Results and Discussion
After introduction into the analysis chamber a survey spectrum
was acquired from the as-received sample. Analysis of the survey
spectrum showed presence of the constituent elements Li, P, O and
N and also a significant amount of carbon which can be attributed
to the presence of adsorbed species from air exposure. To examine
the distribution of Li throughout the film, depth profiling was
performed. Figure 1 shows comparison of conventional monatomic
depth profiling using 4 kV Ar
+
ions and the same sample profiled
with 20 kV Ar
1000
+
ions. Both depth profiles were performed as an
overnight experiment with the cluster mode etch rate >4nm/min.
There is a distinct difference between the two profiles. Under
monatomic bombardment there is an initial increase in Li
concentration after the first etch cycle – a consequence of removing
surface contamination – then a subsequent decrease in Li into the
bulk of the film reaching a steady-state of ~25.5 atomic%. Near
the LiPON interface with Si substrate the Li concentration increases
significantly reaching a maximum of 44%. By contrast when
profiled 20 kV Ar
1000
+
ions the profile shows a different distribution
of Li. An initial increase in Li concentration is shown after the first
etch however there is no subsequent decrease reaching a steady-
state of >31%. This amounts to an increase of 22% Li throughout
the film depth compared to the monatomic profile. Interestingly no
accumulation of Li was seen at the interface with the Si substrate.
A comparison of the Li concentrations profiled using two different
ions is shown in figure 2.
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Previously monatomic Ar
+
bombardment of glasses has shown lower
than expected concentrations of light elements such as sodium [2].
This phenomenon was attributed to positive charge build-up in the
near surface region from implanted Ar
+
ions. The charge build-up
repels the small, mobile positive ions in the surface further into
the bulk resulting in an underestimation in surface quantification.
The same mechanism of Li ion repulsion and bulk migration is
proposed to occur for LiPON thin-films analysed here. Li migration
occurs under monatomic profiling resulting in an underestimation
of Li concentration. Repulsive bulk migration also accounts for the
increase in Li concentration at the interface with the Si substrate.
The light ions are unable to penetrate through into the native
Silicon oxide and instead accumulate at the interface. It must be
noted that this system is particularly susceptible to this process
because the Li+ ions in LiPON by their design are mobile for the
charge transfer process.
Conclusion
It has been demonstrated that the use of XPS combined with
depth profiling techniques provide quantitative information on
the composition of LiPON thin-films created via ALD. It has been
shown that the use of monatomic Ar
+
ions is unsuitable for profiling
materials with mobile, light elements as the build-up of positive
charge can cause migration leading to incorrect stoichiometry. It
is concluded that the use of high energy Arn
+
clusters for depth
profiling this class of materials is vital to mitigate the effects of
ion migration and gain confidence in the validity of the results.
Acknowledgements
Many thanks to the research group of Prof. Gary Rubloff and in
particular Alex Pearse at the University of Maryland.
References
1. A. Kozen, A. Pearse, G. Rubloff, C-F Lin, M. Noked, Chem.
Mater., 2015, 27, 5324–5331
2. Y. Yamamoto, K. Yamamoto, J. Non. Cry. Sol., 2015, 356, 14
Figure 1a Figure 1b
Related documents available online:
Click to download
Sample cleaning
using Ar-GCIS
Applications note
MO395(1)
Smart phone screen –
depth analysis of alkali
and alkali earth metals
Applications note
MO427(0)
Click to download
0
5
10
15
20
25
30
35
40
45
50
0 50
Relative atomic conc.
depth / nm
Li conc.
monatomic
cluster
Si
LiPON
Ar
+ +
+
+
+
+
+
+
+
+ +
Li
Li
Li
Li
Li
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80
Relative atomic conc.
depth / nm
5 kV Ar+
Si 2p
O 1s
N 1s
C 1s
P 2p
Li 1s
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80
depth / nm
20 kV Ar
1000
+
Si 2p
O 1s
N 1s
C 1s
P 2p
Li 1s
Figure 2:
Li ion concentration
comparison between
monatomic (black)
and cluster profile (red)
Figure 1: Depth profile of 50 nm LiPON thin-film
using (a) 5 kV Ar
+
(b) 20 kV Ar
1000
+
Figure 3: Schematic of ion action on light elements.
