A round-robin characterization is reported on the sputter depth profiling of [60(3.0 nm Mo/ 0.3 nm B4C/ 3.7 nm Si)] and [60  (3.5 nm Mo/ 3.5 nm Si)] stacks deposited on Si (111). Two different commercial secondary ion mass spectrometers with time-of-flight and magnetic-sector analyzers and a pulsed radio frequency glow discharge optical emission spectrometer were used. The pros and cons of each instrumental approach are discussed.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3526

Ber, B.

Chapon, P.

Drozdov, M.N.

Tolstoguzov, A.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 1, 01PCSI08 (4pp) (2013) 
 
 
2304-1862/2013/2(1) 01PCSI08 (4) 01PCSI08-1  2013 Sumy State University 
Depth Profiling of Multilayer Mo/Si Nanostructures 
 
A. Tolstoguzov1,*, B. Ber2,†, P. Chapon3,‡, M.N. Drozdov4,§ 
 
1 Centre for Physics and Technological Research (CeFITec), Dept. de Física da Faculdade de Ciências e Tecnologia,  
Universidade Nova de Lisboa, 2829-516 Caparica, Portugal 
2 Ioffe Physical-Technical Institute, Polytechnicheskaya 26, 194021 Saint Petersburg, Russian Federation 
3 HORIBA Scientific, 91165 Longjumeau, France 
4 Institute for Physics of Microstructures of the Russian Academy of Science (IPM RAS), 603950 Nizhny Novgorod, 
Russian Federation 
 
(Received 18 May 2013; published online 30 August 2013) 
 
A round-robin characterization is reported on the sputter depth profiling of [60(3.0 nm Mo/ 0.3 nm 
B4C/ 3.7 nm Si)] and [60   (3.5 nm Mo/ 3.5 nm Si)] stacks deposited on Si (111). Two different commercial 
secondary ion mass spectrometers with time-of-flight and magnetic-sector analyzers and a pulsed radio 
frequency glow discharge optical emission spectrometer were used. The pros and cons of each instrumental 
approach are discussed.  
 
Keywords: Sputter Depth Profiling, Glow Discharge Optical Emission Spectroscopy (GDOES), Mo/Si In-
terferential Mirror, Round-robin Characterization, Secondary Ion Mass Spectrometry (SIMS). 
 
 PACS numbers: 68.49.Sf, 81.70.Jb 
 
 
                                                        
* a.tolstoguzov@fct.unl.pt 
† boris.ber@mail.ioffe.ru 
‡ patrick.chapon@horiba.com 
§ drm@ipm.sci-nnov.ru 
1. INTRODUCTION 
 
Sputter depth profiling is a powerful tool for compo-
sitional analysis of nanometer-thick multilayer struc-
tures. In recent years, evident progress has been made 
in this field, especially regarding secondary ion mass 
spectrometry (SIMS) and glow discharge optical emis-
sion spectroscopy (GDOES) [1].  
The present work is a continuation and further de-
velopment of our previous round-robin study [2]: we 
have analyzed Mo/Si and Mo/B4C/Si stacks. From a 
practical point of view, Mo/Si interferential mirrors are 
of considerable interest for high resolution X-ray li-
thography. The introduction of B4C layers suppresses 
interlayer diffusion, which worsens the reflectivity and 
thermal stability of mirrors. 
Two commercial SIMS instruments – CAMECA 
IMS7f by Cameca SAS at Ioffe Physical-Technical In-
stitute and TOF.SIMS-5 by ION-TOF at Institute for 
Physics of Microstructures, and a modified GD-Profiler 
2 at Horiba Scientific were involved in our inter-
laboratory experiments. This study is driven by neces-
sity to perform accurate and rapid analysis of nanome-
ter-thick periodic multilayers in order to optimize and 
improve their production technology. 
 
2. EXPERIMENTAL DETAILS 
 
2.1 Multilayer samples 
 
The investigated samples are commercially availa-
ble multilayer interferential mirrors produced by mag-
netron sputtering at IPM RAS [3].  
The samples are ca. 420 nm-thick nanostructures 
consisting of 60 layer period superlattices - Mo/B4C/Si 
in the case of the PM596 sample and Mo/Si for the 
PM615 sample (Fig. 1). In both cases, each period is 
7.00  0.05 nm thick. The dimensions of individual Mo 
and Si layers are estimated to be 3.0  0.2 nm (Mo) and 
3.7 ± 0.2 nm (Si) for the PM596, and 3.5  0.1 nm (Mo) 
and 3.5  0.1 nm (Si) for the PM615. The B4C barrier 
layer is a single atomic layer with a nominal width of 
ca. 0.3 nm. This technological parameter is estimated 
via the deposition and shutting rates of the magnetron 
sputtering processing.  
 
