Reconstruction of the SiO
2
structure damaged by low-energy
Universitat
September
ions
implantation
as
Moreover, several techniques for surface analysis use ion
beams to measure physical and structural properties, such as
secondary-ion-mass spectroscopy ~SIMS!, x-ray photoelec-
tron spectroscopy ~XPS!, Auger spectroscopy, etc. These ra-
diation sources affect the structure, properties, and reliability
of the Si/SiO
2
system. Furthermore, doping through oxide
masks and modifications of SiO
2
etch rates are also under-
taken by using ion implantation techniques. Therefore, there
is an intrinsic interest in the basic and applied research re-
lated to the modifications of the structure in amorphous sol-
ids created by ion beams. In this framework, low-dose ion
implantationwithArcausesdamageanddisorderintheSiO
2
layer, but without changing the composition of the layer, as
Ar atoms cannot form stable bonds with the silicon or oxy-
gen atoms.
It is, therefore, essential to understand how the SiO
2
structure is modified after implantation, what kind of defects
are created and what annealing treatments can do to restore
the structure. The structural damage produced in the amor-
phous network of silicon oxide as a consequence of ion im-
plantationischaracterizedbystrainedandbrokenbonds,dis-
tortion of silicon tetrahedra, densification, departure from
local stoichiometry, and, in the case of irradiation with fast
heavy ions, plastic flow phenomena.
similar behavior when other ions are implanted into silica.
Thishasbeenshowntobealmostindependentofthetypeof
ionimplanted.
2?10
Moreover,Ar-implantedoxidelayershave
presented a degree of densification after implantation.
5,7,11
Densification is accompanied by other structural modifica-
tions, such as Si?O bond straining and bond breaking, as a
consequenceofatomdisplacementfrompositionsofequilib-
rium. Indeed, Devine
4
has reported that the creation of de-
fects is greatly enhanced in densified amorphous SiO
2
,as
strained Si?O bonds act as precursors for both oxygen va-
cancy and nonbridging oxygen defects. Infrared spectra give
clear evidence of the reduction in the mean Si?O?Si bond
angle as well as of the presence of low-frequency vibrations
coming from Si?O nonbridging oxygen bonds.
5
The XPS
analysis of the local environments of silicon atoms per-
formed by the authors corroborated the presence of dangling
bonds and allowed their distribution as a function of the
damage created to be calculated.
5
Therefore, one expects ra-
diation damage to induce stable oxygen atom displacements
in the a-SiO
2
network.
In this framework, here we analyze the damage in SiO
2
following very high Ar implantation doses ~.10
17
cm
22
!,
well above the threshold for damage saturation. We also re-
port the experimental annealing experiments performed at
different temperatures with the aim of restoring the structure
of the a-SiO
2
network.
a!
Electronic mail: blas@iris1.fae.ub.es
126 J. Appl. Phys. 81 (1), 1 January 1997 0021-8979/97/81(1)/126/9/$10.00 © 1997 American Institute of Physics
Ar-implanted ions
B. Garrido,
a)
J. Samitier, S. Bota, and J. A. Moreno
EME,DepartamentdeF?
´
sicaAplicadaiElectro
`
nica,Universitat
Barcelona,Spain
J. Montserrat
CentreNacionaldeMicroelectro
`
nica,CNM-CSIC,Campus
Bellaterra,Spain
J. R. Morante
EME,DepartmentdeFisicaAplicadaiElectro
`
nica,Universitat
Barcelona,Spain
~Received 1 March 1996; accepted for publication 23
The damage created in SiO
2
layers by low-energy Ar
structure after various annealing steps have been characterized
dose. Quantitative determinations of the damage produced
spectroscopy.Weshowthattwodosethresholdsfordamage
saturatesandfordosesabove10
17
cm
22
sputteringeffects
~1100°C! restores the structure of the initial nonimplanted
threshold,althoughsomedisorderremains.Electroluminescence
is able to eliminate electrically active defects. For
annealingisunabletorestorethestructurecompletely
layer at the surface and substoichiometric defects appear.
the Ar atoms at such high doses may affect the annealing
Physics. @S0021-8979~97!02001-X#
I. INTRODUCTION
Manystudieshaveanalyzedtheeffectsofradiationdam-
age in SiO
2
/Si systems
1?6
as ion bombardment and other
types of radiation are applied during their manufacture.
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deBarcelona,Diagonal645-647,08028
Auto
`
nomadeBarcelona,08193
deBarcelona,Diagonal645-647,08028
1996!
