56 Revista Latinoamericana de Metalurgia y Materiales, Vo1.12, N° 1 Y 2, 1993.

CHARACTERIZATION OF NITROGEN INCORPORATED CARBON FILMSDEPOSITED BY RF-GLOW-DISCHARGE USING ERD, RBS, NUCLEAR REACTOR, X-RAY DIFFRACTION AND INFRARED SPECTROSCOPY

D.F. Franceschini" and C.A. Achete*, F.L. Freire Jr.** and S.S. Camargo Jr.*

* COPPE, Universidade Federal do Río de Janeiro, Brasil.** Departamento de Física, Pontifícia Universidade Católica do Río de Janeiro, Brasil.

Abstraet .We report a study on nitrogen incorporation into hard a-C: H thin films were deposited by RF self-

bias glow-discharge in a Cl4-N2 atmosphere, under several conditions of total pressure, N2 partial pressureand measured self-bias. Elemental analysis of the obtained films was carried out by Nuclear ReactionAnalysis, Rutherford Back-Scattering Spectroscopy and Elastic Recoil Detection. Film strucníre was studiedby infrared spectroscopy and X-Ray diffraction. Results show that N2 partial pressure is the majar factoraffecting nitrogen incorporation, not being detected any int1uence of the self-bias on nitrogen content.Infrared spectra show that nitrogen is effectively incorporated in the amorphous network.

Keywords: a-C: H; diamond-like carbon; ion beam analysis; infrared spectroscopy; glow-discharge.

INTRODUCTION

Amorphous hydrogenated carbon (a-e: H)films deposited by plasma decomposition ofhydrocarbon gases may be quite hard, withhardness values often higher than that of siliconcarbide [1], insulating and resistant againstcorrosion. These properties make hard a-e: Hfilms a candidate for several applications, speciallyas protective coatings. The structure andproperties of these films, mainly those depositedby self- bias glow-discharge, ha ve beenextensively studied under a wide range ofdeposition parameters [2-5]. These studiesreported on a strong dependence 01' structure andchemical composition, as well as the mechanical,electrical and optical properties on depositionpressure and measured self-bias VB.

Recently, there have been reports of theeffects of nitrogen incorporation into a-C: H films[6-8]. It has been shown that nitrogenincorporation causes the reduction of the electricalresistivity [6] and of the optical band-gap width[7]. None of these works .used a depositionprocess that can be clearly characterized as self-bias glow discharge. In this work we report on astudy on the characterization of nitrogen-incorporated a-C: H films deposited by self-biasglow-discharge in CH4-N2 atmospheres, underseveral conditions of deposition pressure,

measured self-bias and N2 partial pressure.

EXPERIM.ENTAL

The nitrogen incorporated a-e: H filmswere deposited on undoped Si(100) substrates,which were mounted over the stainless steelcathode of a Varian RF(13.56 MHz) diodesputtering systern. Three sets of films wereproduced, the first two at a constant self biasvoltage 01' -370V, with N2 concentration in theplasma ranging from O to 50%, at depositionpressure of 0.8 and 8 Pa. The third set wasproduced at a deposition pressure of 8 Pa and 25%nitrogen concentration in plasma, while varyingthe self-bias from -200 to -950 V. Thethicknesses of the obtained films was determinedby measuring the height of the step produced bythe clamps used to fix the substrates on theelectrode, with a Dektak HA stylus profilometer.

Chemical analysis of the films wasperformed by Me V ion bean techriiques.Hydrogen content was determined by ElasticRecoil Detection of protons using a 2.2 MeV,4He+ incident beam. Nitrogen content wasdetermined by a 14N(d, p)15N nuclear reaction,excited by a-'610 KeV deuteron beam. Thepresence of oxygen in the films was also checkedby nuclear reaction analysis. Carbon content was

Latinémerican Journal al Metallurgy and Materials, VoU2, N° 1, 2, 1993.

