The role of the interstitial oxygen in the recovery and evolution of the
Boron implantation damage
I. Mica1, a, M.L. Polignano1, F. Cazzaniga1, L. Di Piazza1, M. Mariani1,
E. Ricci1, F. Sammiceli1, S. Speranza2
Numonyx, via C.Olivetti 2, 20041 Agrate Brianza – Italy
1
2
MEMC, Via Gherzi, 31, 28100 Novara - Italy
a
isabella.mica@numonyx.com
Keywords: Ion implantation, Boron, Interstitial Oxygen, Defect formation, CZ Silicon wafers.
Abstract. The recovery of the boron implantation damage can be very difficult. Depending on the
energy and the dose many dislocations are generated at the projected range of the boron
implantation. The morphology of these dislocations depends on the silicon substrate. In this work
we demonstrate that the interstitial oxygen concentration ([Oi]) is related with the dislocation
dimension, density end morphology. Particularly long dislocation dipoles were generated by the
boron implantation in substrate with interstitial oxygen, and their density is connected with the [Oi].
concentration.
Introduction
Boron implantation at doses higher than 2-5x1013 cm-2 are prone to dislocations generation [1].
Indeed the implantation heavily damages the silicon substrate and the thermal treatment used to put
the Boron atoms into the substitutional positions could be insufficient to recover the silicon lattice.
In these cases some crystal defects grow near the projected range of the Boron implantation and they
can affect severely the electrical behavior of the devices. Indeed some of these defects can grow up
to the surface region. This effect depends on the dose and on the energy of the implantation.
In this work the role of the Si substrate in the implantation damage recovery is studied. Polished
Czochralski (CZ) wafers of different type were compared.
The defects formed by high dose boron implantation have been observed with transmission
electron microcopy (TEM). Since even a low density of defects in the device junction region may be
very detrimental to device performance, it is essential to increase the statistics of the measurements
introducing an electrical test. To this aim n+/p junctions were formed in the boron-implanted region
and electrically characterized by measuring the junction reverse current as a function of temperature.
Experimental details
The boron implants were carried out in Czochralski grown (100) polished Si substrates. We take in
consideration nitrogen doped wafers conventionally annealed in argon and MDZ® (Magic Denuded
Zone [2]) wafers (see Table 1). [Oi] data were obtained from FTIR measurements by using the
ASTM 83 standard.
Additional treatment on the Czochralski wafers
[Oi] [ppma]
 [cm]
None (as grown)
Conventional denuding treatment (annealing in Ar)
®
< 15
8-12 bf. denuding
8-12
9-14
9-14
9-14
®
12-14.5
9-14
®
8-12 bf. denuding
1.8
MDZ annealed
MDZ annealed
MDZ annealed + 2m denuding treatment
Table 1: Description of the set of different substrate used in the experiment.
The Boron implantation doses were 2x1014 cm-2 with implantation energy of 600 keV or
1.8x1015 cm-2 with implantation energy of 3.5 keV. The recovery annealing was a furnace annealing
at 10000C for 40 minutes for the 600 keV implant and a spike annealing at 11000C for the 3.5 keV
implant.
The wafers were taken after the high energy boron implantation and annealing for microscopy
inspections. Both plan view and in some cases cross view TEM images were obtained to examine
the extended defects. TEM plan views analyses were carried out at the projected range regions of
the implantations. The 600 keV samples were prepared by removing about 1.2 m silicon layer
from the wafer surface, using a dual beam SEM-FIB; the thickness of the lamellas was 250nm. The
3.5 keV samples were manually prepared.
For electrical tests p-n junctions were built only on the wafers with the 600 keV implantation.
The manufacturing process flow was the following. After the boron high energy implantation and
recovery, N+/pwell junctions were formed by the B and As implantation. Then the active area
pattern is defined and is etched to create the STI trenches. The trenches are then filled by oxide
deposition and polished through chemical mechanical polishing (CMP) and finally annealed. Finally
a triple layer of Ti/TiN/AlCu was deposited as a contact metal layer and it was patterned. The
diodes studied in this work have an area of 1.84x10-3 cm2.
