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第２号（2009）

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INFRARED SPECTRA AND THERMALDESORPTION SPECTRA OF ISOTOPICALLYMIXED AMORPHOUS HYDROGENATED CARBON
FILMS PREPARED FROM MIXTURE OF
CD4 AND H2 OR CH4 AND D2Kazunori MARUYAMA1, Wolfgang JACOB2 and Joachim ROTH2
1Department of Materials Engineering, Nagaoka National College of Technology
2Max-Planck-Institut für Plasmaphysik (EURATOM Association), Garching, Germany

Isotopically mixed amorphous hydrogenated carbon films were prepared from 1:1 mixture of CH4 andD2 (a-CH:D). The atomoc ratio of H:D incorporated in the films were 0.83:0.17 for soft films and0.80:0.20 for hard films. Compared the infrared spectra of these films with another spectra of the films
prepared from 1:1 mixture of CD4 and H2 (a-CD:H), it was concluded that deuterium originating from D2
in the plasma is considerede to be incorporated as an isolated aliphatic C-D bonds exclusively. Thermal
desorption spectroscopy of hard a-CH:D films suports this idea because the films release H2 and HD butvery low D2 by heat-treatment under vacuum.
Key Words : amorphous, carbon film, IR spectra, TD spectra

1. INTRODUCTION
One of the common preparation methods ofamorphous hydrogenated carbon films (a-C:H) is thedeposition in a hydrocarbon radio frequency glow
discharge system onto negatively self biased
substrates. It is well known that the application ofnegative bias onto substrates leads to the formationof hard a-C:H films1). The addition of hydrogen or
nobel gases into the source gas also leads to theformation of hard a-C:H films1). Hard a-C:H films
are considered to be formed by chemical etching or
physical sputtering reactions by radicals and positive
ions in the plasma. Chemical etching reactions
subtract hydrogens from carbon atoms to give
unsaturated carbons in the film which provides abonding site for radicals in the plasma and in thefilm. The former leads to the film growth and thelatter leads to carbon-carbon cross-linking bonds.
Soft areas on the surface of deposited films are

removed by physical sputtering. The net growth rateof a-C:H films is a result of the deposition of carbon
materials and the erosion by radicals and ions in theplasma. Küpper et al reported the evidence for thehydrogen abstraction reaction that the impact of D
atoms produced thermaly at surfaces of ultrathin aC:H films bring about the formation of sp3C-D from
sp3C-H through H-D substitution reaction2) and also
D-sp3C-H from sp2C-H by addition reaction of D3).
The present paper gives some information about theetching process of a-C:H films; D atoms originatedfrom D2 are incorporated as an isolated aliphatic CD bond in the films during RF plasma deposition.

2. EXPERIMENTAL
The amorphous carbon films were preparedusing a radio frequency plasma (13.56 MHz)apparatus mentioned in the previous report4). Afterannealing Si substrates at 550K for overnight under― 63 ―

Kazunori MARUYAMA, Wolfgang JACOB, Joachim ROTH

10-5 Pa , then they were treated with H2 plasma (4Pa, -300V bias) for 1 hour in order to reduce
superficial oxide layer. Deposition conditions foramorphous carbon films are summarized in Table
1. The film thickness was monitored by He-Ne

laser interference signal in situ. Silicon substratetemperatures were ambient during deposition. The
films made from CH4-D2 and CD4-H2 are assignedin this paper as a-CH:D films and a-CD-H films,
respectively.

Table 1 Deposition conditions for the preparation of carbon films.

soft a-CH:D
hard a-CH:Dsoft a-CD:H

Source Material

Pressure

6Pa

Bias voltage(Power)-40V(8W)

Depositiontime
8 h 45 min

CH4(30sccm)
D2(30sccm)
CH4(10sccm)
D2(10sccm)
CD4(30sccm)
H2 (30sccm)

4Pa-300V(63W)

53 min6Pa-40V(8W)

11 h 54 min

The absolute concentrations of H and D were
determined by elastic recoil detection (ERD)
analysis using a beam of He+ ions (E=2.6 MeV)
from an accelerator and a scattering angle of 30.
The absolute concentrations of C and O were
determined by proton enhanced cross section
scattering (PES) analysis using a beam of H+ ions
(E=1.5 MeV) with a scattering angle of 165. The
reaction cross sections used for the determination of
the components were taken from references5-7).
Infrared spectra were measured using Perkin-Elmer
Specrometer 1760X. The film thicknesses were
measured mechanically by profilemeter TENCOR
alpha-step 200.

