Modification of wetting properties of SiOx surfaces by Ar implantation

Modification of wetting properties of SiOx surfaces by Ar implantation

Nuclear Instruments and Methods in Physics Research B 193 (2002) 835–845 www.elsevier.com/locate/nimb Modification of wetting properties of SiOx surfa...

434KB Sizes 4 Downloads 33 Views

Nuclear Instruments and Methods in Physics Research B 193 (2002) 835–845 www.elsevier.com/locate/nimb

Modification of wetting properties of SiOx surfaces by Ar implantation M. Chasse, G.G. Ross

*

Institut National de la Recherche Scientifique – Energie et Mat eriaux, Varennes, Qu e., Canada J3X 1S2

Abstract The aim of this experiment is to better understand the mechanism leading to the modification of the wetting properties of insulating (quartz) and non-insulating (Si) surfaces by ion and atom implantation. Coupons of quartz and silicon (with its native oxide layer) have been irradiated by means of 3 keV Ar ions and atoms with a fluence of 1:8  1016 Ar/cm2 . Some exposures to Ar ions have been performed under oxygen partial pressure (5  105 Torr). The samples have been characterized before and after implantation by means of contact angle hysteresis measurements, Rutherford backscattering spectroscopy (RBS) and angle resolved X-ray photoelectron spectroscopy (ARXPS). Irradiation with argon ions or atoms has produced a more hydrophilic surface immediately after implantation. Ar depth profiling by means of RBS has shown that 4% of implanted (atoms and ions) Ar has been retained in quartz, while 13% and 21% of Ar has been retained in silicon after Ar implantation with and without oxygen partial pressure, respectively. However, no difference in the depth distribution has been measured. Characterization by means of ARXPS has shown a noticeable change in the composition of the quartz and silicon oxide surfaces (implantation of Si under an O2 partial pressure producing a thicker oxide layer) which can explain the differences in the reported Ar retention. In general, the irradiation have removed a large portion of oxygen present in the pre-existing carbonaceous layer on the surface of the samples. The ion beam irradiation has been more efficient than atom beam to both, increase the wettability of the quartz surfaces and enhance the concentration of the carbonaceous layer in ‘‘dispersed islands’’ on the surfaces. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.72Ww; 61.80Jh Keywords: Ion implantation; Wetting; XPS; Contact angles; Quartz; Silicon oxide

1. Introduction The ion implantation technique is known to be a powerful tool for modification of tribological, electrical, optical and wetting properties [1–4].

*

Corresponding author. E-mail address: [email protected] (G.G. Ross).

Many applications such as coatings adhesion, antifog and anti-frost treatments require a reliable process for the modification of the wetting properties of surfaces. Up to now, there has been a controversy on the mechanism leading to a modification of wetting properties by ion implantation. This modification has been explained in terms of (1) cleaning and/or ablation of the surface, (2) formation of micropores and/or roughening,

0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 9 1 3 - 8

836

M. Chasse, G.G. Ross / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 835–845

(3) changes in the surface structure and composition, (4) formation of hydrophilic and/or hydrophobic groups, (5) formation of free radicals. In many applications such as anti-fogging glasses and anti-frosting electrical insulator [5,6], the surface composition to be modified is mainly SiOx . However, the complexity of glass, glaze and ceramic composition is an obstacle to well understand the mechanism leading to the modification of the wetting properties. So, the use of more simple materials such as quartz and silicon oxide is more appropriate for this study. The aim of this work is to better understand the mechanism leading to the modification of the wetting properties of insulating (quartz) and non-insulating (Si) surfaces by ion and atom implantation. One important question is what is the respective influence of the molecular modification, the surface erosion and the implanted ion retention on the modification of the surface wettability. Furthermore, the insulating property of those surfaces brings up questions concerning the role of charge accumulation at the surface on the modification of the wetting properties [7]. For that purpose, samples of quartz and silicon (with its silicon oxide surface) were implanted by means of either ionic or atomic argon with an energy of 3 keV and an areal dose of 1:8  1016 Ar/cm2 . Samples were characterized before and after implantation by means of contact angle hysteresis measurements, angle resolved X-ray photoelectron spectroscopy (ARXPS) and Rutherford backscattering spectroscopy (RBS). In this paper, we will present these results and suggest possible mechanisms underlaying the observed changes in wettability.

