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Characterization of complex inter-layer dielectric (ILD) stack by spectroscopic ellipsometry in nanoelectronic manufacturing
Dmitriy V. Likhachev
GLOBALFOUNDRIES, Dresden
Agenda • Interlayer dielectrics (ILDs) in an integrated circuit
• Anatomy of the ILD stack
– Interfaces between dielectrics
– Test pad film stack
• Current problem: inter-layer correlation effects
• Experimental details
– Sample preparation
– Determination of the thin films optical properties
• Determination of the films thicknesses in multi-layer ILD film stack
• Calculated uncertainty of ellipsometric analysis for all dielectric films from complete ILD stack
• Maximum differences for the TEOS and BLOK average thicknesses
• Conclusions
GLOBALFOUNDRIES Confidential 2
Interlayer dielectrics (ILDs) in an integrated circuit
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Cross-section of 64-bit high-performance microprocessor chip (Source: IBM, http://www-
03.ibm.com/press/us/en/attachment/19014.wss?fileId=ATTACH_FILE2&fileName=1057.jpg)
Cu wiring
Interlayer
dielectric
Two types of the interlayer
dielectrics:
• An intermetal dielectric – the
isolation between metal
lines in the same level;
• An interlevel dielectric – the
isolation between two metal
levels.
Anatomy of the ILD stack
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Interfaces between dielectrics
• Interlayer dielectric: low-κ dielectric
materials with dielectric constant κ in the
range of 2.5–3.0 (ordinary SiO2 has the κ-
value of ~4).
• Barrier/etch stop: low-κ dielectric barrier/
etch stop film (BLOk, Barrier LOw κ): lower
dielectric constant κ (~3–5 for SiCxNyHz
vs. ~6–9 for SiNxHy).
Measurements of inter-layer dielectric films
thicknesses and optical properties in the
back-end-of-line (BEOL) process always
have been an important task for interconnect
metrology in semiconductor development and
manufacturing since those properties directly
influence device performance. (Source: C.K. Goldberg and V.S. Wang, Compatibilities of dielectric films, in:
Interlayer Dielectrics for Semiconductor Technologies (Elsevier, 2003), p.79)
Anatomy of the ILD stack
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Test pad film stack
Process engineers face many challenges in BEOL in-line process monitoring
and control. Multi-layer stacks of dielectric films cannot be characterized by
single-wavelength ellipsometry (even with MAI). Even SE still might not
provide sufficient information for unambiguous multi-layer optical analysis.
Current problem: inter-layer correlation effects
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Max d
iffe
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Å
Wafer #
TEOS BLOK
Experimental details
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Sample preparation
LDRTEOS ~200Å
LDRTEOS ~200Å UV-cured ULK ~1800Å
UV-cured ULK ~1800Å BLOK ~250Å
BLOK ~250Å Ta ~160Å
TEOS ~4000Å TEOS ~4000Å
Si Substrate Si Substrate
Two sets of five unpatterned test wafers, each with typical ILD structure on
standard 300 mm Si(100) wafers, were prepared using an Applied Materials
Producer™ SE tool for dielectric chemical vapor deposition and an Applied
Materials Endura™ tool for metal physical vapor deposition. One set of four-
layer test wafers had the following structure: LDR TEOS/UV-cured ULK/
BLOK/TEOS/Si substrate, another set of five-layer test wafers had an
additional thin Ta metal layer between the BLOK and TEOS films.
Determination of the thin films optical properties
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Spectroscopic ellipsometry:
Rotating-compensator spectroscopic ellipsometer (KLA-Tencor, former
Therma-Wave, Opti-Probe® OP9000 metrology tool)
Spectral range: 190–800 nm
AOI: ~65°
Modeling
Critical point with parity (CPP) model for the complex dielectric function
(included in the GO™ (Global Optimizer) software, ver. 1.2a)
− Ta film: 3-oscillator CPP model; ~160 Å layer is optically transparent
− BLOK and UV-cured ULK films: 2-oscillator CPP models
Crystalline Si substrate, Si–SiO2 interface and TEOS: dispersion tables
from Herzinger et al.
Optical characterization of the films by means of an “additive” thin-film
stack approach where the next film optical properties are extracted after
complete characterization of previously deposited films in the stack.
