MOCVD of aluminium oxide films using aluminium P-diketonates as ...

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A. Devi, S.A. Shivashankar' and A.G. Samuelson'. Lehrstuhl fix Anorganische Chemie II, Organometallics and Materials Chemistry,. (?uhr-UniversitZit Bochum ...
J. Phys. IVFrance 12 (2002) 0 EDP Sciences,Les Ulis DOI.

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10.1051/~p420020088

MOCVD of aluminium as precursors A. Devi, S.A. Shivashankar’

oxide films using aluminium

P-diketonates

and A.G. Samuelson’

Lehrstuhl fix Anorganische Chemie II, Organometallics and Materials Chemistry, (?uhr-UniversitZit Bochum, 44780 Bochum, Germany Materials Research Centre, Indian institute of Science, Bangalore 560-012, India

Abstract. Deposltlon of AljO coatmgs by CVD IS of importance because they are often used as abradmg material m cemented carbtde cuttmg tools The conventionally used CVD process for A1203 mvolves the corroswe reactant AK13 In tbls paper, we report on the thermal charactensatlon of the metalorgamc precursors namely alurmmum tnstetramethyl-heptanedlonate [Al(thd),] and ahunmmm tns-acetylacetonate [Al(acac)J and then apphcatlon to the CVD of Al203 films. Crystallme A1203 films were deposited by MOCVD at low temperatures by the pyrolysis of Al(thd), and Al(acac), The films were deposlted on a TIN-coated tungsten carblde (TIN/WC) and SI( 100) substrates m the temperature range 500-l 1OO’C The as-deposlted films were characterised by x-ray dlffrachon, optIca nucroscopy, scannmg and transnusslon electron nucroscopy, Auger electron spectroscopy. The observed crystalluuty of films grown at low temperatures, their nucrostructure, and composltlon may be Interpreted m terms of a growth process that mvolves the meltmg of the metalorgamc precursor on the hot growth surface

1. INTRODUCTION

The deposition of aluminium oxtde films is of importance in various technological fields. For e.g., Its hardnessand high corroston reststancemake A1203 an attractive material for the passivationof metal surfaces[ 11.Sharpand appropnately shapedtools madeof hard matenalssuch ascementedcarbide(WCCo) are wrdely used in metal (stamlesssteel) forming m manufacturing industry, and in aluminium castingprocessesTo increasethe working hfe of thesetools andwork pteces,specialfunctional coatings on the carbide substratesare needed. The primary concern in desrgnmgthese coatmgs (single or multtlayers) IS then ability to meet the increasinglydemandingmetal-formmg requnements.Coatmgsof TIC, Ti(CN), TiN and A1203 are stacked on cemented carbide substratesto meet the requnements successfully[2]. Although then combinedthickness1sonly about 5-15 urn, thesecoatingsincreasethe wear reststanceand durability of the cementedcarbide tools by as much as a factor of ten. The A1203 coatmgsdepositedon cementedcarbidesubstratesgenerallyhave a columnarstructure and, dependmgon the CVD condtttons and on the ‘bonding layers’ that are initially depositedon the substrate,the coating may comprisepredommantlythe K- or the cx-phase[3]. CVD methodshave proven to be excellent processesfor hard coatmgsand offer the advantageof umform coating even of work-pieces of complicated geometry. However, thermal CVD methods for A1203coatingsuseAlCI3 asprecursor,and generallyrequire high deposittontemperatures(950-1200°C). This makesseveredemandson the depostttonsystemand processdue to reactor corrosionand hazardous by-products. Although plasma-assisted CVD can reduce the coatmg temperatureto 60&5OO”C, such a low depositiontemperatureresultsin inadequatemteracttonat the coatmg-substrateinterface, leadingto poor adhesionof the coating to the substrate.A lowering of the depositiontemperaturebelow 400°C by using the chlormatedprecursorsis however not practical, as the chlorme content m the layer increases

