35-word abstract Our data suggest a correlation between near-interface strain in SiOz and the ratio of fixed vs. mobile positive charge generated at the interface during forming gas annealing. A model based on firstprinciples quantum mechanical calculations supports this correlation

H A Kurtz

K Vanheusden

P P Korambath

R D Pugh

S P Karna

W M Shedd

CiteSeerX

I I
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THE EFFECT OF NEAR-INTERFACENETWORK STRAIN
ON THE MOBILITY OF PROTONS IN Si02
K. Vanheusden, P.P. Korambath, S.P. Karna, W.M. Shedd, and R.D. Pugh
US Air Force Research Laboratory, 3550 Aberdeen Ave., SE, Kirtland AFB, NM 87117-5776
H.A. Kurtz
Department of Chemistry, University of Memphis, Memphis, TN 38152 .. .‘>-4?... ,’.W(,,,%)~-~!{ , ~= J &~~.
D.M. Fleetwood --.).!4 ey~u
‘ SandiaNationalLaboratories,P.0. Box5800,Albuquerque,NewMexico87185 .J~y\ ‘jj() 1!3!39
35-word abstract
Our data suggest a correlation between near-interface strain in SiOz and the ratio of fixed vs. mobile
positive charge generated at the interface during forming gas annealing. A model based on first-
principles quantum mechanical calculations supports this correlation.
DISCLAIMER
This report was prepared as an account of work sponsored
byanagency of the United States Government. Neither the
United States Government nor any agency thereof, nor any
of their employees, make any warranty, express or implied,
or assumes any legal liability or responsibility for the
accuracy, completeness, or usefulness of any information,
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herein to any specific commercial product, process, or
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otherwise does not necessarily constitute or imply its
endorsement, recommendation, or favoring by the United
States Government or any agency thereof. The views and
opinions of authors expressed herein do not necessarily
state or reflect those of the United States Government or
any agency thereof.
DISCLAIMER
Portions of this document may be illegible
in electronic image products. Images are
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, ,
1,INTRODUCTION
Hydrogen plays an intriguing role in radiation,
hot-electron, and annealing-induced effects in the
Si/Si02 system [1-5]. In particular, hydrogen
interactions at the Si/Si02 interface play a crucial
role in most of the proposed reaction models. For
example, it has been found that annealing in
hydrogen can generate both fixed and mobile
positive’ charges at the Si/Si02 interface. The
fixed positive species show remarkable
similarities to oxidation induced fixed charge [1]
and have recently been attributed to near-interface
over-coordinated oxygen sites involving trapped
~ ions [4]. The mobile species are only observed
in Si/Si02/Si structures and have been identified
as protons imprisoned inside the oxide of these
layered structures [5]. In another recent study,
radiation induced trapping of mobile protons near
the interfacehas been reported [6]. Interface
strain,resultingfroma mismatchbetweenthe Si
and the Si02 network and from network defects
such as oxygen vacancies, is likely to play an
important role in these hydrogen reactions at the
interface, and in the stability of the hydrogenous
species.
In the present work we report experimental
data suggesting a correlation between near
interface strain in the Si02 network and the ratio
of freed vs. mobile protons generated at the
Si/Si02 interface. Our data suggest that interface
strain may affect proton mobility. First-principles
quantum mechanical modeling has been
performed to investigate the effect of SiOz
network strain, measured in terms of the Si-O-Si
angle, on the mobility of protons near the Si/SiOz
interface. The calculations show that the binding
energy of a proton to a bridging O atom decreases
strongly when increasing the Si-O-Si angle
beyond its calculated equilibrium value (at
-1280). These results may offer new fundamental
insight into the detailed nature of annealing
inducedpositivechargenear the Si02interface,
and may also contribute to a better understanding
of hydrogen chemistry at and near the Si/SiOz
interface.
II, EXPERIMENTALDETAILS
A, Substrates
Both SIMOX and thermal oxide material were
investigated, The SIMOX substrates consist of a
SiOzlayer of - 400-nmthick SiOzlayer buried
below a 200-nmc-Si layer. We also analyzed
400-nm commercially grown thermal oxides. A
20-nm SiOz film was deposited on top of these
oxides, which was subsequently covered by a 300-
nm poly-Si layer. Both layers were sputter
deposited using a Si target and an oxygen partial
pressure of 3.6 x 104 torr and 3 x 10-7 torr,
respectively. A post deposition anneal was
performed at 900 “C in Ar for 50 min.
The local density of both fixed and mobile
charges present at the substrate intetiace of
SIMOX substrates as shown in Fig. 1 was
measured. These substrates were annealed for 600
s in a flow of forming-gas (N2:H2; 95:5) at 700
‘C. After the anneal the remaining top-Si layer
was removed, and small metal gate contacts were
deposited on both sides of the substrate for
capacitance-voltage characterization. This
procedure allowed a comparisonbetween the
oxide charge accumulatedin the area with the
