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Status of Non-contact Electrical Measurements
V.V. Komin, A.F. Bello, C.R. Brundle, Y.S. Uritsky
Defect and Thin Films Characterization Laboratory, Applied Materials Inc., Santa Clara, CA 95054
Abstract. Non-contact electrical metrology includes a variety of characterization techniques used to determine a number
of material/device electrical parameters. These powerful methods, used on full wafers at various processing stages,
complement the traditional device-based contact electrical (capacitance-voltage and current-voltage) measurements of
MO S-based structures. The non-contact electrical techniques are usually built around measurements of surface
photovoltage and surface voltage in combination with illumination and corona charge deposited on a sample. In
principle this allows recombination lifetime, minority carrier diffusion length, and iron contamination density to be
determined for bulk silicon; generation lifetime, doping density and doping profile to be measured for near-surface
silicon; and equivalent oxide thickness, oxide charge density, mobile charge density, total charge density, flat band
voltage, dielectric integrity and other parameters to be obtained for dielectric films. Interface trap density can be used for
qualification of the interface between silicon and dielectric. The non-contact nature of these measurement techniques is
particularly attractive because it makes most of them non-destructive, non-invasive and allows for diagnostics in the
wafer processing stages, rather than waiting for final device characterization. Some methods offer high-resolution fullwafer mapping capabilities. A few of them are destructive by design such as the soft-breakdown field measurements.
Most of the non-contact electrical measurements offer excellent process step isolation, and opportunity for integrated
metrology. They are less expensive, do not require fabrication of the test structures, and require significantly less
preparation and measurement time compared to the traditional MOS-device based analogues. This early and short loop
measurement capability is the most important feature. In some circumstances the fast turn-around of the product
characterization has so high priority that it makes a lower accuracy and/or frequent calibrations, necessary for some noncontact electrical techniques, tolerable (however, if there is no final correlation to device performance they are useless!).
In this paper we review the non-contact electrical measurement techniques most often used in the semiconductor
industry for characterization of bulk silicon, near-surface silicon, dielectrics and interface between silicon and dielectric
films. We provide a comparison of the experimental data with the theory derived from widely accepted publications [15], and discuss the potential sources of discrepancies between the theory of some non-contact measurements and their
implementation in commercial products. These potential discrepancies could cause systematic inaccuracy in
measurements and disagreement between metrology using equipment fabricated by different vendors, resulting in
considerable standardization challenges for the semiconductor industry. We highlight concerns when applying ASTM
standards developed for non-contact electrical measurements [6 - 8] to current characterization of new semiconductor
and dielectric materials. The goal of this paper is to demonstrate advantages and also the challenges in the non-contact
electrical measurements, to define the problem areas and to provide recommendations for possible improvements and
directions to overcome existing problems.
contact electrical measurements provide information
on quality of device components processing before the
device fabrication itself; they have an excellent
process step isolation and short turn-around time; this
makes them especially attractive for use in the
semiconductor industry.
INTRODUCTION
Non-contact electrical measurements have become
important semiconductor characterization tools to
determine a number of material and device-related
parameters, largely because of the availability of
commercial equipment and the non-contact nature of
the measurements. These powerful
methods
complement the traditional device-based electrical
characterization
of
MOS-based
structures
capacitance-voltage
and
current-voltage
measurements, - and also offer new capabilities. Non-
Metrology is playing an important feedback role in
device production. Its evolution is mostly driven by
changes in the device fabrication technology. It is
reasonable to expect that changes in modern devices
will result in changes in the metrology as well. As the
contact methods mostly involve testing of the devices
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
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extreme-low-k dielectrics for back-end-of-line
(BEOL) applications. Even silicon in the device region
is now to be replaced by alternative materials: siliconon-insulator (SOI), strained silicon and SiGe. Noncontact electrical characterization methods need to be
adapted accordingly in order to meet metrology
requirements for these situations (of course this is
difficult when the end users often are unable to clearly
state what it is they need from the metrology!)
or device-related structures, they closely follow any
changes in the device materials or structure via
development of new specifications. Evolution of noncontact metrology is more complicated.
Many of the non-contact methods exploit
approaches that allow, to some extent, determination
of the expected device parameters. If the theoretical
background of a method is well understood and well
developed, and it is applicable over a wide range of the
measured parameters, such a method has a chance to
survive with insignificant modifications within several
device node generations. However, some of the highly
sensitive non-contact electrical measurements operate
reliably only over a narrow range of material situations
and/or have to be recalibrated every time there is a
change in these properties. They are, therefore, useful
for SPC monitoring of well-established processes in
production, but the R&D application of such methods
can be very limited. As a result, a change in the
material or device structure could require the complete
revision of these methods or even their replacement
when industry moves to the next technology node.
In this paper we discuss the current status of noncontact electrical metrology, demonstrate advantages
and challenges in the methodology, define the problem
areas and attempt to provide some recommendations
for possible improvements to address existing
problems. Overall we do not believe the metrology is
yet in good shape for all applications of importance to
the industry.
REVIEW OF MEASUREMENTS, THEIR
IMPLEMENTATION AND THE
OUTCOME
Non-contact electrical measurements are usually
based on different combination of the surface potential
measurements in the dark and under illumination,
surface photovoltage (SPV) measurements, and
applying corona charge to. the sample surface
(exceptions will be also discussed). Historically,
Kelvin performed the first non-contact electrical
measurements in 1881 [9]. Bardeen and Brattain first
described the application of the SPV technique to
characterization of semiconductor in 1953 [10]. In the
period from 1955 to 1961 the increased interest in the
surface voltage and surface photovoltage study of
semiconductors [11-16] resulted in the first full-scale
implementation of a SPV technique in the
semiconductor industry at RCA [17] and, finally, in
the development of ASTM standards for non-contact
measurement of recombination lifetime and diffusion
length [6-8].
