Structural analysis of silicon oxynitride films deposited by PECVD

Materials Science and Engineering B 112 (2004) 123–127 www.elsevier.com/locate/mseb

Structural analysis of silicon oxynitride films deposited by PECVD D. Criadoa,*, M.I. Alayoa, I. Pereyraa, M.C.A. Fantinib a b

Escola Polite´cnica, Universidade de Sa˜o Paulo, CP 61548, 5424-970 Sa˜o Paulo, Brazil Instituto de Fı´sica, Universidade de Sa˜o Paulo, CP 66318, 05315-970 Sa˜o Paulo, Brazil

Abstract We have investigated the SiOxNy films obtained by plasma enhanced chemical vapor deposition (PECVD) technique at low temperature using nitrous oxide (N2O), nitrogen (N2) and silane (SiH4) as precursor gases. Controlling the gas flow ratio during film deposition, it was possible to vary the material stoichiometry from silicon dioxide to silicon nitride. The structure of the films was investigated by X-ray absorption near-edge spectroscopy (XANES) at the Si–K, N–K and O–K edges, Rutherford backscattering spectroscopy (RBS) was utilized to characterize the material composition. The results show the possibility of obtaining a chemical composition varying continuously from SiO2 to Si3N4. The Si–K edge absorption spectra indicate that the basic structural element of the network is a tetrahedron with a central Si atom connected to N and O, consistent with a random bonding model. Analysis at the N–K edge shows the presence of two distinct edges, which are attributed to the different nitrogen neighborhoods in this material, nitrogen in a Si3N4 matrix and nitrogen substituting O in a SiO2 type matrix. # 2004 Elsevier B.V. All rights reserved. Keywords: Silicon oxynitride; Plasma enhanced chemical vapor deposition; X-ray absorption near edge structure (XANES)

1. Introduction Silicon oxynitride films have nowadays assumed great importance for the development of various microelectronic and optical devices, such as: waveguides, TFT transistors, and micro-electro-mechanical systems (MEMS) [1–4]. This material offers some advantages as thin gate dieletric in microelectronic devices in comparison to SiO2, i.e. the more important being suppression of boron penetration, enhanced reliability, high resistance to radiation, low density of interface defects, higher dielectric constant and reduced hot-electron induced degradation [5–9]. For optical devices, the SiOxNy is very attractive mainly due to its transparency in the visible range, and tunability of the refractive index from 1.47 (SiO2) to 2 (Si3N4), allowing a large degree of freedom in waveguides projects [10–14]. Besides, its residual stress can be varied from compressive to tensile by changing the films chemical composition [15,16].

There are many different techniques to obtain silicon oxynitride [1,3,6,13,17,18], but the PECVD technique is particularly interesting due to the possibility of a good control of the chemical composition and a wide range control of the deposition rate by changing the parameters of deposition at low temperatures (320 8C), very important characteristics for the integration of micro-opto-electromechanical structures (MOEMS) [9,19–23]. In this work we focus on the chemical and morphological properties of the silicon oxynitride films. We observe that the chemical composition of the material changes from silicon dioxide to silicon nitride, only by varying the N2/N2O flow ratio and fixing all the other parameters of deposition. Also we present studies on the N, O and Si local order by X-ray near edge spectroscopy (XANES) for films with different composition, as characterized by the Rutherford backscattering spectroscopy (RBS) technique.

2. Experimental * Corresponding author. Tel.: +55 11 3091 5256; fax: +55 11 3091 5585. E-mail address: [email protected] (D. Criado). 0921-5107/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.017

The SiOxNy films studied in this work were deposited in a standard 13.56 MHz RF PECVD capacitively coupled

