Nitrogen Bonding Configurations of SiOxNy Thin Films in Power

Journal of The Electrochemical Society, 155 共1兲 G1-G7 共2008兲

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0013-4651/2007/155共1兲/G1/7/$23.00 © The Electrochemical Society

Nitrogen Bonding Configurations of SiOxNy Thin Films in Power MOSFET Gate Interfaces E. Fazio,a F. Monforte,a F. Neri,a,z F. Bonsignore,a G. Curro,b M. Camalleri,b and D. Calib a

Dipartimento di Fisica della Materia e Tecnologie Fisiche Avanzate, Universitá di Messina, I-98166 Messina, Italy STMicroelectronics, 95121 Catania, Italy

b

The structural properties of silicon oxynitride films grown in a N2O environment at temperatures higher than 900°C and for use as gate dielectrics in vertically diffused power metal oxide semiconductor field effect transistor 共PowerVDMOS兲 technologies have been studied by means of X-ray photoelectron spectroscopy. The progressive modifications of the bonding environments upon reaching the oxynitride–silicon interface have been analyzed as well as of the relation between these modifications and the selected oxynitridation process. The results show that the chemistry of the oxynitride layer is a rather complex one, and it significantly and progressively changes by moving toward the silicon interface, in a way strongly affected by the growth process. In particular, the medium thermal budget processes 共950°C, 20–60 min兲 favor the formation of a relatively uniform distribution of the single oxidized O–N–Si2 bonds both at the interface and throughout its immediate backstage. Such findings can help in assessing the role played by the nitridation process in the quality and reliability performances of the final device. © 2007 The Electrochemical Society. 关DOI: 10.1149/1.2799732兴 All rights reserved. Manuscript submitted April 12, 2007; revised manuscript received September 12, 2007. Available electronically October 29, 2007.

In the last few years, silicon oxynitride thin films have been proposed as an alternative to SiO2 as a thin gate dielectric for microelectronics applications. As the scale of integration increases and the thickness of the dielectric is reduced, the SiO2 dielectric layer properties get less and less sufficient to reliably withstand the increasing electric field. As a consequence, the device degradation due to the high leakage currents and even gate rupture shows up at a higher rate. Silicon oxynitrides 共SiOxNy兲 are materials with a higher dielectric constant and, in thin layer form, exhibit reduced susceptibility to interface state generation with respect to SiO2, higher timeto-breakdown values and reliability, improved I-V and C-V characteristics.1-3 Therefore, silicon oxynitride could eventually replace thin gate dielectrics characterized by a smaller thickness but an equal capacitance, thus assuring an improved robustness of the device. In this context, several analyses have been performed to characterize SiOxNy film quality in terms of both device performance and processing.4,5 A clear understanding of the structure and chemical composition of the oxynitride film could be valuable help in assessing its quality as a gate layer in power devices. Nevertheless, in many works found in the literature, the only relevant parameter taken into account is the overall nitrogen content. In the attempt to disclose the role of the oxynitride structure in the electrical defectivity and reliability, it is of fundamental importance to know the nitrogen bonding configurations present at the silicon substrate interface and their relation with the selected oxynitridation process. In this respect, X-ray photoemission spectroscopy 共XPS兲 is a powerful tool to perform a compositional analysis and also to investigate the relative bonding arrangements of silicon, oxygen, and nitrogen atoms. The aim of the present work is to investigate the progressive modifications of the nitrogen bonding arrangements upon reaching the interface for a set of oxynitride layers grown by means of furnace N2O-based dry processes at different temperatures and times to be used as gate dielectrics in vertically diffused power metal oxide semiconductor field effect transistor 共PowerVDMOS兲 technologies. Changes of the main bonding configurations, moving toward the Si interface, were found to significantly depend on nitridation process parameters, such as the process duration and the furnace temperature at which the oxynitridation processes were performed. The overall picture of the atomic concentrations evolution was checked carrying out a deconvolution of the XPS photoelectron spectra using a fourband model according to existing literature data.3,6-9

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E-mail: [email protected]

