NEUTRON REFLECTION FROM AMORPHOUS HYDROGENATED CARBON FILMS

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NEUTRON REFLECTION FROM AMORPHOUS HYDROGENATED CARBON FILMS

M. Grundy, R. Richardson, G. Beamson, W. Brennan, J. Howard, M. O’Neill

To cite this version:

M. Grundy, R. Richardson, G. Beamson, W. Brennan, J. Howard, et al.. NEUTRON REFLECTION

FROM AMORPHOUS HYDROGENATED CARBON FILMS. Journal de Physique Colloques, 1989,

50 (C7), pp.C7-197-C7-201. �10.1051/jphyscol:1989720�. �jpa-00229884�

COLLOQUE DE PHYSIQUE

Colloque C7, supplkment au nOIO, Tome 50, octobre 1989

NEUTRON REFLECTION FROM AMORPHOUS HYDROGENATED CARBON FILMS

M.J. GRUNDY, R.M. RICHARDSON, G. BEAMSON*, W.J. BRENNAN*, J. HOWARD*

and M. O'NEILL*

Department of Physical Chemistry, University of Bristol, Cantock's close, GB-Bristol BS8 ITS, Great-Britain

ICI, Wilton Materials Research Centre, P.O. Box 90, Wilton, Middlesbrough, GB-Cleveland TS6 8 J E , Great-Britain

Abstract

Neutron reflectivity measurements have been used to study the thickness and composition of amorphous carbon films. Comparison of films made by deposition from CH4 and CD4 doped argon plasmas has enabled the hydrogen content to be determined. Variation in the plasma conditions were found to give a range of X (where the empirical formula is CH,) from 0.4 to 0.7.

Introduction (i) Background

In recent years there has been increasing interest in the properties and applications of amorphous hydrogenated carbon (a-C:H films, which exhibit Ldiamond-like' properties and are therefore known as diamond like carbon (D.L.C. or i-carbon. Amorphous carbon films are a continuum of materials with a wide range of compositions (CH,, X is 0 to 1) structure and properties. The original work of Aisenberg and Chabot(1) has shown that films may be prepared by ion beam deposition. More recent techniques include (2) d.c. and r.f. plasma deposition of a hydrocarbon gas (3); dual beam sputtering (4), hydrocarbon plating (5)and argon ion bombardment of evaporated films (6).

The nature of the amorphous hydrogenated carbon film produced is critically dependent on the deposition conditions (6) which lead to different hydrogen contents and hence different chemical hardness. Due to its technological importance interest has focused on the hard end of the range of materials (X tending t o zero) and a variety of applications are envisaged.(7) Already hard a-C:H is used as a protective coating for Winchester storage disks.(8) Although a wide range of techniques have been used to characterize the chemical structure of such films, glancing angle neutron reflection provides perhaps the best method for quantitative determination of hydrogen content in addition to providing information about film thickness, composition gradients and interfacial mixing.(9

In this work we use neutron reflection to determine the structure of films prepare

d

by different radio frequency (r.f.) plasma deposition conditions and examine how the hydrogen content varies.

Determination of scattering profiles of both hydrogenous and deuterated films prepared using the same dasma conditions allows a determination of composition without knowledge of the film density.

(ii) Theorv

We have already discussed the application of neutron reflection to the study of a-C:H films.(9) Briefly, a well collimated beam of radiation, of wavelength X, is brought onto the sample at a glancing angle B'and the surface reflectivity is measured as a function of (sin O/X ; the reflectivity profile depends on the distribution of scattering density perpendicular to the sur ace. Since the scattering density is determined by material composition, neutron reflection is particularly sensitive to the presence of hydrogen (which has a negative coherent scattering length bH = -3.74~10-15 m c.f. carbon

--

bc = 6.65~10-15 m and deuterium b, = 6.67~10-15 m). A weighted least squares model fitting program has been used to fit the reflectivity profiles determined for hydrogenous and deuterated films. This has enabled determination of

6 ) Film scattering length density, pp

-

Film thickness, d

Interfacial diffuseness, U,* and Usub

A scaling factor and an angular resolution term, AB, which were treated as disposable parameters.

Emeriment a1 (i) S a m ~ l e Pre~aration

Samples were prepared by deposition onto the (111) face of a 75mm diameter single crystal silicon wafer in a parallel plate RF plasma reactor (13.56 MHz). Hydrogenous films were prepared from

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989720

a methane doped argon plasma whilst deuterated films were produced by substitution of methane by da-methane (>99% isotopic purity) supplied by Carnbrian Gases. Deposition conditions were selected to give a range of hydrogen content by varying the argon flow rate, r.f. power and chamber pressure.

The film thickness is controlled by the deposition time and the deposition rate. The latter will also be dependent on process conditions. Films were intended to be ca.400-800

A

thick which is a convenient range for the reflectometry measurements. Samples were stored under ambient conditions for about 8 weeks before the reflection experiment.

