formation hillock

**Hillock Formation** is a **stress-relief mechanism in metal films** — where compressive stress during thermal cycling causes metal atoms to extrude through the surface, forming bump-like protrusions (hillocks) that can short-circuit adjacent metal lines. **What Causes Hillocks?** - **Mechanism**: Metal film expands more than the substrate during heating (CTE mismatch). The resulting compressive stress is relieved by mass transport to the surface. - **Materials**: Common in aluminum (soft, low melting point). Less common in copper (harder, better adhesion). - **Size**: Hillocks can be 100 nm to several $mu m$ tall — large enough to bridge to adjacent metal lines. - **Temperature**: Form during thermal cycling or high-temperature processing (> 300°C). **Why It Matters** - **Short Circuits**: Hillocks bridging to neighboring lines cause catastrophic electrical shorts. - **Aluminum Era**: A major reliability concern for Al interconnects. Mitigated by adding Cu or Ti to Al alloys. - **Passivation**: Strong passivation layers (SiN) help suppress hillock formation by providing mechanical constraint. **Hillock Formation** is **stress acne for metal wires** — unwanted surface bumps that form when thermal stress pushes metal atoms out of their layer.