Thin carbon film deposition using energetic ions of a dense plasma focus

17 February 1997

PHYSICS

ELSEVIER

LETTERS

A

Physics Letters A 226 ( 1997) 2 I 2-2 16

Thin carbon film deposition using energetic ions of a dense plasma focus Chhaya R. Kant, M.P. Srivastava, R.S. Rawat Department

ofPhysics

and Astrophysics, University

ofDelhi,

’

Delhi II 0 007, India

Received 13 March 1996;revised manuscript received 27 August 1996; accepted for publication 23 October 1996 Communicated by M. Porkolab

Abstract

We report the first ever application of the dense plasma focus (DPF) device for thin film deposition. Thin films of carbon have been deposited on glass, silicon and quartz substrates by ablation of the graphite target using highly energetic argon ions of DPF. These films have been analysed for their surface profiles, structure and chemical composition using a surface profilometer, X-ray diffractometer (XRD), Raman spectrometer, and electron spectroscopic composition analyser (ESCA), respectively. PACS: 81.15; 52.75: 68.55. Keywords: Dense plasma focus;

Carbon

The dense plasma focus [ 1,2] is a simple device that makes use of a self-generated magnetic field, for compressing the plasma to very high densities (5 1025-102” rnm3> and high temperatures (l-2 keV). The DPF has been established as a source of highly energetic ions having energies in the range of 25 keV to 8 MeV [3,4], fusion neutrons, X-rays and relativistic electrons. Being such a versatile source, DPF has been found useful for a wide range of applications. The neutrons of DPF have been successfully applied for pulsed activation analysis 151. The DPF has also been used as a spectroscopic source for production of highly ionised species [61, as a pump source for lasers [7], as a high flux X-ray

’ Department of Physics and Electronics, SGTB Khalsa College, University of Delhi, Delhi 110007, India 0375~9601/97/$17.00 PiiSO375-9601(96)00916-4

source for lithography [8] and as highly energetic ion source for processing of materials in the form of thin films [9,10]. In this Letter, we establish carbon thin film deposition on various substrates like silicon, quartz and glass, making use of the impact of energetic argon ions of the DPF device. The preparation of carbon films has recently received considerable attention due to the wide range of its applications. Thin amorphous carbon films are being used as an overcoat to protect the magnetic layers from the abrasive action of the reading head [l 11.Crystalline, highly conducting graphite films have a number of technological applications [ 121. Earlier, thin carbon films have been prepared by various continuous deposition techniques such as rf sputtering [ 11I, ionised deposition [13], chemical vapour deposition 1141, cathodic arc evaporation [15] etc., as well as pulsed systems using laser [16]. A feature common to these

Copyright 0 1997 Elsevier Science B.V. All rights reserved.

C.R. Kant et al./Physic.v Lettrrs A 226 (1997) 212-216

techniques is that the vaporised carbon species arriving at the substrate have signific~tly high energies. The pulsed techniques such as plasma laser deposition [16] have gained importance because of their ability to yield higher deposition rates and deposition of materials having high melting temperatures as compared to the continuous deposition techniques. The DPF device is also a pulsed device having highly energetic ions with high fluence and shares some common features with pulsed laser deposition. In view of this we have used the DPF device to deposit thin films. In the present work, we have used energetic argon ions emitted in each shot from the 3.3 kJ Mather type [9] DPF device, to deposit thin carbon films in a multiple shots experiment. The ions have a pulse length of about 8-10 ns (FWHM), with energies in the range of 25 keV to 8 MeV, The ions are emitted preferably along the axis of the anode in the shape of a fountain spreading outward. The schematic of the DPF device used for thin film deposition is shown in Fig. 1. A perspex box, housing the target and the substrate holder, was inserted by means of a brass cylindrical rod from the top plate of the plasma chamber. The graphite target was stuck to the target holder whose mounting surface was such that the graphite target made an angle of 45” with respect to the anode axis. The whole perspex box could be

PERSPEX

BOX

SUBSTRATE TARGET

FOCUS

CHAMfiER-

Fig. 1. Schematic drawing of the experimental setup. (LVSCR: low voltage SCR; HVSCR: high voltage SCR; HVPT: high voltage pulse transformerI.

