Reoxidization Process Effects on the Nitrogen Bonding Configurations

Journal of The Electrochemical Society, 155 共6兲 G134-G139 共2008兲

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0013-4651/2008/155共6兲/G134/6/$23.00 © The Electrochemical Society

Reoxidization Process Effects on the Nitrogen Bonding Configurations in SiOxNy Power MOSFET Dielectric Gate 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, 1-98166, Messina, Italy STMicroelectronics, 95121, Catania, Italy

b

The structural properties of silicon oxynitride films used at the gate dielectrics interface in Power vertically diffused metal oxide semiconductor technologies have been studied by means of X-ray photoelectron spectroscopy. An overall picture of the interface chemistry evolution as a function of the growth parameters in relation to the effects of the postgrowth reoxidation process is reported. The films were grown in an N2O environment at temperatures higher than 900°C and subsequently reoxidized at 1000°C in a dry oxygen environment. The results show that the chemistry of the oxynitride layer progressively changes by moving toward the silicon interface and, after the reoxidation process, the interface chemical configurations are strongly affected by the initial specific oxynitridation process. In particular, the application of the final reoxidation plays a significative role in determining the distribution of the oxidized O–N–Si2 bonds near the interface. © 2008 The Electrochemical Society. 关DOI: 10.1149/1.2903862兴 All rights reserved. Manuscript submitted January 18, 2008; revised manuscript received February 28, 2008. Available electronically April 23, 2008.

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, because silicon oxynitrides 共SiOxNy兲 are materials with a higher dielectric constant and, in thin layer form, exhibit 共i兲 reduced susceptibility to interface states generation with respect to SiO2, 共ii兲 higher time-to-breakdown values and reliability, and 共iii兲 improved current–voltage and capacitance–voltage characteristics. Then, they assure an improved robustness of the device, especially in the case of Power MOSFETs 共metal oxide semiconductor field effect transistors兲.1-5 In attempting to disclose the role of the oxynitride structure in electrical defectivity and reliability, it is of fundamental importance to know the chemical bonding configurations present at the silicon–substrate interface and their relation to the sequence of the selected preparation processes, i.e., oxynitridation, thermal, and oxidation treatments. In a previous work, we gave a clear description of the chemical bonding and composition of the oxynitride layer, prepared by a selected method, pointing out the significative role played by the thermal budget used in the process.6 If the layers end up with a dielectric/substrate interface characterized by the Si–N bond terminations, one expects an improved layer robustness and reliability at the expense, in the case of a Power MOS device, of a larger resistance in the ON state. This last item is definitely deleterious for the power-device performances. For this reason a reoxidation process is usually used to increase the carriers longitudinal mobility 共parallel to the interface兲 by the removal of nitrogen-related interfacial structural defectivity.7,8 In many works found in the literature the structure and chemical composition of the oxynitride layers has been investigated, but, to our knowledge, a systematic study of the interface chemistry modifications is lacking. In this respect, the aim of this work is to give an overall picture of the interface chemistry evolution as a function of the growth parameters in relation to the effect of the postgrowth reoxidation process. To achieve this goal an extensive analysis of X-ray photoelectron spectroscopy data has been carried out. The results have been analyzed on the basis of a bonding model currently adopted in the literature.3,6,9-12 Optimized oxynitridation and postgrowth reoxidation process parameters have been found in order to grow layers with a high chemical-structural quality, useful to improve the final device performances.

vertically diffused Power MOSFETs with logic level gate driving. The oxynitride films were grown by means of a high-temperature process in N2O flow. N2O nitridation, rather than NO, was preferred in Power MOSFET devices, because the relatively lower nitridation yield allowed an efficient reoxidation of the interface, which was necessary to make the device work properly. Initially, a 20 nm thick SiO2 film was grown at 850°C onto a Si共100兲 n-type epitaxial substrate with a 1 ⍀ cm resistivity. Then, in order to induce the formation of an interfacial nitrided layer between the silicon substrate and the top-lying pure SiO2, a furnace oxynitridation process in N2O gas, for 20 or 60 min, with a temperature ranging from 900 to 1000°C, was performed. So, four different layers were prepared as shown in Table I. A specific set of samples underwent a final and additive reoxidation of the interface. The reoxidation was performed at 1000°C in O2 共dry environment兲 for 10 min. Therefore, a further thin oxide layer was grown at the silicon–oxynitride interface, once again into the silicon. X-ray photoelectron spectroscopy 共XPS兲 measurements of the Si 2p and N 1s photoelectron peaks were carried out in an ultrahigh vacuum 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 analyzer energy mode with a 7 eV pass energy in order to maximize the overall line resolution to about 1.0 eV at full width at half-maximum. Being a surface technique, XPS was able to probe only a few monolayers; hence, to characterize deeper layers, it was necessary to partially remove the SiO2 overlayer. An appropriate and highly controllable chemical etching process was performed at different etch times 共160, 175, 190, 205, 220, and 235 s兲 using a diluted HF acid 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 Without the final reoxidation, upon increasing the etch time, the

