Large Warpage Wafer Saddle and bowl+Wafer-to-Wafer Bonding with Saddle-shaped Wafers

This study investigates the impact of asymmetric wafer shapes, specifically saddle-shaped warpage, on wafer-to-wafer (W2W) bonding accuracy in advanced semiconductor manufacturing. Saddle-shaped wafers, exhibiting curvature in opposite directions along perpendicular axes (typical in 3D NAND devices), introduce significant asymmetric scaling components, leading to increased bonding overlay errors compared to flat or symmetrically warped wafers.

The authors created saddle-shaped wafers using alternating SiO₂ and W stress layer strips, generating a total warpage of ~230 μm (X: 90–100 μm, Y: −130 to −140 μm). Wafers were bonded to flat wafers using dielectric bonding, followed by thinning to 5 μm and post-bond lithography overlay measurements using scanner metrology. A 6-parameter overlay model was applied to capture translation, rotation, scaling, and residual errors separately in X and Y directions.

Key findings include:

1. Positioning effect: Placing the saddle wafer on the bottom chuck of the bonder yields better overlay than on the top chuck, with 80% of sites showing overlay <120 nm versus <200 nm for top chuck placement.

2. Asymmetric scaling compensation: Controlled asymmetric top wafer release reduces the difference in scaling components (ΔS) between X and Y from ±3.4 ppm to ~1 ppm, improving overall bonding overlay, though residual errors may increase slightly.

3. Backside lithography: Applying higher-order corrections per exposure (HOCPE) enables precise backside patterning after wafer thinning. Saddle-shaped bonded wafers required larger nonlinear corrections, yet the 99.7th percentile overlay could be reduced to 12 nm, comparable to flat and umbrella-shaped wafers (page 6, Figure 7).

The study demonstrates that asymmetric wafer shapes must be carefully managed through placement, top wafer release, and post-bond corrections to achieve sub-100 nm overlay accuracy. These insights are critical for 3D NAND-to-logic integration and other applications requiring high-precision W2W bonding.