Nuclear Instruments and Methods in Physics Research B 218 (2004) 375–380

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Mechanical properties of polyhedral oligomericsilsesquioxane (POSS) thin films submitted to Si irradiation

C.E. Foerster a,*, F.C. Serbena a, I.T.S. Garcia b, C.M. Lepienski c,L.S. Roman c, J.R. Galv~ao d, F.C. Zawislak d

a Departamento de F�ısica, UEPG, Al.Nabuco de Araujo sn., 84030-900 Ponta Grossa, PR, Brazilb Instituto de Qu�ımica e Geociencias, UFPEL, 96010-900 Pelotas, RS, Brazilc Departamento de F�ısica, UFPR, CP 19081, 81531-990 Curitiba, PR, Brazild Instituto de F�ısica, UFRGS, CP 15051, 91501-970 Porto Alegre, RS, Brazil

Abstract

The nanoindentation technique was used to measure hardness and elastic modulus of polyhedral oligomeric sil-

sesquioxane (POSS) thin films irradiated with 1 MeV Si ions. The fluences ranged from 1013 to 1016 Siþ cm�2, corre-

sponding to averaged deposited energy densities from 0.3 to 70 eV/A3. Elastic recoil detection analysis, nuclear reaction

analysis, infrared (FTIR) and Raman spectroscopy were used to characterize the compositional changes as a function

of the deposited energy by the Si irradiation. The film starts to decompose at the lowest fluence reaching progressively

an amorphous structure of Si–O–Si at the highest fluence. The hardness and elastic modulus of the pristine film are 0.11

and 5 GPa, respectively. At the highest Si fluence, the hardness and elastic modulus reaches around 6 and 65 GPa,

respectively, that are typical values for silicate glasses (Si–O–Si).

� 2004 Elsevier B.V. All rights reserved.

Keywords: POSS; Ion implantation; Nanoindentation; Mechanical properties

1. Introduction

Si–O bond is stronger than the Si–H bond and

much more than the Si–Si bond. As a result, Si–O–Si–O–Si chains make up the skeleton of the silicate

chemistry. Extensive studies about conversion

processes by using ion irradiation of polymeric

and alkoxide precursors with different C/Si and O/

Si ratios was performed by Pivin and Colombo [1]

in order to obtain Si–O–C bonds. The Si–O–C

* Corresponding author. Tel.: +55-42-220-3044; fax: +55-42-

220-3042.

E-mail address: carlosef@uepg.br (C.E. Foerster).

0168-583X/$ - see front matter � 2004 Elsevier B.V. All rights reser

doi:10.1016/j.nimb.2004.01.011

films were produced from polysiloxanes and

polycarbosilanes. The studied range of deposited

electronic energy (Se � /) by different ions was

varied from 2 to 1000 eV/atom. The kinetics mech-anism of conversion of these pre-ceramic films into

amorphous ceramics coatings was studied as a

function of the ion mass and fluence. In the same

work, mechanical properties measured by nano-

indentation and also its thermal stability were

discussed. Significant hardness and elastic modu-

lus increase were observed in those samples.

A silicate skeleton can also occur in the form ofcages, where the structure is based on Si–O link-

ages with a silicon atom at each vertex. Each sili-

con vertex can accept substituents and the nature

ved.

376 C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 375–380

of the external cage substituent determines its

physical and chemical properties. The cage com-

pounds (RSiO1:5)n, where R is an organic or

inorganic group with n ¼ 6, 8, 10 or 12, are aversatile class of building blocks units for the

synthesis of new materials. Cage structures con-

nected together in a finite 3-D molecular skeleton

are known as polyhedral oligomeric silsesquiox-

anes (POSS). These POSS materials are also re-

ferred to belong to the class of spin-on-glasses

(SOG) materials [2]. Different chemical routes used

to obtain polyhedral silicon–oxygen skeletons ofPOSS can be founded in the literature [3,4]. A very

interesting application for the new based Si–O

materials is in the semiconductor industry, as low

dielectric constant (k) materials. Purely organic

materials do not present mechanical and thermal

stability and do not support elevated processing

temperatures and the high stresses in the inter-

connect structure. Consequently, there are greatinterests in organic/inorganic hybrids, with a rigid

Si–O network allied to a low k characteristic of

polymeric materials [5].