Table I. Nitridation conditions of a pure N2O process.a

Experimental The oxide-sample preparation process was extracted from the standard ST Microelectronics proprietary process for low-voltage

* Electrochemical Society Student Member. z

E-mail: [email protected]

a

Sample

T 共°C兲

t 共min兲

BL AC AL XC

900 950 950 1000

60 20 60 20

T: process temperature; t: process duration.


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Figure 1. Residual thicknesses vs etch time for all samples.

residual thickness decreased from 6.1 down to 1.2 nm, which showed a saturation behavior for the last two etching-time values and indicated that the silicon–substrate interface has been reached. Moreover, the residual thickness increased upon increasing the budget of the oxynitridation process. The difference in residual thicknesses, at a fixed etch time, between reoxidized and nonreoxidized samples decreased upon increasing the selected oxynitridation process 共see Fig. 1兲. In fact, the reoxidation was always less effective upon increasing the budget of the previous oxynitridation process due to the increased nitrogen density at the interface, which limited the diffusion of the oxidative agent and slightly reduced the Si–O reactivity at the interface. In particular, the interfacial oxide growth was more effective after 900°C–60 min and 950°C–20 min nitridation processes. The residual thickness difference between reoxidized and nonreoxidized samples at longer etch times 共220–235 s兲 was representative of the final oxide regrowth, above all in the lower nitridation budget cases 共900°C–60 min and 950°C–20 min兲. This behavior was justified from the interface reaching and from the relative residual thicknesses saturation observed for the corresponding nonreoxidated samples 共see Fig. 1兲. Results and Discussion The chemical-structural analysis was performed by means of XPS in order to study the chemical bonding states of the grown films and the relative atomic content. As already performed for the nonreoxidized samples,6 the quantitative estimation of the film composition has been obtained from the relative values of the areas of the photoemission signals weighted by the known sensitivity factors for each element.6,13 The contribution from the substrate silicon atoms was excluded in the estimation of the nitrogen content 共i.e., only the oxidized silicon phase was taken into account兲. As outlined in a previous work,6 this choice is justified by the strong localization of the nitrogen distribution that is present only within the dielectric layer near the true interface between the dielectric itself and the underlying semiconductor. Moreover, the invariability of the XPS probed volume, at any residual thickness, would induce a loss of correlation among the nitrogen-content values estimated at different residual thicknesses if the underlying semiconductor silicon component was included in the composition calculations. For the reoxidized films, it is fundamental to verify if the reoxidation process would induce a loss of nitrogen content. For a careful estimation it is necessary to consider that 共i兲 the additional thin oxide layer is grown at the silicon/oxynitride interface with about half thickness inside the substrate, 共ii兲 the XPS probe does not distinguish between the “freshly grown” and the original oxide layer, and 共iii兲 the XPS signal intensity decreases exponentially upon increasing the sample thickness and is a function of the inelastic mean free path for the escaping electron in SiO2 共␭ = 2.45 nm兲.

The photoelectrons emitted from the silicon and the oxygen atoms present at the interfacial oxide layer 共grown during postnitridation reoxidation兲 come from a depth larger than the original dielectric thickness. As a consequence, if the nitrogen percentage were estimated as already performed for nonreoxidized samples, an artificial and unphysical increase of its content would be observed. Thus, to account for the above outlined effects, the calculated nitrogen content values have been appropriately scaled by a factor equal to 共⌬e−d/␭ + d兲 d

关1兴

where d, ␭, and ⌬ indicate the residual thickness estimated for the samples without the final reoxidation, the mean free path, and the difference between the residual thicknesses of the samples with and without the final reoxidation, respectively. The correction simply takes into account the existence of an underlying new oxide layer of thickness ⌬, rescaling its intensity contribution by an exponential factor. This correction is only a rough one and must be considered valid only for the shortest etching times 共160 and 175 s兲 or analogously for the larger residual thicknesses of 10 and 7.5 nm. This is the range where the above-cited residual thickness difference ⌬ remains reasonably unchanged. We have carried out the thinning of the top dielectric layer with the same etching sequence for both nonreoxidized and reoxidized samples. The variation of nitrogen

Figure 2. Nitrogen content in percent calculated as the average between the first and second etch-time cases for all the samples, before and after the final reoxidation process.

