A New Characterization Technique for Depth-Dependent
Dielectric Properties of High-k Films by Open-Circuit
Potential Measurement
Koji Kita, Masashi Sasagawa, Kentaro Kyuno and Akira Toriumi
Department of Materials Science, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
Abstract. A new method for characterizing dielectric properties of high-k films was investigated
with an open-circuit potential (OCP) measurement during etching of a film in a solution. The linear
dependence of OCP on etching time was clearly observed. The areal density of the adsorbed ion
charges on the film surface was estimated from the slope in the time dependence of OCP, and it was
shown to be closely related to the electronegativity difference between the atoms on the film surface
and etchant atoms in the solution. This finding implies that the time-dependent OCP measurement
is useful for evaluating the ionic characteristic of dielectric materials. The application of this
technique to a depth-profiling analysis of a multilayer dielectric film was also investigated. The
transition of the etching surface from one layer to another one was clearly observed, which suggests
that the depth-dependent dielectric properties of high-k films can be characterized with this
method.
THEORETICAL BASIS OF THE
MEASUREMENT
INTRODUCTION
Various materials with high dielectric constants
(high-k) have been studied as the possible
candidates for the next generation gate dielectric
film1*. When the high-k films are deposited on Si
substrates, they are often accompanied with a low-k
transition layer at the interface with Si, which
causes a depth-dependent dielectric properties. The
development of a new characterization method,
which is suitable to understand the depth profiles of
dielectric properties of these films, is required. In
this study, we investigate a new method with an
open-circuit potential (OCP) measurement during
chemical etching of a film.
The OCP analysis has been already applied
both for the interface of the chemical oxide 2"3) and
for a few nanometer-thick A12O3 and ZrO2 on Si 4),
however, bulk properties of those films have never
been investigated with the OCP measurement.
When a dielectric film surface grown on Si is
in contact with an HF solution, ions (mainly
negative ions for the present case) are adsorbed on
the film surface and induce charges with an
opposite sign at the interface between Si and the
dielectric film as shown in Fig. 1. As a result, a finite
voltage is generated across the dielectric film,
which is called the open-circuit potential (OCP).
This voltage is given by,
(1)
where Qs, Tox and sox are the areal density of
adsorbed ion charges on the film surface (absolute
value), the thickness, and the dielectric constant of
the film, respectively. 8V is the voltage induced
nearby the film surface in the solution, consisting of
CP683, Characterization and Metrology for VLSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
© 2003 American Institute of Physics 0-7354-0152-7/03/$20.00
166
DC Voltmeter
Reference
electrode
Teflon vessel
/
s
jf -t
/
Sk.
\
Die l(metric film
\
Measurement
HF Solution
FIGURE 2. Schematic of measurement setup. The
HF solution was stirred to keep the etching in a
steady-state.
FIGURE 1. The open-circuit potential (OCP) is the
voltage generated between the Si and the solution,
when a dielectric film on Si is in contact with HF
solution. The negative ions adsorbed on the dielectric
film induce positive charges at Si/dielectric film
interface.
Next, let us consider an application of this
method for depth-profiling analysis. If the film
consists of two layers (Layer 1 stacks on Layer 2),
the OCP can be described as Eq.4, during etching of
the Layer 1.
the voltages across the Helmholtz layer and Guey
layer2).
We consider that Qs will be closely related to
the ionic characteristic of the film. In this paper we
verify this relationship experimentally and show the
OCP measurement can provide a new approach for
the characterization of dielectric properties of the
film. In order to evaluate Qs, the time-dependent
OCP measurement (t-OCP) during the etching of
the film should be examined. If the etching rate is
assumed to be a constant, Eq.l is modified as,
(4)
Note that QS! is determined by Layer 1
irrespective of the existence of Layer 2. Then the
same analysis with t-OCP, discussed above for the
case of a mono-layer film, can be applied for a
multilayer film. Thus characterization of
depth-dependent dielectric properties can be
expected.
In this study, first we apply this method to
various single-layer films to examine and discuss
the validity of our analysis. After that we
demonstrate the applicability of this method for a
depth-dependent analysis of multilayer films.
(2)
where p is the etching rate, T0 is the initial oxide
thickness. If the etching proceeds in a steady-state,
Qs/sox can be evaluated from the relationship
between V and t, because the unknown term 8V can
be assumed to be a constant during etching. In the
case of a steady-state etching of a uniform film
without depth-dependent dielectric properties, Qs
and sox can be considered as constants and V
decreases linearly with time. Then Eq.2 is further
simplified by using p = dTox/dt as,
dV
EXPERIMENTAL
First, the bulk properties of HfO2, Y2O3 and
SiO2 were examined by the OCP measurement. 32
nm-thick HfO2 film was grown on HF-last n+-(100)
Si by RF-sputtering and annealed at 400°C in N2
ambient. 22nm-thick Y2O3 was also grown by
RF-sputtering, and 40nm-thick SiO2 was thermally
grown on the same substrate. Fig.2 shows the
schematic of the OCP measurement setup. The
measurements were conducted in a diluted HF
solution (0.5 wt%). The sample was set into a
(3)
Thus Qs can be determined from the slope of
the linear relationship between V and Tox, if the
dielectric constant of the film £oxis given.