buried oxide exposed to the forming-gas ambient,
and the Si-covered area.
Fom”nggasanneal Top Si etch CVcharacterization
metalgatedep.
FIG. 1. Schematicof SIMOX sample processing
after the forming-gasanneal of the structureon the
left.
Capacitor structures were formed on the thermal
oxide samples by etching part of the top-Si layer.
These substrates were then annealed in forming
gas as described above. Capacitance-voltage
characterization was used to measure the charge
generated in the oxide layer during the anneal
treatment. We also measuredthe initial local
distributionof the chargesinsidethe oxideover
the top and bottom interlace, i.e., the ~
distribution present in the oxide right after the
forming gas anneal is petiormed. This was done
by first obtaining the flatband voltage shift onthe
small capacitors right after the forming-gas
anneal, without disturbing the initial charge
distribution. Subsequently, the voltage shift is
measured after applying a positive (+50 V)
prebias to the gate for 5 minutes. This will
1
accumulate all the mobile positive species at the
substrate interface, resulting in a maximum
voltage shift. Finally a measurement after a
negative (–50 V) prebias is performed, which will
accumulate all the mobile protons at the poly-Si-
gate interface.
B, Calculations
Calculationswere carriedout using ab initio
Hartree Fock (E@ level theory using the 6-3lG**
basis set [7]. The clusters used in this study are
(OH)$i-O-Si(OH), and (SiHJsSi-O-Si(OH)s with
and without a proton attached to the bridging O
atom (see Fig. 2). The first cluster represents a
close-up of the Si02 networks the second cluster
was chosen to model the Si / Si02 interface. The
equilibrium Si-O-Si angle, q, without a proton
attached, using this basis se$ was found to be
131° and 140° for the SiOz and interface clusters,
respectively. Subsequently, the Si-O-Si angle was
fwed at preset non-equilibrium values from 100°
to 175°, and the geomehy was reoptimized for all
other degrees of freedom. The same procedure
was carried out with a proton attached to the
bridging O atom. The equilibrium geome~ for
the Si-O-Si angle in this case was 128° and 130°,
f? p~oton
FIG. 2. Clusters used in the ab initio ‘
Hartree Fock @T) level theory.
n o
Q 1.0g
l(a) TopSp~es6ntd.nng &ming’gasarineal I
- -.
\ > \
\ y;
\ L
\ in~n,,,Ji“applied pre-bia% I i— prs-annssl I :,-- lst+50v I k. . . ..2nd-50V -T, .\,- ~Y_.-— 3rd +50 v . . . . .
-o (b) Top Si removed before forming gas anneal
Q 1.0 -
g -=-%.---*.
>
a \
o “\
.g 0.9 -
.1
‘jo
(0 49.
(0
0 0.8 t I I I I t
-50 -40 -30 -20 -lo 0 10 20
Gate Voltage (V)
FIG. 3. Sequenceof CV data before and after
forminggas annealingof both exposed and Si-
coveredburied oxides.
respectively. The geometry was again
reoptimized for the same Si-O-Si tigle range
fi-om 100° to 175°, again fixing only the Si-O-Si
angle and letting all other degrees of freedom
optimize
III. RESULTS AND DISCUSSION
Figure 3 shows the results for the SIMOX
substrate both for the Si-covered (a) and the
exposed (b) buried oxide. A +50 V bias was
applied to the metal gate for 300 s, followed by a
Cv scan. Subsequently, this procedure is
repeated with a –50 V prebias, and finally again
with a +50 V prebias. The data show that
annealing the closed Si/Si02/Si structure
introduces predominantly mobile positive charge
in the buried oxide. Only a small fraction was
found to be trapped near the substrate interface.
The data fhrther show that, after sweeping the
mobile charge towards the gate, reversing the bias
does not result in reversing the mobile charge drift
towards the substrate interface. This observation
implies that the mobile protons get trapped near or
absorbed by the metal gate electrode. The results
in Fig. 3 (b) show that a forming-gas anneal
performed on the open Si/Si02 structure results in
s I
.
aboutthe sameamountof positivechargein the
oxide. However, unlike in the closed structure, all
the positive charge is fixed near the substrate
interface, Hence, the presence (or absence) of the
top Si layer has a drastic impact on the mobility of
the positive charge being generated at the
substrate interface. We speculate that removing
the top Si layer and subsequently performing a
600 “C anneal may reduce the stress at the
substrate interface. We propose that a reduction
in the local strain at the substrate interface may
favor permanent trapping of the generated protons
near the interface.
The data in Fig. 4 show CV measurements
performed on poly-Si gated capacitors after the
forming-gas anneal. These capacitors have
stacked structures consisting of c-Si / thermal
SiOz / deposited SiOz / poly-Si. The initial CV
curve was taken right after the anneal,without
applyingany bias a;ross the oxide. Applyinga
positive gate bias will drift all mobile positive
charges to the substrate interface, while a negative
bias will drift all the mobile positive charges to
the top Si interface, resulting in a maximum and
minimum flatband voltage shift, respectively.
Comparing the initial curve to the curves taken
after a positive and negative prebias, leads us to
conclude that the positive charge at the substrate
interface is mainly fixed charge, while the positive
charge present near the top-Si interface is largely
mobile. Note that these two interfaces are not
