In-situ FTIR emission spectroscopy in a technological environment: chemical vapour infiltration (CVI) of SiC composites

ELSEVIER

Journal of MOLECULAR STRUCTURE Journal of Molecular Structure 347 (1995) 331-342

In-Situ F T I R E m i s s i o n Spectroscopy in a T e c h n o l o g i c a l E n v i r o n m e n t : C h e m i c a l Vapour infiltration (CVI) of SiC C o m p o s i t e s V. Hopfea, H. Mosebachb, M. Erhard b and M. Meyerc a Fraunhofer-Institut fur Werkstoffphysik und Schichttechnologie, PF 16, 01171 Dresden b Kayser-Threde GmbH (KT), Wolfratshauser Str. 48, 81379 Mtinchen c Dalmler- Benz AG (DB-AG, former: Deutsche Aerospace AG), Zentrales Werkstofflabor, 81663 Mtinchen A method has been established to detect transient species inside a hot wall technological reactor. The CVI plant is used to produce fibre reinforced composite materials with ceramic matrix, in particular to infiltrate carbon fibre woven structures with SiC. The infiltration has been carried out at about 1000*C under reduced pressure with a mixture of CH3SiC13 (methyltrichlorosilane, MTS) and H 2 as the SiC precursor. To investigate the gas reactions near the preform the emissivity has been measured by FTIR spectrometry. Several gaseous species could be detected including MTS, SiC12, (SiC13)n=l,2, SiC14, HSiCI 3, CH 4, CH3C1 and HC1, also possible indications for CH 3 radicals and SiC clusters have been found. The interpretation of the multicomponent high temperature emission spectra has been supported by investigating reference spectra of individual components at working temperature. Band profile changes caused by both temperature and precursor decay have been detected. After radiation calibration, concentration changes and the degree of MTS decay could be roughly estimated. The spectroscopical results are evaluated in comparison with kinetic models of the MTS decay. 1. I N T R O D U C T I O N Chemical vapour infiltration (CVI) is a premier technique for the production of fibre reinforced ceramic composite materials. In principle, fibre woven structures (preforms) are impregnated in a hot wall reactor by pyrolysing a mixture of precursor gases which, as a consequence, precipitates the ceramic material within the pores of the preform. The technique is attractive for forming composites with superiour mechanical properties, but the long processing time of typically some days up to weeks and the run-to-run reproducibility still limit the applicability of that technology. Although the infiltration technique is used on a technological scale, the underlying chemistry is not well understood. Most of all the mechanisms of the gas phase and the surface reactions are speculative yet [1]. In addition, CVI reactors are often operated in a open loop without monitors of precursor feed concentration near the preform to be infiltrated as well as without monitoring exhaust gases. The change in precursor concentration during the infiltration leads to uncontrolled variations of the pore filling rate, and consequently, to unwanted closing of unfilled pores. These deviations from the desired properties are detected only by p o s t mortem of the final infiltration step. Thus, the development of a sensor system for in situ monitoring the concentration of reactants, reaction intermediates and reaction products near the substrate is a key element for improving control of the CVI process. 331-342

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332

Gas phase reactions governing CVI, CVD and related processes have been studied extensively by ex-situ techniques during the last fifteen years. Gas species were sampled from the reactive zone and analyzed using mass spectrometry or Fourier transform infrared spectrometry (FTIR). This methods suffers from having no capability of detecting intermediates with short lifetime and from consecutive reactions of the reaction mixture in the extracting capillary. This drawback can be overcome by in-situ monitoring using optical spectroscopy. Laser induced fluorescence [2], coherent anti-Stokes Raman scattering [3], IR spectroscopy by tunable diode lasers[4], and FTIR [5, 6] are some of the in-situ techniques successfully used for gas-phase reaction studies to understand the mechanisms and kinetics of CVI, CVD, and chemically based physical vapour deposition methods. First attempts were carried out to investigate surface processes in CVD systems by in-situ FTIR reflection and emission spectroscopy [7]. Most of those investigations are focussed on basic problems and measurements in specially designed model reactors. The aim of this work was to establish an in-situ FTIR monitoring method as a leading component for a future process control line on a CVI production reactor 1). This paper describes the spectroscopic problems involved. After a short outline of the experimental method the main part is focussed on the interpretation of the complex multicomponent high temperature gas emission spectra measured within the CVI reactor as well as the identification of reaction intermediates. -

CVI REACTOR

IR SOURCE

/ Focussingmirror I

Globar

/ Graphite Tubes

Figure 1

1)

K300 FTIR-SPECTROMETER

'\ i i '1

Pocussmgmirror 2

Folding mirror

Fieldstop

Reaction Chamber

Schematic diagram of FTIR and CVI reactor system

The CVI plant is used by the DB-AG. The reactor itself was built by Archer Technicoat, UK. The project is funded by the commission of the European community within the BRITE-EURAM research programm under contract no. BREU-CT91-0447.

