Characterization of ion-beam-deposited diamond-like carbon films

584

Diamond and Related Materials, 2 (1993) 584-589

Characterization of ion-beam-deposited diamond-like carbon films V. L i e b l e r , H . B a u m a n n

and K. Bethge

lnstitut fiir Kernphysik, August Euler Strasse 6, W-6000 Frankfurt-am-Main (Germany)

Abstract Diamond-like carbon films were produced on silicon substrates by the ion beam deposition technique. A Penning ion source was operated with different hydrocarbon gases (CH4, C2H6, C3H6 and C6H6). The extracted ions with energies of about l0 keV were decelerated in a retardation system to obtain energies in the range between 0.2 and 2.0 keV. Deposition rates of the order of 5-50 nm min- 1 were achieved with this arrangement depending on the kind of hydrocarbon gas and the ion energy. The films were analysed with accelerated ion beams. Oxygen and nitrogen impurities were detected with the Rutherford backscattering spectroscopy in an amount of some atomic per cent. The hydrogen content was determined with nuclear reaction analysis using the nuclear reaction 1H(15N,~i/4~32keV)12C.Owing to a resonance energy of 6.385 MeV and a ~SN projectile energy of 7 MeV the detection of hydrogen takes place at a depth of 0.35 lam. Measured values of the hydrogen concentration vary from 21 to 36 at%. The effusion of hydrogen during the ion bombardment was observed with multiscaling measurements simultaneously.

1. Introduction In recent years, several techniques have been developed for depositing diamond-like carbon coatings, e.g. plasma beam deposition, magnetron sputtering, r.f. glow discharges, chemical vapour deposition and several ion beam methods [1-8]. Because hydrogen concentrations are in the range between 10 and 60 at.% these coatings are called hydrogenated a m o r p h o u s carbon ( a - C : H ) films. It is a c o m m o n idea that hydrogen plays an important role in the nucleation process and the properties of these films. A parameter influencing the hydrogen content itself is the energy of the deposited particles [9]. Nevertheless, using r.f. plasma deposition it is well known that the structural and chemical properties of the precursor molecules are lost during the deposition [10]. So it remains to be investigated how different hydrocarbon molecule ions (carbon hybridization and carbon-to-hydrogen ratio) of a certain deposition energy influence the properties of a - C : H films. In this contribution, results are presented on a - C : H films deposited by the ion beam deposition technique. Non-mass-selected ions from a Penning ion source operated with different hydrocarbon gases were deposited on silicon substrates. As the ion source emits almost no multiply charged molecule ions [11], a-C : H films could be produced at a defined and fixed energy in the range between 0.2 and 2.0 keV. Accelerated ion beams were used to analyse the films. The deposition rates and impurities were determined by Rutherford backscattering spectroscopy (RBS), whereas the hydrogen content was measured by the nuclear reaction analysis (NRA).

0925-9635/93/$6.00

Furthermore, we made studies on the density of the deposited a-C : H films

2. Experimental set-up 2.1. Penning source

The Penning ion source was operated in a high voltage, low current discharge mode (Vaisc~arge=2.0 kV; Idischar,e= 1.5 mA). The main fraction of ions (greater than 95%) is formed inside the plasma column at nearly anode potential [12]. The cathode of the Penning ion source acts like an internal extraction electrode. Therefore the energy of the extracted ions is well defined and is given by the discharge voltage. The energy spread of the ion beam is about 30 eV. Operating the ion source with hydrocarbon gases causes the synthesis of various species of hydrocarbon molecule ions in the plasma. Measurements of the ion distributions were performed by determining the beam currents of the different extracted ion species in a Faraday cup after a 90 ° bending magnet. Figure 1 shows the ion distribution when the ion source is operated with methane. Obviously, the fraction of molecule ions consisting of more than seven carbon atoms (and several hydrogen atoms) as well as doubly charged ions, e.g. C 2 ÷, is negligible. Measurements of ion distributions were also performed for the hydrocarbon gases ethane, propene and benzene. Table 1 presents the averaged values of the hydrogento-carbon ratios, the ion masses and the number of carbon atoms per ion in the extracted ion beam. Note the ion distributions of the hydrocarbon gases in Fig. 2.