3. RESULTS AND DISCUSSION
3.1 Elemental analyses of a-CH:D and a-CD:H
films
Table 2 shows the elemental densities of a-CH:Dand a-CD:H films determined by PES and ERD
analyses together with the refractive indices of thefilms. Refractive indices of these films were

calculated by using optical film thicknesses
estimated by He-Ne laser interference patteren andthicknesses
measuredmechanicallybyprofilometry. Both soft a-CH:D and a-CD:H films
were prepared under same conditions from 1:1
mixture of methane and hydrogen as shown in
Table 1, however, the amount of incorporatedhydrogen from additive D2 or H2 was different fromeach other, namely, the atomic ratio of H:D in thesoft a-CH:D and hard a-CH:D films are 0.83:0.17
and 0.80:0.20, respectively. The ratio of H:D in thesoft a-CD:H film is 0.23:0.77 as shown in Table 2.
In this paper, the meanings of soft and hard aredefined as whether the film could be scratched by ametal driver or not. The hard a-CH:D film prepared
under -300V bias has a higher density of carbon
atoms than soft films. The atomic ratio of (D+H)/C
in hard films is much lower than soft films. Thissuggests that the concentrations of cross-linking CC bonds in hard films is higher than that in soft
films.

Table 2 Refractive indecis and densities of elements of carbon films obtained.

OpticalThickness by Refractive
thicknessat profilemeter (d) index (n)632 nm (nd)soft a-CH:D 474nm
281nm
1.69
hard a-CH:D 790nm
370nm
2.14
soft a-CD:H 474nm
291nm
1.63

3.2 Infrared spectra
Figures 1(a), 1(b) and 1(c) show infrared spectraof soft a-CH:D, hard a-CH:D and soft a-CD:H
films, respectively. These spectra are presented here
as the function of the extinction coefficient, k.
These k spectra mean the extinction coefficient of
the films including multiple coherent reflections in

Elemental analyses
C O D H
(1022 cm-3)
5.3 0.0 1.1 5.2
8.3 0.0 1.0 4.0
4.9 0.2 4.6 1.4

AtomicAtomic ratio
ratio in H:D in (H+D)/C
0.83:0.17
0.80:0.20
0.23:0.77

1.19
0.60
1.22

the films themselves and taking into account the
Kramers-Kronig-relation between the real andimaginary part of the refractive index. The methodof making k spectra was similar to that reported
previously8).
As can be seen in Figs. 1(a) , 1(b) and 1(c), thesecarbon films show strong C-H stretching bands― 64 ―INFRARED SPECTRA AND THERMAL DESORPTION SPECTRA OF ISOTOPICALLY MIXEDAMORPHOUS HYDROGENATED CARBON FILMS PREPARED FROM MIXTURE OF CD4 AND H2 OR CH4 AND D2

around 2950 cm-1 and C-D stretching bands around2150 cm-1. There is a clear absorption edge at theregion over 3000 cm-1 in Fig.1(a) while noabsorption of the a-CD:H film can be seen in thisregion in Fig. 1(c). The aborption over 3000 cm-1 ischaracteristic for sp2C-H stretching band9).
Therefore, it can be concluded that there is very few
sp2C-H bond formed in a-CD:H films. In otherwords, H originating from additional H2 in theplasma is exclusively incorporated as sp3C-H bond.

0.06(a) (C-H, olefinic)

0.04

(C-H, alifatic)(C-D, alifatic)

0.02

Absorbance

0.00
0.03
0.02

(b)

(CH2) and (CH3)

0.01
0.00
0.06

(c)

-1

2214 cm , (C-D, olefinic)

0.04

(C=O)

(C=C)

0.02
0.00
3000

2500

2000

1500

1000

-1

Wavenumber / cm

Fig. 1 Infrared spectra of amorphous hydrogenated films. (a)Soft a-CH:D film (prepared from mixture of CD4 and H2). (b)Hard a-CH:D film (prepared from mixture of CD4 and H2). (c)Soft a-CD:H film (prepared from mixture of CH4 and D2).
These spectra are presented as the function of the extinction
coefficient, k, see text.

