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Acta Materialia 55 (2007) 1407–1414

Thermomechanical properties of thin organosilicate glass filmstreated with ultraviolet-assisted cure

F. Iacopi a,*, G. Beyer a, Y. Travaly a, C. Waldfried b, D.M. Gage c,R.H. Dauskardt c, K. Houthoofd d, P. Jacobs d, P. Adriaensens e, K. Schulze f,

S.E. Schulz f, S. List a, G. Carlotti g

a IMEC, Kapeldreef 75, B-3001 Leuven, Belgiumb Axcelis Technologies, 108 Cherry Hill Drive, Beverly, MA 01915-1088, USA

c Department of Materials Science and Engineering, 416 Escondido Mall, Stanford University, Stanford, CA 94305-2205, USAd Bio-Engineering Department, Kasteelpark Arenberg 23, 3001 Leuven, Belgium

e IMO, Division of Chemistry, Agoralaan, 3590 Diepenbeek, Belgiumf Center for Microtechnologies, Chemnitz University of Technology, D-09107 Chemnitz, Germany

g Physics Department, University of Perugia, Via Pascoli, 06123 Perugia, Italy

Received 28 July 2006; received in revised form 27 September 2006; accepted 18 October 2006Available online 13 December 2006

Abstract

Ultraviolet (UV)-assisted cure has recently been reported as an efficient method to enhance the mechanical properties of organosili-cate glasses (OSG) at the expense of only minor film densification. In this work we show that, depending on the OSG material, the effectof UV-cure on fracture and thermal conductivity properties can vary to a large extent even though the same extent of stiffness enhance-ment is obtained. When the increase of elastic modulus is mostly due to the rearrangement of the silica matrix into more stable silicabonds, the film cohesion strength and thermal conductivity are also improved. Conversely, these properties remain unaffected whenthe dominant UV-cure mechanism is the increase of matrix connectivity. In the latter case, film adhesion is enhanced. The dominantUV-cure mechanism depends on the initial degree of cross-linking and on the mobility of the OSG matrix.� 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: UV-curing; Organosilicate glasses; Elastic behaviour; Fracture; Thermal conductivity

1. Introduction

Organosilicate glasses (OSGs), also known as C-dopedoxides (SiOC:H), are lower density materials than SiO2

since the silica backbone is interrupted by –CH3 functionalgroups, making the matrix less dense (and lowering connec-tivity). Because of the lower density and also thanks to thelower polarizability of Si–CH3 bonds as compared withSi–O [1], OSG films have a significantly lower dielectricconstant than SiO2. The need for on-chip interconnectsstructures with extremely low dielectric constant (approach-

1359-6454/$30.00 � 2006 Acta Materialia Inc. Published by Elsevier Ltd. All

doi:10.1016/j.actamat.2006.10.008

* Corresponding author.E-mail address: iacopi@imec.be (F. Iacopi).

ing 2.0 and below) has prompted the synthesis of OSG filmswith substantial degrees of porosity. The drastic reductionin film density is very effective in reducing film permittivity[2], whereas thermal and mechanical properties of suchdielectric materials tend to degrade as density decreases.The film elastic modulus E is expected to decrease when filmdensity q is reduced with a power law [3], where C is a con-stant that can be approximated to 1 in most cases, and theexponent n is expected to vary between 1 and 3 [4]:

E=Em ¼ Cðq=qmÞn: ð1Þ

Cohesive film strength is also expected to drastically de-crease with decreasing mass and thus bond density. In asimilar way, the thermal conductivity K tends to decrease

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1408 F. Iacopi et al. / Acta Materialia 55 (2007) 1407–1414

with a power law when film density decreases. For exam-ple, following the differential effective medium (DEM) the-ory, the predicted behaviour [5,6] is