(ii) Reflection Measurement ^S

Neutron reflectivitv of the s a m ~ l e s were determined on the CRISP reflectometer at the ISIS neutron source (10). A p61ychromatic pulsed beam was reflected at fixed low angle (0.25"-0.4') into a single channel detector. The neutron wavelength was determined by time of flight and was in the range 1.5-6.5 A. Although measurements were made at higher glancing angles for some samples, only the low angle runs which allow determination of the critical angle are reported here. The samples were, in most cases, sufficiently thick for the critical angle t o be determined by the a-C:H film not the silicon substrate. Two 0.5 mm slits 2.6 m apart gave an instrumental beam divergence of 0.012" and an 82 mm long footprint for Bi ⁼ 0.35'. The width of the beam was 40 mm. Data normalisation and reduction to profiles of reflectivity vs.(sinB/X) was performed usin the GENIE program.(ll) The program FITLAY was then used to fit the data as in previous work.(9?

Samples measured included a clean silicon substrate in addition to the hydrogenous and deuterated films detailed below in table 1. The silicon scattering length density (psub) btained by data fitting was 0.205~10-5 A-2 which is close to the theoretical value of 0.210~10-5

8i*

obtained by calculation from the density and scattering length of silicon. In subsequent analyses p,,b was fixed to the measured value. The angular resolution width (AB) was found to be 0.035" which is higher than the instrumental resolution (ca.0.015") for the slits used. This is believed to arise from natural curvature of all silicon wafers with a radius of curvature of >l00 m which is invisible to the eye. The angular resolution width, AB, was treated as a disposable parameter in the subsequent analysis but was generally found to be in the range 0.025" to 0.05".

Results

Table 1 shows the thickness and scattering length densities obtained by fitting the data from the films prepared by the plasma conditions shown. The thicknesses are different for the deuterated and hydrogenated films for two reasons. In most cases the deposition rates were the same but the deuterated film was deposited for shorter periods than the hydrogenated. In some cases there was an apparent difference in deposition rate with isotopic substitution. The reason for this difference is not understood at present.

Figure 1 shows typical data sets from a deuterated and hydrogenous film represented by discrete points, whilst the solid line illustrates the model fit. Deuterium has a positive scattering length (b,,) which produces a much hi 'her scattering length density for deuterated samples. This can be seen

by

the higher critical angle ($,it) and greater fringe depth, as the contrast with the silicon substrate is greater. Figure 2 shows an example of a reflectivity profile from a 'hard' carbon film (p, = 0.41~10-5

LL

A-2) and a 'soft' carbon film (pH = 0.32~10-5 A*). It can be seen that the lower pH leads to shallow fringes and lower Bc,it. This illustrates the sensitivity of neutron reflection as a 'probe' for hydrogen.

The different fringe spacings arise from variation in film thickness.

Fig. l .

Table 1. Films Produced and Least Sauares Parameter Fits Preparation Conditions Data Analyses -Least Squares Fit

CH4 AI Pressure Power pH(x10-5fL*) d(A) pD(xl0-5fL*)

sccm Torr Watts

A 50 152 . l 3 70 0.407k0.001 437f 2

*

B 50 152 . l 3 50 0.408f 0.001 760f 3

*

C 50 152 1.0 20 0.2624~ 0.004 618f 3

*

*

Deuterated sample not fabricated.

t

Estimated value. The film could not be treated as a single Iayer.

The interfacial diffuseness determined by model fittin did not vary greatly from sample to sample. The diffuseness of the airjfilm interface (Uair) was found to be 14 f 4

A

while that of the filmlsubstrate interface was 31 f 11

A

Discussion (i) Interfacial diffuseness

Of the two interfaces the airlfilm interface has the lower diffuseness. The values

of the Uair/ilm (14 MA) is very si'milar to the 13.7A value obtained from the bare silicon wafer. This is to be expected although i t has been reported that in the case of a very high energy amorphous film (12) a smoother surface than the substrate may result in order to minimise surface free energy. This may be facilitated by annealing the films at elevated temperatures, which has not been investigated

here.

-

The diffuseness of the substrate interface is typically much higher and exhibits wider variation

(Ufilm/substrate is 31 f l1

A).

Clearly, this is in excess of the substrate diffuseness itself. The high values suggest a grade in composition of the film at the substrate interface. In the model fits the use of a diffuseness introduces an error function profile in the scattering length density with standard deviation U where U is the interfacial diffuseness. The error function profile has been chosen because it is simple t o calculate its effect on the reflectivity, but the calculation is not expected to be sensitive to the detail of the profile used to describe the interfaces.

It seems reasonable to expect a grade of composition at the substrate as the film has been laid down for the greatest time at this interface. Argon bombardment f this interface continues during

B

deposition from the plasma. Typical deposition conditions yielded 50 /min and required ca.10 minutes to produce a suitable film. I t is possible that lighter species or loosely bound moieties may have been removed by the argon bombardment. In addition, the interfacial region may tend to approach thermodynamic equilibrium and a composition profile which minimises the interfacial tension may be formed.

