Plasma polymer films from sputtered polyimide

ARTICLE IN PRESS

Vacuum 81 (2006) 517–526 www.elsevier.com/locate/vacuum

Plasma polymer films from sputtered polyimide A. Choukourov, J. Hanusˇ , J. Kousal, A. Grinevich, Y. Pihosh, D. Slavı´ nska´, H. Biederman Faculty of Mathematics and Physics, Department of Macromolecular Physics, Charles University, V Holesovickach 2, 18000 Prague, Czech Republic Received 13 February 2006; received in revised form 3 July 2006; accepted 3 July 2006

Abstract Deposition of plasma polymer films by r.f. magnetron sputtering of polyimide in an atmosphere of argon, nitrogen and in a selfsputtering mode is studied. In situ analytical techniques are applied to monitor the composition of both plasma and growing film during the deposition. A co-evaporation regime is observed at higher applied powers. Such a regime is characterized by a significant increase in deposition rate of plasma polymer films. In addition CO-based groups appear in the plasma volume and in the resulting films. The films deposited in a pulsed mode are similar in composition to those deposited in the continuous regime at equivalent power. The plasma polymer films are found to be stable to short-term oxidation in air. r 2006 Elsevier Ltd. All rights reserved. Keywords: Thin polyimide-like films; Magnetron sputtering; Glow discharge; Polyimide

1. Introduction Various methods have been applied to deposit polyimide (PI) or PI-like thin films. These are the conventional solution formation method, vapour deposition polymerisation (VDP), ion cluster beam (ICB) deposition and r.f. plasma sputtering. In the solution formation method [1–3], two monomers, pyromellitic dianhydride (PMDA) and oxydianiline (ODA), are mixed together in polar solvent in the form of a polyamic acid solution. The solution is spincoated onto the substrate and then the resulted film is cured at elevated temperature and converted into PI. This method has a disadvantage of involving the environmentally non-friendly substances. In addition, there are difficulties in the thickness and uniformity control of the film, and the retention of the solvent in the films often occurs. VDP is alternative process for the PI film fabrication [4–10]. The precursor monomers are sublimed in the evaporation sources and mixed in the reaction chamber at the pressure of
106 mbar. Upon the condensation on the adjacent surfaces the monomers react to form the polyamic acid which, during subsequent curing, polyCorresponding author. Tel.:+420 2 21 91 2360; fax:+420 2 21 91 2350.

E-mail address: [email protected] (H. Biederman). 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.07.010

merises into PI. Being a dry process, VDP produces the solvent-free films. Vapour deposition allows formation of films with good spacial uniformity on the substrates of the complex shape. But the stoichiometric flux ratio is difficult to control and the film composition is not that of pure PI. Several percent of isoimide [4] and unreacted monomer molecules trapped within [6,9] are reported to be present in the vapour deposited films. The deposition rate is reported to be 8–80 nm/min [8]. Ion cluster beam deposition has also been successfully applied for the fabrication of PI thin films [11]. The precursor monomers sublimed in the evaporation crucibles were ionised by the electron beam from an ionization filament. The resulted ions were accelerated by the high voltage applied between the crucibles and the substrate holder. The working pressure was 106 mbar. The subsequent curing was necessary for imidisation of the deposited films. The cured coatings contain o1% of isoimide, which is much better than in the VDP case. This technique was found to be capable of depositing crystalline PI films. No information on the deposition rate was provided. Only several papers have been published so far on r.f. plasma sputtering of PI in the simple diode or magnetron modes [12–14]. The sputtering was performed in an Ar and N2 atmosphere, or in self-sputtering mode. The PI target

