Potential and Limits of Texture Measurement Techniques
for Inlaid Copper Process Optimization
Holm Geisler, Inka Zienert, Hartmut Prinz, Moritz-Andreas Meyer, and
Ehrenfried Zschech
AMD Saxony LLC & Co. KG, Materials Analysis Department, P.O. Box 110110, D-01330 Dresden, Germany
Abstract For future technology nodes with shrunken interconnect dimensions, a thorough texture analysis of the metal
interconnects becomes increasingly important in order to optimize and to control the inlaid-copper process. In
comparison to plane metal layers deposited on wafers, the microstructure of the metal is more complicated in copper
lines and vias which were produced using an inlaid process. Therefore, advanced texture-measurement techniques like
X-ray microdiffraction, electron backscatter diffraction (EBSD), and TEM combined with automated crystallography
analysis (ACT) are needed to obtain the required microstructure information. These complementary methods are suitable
to pick up local as well as integral information on the crystallographic orientation of the copper interconnects and liner
materials. Potential and limits of the available techniques and the respective instrumentation are discussed in this paper.
Examples of process-monitoring capabilities and of development support, especially with regard to interconnect
reliability, are presented.
in principle nucleate at both the bottom and the
side walls of the trenches and vias. The presence of
sidewall-oriented crystallites was verified by X-ray
texture measurements on copper line test structures,
and the fraction of these grains was found to be
dependent on the geometry of the trenches [2-5]. That
means inlaid copper lines are not characterized by a
bamboo-like structure. The microstructure depends on
the nature of the barrier layer, too [6-8]. The influence
of sidewall-oriented grains on EM has not been proven
so far, however, the existence of sidewall-oriented
grains might be disadvantageous because they disturb
a purely columnar grain morphology, and therefore,
they could produce grain boundaries which are aligned
parallel to the metal lines. In the worst case, this type
of growth behaviour could lead to voids inside the
interconnects if the grains on the sidewalls grew fast
enough to close the structure before it was completely
filled underneath. A high amount of twins is inherent
in inlaid copper interconnects, leading to a different
grain morphology and grain-boundary distribution as
compared with aluminum. A comprehensive
characterization and monitoring of the copper
microstructure on patterned wafers is needed in order
to optimize the inlaid copper process, especially with
regard to reliability [9], and to identify the critical
INTRODUCTION
As long as aluminum-based interconnects were
used in semiconductor industry, it was well established
that a bamboo-like microstructure and a strong {111}
texture of the metal lines result in the best performance
and reliability of the integrated circuits. The bamboo
structure has the advantage that the aluminum grain
boundaries are not parallel to the direction of the
interconnect lines. One of the fast diffusion paths for
directed
material transport, responsible
for
electromigration (EM), could be excluded in this way.
The situation has changed with the transition to the
inlaid copper interconnect technology. On the one
hand, the resistivity and the timing delay of copper
interconnects in combination with low-k dielectrics
were found to be superior to aluminum. On the other
hand, copper metallization could only be integrated in
mass production by using an inlaid technology, i.e.,
pre-existing vias and trenches are filled with metal [1],
A diffusion barrier has to be deposited first, followed
by the deposition of a seed layer and finally by
electrochemical deposition of copper. Since the copper
does not grow on a plane substrate, crystallites could
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
469
by common microdiffraction
methods. The
macrostress (a) can be considered as one contribution
to the term (D) rather than D(x) in equation (1), i.e., an
average over a specific volume [14]. Due to this fact,
the measured stress is influenced by the texture of the
copper inside the probed volume. Hence, a complete
determination of the 3 dimensional stress state of
copper lines by microdiffraction studies requires the
knowledge of the texture of the metallization as well
[15]. This is especially due to the large elastic
anisotropy of copper (Young's modulus £Cu{lll) =
191.1GPa, £Cu(100) = 66.7GPa) and the invalidity of
the biaxial strain model for passivated metal lines [16].
The possible influence of the orientation distribution
on the determination of stress from strain values must
be taken into account. Below, we will show an
example for the combination of texture and stress
measurements. The most complete analysis would of
course imply the simultaneous measurement of both
the orientation of single grains (i.e., g(jc)) and their
local stress state G(JC).
issues of the next generation products with further
shrunken dimensions and new dielectric materials
[10]. It will be discussed here which techniques can
provide quantitative information about texture on a
regular basis (e.g., for microstructure monitoring), and
which additional techniques are available for a deeper
understanding of the critical process parameters and
the film growth behaviour and their impact on the
microstructure as well.
