FAST PLASMA DEPOSITION OF CARBON AND SILICON LAYERS

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FAST PLASMA DEPOSITION OF CARBON AND SILICON LAYERS

J. Beulens, A. Buuron, L. Bisschops, T. Bisschops, A. Husken, G. Kroesen, G.

Meeusen, C. Timmermans, A. Wilbers, D. Schram

To cite this version:

J. Beulens, A. Buuron, L. Bisschops, T. Bisschops, A. Husken, et al.. FAST PLASMA DEPOSITION

OF CARBON AND SILICON LAYERS. Journal de Physique Colloques, 1990, 51 (C5), pp.C5-361-

C5-367. �10.1051/jphyscol:1990543�. �jpa-00230852�

FAST PLASMA DEPOSITION OF CARBON AND S I L I C O N LAYERS

J.J. BEULENS, A.J.M. BUURON, L.A. BISSCHOPS, T.H.J. BISSCHOPS,

A.B.M. HUSKEN, G.M.W. KROESEN, G.J. MEEUSEN, C. J. TIMMERMANS,

A.T.M. WILBERS and D.C. SCHRAM

Department of Physics, Eindhoven University of Technology. P.O. BOX

513, 5600 MB Eindhoven, The Netherlands

Resume-

W a d

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By separating plasma production and plasma deposition and by taking advantage of the high specific ionizing power of thermal plasmas, very high deposition rates on large areas of amorphous C-H and Si-H are obtained. The layers have been analyzed by several methods, among which ellipsometry (band gap, absorption in the visible C-H, Si-H bonding pes i n the infrared), nuclear techniques (hydrogen depth profile), ESCA (type), diffraction Crystall~nlty).

The plasma is produced at high pressure (0.1

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1 bar) in a cascaded arc in a carrier gas (argon). Downstream the feed gases (CH4 resp. SiH4) and additional gases (H,) are admixed to the mainstream. Nearly complete dissociation and charge transfer from ,argon to Si and C takes place whereas the plasma is accelerated to sonic velocities at the h~gh temperature (1 ev).

Since the early seventies many different discharge types have been explored for their applicability in the field of plasma surface modification in general and for plasma deposition in particular. Until about 1985 mainly RF or DC glow discharges were used as a plasma source for deposrt~on. In this work the high ionizing power of a cascaded arc is used for the production of ions and radicals to separate the plasma production and the material deposition spatially. The plasma is produced at high pressure (0.1

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1.0 bar) in a cascaded arc using argon as carrier gas. Down stream the feed gases

&H,

and SiH,, in the nozzle region, and additional gases, about half way the arc, are admixed to the argon main stream. Nearly complete dissociation and charge transfer to Si and C takes place where the plasma is accelerated to sonic velocities in the nozzle at temperatures of 1 eV. The arc plasma and the recombining expanding beam are studied by emission spectroscopy.

This gives us temperatures, electron densities and flow velocities. Also densities of some radicals can be obtained.

The deposition rate in this setup is determined by the gas flow rate, because of the directed beam towards the substrate, in stead of diffusion of radicals in case of RF discharges.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990543

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With this technique amorphous (a-C:H) and crystalline carbon (diamond and graphite) films have been grown [1,2]. By in situ ellipsometry the growth rate, and also the refractive index and thickness, are measured. For amorphous carbon films the growth rate can be up to 200 nm/s.

Amorphous silicon films have been grown recently at rates up to 20 nm/s.

Ex situ several techniques are used to determine film parameters. Spectroscopic ellipsometry, in the wavelength region from 250 nm to 10 pm, provides us the band gap, absorption bands (bond types) and the refractive index. Furthermore techniques like ESCA (type), RBS (hydrogen content and/or depth profile), RAMAN scattering (crystallinity) and diffraction (LEED, crystallinity) are used to get additional information.

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EXPERIMENTAL SET UP Deposition reactor

The deposition system (fig. 1) in this work consists of a cascaded arc in which a DC gas discharge in argon is produced at currents of 10-60 A [l]. This thermal plasma reaches temperatures between 8000 and 13000 ^K. The thermal plasma is allowed to expand supersonically, through a hole (nozzle) in the anode into a vessel at pressures of typically 1 mbar. The cascaded arc consists basically of an anode, a stack of ten insulated copper plates and three cathodes. These components are all water cooled. The cathodes are 1 mm diameter tungsten thorium tips for currents up to 30 ^A per cathode. The cascade plates have an inner bore of 4 mm and a thickness of 5 mm. So ten stacked plates make up a plasma channel of 6 cm length (lmm insulation per plate). Hydrocarbons (or silanes) can be injected just before, inside or just outside a nozzle which is pressed into the anode plate..The pressure inside the vessel depends on the gas flow and the pumping speed. This pumping speed can tuned between 10 11s and 700 I/sec with a stepper motor controlled valve. The sample support can be moved so that the nozzle sample distance can be varied between 2 cm and 80 cm. In table I some typical settings are given.

monomer 1

J.

