Formation of highly tetrahedral amorphous hydrogenated carbon, ta-C:H

ELSEVIER

Diamond and Related Materials

Formation

of highly tetrahedral

M. Weiler”, J. Robertsonb, aFuchbereich b Department

Physik,

4 ( 1995) 268-271

amorphous

hydrogenated

S. Sattel”, V.S. Veerasamyb, Unicersitiit

qf Engineering,

Kaiserslautern, Cambridge

ZiAMOND RELATED MATERIALS

D-67653

Unitlersity,

K. Jung”, H. Ehrhardt”

Kaiserslautem,

Cambridge

carbon, ta-C:H

CB2

GertnanJ> 2 PZ,

UK

Abstract A highly tetrahedral form of hydrogenated amorphous carbon (ta-C:H) with maximum density of 2.9 g cmm3, sp3 fraction of 0.75 and hardness of 61 GPa has been deposited from acetylene using a low pressure plasma beam source. The ion energy dependence of its properties suggests that the subplantation deposition model of a-C also describes the deposition of a-C:H. KQW&:

Diamond-like

films; Ton beam growth;

sp’ bonding;

Introduction Diamond-like carbon (DLC) is a metastable, higher density form of amorphous C (a-C) or hydrogenated amorphous C (a-C:H) containing a significant fraction of tetrahedrally coordinated sp3 C-C bonding [ 11. To date, DLC with the highest fraction of sp3 bonding and the most diamond-like character has been deposited from a filtered C+ ion beam from a solid carbon source [Z-4]. The ion filtering allows deposition from a near mono-energetic beam of a single ion species. Such a-C is often referred to as tetrahedral amorphous carbon (ta-C). In contrast, a-C:H produced by plasma deposition tends to have much less sp3 bonding, lower hardness and lower diamond-like character [ 5-73. This paper describes the plasma deposition of a-C:H with sp3 fractions and hardness values close to ta-C [ S,9]. This ‘ta-C:H’ is deposited from acetylene using a novel plasma beam source first developed by Oechsner and Tomcik [lo]. Three factors are responsible for the highly tetrahedral nature of the resulting a-C:H, (i) a nearly fully ionised plasma beam, (ii) the presence of one dominant ion species and (iii) the mono-energetic character of the beam. These three features allow deposition from a single ion species of well defined energy per C utom, the same conditions as needed for ta-C. It is then possible to confirm that a common mechanismion subplantation-describes the formation of sp3 bonding in both ta-C and ta-C:H.

Experimental

details

The plasma beam source (Fig. 1) consists of a large moveable, 13.6 MHz r.f. powered electrode and a smaller 0925-9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved S.SDI 0925.9635(94)05256-5

sp3 bonding

tungsten grid electrode at ground potential. A hyperbolic magnetic field confines the plasma. Confinement and the low operational pressure of 5 x 10m4 mbar (0.05 Pa) creates a highly ionised plasma. The plasma and the powered electrode develop a positive d.c. bias voltage with respect to the grid electrode. The powered electrode can move vertically, varying its effective area, thereby varying the bias voltage without changing the gas pressure or r.f. power. The ions are accelerated by the bias voltage and are extracted through the grid as a neutralised plasma beam. Acetylene, C,H,, is used as a source gas because of its simple ionisation pattern. It forms almost exclusively C2H,+ ions in low pressure plasmas [7,1 l] because of the relative ease of simple ionisation compared to dissociative ionisation with C-C bond breaking. The mass spectrum (Fig. 2) shows the predominance of C,H,+ ions. Fig. 2 (inset) also shows the sharp ion energy distribution (IED) of the plasma beam (AE/E=0.05). The low pressure has minimised ion energy loss by collisions in the plasma sheath. In contrast, the ions in conventional plasma deposition have a broad TED with a mean ion energy only z 0.4 of the bias voltage [ $7,121. The degree of ionisation of the plasma beam is estimated to over 90%, from the ratio of the carbon ion current to the deposition rate. This high ionisation compares with typical values of lo-50% in conventional plasma deposition [7,13,14]. We now consider the properties of ta-C:H. The density was deduced from the energy of the valence plasmon and also from the film thickness and mass gain. The sp3 fraction was found from the area of the R* and rr* peaks

M. Weiler et al./Diamond and Related Materials 4 (1995) 268-271

faraday cup

InI

I-

-

substrate

holder

1

I

v”‘/l l

plasma beam

extraction

aperture

kradite)

tungsten

Q=z2 grid / [zz

\

.lit

L5L-

,d 1rrfelectrode ceramic

Lzzz2 1; l

movable ceramic pipe

rf powersupply gas sUpply Fig. 1. Schematic

diagram

of the plasma

beam source

in the C K-edge electron energy loss spectra (EELS) [6]. The hardness was measured by a micro-indenter, allowing for elastic recoil [ 151. The mechanical stress in the films was found from the substrate curvature, using Stoney’s equation. The hydrogen content was measured by “N nuclear reaction analysis, elastic recoil detection and combustion.