 
Fig. 1 − Schematic view of the structure of the sample under 
study 
 
The starting layer put directly onto the substrate is 
Mo, and the top surface layer is Si. The B4C layer is 
deposited onto the Si layer in every period of the sam-
ple PM596. The substrates are 0.4 mm-thick commer-
cial Si (111) wafers with an average surface roughness 
of 0.3 nm. 
 
2.2 Instruments 
 
Pulsed radiofrequency (RF) mode is used to power 
the glow discharge optical emission spectrometer GD-
 A. TOLSTOGUZOV, B. BER, P. CHAPON PROC. NAP 2, 01PCSI08 (2013) 
 
 
01PCSI08-2 
Profiler 2 [4, 5]. In GDOES measurements, samples are 
etched by Ar+ ions and accelerated neutrals with very 
low kinetic energies; the sputtered atoms are then ex-
cited by the plasma and de-excited via photons emission 
with characteristic wavelengths, enabling their ele-
mental identification. In our experiments, the operating 
conditions were: argon pressure of 550 Pa, 17 W RF 
power, 5 kHz pulse frequency with 0.25 duty cycle. The 
sputtered area in GD is the largest in comparison to 
other techniques: here a disk of 4 mm in diameter, and 
the useful signals are collected from the whole sputter 
area. 
Time-of-flight secondary ion mass spectrometer 
TOF.SIMS-5 works in the dual beam mode using 
25 keV Bi3+ ions for analysis and 1 keV Cs+ ions for 
sputtering. The angle of incidence was 45° for both ion 
beams. The sputter beam was scanned over an area of 
250  250 μm2, and the analyzed region was ca. 4% of 
the sputter area. Detailed description of the experi-
mental conditions can be found in [6].  
Magnetic-sector secondary ion mass spectrometer 
CAMECA IMS7f employs Cs+ primary ions with an 
impact energy of 5.8 keV and an incidence angle of 32° 
relative to the surface normal (the accelerating and 
sample potentials were 7 keV and 1.2 keV, respectively) 
while the positive secondary ions are detected. The 
primary beam was scanned over a 250  250 μm2 area, 
and the secondary ions were collected from a central 
region of 60 μm in diameter. The “energy window” of the 
spectrometer was fully opened (130 eV), and the mass 
resolution was M/ΔM=400. The crater depths were 
measured by an AMBIOS XP-1 surface stylus pro-
filometer by Ambios Technology. 
Atomic force microscopy (AFM) was used to measure 
the initial surface roughness of the samples and the 
roughness at the bottom of the craters after sputter 
depth profiling. Different instruments were involved in 
this study: a Dimension 3100 by VEECO in tapping 
mode for the magnetic SIMS, a SOLVER by NT-MDT in 
contact mode for the TOF-SIMS, and a NT-MDT NTe-
gra Prima in contact mode for the samples analyzed by 
pulsed RF GDOES. 
 
