Correction of secondary ion mass spectrometry profiles for atom
diffusion measurements
A. Portavoce1,2*, N. Rodriguez1,3, R. Daineche1, C. Grosjean3, C. Girardeaux1
1
Aix-Marseille Université, IM2NP
CNRS, IM2NP, Av. Escadrille Normandie Niemen - Case 142, F-13397 Marseille, France
3
STMicroelectronics, RCCAL, Rousset, France
2
*
Corresponding author. Tel.: +33 (0) 4 91 28 28 04; fax: + 33 (0) 4 91 28 87 75.
E-mail address: alain.portavoce@im2np.fr
Abstract
A simple method is proposed in order to correct experimental secondary ion
mass spectrometry (SIMS) profiles. This method uses only one parameter which is
experimentally accessible. It is tested in the case of As SIMS profiles in Si(001)
acquired with three different primary ion beam energies: 1, 3, and 9 keV. This method
is shown to give consistent corrections. The correction of SIMS profiles measured in a
same sample with different analyze conditions leads to the same As distribution.
Keywords: Characterization methods, SIMS, Diffusion, Simulation, Si(001), As
1. Introduction
The ability to produce sharp and well localized pn-junctions is of major
importance for the fabrication of Si-based microelectronic devices. The dopant
distributions before and after thermal treatments are usually experimentally studied via
secondary ion mass spectrometry (SIMS), since the doping levels are generally less than
1 at %. Typically, the parameters driving dopant redistribution during the process are
determined fitting experimental SIMS profiles using highly elaborated simulators [1].
Consequently, the accuracy of the fitted parameters is highly dependent on the quality
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of the SIMS measurements. Despite that the SIMS technique gives accurate
measurements of doping levels, it is also known to provide distorted profiles due to
mixing effects during ion beam bombardment [2-7]. The slope (or decay length)
observed in the descending parts of SIMS profiles (SIMS) is not the real one but the
combination of the real slope (r) and a mixing-induced slope (m) [8]. This is why the
coefficients of diffusion are preferably determined using the broadening of the
ascending part of the SIMS profiles [9]. Nevertheless, using the model developed by
Zalm and Vriezema [10], Petravić et al.[11] have shown that m can be predicted.
Knowing this length, which is usually given in nanometer (or angstrom) per decade of
concentration in logarithmic scale, diffusion coefficients can be determined with
accuracy using the descending part of SIMS profiles, if m is negligible compared to the
diffusion length of atoms during the experiment. In this case, for symmetric initial
dopant distributions, the diffusion profile measured by SIMS after annealing is
symmetric [12]; otherwise the SIMS profile is not symmetric and the descending part of
the profile cannot be fitted using the same diffusion coefficient as for the ascending part
[13]. Today, since most of dopant implantations are done in the vicinity of the surface,
the ascending part of diffusion profiles cannot be used to determine the diffusion
coefficients, and as the size of microelectronic structures are becoming increasingly
smaller, the experimental observations correspond to short diffusion length. In this case
SIMS profiles cannot be used to accurately determine the simulator parameters, as the
descending slope of the SIMS profiles has no real meaning and varies with the energy
(E) and the impact angle () of the incident ion beam (m ~ E  cos) [10-11]. So far,
the solution used to minimize this problem has been to perform SIMS analyses at the
lowest energy possible [14-16]. For example, SIMS measurements using a beam energy
of 1 keV were shown to provide excellent results [17]. However, performing SIMS
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measurements at low beam energy is technically challenging and causes other
undesirable effects. For example, the SIMS detection limit can be reduced to 1-2
order(s) of magnitude using a 1 keV beam compared to a 3 keV beam. In this letter we
propose a simple way of correcting the SIMS profiles in order to perform accurate
diffusion coefficient measurements using the descending parts of the concentration
profiles. The benefits of this method are i) it is based on a single parameter that can be
measured experimentally, and ii) it is based on laws which have been validated
experimentally.
The slope of descending parts of SIMS profiles can be calculated using the
relation [8]: 2SIMS  2r  2m
(1)
m has been shown to be constant for a given element in a given matrix, for given SIMS
conditions [10-11]. Using Eq. (1), the real slope r can be determined from SIMS
measurements if m is known. Petravić et al. [11] have shown that m can be calculated,
but it can be also measured experimentally. For this purpose, a reference sample is
needed in which the dopant distribution is characterized by an abrupt interface. For
example, a layer of dopant deposited on a Si substrate at room temperature is a good
reference. Indeed, in this case no dopant is contained in the Si bulk. Nevertheless, if
SIMS measurements are performed in this type of sample, a dopant decay length
starting from the sample surface is observed in the Si bulk. This slope is due to the
bombardment-mixing effect and can be measured (after concentration calibration, in the
concentration range lower than 1% to prevent matrix effects): this is m. We propose to
correct dopant SIMS profiles, correcting the descending parts of the profiles using Eq.
(1) and knowing the experimental value of m.