Results and discussion The difference between ta-C:H and conventional plasma deposited a-C:H is seen by plotting the C(sp”), C(sp’) and H contents as in Fig. 3. There is a region at high H content where solid films cannot form because they have no C-C bonded backbone. At higher H content, bounded by the ‘Angus’ line [ 161, lie films with an unconstrained ‘floppy’ network of low C-C coordination. Points for a-C:H C&13,17-191 are seen to lie below 35% sp3 content, and quite close to the Angus line. Points for ta-C:H follow a different trend, reaching quite high sp3 contents and well above the Angus line. The

269

sp3 contents lie below the best ta-C because of the finite H content of ta-C:H. The properties of ta-C:H depend strongly on ion energy; the density, sp3 fraction, hardness, compressive stress and optical gap each passing through a maximum as function of ion energy. We find a peak value of the sp3 fraction (of total C) of 0.75 + 0.06, of density of 2.9 g cmp3, of compressive stress of 8.5 GPa, and hardness of 61 GPa. These values compare well with those attained by ta-C [2-41 and are much greater that those typical of PD a-C:H [S-7]. Each property reaches a peak value at an optimum ion energy of 200 eV, or 92 eV per C atom (Fig. 4). This is similar to the optimum ion energy (140 eV) for ta-C found by Fallon et al. [3]. This similarity suggests that the deposition of ta-C and ta-C:H is controlled by a common process-subplantation [20-221. The depositing species in our ta-C:H is C,H,+. This ion tends to fragment into component ions on impact at the surface, dividing its energy roughly equally between two daughter C+ ions, conserving momentum, so that each C ion has an energy of 0.46E. These C + ions then interact with the growing film. Thus, the density is plotted as a function of ion energy per C atom in Fig. 4. The interaction of the C+ ions with the film depends on their energy. In a-C, ions of energy above a certain penetration threshold penetrate or ‘subplant’ into the film, entering a subsurface interstitial site, where they produce a metastable increase in local density and become sp3 bonded (Fig. 5) [ 213. Slower ions cannot penetrate the film but instead stick to the surface to form sp* bonded a-C. Faster ions penetrate deeper into the film, but their excess energy dissipates as heat in a thermal spike lasting lo-r2 s. This spike can allow a thermally activated relaxation of the density increase. The net density increase is then [21].

AP po

f l/4 -f+0.016p(E/Eo)5’3

(1)

Here, E is the ion energy per C atom, p. is the uncompressed density, 4 is the ion flux ratio, p is a material dependent parameter of order 1 and E, is the activation energy of relaxation. f in Eq. (1) is the penetration fraction, which can be approximated as f=l-

exp[-(E-E,)/E,]

(2)

where E, is the penetration threshold, 25 eV, and E, is a spread parameter. Eq. (1) is found to describe well the ion energy dependence of density of ta-C with p = 0.1, po=2.0gcm-3,E 2~3.1eV,E,=20eVandE,=110eV, as shown by the thin line in Fig. 4. Fig. 4 also shows a fit of Eq. (1) to the density of ta-C:H. A good fit is found retaining p=O.l and with po=1.9gcm~3,Eo=1.46eV,E1=57eVandE2=16eV, each per C atom. We see that the optimum ion energy

M. Weiler et al.jDiamond and Related Matericrls 4 (1995) 268-271

270

6.6 ~10~’ mbar

160

____

1.5 xl O+ mbar

______.2.5 x104 mbar

140 i $g

120 -

5 &

100 -

k % .Z

8o

i$ 8

60-

.E

40 20 -

0

5

10

15

20

25

30

35

40

45

50

55

60

atomic mass units Fig. 2. Mass spectrum of acetylene plasma, at three source chamber pressures. structure arises from r.f. modulation of the d.c. sheath potential [ 1Sj.

l

*D~A

SP3 A

ta-C:H PD a-C:H

Inset: typical

3.2

ion energy distribution

for acetylene.

The double

,

peak

1

I/

rigidity boundary

J 100

Fig. 3. spa, sp’ and H contents of ta-C:H, compared plasma-deposited a-C:H (A, Tamor [6]; ▹, Bustillo [24]; 0, Ref. [24], 0, Kaplan [25]).

to ta-C and [23]; *, Grill

per C atom for ta-C:H is similar to that for ta-C, but the detailed parameter values differ because we are now dealing with a molecular ion which fragments on impact with the a-C [9]. The density is found to vary linearly with sp3 fraction (Fig. 6). The variation is similar to that in a-C [3,17], despite the finite H content of ta-C:H. This suggests that bonded H has a relatively small atomic volume. Ta-C:H films possess high values of intrinsic compressive stress. The stress initially increases with film thickness, then

200

300

400

500

Energy per C atom (eV) Fig. 4. Variation of film density with ion energy per C atom, for a-C:H deposited from the plasma beam source and for a-C deposited from a filtered ion beam [S]. The lines are fits by Eq. (1).

saturates, and finally causes the film to delaminate at high thicknesses [lg]. The stress at moderate thickness is found to vary linearly with sp3 content (Fig. 7) as in ta-C [3]. Stress was used to describe the creation of DLC as a phase transition to a metastable phase [2,19]. The observed linearity of Fig. 7 suggests that the transition occurs as a continuous coordination change of an adaptable random network [Zl], rather than sharp transition from sp2 to sp3 bonding [2].

