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State-of-the-art Characterization For 65 nm CMOS Processes And Beyond
M.J.P. Hopstaken1, M. Juhel2, J.P. Gonchond2, L.F.Tz. Kwakman1, C. Wyon3
1
Philips Semiconductors, 860 rue Jean Monnet, 38920 Crolles, France
2
STMicroelectronics, 850 rue Jean Monnet, 38926 Crolles, France
3
CEA-DRT-LETI CEA/GRE 17, rue des martyrs 38054 Grenoble cedex, France
Characterization of advanced CMOS devices and process control
at different stages in front-end of line (FEOL) areas requires stateof-the-art analytical techniques and measurement protocols. We
employ Dynamic Secondary Ion Mass Spectrometry (D-SIMS) for
in-depth quantitative analysis of both bulk- and dopant
concentrations in GexSi1-x binary alloys. A similar D-SIMS
protocol –in combination with Auger Electron Spectroscopy (AES)
– allows monitoring the different nickel-silicide phase transitions
upon silicidation of (doped) Ni Totally Silicided (TOSI) gates.
Combination of Low-Energy Electron X-ray Emission
Spectroscopy (LEXES) and high-precision DSIMS provides
accurate control of dose, junction depth, and wafer uniformity for
Ultra-Shallow Junctions (USJ) with high analytical throughput,
opening the way to USJ metrology. Similar techniques (LEXES,
SIMS) are employed to control wafer uniformity of deposited NiPt
metal films as USJ contact metal, to monitor NixSi1-x phase
transformations and Pt-redistribution upon silicidation, and to
determine root causes of defectivity at device level (-AES). The
given examples demonstrate the importance of having available
(novel) state-of-the-art characterization techniques –pushing the
analytical limits in terms of resolution, sensitivity, and accuracy
while simultaneously increasing throughput and reproducibility– to
support process development and to speed up yield learning of
advanced CMOS processes.
Introduction
Continuous shrinking of device dimensions and integration of novel materials in
advanced CMOS processes, pose big challenges with respect to physical- and chemical
characterization of such devices. In practice, this demands for continuous improvement
of existing analytical techniques in terms of depth- and lateral resolution and sensitivity,
while maintaining accuracy and analytical throughput (1). The latter is of crucial
importance to support process development, improve yield, and accelerate the
introduction and ramp-up of next-generation CMOS technology nodes. Automation and
full wafer compliance of analytical tools are prerequisite to enable high-volume
throughput and verification of process homogeneity on wafer level. In parallel,
progressive integration of novel non-Si materials requires the development of novel
analytical approaches and evaluation of complementary and novel analytical techniques.
In this paper, we will present and discuss the merits of several analytical techniques and
specific measurement protocols for the characterization of transistor devices and the
control of front-end of line (FEOL) processing.
A growing interest is emerging for channel strain engineering through integration of
GexSi1-x in advanced CMOS processes. Strained epitaxial layers of doped Ge xSi1-x
Source/Drain regions may be applied to control local strain (2). Alternatively, use of
strained Si on a virtual substrate (based on strain-relaxed GexSi1-x on graded buffer) may
provide a way to achieve global strain engineering. Integration of full Germanium layer
(as Germanium-on-Insulator: GeOI) could be an interesting alternative as semi-conductor
alternative to Si and SiGe, for reasons of high carrier mobility and lower bandgap for
lower operating voltages (3). From the analytical point of view, these developments urge
for quantitative in-depth analysis of trace dopant- and bulk Ge-levels in GexSi1-x with
high Ge-content. Quantitative SIMS depth profiling is routinely applied to control such
critical parameters in GexSi1-x Hetero-junction Bipolar Transistors (HBT). Good
quantification based on Si-standards has been demonstrated for limited Ge-content
(GexSi1-x; x <0.3) using an O2+ ion beam under oblique incidence (4). Under these nonoxidizing conditions the sputtering behavior of GexSi1-x alloy is very similar to silicon.
Under (more) oxidizing conditions however, GexSi1-x depth profiling using a low-energy
O2+ ion beam close to normal incidence has been demonstrated to lead to variable altered
(oxide-) layer formation (5). As a consequence, both the sputter yield and the ionization
efficiency become strongly dependent on the analytical conditions (impact energy/angle
of incidence) and Si1-xGex composition, complicating SIMS quantification. In this paper
we evaluate use of low impact energy Cs+ ion to circumvent complex altered layer
formation observed for low energy O2+ bombardment for the quantitative analysis of Si1xGex with high Ge-content and for dopant (B, Ph) profiling in Si1-xGex (up to 100% Ge).
