wide metal rules

**Wide metal rules** are **special design rules** that apply to metal features exceeding a specified width threshold — addressing the unique manufacturing, reliability, and performance challenges that arise when metal conductors are significantly wider than minimum-width wires. **Why Wide Metal Needs Special Rules** - Standard metal design rules are optimized for **minimum-width** routing used in signal interconnects. - Wide metal features (power straps, bus lines, ground planes, I/O pads) behave differently during manufacturing: - **CMP**: Wide features dishing more aggressively → need slotting rules. - **Etch**: Wide features etch differently from narrow lines (different etch bias, edge effects). - **Stress**: Large metal areas create more thermal stress → potential cracking, delamination, or via popping. - **Electromigration**: Current distribution in wide features is non-uniform — current crowding at corners and width transitions. **Typical Wide Metal Rules** - **Slotting Requirements**: Insert slots when width exceeds a threshold (typically 10–20 µm) — see slot rules. - **Maximum Width without Slots**: Hard limit on how wide an unslotted metal feature can be. - **Increased Spacing**: Wide metal may require **larger spacing** to adjacent features than minimum-width wires — due to etch proximity effects and reliability concerns. - **Enclosure Rules**: Via landing pads on wide metal may require different enclosure than on minimum-width wires. - **Corner Rounding**: Sharp 90° corners in wide metal create stress concentrations — corner rounding requirements reduce cracking risk. - **Width Transition**: Rules for transitioning from wide to narrow metal (taper angle, minimum taper length) to avoid abrupt width changes that cause etch and current density issues. - **Minimum Area**: Even wide metal features must meet minimum enclosed area requirements. **Impact on Power Grid Design** - Power grid straps are the primary wide metal features. - Wide metal rules constrain how power straps are designed: - Cannot simply make straps as wide as desired — must comply with slotting. - Spacing to adjacent signal routes must account for wide metal spacing rules. - Corner-turning in power grids requires compliance with corner rules. - Width changes (e.g., from wide strap to narrow via landing) must follow taper rules. **Electromigration in Wide Metal** - Current density in wide metal is **not uniform** — it concentrates at edges, corners, and via connections. - EM checking for wide metal must account for local current density, not just average current density. - Via placement along wide metal must ensure current is distributed evenly. Wide metal rules are **critical for power integrity and reliability** — they ensure that the widest, most current-carrying features on the chip are manufactured with consistent quality and adequate lifetime.