~130 keV! and the reconstruction of the
as a function of the implantation
have been performed from infrared
areencountered:At10
14
cm
22
damage
dominate.Annealingathightemperatures
oxide only for doses below the second
measurementsshowthatannealing
doses greater than 10
17
cm
22
,
sputteringeffectscreateadepletedoxygen
The presence of microcavities created by
behavior. © 1997AmericanInstituteof
We have previously reported
5,7
that the damage pro-
duced by Ar ions in SiO
2
layers increases with the implan-
tation dose, for low doses; however, for doses higher than
10
14
cm
22
damage saturates. Various authors have reported
II. EXPERIMENTAL PROCEDURE
The substrates used were ^100& n-type, phosphorus-
doped ~1.0310
15
cm
23
! silicon wafers, with resistivity be-
tween 3 and 5 V cm. The oxide layers were grown by wet
oxidation at 950°C. The thickness of these layers before the
implantationandtheannealingprocesseswasaround2500Å
as measured by ellipsometry. In measuring the oxide thick-
ness it is important that the implanted ions do not reach the
silicon substrate. In previous studies we analyzed as-
implantedsamplesuptodosesnearthethresholdfordamage
saturation ~10
14
?10
15
cm
22
!.
5,7,11
Here, we increased the
rangeofdosesupto3.2310
17
cm
22
,andweperformedrapid
thermalannealing ~RTA! inN
2
andO
2
forallthesamples,at
different temperatures ~from 400 to 1100°C! and duration
times ~from1sto1min!.
The energy of Ar ions ~130 keV! was chosen so that the
maximum distribution of the implanted ions was in the
middle of the layer ~this was previously calculated by com-
puter simulation!. The implantation was performed by using
an Eaton Nova 4206 implanter; cooling and a low-beam cur-
rent were used to avoid heating.
For the Fourier transform infrared spectroscopy ~FTIR!
measurements, the SiO
2
films on the reverse side of the sub-
strate were etched off in order to prevent infrared absorption
by the nonimplanted film. Both reflection and transmission
operationmodeswereusedinaBOMEMDA3spectrometer.
Infrared spectra were measured for all the samples in the
midinfrared range ~4000?400 cm
21
! at a resolution of 2
cm
21
.Rawspectrawereappropriatelyrationedtoabaresili-
con spectrum.
The electroluminescence ~EL! measurements were per-
formed with an alternative technique which is based on the
useofanelectrolyte?oxide?semiconductor ~EOS! system,to
carry out a controlled injection and heating of electrons in
SiO
2
films depending on the applied electric field. This tech-
nique allowed us to perform defect characterization without
the need to deposit any metal or polycrystalline silicon con-
tact. In contrast with other techniques, EL gives information
of electrically active defects in the oxide. Further details of
this last technique are explained in the literature.
12
Further-
more, some etching-rate measurements in a buffered HF
mixture were selectively performed in some of the samples.
III. INFRARED AND ELLIPSOMETRY RESULTS
A. Samples as implanted with Ar
The infrared spectra of the implanted samples show a
strong modification of the SiO
2
stretching band. This is the
most intense band of the SiO
2
infrared spectrum, and in
transmissionatnormalincidenceitiscomposedoftwotrans-
verse optic ~TO! vibrational modes, the TO
3
or asymmetric
stretching mode and the TO
4
or symmetric stretching mode,
which in nonimplanted thermal oxides are centered around
1080 and 1200 cm
21
, respectively.
13,14
After the implanta-
tion, the whole stretching band becomes shifted markedly
towardlowerwavenumbers,itbecomesbroaderandbothits
intensity and area decrease. Figure 1 shows the absorption
spectra of SiO
2
after implantation doses ranging from
3.2310
12
to3.2310
17
cm
22
.Theshiftandbroadeningofthe
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stretchingbandincreaseswithimplantationdose,butreaches
a saturation regime after a dose threshold of 10
14
cm
22
,
which corresponds to a nuclear deposited energy of
4.6310
23
eV/cm
3
, in agreement with previous works.
2,6
This
energy is only slightly higher than the Si?O bond energy
density, which is 3.8 eV/bond or 3.4310
23
eV/cm
3
, which
suggests that structural damage efficiency for nuclear colli-
sions is higher than for electronic losses. This assumption is
corroboratedbytheamountofenergyrequiredtoreachsatu-
ration when using MeV proton or electron irradiation?
values around or higher than 10
26
eV/cm
3
.
9,10
From 10
14
cm
22
to doses near 10
17
cm
22
, the spectra of
the implanted oxides are identical, suggesting that for three
orders of magnitude of dose rise the structural damage does
notincrease.ThevaluesofthepositionandwidthoftheTO
3
band for thermal nonimplanted oxides and for implanted ox-
ides into this saturation regime are given in Table I. Never-
theless, for doses above 10
17
cm
22
~second threshold!, other
band modifications are observed. Although the position and
widthofthebandsarenolongermodified,theirintensityand
area decrease markedly.