M. Weiler et al./Diamond and Related Materials 4 (1995) 268-271

subplantation of molecular ion lo

penetratio;

Fig. 5. Schematic relaxation during

illustration the thermal

Conclusion

O _

In conclusion, the use of acetylene in a novel plasma beam source allows the deposition of a highly tetrahedral form of a-C:H with properties much superior to those of conventional plasma deposited a-C:H, comparable to those of ta-C prepared from filtered C+ ion beams, and approaching those of diamond itself. The similarity has allowed us to demonstrate that the deposition of a-C:H is controlled by the same ion subplantation process that controls deposition of ta-C.

- ions separate

of (a) ion spike.

271

subplantation

Acknowledgement

(b) density

The authors thank the Deutschen Forschungs Gemeinschaft (DFG) for sponsoring this work. 31

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sp3 fraction Fig. 6. Variation ta-C [3].

oL:‘* 0

of density

1 2

with

sp3 fraction,

including

data

I,,,,,,,.

4

6

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Stress, Fig. 7. Variation of compressive data for ta-C [2,3].

stress

for

1,

10

12

14

GPa with

sp3 fraction,

including

References Cl1 J. Robertson, Prog. Solid State Chem., 21 (1991) 199; Surf. Coat. Technol., 50 (1992) 185. PI D.R. McKenzie, D. Muller and B.A. Pailthorpe, Phys. Rev. Lett., 67 (1991) 773. C.A. Davis, J. Robertson, G. c31 P.J. Fallon, V.S. Veerasamy, Amaratunga, W.I. Milne and J. Koskinen, Phys. Rev. B, 48 (1993) 4777. c41 F. Xiong, Y.Y. Chang and R.P.H. Chang, Phys. Rev. B, 48 (1993) 8016. c51 P. Koidl, C. Wild, B. Dischler, J. Wagner and M. Ramsteiner, Mater. Sci. Forum, 52 (1990) 41. C61 M.A. Tamor, W.C. Vassell and K.R. Carduner, Appl. Phys. Lett., 58 (1991) 592. R. Kleber, A. Kruger, W. Dworshak, K. Jung, I. c71 H. Ehrhardt, Muhling, F. Engelke and H. Metz, Diamond Relat. Mater., I (1992) 316. and V.S. Veerasamy, PI M. Weiler, S. Sattel, K. Jung, H. Ehrhardt Diamond Relat. Mater., 3 (1994) 608. V.S. Veerasamy and c91 Weiler, S. Sattel, K. Jung, H. Ehrhardt, J. Robertson, Appl. Phys. Lett., 64 (1994) 2797. Cl01 H. Oechsner and B. Tomcik, Surf. Coat. Technol., 47 (1991) 162. Cl11 R. Kleber, M. Weiler, A. Kruger, S. Sattel, G. Kunz, K. Jung and H. Ehrhardt, Diamond Relat. Mater., 2 (1993) 246. Cl21 C. Wild and P. Koidl, J. Appl. Phys., 69 (1991) 2909. Cl31 W. Moller, Appl. Phys. A, 56 (1993) 527. Cl41 W. Jacob and W. Moller, Appl. Phys. Lett., 63 (1993) 1771. J. Appl. Phys., 66 Cl51 X. Jiang, K. Reichelt and B. Stritzker, (1989) 5805. Cl61 J.C. Angus and F. Jansen, J. Vat. Sci. Technol., 6 1778 (1988); J. Robertson, Phys. Rev. Lett., 68 (1992) 220. Cl71 J.J. Cuomo, J.P. Doyle, J. Bruley and J.C. Liu, Appl. Phys. Lett., 58 (1991) 466. in Proc. Cl81 S. Sattel, M. Weiler, K. Jung and H. Ehrhardt, CIMTEX, Florence, 1994. [I91 D.R. McKenzie, J. Vat. Sci. Tech&. B, 11 (1993) 1928. PO1 Y. Lifshitz, S.R. Kasi and J.W. Rabalais, Phys. Rev. L.&t., 68 (1989) 620. t-211 J. Robertson, Diamond Relat. Mater., 2 (1993) 984;; 3 (1994) 361. c221 CA. Davis, Thin Solid Films, 226 (1993) 30. ~231 K.C. Bustillo, M.A. Petrich and J.A. Reimer, Chem. Mater., 2 (1990) 202. c241 A. Grill and V.V. Patel, Appl. Phys. Lett., 60 (1992) 2089. Appl. Phys. Lett., 47 ~251 S. Kaplan, F. Jansen and M. Machonkin, (1985) 750.