DEPOSITION OF Al2O3 ON CERAMIC SUBSTRATESBY PECVD METHOD

Lucie Špirková aVlastimil Brožek a

Jean Durand b

a) Institute of Chemical Technology, 166 28 Prague, Czech Republicb) Laboratoire des Matériaux et Procédés Membranaires (CNRS UMR 9987), Ecole National Supérieure de Chimie, 340 96 Montpellier, France

AbstractLow temperature deposition of aluminum oxide films using a plasma enhanced metal-

organic chemical vapor deposition technique is described. The plasma was generated by a 40kHzRF. A single molecular source reagent trimethylaluminum, Al(CH3)3 was carry away into thereacting chamber by He-carrier gas. The films were deposited over the temperature range 150-450°C. Films were deposited on silicon wafers and porous ceramic plates. The thickness,structure and surface morphology were observed using scanning electron microscopy.

1 INTRODUCTION

Aluminum oxide offers high radiation resistance [1], high thermal conductivity and verylow permeability of alkali ions and other impurities [2]. The chemical stability against almost allknown media makes it nearly ideal material for wear-protective coating on cutting tools for non-aluminum workpieces. Thin films-layers of Al2O3 have been extensively used in optical andelectronic industries, as protective films for metal reflectors, laser mirrors, masks againstimpurities, and as passivation layers in metal/oxide/semiconductor devices [3]. Aluminum oxidefilms on porous ceramic substrates can find an application as a top separating layers inmembranes for ultrafiltration.

In such applications, aluminum oxide films have been prepared by means of chemicalvapor deposition (CVD) and physical vapor deposition (PVD) methods [4-6]. High-temperatureCVD processes often cause unwanted thermal strain and diffusion problems. PVD processes,such as evaporation or sputtering, do not provide good coverage of surface steps, and aretherefore not suitable for multilevel devices having complicated surfaces morphology. Aconsiderable interest has been taken in low-temperature deposition process having good stepcoverage. Plasma-enhanced chemical vapor deposition (PECVD) process satisfies the aboverequirements, although it has some limitations [7].

Recently several researches adopted the low-temperature PECVD method for thepreparation of aluminum oxide films using trimethylaluminum (TMA) source [8-10].

In the present work, aluminum oxide films were deposited on two types of supports:commercial silicon wafers and ceramic plates prepared by thermal spraying of gray aluminaAl2O3/ 3%wt TiO2.

2 EXPERIMENTAL

2.1 Apparatus

The apparatus for the plasma-enhanced chemical vapor deposition of aluminum oxide isshown in Fig.1.

Fig.1 Scheme of the reacting apparatus

It consists of a reaction tube chamber made of glass, an RF. generator (40 kHz),a pumping system, and a gas manifold. The reactor is furnished by two aluminum electrodeplates. In contrast to high frequency discharge (13.56, 40 or 80 MHz) in previous works [7,8],the excitation frequency of 40 kHz produces plasma with a small potential drop. This frequencycorresponds to a higher growth rate of the deposited layer. The support holder, aluminum disk,was located in the middle of the reactor, about 2 cm under the bottom of the electrodes. Duringthe deposition, the temperature of the substrate was maintained with a thermocouple in the rangeof 150-400 °C. The apparatus is evacuated with a rotary oil pump, the pressure in the reactor ismonitored with a Pirani gauge. The precursor trimethylaluminum TMA was kept in the reservoirin an inert atmosphere at the temperature range 20-35°C, in order to avoid condensation there.The gas flow is regulated with needle valves. N2O was used as plasma ionization gas. Theprecursor is introduced into the reactor in a carrier gas-He stream. The pressure conditions were

������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������

Pirani gauge

������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������

������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������

���������������������������������������������������

rotary pump

��������������������������������������������������������������������������������������������������������

precursorreservoir

He N2Oheating andtemperaturecontroller

aluminumelectrodes

sample holder

samplesupport

40 kHzgenerator

gas manifold

40 kHzgenerator TMA

chosen as follows: p(He,TMA) = 0.1 mbar, p(He,N2O,TMA) = 0.5 mbar. The deposition timewas 60-120 min and the thickness of the layers varied from 100 nm to 2 µm.

2.2 Substrates

Silicon wafers (30x10x0.7 mm) and alumina ceramic substrates (30x10x1.5 mm) wereused. Commercial silicon is a readily available material which has a smooth surface and, owingto its infrared transparency, allows additional characterization of the deposited films. For somecomparisons, rough alumina ceramic substrates prepared by thermal spraying of gray alumina(Al2O3/ 3%wt TiO2) powder (feedstock size in the range 40 -70 µm), were used. The porosityof the second type of substrates varies from 7–12 %, and the mean pore size is about 10 µm.

