Thermodynamic analysis and deposition of refractory materials

208

Surfi,ce and Coatings Technology, 49 (1991) 208— 214

Thermodynamic analysis and deposition of refractory materials Claude Bernard Institut National Polytechnique de Grenoble, Ecole Nationale Supérieure d’Electrochimie el d’Electrométallurgie cle Grenoble (Unite de Recherche associCe au CNRS 29), UMS 818, Domaine Universitaire, B.P. 75, 38402 Saint-Martjn-d’Heres (France)

R. Madar Institut National Polytechnique dc Grenoble, Ecole Nationale Supérieure c/c Physique de Grenoble (Unité cie Recherche associCe au CNRS 1109), Domaine Unjversjtajre, B.P. 46, 38402 Saint-Martin-d’Hères (France)

Abstract This article does not pretend to be an exhaustive review of all publications in which a thermodynamic analysis has been used to analyse the chemical vapour deposition of refractory materials. It simply covers a certain number of experiments in which the present authors made use of various aspects of this method: overall or partial optimization of a process, determination of the deposition material most suited to fulfilling a given role and approach to mechanisms governing the chemical deposition reaction. A thermodynamic analysis is presented here from a more unusual angle, by examples of localized and varied intervention, in order to demonstrate the multiple uses of the method.

1. Introduction

There exist 16 different transition metals which have

and a high degree of purity, most experimental workers have preferred the halide approach whenever possible. Some of these halides, such as titanium tetrachloride or tungsten hexafluoride, are liquids at room temperature, and they have sufficiently high vapour pressures to allow known transport rates for a deposition process. On the contrary, many of these halides are solids at room temperature and sublimation tests have encountered monitoring and reproducibility problems [6, 7]. It may therefore be advantageous to perform in-situ halogenation immediately upstream of the deposition chamber. It is standard practice to use this method for synthesizing Ill—V materials by chloride CVD, a process that in certain cases is facilitated by the well-established predominance of the oxidation of the Group III metal [8]. As far as refractory metals are concerned, there are multiple degrees of oxidation states and their presence in the gas phase depends on experimental parameters (e.g. temperature, pressure and flow rate). In this case, the data must be critically examined and the halogenation reaction simulated before checking the accuracy of the choice by means of an in-situ analysis. The in -situ chlorination of tungsten is a good example to illustrate this procedure [9]. In the preparation of for electronic device applications [10],

melting temperatures higher than 1940 K and these metals are termed the refractory metals. Each of them has been prepared by CVD from various precursors [5] (e.g. halides, oxyhalides or organometallic compounds of varying degrees of complexity). Given the difficulties in obtaining a sufficient quantity of organometallic precursors with satisfactory stability

such a procedure is used. The goal here is to find a substitute for WF6, which is extremely aggressive with respect to the underlying doped silica. Figure 1 shows that, under a pressure of l0~Pa and in the presence of chlorine, excess tungsten is covered with at least up to 1200 K and that the WC14 and WC12 equilibrium partial pressures are relatively small. To analyse

Over the past 4—5 years, the thermodynamic approach has been adopted by an increasing number of experimental workers prior to conducting chemical vapour deposition (CVD) experiments by minimization of the Gibbs energy of the overall chemical system involved in the reactors. There is no point in dwelling on the choice of data, though it is a crucial factor in determining the confidence level of the theoretical predictions. Various sources of data are available including compilations and databanks [1]. No details will be given of recent efforts to model deposition reactors by combining thermodynamics, the equations of change and the kinetic rate expressions [2—4].Some examples will simply be given on the type of information that can be expected from such an approach, based on specific case histories taken from the family of refractory materials. Whenever possible, a comparison will be made between theoretical predictions and experimental results.

2. in-situ halogenation

0257-8972/91/$3.50

~ 1991

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Elsevier Sequoia. Lausanne

C. Bernard, R. Mac/ar

I Thermodynamic analysis of re/raclory

mat 101

materials

209

approach, namely total consumption of the original chlorine and production of WCI4 as the majority vapor species. A similar example, discussed below, concerns the case where the halide is not produced in situ but is trans-

_______________________________.~.

100 ~WCL WCL

CL 2

WCL5

wci3

formed in situ. This tactic has been applied to reduce the deposition temperature when TiCl4 is the titanium carrier gas (conventional method). The thermodynamic analysis and subsequent experimental results have shown that in-situ pre-reduction of TiCl4 on titanium could give rise to high adherence, good quality deposits on cemented carbide above 1100K [11].