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was placed on the working electrode/magnetron and r.f. power was applied between the electrodes. The positive ions move to the target, are accelerated in the sheath and produce the sputtering. The intensive ion bombardment leads to the cleavage of the polymer chains in the target. The sputtered fragments deposit on the surfaces and form the film, which has little in common with PI. The loss of benzene, imide and ether structures is reported. Nevertheless, the wear characteristic and the friction coefficient of these films were found to be very good. Reportedly, they are even better than those of conventional PI [14]. The deposition rate was 3–4 nm/min [12]. Recently, a combined version of r.f. magnetron sputtering and VDP has been applied [15,16]. The authors refer to this method as a glow-discharge-induced vapour deposition polymerisation (GVDP). As in the VDP, the vapours of two monomers, PMDA and ODA, are deposited on a solid substrate. However, the evaporation of the monomers is achieved by r.f. magnetron sputtering. The monomer powders were placed on the top of the magnetron. The sputtering was performed in He (10–20 Pa) at low power (10–30 W) and bias (50 to 150 V) to minimise the fragmentation of the monomer molecules. These parameters enabled the preferential sublimation of volatile species from the target surface. Subsequent curing was applied. The properties of the resulting films are reported to be comparable to those obtained by other deposition methods. The deposition rate was found to be 60–200 nm/min. Our recent research found that thermal degradation of PI occured when heating conventional PI in vacuum above 450 1C [17]. The emitted lower molecular weight species formed PI-like films on adjacent surfaces. The simultaneous activation of the growing film with a glow discharge enhanced cross-linking, although at expense of losing its specific PI character. The objective of this work is to study the process of r.f. magnetron sputtering of PI. 2. Experimental PI with a structural formula shown in Fig. 1 (Goodfellow Cirlex CL-HN) was sputtered by r.f. magnetron discharge in continuous wave (CW) and pulsed modes. The

target surface was cleaned with abrasive paper and alcohol before each experiment. The parameters of the deposition are: power 15–250 W; working gas flow rate 8 sccm; working pressure 2 Pa; distance target-substrate 40 mm. Argon and nitrogen were used as working gases. The films were deposited on glass, silicon and aluminum foil substrates. The experimental setup (Fig. 1) was equipped for simultaneous in situ minitoring with Fourier transform infra red (FTIR), quadrupole mass spectrometry (QMS), optical emission spectroscopy (OES) and quartz crystal microbalance (QCM) techniques. A FTIR spectrometer (Bruker Equinox 55) was operated in reflection–absorption mode. One hundred scans per spectrum with a resolution of 2 cm1 were collected. Glass precoated with silver was used as a reflective surface. Mass spectroscopy of neutral species (Hiden HAL 301 RC) and OES (Acton Research SpectraPro 300i) was used for analysing the plasma composition. For the in situ measurements the chamber was evacuated to a pressure o103 Pa, the working gas was introduced and the pressure/flow rate were adjusted. The acquisition of the FTIR and MS spectra was started. The reference spectra were collected before the discharge was initiated. Thus, series of spectra from the initial stage of the film formation up to the end of the deposition were acquired. The deposition rate was monitored by means of a 5 MHz quartz-crystal microbalance (QCM, Maxtek, Inc) sensor. Ex situ analysis included X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and contact angle measurements. The XPS of the 2.5-month aged films were acquired using a high-resolution monochromatic Al-Ka X-ray source (VSW MX10, 15 keV, 25 mA emission current) and a 150 mm concentric hemispherical electron energy analyser (VSW Class 150) operated in constant energy analyser mode. The photoemission angle was normal to the surface and the pass energy was 22 eV. The spectra were charge corrected by referencing the first component in the C1s peak to 285 eV (C–C, C–H bonds). The chemical derivatisation reactions were performed as described elsewhere [18]. Briefly, the chamber with plasma polymer samples was evacuated and filled with gaseous trifluoroacetic anhydride (TFAA). TFAA is a well-known tag molecule for the hydroxyl detection. After the

Fig. 1. Experimental arrangement for r.f. magnetron sputtering of polyimide.