Revista Latinoamericana de Metalurgia y Materiales. Vol.12. N° 1 Y 2. 1993. 57

150

c: ( o ) : i:

e ' e." 300

"-z z

V = -370 V oo Ir::100 B 1-

Ulen t!]t!]

~ 200z<t

LIJLIJ 0,8 Po 1-1- <t<t ~o:0:50z ~ IDOo1- 1-

Ul Ulo oel. el.LIJ LIJo O o

°0.00 12.50 25.00 37,50 80.00 Ov, NITROGEN IN PLASMA

(a)

deterrnined by Rutherford Back-ScatteringSpectroscopy (RBS).

Film structure was studied by submittingthe samples to FTIR spectroscopy and X-raydiffraction. Infrared spectroscopy was performedby a Perkin-Elmer 1700 FTIR spectrorneter.X-ray diffraction analysis was made in a Rigakugoniometer coupled to a Phillips X-raygenerator,using Mo Ka radiation.

RESULTS AND DISCUSSION

The deposition rate dependences on N2partí al pressure in the plasma, and self bias VB areshown in figure 1. In figure 1(a) the depositionrate is plotted against N2 partial pressure, for filmsobtained at 0.8 and 8 Pa, with VB fixed at -370VThe two curves show a dramatíc decrease indeposition rate upon increasing of N2 partíalpressure. This decrease cannot be attributed to asimple dílution effect, as it is suggested bycomparison between deposition rate valuesobtained from films deposited with 0% N2, whichshow a factor of two reductions in deposition ratefor a ten-fold decrease of total pressure, although

the addition 0[50% N2 in the plasma resulted innearly vanishing the deposition rate. In figurel(b) the deposition rate is plotted against themeasured self-bias, for films obtained with S Paand 25% N2 in plasma. The curve shows amaximum for VB equal to -800 V Such abehavior was previously reported for undoped a-C: H films and has been attributed to the onset ofsputtering induced by the more energetic ionsproduced at higher self-bias values [2].

Concerning to the chemical composition,the films studied in this work were found to beoxygen-free as deterrnined by nuclear reactionanalysis, and heavy elernent-free as determinedRBS. The hydrogen content of the films hasfound to be constant within experimental error,upon nitrogen incorporatión and self-biasvariation,as shown on table 1. The constantbehaviour of the hydrogen content upon nitrogenincorporation is consistent with results reported byKaufrnan et al [8] As expected, the nitrogencontent was found to increase upon increasing N2partial pressure, as shown in figure 2(a). Nochanges on nitrogen content were found upon self-bias variation, as shown in figure 2(b).

( b )

e Po25 "lo N

2

200 400 500 800 1000H LF - BIAS ( - v )

Figure l. Deposition rate asa function of (a) N2 partial pressure, with fixed VB and total pressure,and of (b) VB, with fixed total pressure and N2 partial pressure.

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5 8 Revista Latinoamericana de Metalurgia y Materiales, Vol.J 2, N° 1 Y 2, 1993.

Table 1. Hydrogen content detennined by Elastic Recoil Detection(1) Films deposited with fixed VB, at total pressures indicated.(II) Films deposited at 8 Pa total pressure and 25% N in plasma.

(1) (II)%N2in Hydrogen content self-bias Hydrogenplasma (%) (V) content (%)

(0.8 Pa) (8 Pa)O 15.3 12.8 -200 -

12.5 13.7 15.8 -370 13.725.0 12.0 13.7 -6()() 17.1337.5 - 12.0 -800 16.650.0 - 16.6 -95() 16.3

25~-------------------------'o¡:: 20c:t:a::

<.)~ 15o1-<{

-1z+(.)

