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Stiffening and hydrophilisationof SOG low-k material studied
by ellipsometric porosimetry, UV ellipsometry
and laser-induced surface acoustic waves
A. M. Urbanowicz1, B. Meshman2, D. Schneider3, and M. R. Baklanov1
1

IMEC, Leuven, Belgium
IRSET, San Jose, USA
3
Fraunhofer Institute for Materials and Beam Technology, Dresden, Germany

2

Received 7 June 2007, accepted 20 November 2007
Published online 20 March 2008
PACS 43.35.Pt, 61.80.Ba, 61.82.Pv, 77.55.+ f, 78.20.Ci, 78.66.Qn
In this paper we evaluate the correlation between UV lightabsorption of SOG (spin-on glass) low dielectric constant(low-k) films and chemical changes induced by an excimer
UV source at several wavelengths: 172 nm, 222 nm, and308 nm. Low-k absorption in the UV range was measured bynitrogen purged UV ellipsometry. It is shown that im-provement of mechanical properties is accompanied by degradation (hydrophilisation) of low-k films. Changes in hydrophobic properties were evaluated by water-source ellipsometric porosimetry (WEP). The Young’s moduli of the films
were measured by laser-induced surface acoustic waves(LAwaves).

phys. stat. sol. (a) 205, No. 4, 829–832 (2008) / DOI 10.1002/pssa.200777749

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phys. stat. sol. (a) 205, No. 4, 829–832 (2008) / DOI 10.1002/pssa.200777749

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applications and materials science

Stiffening and hydrophilisationof SOG low-k material studied
by ellipsometric porosimetry, UV ellipsometry
and laser-induced surface acoustic waves
A. M. Urbanowicz*, 1, B. Meshman2, D. Schneider3, and M. R. Baklanov1
1IMEC, Leuven, Belgium
IRSET, San Jose, USA
3
Fraunhofer Institute for Materials and Beam Technology, Dresden, Germany
2

Received 7 June 2007, accepted 20 November 2007
Published online 20 March 2008
PACS 43.35.Pt, 61.80.Ba, 61.82.Pv, 77.55.+ f, 78.20.Ci, 78.66.Qn*

Corresponding author: e-mail urbano@imec.be, Phone: +32 1628 1469, Fax: +32 1628 1214

In this paper we evaluate the correlation between UV lightabsorption of SOG (spin-on glass) low dielectric constant(low-k) films and chemical changes induced by an excimer
UV source at several wavelengths: 172 nm, 222 nm, and308 nm. Low-k absorption in the UV range was measured bynitrogen purged UV ellipsometry. It is shown that im-provement of mechanical properties is accompanied by degradation (hydrophilisation) of low-k films. Changes in hydrophobic properties were evaluated by water-source ellipsometric porosimetry (WEP). The Young’s moduli of the films
were measured by laser-induced surface acoustic waves(LAwaves).

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Integration of low-k materials is acontinuing issue in the microelectronics industry. To reduce the RC delay and improve the operating speed ofmultilevel Cu interconnects in modern ultra large scale integrated (ULSI) circuits, the k-value of dielectric layers has
to be as low as possible. To decrease the k-value, porosity
has been implemented to a new generation of dielectrics[1]. However, porous low-k materials have lower mechanical strength compared to conventional dielectric films. Improving mechanical properties (such as Young modulus) of
new low-k materials becomes a challenge. The low-k dielectrics UV curing processes could solve this issue. The
combined action of UV radiation and thermal activation isshown to generate a rearrangement in the bonding structureof the SiCOH based low-k [2]. As a result, the mechanicalproperties are improved, but the hydrophilicity increases.
Subsequent moisture absorption into the porous structure
significantly increases the k-value of dielectric film because of high polarizability of water molecules.
Most of results of low-k dielectrics UV curing reported
so far are related to wideband UV sources. It is expectedthat narrow band UV sources are more efficient for understanding of precise mechanisms of modification and suchdata are important for optimisation of technological processes. Optical transmission measurements of SiCOH
based layers showing transparency in the far UV range
with increasing absorption in the VUV suggest the use ofexcimer light source. The main advantage of an excimer
light source is that it can emit light with very narrow spectral distribution.
In this paper we analyse the effect of UV and VUV excimer light of different wavelengths (172 nm, 222 nm and308 nm), different power and exposure time to study amechanism of photo induced changes of SOG low-k. The
original (pristine) and UV treated SOG low-k films werecharacterized by nitrogen-purged ultra-violet (PUV) ellipsometry for determination of absorption in the UV range,
ellipsometric porosimetry (EP) for determination of porosity and pore size [3], and ellipsometric porosimetry withwater source (WEP) for evaluation of hydrophobic properties [4]. The mechanical properties of the films wereevaluated by laser-induced acoustic wave spectroscopy© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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pssa830

