The adhesion of hydrogenated amorphous carbon films on silicone

Thin Solid Films 394 Ž2001. 102᎐108

The adhesion of hydrogenated amorphous carbon films on silicone K. Donnelly a,U , D.P. Dowling a , M.L. McConnell a , M. Mooney b a

Surface Engineering Group, Materials Technology Department, Enterprise Ireland, Glasne¨ in, Dublin 9, Ireland b National Microelectronics Research Centre, Lee Maltings, Uni¨ ersity College, Cork, Ireland Received 17 August 2000; received in revised form 10 April 2001; accepted 17 May 2001

Abstract A number of hydrogenated amorphous carbon films, the properties of which were varied by adjusting the RF power, were deposited from a 13.56-MHz plasma onto silicone and glass substrates. Deposition was monitored by in situ ellipsometry performed on the glass substrates and the refractive index was used as a figure of merit for each film. Film adhesion was measured by a pull adhesion technique and films were found to have adhesion of 12 MPa, except in a region Ž1.75- n - 1.83. where the adhesion was 20 MPa. No link was observed between adhesion and surface morphology or roughness; however, the increase in adhesion was accompanied by a change in the Raman spectrum. In the higher refractive index ŽG 1.88., lower adhesion Žf 12 MPa. films, the G- and D-bands are evident in the Raman spectra. However, in the lower refractive index ŽF 1.83., higher adhesion Žf 20 MPa. films, the Raman spectra are dominated by a featureless fluorescence background, with no G- or D-bands evident. The higher adhesion of the lower refractive-index films is combined with a low friction that makes these films ideal for overcoming the inherent tackiness of silicone and may provide suitable coatings for biomedical applications. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond-like carbon; Adhesion; Raman scattering; Surface morphology; Physical vapour deposition

1. Introduction Hydrogenated amorphous carbon Ža-C:H. films exhibit a wide range of mechanical and physical properties, based on the ratio between sp 3 and sp 2 hybridised carbon᎐carbon bonds and on the hydrogen content w1x. The sp 3rsp 2 ratio and hydrogen content are difficult to determine on a routine basis, but can be approximated by a measure of the refractive index Ž n. of the films, as determined by in situ ellipsometry. Previous studies w2,3x have shown there to be a close correlation between refractive index Ž n. and extinction coefficient Ž k . for a-C:H films, and while refractive index Ž n. values are used in this work, values of extinction coefficient Ž k . are implicit therein and could equally have been used. U

Corresponding author.

The reason for quoting refractive index Ž n. values is that this parameter has been shown to be indicative of a-C:H film quality, as defined by cross-linking w4x, hardness w4,5x, wear resistance w4,5x and laser damage threshold w2x. In general, films with a high refractive index Ž n ) 1.9 at ␭ s 675 nm. are hard, rigid, diamond-like films, and those with low refractive index Ž n - 1.75 at ␭ s 675 nm. are soft, flexible polymeric films w2,5x. One important property of a-C:H is biocompatibility, and the potential of DLC coatings for biomedical applications has been extensively reported in the literature w6᎐9x. In this work, all the films studied contain significant amounts of hydrogen, and so fall into the category of diamond-like carbon ŽDLC. films, more correctly referred to as hydrogenated amorphous carbon; however, for simplicity, the generic term DLC is used throughout. The aim of this work was to provide a low-friction,

