{SiO}_{2}$ for which this reaction would start only by subjecting the bonded pair to an anneal of $150^{\circ}\mathrm{C}$ . As concerns the impact of shapes on final bonding results, bonding experiments are carried out by combining different wafer shapes. Tools such as patterned wafer geometry (PWG) and a lithography scanner were used to measure the distortion signature of the wafers before and after bonding. Bonding recipe parameters were optimized to minimize overlay errors, on two different bonding tool configurations, for nominally flat wafers. The same parameters were used to bond the warped wafers to investigate the impact of wafer warpage. By using one of the two tool's configurations, overlay results can be significantly reduced for flat wafers. When wafers with different shapes are bonded, recipes must be optimized to obtain tighter overlay specifications. Such optimization is needed to counteract the effect of the incoming distortion caused by the different incoming shapes and measured by the scanner. However, it should be noted that the incoming distortion is not solely determined by the shapes, but also by the kind of dielectric stack used, meaning that the same shape with a different stack will result in a different incoming distortion, as seen in the two different test vehicles used in this study. Tendency of the bond wave to propagate according to the Si-orientation is visible in the distortion signatures after bonding.