© 1993 - - Elsevier Sequoia. All rights reserved

V. Liebler et al. / Ion-beam-deposited diamond-like carbon films 1ion " source 'I extraction I

I0 °

I

10 -1

l

f

~

Einzel lens

q

585 I

drift

i

.

t

I

irelardahomsubstrete I system Ir

I

i

I

/

- - A Vefl

O . ,,..d .~ i0 -z

10 -a 10 -4

o 10 -5 10 -e 0

50

tO0

150

200

Mass/Charge [amu/e]

15-2.0kV Vett :_+ 7.5 kV

Fig. 1. Ion distribution of a Penning source operated with methane. Fig. 3. Schematic illustration of the IBD system. TABLE 1. Characterizaton of the hydrocarbon ion distributions Hydrocarbon gas

H-to-C ratio

Mass (amu)

Number of C atoms per ion

Methane (CH4) Ethane (C2H6) Propene (C3H6) Benzene ( C 6 H 6 )

2.84 1.81 1.35 0.86

17.5 26.7 35.1 58.0

1.18 1.93 2.63 4.51

1.0

I

I

I

I

I

I

I

A 0.8

.0 "~

0 Methane A Ethane

/

~

/'

/

/

•Propene

/

/~

0.6

~ Benzene

/

•

/

X

- ~~

XX

~7~0"4 ~

/

\d

k

2.2. Deposition system The ion beam deposition (IBD) system is schematically illustrated in Fig. 3. The Penning ion source forms ions at the anode potential, which has a defined value l / e ft VS. ground potential. To obtain intensive ion currents the beam has to be extracted with a minimum voltage of 7 kV. After passing through an Einzel lens the hydrocarbon ions are decelerated by a retardation system with cylindrical geometry. This system is positioned in front of the substrate where the film deposition takes place at ground potential. Therefore the ions are only accelerated with V~ffeffectively. Suitable ion optical conditions were chosen to obtain films of large diameter (about 15 mm). In order to prevent space-charge-compensating electrons from accelerating out of the drift section towards the substrate a negatively biased electrode ( - l 0 kV) is used. In Table 2 the experimental parameters of the deposition process are listed.

.

"

//

\,

3. Measurements 0 "-'

W

0.2

j /

~ .

\

//~

\

/C.3~ \

.v~k

\

/.

\

/

\

./

.\

3.1. Rutherford backscattering spectroscopy Applying RBS the samples are bombarded with a 4He

o 0

0

I

I

I

I

1

2

3

4

-

-

I

I

I

5

6

7

C a r b o n A t o m s per Ion Fig. 2. Ion distributions of a Penning source operated with various hydrocarbon gases. The intensities of the molecule ions having the same number of carbon atoms are summed up.

The intensities of molecule ions having the same number of carbon atoms are summed. All gases with the chemical formula C.Hm lead to the dominant molecule ion species C.H~+ except for benzene which is cracked partly.

ion beam of a certain energy at an angle of 0 ° with respect to the normal of the sample surface. With a TABLE 2. Deposition parameters Residual gas pressure Pressure during deposition Accelerating voltage V¢fr Current Current density Deposition rate Film thickness

2 x 10-5 Pa (2-5) × 10-4 Pa 0.2-2.0 kV 100-600 IxA 50-300 pA cm-2 5-50 nm min0.2-5 pm