KDEM

Km

¼ qqm

� �3=2

: ð2Þ

In both Eqs. (1) and (2) the subscript m refers to the pris-tine properties of the material. Therefore, there is greatinterest in exploring possibilities for enhancing the thermo-mechanical properties by modifying the organosilicate ma-trix structure while preserving the initial film porosity. In aprevious study, we had shown how the combined action ofultraviolet (UV) radiation and thermal activation can gen-erate a rearrangement in the bonding structure of organos-ilicate glass films accompanied by an enhancement of thefilm’s mechanical properties while preserving the organosi-licate nature and pristine porosity of the films [7]. Therearrangement results from a transition of the large angleSi–O–Si bonds towards the more stable small angle or ‘net-work’ configuration (�144�), as well as an increase of thenetwork connectivity. Here we will show that, dependingon which one of these two channels is dominating, eventhough a similar gain in stiffness is achieved, the effectson thermal and fracture properties of the films can be verydifferent.

2. Experimental

Two types of OSG films were chosen for comparisonand deposited directly on Si. The OSG A material is depos-ited by chemical vapour deposition (CVD) at 350 �C. OSGA reference films (Black Diamond, a trademark of AppliedMaterials, Santa Clara, CA) contain about 7% porosityand possess a relative dielectric constant er of about 3.0.OSG B films are spin-on cast and cured in a furnace for1 h at 400 �C. The furnace-cured (reference) films (NCS,a trademark of CCIC, Japan) contain 30% porosity andhave a permittivity of around 2.45.

In this work the as-deposited CVD films are the refer-ence films for the OSG A, whereas OSG B reference filmsare those subjected, after spin-coating, to a 60 s hotplatebakes at 150 and 350 �C, and subsequently to furnace cureat 400 �C. Reference films were included in all characteriza-tion experiments for direct comparison. A series of as-deposited OSG A films and OSG B films without furnacecure were prepared on Si substrates and exposed to UVradiation under the same conditions at 400 �C in an inertenvironment at atmospheric pressure (Axcelis RapidCureUV system) [8]. The OSG B films underwent only the60 s hotplate bake at 150 �C to evaporate the solvents priorto UV-assisted cure. A smaller subset of OSG B films wasalso prepared by including an additional 60 s film bake at350 �C prior to UV-cure.

A microwave-driven electrodeless bulb emitting broad-band UV radiation roughly in the 200–400 nm range wasselected [9]. The cure system is provided with redundantconfinement cages to contain the microwave field. While

the microwave power was kept constant, the exposureduration was varied between 60 and 2400 s, and the evolu-tion of properties such as elastic modulus and film thick-ness and density was measured. Subsequently, an optimalcure time was arbitrarily defined as the duration yieldingthe maximum increase in stiffness for a maximum 5–6%increase in film density. OSG A and B films cured withthe optimal UV-cure time were fully characterized in termsof matrix structure, mechanical properties and thermalconductivity, and compared with the reference films.

Most of the characterization was performed on 300 nmfilms. Thicker films (800 nm) were used for nanoindenta-tion measurements and Nuclear Magnetic Resonancespectroscopy.

Thickness measurements were performed with ellipsom-etry at a wavelength of 633 nm. Structural and chemicalinformation about the matrix materials was deduced fromFourier-transform infrared spectroscopy (FTIR) in trans-mission mode, solid-state magic angle spinning nuclearmagnetic resonance spectroscopy (MAS NMR) and X-ray absorption near edge structure (XANES). 29Si NMRspectra were recorded with a Bruker AMX300 spectrome-ter with a magnetic field of 7.0 T and a spinning frequencyof 4 kHz.

Solid-state 13C cross polarization MAS NMR spectrawere recorded at room temperature on a Varian Unity200 spectrometer operating at a static magnetic field of4.7 T. Magic angle spinning was performed at 4.7 kHz.Ceramic Si3N4 rotors were filled with about 100 mg of filmpowder for performing NMR measurements. Chemicalshifts in the NMR spectra yield information on the short-range atomic structure of the OSG matrix [10], but powderpreparation may induce an artificial increase in the numberof defect sites and Si–OH groups in the matrix. XANESalso probes the local structure and chemistry of the mate-rial and can be performed on films. The measurementswere performed at the PTB PGM ondulator line at BESSY(Berlin, Germany) with fluorescence detection from syn-chrotron light. The position of the absorption edge thresh-old can provide information on the chemical environmentof the chosen element [11], and the amplitude and sharp-ness of the white line resonance can provide informationconcerning the short range order around the chosen ele-ment [12].