333 2. E X P E R I M E N T A L

SYSTEM AND PROCEDURES

The full description of the measuring system is given elsewhere [8]. The FTIR instrumentation a K300 spectrometer which was initially designed by KT for outdoor measurements of environmental pollutants. This double-pendulum spectrometer with a maximum resolution of 0.01 cm-1 (unapodized) is equipped with a KBr beamsplitter and a MCT detector which is designed for the 450-4000 cm-1 region. The auxilliary optical path is fitted through two inert gas purged KBr windows to the CVI plant by a low f-number focussing mirror (Fig. 1). In principle, the optical design supports both measurements in transmission as well as in emission. After a preliminary attempt using a high temperature glowbar source for transmission measurements, better spectra have been collected in the emission mode. To get emission spectra with highest SNR the background radiation from the hot walls has to be suppressed as far as possible. Therefore the second view port at the reactor was closed by a KBr window and the optical path within the reactor had to be carefully aligned. In order to eliminate spurious effects from the measured spectra which are not caused by the gaseous species of interest inside the reactor, reference measurements must be performed. This can be done by tuming off the incoming precursor gas flow while keeping all other parameters and configurations of the whole system unchanged. By forming the ratio measurement / reference (calculating of the transmittance) spurious effects in the IR spectra are removed such that almost all signatures resulting from gaseous species in the CVI reaction zone are still present. The inner walls of the reaction chamber are made from graphite resulting in a near blackbody characteristic of the scattered background radiation. When carrying out emission measurements for quantitative analysis, a radiation calibration must be performed since the radiaton collected by the FTIR spectrometer must be transformed into emittance units. Using a calibrated blackbody radiation source the emittance (E) of the hot wall background have been estimated as E= 0.1 or less. The SiC infiltration processes are performed in the DB-AG CVI plant in the low pressure region of typically 5-100 mbar and at temperatures between 950°C and 1250°C. The total pressure is controlled by a butterfly valve and a liquid ring pump which has a good longterm stability against the very corrosive reaction products. Methyltrichlorosilane (MTS) donated by Merck Chemicals was employed as a precursor for the SiC deposition. This liquid precursor is fed into the reactor by a peristaltic dosage pump. Pure hydrogen and argon are used as reactants and carrier gases. The spectra are scanned at different resolution (0. lcm-1...5cm-1). If not otherwise indicated the spectra are plotted with lcm-1 resolution.

3. I D E N T I F I C A T I O N O F G A S E O U S S P E C I E S A series of spectra within the MTS and the MTS / H 2 system were measured by changing the temperature and total pressure in the CVI reactor as well as the flow rates of MTS, hydrogen and argon (Fig. 2.-5). From many previous experiments [9] it is a wellknown fact that crystalline B-SiC, the target product for high temperature composites, may be deposited above 1000*C. This deposit can contain a second phase, either free carbon (graphite), when pyrolysing MTS in an inert atmosphere, or some free silicon, when a hydrogen rich atmosphere is used. Pure SiC is deposited typically at a molar ratio of nMT S / n H2 of 0.5... 1.0. The above shown spectra are centred around these parameters.

334

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180 805 cm

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illl

1100

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1200

i i F ~,

1300

1400

r j

1500

Wavenumber [cm -1]

IR emission spectra of M T S / H 2 (10.5slm / 7.5slm) at 600"C and 800"C, Ptot = 10mbar, flow rates: MTS= 10.5slm H2=7.5slm

Figure 2

220 I-

d 125 ~-

•

200 ~1

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140

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120

100

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~,k.,L~/

100 95

160

I

. 800

.

.

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.

.

.

1000

_ 1200

1400

Wavenumber [cm "1]

Figure 3

IR emission spectra of M T S / A r (flow rate MTS: 10.5slm) and M T S / H 2 at 1050"C respectively flow rates: M T S = 10.5slm H2=7.5slm