In the present work methyl [3,5,7,9,11,13,15]

heptacyclopentylpentacyclo [9.5.1.13;9.15;15.17;13]

octasiloxane-1-propionate (methyl propionate –

POSS) thin films were study to verify the effects of

deposited ion energy by Si irradiation on themechanical properties.

2. Experimental procedure

The POSS (C39H70O14Si8) was supplied by

Sigma–Aldrich Co. and diluted into chloroform at

110 mg/ml under nitrogen atmosphere. The filmswere deposited by spin coating on clean silicon

wafers with thickness between 700 and 800 nm on

Si(1 0 0). A silicon oxide layer of the order of 50

nm is always present on the Si surface. X-ray dif-

fraction investigation showed typical Si–O cage

peaks indicating a crystalline structure for the

POSS films [4].

Si ion irradiations were performed at 1 MeVand the fluences ranged from 1013 to 1016 Siþ cm�2

at room temperature using the Tandetron )3 MeV

implanter at the Instituto de F�ısica, Porto Alegre,

Brazil. The ion beam current densities were lower

than 50 nA/cm2 in order to avoid local heating and

film damage. The ion irradiation parameters were

determined by using the TRIM code and the

atomic composition of the POSS structure. Therange of 1 MeV Si ions was larger than the pristine

film thickness. The hydrogen and oxygen profiles

were determined by elastic recoil detection analysis

(ERDA) by using a 4He beam at 2.8 MeV, and

nuclear reaction analysis (NRA) by using the

reaction 16O(a, a)16O at 3.035 MeV, respectively.

Infrared measurements (FTIR) were carried out

in a FTIR 8400/8900 Shimadzu equipment intransmittance mode. A scanning of 32 accumula-

tions with a resolution of 4 cm�1 was used to ob-

tain the spectra in the range from 400 to 4000

cm�1. Raman spectra were obtained using a 10

mWHe–Ne laser for excitation with k ¼ 632:9 nm.

The measured shift energy range was 500 < Dm <2000 cm�1. The micro-beam, of about 2 lm, was

moved rapidly in a random pattern over thesamples to avoid local heating and photo damage

of the film by the laser.

Hardness (H ) and elastic modulus (E) mea-

surements were performed using the nanoinden-

tation technique using a Berkovich diamond tip

following the Oliver and Pharr method [12] by a

Nanoindenter-XP at the Departamento de F�ısica –

UFPR. Using a Veeco/Sloan DEKTAK3 device,surface profiles were performed in track lengths of

about 50 lm in different sample regions.

3. Results and discussion

3.1. Composition and thickness changes

It is well known that carbon-based polymers

loose O and H during the irradiation process, be-

cause the deposited high-energy by the ions ionize

the electrons in the target, producing a significant

destruction of bonds and a corresponding disso-

ciation process. Fig. 1(a) shows the hydrogen

profiles for pristine film and selected Si POSS

irradiated films. The data show that a significantreduction in hydrogen content occurs at high Si

fluence and the film becomes thinner. The oxygen

profiles are shown in Fig. 1(b). Polymers tend to

oxidize and hydrolyze when exposed to the ambi-

Fig. 1. (a) Hydrogen profiles obtained by ERDA for pristine

and irradiated POSS films at 1· 1014 Siþ cm�2 and 1· 1015 Siþ

cm�2; (b) Oxygen profiles obtained by NRA for pristine and

irradiated POSS films at 1· 1013 Siþ cm�2, 1· 1014 Siþ cm�2 and

1 · 1015 Siþ cm�2.

Fig. 2. Relative oxygen and hydrogen loss as a function of

deposited energy (/ðSn þ SeÞ) due the Si irradiation. The lines

are a guide for the eyes. Estimated error is around 5%.

C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 375–380 377

ent atmosphere after the irradiation if all free

radicals are not stabilized [1,6]. This may explain

the oxygen increase between 1013 and 1014 Siþ

cm�2 irradiation. It is very difficult to subtract this

effect from the obtained spectra, consequently onlya qualitative interpretation on these spectra can be

performed.