2. Experimental

(15 min), trichloroethylene (5 min), acetone (15 min), methanol (15 min) and ethanol (15 min). Samples were dried at least 2 h in free air. The sample surfaces were examined by optical microscopy and only clean surfaces without scratches were used. Reproducibility of the contact angles measurements (see below) insures the efficacy of the cleaning procedure. The cleaned samples were introduced in the implantation chamber pumped down to a pressure of 1  107 Torr by means of a turbomolecular pump. Then they were implanted with mass-analysed 3 keV Ar to an areal dose of 1:8  1016 Ar/cm2 . Either atomic neutral Ar or Ar ions were used for implantation. For some experiments the Ar ions were implanted under an O2 partial pressure (5  105 Torr); this process is known to be very efficient in polymers [4] for obtaining very wetting surfaces. No variation in the measure of the ion beam current was observed meaning that only a negligible part of the ions were neutralized by the O2 partial pressure. On the other hand, the use of atomic Ar beam was justified because the atoms are not affected by the occurrence of an electric field due to ion accumulation and secondary electron emission. The atomic Ar beam was obtained by passing the Ar ions through a hydrogen gas at a pressure of 9  105 Torr for a neutralization efficiency of 90%. After implantation, the samples were immediately brought to atmosphere by filling the vacuum chamber by means of either nitrogen, for implantations performed under vacuum, or oxygen for implantations done under an O2 partial pressure. The samples were then characterized in the following minutes by means of contact angle hysteresis measurements, ARXPS or RBS. Finally, the sample surfaces were examined by optical microscopy.

2.1. Sample preparation 2.2. Sample characterization Clear fused quartz and silicon wafers were used as coupon materials. Quartz was 99.99% pure, amorphous, 1 mm thick and samples were cut in pieces of 10  10 mm2 . Silicon was non-doped, n-type, h1 0 0i (0.50°) orientation, polished on both faces and 0.25 mm thick. All samples were cleaned ultrasonically in soapy distilled water

2.2.1. Wetting properties The wetting properties of surfaces were determined by the measurement of the hysteresis of the contact angle. The advancing (ACA) and receding contact angles (RCA) are measured by increasing and then decreasing the volume of a drop of liquid

M. Chasse, G.G. Ross / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 835–845

(distilled water) deposited on the sample surface. Recorded images are digitized and analysed with a software routine that evaluates the tangent at the point of contact between the drop and the surface (i.e. the contact angle). Typically, the maximum volume of the drop is 10 ll and the injection and withdrawal speed is 6 ll/min. 2.2.2. Physicochemical properties The physicochemical properties of the samples were obtained by means of a VG 220i X-ray photoelectron spectrometer in Al Ka radiation. A non-monochromatized X-ray source operating at 400 W was employed to minimize problems with sample charging. The principal photoelectron peaks were measured at take-off angles (relative to the normal to the sample surface) of 0°, 15°, 30°, 45° and 60° in order to get information in depth [8]. In the model used to interpret that data, it was assumed that the substrate material was covered with a partial layer of uniform thickness. The aspect ratio of the fractional layer is assumed to be very low, so that shadowing effects are ignored. Within the partial layer, or the substrate, the composition is assumed to be uniform. For a given element, the photoelectron intensity I from a given orbital is then

2.2.3. Ar depth profiling The depth profiles were measured by RBS described in details elsewhere [9]. Very briefly, a 350 keV 4 Heþ ion beam is incident at an angle of 65° with respect to the normal of the sample and the backscattered 4 He are energy-analysed at an angle of 145° to the beam, in a low-noise thin-window ion-implanted detector with a resolution of 5.1 keV (standard deviation). The beam current is measured by a Faraday cup and, during analysis, by counting the backscattered yield from a rotating target consisting of tungsten spokes. The 4 He spectrum is deconvoluted into a depth profile with our own computer code Alegria [10]. 2.3. Simulation The Monte Carlo computer code SRIM-2000 [11] was used for the calculation of the depth distributions. They use an amorphous target structure and the binary collision approximation. The distributions were characterized by their mean projected range and variance.