Information about the total porosity of the films wasretrieved from ellipsometric porosimetry (EP; [13]) mea-surements using toluene as probe molecule. The dielectricconstant of the organosilicate films was extracted frommeasurements at 100 kHz of metal–insulator–metal planarcapacitors. The use of the lithographic definition for thecapacitors area allows for good precision in the calculationof the permittivity.

The stiffness or elastic modulus E was measured withnanoindentation with a Berkovich tip on 800 nm thickfilms, to avoid the influence from the backing Si substrate[14]. Poisson’s ratio m was retrieved by means of an opto-acoustic technique, Brillouin light scattering spectroscopy

F. Iacopi et al. / Acta Materialia 55 (2007) 1407–1414 1409

(BLS). In BLS measurements the induced surface waves areprobed relying on inelastic photon–phonon scattering. Thesetup used allows the probing of acoustic waves with wave-lengths comparable to that of the laser light (532 nm) andfrequencies in the GHz range. A multipass tandemFabry–Perot interferometer is used to extract the weakinelastic components of the total amount of light scatteredwithin a given solid angle [15,16]. BLS and surface acousticwaves spectroscopy (SAWs) induced with the impulsivestimulated thermal scattering technique (ISTS; [17]) werealso used to extract the elastic modulus of the films in anon-contact fashion and to determine the total film density.Film thickness data from ellipsometry were used as inputparameters for fitting the acoustic dispersion curves in bothtechniques.

Fracture properties were determined by capping the300-nm-thick blanket reference and UV-cured films with20 nm physical vapour deposition Ta(N) layers and form-ing a sandwiched structure between two Si wafers of thetype described in Ref. [18]. A four-point bend loadinggeometry [19] was used to compare the adhesion behaviourat the OSG/Ta(N) interface, and a double cantilever beam(DCB, normal loading) was used to compare the cohesivecrack behaviour of the reference and UV-cured OSG films.Subcritical cohesive crack propagation was monitored inambient conditions according to the method presented in[20]. The failure location was verified with X-ray photoelec-tron spectroscopy.

Finally, thermal conductivity K measurements were con-ducted with the 3x method [21]. OSG films 800 nm thickwere capped by a 50-nm-thick SiC layer above which aconductive microstrip was placed following the specimenpreparation described in Ref. [22]. The conductivity ofthe SiC layer was determined on additional specimenswhere the layer was deposited directly on Si.

3. Results

3.1. Trends vs. cure time

The curves in Fig. 1(a) and (b) display the trends for theincrease in elastic modulus and film increase in density with

0%

20%

40%

60%

80%

100%

120%

140%

0 500 1000 1500 2000 2500

UV cure time (s)

E in

crea

se %

OSG A

OSG B

Fig. 1. Comparison of the trends for the increase of elastic modulus (a) and fi

respect to the reference films vs. cure duration time forboth OSG A and B types. For the OSG B, the curve isrelated to films with only a 150 �C bake before theUV-cure.

The properties of the reference films are reported inTable 1. The reference OSG A film has low porosity withstiffness around 11 GPa, and the OSG B has a 30% porousvolume with an elastic modulus of 7 GPa as measured withnanoindentation. In Table 1 are presented the film proper-ties of the films UV-cured for the ‘optimal’ duration. Basedon the curves in Fig. 1, for both OSG types an optimal curetime was defined for 300 s duration. The optimal cure timecorresponds to a modulus enhancement of around 40–45%for a film densification below 5–6%. Interestingly, weobserve that the UV-assisted cure has considerably less effi-ciency on the OSG B film series which included an addi-tional 60 s film bake at 350 �C prior to cure (full bake). Inthis case, for a 300 s UV-cure duration only a 14% increasein elastic modulus is obtained, compared with a 40%increase for the optimal condition (Table 1). It is worthmentioning that the E values retrieved with BLS and SAWs,even though they show identical increase trends, are system-atically about 40% lower than the values measured withnanoindentation reported in Table 1. The origin of suchan offset is attributed to the absence of contact artifactsfor the acoustic techniques, as discussed in Ref. [23].