Hydrogen and oxygen contents variations rel-

ative to the pristine film as a function of the

deposited energy density by the Si ions are shown

in Fig. 2. Hydrogen loss is around 40% for low Si

fluences and reaches approximately 90% at the

highest fluence. By increasing the Si fluence, acompaction and erosion of the films takes place

and the measured values are close to the ones re-

ported to silicate glasses. In respect to the oxygen

loss a reduction occurs at 1013 Siþ cm�2, but for

1014 Siþ cm�2 incorporation is observed. At higher

Si fluences an oxygen loss takes place, reaching a

maximum of �30%. Probably at 1014 Siþ cm�2 a

highly density of ‘‘dangling bonds’’ at surface fa-vors oxygen incorporation when the sample is

exposed to air or this observed effect is a result due

the film compaction.

FTIR analyses of the pristine and irradiated

films are shown in Fig. 3. The C–H symmetric and

asymmetric stretching bands at 2860 and 2960

cm�1 [7–9] related to cyclopentyl and methyl pro-

pionate structures are present in the pristine film asshown in Fig. 3(a). Si irradiation at 1013 Siþ cm�2

produces a small reduction for these peaks, but

irradiation higher than 1014 Siþ cm�2 show that

these C–H structures are drastically affected by the

irradiation process. Fig. 3(b) shows the fingerprint

regions situated from 400 to 1300 cm�1 of the

molecule POSS for different irradiation conditions

and in addition for the pristine film. The methylband at 1252 cm�1 disappears after fluences higher

than 1013 Siþ cm�2. Si–O stretching bands are re-

lated to the octasiloxane structure (SiO1:5) and

SiO2 substrate. Absorption around 1000 cm�1

corresponds to the asymmetric stretching of the

Si–O–Si bonds [7–9]. However, bands situated at

650 cm�1 can be associated with the oxygen dis-

placement in the Si–O–Si bonds, meaning that theSi irradiation at high fluences produces damage to

Fig. 3. Normalized infrared spectra for pristine and irradiated

POSS films. The spectra were displaced in the transmittance

axis in order to permit follow the changes.

Fig. 4. Raman spectra for pristine and irradiated POSS films.

Peaks D and G are indicated in order to identify the C–C sp2

bonds. The spectra are displaced to permit follow the changes.

For pristine film because the intensive fluorescence the acqui-

sition time was three times lower.

378 C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 375–380

the octasiloxane structure. This fact was also ob-

served by X-ray diffraction analysis for fluences

higher than 1015 Siþ cm�2, but not shown here. Si–

H stretching bands were not observed in the region

of 2200 cm�1 as well the Si–OH groups could notbe identified at 3900 cm�1, meaning that the

hydrogen originated from the breaking of the C–H

bonds do not recombine to form other groups.

These FTIR results agree with ERDA analysis

(Fig. 2). However, the oxygen increasing content

observed by NRA for the sample irradiated at 1014

Siþ cm�2, could not be observed in the FTIR

spectrum. It was not possible to observe increaseof Si–O bonds, neither the capture of H2O by the

dangling bonds.

Raman analyses showed in Fig. 4 were per-

formed in order to study the effect of Si irradiation

on the C–C bonds. The main feature of the spectra

is a broad band in 1000–1700 cm�1 region that

correspond to the C–C vibration modes, which are

referred in the literature as D (1350 cm�1) and G(1580 cm�1) [10] peaks, indicated in the figure. It is

our aim here to present only a qualitative discus-

sion on the Raman results. For fluences lower than

1014 Siþ cm�2 an intensive and typical polymeric

fluorescence is present as shown for the pristine

film. Increasing the Si fluence the D and G peaks

start to appear. Initially the G peak is predomi-

nant and at the higher fluence the films present a D

peak predominance. This behavior has been ob-

served as typical for irradiated polymeric struc-

tures, as well as for amorphous carbon structures.