Application of metal gate electrodes is required for future sub-45 nm CMOS
technology nodes to suppress boron penetration and gate depletion associated with polysilicon gate. For reasons of materials compatibility in CMOS process flow, the totally
nickel silicided (TOSI or FUSI) gate seems the most attractive candidate to replace polysilicon gate (6, 7). An integration issue arises as high performance CMOS technology
requires both n- and p-type gate materials, imposing the need for work-function
engineering at the metal-gate/gate dielectric interface. This may be achieved by predoping the poly-Si prior to Ni-deposition and subsequent silicidation. Significant work
function modulation of Ni-TOSI gate from 4.3 to 5.0 eV has been demonstrated using
conventional dopants (i.e. B, Ph, As) (8). SIMS analysis reveals pile-up of dopants at the
metal-gate/gate oxide interface, possibly caused by segregation in front of the NixSi1x/poly-Si reaction front during silicidation (´snowploughing´).
Measurement of thickness and composition of thermally grown (doped) silicide films
is a challenging metrology task, since the metal-Si reactions can lead to inhomogeneous
layers with incomplete phase transformation or to complete silicidation that produces
significant surface roughness. In previous work, X-Ray Reflectivity (XRR) has been
shown to be well suited to follow different phase transitions on wafer level in the solid
state reaction between Ni and Si for thin layers (up to 15 nm of deposited Ni) on
crystalline Si (9). Application of XRR to follow NiSi formation on poly-Si is limited
because of the inherent roughness, requiring alternative analytical approaches. Here we
explore the possibilities of available depth profiling techniques, e.g. DSIMS and AES for
characterization of thin (doped) NixSiy films in both large capacitors and at sub-micron
lateral scale, respectively. Compositional in-depth analysis has been performed on totally
silicided NiSi gate electrodes with well-defined NixSi1-x phases, as evidenced from
external calibration.
Advanced 65 nm CMOS technology and beyond requires ultra-shallow junction
(USJ) depths of less than 15–10 nm for source and drain (S/D) junctions (10). This
demands accurate SIMS depth profiling of USJ’s in terms of dose, junction position,
abruptness, and wafer uniformity. State-of-the art SIMS depth profiling using low (150 550 eV) impact energy ion beam may accurately provide both the shape and abruptness
of USJ with sufficiently good depth resolution (1-2 nm/decade), at least beyond the
surface transient (11). Surface transient effects give rise to distortion of the profile in the
near surface region, even at the lowest achievable ion impact energies. This seems to
pose insurmountable problems for the accurate determination of profile shape and thus
integrated dose in the very near surface region (12). Here we demonstrate feasibility of
LEXES as an alternative technique to obtain absolute doses with good accuracy and
reproducibility. Also, we employ high-precision SIMS metrology to show subtle shifts in
USJ depth with almost 1 Å precision.
Progressive downscaling of dimensions requires SALICIDE (self-aligned silicide)
technology with lower Si-consumption to maintain ultra-shallow S/D junctions, low
contact resistance, and low sheet resistance in narrow features. For these reasons thin
nickel mono-silicide film is emerging as the source/drain contact metal of choice for
CMOS transistors of the 65 nm technology node and beyond (13, 14). Current salicide
technology typically comprises a first low temperature anneal to form Ni-rich Ni2Siphase, followed by selective etch of excess metal. The desired low-resistivity mono-NiSi
phase is formed during a second higher temperature anneal. The purpose of this two-step
thermal process is to prevent excessive silicidation at the edges of poly-gates and source
drain lines (14). However, the temperature budget for silicide formation is limited to
prevent the formation of high-resistivity NiSi2 di-silicide at higher temperatures (500 –
700°C), especially on poly-Si (15). Application of NiPt alloy with a few at.% platinum
instead of pure Ni has been demonstrated to improve the thermal stability of the monoNiSi phase by retarding nucleation and agglomeration of the NiSi2 phase (16, 17). This is
highly beneficial to increase the thermal processing window for mixed devices.
Addition of Pt increases the complexity of the salicide process and challenges
physical characterization, requiring (a combination of) novel characterization techniques.
