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Effects of nitrogen ion irradiation on plasma polymerized films produced from titanium tetraisopropoxide–oxygen–helium mixtures
Surface & Coatings Technology 203 (2008) 534–537
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Effects of nitrogen ion irradiation on plasma polymerized films produced from titanium tetraisopropoxide–oxygen–helium mixtures Nilson C. Da Cruz a, Bruno B. Lopes a, Elidiane C. Rangel a, Mario A.B. de Moraes b, Steven F. Durrant a,⁎ a b
Laboratório de Plasmas Tecnológicos, Campus Experimental de Sorocaba, Universidade Estadual Paulista (UNESP), Av. Três de Março, 511, Alto de Boa Vista, 18087-180 Sorocaba, SP, Brazil Laboratório de Processos de Plasma, Departamento de Física Aplicada, Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas (UNICAMP), 13083-970 Campinas, SP, Brazil
A R T I C L E
I N F O
Available online 25 May 2008 Keywords: Thin films Ion irradiation PECVD XPS Ti
A B S T R A C T In this work films were produced by the plasma enhanced chemical vapor deposition (PECVD) of titanium tetraisopropoxide–oxygen–helium mixtures and irradiated with 150 keV singly-charged nitrogen ions (N+) at fluences, φ, between 1014 and 1016 cm− 2. Irradiation resulted in compaction, which reached about 40% (measured via the film thickness) at the highest fluence. Infrared reflection–absorption spectroscopy (IRRAS) revealed the presence of Ti–O bonds in all films. Both O–H and C–H groups were present in the as-deposited films, but the density of each of these decreased with increasing φ and was absent at high φ, indicating a loss of hydrogen. X-ray photoelectron spectroscopy (XPS) analyses revealed an increase in the C to Ti atomic ratio as φ increased, while the O to Ti ratio hardly altered, remaining at around 2.8. The optical gap of the films, derived from data obtained by ultraviolet–visible spectroscopy (UVS), remained at about 3.6 eV for all fluences except the highest, for which an abrupt fall to around 1.0 eV was observed. For the irradiated films, the electrical conductivity, measured using the two-point method, showed a systematic increase with increasing φ. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Titanium oxide films find application as anti-reflective coatings [1], electrochromic devices [2], interface filters [3], waveguides [4], biosensors [4], photocatalysts [5], biocompatible coatings [6], and layers of high dielectric constant [7]. As summarized in a recent publication [4], among the techniques employed to fabricate titanium oxide films are sputtering, chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD). The latter has been used by Szymanovski et al. [5] to produce TiO2 films from titanium tetrachloride. More recently, using titanium tetraisopropoxide (TTIP), Ti(OC3H7)4 [8] and titanium (IV) ethoxide–oxygen– helium mixtures [9], films bearing a high density of Ti–O bonds have been produced by PECVD. Depending on the deposition parameters, these films have a polymer-like structure containing C–H, C–O and O–H bonds. X-ray photoelectron spectroscopy (XPS) analysis has shown that the Ti atoms in the films are predominantly in the +4 oxidation state. For this reason, the term TiO2-like can be applied to these films. Properties of TiO2-like films such as their refractive indices may be modified by thermal treatment [8]. Apart from heating, a possible alternative procedure to change the TiO2-like film properties is by ion irradiation. This technique has been used on silicon-based polymers synthesized by PECVD from various organosilicone monomers. These
⁎ Corresponding author. Tel.: +55 15 3238 3406; fax: +55 15 3228 2842. E-mail address: [email protected] (S.F. Durrant). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.05.028
films contain a high density of SiO bonds and thus may be considered SiO2-like films. In some of these investigations, 170 keV He+ ions with fluences between 1 × 1014 and 1 × 1016 cm− 2 have been used [10–13]. Loss of hydrogen and a remarkable structural rearrangement, with large increases in the film compaction and surface hardness were observed. The above-mentioned SiO2-like films may be considered as a counterpart of the TiO2-like films as both usually contain organic groups and a large density of M–O bonds (MfTi, Si). Thus, the ion irradiation of the plasma films studied here might show some similar effects to those observed for SiO2-like films. This is a somewhat crude comparison, however, since the films examined here are not exactly TiO2-like but may rather be considered as a-C:H:O:Ti. In previous studies the effects of low energy ion irradiation (at substrate biases of −600 and +600 V) on films derived from TTIP plasmas were examined [14]. In the present investigation, the effects of high energy ion irradiation on the structure, composition, volume, optical gap and electrical conductivity of a-C:H:O:Ti films deposited from the same plasmas are reported. To our knowledge, this is the first study of its kind. 2. Experimental Films were deposited for one hour from plasmas fed titanium tetra-isopropoxide (TTIP)–oxygen–helium mixtures. The RF PECVD system is described in the literature [14]. In brief, for depositions, substrates were placed on the lower of two horizontal parallel-plate electrodes within a cylindrical steel chamber continuously evacuated