0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 1 6 2 - 2

K. Donnelly et al. r Thin Solid Films 394 (2001) 102᎐108

potentially biocompatible coating on silicone rubber to overcome the inherent tackiness that limits certain biomedical applications, despite its favourable mechanical properties w10,11x. We have previously reported on the coefficient of friction of a range of DLC films on silicone, showing that while the DLC-coated silicone had a lower friction coefficient Ž␮ s 0.40" 0.05. than the uncoated silicone Ž␮ s 1.20" 0.20., there was no observable correlation with refractive index w12x. In the present work, we report on the effect of refractive index on the adhesion, Raman spectrum, morphology and surface roughness of a range of DLC coatings on silicone. 2. Experimental details 2.1. DLC deposition DLC films were deposited in a cylindrical stainless steel chamber, as described elsewhere w2,5,6x, onto silicone substrates that were placed on a 10-cm-diameter electrode driven at 13.56 MHz. Prior to deposition, the silicone substrates were pre-treated for 1 min in a 25-Pa oxygen plasma, which was found to significantly improve film adhesion. Films were deposited from a 30:1 HerC 2 H 2 mixture at 15 Pa. Film growth was monitored by in situ ellipsometry Žat ␭ s 675 nm., which was used to measure the refractive index Ž n., extinction coefficient Ž k . and thickness of the DLC films on 25 = 25-mm2 soda glass slides placed next to the silicone samples. Changing the RF power, and the commensurate change in the self-bias voltage Ž V b . on the driven electrode Žwhich acts as a substrate holder., facilitates the deposition of a range of DLC films, from soft polymeric material Žlow refractive index. to hard, wear-resistant films Žhigh refractive index.. The RF power ranged from 20 Ž n s 1.74, V b s y132 V. to 130 W Ž n s 2.4, V b s y444 V.. Films used in this study were 140 " 10 nm thick and are listed in Table 1. 2.2. Raman spectroscopy Films were examined by Raman spectroscopy recorded at room temperature using a Renishaw Ramascope spectrometer, consisting of an air-cooled 25mW argon ion laser source Ž ␭ s 514.5 nm., an Olympus microscope and a Peltier, cooled CCD detector. The laser power delivered at the sampling point in all experiments detailed was no more than 3 mW, with a typical spot size of 1 ␮m. 2.3. Surface roughness Surface roughness was determined by scanning white-light interferometry. Readings were taken over a number of areas measuring 0.14= 0.18 mm2 and aver-

103

Table 1 The films used in this study RF power ŽW.

Self bias ŽV.

Refractive index

Roughness Ž␮m.

20 30 40 50 60 65 70 80 90 100 130

y132 y140 y145 y187 y194 y209 y231 y243 y277 y318 y444

1.74 1.75 1.80 1.83 1.88 1.94 1.96 2.03 2.07 2.15 2.40

0.21" 0.05

0.25" 0.11

0.26" 0.07 0.23" 0.02

The surface roughness data were determined by white light interferometry on the films deposited onto the silicone substrates and illustrated in Fig. 5 Žthe error is the standard deviation calculated from four measurements across each sample.. All films were 140 " 10 nm thick.

aged. There were a considerable number of gross nonuniformities attributable to the underlying substrate, and the roughness measurements were performed on relatively smooth areas in order to isolate the contribution of the DLC refractive index to the roughness. 2.4. Electron microscopy For examination by electron microscopy, the samples were prepared onto aluminium holders and introduced into an electron probe X-ray microanalyser ŽEPMA. JEOL JXA 8600. Sample preparation included sputter-coating of the specimen with a thin layer of gold ŽAu. in order to aid surface electron conductivity. 2.5. Adhesion testing So-called practical adhesion, the force or work required to remove or to detach a film or coating from the substrate w13x, is one of the fundamental properties possessed by any film. In practice, there is a myriad of methods for measuring adhesion, all with their merits and demerits. In this study, it was decided that a simple pull test would provide the most reproducible and interpretable indication of the force required to separate the film ŽDLC. from the substrate Žsilicone.. Adhesion was measured using a force cell to determine the force required to separate two pieces of coated polymer that had been bonded together. One piece of polymer was glued to a steel plate to provide mechanical support, and the second piece was glued to a stainless steel pin Žthe coated face was exposed in both cases.. A small spot of adhesive was then used to join the two coated polymers Žcoated surface to coated surface ., as shown in Fig. 1. The adhesive used was standard two-component epoxy Žprofessional grade

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K. Donnelly et al. r Thin Solid Films 394 (2001) 102᎐108

Fig. 1. Schematic diagram of the adhesion test used in this work. Two pieces of the coated polymer were glued to a steel base plate and a metal pin, respectively. A spot of glue was applied between the pieces of polymer and the force required to separate the two pieces was measured.