A. M. Urbanowicz et al.: Stiffening and hydrophilisation of SOG low-k material(LAwave) [5]. Fourier transform infrared (FTIR) spectroscopy was used for characterisation of the chemical composition before and after UV irradiation.
2 Experimental
2.1 Samples preparation The samples used in thisstudy were silica-based low-k films deposited on 200 mm
silicon wafers by SOG technology. The films had 10–12%
carbon-containing-hydrophobic groups, 32% porosity, anda pore size close to 2 nm after a standard curing (thermalannealing). The k-value of the films was in the rangeof 2.3–2.4. Such prepared samples were irradiated byplasma-based, planar VUV (vacuum ultra-violet) source,which provides single or multiple bands between 120 nm
and 350 nm (Fig. 1). For this evaluation, 172 nm, 222 nm
and 308 nm were used. The plasma is induced in achamber with excimer gas mixture. The light fromthe plasma is emitted into processing chamber through aspecial quartz window. The spectral distribution of
the emitted light depends on excimer gas composition asshown in Fig. 2. During the UV exposure, the sample
chamber is flowing with N2 (99.99% purity) and continuously pumped down. N2 pressure is about 1 Torr with flow
0.1 liter per minute.
2.2 Metrology The refractive index and extinction coefficient of the film studied were measured by a Sopra
PUV-EP ellipsometric system (λ = 150–650 nm). Measurements were done in nitrogen ambient at atmosphericpressure. Amount of H2O and O2 during measurement wasbelow the level of 5 ppm. Porosity, pore size distribution
and bulk hydrophilicity were measured by using ellipsometric porosimeter EP-10 equipped with SENTECH 801
spectroscopic ellipsometer (λ = 350–850 nm). The ellipsometer is mounted on a vacuum chamber that can be
filled with a solvent vapor (such as toluene or water) in acontrollable way [3, 4].
The elastic modules of the films studied were measured by LAwave. This technique is based on high frequency surface acoustic waves, which are very sensitive tosurface film, even if their thickness is much thinner thanthe penetration depth of the waves. The acoustic waves aregenerated by a short laser pulse in a wide spectral range.
The phase velocity of the wave propagation is measured
depending on a frequency. Fitting the theoretical curve toFigure 2 Power versus wavelength of emitted light for different
excimer mixtures.

the curve measured yields the elastic modulus (Young’smodulus) of the film [5].
The chemical composition of low-k films before andafter UV irradiation was analyzed using a Biorad QS2200
ME FTIR system.
3 Results and discussion Figure 3 shows the refractive index and the extinction coefficient of the pristine low-k
dielectric film (no UV treatment) in a range from 150 nm
to 650 nm. The pristine sample has strong absorption below 180 nm. It indicates that the UV region below 180 nm
is photochemically active according to a fundamental lawof photochemistry [6]. Figure 4 reflects changes of Youngmoduli (YM) after UV irradiation. The y-axis presents exposure. The exposure is product of UV power, time oftreatment and light frequency. The highest changes in YM
are observed for 172 nm. This improvement of mechanicalproperties indicates that wavelength of light below 180 nm
causes the highest efficiency of photochemical reactions.
Dielectric films treated with 222 nm and 308 nm UV reveals small improvement of YM. This correlates well withFTIR and EP data as presented in Figs. 5 and 6.

Figure 1 Scheme of the experimental setup.

Figure 3 Refractive index and extinction coefficient of the low-k(as measured by PUV) versus wavelength.

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Original
Paperphys. stat. sol. (a) 205, No. 4 (2008)

Figure 4 Young’s moduli (as measured by LAwave) versus UVexposure. The numbers on the graph correspond to the sample
numbers in Table 2.

Figure 5 shows ratio Si–O–Si (1050 cm–1) cage bondto Si–CH3 (1276 cm–1) FTIR peaks versus exposure. Thisratio reflects well the compromise between hydrophilicity
and mechanical properties. Increasing the number of Si–O–Si groups gives better mechanical properties. While thenumber of Si–O–Si groups increases, a reduction of thenumber of CH3 (methyl) groups is observed [7]. Themethyl groups are hydrophobic due to low polarizability.
Depletion of CH3 groups leads to hydrophilisation of thelow-k. The degree of hydrophilisation is shown in Fig. 6.
The hydrophilisiation of UV treated low-k’s can be related
to photo-dissociation of carbon containing groups and across-linking mechanism of the Si–O–Si network. Possible reactions for UV and VUV irradiation are presented inTable 1.
Another reason of hydrophilisation can be related to influence of residual oxygen in the UV vacuum chamber.
Excited oxygen can remove carbon-containing hydrophobic groups.

831

Figure 6 Hydrophilicity (as measured by EPW) versus exposure.