2.3 Characterization of the deposited films

For the SEM analysis, samples about 5x5 mm were prepared and sputtered with Pt. Thesamples were characterized using scanning electron microscopy (SEM) to determine themorphology, thickness and grow rate of the layers. The chemical composition and carboncontent were measured with an energy dispersion X-rays spectrometer (EDX). The layers werenot sufficiently thick to determine the crystal structure by X-ray diffraction.

3 RESULTS

The chemical composition of the deposited samples is shown in Fig. 2. EDX analysis provedthat the samples contain no carbon. It can be, therefore, assumed a complete decomposition oforganic precursor in the plasma medium.

Fig.2. EDX analysis of Al2O3 film on ceramic substrate

On the silicon substrates, smooth and uniform aluminum oxide films were deposited. Fordeposition on substrates at the low temperature (100-150°C), compact films with a grow rateof maximum 20 nm min-1 were obtained. The free surface is covered with a ball-shapedstructure, growing out of the 0.4 -1.2 µm at the surface (Fig.3). After cooling down, a crackingoccurs and the layer starts to detach from the substrate. The higher temperature of the substrate,the larger the grains of the deposits.

Fig.3 The free surface covered with a ball-shaped structure. After cooling down, a cracking occurs and the layer starts to detach from the substrate.

Fig.4 For the substrate temperature250°C,

columnar structure with no cracks.

Fig.5 Structural features of 1,5 µm in diameter with a different shapes are on free surface of ceramic substrate.

Fig.6 Layer formation on the surfaceoverlaps existing narrow pores

For the substrate temperature between 250-300°C, the structure becomes columnar, nocracks appear after cooling down, and films are more adherent (Fig.4). There is a strongdecrease of the grow rate for the substrate temperature over 350°C.

In the second case of ceramic substrate, the influence of the porous, roughceramic surface on the film formation can be observed. The surface is, however, entirelycovered by the deposits, but structural features of some 0.5-2 µm in diameter witha different shapes are on free surface. It can be supposed, that in these places, the

nucleation and grain growth were preferential, due to the presence of impurities (fence),in the feedstock gray alumina powder (Fig.5). The nucleation starts on an active centersin the form of islets, which gradually enlarge and when they reach up to 100 µm, theystart to coalescent with each other. Fig.6 shows that a layer formation on the surfaceoverlaps existing narrow pores (in our case the pore width is 1 µm).

4 CONCLUSION

Homogenous aluminum oxide films were deposited at by the plasma enhancedchemical vapor deposition. The temperature of the substrate dominates the growth rateand the grain size of deposits. The morphology observations indicate that to obtainadhesive, granulated compact films, at the frequency of RF 40 kHz, the substratetemperature should be between 250 - 300°C. Impurities on the ceramic surface probablylead to the formation of deposits with a large roughness.

5 REFERENCES

[1] Zaininger K.H., Waxman A.S., IEEE Trans. Electron Devices, ED-16, 1969, p.333[2] Abbott D.A. and Kamins T.I., Solid State Electron, 13, 1970, s.565[3] Mansour S. Al-Robaee and Ghanashyam Krishna M., Subanna G.N., Narasimha Rao K. and Mohan S. Properties of Al2O3 films prepared by argon ion assisted

deposition, J.Mater. Res., Vol.9, No. 10, Oct 1994[4] Fournier J., DeSisto W., Brusasco R., Sosnowski M., Kershaw R., Baglio J., Dwight K., Wold A., Mater. Res. Bull., 23, 1988, s.31[5] Maruyama T., Nakai T., Appl. Phys. Lett. 58 (19), 1991, s.2079[6] Kumagai H., Toyoda K., Matsumoto M., Obara M., Jpn. J. Appl .Phys. 32, 1993, s.6137[7] Yong-Chun Kim, Hyung-Ho Park, John S. Chun and Won-Jong Lee,

Thin Solid Films, 237, 1994, s.57[8] Wang H.L., Lin C.H., Hon M.H. The dependence of hardness on the density of amorphous alumina thin films by PECVD, Thin Solid Films 310, 1997, s.260[9] Meiners L.G., Thin Solid Films, 113, 1984, 85-92[10] Catherine Y., Talebian A., J. Electron Mat., 17, 1970, 127