Araldite, from Evode Ltd. at the polymer metal interface and cyanoacrylate ŽLoctite 415. at the DLC᎐DLC interface. It was found that gluing the metal pin directly onto the silicone resulted in a high rate of failure at the metal᎐glue joint if cyanoacrylate was used and the DLC᎐glue joint if epoxy was used. Therefore, the arrangement shown in Fig. 1 was adopted. In the arrangement shown in Fig. 1, there are four glue joints at which failure can occur: the pinrsilicone joint; the DLCrsilicone joint; the DLCrDLC joint; and the siliconersteel plate joint. In addition, there are occasions when the polymer itself tears, making a total of five observed failure modes. Obviously, only the DLCrsilicone failure mode gives information about the adhesion of DLC on silicone, and hence every effort was made to make all the other joints stronger than this bond. To this end, a number of adhesives were examined and the silicone was treated on the uncoated side by passing a flame over the surface. Flaming increases the polymer surface energy w14x and increases the metal᎐silicone bond strength considerably; there was no detectable effect on the DLC. Even though much care was exercised in ensuring that all the glue joints were stronger than the DLCrsilicone bond, many pull tests showed failure at other locations. In order to overcome this problem and to ensure a reliable determination of adhesion, 15 individual adhesion measurements were performed on separate areas of each coated polymer. Over the course of approximately 250 experiments, the five modes of failure mentioned above were observed with their corresponding frequency as follows: 1. 2. 3. 4. 5.

curred at the DLCrsilicone interface Žfailure mode II.. The adhesion Žin MPa. was then determined as the ratio of the force required to separate the film from the silicone to the area of film removed. The adhesion obtained for each of these pull tests was averaged in order to obtain the corresponding DLCrsilicone adhesion. 3. Results 3.1. Adhesion Fig. 2 shows a plot of adhesion versus refractive index Ž n. for DLC films deposited on silicone. Each point is an average of the five᎐eight successful measurements obtained from 15 separate experiments on each film Žthe error bars represent one standard deviation.. Two distinct regions are evident where the adhesion is approximately constant. The first region, below a refractive index of 1.85, exhibits adhesion of approximately 20 MPa; the second region, above n s 1.85, has an adhesion of approximately 12 MPa. There is also one film with a refractive index of 1.75 that has an anomalously low adhesion value of 12.4" 1.5 MPa.

pinrsilicone failure Ž19%.; DLCrsilicone failure Ž42%.; DLCrDLC failure Ž13%.; siliconersteel plate failure Ž17%.; and failure within the silicone bulk, or failure mode unclear Ž9%.

In general, of the 15 tests on each polymer, there were 5᎐8 Ž33᎐55%. useful examples where failure oc-

Fig. 2. A plot of adhesion versus refractive index for a range of a-C:H films deposited onto silicon. The error bars represent one standard deviation in the data.

K. Donnelly et al. r Thin Solid Films 394 (2001) 102᎐108

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Table 2 Raman data-fitting results for films deposited on silicone and glass substrates Refractive index

Centre Žcmy1 .

Area Dpeak

Gpeak

Dpeak

Width Žcmy1 . Gpeak

Dpeak

Integral Gpeak

Silicone 1.74 1.755 1.83 1.88 1.96 2.03 2.07 2.15 2.40

16078 45950 13161 25306 13793 33566

76844 78962 63765 65307 56637 56037

1391 1406 1397 1409 1383 1419

1589 1578 1572 1576 1576 1576

56 194 103 150 98 186

77 86 93 93 90 90

912602 2184296 1248140 790156 214253 115322 88566 83961 61397

Glass 1.74 1.755 1.83 1.88 1.96 2.03 2.07 2.40

36476 80185 55938 71752 117572 99178 105495

26111 77619 308256 251217 121084 128224 88363

1389 1423 1344 1345 1434 1412 1444

1587 1570 1530 1533 1539 1544 1546

267 301 167 209 325 304 318

93 122 156 166 156 162 152

1140023 1825216 759808 745582 288618 374589 266137 184661

The integral refers to the integrated area under the spectra, excluding the G and D peaks if present. The integral is the area under the Raman plot between 1150 and 1800 cmy1 .

Fig. 3. Normalised Raman spectra of DLC coatings on: Ža. silicone; and Žb. glass as a function of refractive index wthe peak at 1120 cmy1 in Ža. is from the silicone substrate x.

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K. Donnelly et al. r Thin Solid Films 394 (2001) 102᎐108

Fig. 4. Ža. Raman G- and D-peak position; Žb. Raman G- and D-peak width; Žc. I D rIG ratio; and Žd. integrated background intensity Žafter Dand G-peak removal. as a function of refractive index for films on glass and on silicone.