N.C. Da Cruz et al. / Surface & Coatings Technology 203 (2008) 534–537
by a mechanical pump. Glass, aluminum-coated glass slides and aluminum substrates were used for films to be analyzed by profilometry, infrared reflection–absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS), respectively. A substrate bias of +200 V was used. Oxygen and helium were fed to the system via precision mass flow controllers. Vapor of TTIP was fed to the chamber from a purpose-built, resistively heated, introduction system. Films were irradiated with 150 keV singly-charged nitrogen ions, at fluences of 1014 to 1016 cm− 2, using an Eaton GA-4R model 4240 ion implanter. To increase heat dissipation during irradiation, the films were placed on a copper block. An ion current of 100 μA was used. Film thicknesses were determined from a step-height, produced by a mask during the deposition, using a Tencor Alphastep 200 profilometer. Infrared spectra of the films deposited were obtained using a Bomen MB101 spectrophotometer. The XPS analyses were undertaken, within 10 weeks of irradiation, using a McPherson ESCA 36 spectrometer with an aluminum Kα radiation source for photoelectron excitation. Atomic ratios were determined using the procedure reported earlier [15]. Ultraviolet– visible spectra were obtained using a Perkin-Elmer model lambda 9 spectrophotometer. Electrical conductivity was determined via Ohm's law using the conventional two-point method, in which the potential difference is plotted as a function of the current when an increasing voltage is applied between two pin electrodes in contact with the film surface. The whole film thickness was used to calculate the conductivity. 3. Results and discussion Fig. 1 shows the spectra of the as-deposited, or pristine, and irradiated films for the wavenumbers between 5000 and 400 cm− 1. The thickness of each film is given in the caption. The spectrum of the as-deposited film reveals a strong broad band peaked at ~3300 cm− 1, which is attributed to stretching in OH groups [16]. Hydroxyl groups do not exist in the monomer molecule but their presence in the film can be explained by the recombination of molecular fragments resulting from the dissociation of the TTIP molecules in the plasma. Furthermore, OH groups may result from water or oxygen uptake by the film from ambient air [17]. In plasma-deposited polymers, CH groups are usually present. Since their C–H stretching absorptions are in the 3000–2900 cm− 1 region [18], the lineshape of the band peaked at ~ 3300 cm− 1, for the pristine film and for the film irradiated at the lowest ion fluence, strongly indicates that these absorptions are present, overlapping with the bands at their low energy sides. The low
535
intensity of these C–H absorptions, however, reveals a low CH group content in the films. It can be observed that there is an overall decrease of the absorption intensities of the spectra in the interval 3000–2900 cm− 1 as the ion fluence is increased, consistently with the depletion of C–H bonds. Such a behavior is attributed to rupture of the H bonds by the ion beam and is commonly observed in the infrared spectra of irradiated organic [19,20] and organosilicone compounds [11,13,21]. The concomitant disappearance of the OH bonds observed in Fig. 1 at higher fluences is due to the same effect. The peaks at 1530 and 1390 cm− 1 can be assigned, respectively, to asymmetric and symmetric stretching in carboxylate groups [18,22] while the shoulder at about 1730 cm− 1 is probably due to CfO bond stretching [23]. Furthermore, absorptions due to CfC bonds, usually occurring in the 1650–1600 cm− 1 region [18], are expected to contribute to the spectra, overlapping with the bands due to C_O stretching and carboxylate asymmetric stretching. In the ion irradiation of polymer films containing CHx groups, the formation of CfC bonds is commonly observed [19,24] and attributed to the scission of the C–H bonds and further recombination of the C atoms. The band in the 1000 to 400 cm− 1 region is due to the Ti–O stretching mode [8,25] and is present in all films, while the other bands in the spectrum of the as-deposited film are either absent or