IR laser induced decomposition of tetrakis(dimethylamido)titanium for chemical vapor deposition of TiNx

INFRAREDPHYSICS &TECHNOLOGY ELSEVIER

Infrared Physics& Technology37 (1996) 727-731

IR laser induced decomposition of tetrakis(dimethylamido) titanium for chemical vapor deposition of TiN x M. Janovskfi, Z. Bastl * J. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 182 23 Prague 8, Czech Republic

Received 13 December1995

Abstract

The CO 2 pulsed laser decomposition of tetrakis(dimethylamido)titanium yields methane and dimethylamine in the gas phase and solid deposits which contain titanium nitride, significant amounts of -C---N group containing species and oxygen, the presence of which is caused by sensitivity of the deposit to air.

1. Introduction

Titanium nitride layers are regarded at present as one of the most promising materials for coatings, with unique chemical and physical properties, They exhibit exceptional wear and corrosion resistance, are chemically inert and have many important applications in microelectronics, as hard coatings for tools, as solar control coatings, sensors or as catalysts [1]. The two most frequently used techniques of TiN films preparation are reactive sputter deposition and chemical vapor deposition. For metallo-organic chemical vapor deposition (MOCVD) of TiN films tetrakis(dimethylamido)titanium, Ti[N(CH 3)]4 (TDMT) is frequently used as a single-source precursor. The TiN films can be prepared by thermolysis of TDMT at rather low substrate temperatures (200400°C) but are usually contaminated by carbon which has an adverse effect on their properties. Our previ-

* Correspondingauthor.

ous study of TDMT decomposition on several different substrates showed that heterogeneous pyrolysis of TDMT is influenced by chemical composition of the surface on which it occurs [2]. In a continuation of our studies of IR laser-driven gas phase reactions for CVD of hard coatings we report in this paper on the CO 2 laser powered decomposition of TDMT and on the properties of solid material produced by this decomposition. We show that the IR laser induced decomposition of TDMT affords solid deposits containing titanium nitfide. The relative content of TiN x depends on experimental conditions during irradiation. Deposits were studied by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). 2. Experimental

The experiments were performed with a TEA CO 2 laser (Medicom a.s.) using the P(24) line at 940.55 cm -l. The laser was operated at a repetition

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M. Janovsk~, Z. Bastl /lnfrared Physics & Technology 37 (1996) 727-731

rate of 0.7 Hz with a pulse energy of 200 mJ and a pulse duration (FWHM) of about 70 ns. The beam radiation wavelength was checked using a CO 2 laser spectrum analyzer (Division of Lasercraft, Optical Engineering, Santa Barbara, CA) and its energy was measured with a Coherent Labmaster pyrometer. TDMT was irradiated for about 5 min in a cylindrical glass cell 14 cm long with 2.5 cm internal diameter, equipped with NaC1 or KBr windows on both sides. The cell was attached to a vacuum manifold (base pressure --, 10 -6 mbar) and a glass bottle containing TDMT. The whole apparatus was placed into the thermostat, kept at 85 +__5°C. The temperature of the thermostat determined the pressure of TDMT in the reaction cell and at the above temperatures it was 4 + 1 mbar. TDMT (99.8%, Schumacher Chemicals) was used without further purification. The substrates used in the experiments were gold plates, approx. 15 × 10 mm in size. Prior to their insertion into the reaction cell they were carefully cleaned in ultrasonic bath. The properties of the deposits were evaluated using X-ray photoelectron spectroscopy (XPS), F U R spectroscopy and SEM technique. The photoelectron spectra were recorded in a VG ESCA 3 Mk II electron spectrometer with electrostatic hemispherical analyzer operated in the fixed analyzer transmission mode. The spectra were measured using AI K ot radiation. Detailed spectral scans were taken over Ti 2p, N Is, C Is and O 1s regions. Quantification of the surface concentrations of elements was accomplished by correcting the photoelectron peak areas for their cross-sections. Curve fitting of the overlapping spectral lines was carried out using mixed Gaussian-Lorentzian peak shapes [3-6]. During the transport from the reaction vessel to the electron spectrometer the samples were kept under nitrogen atmosphere. The IR spectra of the gas phase and deposits were measured with a Philips PU9800 F H R spectrometer with resolution 4 cm -1 . Scanning electron microscopy studies were performed on a JEOL JSM-U3 instrument. 3. Results and discussion

The irradiation of TDMT results in the production of gaseous methane and dimethylamine, which yield

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BINDING ENERGY (eV) Fig. 1. Typical X-ray photoelectron spectra of the deposits obtained by laser irradiation of TDMT using focused (1) and unfocused (2) laser radiation.