The next paragraph will shortly summarize
the general concept of a complete microstructure
characterization of interconnects. The following
chapters will demonstrate which information the
available experimental techniques for texture analysis
(X-ray (micro)diffraction, EBSD, ACT) can provide,
and where the limits are.
MICROSTRUCTURE
CHARACTERIZATION OF COPPER
INTERCONNECTS
X-RAY TEXTURE ANALYSIS CLASSICAL ORIENTATION
DISTRIBUTION
The following microstructure parameters of
interconnect materials are of importance: grain size,
texture, stress, and all kinds of defects of filled
trenches and vias. In addition, seed layer and diffusion
barrier have to be characterized. A complete
microstructure characterization should therefore imply
the determination of a general microstructure function
as defined by Bunge [11]:
1
/(;c) phase
g(#) orientation
D(x) defects, lattice strain
X-ray microdiffraction is one of the most suitable
nondestructive methods for classical texture analysis
of inlaid copper interconnects. Several (hkl) pole
figures of the material (e.g., (Ill), (200), and (220) for
copper) have to be recorded in order to obtain the
necessary information about the orientation
distribution function fig) (ODF) of the crystallites
[17]:
(1)
dVzIV
-j—
dg
G(x) specifies phase /, crystal orientation g, as well
as lattice defects D (including local residual stress) in
any volume element of the material at the position x of
the sample. The first part of this function, i(x\ is of
concern if the microstructure of the barrier metal has
to be included in the characterization of the
interconnects (e.g., determination of the fractions of ocTa and (3-Ta or other phases like TaN) or when copper
alloys are used instead of pure copper. The whole
information on texture is contained in the term g(x).
The experimental determination of g(jc), including a
discussion of the current prospects and limits will be
the main topic of the next paragraphs. Additionally,
the mechanical stress is of particular interest with
regard to stress-induced migration that results in stress
voiding [12, 13]. The first-order strain or macrostrain
(E) is the quantity which is experimentally accessible
(2)
The crystal orientation g (i.e., the orientation of the
crystallographic axes with respect to the chosen
sample reference axes) is expressed by the Euler
angles ^, O, <fa [18]. The ODF considers the volume
fractions dVg of crystallites with orientation g in the
irradiated sample volume V with an orientation spread
dG. Software for the calculation of the ODF is
commonly based on one of two general approaches,
the harmonic method or the direct methods, e.g., the
ADC algorithm (Arbitrarily Defined Cells method)
[19], and WIMV (Williams-Imhof-Matthies-Vinel)
algorithm [20]) [21]. The direct methods provide an
improved ODF approximation in the case of sharp
textures. Quantitative information about the volume
470
texture of one single metallization layer can be
determined. Particularly, the monitor wafers were
processed up to the first metal layer only, and are
passivated by an etch stop layer (SiN or SiCN). One of
the advantages of X-ray texture analysis is that
common passivation layers on top of the interconnects
do not disturb the measurements. The X-rays also
penetrate a relatively thick ILD layer on top of copper.
This is advantageous if the influence of further process
steps (e.g., thermal treatments, influence of capping
layers) on the copper texture has to be studied or
monitored.
fractions of certain texture components can be
extracted from the calculated ODF. Some
complementary information is already contained in the
pole figures themselves (e.g., the full width at half
maximum (FWHM) of a pole which gives the
orientation spread of a texture component). From the
experimental point of view, the measurement of pole
figures has become more convenient and less time
consuming since sensitive X-ray area detectors were
available [22, 23]. Area detectors with a high quantum
efficiency (e.g., General Area Detector Diffraction
System, GADDS) allow detection of very small
diffracted signals which is beneficial for test structures
with a low density of Cu lines, and reduce the required
set of angles {%;()>} for the acquisition of a pole figure
considerably due to the fact that a whole segment of
the reciprocal space is recorded in just one frame.