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Figure 3. Experimental 共a兲 Si 2p and 共b兲 N 1s XPS spectra of SiOxNy thin films grown at 900°C–60 min at different etch times, after the final reoxidation process.


content as a function of residual thickness for each sample, before and after the final reoxidation, is shown in Fig. 2. It is evident that, upon increasing the nitridation thermal budget, the nitrogen content remains unchanged after the final reoxidation for each type of sample. In other terms, no significative nitrogen loss is detected due to the final reoxidation in all the examined cases. In an attempt to disclose the role of the oxynitride structure in the electrical defectivity and reliability for each sample, the evolution of the different bonding configurations as a function of residual thickness has been examined to achieve a sort of depth profiling of the nitrogen chemistry within the dielectric layer. We have used the same model adopted previously for the nonreoxidized cases.6 Modifications in both the subband structure and the chemical shift of the Si 2p and N 1s core-level photoemission peaks, varying the growth parameters in relation to the effect of postgrowth reoxidation, have been observed. The XPS core-level binding energies have been referred to the position of the semiconductor Si 2p photoemission peak 共99.6 eV兲 in order to eliminate possible charging effects. From Fig. 3-6, the Si 2p and N 1s XPS spectra of all the samples after the final reoxidation and for each etch time are shown. Similar to what was observed in the nonreoxidized samples6 and despite the

low signal/noise ratio, readily recognizable modifications of the Si 2p and N 1s core-level photoemission peaks are evidenced as a function of the nitridation parameters and of the distance from the substrate interface. The effect to be highlighted is the progressive modification of the N 1s band that, within our experimental resolution, shows a multicomponent structure. The modifications and the chemical shifts for the N 1s photoemission peaks have been investigated by means of a subband deconvolution with four GaussianLorentzian bands, already adopted for the nonreoxidized samples.6 The four subbands considered in the model represent 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.9-12,14,15 The fitting results are shown in Fig. 7 and 8 for two samples, before and after the final reoxidation, at etch times of 160 and 235 s. It is evident that the nitrogen bonding configurations are progressively changing their relative weights upon reaching the dielectric/ silicon interface, for each of the oxynitridation processes. The latter ones, in turn, affect the way in which the four nitrogen bonding configurations are modified by the final reoxidation process 共see Fig. 7 and 8兲.




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Figure 7. N 1s spectra of SiOxNy films grown at 900°C–60 min at the etch time of 共a and c兲 160 and 共b and d兲 235 s, before and after the final reoxidation process. The dashed lines represent the fitting of the peaks related to the Si3wN, N共−SiO3兲x, O–N–Si2, and O2–N–Si bonds.

Figure 8. N 1s spectra of SiOxNy films grown at 1000°C–20 min at the etch time of 共a and c兲 160 and 共b and d兲 235 s, before and after the final reoxidation process. The dashed lines represent the fitting of the peaks related to the Si3wN, N共−SiO3兲x, O–N–Si2, and O2–N–Si bonds.

The Power VDMOS reliability performance significantly depends on the quality of the oxynitride layers. It is known that the bonding environment near the oxynitride/silicon interface could be somehow related to the amount and type of point defects acting as carrier traps and affecting, for example, the channel region transport properties of a Power MOSFET device or the electrical susceptibility of some gate dielectric. Thus, an accurate study of the change of the nitrogen content and, in particular, of the main bonding configurations upon reaching the Si interface is certainly of interest. The depth profile of the overall nitrogen content for the four oxynitridation processes, before6 and after the final reoxidation, is shown in Fig. 9. A withdrawal of the nitrogen distribution from the interface is evident. Moreover, in the largest nitridation budget case, the nearinterface piling up of nitrogen is partially preserved even after the reoxidation, even if it is obviously moved away from the silicon substrate due to the added interfacial SiO2 layer. Concerning the local bonding configurations 共see Fig. 10兲, the emerging picture shows the following. 1. In the simply nitrided samples, the Si3wN contribution remains negligible throughout the whole film thickness in all cases, even if it is significantly present only at the interface for the highertemperature oxynitridation process. When the samples are reoxidized, the Si3wN contribution is totally absent for all four oxynitridation processes. 2. The N共−SiO3兲x component, in which the N atom is connected to the O atoms through a silicon bridge and x = 1, 2, or 3, does not substantially change with respect to the behavior shown in the nonreoxidized samples. In each examined case, the whole profile moved away from the interface after the reoxidization process. Moreover, it is evident that, upon increasing the initial nitridation budget, its profile broadens with a larger tail close to the interface. These effects are clearly driven by the thickness of the postannealing oxide, while, through all the nonreoxidized samples, the maximum peak

position remains almost unchanged. This component behavior almost reflects the modifications observed in the overall nitrogen content 共see Fig. 9兲. 3. The total contribution of the O–N–Si2 and O2–N–Si components remains negligible throughout the whole film thickness in the reoxidized samples with the lower oxynitridation budgets 共900 and 950°C–20 min兲. On the contrary, for the higher oxynitridation budget, this contribution is relevant if compared to that observed in the