For instance, NiPt based salicide technology demands tight control over PVD metal
deposition uniformity in terms of both thickness and NiPt composition. Moreover, novel
analytical approaches are required to understand the effect of Pt on different NixSi1-x
phase transitions and the in-depth distribution of Platinum. XRR has proven to be well
suited to follow silicidation reactions with pure Ni (9). Addition of Pt, however, demands
construction of a new multilayer model to permit accurate and realistic fitting of
reflectivity curves. Finally, addition of Pt may lead to various kinds of sub- defectivity
on device wafers. Interpretation of Energy Dispersive X-Ray (EDX) analysis is often not
unambiguous, due to the relatively large probed volume. Here we employ full wafer AES for the characterization of defectivity at device level.
Germanium-Silicon Alloy Channel Engineering
Bulk Germanium content in GexSi1-x
Latest generation Dynamic SIMS equipment (300 mm Cameca Ultra Wf) with
floating column design enables low energy impact of Cs+ ions (down to 550 eV at ~4565° incidence angles) while detecting either negative- or positive (Cs-cluster) ions. Good
linear correlation of ion intensity with Ge-fraction up to ~40 % Ge-fraction for negative
cluster-ions (GeSi-) has been reported by Juhel et al. (18).
Here we have explored an alternative SIMS protocol using low energy Cs+ impact
energy while analysing CsnGe+ and CsnSi+ (n=1, 2) cluster ions. For this purpose three
different GexSi1-x staircase samples have been analyzed with varying Ge-fraction (up to a
maximum of 75 at%), epitaxially grown on a Si(100) substrate. Ge-concentrations of
different GexSi1-x layers have been derived using model fitting of high resolution X-Ray
Diffraction (XRD) rocking curves with an accuracy better than 1 at% (19). Thicknesses
of different GexSi1-x layers have been determined from SEM cross-section. This allows to
establish a correlation with SIMS ion intensities and erosion rate as a function of Gecontent. Although absolute ion intensities for CsnGe+ and CsnSi+ cluster ions show rather
strong variations, we find good linear correlation between SIMS ion intensity ratios (e.g.
point-to-point normalization: I(Csn70Ge+)/I(Csn28Si+) for n= 1, 2) and atomic composition
from XRD. Figure 1 shows the [Ge]/[Si] atomic ratio as determined from XRD, plotted
against SIMS ion intensity ratio I(Cs70Ge+)/I(Cs28Si+).
CsSi+
3
Cs70Ge+
2.5
Ion Intensity (a.u.)
XRD [Ge]/[Si] atomic concentration ratio
3.5
Linear regression:
y = 2.7486·x
2
1.5 0
1500
3000
R2 = 0.9957
4500
Sputter time (s.)
1
0.5
0
0.00
0.20
0.40
0.60
0.80
1.00
1.20
SIMS Ion Intensity ratio I(CsGe+)/I(CsSi+)
Figure 1.
Comparison of XRD [Ge]/[Si] atomic concentration ratio versus
70
+
I(Cs Ge )/I(Cs28Si+) SIMS ion intensity ratio. Inset shows SIMS Cs28Si+ and Cs70Ge+
cluster ion profiles (4 keV Cs+ impact energy) for one of the GexSi1-x staircase samples
This shows a good linear correlation at least up to a [Ge]/[Si]-ratio of 3,
corresponding with a Ge-content of ~75 at%. Hence, Ge-content in binary GexSi1-x alloys
may be easily derived using equation [1]. Calibration of sensitivity factor k is established
by consistently using one of the GexSi1-x staircase samples as a standard.
Ge  x  k  I (Cs70Ge )
Si 1  x
I (Cs 28Si  )
[1]
With respect to depth scale calibration, we have derived the SIMS erosion rate using
4 keV Cs+ sputtering from SEM thicknesses for the very same staircase samples. This
shows only a moderate and practically linear increase of erosion rate as a function of Geconcentration up to 65 at% Ge, in agreement with previous observations for lower Geconcentrations (18). Applying this linear correlation allows correction for these slight
erosion rate changes as a function of Ge-concentration.
Dopant profiling in GexSi1-x
Boron doping levels in GexSi1-x (0< x< 1) can be monitored by analyzing negative
BSi- and BGe- cluster ion intensities. However, the latter intensities are expected to
depend greatly on GexSi1-x matrix composition. This imposes the use of gauge
implantation calibration samples with similar matrix composition. In full Germanium
layers, analysis of B-concentrations through negative BGe- cluster ion is feasible with
5E16 at.cm-3 detection limit (2 keV Cs+ impact energy; mass resolution m/m= 300).