Additionally, the segments of several (hkl) diffraction
cones (e.g., Cu (111) and (200)) can be captured
simultaneously with a GADDS system. A limitation of
X-ray texture analysis is that it cannot spatially resolve
the location of individual crystallites which contribute
to the diffraction pattern, i.e., it does not resolve the
coordinate x in equation (1) on a scale which is of the
order of the mean grain size in inlaid copper
interconnects. Modern X-ray microdiffraction tools
with suitable primary-beam optics and small beam size
(typically 50 - lOOjam at normal incidence) facilitate
the measurement of pole figures on small test patterns
consisting of inlaid copper interconnects, and the
microstructure can also be mapped over whole wafers
at specified positions. But the beam shape and
intensity conditions are not sufficient to strike single
crystallites only. For this purpose, synchrotron
radiation with submicron beam diameter is needed.
Encouraging results have been demonstrated recently
[24,25].
FIGURE 1. {111} pole figures of inlaid copper lines (line
width w = 180nm) for (a) ILD #1 (Si(F)O) with SiN
passivation, (b) ILD #2 (SiCOH) with SiCN passivation. A
broadening of the {111} pole perpendicular to the length of
the trench (direction TD\ as explained by the inset in Fig.
2) is clearly visible for ILD #2.
The main component of the texture in 180nm wide
lines was always a strong {111} fiber with the (111)
direction along the wafer normal z (Fig. 1).
Additionally, the {111} planes of {511} twins and an
engaged {111} component with preferred in-plane
orientation ((110) along the metal lines) were
identified. The fraction of grains with their {111}
planes parallel to the sidewalls of the trenches are
negligible. The fiber texture identified here is
consistent with the observation of earlier studies on
larger (i.e., several mm wide) test structures and
copper-line widths between Ijam and 350nm [4, 5].
But the fraction of sidewall-oriented grains is
significantly lower in the 180nm wide trenches as
compared to the 350nm wide trenches. The main
reason for this observation is that different platingprocess conditions were used for the production of the
inlaid copper lines in [4, 5]. The 180nm and l.Sjam
wide lines which are the subject of the present study,
were filled using process conditions which were
especially designed to prevent the growth of
crystallites on the sidewalls of the trenches. The results
from the microstructure monitoring prove that the
X-ray Microstructure Monitoring
The following example will demonstrate the
applicability of X-ray microdiffraction to monitor the
texture of inlaid copper lines on test patterns with
different interconnect geometries. Pole figure analysis
is needed to characterize the texture of inlaid
interconnects with sufficient accuracy, and to detect
the fraction of sidewall-oriented crystallites. The
results shown here were collected over several weeks
on arrays of parallel copper lines with line widths of w
= 180nm (pitch p = 360nm, depth d = 450nm) and
1.8pm (p = 3.6jam, d = 450nm), respectively. The
diameter of the X-ray beam of SOjam allowed analysis
of test structures with small lateral dimensions
(between 150jjm and some hundred micrometers). The
test structures used are designed in such a way that the
471
The orientation spread of the {111} fiber texture,
monitored over several weeks, is plotted in Fig. 2 for
lines with w = 180nm and l.Sjjm. The FWHM of
the {111} component was determined using %-cuts
through the center of the pole figures in the directions
along and transverse to the metal lines (directions
'RD' and TD', respectively, as explained by the inset
in Fig. 2). The FWHM of the {111} component was
constant over time for the standard process using
ILD#1 (Si(F)O) with a standard etch-stop layer (SiN),
demonstrating the stability of the process. The FWHM
values for the 180nm and l.Sjam wide lines are almost
the same, so that the line width does obviously not
have a significant influence on the strength of the
{111} texture in this range. However, a significant
increase of FWHM(TD) was observed for ISOnm wide
metal lines when the copper was inlaid in ILD#2
(SiCOH) which was covered with a SiCN layer (Fig.
2, week 10), leading to an 'anisotropic' texture with
FWHM(TD) « 2 • FWHM(RD). This change was not
found for 1.8|am wide lines, as shown in Fig. 2b.
texture of these inlaid interconnects is in accordance
with this process.
$3H
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2,8-1
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FIGURE 2. FWHM of the Cu {111} pole for copper lines
with w = ISOnm (a) and w = l.Sum (b), representing the
orientational spread along (*RD') and transverse ('TD') to
the metal-line direction. Other ILD and etch-stop layers
(SiCOH/SiCN) were used in week 10. Weeks 1-9:
Si(F)O/SiN.
I/ZZZJ {111} total
••{111}
1{111}[10-1]
40
10
9
Barrier Layer
10
Week
FIGURE 3. Volume fractions of the texture components
for narrow copper lines (w = 180nm), calculated from
the ODF. Weeks 8-9: ILD #1, week 10: ILD #2. (Ill)
[0 1 -1] and (111) [1 0 -1] characterize the grains with
engaged in-plane orientation, i.e., (110) along the lines.