The trends related to the modulus increase (Fig. 1(a))both show an almost stepwise increase for short cure dura-tions, after which the increase over time becomes less pro-nounced. The densification trend vs. cure time for the OSGB (Fig. 1(b)) follows a very similar trend as the modulusincreases, as opposed to the densification trend for theOSG A, which appears much smoother. This indicatesthat, even though the stiffness increase for equal cure dura-tion is comparable for both types of OSG films, the funda-mental mechanisms behind the increase are not necessarilythe same.

All further characterization presented in this work wasperformed comparing the reference OSG A and B filmsto the films cured with UV with the ‘optimal’ conditions,i.e. 300 s duration and using the 150 �C pre-baked OSGB films.

UV cure time (s)

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

0 500 1000 1500 2000 2500

Den

sity

incr

ease

%

OSG A

OSG B

lm densification behaviour (b) vs. UV-cure time for OSG A and B films.

Table 1Porosity, dielectric constant, film density, elastic modulus, Poisson’s ratio and thermal conductivity values for reference OSG A (pristine) and OSG B(furnace-cured) films compared with the properties of films that underwent UV-cure

Material Condition Porosity (±1%) Density (g/cm3) (±0.02) er (±0.05) E (GPa) (±1) m (±0.02) K (W/m K) (±0.01)

OSG A Reference 7 1.48 3.0 11 0.26 0.32OSG A Optimal UV 8 1.52 3.15 16 0.23 0.34OSG B Reference 30 1.17 2.45 7 0.26 0.25OSG B Full bake, UV 30 1.18 2.45 8 0.21 0.27OSG B Optimal UV 28 1.23 2.6 10 0.20 0.29

1410 F. Iacopi et al. / Acta Materialia 55 (2007) 1407–1414

3.2. Structural characterization for the optimal cure

condition

FTIR spectra for a reference (pristine) and a UV-curedOSG A film, and for a reference (furnace-cured) and a UV-cured OSG B film are compared in Fig. 2(a) and (b),respectively. The figures focus on the absorption regionof the Si–O–Si stretch bands. Fig. 2(a) indicates no majordifference between the pristine and UV-cured OSG A sam-ples. A minor increase in the Si–O–Si stretch band absorp-tion peak around 1040 cm�1, related to the network/smallangle silica structure, together with some reduction in thetotal absorption of the Si–CH3 (800 and 1270 cm�1) andSi–OH (890 cm�1) absorption bands are observed afterUV-cure. On the other hand, Fig. 2(b) shows a significantdifference in the Si–O–Si stretch absorption bands between

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

400600800100012001400

Wavenumber (cm-1)

Wavenumber (cm-1)

Abs

orb

ance

A

bso

rban

ce

400600800100012001400

Reference

UV -cure

Reference

UV -cure

a

b

Fig. 2. Comparison of the FTIR spectra for reference and UV-cured OSGA films (a) and OSG B films (b). While no significant difference is observedbetween the spectra for the OSG A films, the Si–O–Si stretch bands of theUV-cured OSG B show less absorption in the region corresponding to thelarge angle silica bonds (�1100 cm�1).