Ellipsometry showed that layer thicknesses were modi-
fiedaftertheimplantationprocess.Weinterpretedthereduc-
tion in thickness as a densification and not as a sputtering
FIG. 1. Infrared-absorbance spectra of the implanted samples for different
doses ~in cm
22
! in the region of the TO
3
and TO
4
modes. 1: Reference
nonimplanted sample; 2: 3.2310
12
;3:1.0310
13
;4:3.2310
13
;5:1.0310
14
;
6: 3.2310
14
;7:3.2310
15
;8:1.0310
16
;9:3.2310
16
; 10: 1.0310
17
; and 11:
3.2310
17
.
TABLE I. Values ~in cm
21
! for the peak frequency and peak width of the
TO
3
band of thermal nonimplanted reference oxides and implanted oxides
well into the saturation regime of structural damage.
Thermal oxide Implanted oxide
TO
3
frequency ~cm
21
! 1083 1045
TO
3
width ~cm
21
! 78 105
127Garrido et al.
TABLE II. Percentage of decrease in thickness for the higher doses ana-
effectbecauseitincreasedfirstforlowdosesandmaintained
at a constant value ~1.5%! through the three orders of mag-
nitude of dose increment ~10
14
?10
17
cm
22
! at which the re-
gime of damage saturation was extended. However, some
dramatic modifications were observed for doses equal to and
higher than the second dose threshold previously mentioned
~10
17
cm
22
!. A major color change in the samples was
clearly visible to the naked eye. As deduced from the ellip-
sometry results, the modifications were due to a marked re-
duction in thickness and not to a variation of the refractive
index. These results are in accordance with the infrared re-
sults presented above inasmuch as the evolution of infrared
spectrafordoses .10
17
cm
22
onlyconsistedofadecreasein
theintensityandareaofthebands ~smallerthickness! butnot
inposition,width,orshapemodifications ~relatedwithstruc-
ture!. In view of these results, one can conclude that above
this second threshold sputtering effects dominate. The per-
centage reductions in thickness, as measured by ellipsom-
etry, for the higher doses reported in this work, are given in
Table II.
B. Annealed samples
All the samples in the range of implantation doses from
10
13
to 10
17
cm
22
were annealed in a RTA furnace ~N
2
at-
mosphere! at different temperatures from 400 to 1100°C in
order to evaluate the recovery from structural damage after
the implantation process. From the infrared spectra it was
clearly observed that the modifications of the absorption
bands were in the opposite direction with respect to their
change following implantation, i.e., the peak frequency of
theTO
3
modeshiftedtowardhigherwavenumbers,thepeak
width decreased, and the intensity and area both increased.
Nevertheless,thedegreeofrecoverywasfoundtodependon
the annealing temperature and on the implantation dose.
Fordosesbelow10
17
cm
22
,theTO
3
peakfrequencywas
restored to that of the original thermal oxide ~1083 cm
21
!
only after annealing at 1100°C. For doses equal to or above
10
17
cm
22
, the TO
3
band was not restored to the original
position. Figure 2 shows the TO
3
peak frequency as a func-
tion of implantation dose, for different annealing tempera-
tures. Indeed, for the highest dose shown in Fig. 2 ~10
17
cm
22
!, the curves deviate from the negative slope character-
istic of lower doses, suggesting a different annealing behav-
ior and an incompletely restored structure.
The evolution of the TO
3
peak width with annealing
temperature showed a similar pattern ~Fig. 3!. For doses be-
low10
17
cm
22
therecoveryisimportantonlyafterannealing
at 1100°C, but there were some difference between the final
lyzed in this work as measured by ellipsometry.
Dose ~cm
22
! Thickness decrement ~%!
1.0310
15
1.5
1.0310
16
1.5
3.2310
16
1.5
1.0310
17
2.4
3.2310
17
14.2
128 J. Appl. Phys., Vol. 81, No. 1, 1 January 1997
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value reached after annealing ~80 cm
21
! and the value of the
thermal nonimplanted oxide ~78 cm
21
!. This is consistent
with the presence of residual disorder in the structure ~wider
band! as we discussed below.
IV. ELECTROLUMINESCENCE RESULTS
The EL spectrum for the nonimplanted reference sample
is plotted in Fig. 4~a!. The most significant result is the pres-
ence of a predominant band located at 1.9 eV and a less
intense band at 4.3 eV. We examined the 1.9 eV band pre-
viously and determined that it was closely related to the sili-
FIG. 2. Peak frequency of the TO
3
band as a function of the implantation
dose, for different annealing temperatures. Curves are only a guide for the
eye.
FIG. 3. Peak width of the TO
3
band as a function of the implantation dose,
for different annealing temperatures. Curves are only a guide for the eye.
Garrido et al.
