Compositional characterization of silicon nitride thin films prepared by RF-sputtering

Vacuum 67 (2002) 513–518

Compositional characterization of silicon nitride thin films prepared by RF-sputtering !ına, C. Prietoa, P. Miranzob, * M. Vilaa,*, J.A. Mart!ın-Gagoa, A. Munoz-Mart c ! M.I. Osendib, J. Garc!ıa-Lopez , M.A. Respaldizac a

Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain b ! Instituto de Ceramica y Vidrio, CSIC, 28500 Arganda del Rey, Spain c ! Centro Nacional de Aceleradores, Parque Tecnologico Cartuja’93, 41092 Seville, Spain

Abstract Silicon nitride thin films have been prepared by RF reactive and non-reactive sputtering over several substrates. Five different working sputtering gasses have been selected to study its influence on the composition and properties of the obtained thin films (Ar, N2, Ar/N2(50%), N2/H2(5%), Ar/H2(5%)). Optical absorption spectroscopy characterization has shown that these thin films have different macroscopic properties as the refractive index and the position of the absorption edge. In order to explain these differences, Fourier transform infrared, Auger electron, X-ray photoemission and Rutherford backscattering spectroscopies have been used to study the dependence of the working sputtering gas on the stoichiometry of the samples. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Silicon nitride thin films stoichiometry; Reactive sputtering

1. Introduction Silicon nitride films are known to be very useful for many different aspects in materials science. In electronics and optoelectronics [1,2], silicon nitride is used in silicon and III–V semiconductor device technology, in which, for some applications, the deposition techniques need to be performed at low temperatures and with low-energy particles. Preparation of silicon nitride thin films has been reported by different chemical vapour deposition (CVD) techniques [3,4], as well as those by sputtering [5,6]. *Corresponding author. Tel.: +34-91-334-90-00; fax: +3491-372-06-23. E-mail address: [email protected] (M. Vila).

On the other hand, crystalline silicon nitride has a wide range of transparency in the UV to the IR region and good protective passivating properties that makes it a suitable candidate material for many optical applications requiring low optical losses in the films. Low-temperature processing is also required, which has led to extensive studies into the deposition of Si3N4 by plasma-enhanced CVD [7,8], or assisted ionbeam sputtering deposition [9]. We have chosen RF-sputtering because although the prepared silicon nitride thin films are amorphous, the technique is suitable for all low process temperature applications while high deposition rates can be obtained. In this work we present the compositional characterization, carried out by several techniques,

0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 2 2 1 - X

M. Vila et al. / Vacuum 67 (2002) 513–518

of a set of silicon nitride thin film samples prepared by RF-operated magnetron sputtering under different sputtering working gases. Due to the gas mix up of the growth atmosphere it is known the possibility of modifying the composition of the films by changing the deposition conditions [10]. The aim of the present paper is to correlate the optical and electrical characterization with the thin films stoichiometry.

2. Experimental Silicon nitride thin films were grown by using a planar 200 magnetron source (Angstrom Science). The sputtering system is equipped with a Huttinger RF-power supply. An Si3N4 circular target of 200  6 mm was mechanically clamped to the watercooled RF electrode. Silicon nitride discs are sintered from a mixture of silicon nitride and 10 wt% yttrium oxide powders. In our preparations, ceramic blocks are characterized by X-ray diffraction, composed of b-Si3N4 as a majority phase and yttrium silicate as a secondary phase located at the grain boundaries. The vacuum system provides a residual pressure near 1  107 mbar. Ar, Ar/H2(5%), N2, N2/Ar N2/H2(5%) with the purity of 99.999%, were used as sputtering gases, the gas flow rate was adjusted with a mass flow controller to obtain a pressure of 5.0  103 mbar for the sputtering deposition. Typical RF power was 100 W, obtaining the ( deposition rates of 54, 44 and 22.2 A/min depending on the mixture of gases used (Ar, Ar/N2 and N2, respectively). All the samples were grown at room temperature. Ex situ Auger electron spectroscopy (AES) and X-ray photoemission spectroscopy (XPS) experiments were carried out in a UHV chamber. This chamber is equipped with an Mg anode in the Xray source (1253.6 eV photons) and a double-pass cylinder mirror analyser with an energy resolution about 0.9 eV. Silicon nitride thin films were characterized by ion beam techniques at the 3 MV tandem accelerator of the Centro Nacional de Aceleradores at Sevilla.