Staircase samples with different B-doped regions in full Germanium layer –epitaxially
grown on a graded GexSi1-x buffer layer on Si– have been analyzed while monitoring
10 70
B Ge- and 11B70Ge- cluster ions. Integral doses derived for both B-isotopes are in
excellent agreement with natural isotopic abundance, hence no major mass interferences
are observed using 70Ge isotope cluster ions. Depth profiling of phosphorus in GexSi1-x
(0< x< 1) is performed straightforwardly by monitoring P- ion intensity. We remark that
mass resolution requirements (30SiH mass interference in Si) become less stringent with
increasing Ge-fraction. Hence, use of low energy Cs-sputtering may provide an attractive
alternative for dopant profiling in GexSi1-x alloy, up to full Germanium.
Totally Silicided (TOSI) gate stack
Quantitative bulk NixSi1-x composition
Nickel TOSI gates have been fabricated from poly-Si, deposited on a thin SiO2 gate
dielectric. At this stage, poly-Si may be pre-doped by ion implantation, prior to Nideposition and subsequent silicidation reactions. The latter is achieved in a subsequent 2step thermal treatment: after a first low temperature anneal the Ni reacts with poly-Si to
form a Ni-rich phase (Ni2Si). After selective etch of unreacted Ni-metal, silicidation
reactions may proceed to form mono-NiSi in a second higher temperature anneal step. As
silicidation kinetics is very sensitive to temperature budget, different NixSi1-x phases have
been observed to (co-)exist upon different thermal treatment (6).
We have evaluated use of low-energy cesium ion impact energy for the quantitative
analysis of in-depth Ni-concentration in NixSi1-x. For this purpose different well-defined
NixSi1-x phases (Ni31Si12, Ni2Si, mono-NiSi) have been characterized using external
techniques. Identification of crystalline phases and quantification of Ni-concentrations
has been established by high resolution XRD and Rutherford Backscattering
Spectroscopy (RBS), respectively. The latter provides standard-free external calibration
with sufficiently good depth resolution, but is not feasible from an economic point of
view and for reasons of low analytical throughput. In case of cesium ion bombardment,
we have regarded emission of secondary ions in both polarities (i.e. detection of negativeand positive species). A striking similarity with behavior of GexSi1-x binary alloy is
observed in case of low energy Cs+ ion bombardment in combination with analysis of
positive Cs-cluster ions. In analogy to GexSi1-x binary alloy, Ni-concentration may be
derived according to equation [2]:
Ni  x  k  I (Cs 60Ni  )
Si 1  x
I (Cs 28Si  )
[2]
We remark that Cs60Ni+ cluster ion is least affected by CsSi2+ mass interferences.
Using the thus proposed SIMS quantification protocol, we obtain bulk Ni-concentrations
in NixSi1-x with an accuracy well within 1 at% (for Ni-concentrations between 50 and 72
at.%). This is sufficient to discriminate between the different phases known to exist for
NixSi1-x (e.g. Ni31Si12, Ni2Si, mono-NiSi).
Undoped
70
Fluorine
Ni-concentration (at%)
60
Indium
Gallium
50
Antimony
40
Aluminium
30
20
10
0
0
250
500
750
1000
1250
1500
1750
Sputter time (s.)
Figure 2.
In-depth atomic Ni-concentration profiles in pre-implanted (nominal dose
1e15-5e15 at.cm-2) TOSI gate electrode (in large 2500 x 2500 m2 capacitors).
Concentration profiles have been derived from Cs60Ni+ and Cs28Si+ SIMS cluster ion
intensities using 2 keV Cs+ impact energy
AES depth profiles (using 2 keV Ar+ ion sputtering and surface analysis in alternating
fashion) and may be obtained both from peak-to-peak intensities (derivative spectra) as
well as by integration of direct spectra, using the very same calibration standards,
qualified by RBS. NixSi1-x compositions derived from AES depth profiling are in good
agreement with SIMS, though the accuracy is slightly worse within ~4 at%. We suggest
that this quantification problem may due to intensity transfer between different Auger Nitransitions and/or electron backscattering effects because of the heavier Ni. The most
important benefit for semi-quantitative AES is its high spatial resolution (down to 15 nm),
which proves useful for characterization of in-depth NixSi1-x composition in small areas
and for identification of different discrete NixSi1-x phases in case of lateral segregation at
the sub- scale. The latter is frequently observed for apparently non-stoechiometric
NixSi1-x concentrations (i.e. with average composition from SIMS not corresponding to
Ni31Si12, Ni2Si, mono-NiSi).