identical. The substrate interface is a c-Si /
thermal-oxide intefiace, while the top interface is
a sputtered-oxide / poly-Si interface. The sputter
deposition technique used to deposit the thin
oxide film is likely to yield oxygen deficient SiOz.
Thus, the top Si02/Si interface is likely to be more
oxygen deficient than the lower interface. This
excess Si is likely to cause additional local strain
in the near-interfaceoxidenetworkas compared
to a stoichiometricthermaloxideinterface. In
fac~ it is not unreasonable to assume that this
interface has certain similarities to the buried
oxide/Si interfaces in SIMOX material, as it has
been suggested that the high temperature anneal
induces oxygen depletion of the buried oxide
intefiaces [8,9].
The results of the two sets of Hartree Fock
calculation for the (OH)3Si-O-Si(OH)3 cluster are
shown graphically in Fig. 5 where the difference
in HI? energy with respect to the equilibrium
geometry was plotted against the Si-O-Si angle.
The results in Fig. 5 show that the angular strain
t 1 & I I I I I 8
~ 10 -E22i!!??z!!l
Mobile pmtone
Low.T deposited S102
-: Thermally grown S1O= ; /
~1
++++WP**+ -1.+.*;,1
g%0.9
c) -Pi-
GSJ51iJs!rate/
I=
I
+-a-~-~ . . . .0.8 - -_ #-- . . . . . . . — initial
-- +5(IV
1 I , t I t
-50 -40 -30 -20 -lo 0
Gate Bias (V)
FIG. 4. Sequenceof CV.data before and after
applying positive and negative biases to the
gate.
associated with stretching the Si-O-Si angle
towards 180° is much larger when a proton is
attached to the oxygen. In the absence of the .
proton, the HF energy increase with increasing
angle is small. Figure 5 also shows the difference
in HF energy between the two clusters (with and
without the proton), which is essentially the
change in the proton binding energy vs. Si-O-Si
angle. This curve shows that the binding energy
between the proton and the cluster decreases
significantly for large Si-O-Si angles. Such non-
equilibrium angles only exist in the presence of
network strain. Similar results were obtained for
the (SiH3)3Si-O-Si(OH)3cluster, as will be shown
at the conference. Hence, these calculations offer
support for our suggested correlation between the
presence of strained bonds at or near the interface
and the mobility of the protons generated at that
interface. The loweringof the ~ bindingenergy
in the presenceof strain near the interfacemay
explain why we observe mobile proton generation
at the buried oxide interfaces and at sputter
deposited Si02/Si interlaces.
IV. SUMMARY AND CONCLUSIONS
It has been shown that both &ed and mobile
positive charge buildup can occur at the Si / SiOz
interface during annealing in forming gas at 600
“C. Our data show that mobile charges are
typically generated at oxygen deficient interfaces,
● ▼
1.0
-0.6
1 1 I I 1 1 I 1 I (
L , .*I 1 I I I ! I
100 120 140 160 180
Si-O-Si angle ( 0 )
FIG.5. Results of the Hartree-Fock level
cluster calculations for clusters with the
proton at its equilibrium position and at
infinity.
found in SIMOX or sputter deposited SiOz. Fixed
positive charge buildup was observed at a
standardthermaloxide interface,under identical
experimental conditions. We tentatively attribute
this remarkable difference to a difference in the
presence of strained bonds at or near the interface.
This is fhrther substantiated by the observation
that freed protons were generated near the
substrate interface when pefiorming a forming-
gas anneal of SIMOX after the top Si layer was
stripped off. We suggest that this maybe due to a
reduction of strain at the interface caused by
annealing in the absence of the top Si layer.
First-principle Hartree-Fock calculations were
performed using (OH)sSi-O-Si(OH)s and
(SiHq)$i-O-Si(OH)s clusters with and without a
proton attached to the bridging O atom. HF
energies were calculated as a fi.mction of the
bridging Si-O-Si angle. The results show that the
binding energy between the proton and the cluster
decreases as the Si-O-Si angle increases above its
equilibrium value. Together, these data seem to
support a model in which the generation of mobile
protons at the interface is facilitated by the
presence of near-interface network strain. These
experimental results and particularly our clust~r
calculation data provide a new elementfiat ~
aid to understand the complex hydrogen
interactionschemeattheSi/ Si02interface.
The authors would like to thank J.A. Ruffner for
preparing the sputter deposited films. The part of
this work performed at US Air Force Research
Laboratory was supported under contract
PE62601, Project 8809. H,A, Kurtz
acknowledges support horn NSF through grants
STI-9602656 and CHE-9708517. The portion of
this work performed at Sandia National
Laboratories was supported by the US Department
of Energy (DOE) and by DARPA. Sandia is a
multiprogram laboratory operated by Sandia
Corporation, a Lockheed Martin Company, for the
US Department of Energy under contract DE-
AC04-94AL85000.
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THE EFFECT OF NEAR-INTERFACE NETWORK STRAIN ON THE MOBILITY OF PROTONS IN Si02

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THE EFFECT OF NEAR-INTERFACE NETWORK STRAIN ON THE MOBILITY OF PROTONS IN Si02

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