greatly diminished in the spectra corresponding to φ = 5 × 1015 or 1 × 1016 cm− 2. Inspection of the spectra of the films corresponding to the two latter fluences, however, shows that the bands in the interval 1860–1160 cm− 1, although very weak, are still present. Consistent with the above findings, preliminary XPS investigations of the pristine film and of that irradiated at the highest fluence, reveal the presence of Ti, O and C. From the intensities of the Ti2p, O1s, and C1s photoelectron peaks and following the calculation procedure of Ref. [15], the O:Ti and C:Ti atomic ratios were calculated. The results are given in Table 1. As the highest oxidation state of Ti is +4, the O:Ti ratios of 2.6 and 2.8 imply that some of the O atoms are not bound to Ti. Such an “excess” of oxygen arises in part from the deposition process. Since the film is formed from molecular fragments of the monomer molecule, in which the O:Ti atomic ratio is initially 4, it is not surprising to find an O:Ti ratio larger than 2 in the films. In addition, post-deposition oxygen incorporation may also be significant as large concentrations of free radicals are observed in plasmadeposited polymers [17]. Oxygen incorporation arises from recombination of the free radicals with oxygen or water vapor from the atmosphere [17,26]. Such incorporation is also observed in ion irradiated polymers [13,21,27], as some of the free radicals formed by bond breaking survive the irradiation process and are thus able to react with ambient air. Indeed, the higher O:Ti atomic ratio of the irradiated films, compared to that of the pristine film, as shown in Table 1, is consistent with this interpretation. It is well known that ion irradiation of polymer films often results in a reduction in film thickness. Fig. 2 shows the thickness decrease (%) for the films studied here as a function of the ion fluence. As can be seen, the decrease is relatively low for the lower ion fluences, but abruptly changes to higher values at about 2 × 1015 cm− 2, being nearly 40% for the highest ion fluence. Because the film surface area is not changed by ion irradiation, the percent thickness decrease is the same as the percent volume decrease. Volume decreases, in some cases as high as 50%, have been reported for various Si-based polymers irradiated with ions in the hundreds of keV range [11,13,21] and are associated with intense H loss due to H bond breaking in the abundant
Table 1 Atomic ratios in the films as determined by XPS for the as-deposited film and that irradiated at the highest ion fluence Fig. 1. IRRAS spectra of the pristine film and of films irradiated at various ion fluences. Film thicknesses of 862, 845, 808, 730 and 516 nm were obtained for the films deposited without irradiation (as-deposited) and at fluences, φ, of 1014, 1015, 5 × 1015 and 1016 cm− 2, respectively.
Fluence (cm− 2)
O:Ti
C:Ti
0 (pristine) 1 × 1016
2.6 2.8
0.63 1.1
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N.C. Da Cruz et al. / Surface & Coatings Technology 203 (2008) 534–537
Fig. 2. Percent film thickness decrease as a function of the ion fluence.
Fig. 3. Optical gap as a function of the ion fluence.
C–H and O–H bonds existing in the pristine film. The volume change arises from the structural rearrangement due to recombination of the dangling bonds formed by H detachment since a dense closed packed structure is formed. As the concentration of CHx groups in our films is low, however, dehydrogenation occurs mainly by cleavage of O–H bonds, implying that O radicals play an important role in the structural reconstruction. Another physical effect may contribute to volume change. As revealed by mass spectrometric measurements, species such as CHx (x = 1,2,3,4), C2H2, C3H8, CO, CO2 and CONCH3 can be emitted from organic polymers during ion irradiation [19,28,29]. Such an effect is sometimes termed electronic sputtering [30] and may occur in our films, depleting their C, O, and H contents and thus decreasing the film volume. Furthermore, the N atoms transported in the ion beam and implanted in the films may react with O and H atoms forming volatile species such as NOx and NHx that may be released, further depleting the film mass. Such species, together with atomic or molecular nitrogen can also be emitted by the films and may explain why N and N-related groups were not detected, respectively, by XPS and by IR spectroscopy, in the spectra of the irradiated films. The contribution of sputtering due to the N+ ions to the decrease in the film mass was investigated using the SRIM [31] simulation program. The simulations were carried out for 150 keV N+ ions and for films of the same O:Ti and C:Ti ratios as the pristine film but with variable H:Ti ratios. In none of the simulations undertaken does sputtering significantly contribute to film mass loss. The absorption coefficient, α, of the films was determined as a function of the photon energy, E, from the UVS data. As for our films no linear relationship was observed between (αE)1/2 and E, the optical gap could not be determined using the Tauc model [32], according to which ðαEÞ1=2 ¼ BðE−EG Þ
