Specific Process Knowledge/Thin film deposition/Deposition of Aluminium/Thermal deposition of Al
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Thermal evaporation of Aluminum
Aluminum can be thermally evaporated at DTU Nanolab in the Thermal Evaporator.
We recommend depositing a thin layer of thermally evaporated aluminum on top of e-beam resist before e-beam lithography. It is also possible to use Cr, see more info here. The advantage of Al is that it can be removed simultaneously with the e-beam resist. However Cr may allow even better resolution of the lithography.
For deposition onto the unexposed e-beam resist, you need to use thermal evaporation as it will be damaged by e-beam evaporation.
Using the Thermal Evaporator
The Thermal Evaporator (Lesker) is easy to use and rather quick for processing only a few samples and/or small samples, as it has a small chamber. The small chamber also means that the deposited thickness may vary quite a bit across large samples (4-14 % variation across a 4" wafer if not using rotation, similar with rotation).
See a few more details comparing the instruments including particle count and uniformity on 6" wafers here (without rotation): File:thermal evap wtec vs lesker nano uniformity and particles.pdf
As usual, you need a training session before using the instrument independently.
Time requirements for Al deposition (approx.)
- Vent the chamber (3 min)
- Loading samples and adding Al pellets to the crucible as necessary (5 min)
- Pump down (15-25 min)
- Run the process (soaking/ramping 7 minutes plus time for the deposition rate to stabilize after opening the source shutter, time for the deposition itself and about 1 minute for running down the power afterwards). The deposition rate is 0.5-2 Å/s.
In total set aside at least 45 min PLUS time for the deposition itself.
Quality control (QC) of Al evaporation on the thermal evaporator
| Quality control (QC) for the Thermal evaporator | ||||||||||||||||||
Thicknesses are measured in 5 points with one of the stylus profilers. |
Using the Wordentec
The Wordentec is useful if you need to deposit a thin layer on more than three wafers: You can load up to six 6" wafers at a time, but you can only add 8 pellets at a time, which in the Wordentec gives a total of approx. 150 nm (or 20 nm per wafer on six wafers, as extra material is used up for each soaking step). The pumping time for the Wordentec is at least 1 hour, which means that this is much slower than the Thermal Evaporator unless you are depositing on many wafers.
Use process 9 in Wordentec to deposit thermal Aluminum. Follow the manual.
Observe that you need to get a special training to use the thermal deposition source.
Set-up
- Two "boats" of Tantalum are mounted (on top of each other) in the thermal source each time.
- We have also earlier seen that two boats give stable depositions, since the lower boat gets hot (and heat the top boat), even if there if a thick layer of aluminum all over the top boat.
- If the last deposition from the source was Aluminum, the same boats can be used, otherwise they should be changed.
- "Small" Al pellets (diameter 1/8", height 1/8") are used. Use these small pellets to get a more uniform deposition with stable deposition rate.
- The boat should be placed as horizontal as possible, so the Aluminum metal will be evenly distributed in the boat after it is melted. If it is a problem to mount the boats horizontal, contact equipment responsible persons.
Program settings
- The deposition rate is normally 1 or 2 Å/s.
The settings saved in the material file for thermal aluminum are these:
- Soak power 1: 5%
- Soak power 2: 7%
These are already saved in the process recipe.
Notes
This files contains some short notes regarding thermal Al evaporation: Short Tests thermal deposition of Al in Wordentec
Roughness of thermally evaporated aluminiu
A study by AFM was performed to examine Al films deposited with thermal evaporation in the Wordentec. See details here.