(Q. ) 0·8 Po

e Po

V :-37~ V

o~~~.~~~-T~~.B~~~0.00 12.50 25.00 37.50 50.00

o/.;NITROGEN 111I PLASMA

(a)

2~--------------------------~o¡:: 20<{

cr::

u~ 15o~el:

;- 10

+o

( b )

8 Pa'25 °/0 N

2

O~~~~~~~~~'-rT~~-Mo 200 400 600

SELF - BIAS ( - v )000 1000

(b)

Figure 2. Nitrogen content as a function of (a) N2 partial pressure, and of (b) self-bias.(Other parameters as in fig. 1).

lnfrared spectra of .films obtained at 8 Patotal pressure, with a -370 V self bias, for N2partial pressure of O and 37.5% N2 are shown infigure 3. These spectra are representative of thewhole range of deposition parameters studied. Ascan be seen in the figure, absorption bands aresuperimposed to an interference pattern originatedfrom reflection at the interfaces. The main featuresof the pure a-e: H film spectrum are the C-Hstretching (2920 cm-1) andbending (1450 cm!)modes. This spectrum also displays a small band

at 1600 crrr Í, which can be assigned to de C=Cstretching mode. The main effects of nitrogenincorporation, as shown by the a-C, N: H filmspectrum, arethe appearance of the NH stretchingband at 3370 crrr+, the reduction of the CHstretching band intensity, and the growth of astrong band at 1575 crrr+, which has beenassigned to aromatic C=C stretching mode [8]. Inreference [8] this band was correlated to aninfrared observation 01' Raman vibrational modescharacteristic of graphitic micro-domains, which

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Revista Latinoamericana de Metalurgia y Materiales. Vo1.l2. N° 1 Y 2. 1993. 5 9

o

J

I450cml _1

.1575cm

-1f3 70 cm

wüz«OJcro(f)

OJ« 8 Pa

- 370 V37.5 % N

2

1300 1800 2300 2800 3300 3800

W ( cm 1)

2920 cml

¡

woz«alcroVlID« 8 Po

- 370 Vo D/D NZ

1300 1800 2300 2800 3300 3800

W (e ni 1 )

(a) (b)Figure 3. Infrared spcctra 01' Iilms dcpositcd with O and 37.5 %N2 partial pressures.

(Other parameters indicated in figure ).

was rnade possible by the symrnetry hrcakingdiffraction patterns taken Irom filrns obtained at 8Pa total pressure, with -:no V self-bias andvarying N2 concentrution, showcd only dilf'uxebands charucteristic 01' arnorphous solids, withvery small changes upon nitrogen incorporation.

CONCLUSIONS

In conclusión, results reponed above areindicative that the N2 partial pressure is the majorfactor affecting nitrogen incorporation into a-C: Hfilms deposited by self-hias glow-discharge.Variation of N2 partial pressure from O to 50%lead to nitrogen content variations ranging from Oto 20% and from O to 13% for 0.8 and 8 Pa totalpressures respectively, whereas no variation wasdetected for self-bias variation from -200 to -950V. The infrared spectra of the films show thatnitrogen is being effectively incorporated into theamorphous network. It seems that carbon atornslocated at different bonding environments arebeing replaced by nitrogen, as it is evidenciated bythe intensity reduction of the C-H stretching bandsand by the growth of the N-H and C=C stretchingbands.

Work on the determination 01' themechanical properties and on Raman and Auger

spectroscopy analysis of the films studied in thiswork is now in course.

REFERENCES

1. Angus, J.c. and Hayman, c.c.- Science 241(1989),913

2. Zou, J.W. et al - J. Appl. Phys .. n2 (1989),3914

3. Zou, J.W. et al - J. Appl. Phys. ti (1990),487

4. Tamor, M.A. et al - Appl. Phys.Lett. ~(1989), 123

5. Varhue et al - J. Apply. Phys. fil... (1990), 4590

6. Kalish, R. et al - J. Appl. Phys. A.5.1 (1991),48

7. Han, H. -X. and .Feldrnan, B.J. - Salid StateComm . .62 (1988), 921 .

8. Kaufman, J.H. et al - Phys. Rev. B 32 (1989),13053

Latinémerican Journal of Metallursy al/ti Mnterials, Vol. 12. N° 1, 2. 1993.