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Two-step processes for bimodal N concentration profiles in ultra-thin silicon oxynitrides
Thin Solid Films 436 (2003) 162–167
Two-step processes for bimodal N concentration profiles in ultra-thin silicon oxynitrides Anindya Dasgupta, Christos G. Takoudis* Advanced Materials Research Laboratory, Department of Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60607-7000, USA Received 24 September 2002; received in revised form 27 February 2003; accepted 2 April 2003
Abstract The presence of nitrogen in the dielectric films is known to impart highly desirable properties in the ultra-large-scale-integration era; the position, amount and concentration profiles of nitrogen are of great interest. In this work, we have studied two-step processes leading to bimodal nitrogen concentration profiles, with one nitrogen peak near the Siydielectric interface and the other at the dielectric surface. Secondary Ion Mass Spectroscopy and Angled Resolved X-Ray Photoelectron Spectroscopy studies suggest that an ammonia nitridation step (at 1000 8C and 1 atm) following silicon oxynitridation in N2 O (or possibly following oxidation in O2) at the same conditions results in a bimodal nitrogen profile when short nitridation times are used; increasing the duration of the nitridation step is found to completely nitridate the initially grown oxynitride (or oxide). Post nitridation of NOgrown oxynitrides that are not found to result in bimodal N concentration profiles at the conditions studied. The experimental findings are in agreement with theoretical predictions of preliminary modeling studies. The engineering of desired bimodal nitrogen concentration profiles in nano-dielectric materials of interest becomes, therefore, possible through two-step processes. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Bimodal nitrogen concentration profile; Silicon oxynitride; Secondary ion mass spectroscopy; X-ray photoelectron spectroscopy
1. Introduction Silicon oxynitrides have been the focus of attention in ultra-large-scale-integration, because the incorporation of N in the dielectric film results in higher dielectric constant than that of silicon oxides w1–12x. Oxynitrides provide enhanced thickness control due to their selflimiting growth; they are also reported to improve performance as a diffusion barrier of boron from the pq-gate layer, during subsequent high temperature processing steps w1x. Since N imparts properties like improved hot-carrier resistance, reduction of interface state generation and longer device lifetime, it would be imperative to have N at the substrateydielectric interface and near the dielectric surface. Moreover, silicon oxynitrides (SiOxNy) combine the basic merits of both SiO2 and Si3N4. Therefore, for optimal performance of *Corresponding author. Tel.: q1-312-355-0859; fax: q1-312-9960808. E-mail address: [email protected] (C.G. Takoudis).
such dielectric layers, it would be important to obtain desired N concentration profiles. Silicon oxynitrides can be grown using ammonia (NH3), nitrous oxide (N2O) or nitric oxide (NO) ambient w1–12x. Oxynitrides grown with N2O or NO show relatively low increase in the dielectric constant of the film; NH3, which is known to incorporate increased amounts of N, can be used as an alternative precursor. Thermal growth in N2O w1,4,9–11x or NO w12–14x has been generally reported to incorporate N near the oxynitrideysubstrate interface. Three-step processes involving an oxynitridation (in N2O or NO) followed by oxidation (in O2) and oxynitridation (in N2O or NO) steps have already been reported to result in bimodal nitrogen profiles w1x. Engineering the nitrogen concentration profile in ultra-thin dielectrics is of paramount importance in designing nano-dielectric materials of interest. The objective of this study is to obtain and fundamentally study two-step processes leading to desired bimodal nitrogen concentration profiles within nano dielectric
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 3 . 0 0 6 0 8 - 4
A. Dasgupta, C.G. Takoudis / Thin Solid Films 436 (2003) 162–167