3. Results 3.1. Non-implanted samples

I ¼ f ½cL kL ð1  expð  t=kL cos hÞÞ /TAr cos h þ cS kS expð  t=kL cos hÞ þ ð1  f ÞcS kS ;

837

ð1Þ

where / is the X-ray flux, T the transmission/detector efficiency, A the analysis area, r the crosssection for photoelectron emission, f the fraction of the substrate surface covered by the layer, t the thickness of the layer, cL the concentration of the element in question in the layer, kL the photoelectron inelastic mean free path through the layer, h the photoelectron take-off angle relative to the normal to the sample surface, cS the concentration of the element in question in the substrate and kS the photoelectron inelastic mean free path in the substrate. In this equation, f, c and t can be treated as adjustable parameters. All other values are either experimental parameters or found in the literature.

It is of prime importance to characterize the non-implanted samples in order to understand the implantation effect in comparing characterization result before and after implantation. The nonimplanted quartz samples have ACA and RCA of 38° and 13°, respectively, while the silicon samples, including its oxide layer, have ACA and RCA of 54° and 32°, respectively. The precision of the measurements is 2°. Fig. 1 shows the relative atomic concentrations of C, O and Si in silicon (top) and quartz (bottom) as measured by ARXPS. The X-rays were detected at different angles (0°, 15°, 30°, 45° and 60° relative to the surface normal) which corresponds to different depths, 60° being more sensitive to the very first atomic layers. As expected, in quartz, only the C concentration increases with the angle while both O and C increased in silicon. Using the Eq. (1) and the procedure briefly described above, a fit has

838

M. Chasse, G.G. Ross / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 835–845

Fig. 1. Atomic concentration measured at different X-ray detection angles for non-implanted silicon (a) and quartz (b) surfaces. The right hand side illustrates the result of the model briefly described in Section 2.2.2.

been adjusted to the experimental points and the results are illustrated by the lines in Fig. 1. We have made the hypothesis that the bulk has no significant quantity of carbon and is covered by a surface layer containing all the detected carbon. This hypothesis has been confirmed by using an ion beam to sputter the surface during the XPS measurements; after a few seconds, the carbon level was below the detection limit (1%). According to this fit, the silicon and the quartz samples are 93% and 79% covered with a 1.7 and 0.7 nm thick layer containing 57% and 84% of C, respectively. After subtraction of the surface effect, this measurement gives in quartz an amazing O/Si ratio of 1.5. It is important to note that the value of 1.2 obtained by a simple division of O and Si concentrations in Fig. 1 is affected by the O contain in the surface layer and, consequently, does not represent the real O/Si ratio in the quartz. The C 1s (up) and Si 2p (bottom) spectra measured by XPS are shown in Fig. 2. All spectra shown in this paper have been measured with a detection angle of 45°. An example of the deconvolution of the spectra into its components is illustrated for the quartz C 1s spectrum. In our deconvolution procedure, we have minimize the number of peaks necessary for the reconstruction of the spectra while maintaining constant the width at half maximum of the gaussian component. Because of the proximity of atoms close to

Fig. 2. C 1s (a) and Si 2p (b) XPS spectra of non-implanted silicon and quartz surfaces. In (a) there is an example of the deconvolution procedure of a spectrum into different components.