Dopant profiling in Ni-TOSI
As stated before, segregation of pre-implanted dopants upon silicidation is of key
importance to achieve the desired work-function (m) modulation at the metal gate/gateoxide interface. Accurate work-function engineering requires simultaneous in-depth
analysis of both bulk- and trace (dopant) levels in NixSi1-x binary alloys. In order to
extend work-function modulation even further, behavior of various more exotic
implantations has been investigated for application in Ni-TOSI gate on SiO2 dielectric.
The proposed SIMS cesium cluster approach offers a clear advantage as it allows to
monitor both electropositive species such as Al, Ga, and In (predominantly emitted as
CsM+ cluster ion) and electronegative species such as F, Sb, and Se (predominantly
emitted as Cs2M+ cluster ion) (20). This enables the acquisition of both CsNi+ and CsSi+
bulk traces –allowing to derive quantitative NixSi1-x composition– and CsnM+ (n=1, 2)
dopant trace in one single depth profile. Ni-concentration profiles –derived from SIMS
CsNi+ and CsSi+ cluster ions according to equation 2– in TOSI gates, pre-doped with
exotic dopants are shown on Figure 2.
SIMS dopant profiles (not shown) show very different behavior for the various
implanted species: for the majority of species (O, C, Co, Er, Se, Sb, V), dopant profiles in
TOSI gate closely resembles an as-implanted profile with no/or only small pile-up of
dopant at the gate-oxide interface. In addition, NixSi1-x composition is hardly affected by
presence of dopants. Hence, presence of these species exerts little or none effect on
silicidation kinetics and effects no significant work-function modulation (21).
Other species such as In, Cd, and F are strongly segregated towards the gate-oxide
interface, while Se, S, and Sb show only partial segregation of the implanted dose.
However, according to electric measurements no significant breakthrough is observed in
the NiSi work-function compared to m modulation achieved by conventional dopants (8,
21). Again, NixSi1-x composition is hardly affected by presence of dopants. Hence,
presence of these species exerts little or none effect on silicidation kinetics, except for Sb.
In case of presence of Al and Sb, a strong interaction is observed on Ni-concentration
profile, with incomplete silicidation of the poly-Si and only a thin and relatively Si-rich
NixSi1-x phase being formed. This implies that silicidation is strongly slowed down with
Sb-implant (3E15 at.cm-2), and to a lesser extent with Al-implants (≤ 5E15 at.cm-2). In
both cases, the silicidation has been blocked and the poly-Si gate has not been entirely
silicided. On one hand, a uniform but thin NiSi layer was observed with Sb, in agreement
with cross-section TEM. On the other hand, Auger mapping clearly evidenced lateral
phase segregation into mono-NiSi matrix with poly-Si domains with Al, consistent with
the low global Ni-concentration (~25 at.%) from SIMS.
Another peculiarity is observed in case of pre-doping with some of the metallic
species (i.e. Ga, Er, Ti, Mg). For these species, SIMS CsM+ cluster ion profiles reveal
strong segregation to the outer surface. Additional -AES analysis (PHI670 -Auger) for
Ga-implanted TOSI gate suggests that the surface composition consists mainly of Ni2Siand NiSi-domains with local presence of Ga-precipitates, surrounded by Si-rich halo, see
figure 3. This implies that implanted Ga has segregated during/after silicidation anneals,
locally affecting the silicidation kinetics.
C5330_12.map: NMW-5 (Al-doped)
STM
05 Jul 13 10.0 keV 0 0.0 µ FRR
27.31 min
Ga1/Full
Nickel mapping
Ni1
128140
87280
0.500 µm
Si1
0.500 µm
Silicon
0.500 µm
0.500 µm
0
44000
Ga1
SEM-image
0.500 µm
Gallium
mapping
0
0.500 µm
0
Figure 3.
SEM image (bottom right) of Ga-doped TOSI gate and Auger mappings
(10 nA electron beam at 10 keV acceleration voltage). Sample surface has been slightly
pre-sputtered with Ar+ ion beam to remove native oxide layer
Hence, these examples show that -besides thermal budget and silicide thicknesssilicidation kinetics may be greatly influenced by presence of only minor dopant
concentrations. This raises an important integration issue for advanced CMOS dual gate
technology, requiring optimization of thermal processing conditions for p+ and n+ type
gate materials with different implanted species.