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films; these processes include a knowledge-based understanding of the post-annealingyreacting an oxide or oxynitride layer with NH3. Such process–structure relations are pursued through data obtained from Secondary Ion Mass Spectroscopy (SIMS), Angled Resolved XRay Photoelectron Spectroscopy (AR-XPS), and spectroscopic ellipsometry coupled with preliminary mathematical modeling of these systems. 2. Experimental A hot-wall furnace reactor with an internal diameter of 2-inch was used for the oxynitridation and nitridation processes. The furnace temperature during the oxidation, oxynitridation and nitridation was maintained at 1000 8C. The film thickness was measured with an M-44 Spectroscopic Ellipsometer (J.A. Woollam Co., Inc.). Secondary ion mass spectroscopy (Cameca IMS 5f SIMS) was used for the N, O and Si depth profiles within the dielectric films, while the bonding states at different depths within the dielectric film were obtained with a angle-resolved X-ray photoelectron spectroscope (Kratos AXIS Ultra XPS). A monochromatic Al Ka source at 1486.6 eV was used and a pass energy of 40 eV was set at take-off angles of 15 and 908. At 158, electrons emit from the near surface region, while at 908 information from the bulk (and the interface) of the dielectricysubstrate region can be obtained. All peaks were charge referenced to the Si 2p spectrum w15x. The adventitious C 1s peak observed in the survey spectrum (at 158 take off angle and 100 eV pass energy) is due to surface contaminants and does not effect the characterization of the dielectric film (e.g. w16x). Curve fitting of the N 1s spectra is performed on top of linear backgrounds. The GaussianyLorentzian values for the fitted peaks were the same in all photoemission spectra. Full width-half maxima (FWHM) values of each N 1s peak are comparable to literature data w17x. The dielectric films characterized with SIMS were approximately 5–28 nm thick, while those used in ARXPS were 5–8 nm thick. All films were grown at 1 atm NO (Matheson, 99%), N2O (AGA, 99.5%), O2 (AGA, 99.96%), NH3 (Matheson, Anhydrous) and a feed flow rate of 1 standard liter per minute (slpm). Approximately, 2=1 cm2 double polished p-type CZ Si(100) wafer samples with a resistivity of 5–15 V were used. Prior to growth the samples were processed in (H2O2:H2SO4, 2:1) for 30 min, DI water rinsed, N2 dried followed by 1 min buffered HF etch to remove the native oxide. The etched sample wafers were DI water rinsed, N2 dried and immediately loaded in the furnace, which was maintained at 1000 8C in Argon ambient. 3. Results–discussion The SIMS N and O depth profiles for the films grown with the two-step processing sequence N2O (2 h)™
Fig. 1. N and O SIMS depth profiles for the two-step processing sequence N2O (2 h)™NH3 (15 min) (a) and N2O (2 h)™NH3 (1 h 15 min) (b). Ts1000 8C, Ps1 atm, feed flow rates1 slpm.
NH3 (15 min) are shown in Fig. 1a. The intensities on the vertical axis are direct measures of the N and O concentrations in the film; the sputtering cycles on the horizontal axis are monotonically related to the film depth as measured from the top surface. Each cycle is approximately 10 s long. The dielectricysilicon substrate interface can be approximately located at the point where the O signal becomes half of its maximum signal w1x. It is seen that the N2O (2 h)™NH3 (15 min) processing results in two peaks, one near the dielectric surface and the other near the interface. The film thickness measured ˚ with spectroscopic ellipsometry is approximately 275 A. The N peak at the interface is due to the first oxynitridation step with N2O w1,4,9–11x. The other N peak near the dielectric surface appears to be the result of the nitridation step with NH3 at 1000 8C; further, there is nitrogen in the bulk of the thin dielectric as well. Fig. 1b shows the N and O depth profiles of films grown in the two-step processing sequence that includes the same steps as in Fig. 1a, while the second step is one hour longer. The N intensity near the dielectric surface is seen to increase by more than a factor of 2 (Fig. 1a and b); the N peak concentration near the dielectricysubstrate interface, however, increases only slightly with the increased processing of the film in NH3. The appreciable increase in the N peak intensity
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Fig. 2. N 1s ARXP Spectra for the two-step processing sequence N2O (10 min)™NH3 (1 h 25 min) at 158 (a), 908 (b) and survey (c). Other conditions are the same as in Fig. 1.