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rapidly with decreasing temperature and chlorine causes undesired side effects on the layer. The hazardous nature of chlorine can be avoided by using non-chlorinated metalorganic (MO) compounds as precursors. At the same time, deposition temperature can be lowered considerably due to the easy dissociation and the high reactivity of the metalorganic compounds. A low temperature process can also be reahzed by means of MO-PACVD, although the aforementioned problem of poor adhesion would probably arise. Some attention has been paid recently to the MOCVD of alumina films using MO precursors such as altmnnium-tri-isopropoxide (ATI), aluminium 2-ethylhexanoate, aluminium acetylacetonate [Al(acac)j] and alummium hexafluoroacetylacetonate [Al(hfac)x]. Alumina crystallises in several metastable allotropic modtfications in addition to the stable a-Al203 phase, usually referred to as corundum. In wear-resistant alumina coatings prepared by CVD by the “chloride process”, the most commonly occurrmg metastable modification is K-AlzOr. Even so, its structure has been reported only recently [4]. In addition to their thermodynamic differences, a-Al203 and ~-Al203 exhibit pronounced microstructural and morphological pifferences when produced by CVD. Even though these differences are probably reflected in the cutting performance of the coated tool bit, not much is reported in the literature on how the nucleation stage should be controlled to obtain the desired alumina phase. In a CVD process, we may expect the molecular structure of the precursor to affect strongly the deposition temperature, growth rate, as well as the microstructure of the as-deposited films. The main objective of our investigation ‘is to grow Al203 films from two different P-diketonate complexes, and to study their growth, elemental and phase composition, and microstructure as a function of various CVD process parameters i.e., substrate temperature, reactor pressure, and substrate material. There has been one report in the literature on the use of Al(thd)j as precursor for the CVD growth of alumma films on Si( 100) and glass substrates, wherein it was reported that amorphous alumina films were obtained [5] Here, we report the thermal characterisation of the metalorganic complex alumimum tristetramethyl-heptanedionate [Al(thd)3] and compare it with aluminium acetylacetonate [Al(acac)3]. The two complexes were then used as CVD precursors to grow Al203 films on cemented carbide substrates and Si(lO0).

2. EXPERIMENTAL The metalorgamc precursor Al(thd)s, and Al(acac)3 are crystalline complexes that sublime and were synthesised by modifying the procedures’reported in the literature [6]. Thermogravimetry (TG) and differential thermal analysts (DTA) were carried out using a simultaneous TG/DTA system (STA1500, Polymer Laboratories) at atmospheric pressure over the temperature range 25500°C with a heating rate of lO”C/mm, using - 5 mg of finely powdered sample in an argon atmosphere (99.995%, 20sccm). Depositions were carried out in a hot wall, horizontal flow quartz tube reactor using high purity argon as the carrier gas and oxygen as the reactant gas. The precursor was evaporated from a stainless steel vaporizer maintamed at 120-150°C and was fed into the reactor through heated lmes to prevent condensation A rotary pump was used to conduct depositions at reduced pressures. The total pressure of the reactor was measured by a capacitance manometer and controlled with a manual throttle valve. Films were deposited simultaneously on TiN-coated cemented carbide (TIN/WC) and Si(100) substrates, at different temperatures (400-I 100°C) and,(total) reactor pressures (2.5-100 Tot-r). Average film thickness was calculated from the increase m substrate weight, due to the film, measured on a semi-microbalance (resolution: 10 pg); the weight gam was of the order of 200-500 pg. (Film thickness was assumed to be uniform and film density assumed to be that of alumina). Films were characterised optical microscopy, XRD, SEM, TEM and Auger electron spectroscopy (AES).

EUROCVD 3. RESULTS

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AND DISCUSSION

3.1 Thermal analysis of Al(thd)s and Al(acac)x A detailed thermal analysis of Al(thd)x and Al(acac)3 was carried out to evaluate their suitability as CVD precursors. The comparative TGIDTA of Al(acach and Al(thdh is shown in Figure 1. The TG data of Al(acac)j and Al(thd)3, show a monotonic weight loss as a function of temperature. At about 250°C, the weight loss is complete in both cases. However, it can be seen that the temperature for the onset of weight loss is slightly higher in the case of Al(acach (-15O’C) compared to Al(thd)j (-140°C). There are no signatures of decomposition in the TG curves although, as metalorgamc complexes, both the precursors are expected to decompose below -350°C. In the DTA curves, there are sharp dips at 192°C and 216°C for Al(acac)3 and Al(thd)3, respectively, representing their melting points.