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derivatisation the samples were analysed by XPS and the surface concentration of hydroxyls was calculated. 3. Results and discussion 3.1. Deposition rate PI was sputtered at different powers in (CW) and pulsed mode, other parameters being held constant. The time dependence of film thickness is shown in Fig. 2a. It has also been recalculated to deposition rate and shown in Fig. 2b. For lower powers, after the initial increase the deposition rate decreases and reaches an almost constant value. For the 75 W power, the initial trend is the same. However, the deposition rate does not level off. After approximately 8 min of discharge operation, a considerable increase of the deposition rate is observed. This can be attributed to the intensive heating of the PI surface leading to thermal cracking and evaporation of fragments of polymeric

400 N2 Ar

Thickness, nm

Thickness, nm

chains. The effect of thermally induced decomposition of the same PI with emission of lower molecular weight fragments has been observed recently [17]. The operation in such co-evaporation regime leads to rapid degradation of the target surface. It becomes black and rigid. The sputtering of PI in nitrogen proceeds with higher deposition rate (Fig. 3). The thickness of the film sputtered in N2 for 30 min is approximately five times higher than that of sputtered in Ar at the identical conditions. Such effect was noticed before for the sputtering of PTFE [21]. The sputtering was also performed in a self-sputtering mode. In this case, the discharge is initiated in an atmosphere of argon with the subsequent closure of the gas supply. The plasma is sustained by the volatile species coming from the target. The self-sputtering proceeds with five times lower deposition rate as seen in Fig. 4.

300

75 W 40 W 20 W 15 W pulsed ton = 0.2 ms, D =0.2

150

519

100

200

100 50 0 0

10

20

30

Time, min 0 0

10

(a)

30

Fig. 3. Deposition rate of PI sputtered in argon and nitrogen under identical conditions (15 W, 2 Pa, 8 sccm).

Time, min 30

400

75 W 40 W 20 W 15 W pulsed ton = 0.2 ms, D = 0.2

25 20

300 Thickness, nm

Deposition rate, nm/min

20

15 10

sputtering in Ar, 2 Pa, 100 W, -520 V self-sputtering, 2 Pa, 100 W, -500 V

200

100 5 0

0 0

(b)

10

20

30

Time, min

Fig. 2. (a) Film thickness and (b) deposition rate of PI sputtered in argon (2 Pa, 8 sccm).

0

10

20

30

Time, min Fig. 4. Deposition rate of PI sputtered in argon and in self-sputtering mode under identical conditions (100 W, 2 Pa).

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520

3.2. Plasma diagnostics 3.2.1. Optical emission spectroscopy (OES) OES spectra of the PI sputtering process were acquired in the 200–1000 nm wavelength range. Sputtering in argon was monitored at the different powers. Argon lines

1200

dominate the region of spectra with wavelengths higher than 400 nm and no significant differences are observed. Hence, the most attention is paid to the 200–400 nm region. The spectrum in this region consists of several bands, which can originate from different species (Fig. 5a). The bands in the 200–280 nm range cannot be assigned to the

306.1 nm and 308.8 nm OH A2Σ+ - X2Π Ar

Intensity, a.u.

CO 3d positive 800 CH

400

(a)

CO 3d positive CO CO Cameron bands 5B

0 200 220 240 260 280 300 320 340 360 380 400 420 440 Wavelength, nm

Intensity, a.u.

1200

CO b3Σ - a3Π 3d positive CO b3Σ - a3Π and CH 3143A system

800

CO Triplet bands

400

(b)

388.3 nm CN violet system

CO CO Cameron bands 5B

0 200 220 240 260 280 300 320 340 360 380 400 420 440 Wavelength, nm 1200

Intensity, a.u.

N2, second positive system

800

400

(c)

388.3 nm CN B2Σ-X2Ε

contribution of OH

0 200 220 240 260 280 300 320 340 360 380 400 420 440 Wavelength, nm

Fig. 5. OES spectra of PI sputtering in: (a) argon (100 W, 2 Pa, 8 sccm); (b) self-sputtering mode (100 W, 2 Pa); (c) nitrogen (15 W, 2 Pa, 8 sccm).