335

Many overlapping gas-band contours can be distinguished within the IR emission spectra of the pyrolysis atmosphere (Fig. 5). Only the spectra of rather simple and stable molecules are clearly detectable. HC1 is identified by their P-branch of the rovib structure around 2800cm- 1 and methane by the P-Q-R rovib structure of the deformation band around 1306cm-1 (Tab.2). All other spectral signatures are not interpretable without using additional information and reference spectra. One starting point for a systematic search of species is the thermodynamic approach. Some representive results of minimum free energy calculations are given in Table 1. The most abundant gas phase species in the MTS system are HC1, H 2 (not measurable), perchlorosilanes, HSiC13 and CH4. A premixing of hydrogen leads to more or less pronounced deviations of the concentration but not to other species. The concentration of the precursor itself should be very low in thermodynamic equilibrium. As the residence time of the gas mixture inside the through-flow CVI reactor is rather low (< ls) kinetic considerations should be included in a systematic search of reaction intermediates. From the kinetic approach (see Ch. 4.) the most abundant intermediates from primary bond cleavage are the radicals CH 3, SiC13, SiC12, CH3SiC12 and the neutral CH3C1. Including consecutive reaction pathes the number of species is much higher. Unfortunately a literature check clearly shows a lack of reliable IR reference spectra of the above mentioned species, particularly at high temperatures. Spectra of radicals are measured typically in solid matrices at cryogenic temperatures. Temperature dependent band shifts and band contours are difficult to investigate because of the instability and reactivity of the most of the above mentioned molecules. Therefore we are forced to combine several methods for species identification : obtaining a set of reference spectra at the CVI-FTIR reactor at high temperatures where gases are no longer stable (Ch.3.1) studying literature data deconvolution of mixture spectra by spectra subtraction (Ch. 3.2) spectra simulation(Ch. 3.3) Table 1 Thermodynamical Calculations: Most abundant species in the gas phase T= 1300K Ptotal = 20mbar p0prec = 2mbar, partial pressure is given in mbar SIC14 / Ar

SiC14 / H2

SiC14 SiC13 C1 C12 SiC12

HC1 SiC12 SIC14 SiC13 HSiC13 H2SiC12

2 5"10-3 5"10-3 3 * 10-4 1"10-4

5 2 0.5 0.4 0.1 6"10-3

CH3SiC13 / Ar

CH3SiC13/ H 2

HC1 3 H2 1 SiC14 4"10-1 SiC13 7 * 10-2 SiC12 7"10-2 HSiC13 7"10-3 ....................... CH3 SiC13 7"10-7

H2 15 HC1 5 SiC14 1"10-2 SiC12 1* 10-2 SiC13 6"10-3 H S i C 1 3 2"10-3 CH4 2"10-3 ........................... CH3 SiC13 3" 10-6

336

MTS

100

MTS compensation

50

..Q

0 660 680 700 720 740 760 780 800 820 84( o~

-50

-100 [500

600

700

800

900

1000

1100

1200

1300

1400

1500

Wavenumber [cm1] Figure 4

IR emission spectrum of MTS at 1050"C: difference spectrum [MTS 10.5slm/Ar ] - F* [MTS(10.5slm) / H2(15slm)] compensation factor F=1.98

200 130 180

MTS/H 2

120 110

160 100 140

E

2100

2150

2200

2250

2300

2350

2400

2450

120

100 800

1200

1600

2000

2400

2800

3200

Wavenumber [cm"1] Figure 5

IR emission spectra of MTS/H 2 (3.7slm/7.5slm) at 950°C and Ptot=20mbar

337

3.1 R E F E R E N C E S P E C T R A At typical temperatures of the deposition process none of the above listed molecules is really stable (except HC1). Therefore the following reference spectra have been measured in a broad temperature interval to distinguish between temperature dependent band shifts and, on the other side, bands arising from products of thermal decay. SiCI4 Spectra of pure SiC14 which are expanded to monitor band profile deviations of the Si-C1 valence vibration are given in Fig. 6. At normal temperature this band is centered around 615cm-1 [10]. Again the spectra at 400°C and 600"C (Fig. 5) show emission maxima near this wavenumber. Caused by the occupation of higher excited states and anharmonicity, the band intensity increases with temperature and the band profile shows an asymmetric "red-shadowed" broadening. But the spectrum at 1050°C is in disagreement with commonly accepted models. The maximum is now downshifted to 596cm-1 and the band profile becomes less assymetric and broader (Fig. 5). The vl+v4 combination band which forms a shoulder at 641cm-1 shows no shift. This unusual behaviour could be caused by: (i)temperature dependent shifts, (ii)influence of radiation reabsorption, (iii) partial decay of SiC14. The first cause can be excluded. Band profile simulation on similiar species show no band shift with increasing temperature, see Fig.6 [7]. The second cause might also be excluded. Reabsorption phenomena are normally restricted to higher intensities, e.g. flattening of band heads or band inversions. But concentration dependent spectra, which are performed at 1050"C (not shown), give no indication of changing band profiles. Thus the reactivity of SiC14 (iii) should be taken into account. From thermodynamical estimations a high stability of the SiC14 in an inert atmosphere is evident. But the reaction chamber is far from being inert. The reactor walls (graphite) are partially covered with SiC and free silicon which could act as reducing agents against SiCI4. In a hydrogen-rich atmosphere, a typical reducing ambient, the equilibrium is characterized by a much higher degree of decay, where silicon subchlorides and partially chlorinated silanes are the most abundant products (Tab. 1). In accordance with literature [11] the bands centered near 590cm-1 result from a mixture of SiC13 with its higher oligomeres (SiC13)n, see Tab. 2. 300

