OpenROAD - GDS Export

GDS export is the final stage of the physical implementation process. After placement, routing, and parasitic extraction, the design is converted from the internal OpenDB database or DEF representation into a GDSII file, which is the standard format for tape-out and mask generation.

Official documentation:

• OpenROAD GDS Export (write_gds)
• OpenROAD-flow-scripts GDS Export Tutorial
• OpenDB API Reference

1. Overview

The GDS export step consolidates all physical data — cell placements, routing geometries, and technology layers — into a single GDSII file.

Input	Description
*.lef	Technology and standard-cell layer definitions
*.def	Final placed and routed layout
*.lib	Liberty timing library (optional for annotation)
OpenDB	Internal database built by OpenROAD

Output	Description
design.gds	Final GDSII file for sign-off and tape-out

2. Basic GDS Export Example

# Load tech and design data
read_lef tech.lef
read_lef stdcells.lef
read_def results/routed.def

# Write final GDSII layout
write_gds results/design.gds

This creates results/design.gds that can be opened in KLayout or Magic for verification and inspection.

👉 Reference example: write_gds1.tcl

3. Including Macro GDS Files

If your design contains hard macros or IP blocks (e.g., SRAMs, PLLs), their GDS files must be included before export to ensure proper hierarchy.

read_lef tech.lef
read_lef stdcells.lef
read_lef sram_macro.lef
read_def results/routed.def

# Load macro GDS
read_gds macros/sram_macro.gds

# Export complete chip layout
write_gds results/top_with_macros.gds

Important:

• The macro’s LEF and GDS must share consistent cell names.
• All included GDS files are merged into the final top_with_macros.gds.

4. Hierarchical vs. Flattened GDS

By default, OpenROAD exports a hierarchical GDS, preserving cell structure. For smaller test chips, you can flatten the hierarchy:

write_gds -flatten results/design_flat.gds

Mode	Description
Hierarchical	Recommended for large designs; smaller file size.
Flattened	Easier for inspection but much larger; used for simple blocks.

5. Merging with Padframe or Top-Level

If your core design is part of a larger SoC or includes an IO ring, you can merge multiple GDS files using:

merge_gds -input {core.gds pad_ring.gds} -output results/chip_top.gds

This command combines layouts hierarchically under a new top cell.

6. Optional — Write OASIS Format

For modern tape-outs, OASIS format may be preferred due to smaller file sizes.

write_oas results/design.oas

OASIS files can be loaded directly into KLayout and many foundry tools.

7. Example Full Script

# === GDS Export ===
read_lef tech.lef
read_lef stdcells.lef
read_def results/routed.def
read_gds macros/sram_macro.gds

# Optional flatten
write_gds -flatten results/design_flat.gds

# Standard hierarchical export
write_gds results/design.gds
write_oas results/design.oas

# Save OpenDB snapshot
write_db results/final.odb

8. Outputs

File	Description
design.gds	Final hierarchical GDSII layout
design_flat.gds	Optional flattened layout
design.oas	Optional OASIS layout
final.odb	OpenDB snapshot for later analysis
gds_export.log	Export log (layer mapping and cells processed)

9. Post-Export Verification

After generating the GDS file, verify it with KLayout or Magic before sign-off.

Example: Open GDS in KLayout

klayout design.gds &

Example: Run DRC/LVS

klayout -b -r drc_check.lydrc -rd layout=design.gds -rd report=drc.lyrdb
netgen -batch lvs "layout/design.spice top" "schematic/top.spice top" setup.tcl

Checks to perform before tape-out:

• DRC violations (width, spacing, enclosure)
• LVS comparison against schematic
• Antenna violations
• Layer naming consistency

10. Troubleshooting Tips

Issue	Cause	Solution
Missing macros in GDS	Macro GDS not loaded	Add read_gds <macro>.gds before export
Wrong layer mapping	PDK layer map mismatch	Ensure correct .map or .lef from PDK
File too large	Flattened export	Use hierarchical mode or OASIS format
LVS mismatch	Cell name differences	Verify symbol names in LEF and GDS
DRC errors	Routing incomplete	Re-run detailed routing before export

11. Summary of Key Commands

Command	Description
write_gds	Export layout in GDSII format
write_oas	Export layout in OASIS format
read_gds	Import macro or top-level GDS files
merge_gds	Combine multiple GDS files into one
write_db	Save OpenDB snapshot for reuse

12. Next Step

After exporting the GDS, proceed to Verification and Sign-off, where you perform:

• DRC/LVS using KLayout or Magic
• Timing and parasitic re-validation
• Final tape-out packaging

👉 See also: {doc}verification