V. Liebler et al. / Ion-beam-deposited diamond-like carbon films

586

surface barrier detector positioned in the backward direction the energy of the scattered ions is determined at an angle of 9 ° with respect to the beam axis. To obtain the sample composition the measured spectra are simulated by the computer code RUMP [13]. The deposition rates are determined by measuring the accumulated charge during the deposition process in comparison with the results of RBS lateral scans using a 3.5 MeV 4He ion beam. This energy is in the range of the resonance region of the ~2C(~, ~')~2C scattering [14]. Figure 4 shows a typical thickness profile of an a-C : H film prepared with propene (V~rf=2.0kV) and as an example an RBS spectrum with a RUMP simulation. The error bars represent an inhomogeneity in the film thickness within the analysing beam spot (0.7 mm x 0.7 mm). In addition, film densities between 1.2 and 1.9 g cm -~ (Fig. 5) were determined by comparison with mechanical step measurements because RBS is sensitive to the particle area density and the step measurement on the geometrical thickness. If we take into account the different hydrogen contents of the a-C : H films and the self-sputtering yield of carbon on graphite [15], the deposition rates increase linearly with the number of carbon atoms per hydrocarbon molecule as seen in Fig. 6. The data point for benzene does not fit this correlation, because benzene molecules are cracked in the plasma, leading to a relatively small amount of deposited C 6 H ~- molecules (see Fig. 2). For the detection of impurities in the a-C:H films by RBS a 2.0 MeV '*He ion beam was used. The impurities detected are oxygen and nitrogen in amounts of approximately 2 at.% and 1 at.% respectively.

i,,,i,,,,

1.0

~D

RBS 3 5

----Step

Measurement

20

2.0

-~ 15

1.5

i0

.~O

.

~[~ 0 . 5

I"

MeV 4He

~C

'

~:~,..

,

.

.

.

.

4~

[

c°..

0.5 /,,Jtl~,,,I,|/,,,,]lll 0.0 1.0 2.0

I /

0.0

Ion E n e r g y

1.0

2.0

[keV]

Fig. 5. Density of a-C : H films prepared by IBD.

2.0

i

,

I

I

I

o

rD

//

1.5

C3H6

C6H6 /

/

1.0

C2H6./~/ CH4 / /ff

0.5

/

/ / / J

0 1 Carbon

,

. . . .

1.0

0.0

~.

. . . .

1.5

O

I

!

2 3 Atoms

J

4 per

I

I

6 7 Molecule

5

Fig. 6. IBD deposition rates for various hydrocarbon gases. The data are normalized: p = 1.8 g cm-3; 30 at.% H; self-sputtering yield, 0.22 (1 keV C on graphite).

Si

5 - RUMP s i m u l a t i o n ', , /

i

0

200

400

I

0.0 0

. . . .

2.0

.r--t

O RBS

2.5

C2H8

CH 4 0.5

O • p,,,I 25

,,,i1,,,,

1.5

3.2. Nuclear reaction analysis The hydrogen content was determined by NRA using the resonant reaction 1H(~SN, 0t7)12C [16]. This com3.0

I

2.0

5

10

15

Position

20

25

30

[mm]

Fig. 4. Thickness profile of an a-C : H film (propene; deposition energy, 2.0 keV). The inset presents a single RBS measurement of this film and the RUMP simulation (--).

pound reaction produces a 160 nucleus which decays to 12C under emission of an 0c particle. The 4.433 MeV 7 radiation of the remaining 12C nucleus was detected with NaI(T1) scintillators. Owing to a resonance energy of 6.385 MeV and a bombarding energy of 7.0 MeV the detection of hydrogen in an a-C:H film (hydrogen-tocarbon ratio, 0.4; p = 1.8 g cm - 3) takes place at a depth of 0.35 lam [17].