250

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640

660

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580

600

Wavenumber [cm-1] Figure 6

IR emission spectra of SiC14 / Ar at different temperatures, Ptot = 10mbar, flow rate SIC14: 5.5slm

680

338 SiCl4 / H2, silanes As expected (Tab. 1), SiC12, (SiC13)n and HSiC13 dominate in hydrogen-rich atmospheres. In Fig.7 a weighted difference spectrum is given where the stretching region is balanced out by subtracting the "pure" SiC14 spectrum (see above). This is a reasonable reference spectrum for SiC12 as well as for HSiC13 (Tab.2). The expected Si-H streching vibration at 2259 cm-1 [21,12] drops below the detection limit in this gas mixture. But this band is clearly visible in the pyrolysis gas of the MTS decay at higher H 2 flow (Fig.5). This band comes along with two other Si-H bands at 2235cm-1 and 2247cm-1 whose origin is not yet known. The band at 2235cm-1 might arise from H2SiC12 but the expected deformation band at 877cm-1 [12] has not been detected. The band at 2247cm-1 which probably arises from a Si-H containing molecule is overlapped by a HC1 rotational peak, see high resolution spectrum in Fig. 5. From the compensation factor the degree of decay of SiC14 is estimated as 0.8, which, in comparison with thermodynamic results (Tab. 1), gives evidence for the kinetic control of the decay process.

Reference spectra of methane were measured at process temperature of 1050"C. The degree of decay is near 0.75 but reaction products are not detectable. This molecule can easily be identified using the Q-branch at 1306cm-1 and some additional rovib-peaks (Tab.2) which are sharp, even at high temperatures.

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700 800 900 Wavenumber [cm1]

1000

1100

IR emission spectra of SiC14 / H2 at 1050°C, difference spectrum: [SiCI4 10.5slm/H2(7.5slm)] - F* [SiCl4(10.5slm) / Ar] compensation factor F=0.216

339

CH3SiCI3 (MTS) The MTS spectrum have been measured in the temperature region between 600"C and 1050"C (Fig 2). This molecule is thermodynamically unstable within this region and the concentration should drop below detection limit at 1050"C (Tab. 1). In reality, at 600"C there is virtually no decay and the peak positions well fit literature data [13, 14]. Even at the highest temperature, the characteristic peaks are clearly detectable (Tab.2). Thus these spectra can be used for reference purposes, e.g. for studying temperature dependent effects on band profile and for difference spectroscopy, see Ch. 3.3. SiC (cluster) The upper boundary of the deposition process is limited by the homogenous nucleation resulting in cluster formation within the gas phase (aerosol) which drastically diminishes the quality of the deposits. Thus, in-situ monitoring of such particles would be very helpful for process control but reference spectra are not available. Therefore the spectrum of SiC particulates dispersed in an inert ambient has been simulated using the effective media approximation and optical multilayer modelling [ 15,16]. The dielectric function of SiC (at normal temperature) is modelled using a set of damped oscillators derived from the parametes of the bulk material. The resulting emission spectra depend on particle shape and density. Near the expected detection limit of this species (particle density 10 -8) the simulated spectrum in Fig.7 may be used as a reference. CH3 The spectrum of the methyl radical has been discussed in literature very controversely. The molecule has a planar structure with an "umbrella mode" deformation vibration which is very strong [17]. This mode with a ground state band head probably near 650cm-1 is characterized by an extremly negative anharmonicity [18]. This gives rise to a series of hot-bands shifted to higher wavenumbers in intervalls of roughly 40cm- 1. Unfortunately the detectibility of this very interesting intermediate is restricted by interferences with other species, see Tab.2. 1,0

0,8

0,6

E

0,4

0,2

0,0 600

650

700

750

800

850

900

950

1000

1050

1100

Wavenumber [cm1] Figure 8

Simulated IR emissions spectrum of SiC particulates dispersed in inert gas in dependance of particle density (filling factor)

340 O-130 Reference spectra have been extracted from literature [ 14]. This molecule is rather easy to detect by using a band near 730cm-1 with a characteristic P-Q-R profile but interferences with the CH 3 radical can not be excluded.