Experimental The oxynitride films were grown by means of a dry hightemperature process in N2O flow. Initially, 20 nm thick SiO2 films was grown at 850°C onto a Si共100兲 n-type epitaxial substrate with a 1 ⍀ cm resistivity. Then, in order to produce the formation of an interfacial nitrided sublayer between the silicon substrate and the top-lying pure SiO2, a furnace oxynitridation process in N2O gas, for 20 or 60 min, with temperature ranging from 900 to 1000°C, was performed. Thus, four different layers were prepared as shown in Table I. Film thicknesses of 21 nm 共for the samples BL-AC兲 and 22 nm 共for the samples AL-XC兲 were estimated from the analysis of ellipsometric data. The thickness values depend on the balance between N2O diffusion through the initial oxide film and the reaction rate at the interface, the latter being strongly dependent on the nitrogen concentration embedded in the near interface. Ex situ ellipsometric measurements were performed with a Jobin–Yvon UVISEL phase modulated spectroscopic ellipsometer in the energy range 0.75–4.75 eV at a fixed incident angle of 70.6°. XPS measurements of the Si 2p and N 1s photoelectron peaks were carried out in an ultrahigh vacuum 共UHV兲 system using the Mg K␣ radiation of a conventional achromatic twin-anode Al/Mg K␣ source. The photoelectron spectra were collected by means of a hemispherical sector analyzer 共CLAM 100兲 from VG Instruments used in constant analyze energy 共CAE兲 mode with a 7 eV pass energy in order to maximize the overall line resolution to slightly less than 1.0 eV full width at half-maximum 共fwhm兲. Being a surface technique, XPS is able to probe only a few monolayers; hence, to characterize deeper layers, it is necessary to remove the SiO2 overlayer. An appropriate chemical etching process was performed at different etch times 共160, 175, 190, 205, 220, and 235 s兲 using a diluted HF 共1:100兲 solution in H2O, with a 0.9 Å/s etch rate. For each etch time, the residual oxide thickness was estimated from the Si 2p4+ /Si 2p0+ peak intensity ratio, using the well-known “overlayer model.”3 In Fig. 1 is shown the residual oxide thickness as a function of the etch time. Upon in-

Table I. Growth conditions and films thicknesses as derived through ellipsometry measurements.

Sample

T 共°C兲

t 共min兲

d 共nm兲

BL AC AL XC

900 950 950 1000

60 20 60 20

21 21 22 22

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Journal of The Electrochemical Society, 155 共1兲 G1-G7 共2008兲 of the photoemission signals weighted with the corresponding atomic sensitivity factors.10 In particular, we choose to analyze the relative nitrogen concentration Ntot with respect to those of silicon and oxygen in the oxide and in the nitrurated phase 共i.e., we excluded the substrate contribution兲 Ntot共%兲 =

Figure 1. Residual dielectric thickness vs etch time.

creasing the etch time, the residual thickness decreases from 6.1 nm down to 1.2 nm, showing a saturation behavior for the last two etching time values, i.e., indicating that the silicon substrate interface has been reached. Results and Discussion XPS was the experimental technique adopted to study the chemical bonding states of the grown SiOxNy thin films and the relative atomic content. From the literature data, it is evident that it is important to study, not only the effect of the overall nitrogen content near the interface, but also the specific bonding configurations present and their relation with the selected oxynitridation process, in order to optimize the SiOxNy film quality. We obtained a quantitative estimation of the film’s composition from the relative intensities

IN /SN ⫻ 100 ISiox /SSi + IO /SO + IN /SN

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being IN, IO, and ISiox the integrate intensities of the N 1s, O 1s, and Si 2p peaks and SN, SSi, and SO the sensibility factors of each element. The choice is justified by the strong localization of the nitrogen distribution, present only near the real interface between the dielectric and the semiconductor. Otherwise, the invariability of the XPS probe volume as a function of the etch time would induce a loss of correlation between the nitrogen atomic content for the different residual thicknesses. Then, we looked at the modifications occurring in the subband structure of the Si 2p and N 1s core level photoemission peaks as a function of the growth parameters. Moreover, for each sample, the evolution of the different bonding configuration as a function of the residual thickness was analyzed. These results are reported in Fig. 2-5. Despite the low S/N ratio, readily recognizable modifications of the Si 2p and N 1s core level photoemission peaks are evident as a function of both the nitridation parameters and the distance from substrate interface. In particular, the following effects are evident: 共i兲 the Si 2p bands show the presence of two distinct components at about 99.5 eV 共silicon bonded to silicon, i.e., the substrate contribution兲 and at ⬃104 eV 共silicon atoms bonded to oxygen兲; 共ii兲 the Si 2p lineshapes show overall widths exceeding 0.9 eV with a marked and increasing asymmetry upon reaching the interface 共particularly evident in the sample grown at 1000°C, 20 min兲; 共iii兲 a progressive modification of the N 1s band that, within our experimental resolution, shows a multicomponent structure. In order to investigate these modifications and the chemical shifts for the N 1s photoemission peaks, we performed a subbands deconvolution based on a best-fit procedure. According to the literature data about the chemical structure of nitrogen atoms within the

Figure 2. Experimental Si 2p 共a兲 and N 1s 共b兲 XPS spectra of SiOxNy thin films grown at 900°C, 60 min at different etch times.