expected to occur in the irradiation of plasma polymers. The optical gap of conventional polymers typically falls with increasing ion fluence, φ. In polyimide films, for example, bombardment with 40 keV Ar+ causes EG to fall from about 0.7 eV to about 0.5 eV as φ is increased from 1015 to 1016 cm− 2 [35]. Similarly, Sharma et al. [36] recently showed that EG of the polymer CR-39 decreases from 3.4 eV in untreated samples to 0.82 eV following implantation with N+ at φ of 1016 cm− 2. Fig. 4 shows the electrical conductivity σ of the films as a function of φ. A fall in σ is observed at low fluences but a very strong increase is observed at higher φ (equivalent to four orders of magnitude at φ = 1016 cm− 2 compared to that of the unirradiated film). Inspection of the literature reveals that σ of conventional polymer films may be increased by 15 orders of magnitude or more [24,37] by ion implantation. This is due to the radiation-induced formation of conducting carbon-rich structures, and increased cross-linking [38]. Considering the effect of N+ implantation, it is known that the sheet resistance of polyimide films can be reduced by about 9 orders of magnitude under irradiation with 90 keV N+ (φ = 1016 cm− 2) [34]. Nitrogen ion implantation of polyetheretherketone (PEEK) increases its surface conductance by 10 orders of magnitude [39]. Clearly, significant reductions in the EG and increases in σ are produced by N+ implantation of conventional polymers. Previous studies by some of the present authors have also shown that ion implantation of plasma polymerized benzene films with 170 keV N+2 at φ between 1013 and 1017 cm− 2 reduces the electrical resistivity, ρ, by 15 orders of magnitude and also decreases EG from 1.6 to 0.9 eV [40].
ð1Þ
where EG is the optical gap and B is a constant. Consequently, we defined EG as the photon energy corresponding to an absorption of α = 104 cm− 1. This procedure is commonly employed to determine the optical gap of amorphous films [33]. Fig. 3 shows EG as a function of φ. The values of EG are stable at about 3.7 eV but fall abruptly at fluences above 1 × 1015 ions cm− 2 and reach about 1 eV for the fluence of 1 × 1016 ions cm− 2. Švorčik et al. [34] listed the major degradation processes observed in ion irradiation of conventional polymers as excitation and ionization, formation of free radicals, macromolecular chain scissioning, cross-linking, and the outgassing of volatile degradation products. Other effects of irradiation include carbonization, graphitization, and the creation of dangling bonds. Similar complex effects can be
Fig. 4. Electrical conductivity of the films as a function of ion fluence.
N.C. Da Cruz et al. / Surface & Coatings Technology 203 (2008) 534–537
For the same plasma polymers, at a constant φ of 5 × 1015 cm− 2, ρ falls by at least 6 orders of magnitude, and EG is reduced from about 2.8 to 1.1 eV, when the ion energy is increased from 0 to 150 keV [41]. The results of the present study are consistent with these findings. A qualitative explanation for the observed dependence of EG on φ may be the associated increase in the density of unsaturated carbon bonds. Unsaturated carbon bonds add π-electrons to the polymer structure. Owing to the relatively low energy gap between π-bonding and π⁎antibonding levels, the photon energy required to promote electronic transitions is reduced. A key factor influencing the dependence of σ on φ is that the highly cross-linked structures produced by implantation furnish longer conduction paths. In addition, unsaturated carbon bonds supply delocalized π-electrons, which act as charge carriers within the film structure. Both mechanisms increase conduction. 4. Conclusions In the plasma-deposited titanium-containing films examined here, some effects of ion implantation similar to those observed in the ion irradiation of organosilicone films were observed, such as a hydrogen loss, a pronounced structural modification, and compaction. The optical gap falls abruptly from ~ 3.6 eV to ~1.0 eV as the fluence approaches 1 × 1016 cm− 2. In the range of fluence studied here (1014 to 1016 cm− 2), increases of up to six orders of magnitude were induced in the electrical conductivity of the films. Acknowledgements The authors thank the Fundação para o Amparo da Pesquisa do Estado de São Paulo (FAPESP), and the Ministerio de Ciência e Tecnologia/Conselho Nacional de Desenvolvimento Científico e Tecnológico (MCT/CNPq) for financial support. References [1] [2] [3] [4] [5]