V. Liebler et al. / Ion-beam-deposited diamond-like carbon films

3.2.1. Hydrogen content The dependence of the hydrogen content on the ion energy (effective accelerating potential V~ef) as well as on the hydrocarbon gas is shown in Fig. 7. It is obvious that up to an C,Hm ion energy of approximately 39 eV amu -1 (v=8.7 x 104m s -1) the hydrogen content increases. For the different gases this value is indicated by an arrow. In the case of benzene the energy per mass unit corresponds to a C 6 H 6 ion energy of 3.0 keV. A fully constrained covalent network model has been proposed by Angus and Jansen [18] to describe the structure of a-C:H films. From this model they derived an expression for the sp3-to-sp 2 carbon site ratio as a function of the hydrogen atomic fraction for a fully constrained non-crystalline network. In ref. 19 the sp 3to-sp z ratios from experimental data are described in general, but not exact, agreement with the prediction of this model. So the averaged sp3-to-sp 2 ratios for the different hydrocarbon gases (Fig. 7) can be calculated only approximately. The results are listed in Table 3. Although these data might not be absolutely reliable, a strong dependence of the spa-to-sp 2 ratio on the hydrocarbon gas used for the IBD can be observed. As the amount of sp 2 carbon sites of the deposited molecule ions increases, there is also an increase in the sp3-to-sp 2 ratio in the a-C:H film, meaning that the structural property of the precursor molecules can somehow be found in the film. A convenient way for categorizing different classes of hydrocarbons such as a-C : H is to plot the atom number density vs. the atomic fraction of hydrogen [20]. This I

40-

I

I

I '

35

587

TABLE 3. Averaged sp3-to-sp 2 ratios calculated by the fully constrained network model in comparison with the carbon hybridization of the hydrocarbon gases and fluence threshold of hydrogen effusion Hydrocarbon gas

Methane (CH4) Ethane (C2H6) Propene (C3H6) Benzene (C6H6)

sp3-to-sp 2 ratio

Threshold (lSN cm -2)

Molecule

a-C : H film

Minimum

Maximum

oc oo ~ 0

1.5 + 0.7 1.5 ___0.5 0.7 _ 0.2 0.2 + 0.1

4.0 x 1.0 x 3.5 x 1.8 x

2.1 x 3.5 x 1.1 x 6.5 x

1013 1014 1014 1015

101¢ 1014 101~ 101~

kind of plot can be derived by combining the results of NRA, RBS and mechanical step measurements (Fig. 8). The data points of a-C : H films deposited with methane and ethane (sp 3) as well as those produced with benzene (sp 2) will be found to cover different areas in the diagram. The data points of films deposited with propene (sp 3 and sp 2) are positioned between those mentioned above. The explanation of this phenomenon is the same as found for the averaged sp3-to-sp 2 carbon site ratios. There is no independence of the film properties on the precursor molecules observable. The atom number density of films deposited with propene increases with increasing hydrogen content. This can be explained as an increase in the averaged coordination number of carbon from 3 to 4, i.e. a decrease in the sp2-to-sp 3 ratio. 3.2.2. Hydrogen effusion Irradiation of hydrogen-containing materials with ions in the kiloelectronvolt and megaelectronvolt energy range can lead to hydrogen release. This phenomenon was observed during the analysis of hydrogen by NRA for different materials, e.g. diamond [21], frozen methane films [22] and a-C:H films [23]. It is well known that the effusion of hydrogen is correlated to the rate of the

3O 0 •.~ 25 2O

,- ....

4O 0 r..D

E 2:

....

I

I '~

0.25

I

~F

-

I

I '

0.20

C6H8

Call8

I

1

0 Methane A Ethane • Propene Benzene OAA

•

35

Ill

0~

q~ o

I

~rCt

3O

00

0.15

~" ~

"

mO

A

~

0 0 0

n

Z 20

- ~

0.0

1.0

2.0

0.0

:

~

~"

1.0

2.0

O

0.10 0.0

I o n E n e r g y [keV] Fig. 7. Hydrogen concentration of a-C : H films prepared by IBD. A C,H,, ion energy of 39 eV amu- 1 is indicated by arrows.

0.2 Atomic

0.4 Fraction

0.6 of Hydrogen

Fig. 8. Atom number density divided by Avogadro's number N, vs. atomic fraction of hydrogen.