pumping system

0

Fig. 1 Outline of the reactor for plasma deposition used in these experiments. The plasma is generated in the cascaded arc and transported towards the substrate by means of a supersonic expansion and a subsonic plasma beam. The monomers can be injected in the arc or through a ring downstream the expansion.

arc pressure (bar) 0.1-1

gas flows (sl/min) Ar 0.5-10

H2 0-10

%H 0-1

5% in Ar 0-1

system pressure (mbar) 0.1-130

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CARBON DEPOSITION

By varying plasma parameters (including vessel pressure, see table I) it is in principle possible to grow every kind of carbon layer, symbolized by the triangle formed by the extremes: diamond, graphite and polymers. The material in between is called amorphous hydrogenated carbon (a-C:H, see fig. ^2). The corners of the triangle are formed by material with a well defined structure either crystalline (diamond, graphite) or regular cross links between molecules.

diamond

graphite polymers

Fig. 2 Schematic representation of the possible types of carbon films produced by plasma deposition with the set up of fig. 1. The Q, defined as the carbon flow rate (the total monomer flow rate multiplied by the number of carbon atoms per monomer molecule devided by the argon flow rate and the arc power, denotes the relation of the film quality to the arc plasma parameters. The symbol H is the hydrogen content of the produced film. The Q and the H decrease and increase respectively in the direction of the arrows.

W : H films

The amorphous carbon films are produced using only argon (20-400 scc/s) and a hydrocarbon (0-8 scc/s) in the setup. The substrate temperature was always between room temperature and 100 OC (due to water cooling), and the vessel pressure about 1 mbar. The nozzle sample d~stance was about 70 cm.

Diraction (Low Energy Electron Diffraction, LEED) and SEM photo raphy do not show any crystalline structures in these amorphous films. RAMAN spectra s%ow very broad phonon absorption bands which are characteristic for amorphous non crystalline materials. The main film

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diagnostic tool used in our work is ellipsometry [3,4]. With a He-Ne ellipsometer in situ measurements of the refractive index (n,k) and the film thickness can be performed. In fig. 3 the refractive index of the amorphous films is shown as a function of the 'energy' coefficient Q (W-') defined by:

Q = [Cf l o w 1

ml,wT-FEz

in which [ G o w ] and [Arflow] stand for the carbon and argon flow rates respectively, and [p,,] is the arc power in watts. The quantity Q-' can be seen as the available power, transported by argon, per injected carbon atom.

Ex situ these films are studied by spectroscopic ellipsometry [5]. This apparatus is in principle equal to the He-Ne ellipsometer with this respect that here a cascaded arc is used as a light source [6], with a high spectral intensity in a small angle in a very broad wavelength region, and of course a different detection system. The detection now takes place by a monochromator plus a photon muftiplier (250-800 nm) or a InSbjHgCdTe sandwich

1R

detector (2-8.5 hm). With this setup the optical band gap, which can be determined from a Tauc plot, and the C-H absorption bands can be measured. The band gap is also plotted vs the factor Q in fig. 3. A typical measurement of the absorption bands is given in fig.4. In this figure the plasma parameters were so that the factor Q was small @ 2 10'6 W-'), which im lies a soft polymer like a-C:H film. The absorption peak at 1450 _cm-' may refer to sp2 CH, or to sp CH,. As also

P

a peak is found at 2946 cm-' (sp2 CH,) and not at 2920 cm-' (sp3 CH,) the conclusion is that sp2 CH, is one of the revalent binding forms. The

P

peaks at 1370 cm-' and 2875 cm-' can be attributed to sp3 CH and sp CH3. respectively [7].

Wih Rutherford Backscattering the hydrogen content and or depth prof~le can be measured. A detailed study still has to be done, but some tests reveal that for polymer like films the H content is about 60 at% (Q <4 10-6 W-') whereas the diamond like coatings (Q > 10-5 W-') show a content of 20-30 at%. Also the mechanical property hardness pickers) can be plotted vs Q in fig. 3 whlch gives a simple relation between the optical and mechanical properties.

Energy coefficient (10-6 W - l )

Fig. 3 Plot of some film parameters vs the energy factor Q (W-'). m: Refractive index obtained by in situ He-Ne ellipsometry. A : he optical bandgap (eV) obtained by ex situ spectroscopic ellipsometry. +? Vickers hardness (GPa).

vs wave number U (cm-'). The parameter @ is strongly coupled with the absorption coefficient of the film.

Diamond

With the same experimental setup it is also possible to grow diamond films [8]. The parameters are quite different from the ones for amorphous films. Concerning substrate temperature, pressure and h drogen/carbon ratio however there is no difference with other deposition techniques like RF pkmas, hot filament or microwave plasmas. In this work a mixture of 1% methane in hydrogen was used with argon used as carrier gas. (Ar:H,:CH4=100:100:1). The pressure in the substrate chamber must be (in our case) larger than about 45 mbar, and the substrate temperature, monitored by a pyrometer, was between 850 and 1000 OC. The nozzle to substrate distance in this setup was between 2 and 10 cm. Diamond was deposited on silicon wafers (deposited area about 4 cm2). The diamond quality was examined by SEM photography and (micro) RAMAN. Without special pretreatment of the substrate surfaces faceted individual crystals of 15-25 km, maximum up to 65 p, were deposited after 1 hour. h the center of the deposit (center of the beam) the particles are well faceted while more to the edge the particles are less and less faceted and end up in ball shaped structures. RAMAN spectra of these particles reveal that the center particles are 'pure' diamond and more to the edge graphitic and amorphous particles are formed. After scratching the substrate surfaces with diamond powder (1 pm particle size) continuous diamond films were grown.