D. Criado et al. / Materials Science and Engineering B 112 (2004) 123–127

system described elsewhere [24], from appropriate gaseous mixtures of electronic grade (99.999%) silane (SiH4), nitrous oxide (N2O) and nitrogen (N2). The films were obtained utilizing different nitrogen and nitrous oxide gaseous flows but maintaining the total N2O + N2 flow equal to 75 sccm for samples grown with high deposition rate. For obtaining low deposition rate the N2O + N2 flow was maintained in 39 sccm. The thickness for all samples was 100 nm. All the samples studied were deposited at 320 8C, [25] to prevent undesirable Si–OH bonds, the RF power density for samples with high deposition rate was kept at 500 mW cm 2. The SiH4 flow was fixed at 15 sccm, value high enough to lead to appropriate deposition rates for thick films production [26] but sufficiently low as to prevent undesirable gas phase reactions. For these conditions, the deposition rate was between 15 and 25 nm/min. In order to obtain low deposition rate, we use RF power density of 250 mW cm 2 and 3 sccm silane flow [25]. The deposition rate was between 0.26 and 0.30 nm/min. All the studied films were deposited onto p-type, (1 0 0), single crystalline silicon substrates in the 1–10 V cm resistivity range, for XANES measurements, and onto ultra dense amorphous carbon for Rutherford backscattering spectroscopy (RBS) measurements. The thickness of the samples was determined with a Tencor 500 profilometer. The RBS experiments were done at LAMFI/USP, Sa˜ o Paulo, using a He+ beam with energy of 1.7 MeV, charge of 30 mC, current of 30 nA and detection angle of 1708. The XANES measurements were conducted at the Brazilian Synchrotron Light Laboratory (LNLS, Campinas, Brazil), performed at room temperature under 108 mbar. Two samples were measured before and after a sputter etch process and negligible changes were detected at the O and N–K absorption edges. The films measurements at the Si–K edge were done at the SXS beamline [27] in the 1800–2300 eV range, using a channel cut InSb (1 1 1) monochromator. The XANES spectra were collected in the 1835–1855 eV energy range with 0.2 eV step. The N–K and O–K edge spectra were recorded at a SGM beamline using a 0.5 mm2 spot in the 390–600 eV range with steps of 0.3, 1.0 and 0.3 eV for the 390–420, 420–450 and 550–600 eV energy ranges, respectively. The spectra were colleted in total current yield detection mode. In Table 1 the deposition conditions utilized Table 1 Deposition conditions for all the measured samples by XANES Name

N2O (sccm)

N2 (sccm)

SiH4 (sccm)

K-edges measures

A B C D E F G H I

75 60 45 15 0 39 7 3 0

0 15 30 60 75 0 32 36 39

15 15 15 15 15 3 3 3 3

O and N–K Si–K O and N–K Si, O and N–K Si–K Si–K O and N–K O and N–K Si, O and N–K

for samples growth and the different XANES edges measurements accomplished in each one of them are shown.

3. Results In Fig. 1 the nitrogen, oxygen and silicon atomic concentration as a function of the N2 and N2O gaseous flow are shown for samples grown with high deposition rate. It is observed that the highest oxygen incorporation in the solid phase occurs, as expected, for the highest N2O gaseous flow utilized. The material obtained under these deposition conditions presents a chemical composition similar to stoichiometric silicon dioxide (67 at.% for oxygen and 33 at.% for silicon). Furthermore, increasing the nitrogen gaseous flow (and diminishing therefore the nitrous oxide one) increases the nitrogen concentration and diminishes the oxygen concentration monotonically until a composition similar to stoichiometric silicon nitride is attained. Similar results were reported for films obtained by the remote plasma enhanced chemical vapor deposition (PECVD) technique utilizing SiH4, N2, N2O and He gaseous mixtures [8]. Also, we can observe in Fig. 1 that the nitrogen concentration in the material does not present a linear increase with the nitrogen gaseous flow, and high nitrogen flows are necessary in order to obtain efficient N incorporation. This can be attributed to the different reactivity between nitrogen, oxygen and silane radicals in the plasma. It is known that silicon radicals react preferentially with oxygen radicals to form Si– O bonds [16,25], and only when all the oxygen radicals have been consumed Si–N, Si–H and N–H bonds are formed. In this way, when high nitrous oxide gaseous flows are utilized, many oxygen radicals are present in the plasma, and so, almost all the silane radicals will react with oxygen radicals, resulting in few Si–N bonds in the solid phase. Only when low nitrous oxide gaseous flows (and therefore, low quantity

70

Atomic Concentration (%)

124

Si O N

60 50 40 30 20 10 0 0

10

20

30

40

50

60

70

80

N2 Gaseous Flow (sccm) (

N 2O flow)

Fig. 1. Atomic concentration of oxygen (*), nitrogen (~) and silicon (&) in the films deposited with different N2 and N2O gaseous flows. The SiH4 gaseous flow was maintained constant in 15 sccm.

D. Criado et al. / Materials Science and Engineering B 112 (2004) 123–127

Atomic Concentration (%)

70

125

Si O N

60 50 40 30 20 10 0 0

4

8

12 16 20 24 28 32 36 40 44

N2 Gaseous Flow (sccm)

(

N2O flow

)

Fig. 2. Atomic concentration of oxygen (*), nitrogen (~) and silicon (&) in the films deposited with different N2 and N2O gaseous flows. The SiH4 gaseous flow was maintained constant in 3 sccm.

