BEHAVIOUR OF LOW DOSE ARSENIC IMPLANTS
IN SILICON
F. Cappellani, M. Henuset, G. Restelli
To cite this version:
F. Cappellani, M. Henuset, G. Restelli. BEHAVIOUR OF LOW DOSE ARSENIC IMPLANTS IN SILICON. Journal de Physique Colloques, 1973, 34 (C5), pp.C5-145-C5-149.
<10.1051/jphyscol:1973529>. <jpa-00215316>
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JOURNAL DE PHYSIQUE
Colloque C5, suppldment au no 11-12, Tome 34, Novembre-Ddcembre 1973,page C5-145
BEHAVIOUR OF LOW DOSE ARSENIC IMPLANTS IN SILICON
F. CAPPELLANI, M. HENUSET AND G . RESTELLI
Electronics
Joint Research Centre, Ispra, Italy
RBsumb. - Le comportement d'ions d'arsenic de 80 keV implantks ?
temperature
i
ambiante
dans des cristaux de silicium orientks ou non, en dessous de la dose d'amorphisation, a kt8 Btudik
en mesurant les profils physique et tlectrique suivant des recuits isochrones.
A partir de l'analyse du profil physique, les paramhtres de pknktration, Rp, AR, et Rmax pour des
ions canalisks le long de l'axe < 111 >, ont kte mesurks.
Aprhs un recuit isochrone d'kchantillons implantks hors canalisation, un coefficient de diffusion
effectif k 900 oC pour l'arsenic a ktt calculk Cgal k 5 x 10-16 cm2.s-1.
11 est montrk que pour des ions d'arsenic implant& dans le silicium hors canalisation, la fraction
klectriquement active, qui est d'environ 0,4 aprhs un recuit de 30 min a 600 OC, augmente continuellement jusqu'k ce qu'une activation Bectrique des ions arsenic de 100 % soit atteinte B environ 900 oC. Une tendance similaire est observke pour les implantations en canalisation < 111 >.
Le procCdt de recuit semble suivre une reaction cinktique du premier ordre avec une Cnergie
d'activation kgale a 0,4-0,5 eV.
I1 est donne un essai d'interprktation du mCcanisme impliquk.
Abstract. - The behaviour of 80 keV arsenic ions implanted at room temperature in oriented
and disoriented silicon crystals below the amorphization dose, has been studied by measuring
the physical profile and the electrical profile following isochronal anneals.
From the analysis of the physical profile, the penetration parameters, ED,
A& and R,,, for
ions channeled along < 111 > axis, have been measured.
After isochronal annealing of off-channeling implants, an effective diffusion coefficient at 900 oC
for arsenic equal to 5 x 10-16 cm2.s-1 has been calculated.
It is shown that for arsenic ions implanted in disoriented silicon, the electrically active fraction,
which is about 0.4 after 30 min annealing at 600 o C increases continuously until 100 % electrical
activation of the arsenic ions is reached at about 900 o C . Similar trend is observed for < 111 >
channeled implants. The annealing process appears to follow a first order reaction kinetics with
an activation energy equal to 0.4-0.5 eV. A tentative interpretation of the mechanism involved
is given.
1. Introduction. - Extensive investigations on the
behaviour of boron and phosphorous implants in
silicon have been reported ; much less attention has
been devoted to arsenic.
Previous studies on atom location and electrical
properties of low energy (20-40 keV) arsenic implants
have been published before 1970 ; the results can be
found in the book by Mayer, Eriksson and Davies [I].
Detailed studies on the electrical behaviour of 280 keV
arsenic implants in silicon have been published by
the IBM group [2]-[6] and data on the penetration
parameters of arsenic implants by Kleinfelder et
al, [7] and by E. Davies [8]-[9].
The annealing behaviour of arsenic implants in
silicon for doses below the amorphization limit has
been the subject of the present research. Since the
implantation energy around 100 keV has not been
investigated, the value of 80 keV was chosen. At this
energy the range of arsenic ions allows the determination, with sufficient precision, of the penetration
profile, using anodic sectioning techniques.
The determination of the total concentration distribution using radiotracer techniques has allowed
the evaluation of the penetration parameters : mean
projected range %, standard deviation of the projected range A%, and maximum range R,,, for As'
ions channeled along < 111 > axis.
An effective diffusion coefficient for arsenic at
9000 in off-channeling implants has been calculated
equal to 5 x 10-l6 cm2.s-l.
The number of electrical carriers has been calculated by integration of the carrier concentration profile obtained from differential sheet resistivity and
Hall effect measurement, together with anodic sectioning, after each isochronal annealing step. Attributing the carriers to substitutional uncompensated
arsenic atoms, the electrically active fraction of
implanted atoms which is about 0.4 after 30 min
annealing at 6000C increases continuously until
100 % electrical activation is reached at about 900 OC.