3. RESULTS AND DISCUSSION 
 
Raw depth profiles, in a semi-logarithmic scale, ob-
tained by pulsed RF GDOES and SIMS are shown in 
Fig. 2 for the sample PM596 and in Fig. 3 for the sample 
PM615. In the case of pulsed RF GDOES (Fig. 2a and 
Fig. 3a), the bombarding ions were Ar+ with mean im-
pact energy of 50 eV. We measured the intensity of the 
light emission of excited sputtered atoms at wavelengths 
of 381.411 nm (Mo), 251.611 nm (Si), 249.678 nm (B), 
and 156.144 nm (C).  
For the PM596 sample (Fig. 2a), the number of fully-
resolved periods was estimated to be 59. With deeper 
penetration into the sample structure only the depth 
profile of carbon exhibits a continuous decrease in inten-
sity, however without decay of the signal oscillations. In 
our opinion this profile can be considered as a result of 
two contributions - one of them relating to carbon origi-
nating from the B4C barrier layers and the other one 
from contamination. For the PM615 sample (without the 
B4C barrier layers), an evident degradation of both Mo 
and Si profiles versus the sputter time (the depth of 
sputtering) is observed (Fig. 3a).  
For the TOF-SIMS, only the first 10 periods of the 
structures were analyzed (Fig. 2b and Fig. 3b). The 
depth distributions of positively charged atomic Mo+ 
(sum of all isotopes), Cs211B+ and 28Si+ secondary ions are 
shown in Fig. 2b (PM596) and Fig. 3b (PM615). As com-
pared with pulse RF GDOES, the introduction of B4C 
layers does not induce significant difference between the 
measured profiles.  
For CAMECA IMS7f with a magnetic sector mass 
analyzer (hereinafter this instrument will be called the 
magnetic SIMS or M-SIMS) we chose molecular ions 
MCs+ (where M is B, Si or Mo) as the characteristic ions. 
In spite of the fact that the mechanisms of secondary ion 
formation under Cs+ bombardment and deposition are 
still debated, Cs-based SIMS allows for the minimization 
of matrix effects, which greatly influence the secondary 
ion yields. In our case, non-degraded profiles were ob-
tained for both samples with 58 resolved periods (Fig. 2c 
and Fig. 3c). No significant difference between the data 
of the PM596 and PM615 samples was observed. How-
ever, the signals of Si and Mo within each period over-
lapped, and in case of SiCs+ were split into two parts (see 
the inserts in Fig. 2c and 3c) – phenomena which was 
not observed for pulsed RF GDOES and TOF-SIMS. One 
possible explanation could be the ion-induced distortion 
of the interfacial region by Cs+ ions with high impact 
energy (5.8 keV) – the highest in our experiments. In 
principle, in CAMECA IMS7f this energy can be reduced 
to 1 keV or lower, however, it results in the decrease of 
the sputter rate and increase of the analysis time. 
The final crater depth in the magnetic SIMS was 
measured for both samples using an AMBIOS XP-1 
profilometer. Assuming a constant sputter rate, we ob-
tained the values of 5.71 and 6.88 nm/min for the PM596 
and PM615 samples, respectively. This is only average 
since for our samples the sputter rate of individual lay-
ers varies significantly from each to other. The B4C lay-
ers in the sample PM596 cause a decrease of the overall 
sputter rate as compared to the sample PM615. Using 
this data, we converted sputter time into depth of sput-
tering in Fig. 2c and Fig. 3c (upper X-axis). 
The initial surface roughness Ra of the both samples 
is found to be similar, within the range from 0.13 nm to 
0.22 nm. For the PM596 sample (with B4C barrier lay-
ers) after the depth profiling using magnetic SIMS, the 
average roughness increased by a factor of 1.7, and for 
the PM615 sample (without B4C barrier layers) the 
roughness increased 9 times. For the samples analyzed 
by TOF-SIMS using 1 keV Cs+ sputter ions, no signifi-
cant difference in the roughness before and after depth 
profiling was observed.  
In case of pulsed RF GDOES, we measured the 
roughness after sputtering of 30 periods since with com-
pleting depth profiling the bottom of crater was deep in 
Si substrate. The average roughness of both initially flat 
samples increased and reached 0.7 nm and 1.0 nm for 
the PM596 and PM615 samples, respectively. However, 
such little difference in our opinion cannot be considered 
the only root of the abovementioned difference between 
the depth profiles of these samples shown in Fig. 2a and 
Fig. 3a.  
 DEPTH PROFILING OF MULTIYEAR MO/SI NANOSTRUCTURES…  PROC. NAP 2, 01PCSI08 (2013) 
 
 
01PCSI08-3 
 
The pulsed RF GDOES and TOF-SIMS data show 
that the relative depth resolution (interface width) is 
better for the layers composed of light atoms (B and 
Si) than heavier one (Mo), especially for the 2d period, 
which is less contaminated. This confirms the 
influence of cascade collision anisotropy on the 
sputtering of multilayer stacks. One should take into 
account that the cascade anisotropy results in recoil 
implantation (Mo in Si and vice versa). Our rough 
estimate using the SRIM-2008.04 code [7] shows that 
0.3 nm-thick B4C layer can fully suppress such 
implantation in the case of the pulsed RF GDOES 
experiments. Besides, the analyzed samples may be 
heated by the intense bombardment from the plasma, 
and B4C layers can also suppress interlayer diffusion.   
Overall, the mechanism(s) of sputtering with 
ultra-low energy particles (ion and neutrals from the 
plasma) differs from the mechanism of isotropic linear 
cascade collisions that dominate at bombarding 
energies above 300-500 eV. This is one of the reasons 
why for the more energetic bombarding ions (1 keV 
and 5.8 keV Cs+) used in the TOF- and magnetic 
SIMS, respectively, this layer cannot serve as an anti-
mixing barrier to suppress broadening of the 
0 10 20 30 40 50 60 70
10
-1
1
10
10
2
Si
B
C
IN
T
E
N
S
IT
Y
 (
a
rb
. 
u
n
it
s
)
TIME of SPUTTERING (s) 
PM596
Mo
(a)
0 2 4 6 8 10 12
1
10
10
2
10
3
 Mo
+
 Si
+
 Cs
2
B
+
PM596
IN
T
E
N
S
IT
Y
 (
a
rb
. 
u
n
it
s
)
TIME of SPUTTERING (min) 
(b)
0 17.5 35.0 52.5 70.0
1
10
10
2
10
3
10
4
10
5
0 100 200 300 400
DEPTH of SPUTTERING (nm)
CsB
+
CsMo
+
IN
T
E
N
S
IT
Y
 (
c
o
u
n
ts
/s
)
TIME of SPUTTERING (min)
PM596
CsSi
+
(c)
0 1 2 3 4 5
0
2
4
 