4~b!#. The band at 2.7 eV exhibited a significant increase in
two orders of
This band is
of oxy-
ofthe1.9,
dosesfor
fordosesofthe
led to an
?SiO
2
?Si
of a negative
charge in the oxide.
As an illustration of the recovery of defects observed
10
band
4
4
3
3
3
con oxidation mechanism.
11
We reported that it is a direct
consequence of the wet oxidation process and it is related to
the presence of silanol groups, Si?OH, which act as precur-
sors of a negatively charged nonbridging oxygen-hole
~NBOH! centers.Thebandlocatedat4.3eVisrelatedtothe
presence of interface states, as discussed previously.
11
The
relatively low intensity of the 4.3 eV band is a consequence
of the local distribution of SiO
2
/Si interface states.
The Ar ion implantation process produced a marked
modification of the previous spectrum. First, a new band
located at 2.7 eV appeared, which was accompanied by an
increase of the intensity of the 1.9 and 4.3 eV bands @Fig.
FIG. 4. ~a! Electroluminescence spectrum for the nonimplanted structure.
~b! Electroluminescence spectrum after implantation at a dose of 1.0310
14
cm
22
.
TABLE III. Intensities ~in a.u.! of electroluminescence
annealed samples were implanted with a dose of 1.03
Sample 1.9 eV
Nonimplanted 1.0310
Implanted with 1.0310
14
cm
22
8.0310
1 s annealing ~1100°C! 8.0310
10 s annealing ~1100°C! 4.5310
60 s annealing ~1100°C! 2.7310
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after the annealing at 1100°C, Table III shows the evolution
of the three EL bands for the sample implanted with a dose
of 1.0310
14
cm
22
, for different annealing times. Increasing
annealing temperature had the same effect as increasing an-
nealing time for a given temperature, although the behavior
of each band was different. First, the intensity of the 1.9 eV
band decreased after the annealings even to a lower value
thanbeforeimplantation.Thisresultindicatesthattherecov-
ery of the oxide not only is related to the elimination of
defects produced by the implantation but, also, for the de-
crease of defects, it is present before the implantation. Then,
the quality of the initial oxide, at least for this electrically
activedefect,wasimproved.Second,astrongdecreaseinthe
2.7 eV band was also observed, but some defects remained
even after an annealing at 1100°C with a duration of 60 s.
Finally,wealsoobservedatotalrecoveryofthe4.3eVband
related to the SiO
2
/Si interface.
V. DISCUSSION
The structure of amorphous SiO
2
is built up from basic
SiO
4
tetrahedral units centered around silicon atoms. How-
ever, unlike in the crystalline form, the Si?O?Si intertetra-
hedral angle of the amorphous material varies from one tet-
rahedron to another between 120° and 180°.
13,14
The mean
value of this Si?O?Si angle distribution is found to be
144°.
15,16
One of the effects of the ion implantation ~the
same as that found after x-ray or neutron irradiation! is the
varied modification of this angle distribution. The densifica-
tion of the structure found from ellipsometry is a conse-
quenceoftheSi?O?Simeananglereduction.Anothereffect
of implantation is to widen the bond angle distribution, and
hence increase the dispersion of intertetrahedral angles,
which can be regarded as an increment of disorder. These
modifications led to shifts and broadenings of the vibrational
spectrabandsfromwhichcompactionandstructuraldisorder
could be quantified if no other perturbing effects were modi-
bands. The noise level is located at 1.0310
3
. All the
14
cm
22
.
2.7 eV band 4.3 eV band
{{{ 2.0310
3
1.6310
5
2.0310
4
5.3310
3
4.5310
3
2.0310
3
5.0310
3
1.6310
3
1.5310
3
129Garrido et al.
all the implanted structures. It rose more than
magnitude with respect to the noise level.
thought to be a direct consequence of the production
genvacanciesbyionimplantation.Theintensities
2.7,and4.3eVbandsincreasedwiththeimplanted
lowdoses,andshowedatendencytosaturate
order of 10
14
cm
22
. Furthermore, Ar implantation
increase of the flatband voltage of the electrolyte
system, which is an indication of the presence
fying the bands. Previous experimental studies of silicon ox- fuse over a sufficient distance before recombining with sili-
ideimplantedwithAr ~andotherheavyions! haveconfirmed
some degree of densification.
3,5,6
However, as well as compaction and disorder, another
relevant phenomenon produced after high-dose implantation
is the creation of point defects, which arise mainly from
broken Si?O bonds and displaced O atoms. In general, it is
thepresenceofthesepointdefects,whichareconnectedwith
the alteration of the environment of the silicon atom, that is
mainly responsible for modifications of the vibrational spec-
tra of implanted samples. This fact can be demonstrated by
considering the results of the central force model for SiO
2
,
17
in which Si?O?Si bond angle u and infrared-absorption
wave number k are related by
k5k
0
sin~u/2!, ~1!
where k
0
51134 cm
21
.