near the dielectric surface coupled with the corresponding marginal increase near the interface suggest that a nitridation front moves from the surface towards the bulk of the film, during the processing in ammonia. The two N concentration peaks could, thus, be engineered and optimized through both the first oxynitridation step in N2O and the subsequent nitridation step in NH3. Decreasing the duration of the nitridation step in NH3, for example, could separate more distinctly the two N concentration peaks. Although these experiments have been performed at 1000 8C, lower temperatures or combination of different temperatures and variable– duration processing sequences could further facilitate the engineering of the two N concentration peaks, during these two-step processes. Binding energies (BE), peak widths and intensities from the ARXP Spectra can be related to the chemical and physical properties of the dielectric materials. Fig. ˚ 2a and b illustrate the ARXP N 1s spectra of a 57 A thick oxynitride, fabricated by thermal oxynitridation in N2O for 10 min followed by nitridation in NH3 for 1 h 25 min. The nitridation duration has been estimated based on the time needed to nitride the entire oxynitride film. Indeed, Fourier transform infrared spectroscopy
reveals that the 1060 cmy1 Si–O transverse optical mode intensity decreases with increasing nitridation time. This is due to substitution of O atoms by N atoms during the nitridation step. The nitridation duration of 1 h 25 min is, therefore, the time beyond which the 1060 cmy1 Si–O peak does not decrease any further (indicative of the end of nitridation). At 158 (Fig. 2a), an intense N 1s peak is observed at 398.1 eV with a FWHM of 1.5 eV. The NIST w18x databank suggests that the 397.9 eV feature correspond to SiN1.33, while the 398.0 eV to SiN1.7. Fig. 2b illustrates the ARXP spectra of the same sample at 908. Here, the N 1s at 398.6 eV peak (FWHM 1.5 eV) is observed to have higher binding energy than that in Fig. 2a by about ;0.5 eV. Multiple bonding arrangements have been attributed to the N 1s peak at 398.6 eV. Bhat et al. w19x have assigned it to Si–N_ H2 in NH3-nitrided oxides. Prabhakaran et al. w20x suggested that this peak may be due to (Si–)2N–O, but this structure is reported to occur at a binding energy shift of 1.5–2.0 eV higher than that of N(–Si)3 at 397.5 or 398.0 eV. Thus, the feature at 398.6 eV may be assigned to the Si–N_ H2 moiety due to the NH3 nitridation, during which N and H atoms replace the initial O atoms present in the oxynitride, or to non-stoichiometric SiOxNy due to incomplete incorporation of oxygen atoms w19–25x. No peaks are observed at )399 eV that have been attributed to the stoichiometric (Si–)2N–O compound w18,19,25x. A comparison of the N 1s spectra in Fig. 2a and b suggests that the oxynitride layer near the dielectricy substrate interface has been nitridated to a lesser extent than the dielectric surface. Lower N 1s BE at 158 ARXP suggests presence of more stoichiometric and nonstoichiometric silicon nitride near the surface, while higher BE at 908 ARXP suggests presence of more nonstoichiometric SiOxNy near the dielectricysubstrate interface. These data corroborate the SIMS data (Fig. 1a and b), in which increased NH3 nitridation duration suggests an N front moving from the top dielectric surface towards the bulk. Such data appear to be in good agreement with theoretical predictions of preliminary quantitative modeling that account for solid phase diffusion and reaction within the dielectric films of interest, at quasi–steady state conditions. Further, experimental (and preliminary theoretical) results suggest that variation of the duration or temperature of oxynitridation and nitridation processing steps may indeed result in a twostep bimodal nitrogen concentration profile within such nano dielectric layers. Electrical characterization of the resulting dielectric films has also been performed. It is observed that the NH3 nitridation step does not deteriorate the electrical performance of the dielectric oxynitride layer grown in N2O w26x. The Si3N4 thermal growth in NH3 is known to be susceptible to interfacial problems attributed to the H atom incorporation during nitridation w1,26x. Since in
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Fig. 3. N and O SIMS depth profiles for the two-step processing sequence NO (3 h)™NH3 (15 min) (a), NO (3 h)™NH3 (1 h) (b). Ts1000 8C, Ps1 atm, feed flow rates1 slpm.
the above processing sequence, minimal nitridation takes place near the interface, the interface trap density apparently remains similar to that of a dielectric oxynitride grown in N2O. The N and O depth profiles for films grown with the two-step processing sequence NO (3 h)™NH3 (15 min) are shown in Fig. 3a. The thickness of the dielectric ˚ A thick oxynitride is oxynitride grown in NO is 50 A. difficult to grow thermally in NO, due to its substantial self-limiting growth w1x. The NO oxynitridation results in a N peak at the dielectricysubstrate interface w1,12– 14x. Fig. 3a suggests N is distributed equally throughout the film, after 15 min post-nitridation in ammonia. Fig. 3b shows results from a similar two-step processing sequence with a longer second step. Therefore, increasing the NH3 nitridation duration from 15 min to 1 h does not appear to change the overall N concentration profile, although more N is probed in the film after the longer NH3 nitridation step. The ARXP N 1s spectra of a silicon oxynitride film grown in NO for 3 h followed by a post nitridation step with NH3 for 1 h is illustrated in Fig. 4. The film ˚ The N 1s peaks at both 158 (Fig. 4a) thickness is 61 A. and 908 (Fig. 4b) appear at 398.6 eV, and have similar FWHM of 1.4 eV. Identical N1s peak positions and FWHMs for both ARXP angles suggests that N bonding states near the dielectric surface and at the dielectricy substrate interface are similar in nature. This observation
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corroborates, therefore, the SIMS data (Fig. 3), in which N is most likely evenly distributed across the entire thickness of the dielectric after the post-nitridation process. This is different from the nitridation mechanism of the N2O-grown oxynitrides, in which a nitridation front was found to proceed from the dielectric surface towards the bulk. Indeed, a critical N concentration within the dielectric layer and the initial oxynitride thickness seem to play an important role in the final N concentration profile. SIM spectra of NO-based oxynitrides suggest that the FWHM of the N concentration profile is approximately 65–70% of the total film thickness; this is higher than that of oxynitrides grown in N2O at similar conditions w1,12–14x. Thus, a short nitridation in NH3 appears to be sufficient to yield a rather uniform N concentration profile across the dielectric film. Increasing the nitridation time only increases the amount of nitrogen in the dielectric film. For a thicker oxynitride film in NO ambient, a SiO2-based oxide of desired thickness could be grown first, followed by a NO anneal. Since, N would incorporate near the interface in the thicker dielectric film; the N front moving from the surface during a NH3 nitridation step would not readily merge with the N peak near the
Fig. 4. N 1s ARXP Spectra for the two-step processing sequence NO (3 h)™NH3 (1 h) at 158 (a), 908 (b) and survey (c). Other conditions are the same as in Fig. 3.