3. Organometallic chemically vapour-deposited coatings For various reasons, the transport and deposition temperature (K) of refractory metals by halides are sometimes abanFig. I. Tungsten chlorination [91:calculated concentrations of tungdoned in favour of organometallic carriers. Apart sten chlorides vs. chlorination temperature l0~Pa; [w], from the fact that they do not generate any corrosive 10 mol; (EAr 9 x l0~Pa; P~.1, l0~Pa). species, organometallic carriers easily thermally decompose to offer the possibility of working at low temperature, thereby suggesting new applications such as the operation of the chlorinator, the plus chlo- deposition on polymers or low melting temperature rifle equilibrium must be studied [10]. Another solution light alloys. is to lower the pressure. In such case (Fig. 2) it can be Although the decomposition of the organometallic seen that, at 133 Pa, disappears above 820 K compound is in itself an irreversible phenomenon, the and that the WCI2 and, in particular, WCI4 partial nature and proportions of the deposited phases are pressures are then sufficient to initiate the deposition none the less in agreement with predictions made on process. Figure 3 gives a spectra obtained by in -situ the basis of a thermodynamic analysis [12], at least for mass spectrometry, confirming the thermodynamic average substrate temperatures. An example worthy of mention is the deposition of chromium carbonitride at 800 K from gaseous mixtures such as bis(benzene) moL chromium (BBC)—NH3 [13]. For this study, the ther500

1000

‘

1500

(~(~,L =

=

101 0

=

______________________________

10 ~.1

.WCL~

WCL2

/

/ / / / / / / / / / f /I // // 4 ~

500

/ / fl

CLI

______

WCL _____

WCL

~

I

temperature (K)

Fig. 2. Tungsten chlorination [91:calculated concentrations of tungsten chlorides vs. chlorination temperature (~~01 = 133 Pa, ~“J 10 mol; ~A. = 119.7 Pa; P~1, = 13.3 Pa).

modynamic data had of thetoCr—N—C system at and high ternperature first be modelled then extrapolated to 800 K. Figure 4 indicates the phases likely to be found in equilibrium at this temperature, and Fig. 5 shows the corresponding deposition diagram. Experiments were conducted for gas mixture compositions corresponding to each of the five domains of the theoretical deposition diagram. Table 1 gives a comparison between thermodynamic predictions and experimental results. The results of the electron probe microanalysis (EPMA) were standardized with respect to the chromium content in order to facilitate the comparison with X-ray diffraction (XRD) data. Since the X-ray photoelectron spectroscopy (XPS) results confirm the presence of free carbon in every sample, these results can be considered to be in good agreement, except for the final test, for which the analyses predicted the coexistence of small amounts of Cr7C3 with Cr3C2 and carbon, a coating which is therefore not entirely at equilibrium.

C. Bernard, R. Mac/ar / Ther,nodvnamic analysis of rcftactory ,naterials

2)0 Intensity

Ar+

1+ Ar 2+ cI+ HCI

C1

0~

W+

WCI+ WO~

+

SO

WCl~

WCI

WOCI~

WOCI+

WOCI

8~ Mass (Atomic Unit

Fig. 3. Mass spectrum of chlorinated tungsten 2.5 cm3 min~).

[91 (T

=

3 min’: argon flow rate.

1025 K: P

101

Cr

=

133 Pa: chlorine flow rate, 0.5 cm

E-2

CrN- Cr 2(N.Cl. C

I~-~ Fig. 4. Calculated isothermal section of the Cr—N—C system at 800K[13].