18
The distance between adjacent sili-
cons can be expressed as
d
Si?Si
52r
0
sin~u/2!, ~2!
wherer
0
~Si?Odistance! takesthevalueof1.6Å.Therefore,
as density r is inversely proportional to the cube of this
distance, an approximate determination of the relative local
densification takes the form
Dr
r
rel
5
S
k
rel
k
D
3
21, ~3!
where the subscript rel stands for a relaxed oxide. In the
same way, we can define the relative strain E in connection
withthedistancebetweensiliconatomsrelativetothatofthe
relaxed oxide. Then, by using Eq. ~2!, we arrive at
E5
k
k
rel
21. ~4!
Thus, for the 1.5% of densification measured from ellip-
sometry, the corresponding shift in frequency of the TO
3
mode obtained from Eq. ~3! is 5.4 cm
21
. Hence, this low
degree of densification cannot explain the shifts near 40
cm
21
that are measured from infrared spectroscopy ~see
Table I!. Therefore, as expected from the high nuclear dam-
age efficiency of low-energy ion implantation, the most im-
portanteffectsarethoseconnectedwithpointdefectcreation
as a consequence of atom displacement from their equilib-
rium positions. Another argument is the following: We have
shown that the infrared TO
3
mode is at 1083 cm
21
for a
thermaloxide ~nonimplanted!,butwhenoxygenisnonbridg-
ing two tetrahedra as a consequence of one broken bond, the
value given in the literature for the same stretching vibration
is around 1000 cm
21
,
18
so the infrared band must have a
low-frequency contribution, increasing with the implantation
dosage, which makes the whole band shift toward lower fre-
quencies.
For doses above the saturation threshold of 10
14
cm
22
,
the TO
3
band did not shift to lower wave numbers so we
concluded that no further net broken bonds were created.
However, we expect the situation is somewhat more compli-
cated because additional implanted ions create defects, but
damageisrepairedatthesamerateasitisproduced.Wecan
depict the situation as having oxygen interstitials which dif-
130 J. Appl. Phys., Vol. 81, No. 1, 1 January 1997
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con atoms and hence repairing the previously Si?O broken
bonds. For a given oxygen displaced atom this would be
easier when the amount of damage is sufficiently high. Nev-
ertheless, ion implantation produces a local increment of
temperature of at least some hundreds of degrees within a
periodofsecondswhichisprobablyresponsibleforthebond
restoring equilibrium reached at saturation. This mechanism
is valid even after sputtering effects begin to appear because
no further net damage is created inside the layer as deduced
fromthelackofshiftsorbroadeningsoftheinfraredspectra.
The creation of large numbers of oxygens displaced
fromtheirequilibriumpositionssuggeststhatadamagedsur-
face layer poor in oxygen must be created. Indeed, surface
deviations from bulk concentration are expected to be the
result of contamination and oxygen depletion after ion im-
plantation. To corroborate the existence of this layer we per-
formed etching rate measurements in a buffered HF mixture
and found that the etching rate increased across the whole
range of doses except at higher doses ~10
17
and 3.2310
17
cm
22
!; these samples seemed to remain unetched at the be-
ginning of the experiment. This is consistent with a silicon-
rich layer at the surface of implanted samples and the pres-
ence of substoichiometric defects. Nevertheless, XPS
profiling of the O1s and Si2p atomic levels through the
thickness of the layers clearly showed that the composition
of the surface SiO
x
layer was x?1.9160.03, whereas the
mean value obtained for the bulk was the stoichiometric
value2.0160.03.Then,althoughnonetmeanbulkstructural
damage is expected after the threshold for saturation ~as
measured from infrared experiments!, increasingly important
surface stoichiometric deviation is encountered. For greater
implantation doses, a high quantity of oxygen atoms may be
sputteredoutordisplaceddeepestinthelayer.Thisbehavior
explains the reduction in thickness observed.
Nevertheless, the samples implanted with doses above
the second threshold of 10
17
cm
22
, showed different anneal-
ingbehavior.Wefoundthattherecoveryofthestructurewas
not complete even after the annealing at 1100°C. As a good
illustration, the sample implanted with 3.2310
17
cm
22
Ar
atomsshowedafrequencyabsorption ~TO
3
mode! situatedat
1045 cm
21
. After the annealing at 1100°C, the position of
thebandmovedto1075cm
21
,whichisfarfromthethermal
oxidepeakfrequency ~1083cm
21
!.Theseresultscanalsobe
explained by the reduction of thickness and the existence of
a surface modified layer poor in oxygen. In fact, the fre-
quency of the TO
3
mode decreases with the thickness of the
layer due to optical interference effects.