Ultra-Shallow Junctions (USJ) and Metal Contacts
Ultra-Shallow Junction (USJ) metrology
As stated before, transient effects in SIMS greatly complicate the determination of
USJ absolute dose (12). The latter is crucially important for wafer uniformity –translating
in device uniformity– and implanter tool matching. Integral dose may be alternatively
determined using Low Energy X-Ray Emission Spectroscopy (LEXES), which is capable
of accurate and absolute dose determination with high analytical throughput and 300 mm
wafer mapping capability. The LEXES technique is generally applicable to major dopants
(B, Ph, As) with excellent precision (e.g. relative standard deviation RSD better than 1%
at 1E15 at.cm-2 dose level) (22). For instance, 49 points LEXES mapping of an Asdimere implanted 300 mm Si(100) wafer (4 keV 7E14 As2+/N0) shows no significant
dispersion (Mean dose= 1.40e15 ± 7.3e12 at.cm-2) over full wafer level. High accuracy
and reproducibility, full wafer mapping capability, and high analytical throughout make
LEXES a highly useful metrology tool for implanter tool matching and wafer uniformity
control.
Similarly, precise determination of ultra-shallow junction depths necessitates highaccuracy SIMS metrology with respect to depth scale calibration. Depth scale correction
in SIMS depth profiling for accurate junction depth determination may be performed by
using a linear correction of the erosion rate for beam current drift. This is based on
continuous ion beam current measurement during analysis, available on latest generation
SIMS equipment (300 mm Cameca Ultra Wf). This procedure greatly improves SIMS
depth scale accuracy (measurement precision) down to a few Å precision in junction
depth compared to conventional crater depth measurement. In combination with full 300
mm wafer compatibility and high automation level of the SIMS tool, this opens the way
to use SIMS as a metrology tool to detect and eventually control small variations in USJ
junction depth variation across wafer level due to a thermal gradient during annealing
(RTA).
1.E+22
d= 1.25±0.1 nm
as-implanted
@5e18 cm-3
RTA ; low
ramp rate
-3
As-concentration (at.cm )
1.E+21
1.E+20
1.E+19
RTA ; high
ramp rate
1.E+18
15
20
25
1.E+17
1.E+16
0
10
20
30
40
50
Depth (nm)
Figure 4.
Arsenic Ldd SIMS profiles (in duplicate) for both as-implanted wafer (GePAI + 1 keV 6.5E14 cm-2 As /N0 + BF2 pocket implant) and after different RTA’s (550
eV Cs+ impact energy at ~50°; AsSi- ion detected)
A second application demanding high accuracy depth scale calibration is the
evaluation of thermal treatment (RTA) on junction depth. Figure 4 shows SIMS depth
profiles for Ldd Arsenic USJ (Ge-PAI + 1 keV 6.5E14 cm-2 As /N0 + BF2 pocket
implant) for CMOS065 (PMOS) using low energy Cs+ ion at oblique incidence. This
reveals a significant effect of ramp rate on junction position after RTA with a 1.25 ± 0.1
nm difference in junction position (defined at 5E18 at.cm-3 concentration level). Hence,
increasing the ramp rate produces shallower junction, which may be assigned to reduced
transient enhanced diffusion effects during temperature ramping.
Source/Drain Contact Metal
Deposition of thin (50-150 Å thickness range) NiPt-metal film is achieved through
PVD sputter deposition using a NiPt sputter target. Atomic composition of the asdeposited NiPt film before (or after) silicidation can be accurately determined through the
integral doses of both Ni and Pt obtained by LEXES wafer mapping. This allows
optimization of wafer uniformity, both in terms of thickness and NiPt atomic composition.
Normalised Yield
20
Ni
(× 2.5)
15
10
Pt
RTA low T
RTA high T
O
Si
5
0
500
550
600
650
700
750
800
850
900
Channels
Figure 5.