CsB
+
CsMo
+
CsSi
+
0 5 10 15 20 25 30 35 40 45 50 55
10
-1
1
10
10
2
Si
PM615
IN
T
E
N
S
IT
Y
 (
a
rb
. 
u
n
it
s
)
TIME of SPUTTERING (s) 
Mo
(a)
0 2 4 6 8 10 12
1
10
10
2
10
3
(b)
 Mo
+
 Si
+
PM615
IN
T
E
N
S
IT
Y
 (
a
rb
. 
u
n
it
s
)
TIME of SPUTTERING (min) 
0 14.5 29.1 43.6 58.1 72.7
1
10
10
2
10
3
10
4
10
5
0 100 200 300 400 500
DEPTH of SPUTTERING (nm)
CsSi
+
CsMo
+
IN
T
E
N
S
IT
Y
 (
c
o
u
n
ts
/s
)
TIME of SPUTTERING (min)
PM615 (c)
0 1 2 3 4 5
0
2
4
CsSi
+
CsMo
+
Fig. 2 − Depth profiles of the PM596 sample by pulsed RF 
GDOES (a), TOF-SIMS (b) and magnetic SIMS (c) 
Fig. 3 − Depth profiles of the PM615 sample by pulsed RF 
GDOES (a), TOF-SIMS (b) and magnetic SIMS (c) 
 A. TOLSTOGUZOV, B. BER, P. CHAPON PROC. NAP 2, 01PCSI08 (2013) 
 
 
01PCSI08-4 
interfaces with deeper penetration into the sample 
structure. 
 
4. CONCLUSIONS 
 
Comparative sputter depth profiling of the periodic 
nanometer-thick Mo/B4C/Si and Mo/Si multilayers has 
been carried out at three laboratories using two differ-
ent commercial SIMS instruments and a pulsed RF 
GDOES. The pros and cons of each instrumental ap-
proach can be summarized as follows: 
(1) The modified glow discharge optical emission 
spectrometer GD-Profiler 2 from Horiba Jobin Yvon 
with pulsed RF operation provides a rapid (less than 
minute for the complete profiling) and sensitive analy-
sis. The fully resolved signal oscillations with approxi-
mately constant relative peak width were obtained for 
Mo/B4C/Si structure (the PM596 sample); in case of 
Mo/Si stack (the PM615 sample) damped profiles were 
observed. In our opinion, B4C layers act as a barrier for 
the ballistic atomic mixing and ion-simulated interlay-
er diffusion, suppressing progressive broadening of the 
interfaces with deeper penetration into the PM596 
sample.  
(2) Using a TOF.SIMS-5 by IONTOF with 1 keV Cs+ 
for sputtering and 25 keV Bi3+ for analysis, the best 
modulation for the surface peaks was obtained. Howev-
er, the complete depth profiling of both samples was 
not carried out; only the first 10 periods were analyzed, 
and no significant difference between the data of the 
PM596 (with B4C layers) and PM615 samples was 
observed. Since the modulation and relative layer 
thickness were becoming worse with deeper penetra-
tion into the samples structure, their final values (close 
to Si substrate) remain unknown.  
(3) A Cameca IMS7f with 5.8 keV Cs+ primary ions 
took approximately 70 minutes for the completion of 
the depth profiling of each sample. All peaks in the 
profiles of cesiated secondary ions were resolved. How-
ever, the peaks pertaining to different layers were 
overlapped, and in the case of SiCs+ were split. It is 
evident that the Cs+ impact energy should be lowered 
in order to minimize ion-induced atomic mixing.  
 
ACKNOWLEDGEMENTS 
 
This work was partly supported by: (i) the Russian 
Fund for Basic Research and Agency for Education of 
Russian Federation (Rosobrazovanie), (ii) the Center of 
Shared Scientific Equipment “Material Science & 
Characterization for Advanced Technologies” and the 
Ministry of Education and Science of Russian Federa-
tion (contract No. 16.552.11.7002), (iv) the Portugal 
Fundação para a Ciência e a Technologia (FCT) via the 
Strategic Project UI 68-2011-2012 (PEst-
OE/FIS/UI0068/2011), and (v) the Ministry of Educa-
tion and Science of Russian Federation (contracts No. 
14.132.21.1713 and 14.B37.21.0895). 
 