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Effects of nitrogen ion irradiation on plasma polymerized films produced from titanium tetraisopropoxide–oxygen–helium mixtures Nilson C. Da Cruz a, Bruno B. Lopes a, Elidiane C. Rangel a, Mario A.B. de Moraes b, Steven F. Durrant a,⁎ a b
Laboratório de Plasmas Tecnológicos, Campus Experimental de Sorocaba, Universidade Estadual Paulista (UNESP), Av. Três de Março, 511, Alto de Boa Vista, 18087-180 Sorocaba, SP, Brazil Laboratório de Processos de Plasma, Departamento de Física Aplicada, Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas (UNICAMP), 13083-970 Campinas, SP, Brazil
A R T I C L E
I N F O
Available online 25 May 2008 Keywords: Thin films Ion irradiation PECVD XPS Ti
A B S T R A C T In this work films were produced by the plasma enhanced chemical vapor deposition (PECVD) of titanium tetraisopropoxide–oxygen–helium mixtures and irradiated with 150 keV singly-charged nitrogen ions (N+) at fluences, φ, between 1014 and 1016 cm− 2. Irradiation resulted in compaction, which reached about 40% (measured via the film thickness) at the highest fluence. Infrared reflection–absorption spectroscopy (IRRAS) revealed the presence of Ti–O bonds in all films. Both O–H and C–H groups were present in the as-deposited films, but the density of each of these decreased with increasing φ and was absent at high φ, indicating a loss of hydrogen. X-ray photoelectron spectroscopy (XPS) analyses revealed an increase in the C to Ti atomic ratio as φ increased, while the O to Ti ratio hardly altered, remaining at around 2.8. The optical gap of the films, derived from data obtained by ultraviolet–visible spectroscopy (UVS), remained at about 3.6 eV for all fluences except the highest, for which an abrupt fall to around 1.0 eV was observed. For the irradiated films, the electrical conductivity, measured using the two-point method, showed a systematic increase with increasing φ. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Titanium oxide films find application as anti-reflective coatings [1], electrochromic devices [2], interface filters [3], waveguides [4], biosensors [4], photocatalysts [5], biocompatible coatings [6], and layers of high dielectric constant [7]. As summarized in a recent publication [4], among the techniques employed to fabricate titanium oxide films are sputtering, chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD). The latter has been used by Szymanovski et al. [5] to produce TiO2 films from titanium tetrachloride. More recently, using titanium tetraisopropoxide (TTIP), Ti(OC3H7)4 [8] and titanium (IV) ethoxide–oxygen– helium mixtures [9], films bearing a high density of Ti–O bonds have been produced by PECVD. Depending on the deposition parameters, these films have a polymer-like structure containing C–H, C–O and O–H bonds. X-ray photoelectron spectroscopy (XPS) analysis has shown that the Ti atoms in the films are predominantly in the +4 oxidation state. For this reason, the term TiO2-like can be applied to these films. Properties of TiO2-like films such as their refractive indices may be modified by thermal treatment [8]. Apart from heating, a possible alternative procedure to change the TiO2-like film properties is by ion irradiation. This technique has been used on silicon-based polymers synthesized by PECVD from various organosilicone monomers. These
⁎ Corresponding author. Tel.: +55 15 3238 3406; fax: +55 15 3228 2842. E-mail address: [email protected] (S.F. Durrant). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.05.028
films contain a high density of SiO bonds and thus may be considered SiO2-like films. In some of these investigations, 170 keV He+ ions with fluences between 1 × 1014 and 1 × 1016 cm− 2 have been used [10–13]. Loss of hydrogen and a remarkable structural rearrangement, with large increases in the film compaction and surface hardness were observed. The above-mentioned SiO2-like films may be considered as a counterpart of the TiO2-like films as both usually contain organic groups and a large density of M–O bonds (MfTi, Si). Thus, the ion irradiation of the plasma films studied here might show some similar effects to those observed for SiO2-like films. This is a somewhat crude comparison, however, since the films examined here are not exactly TiO2-like but may rather be considered as a-C:H:O:Ti. In previous studies the effects of low energy ion irradiation (at substrate biases of −600 and +600 V) on films derived from TTIP plasmas were examined [14]. In the present investigation, the effects of high energy ion irradiation on the structure, composition, volume, optical gap and electrical conductivity of a-C:H:O:Ti films deposited from the same plasmas are reported. To our knowledge, this is the first study of its kind. 2. Experimental Films were deposited for one hour from plasmas fed titanium tetra-isopropoxide (TTIP)–oxygen–helium mixtures. The RF PECVD system is described in the literature [14]. In brief, for depositions, substrates were placed on the lower of two horizontal parallel-plate electrodes within a cylindrical steel chamber continuously evacuated