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specific fragments but come from various hydrocarbons. The band with the highest peak at 283.3 nm consists of emission from CO and OH groups. The very intensive band at 306.1 nm degrading to red is attributed mainly to OH emission with a slight contribution from CO. CN groups are responsible for the band with the most intensive peak at 388.3 nm degrading to violet. The detailed assignments of emission bands are given in Ref. [17]. All the assignments have been performed according to Pearce et al. [19]. Fig. 5 depicts the OES spectra of PI sputtering at 100 W in argon (Fig. 5a), self-sputtering mode (Fig. 5b), and in nitrogen (Fig. 5c). As expected, self-sputtering is characterised by the absence of emission from argon. Furthermore, the CN band is broadened and not resolved. The OH emission at 306.1 and 308.8 nm disappears (compare with the strong intensity of these peaks for sputtering in Ar, Fig. 5a). On the contrary, there is a significant increase in CO species as can be seen from an enhancement of their Cameron bands (200–250 nm) and the third positive system (280–320 nm). The peak at 315 nm of the latter is most

15 W 100 W

Intensity, a.u.

300

521

probably contributed by the CH 3143 A˚ system, which is also present for the sputtering in Ar. Furthermore, the CO triplet bands dominate the self-sputtering spectrum in the 340–440 nm range. The spectrum of sputtering in nitrogen is remarkable for the very intensive emission of the neutral N2 second positive system with less intensive contributions from OH and CN groups. However, the hydrocarbon bands observed in the spectra for argon and self-sputtering are absent here. The sputtering performed at different powers in argon produces similar spectra, which differ only in intensity of the CN and the argon bands. The emission of the CN groups is much higher with respect to argon in the case of 100 W sputtering, whereas these groups are minor at 15 W deposition (Fig. 6). The time dependence of various peaks for sputtering in argon is given in Fig. 7. The emission from the Ar atoms and the CN species increases steadily during the first minute of discharge operation and becomes stable later. On

CN violet system B2Σ-X2Ε 388.3 nm

200 Ar 393.5 nm 100

0

360

370

380 Wavelength, nm

390

400

Fig. 6. OES of polyimide sputtered in Ar at different powers (2 Pa, 8 sccm).

1500

OH, A2Σ+−X2Π and CO b3Σ−a3Π, 283.3 nm

OH, A2Σ+−X2Π, 306.1 nm

Intensity, a.u.

OH, A2Σ+−X2Π, 308.8 nm CN, B2Σ−X2Σ, 388.3 nm Ar I, 394.9 nm

1000

500

0 0

2

4 Time, min

6

8

Fig. 7. Time dependence of emission of various bands during PI sputtering in argon (100 W, 2 Pa, 8 sccm).

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522

Intensity, a.u.

1200

CO, OH 283.3 nm OH 308.8 nm OH 312 nm CN 388.3 nm Ar 420.1 nm

800

400

0

1

2

3

4

5 6 Time, min

7

8

9

10

Fig. 8. Time dependence of emission of various bands during PI sputtering in self-sputtering (100 W, 2 Pa).

no discharge

1E-8

I [a.u.]

the contrary, there is an abrupt increase in emission from the hydroxyls during the initial period of sputtering. The OH groups dominate spectra after 1 min of the sputtering. However, their concentration decreases further with time. The same trend is observed for the self-sputtering mode (Fig. 8). Hydroxyls reach maximum after the first minute of sputtering with a subsequent decrease in their emission. However, in this case the most intensive emission comes from CO species as was shown in Fig. 5b.

1E-9

1E-10

1E-11

3.3. Mass-spectroscopy

0

3.4. Film characterisation

30

40

50

60

70

80

I [a.u.]

90

100

40 W

1E-9

1E-10

1E-11 0

10

20

30

40

50

60

70

80

90

100

100 W

1E-8

1E-9

1E-10

1E-11 0

The above considerations are supported also by the analysis of the resultant films. The in situ FTIR spectra of the films sputtered in Ar are given in Fig. 10. With increasing power the intensity of NH/OH band increases

20

1E-8

I [a.u.]