all samples, in spite of the huge variation in oxygen content (65–9%), with the edge absorption at 533 eV (+0.1 eV), which indicates that the O atoms only bond to Si atoms. The N–K edge normalized spectra are depicted in Fig. 5. The spectrum for a Si3N4 sample deposited by CVD technique is also shown for comparison. All the spectra present an absorption edge at approximately 398.5 eV, similar to the Si3N4, which is related with the N–Si bonds. However, all the PECVD samples present a resonance line (RL) at lower energy, approximately 398.7 eV, which increases in intensity for higher oxygen content in the sample. Other authors [5,31–33] have observed this resonance line in amorphous non-hydrogenated SiNx films formed by ion implantation and attributed it to N dangling bonds due to the fact that this atom is undercoordinated in these samples. These authors

thermal SiO2

Absorption (a. u.)

of oxygen radicals), Si–N bonds will be incorporated in the material, these results are totally compatible with the FTIR characterizations presented in previous works [28]. In Fig. 2 are shown the RBS measurements for the samples grown with low deposition rate. We can observe that for the three samples deposited with lower nitrogen flows, the chemical composition is practically the same of silicon dioxide (33 at.% for silicon and 66 at.% for oxygen), and only for samples grown with N2O flow 7 sccm, it is possible to notice an appreciable nitrogen incorporation. This fact is similar to that explained for high deposition rate grown samples. For the higher nitrogen flow, a nitrogen concentration (53 at.%), value very similar to stoichiometry Si3N4, is obtained even though this material presents 9 at.% O, which we attributed to residual N2O from previous depositions. The XANES spectra at the Si–K edge of thermal silicon dioxide, amorphous silicon nitride and amorphous silicon oxynitride films are presented in Fig. 3. We can see that all samples have an abrupt absorption edge and with the decrease in the nitrogen and consequent increase in oxygen incorporation, the absorption edge changes from the energy typical of silicon nitride to that typical for silicon dioxide. It is very important to note that the samples do not present two absorption edges, corresponding to SiO2 and Si3N4 absorption edges suggesting that Si bonds randomly with nitrogen and oxygen in the material studied in this work. Similar results were obtained in previous works for films grown from N2O and SiH4 gaseous mixtures [29,30]. In Fig. 4 the O–K absorption edge is shown, all the spectra were normalized and the spectrum for thermal SiO2 is also shown for better comparison. The obtained results demonstrated that the local oxygen structure is the same for

F (Si=36% O=64% N=0%) B (Si=36% O=60% N=4%) D (Si=39% O=30% N=31%) I (Si=38% O=09% N=53%) E (Si=43% O=02% N=55%)

1840

1845

1850

1855

1860

1865

Energy (eV) Fig. 3. The Si–K edge XANES espectra of the under study films and the reference thermal silicon dioxide.

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to N–Si bonds in a Si3N4 type matrix and to a resonance line at lower energy attributed to N dangling bonds originated by N atoms substituting O in the Si–O–Si bridges.

thermal SiO2

Absorption (a. u.)

A (Si=34% O=65% N=0%) C (Si=38% O=55% N=06%) G (Si=37% O=54% N=09%)

Acknowledgments

H (Si=38% O=34% N=28%) D (Si=39% O=30% N=31%) I (Si=38% O=09% N=53%)

520

530

540

550

560

570

580

590

600

Energy (eV)

Thanks are due to Brazilian Synchroton Light Laboratory—LNLS/Brazil and Ion Beam Materials Analyze Laboratory—LAMFI-IF/Brazil for the RBS measurements. The authors are grateful also to Brazilian agencies FAPESP (Process numbers: 03/04523-6 and 00/10027-3 and 01/ 06516-1) and CNPq for financial support.

Fig. 4. The N–K edge XANES espectra of the under study films and the reference silicon nitride.

References C (Si=38% O=55% N=06%)

Absorption (a. u.)

G (Si=37% O=54% N=09%) H (Si=38% O=34% N=28%) D (Si=39% O=30% N=31%) I (Si=38% O=09% N=53%) stoichiometry Si 3 N4

390

400

410

420

430

440

450

460

470

Energy (eV) Fig. 5. The O–K edge XANES spectra of the under study films and the reference thermal silicon dioxide.

combined XANES and EXAFS measurements and observed a correlation between N coordination in N-rich SiNx and the evolution of the RL in the XANES spectra [31]. The RL observed in our samples can also be related to N dangling bonds originated by two fold coordinated N atoms substituting O in Si–O–Si bridges on a SiO2 matrix.

4. Conclusion In this work the structure of silicon oxynitride films deposited by RF PECVD with composition changing from silicon dioxide to silicon nitride is studied. XANES spectroscopy at the Si–K edge shows that both oxygen and nitrogen atoms bond to silicon atoms in a proportion consistent with the RBS measurements. These results indicate that the best description for the material network is the random bonding model (RBM). The results for the O–K edge show that O bonds only to Si atoms. The N–K edge XANES results lead to an absorption edge similar to stoichiometric Si3N4, related

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