588

V. Liebler et al. / Ion-beam-deposited diamond-like carbon films

electronic energy deposition of the ions [24]. Furthermore, another parameter might be the properties of the irradiated films themselves. For that purpose, the effusion caused by the analysing 7 MeV 15N beam was studied by applying the multiscaling measurement method. The dependence of the hydrogen content on the irradiation fluence Dr~ of the ~SN ions was determined. The hydrogen effusion of an a-C:H film deposited with propene and a deposition ion energy of 0.6 keV is demonstrated in Fig. 9. A function p(DN) = pp + ~,2=oa i exp( - CiDN) describes the effusion curve well [24]. p, is the constant residual hydrogen content; a~ and c~ are the coefficients of the three exponential functions. The inset in Fig. 9 shows the deviation of measured and fitted data. Samples deposited with various deposition ion energies for methane as well as various gases at a fixed deposition ion energy of 0.6keV (Vcff=0.6kV) were irradiated. As plotted in Fig. 10, both variations are lead to a correlation between the coefficients c~ and the deposition ion energy. The decrease in the coefficients c~ indicates a stronger binding of hydrogen when the initial hydrogen content decreases (Fig. 7). It should be mentioned that these coefficients satisfy the equation q=cob%= 0.21 4- 0.02). The hydrogen content p~ at ~SN fluences of approximately 1017 cm -2 ranges from 5.0 to 6.5 at.%. The hydrogen release occurs only after a defined threshold ion fluence. The values are listed in Table 3. There is a strong dependence on the kind of hydrocarbon gas, but no dependence on the deposition energy within the accuracy of the measurements observable. This might be explained as arising because the threshold is dominated by the kind of hydrocarbon gas and not the deposition energy.

0.5

'

'

I

0 "0

].

' .

' .

'

I

.

'

1

1

' 1

I i

I

'

.

'

'

I

i

'

)

'

' l

o.,

l (D

0.3

".

% -o.o~

•

-

"

_ o 0 3 r l

l

0

l

~

l

l

2

[

[

4

l

l

l

l

6

,

l

l

~

8

10

2 4 15N F l u e n c e

6 8 [101ecm-2]

'

I

II

I

[

'

'

'

'

I

'

'

I

'

'

'

I

10-15

CH4

n------~n

n

1 - - - - - . - - - - - - - - - l ~

I

i0-Ie

10_17

, , I ,

, I ,

I

' ' 1 '

' I '

I ' ~ ' 1 ~ ' ' 1

CB 10-15

,

,

,

,

,

,

I

o.o/'°

Ve~~ = 600 V

zx,,A- / ' a

A - OH4

I/m--'-'--m 0

1

A

10-16

1 0 - 1 7

,

,

I

,

20

,

,

I

40

L

I

60

,

B -

CzH e

C -

C3H8

,

,

J

,

,

,

80

I

100

Ion E n e r g y [ e V / a m u ] Fig. 10. Effusion coefficents q (i=0, 1, 2) vs. deposition ion energy per mass unit.

4. Conclusion Using the IBD method with hydrocarbon ions in the energy range 0.2-2.0 keV it is possible to form a-C:H films. In general, the densities are not as high as those obtained by other deposition methods (0,19NACm -3 [20]). The deposition rates show a linear increase with increasing number of carbon atoms per hydrocarbon gas molecule. The impurities detected are oxygen and nitrogen in amounts of some atomic per cent. The hydrogen content varies between 21 and 36 at.% depending on the ion energy as well as on the kind of hydrocarbon gases. A maximum hydrogen content %ems to occur at a certain velocity of the deposited particles. Studying the hydrogen release in a-C:H films by ion beam irradiation we observed only a strong correlation between the threshold fluence and the kind of hydrocarbon gas. In the ion energy range under consideration the structural properties of the precursor molecules are not lost during the IBD process, contrary to the results obtained by r.f. plasma deposition.

Acknowledgment

~

0

'

. . . .