2. Experimental
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Two samples were built. They consist of a Si(001) wafer implanted with As
atoms using an implantation energy of 30 keV. The As dose was 1.75  1014 and 1.75 
1015 at cm2 in the first and second samples, respectively. SIMS measurements were
performed using a CAMECA IMS-7f system operated at 1, 3, or 9 keV with a Cs+
primary ion beam. At 1 keV AsCs2+ ions were detected with applied voltages on the ion
source (Vi) and the sample (Vs) equal to 2 and 1 kV, respectively. At 3 and 9 keV AsSi
ions were detected with Vi = 2 kV and Vs = 1 kV, and Vi = 6 kV and Vs = 3 kV,
respectively. These conditions give an impact angle of ~45° at 1 keV, and ~23.5° at 3
and 9 keV. The mixing effect depends on both E and , but in our experimental
conditions the mixing effect is lower for the lower energy conditions, thus for
convenience we decided to use in the text the energy only to differentiate the three
SIMS settings. The experimental SIMS profiles were corrected using a homemade
program written in the FORTRAN language. This program calculates the local slope in
the vicinity of each point of the SIMS profile and corrects it using Eq. (1).
3. Results and discussion
In order to judge the relevance of the correction three facts can be considered: i)
lower is the primary beam energy, and closer are the real profile and the SIMS profile.
Consequently, the higher the SIMS beam energy is, and greater should be the
correction. ii) The integration of the real profile should be equal to the As implanted
dose (SIMS profile concentration calibrations were performed using a reference sample
in which the concentration was known, and not using the knowledge of the dose).
Indeed, since the ionization and mixing steady-state-conditions during SIMS are
reached for greater depth at higher impact energies, the integration of SIMS profiles
measured in the same sample but for different beam energy gives different values. iii)
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The correction of SIMS profiles measured in a same sample using different SIMS
conditions should give the same profile (ideally the real one).
FIG. 1. Comparison between as-measured (solid
symbols) and corrected (open symbols) SIMS
profiles obtained in a same sample, using a beam
energy of (a) 1 keV, and (b) 9 keV.
FIG. 2. Comparison between corrected SIMS
profiles measured in a same sample using 3
different energies: 1 (open circles), 3 (open
squares) and 9 keV (solid squares). (a) As dose of
1.75  1014 at cm2, and (b) As dose of 1.75 
1015 at cm2.
The measured values of m in our As reference sample gave for our SIMS
measurement conditions m ~ 3.6, 4.5, and 8.0 nm /decade for beam energies of 1, 3,
and 9 keV, respectively. Fig. 1 presents the SIMS profiles measured using a beam
energy of 1 and 9 keV in the same as-implanted sample with a dose of 1.75  1015 As at
cm2, as well as their corresponding corrected profiles. The amplitude of the correction
was found to increase with the incident beam energy. As expected, the SIMS profiles
that need the minimum correction are the profiles obtained using the energy beam of 1
keV. The integrals of the SIMS profiles measured in the as-implanted samples and in
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their corresponding corrected profiles are presented in Tab. I. The percentage errors of
the doses measured using the profiles with and without correction are given compared
to the predicted implanted dose. In general, for both types of profiles with and without
correction, the error on the dose is smaller for small primary beam energies, and the
error on the As dose is smaller after correction.
TAB. I. Integrated doses measured using the SIMS profiles before and after correction. The percentage
error of the measured doses is given compared to the predicted implanted dose.
Nevertheless, there are a few exceptions. This can be explained by the fact that the
ascending and the flat parts of the profiles are not corrected. Thus the errors made on
these parts of the SIMS profiles will subsist after correction. However, the deviation of
the dose error between the profiles obtained for the three energies 1, 3 and 9 keV is ~ 2
smaller after correction, showing that the integrated doses measured after correction are
better converging. Fig. 2 shows all the corrected profiles obtained for the two samples.
For each sample the corrected profiles are quasi-superimposed. This clearly shows that
the correction leads to the same profile independently of the SIMS measurement
conditions. Finally, Fig. 3 presents SIMS measurements obtained in the sample
implanted with an As dose of 1.75  1015 at cm2 and annealed at 950 °C for 300 s,
using an incident beam energy of 1 and 9 keV. The corrected profiles are presented in
the same figure. The descending parts of the as-measured profiles are different.
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Consequently, the measurement of the diffusion length of As atoms yields different
values depending on the profile. In contrast, the corrected profiles are superimposed.
The error on the diffusion depth of As at a concentration of ~ 1018 at cm3 is ~ 13% and
27% on the SIMS profiles measured at 1 and 9 keV, respectively, compared to the
corrected profiles. The diffusion depth is at least 10 nm deeper in the experimental
profiles without corrections. The larger difference between measured and corrected
profiles at 1 keV on Fig. 3 compared to Fig. 1 is due to the stronger curvature of the
profiles on Fig. 3, because of the non-Fickian diffusion of As.
FIG. 3. As-measured (solid symbols) and corrected (open symbols) SIMS profiles measured in a same
sample using two different energies: 1 (square) and 9 keV (circle). After implantation the sample has been
annealed under N2 gas at 950 °C for 300 s.
4. Conclusion
In order to accurately measure diffusion coefficients using the descending part of
SIMS profiles, a method allowing for the correction of SIMS profiles is proposed. This
method, based on the measurement of a single parameter, is shown to provide consistent
corrections. The correction of SIMS profiles obtained in a same sample via different
measurement conditions gives a similar profile. This type of correction is shown to be
useful for the measurement of junction depth if a precision better than 10 nm is
required.
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