We would like to group non-contact electrical
measurements
into
three
categories:
the
characterization of bulk substrate material, the nearsurface region and dielectric films. Characterization of
the interface between substrate and dielectric will be
placed in the last category.
Characterization of bulk substrate
material
Typical parameters for this category of non-contact
electrical measurements are minority carrier lifetime
(r), diffusion length (L), and iron concentration [Fe\.
Typical applications are qualification of the incoming
wafer material by estimation of lifetimes and the
concentration of heavy metals - lifetime-killers that
form deep levels in silicon.
Today, non-contact electrical metrology, which is
well-established for the semiconductor products based
on traditional silicon (Si) / silicon dioxide (SiO2)
technology, faces new challenges. The gradual
transition of the industry from the 130 nm to the 65 nm
node is associated with deployment of new materials,
film thickness, and device structures not seen before.
Front-end-of-line (FEOL) features have approached
atomic dimensions. The term "dielectric" is not only
associated with thermally grown SiO2 anymore. It has
already partially been replaced by other "advanced"
dielectrics with a broad variety of materials from highk advanced gate dielectrics for FEOL to low-k and
Lifetime and diffusion length
Lifetime measurements are widely used for
qualification of semiconductors since the basic theory
of electron-hole pair recombination via recombination
centers (also called traps) was introduced in 1952 by
Hall [18], and Shockley and Read [19]. The
recombination lifetime r is the average time for an
electron-hole pair to recombine. Charge carriers move
randomly through the semiconductor lattice (being
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Estimation of heavy metal concentration
scattered all the time) and this movement can be
described by a diffusion coefficient D. A minority
carrier with a specific lifetime T thus will be found at
an average distance from the point were it was
generated given by equation (1):
The method for estimation of iron concentration in
the bulk region of silicon using a combination of
standard SPV measurements [6] and dissociation of
FeB pairs in />-type silicon was developed by Zoth and
Bergholz in 1989 [1]. Their widely accepted formula
for iron concentration is represented by equation (3):
(1)
'bulk'
This specific distance is called the diffusion length L
of the minority carrier [2].
1
L
Recombination processes at the surface in general,
affect measurements of bulk lifetime and diffusion
length. Diffusion length measurement based on the
ASTM F 391 - 96 standard, however, is affected by
only back-surface recombination while lifetime is
affected by both the front- and the back-surface
recombination.
aft
T
eff
\
b
bef
Similarly, the iron concentration can be determined
using the minority carrier lifetime measured before
and after the FeB pairs dissociation:
[Fe] = Const
1
Rafter
1
(4)
*before ,
Attempts to estimate concentration of other heavy
metals using diffusion length or lifetime measurements
have been recently made. For example, Semiconductor
Diagnostics Inc (SDI) has developed the SPV-Cu
method for measurements of copper concentration by
analysis of the drop of diffusion length due to
dissociation of Cu-Cu pairs. However, this method is
not sensitive to the presence of copper in any form
other than Cu-Cu complex deep level. SEMILAB is
also developing similar approaches for measurements
of concentration of other metals-lifetime killers
primarily based on the PCD by microwave reflectance
(uPCD) method.
S +S
f
(3)
L
where Lbef and Laft are the diffusion length before and
after dissociation of FeB pairs.
To illustrate the effect of the surface recombination
on lifetime measurements performed, for example, by
photoconductivity decay (PCD), we will consider the
simplified case of the large lifetime values equivalent
to condition L > T, where T is the substrate thickness.
The effective lifetime ieff can be described by the
equation (2):
1
1
(2)
T
bulk
In equation (2), the bulk lifetime Tbuik represents the
recombination in the bulk region of the silicon
substrate; the front-surface recombination velocity Sf
represents the recombination at front-surface; and the
back surface recombination velocity S^ represents the
recombination at the back surface. If the Sf and 5^
values are known, we can obtain the bulk lifetime
from measured effective lifetime. Otherwise, the
assumption that the measured effective lifetime gives
to us the value of bulk recombination lifetime will lead
to unpredictable inaccuracy [5].
Implementation of the measurements in commercially
available metrology equipment
Recombination lifetime and diffusion length
measurements have become ubiquitous in the
semiconductor industry in the last decade, mainly
because they are actual electrical properties of
importance. Bulk wafer contamination, either during
wafer growth or subsequent processing, directly
affects their values. Among the recombination lifetime
measuring methods, the most commonly used
techniques are the photoconductance decay (PCD) and
the surface photovoltage (SPV). The major strength of
these techniques is the non-contact nature, rapid
measurement, and the ability to map full wafers. Their
major weakness is the unknown surface recombination
In the case of diffusion length measurements using
approach described in ASTM F 391 - 96 standard, the
back surface recombination begins to significantly
affect the accuracy of the minority carrier diffusion
length measurements once the substrate thickness, T, is
less than four times the diffusion length, L (ASTM F
391 - 96, statement 5.2) [6], a situation uncommon in
1996, but quite ordinary now [5].
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In case of lifetime measurements, both the frontand the back-surface recombination can affect the
accuracy of the measurements. An attempt to resolve
the problem is made in the Elymat technique based
upon photocurrent measurement and introduced in
1988 [21]. In Elymat, a wafer is placed between two
baths of electrolyte that removes the oxide from the
substrate surface and significantly reduces the surface
recombination velocities when the surface is in
accumulation at applied bias. This method, however,
does not provide the measurements of the actual
surface recombination velocity and obviously does not
belong to the category of non-contact metrology. In
addition, similar to f^PCD, the high level of excitation
makes it less sensitive to the iron presence in silicon
[2].
velocity, which may compromise the results. SPV is
widely accepted in the semiconductor industry for
specific determination of iron concentration in p-Si.
