Plasma Technology  

Aluminium oxide deposition for surface passivation

Al2O3 differs from conventional surface passivations,
such as Si3N4, by its high fixed negative charge density.
This negative charge shields the charge carriers of
p-type c-Si from the surface and leads to higher measured
effective lifetimes and a better level of surface passivation.
In particular plasma assisted ALD films used in the
passivation of highly doped p-type emitters on
n-type solar cells yielded a cell efficiency of 23.2%.

with kind permission
TU Eindhoven, Dr Erwin Kessels

ZnO:Al with DEZ, TMA (~few % by cycle ratio) and H2O as
alternative transparent conductive oxide (TCO) can also
be grown in the ALD chamber.



Linear self limiting growth measured by in situ spectroscopic
ellipsometry at 25 300 °C deposition temperatures

Al2O3 remote plasma ALD for solar cell passivation


metal precursor: TMA (tri methyl aluminium)


non metal precursor: O radicals and O2

temperature controlled vapour draw

dose control by fast pulse ALD valve

TMA + O2 plasma recommended for surface passivation
(rather than H2O thermal)

Deposition temperature range: 200°C recommended
for surface passivation

1.2 Å/cycle (saturated dose @ 200 °C)

Rate: 2 nm/min
Repeatability < ±1%

Carbon impurity < 2 at% @ 200 °C

Hydrogen impurity < 3 at% @ 200 °C

Post deposition anneal required at 425°C for
highest fixed negative charge density

 

growth rate vs TMA dose time at 200° C

 

growth rate vs plasma time:
A decreasing growth rate with decreasing
plasma time indicates incomplete surface
activation (removal of CH3 groups).

 

AFM RMS analysis shows very smooth films:
c-Si substrate: 0.0555 nm
20 nm Al2O3: 0.0589 nm

 

 

Why remote plasma ALD ?

A "remote plasma" makes sure, the substrates
are NOT in contact with the plasma !

The remote plasma just cracks molecules,
so that very reactive species can be used
for the growth process.

Such reactive species often enable a very
efficient plasma preclean/ conditioning of the
substrates, lead to cleaner films and lower
the deposition temperature.

In Oxford systems it is possible to run
ALD processes using
- the thermal only method
- ozone assisted processes
- remote plasma assisted processes
together without any hardware change.
Multiple step processes using all technologies
can be chosen from the software.

low temperature ALD down to 25° C
is enabled by the use of O radicals;
this is not practical using thermal
H2O processes.

 


FlexAl

 


ALD schematic
valve between remote ICP source
and chamber,
spectroscopic ellipsometry optional

 

At a given temperature radical assisted
ALD gives higher film densities than
purely thermal ALD.

 

radical assisted Al2O3 ALD at 20° C
very low water vapour tansmission rate

 

OpAl

 

 

 

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