Z

e—~

0-S

.........i

SOC

0-5

PARTIAL

i 0-4

PRESSURE

~

0-3

(atm)

0-2

Fig. 5. CVD diagrams calculated at 800 K with a total pressure of

4. Determination of Si02-based planarizing compositions for coatings While a thermodynamic analysis is most generally used to optimize all or part of a process, as above, it can also be used to determine the best coating material. The main concern of industries working on the production of new-generation integrated circuits (very-

506 Pa and an initial gas phase of BBC—NH3 — He [13].

large-scale integration) is to achieve device miniaturization in order to increase information density, to obtain high speed and to minimize power consumption. One of the restrictions arising from this development is the fact that the conducting or insulating layers deposited must fill holes that are becoming increasingly

C. Bernarc/, R. Madar / Thermodynamic analysis of refractory materials

211

TABLE I. Comparison between experimental results and thermodynamic prediction where k is the free carbon ratio determined from X-ray photoelectron spectroscopy

k

Stable phase

XNFI,/XBBC

(%)

240 50 20 12.5 0

Predicted

XRD

EPMA

CrN + C

CrN

CrN

CrN + Cr2(N,C) + C Cr,( N,C) + C Cr,(N,C) + Cr5C, + C Cr3C, + C

CrN + Cr2(N,C) Cr( N1156C044) Multiphase Cr7 C3 + Cr3 C,

CrN097C1514 Cr,( N55 47C5, ~ 1.1 CrN5515C1535 Cr556,C5555

1()3C~()5

deeper and narrower and must cover abrupt steps while at the same time creating the least number of defects and avoiding the formation of cavities. As a result, the CVD technique has experienced considerable development in recent years at the expense of pl~ysicaldeposition methods, which are too directional. This change in methods is accompanied by research into new so-called “planarizing” materials which provide an excellent degree of device planarization during the deposition process and by post-annealing. Insulating films were initially made of silica, then silica and boron oxide (BSG), and more recently a mixture of silicon, boron and phosphorus oxides (BPSG). Film compositions and deposition parameters had to be optimized in this manner with a view to selecting the gas mixtures resulting in the best coverage of the step.

100 C present but not analysed 52 C present but not analysed 29

This thermodynamic optimization was achieved by establishing relations between the coverage of the step during deposition, the minimum planarization annealing temperature and the characteristic temperatures of the deposited mixed oxide, i.e. the vitreous transition temperature, creep temperature and liquidus temperature [14]. For the BPSG coating example, the thermodynamic properties of the phases present had to be modelled, and especially those of the liquid oxide mixture. This was necessary in order to calculate the isothermal sections in the SiO2 —B703 —P2 05 ternary diagram, like that shown in Fig. 6, and to locate the ternary eutectic composition domain which, as confirmed by later experiments, proved to be the domain where coatings exhibit the best planarizing behaviour. The same approach can be used to propose precise

Si112 0 isotherm 1500 K .isotherm 1400 K temazy eutectic pOint

isotherm 1300 K

____________________

a +liq a

eutectic valley quasi-binary section

~r + liq

/

+

Y +liq

CC -

a

+

+ ‘y~

-

B2/30

P2,5 0

Fig. 6. Calculated isothermal section of the Si1120—B,150—P,150 system at 1300 K and isotherms in the silica-rich corner [141: ~~[Si3(P04)41; 3 !j[SiP,07]; .y = ~[BPO4].

C. Bernc,rcl, R. Mac/ar

212

I

Thermoc/ynamic cinalysis of refractory materials

a-priori compositions in new systems that are likely to offer significantly lower post-deposition annealing ternperatures. For example, the deposition of a coating containing 80 mol.% SiO2, 16 mol.°AB2 03 and 4 mol.% GeO2 should lead to the lowest possible creep temperatures in this system, taking into acount the doping agent limitations specific to the process [14].

5. Reaction mechanism approach

1

+

~

,~A

(~1)

However, depending on the temperature, pressure or the inlet gas composition, the comparison of results indicates that two different types of mechanism can be proposed, which will obviously have to be confirmed by more detailed experimental studies: either the homogeneous reaction according to CH3SiC13 + H, -s CH4 + SiC12 + HCI -

(2)

followed by the heterogeneous reaction ~1i4

+

1 /C’I~’\ SiC12—s \~5i~/

+

~J(~ ~“—~

+

“2

~ (.‘,

or, at higher temperatures, lower pressures and lower MTS-to-H2 ratios, the homogeneous reaction CH3SiC13 + H2