the furnace- and UV-cured OSG B films. The absorption inthe 1100–1200 cm�1 region, related to the large angle Si–O–Si stretch, is far less pronounced for the UV-cured film.This indicates that there are significantly more small angle/network Si–O–Si bonds in the UV- than in the furnace-cured OSG B. No significant changes in the Si–CH3

absorption bands are observed.Data from NMR spectroscopy are discussed using the

nomenclature described by Mabboux and Gleason for29Si bonding configurations [10]. Si atoms bonded to oneO and three CH3 groups are termed ‘‘M’’. ‘‘D’’ corre-sponds to Si bonded to two O and two CH3 groups, ‘‘T’’refers to Si bonded to three O and one CH3, and ‘‘Q’’denotes Si bonded to four O. When CH3 groups arereplaced by OH or O–CH3 groups the notation is modifiedto ‘‘TOH’’ to denote bonding to three O and one OH group,‘‘DOH’’ to denote bonding to two O, one CH3 and one OH,or ‘‘DOMe’’ to denote bonding to two O, one CH3 and oneO–CH3 group.

29Si NMR spectra indicate that UV-cured OSG A filmpossess 6% more Q groups (fully cross-linked Si atoms)and 4% more T groups than pristine films, at the expenseof the least cross-linked D and M groups (see Table 2).Also, UV-cured films show about 6% loss of –OH contain-ing moieties, as calculated from NMR spectra. The NMRspectra for the reference and UV-cured OSG A materialsare reported in Fig. 3(a). NMR 29Si spectra from OSG Bfilms show no significant difference between reference andUV-cured films, and that only T and Q groups are present

Table 2Overview of the organosilicate unit groups present in the matrix of OSG Aand B for reference and UV-cured films

Position(ppm)

OSG A OSG B

Reference(%)

UV-cured(%)

Reference(%)

UV-cured(%)

M �109 1.7 0 0 0D �102 16.2 12.4 0 0DOH/DOMe �65 14.2 9.9 8.3 8.5TOH �56 7.8 6.7 12 11.8T �18 43.3 47.3 41.2 39.3Q 9 16.9 23.6 38.5 40.4

Averageconnectivity

2.31 2.42 2.47 2.47

The percentages are retrieved by deconvolution calculations from the 29SiNMR spectra, and the estimated error is about ± 0.2%. The last rowreports the average connectivity number for the different matrices.

Chemical shift (ppm)-150-130-110-90-70-50-30-1010

UV -cured

-130-110-90-70-50-30-1010

M

D

Q

UV cured

T

Chemical Shift (ppm)

T Q

DM

Reference

Reference a

b

Fig. 3. Comparison of the 29Si NMR spectra for reference and UV-curedOSG A films (a) and OSG B films (b). UV-cured OSG A films show ahigher degree of connectivity than reference films (loss of M and Dgroups), whereas no significant difference is found between reference andUV-cured OSG B films.

0

1

2

1830 1840 1850

Photon Energy (eV)

No

rmal

ized

ab

sorp

tio

n

OSG Aref

OSG Auv

OSG Bref

OSG Buv

SiC

Si02

Fig. 4. Comparison of the Si K edge absorption spectra for OSG A and Bfilms with both UV and reference cures. Spectra from CVD SiC and SiO2

are also included for reference. Spectra correspond to fluorescence yielddetection and are normalized to equal intensity at 1850 eV. OSG A spectraare affected by the cure conditions, while OSG B spectra are independentof them.

F. Iacopi et al. / Acta Materialia 55 (2007) 1407–1414 1411

in the structure of this highly cross-linked organosilicatematerial. Details about the chemical shifts for organosili-cate glasses are found in Ref. [24].

A ‘connectivity number’ is defined as the number ofbonds per network-forming atoms in an amorphous silicamatrix according to the continuous random network the-ory [25]. Based on the relative amounts of D, M, T andQ groups and their -OH containing counterparts [26], aver-age connectivity numbers of 2.31 and 2.42 are calculatedfor the pristine and UV-cured OSG A films (Table 1). Notethat the percolation of rigidity number for organosilicateglasses was reported to lie between 2.35 and 2.4 by Rossand Gleason [27], so that the steep increase in E reportedin Fig. 1(a) corresponds roughly to approaching near tothe reported rigidity threshold. Opposed to that, the matrixconnectivity number for the OSG B is 2.47, equal for bothreference and UV-cured films (Table 2).

