2007 International Conference on Frontiers of Characterization and Metrology for Nanoelectronics
Ultra Low-κ metrology using X-Ray Reflectivity and Small-Angle X-ray
Scattering techniques
L.Plantier
a
a*
, J-P.Gonchond b, F.Pernot c, A.Peledc, C.Wyon d, J-C.Royerd
Freescale Semiconductor, b STMicroelectronics, 850, rue Jean Monnet, 38926 Crolles, France
c
Jordan Valley, Zone #6 Ramat Graviel, Industrial zone, Migdal Ha’Emek 23100, Israel
d
CEA/LETI, 17 rue des martyrs, 38000 Grenoble, France
*e-mail : lise.plantier@freescale.com, ph: 33 4 38 92 31 21, fax: 33 4 38 92 21 22
Introduction
The introduction of Ultra low-κ (ULK) as interlayer
dielectric (IMD) brings some new challenges for metrology.
Indeed several parameters are representative of their
chemical, mechanical and physical properties such as
thickness, porosity or pore size [1]. A non-destructive
technique using X-ray has been used to monitor ULK film
density and thickness using X-Ray Reflectivity (XRR) and
Small Angle X-ray Scattering (SAXS) to enable average
pore size measurements in a fab environment.
Conclusion and Perspectives
The single metrology tool, combining XRR and SAXS has
been demonstrated as a capable equipment to assess
standard porous ULK metrology, in industrial platform.
Therefore those techniques would enable further
characterizations of ULK such as post-processing impacts
on the material properties, and thus allow a deeper
understanding of the low-κ evolutions during processing on
both 300mm monitor and product wafers.
Experimental details
Porous ULK films are prepared by a porogen approach, with
a subsequent UV post-deposition treatment for porogen
removal and cross linking of the matrix [2]. Measurements
have been performed in a unique metrology platform,
gathering XRR and SAXS techniques. ULK film density is
deduced from the measurement of the critical angle from the
XRR spectra and the average pore diameter is deduced from
SAXS spectra with the primary assumption that pores have a
spherical shape and a log-normal distribution of the pore
size distribution. Results were compared with current
metrology equipments used to determine the ULK layers
characteristics. Spectroscopic Ellipsometer was used for
thickness measurements, Corona charge method for
Electrical Oxide Thickness (EOT) measurements [3] and
Ellipsometric Porosimetry (EP) for pore size determination.
These works have been performed with the financial support of the
European SEA-NET project “MUXT” for the development,
assessment and improvement of a fully automated x-ray metrology
platform (IST-027982)
References
International Sematech, 25th march 2003
2 A.Humbert et al., Microelectronic Eng. 82, p399-404, Aug 2005
3 D. Fossati et al., Proceeding MFMN 2006.
THICKNESS COMPARISON
800
Corona charge (EOT)
thickness [nm]
700
600
500
XRR
400
300
200
200
250
300
350
400
450
optical thickness [nm]
500
550
Fig 1: comparison of thickness measured with XRR, Ellipsometer
and Corona charge techniques.
Techniques
XRR
Ellipsometer
Corona charge
%P/T
2.85
3.53
3.03
CPM
35
> 50
33
acquisition time [sec]
30
5
30
Table 1: Statistical value of the measurement system variation for
the different technique and respective acquisition time for one site
measurement.
Pore radius (A)
16
14
EP (Supplier A)
Results and discussion
XRR specific spectrum of thick ULK layers usually exhibits
a high number of fringes as well as two critical angles
characteristics of films with different densities. Inter-fringes
are characteristics of the film thickness. The XRR spectrum
of ULK films after porogen removal is modeled as a single
SiO2C layer. Thickness determined by XRR has been shown
to correlate standard thickness measurements using
Ellipsometer, as well as EOT values using Corona charge
method (Fig. 1). Regarding either the statistical value of the
measurement system or the typical acquisition time
associated to each technique, the three techniques fulfill
industrial requirements (Table 1). XRR technique can
provide additional measurements such as density and thus
porosity. Indeed the latter is deduced from the density, but
the calculation needs implementation of the skeleton density
assuming that the skeleton is identical to the dense, nonporous prototype.
Moreover, the SAXS available technique allowed us to
monitor the average pore size of porous low-κ. Without any
contribution from specular X-ray reflectivity, SAXS
intensity, increases with the ULK film porosity and average
pore size (Fig. 2.a). The SAXS spectrum shifts to higher
incidence angle values for smaller pore size. Measurements
obtained from SAXS spectra were compared with the pore
size provided by EP technique (Fig. 2.b). Both techniques
are shown to be in good agreement for typical 65-nm node
film thickness but SAXS shows weaknesses for thinner
films due to low intensity when measuring.
Acknowledgements
12
10
8
ULK
VERY THIN ULK
6
a.
b.
4
4
6
8
10
12
SAXS (JVX)
Fig.2: (a) SAXS spectra and (b) correlation of measured pore
radius between SAXS and EP techniques, for two ULK
thicknesses.
Key words: Low-k, X-ray reflectivity (XRR), small-angle X-ray
scattering (SAXS)
14