characteristic peaks at 1309, 3018 and 731 c m - i in the FTIR spectra. A brown solid deposit is formed simultaneously on the inside walls of the reaction cell. XPS survey spectra of the deposits (Fig. 1) reveal the occurrence of not only Ti, N and C but also of oxygen and trace amounts of silicon, the latter originating likely from the resin used to fix the windows to the glass cell. The overall stoichiometry of the deposits produced with focused and unfocused laser radiation as well as the stoichiometry of the layer produced using unfocused radiation and subsequently sputtered by argon ions are displayed in Table 1. To avoid large changes in sample chemistry the sputtering was carried out under rather mild conditions, leading to the removal of about 10 monolayers of material. The photoelectron spectra of N 1s electrons are shown in Fig. 2. The measured core level binding energies, peak widths, their assignment s and calculated comTable 1 Relative atomic concentrations of elements present in the deposits produced by decomposition of TDMT (pressure = 4 mbar) by CO 2 laser radiation (Concentrations normalized to the total amount of nitrogen set to 1) Conditions

Ti

C

O

Focused radiation Unfocused radiation After ion sputtering a

0.8 0.7 1.2

2.9 3.4 3.1

1.2 1.3 1.1

aE=4keV,

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M. Janovsk6, Z. Bastl / lnfrared Physics & Technology 37 (1996) 727-731

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BINDING ENERGY (eV) Fig. 2. N Is core level spectra of deposits produced using focused (1) and unfocused (2) laser radiation. Spectrum (3) was obtained after argon ion sputtering (energy 4 keV, 30 s) of the sample obtained with unfocused radiation. The spectra are normalized to the same height.

positions of the prepared layers are summarized in Table 2. Layers contain titanium nitride with N ls binding energy of ~ 398.6 eV and Ti 2p3/2 binding energy of ~ 455.7 eV. These values agree well with those reported in the literature for Ti nitride [7]. The nitrogen peak centered at ~ 398.7 eV and the carbon line with C 1s binding energy ~ 285.4 eV can be assigned to the compound containing N - C bond. It follows from the spectra, that ion sputtering causes significant increase of nitride concentration which is probably a consequence of removal of the species,

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adsorbed on the deposit surface. The C ls line with binding energy ~ 286.6 eV can be assigned to carbon atoms bonded in the triple C = N bonds [7,8]. We will show later that the presence of such species is corroborated by our IR spectra. Nitrogen with N ls binding energy ~ 400.5 eV corresponds to nitrogen bonded in the triple C - N bonds [8]. However, the concentration of nitrogen in this chemical state is lower than concentration of the corresponding carbon. A possible explanation of this disagreement is that the N 1s binding energy of nitrogen in the N=-C group depends also on the atom to which this group is linked. If the N--C group is bonded to some organic moieties like CH x, N(CH3) 2, the corresponding N 1s binding energy is 400.5 eV while if it is bonded to the metal atom (in our case Ti), corresponding N ls energy is 396.8 eV. This assignment agrees also with observed high concentration of nitrogen with binding energy of N ls electrons 396.8 eV compared with the concentration of titanium in the nitride form. We assign the binding energy of Ti 2p3/2 electrons 457.3 eV to titanium bonded to the N - C group. Layers contained also Ti oxide (Ti 2p3/2 ~ 459.0 eV, O ls ~ 530.8 eV), the presence of which is likely the result of partial sample oxidation by air during its transport from the reactor to the spectrometer, although it was done in atmosphere of nitrogen (99.9%). This result points to sensitivity of the deposit a n d / o r of the adsorbed surface species to oxygen. We found, that the layers prepared under different experimental conditions (precursor pres-

Table 2 Core level binding energies (BE), full widths at half maxima of photoemission lines, (FWHM) in eV, composition of the deposits and assignment of the chemical states of elements Core level

BE

FWHM

Concentration

Assignment

Focused radiation

Unfocused radiation

After ion sputtering

Ti 2P3/2

455.7 457.3 459.0

1.9 1.9 1.9

0.14 0.15 0.51

0.10 0.13 0.52

0.24 0.38 0.62

Ti-N Ti-C-=N TiO,

N Is

396.8 398.7 400.5

1.8 1.8 1.8

0.41 0.14 0.45

0.26 0.10 0.64

0.41 0.29 0.30

TiN, T i - C m N N-CN-=C-CH x

C Is

285.4 286.6

2.0 2.0

1.76 1.10

1.98 1.41

2.35 0.80

N - C , (CH~-Si-O) N-~C-

O ls

530.8

2.4

1.17

1.26

1.14

TiO~

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M. Janovsk6, Z. Bastl / lnfrared Physics & Technology 37 (1996) 727-731

to titanium while the vibration at 2200 c m - t corresponds to C---N species bonded to some organic group (CH x, N(CH3)2). We did not see either the peak at 1273 cm -t observed by Dubois et al. [11] and assigned to three-membered T i - N - C metallacycles or the vibration band at 1590 c m - t reported by Truong et al. [13] and attributed to Ti-imine complex. This again shows that the chemistry taking place during gas phase thermolysis by CO 2 laser radiation differs significantly from that of conventional pyrolysis.