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surface helps prevent dopant diffusion during subsequent high temperature processing steps. A similar trend is also observed in the ARXP N 1s ˚ oxide, grown in O2 at 1000 8C for 4 spectra of a 84 A min and post nitrided in NH3 for 1 h 25 min (Fig. 6). At 158 (Fig. 6a), the N 1s peak is observed at 398.0 eV, while at 908 (Fig. 6b) the N 1s binding energies is observed at higher BE by approximately 0.8 eV. Both have similar FWHMs of 1.5 eV. At 158, the peak at 398.0 eV can be assigned to SiN1.33 or SiN1.7, while at 908, the peak at 398.8 eV can most likely be assigned to non-stoichiometric SiOxNy or Si–N_ H2. The presence of more stoichiometric silicon nitride related compounds at 158 (i.e. near the surface) suggests that the bulk and interface have not been nitrided to the same extent as the top surface of the dielectric, when a pre-grown oxide is nitrided in NH3. Yet, a bimodal N concentration profile appears to be possible with controlled ammonia post nitridation of an oxide layer, although the N peak near the dielectricysubstrate interface may be weak. A close comparison of the N 1s spectra of all three different processing sequences show that there is an N
Fig. 5. N and O SIMS depth profiles for the two-step processing sequence O2 (10 min)™NH3 (1 h) (a), O2 (8 min)™NH3 (4 h) (b). Ts1000 8C, Ps1 atm, feed flow rates1 slpm.
interface. This could be another method to grow a bimodal N concentration profile with higher N concentrations at both the interface (due to NO oxynitridation) and surface (due to NH3 nitridation), although it would really be a three-step process. Fig. 5a includes SIM Spectra of N and O for films ˚ 184 A-thick grown in O2 (10 min)™NH3 (1 h). The resulting nitrogen concentration profile yields one peak near the dielectric surface and another, albeit very weak, near the dielectricysubstrate interface. When the duration of the nitridation step is increased to 4 h, the entire dielectric film is nitridated (although this film was oxidized for 8 min (Fig. 5b)). It is interesting to point out that NH3 nitridation suggests that some N reaches the interface relatively early (Fig. 5a). This is possibly due to the absence of N in the oxide layer, since N is known to retard diffusion. Such a N concentration profile indicating higher N incorporation near the dielectric surface and minimal at the dielectricysubstrate interface could be important in the context of mobility degradation. It is known that excessive N near the interface could lead to mobility degradation w27x. Moreover, lower N near the interface suggests lower H incorporation at the interface from the NH3 nitridation step w26x and better electrical properties (lower interface trap densities), while higher N concentration near the dielectric
Fig. 6. N 1s ARXP Spectra for the two-step processing sequence O2 (4 min)™NH3 (1 h 25 min) at 158 (a), 908 (b) and survey (c). Other conditions are the same as in Fig. 5.