N.C. Da Cruz et al. / Surface & Coatings Technology 203 (2008) 534–537
by a mechanical pump. Glass, aluminum-coated glass slides and aluminum substrates were used for films to be analyzed by profilometry, infrared reflection–absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS), respectively. A substrate bias of +200 V was used. Oxygen and helium were fed to the system via precision mass flow controllers. Vapor of TTIP was fed to the chamber from a purpose-built, resistively heated, introduction system. Films were irradiated with 150 keV singly-charged nitrogen ions, at fluences of 1014 to 1016 cm− 2, using an Eaton GA-4R model 4240 ion implanter. To increase heat dissipation during irradiation, the films were placed on a copper block. An ion current of 100 μA was used. Film thicknesses were determined from a step-height, produced by a mask during the deposition, using a Tencor Alphastep 200 profilometer. Infrared spectra of the films deposited were obtained using a Bomen MB101 spectrophotometer. The XPS analyses were undertaken, within 10 weeks of irradiation, using a McPherson ESCA 36 spectrometer with an aluminum Kα radiation source for photoelectron excitation. Atomic ratios were determined using the procedure reported earlier [15]. Ultraviolet– visible spectra were obtained using a Perkin-Elmer model lambda 9 spectrophotometer. Electrical conductivity was determined via Ohm's law using the conventional two-point method, in which the potential difference is plotted as a function of the current when an increasing voltage is applied between two pin electrodes in contact with the film surface. The whole film thickness was used to calculate the conductivity. 3. Results and discussion Fig. 1 shows the spectra of the as-deposited, or pristine, and irradiated films for the wavenumbers between 5000 and 400 cm− 1. The thickness of each film is given in the caption. The spectrum of the as-deposited film reveals a strong broad band peaked at ~3300 cm− 1, which is attributed to stretching in OH groups [16]. Hydroxyl groups do not exist in the monomer molecule but their presence in the film can be explained by the recombination of molecular fragments resulting from the dissociation of the TTIP molecules in the plasma. Furthermore, OH groups may result from water or oxygen uptake by the film from ambient air [17]. In plasma-deposited polymers, CH groups are usually present. Since their C–H stretching absorptions are in the 3000–2900 cm− 1 region [18], the lineshape of the band peaked at ~ 3300 cm− 1, for the pristine film and for the film irradiated at the lowest ion fluence, strongly indicates that these absorptions are present, overlapping with the bands at their low energy sides. The low
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intensity of these C–H absorptions, however, reveals a low CH group content in the films. It can be observed that there is an overall decrease of the absorption intensities of the spectra in the interval 3000–2900 cm− 1 as the ion fluence is increased, consistently with the depletion of C–H bonds. Such a behavior is attributed to rupture of the H bonds by the ion beam and is commonly observed in the infrared spectra of irradiated organic [19,20] and organosilicone compounds [11,13,21]. The concomitant disappearance of the OH bonds observed in Fig. 1 at higher fluences is due to the same effect. The peaks at 1530 and 1390 cm− 1 can be assigned, respectively, to asymmetric and symmetric stretching in carboxylate groups [18,22] while the shoulder at about 1730 cm− 1 is probably due to CfO bond stretching [23]. Furthermore, absorptions due to CfC bonds, usually occurring in the 1650–1600 cm− 1 region [18], are expected to contribute to the spectra, overlapping with the bands due to C_O stretching and carboxylate asymmetric stretching. In the ion irradiation of polymer films containing CHx groups, the formation of CfC bonds is commonly observed [19,24] and attributed to the scission of the C–H bonds and further recombination of the C atoms. The band in the 1000 to 400 cm− 1 region is due to the Ti–O stretching mode [8,25] and is present in all films, while the other bands in the spectrum of the as-deposited film are either absent or greatly diminished in the spectra corresponding to φ = 5 × 1015 or 1 × 1016 cm− 