Study of thinned Si wafer warpage in 3D stacked wafers

3D (three-dimensional) wafer stacking technology has been developed extensively recently. One of the many technical challenges in 3D stacked wafers, and one of the most important, is wafer warpage. Wafer warpage is one of the root causes leading to process and product failures such as delamination, cracking, mechanical stresses, within wafer (WIW) uniformity and even electrical failure. In this study, the wafer warpage of thinned Si wafers in stacked wafers has been evaluated. Si wafer or glass was used as a thick substrate, and Cu or polyimide was used as the bonding material. The top Si wafer in the bonded stack was ground down to 20-100 μm, and wafer curvature was measured. Wafer curvature and how it relates to bonding material, substrate material of the stacked layers, and thickness of thinned Si wafer will be discussed.