the different identified molecular species, this procedure, is not rigorous for this kind of material which is not polymeric. However, it allows to better understand the relation between the wettability and the surface chemistry. Fig. 2(a) shows that especially in quartz, a notable quantity of C atoms make stronger bonds than the simple C–C and C–H bindings, probably with more electronegative O atoms. As expected, Fig. 2(b) shows that all the Si atoms are bound to O atoms in quartz while the majority of the Si atoms are linked to other Si atoms in silicon. The quartz samples have also been analysed by means of RBS. An O/Si ratio of 1.7 was found which is slightly higher than the ratio obtained

M. Chasse, G.G. Ross / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 835–845

with ARXPS (1.5). A plot of the O and Si depth profiles for the first 30 nm will be shown further in Fig. 5. 3.2. Implanted samples 3.2.1. Silicon samples Immediately after the implantation, all contact angles have decreased. The Arþ implantation under vacuum has reduced the contact angles from 54° to 39° in advancing and from 32° to 27° in receding. The use of an O2 partial pressure has strongly improved the efficacy of the implantation, reducing the contact angles below 5° (limit of our apparatus) for both advancing and receding. The relative concentrations of C, O and Si as measured by ARXPS on the Ar implanted silicon samples are shown in Fig. 3. As noted for the nonimplanted samples, an increase of C and O concentrations in silicon is observed at large detection angles. The implantation of Ar under an O2 partial pressure has strongly increased (by a factor of 1.8), the O concentration which has induced a relative decrease of the Si and particularly the

Fig. 3. Atomic concentration measured at different X-ray detection angles for silicon surfaces implanted with Arþ under vacuum (filled symbols) and O2 partial pressure (textured symbols); the lines have been calculated with Eq. (1). The bottom part illustrates the result of the model after Arþ implantation under vacuum (left) and O2 partial pressure (right).

839

C concentrations. The fit procedure described above and the Eq. (1) were used to calculate the dashed lines shown in Fig. 3. This procedure has given a covering fraction of 53% after implantation, compared to 93% before it. The relative concentration of C in the surface layer has been established to 49% for the implantation under vacuum and to 29% when an O2 partial gas pressure was used. The thickness of the surface layer has been reduced from 1.7 nm before implantation to 1.4 nm when the O2 partial pressure was used. Otherwise, the Ar implantation has increased the layer thickness to 2.9 nm. In both cases, the total quantity of C (product of t, f and C%) has been reduced by the implantation, being 1.1 and 4.3 lower with implantation under vacuum and O2 partial pressure, respectively. Fig. 4 shows the C 1s and Si 2p spectra measured by XPS. The implantation of Ar under

Fig. 4. C 1s (up) and Si 2p (bottom) XPS spectra of non-implanted silicon surface and surfaces implanted with Arþ under vacuum and O2 partial pressure.

840

M. Chasse, G.G. Ross / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 835–845

vacuum has slightly increased the relative concentration of the Si–O bonds. It has also slightly moved the Si–O binding energy toward the high energy and the Si–Si binding energy which means that the Si feels more the presence of the neighbouring Si atoms. The Ar implantation under an O2 partial pressure has strongly increased the relative concentration of the Si–O bonds, probably in increasing the oxide layer thickness. On the other hand, the Ar implantation has induced a peak around 284 eV in the C 1s spectrum, which corresponds to a Si–C bond. The result of the deconvolution (not shown in Fig. 4 for clarity purposes) is presented in Table 1 together with that of the O 1s spectra. Note that the O 1s spectra have not been shown to save space and because of the lack of visible structure in its shape. A glance at this table informs us that both implantations have induced the formation of C–Si bonds. Both implantations have also reduced the relative concentration of the C–C and C–O as well as the bond at 534.8 eV (which has not been identified). Finally, the implantation under an O2 partial pressure has induced the formation of bonds at 281.6 eV in the C 1s spectrum and at 530.6 eV in the O 1s spectrum which corresponds to O–O bonds. Fig. 5(a) shows the depth profile of Ar implanted in silicon. For an implantation dose of 1:8  1016 Arþ /cm2 , only areal densities of 3:8  1015 and 2:3  1015 Ar/cm2 were found in samples implanted under vacuum and under an O2 partial pressure, respectively. This represents retained percentages of 21% and 13%, for the implantation under vacuum and O2 , respectively. The depth