Figure 9. Behavior of the total nitrogen content 共a兲 before and 共b兲 after the final reoxidation process vs the residual thickness for the four oynitridation processes.

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Figure 10. Behavior of the nitrogen bond fractions 共a–c兲 before and 共d–f兲 after the final reoxidation process vs the residual thickness for the four oynitridation processes.

simply nitrided samples. 共The O–N–Si2 and O2–N–Si bond fractions have been summed up together because the weight of the O2–N–Si phase is small within our experimental resolution兲. A shift of the O–N–Si2 and O2–N–Si distributions away from the interface is also observed in this case, and the interfacial piling up observed in the simply nitrided samples with a larger thermal budget is smeared out after the reoxidation. Altogether, it is evident that the final reoxidation process induces a redistribution of the nitrogen bonding configurations and, in particular, the nitrogen of the initial Si3wN component seems to be converted to the oxidized phase with a more uniform distribution far from the interface. The effectiveness of the final annealing in growing a new interfacial oxide suggests that any possible Si3wN contribution is likely to be negligible 共see Fig. 10兲. The absence of the Si3wN contribution after the final reoxidation can be explained by the strain-energy minimization principle, as already proposed for the simply nitrided samples.6,17 In this case, the interface is less strained even without Si3wN bonds as a buffer layer. On the other side, the strongly cross-linked oxynitride layer would push the too-dense Si3wN bonds within the less-dense oxide matrix, which is a thermodynamically unfavored situation. This justifies the nitride-tooxynitride transformation of the interfacial Si3wN bonds during the new oxide growth.16,17 Finally, by means of this study, it is possible to identify the potentially optimal parameters of the nitridation process for a 20 nm gate oxide that, after a final reoxidation, can improve the interfacial electrical quality and final Power MOSFET device performance. In particular, nitridation thermal budgets at 950°C–60 min and 1000°C–20 min, after a reoxidation process at 1000°C for 10 min in a dry O2, shows the larger concentrations of the oxidized nitrogen phases, from 3 to 5 nm from the interface, as a result of an average shift of the bonding distribution away from the interface and a partial contribution due to bond conversion to oxidized phases. At these depths, an influence of carrier trapping defects on oxide quality, with different roles played by hole and electron traps, can be expected, as known from consolidated literature data18 and also from indications found in a preliminary electrical characterization analysis on these samples.19 Conclusion The progressive modifications of the bonding environments upon reaching the oxynitride/silicon interface as well as the relation of the observed bonding configurations with the selected oxynitridation process and the final reoxidation were investigated by means of XPS spectroscopy.

The following was found. 1. The chemistry of the oxynitride layer is a rather complex one and it changes progressively, moving toward the silicon interface. The final reoxidation mainly acts in moving away the nitrogen distribution from the interface. 2. For the reoxidation process examined here, no loss of nitrogen seems to occur for all the nitridation cases. 3. The final reoxidation plays a significative role in the Ox–N–Siy bond reorganization near the interface. It seems to be strongly affected by the interfacial chemical configuration determined by the specific oxynitridation process. 4. The initial nitridation process at 1000°C favors the formation of the Si3wN and the singly oxidized O–N–Si2 bonds at the interface. The final reoxidation induces a further growth of the O–N–Si2 phase at the expense of the Si3wN phase, assuring the prevalence of the oxidized phase, which may help in limiting the formation of trapping defect and improving the interface electrical quality and the final device performance. In conclusion, on the basis of the above outlined considerations, a set of optimized oxynitridation process parameters for a 20 nm gate SiO2 has been found that, after a subsequent final reoxidation process, could guarantee improved device performances for highreliability applications. In particular, the samples nitrided at 950°C–60 min and 1000°C–20 min, with a final reoxidation at 1000°C–10 min in a dry O2 environment, show a larger oxidized nitrogen concentration from 3 to 5 nm from the interface, a distance at which the influence of the trapping defects on the gate oxide quality can be significative to define the device performances. University of Messina assisted in meeting the publication costs of this article.

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