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oxynitride films,6-9,11,12 the dominant and characteristic components of the N 1s line shape are the Si3⬅N contribution 共related to the N species with three Si nearest neighbors兲 and the N共–SiO3兲x one. This latter component differs from the previous one only in the second-nearest neighbors, where the oxygen atoms, substituting Si, induce a binding energy difference of 0.7 eV.9 That is, the Si3⬅N

bonds corresponds to the nitrogen atoms dispersed within the SiO2 matrix, while the N共–SiO3兲x configuration corresponds to those incorporated into the Si interfacial layers.12 As reported in the literature, there are quantitative disagreements in the binding energy of each specific nitrogen specie, which made the assignments uncertain. Nevertheless, according to theoretical expectations and on the


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basis of high-resolution measurements performed using synchrotron radiation,8,13 the occurrence of two other different bonding configurations, namely, the O–N–Si2 and O2–N–Si ones, have been proposed for the deconvolution of the N 1s photoelectron band.6 On the basis of the above considerations, we carried out the spectral deconvolution of the N 1s line shape with four Gauss– Lorentzian bands representing the following bonding configurations in order of increasing binding energy: Si3⬅N, N共–SiO3兲x, O–N–Si2 and O2–N–Si bonds; located respectively at 397.5, 398.8, 400.5, and 401.7 eV. The results are shown in Fig. 6 and 7 for all the grown samples at the etch time of 160 and 235 s, respectively. It is evident that the nitrogen bonding configurations are progressively changing their relative weight with the selected oxynitridation processes. An examination of the line-shape trends in Fig. 3-5 already shows a sudden and, by far, not obvious redistribution of the spectral weight

among all the nitrogen bonding components upon moving toward the substrate. When the growth process occurs at temperatures in the 900–950°C range, a film is produced that exhibits a progressive transfer of weight from the embedded N共–SiO3兲x to the singly oxidized O–N–Si2 configuration as the true interface is approached. A more complicated evolution of the bonding distribution within the probed depth is observed in the case of the 1000°C grown layer, which could suggest a significant modification of the growth mode in such a case. A better understanding of the change of the SiOxNy chemistry can be obtained by examining the progressive modifications of the bonding environment upon reaching the oxynitride– silicon interface. This is of great importance because these local bonding configurations could be somewhat related to the amount and distribution of point defects, which act as carrier traps affecting, for example, the channel region transport properties of a Power MOSFET device. The overall picture of the bond fractions evolution

Figure 6. N 1s spectra of SiOxNy films grown at 900°C, 60 min 共a兲, 共c兲 and at 950°C, 20 min 共b兲, 共d兲 at the etch times of 160 and 235 s, respectively. The dashed lines represent the fitting of the peaks related to the Si3⬅N, N共–SiO3兲x, O–N–Si2 and O2–N–Si bonds, respectively.


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Figure 7. N 1s spectra of SiOxNy films grown at 950°C, 60 min 共a兲, 共c兲 and at 1000°C, 20 min 共b兲, 共d兲 at the etch times of 160 and 235 s, respectively. The dashed lines represent the fitting of the peaks related to the Si3⬅N, N共–SiO3兲x, O–N–Si2 and O2–N–Si bonds, respectively.

共see Fig. 8兲, as a function of the residual thickness, shows that: 共i兲 the overall nitrogen content varies from 1 to 1.5%, for a distance from the interface of ⬃6.0 nm, to 3% and, for the sample grown at 1000°C, up to ⬃5% at the interface; 共ii兲 the Si3⬅N configuration, as expected, is significantly present only at the interface and for the highest temperature processes; 共iii兲 the N共–SiO3兲x component 共N atom connected to O through a silicon site, with x = 1, 2, or 3兲 is the main one; 共iv兲 the N共–SiO3兲x contribution abruptly decreases when the interface is almost reached, especially for the high thermal budget process; 共v兲 the O–N–Si2 and O2–N–Si components 共N bonded to one or two O nearest neighbors兲 remain negligible throughout all the film thickness and suddenly increase when the interface is approached 共the former being the prevalent one兲. Such evidence suggests the occurrence of a prevalent oxidation mode at the true interface at the expense of the N共–SiO3兲x one. As a consequence, in such cases, a certain degree of interface disorder can be envisaged in the layer near the interface with the inclusions of oxynitrided bonds in the substrate and largely diffused throughout the interface plane. However, when a high thermal budget process 共1000°C, 20 min兲 is used 共see Fig. 8b兲, a Si3⬅N termination at the interface, followed by an oxidized nitrogen backstage, is produced. At least two mechanisms are likely necessary to explain this finding. First, nitrogen, brought into the film during oxynitridation, reacts only with Si–Si bonds at or near the interface, not with Si–O bonds in the bulk 共i.e., nitrogen is present in a nonequilibrium state where the rate of transition to equilibrium is slow and some nitrogen is