Scattering density

In the case where both the deuterated and hydrogenous pfil, have been determined it is possible to evaluate the film composition in terms of a CH, ratio where X is determined. E.S.C.A.

measurements have previously shown minimal presence of oxygen and other elements. Since the scattering length density, pH for a CH, film is given by

PH = N(bC xbH) ( l )

where the b's are the scattering lengths of the atoms and N is the number of formula units per unit volume, (assumed independent of isotope) it can be shown that:

b C ( p D

-

p H )

X =

P H ~ D

-

P D ~ H ⁽²⁾

This equation has been evaluated using the values of scattering length density determined from hydrogenous (pH) and deuterated (pD) films, the results are shown below.

Table 2. Calculated C:R Ratios

CH4 Ar Pressure Power X

seem (Torr) (Watts)

D 10 0 0.5 20 0.692rt0.004

E 10 15 0.5 20 0.6605 0.009

F 10 20 0.5 20 0.678f 0.011

G 10 30 0.5 20 0.635f 0.007

H 10 40 0.5 20 0.557zk0.004

I 10 50 0.5 20 0.543?~0.006

J 10 100 0.5 20 0.5475 0.010

This is more clearly illustrated in Figure 3 which shows the variation of X with both argon flow rate and r.f. power. It can be seen that as the argon flow increases the proportion of hydrogen falls to an apparently constant level. Similarly, an increase in the power produces a fall in hydrogen content and therefore an increase in [hardness' of the film. It is also worth noting that with all other conditions constant a reduction in pressure produces a reduction in X from 0.692 to 0.422. This is consistent with

work by both the authors (9) and others(l3), viz: [Hard' carbon is favoured by High argon flow rates

Hieh Dower (iii

j LOG

iressure

Amorphous carbon consists of a mixture of sp2 and spGydbridised carbon and it is known that as the hydrogen content decreases the hardness and density increase as does the sp2/sp3 ratio. The increasin sp2 content has been associated with the formation of fused benzene clusters connected by hydroear%on chains (14) with random orientation leading to hardness. These clusters are larger than sp3 type moieties and are less likely t o be removed by argon bombardment. It is also probable that increasing the power produces a more energetic plasma with larger species preferably retained in the film. In addition, the ratio of ion reactive neutral bombardment of the substrate under the plasma increases with decreasing pressure and greater ion bombardment is also believed (9) to produce an increase in the graphite type moieties in the film.

VARIATION OF CnH RATIO WITH PLASMA CONOITIONS 0 . 7 0 ~

CH4

-

^lOsssm

Ar

-

^X COOROINASE POWER

-

^{20 WATTS}

PRESSURE

-

^{0.5 TORR}

I ^\

^POWER

-

^X COORDINATE

F i g . 3 (b)

0.501 J

0. 0 0 . 10.00 20.00 30.00 40.00 SO. 00 80.00 70.00 BO.00. Q 0 0 0 100.00 POWER

Conclusion

We have used neutron reflection t o characterize amorphous hydrogenated carbon films and demonstrated a quantitative method for determination of hydrogen content. This will allow deposition conditions to be characterized to enable future 'tuning' of the plasma to produce the desired properties in the a-C:H film.

References

1. S. Aisenberg and R.Chabot; J.Appl.Phys.42, 2953 (1971).

2. B. M erson and F.W. Smith; Solid State Commun. 3, 531 (1980); J.Non.Cryst.So1id S, 435 (1980j'.

3. See for example: L.M. Holland and S.M. Ojaha, T.S.F.

48,

L21 (1978).

4. See for example: B.A. Banks and S. Rutledge; J.Vac.Sci.Techno1. 21, 807(1982).

5 . See for example: T. Mori and Y. Namba, J.Vac.Sci.Techol.~, 23 (1983).

6. S. Fujimori and K. Nagai, Jpn.J.Appl.Phys.20, L194 (1981).

7. M. Simpson, New Scientist

1603,

pq.50-53, March 1988.

8. H.C. Tsair and D.B Bogy, J.Vac.Sc~.Technol.&, 3287 (1987).

9. M.J. Grundy, R.M. Richardson, S.J, Roser, G. Beamson, W.J. Brennan, J. Howard, M. O'Neill, J. Penfold, C. Shackleton and R.C. Ward, Thin Solid Films, in press 1989).

10. J. Penfold, R.C. Ward and W. G. WilLiams, J.Phys,E. 20, 1411 (19815

11. Punch Genie Manual, Rutherford Appleton Laboratory Report RAL-86-102 (1986).

12. S. Aisenberg, J.Vac.Sci.Techno1. A2, 369 (1984).

13. See for example: F. Jansen, M. Machonkin, S. Kaplan and S. Hark, J.Vac.Sci. Technol.

B,

605 (1985).

14. J . Robertson, Adv.Phys. 35, 317 (1986) and J. Perrin, EMRS Symposium Proc.

U,

105 (1987).