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.