FIGURE 4. Dependence of copper texture in ISOnm wide
copper lines on the type of barrier layer, (a) volume fractions
of texture components, (b) FWHM(TD) and FWHM(RD).
All samples are inlaid in ILD #2 (SiCOH), except 'Ta
TEOS\ which is inlaid in Si(F)O.
472
Additionally to the FWHM, the different texture
components were quantified by calculating the ODF.
The software LaboTex, based on the ADC algorithm,
was used for this purpose [26]. The resulting data of
the 180nm wide lines are summarized in Fig. 3 for
weeks 8-10. The volume fraction of the {111} fiber,
including the engaged component {111}{110), was
larger than 45%. The individual volume fractions of
the twins and the engaged {111}(110) grains are
below 10%. In contrast to the broadening of the
FWHM(TD) for ILD #2, there is no big difference
visible in the volume fractions of the texture
components when copper is inlaid in ILD #2 (SiCOH)
as compared with ILD #1 (Si(F)O). Due to the
difficulty of determining the background in the pole
figures precisely, the amount of randomly oriented
crystallites cannot be quantified accurately. This leads
to a large uncertainty with respect to the fraction of the
random texture component and, therefore, the sum of
{111}, engaged {111}, {511}, {611}, and {5 7 13}
components (according to Fig. 3) is uncertain as well.
Nevertheless, the relative volume fractions of those
texture components are reliable (except for the
randomly oriented crystallites).
Copper Texture in Trenches with Different
Barrier Layers
The integration of new and reliable diffusion
barriers will be an important challenge for future
technology nodes, especially for inlaid structures with
low-k dielectric materials. The volume fraction of the
{111} fiber, including the engaged components
{111}{110), can be considerably changed if another
type of barrier is used or if the deposition process of
the barrier layer is different. For instance, a differently
deposited Ta barrier (Ta resputter', Fig. 4) leads to a
remarkable reduction of the Cu {111} volume fraction
as compared with the process of record Ta Std.' (both
with ILD #2) and Ta on Si(F)O (Ta TEGS'), whereas
the fraction of second-generation {5 7 13} twins is
increased. The FWHM(TD) is also considerably
broadened for the Ta resputter' process (Figs. 4b. and
5b), but FWHM(RD) is almost unchanged. The
Ta/TaN/TiSiN/Ta barrier layer stack results in an
almost isotropic broadening of the FWHM of Cu{111} (Fig. 5d), and, compared to Ta Std', the
volume fraction of the {111} texture decreases as well.
An influence of the barriers on the engaged Cu {111}
component is also observed. As a matter of fact, the
use of both Ta/TaN/TiSiN/Ta' and Ta resputter'
barrier layers resulted in different EM behaviour
compared to the Ta Std.' barrier, which is probably
related to the change of texture. It can be concluded
from the pole figures that the barrier layer was
modified on the bottom of the trenches and at the
lower part of the sidewalls, i.e., close to the edges
formed by the bottom and the sidewalls, for the
process Ta resputter'. As a result, a fraction of copper
crystallites grew similar to sidewall-oriented grains,
but with a slight tilt (smaller than about 10 deg.) of
their {111} lattice planes relative to the sidewalls. This
leads to the two additional bright spots in direction
TD' (at positions 9 o'clock and 3 o'clock) close to the
outside margin of the {111} pole figure in Fig. 5b. The
additional spots are only present in direction TD'
because of the infinitely long lines with the line axis
oriented along 'RD'. The considerable broadening of
the {111} pole at the centre of the pole figure in Fig.
5b in direction TD' can also be explained by this type
of crystallites. It is caused by the symmetry-equivalent
{111} lattice planes of this group of crystallites.
FIGURE 5. Cu {111} pole figures of 180nm copper lines in
ILD #2 (SiCOH) with (a) standard Ta barrier (Ta Std'), (b)
differently deposited Ta (Ta resputter'), (c) TaN resputter',
and (d) Ta/TaN/TiSiN/Ta.
473
Additionally, the (111), (200), and (220) pole
figures can be recorded during the same measurement
series. This set of pole figures is sufficient to calculate
the ODF and to reconstruct the {311} pole figure from
this ODF (Fig. 6b). As an advantage, the texture of the
inlaid structure is characterized at exactly the same
position where the stress measurements are performed.