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Fig. 3. FT[R spectra of: gas phase TDMT before irradiation (l), material deposited on NaCI window and gas phase after laser irradiation (2), deposit (3).

sure, focused resp. unfocused radiation) contained always the same chemical states of atoms, but their relative concentrations were different. Let us mention that titanium carbide which is observed in the products of heterogeneous pyrolysis TMT on various heated surfaces [2,9,10] is not present in our deposits. The IR spectrum of the precursor used in our experiments (Fig. 3, spectrum (1)) agrees with that reported in the literature [11]. IR spectra measured after irradiation of TMT by CO 2 laser show, in addition to bands of undecomposed TMT and bands belonging to the deposit, the absorption bands characteristic of dimethylamine and methane - - the gas phase products of the decomposition TMT. We believe they are produced by cyclic intermolecular hydrogen transfer and subsequent elimination. The IR spectrum of the deposit shows vibrations characteristic of N - C bonds (953 cm -1) and C H x - S i - O groups (808, 1047, 1264 cm-~). The latter are assignable to traces of the silicone resin. Our angle resolved photoelectron spectra indicate that it is present on the substrate surface but not in the deposit. Besides, the intensive vibrations at 2140 and 2200 cm-t are also observed which can be assigned [12] to the C---N triple bond. As follows from the XPS results mentioned above the C=--N groups are likely bound to two different groups of atoms. Vibration at 2140 c m - t corresponds to the C ~ N group bonded

Fig. 4. Typical scanning electron micrograph of deposit produced with focused (a) and unfocused (b) laser radiation.

M. Janovskt, Z. Bastl / lnfrared Physics & Technology 37 (1996) 727-731

The morphology of deposits produced depends on whether the focused or unfocused radiation is used. SEM images of the layers deposited with focused radiation are consistent with fluffy agglomerates in which individual grains are not resolvable (Fig. 4a) while the deposits produced with unfocused radiation contain grains of material with size of a few Ixm (Fig. 40).

4. Conclusions

The layers deposited by decomposition of TMT in the gas phase initiated by CO 2 laser radiation were studied by XPS, IR spectroscopy and SEM. The deposits contain titanium nitride species containing N-C, N--C, groups and titanium oxide. The concentration of nitride is higher in the deposits produced with focused laser radiation which are, at the same time, more dispersed. The dominant gas phase products of the TMT decomposition were methane and dimethylamine.

Acknowledgements The authors wish to thank Dr. V. Masarhk for the SEM images of the deposits. This work was sup-

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ported by the Grant Agency of the Academy of Sciences of the Czech Republic (Grant No. A440109).

References [1] M. Kn~ov~i, Chem. Listy 88 (1994) 501. [2] M. Janovsk~i and Z. Bastl, Collect. Czech. Chem. Commun. 60 (1995) 372. [3] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [4] D. Briggs and M.P. Seah, eds., Practical Surface Analysis, Vol. 1: Auger and X-ray Photoelectron Spectroscopy (Wiley, Chichester, New York, 1990). [5] J.H. Scofield, J. Electron Spectrosc. Rel. Phenom. 8 (1976) 389. [6] A.E. Hughes and B.A. Sexton, J. Electron Spectrosc. Rel. Phenom. 46 (1988) 31. [7] NIST X-ray Photoelectron Spectroscopy Database (U.S. Dept. of Commerce, Gaitthersburg, MD, 1989). [8] B.A. Sexton and N.R. Avery, Surf. Sci. 129 (1983) 21. [9] R.M, Fix, R.G. Gordon and D.M. Hoffman, Chem. Mater. 2 (I 990) 235. [10] A. Katz, A. Feingold, S.J. Pearton, S. Nakahara, M. Ellington, U.K. Chakrabarti, M. Geva and E. Lane, J. Appl. Phys. 70 (1991) 3666. [1 l] L.H. Dubois, B.R. Zegarski and G.S. Girolami, J. Electrochem. Soc. 139 (1992) 3603. [12] M. Horhk and D. Papou~ek, Infrared Spectra and Structure of Molecules (Academia, Praha, 1976) pp. 536-545. (in Czech) [13] C.M. Truong, P.J. Chert, J.S. Corneille, W.S. Oh and D.W. Goodman, J. Phys. Chem. 99 (1995) 8831.