PCD is found to have lower sensitivity to iron
contamination due to the high photon injection level
compared to SPV approach [2].
Diffusion length measurements are usually based
on Method B of the ASTM F391-96 standard
exploiting the multi-wavelength illumination source.
Several vendors including SDI and SEMILAB have
developed a rapid full-wafer mapping of the diffusion
length and the iron concentration obtained from
equation (3).
Three non-contact approaches are most commonly
used for recombination lifetime measurements: (1)
lifetime obtained from photoconductivity decay or
uPCD, (2) lifetime calculated from the measured
diffusion length based on the ASTM F 391 - 96
standard, and (3) lifetime from the analysis of SPV
decay after the excitation by a Xenon flash lamp used
in KLA-Tencor Quantox.
Exactly the same approach is used by SEMILAB in
lifetime measurements by jiiPCD. SEMILAB in-situ
(built-in bath) chemical treatment of the wafer surface
is designed to significantly reduce the surface
recombination and allow accurate measurements of the
lifetime using uPCD. Again, this method does not
provide the measurements of the actual surface
recombination velocity. The assumption that for the
surface in accumulation, Sb = 0 is not always valid and
Sb can vary substantially [22, 23]. Hence, this
approach does not provide the solution of the problem.
Surface recombination problem for lifetime and
diffusion length measurements
The effect of the surface recombination is a
common problem for lifetime and diffusion length
measurements including iron measurements.
For non-contact methods, most of the electrical
metrology vendors do use a front-surface conditioning
before lifetime / diffusion length measurements, for
example using corona biasing (SDI, KLA-Tencor,
SEMILAB). However, the implementation of a back
surface conditioning is often a real challenge and
though a surface pretreatment is advised by the
vendors as a remedy to any potential back-surface
effect problem none is implemented. SDI recommends
to use the Enhanced SPV formula and to input Sb
value correction if L > 500 jam, but the user has to
provide the correct back surface recombination
velocity!
The advantage of the diffusion length measurement
method is that the front-surface recombination does
not have any significant effect on the measurements by
design. However, back-surface recombination effect
becomes noticeable at long diffusion lengths. The last
revision of ASTM F 391-96 standard for SPV
measurement was issued in 1996 when typical
diffusion lengths in silicon wafers were in agreement
with the "L < 774" rule for "the most accurate diffusion
length measurements" [6]. Since then, the quality of
silicon wafers has been drastically improved. Today
the typical diffusion length can be up to 3 times larger
than the substrate thickness. The ASTM F 391-96
standard states "an estimate of the diffusion length is
possible when the diffusion length exceeds twice the
thickness" and the diffusion length estimate procedure
was developed for the case where the back-surface
recombination velocity is known [20]. However, a
back-surface recombination velocity measurement is
not commercially available and it also must be
estimated (guessed?). This makes the estimate of the
diffusion length very complicated, with an increased
uncertainty (which we will attempt to determine in the
next section) for the case of L > T.
Since commercial tools for back surface
recombination velocity measurement are currently not
available, and these rates can vary substantially [22,
23], this SDI "SZ? correction" capability does not
provide the solution to the problem.
KLA-Tencor does not even mention any special
treatment of, or correction for, the back surface of the
specimen during the lifetime measurements by
Quantox. Based on considerations described above,
one can conclude that the back-surface recombination
could affect the Quantox lifetime measurements at
high lifetime values equivalent to large L [5].
785
From this data we plotted the values of the measured
diffusion length, incorporating an assumed Sb of 102
cm/s (often considered as typical value for the
oxidized wafers), against what the true L would be for
a variety of actual Sb values. This is shown in Figure 2
(left). Strong deviations are observed at higher L
values. Figure 2 (right) expresses this as percentage
variation in the determined value. It can be seen that
even for what is currently quite modest values of L,
large percentage variations are seen (e.g. 40% at L =
750 um for a true value of Sb of 104 cm/s). At higher L
values the errors become catastrophic. Figure 3
presents similar modeling results for a bare Si wafer,
where an assumed Sb value of 104 cm/s is often
considered typical. Again significant deviations in true
L from measured L are observed at higher L if the real
Sb deviates from the assumed value of 104 cm/s.
Evaluation of the effect of back surface recombination
on lifetime and diffusion length measurements
In order to evaluate the rough magnitude of the
effect of the back-surface recombination on the
diffusion length measurements, the following study
was carried out. A wafer with 150A of thermal oxide
was measured using a commercially available SDI
FAaST-330 tool. Measurements were performed using
"Standard" mode without back-surface recombination
correction and "Enhanced" mode with different Sb
correction values introduced in SDI software in the
range from 10° to 104 cm/s reported elsewhere [22,
23]. The experimental data was compared to results of
the modeling based on the widely accepted publication
of Schroder [4]. In our model, we used equation (7.53)
on page 445 of this reference for a fit for absorption
coefficient vs. wavelength data for silicon.
1000
900
800
700
600
500
400
300
200
100
We have developed a formula that accounts for
effect of the back-surface recombination during
calculations of diffusion length using equation (A7.4)
on page 486 [4]. Diffusion length data from
"Standard" measurements is also used in this
theoretical calculations.
The experimental and theoretical data are plotted
on the same graph illustrated in Figure 1. Analysis of
data in Figure 1 shows that data produced by SDI
"Enhanced" mode is in good agreement with our
theoretical data. So, provided Sb is known we are in
good shape.