19
Moreover, follow-
ing the work of Nakamura, Mochizuki, and Usami,
19,20
who
correlated the vibrational response of substoichiometric ox-
ides with their composition, we expect the oxide vibrational
bandtobeshiftedfromthevalueofthethermaloxidedueto
the contribution of the surface SiO
x
layer poor in oxygen.
Furthermore, added to this previous effects there is the pos-
sibility of formation of microcavities for the samples irradi-
ated at very high doses. After the annealing, the Ar trapped
in these microcavities evolves, leaving an oxide layer with a
certain porosity. Some authors have reported that porous ox-
ide layers present the infrared-absorption band displaced to
Garrido et al.
amount to the dielectric function:
De~k!5
E
0
?
g~j2k
0
!5
s
whereF istheoscillator
absorption, g~j!5
functiong~j2k
0
!
k
0
and with s as
dispersionanalysis
taltransmittancespectra
in Table IV. This
dielectric function
sample which has
reported by Naiman
absorbing Si?O
TABLE IV. Parameters obtained after the dispersion analysis of implanted oxides. Both TO
3
and TO
4
bands
indicated
parameter,
10
2
35.1
30.3
11.0
29.6
the initial number of
implantationdose.
points. The
to the implanted
131J. Appl. Phys., Vol. et al.
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4pF
j
2
2k
2
2ig~j!k
g~j2k
0
!dj, ~5!
1
Ap
exp2
S
j2k
0
s
D
2
, ~6!
strength, k
0
isthecentralpositionof
g
0
j/k
0
is the oscillator damping, and the
isabroadeningGaussiancurvecenteredat
full width at half-maximum. The results of
wereobtainedafterfittingtheexperimen-
withEqs. ~5! and ~6! andareshown
Table IV include the parameters of the
obtained for a nonimplanted sample, a
reached damage saturation, and the values
etal.for SiO
2
.
22
Finally, the number of
bonds is calculated from
FIG. 5. Number of Si?O broken bonds relative to
absorbingbondsbeforeimplantation,asafunctionofthe
The solid line is the theoretical curve fitted to the experimental
dashed line is the evolution if damage was proportional
dose at the initial rate of damage.
81, No. 1, 1 January 1997 Garrido
lowerwavenumbers.
21
Thisisnotincontradictionwithden-
sification of the whole layer only if local densification is
important enough to account for the formation of microcavi-
ties. The XPS experiments have shown traces of Ar before
annealing and none afterward.
As the infrared response of oxides depends on the
amount of absorbing Si?O bonds, we performed approxi-
mate calculations of the mean bulk number of Si?O broken
bonds as a function of the implantation dose and the anneal-
ing temperature. To achieve this we obtained the oscillator
strengths and complex dielectric functions of the layers from
the frequency dispersion analysis of their transmittance and
reflectance spectra. The model for dielectric functions best
suited for our purposes here is the one developed by Naiman
etal.
22
for amorphous materials, which considers that
strengths and maximum frequencies of vibrations in amor-
phous solids are spread in frequency through gaussian
shapes. So, each absorption band contributes the following
were fitted with two Gaussians each. The parameters
strength; and s, dispersion parameter. g, or damping
mately equal to 8 cm
21
for all the fittings. All the quantities
nonimplanted; ~b! samples implanted with a dose of
Ref. 22!.
Parameters
Nonimplanted
SiO
2
~cm
TO
4
Peak 1 k
0
1217.9
F 2091.3
s
Peak 2 k
0
1163.2
F 3118.9
s
TO
3
mode Peak 1 k
0
1092.1
F 4700.0
s
Peak 2 k
0
1068.5
F 42010.7
s
N}d
ox
E
band
ve9~v!dv, ~7!
whered
ox
is the thickness of the oxide and the proportional-
ity constant is obtained from the spectra of the thermal non-
implanted oxide used as reference.
The evolution of the density of broken bonds @broken
bonds (N
R
)5initial bonds2absorbing bonds detected after
implantation# asafunctionoftheimplantationdoseisshown
inasemilogarithmicplotinFig.5,relativetothetotalinitial
number before implantation. As was previously deduced
from the qualitative analysis of the infrared bands, the
are k
0
, frequency of resonance; F, oscillator
which is not indicated, was always approxi-
are given in cm
21
. ~a! Reference samples of SiO
2
15
cm
22
; ~c! results reported by Naiman et al. ~see
1
!~a!
Implanted
SiO
2
~cm
21
!~b!
Naiman et al.
~cm
21
!~c!
1218.8 1218
1903.6 2200
43.8 35
1156.3 1164
3191.5 3200
39.4 30
1061.5 1092
5581.5 3900
38.5 10.8
1034.8 1067
35741.4 40100
50.2 30.4
TABLE V. Shift in peak frequency in ~a! and differences in peak width in
amount of broken bonds remains constant after a dose of
10
14
cm
22
. We have also represented in Fig. 5 the evolution
expected as given by an equation of the type
N
R
5AN
T
~12e
2B~D/N
T
!