 
REFERENCES 
 
 
1. R. Escobar Galindo, R. Gado, D. Duday, C. Palacio, Anal. 
Bioanal. Chem. 369, 2725 (2010). 
2. A.B. Tolstoguzov, J. Anal. Chem. 65, 1370 (2010).   
3. S.S. Andreev, S.V. Gaponov, S.A. Gusev, M.N. Haidl, E.B. 
Kluenkov, K.A. Prokhorov, N.I. Polushkin, E.N. Sadova, 
N.N. Salashchenko, L.A. Suslov, S.Yu. Zuev, Thin Solid 
Films 415, 123 (2002). 
4. P. Belenguer, M. Ganciu, P. Guillot, T. Nelis, Spectrochem. 
Acta B 64, 623 (2009). 
5. L. Lobo, J. Pisonero, N. Bordel, R. Pereiro, A. Tempez, P. 
Chapon, J. Michler, M. Hohl, A. Sanz-Medel, J. Anal. At. 
Spectrom. 24, 1373 (2009). 
6. M.N. Drozdov, Yu.N. Drozdov, M.M. Barysheva, V.N. 
Polkovnikov, N.I. Chkhalo, J. Surf. Investigation. X-ray, 
Synchrotron and Neutron Techn. No.5, 807 (2010). 
7. Available (2013): http://www.srim.org 
 