Fig. 5. RBS depth profiles of O and Si (up) in quartz and Ar (bottom) in silicon and quartz after implantation of Arþ (3 keV, 1:8  1016 Ar/cm2 ) under vacuum (solid lines) and O2 partial pressure (dashed lines).

distributions are similar in both cases with a mean depth of 5 nm, slightly shallower than 6.5 nm given by the SRIM-2000 simulation. The surface erosion induced by the ions could be responsible for this discrepancy.

Table 1 Relative concentration (%) of different molecular bonds forming the C 1s and the O 1s spectra of the silicon samples (ha /hr )

C 1s spectra (281.6)

Non-implanted (54°/32°) Arþ (vacuum) (39°/27°) Arþ (O2 ) <5°

O 1s spectra C–Si (283.7)

C–C, C–H (285.0)

C–O (286.8)

[email protected] (288.3)

O–[email protected] (289.2)

O–O (530.6)

SiO2 (532.6)

(534.8)





75.1

18.7



6.2

2.8

92.7

4.5



14.8

62.7

13.6

4.1

4.7



97.1

2.9

3.1

10.9

68.8

10.9



6.3

8.5

89.1

2.4

Eb (eV) are given in parantheses.

M. Chasse, G.G. Ross / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 835–845

3.2.2. Quartz samples Immediately after the implantation, all contact angles have also decreased. The Arþ implantation under vacuum has reduced the contact angles from 38° to 29° in advancing and from 13° to 7° in receding. The use of an O2 partial pressure has once again strongly increased the efficiency reducing the contact angles below 5° (limit of our apparatus) for both advancing and receding. The implantation of atomic Ar0 has been more efficient than the ion implantation, reducing the angles from 38° to 16° in advancing and from 13° to 5° in receding. Fig. 6 shows a relative increase of the C concentration for larger detection angles. The fit procedure described above and the Eq. (1) were used to calculate the dashed lines shown in Fig. 6. The results from the fit procedure show that the Ar ion implantation has strongly reduced the covering fraction of quartz by the surface layer from 79% before implantation to 35% after implantation. However, the use of atomic Ar has been less efficient, leaving the covering fraction to 68%. Also, an increase of the relative C concentration in the surface layer, from 84% to 100%, is observed after the implantation with ions. However, the atomic Ar implantation has left unchanged this concen-

Fig. 6. Atomic concentration measured at different X-ray detection angles for quartz surfaces implanted with Arþ (filled symbols) and Ar0 (textured symbols); the lines have been calculated with Eq. (1). The bottom part illustrates the result of the model after Arþ (left) and Ar0 (right) implantation.

841

tration. Finally, the layer thickness has not been strongly affected by the implantation; the original value was 0.7 nm before implantation and thicknesses of 0.9 and 0.7 nm were deduced for Arþ and Ar0 implantations, respectively. Once again, the total quantity of C has been decreased by the implantation by a factor of 1.5 for ions and 1.2 for atoms. The C 1s and Si 2p spectra obtained by means of XPS are presented in Fig. 7. The implantation of Ar (both ions and atoms) under vacuum has moved the Si 2p spectrum more towards the high energy than the implantation under an O2 partial pressure for which the displacement is very small. As expected, all Si atoms are bound to O atoms. On the other hand, the Ar implantation under an O2 partial pressure has induced a peak around 290 eV in the C 1s spectrum, which corresponds to

Fig. 7. C 1s (up) and Si 2p (bottom) XPS spectra of nonimplanted quartz surface and surfaces implanted with Arþ (under vacuum and O2 partial pressure) and with Ar0 .