Figure

1: Simultaneous

TGiDTA

of (a) Al(acac)s

and (b) Al(tbd),

3.2 Thin film deposition and characterisation 3.2.1 Growth rates The growth of thm films by CVD IS strongly dependent on different process parameters. Of these, the most important is the growth (substrate) temperature, because surface reactions are temperatureactivated, and because reaction pathways can be altered by temperature. Therefore, the growth rate of

GO Figure reactor

2: Arrhemus

plots of the growth

rates of films deposlted

from (a) Al(tbd)3

(b)

and (b) Al(acac),

on TIN/WC

at a total

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alumma films deposited from Al(thd)s on TiN/WC and Si(lO0) substrates was measured as a function of substrate temperature, at a total reactor pressure of 10 Torr. It was necessary to use oxygen to pyrolyse the Al(thd)z that IS carbon-rich due to the presence of t-butyl groups and in order to obtain crystalline films Figure 2a shows the Arrhemus plot of growth rates of Al203 on TiN/WC from Al(thd)3. We see that the temperature dependence is typical of CVD reactions: m the (low) temperature range from 60&85O”C, with the reaction leading to film growth associated with an activation energy E, of (73 0+8) kJ/mol. The growth rate saturates in the temperature range 80&85O”C and the maximum growth rate obtained was 630 &min at 850°C and, above 900°C the growth rate drops gradually. A similar set of experiments was carried out using Al(acac)3 as precursor and the activation energy estimated, i.e , 35.6 kJ/mol (Figure 2b), is comparable to the activation energy reported by Maruyama et al. for A1203 deposttion on Si substrates [7], though it is significantly lower than for Al(thd)x. However, the growth rate obtained m this study was much higher (1200 A/mm) than reported by Maruyama. It should be noted that there was no need to use oxygen during film growth to obtain films compnsmg crystalline alumina, unhke m the case of Al(thdh.

3.2.2 Morphology of MOCVD-grown Al203 coatings As deposited, the coatings appear blackish and shiny. The surface morphology of the films deposited from the two precursors on TiN/WC and Si(100) was examined by optical microscopy (400X) and SEM. The top surfaces of the films are characterised by spherical, droplet-hke formations measuring a few micrometers m diameter. Such “spherulitic formations” in MOCVD-grown Al203 coatings have been reported previously [8]. Figure 3 shows the SEM micrographs of the Al203 coatings obtained from the two precursors at a growth temperature of 600°C. These features are formed at growth temperatures up to 9OO”C, hmtmg that the nucleation and growth of the films mvolves a process akin to melting. The SEM micrographs of “spheruhtes” in films grown at 900°C reveal “graininess” that suggest that crystallisation of alumma occurs within these droplet-like features.

(a) Figure

3:

SEM nncrographs

@‘I of films deposlted

on

TIN/WC at 600°C

from (a) Al(thd)s

and (b) Al(acac)J

3.2.3 Crystallinity of AlzOJ coatings The composition and crystallinity of the coatings obtained by the MOCVD process from the pdiketonates determines their mechanical properties and hence their utility. Electron microscopy was used to examine the microstructure of the films, The ring pattern in the selected area diffraction (SAD) data (Fig. 4b) reveals that the films are polycrystalhne although the growth temperature is only 600’CThe phase formed is K-&(&. Indeed, films grown at temperatures a!! low as 500°C exhibit noticeable crystallimty. The discontinuous ring pattern observed m the SAD indicates grain growth m a few preferred crystallographic directions. The cross-sectional TEM image shows clearly that tine columnar

EUROCVD

Figure

4: (a) Cross-sectlonal

TEM

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unage of A&O3 films grown from Al(acac)3 SAD pattern

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on SI( 100) at 600°C and (b) correspondmg

grains of alumma(-300-700 A) are present;the columnar gramsare generally parallel to the surfaceof the Si( 100) substrate.It is also clear from the TEM image that the alumina grams in the film are not necessarilyconnected,and are embeddedin an amorphousmatrix of density less than that of alumma. The films obtainedusingAl(thd)a andAl(acac)J at -6OO-85O’Ccomprisedprimarily of k-A1203.