English

Depth Profiling of Multilayer Mo/Si Nanostructures

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 2 No 1, 01PCSI08 (4pp) (2013)   2304-1862/2013/2(1) 01PCSI08 (4) 01PCSI08-1  2013 Sumy State University Depth Profiling of Multilayer Mo/Si Nanostructures  A. Tolstoguzov1,*, B. Ber2,†, P. Chapon3,‡, M.N. Drozdov4,§  1 Centre for Physics and Technological Research (CeFITec), Dept. de Física da Faculdade de Ciências e Tecnologia,  Universidade Nova de Lisboa, 2829-516 Caparica, Portugal 2 Ioffe Physical-Technical Institute, Polytechnicheskaya 26, 194021 Saint Petersburg, Russian Federation 3 HORIBA Scientific, 91165 Longjumeau, France 4 Institute for Physics of Microstructures of the Russian Academy of Science (IPM RAS), 603950 Nizhny Novgorod, Russian Federation  (Received 18 May 2013; published online 30 August 2013)  A round-robin characterization is reported on the sputter depth profiling of [60(3.0 nm Mo/ 0.3 nm B4C/ 3.7 nm Si)] and [60   (3.5 nm Mo/ 3.5 nm Si)] stacks deposited on Si (111). Two different commercial secondary ion mass spectrometers with time-of-flight and magnetic-sector analyzers and a pulsed radio frequency glow discharge optical emission spectrometer were used. The pros and cons of each instrumental approach are discussed.   Keywords: Sputter Depth Profiling, Glow Discharge Optical Emission Spectroscopy (GDOES), Mo/Si In-terferential Mirror, Round-robin Characterization, Secondary Ion Mass Spectrometry (SIMS).   PACS numbers: 68.49.Sf, 81.70.Jb                                                           * a.tolstoguzov@fct.unl.pt † boris.ber@mail.ioffe.ru ‡ patrick.chapon@horiba.com § drm@ipm.sci-nnov.ru 1. INTRODUCTION  Sputter depth profiling is a powerful tool for compo-sitional analysis of nanometer-thick multilayer struc-tures. In recent years, evident progress has been made in this field, especially regarding secondary ion mass spectrometry (SIMS) and glow discharge optical emis-sion spectroscopy (GDOES) [1].  The present work is a continuation and further de-velopment of our previous round-robin study [2]: we have analyzed Mo/Si and Mo/B4C/Si stacks. From a practical point of view, Mo/Si interferential mirrors are of considerable interest for high resolution X-ray li-thography. The introduction of B4C layers suppresses interlayer diffusion, which worsens the reflectivity and thermal stability of mirrors. Two commercial SIMS instruments – CAMECA IMS7f by Cameca SAS at Ioffe Physical-Technical In-stitute and TOF.SIMS-5 by ION-TOF at Institute for Physics of Microstructures, and a modified GD-Profiler 2 at Horiba Scientific were involved in our inter-laboratory experiments. This study is driven by neces-sity to perform accurate and rapid analysis of nanome-ter-thick periodic multilayers in order to optimize and improve their production technology.  2. EXPERIMENTAL DETAILS  2.1 Multilayer samples  The investigated samples are commercially availa-ble multilayer interferential mirrors produced by mag-netron sputtering at IPM RAS [3].  The samples are ca. 420 nm-thick nanostructures consisting of 60 layer period superlattices - Mo/B4C/Si in the case of the PM596 sample and Mo/Si for the PM615 sample (Fig. 1). In both cases, each period is 7.00  0.05 nm thick. The dimensions of individual Mo and Si layers are estimated to be 3.0  0.2 nm (Mo) and 3.7 ± 0.2 nm (Si) for the PM596, and 3.5  0.1 nm (Mo) and 3.5  0.1 nm (Si) for the PM615. The B4C barrier layer is a single atomic layer with a nominal width of ca. 0.3 nm. This technological parameter is estimated via the deposition and shutting rates of the magnetron sputtering processing.    Fig. 1 − Schematic view of the structure of the sample under study  The starting layer put directly onto the substrate is Mo, and the top surface layer is Si. The B4C layer is deposited onto the Si layer in every period of the sam-ple PM596. The substrates are 0.4 mm-thick commer-cial Si (111) wafers with an average surface roughness of 0.3 nm.  2.2 Instruments  Pulsed radiofrequency (RF) mode is used to power the glow discharge optical emission spectrometer GD- A. TOLSTOGUZOV, B. BER, P. CHAPON PROC. NAP 2, 01PCSI08 (2013)   01PCSI08-2 Profiler 2 [4, 5]. In GDOES measurements, samples are etched by Ar+ ions and accelerated neutrals with very low kinetic energies; the sputtered atoms are then ex-cited by the plasma and de-excited via photons emission with characteristic wavelengths, enabling their ele-mental identification. In our experiments, the operating conditions were: argon pressure of 550 Pa, 17 W RF power, 5 kHz pulse frequency with 0.25 duty cycle. The sputtered area in GD is the largest in comparison to other techniques: here a disk of 4 mm in diameter, and the useful signals are collected from the whole sputter area. Time-of-flight secondary ion mass spectrometer TOF.SIMS-5 works in the dual beam mode using 25 keV Bi3+ ions for analysis and 1 keV Cs+ ions for sputtering. The angle of incidence was 45° for both ion beams. The sputter beam was scanned over an area of 250  250 μm2, and the analyzed region was ca. 4% of the sputter area. Detailed description of the experi-mental conditions can be found in [6].  