0.1
Theoretical Curve
o Measured Data Points
1000
10
100000
Sb [cm/s]
We have estimated how much the diffusion length
L value would deviate from true L, if the "guessed at"
Sb value used for these calculations were different
from the true back surface recombination velocity Sb.
FIGURE 1. Results of the diffusion length L measurements
for "enhanced mode" and different Sb correction values
performed using SDI FAaST-330 tool.
101
2000 -,
500
1000
1500
500
2000
1000
1500
2000
TrueL [jam]
True! [jam]
FIGURE 2. Results obtained using Sb=\Q2 cm/s (typical for oxidized wafer and recommended by vendor) in case if true Sb is
different from the assumed one (left). The results on the left are used to calculate inaccuracy of the diffusion length
measurements AL/L (right) as a function of the L. The wafer thickness used for calculations is 725 urn.
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500
500
1000 1500 2000
1000 1500 2000
True! [um]
True/, [urn]
FIGURE 3. Results obtained using Sb=\04 cm/s (typical for bare wafer and recommended by vendor) in case if true Sb is
different from the assumed one (left). The results on the left are used to calculate inaccuracy of the diffusion length
measurements AL/L (right) as a function of the L. The wafer thickness used for calculations is 725 um.
Figure 5. This Figure demonstrates that noticeable
discrepancies for iron detection at different Sb values
appear at low iron concentrations.
Analysis of these modeling results supports the
ASTM F 391-96 statement 5.2: for L < T/4 (T = 725
jim) the measured diffusion length equals the actual L
regardless the Sb value. However, at large L values,
the inaccuracy of the measurement increases if the
actual Sb is different from the assumed one.
The important bottom line question here, for
which the metrology community needs an answer
from the semiconductor industry, is what
uncertainty in true L is acceptable. Are 40%
variations in determined numbers, associated
purely with the sensitivity of the metrology to Sb,
acceptable?
—Theoretical Curve
o Measured Data
' O.OE+00
l.E+00
Evaluation of the effect of back surface recombination
on iron concentration measurements
Using the same approach as for the diffusion length
modeling, we carried out a study of the Sb effect on
the iron concentration measurements. Diffusion length
measurements were performed for p-type wafer with
introducing different values of Sb correction into SDI
FAaST-330 software. Immediately after the FeB pairs
dissociation, diffusion length was measured for the
same set of Sb values. The iron concentration was
calculated using equation (3). Experimental and
modeling results are presented in Figure 4.
l.E+02 l.E+04
Sb [cm/s]
l.E+06
FIGURE 4. Results of the iron concentration measurements
performed using different Sb correction values. Theoretical
curve is derived using equation (3).
Analysis of results illustrated in Figure 5 (right)
shows that inaccuracy of the iron measurements can be
more than 30% for a Fe concentration of 1 x 1010 cm"3 if
back-surface recombination is not accounted. Today
this may be a tolerable inaccuracy for most of the
semiconductor products (?). However, the next
technology nodes will require lower Fe concentrations,
IxlO 9 cm"3 or better, where the inaccuracy becomes
much worse. Again real guidance from the industry
is needed as to what is acceptable.
Good agreement between experimental and
theoretical data allows us to use this model to estimate
the effect of the back-surface recombination on the
iron measurements. Results of our study are shown in
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1E+10
1E+11
1E+12
Sb=50cm/s
e
o
l.E+1.1 -.
l.E+10 J
S*=10 cm/s
— l.E+09
1E+09
1E+10
1E+11
1E+12
True [Fe ], [cm ]
Iron Concentration [Fe ], [cm"
FIGURE 5. Modeling results of iron concentration [Fe] (left) calculated for 5^=100 cm/s (recommended by vendor for oxidized
wafers and assumed to be constant) vs. iron concentration [Fe] corresponding to Sb values that are different from recommended.
Theoretical curve is derived using equation (3). Inaccuracy of iron concentration [Fe] measurements (right) for different iron
concentrations [Fe] is derived from Figure on the left.
Charge Region (SCR) near to the surface. As with
other lifetime measurements, it is sensitive to various
sources of contamination and defects, particularly
metallic contamination. Metals such as Fe, Cu, Ni in
Characterization of the near-surface
region
certain states form deep levels in silicon and
effectively capture the minority carriers causing
significant reduction of the recombination lifetime.
Typical parameters characterizing the near-surface
region are generation lifetime,
near-surface
recombination lifetime and near-surface doping. It is
the near surface region properties that are most
important in device performance.
Lifetimes
fall
Generation lifetime
into two primary categories:
Generation lifetime is the rate at which minority
carriers are thermally generated when there is a deficit
of carriers. This is a parameter useful for
characterization of contamination by metals (Fe, Ni,
Cr, Co) that could precipitate at the surface and impact
the quality of the device region, especially the CMOS
transistor channel. Generation lifetime is sensitive
mostly to metals-lifetime-killers that form deep-level
defects in the vicinity of the midgap ± 2kT/q.
recombination lifetimes and generation lifetimes. The
concept of recombination lifetime, Tr> holds when
excess carriers, introduced by light or by forward-
biased p-n
junction,
decay
as
a result
of
recombination. Generation lifetime, rgif applies when
there is a deficit of carriers and equilibrium is reached
by thermal generation of the electron-hole pairs [3].
When recombination and generation events occur in
the bulk, they are characterized by rr and %. When it
occurs at the surface, they are characterized by the
surface recombination velocity sr and the surface
generation velocity sg. Knowledge of sr and sg,
Doping concentration
however, is mostly important for metrology in order to
Another device parameter of the near-surface
silicon region is the silicon active doping
concentration. This is the net concentration of ionized
impurities (i.e. the difference between the
concentration of ionized acceptors and donors) in the
improve an accuracy of the lifetime measurements.