3.2.4 Film composition The chemicalcompositionof A1203coatingsobtainedby MOCVD can be expectedto be different from those preparedby the ‘chloride process’.The incorporation of carbon into the coatings is likely, especially when low pyrolysu temperaturesare employed,and when no oxidismg ambientis used.This carbon content can affect the microstructure as well as the physical properties of the coatings. The elementalcompositionof the MOCVD-grown Al203 coatingswas therefore mvestigatedby quantitative Auger electron spectroscopy.Figure 5 showsthe AES atomic concentrationdepth profile asa function of sputtenngtime of a film grown from Al(thd)s on TiN/WC at 800°C. It is cIear from the AES data that, apart from the aluminium and oxygen in the film, carbon is alsopresentto a significant degree.It is seen that in the bulk of the film, 42 at.% of aluminiumand 30 at.% of oxygen are present.The carboncontent is about 13 at.%, and remainsalmostconstantthroughoutthe film, until the interface is reached.It is also seenthat Al is presentin a proportion m excessof that required for the formatton of A&03. It IS to be noted that someamount of interaction betweenthe Al203 coating and the underlying TiN/TiC substrate hastaken place. This is often an indication of the strengthof adhesionof the film to the substrate.As no Auger spectrumwas recordedm the mterfacial region, the chemicalbondingat the interface is not known.

SPUTTER

Figure

5: Auger

depth profile

of alurnma

film grown

on TIN/WC

TIMEWIN

from Al(thdX

at 800°C and 10 TOIT total reactor pressure

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AES depth profile of composition was also obtained for films deposttecl from Al(acac)j on TiN/WC substrate at 800°C (Figure 6). It is seen that, at the top surface of the film, the carbon content is high (65 at.%), but drops down to about 40 at.% and remains steady to a depth of about 1200 A. The presence of alummmm and oxygen with atomic concentrations of about 25 at. % and 30 at. % respectively, was also observed. It IS apparent that, in the absence of oxygen flow during the pyrolysis, the film contains a large amount of residual carbon.

SPUTTER

Figure

6: Auger

depth

profile

of ahmma

film grown

ori TNWC

TIME(MIN)

from Al(acac)3

at 800°C and 10 Torr total reactor pressure

Comparing the data obtained from the two precursors, it may be noted that, as oxygen was used in the depositton of Al203 from Al(thd)3, the carbon content of these films was lower because of the reaction between carbon and oxygen and the removal of the reaction products. On the other hand, Al203 was deposited from Al(acac)3 in the absence of any external source of oxygen. Hence, a large proportion of the carbon from the decomposition of the ligand gets mcorporated into the growing film. This results in a higher growth rate - nearly double the growth rate obtained from Al(thd)s under similar CVD conditions. 4. PROPOSED GROWTH MODEL The morphological features shown in the various micrographs may now be considered together with the AES data to understand the growth process in the MOCVD of aluminum oxide from Al(acac)3. Compositional and microscopic analyses show that the films contain significant proportions of carbon. The spherulittc surface features revealed by optical and scanning electron microscopy, as well as the density and mirror-like sheen of the films, suggest that a process akin to melting probably occurs during the pyrolyses of the precursor. Such melting might provtde a carbonaceous, Al-containing entity acting as the matrix in which the nucleation and growth of crystalline Al203 could take place, as in the flux growth method for crystals. To explain the formation of spherultic features (or “droplets”) on the substrates, a nucleation and growth model may be proposed as follows. Taking the observed spherulitic features into account and noting that the metalorgamc complex melts at -2OO’C, we hypothesise that these spheruhtes are formed due to the melting of the precursor or its fragments on the hot substrate surface. The sublimatton of the precursor produces vapours comprismg clusters of precursor molecules whose size depends on the pressure in the vapouriser, but may be expected to be of the order of micrometers As the clusters enter the CVD reactor, they (embedded in argon, the tamer gas) are approximately at the temperature of the heated precursor transport tubes connecting the vaporizer to the reactor, ie.,-15OT. Given that the reactor IS pumpedto maintain low pressureconditions(10 Torr), the velocity of the vapour streamwould be htgh, and its passageinto the hot zone of the reactorrapid The precursorclusters,therefore,experiencea rather