2. Inspection of the spectra of the films corresponding to the two latter fluences, however, shows that the bands in the interval 1860–1160 cm− 1, although very weak, are still present. Consistent with the above findings, preliminary XPS investigations of the pristine film and of that irradiated at the highest fluence, reveal the presence of Ti, O and C. From the intensities of the Ti2p, O1s, and C1s photoelectron peaks and following the calculation procedure of Ref. [15], the O:Ti and C:Ti atomic ratios were calculated. The results are given in Table 1. As the highest oxidation state of Ti is +4, the O:Ti ratios of 2.6 and 2.8 imply that some of the O atoms are not bound to Ti. Such an “excess” of oxygen arises in part from the deposition process. Since the film is formed from molecular fragments of the monomer molecule, in which the O:Ti atomic ratio is initially 4, it is not surprising to find an O:Ti ratio larger than 2 in the films. In addition, post-deposition oxygen incorporation may also be significant as large concentrations of free radicals are observed in plasmadeposited polymers [17]. Oxygen incorporation arises from recombination of the free radicals with oxygen or water vapor from the atmosphere [17,26]. Such incorporation is also observed in ion irradiated polymers [13,21,27], as some of the free radicals formed by bond breaking survive the irradiation process and are thus able to react with ambient air. Indeed, the higher O:Ti atomic ratio of the irradiated films, compared to that of the pristine film, as shown in Table 1, is consistent with this interpretation. It is well known that ion irradiation of polymer films often results in a reduction in film thickness. Fig. 2 shows the thickness decrease (%) for the films studied here as a function of the ion fluence. As can be seen, the decrease is relatively low for the lower ion fluences, but abruptly changes to higher values at about 2 × 1015 cm− 2, being nearly 40% for the highest ion fluence. Because the film surface area is not changed by ion irradiation, the percent thickness decrease is the same as the percent volume decrease. Volume decreases, in some cases as high as 50%, have been reported for various Si-based polymers irradiated with ions in the hundreds of keV range [11,13,21] and are associated with intense H loss due to H bond breaking in the abundant
Table 1 Atomic ratios in the films as determined by XPS for the as-deposited film and that irradiated at the highest ion fluence Fig. 1. IRRAS spectra of the pristine film and of films irradiated at various ion fluences. Film thicknesses of 862, 845, 808, 730 and 516 nm were obtained for the films deposited without irradiation (as-deposited) and at fluences, φ, of 1014, 1015, 5 × 1015 and 1016 cm− 2, respectively.
Fluence (cm− 2)
O:Ti
C:Ti
0 (pristine) 1 × 1016
2.6 2.8
0.63 1.1
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Fig. 2. Percent film thickness decrease as a function of the ion fluence.
Fig. 3. Optical gap as a function of the ion fluence.
C–H and O–H bonds existing in the pristine film. The volume change arises from the structural rearrangement due to recombination of the dangling bonds formed by H detachment since a dense closed packed structure is formed. As the concentration of CHx groups in our films is low, however, dehydrogenation occurs mainly by cleavage of O–H bonds, implying that O radicals play an important role in the structural reconstruction. Another physical effect may contribute to volume change. As revealed by mass spectrometric measurements, species such as CHx (x = 1,2,3,4), C2H2, C3H8, CO, CO2 and CONCH3 can be emitted from organic polymers during ion irradiation [19,28,29]. Such an effect is sometimes termed electronic sputtering [30] and may occur in our films, depleting their C, O, and H contents and thus decreasing the film volume. Furthermore, the N atoms transported in the ion beam and implanted in the films may react with O and H atoms forming volatile species such as NOx and NHx that may be released, further depleting the film mass. Such species, together with atomic or molecular nitrogen can also be emitted by the films and may explain why N and N-related groups were not detected, respectively, by XPS and by IR spectroscopy, in the spectra of the irradiated films. The contribution of sputtering due to the N+ ions to the decrease in the film mass was investigated using the SRIM [31] simulation program. The simulations were carried out for 150 keV N+ ions and for films of the same O:Ti and C:Ti ratios as the pristine film but with variable H:Ti ratios. In none of the simulations undertaken does sputtering significantly contribute to film mass loss. The absorption coefficient, α, of the films was determined as a function of the photon energy, E, from the UVS data. As for our films no linear relationship was observed between (αE)1/2 and E, the optical gap could not be determined using the Tauc model [32], according to which ðαEÞ1=2 ¼ BðE−EG Þ