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1s peak shift of approximately 0.5 eV for the processing N2O™NH3, 0.8 eV for O2™NH3, and no shift for NO™NH3, when the ARXP angle changes from 15 to 908. Unlike oxynitrides, absence of any N near the interface before the NH3 nitridation step can explain the apparent maximum peak shift for the O2™NH3 sequence. These observations suggest that N2O grown oxynitrides and O2 grown oxides are more favorably nitridated near the top surface compared to the bulk and the dielectricysubstrate interface. On the other hand, there is practically no change in NO-grown oxynitrides post-nitrided by ammonia. 4. Summary–conclusion SIMS and ARXPS studies suggest that bimodal N concentration profiles can be obtained with a controlled nitridation of an oxynitride grown in N2O or perhaps an oxide grown in O2, although the bimodal N profile appears to be much less pronounced in nitrided oxides. This process does not appear to be effective with oxynitrides grown in NO, where the nitridation process is found to take place all across the film. Shorter durations of the nitridation step enhance the separation of the two N peaks at the dielectricysubstrate interface and the top dielectric surface. Increasing the nitridation duration in such two-step processes increases the N content near the dielectric surface with an apparent N front moving towards the bulk of the dielectric film. These experimental findings are in good agreement with theoretical predictions of preliminary modeling studies. In contrast to three-step processing, two-step processes eliminate an entire step and contribute to savings of the chemical (and thermal) budgets. Unlike threestep processes, the two-step ones eliminate an oxide step in between the two oxynitridation steps and, therefore, do not contribute to undesirable nitrogen removal or to the formation of potentially thick films. Also, the choice of precursor gases for the initial oxide or oxynitride growth, the processing temperature, and variation of the post-nitridation duration lend added degrees of freedom for tailoring desirable bimodal N concentration profiles in ultra-thin dielectrics of interest. Acknowledgments We thank Mr Rick Haasch and Ms Judy Baker at the Center for Microanalysis of Materials at UIUC for helping us with the Angles Resolved X-Ray Photoelectron Spectroscopy and Secondary Ion Mass Spectroscopy data; that facility is supported by the US Department of Energy under grant DEFG02-96-
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ER45439. Partial funding from the NSF and the University of Illinois at Chicago is also acknowledged. References w1x S.S. Dang, C.G. Takoudis, J. Appl. Phys. 86 (1999) 1326. w2x M.L. Green, E.P. Gusev, H.L. Lu, E. Garfunkel, T. Gustafsson, W.N. Lennard, D. Bransen, Electrochem. Soc. Meet. Abst. 98 (1) (1998) 330. w3x E.P. Gusev, H.L. Lu, E. Garfunkel, T. Gustafsson, M.L. Green, W.N. Lennard, D. Bransen, J. Appl. Phys. 84 (1998) 2980. w4x Y. Okada, P.J. Tobin, V. Lakhotia, W.A. Feil, S.A. Ajuria, R.I. Hedge, Appl. Phys. Lett. 63 (1993) 194. w5x K.A. Ellis, R.A. Buhrman, Appl. Phys. Lett. 68 (1996) 1696. w6x M. Bhat, L.K. Han, D. Wristers, J. Yan, D.L. Kwong, J. Fulford, Appl. Phys. Lett. 66 (1995) 1225. w7x E.C. Carr, R.A. Buhrman, Appl. Phys. Lett. 63 (1993) 54. w8x D. Wristers, L.K. Han, T. Chen, H.H. Wang, D.L. Kwong, M. Allen, J. Fulford, Appl. Phys. Lett. 68 (1996) 2094. w9x S. Shingvi, C.G. Takoudis, J. Appl. Phys. 82 (1997) 442. w10x R.I. Hegde, B. Maiti, P.J. Tobin, J. Electrochem. Soc. 144 (1997) 1081. w11x N.S. Saks, D.I. Ma, W.B. Flower, Appl. Phys. Lett. 67 (1995) 374. w12x R.I. Hegde, P.J. Tobin, G.K. Reid, B. Maiti, S.A. Ajuria, Appl. Phys. Lett. 66 (1995) 2882. w13x A. Kamath, D.L. Kwong, Y.M. Sun, P.M. Blass, S. Whaley, J.M. White, Appl. Phys. Lett. 70 (1997) 63. w14x Y. Okada, P.J. Tobin, V. Lakhotia, S.A. Ajuria, R.I. Hedge, J.C. Liao, P.P. Rushbrook, L.J. Arias, J. Electrochem. Soc. 140 (1993) L87. w15x J.R. Shallenberger, D.A. Cole, S.W. Novak, J. Vac. Sci. Technol. A 17 (1999) 1086. w16x M. Bersani, L. Vanzetti, M. Sbetti, M. Anderle, Appl. Surf. Sci. 144–145 (1999) 301. w17x N. Miyata, H. Watanabe, M. Ichikawa, Appl. Phys. Lett. 76 (2000) 3561. w18x C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Rumble, Jr., NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, Version 3.2 (Web Version) (2002) whttp:yysrdata.nist.govyxpsy x. w19x M. Bhat, J. Ahn, D.L. Kwong, M. Arendt, J.M. White, Appl. Phys. Lett. 64 (1994) 1168. w20x K. Prabharakaran, Y. Koboyashi, T. Orino, Appl. Surf. Sci. 130–132 (1998) 182. w21x H.-W. Chen, D. Landheer, T.-S. Chao, J.E. Hulse, T.-Y. Huang, J. Electrochem. Soc. 148 (2001) F140. w22x Z.H. Lu, R.J. Hussey, M.J. Graham, R. Cao, S.P. Tay, J. Vac. Sci. Technol. B 14 (1996) 2882. w23x D. Bouvet, P.A. Clivaz, M. Dutoit, C. Coluzza, J. Almeida, G. Margaritondo, F. Pio, J. Appl. Phys. 79 (1996) 7114. w24x C.H.F. Pedan, J.W. Rogers Jr, N.D. Shinn, K.B. Kidd, K.L. Tsang, Phys. Rev. B 47 (1993) 15 622. w25x G.M. Rignanese, A. Pasquarella, J.-C. Charlier, X. Gonze, R. Car, Phys. Rev. Lett. 79 (1997) 5174. w26x A. Dasgupta, C.G. Takoudis, MRS Symp. Proc. 716 (2002) 177. w27x M.L. Green, E.P. Gusev, R. Degraeve, E.L. Garfunkel, J. Appl. Phys. 90 (2001) 2057.