I

-~

5C2H2 + SiC12 + HCI + ~H2

(4)

followed by the heterogeneous reaction ~C2H2+ SiC12 + ~H2

—~


the difference (homogeneous pressure heterogeneous pressure) for each gas species and then, depending on whether this difference is positive or negative, to deduce whether this species is a reactant or a product with respect to the overall deposition reaction. By combining the products and reactants having the highest values of the ratio (homogeneous pressure heterogeneous pressure)/homogeneous pressure, it is also possible to imagine which reaction mechanisms are likely to lead to a return to equilibrium [16]. Finally, there is another CVD domain where a thermodynamic analysis can be used to optimize the process and to understand the transport mechanism. This is the activated cementation process. This process can be broken down into four steps: (1) formation of gaseous metallic halides at the surface of a donor pre-alloy (or cement); (2) gas diffusion transport of these metallic halides from the cement to the substrate: (3) deposition of this element at the substrate surface by chemical reaction; (4) solid phase diffusion of the element into the substrate. In each of these steps, the thermodynamic approach plays an important role, two aspects of which will be presented here. The first aspect is the choice of cement which must ensure that a stable gas phase is made available throughout the process time. The partial pressures of the gaseous metallic halides in this gas phase must be constant. Consequently, the domain must be two phase if the donor pre-alloy is binary, or three phase if the pre-alloy is ternary. The importance of this choice can be illustrated by the codeposition of aluminium and hafnium on felts made of nickel alloy with 20 at.% Cr [17]. In order to be able to fix the aluminium and hafnium activities within the desired ranges, a ternary pre-alloy selected from the Al—Hf—Ni system was considered. This study thus required modelling of the three constituent binaries as well as the ternary compounds involved. The three-phase domain selected for the cement is indicated on the calculated ternary —

—

It would be wholly unreasonable to claim that the reaction mechanisms can be described from a simple thermodynamic analysis, especially when operating at low temperatures. The controversies that have arisen concerning the “simple” pyrolysis of silane clearly illustrate the complexity of the subject. None the less, for relatively high temperature deposits, corresponding to the preparation conditions of numerous refractory materials, a thermodynamic analysis can provide a number of useful indications, Two types of calculation can be performed: calculalion of the heterogeneous equilibrium, which gives an overall account of the deposition reaction, and calculation of the homogeneous gas phase equilibrium, which describes this phase well in a hot-walled reactor. Different types of information can be deduced from these two series of calculations, In the case of SiC deposition from a CH,SiCl, (MTS) —hydrogen mixture, Langlais and Prebende [15] found that, in all cases, the overall deposition reaction corresponds to the equation —~

process which seemed thermodynamically favourable does not take place. In this case, the two abovementioned types of calculations can be used to establish

(5)

Other possible uses of these two series of calculations can also be considered. For example, for some kinetic reason such as surface contamination, a deposition

.

.

.

section shown in Fig. 7. This domain corresponds to the equilibrium between Ni3Al—Al20Hf16Ni64 and the nickel-rich limit composition NiAl at 1273 K [18]. Another advantage of the thermodynamic approach concerns the transport mechanism between the cement and substrate. This can be deduced from the flux calculation of each species, which is expressed by two terms: .

.

.

.

.

.

.

Stefan flow and Fickian flow. In these equations, the composition gradients between substrate and cement are assumed to be linear and are obtained from the calculated equilibria at each of these points. The modelling result between the previously selected cement and a solid nickel substrate, with a surface concentration of .

C. Bernard, R. Mac/ar

/

Thermodynamic ana!~vsisof refrc,ctory materials

21 3

Al

A)- Hf ~Ni T 1273 K 0.8

0.8

0.6

0.6 / 11g.

/

\~AI

-~‘

AI

3Hf4

~

~ ~

AIHf2

Al Hf3 ______

Hf

0

11g.

0.2

~AIHfNi2 ~ AI20H116N164

0.4

N13AI 0.2

____\

Ni ~4

0.6

4

4

0.8 Hf2NI5 HfN)5 Hf2 N)7

0 Fig. 7. Calculated isothermal section of the Al—Ni—Hf system at 1273 K [18].

shifted CVD diagrams [21, 22], have not been dealt with. This gives proof, if any were needed, of the scope

Al F Al F2

and power of this method.