Magnetic-sector secondary ion mass spectrometer CAMECA IMS7f employs Cs+ primary ions with an impact energy of 5.8 keV and an incidence angle of 32° relative to the surface normal (the accelerating and sample potentials were 7 keV and 1.2 keV, respectively) while the positive secondary ions are detected. The primary beam was scanned over a 250  250 μm2 area, and the secondary ions were collected from a central region of 60 μm in diameter. The “energy window” of the spectrometer was fully opened (130 eV), and the mass resolution was M/ΔM=400. The crater depths were measured by an AMBIOS XP-1 surface stylus pro-filometer by Ambios Technology. Atomic force microscopy (AFM) was used to measure the initial surface roughness of the samples and the roughness at the bottom of the craters after sputter depth profiling. Different instruments were involved in this study: a Dimension 3100 by VEECO in tapping mode for the magnetic SIMS, a SOLVER by NT-MDT in contact mode for the TOF-SIMS, and a NT-MDT NTe-gra Prima in contact mode for the samples analyzed by pulsed RF GDOES.  3. RESULTS AND DISCUSSION  Raw depth profiles, in a semi-logarithmic scale, ob-tained by pulsed RF GDOES and SIMS are shown in Fig. 2 for the sample PM596 and in Fig. 3 for the sample PM615. In the case of pulsed RF GDOES (Fig. 2a and Fig. 3a), the bombarding ions were Ar+ with mean im-pact energy of 50 eV. We measured the intensity of the light emission of excited sputtered atoms at wavelengths of 381.411 nm (Mo), 251.611 nm (Si), 249.678 nm (B), and 156.144 nm (C).  For the PM596 sample (Fig. 2a), the number of fully-resolved periods was estimated to be 59. With deeper penetration into the sample structure only the depth profile of carbon exhibits a continuous decrease in inten-sity, however without decay of the signal oscillations. In our opinion this profile can be considered as a result of two contributions - one of them relating to carbon origi-nating from the B4C barrier layers and the other one from contamination. For the PM615 sample (without the B4C barrier layers), an evident degradation of both Mo and Si profiles versus the sputter time (the depth of sputtering) is observed (Fig. 3a).  For the TOF-SIMS, only the first 10 periods of the structures were analyzed (Fig. 2b and Fig. 3b). The depth distributions of positively charged atomic Mo+ (sum of all isotopes), Cs211B+ and 28Si+ secondary ions are shown in Fig. 2b (PM596) and Fig. 3b (PM615). As com-pared with pulse RF GDOES, the introduction of B4C layers does not induce significant difference between the measured profiles.  For CAMECA IMS7f with a magnetic sector mass analyzer (hereinafter this instrument will be called the magnetic SIMS or M-SIMS) we chose molecular ions MCs+ (where M is B, Si or Mo) as the characteristic ions. In spite of the fact that the mechanisms of secondary ion formation under Cs+ bombardment and deposition are still debated, Cs-based SIMS allows for the minimization of matrix effects, which greatly influence the secondary ion yields. In our case, non-degraded profiles were ob-tained for both samples with 58 resolved periods (Fig. 2c and Fig. 3c). No significant difference between the data of the PM596 and PM615 samples was observed. How-ever, the signals of Si and Mo within each period over-lapped, and in case of SiCs+ were split into two parts (see the inserts in Fig. 2c and 3c) – phenomena which was not observed for pulsed RF GDOES and TOF-SIMS. One possible explanation could be the ion-induced distortion of the interfacial region by Cs+ ions with high impact energy (5.8 keV) – the highest in our experiments. In principle, in CAMECA IMS7f this energy can be reduced to 1 keV or lower, however, it results in the decrease of the sputter rate and increase of the analysis time. The final crater depth in the magnetic SIMS was measured for both samples using an AMBIOS XP-1 profilometer. Assuming a constant sputter rate, we ob-tained the values of 5.71 and 6.88 nm/min for the PM596 and PM615 samples, respectively. This is only average since for our samples the sputter rate of individual lay-ers varies significantly from each to other. The B4C lay-ers in the sample PM596 cause a decrease of the overall sputter rate as compared to the sample PM615. Using this data, we converted sputter time into depth of sput-tering in Fig. 2c and Fig. 3c (upper X-axis). The initial surface roughness Ra of the both samples is found to be similar, within the range from 0.13 nm to 0.22 nm. For the PM596 sample (with B4C barrier lay-ers) after the depth profiling using magnetic SIMS, the average roughness increased by a factor of 1.7, and for the PM615 sample (without B4C barrier layers) the roughness increased 9 times. For the samples analyzed by TOF-SIMS using 1 keV Cs+ sputter ions, no signifi-cant difference in the roughness before and after depth profiling was observed.  In case of pulsed RF GDOES, we measured the roughness after sputtering of 30 periods since with com-pleting depth profiling the bottom of crater was deep in Si substrate. The average roughness of both initially flat samples increased and reached 0.7 nm and 1.0 nm for the PM596 and PM615 samples, respectively. However, such little difference in our opinion cannot be considered the only root of the abovementioned difference between the depth profiles of these samples shown in Fig. 2a and Fig. 3a.   