Wafer edge overlay control solution for N7 and beyond

Improving the on-product overlay performance after optimization of the etch-induced contributions

“…By doing so, the chemical discontinuities can be mitigated. Subsequently, the overlay penalties at the wafer edge due to electrical discontinuities can be mitigated by the application of a tunable focus ring [3].…”

“…With tighter overlay requirements etch step contribution to overlay gained more attention and solutions for mitigation have been implemented. One major aspect is the high etch contribution to overlay at the wafer edge [3]. During etching a boundary is formed between the wafer and the plasma which is known as "sheath" [4].…”

“…The flatness of the sheath enables the incoming trajectory of ions to be straight (perpendicular to wafer plane). Due to the sharp transition of the electric field and plasma at the wafer edge the sheath bends around the wafer edge and causes non-normal ion trajectory [4,5] eventually causing high overlay at the wafer edge [3]. To mitigate the electrical field discontinuity, etch tools are equipped with a focus ring (FR) placed around the wafer, which improves the overlay at wafer edge [3,6].…”

“…Due to the sharp transition of the electric field and plasma at the wafer edge the sheath bends around the wafer edge and causes non-normal ion trajectory [4,5] eventually causing high overlay at the wafer edge [3]. To mitigate the electrical field discontinuity, etch tools are equipped with a focus ring (FR) placed around the wafer, which improves the overlay at wafer edge [3,6]. The focus ring has a limited lifetime and its performance drifts due to wear (Figure 1).…”

“…Overlay after development (ADI) is usually different from the overlay measured after etch (AEI) level. Etch contribution can be quantified simply from ADI-AEI overlay difference, both at wafer [3] and intra-field level [7] as shown in Figure 2. This approach enables isolating the pure etch contribution regardless of the overlay contributions of mask and lithography at ADI level (mask writing errors, lens and reticle heating, aberration impact).…”