3. Results and discussion From the macroscopic point of view, there are several physical properties that present differences between samples prepared with different sputtering gases, for instance, it has been reported that, at room temperature, the electrical resistivity of these samples may change more than three orders of magnitude [11]. Additionally, these samples are optically different when deposited over transparent substrate thin films grown in Ar-rich atmosphere which have a typically brown colour. In order to characterize this optical behaviour, we performed optical absorption measurements. Fig. 1 presents the absorbance of the five studied samples. These optical measurements in the UV– Vis region exhibit a big difference between the samples grown in the presence of nitrogen and without it; there is a shift in the absorption band edge from 400 to 225 nm when nitrogen-reactive sputtering is used, which is responsible for the absence of brown colour. Additionally, local maxima and minima can be observed in the transparent wavelength region. They are originated from the interference between the upper and the lower faces of the thin film. The position of these maxima and minima depends on the optical path (thickness d and refractive index n product), 2nd ¼ ml=4; m being the order number (even for maxima and odd for minima). For such thin films

2.0

Ar/H2 1.5

Absorbance

514

Ar 1.0

0.5

N2 N2/Ar N2/H2

0.0 200

300

400

500

600

700

800

Wavelength (nm) Fig. 1. UV–Vis absorbance spectrum of the silicon nitride films prepared with different sputtering gase mixtures on fused silica substrate, all the films have a thickness of 0.4 mm.

M. Vila et al. / Vacuum 67 (2002) 513–518

of well-known thickness, the refractive index can be obtained and the results are 2.5, 1.5, and 2.4 for the Ar, Ar/N2 and N2 preparation gases, respectively. The addition of small amounts of the hydrogen to the sputtering working gas has a very small influence in the refractive index, resulting in a very weak decrease (2.4 and 2.3 for Ar/H2 and N2/H2, respectively). Even the measurement of interference maxima and minima to obtain the refractive index is very precise; the thickness measurement may have a 5% error that gives a 5% error in the obtained refractive index. IR absorption spectroscopy has been used to detect the presence or absence of silicon dioxide in the films and also to have information about the influence of the preparation conditions on the amount of Si–N bonds present at the sample. Fig. 2 shows the IR absorption (obtained from a Perkin–Elmer FTIR spectrometer with 4 cm1 resolution) for the five studied samples of the same thickness (0.4 mm), prepared over two-face polished Si(1 0 0) wafers. A peak appearing at 600 cm1 is characteristic from the Si substrate. It can be noted that samples do not have the characteristic absorption band at 1040 cm1 of the asymmetric stretch mode of the Si–O–Si bond. This proves that there is no significant amount of silicon dioxide as a separate phase in the samples. Additionally, the shape of such a wide band, which appears in the 700–1200 cm1 region, is character-

1.0

Absorbance

Ar/H 2 0.8

0.6

0.4

Ar N 2/H 2 N2 Ar/N 2

0.2 400

600

800

1000

1200

1400

-1

Energy (cm ) Fig. 2. Infrared absorbance spectra of the films prepared with different sputtering gas mixtures. Spectra have been shifted vertically to a better observation. Film thickness is 0.4 mm for all the samples.

515

istic of an amorphous structure of the silicon nitride films [12]. Films exhibit two absorption peaks near 475 and 835 cm1, which are two intrinsic first-order IR absorptions being a silicon atom breathing mode and an Si–N stretching mode, respectively. The variation of the main peak frequency value (835 cm1) has been related to the formation of an homogeneous SiOxNy alloy [13], the shift in this value to higher energies is related to the increase of the oxygen in the sample. In our samples, the maxima move from 865 to 890 cm1 for the Ar/H2- and N2/H2-prepared samples. The three other samples give maxima in the 870– 875 cm1 range, which compared to the given value for pure Si3N4 in Ref. [13] lies in the low oxygen concentration region, but indicates that samples should be considered as a silicon oxinitride. In the hypothesis that silicon oxinitrides can be formulated as an (SiO2)x(Si3N4)1x alloy, the IR peak position will vary linearly between the nitride and oxide values (835 and 1055 cm1, respectively); under this assumption, the oxide fraction moves from x ¼ 0:14 to x ¼ 0:18: As can be observed in Fig. 2, samples grown with a certain amount of hydrogen contribute a little at 1170 cm1 that corresponds to the N–H bending and stretching bonds. The conclusion from the IR-absorption spectroscopy is that no big differences exist between samples, from the Si–N bonds’ point of view, to explain the observed optical and electrical behaviour. We have used AES in order to check the homogeneity of the samples. AES spectra have been collected before and after cleaning the surface by argon bombardment, as well as, at different depths and, except because of the carbon contamination at the surface, no differences can be observed in the atomic composition concentration neither laterally nor in depth. Fig. 3 shows the AES signal obtained from the Ar- and Ar/ N2(50%)-prepared samples. Spectra have been normalized to the nitrogen signal and show that samples have an oxygen content, in agreement with the IR absorption data. This contamination comes from the yttrium oxide needed to prepare the ceramic target with a proper density. On the other hand, XPS experiments have been performed in order to quantify the stoichiometry