842

M. Chasse, G.G. Ross / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 835–845

Table 2 Relative concentration (%) of different molecular bonds forming the C 1s and O 1s spectra of the quartz samples (ha /hr )

C 1s spectra C–Si (284.3)

Non-implanted (38°/13°) – Arþ (vaccum) (29°/7°) – Ar0 (H2 ) (16°/5°) – Arþ (O2 ) <5° 18.5

O 1s spectra

C–C, C–H (285.0)

C–O (287.2)

O–[email protected] (289.2)

(O–[email protected])–O (289.8)

(296.2)

O–O (530.0)

SiO2 (532.7)

(534.8)

(537.4)

64.4

18.4



7.4

8.1

1.7



77.6

17.8

4.6

53

13.9

7.8



21.7

3.5

0.6

87.4

10.5

1.5

66.2

13

7.3



12

1.5



81.7

15.8

2.5

29

19.4



3.2



71

25

4

25

(292.7)

4.8

Eb (eV) are given in parentheses.

a (O–[email protected])–O bond while the implantations under vacuum have increased the relative concentration of the higher energy bonds. The result of the deconvolution (not shown in Fig. 7 for clarity purposes) is presented in Table 2 together with that of the O ls spectra. The formation of C–Si bonds have been observed only after Ar implantation under an O2 partial pressure. This implantation has also strongly reduced the relative concentration of both the C–C bonds and the bonds at 292.7 eV, but increased that of the bonds at 534.8 eV. Comparison of the effects induced by atoms and ions suggest that the ions are more efficient to increase the higher energy components of the C 1s spectrum and to decrease those of the O 1s spectrum. However, the C–C concentration has been slightly increased by the use of Ar atoms. Fig. 5 shows the depth profile of O and Si (up), and Ar (bottom) measured by RBS after Ar implantation in quartz. For an implantation dose of 1:8  1016 Arþ /cm2 , Fig. 5(a) shows that only areal densities of 7  1014 Ar/cm2 was found in all implanted samples (Arþ , Ar0 and Arþ (under O2 )). This represents retained percentages of only 4%. However, the Ar depth distributions are slightly shallower to those measured in the Si samples (mean range of 3.2 nm in quartz compared to 5 nm in Si). As a comparison, the SRIM-2000 simulation gives also a shallower mean range in quartz (6 nm in quartz compared to 6.5 nm in Si). Nevertheless, that difference is not sufficient to explain an eventual repel of the ions by charge accumu-

lation at the surface which could be responsible for the low concentration that is also confirmed by the results from the implantation of atomic Ar. As shown in Fig. 5(b), the Si depth profiles are similar for all the implanted silicon samples while the O profile has been kept to the original concentration only by the implantation under an O2 partial pressure.

4. Discussion 4.1. Wetting of silicon samples A glance to Figs. 1 and 3 convinces us that the water contact angles decrease with the fraction covered by the surface layer. However, this explanation seems not sufficient. Therefore, the relative concentrations of Ar, O and C normalized to that of Si have been plotted as a function of the ACA. The plots are shown in the upper part of Fig. 8. According to this figure, more wetting surfaces are obtained when the concentrations of O increase. On the opposite, larger concentrations of Ar and C are favorable to larger contact angles. Analysis of the ARXPS measurements by means of the model described in Section 2.2.2, allows to consider, on the one hand, the molecular composition of the layer surfaces and, on the other hand, the molecular composition of the substrate (the silicon oxide layer in this case). The relative concentration of the principal molecular bonds have been plotted as a function of the ACA and

M. Chasse, G.G. Ross / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 835–845

843

can link to another atom (e.g. C or O) and form a new molecule. When the implantation of Ar is done under an O2 partial pressure, the Si atom can link to an O2 molecule and form a very wetting SiO2 molecule. So, this mechanism could explain the increase of the surface wettability by an augmentation of the O concentration and the formation of SiO2 bonds. 4.2. Wetting of quartz samples