expected to occur in the irradiation of plasma polymers. The optical gap of conventional polymers typically falls with increasing ion fluence, φ. In polyimide films, for example, bombardment with 40 keV Ar+ causes EG to fall from about 0.7 eV to about 0.5 eV as φ is increased from 1015 to 1016 cm− 2 [35]. Similarly, Sharma et al. [36] recently showed that EG of the polymer CR-39 decreases from 3.4 eV in untreated samples to 0.82 eV following implantation with N+ at φ of 1016 cm− 2. Fig. 4 shows the electrical conductivity σ of the films as a function of φ. A fall in σ is observed at low fluences but a very strong increase is observed at higher φ (equivalent to four orders of magnitude at φ = 1016 cm− 2 compared to that of the unirradiated film). Inspection of the literature reveals that σ of conventional polymer films may be increased by 15 orders of magnitude or more [24,37] by ion implantation. This is due to the radiation-induced formation of conducting carbon-rich structures, and increased cross-linking [38]. Considering the effect of N+ implantation, it is known that the sheet resistance of polyimide films can be reduced by about 9 orders of magnitude under irradiation with 90 keV N+ (φ = 1016 cm− 2) [34]. Nitrogen ion implantation of polyetheretherketone (PEEK) increases its surface conductance by 10 orders of magnitude [39]. Clearly, significant reductions in the EG and increases in σ are produced by N+ implantation of conventional polymers. Previous studies by some of the present authors have also shown that ion implantation of plasma polymerized benzene films with 170 keV N+2 at φ between 1013 and 1017 cm− 2 reduces the electrical resistivity, ρ, by 15 orders of magnitude and also decreases EG from 1.6 to 0.9 eV [40].
ð1Þ
where EG is the optical gap and B is a constant. Consequently, we defined EG as the photon energy corresponding to an absorption of α = 104 cm− 1. This procedure is commonly employed to determine the optical gap of amorphous films [33]. Fig. 3 shows EG as a function of φ. The values of EG are stable at about 3.7 eV but fall abruptly at fluences above 1 × 1015 ions cm− 2 and reach about 1 eV for the fluence of 1 × 1016 ions cm− 2. Švorčik et al. [34] listed the major degradation processes observed in ion irradiation of conventional polymers as excitation and ionization, formation of free radicals, macromolecular chain scissioning, cross-linking, and the outgassing of volatile degradation products. Other effects of irradiation include carbonization, graphitization, and the creation of dangling bonds. Similar complex effects can be
Fig. 4. Electrical conductivity of the films as a function of ion fluence.
N.C. Da Cruz et al. / Surface & Coatings Technology 203 (2008) 534–537
For the same plasma polymers, at a constant φ of 5 × 1015 cm− 2, ρ falls by at least 6 orders of magnitude, and EG is reduced from about 2.8 to 1.1 eV, when the ion energy is increased from 0 to 150 keV [41]. The results of the present study are consistent with these findings. A qualitative explanation for the observed dependence of EG on φ may be the associated increase in the density of unsaturated carbon bonds. Unsaturated carbon bonds add π-electrons to the polymer structure. Owing to the relatively low energy gap between π-bonding and π⁎antibonding levels, the photon energy required to promote electronic transitions is reduced. A key factor influencing the dependence of σ on φ is that the highly cross-linked structures produced by implantation furnish longer conduction paths. In addition, unsaturated carbon bonds supply delocalized π-electrons, which act as charge carriers within the film structure. Both mechanisms increase conduction. 4. Conclusions In the plasma-deposited titanium-containing films examined here, some effects of ion implantation similar to those observed in the ion irradiation of organosilicone films were observed, such as a hydrogen loss, a pronounced structural modification, and compaction. The optical gap falls abruptly from ~ 3.6 eV to ~1.0 eV as the fluence approaches 1 × 1016 cm− 2. In the range of fluence studied here (1014 to 1016 cm− 2), increases of up to six orders of magnitude were induced in the electrical conductivity of the films. Acknowledgements The authors thank the Fundação para o Amparo da Pesquisa do Estado de São Paulo (FAPESP), and the Ministerio de Ciência e Tecnologia/Conselho Nacional de Desenvolvimento Científico e Tecnológico (MCT/CNPq) for financial support. References [1] [2] [3] [4] [5]
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