With increasing discharge power the composition of the plasma and of the resulting film changes. There is a clear ‘‘transition’’ between the mass-spectra obtained at 40 and 75 W (Fig. 9). The peaks of the CxHx fragments sputtered from the target strongly increase with power. The peaks with massto-charge ratio m/q ¼ 28 and 44 are the most prominent at high power (apart from Ar and residual water). The former can be assigned to CO+, C2H+ and N+, and the latter + corresponds to CO+ 2 and C3H8 . Considering the development of other hydrocarbon peaks (Fig. 9) it can be assumed that there is a significant contribution of either CO or N2. When the optical spectra (Fig. 5) are considered showing the drastic increase in the CO emission, we can conclude that the main contribution to the peak m/q ¼ 28 increase is from carbon monoxide. This species may originate both from the target and from the reaction of hydrocarbon fragments with residual water and oxygen in the reactor.

10

10

20

30

40

50

60

70

80

90

100

m/q [amu] Fig. 9. Neutral species mass spectra of the r.f. magnetron discharge with a polyimide target in argon for various powers. Note the changes in m=q ¼ 12 (C+), m=q ¼ 28 (CO+, C2H+, N+), m=q ¼ 44 (C3H+, CO+).

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incr.power

Transmittance [a.u.]

C-H N-H, O-H

C

C

C

N

C-H 40 W 20 W 150 W

75 W

more C=O 4000

C=O C=N C=C

523

3500

3000

2500

less C=O

2000

1500

1000

Wavenumber [cm-1] Fig. 10. The transmission-reflection FTIR spectra of films deposited by magnetron sputtering of polyimide. The arrow indicates changes in spectra with increasing power.

and triple CC/CN bonds decreases with respect to the hydrocarbon peak. Especially interesting is the region 1750–1600 cm1, where the peaks of CQO, CQN, CQC and deformation vibrations of NH2 and OQC–NH2 are overlapping. Although it is difficult to distinguish these peaks directly, the CQO peak is usually found at slightly higher wavenumbers than the CQN and CQC peaks. Therefore, the apparent shift of the band maximum in dependence on power in this spectral region can be assigned to the increase of CQO concentration that is in agreement with the plasma composition obtained by OES and MS. The XPS results for the films deposited in Ar in CW mode at the different r.f. powers are shown in Fig. 11. The results of the fitting of the C1s spectra are given in Table 1a. The C1 s spectra are fitted with three components. The first is referenced to 285.0 eV and attributed to C–C and C–H bonds. The second peak at
286 eV is broad and may be contributed by various species. The most probable are C–O (286.4 eV) and C–N (285.9 eV) functionalities [20]. The presence of triple CN bonds (binding energy for CRN is 286.7 eV, [20]) was confirmed by FTIR. However, taking into account the low nitrogen concentration, their contribution is minimal. The third peak is shifted by about 3 eV with respect to the reference point and can be assigned mainly to CQO (287.9 eV), O–C–O (287.9 eV) and N–CQO (288.1 eV) bonding environments [20]. The ratio of the third component in the total C1s peak is almost constant as the power increases. Nevertheless, the position of its centroid shifts in the higher binding energy direction, which indicates that the amount of higher oxidized carbon atoms increases. The position of the second peak is stable regardless the applied power. It is located at 286.0 eV for 20 PW sputtering and 286.2 eV for 100 and 150 W sputtering. The amount of CN and C–O functionalities increases,