Hf F4 H2 HF

AIF3 A12F6

References Refractory

I C. Bernard and R. Madar, Proc. Chemical Vapor Deposition of

_______

search Society, Pittsburgh, PA, 1990, p. 3. 2 C. Bernard, High Temp. Sci., 27(1990) 131. 3 K. J. Jensen, Chem. Eng. Sci., 42(1987) 923. 4 D. W. Hess, K. F. Jensen and T. J. Anderson, Rev. C/scm. Eng.,

_______

Cement

Metals and Ceramics Symp., Vol. 168, Materials Re-

Substrate

Fig. 8. Migration direction of gas species during an Al — Hf codeposition process [18].

25 at.% Al and 2 at.% Hf, is given in Fig. 8 for a temperature of 1273 K. The deposition reaction on the substrate is probably equivalent to dismutation for aluminium and reduction for hafnium. 6. Conclusion This brief review of the advantages that can be legitimately expected from a thermodynamic analysis, with a view to optimizing the deposition of refractory materials, is obviously incomplete. Such important points as the selectivity approach [19, 20], the prediclion of metastable coatings [I] or the calculation of

3(1985) 97. 5 Y. Pauleau, Mater. Tech., 9—10 (1989) 31. 6 C. M. Hollabaugh, R. D. Reiswig, P. Wagner, L. A. Wahman and R. W. White, J. NucI. Mater., 56(1975) 325. 7 P. Salles, C. Bernard, M. Ducarroir and M. Nadal, in G. W. Cullen (ed), Proc. 10th mt. ~onf. on Chemical Vapour Deposition, Vol. 87—8. Electrochemical Society, Pennington, NJ, 1987, p. 1129. 8 J. L. Gentner, C. Bernard and R. Cadoret, J. Cryst. Growth, 56 (1982) 332. 9 N. Thomas, E. Blanquet, C. Vahlas, C. Bernard and R. Madar, Proc. (‘hemical Perspectives of Microelectronics Materials (II) Symp., Vol. 204, Materials Research Society, Pittsburgh, PA, 1990, p. 451. 10 E. Blanquet, N. Thomas, C. Vahlas, J. C. Oberlin, J. Torrés, R. Madar and C. Bernard, in K. E. Spear and G. W. Cullen (eds.) Proc. 11th In!. Conf on Chemical Vapour Deposition, Vol. 90, Electrochemical Society, Pennington, NJ, 1990, p. 474. II B. Drouin Ladouce, Thesis, Université d’Orléans, 1990. 12 M. A. Tromson-Carli, P. Gibart, C. Vérié, C. Bernard and Leroux, M. C. Schouler, J. Cryst. Growth, 48 (1980) 367. 13 F. Schuster, F. Maury, J. F. Nowak and C. Bernard, Surf Coat. Technol, 46(1991) 275.

214

C. Bernard, R. Mcic/ar / Therniodyncimic analysis of refiactory mc,terials

14 G. Baret. Thesis, lnstitut National Polytechnique de Grenoble, 1990. IS F. Langlais and C. Prebende, in K. E. Spear and 0. W. Cullen (eds.) Proc. lit/s In!. c’onf: on (‘liemictil Vapour Deposition. Vol. 90—12, Electrochemical Society. Pennington. NJ. 1990. p. 686. 16 J. L. Gentner, Thesis, Université de Clermont Il, 198). 17 G. Leprince, Thesis. Université d’Orléans. 1989. 18 P. Y. Chevalier, T/,ermoc/citc,. Therma. Grenoble, 1988. 19 R. Madar and C. Bernard, J. Vac. Sci. Technol. A, 8(1990) 1413.

20 J. 0. Carlsson. in J. W. Hastie (ed). Matericds Chemistry at High Temperature. Vol. 2, Humana Press, Clifton, NJ. 1990. p. 209. 2) D. E. Rosner and J. Collins. in K. E. Spear and 0. W. Cullen (eds.) Proc. lit/i In!. (‘ant: ass Oze,nical Vajsour Deposition. Vol. 90—12. Electrochemical Society, Pennington, NJ. 1990. p. 49. 22 K. E. Spear. in Mc. D. Robinson, 0. H. J., Van den Brekel, G. W. Cullen and J. M. Blocher (eds.). Proc. 91/i mt. (‘o,~fon O,emiccd Vapour Deposition. Vol. 84—6, Electrochemical Society. Pennington, NJ, 1984. p. 8).