!, ~8!
whereD is the implanted dose,N
T
is the density of the total
initial number of bonds and A and B are constants which
were varied during the fitting. This equation is similar to the
one obtained if we assume that the process of damage satu-
ration begins as soon as ion tracks overlap for higher doses,
similar to the model of thermal and displacement spikes for
ion implantation in crystals.
23
After the threshold dose of
10
17
cm
22
~not shown in Fig. 5!, the number of absorbing
bonds diminishes as a consequence of sputtering, but if
thickness corrections are performed, the same amount of
damage is observed as in saturation.
InFig.6wehaveplottedthenumberofabsorbingbonds
normalized to the bond density of thermal oxide ~taken as
100%! for the different series of annealed samples. The re-
covery of the broken bonds of the structure is complete even
afterannealingat900°C ~electricaldefectswereobservedas
being restored from the EL measurements! except for the
highestdoseforwhichweexpectsomeoxygenlossfromthe
layer. This behavior clearly means that broken bonds are
restored after moderate temperatures. This phenomenon can
beunderstoodifweconsiderthatthediffusioncoefficientfor
O into SiO
2
is strongly dependent upon temperature and its
value is high at moderate annealing temperatures
~D52.7310
24
e
21.16/kT
cm
2
/s! so oxygen interstitials can
move faster and repair damaged tetrahedra with oxygen va-
cants.
Incontrast,aspointedoutabove,thepeakfrequencyand
peak width of the TO
3
band are only completely recovered
for the highest annealing temperature ~not significantly for
900°C!. Therefore, for oxide strain relaxation to occur, suf-
FIG. 6. Number of absorbing bonds relative to the bond density of thermal
oxide ~taken as 100%! as a function of implantation dose, for different
annealing temperatures. Curves are only a guide for the eye.
132 J. Appl. Phys., Vol. 81, No. 1, 1 January 1997
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ficiently high temperatures and annealing times are needed,
so the oxide can flow ~viscoelastic relaxation mechanism!.
ForalltheresultsinFigs.2and3theannealingdurationwas
10 s, and from the work of Irene, Tierney, and Angiletto
etal.
24
theviscoelasticrelaxationtimesare: .2hfor900°C
and 10 s for 1100°C. From this analysis it is clear that only
the annealings performed at the highest temperature were
capable of restoring both point defects and distortion and
densification of the oxide structure.
Therefore, because of the coexistence of two mecha-
nisms of damage?i.e., creation of point defects and
distortion-disorder structural modifications?it is difficult to
differentiatethetwo.Astheviscoelasticrelaxationtimeisso
much greater for an annealing temperature of 900°C, and
with the result that all broken bonds are restored for this
annealing time, then, the differences between samples an-
nealed at 900°C and thermal oxides come about only from
distortioninducedeffects.WegiveinTableVthedifference
between peak frequency and width of the TO
3
band for ther-
maloxidesandtheoxidesimplantedandannealedat900°C.
We also include in this table the relative intrinsic strain and
densification values calculated following the central force
modelfortheamorphousSiO
2
@seeEqs. ~1!?~4!#.
17
Thelocal
densification values in saturation are between 2% and 4%,
which are slightly higher than the 1.5% densification experi-
mentally obtained from ellipsometry; but, we have to take
into account that the values obtained from infrared experi-
mentsrepresentlocaldensityvaluesintheneighborhoodofa
Si?O bond and do not take into account the presence of
porosity or microcavities. The more important departures of
local densification from measured densification for the
higherdosesmaybeexplainedbythepresenceofmicrocavi-
ties created by the implanted Ar atoms.
It is very interesting to show how the peak frequency
and width of the TO
3
mode evolve together after implanta-
tion and annealing. To do so, we show in Fig. 7 the experi-
mental results of peak frequency versus peak width. Several
points should be highlighted. The path followed by the im-
planted samples ~damage path! is different from that of the
annealing samples ~recovering path!. The curve for the im-
planted samples has a higher slope and a significant change
occurs near saturation ~modification in peak frequency but
not in width!. Otherwise, the curve for the annealing recov-
ery is smoother and it seems as if peak frequency ~related
~b!, between the TO
3
band of a thermal oxide and the layers implanted and
annealed. Also given is the relative intrinsic strain in ~c! and the local
densification in ~d! as calculated from the deviations in the peak frequency,
following the central force model ~see Ref. 17!.
Dose
~cm
22
!
Shift in
position ~a!
~cm
21
!
Difference in
width ~b!
~cm
21
!