167
Teflon vessel with a 5 mm-diameter window. The
film surface was arranged to contact with the
solution through the window. The solution was
stirred during measurement, in order to keep the
etching condition in a steady state. A bare Si was
used as the reference electrode, which was
confirmed to show a stable potential of -0.23V
against an Ag/AgCl standard electrode. The voltage
between the back contact of Si and the reference
electrode was measured continuously by a digital
voltmeter.
Next, the application of t-OCP measurement
for a depth-dependent profiling analysis was
examined. As an example of multilayer films, an
HfO2/SiO2 stack was examined. The sample was
prepared by a deposition of HfO2 (32nm-thick) on
thermally grown SiO2 (12nm-thick) followed by an
annealing at 400°C in N2 ambient.
that such assumption is generally considered to be
valid for SiO2 etching in an HF solution, and was
determined by the ratio of initial oxide thickness T0
~ 1.2
1000
2000
3000
4000
5000
Time [s]
FIGURE 3. t-OCP curve for a HfO2 film on Si in
0.5 wt% HF solution. A straight line is shown clearly
in the middle stage of etching, before a sudden
potential drop (the endpoint of etching).
RESULTS AND DISCUSSION
Characterization of Bulk Properties with
t-OCP Measurement
The t-OCP for HfO2 film on Si during etching
is shown in Fig.3. It consists of three stages. The
OCP does not reach a steady-state in the early stage,
however, it shows a straight line clearly in the
middle stage. This linear dependence assures a
steady-state etching. In the final stage, the OCP
shows a sudden drop followed by a gradual
recovery, similar to the OCP behavior reported for a
chemical-oxidized surface dipped into an HF
solution 2). The etching endpoint (tencj) can be
detected from these drop and recovery of OCP.
The t-OCPs for HfO2, Y2O3, and SiO2 are
shown in Fig.4. The time-axis is converted to the
decrease in film thickness with etching (pxt),
considering the etching rate (p) difference for each
film, p was assumed to be a constant from the
beginning to the end of etching, referring to the fact
0
10
20
30
40
50
Change in Film Thickness ( T- TQX) [nm]
FIGURE 4. t-OCP curves for HfO2, Y2O3, and
SiO2 measured in 0.5 wt% HF solution. The
time-axis is converted to the decrease in film
thickness with etching, by considering the etching
rate difference for each material. The slope in the
middle stage corresponds to Qs/sox.
Table 1. Comparison of the estimated values of areal density of adsorbed ion charges Qs. High-k films show
higher Qs than SiO2. The errors of Qs come from the imperfection of the linear relationship in Fig.4.
Sample
Dielectric constant sox
Pauling's electronegativity XM (kcal/30)T72~
______(M = Hf, Y, and Si)
Pauling's electronegativity difference
AX=XF-XM (kcal/30)1'"
Areal density of adsorbed ion charges Qs (C/cm2)
Hf02
20
15
1.3
1.2
SiO2
2.7
1.1
2.8
7
1.2 (±0.16)X 1Q'
168
1.9
1.4 (±0.34) X IP'
7
4.7 (±1.2)X 1Q'8
to the total etching time tend. Then Qs/sox for each
material can be determined from the slope in Fig. 4.
The Qs values are listed in Table 1, estimated by
employing typical reported values of dielectric
constants, 4, 20, and 15 for SiO2, HfO2, and Y2O3,
respectively. The estimation errors indicated in
Table 1 come from the slight imperfection of the
linear relationship between the OCP and the
decrease in film thickness. They are considered to
be caused by the experimental instability of the
measurement is a new method to characterize the
ionic characteristic of high-k films.
The electronegativities of the constituent atoms
are closely related to the polarizability of the film.
Although the slight difference of molar volume for
each material is not considered, Qs obtained by our
method is considered to reflect the polarizability of
the film surface. Thus this method is extendable to
the depth-profiling analysis of the polarizability of
high-k films, which often exhibit depth-dependent
dielectric properties including the formation of
low-k layers at the interface with Si. We discuss
such application of t-OCP in the next section. In
addition, this method of characterizing the
polarizability will be powerful from the viewpoint
of understanding the reliability of high-k gate
dielectrics, because the polarizabiltiy critically
affects the dielectric breakdown through the local
electric field enhancement 5) .