13C NMR spectra indicate both for the OSG A and Bthe appearance of C=C bonds with UV-cure which are

not present in the reference materials (not shown). Theamount of those bonds is estimated in about 3% of thetotal C content in the matrices.

Fig. 4 displays the Si K edge XANES spectra of boththermally and UV-cured OSG A and B, as well as referencespectra from a thermal SiO2 and SiC:H. The systematic shiftof the Si absorption edge to higher photon energy withincreasing O and decreasing C coordination is clearly dis-cernable in the series SiC:H, OSG A, OSG B and SiO2,which is consistent with the NMR results with respect tothe M, D, T and Q peak assignments. In addition, theUV-cure appears to increase the OSG A local order, asmanifested by a sharper peak, while the OSG B spectrawith thermal and UV-cure were virtually identical. Theseobservations also appear to be in agreement with the NMRdata.

3.3. Thermomechanical characterization for the optimal cure

condition

As indicated in Table 1, UV-cure applied for 300 s (opti-mal cure time) leads to about 45% and 40% increase in filmstiffness for the OSG A and B, respectively, correspondingto only a minor loss of porosity (below 6% film densifica-tion). It is interesting to notice that the Poisson’s ratio,defined as the negative ratio of the transversal over the lon-gitudinal strain

v ¼ �etrans=elong ð3Þdecreases when UV-cure is applied for both OSG A and Bfrom 0.26 to 0.23 and 0.20, respectively. The effect is thusmost pronounced for the OSG B. For isotropic materialsE and m are related to the shear modulus G and the com-pressional or bulk modulus K according to the followingexpressions

G ¼ E=ð2ð1þ vÞÞ ð4ÞK ¼ E=ð3ð1� 2mÞÞ: ð5Þ

1 1.5 2 2.5 3 3.5

G (J/m2)

V(m

/s)

10-3

10-4

10-5

10-6

10-7

10-8

10-9

Reference

UV cure

Fig. 6. Comparison of subcritical crack growth rate (V) vs. the appliedstrain energy release rate (G) in ambient conditions for reference and UV-cured OSG B films measured with double cantilever beam geometry. UV-cure yields a significant enhancement of the cohesive strength of OSG Bfilms.

1412 F. Iacopi et al. / Acta Materialia 55 (2007) 1407–1414

The increase in E and decrease in m as a result of UV-cure indicate that the shear modulus G increases at a higherrate than the compressional modulus.

For example, from the E and m values reported in Table 1we can calculate that the increase of about 45% in elasticmodulus for the OSG A with UV-cure corresponds to anincrease of about 50% of the shear modulus (from 4.4 to6.5 GPa) but only a 30% increase of the compressionalmodulus (from 7.6 to about 10 GPa). Film isotropy is a fairassumption, since significant anisotropic behaviour wouldhave shown in the SAWs acoustic dispersion [17]. Thismeans that the UV-cure is considerably more efficient inimproving the film elastic properties for longitudinal strainand shear than for a compression type of loading.

Investigation of the thermal conductivity K of the OSGmaterials shows a tendency to increase upon UV-cure forboth OSG types. Nevertheless, for a similar 40–45%increase in stiffness, the increase in K is 6% and 16% forOSG A and B, respectively (Table 1). On the other hand,even the less efficient UV-cure of fully baked OSG B filmsproduces an 8% increase in thermal conductivity.

Results from fracture measurements with four-pointbending geometry are reported in Fig. 5. The uncertaintyestimation is deduced from the standard deviation on atypical set of 6–10 measurements. X-ray photoelectronspectroscopy (XPS) analysis confirmed that the failuretook place within a few nanometers from the OSG/Ta(N)interface. Interestingly, UV-cure leads to an increase ofabout 30% of the critical strain energy release rate Gc forthe OSG A/Ta(N) interface, but does not affect the Gc

value for the OSG B/Ta(N) interface. On the other hand,fracture studies with double cantilever beam geometry,where the crack velocity V is driven across several ordersof magnitude, gives the opposite indication. No significantdifference is found between the V–G curves of reference andUV-cured OSG A/Ta(N) film stack (not shown), whereas aconsiderable shift is observed between the behaviour forthe reference and UV-cured OSG B material, as shown inFig. 6. XPS failure analysis indicated that when the DCB