M. Vila et al. / Vacuum 67 (2002) 513–518

516

of the silicon nitride films prepared with Ar and Ar/N2 as sputtering gases. To determine the nitrogen–silicon relative content, the intensity of the N 1s and the Si 2p core level peaks showed in Fig. 3b has been analyzed. We obtain I(N 1s)/

Intensity (arb. units)

Ar/N 2 Ar

100

C

Ar

Si

200

O

N

300

400

500

600

Kinetic energy (eV)

(a)

O 1s

Intensity (arb. units)

N 1s

Si 2p

Si 2s C Ar

Y

Ar/N2

Ar-prepared 0

100

(b)

200

300

400

500

600

Binding energy (eV)

Fig. 3. (a) AES signal obtained from samples prepared with Ar and Ar/N2(50%) sputtering gases. Spectra have been normalized to the N signal. (b) XPS signal obtained for the same two samples. Spectra have been corrected by the charge accumulation effect by using the residual carbon peak.

I(Si 2p)=1.38 and 1.64 for the Ar- and Ar/N2prepared samples, respectively. For a well-known stoichiometric Si3N4 sample, the I(N 1s)/I(Si 2p) ratio can be calculated [14] and the obtained value for SiNx with x ¼ 1:33 is 1.95. The comparison provides an SiNx stoichiometry of x ¼ 0:94 and 1:12 for our films prepared with Ar and Ar/N2, respectively. The error in the N/Si ratio can be estimated as 10%, being the sum of Si and N determination errors. Finally, ion beam techniques have been used to characterize the composition and density of the amorphous silicon nitride thin films. The absolute amount of nitrogen in the films was measured by Nuclear Reaction Analysis (NRA). In particular, the well known 14N(d; a1 )12C reaction [15] at 1390 keV was employed, with the NRA detector placed at a laboratory angle of 1501. This method allows quantitative and simple determination of nitrogen up to depths of about 1 mm. An extensive report of data analysis will be performed elsewhere [16]. The obtained nitrogen atomic density is given in Table 1. Additionally, the global composition and thickness of the films were determined by Rutherford backscattering spectrometry (RBS) using a 2 MeV He+ beam. Spectra were analyzed using the RUMP simulation code [17]. Experimental data as well as simulation are showed in Fig. 4 for the Ar/N2-prepared sample. The simulation does not correctly reproduce the Si(1 0 0) substrate signal because the channelling effect was not completely avoided, but this error does not have a direct influence on the determination of signal coming from oxygen and nitrogen because the step height is equal in the experimental data as in the

Table 1 Nitrogen density results from the nuclear reaction analysis and atomic density and sample chemical composition obtained from the simulation of the RBS by the RUMP code. [N]/[Si] and [O]/[Si] ratios are given to obtain the oxinitride chemical formula Sputtering gases

N-density (at/cm2)

Atomic density (at/cm2)

[Si]

[N]

[O]

[Y]

[Ar]

[N]/[Si] ratio

[O]/[Si] ratio

Ar Ar/H2 Ar/N2 N2 N2/H2

1.25  1018 1.18  1018 1.47  1018 2.23  1018 2.27  1018

3.20  1018 3.05  1018 2.50  1018 3.50  1018 3.80  1018

3.8 3.8 2.6 1.9 1.9

4 4 4 4 4

1.4 1.5 1.0 0.8 0.8

0.18 0.18 0.1 0.07 0.07

0.1 0.1 0.08 — —

1.05 1.05 1.5 2.1 2.1

0.3 0.3 0.3 0.7 0.7

M. Vila et al. / Vacuum 67 (2002) 513–518 N

Experimental Simulation

O

Yield

Si(100) wafer

Si

Ar

100

200

300

Y

400

Channel Fig. 4. RBS pared film. comparison. the signal of

spectra corresponding to the Ar/N2(50%)-preSimulation by the RUMP code is given for Chemical symbols and arrows are given to show each element placed at the film surface.