3.2. Raman Fig. 3 shows the Žnormalised. Raman spectra of the DLC films in the range 1000᎐1800 cmy1 . The spectra are presented as a function of refractive index on the silicone substrates on which the adhesion was determined, and on the glass slides on which the refractive index was determined. The spectra in this region are characterised by a pair of bands at 1400 and 1540 cmy1 , designated the D- and the G-bands, respectively w15,16x. In addition, there is a broad background, monotonically increasing with frequency, that has been associated with fluorescence, which indicates the formation of a polymerised organic structure with a high hydrogen content w17,18x. The spectra were fitted to two Gaussian line shapes attributed to the G- and D-bands and a quadratic polynomial background was used to subtract the fluorescence. The fit parameters are presented in Table 2 and some trends in the data are presented in Fig. 4. From Fig. 3a, it is evident that the G-band, and to a lesser extent the D-band, is clearly visible in some of the spectra, indicating the presence of DLC on both the silicone and glass substrates. It should also be noted that as the refractive index of the films fall, the G- and D-band features become less pronounced and the fluorescence background increases, until at low refractive index Ž- 1.755 on glass and - 1.88 on silicone. the G- and D-bands disappear and only the fluorescence remains ᎏ the films can no longer be considered as DLC. All films are characterised by the

refractive index, as measured by ellipsometry on the glass substrate. It is clear from Fig. 4 that the DLC grown on glass is not the same as the DLC grown on silicone placed beside it on the driven electrode, as described above. In general, the silicone DLC G-band is shifted to higher frequencies ŽFig. 4a., exhibits narrower Raman bands ŽFig. 4b. and the I D rIG ratio is lower ŽFig. 4c.. The G-band shift to higher frequency indicates that the DLC on the silicone is more graphitic in nature Žincreased sp 2-bonded aromatic sites w16x., although the reduced I D rIG ratio would suggest otherwise w19,20x. At this point it is unclear whether the differences in the Raman spectra between the films grown side by side on the different substrates are due to the mechanical behaviour of the substrates Že.g. stress relief on the softer silicone may be more efficient and lead to less disorder and narrower Raman bands., or to a different DLC structure, as suggested by Zhou et al. w18x, who observed that a-C:H grown on polymethyl methacrylate ŽPMMA. exhibited less DLC character and more polymeric character than similar films grown on silicon. What is clear is that as the refractive index of the films Žand the self-bias voltage on the driven electrode. falls, the films lose their DLC character and become more polymeric. This conversion to polymeric material occurs at lower refractive index for the DLC on glass. The fact that the fluorescence background is independent of the substrate material ŽFig. 4d. indicates that the unbound hydrogen content of the films depends only on the growth conditions.

K. Donnelly et al. r Thin Solid Films 394 (2001) 102᎐108

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Fig. 5. Scanning electron micrographs of DLC films on silicone substrates. a: n s 1.74, R a s 0.21" 0.05 ␮m; b: n s 1.83, R a s 0.25" 0.11 ␮m; c: n s 1.96, R a s 0.26" 0.07 ␮m; and d: n s 2.07, R a s 0.23" 0.02 ␮m. The scale bars represent 50 ␮m.

3.3. Surface roughness Having observed that the structure of the DLC varies with refractive index, it is important to investigate whether the change in adhesion observed in Fig. 2 could be due to film morphology. Fig. 5 presents some micrographs of coated silicone samples, with roughness data for these samples reported in the figure legend and in Table 1. As all the R a values are approximately the same, there is no evidence that the structure changes observed in the Raman data manifest themselves macroscopically. 4. Conclusions

adhesion and has been previously been shown to reduce the high inherent friction of silicone. Further work is being undertaken in order to determine whether these carbon films share the biocompatibility and longterm stability of DLC, which, combined with their stronger adhesion, would make them excellent candidates for biomedical applications. Acknowledgements This work was part funded under the ‘Access to Large-Scale Facility’ Programme, part of the Training and Mobility of Researchers sponsored by the European Commission. References

This work presents data on a number of carbon films deposited onto silicone substrates. Adhesion measurements indicate separate regions, depending on the refractive index of the films, as determined by ellipsometry on a nearby glass substrate. At low refractive index, the films appear to have significantly increased adhesion over films with a higher index. Within the low and high adhesion ranges, there does not appear to be any correlation between the adhesion and the refractive index. There is no observable morphological change in the films, but low and high adhesion films are characterised by distinctive Raman spectra. The loweradhesion, higher refractive-index films show Raman spectra typical of DLC, whereas the higher-adhesion regime is characterised by featureless Raman spectra that do not indicate the presence of DLC. Therefore, it is concluded that we have observed a carbon film that forms a coating on silicone, the film exhibits improved

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