The t-OCP measurement was applied for a film
consisting of two layers, HfO2/SiO2, in order to
demonstrate an applicability of t-OCP measurement
for depth-profiling analysis. As indicated in Fig.6,
t-OCP shows two straight lines sequentially in the
middle stage of etching. The estimated values of Qs
\f"";
^4
-»4
1
00
§o ?o § c !
Depth-Profiling Analysis of Dielectric
Properties with t-OCP Measurement
Hfl
,'
o
D
D
N
-».
-*•
Ol
al Density of Adsorbed Ions Qs [C/c
etching conditions or the depth-dependent
properties of the films.
In our measurement, the adsorbed ions on the
surface are mainly considered to be HF2", because
their adsorption process would be the first step of
the etching, on the analogy of a well-known
mechanism for SiO2 etching in a diluted HF solution.
Thus Qs is considered to reflect the difference of
Pauling's electronegativity between the surface
metal atoms (Hf, Y, and Si) in each oxide (shown in
Table 1) and fluorine (= 4.0). The relationship
between the electronegativity difference (AA) and
Qs is shown in Fig.5. The horizontal axis is taken as
AA'2, taking account that their Coulomb energy is
proportional to the square of the amount of charge.
The linear relationship through the origin between
Qs and AX2 is observed, which supports our
consideration that Qs reflects the ionic characteristic
of the film. Thus it is concluded that t-OCP
f
00.1
Si02
A ...
0
2
4
6
8
(XF~XM)2 [kcal/30]
10
500
1000
1500
Etching Time [sec]
2000
2500
FIGURE 6. t-OCP curve for HfO2/SiO2 multilayer
film (data for the middle stage of the etching). The
horizontal axis shows the etching time. Two slopes
were observed sequentially, corresponding to HfO2
and SiO2 etching respectively, as indicated in the
figure. A voltage step between two slopes is
considered to come from the sudden change of Qs at
the interface of HfO2/SiO2.
2
FIGURE 5. Relationship between Qs and AY . AY
is the Pauling's electronegativity difference between
metal atom (Hf, Y, and Si) in the film and fluorine
(=4.0). AX2 is related to their Coulomb energy. A
linear relationship between Qs and AX2 was
suggested (The dotted line is a guide for the eye).
169
from these two slopes, QHf02 and QSi02 are almost
identical to the values obtained from the t-OCP of
single layer films (Table 1). t-OCP shows a step at
the moment of the transition from HfO2-etching to
SiO2-etching, which is considered to occur by a
sudden change of areal density of adsorbed ion
charges from QHf02 to Qsw- The observed voltage
step height is about 0.08 V, which is less than the
expected value (0.20V) from AQs = QHJOI - Qsi02The decrease of the step height and the broadening
of the step are considered to be caused by the
existence of the physical roughness and
compositional intermixing at the interface.
306-312(2001).
4 Okorn-Schmidt, H. R, Gusev, E. P., Carrier, E.,
Buchanan, D. A., Rath, D. L., Callegari, A., Guha, N.
Bojarczuk, A., Gribelyuk, M. and Copel, M.,
"Characterization of Bulk and Interface Properties of
Dielectric Layers and Stacks," in the 4th Physics and
Chemistry ofSiO2 and the Si-SiO2 Interface, edited by
H. Z. Massoud et al, The Electrochemical Society,
2000-2, Pennington, 2000, pp. 505-511.
5 McPherson, J. W., Kim, J., Shanware, A., Mogul, H.
and Rodriguez, J., "Proposed Universal Relationship
Between Dielectric Breakdown and Dielectric Constant"
in Tech. Dig. Int. Electron. Devices Meet. 2002, IEEE,
New York, 2002, pp.633-636.
CONCLUSIONS
A new method for characterizing the dielectric
properties using the t-OCP measurement was
successfully applied both for single-layer and
multilayer dielectric films. The areal density of
adsorbed ion charges Qs for HfO2, Y2O3 and SiO2
were estimated from t-OCP for single-layer films.
They were closely related to the electronegativity
differences between the atoms of the film surface
and the etchant atoms in the solution, which shows
the t-OCP analysis can characterize the ionic
characteristic on the surface of high-k films. From
the t-OCP for HfO2/SiO2 stacked film, the transition
from the first layer etching to the second layer one
was clearly observed. These results show that this
method can be used to examine the depth-dependent
ionic characteristics of high-k films.
ACKNOWLEDGEMENTS
This work was supported in part by a Grant for 21 st
Century COE program, "Human-Friendly Materials based
on Chemistry" and a Grant-in-Aid for Scientific Research,
from the Ministry of Education, Culture, Sports, Science
and Technology in Japan, and by NEDO/MIRAI project.
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170