213

moved along the anode axis so that the graphite target could be adjusted at different distances from the top of the anode. Films have been prepared by means of multiple shots. During each shot, the filling argon gas pressure and the charging voltage of the capacitor were kept at 80 Pa and 14 kV, respectively, so as to have good focusing. A shutter was provided between the anode and the box to avoid striking of ions on the graphite target, when no proper focusing was obtained. The shutter was removed after we obtained a sharp peak in the voltage probe signal, as observed on the oscilloscope indicating strong focusing. The deposition of the films was carried out for two different orientations of the substrate with respect to the target. In the first orientation, the substrate was placed parallel to the axis of the anode and at a distance of 1 cm opposite to the center of the graphite target as is shown in the inset of Fig. 1. In the second orientation the substrate was placed parallel to the target making an angle of 45” with the anode axis at a distance of 1 cm from the center of the graphite target. The latter orientation is shown in Fig. 1. We deposited thin films of carbon on silicon, quartz and glass substrates keeping the the target at different distances (like 1.3, 2.3 and 3.3 cm> above the anode and also varying the number of shots (like it = IO, 20 and 30). Visual inspection of the film shows a clearly distinct upper portion having a transparent, uniform brownish thin film on the substrate and a lower portion having a greyish hazy patch due to the deposition of macrop~icles. Lossy et al. [IS] have used cathodic arc evaporation for deposition of hydrogen free amorphous carbon films where they have filtered the macroparticles by a magnetic method In our work, since a thin film is formed in the upper portion of the substrate we have characterized that upper uniform portion of the film. The surface profiles of the films deposited on silicon have been studied using a Dektak profilometer. The thickness
214

C.R. Kant et al./ Physics Letters A 226 (1997) 212-216

substrate at a target distance of 1.3 cm is shown in Fig. 2. The roughness average of the film was 5.3 nm. The average surface roughness of the film across the substrate varied up to 20 nm. The X-ray diffraction spectra have been obtained on a Phillips PW- 1840 diffractometer. The diffraction spectra of the films deposited on film substrates at target distances of 1.3, 2.3 and 3.3 cm from the top of the anode are depicted in Figs. 3a, 3b and 3c, respectively. In Figs. 3a and 3b, the two sharp peaks at 2 8 = 26.6” and 59.8” correspond to graphite interlayer distances along the c-direction d(002) of 0.335 nm and d(103) of 0.1545 nm, respectively. Both these peaks are highly symmetrical and very sharp with full width at half maximum (FWHM) = 0.2”. The sharpness of the peaks indicates a highly textured structure of graphite in the deposited film. The highly symmetrical shape of the peak suggests the existence of hexagonal ABAB . . . stacking of graphite sheets as has also been inferred by Ata et al. [ 171. The XRD peaks correspond to reflection due to the planes with Miller indices of [(hkl), l# 01 type i.e. (002), (103) planes having 1# 0. The presence of such a type of diffraction planes [(hkl), 1 # 01 suggests that successive layers of (001) carbon sheets are stacked parallel to the substrate without rotational or translational shift to form almost perfect crystallites [14]. It can be seen from Fig. 3c that there is no peak at 28 = 26.6”. The XRD spectrum of the film deposited on quartz substrate at a target distance of 1.3 cm above the anode is shown in fig. 3d.

Fig distance of 1.3 cm above the anode.

24

30

36 42 48 29 (DEGREES)

54

60

Fig. 3. XRD patterns for the films deposited on silicon substrates when the target is at (a) Z = 1.3 cm (b) Z = 2.3 cm and (c) Z = 3.3 cm. (d) XRD pattern for quartz substrate at target distance Z= 1.3 cm.