Rutherford Backscattering Spectra (RBS) (2 MeV He+; 85° scattering
angle) for two thin NiSiPt-films after different anneals and selective etch. Note that
vertical scale has been expanded (×2.5) for channels< 675 for ease of comparison
XRR has been shown to be well adapted to monitor the various phase transformations
of thin NixSi1-x films on Si-substrate at different annealing temperatures (9). High
analytical throughput and wafer mapping capability (Jordan Valley JVX 5200T) truly
turns XRR in a metrology tool, which is a key enabler to optimise the two-step annealing
process to obtain the low resistivity mono-Si phase with good wafer uniformity (<1%
variation in mono-NiSi thickness). Explorative XRR experiments on thin NiSiPt films
show a clear shift of critical angle to higher grazing angle and enhanced fringe amplitude
in comparison with Ni-only case, which is indicative for pronounced density gradients,
associated with presence of (multiple) Pt-containing phases. We remark that accurate and
realistic model fitting of XRR reflectivity curves for complex multilayer relies heavily on
choice of the correct multilayer model. We have employed RBS as an external calibration,
because of the good depth resolution (down to several nm’s at high scattering angle) and
its particularly high sensitivity towards the heavy Pt species. Figure 5 shows RBS spectra
for two thin NiSiPt-films, after first anneal step (RTA1) and selective etch. Clearly, the
major part of Pt (areal dose 2-3E15 at.cm-2) resides at the outer surface. Model fitting
(RUMP) of spectrum for the lower anneal temperature are consistent with presence of a
Ni2Si phase on top of a NixSi1-x layer, decreasing in Ni-concentration towards mono-NiSi.
After the higher anneal temperature, silicide layer is seen to consist of a thinner Ni2Si
phase on top of a mono-NiSi. In addition, additional Pt (~1E15 at.cm-2) is observed (see
figure 5; around channel 839) at the Ni2Si/NiSi-interface. These data provide valuable
input for the construction of a realistic model for fitting of XRR reflectivity curves.
active area
(P+)
active area
(N+)
active area
(N+)
Auger Ni-mapping
Figure 6.
SEM image (left) of large test structures with indicated positions of active
+
areas (P and N+) after NiPt silicidation. Right panel shows the -Auger Ni-mapping on
silicided N+ doped active area
Different kinds of sub-100 nm defectivity have been observed in C065 test chips on
300 mm wafer upon introduction of Pt in the NiSi baseline. Limitations in both lateraland depth resolution for EDX analysis –as available on defectivity tools– necessitates
wafer transfer for more detailed -AES analysis. State-of-the art Auger equipment (300
mm FEI 1250A -Auger with Hemi-Spherical Analyzer) is compatible with KLA/Tencor
inspection tool files, which enables to navigate to the very same defects on full wafer
using defectivity coordinates. Efficient and rapid defect retrieval and the subsequent
high-resolution (down to 15 nm lateral resolution) Auger chemical analysis are essential
for root cause determination, in order to define corrective actions to eliminate the
observed defectivity. Figure 6 shows an SEM image of test structures with apparent
defectivity at the edges of active areas after formation of thin NiSiPt silicide, in particular
for active area on N+ substrate. Auger Ni-mapping and the anti-correlated Si-mapping are
consistent with presence of Ni-depleted zones at the edges of silicided areas, due to
incomplete silicidation. Hence, Auger mapping unambiguously identifies the root cause
of defects, being incomplete silicidation reaction at the edges of active areas, in
particularly on N+ doped active area. The latter fact hints in the direction of dopant
interaction, locally blocking silicidation at edges of active area.
Conclusions
The given examples demonstrate the importance of having available (novel) state-ofthe-art characterization techniques –pushing the analytical limits in terms of resolution,
sensitivity, and accuracy while simultaneously increasing throughput and
reproducibility– to support process development, improve yield, and to speed up yield
learning of advanced CMOS processes. The need for enhanced resolution and sensitivity
is mainly driven by progressive downscaling of advanced devices, whereas integration of
novel non-Si components requires continuous improvement and development of
analytical protocols and evaluation of novel analytical techniques. At the same time, pilot
production of advanced semiconductor devices in an industrial environment poses the
seemingly contradictory need for increased analytical throughput and reproducibility,
merely shifting the physical- and chemical characterization in the direction of metrology.
For the latter, full wafer compliance and automation of analytical tools are of paramount
importance to control wafer process uniformity, tool matching, and defect localization.
This is essential to provide an efficient and rapid analytical support, in order to accelerate
the introduction of next CMOS technology nodes.
Acknowledgements
The authors are greatly indebted to Y. Tamminga (Philips Research Natlab,
Eindhoven, Netherlands) for RBS measurements and to D. Evans and C. Hitzman
(Materials Analytical Services, Santa Clara (CA), United States) for LEXES
measurements.
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