Slot rules are design rules that require openings (slots) to be inserted into wide metal features — breaking up large continuous metal areas to improve CMP planarity, prevent dishing, and reduce stress-related reliability risks.
Why Slots Are Needed
- CMP Dishing: Wide metal features (power straps, ground planes, bus lines) are polished more aggressively at their center during CMP, creating a concave (dished) surface. Dishing increases with metal width.
- Dishing Impact: A dished metal feature has reduced thickness at its center → higher resistance, worse electromigration lifetime, and potential via connection problems.
- Stress Relief: Large continuous metal areas generate significant thermal stress during processing — slots reduce the effective area and allow stress relief.
Slot Rule Specifications
- Trigger Width: Slotting is typically required for metal features wider than a threshold (e.g., 10–20 µm depending on the process and metal layer).
- Slot Dimensions: Minimum and maximum slot width (e.g., 1–3 µm) and length.
- Slot Pitch: Maximum distance between adjacent slots — ensures that no large unslotted area remains.
- Slot Orientation: Slots are typically oriented perpendicular to the current flow direction to minimize their impact on current carrying capacity.
- Border Spacing: Minimum distance from slots to the edge of the metal feature.
How Slots Work
By inserting openings in wide metal, the feature is effectively converted from one wide metal region into multiple narrower parallel conductors connected at the ends. Each narrow section experiences less CMP dishing — resulting in a more uniform metal thickness.
Electrical Impact
- Increased Resistance: Slots reduce the effective metal cross-section, increasing sheet resistance. For power grid wires carrying high current, this must be accounted for.
- Changed Current Flow: Current must flow around the slots — current density increases at slot corners, potentially creating EM hot spots.
- Parasitic Changes: Slot geometry affects wire capacitance and inductance.
Design Considerations
- Power Grid: Wide VDD/VSS straps are the most common candidates for slotting. Must balance CMP needs against IR drop requirements.
- Automated Insertion: EDA tools (Calibre, IC Validator) automatically insert slots in wide metals as part of DRC/DFM processing.
- Custom Handling: Critical power paths may need manual slot optimization to balance CMP requirement against electrical performance.
Slot rules are a manufacturing-driven constraint that ensures wide metal features maintain uniform thickness after CMP — without them, power grid resistance would be unpredictable and via connections unreliable.