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Gc

(J/m

2 )

OSG B

OSG A

Reference UV –cureReference UV –cure

Fig. 5. Comparison of the critical strain energy release rate Gc for OSG/Ta(N) interfacial failure for reference and UV-cured OSG A and B films(four-point bending measurements). UV-cure yields about 30% enhance-ment of the adhesive fracture energy for OSG A films, but does not affectthe fracture energy of the OSG B/Ta(N) interface.

geometry is used (normal loading mode), the crack propa-gated cohesively in the OSG films in all the cases. Fig. 6shows that for the same range of the applied G the crackpropagates faster in the reference (furnace-cured) OSG Bfilm as compared with the UV-cured condition.

4. Discussion

It was observed that by applying the same UV-assistedcure process with a broadband spectrum on OSG A andB films a similar increase in film stiffness is obtained. Wehad already reported that the stiffness enhancementthrough UV exposure and the simultaneous supply of ther-mal energy is a result of both the increase in the degree ofconnectivity of the organosilicate matrix on the one hand,and the enhancement of the Si–O–Si network bond struc-ture, indicating the formation of more energetically stablesilica bonds [7], on the other.

From the structural characterization comparing refer-ence to UV-cured OSG A films, we observe that in the firstcase the mechanism of increase in the degree of connectiv-ity of the matrix is dominating, as indicated by both 29SiNMR and XANES analysis of the Si K edge. Conversely,the dominant mechanism for the OSG B films is linked tothe transition of large angle to small angle silica bondsupon UV-cure, as indicated by the FTIR spectra inFig. 2(b), and by the fact that 29Si NMR spectroscopy doesnot show significant changes in the connectivity of the silicamatrix.

As a result of the different dominant mechanisms, theeffect of UV-cure on other thermomechanical propertiesbeyond stiffness is different for OSG A and B. In particular,it is found that when the increase in E is mainly caused bythe transition from the large to the stronger small angle sil-ica bonds, as in the case of OSG B, other properties like thecohesive strength and thermal conductivity are also consid-erably enhanced. On the other hand, adhesive fractureproperties remain unaffected.

F. Iacopi et al. / Acta Materialia 55 (2007) 1407–1414 1413

Conversely, when the increase in stiffness is mostly theresult of the increase in matrix connectivity, as seen forthe OSG A films, cohesive strength and thermal conductiv-ity show no improvement. Adhesive strength does benefitfrom UV-cure in this case.

Adhesive properties can only be enhanced by increasingthe amount of cross-layer bonds between the dielectric andbarrier layers. This is the case for the OSG A film, where thebreakage of O–H and C–H bonds of –OH and –CH3 termi-nal groups can lead to the formation of new Si–O–Si or C-based cross-links inside the glass matrix, as indicated byNMR studies, but also to the formation of, for instance,Si–O–N and Ta–C [28] across the OSG/Ta(N) layers.

When fracture occurs adhesively under mixed-modeloading conditions, the crack is typically constrained to arelatively flat fracture surface, so that the creation of newbonds across the layers has an immediate impact inenhancing the adhesive strength, as schematically depictedin Fig. 7(a). In contrast, Fig. 7(b) shows a qualitative expla-nation of why an enhancement of the cross-linking of thematrix does not necessarily lead to better cohesive strength.The path for cohesive failure under mode I loading condi-tions is not constrained, so the crack is free to undergosmall-scale deflections toward areas of lower density and/or connectivity within the glass network. As a consequence,the amount of increase in network connectivity needed fora significant improvement in the cohesive properties mustbe considerably higher than the threshold needed for theimprovement of adhesion.