Experimental results
High energy B implantation – 1st experiment. First of all the conventionally denuded CZ wafers
were compared with the MDZ® annealed wafers. The high energy implantation was carried out
(2x1014cm-2 600keV) and studied. It was shown that the dislocation morphology is different in the
two substrates.
A
B
Figure 1: A. TEM plan views of the conventional annealed wafer. B. TEM plan views of the MDZ ® annealed wafer.
The TEM plan views of the conventionally annealed wafers show many dislocations at the
projected range of the boron implantation after the annealing. The morphology of these dislocations
is shown in the Figure 1A. The morphology of the dislocations at the projected range of the MDZ®
CZ substrate is instead completely different. The Figure 1B shows an example of this. The
dislocations are long dipole elongated on <1,1,0> directions. A more detailed analysis of all the
TEM plan views shows some interesting difference between the two substrates. The dislocations
density is very similar in both the cases (1.3m-2 in MDZ® CZ and 1.4m-2 in conventionally
annealed CZ). However the length of the dislocation segments is obviously very different. In the
MDZ® CZ the 38% of the dislocation are longer than 1.21m (the projected range of the
implantation), whereas in the conventionally annealed CZ only the 2% of the dislocation grow more
than the projected range. The TEM cross view of the MDZ® CZ sample reveals an interesting
characteristic. The dislocations are locked at planes located close to the boron implantation
projected range. Most of the dislocations grow in the (1,0,0) plane along the <1,1,0> directions.
Only few dislocations grow towards the surface, and these dislocations are the shortest.
The electrical measurements add statistics to this observation. The Figure 2 shows the boxplots
of the reverse current at 4.5Volt of the diodes on the conventionally annealed CZ substrate and on
the MDZ® CZ ones. The leakage current in the diodes, made on the conventionally annealed CZ, is
higher than in MDZ® wafers on all the measured sites.
Figure 2: The boxplots of the reverse current at 4.5Volt of
the diodes on the conventionally annealed CZ substrate
(straight line) and on the MDZ® CZ ones (dotted line).
The leakage current is collected at different temperature (from 250C to 1350C), because the study
of the Arrhenius plot can give the electrical signature of the dislocations [3,4]. The Figure 3 shows
the Arrhenius plots of the leakage current at 3V for a couple of sites for every substrate.
A
B
Figure 3: The Arrhenius plots of the leakage current at 3V for a couple of site for CZ conventional annealed(A) and
MDZ® annealed(B) wafers. The calculated values of Egen and Ediff are showed near the charts.
According to the SRH theory, the junction reverse current Ir can be written as the sum of a
generation contribution Igen and a diffusion contribution Idiff, with different temperature behaviors:
Ir = Idiff + Igen, with I diff  T 3 exp Ediff / kT  and I gen  T 3 / 2 exp E gen / kT 
(1)
where T is the measurement temperature, k is the Boltzmann constant, Ediff and Egen are the
activation energies of the diffusion and of the generation contribution. The theory puts some
constraints on Ediff and Egen, namely Ediff should be the same as the energy gap Eg (about 1.1 eV) and
Egen can range from Eg/2 to Eg. A best-fit of eq. (1) to experimental data of reverse current vs.
temperature yields Egen from the low temperature part of the charts and Ediff from the high
temperature part of the charts. In junctions with dislocations in the space charge region the reverse
current includes a “soft” contribution, due to a local tunnel effect [5] and hence very weakly
dependent on temperature. An attempt to fit the reverse current of this sort of junctions with Eq.1
results in Ediff and Egen data lower than required by the non-defective junction theory. Fig. 3 shows
that all the measured sites of the MDZ® annealed substrate have low leakage and their Egen is 0.5eV,
but the measured sites of the CZ conventional annealed substrate show an Egen significantly lower
than 0.55eV. So we can conclude that the electrically active dislocations are revealed only in the CZ
conventionally annealed substrate. This result is consistent with TEM observations, as the
dislocation dipoles found in MDZ® wafers do not grow in the surface direction, and hence do not
reach the junction depletion region.
High energy B implantation – 2nd experiment. Then we proceeded with a further investigation
about the mechanism that allows the dislocation dipoles generation in the MDZ® annealed
substrates.