The energies of the reactions listed in Table 1 werecalculated by Ushio et al. who used molecular models of
the reactants and products and a first-principles molecularorbital program, Gaussian 03 [8]. Since the reaction energy
corresponds to the driving force of a reaction in a thermodynamic sense, reactions 1 and 2 are the most probableto occur. Reaction 4 is insignificant, because the reactionenergy is negligible. Considering the reaction energies
and comparing the changes in measured FTIR spectra, we
can conclude that a Si–O–Si cross-link generation froma CH3 group and OH group is the primary reaction.
Methane (CH4) releases as a reaction product of this reaction. Therefore, Si–O–Si cross-link generation leads
to CH3 groups depletion and, as a result, strong hydrophilisation occurs.
The results of our tests and settings of the UV system
are summarized in Table 2. These data show that the highest modification of low-k films occurs after 172 nm irradiation due to strong absorption of our material in this region(Fig. 3). This correlates with a reduction of porosity and amean pore radius (as measured by EP). Moreover the ratio
Si–O/Si–CH3 (as calculated from FTIR) reflects that improvement of mechanical properties leads to reduction of
the number of hydrophobic CH3 groups. This correlateswith WEP data. Mechanical properties are improved buthydrophobic properties become significantly worse. In theregion of 200–400 nm the impact of photochemical reacTable 1 Possible reactions during UV and VUV irradiation andtheir reaction energies (i.e., differences between energies of reactants and those of products) [2].
reaction

Figure 5 Si – O – Si/Si – CH3 ratios (as measured by FTIR) versus
exposure.
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12
34energy [eV]O3Si – CH3 + HO – SiO3 → O3Si – O – SiO3 + CH4
O3Si – H + HO – SiO3 → O3Si – O – SiO3 + H2O3Si – CH3 + H2 → O3Si–H + CH4
O3Si – OH + HO – SiO3 → O3Si – O – SiO3 + H2O

1.64
1.43
0.21
0.12

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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A. M. Urbanowicz et al.: Stiffening and hydrophilisation of SOG low-k material studied

Table 2 Summary of experimental conditions and results.
samplewavelength (nm)time (min)power

FTIR Si – O/Si – CH3

EP pore radii (nm) EP porosity (%) EPW water (%)

1234567891011172
172
172
172
222
222
222
308
308
308
pristine

105103105105105–lowmedmed
highlowhighhighlowlowhigh–

8.90
7.24
34.14
9.15
6.68
5.90
5.94
4.83
6.52
5.94
4.61

0.80
0.75
0.60
0.60
0.85
0.90
0.80
0.90
0.80
0.90
1.00

tions is low because of low low-k absorption. However, we
still observed some changes after 222 nm and 308 nm UVirradiation. We can assume that it may be related to “hotplate nature”. When a sample is irradiated with UV wavelength from “hot plate” region, most of the light is transmitted through low-k and warms up the Si substrate due tostrong absorption of silicon for the wavelengths shorter
than 400 nm. This effect could be related to multi-photon
absorption on the Si substrate near interlayer with thelow-k. Temperature of the Si substrate may play a role inphysical and chemical processes occurring during the UV
treatment of the low-k.
4 Conclusions The largest improvement of the mechanical properties of the low-k studied occurs during
172 nm UV irradiation due to the fact that the absorption
edge of the SOG low-k matrix lies below 180 nm. However, improving the mechanical properties of low-k filmsby UV light leads to significant hydrophilisation. Thisphenomenon is related to removing of hydrophobic carbon-containing groups as a result of UV induced photodissociation and Si–O–Si cross-linking mechanism. Thedegree of hydrophilisation of 172 nm was higher than in
the case of 222 nm and 308 nm. There are two possible
reasons. One reason could be related to the smaller degreeof photo-chemical reactions due to lower UV light absorption for 222 nm and 308 nm wavelengths. Another reason
might be related to influence of residual oxygen in UV© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3226273233333232343132

17223032
1513161015125vacuum chamber which could be excited by 172 nm radiation. Therefore, an optimal balance between improvementof mechanical properties and hydrophilisation of a low-k
must be established.
Acknowledgement It is our pleasure to thank Dennis
Shamiryan for fruitful discussions.

References[1] K. Maex, M. R. Baklanov, D. Shamiryan, F. Iacopi,
S. H. Brogersma, and Z. S. Yanovitskaya, J. Appl. Phys.
93(11), 8793 (2003).
[2] J. Ushio, T. Ohno, T. Hamada, S.-I. Nakao, K. Yoneda,M. Kato, and N. Kobayashi, Jpn. J. Appl. Phys. 46, 17 (2007).
[3] M. R. Baklanov and K. P. Mogilnikov, Microelectron. Eng.
64, 335 (2002).
[4] M. R. Baklanov, K. P. Mogilnikov, and Q. T. Le, Microelectron. Eng. 83, 2287 (2006).
[5] D. Schneider, P. Siemroth, T. Schülke, J. Berthold,B. Schultrich, H. H. Schneider, R. Ohr, B. Petereit, andH. Hilgers, Surf. Coat. Technol. 153, 252 (2002).
[6] J. G. Calvert and J. N. Pits, Photochemistry (Wiley, NewYork, 1967), pp. 10 – 50.
[7] K. Yoneda, M. Kato, N. Kobayasi, N. Matsuki, K. Matsushita,N. Ohara, A. Fukazawa, and T. Kiura, Proc. 2005 Inerconnect
Technology Conf., p. 220.
[8] M. J. Frisch et al., Gaussian 03 (Gaussian, Inc., Wallingford,CT, 2004), Revision B.05.

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