The number of functional groups in the OSG B films ispreserved upon UV-assisted cure, as indicated by the datain Table 2, and as a result we observe no improvement inthe adhesive properties. On the other hand, the transitionof the large angle Si–O–Si bonds into small/network bondsyields to the enhancement of the cohesion of the matrix,thanks to a net average strengthening of the bonds in thematrix skeleton.

Finally, a note on why OSG A and B behave differ-ently upon UV-cure. First of all, the OSG B films alreadyhave a highly cross-linked matrix from the start (only the

OSG film

OSG film

Ta(N) film

Ta(N) film

a

b

Fig. 7. (a) For an interfacial failure (four-point bend loading case) thecrack path is spatially confined, so that an increase of cross-layer bonddensity is very effective in enhancing adhesion strength. (b) When thefailure path is cohesive in the OSG film (DCB loading case), there is muchless spatial confinement and the crack tip can more easily deflect to find apath with low cross-link density.

T and Q groups are present) as compared with the OSGA material, as shown by 29Si NMR data. So there is notmuch margin for increasing the matrix connectivity ofOSG B films upon UV-cure. Also, OSG B films arespin-on cast at room temperature and experience only alow temperature bake (150 �C) before being UV-cured.Conversely, the OSG A films is deposited by plasma-enhanced CVD at 350 �C, so that the films experiencehigher temperature and a higher energetic (plasma) pro-cess upon deposition, so that the silica skeleton is alreadyin a more ‘frozen state’ as compared with the OSG Bmatrix before undergoing UV exposure. This explainswhy, for the same condition and amount of energy sup-plied by the UV-assisted cure (UV spectrum and temper-ature), the conversion of silica bonds from large to smallangle is more effective in the case of the OSG B [29]. Theimportance of the mobility of the initial matrix is alsoconfirmed by the fact that when the OSG B films arebaked at higher temperature (350 �C) prior to UV-cureonly a minor increase in stiffness and other thermome-chanical properties is observed (full bake, UV-cure inTable 1).

Finally, both OSG materials show a decreasing trend forPoisson’s ratio when exposed to UV-cure, which is mostpronounced for the OSG B. The decrease of Poisson’s ratiopoints to a larger rate of increase of the shear modulus (ormodulus of rigidity) as compared with the increase of bulkor compressional modulus. In turn, this indicates that theOSG film resistance to shear stresses improves more thanthe resistance to hydrostatic pressure or forces that wouldlead to volumetric change.

5. Conclusions

In this work we confirm UV-assisted cure as an effectivemethod for stiffening thin organosilicate glass films withminimal material densification. Nevertheless, we show that,even though equivalent cure conditions are applied and thesame extent of increase in stiffness is obtained, the effect ofUV-cure on thermal and fracture properties dependsstrongly on the initial condition (degree of cross-linkingand mobility) of the chosen organosilicate glass matrix.Also, the Poisson’s ratio of the OSG films systematicallydecreases as a consequence of UV-cure, indicating thatthe increase of the shear modulus of the films dominatesover the increase of the bulk or compressional modulus.

The enhancement of elastic modulus of the filmsthrough UV-cure is attributed to both or either the increasein matrix connectivity and the rearrangement of silicabonds from large to small angle. On the other hand, otherthermomechanical properties, such as cohesive strengthand thermal conductivity, are significantly enhanced onlywhen the change in type of silica bonds is the dominatingeffect upon UV-cure. When the increase in stiffness is duemostly to an increase in matrix connectivity, cohesionand thermal conductivity of the films remain unaffected,whereas interfacial adhesion is enhanced.

1414 F. Iacopi et al. / Acta Materialia 55 (2007) 1407–1414

Acknowledgements

The authors thank Dr. Burkhard Beckhoff from thePhysikalisch-Technische Bundesanstalt in Berlin for helpin setting up the measurements at the BESSY synchrotron.Rudy Caluwaerts and Fred Loosen at IMEC are acknowl-edged for their contribution on dielectric film depositionand characterization. This work was partially supportedby the European Commission through the NANOCMOSand PULLNANO projects.

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