Atomic Layer Deposition (ALD) is prized for coating deep, high-aspect-ratio (HAR) features. Using Chipmetrics’ PillarHall® lateral HAR test structures, we have now identified an unexpected second growth front that starts at the closed end of the trench and advances toward the entrance, meeting the primary front in the middle.
How we saw it
- PillarHall® geometry lets us polish and inspect the entire trench in-plane.
Imaging spectroscopic ellipsometry (ISE) and SEM resolve thickness variations down to a few nanometres, revealing the counter-propagating front that conventional vertical cross-sections miss.
Why it forms
Dr Simon Elliott at Schrödinger has developed models that links the effect to transient gas transport and surface-reaction dynamics:
- Precursor arrival – Molecules diffusing past the entrance accumulate at the blind end during a pulse.
- Surface saturation lag – That build-up triggers fresh reactions from the rear, creating a second nucleation zone.
- Periodic pressure swings – Pulse / purge timing governs how far and how often the rear front advances.
What it means for ALD engineers
| Challenge | Take-away | Action |
| Hidden thickness gradients in deep HAR features | Conformality isn’t a single-front problem | Map both fronts with lateral test chips before finalizing recipes |
| Pulse / purge balance | Rear front speed depends on transient pressure | Fine-tune pulse lengths, purge efficiency and total pressure |
| Metrology coverage | Standard top-down or cleaved views can miss the effect | Add PillarHall® or similar LHAR structures to routine QC |
Next steps
- Modelling: Extend transient-flow simulations to other chemistries and aspect ratios.
- Experiments: Vary temperature, pressure and precursor dose to isolate key variables.
- Collaboration: We invite tool makers and process engineers to test the phenomenon in their own flows and share data.
With these findings, we see that PillarHall® continues to expose the nuances of conformal film growth – now including a surprising second growth front. Understanding and controlling it will be crucial for the next generation of ultra-dense 3D devices.
Interested in learning more about this second front phenomenon? We’re happy to hear your thoughts, and plan to follow up on this subject as we learn more.
Want to find out for yourself? Contact our sales team to see what our PillarHall® test chips can do for you.