During the 2D- or 3D-stress analysis, the distortions of
the segments of the {311} diffraction cones are
evaluated for multiple pairs of angles (%; <(>), and the
complete stress matrix is calculated, including the
shear-stress components. The anisotropy factor of
copper is also taken into account. The advantages of
this method are obvious. The option exists to correlate
the obtained stress values with possibly observed
changes of the texture in the investigated inlaid
copper-line test structures. Since the {311} pole figure
has been measured, it is known which groups
of crystal orientations contribute to the stress
measurement at a certain set of angles (%; (j>), e.g., if
the diffracted intensity includes the fiber as well as the
engaged texture component or if it includes the fiber
only. A limitation of this type of analysis is the
relatively low intensity due to the small amount of
copper grains in the irradiated area of typically lOOjjm
x lOOjiim.
Characterization of the Texture for X-ray
Stress Measurements
The use of an X-ray microdiffractometer with an
area detector (Bruker D8 with GADDS in our case), a
precise V4 circle Eulerian cradle, and a laser-video
microscope for an accurate sample adjustment make it
possible to perform both stress and pole-figure
measurements on patterned wafers on the same test
structures directly in series. The use of a GADDS
system allows a 2-dimensional (triaxial) stress data
analysis [27]. This is needed for encapsulated inlaid
Cu lines for which a biaxial stress model cannot be
assumed to be valid. In principle, the pole figure of a
higher order (hid) reflection of Cu (e.g., {311}) has to
be measured at first to find the orientation distribution
of the {311} crystallites as a function of the angles (%;
(|>) (Fig. 6). The stress measurements can be optimized
then by choosing the optimum angles where the
highest possible intensities will be obtained. This
concept is indicated by red lines in Fig. 6a.
In-Line Application
A limited number of (split) lots for technology
development and optimization, the high costs
particularly of 300mm wafers and of SOI wafers as
well as the reduction of the response time in case of
process excursions are driving forces for the
development and implementation of in-line tools for
interconnect microstructure analysis using an X-ray
based metrology [28]. Monitoring of process stability
for liners and interconnects and measurement of timedependent microstructure evolution (e.g., texture
evolution in interconnects upon annealing [29]) are
typical examples for in-line applications. It has been
described in which way the experimental setup, the
data acquisition and the evaluation method should be
designed and optimized to obtain a reasonable
compromise on accuracy and speed. Such an
experimental setup is normally optimized for certain
materials combinations like Cu, Ta, Ta(N), ILD. This
strategy aims at in-line application of X-ray tools for
copper and barrier microstructure monitoring with the
highest possible automation, whole wafer capability,
high throughput, including wafer mapping and trend
chart capabilities for texture [30]. Furthermore, X-ray
metrology tools with the additional capability to
FIGURE 6. (a) Cu {311} pole figure of copper lines with
w = 180nm. The positions for stress measurements are
indicated by red lines, (b) reconstructed {311} pole figure,
calculated from the ODF, rotated by 90°. Contributions to
the {311} pole figure from the {111} oriented grains (fiber)
and {511} oriented grains (twins) are indicated.
474
perform a concomitant residual stress analysis are
desirable, as shown in the previous paragraph.
EBSD AND ACT - ORIENTATION
STEREOLOGY
b)
GOOnm = 15 steps
P01]
600nm = 20 steps
111
FIGURE 7. EBSD inverse pole-figure (IFF) map on 200nm
wide copper lines, plan view (a). EBSD IFF map on crosssections through vial, via3, via5 chains (b), showing the
orientation of the Cu crystallites inside the vias. Orientations
are defined by the color coding of the orientation triangle at
the bottom margin of the figure, and are referenced to the
wafer normal for both (a) and (b). 23 twin boundaries are
marked by black lines. The orientation stereology g(x) is
obtained with this technique.
475
The main advantage of OIM (Orientation Imaging
Microscopy), including EBSD (Electron Backscatter
Diffraction) and ACT (Automated Crystallography for
the TEM), is that these methods directly provide the
orientation-location distribution g(x). This is
complementary to X-ray (micro)diffraction, which
provides information over an integrated sample
volume (resulting in (g) instead of g(x)) with
penetration depths of at least several microns,
containing a larger number of crystallites. Critical
locations for stress migration or electromigration are in
particular the vias and their neighborhood. Therefore,
the orientation of single grains inside specific vias or
via chains is of primary interest. To get access to this
location and to measure the local texture exactly there,
EBSD or ACT can be performed on cross sections.