whereas CC and CH decrease, when the power rises from 20 to 150 W. It corresponds to the changes in the element composition (see Fig. 11 and Table 2a). The low-power film contains 71% of C, 26.5% of O and only 2.5% of N, which results in high ratio C/N ¼ 28.4. In the 150 W film, the carbon concentration decreases to 59.0%, while nitrogen and oxygen content grows up to 6.0% and 35.0%, respectively. The C/N ratio decreases to
10. Thus, the films deposited at higher powers are richer with nitrogen and oxygen. The original PI material has the composition of 75.8% C, 6.9% N and 17.3% O (without hydrogen being taken into account), and C/N ¼ 11. The nitrogen content and C/N in the 100 and 150 W films approach the original PI values. However, the concentration of oxygen in the films is exceedingly high. For the 150 W film it is twice the value of the original material used as the target. 3.5. Pulsed mode The pulsed mode is known to be effective in plasma polymerisation of unsaturated organic vapours in terms of retention the monomer functional groups [21]. Since PI is composed of aromatic and imide structures it was assumed that pulsing could affect the film composition. Several depositions were performed in Ar in a pulsed mode with average powers 15 and 25 W (duty cycle 0.1, ton ¼ 1 ms). The C1s spectrum of the 15 W film is given in Fig. 12 and it is similar to that of the 20 W film prepared in CW mode. The element composition of the pulsed sputtered films is also very close to that for the CW sample (Table 2b). These films are rich in carbon and have low (1–3%) content of nitrogen. As in the case of the CW films, the higher power results in the higher nitrogen concentration. The oxygen concentration is also very close to the low-power CW film and does not exceed by much the value of original PI. The first two components of the pulsed C1s spectra are

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524

287.9

286 285

CC , CH C-O, C- N, C=N C=O, N- C=O 20 W Element composition: C,% N,% O,% 71.0 2.5 26.5

296

294

292

290

288 288.3

286

284

282

280

286.2 285

100 W Element composition: C,% N,% O,% 62.8 5.4 31.8

296

294

292

290

288 288.4

286

284

282

280

286 285

150 W Element composition: C,% N,% O,% 59.0 6.0 35.0

296

294

292

290 288 286 Binding energy, eV

284

282

280

Fig. 11. High resolution C1s spectra of polyimide films sputtered in CW at different powers.

positioned similarly to the CW samples. The position of the third peak in the C1s spectrum of the pulsed sputtered film is the same as for the 20 W CW film and lower than for the 100 and 150 W CW films (Table 3). The film prepared at 15 W average power (150 W peak power)

has lower content of CO and CN species than for the 150 W CW film (peaks C1s2 and C1s3, Table 1a, b). In general, pulsed sputtering of PI produces films with composition very similar to that of films deposited in CW mode at equivalent power.

ARTICLE IN PRESS A. Choukourov et al. / Vacuum 81 (2006) 517–526 Table 1 The results of the fitting of the C1s spectra of polyimide sputtered thin films

Table 3 The XPS peak positions for the r.f. magnetron sputtered polyimide films Peak

Bonding environment

20 W

100 W

150 W

56.6 26.5 16.9

45.9 37.3 16.8

42.9 39.5 17.6

(a) CW C1s1: C–C, C–H C1s2: C–O, C–N C1s3: CQO, N–CQO N1s O1s

(b) Pulsed mode (duty cycle 0.1; ton ¼ 1 ms) Bonding 15 W 25 W environment (average); (average); 150 W (peak) 250 W peak C–C, C–H C-O, C–N CQO, N–CQO

59.4 27.1 13.5

63.5 21.3 15.2

Table 2 Element composition of sputtered polyimide films Power, W

Element composition (at%) N

O

C/N

71.0 62.8 59.0

2.5 5.4 6.0

26.5 31.8 35.0

28.4 11.6 9.8

(b) Pulsed mode (duty cycle 0.1; ton ¼ 1 ms) Average power, W C N

O

C/N

15 W 25 W

24.0 25.6

54.5 25.6

(a) CW 20 W 100 W 150 W

74.7 71.6

1.3 2.8

287.9

285.9 285

(a)

15 W(aver.) CC, CH C-O, C-N, C=N C=O, N-C=O

292

290

288

286

20 W

100 W

150 W

285.0 286.0 287.9 399.1 530.5

285.0 286.2 288.3 398.9 529.6

285.0 286.0 288.4 398.9 529.5

(b) Pulsed mode (duty cycle 0.1; ton ¼ 1 ms) Bonding environment 15 W (average); 150 W (peak)