Carrying over this consideration to the a-CH:D
film, it is expected that the a-CH:D film has asimilar structure. Namely, the a-CH:D film has
almost no sp2C-D bond. Küpper et al reported about
the infrared spectra of a-C:H and a-C:D films andthey assigned the peak around 2150 cm-1 to be themixture of stretching sp2C-D and sp3C-D bonds.
They estimated sp2C-D absorption band to be 2274
cm-1 by a fitting process using spectra of a-C:H
films and isotope factor (1/1.36)3). Using thisisotope factor, aromatic C-H bond at 3057 cm-1
shifted to 2248 cm-1 and olefinic C-H bond at 3010
cm-1 similarly shift to 2214 cm-1. As can be seen in

Fig. 1(c), the a-CD:H film has a strong absorption
peak at 2214 cm-1 while the a-CH:D film has not asshown in Fig. 1(a). The lack of sharp absoprtion at
2214 cm-1 for a-CH:D in Fig. 1(a) is referred to thelack of the absorption over 3000 cm-1 for the aCD:H film in Fig.1(c).
If the absorption centered at 2925 cm-1 in theinfrared spectrum of the a-CD:H film in Fig. 1(c)contains H-sp3C-H stretching, there should appear
remarkable H-sp3C-H deformation absorptions ataround 1470 and 1380cm-1, which is observed
clearly in the infrared spectrum of the a-CH:D filmin Fig. 1(a). However there is no such absorptions
in such region. Therefore, the absorption centered at2925 cm-1 is considered to be isolated methyn typesp3C-H stretching mode. In other wards, sp3 carbon
does not bond with more than one H. On the other,absorption band centered at 2175 cm-1 in Fig. 1(a)is considered to be isolated sp3C-D stretching.
These discussion is supported by TDS spectra
mentioned later. The absorption intensity of C=C
stretching around 1515 cm-1 in Fig. 1 is rather low.
However, the infrared spectrum of some disubstituted olefine, for example trans-C3H7CH=CH-CH3, often lacks C=C stretchng bandaround 1500 cm-1.
Another characteristic difference between thetwo infrared spectra of two soft films in Fig. 1 isthat the a-CD:H film shows rather strong absorption
around 1720 cm-1 (Fig. 1(c)) while the a-CH:D
films does not. Since this band is assignable to C=O
stretching (Fig. 1(a)), the a-CD:H films are likely
to be oxidized easily while a-CH:D films are not.
This result is confirmed by RBS analysis shown in
Table 2. It is not clear from what this different
property against oxidation originates. However, as
can be seen in Table 2, the soft a-CH:D film has alittle higher carbon density than the a-CD:H film.
As mentioned previously, hard films with highcarbon density over 81022 cm-3 show highresistivity against oxidation. It was suggested in thepaper that the small pore size of hard films prevents
the penetration of O2 and H2O into the film10) which
results in the high resistivity against erosion in air.
The critical carbon density showing the resistivity
against oxidation might lie in the region between
that of these two soft films.
3.3 Heat treatment under vacuum (TD spectra)
As was reported previously, when soft a-C:D
film are heated in air, its releasing rate of deuterium
is much higher than that of carbon and after
deuterium depletion the films show a resistivity
against further erosion due to annealing in air11). It― 65 ―

Kazunori MARUYAMA, Wolfgang JACOB, Joachim ROTH

(b)

20
10

5

10 20 30 40 50 60 70 80 90 100
Mass number / amu

(a)

m28

m18
m44
m20

Count / a.u.

10

(b)

Count / a.u.

Count / a.u.

m2

m4

m32
0
300 400 500 600 700 800 900 1000
Temperature / K
12

10
0

m44

Fig.2 (a)Thermal desorption (TD) spectra of the soft a-CH:D
film monitored at mass numbers of 2(indicated as m2 in the
figure), 16, 28 and 40 amu. (b)Mass spectrum of the soft aCH:D film obtained at 540K.

0300 400 500 600 700 800 900 1000
Temperature / K

20

30

0

m4

30

m28

0
300 400 500 600 700 800 900 1000
Temperature / K
40

m3

m18

m16

10

m16

m2

(a) m18

20

Count / a.u.

10

30

10

(a)

Count / a.u.

20

40

Count / a.u.

is well known that amount of double bonds in aC:H films increases during heat treatment. Craig et
al reported that heat treatment under 1 mPa at 770K
caused the increase in absorption of 1620 cm-1 12).
In the present study this was confirmed by theincrease in 1610 cm-1 absorption in infrared spectra
when the film was heated in air. However there wasno clear increase detected in this infrared reagion
when the film was heated under vacuum below 10-7
Pa in the present study. Therefore less formation of
C=C double bond during heat treatment undervacuum may responsible for sublimation of soft
carbon films.
Figures 2, 3 and 4 show thermal desorption
spectrometry (TDS) spectra for soft a-CH:D, harda-CH:D and soft a-CD:H films with mass spectra,
respectively. Herein, some of the signals of
quadrupole mass analyzer (QMS) are plotted as afunction of the substrate temperature. As wasreported in the previous paper, most strong signal
besides mass number 18 amu for TDS spectrum forsofta-C:D films are mass number 2811). In thepresent study for soft a-CH:D and a-CD:H films,
the signals for mass number 28 also shows strong
peak as shown in Figs. 2(b) and 4(b). The
respossible molecules for mass number 28 are CO,
C2H4 and C2D2. If C2H4 is exclusively responnsible

8
6
4
2
0

10 20 30 40 50 60 70 80 90 100
Mass number / amu

Fig.3 (a)Thermal desorption (TD) spectra of the hard a-CH:D
film monitored at mass numbers of 2(indicated as m2 in the
figure), 3 and 4 amu. (b)Mass spectrum of the soft a-CH:D
film obtained at 760K.