Other parameters such as near-surface recombination
lifetime and generation lifetime are used for
characterization of the near-surface region of silicon
similar to the bulk T& and rr.
near-surface SCR.
Near-surface recombination lifetimes
Near-surface recombination lifetime is the rate at
which excess minority carriers recombine in the Space
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Semiconductor Diagnostics Inc (SDI)
Implementation of the measurements in commercially
available metrology equipment
SDI offers a minority carrier recovery time (socalled "£/?/-r") as another approach to estimate the
defectiveness of near-surface silicon. Epi-r is
controlled by generation-recombination processes in
SCR [27], sensitive to deep levels with activation
energy in the vicinity of the midgap ± 2kTlq, It can be
used for an indication of the presence of metal
contamination in the SCR. The Epi-r method is based
on the analysis of small signal AC-SPV. The
principles of this method were originally developed by
Nakhmanson [28] in 1975.
Commercially available metrology equipment from
KLA-Tencor, SDI, SemiTest, QC-Solutions and other
metrology vendors provides capabilities for nearsurface silicon characterization. We will review some
of the solutions provided by these manufacturers.
KLA-Tencor
Direct measurement of generation lifetime is
implemented in Quantox from KLA-Tencor. It is
performed using the corona-oxide-semiconductor
(COS) technique [24] similar to the generation lifetime
measurements for an MOS-capacitor (MOS-C, defined
in ASTM Standard F 1388 [25], that was developed in
1992. Similarly to MOS-C measurements, a guard
ring of corona charge is created around the
measurement site, biasing the silicon to accumulation.
This results in the suppression of injection of minority
carriers laterally into the large SCR, instantly created
by biasing the near-surface region into strong
inversion. Generation lifetime measurements are taken
using charge-time (Q-t) pulsed measurements. The
surface voltage is then determined as a function of
time. The slope of voltage vs. time as carriers are
generated in the non-equilibrium state of deep
depletion is used to calculate the generation lifetime,
Glifetime, as shown in equation (5) [26]:
SDI has also implemented surface doping
measurements (SD) based on principles developed by
Nakhmanson [28]. Surface Doping is determined from
equation (6):
SD =
2\V.
SB
(6)
where VSB is determined from surface voltage
measurements in the dark and under illumination and
capacitance, C, is determined from the following
equation (7):
C = Const -
GLifetime = -
Si
dV
_kT_]
1
colmV,
SPY
(7)
(5)
where co is the angular frequency of light modulation
and Const is a function of the photon flux and
dependent on the geometry of the SPY probe [27].
dt
where dV/dt is the initial slope of the surface voltage
vs. time curve, ssi is the dielectric constant of the
silicon, Wdd is the maximum depth pulsed to during the
deep depletion pulse, and Winv is the equilibrium
depletion depth.
QC Solutions
QC Solutions provide the surface charge, the
doping concentration (resistivity), the near-surface
recombination lifetime, and the depletion layer width
measurements based on SPY [29].
Near-surface recombination lifetime, SRLifetime, is
obtained from the analysis of voltage transient for
weak inversion condition and after the forward biasing
by a visible light source. The recombination lifetime is
determined from the surface voltage decay [26].
SemiTest
Near-surface doping, NSDoping, is the silicon
doping value averaged over the SCR. The
measurement procedure is very similar to that for
GLifetime
measurements. Known interferences
reported by KLA-Tencor for NSDoping measurements
are high density of interface traps resulting in high
surface recombination, dielectric leakage and high
dielectric charging.
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Epimet from SemiTest provides measurement of
the resistivity profile of epitaxially grown silicon
layers. These measurements are similar to CVSchottky or Hg-probe but performed in non-contact
manner [30].
To our best knowledge, these methods for
characterization of near-surface region offered by
different metrology vendors are providing adequate
capabilities that are expected by industry. There is
always room for improvements but at least this
metrology is not a showstopper.
Equivalent oxide thickness (EOT)
Characterization of dielectrics and
interfaces
EOT is one of the widely used parameters for
characterization of dielectrics. EOT is the thickness
that perfect SiO2 would have for a given capacitance
value. In non-contact electrical metrology EOT is
derived from the dielectric capacitance that is obtained
from the slope of the curve of surface voltage as a
function of corona charge ("Q-V curve").
Typical parameters for this category of
measurements are: equivalent oxide thickness (EOT),
leakage current, total charge, flatband voltage, density
of interface traps (Dit\ soft-breakdown field, and
mobile ions concentration.
Corona-oxide-semiconductor (COS) techniques are
used for non-contact characterization of dielectrics and
interfaces [24]. COS, as implemented in KLA-Tencor
Quantox, is based on analysis of the dependence of the
surface voltage and SPY as a function of the corona
charge. SDI has introduced the Corona-Oxide
Characterization of Semiconductors (COCOS)
technique, which is based on measurement of the
contact potential difference (CPD) voltage in dark and
under illumination [31, 32].
Usually, dielectric capacitance measurements are
performed at conditions when the silicon surface is in
accumulation. This is a way to reduce the contribution
of capacitance from the silicon space charge region
(SCR). However, if the gate dielectric EOT is less than
about 10-15 A (depends on material), contribution of
the SCR capacitance cannot be neglected. Moreover,
at deep accumulation, the quantization of charge
carriers in the narrow potential well near the surface
should be taken in account as well. These quantum
mechanical corrections could be in the range of 4-6A
[31], a substantial fraction of the total.