Additionally, OIM provides important information
about the grain sizes. If EBSD is made in plan-view on
inlaid copper interconnects, the in-plane grain sizes
can be measured, and, due to the fact that the relative
orientations of neighboring grains are resolved, twin
boundaries (e.g., £3) as well as small angle and high
angle grain boundaries can be identified. As an
advantage compared to X-ray texture analysis, OIM
permits the investigation of the distribution of the
different types of grain boundaries in inlaid copper
lines or vias. On the one hand, it is desirable to record
OIM images directly on EM test structures before and
after (or even during) an EM test, and to correlate the
EM behavior directly with the grain orientation, grain
size distribution, and the types of prevailing grain
boundaries. On the other hand, the test structures have
to be encapsulated by passivation layers in order not to
falsify the EM results by the presence of any free
copper surfaces. This can be a limiting experimental
factor for EBSD measurements, since this technique
does not work if the interconnects are covered with
passivation layers thicker than about SOnm,
particularly if narrow copper lines with line widths less
than 200nm are studied. It also depends on the line
width of the passivated copper lines whether the
quality of the obtained Kikuchi patterns is sufficient
for an orientation analysis or not. Larger copper
structures (e.g., contact pads) can still be imaged
through slightly thicker passivation layers than narrow
copper lines using EBSD.
The mean grain diameters can be extracted from an
EBSD image if the grain sizes are larger than about
30-50nm [32]. This allows a grain-size monitoring
of inlaid copper interconnects, in addition to the
monitoring of the texture using X-ray microdiffraction
and OIM. Thus, the combination of OIM and
X-ray techniques leads to a more comprehensive
microstructure monitoring of inlaid copper
interconnects. In principle, X-ray diffraction also
contains information on the grain size, however, in the
case of the commonly used diffraction geometry, in
the direction perpendicular to the surface of the wafer.
Furthermore, the calculation of grain sizes from X-ray
diffraction data is more challenging due to the possible
influence of microstrain, grain shape and instrumental
parameters on the measurements.
Microstructure Monitoring using OIM g(x) and Grain Size
Fig. 7a shows an example for a plan-view EBSD
inverse pole figure (IFF) map of an array of 200nm
wide inlaid copper lines after plating, polishing and
annealing. The measurements were performed at a
LEO 1550 SEM (Scanning Electron Microscope) with
a thermal field emission gun. Typical experimental
parameters are an operating voltage of £/B = 20kV, 70°
sample tilt, 3jjm x lOjum scan size, and 40nm step
width. The EBSD system used here is from TexSem
Laboratories (TSL). Most of the copper grains have a
{111} texture as clearly seen in Fig. 7a. This is
consistent with the results from X-ray microstructure
monitoring shown in the previous paragraphs. The
twin boundaries are marked by solid black lines on the
EBSD image. Besides {111} oriented grains, a certain
amount of {511} and {611} twins was identified.
There is only a very small number of {100} or {101}
oriented crystallites. Pole figures can be calculated
from the EBSD patterns on inlaid copper lines as well
(see, e.g., [31]), but their angular resolution is worse
than that from X-ray texture analysis (Fig. 8). A
typical EBSD scan on arrays of inlaid copper lines,
like the one shown in Fig. 7a, covers an area of several
tens of pin2, containing a much smaller number of
crystallites compared to X-ray microdiffraction with a
beam diameter of about lOOjam. Therefore, the volume
fractions of texture components that were obtained
with X-ray diffraction are statistically more significant
than those from EBSD, leading to more detailed pole
figures. On the other hand, EBSD provides the
orientation of single crystallites together with their
coordinates, and thus directly determines the
orientation function g(x) as defined in equation (1).
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twins as separate grains
twin boundaries (s3) removed
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FIGURE 9. Monitoring of the mean grain diameter on
arrays of 200nm wide copper lines. The data were obtained
from EBSD images as shown in Fig. 7a. For the solid
squares, £3 twins are treated as separate grains, for the
empty circles, the twin boundaries have been removed [33].
Weeks 1-7 correspond to weeks 1-7 in Fig. 2, respectively.
The results of grain-diameter monitoring with
EBSD for seven weeks on test structures with arrays of
200nm wide inlaid copper lines are plotted in Fig. 9.