A similar trend is observed for ions implanted along
< 111 > axis in channeling conditions. In both
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973529
C5-146
F. CAPPELLANI, M. HENUSET AND G. RESTELLI
cases from an Arrhenius plot an activation energy
equal to 0.4-0.5 eV has been calculated.
2. Experimental procedures. - 1-100 a.cm P-type
silicon slices, cut normally to the < 111 > axis with
a precision of f 0.50, were implanted at room temperature at an energy of 80 keV using a 100 keV ion
accelerator [lo]. The dose was fixed at
neled fraction calculated by subtracting an area corresponding to a gaussian curve centered at % and
having a standard deviation equal to AX,, accounts
for 12-15 % of the implanted dose (Fig. 1).
below the critical value required to form an amorphous layer [Ill. The dose rate was in the range
0.1-0.3 ~ A . c m -to~ avoid heating effects ; secondary
electron suppression was provided.
For off-channeling implants, the specimens were
mounted with the beam direction 80 to the < 111 >
I
I
1
I
I
500
1000
1500
2000
axis.
DEPTH ( A )
Scanning of the specimen over a diameter of 20 mm
was provided using electrostatic deflection giving a FIG. 1. - Total concentration distribution curve of 80 keV
1.50 ; the critical arsenic ions implanted a t room temperature in disoriented
maximum beam divergence of
angle for 80 keV As ion channeling along < 111 > silicon crystals, after 30 min isochronal annealing at 500 and
900 OC respectively. Implanted dose 7 x lo13 ions.cm-2.
axis is about 3.50.
The silicon slices were chem-mechanically polished ;
a final cleaning procedure (HF dip, wash in methanol
The analysis of the total concentration distribution
and blow-dry with nitrogen) was provided immedia- of Asf ion implanted along < 111 > in channeling
tely before mounting the slices in the target chamber. conditions, shows a nearly exponential tail extending
Annealing in 30 min steps was carried out in a flow up to 4 000 f 200 A (Fig. 2). This value of R,,
of dry nitrogen.
is in fairly good agreement with that obtained extraThe total arsenic concentration distribution vs polating the data of Kleinfelder et aZ. [7].
penetration depth was obtained by radioactivation
in conjunction with layer removal by anodic oxidation and HF stripping using an automated
machine 1121, 1131.
Carrier concentration profiles were obtained by
differential sheet resistivity and Hall effect measurements. A Van der Pauw pattern was defined using
photomasking and mesa-etching ; this procedure was
found preferable to the direct implantation of the
pattern using mechanical collimators.
+
3. Results. - 3.1 PENETRATION
PARAMETERS.
From the physical profile, the mean projected range
500
1500
2500
3500
4500
for 80 keV arsenic ions in disoriented silicon has
DEPTH (A)
been found equal to 450 50 A with a standard
FIG. 2. - Total concentration distribution curve of 80 keV
deviation A% equal to 230 f 25 A.
arsenic ions implanted at room temperature along < 111 >
The mean projected range agrees with the value
axis in silicon (full line).
calculated according to the LSS theory [14], while Circles represent the carrier concentration profile after 30 min
at 550 OC anneal.
the standard deviation shows a large discrepancy with
respect to the calculated value of 169 A [14]. This
Implanted dose :7 x 1013 ions .cm-2.
Beam divergence : f 1.5".
situation is reversed if a comparison is made with
the experimental values of reference [6] :
&
+
i?,
=
600
+ 60 A and AX, = 250 + 25 A .
The range distribution profile fits a gaussian curve
up to Rp and then exhibits a nearly exponential
penetrating tail most probably due to channeling
effects favoured by the large critical angle, the beam
divergence and the low implantation dose. The chan-
3.2 ENHANCED
DIFFUSION. - Figure 1 shows the
physical profiles of 80 keV arsenic implants in disoriented silicon after isochronal annealing for 30' at
5000 and 900 0C respectively. The 5000 annealing
was required by the necessity to anneal out fast
neutron irradiation defects occurring during the
reactor activation of arsenic atoms. The agreement
BEHAVIOUR OF LOW DOSE ARSENIC IMPLANTS I N SILICON
in the projected range standard deviation with the
experimental value reported by Crowder [5], can be
used to exclude diffusion phenomena after a 500 OC
anneal. On the other hand, the broadening of the
distribution after 900 0C annealing is evident and
must be attributed to arsenic diffusion.