3.2 SPECIES IDENTIFICATION IN THE MTS SYSTEM A set of spectra have been taken while preforms were being infiltrated in the CVI plant. The parameter window is given in Ch. 2. Based on the reference spectra given above, a correlation table for the Si-C-C1-H system was established for species identification (Tab.2). The following species could be detected

CH3SiCI 3, SiCl4, SiCI3, (SiCl3)n, SiCl2 HSiCI3, H2SiC! 2 (?), HC! sic (9.)

Ca4, CH30, CH3 (9.) CO, NH3, chlorosiloxanes Because of band overlap, the detection of H2SiC12, SiC cluster and of the CH 3 radical is not sure. The detected CO and chlorosiloxanes (Tab.2) arise from the precursor handling system, NH3 comes from the reactor purge system using ammonia as a purge gas.

4. KINETICS OF THE MTS DECAY A deeper look into the mechanism of the MTS decay is out of scope of this paper. However for planning future experiments it might be very interesting to compare the above given results with some very recent theoretical attempts [ 19, 1]. Supported by RRKM calculations the most abundant reaction channels of the primary decay of MTS are given by equation (1) - (3). Reaction (4) is less probable because of its higher activation entropy. CH3SiC13 ---> CH 3 + SiC13 (1) CH 3 SiC13 ---> CH2SiC13 + H (2) CH3SiC13 ---> H2C=SiC12 + HC1 (3) CH 3 SiC13 ---> SiC12 + CH 3C1 (4) The products of the primary decay can undergo several consecutive reaction channels. In a hydrogen rich mixture, equation (5)-(6) for instance are dominant but in an "inert" atmosphere the reaction paths (7)-(8) seem to be more probable. CH 3 SiC13 SiC13 SiC13

+ + +

H2 H2 CH 3

---> ---> ---> --->

CH4 HSiC13 SiC12 SiC12

+ + + +

H H CH3C1 C1

(5) (6) (7) (8)

In comparing the kinetic model with the spectroscopic results, the conclusions can be drawn that most of the species proposed by kinetic theory have been identified in the experiments. Because of the variety of reaction channels it cannot be distinguished whether some key species, as SiC12 and CH3C1, are arising from primary bond cleavage or from secondary reactions.

341 Table 2

Assignment table of gaseous species in the system Si-C1-C-H

Peak position I. . . . . . . . Appearance in System ....... I Assignment /cm-1/ SiC14 SiC14/H2 MTS MTS/H 2

490...510 572 585 592 603 619 641 691 720 730 730 738 752 762 802 805 938 1000 1185 1270 1291 1306 1405 2259 2500 .... 2900 near3000

# # # # #

# # # #

# # # #

# #

#

# #

#

# # # # # #

#

# # # # # # # # #

# #

#

# #

# # # # # # #

SiC12 MTS SiC13 Si2C16 HSiC13 SiC14 SiC14 CH 3 (?) CH3CI SiC14 (?) CH3C1, CH 3 (?) MTS MTS MTS HSiC13 MTS SiC (cluster) chlorosiloxanes chlorosiloxanes MTS CH4 CH4 MTS HSiC13 HC1 CH4

Table 3

Band positions for analytics of individual species

Species

Peak Position /cm-1/

Interferences with other Species

CH3SiC13

754 8O5 572 641 580 505 2259 1306 2652

CH3C1, partially overlapped HSiC13 SiC13

SiC14 SiC13 SiC12 HSiC13 CH4 HC1

SinC12n+2, partially overlapped

Int. seeFig.

Spectrum

vs vs vs ? vs vs m

2, 2 3, 3, 5 3, 6 4 4 4 4 2 2 2 7 2 4, 4, 4, 2 3, 3,

m, sh vw m, sh m w vs s vs, br w, br. w, br. s m, sh m, sh, w m s m

5 5 5

3, 5, 7 5, 6, 7 5, 6, 7 5, 6, 7

7 7 7 5 5

342 4. C O N C L U S I O N The in-situ FTIR method developed is well suited for monitoring the gas atmosphere in the reaction chamber of a technological CVI plant. Several gaseous species and intermediates have been identified. A continous monitoring of their transient concentrations might be the starting point of future process control. The identification of individual species in high temper. ature gas atmospheres has been supported by a combination of methods including measuring temperature dependent reference spectra, difference spectroscopy, literature check, and spectra simulations. But because of the high reactivity of most of the reactants these procedures are far from being trivial and will be accomplished by chemometric methods in near future.

6. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

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