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

Wed Aug 20 20:25:30 EDT 2025

Hi Guoliang,

It looks like you may be seeing spitting of Al from the crucible.  I have
not personally seen this before with electron beam evaporation of Al, but I
have seen the same result with sputtering.  In the case of sputtering, you
can get micro-arcing on the surface of an Al target.  The high energy arcs
will generate molten Al that spits from the target and deposits on the
surface of the wafer.  In those cases, we saw the same behavior with
thickness.  Thinner films looked pristine, while thicker films that
underwent continuous deposition showed the same black dots.  Our analysis
also showed that such dots were pure Al.  The black dots were present
regardless of sputter power and seemed mainly dependent on the overall time
during which the plasma is present at the target.  Once we broke the
deposition into cycles, the issue went away, and we achieved pristine films
up to and beyond 1um thick (each cycle was 200nm or less of
deposition followed by a wait time and target cool down with the plasma
off).

In the case of electron beam evaporation, there are several factors that
can cause similar spitting of molten material, such as

   - Non-uniform heating:  If the sweep of the electron beam is too small
   and you are only heating a localized spot within the crucible, it can
   overheat that region, leading to spitting.  The fix is to increase the area
   over which the beam sweeps.

   - Material purity:  If there are impurities in the metal, especially
   carbon impurities, it can increase the tendency to spit.  At one point in
   time, we evaporated Au from a graphite crucible and had this issue.  After
   switching to tungsten crucibles, the problem went away.

   - Outgassing:  If the target material is heated too quickly, it can lead
   to outgassing and spitting.  Given that you are only seeing this as you
   increase thickness, outgassing is likely not the issue, but I figured it
   may be worth mentioning.

In addition, you mentioned that your throw distance is 1m.  While that is a
relatively long throw, molten particles can travel such distances.  We have
a similar distance in one of our evaporators, and we did see the effect
with other materials in the past.

Best Regards,

Matt

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. 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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-- 

--
Matthew T. Moneck, Ph.D
Executive Director, Claire & John Bertucci Nanotechnology Laboratory
Electrical & Computer Engineering | Carnegie Mellon University
5000 Forbes Avenue, Pittsburgh, PA 15213-3890
Phone: 412-268-5430
ece.cmu.edu
nanofab.ece.cmu.edu
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