Both COS and COCOS techniques exploit
deposition of the corona charge on the dielectric
surface as a non-contact way to apply bias to the
dielectric and to the semiconductor. If dielectric
leakage is substantial, both methods require
corrections for the leakage current through dielectric.
Figure 6 depicts typical distortion of the CPD curves
caused by dielectric leakage. Substantial leakage could
be due to poor integrity of dielectric material, high
charge on the surface that could result in FowlerNordheim (F-N) tunneling or due to direct tunneling
(DT) that occurs at small thickness regardless of the
quality of the dielectric material.
Another challenge arises because of DT of the
carriers through dielectrics with an EOT less than 25 A.
This results in the distortion of the Q-V curve that
leads to an inaccuracy in capacitance and EOT
measurements.
Different vendors are trying to resolve the EOT
measurement challenges for leaky dielectrics.
For example, KLA-Tencor has introduced the
ACTIV technology to correct the Q-V curve for the
dielectric leakage. According to KLA-Tencor, the
ACTIV is designed to improve the accuracy of the
numerous parameters affected by leakage such as
capacitance, EOT, Dit, Vfb, Vt, and others. We have no
publishable information on this technology.
SDI has introduced two approaches to estimate the
dielectric thickness at substantial leakage current. We
will designate them as "SASS Tox-1" and "SASS
Tox-2". According to SDI [33, 34], SASS Tox-1
determines the surface voltage at condition when
corona current equal to the dielectric leakage current
("SASS" conditions), that could be used as an
extremely sensitive indicator of the thickness of the
particular material under proper calibration procedure
[33, 34]. Modeling of the measurements is quite a
challenge, but even our simplified calculations for the
F-N tunneling case based on formulas (6.76)-(6.79) on
page 392 of the reference [4] demonstrate that SASS
Tox-1 has very weak dependence on the dielectric
constant. On the other hand, it strongly depends on the
dielectric barrier height variation, which is another
FIGURE 6. Effect of the dielectric leakage on the Q-V
measurements. Dot line depicts the curve of CPD voltage
under illumination vs. corona charge without leakage. SDI
COCOS technique was used to measure a 45A HfO2 film
[31].
790
challenge and it remains to be seen what the results
are.
parameter of dielectric material indirectly related to
dielectric constant. The dielectric barrier height cannot
be obtained from SASS Tox-1 measurements.
Therefore, SASS Tox-1 requires calibration for every
new dielectric material. The situation when DT is
significant is even more complicated. First, the
spectrum of distribution of the surface states is
dynamically changing and, generally speaking, is
unknown, and, secondly, the carrier on the surface
state is confined and cannot be considered to be in a
free state. These circumstances do not allow using the
traditional solution of the quantum-mechanical
problem for the tunneling through the potential barrier
of finite height from one free state to another free
state. Therefore, the task of development of a reliable
theoretical model for the carrier tunneling between
silicon conduction or valence band and the surface
state is complicated (details on such estimates are
beyond the scope of this review).
°
o
SDI: y=1.0x+0.1
SSM: y=1.3x-4.2
.a 15 8 9S
7
9
11
13
15
17
19
'Optical" Thickness [A]
FIGURE 7. "Electrical" thickness measurements of DPN
gate oxide using "capacitance" approach, SSM-6200 and
"non-capacitance" approach, SDI SASS Tox-1 vs. "optical"
thickness by ellipsometer Therma-Wave OP-5340.
We described the SASS Tox-1 methodology in
some detail above in order to understand the results of
the following experiment. Measurements of nitrided
gate oxide films of different thicknesses were carried
out using Therma-Wave Opti-Probe OP-5340
spectroscopic ellipsometer (using recipes specifically
developed for DPN ISSG oxides), the SDI SASS Tox1 tool and the SSM-6200 EM-Gate. The SSM-6200
elastic metal gate is a commercially available product
of Solid State Measurements Inc (SSM) providing
virtually preparation free, CV-tests of ultra-thin gate
dielectrics using the traditional CV approach with
sophisticated modeling and corrections developed to
improve the accuracy of the capacitance, EOT and
leakage current measurements [35]. However, this is
of cause a contact method also not a subject for further
discussion here. Results of the comparison of the SDI
determined EOT to the SSM EOT, both plotted against
the ellipsometry-determined thickness, are illustrated
in Figure 7. They are consistent with our evaluation of
the SASS Tox-1 capabilities discussed above, taking
in account that according to SDI, the SASS Tox-1 tool
was calibrated for the optical thickness of thermal SiO2
obtained using ellipsometry. The unity slope for the
SDI curve shows that it is measuring essentially
physical thickness, not a true EOT.
Interface Trap Density (Dit)
Density of interface traps, Dit, is an important
parameter for qualification of the interface between
dielectric and semiconductor. Generally speaking, the
lower Dit, the better quality of the interface. Dit
increase can be caused by different reasons: interface
roughness, presence of intrinsic defects or extrinsic
impurities, and so on.
In order to compare the Dit measurements from
different vendors, we have performed measurements
on a set of 60A HfO2 samples treated using different
pre- and post-deposition conditions. KLA-Tencor
Quantox and SDI FAaST-330 tools were used for the
metrology. Some of the results are shown in Figure 8.
In order to investigate such a discrepancy (one
might suggest an anti-correlation) in the results of Q-V
measurements obtained from different tools, we have
scrutinized the raw data provided by both tools. Figure
9 illustrates the dependence of SPY as a function of
corona charge that is used by Quantox for
determination of Vfb; the dependence of the surface
barrier voltage Vsb as a function of corona charge that
is used in the SDI COCOS technique to determine Vfb,
and Dit; and the theoretical curve of the surface barrier
simplified to the case of an ideal interface (Dit = 0).