C.M. Herzinger, B. Johs, W.A. McGahan, J.A. Woollam, W. Paulson, Ellipsometric determination of optical
constants for silicon and thermally grown silicon dioxide via a multi-sample, multi-wavelength, multi-angle
investigation, J. Appl. Phys., 83, 3323–3336 (1998).
Comparison of Ta film optical properties from different studies
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H.G. Tompkins, T. Zhu, and E. Chen, Determining thickness of thin metal films with spectroscopic ellipsometry for
applications in magnetic random-access memory, J. Vac. Sci. Technol. A 16, 1297–1302 (1998).
T. Waechtler, B. Gruska, S. Zimmermann, S.E. Schulz, T. Gessner, Characterization of sputtered Ta and TaN films by
spectroscopic ellipsometry, in: T.-A. Tang, G.-P. Ru, Y.-L. Jiang (Eds.), ICSICT-2006 - 2006 8th International Conference
on Solid-State and Integrated Circuit Technology Proceedings, Shanghai, China, October 23–26, 2006, IEEE Conf. Proc.,
pp. 2184–2186 (2006).
Determination of the films thicknesses in multi-layer ILD film stack
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Normalized residual uniqueness test from thin LDR
TEOS layer deposited on the multi-layer stack with
and without intermediate thin Ta metal layer. The
presence of the Ta layer reduces the uniqueness
range (usually, the parameter range that keeps the
residual values within 10% of the minimum value).
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No
rma
lize
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Normalized Film Thickness
no Ta layer
with Ta layer
Uniqueness
range
No Ta layer
Calculated uncertainty of ellipsometric analysis for all dielectric films from complete ILD stack
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0.0
0.5
1.0
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2.5
3.0
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4.0
4.5
5.0
LDR TEOS ULK BLOK TEOS
Un
cert
ain
ty (
Å)
no Ta layer
with Ta layer
In modern semiconductor manufacturing the global trend of miniaturization dictates
continuous decrease in uncertainty for all thickness measurements and the
measurements should match stricter requirements for 2X and 1X process nodes.
Currently, one of the most important problems is to achieve required tool-to-tool
matching for routine process control in semiconductor processing. For instance,
current specification for the matching of ellipsometric tools on the ONO stack is 0.3%
for the bottom oxide thickness which results in a value of 3 Å for 1000 Å oxide layer.
Strong correlation between
the model fit parameters is
synonymous with large
parameter uncertainties. As
can be seen, the uncertainties
for all dielectric films were
greatly reduced by inclusion
of the thin metal layer into the
multi-layer film stack, which
indicates a decline in inter-
layer correlation effects.
Maximum differences for the TEOS and BLOK average thicknesses
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x d
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with Ta layer
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Ma
x d
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ÅWafer #
no Ta layer
with Ta layer
Maximum differences for the TEOS (a) and BLOK (b) average thicknesses
obtained during “additive” thin-film stack characterization (with and without
Ta thin metal layer).
a b
Conclusions
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A method to reduce thicknesses correlations between the dielectric films in
the multi-layer ILD film stack has been presented. The method is based on
inclusion of additional thin absorbing metal layer into the multi-layer structure
in order to suppress the inter-layer correlations, thereby allowing an
accurate determination of the thicknesses of individual films in the stack.
We reported the characterization of the film stack with up to five-layer film
structure in which four of the films are dielectric by an ex situ single-angle
spectroscopic ellipsometer. The optical characterization of all layers in the
spectral range of 200–800 nm (6.20–1.55 eV) has been performed using an
“additive” thin-film stack approach.
The uniqueness of obtained ellipsometric solutions and accuracy of
established optical models for the multi-layer thin-film stack have been also
investigated. The reduction in uncertainties for all dielectric films clearly
indicates a decrease in cross-correlations of the model parameters.
Publication: D.V. Likhachev, Characterization of complex inter-layer dielectric
stack by spectroscopic ellipsometry: A simple method to reduce parameters
correlations// Thin Solid Films 550, 305–311 (2014)
doi: 10.1016/j.tsf.2013.11.082
Acknowledgements
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Author wish to thank
Dr. Denis Shamiryan
(MAPPER Lithography,
The Netherlands) and
Mr. Steffen Brunner
(GLOBALFOUNDRIES,
Germany) for their
technical advice and
support.