0.2

0.1

10-14

10

Fig. 9. Hydrogen effusion of an a-C : H film (propene; 0.6 keV) caused by irradiation with 7 MeV tSN:--, fitted function p(DN); ---, singleexponential functions.

The cooperation of the accelerator staff of the Institut f/Jr Kernphysik of the University of Frankfurt is gratefully acknowledged. This work was supported by the DFG, Bonn.

References 1 F. M. El-Hossary, D. J. Fabian and A. P. Webb, Thin Solid Films, 192 (1990) 201.

V. Liebler et al. / Ion-beam-deposited diamond-like carbon films

2 S. Aisenberg and R. Chabot, J. Appl. Phys., 42 (1971) 2953. 3 A. C. Greenwald, J. K. Hirvonen and N. K. Jaggi, in L. E. Rehn, J. Greene and F. A. Smidt (eds.) Processing and Characterization of Materials Using Ion Beams, Materials Research Society Syrup. Proc., Vol. 128, Materials Research Society, Pittsburgh, PA, 1989, p. 109. 4 R. L. Wu, Synthesis and characterization of diamond-like carbon films using ion beam technique, in D. B. Poker and C. Ortiz (eds.), Optical Materials: Processing and Science, Materials Research Society Symp. Proc., Vol. 152, Materials Research Society, Pittsburgh, PA, 1989 p. 33. 5 H. Vora and T. J. Moravec, J. Appl. Phys., 52 (10) (1981) 6151. 6 J. A. Thornton, J. Vac. Sci. Technol., 12 (1975) 830. 7 A. Imamura and T. Tsukamoto, Surf Coat. Technol., 36 (1988) 16l. 8 S. Schiller, U. Heisig and K. Goedicke, Thin Solid Films, 40 (1977) 327. 9 J. Kessler, B. Tomcik, J. Waldorf and H. Oechsner, Vacuum, 42 (1991) 273. 10 P. Koidl, C. Wild, R. Locher and R. E. Sah, Amorphous, hydrogenated carbon films and related materials: plasma deposition and film properties, in R. E. Clausing, (ed.) Diamond and Diamond-like Films and Coatings, Plenum, New York, 1991, p. 243.

589

ll H. Baumann and K. Bethge, Nucl. Instrum. Methods, 189 (1981) 107. 12 P. Rohwer, H. Baumann, K. Bethge and W. Schfitze, Nucl. Instrum. Methods, 204 (1982) 245. 13 L. R. Doolittle, Nucl. Instrum. Methods B, 15 (1986) 227. 14 F. Ajzenberg-Selove, Nucl. Phys. A, 375 (1982) 1. 15 T. Miyazawa, S. Misawa, S. Yoshida and S. Gonda, J. Appl. Phys., 55 (1984) 188. 16 C. Rolfs and W. S. Rodney, Nucl. Phys. A, 235 (1974) 450. 17 J. F. Ziegler, J. P. Biersack and U. Littmark, IBM Res. Rep. RC9250, 1982. 18 J. C. Angus and F. Jansen, J. Vac. Sci. Technol. A, 6 (1988) 1778. 19 Y. Wang, H. Chen and R. W. Hoffman, J. Mater. Res., 5 (ll) (1990) 2378. 20 J. C. Angus, Diamond Relat. Mater., 1 (1991) 61. 21 J. P. F. Sellshop, C. C. P. Mabida and H. J. Annegarn, Nucl. lnstrum. Methods, 168 (1980) 529. 22 W. L. Brown, L. J. Lanzerotti, J. E. Bower and K. J. Marcantonio, Nucl. Instrum. Methods B, 24-25 (1987) 512, 23 W. Varhue and P. Pastel, J. Mater. Res., 5 (ll) (1990) 2441. 24 H. Baumann, T. Rupp, K. Bethge, P. Koidl and C. Wild, Proc. Eur. Materials Research Society Meet., in 17 (1987) 343.