Two-step processes for bimodal N concentration profiles in ultra-thin silicon oxynitrides Anindya Dasgupta, Christos G. Takoudis* Advanced Materials Research Laboratory, Department of Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60607-7000, USA Received 24 September 2002; received in revised form 27 February 2003; accepted 2 April 2003
Abstract The presence of nitrogen in the dielectric films is known to impart highly desirable properties in the ultra-large-scale-integration era; the position, amount and concentration profiles of nitrogen are of great interest. In this work, we have studied two-step processes leading to bimodal nitrogen concentration profiles, with one nitrogen peak near the Siydielectric interface and the other at the dielectric surface. Secondary Ion Mass Spectroscopy and Angled Resolved X-Ray Photoelectron Spectroscopy studies suggest that an ammonia nitridation step (at 1000 8C and 1 atm) following silicon oxynitridation in N2 O (or possibly following oxidation in O2) at the same conditions results in a bimodal nitrogen profile when short nitridation times are used; increasing the duration of the nitridation step is found to completely nitridate the initially grown oxynitride (or oxide). Post nitridation of NOgrown oxynitrides that are not found to result in bimodal N concentration profiles at the conditions studied. The experimental findings are in agreement with theoretical predictions of preliminary modeling studies. The engineering of desired bimodal nitrogen concentration profiles in nano-dielectric materials of interest becomes, therefore, possible through two-step processes. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Bimodal nitrogen concentration profile; Silicon oxynitride; Secondary ion mass spectroscopy; X-ray photoelectron spectroscopy
1. Introduction Silicon oxynitrides have been the focus of attention in ultra-large-scale-integration, because the incorporation of N in the dielectric film results in higher dielectric constant than that of silicon oxides w1–12x. Oxynitrides provide enhanced thickness control due to their selflimiting growth; they are also reported to improve performance as a diffusion barrier of boron from the pq-gate layer, during subsequent high temperature processing steps w1x. Since N imparts properties like improved hot-carrier resistance, reduction of interface state generation and longer device lifetime, it would be imperative to have N at the substrateydielectric interface and near the dielectric surface. Moreover, silicon oxynitrides (SiOxNy) combine the basic merits of both SiO2 and Si3N4. Therefore, for optimal performance of *Corresponding author. Tel.: q1-312-355-0859; fax: q1-312-9960808. E-mail address: [email protected] (C.G. Takoudis).
such dielectric layers, it would be important to obtain desired N concentration profiles. Silicon oxynitrides can be grown using ammonia (NH3), nitrous oxide (N2O) or nitric oxide (NO) ambient w1–12x. Oxynitrides grown with N2O or NO show relatively low increase in the dielectric constant of the film; NH3, which is known to incorporate increased amounts of N, can be used as an alternative precursor. Thermal growth in N2O w1,4,9–11x or NO w12–14x has been generally reported to incorporate N near the oxynitrideysubstrate interface. Three-step processes involving an oxynitridation (in N2O or NO) followed by oxidation (in O2) and oxynitridation (in N2O or NO) steps have already been reported to result in bimodal nitrogen profiles w1x. Engineering the nitrogen concentration profile in ultra-thin dielectrics is of paramount importance in designing nano-dielectric materials of interest. The objective of this study is to obtain and fundamentally study two-step processes leading to desired bimodal nitrogen concentration profiles within nano dielectric
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 3 . 0 0 6 0 8 - 4
