[labnetwork] Troubleshooting defects in thick Al film (750 nm, 200 mm Si, e-beam evap)

[Wang Guoliang]

Dear All from the Labnetwork community,

Recently, our lab received a request to deposit a 750 nm aluminum film on a 200 mm Si substrate using e-beam evaporation. The deposition was carried out with an aluminum slug in a 40 cc water-cooled pocket. The process proceeded smoothly with stable rate, power, and pressure.

However, upon initial microscopic inspection, the deposited aluminum film exhibited numerous black dots distributed across the entire wafer. We attempted several approaches to eliminate these defects, but so far have been unsuccessful.

I am wondering if anyone has encountered similar issues when depositing thick aluminum films. Any insights or suggestions on how to address this problem would be greatly appreciated.

Details of the evaporation:

  *   Deposition rate: 1 A/s
  *   Deposition power: ~20%
  *   Deposition pressure: ~5E-7 mbar
  *   Substrate cooling: No (Wafer were placed on a planar wafer holder)
  *   Throw distance: 1 m

Approaches we have already tried:

  *   AFM/SEM characterization – The black dots varied in size, with the largest reaching ~200 nm in diameter. Their morphology was pyramid-like, featuring a central peak surrounded by some trenches.
  *   EDX analysis – The black dots were identified as aluminum.
  *   FIB analysis – There was no cavity inside the black dots.
  *   Adhesion layer – Adding a 75 nm Ti adhesion layer yielded no improvement.
  *   Different deposition rate at 5 A/s and 0.5 A/s – Both yielded no improvement.
  *   Thickness variation – Depositions at 200 nm, 400 nm, and 600 nm were tested. Defects only appeared in the 600 nm Al layer. All wafers exhibited similar film stress.
  *   Pre-melted aluminum pellets – Using a carbon crucible liner, reduced deposition power, and the same rate resulted in an increased number of black dots.
  *   Multiple-stage deposition – Depositing 400 nm with a liner, pausing for 30 minutes, and then depositing an additional 350 nm yielded no improvement.
  *   Alternative substrates – Depositions on Si substrates with thermal oxide and on fused silica showed no improvement.

Attached are the microscope image, AFM image, and SEM image for your reference.

Thanks and best regards,
Guoliang

Guoliang​​

Wang MSc
Process Engineer
SAL MicroFab
Silicon Austria Labs GmbH
High Tech Campus Villach - Europastraße 12
A‑
9524

Villach
,
AT
M: +4366488843743
guoliang.wang at silicon-austria.com<mailto:guoliang.wang at silicon-austria.com>



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mtl.mit.edu/pipermail/labnetwork/attachments/20250820/20c4d73d/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Microscope image.bmp
Type: image/bmp
Size: 17694774 bytes
Desc: Microscope image.bmp
URL: <https://mtl.mit.edu/pipermail/labnetwork/attachments/20250820/20c4d73d/attachment.bmp>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: AFM image.png
Type: image/png
Size: 443701 bytes
Desc: AFM image.png
URL: <https://mtl.mit.edu/pipermail/labnetwork/attachments/20250820/20c4d73d/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: SEM image.JPG
Type: image/jpeg
Size: 87072 bytes
Desc: SEM image.JPG
URL: <https://mtl.mit.edu/pipermail/labnetwork/attachments/20250820/20c4d73d/attachment.jpe>