Also for the films the morphology is changing from diamond in the center to a mixture of diamond, graphite and amorphous carbon at the edge. This change in morphology can be caused by two parameters: substrate temperature and plasma composition. The substrate temperature decreases towards the edge because the substrate is heated by the plasma itself, and the particle densities also change towards the edge which can lead to another deposited material anyway. As an example also the pressure dependence to the morphology is given in fig. 5. In this figure one can see that when the pressure is decreased the graphitic layers are formed. The 1 mbar setting

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therefore is the starting point for producing graphite layers.

Fig. 5 Cascaded arc plasma deposition at different pressure levels. In a series of experiments all other deposition parameters were kept constant, while the pressure in the reactor vessel has been varied. Drastically different materials are deposited. At 1 mbar, only graphite material can detected by means of Raman spectroscopy. At 22 mbar the nanocrystalline diamond or diamond precursor peaks at 1150 cm-1 and at 1470 cm-' become clearly distinctable. At 34 mbar a narrow diamond peak is already detected and at 45 mbar the common Raman spectrum of a continuous diamond coating (with some impurities) is measured.

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[arb.units] ^Intensity

I

downstream the nozzle. The samples are supplied to the vacuum vessel through a load lock system which prevents the vessel of being polluted by other gases than argon silane or hydrogen.

We used four different types of substrates: gold coated stainless steel, gold coated quartz, quartz and silicon substrates. The substrate temperature has been varied between 100% and 300%. The plasma settings were in both cases the same: 3 scc/s SiH4/Ar mixture in 70 scc/s argon. We have not varied the plasma settings until now. The silane content will be increased as well as additional hydrogen in further experiments planned this spring.

Until now we used spectroscopic infrared ellipsometry to investigate the gold coated samples. The other samples will be examined with infrared transmission experiments and resistance measurements. We have detected polysilane chains: [(SiH,),], SiH, and SiH bonds depending on the substrate temperature 191. With a temperature of roughly 25@C we have detected the SiH bond accompanied by a SiH, bond of the same strength. Compared to the other samples the concentration of these bonds is relative low (a factor 10 lower). Also Auger profile measurements have been performed on the gold coated samples. This experiment shows that the gold silicon boundary is smaller than 10 nm. Also a constant concentration of oxygen 2% throughout the layers ( 6 F nm) produced at lower temperatures has been measured. The refractive index of the films is est~mated to be 1.70 a0.05 (between 3 and 5 pm) for the samples produced at temperatures below 150%. This value lies between those for a-Si:H (3.68, ref. 10) and amorphous SiOz (1.45 ref. 11).

The samples produced at higher temperatures have a refractive index of 3.68. The highest deposition rate achieved until now is approximately 10 nm/s, which is about 30 times as high as for 'conventional' RF plasma and CVD methods.

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REFERENCES

1 G.M.W. Kroesen, Plasma deposition, 'investigations on a new approach', Thesis, Eindhoven University of Technology (1988).

2 J.J. Beulens, P.C.N. Crouzen, G.M.W. Kroesen, H. Vasmel, C.B. Beijer, D.C. Schram, H.J.A.

Schuurmans, C.J. Timmermans and J. Werner, Proceedings of the Symposium on Plasma Polymenation and Plasma Interactions with Polymeric Materials, USA.

3 G.M.W. Kroesen, internal report VDF-NT/87-02, Eindhoven University of Technology, Dept. of Physics, the Netherlands (1987).

4 R.M.A. Azzam and N.M. Bashara, Ellipsometry and polarized light, North Holland, Amsterdam, the Netherlands (1977).

5 M.S. de Wit , internal report, VDF-NT/89-07, Eindhoven University of Technology, Dept. of Physics, the Netherlands (1989).

A.T.M. Wilbers, G.M.W. Kroesen, C.J. Timmermans and D.C. Schram, to be published in JQSRT.

7 B. Dischler, A. Bubenzer and P. Koidl, J. Appl. Phys. 54, 4590, 1983.

8 P.K. Bachmann, H. Lydtin, D.U. Wiechert, J.J. Beulens, G.M.W. Kroesen and D.C. Schram, Proceedings of the Third International Conference of Surface Modification Technologies, Neuchatel, Switzerland (1989).

9 J. Mort and F. Jansen, Plasma deposited thin films, CRC press Inc. 1986.

10 The Physics of Hydrogenated Amorphous Silicon II; Electronic and Vibrational Properties, J.D.

Joannopoulos and G. Lucovsky, Springer-Verlag (1984).

11 Handbook of optical Constants, Edward D. Palik, naval research laboratory, Washington DC, Academic Press.