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.