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films; these processes include a knowledge-based understanding of the post-annealingyreacting an oxide or oxynitride layer with NH3. Such process–structure relations are pursued through data obtained from Secondary Ion Mass Spectroscopy (SIMS), Angled Resolved XRay Photoelectron Spectroscopy (AR-XPS), and spectroscopic ellipsometry coupled with preliminary mathematical modeling of these systems. 2. Experimental A hot-wall furnace reactor with an internal diameter of 2-inch was used for the oxynitridation and nitridation processes. The furnace temperature during the oxidation, oxynitridation and nitridation was maintained at 1000 8C. The film thickness was measured with an M-44 Spectroscopic Ellipsometer (J.A. Woollam Co., Inc.). Secondary ion mass spectroscopy (Cameca IMS 5f SIMS) was used for the N, O and Si depth profiles within the dielectric films, while the bonding states at different depths within the dielectric film were obtained with a angle-resolved X-ray photoelectron spectroscope (Kratos AXIS Ultra XPS). A monochromatic Al Ka source at 1486.6 eV was used and a pass energy of 40 eV was set at take-off angles of 15 and 908. At 158, electrons emit from the near surface region, while at 908 information from the bulk (and the interface) of the dielectricysubstrate region can be obtained. All peaks were charge referenced to the Si 2p spectrum w15x. The adventitious C 1s peak observed in the survey spectrum (at 158 take off angle and 100 eV pass energy) is due to surface contaminants and does not effect the characterization of the dielectric film (e.g. w16x). Curve fitting of the N 1s spectra is performed on top of linear backgrounds. The GaussianyLorentzian values for the fitted peaks were the same in all photoemission spectra. Full width-half maxima (FWHM) values of each N 1s peak are comparable to literature data w17x. The dielectric films characterized with SIMS were approximately 5–28 nm thick, while those used in ARXPS were 5–8 nm thick. All films were grown at 1 atm NO (Matheson, 99%), N2O (AGA, 99.5%), O2 (AGA, 99.96%), NH3 (Matheson, Anhydrous) and a feed flow rate of 1 standard liter per minute (slpm). Approximately, 2=1 cm2 double polished p-type CZ Si(100) wafer samples with a resistivity of 5–15 V were used. Prior to growth the samples were processed in (H2O2:H2SO4, 2:1) for 30 min, DI water rinsed, N2 dried followed by 1 min buffered HF etch to remove the native oxide. The etched sample wafers were DI water rinsed, N2 dried and immediately loaded in the furnace, which was maintained at 1000 8C in Argon ambient. 3. Results–discussion The SIMS N and O depth profiles for the films grown with the two-step processing sequence N2O (2 h)™
Fig. 1. N and O SIMS depth profiles for the two-step processing sequence N2O (2 h)™NH3 (15 min) (a) and N2O (2 h)™NH3 (1 h 15 min) (b). Ts1000 8C, Ps1 atm, feed flow rates1 slpm.
NH3 (15 min) are shown in Fig. 1a. The intensities on the vertical axis are direct measures of the N and O concentrations in the film; the sputtering cycles on the horizontal axis are monotonically related to the film depth as measured from the top surface. Each cycle is approximately 10 s long. The dielectricysilicon substrate interface can be approximately located at the point where the O signal becomes half of its maximum signal w1x. It is seen that the N2O (2 h)™NH3 (15 min) processing results in two peaks, one near the dielectric surface and the other near the interface. The film thickness measured ˚ with spectroscopic ellipsometry is approximately 275 A. The N peak at the interface is due to the first oxynitridation step with N2O w1,4,9–11x. The other N peak near the dielectric surface appears to be the result of the nitridation step with NH3 at 1000 8C; further, there is nitrogen in the bulk of the thin dielectric as well. Fig. 1b shows the N and O depth profiles of films grown in the two-step processing sequence that includes the same steps as in Fig. 1a, while the second step is one hour longer. The N intensity near the dielectric surface is seen to increase by more than a factor of 2 (Fig. 1a and b); the N peak concentration near the dielectricysubstrate interface, however, increases only slightly with the increased processing of the film in NH3. The appreciable increase in the N peak intensity
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Fig. 2. N 1s ARXP Spectra for the two-step processing sequence N2O (10 min)™NH3 (1 h 25 min) at 158 (a), 908 (b) and survey (c). Other conditions are the same as in Fig. 1.
