Custom Layout Design

Custom / Full-Custom Layout — manual, transistor-by-transistor layout design where engineers hand-optimize every feature for maximum performance, density, or analog precision.

When Custom Layout Is Used

• SRAM bitcells: Must be absolute minimum area. Every nanometer matters
• High-speed I/O: SerDes analog front-end, clock buffers — timing-critical
• Analog blocks: Op-amps, ADCs, DACs, bandgap references — require precise matching
• Standard cells: The cells themselves are custom-designed (then instantiated millions of times)
• Critical datapaths: CPU ALU, multiplier — when automated PnR isn't good enough

Custom Layout Process

1. Circuit simulation and sizing (SPICE)
2. Manual polygon-level layout in Cadence Virtuoso
3. DRC check → fix violations iteratively
4. LVS check → ensure layout matches schematic
5. Parasitic extraction → re-simulate with parasitics
6. Iterate until performance targets met

Skills Required

• Deep understanding of process technology and design rules
• Knowledge of parasitic effects and their impact on performance
• Spatial reasoning and pattern optimization
• Years of experience to become proficient

Productivity

• Custom layout: ~10–50 transistors per engineer-day
• Automated PnR: Millions of cells per hour
• Only used where the performance/area benefit justifies the enormous time investment

Custom layout is the most labor-intensive part of chip design — but for the few critical structures that demand it, nothing else achieves the same results.