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

[Jacob T. Trevino]

Hi Guoliang,

To me it sounds a lot like classic Al hillock formation from stress relief
in thick films. As the Al film grows thicker, compressive stress builds due
to differential thermal expansion between the Al and Si. Without active
substrate cooling there is localized heating that gives the Al atoms enough
mobility to relieve stress by extruding outwards.

If you have any active cooling capabilities that would likely help. If not,
you could also try breaking the deposition into more steps with cooldowns,
or even tossing in a thin Ti layer mid-stack. Here is a reference that
might be worth checking out.

Ericson, Fredric, et al. "A transmission electron microscopy study of
hillocks in thin aluminum films." Journal of Vacuum Science & Technology B:
Microelectronics and Nanometer Structures Processing, Measurement, and
Phenomena 9.1 (1991): 58-63.

Best,
Jacob
-------------------------------------
*Jacob Trevino, PhD*
Senior Director, Columbia Nano Initiative Labs
Columbia University
530 W120th Street, Room 1015/MC 8903
New York, NY 10027
Email: jt2900 at columbia.edu
Cell: 734-787-2171
Desk: 212-854-9927
https://cni.columbia.edu/
https://www.linkedin.com/in/jacob-t-trevino/


On Wed, Aug 20, 2025 at 3:50 PM Wang Guoliang <
guoliang.wang at silicon-austria.com> wrote:

> 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.
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> 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
>
>
>
>
>
>
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