trapped兲. Alternatively, the Si3⬅N piling up could be due to the strong mismatch stress at the Si/SiO2 interface for larger thermal budget. The strain-related mechanism can be assumed to determine the abrupt increase of the silicon nitride phase and, at the same time, a corresponding increase of the oxidized nitrogen backstages close to the true interface.14,15 Such a different nitrogen bonding configuration scenario can dramatically impact the electrical quality of the interface and the effective type and distribution of electrically active point defects, contributing to the final reliability of the device. In fact, according to the literature data, the interstitial silicon, generated at the interface during the oxynitride process, does not spread over the oxide but remains close to the interface. Our experimental finding of such an occurrence may be of great importance when related to the detrimental role played by the silicon enrichment as a precursor of electrical defectivity.5 A limited number of reports exists in the literature concerning a systematic correlation between the bonding configuration of silicon oxynitride films and the growth parameters characterizing the specific preparation method. To further investigate about the chemistry of the oxynitride layers, we analyzed the behavior of each of the above identified contributions in terms of the nitridation parameters. In detail, the behavior of the bond fractions and of the overall nitrogen content was considered at the interface distance values of 3.4 and 1.2 nm, respectively 共see the trend in Fig. 9兲. Such values were chosen to be, in one case, near the substrate interface and, in the other case, in a dielectric region whose distance from the interface is

Figure 8. Behavior of the nitrogen content 共a兲 and the bond fractions 共b兲, 共c兲, and 共d兲 vs the residual thickness for the four oynitridation processes.

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Figure 9. Nitrogen content and bond fractions for two residual thicknesses: 1.2 nm 共full bar兲 and 3.4 nm 共open bar兲.

comparable to the tunnel length typical of the electrons injected from the substrate into the dielectric layer applying an appropriate electric field 共a procedure generally adopted to investigate the electrical quality of the gate layer in MOS devices兲. From the trends shown in Fig. 9, it appears that the sample grown at the highest thermal budget process shows the highest interfacial nitrogen content, whereas for all the other samples, it is comparable to the bulk one. Concerning the nitrogen bonding configurations, the following observations can be made. Near the interface, it seems that the oxynitride process at 1000°C favors the formation of the Ox–N–Siy phase and of the Si3⬅N bonds. 共The O–N–Si2 and O2–N–Si bond fractions have been considered together because the latter has a lower weight.兲 Far away from the silicon interface 共at ⬃3.0 nm兲, the nitrogen content is comparable in all the grown samples and the Si3⬅N bond fraction values are very low. However, it seems that, at 1000°C, the formation of the O–N–Si2 phase is less favored since the N共–SiO3兲x component is the prevalent one. Moreover, the N共–SiO3兲x bond fraction values at 3.0 nm from the interface are comparable or higher than the ones estimated near the interface 共see also Fig. 8兲, for each of the nitridation growth processes. From the overall trend shown in Fig. 9, it is evident that the bonding distribution is complex and significantly different for each growth mode 共in particular, when high-temperature processes are used兲 and that it changes dramatically as a function of the distance from the interface, for the selected nitridation growth parameters. By means of this study, it has been possible to identify the potentially optimized parameters of the nitridation process. Medium thermal budget processes 共temperature higher than 900°C, but not 1000°C, and times ⬎20 min兲 are required in order to prepare oxynitride layers where the O–N–Si2 phase is significantly present both at and near 共3 nm兲 the silicon interface. This chemical configuration, which tends to stabilize oxygen, is generally considered necessary to avoid the formation of trapping defects and, then, is useful to improve the interface electrical quality and the final device performance.16,17 Conclusion The oxynitride/silicon interface for a series of differently prepared samples was investigated by means of XPS spectroscopy in order to identify all the bonding configurations observed. The following was found:

1. The chemistry of the oxynitride layer is a rather complex one; it changes progressively moving toward the silicon interface. 2. The nitrogen bonding configurations, their distribution, and also the overall nitrogen content change significantly as a function of the selected oxynitridation thermal budget process 共i.e., temperature and time兲. 3. For the higher thermal budget process 共1000°C, 20 min兲, the Si3⬅N pinning up at the true interface is needed to reduce a strong mismatch stress. 4. The medium thermal budget processes 共950°C, 20–60 min兲 favor the formation of a relatively uniform distribution of the single oxidized O–N–Si2 bonds both at the interface and throughout its immediate backstage. This chemical configuration is generally considered necessary to avoid the formation of trapping defects and, then, useful to improve the interface electrical quality and the final device performance. In conclusion, the above-outlined results are representative of a systematic study of the progressive modifications of the nitrogen bonding configurations upon reaching the oxynitride silicon interface as well as of the relation between these modifications and the growth parameters. A check of the possible correlation between the observed nitrogen chemistry at the interface and the electrical quality of the dielectric layer is currently under investigation. University of Messina assisted in meeting the publication costs of this article.

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