Relative
strain ~c!
~%!
Relative
densification ~d!
~%!
10
13
2.2 22.6 0.20 0.6
10
14
6.3 27.5 0.58 1.7
10
15
7.4 28.2 0.68 2.1
10
16
13.0 211.8 1.20 3.6
10
17
14.3 212.6 1.32 4.0
Garrido et al.
nealed samples. These discrepancies become more signifi-
with strain! and peak width ~related with disorder! were
similarly recovered. If we subtract two absorbance spectra
coming from one as-implanted sample and one annealed
sample,bothofthemwiththesamevalueinpeakfrequency,
we can expect the difference ~Fig. 8! to be the contribution
coming from the nonbridging oxygens because these are the
first to be recovered after the annealing ~broken bonds are
recovered after a 900°C annealing!. We observed a band
centered at 990 cm
21
which can be associated with the con-
tribution coming from the nonbridging oxygen vibration.
All these results are in agreement with the EL results
presented above. The signal coming from all the electrically
active defects decreases a point near that of the initial levels.
However, some remaining damage may exist, indicated by
small differences observed between nonimplanted and an-
FIG. 8. Substraction of two absorbance spectra coming from one as-
implanted sample ~1! and another annealed ~2! but both of them with the
samevalueforthepositionofthepeak.Thedifferenceisabandcenteredat
990cm
21
~3! whichisthecontributionofthenonbridgingoxygenvibration.
FIG. 7. Peak frequency vs peak width of the TO
3
absorption band for
samples implanted and annealed. Additional samples annealed under O
2
atmosphere have been included for comparison.
J. Appl. Phys., Vol. 81, No. 1, 1 January 1997
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cant as the implantation dose is raised. Finally, the etching
experiments have also shown that samples implanted at high
doses ~.10
16
cm
22
! andannealed,donotcompletelyrecover
the structure of the initial nonimplanted oxide because the
etching rate obtained is near 150 Å/min, in contrast with the
130 Å/min obtained for a thermal oxide layer. These differ-
ences are clearly indicative of a structure which retains point
defects and microcavities.
VI. SUMMARY AND CONCLUSIONS
Implantation of low-energy ~130 keV! Ar ions in a large
range of doses was carried out in thermal silicon oxide lay-
ers. The extent of structural damage and its subsequent re-
coveryasafunctionoftheimplantationdoseandtheanneal-
ing temperature were characterized by means of
ellipsometry, infrared spectroscopy, etching rate measure-
ments, and EL using a new technique based on an EOS sys-
tem.
Two dose thresholds for damage were encountered: The
first at 10
14
cm
22
at which there was a saturation of damage
inducedbytheionimplantation.Adynamicregimewaspro-
ducedatwhichforhighdosesdamageisrepairedatthesame
rate as it is produced. The second threshold occurs at a dose
of 10
17
cm
22
; for doses above this, sputtering dominates.
We have demonstrated the coexistence of two damage
mechanisms: Creation of point defects and distortion-
disorder structural modifications. From the infrared analysis
we were able to separate the effects of several annealed
samples and calculated that the local densification in the
saturation regime is between 2% and 4%, which is slightly
higher than the 1.5% densification obtained by ellipsometry.
The presence of microcavities might explain the differences
observed.
Wehavealsoshownthattheevolutionofdamageduring
implantation differs from that of recovery during annealing.
Annealing led to the smooth recovery of the structure de-
pending on the temperature and duration. Spectral substrac-
tion of two samples has shown that the difference between
implanted and annealed samples occurs within a small band
centered at 990 cm
21
originating from the nonbridging oxy-
gen bonds.
The production of a large number of displaced oxygen
atoms suggests that a damaged surface layer poor in oxygen
is created. Etching rate measurements show that the etching
ofthissurfacelayerwasveryslow,consistentwithasilicon-
rich layer. XPS experiments show a thin surface layer with a
composition of x51.91. Therefore, for such highly im-
planted samples the complete recovery of the structure after
annealing is not possible even at the highest temperature
~1100°C!.
Annealing at high temperature for a short period has
been shown to be effective in restoring the structure of an-
nealed samples. The oxides implanted at moderate doses
completely recovered their structure after annealing at
1100°C, although some disorder remained, as deduced from
the vibrational analysis. The viscoelastic relaxation time for
this temperature is small enough for the oxides to flow and
recover a stressed tetrahedra pattern.
133Garrido et al.
Electrically active defects are nearly completely recov-
ered, as detected by EL, at moderate doses. Nevertheless,
annealed sample layers implanted at higher doses ~.10
16
cm
22
! do not recover the same etching rate of the original
thermal oxide, which is also consistent with the presence of
certain substoichiometric defects. This might suggest also
the presence of microcavities created by the Ar atoms.
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Garrido et al.