One can see that both experimental curves are
saturated at high positive corona charge and do not
reach the theoretical surface barrier values that is,
typically, about 0.8V. The explanation is that in the
real measurements, the illumination intensity is finite
In order to address the increased demand in EOT
measurements for gate dielectrics at substantial
leakage, SDI plans to develop SASS Tox-2, designed
to measure a capacitance calculated from the
derivative of the surface voltage decay taken in the
initial moment of turning the corona current off [33,
34]. The idea of the method is elegant and
straightforward. However, doing this with the required
precision and speed in that "initial moment" is a
791
and, hence, the theoretical surface barrier could not be
reached. The second observation is that the theoretical
curve changes sign for V corresponding to the flat
band condition. Based on this, the flat band voltage
value is established in both COS, used in Quantox, and
COCOS, used in SDI FAaST-330. However, one can
see that the COCOS, Vsb curve does not change its
sign in the experimental data. This brings up the issue
of accuracy of the flat band voltage measurement.
Another issue is that in the COCOS technique, the Dit
spectrum is calculated using the slope of the Vsb vs.
Qc curve. Therefore, one can expect that COCOS Dit
spectrum, i.e. Dit as a function of Vsb, will be
"compressed" because the COCOS Vsb nearly reach
approximately 2/3 of the theoretical surface barrier
height.
SDI COCOS
middle of silicon band gap while COCOS uses the
minimum value found in COCOS Dit spectrum
regardless its position in band gap. At this point we
realize that there is little reason to expect the results of
the COCOS and Quantox measurements to agree,
since different things are actually being measured,
even though they are being given the same names.
Only standardization of the terminology definitions
and Q-V technique procedures can resolve this
situation. While it is interesting to note the anticorrelation observed between the two different tools,
we have no idea what physics is behind this. This anticorrelation is so striking one could almost calibrate
one tool against the other!
>"
g>
.2n
Q l.E+12
KLA-Tencor Quantox^
Theory
(no DIT)
0.8 q
KLA-Tencor
SDI Vsb
I
0)
o
1 2 3 4 5 6 7 8 9 10 11 12 13
Slot No
-2.E+12
0.80 -i
SDI COCOS
FIGURE 9. Comparison of the theoretical surface barrier
voltage with SPV measured by KLA-Tencor Quantox and
the surface barrier voltage measured by SDI FAaST-330.
£0.00
>-0.40
-0.80 J
2.E+12
KLA-Tencor Quantox
In order to improve the Q-V measurements, SDI
has released COCOS-II and VC COCOS technologies.
In COCOS-II the measurement of the CPD voltage
under illumination is not used any more. According to
SDI, the new Variable Corona (VC) COCOS is
designed to improve the flat band voltage
measurement accuracy by reducing the charge quanta
at approaching the flat band conditions.
1 2 3 4 5 6 7 8 9 10 11 12 13
Slot No
FIGURE 8. Results of the interface state density Dit and
flat band voltage Vfb measurements of the set of 60A HfO2
samples using SDI FAaST-330 and KLA-Tencor Quantox
for different pre- and post-treatment conditions.
This makes it quite challenging to convert the
COCOS Dit spectrum to a traditional Dit spectrum, i.e.
Dit as a function of energy through the band gap of the
semiconductor (see Figure 6.31 of the reference [4]). It
could also be confusing for users of COCOS method
that SDI defines difference of the CPD voltage in dark
and under illumination as a surface barrier voltage,
despite the fact that this AVCpD does not actually
reaches the theoretical value of the surface barrier
voltage. COS defines this AVCPD simply as SPV, which
is an accurate terminology in this situation.
Analyzing the SDI Vsb curve behavior shown in
Figure 9, we came to the conclusion that "corona
charge fine tuning" by VC COCOS could be
potentially not enough to resolve the Vfb accuracy
problem. Something else causes the distortion of the
Vsb vs. Qc curve. Our wild guess would be that the
problem is in the light source that does not provide
enough intensity for successful flat band voltage
determination.
Another confusion is in different definitions of Dit
used in COS and COCOS: COS calculates Dit at
792
measurements corrected for "optical" thickness in
combination with Vfb and Dit measurements: Vs is
mostly sensitive to charge on the surface of the film,
Vfb is sensitive to charge in the dielectric bulk located
closer to interface and Dit is a characteristic parameter
of the interface. We would also recommend using the
GLifetime measurements if the effect of the plasma
damage on the underlying silicon is important.
Soft-breakdown and mobile charge
Soft-breakdown and mobile charge measurements
are well understood and well implemented in different
commercially available metrology tools provided by
KLA-Tencor, SDI, SEMILAB and other vendors.
Soft-breakdown tests [36] are definitely destructive
by design and the properties of the dielectrics are
modified during these measurements because high
corona charge is deposited on the surface of dielectric
until it reaches onset of F-N tunneling. The
consequently measured voltage is the soft-breakdown
voltage that is a characteristic parameter for dielectric
strength of the film material.
Stress Induced Leakage Current
Stress Induced Leakage Current (SILC, introduced
by SDI) provides measurements of the leakage I-V
curve for dielectrics. This method is useful for
dielectric integrity characterization of dielectric films
with thickness more than 40A. SILC loses sensitivity
to the interface, dielectric and surface defects when the
contribution of DT becomes substantial. Another issue
is poor correlation to device level leakage data, mostly
because of differences in MOS and COS physics.