59.8” corresponding to reflections due to the (0021 and (103) planes, respectively. The intensity of the peak at 2 0 = 26.6” for the film deposited on quartz substrate is almost four times greater as compared to the film deposited on silicon substrate. This suggests that the quartz is a good substrate for deposition of crystalline graphite films. The films deposited on glass substrates were found to be amorphous. The films deposited on silicon substrates were analyzed for chemical composition at the surface using a Shimadzu [ESCA]-750. A depth composition analysis of the film deposited at a target distance of 1.3 cm above the anode was done by in situ argtn ion etching of the film to depths of 70 A and 140 A. The quantitative results of the ESCA study are summarized in Table 1. The ESCA study of the film reveals the presence of carbon and oxygen. The presence of oxygen is due to the oxidation of carbon film on being exposed to atmosphere. A careful study of Table 1 reveals that the relative carbon concentration on the surface of the film decreases with the increasing target exposure height. This decrease is expected because the ion fluence reaching the target decreases, leading to lesser ablation of the graphite target. Figs. 4a, 4b and 4c depict the Cls

C.R. Kant et u~./Phys~~.~ Lerrers A 226 (19971212-216 Table

215

I

Surface and depth composition analysis of carbon films by ESCA Distance from the top of the anode Z (cm)

ESCA region of the film

Element

Peak height

Peak area

1.3

surface layer

Cls 01s

69.88 30.12

61.76 38.24

at a depth of 70 A from the surface

CIS

85.71

01s

14.29

76.8 1 23.19

atadepthof l4Ow from the surface

Cls 01s

87.58 12.42

80.00 20.00

surface layer

CiS

ois

59.12 40.88

54.17 45.83

ClS 01s

60.26 39.74

53.33 46.67

2.3 3.3

surface layer

ESCA spectra at different depths of the film deposited at a target distance of 1.3 cm above the anode. The binding energy of Cls was found to be 289.1 eV from Fig. 4a. The C 1s ESCA spectra show that the line shape of carbon is asymmetrical and has a broad energy loss feature of FWHM = 2 eV. This broad energy loss feature can be assigned to an

(al

!\: 6

1

292

6tNDiNG

III

288

ENERGY

ICI

1% 284 I eVf

:zao

Fig. 4. ESCA spectra for tire tiim deposited at Z= I .3 cm for carbon Is at (a) the surface, (b) a depth of 70 f and (c) a depth of 140 A.

interband transition involving the n states mat is excited by some photoemitted electrons 1181. The observed asymme~y of the line shape of C 1s can be at~ibuted to various factors. Graphite has layers stacked together roughly parallel and equidistant in which the carbon atoms are trigonally coordinated as sp* hybrids and the rr electrons are delocalised throughout the layer structure. The delocalisation of the rr electrons leads to an asymmetric line shape. The tail in the Cls line shape towards the higher binding energy side is also a signature of sp2 hybridised carbon. This was confined by the Raman spectrum ~Jasco-~45) which showed the peak corresponding to sp2 hybridised carbon. Table 1 shows that oxygen is present in a fairly large amount. The oxygen present may be in the form of physisorbed species as well in the form of functional groups like -C-O-, >C=O etc. These functional groups lead to a skew line shape in Cls ESCA spectra. The carbons in these functional groups have higher binding energies than those which bond to other carbon atoms [I91 which justifies the observation of the binding energy value of Cls at 289.1 eV. A considerable decrease in oxygen content was seen from the data of Table 1 as the sample was etched to greater depths. This decrease in the oxygen content led to the lowering of the binding energy of C 1s which can be seen in Figs. 4a, 4b and 4c in the form of a chemical shift towards the lower enkrgy side as the sample was etched to depths of 70 A and 140 A.

216

C.R. Kunt et d/Physics

In conclusion, we have established that the dense plasma focus device can be used to deposit thin films of carbon on different substrates. These films were found to be crystalline graphite films when deposited on silicon and quartz substrates but amorphous in nature when deposited on glass substrates. One of us (Chhaya R. Kant) thanks the University Grams Commission, India, for the award of a senior research fellowship. Professor S. Lee is gratefully acknowledged for providing the ICTP-UM collaboration for the fabrication of the DPF at Delhi University.

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Letters A 226 11997) 212-216

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