25 W (average); 250 W peak

C1s: C–C, C–H C1s2: C–O, C–N C1s3: CQO, N–CQO N1s O1s

285.0 286.1 288.0 398.6 529.9

285.0 285.9 287.9 398.8 530.4

3.6. Short-term oxidation effects

C

294

Binding energy (eV)

% in total C1s peak

(a) CW C–C, C–H C–O, C–N CQO, N–CQO

296

525

284

282

280

Fig. 12. C1s spectra of polyimide films sputtered in pulsed mode (average power 15 W, duty cycle 0.1, ton ¼ 1 ms).

The oxidised species detected in the films may originate both from the PI target material which is rich with oxygen, and as a result of post-deposition oxidation reactions. The mass-spectral data of PI r.f. magnetron sputtering show that one of the most abundant species during the process is CO. The effect of the short-term oxidation was studied by FTIR spectroscopy. In this case, the spectra were collected immediately after deposition without exposure to air. Then the experimental chamber was vented with ambient air and the spectra were re-collected within 10 min. Finally, the system was evacuated again and the third series of spectra were acquired (Fig. 13). It has been shown previously that, when exposed to air immediately after deposition, hydrocarbon plasma polymer intensively oxidises to form hydroxyls and carbonylbased species [22]. Half an hour incubation under vacuum was necessary to reduce the oxidation process. Here, after venting the chamber, a broad band in 3000–3500 cm1 region immediately increases as a result of water vapour absorption. However, when the chamber is reevacuated the spectrum almost returns to its pre-exposure state as the absorbed water escapes. This indicates that hydroxyls are unlikely to form during the short-term contact with air. These findings are further supported by the chemical derivatisation results. The derivatization reactions with TFAA were performed 1 day after the deposition. The samples treated were those deposited at 15, 75 W in CW mode and at 15 W (average power), duty cycle 0.2 in the pulsed mode. Regardless of the deposition conditions all the samples showed very small (o1 OH group per 100 carbon atoms) surface hydroxyl concentration.

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526

a Transmittance [a.u.]

c

4000

b

3800

3600

3400

3200

Wavenumber

3000

2800

[cm-1]

Fig. 13. The FTIR of the PI sputtered film (50 W, Ar, 2800-3600 cm1- OH/NH group, 2800–3000 cm1- CH2/CH3 group): (a) immediately after the deposition and before exposure to air; (b) exposed to air, the film spectrum is overlapped by the water vapour spectrum; (c) under vacuum after exposure to air.

4. Conclusions Polyimide (PI) magnetron sputtering is characterised by the emission of low molecular weight fragments from the target which deposit on adjacent surfaces to form a plasma polymer. The initial stages of sputtering cause an abrupt release of hydroxyls, which originate from water absorbed by PI material. When the power applied exceeds a certain limit PI co-evaporation occurs. This results in a significant increase of the deposition rate. A substantial difference between the conventional sputtering of the target and sputtering with co-evaporation is confirmed by all of the diagnostic methods. Carbon monoxide is one of the most important species detected in the discharge zone. Plasma composition obtained by means of mass spectroscopy and optical emission spectroscopy is in a good agreement with the film composition obtained by infrared spectroscopy. A lack of hydroxyls was detected in the films and oxygen is bound mainly to carbon. An increase of the CQO component was observed under co-evaporation conditions. Acknowledgement This work is a part of the research plan MSM 0021620834 that is financed by the Ministry of Education of the Czech Republic and partly was supported by the Grant OC 527.10 and ME 554, both from the Czech Ministry of Education, Youth and Sports of the Czech Republic. References [1] Liou HC, Ho PS, Tung B. J Appl Polym Sci 1998;70:261–72. [2] Liou HC, Ho PS, Tung B. J Appl Polym Sci 1998;70:273–85.

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