(b)

10 20 30 40 50 60 70 80 90 100
Mass number / amu

Fig.4 (a)Thermal desorption (TD) spectra of the soft a-CD:H
film monitored at mass numbers of 4(indicated as m4 in the
figure), 18, 20, 28, 32 and 44 amu. (b)Mass spectrum of thesoft a-CH:D film obtained at 570K.

― 66 ―INFRARED SPECTRA AND THERMAL DESORPTION SPECTRA OF ISOTOPICALLY MIXEDAMORPHOUS HYDROGENATED CARBON FILMS PREPARED FROM MIXTURE OF CD4 AND H2 OR CH4 AND D2

for the intensity at mass number 28 in Fig. 2(b) for
a-CH:D, C2D4 (mass number 32) for a-CD:H in Fig.
4(b) should samely show strong intensity.
However, Fig. 4(b) does not show strong intensity
for mass number 32 but 28. In addition, the profile
of mass number 28 makes large peaks whichindicates that this signal does not come from background CO as shown in Fig. 2(a). Besides the onset
temperatures for mass number 18, 28 and 44 amu
are almost the same. Then a part of H2O(m18), CO(m28) and CO2 (m44) are assumed to be evolved bysample itself.
As was mentioned previously, soft a-C:H films
release hydrocarbon species exclusively during
annearing under vacuum, while hard a-C:H films
release predominantly H2 molecules to give semihard a-C:H films with very low hydrogenconcentration10). We assumed in relation to theinfrared spectra that sp3 carbon does not bond withmore than one D in the a-CH:D films. TDS
spectrum of hard a-CH:D film in Fig.3 supports thisidea. Namely, it releaes H2, HD with about a half
signal intensity of H2, and very low D2. In addition,
the intensity of mass number 16 show peak whichindicates that CH4 must be evolved from the bulk of
the hard films. However, it is not clear at present
whether CH4 was produced by the thermal
degradation of the film itself or by the simple
diffusion of free gaseous CH4 incorporated in thefilms.
As mentioned above, H originating from H2 isexclusively incorporated as aliphatic C-H bonds in
a-CD:H films. As can be seen in Table 2, the ratio
of incorporated H isotope is estimated as an atomic
ratio of H/D of 0.23/0.77. Taking into theconsideration that there is little sp2C-H bond in thea-CD:H films, this atomic ratio seems to be rather
high. Küpper et al reported that D generated bythermal dissociation of D2 reacts with sp3C-H togive sp3C-D through H-D substitution reaction2) andalso reacts with sp2C-H to give D-sp3C-H byaddition reaction of D3). If the addition reaction of
H radicals and/or H+ ions to C=C double bonds
formed in the film is the exclusive main reaction toform C-H bonds, it is clear that there is no sp2C-H
bond in the film, because the addition reacion of H
convert sp2C to sp3C spontaneously. In order toestimate this addition reaction, a soft a-C:D film
without C-H bond prepared from CD4 was treatedin a H2 plasma with self bias of -40, -80, and -120V.
However there is no increase in the absorption
region of sp3C-H around 2950 cm-1 nor decrease of
sp2C-D at 2214 cm-1 in infrared spectra. Only
erosion with the decrease of film thickness could beobserved. So this experiment is failed to show thedirect evidence of addition reaction or substitution
reaction of H on the film surfaces. Therefore it is
not clear at present why there is so little sp2C-H
bond in the a-CD:H films.

4. CONCLUSIONS(1) Compared the infrared spectra of a-CH:D and aCD:H films, it was concluded that the strong
absorption peak at 2214 cm-1 was assigned toolefinic C-D stretching band.
(2) In the soft a-CD:H film, H originating from H2in the plasma was incorporated as an isolated
aliphatic C-H bond exclusively. Because -CH2- and-CH3 deformation absorptions at 1470 and 1380
cm-1 were missed in the infrared spectrum of the aCD:H film.
(3) Thermal desorption spectroscopy of hard aCH:D films suports above conclusion because thefilms release H2 and HD but very low D2 by heattreatment under vacuum.
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Kazunori MARUYAMA, Wolfgang JACOB, Joachim ROTH

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(Received September 25, 2009)― 68 ―