The mean grain diameter was determined considering
the S3 twin boundaries as grain boundaries as well as
by excluding the Z3 twin boundaries. The week-toweek variation of the mean grain diameter is marginal,
documenting the high stability of the inlaid copper
process. The values of the mean grain diameter are
close to the copper line width, and, as expected, they
are systematically larger if the £3 twin boundaries are
removed during the analysis. The grain sizes were
evaluated on EBSD images like the one shown in Fig.
7a. Sidewall-oriented copper grains were not detected
with EBSD in 200nm wide inlaid copper lines (see,
e.g., Fig. 8a), which is consistent with the X-ray
diffraction results shown in the previous paragraphs.
RD
FIGURE 8. (a) {111} pole figure, calculated from a 3|im x
10pm EBSD map of an array of 200nm wide copper lines,
like shown, e.g., in Fig. 7a, and (b) {111} pole figure,
obtained from X-ray microdiffraction with SOjam beam
diameter on an array of 180nm wide copper metal lines.
476
Grain Orientation in Vias
SUMMARY
Fig. 7b shows three examples for EBSD IFF maps
of via-chains (vial, via3, via5) in cross-sectional view.
The cross-section samples were prepared by Focused
Ion-Beam milling (FIB), and they were subsequently
transferred to the SEM/EBSD system. The EBSD
images were recorded with step widths of 20nm 40nm. The reference axis for the color coding of the
orientations is the wafer normal so that the same colors
are related to the same grain orientations on both the
cross-sections and the plan-view images (e.g., blue
grains have their (111) direction parallel to the wafer
normal). The types of grain boundaries can be derived
from the EBSD images. Especially, the EBSD images
reveal if there exist grain boundaries which are
oriented parallel to the direction of the current flow
through the via chains. Since the information depth of
EBSD is confined to a very thin surface layer (in the
order of some tens of nanometers), the inlaid copper
structures are not probed through their whole
thickness. A future perspective of this type of
measurement is to perform sequential FIB cuts and to
take a series of EBSD images. A 3-dimensional
orientation image of the inlaid structure is created then
by a 3D reconstruction procedure.
The capabilities of X-ray microdiffraction and OIM
techniques for microstructure analysis on inlaid copper
interconnects have been demonstrated. Texture, grain
size, and the first order stress of arrays of filled inlaid
copper lines (as well as of arrays of short inlaid line
segments) can be thoroughly characterized using X-ray
microdiffraction and EBSD. The influence of the line
geometry of the inlaid line structure, the deposition
conditions, and the type of ILD and barrier layers on
the copper texture in filled trenches can be studied. Xray diffraction and EBSD are complementary
techniques since the first method integrates over a
larger sample volume, resulting in statistically more
significant texture data (e.g., engaged texture
components can be directly identified in pole figures),
whereas EBSD additionally provides grain boundary
distributions, grain sizes, and the grain orientation
stereology g(x). EBSD is also a suitable method for
studying the local grain orientation, grain boundary
distribution, and grain size distribution in via chains.
X-ray texture analysis requires arrays of inlaid copper
structures.
ACT will be needed for studies of texture and grain
size in barrier and seed layers for inlaid interconnects
since EBSD and X-ray diffraction reach their limits in
this case.
ACT for Microstrncture of Diffusion
Barriers and Seed Layers
Until now, only the copper microstructure of
completely filled inlaid interconnects has been
discussed. For diffusion barriers and nucleation (seed)
layers, EBSD and X-ray diffraction reach their limits
with respect to texture and grain-size analysis because
of the tiny grain sizes. Therefore, a complete
microstructure analysis of inlaid metal lines or vias,
including the barrier and seed layers, will only be
possible using ACT as complementary technique, even
though it is much more time-consuming. EBSD will
not have the capability to resolve the small crystallites
of the nanocrystalline barrier layers anymore, and Xray measurements on inlaid copper interconnect
structures result in very poor diffracted intensities
from the barrier layers, since typical beam diameters
are less than lOOpm. The situation is similar for seed
layers. The lateral resolution of EBSD is close to the
mean grain size of copper seed layers, so that this
method could in principle still work in this case. But it
will be critical to image the seed layers in inlaid
structures with EBSD, in contrast to blanked thin films
of comparable thickness.
ACKNOWLEDGMENT
Discussions with Holger Saage (AMD Saxony,
Dresden, Germany) about the ACT method are
gratefully acknowledged.
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