DEPTH PROFILING OF MULTIYEAR MO/SI NANOSTRUCTURES…  PROC. NAP 2, 01PCSI08 (2013)   01PCSI08-3  The pulsed RF GDOES and TOF-SIMS data show that the relative depth resolution (interface width) is better for the layers composed of light atoms (B and Si) than heavier one (Mo), especially for the 2d period, which is less contaminated. This confirms the influence of cascade collision anisotropy on the sputtering of multilayer stacks. One should take into account that the cascade anisotropy results in recoil implantation (Mo in Si and vice versa). Our rough estimate using the SRIM-2008.04 code [7] shows that 0.3 nm-thick B4C layer can fully suppress such implantation in the case of the pulsed RF GDOES experiments. Besides, the analyzed samples may be heated by the intense bombardment from the plasma, and B4C layers can also suppress interlayer diffusion.   Overall, the mechanism(s) of sputtering with ultra-low energy particles (ion and neutrals from the plasma) differs from the mechanism of isotropic linear cascade collisions that dominate at bombarding energies above 300-500 eV. This is one of the reasons why for the more energetic bombarding ions (1 keV and 5.8 keV Cs+) used in the TOF- and magnetic SIMS, respectively, this layer cannot serve as an anti-mixing barrier to suppress broadening of the 0 10 20 30 40 50 60 7010-1110102SiBCINTENSITY (arb. units)TIME of SPUTTERING (s) PM596Mo(a)0 2 4 6 8 10 12110102103 Mo+ Si+ Cs2B+PM596INTENSITY (arb. units)TIME of SPUTTERING (min) (b)0 17.5 35.0 52.5 70.01101021031041050 100 200 300 400DEPTH of SPUTTERING (nm)CsB+CsMo+INTENSITY (counts/s)TIME of SPUTTERING (min)PM596CsSi+(c)0 1 2 3 4 5024 CsB+CsMo+CsSi+0 5 10 15 20 25 30 35 40 45 50 5510-1110102SiPM615INTENSITY (arb. units)TIME of SPUTTERING (s) Mo(a)0 2 4 6 8 10 12110102103(b) Mo+ Si+PM615INTENSITY (arb. units)TIME of SPUTTERING (min) 0 14.5 29.1 43.6 58.1 72.71101021031041050 100 200 300 400 500DEPTH of SPUTTERING (nm)CsSi+CsMo+INTENSITY (counts/s)TIME of SPUTTERING (min)PM615 (c)0 1 2 3 4 5024CsSi+CsMo+Fig. 2 − Depth profiles of the PM596 sample by pulsed RF GDOES (a), TOF-SIMS (b) and magnetic SIMS (c) Fig. 3 − Depth profiles of the PM615 sample by pulsed RF GDOES (a), TOF-SIMS (b) and magnetic SIMS (c)  A. TOLSTOGUZOV, B. BER, P. CHAPON PROC. NAP 2, 01PCSI08 (2013)   01PCSI08-4 interfaces with deeper penetration into the sample structure.  4. CONCLUSIONS  Comparative sputter depth profiling of the periodic nanometer-thick Mo/B4C/Si and Mo/Si multilayers has been carried out at three laboratories using two differ-ent commercial SIMS instruments and a pulsed RF GDOES. The pros and cons of each instrumental ap-proach can be summarized as follows: (1) The modified glow discharge optical emission spectrometer GD-Profiler 2 from Horiba Jobin Yvon with pulsed RF operation provides a rapid (less than minute for the complete profiling) and sensitive analy-sis. The fully resolved signal oscillations with approxi-mately constant relative peak width were obtained for Mo/B4C/Si structure (the PM596 sample); in case of Mo/Si stack (the PM615 sample) damped profiles were observed. In our opinion, B4C layers act as a barrier for the ballistic atomic mixing and ion-simulated interlay-er diffusion, suppressing progressive broadening of the interfaces with deeper penetration into the PM596 sample.  (2) Using a TOF.SIMS-5 by IONTOF with 1 keV Cs+ for sputtering and 25 keV Bi3+ for analysis, the best modulation for the surface peaks was obtained. Howev-er, the complete depth profiling of both samples was not carried out; only the first 10 periods were analyzed, and no significant difference between the data of the PM596 (with B4C layers) and PM615 samples was observed. Since the modulation and relative layer thickness were becoming worse with deeper penetra-tion into the samples structure, their final values (close to Si substrate) remain unknown.  (3) A Cameca IMS7f with 5.8 keV Cs+ primary ions took approximately 70 minutes for the completion of the depth profiling of each sample. All peaks in the profiles of cesiated secondary ions were resolved. How-ever, the peaks pertaining to different layers were overlapped, and in the case of SiCs+ were split. It is evident that the Cs+ impact energy should be lowered in order to minimize ion-induced atomic mixing.   ACKNOWLEDGEMENTS  This work was partly supported by: (i) the Russian Fund for Basic Research and Agency for Education of Russian Federation (Rosobrazovanie), (ii) the Center of Shared Scientific Equipment “Material Science & Characterization for Advanced Technologies” and the Ministry of Education and Science of Russian Federa-tion (contract No. 16.552.11.7002), (iv) the Portugal Fundação para a Ciência e a Technologia (FCT) via the Strategic Project UI 68-2011-2012 (PEst-OE/FIS/UI0068/2011), and (v) the Ministry of Educa-tion and Science of Russian Federation (contracts No. 14.132.21.1713 and 14.B37.21.0895).   REFERENCES   1. R. Escobar Galindo, R. Gado, D. Duday, C. Palacio, Anal. Bioanal. Chem. 369, 2725 (2010). 2. A.B. Tolstoguzov, J. Anal. Chem. 65, 1370 (2010).   3. S.S. Andreev, S.V. Gaponov, S.A. Gusev, M.N. Haidl, E.B. Kluenkov, K.A. Prokhorov, N.I. Polushkin, E.N. Sadova, N.N. Salashchenko, L.A. Suslov, S.Yu. Zuev, Thin Solid Films 415, 123 (2002). 4. P. Belenguer, M. Ganciu, P. Guillot, T. Nelis, Spectrochem. Acta B 64, 623 (2009). 5. L. Lobo, J. Pisonero, N. Bordel, R. Pereiro, A. Tempez, P. Chapon, J. Michler, M. Hohl, A. Sanz-Medel, J. Anal. At. Spectrom. 24, 1373 (2009). 6. M.N. Drozdov, Yu.N. Drozdov, M.M. Barysheva, V.N. Polkovnikov, N.I. Chkhalo, J. Surf. Investigation. X-ray, Synchrotron and Neutron Techn. No.5, 807 (2010). 7. Available (2013): http://www.srim.org  

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Depth Profiling of Multilayer Mo/Si Nanostructures

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