As discussed by Ferrari and Robertson [10] fora : C, the IðDÞ=IðGÞ ¼ CðkÞ=La ratio, where CðkÞis a function of the Raman light and La is the

cluster diameter, represents a competition between

a clustering C–C sp2 bonds mechanism and a dis-

ordered character of these bonds due the contin-

uous irradiation process. In our samples this ratio

increases with the irradiation indicating a probable

diminishing clusters size.The Raman spectra confirm the results of

ERDA and NRA analysis that the increasing

fluence produces the breaking of the C–H and

C–O bonds, with elemental loss of H and O.

The Raman light used in the present work is not

efficient to observe Si–O–C structures. Pignataro

et al. [11] observed by X-ray photoelectron spec-

troscopy (XPS) that the irradiation of polysilox-ane films produces a ceramic-like SiOxCyHz phase.

In our case it is possible that this phase has been

formed.

The ERDA, NRA and FTIR analysis reveal a

significant compaction of the films due to the lost

of hydrogen and oxygen as a function of the Si ion

fluence. The relative film compaction in relation to

the pristine film is around 8% for the lowest flu-

Fig. 6. Hardness and elastic modulus at 20% of film thickness

[12–14] as a function of deposited energy by the Si irradiation.

The lines are a guide for the eyes. The values are obtained by

interpolation of curves in Fig. 5.

C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 375–380 379

ence, reaching 50% at the highest Si fluence,

respectively.

3.2. Mechanical properties

The hardening process of the POSS film occurs

after Si irradiations even at low fluence. Fig. 5

shows hardness (Fig. 5(a)) and elastic modulus

(Fig. 5(b)) profiles for different Si fluences. These

figures also show the effect of substrate on hard-

ness and elastic modulus for tip penetrations

higher than the film thickness. For the pristine filmthe hardness and elastic modulus are approxi-

mately 0.11 and 5 GPa, respectively. After Si

irradiation, these values increase reaching around

6 and 65 GPa for H and E, respectively at the

maximum Si fluence. It is important to observe

that Si irradiation at 1014 Siþ cm�2 produces lower

hardness and elastic modulus than the sample

irradiated at 1013 Siþ cm�2. This fact is probablycorrelated to the oxygen incorporation verified by

NRA (Fig. 1(b)) indicating a low cross-linking

process at this fluence. Si irradiation at higher

fluences increases film hardness reaching it highest

value around 6 GPa.

The skeleton cage structure of the films have a

stoichiometry SiO1:5 what is similar to the quartz

(SiO2) structure that present hardness around �9GPa [12]. However, we have organic radicals at-

Fig. 5. Hardness and elastic modulus profiles for pristine and

selected irradiated samples as a function of the contact depth

[12]. For depths higher than the film thickness, values tend to

reach Si substrate values (H � 11 GPa and E � 180 GPa).

tached to the silicon vertices that after the irradi-

ations transforms the films to a complex structure

of Si–O–C–H bonds, among these silicate [11] and

a : C phases [10], that also contribute to the ob-

served hardness.To compare hardness and elastic modulus at a

depth where substrate effects are minimized

[12,13], in Fig. 6 hardness and elastic modulus are

shown as a function of the deposited energy den-

sity at a depth of 20% of the film thickness, taking

into account the compaction of the films due to the

Si irradiation.

4. Conclusions

Si irradiation of methyl propionate – POSS,

produces loose of hydrogen and oxygen contents

as observed by ERDA, NRA and C–C cluster that

are also present as Raman analysis. Concerning

the mechanical properties of the films, an increasein hardness and elastic modulus is observed as a

function of the Si fluence. However, this increase is

not as high as in pure polymeric irradiated mate-

rials on that hardness of 12–15 GPa is obtained at

high ion fluences [14,15]. Irradiated POSS samples

show maximum hardness of about 6 and 65 GPa

for the elastic modulus at higher Si fluences. These

values are characteristic of silicate glasses and

380 C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 375–380

probably related to a high content of Si–O bonds

since the remaining oxygen is about 70% at highest

fluence.

Acknowledgements

The authors acknowledge the financial supportfrom PADCT/CNPq; PROCAD/CAPES. The au-

thor L.S. Roman acknowledges the CNPq for the

fellowship (PROFIX).

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