Fig. 8. Ar, O and C concentrations in silicon normalized to the Si concentration as a function of the ACA (up). C–O, O–[email protected] and C–Si bond concentrations in the silicon surface layer normalized to the C–C and C–H bonds, as well as the Si–Si/SiO2 bond ratio on the silicon surface as a function of the ACA (bottom). Lines are the result of linear fits.

the plots are shown in the lower part of Fig. 8. It is clear that the S–Si/SiO2 ratio influences greatly the wetting properties of the silicon surface. The wettability of the surface decreases with an increase of the Si–Si/SiO2 ratio. This result is not surprising because the SiO2 bonds are known to be very wetting. On the other hand, the molecular bonds including carbon atoms have not a notable influence on the wettability of the silicon samples. Finally, the behavior of the atom and molecular bond concentration as a function of the RCA follows the same tendency. The O concentration and the increase of the relative concentration of SiO2 bonds are the dominant parameters leading to a strong increase of the wetting properties of Si after Ar irradiation. It is known that Ar ions of 3 keV have enough energy to break the Si–Si bonds. Then the Si atom

Comparison of results in Figs. 1 and 6 shows that the Ar implantation has reduced both the contact angles and the fraction covered by the surface layer. The implantation of ions has been more efficient then the implantation of atoms to reduce the covered fraction (twice lower) but has less reduced the contact angles. So, it seems that the lone fraction cannot explain the observed modification in wetting properties. However, it worthies mentioning that the surface layer of the Arþ implanted surface contains no oxygen while the O concentration is 15% after the Ar0 which is similar to the non-implanted quartz. We still do not understand the mechanism leading to an increase of the O concentration after an Ar0 implantation but the presence of the H2 molecules used for the neutralization could play a role. The relative concentrations of Ar, O and C normalized to that of Si have been plotted as a function of the ACA. The plots, shown in the upper part of Fig. 9, do not disclose any sizeable change with the ACA although a slight increase of Ar and C concentrations seems to appear at larger contact angles. Fig. 9 (bottom) shows the behavior of the molecular composition of the surface layer, as obtained by means of analysis of the ARXPS measurements (and the model described in Section 2.2.2) and normalized to the C–C and C–H bonds. According to these plots, an increase of the C–O and 296 eV bonds is observed on more wetting surfaces. On the opposite, and increase of the 293 eV bonds leads to a more hydrophobic surface. It was shown in Section 4.1 that the S–Si/SiO2 ratio influences greatly the wetting properties of the silicon surface. According to Fig. 9, it seems that the bond at 105 eV is even more wetting that

844

M. Chasse, G.G. Ross / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 835–845

the surface layer and the decrease of the covered fraction following Arþ implantation and a change in the molecular composition following the Ar0 implantation.

5. Conclusion

Fig. 9. Ar, O and C concentrations in quartz normalized to Si concentration as a function of the ACA (up). C–O, 293 and 296 eV bond concentrations in the quartz surface layer normalized to the C–C and C–H bonds, as well as the 105 eV/SiO2 bond ratio on the quartz surface as a function of the ACA (bottom). Lines are the result of linear fits.

the very wetting SiO2 molecule. As mentioned for silicon, the behavior of the atom and molecular bond concentration as a function of the RCA follows the same tendency. So, its seems that in the case of quartz samples, the composition of the surface layer is the dominant factor in the modification of surface wettability. The O concentration, which was the dominant parameters for the silicon samples, could not be so important for quartz because of the relative high O concentration in the non-implanted quartz which could be close to the saturation level. On the other hand, the thickness of the surface layer has not been reduced by the implantation which exclude the erosion/sputtering as a major parameter. Briefly, a possible mechanism is the selective ion beam induced erosion/desorption [12] of O from