Mobile charge measurements of alkali metals
exploit the drift of the ions through the dielectric
lattice at elevated temperature in the range of 130170°C in the electric field created by corona charge
deposited on the surface of dielectric. This biasthermal stress technique is common and widely
implemented in commercial metrology tools. SDI has
expanded the mobile charge technique to measurement
of mobile copper at higher temperature and at higher
electric fields if necessary.
CONCLUSION
In this paper we have reviewed the non-contact
electrical metrology that has become commercially
available in the semiconductor industry. The goal of
the review was to describe the existing capabilities of
the non-contact electrical measurements, to highlight
challenges associated with their applications to new
materials and device structures, and to provide
recommendations for further improvement of this
promising metrology.
The mobile charge measurements provide a fast
estimate of the mobile charge concentration in oxide
and usually are designed for measurements of 1000A
thick oxides. Such measurements on thinner dielectrics
are usually not recommended, primarily because of
lack of sensitivity of the measuring probes to small
variation of the oxide voltage before and after biasthermal stress. Another issue associated with the
mobile copper measurement is poor repeatability
because copper tends to diffuse through the interface
into the silicon while most of the alkali metals stay in
the oxide. However, fast full mapping of blanket
wafers makes mobile charge measurements a very
attractive method for root cause analysis and
troubleshooting the semiconductor processing
equipment.
We provided qualitative and some quantitative
estimates of the accuracy of diffusion length and iron
concentration measurements. We demonstrated that
surface recombination does affect the accuracy of the
lifetime, diffusion length and iron concentration
measurements. Lifetime and diffusion length are the
fundamental parameters for characterization of bulk
substrate material quality (if measured accurately!!!).
In order to understand what measurement accuracy is
actually needed, input from industry is required. This
would help with standardization and expansion of
ASTM standards for new materials, important to
improve the accuracy of the measurements. Only then
can the lifetime and diffusion length methods
effectively be included in a Metrology Roadmap.
Surface voltage
Surface voltage (Vs) measurements play the role of
the basic element to construct more sophisticated
measurements like soft-breakdown, mobile charge
measurements and others. Vs measurements alone
have been used by SDI as a "Plasma Damage
Monitor" ' (PDM) voltage [37]. However, Vs is
sensitive not only to presence of the charge on the
surface of dielectric but also to variation of dielectric
thickness. KLA-Tencor recommends using the Vs
Metrology for characterization of near-surface
region provided by different metrology vendors, in
general, is providing expected capabilities. There is
793
always room for improvements
measurements are not showstoppers.
but
4. Schroder, D. K., Semiconductor Material and Device
Characterization, 2nd Edition, New York: John Wiley &
Sons, 1998, p. 481-484.
these
The issue of the EOT measurement of gate
dielectrics at substantial leakage current through the
dielectric still remains an unresolved problem despite
the numerous attempts to expand the existing
metrology capabilities for EOT measurements below
20A.
5. Schroder, D. K., Meas.Sci.Technol 12, R16-R31 (2001).
6. ASTM F 391 - 96, Am. Soc. Test. Mat., 1996.
7. ASTM F 28 - 91 (1997), Am. Soc. Test. Mat., 1997.
8. ASTM F 1535 - 94, Am. Soc. Test. Mat., 1994.
Dit is widely accepted in industry as an important
parameter for qualification of dielectrics and
interfaces. However, existing discrepancies in the Dit
definitions used by different metrology vendors need
to be resolved by standardizing the Dit definition and
the Dit measurement procedure.
9. Lord Kelvin, Nature (1881).
10. Brattain, W. H., and Bardeen, J., Bell Syst. Tech. J. 32,
1-41 (1953).
11. Garrett, C. G. B, and Brattain, W. H., Phys. Rev. 99, 376387(1955).
Non-contact
electrical
metrology
expands
capabilities of traditional electrical testing, reduces
measurement turn around and cost of ownership.
However, we do not believe it is yet in good shape and
additional efforts are still needed to make this
promising metrology reliable and standardized in order
to address metrology needs of semiconductor industry.
12. Moss, T. S., J. Electron. Control I , 126-138 (1955).
13. Brattain, W. H., and Garrett, C. G. B., Bell. Syst. Tech. J.
35,1019-1040(1956).
14. Johnson, E. O., J. Appl. Phys. Letters 28, 1349-1353
(1957).
15. Quillet, A., and Gorsar, P., J. Physique Radium 21, 575580(1960).
ACKNOWLEDGMENTS
16. Goodman, A. M., J. Appl. Phys. 32, 2550-2552 (1961).
We appreciate the efforts of the suppliers that have
taken tremendous business risks in developing this
specialized metrology equipment in the fast changing
environment of the semiconductor industry needs.
17. Goodman, A. M., Goodman, L. A., and Grossenberg, H.
F., RCA Rev. 44, 326-341 (1983).
18. Hall, R. P., Phys. Rev. SI, 387 (1952).
We would like to acknowledge KLA-Tencor
Corporation, Solid State Measurements Incorporated
(SSM) and Semiconductor Diagnostics Incorporated
(SDI) for provided opportunity to use their recent
metrology for advanced dielectrics characterization.
19. Shochley, W., and Hall, R. P., Phys. Rev. 87, 835-842
(1952).
20. Lagowski, J., Kontkiewicz, A. M., Jastrzebski, L. L., and
Edelman, P., Appl. Phys. Letters 63, 2902-2904 (1994).
21.Lehmann, V., and Foil, H., J. Electrochem. Soc. 135,
2831 (1988).
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DISCLAIMER
Certain commercial instruments are identified in this
review in order to adequately specify the metrology
procedures. Such identification does not imply any
recommendation or endorsement by Applied
Materials, nor does it imply that the instruments
identified are necessarily the best available for the
purpose.
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795