Because of the presence of a channeled fraction,
the calculation of the effective diffusion coefficient Dx
was performed using the gaussian portion of the
curve. A Dx value equal to 5 x 10-l6 cm2.s-I at
900 OC was calculated [15]. This value is only about
an order of magnitude higher with respect to the
diffusion coefficient obtained from extrapolation of
the values measured at higher temperatures in radiation damage free silicon [16]. From our results there
is no evidence of a temperature independent enhanced
diffusion coefficient as it has been observed for antimony implants [17]-[19]. Both these results seem to
support an hypothesis of absence of enhanced diffusion
in low dose arsenic implants.
3 . 3 ELECTRICALLY
ACTIVE FRACTION. - Figure 3
shows the sheet resistivity us anneal temperature of
RT 80 keV off-channeling arsenic implanted silicon
Iayers for different implantation doses of loL3, loi4
and lo1' i ~ n s . c m - ~The
. duration of the annealing
at each temperature was always 30 min. The measurements were performed using four point probe technique.
ANNEALING TEMPERATURE
C5-147
loi3 and 1014ions.cm-2 implants which are below the
amorphization dose, exhibit non saturating characteristics with a continuous increase of the layer
conductivity. This trend is in agreement with the
results obtained for 280 keV arsenic implants by
Crowder and Morehead [2].
In this experiment the attention was concentrated
on the annealing behaviour of layers implanted with
a dose (7 x 1013Asf .cm-') below the amorphization limit.
The determination of the number of carriers from
sheet conductivity and sheet Hall effect measurement
can be imprecise because of the effect of non-uniform
distribution of carrier density and mobility. Therefore the total number of carriers has been calculated
by integration of the carrier concentration proaes,
obtained from sheet resistivity and sheet Hall coefficient measurements in conjunction with sectioning
by anodic oxidation and stripping.
The mobility values obtained after 550 OC annealing
agree within 20 % with the data reported in
reference [20] for diffused silicon samples.
Assuming that the carriers are due to substitutional
uncompensated arsenic atoms, by comparing the
carrier concentration with the total arsenic concentration obtained from the physical profile, an electrically active fraction after isochronal annealing
steps has been calculated (Table I).
Figure 4 shows a set of carrier concentration profiles determined after 30 min annealing at 650, 700,
800 and 900 OC. After the 900 OC annealing the electrical and the physical profiles coincide.
Assuming a first order reaction mechanism for
the electrical activation of the arsenic atoms, an
Arrhenius plot has been drawn for the temperature
+
( "C )
FIG.3. - Sheet resistivity vs anneal temperature for room
temperature 80 keV arsenic implanted layers with doses of
1013, 1014 and 10'5 ions.cm-2 respectively. The duration of
each annealing was 30 min.
m
500
It is evident that the 10'' i o n ~ . c m -implant
~
shows
a predominant annealing step before 550 OC in correspondence with the reordering of the amorphous
silicon layer formed at these doses. On the contrary, the
1000
DEPTH
(A)
1500
2000
FIG. 4. - Set of carrier concentration vs penetration depth
profiles after 30 min isochronal annealing at 650, 700, 800 and
900 OC.
Electrically active fraction of implanted Asf .ions
0.38 f 10 % 0.46
+ 10 %
0.56
+ 10 %
0.78
+_
10 % 0.92
+_
10 %
C5-14 8
F. CAPPELLANI, M. HENUSET AND G. RESTELLI
range 600-900 OC. The points lie close to a straight
line and from the gradient an activation energy
AE equal to 0.4 eV has been calculated [21], [22].
The same analysis of the data obtained from the
comparison of the physical profile and the electrical
profile after isochronal annealing steps, in the same
temperature range, has been performed according
to Ishino et al. [21] for the samples implanted in
channeling conditions along < 111 >. Also in this
case the data fit a first order reaction mechanism with
an activation energy calculated from the Arrhenius
plot equal to 0.5 eV.
4. Discussion. - As an implanted ion slows down
and comes to rest in a crystal, it suffers collisions
with lattice atoms which result in the production of
highly disordered regions around the path of each
ion. At sufficiently high doses a layer is formed which
is characterized by absence of long range order
(amorphous layer). In silicon this amorphous layer
anneals by epitaxial reordering on the underlying
crystalline material at temperatures between 550 and
6500C. The presence of the amorphous phase has
a pronounced effect on the anneal characteristics of
ion implanted layers as observed by electrical measurements.
When the implantation dose is such that an amorphous layer is not formed, the correlation between
disorder anneal and electrical characteristics is not
observed and the achievement of electrical activity
of the implanted ions requires temperatures as high
as 1 000 OC as well known for many dopants and
demonstrated by these results in the case of arsenic.
The temperature range 600-1 000 OC is well above
any identified primary defect anneal range [23]-[25].