The same high energy and high dose boron implantation (2x1014cm-2 600keV) as in the previous
experiment was used to study the implantation defect growth in a set of different substrates (see
Table 1). If we compare as-grown CZ wafers, wafers treated with the MDZ® annealing, wafers
annealed with conventional oxygen denuding and MDZ® wafers with an additional thermal
treatment inducing about 2μm oxygen denuding, we found dislocation dipole only in the first two
types of substrates. The common characteristic between these two substrates is the presence of the
oxygen interstitials ([Oi]) in the implanted region. Indeed the thickness of silicon depleted from
interstitial oxygen is well deeper than the projected range of the B implantation in the CZ
conventional annealed and in the MDZ® wafers with the additional denuding treatment.
In order to confirm this assertion the same implantation is carried out on a MDZ® annealed wafer
with a higher [Oi] concentration. The dislocation dipole density increases, thus confirming the role
of interstitial oxygen in the formation of dislocation dipoles (Fig.4).
Figure 4: TEM plan views of the MDZ®
annealed wafers with [Oi]=12-14.5ppma.
All these results are collected in the chart of figure 5. The number of dislocation per m2 is
showed for the different substrates of the set. The dislocations are divided into two types: the
dislocation loops and the dislocation dipoles. The dislocation dipoles are not found in the substrates
where [Oi] in the implanted zone was reduced by a denuding treatment; in addition the number of
the dislocation dipoles is the higher the higher the [Oi] is.
Figure 5: Number of dislocation per m2 for the different
substrates of the experimental set.
Low energy B implantation. On the same set of wafers we performed a shallower implantation at a
higher dose (1.8x1015cm-2 3.5keV). This implantation is carried out through a screen oxide of 3nm.
The screen oxide growth involves a thermal treatment long enough to deplete from interstitial
oxygen a silicon thickness of the order of the projected range of the implantation. Indeed in all the
samples there are only the dislocation loops. The plan TEM images are all similar to Figure 6.
Figure 6:TEM plan view of the low energy B
implantation. The lamella was prepared
manually.
The fact, that in all the substrates there are dislocation loops, confirms the connection between
the interstitial oxygen and the long dislocation dipole generation. This result indicates also that there
isn’t a relevant contribution of the specific point defect profile of MDZ® treated wafers.
Discussion. All these experiments consistently show that the long dipole dislocation generation is
connected with the interstitial oxygen presence.
In order to give an explanation of this behavior we recall the fundamental properties of the
isolated oxygen atoms in silicon. At room temperature the interstitial position of isolated oxygen
atoms dissolved in silicon is nowadays widely accepted [6]. Moreover infrared (IR) measurements
on oxygen-doped silicon led to conclude that oxygen is covalently bonded with two neighboring
silicon atoms [7,8] along a <111> direction. So we suggest that the presence of an interstitial
oxygen atom inhibits the dislocation growth along that <111> direction. The dislocation grows all
the same in the region where a high interstitial excess was created by the implantation by
proceeding along another <111> direction. This results in a seeming <110> growth. A high
resolution TEM show this zigzag path of the crystal defects (Fig. 7).
Figure 7:TEM plan high resolution filtered image of a bit of
a dislocation dipole. In the circle is showed the zigzag path.
Conclusion
In this work conventionally annealed CZ wafers were compared with the MDZ® annealed wafers
after Boron implantation at high dose. We found that the MDZ® annealed wafers develop some long
dislocation dipoles elongated on <110> directions and parallel to the (100) plane. By the analysis of
a set of different MDZ® substrates we conclude that the dislocation dipoles develop only in the
wafers where the interstitial oxygen [Oi] in the implanted region is not reduced with respect to the
concentration in the as-grown crystals.
This effect of MDZ® has so far been masked by the fact that device processes always included
cycles at temperatures higher than 1000 deg that caused some oxygen depletion from the surface.
Lower thermal budget processes associated with RTA will minimize the oxygen out-diffusion in the
device process and will allow to appreciate this positive secondary effect of MDZ®. Of course this
beneficial effect is active only for implantations deep enough that the projected range is higher than
the zone denuded by the process thermal budget.
Acknowledgments
We would like to thank G. Borionetti (MEMC – Novara, Italy) for the helpful discussion.
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