simulation, but needs a careful measurement of the step height. The obtained film atomic density and the relative amounts of their different components normalized to 4 for the nitrogen are given in Table 1. Under these conditions, the precision on the film stoichiometry thus determined is about 5%. [N]/ [Si] and [O]/[Si] ratio concentrations are also given in Table 1; in order to calculate the [O]/[Si] ratio, the oxygen quantity that forms small amounts of yttria in the film has been subtracted. It should be noted that there is a vague agreement in the sample stoichiometry when it is determined by the XPS measurements and when it is calculated by the combined use of the NRA and RUMP simulation of the RBS data. Nevertheless, the obtained chemical compositions do not allow a formulation of this oxinitride in the form of an (SiO2)x(Si3N4)1x alloy as suggested when oxinitride is intentionally prepared. From our point of view, the reason is that both silicon oxide and silicon nitride are stable compounds with nonstoichiometric content of oxygen and nitrogen, respectively. It should be noted that surface atomic density obtained for the Ar/N2-prepared sample is considerably smaller than that obtained for the other samples. Nevertheless, thickness has been determined to be 0.4 mm for all the samples by a talystep; thus, the actual volume atomic density for the Ar/N2-prepared film is smaller than for the

517

other samples and implies that this film has a worst packing of their grains. This is because the obtained refraction index by the interferences in the absorption spectra is quite small. It is worth mentioning that the obtained values for [N]/[Si] ratio explain the optical behaviour of these films. The sample prepared with Ar/N2(50%) results to be stoichiometric in agreement with that of Li et al. [18]. This film has the lowest refractive index value and a very good transparency. When samples are prepared with an Ar-rich gas, films have an excess of silicon that provokes an increase in the refractive index as well as an increase in the extinction coefficient for wavelengths below 400 nm. Samples prepared with an N2-rich gas have nitrogen in excess that changes the refractive index with respect to the stoichiometric sample, but does not change the transparency of the film.

4. Conclusions Silicon nitride thin films have been prepared from a dense Si3N4 target by RF magnetron sputtering within different working gases. The nature of gases used in the preparation has a strong influence on the optical properties of the thin films, giving values for the refractive index from 1.5 to 2.5. The causes for this variation should be found in the atomic composition of the films, while their actual composition has been determined by the RBS technique. The sample prepared with Ar/N2(50%) presents a composition with an [N]/[Si] ratio near stoichiometric value but with an oxygen content, samples prepared with Ar-rich gases present the defect of nitrogen and samples prepared with N2-rich gases have nitrogen in excess with respect to the stoichiometric Si3N4 compound and present a higher oxygen content.

Acknowledgements This work has been supported by CICyT under grants’ number MAT2000-0767-C03-01 and MAT2000-0767-C03-02 as well as by CAM 07N/ 055/1998.

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References [1] Ouacha A, Willander M, Hammarlund B, Logan RA. J Appl Phys 1993;74:5602. [2] Shi L, Steenberger AM, de Vreede AH, Smit MK, Scholtes TLM, Groen FH, Pedersen JW. J Vac Sci Technol A 1996;14:471. [3] Lapeyrade M, Besland MP, Meva’a C, Sibai A, Hollinger G. J Vac Sci Technol 1999;17:433. [4] Hugon MC. J Vac Sci Technol 1999;17:1430. . HP, Huppertz M. Thin Solids Films 1998;317:153. [5] Lobl [6] Awan SA, Gould RD, Gravano S. Thin Solids Films 1999;355–356:456. [7] Dodabalapur A, Rothberg LJ, Miller TM, Kwock EW. Appl Phys Lett 1994;64:2486. [8] Shokrani M, Kapoor VJ, In: Pouch JJ, Alterovitz SA, editors. Plasma properties, deposition and etching, vols. 140–142 of materials science forum. Switzerland: Trans Tech Publications Aadermannsdorf, 1993.

[9] Lambrinos MF, Valizadeh R, Colligon JS. Appl Opt 1996;35:3620. [10] Hirohata Y, Shimamoto N, Hino T, Yamashima T, Yabe K. Thin Solid Films 1994;253:425. [11] Vila M, Prieto C, Miranzo P, Osendi MI, Ram!ırez R. Surf Coatings Technol 2002;151–152:67. [12] Wada N, Solin SA, Wong J, Prochazka S. J Non-Cryst Solids 1981;43:7. [13] Tsu DV, Lucovsky G, Mantini MJ, Chao SS. J Vac Sci Technol A 1987;5:1998. [14] K.archer R, Ley L, Johnson RL. Phys Rev B 1984;60:1898. [15] Amsel G, Nadai JP, D’Artemare E, David D, Girard E, Moulin J. Nucl Instrum Methods 1971;92:481. ! [16] Vila M, Garc!ıa-Lopez J, Prieto C, Respaldiza MA, to be published. [17] Doolittle LR. Nucl Instrum Methods B 1985;9:344. [18] Li B, Fujimoto T, Fukumoto N, Honda K, Kojima I. Thin Solid Films 1988;334:140.