Above 600 OC in fact, only secondary defects such
as dislocation loops, stacking faults and dislocation
networks have been observed.
Annealing in the temperature range of 600-1 000 OC
has been investigated in low dose room temperature
arsenic implants by EPR studies indicating an increase
in substitutionality of arsenic atoms [26].
Measurements by channeling technique, previously
performed by Eriksson et al. [27] have been recently
revised by Haskell et al. [28] because of the ascertainment of a beam induced off lattice motion of arsenic
atoms. However, no conclusive results can be derived
from atom location measurements since substitutional
atoms can be electrically inactive due to the presence
of compensating defects.
An interpretation of the physical process governing
the annealing can be inferred from the activation
energy AE obtained from the Arrhenius plot. Such
an analysis has been performed e. g. for boron and
phosphorous implants [29]-[31].
Ishino et al. [21] have indicated how the activation
energy AE and order of reaction may be determined
from isochronal studies. Using their treatment the
experimental data fit a first order reaction of the
type Dl1
Ci-I = k , t exp - E
In ci
k%
where
Ci-,and Ci are the defect concentrations before and
after the i th anneal of duration f ,
is the temperature of the i th anneal,
Ti
is the rate constant for the reaction,
ko
AE
is the activation energy of the process
which comes out equal to 0.4-0.5 eV
(as reported in the preceeding paragraph).
A second calculation of the activation energy of the
process can be performed following the treatment
outlined in the paper by Gusev and Titov [29].
In the approximation given by these authors the
following equation holds
AH
Ace C,(T) = B exp - kT
where
Ce(T) = concentration of electrically active defects
at temperature T,
=
change
in the concentration of electrically
AC,
active defects in a time t,
B
= quantity which is constant for each annealing
stage (for constant short annealing duration),
AH = annealing activation energy.
-
From the Arrhenius plot AH comes out to be equal
to 0.25 eV. However in this second case the interpretation of AH follows from the mechanism assumed
for the physical process.
Infact, the creation of an electrically active arsenic
atom (assuming that the carriers are due to substitutional uncompensated arsenic) can result from two
possible processes :
a) the dissociation of a complex defect ;
b) the combination of two defects during an encounter.
In the case that the process is the dissociation of
a complex defect which leaves a substitutional uncompensated arsenic atom the annealing activation energy
AH is equal to the dissociation activation energy AEd
of the complex defect.
The second annealing process, i. e. the combination of two defects during an encounter, has been
discussed by Waite [32] and by Jurkov [33]. The
migration of defects is similar to diffusion and can be
represented by a diffusion coefficient with an activation energy for defect migration AE,. Assuming that
the electrically active atom is created by the encounter
of an interstitial type and a lattice type defect and that
the diffusion of the interstitial defect is much faster,
it can be shown that the annealing activation energy
AH is equal to AEm/2.Therefore the activation energy
BEHAVIOUR OF LOW DOSE ARSENIC IMPLANTS IN SILICON
of the process would be equal to 0.25 eV in the first
case and 0.5 eV in the case of the combination of
two defects.
Experimental data are available only for the E-like
center, substitutional arsenic atom vacancy pair ;
the dissociation energy has been measured equal to
1.27 eV [34] and the activation energy for motion
equal to 1.07 eV [35]. No data are available for
defects like a closely spaced vacancy interstitial pair
or interstitial associated with an impurity atom or
complexes of two arsenic atoms [28]. The agreement
between the value obtained according to Ishino
et al. (0.4-0.5 eV) and that obtained assuming that
the electrical activation results from a combination
of two defects (0.5 eV) can be used as a criterion of
choice.
In this case the migrating defect would be an interstitial arsenic atom which drifts during the annealing
to a lattice type defect and the activation energy of
the process would be the migration energy of interstitial arsenic.
As pointed out by Gusev and Titov 1291 the lattice
C5-149
type defects are not necessarily single vacancies but
they may be more complex defects. When they
combine with an interstitial impurity atom, an unstable
complex is formed which then dissociates rapidly,
producing an electrically active center.
The activation energy for interstitial arsenic migration (0.4-0.5 eV) is in rather good agreement with
the values theoretically estimated for insterstitialcy
diffusion mechanism [16], [36].
Further studies are in progress to elucidate the
reliability of this mechanism, however it appears that
current concepts and inventory of identified defects
are not sufficient for a clear interpretation either of
the annealing mechanism either of the defects involved.
Acknowledgments. - The authors gratefully
acknowledge the continuous encouragement of Professor G. Bertolini and the help of F. Mousty,
R. Adam and G. Melandrone for the experimental
techniques.
Preliminary results of this work were obtained by
E. Baldo in the course of his thesis.
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