The implantation of Ar with an energy of 3 keV and a dose of 1:8  1016 Ar/cm2 has reduced the ACA and the RCA in both Si and quartz. When the Ar implantation was performed in an O2 partial pressure (5  105 Torr), the effect was strongly increased, both contact angles decreasing below 5°. RBS depth profiling of implanted Ar has shown that only a few percentage of Ar has been retained into Si (21% and 13% for implantations under vacuum and an O2 partial pressure, respectively) and quartz (4% for all implantations). Also, the depth distributions are shallower in quartz (Rp ¼ 3:2 nm) than in Si (Rp ¼ 5 nm) and both are smaller than the prediction of SRIM-2000 (Rp ¼ 6 and 6.5 nm for quartz and Si, respectively). The discrepancy cannot be explained by the accumulation of electrical charges at the surface of the insulating quartz. According to our results, the erosion/sputtering is not a major parameter for the wetting property modification. The dominant factor seems to be the modification of the chemical composition of the surface layer. For both samples, the contact angles decrease with the fractions covered by the surface layer. However, the increase of both the O and the SiO2 /Si–Si concentrations favors greatly the hydrophobicity of the silicon surfaces while no sizeable influence is observed for the quartz surfaces. The latter seems to be more influenced by the C–O/ C–C concentration, becoming more wetting with its increase. The increase of the relative concentration of the bonds at 296 and 293 eV seems to favor a higher hydrophilicity and hydrophobicity, respectively. Finally, the use of atomic Ar (as compared to the ionic Ar) has been more efficient to increase the wettability of the quartz surfaces. However, no difference has been observed in the Ar depth profiles which suggest that the repel of the ions by a possible accumulation of electrical charges at the

M. Chasse, G.G. Ross / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 835–845

surface cannot explain this discrepancy. A larger O concentration (15% versus 0%) in the Ar0 implanted surface layer could explain this higher hydrophobicity. However, the mechanism is still not understood.

Acknowledgements The authors are grateful to M. Chartre and P.P. Mercier for their excellent operation and maintenance of the implanter and accelerator. M. Chasse thanks the Fonds FCAR for financial support. This work has been supported by the Natural Sciences and Engineering Research Council of Canada.

References [1] L. Boudoukha, S. Paletto, F. Halitim, G. Fantozzi, Nucl. Instr. and Meth. B 122 (1997) 233.

845

[2] Y.Q. Wang, R.E. Geidd, Nucl. Instr. and Meth. B 79 (1993) 659. [3] S. Lombardo, S.U. Campisano, G.N. van den Hoven, A. Cacciato, A. Polman, Appl. Phys. Lett. 63 (14) (1993) 1942. [4] W.-K. Choi, S.-K. Koh, H.-J. Jung, J. Vac. Sci. Technol. A 14 (4) (1996) 2366.  . Jean, G. Veilleux, N. [5] G.G. Ross, R.W. Paynter, E Boussaa, A. Poirier, SPIE 3413 (1998) 45. [6] G.G. Ross, G. Abel, M. Bolduc, H. Bourque, M. Chasse, O. Couture, A. Hallil, K. Laaziri, in: D.L. Zhang, K.L. Pickering, X.Y. Xiong (Eds.), ICAMP Proc., IMEA Publishing, 2000, p. 51. [7] V.M. Moreno-Villa, M.A. Ponce-Velez, E. Valle-Jaime, J.L. Fierro-Chavez, IEE Proc. Gener. Transm. Distrib. 145 (6) (1998) 675. [8] R.W. Paynter, Surf. Interface Anal. 3 (4) (1981) 186. [9] G.G. Ross, L. Leblanc, B. Terreault, J.F. Pageau, P.A. Gollier, Nucl. Instr. and Meth. B 66 (1992) 17. [10] F. Schiettekatte, G.G. Ross, in: J.L. Duggan (Ed.), Application of Accelerators in Research and Industry, AIP Press, New York, 1997, p. 711. [11] SRIM-2000 J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. B 174 (1980) 257. [12] F. Schiettekatte, G.G. Ross, A. Chevarier, N. Chevarier, A. Plantier, Nucl. Instr. and Meth. B 132 (1997) 607.