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sudden rise m temperature as they “land” on the hot substrate, situated about 30 cm from the edge of the silica reactor tube. This rapid heatmg process may be expected to lead to the melting of the precursor, or of fragment(s) of the decomposing precursor, on the substrate surface held at -800°C. A deposition process involving such melting can lead to the spherulitic features of the type observed. We have attempted experimentally to verify the validity of this hypothesis. To mimic the rapid heating of the precursor vapour, finely powdered Al(acac)3 was taken in a fused quartz boat and inserted rapidly into a preheated furnace at 600°C in open air, and held m the hot zone for about a minute. When the boat was being withdrawn from the furnace, it contained a hquid, Indicating that either the alumimum complex, or fragment(s) resulting from its decomposition, had melted. The melt solidified rapidly upon coohng. The surface of this rapidly frozen material has a dense, pore-free structure, as may be expected, and as verified by SEM analysis. This solidified material was subjected to elemental analysis as well as IR spectroscopy, which showed that it was identical in elemental compositton as well as in its IR spectrum to the Al(acac)j complex. It was therefore concluded that the complex melts congruently if tt is heated so rapidly that it does not sublime completely during this period. By contrast, during thermal analysis in the TGDTA apparatus, a small amount of the aluminium complex (-10 mg) is heated relatively slowly (10”Uminute). This leads to a complete sublimation of the precursor, and explains the absence of any evidence from such analysis for the congruent melting of the precursor. A similar congruent melting was found to occur when Al(thdh was heated rapidly. Based on these observattons, the CVD growth of alumina from Al(acac)s and Al(thd)s may now be understood. In the low-pressure CVD process, the precursor clusters are carried rapidly to the hot substrate surface, where they undergo congruent melting. The congruent melting of the precursor clusters provides a carbonaceous environment for the growth of aluminum oxide grains, even in an argon atmosphere, because the aluminium atoms in the precursor molecule are bonded directly to oxygen atoms. The growth process may therefore be compared to “flux growth”, which results m crystallisation at a lower temperature than that required for crystal growth from a melt of the matenal to be crystallised (such as the Czochralsh method). The congruent melting of the precursor gives rise also to the mirror-like sheen of the dense, pinhole-free deposit containing crystalhtes of alumina, which are very small because of the low growth temperature. Although congruent melting of the rapidly heated precursor was observed to occur m air in the present experiments, it seems reasonable to assume that similar melting takes place at low pressure m the CVD reactor. The model proposed here can explain the amorphous nature of the aluminium oxide films deposited previously by CVD using Al(acac)s and other P-diketonate complexes of alumimum as precursor [7]. These depositions were carried out at atmospheric pressure and would, therefore, have involved smaller clusters of the precursor than possible at a low pressure. Moreover, experimental CVD systems operating at atmospheric pressure do not necessarily involve pumping. If SO, a rapid transit of precursor clusters into the hot zone - necessary for the rapid heating of the precursor clusters - does not take place. This may mean that the growth of the film involves decomposed fragments of the alummium complex arriving at the substrate, and not its congruent melt. Furthermore, the crystallites of alumina present in the films formed by MOCVD at low substrate temperatures (-600°C) are small, as is apparent from this study, and would require transmission electron microscopy, rather than XRD, to be observed.

5. SUMMARY

AND CONCLUSIONS

The low pressure CVD of alumina films usmg alummium fl-diketonates as precursors has been studied. It is found that films comprising crystallites.of-tc-AlzOj can be deposited from both Al(acac)s and Al(thd), on Si(100) and TiN/WC at temperatures as low as 500°C. The crystallites grow in carbon-rich matrix formed by the melting of the precursor clusters when they are heated rapidly on the substrate surface. The observed surface morphology of the films and their composition support such a proposed mechanism for the MOCVD at low temperatures of films containing crystalline alumina.

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Acknowledgements The authors would like to thank Widia (Indta) Ltd. for funding the project. The AES analysis provided by D. Cahen (Weizmann Institute of Science, Israel) IS gratefully acknowledged. REFERENCES 1. V.A.C. Haanappel, H.D Van Corbach, T. Fransen, and P.J. Gellings, Thin Solid Films, 230(1993)138. 2. C. Colombier and B. Lux, J. Mater. Sci., 24( 1989)462. 3. M. Halvarsson, S. Vuorinen, and H Norden, Surf. Coat. Technol., 61(1993)177 4. B. Olhvter, R. Retoux, P. Lacorre, and G. Ferey, J. Mater. Chem., 7(1997)1049. 5. E. Ciliberto, I. Fragala, R. Rizza, G. Spoto, and GC. Allen, Appl. Phys. Lett. 67( 1995)1624. 6. E.W. Berg and N.M. Herrera, Anal. Chem Acta., 60( 1972) 117. 7. T. Maruyama and T. Nakai, Appl Phys. Lett., 58(1991)2079. 8. B. Lux, C. Colombier, and H Altena, and K. Stjernberg, Thin Solid Films, 138(1986)49. 9. AnJana Devi, Ph.D Thesis (1997) Indtan Institute of Science, Bangalore, India.