near the dielectric surface coupled with the corresponding marginal increase near the interface suggest that a nitridation front moves from the surface towards the bulk of the film, during the processing in ammonia. The two N concentration peaks could, thus, be engineered and optimized through both the first oxynitridation step in N2O and the subsequent nitridation step in NH3. Decreasing the duration of the nitridation step in NH3, for example, could separate more distinctly the two N concentration peaks. Although these experiments have been performed at 1000 8C, lower temperatures or combination of different temperatures and variable– duration processing sequences could further facilitate the engineering of the two N concentration peaks, during these two-step processes. Binding energies (BE), peak widths and intensities from the ARXP Spectra can be related to the chemical and physical properties of the dielectric materials. Fig. ˚ 2a and b illustrate the ARXP N 1s spectra of a 57 A thick oxynitride, fabricated by thermal oxynitridation in N2O for 10 min followed by nitridation in NH3 for 1 h 25 min. The nitridation duration has been estimated based on the time needed to nitride the entire oxynitride film. Indeed, Fourier transform infrared spectroscopy
reveals that the 1060 cmy1 Si–O transverse optical mode intensity decreases with increasing nitridation time. This is due to substitution of O atoms by N atoms during the nitridation step. The nitridation duration of 1 h 25 min is, therefore, the time beyond which the 1060 cmy1 Si–O peak does not decrease any further (indicative of the end of nitridation). At 158 (Fig. 2a), an intense N 1s peak is observed at 398.1 eV with a FWHM of 1.5 eV. The NIST w18x databank suggests that the 397.9 eV feature correspond to SiN1.33, while the 398.0 eV to SiN1.7. Fig. 2b illustrates the ARXP spectra of the same sample at 908. Here, the N 1s at 398.6 eV peak (FWHM 1.5 eV) is observed to have higher binding energy than that in Fig. 2a by about ;0.5 eV. Multiple bonding arrangements have been attributed to the N 1s peak at 398.6 eV. Bhat et al. w19x have assigned it to Si–N_ H2 in NH3-nitrided oxides. Prabhakaran et al. w20x suggested that this peak may be due to (Si–)2N–O, but this structure is reported to occur at a binding energy shift of 1.5–2.0 eV higher than that of N(–Si)3 at 397.5 or 398.0 eV. Thus, the feature at 398.6 eV may be assigned to the Si–N_ H2 moiety due to the NH3 nitridation, during which N and H atoms replace the initial O atoms present in the oxynitride, or to non-stoichiometric SiOxNy due to incomplete incorporation of oxygen atoms w19–25x. No peaks are observed at )399 eV that have been attributed to the stoichiometric (Si–)2N–O compound w18,19,25x. A comparison of the N 1s spectra in Fig. 2a and b suggests that the oxynitride layer near the dielectricy substrate interface has been nitridated to a lesser extent than the dielectric surface. Lower N 1s BE at 158 ARXP suggests presence of more stoichiometric and nonstoichiometric silicon nitride near the surface, while higher BE at 908 ARXP suggests presence of more nonstoichiometric SiOxNy near the dielectricysubstrate interface. These data corroborate the SIMS data (Fig. 1a and b), in which increased NH3 nitridation duration suggests an N front moving from the top dielectric surface towards the bulk. Such data appear to be in good agreement with theoretical predictions of preliminary quantitative modeling that account for solid phase diffusion and reaction within the dielectric films of interest, at quasi–steady state conditions. Further, experimental (and preliminary theoretical) results suggest that variation of the duration or temperature of oxynitridation and nitridation processing steps may indeed result in a twostep bimodal nitrogen concentration profile within such nano dielectric layers. Electrical characterization of the resulting dielectric films has also been performed. It is observed that the NH3 nitridation step does not deteriorate the electrical performance of the dielectric oxynitride layer grown in N2O w26x. The Si3N4 thermal growth in NH3 is known to be susceptible to interfacial problems attributed to the H atom incorporation during nitridation w1,26x. Since in
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Fig. 3. N and O SIMS depth profiles for the two-step processing sequence NO (3 h)™NH3 (15 min) (a), NO (3 h)™NH3 (1 h) (b). Ts1000 8C, Ps1 atm, feed flow rates1 slpm.
the above processing sequence, minimal nitridation takes place near the interface, the interface trap density apparently remains similar to that of a dielectric oxynitride grown in N2O. The N and O depth profiles for films grown with the two-step processing sequence NO (3 h)™NH3 (15 min) are shown in Fig. 3a. The thickness of the dielectric ˚ A thick oxynitride is oxynitride grown in NO is 50 A. difficult to grow thermally in NO, due to its substantial self-limiting growth w1x. The NO oxynitridation results in a N peak at the dielectricysubstrate interface w1,12– 14x. Fig. 3a suggests N is distributed equally throughout the film, after 15 min post-nitridation in ammonia. Fig. 3b shows results from a similar two-step processing sequence with a longer second step. Therefore, increasing the NH3 nitridation duration from 15 min to 1 h does not appear to change the overall N concentration profile, although more N is probed in the film after the longer NH3 nitridation step. The ARXP N 1s spectra of a silicon oxynitride film grown in NO for 3 h followed by a post nitridation step with NH3 for 1 h is illustrated in Fig. 4. The film ˚ The N 1s peaks at both 158 (Fig. 4a) thickness is 61 A. and 908 (Fig. 4b) appear at 398.6 eV, and have similar FWHM of 1.4 eV. Identical N1s peak positions and FWHMs for both ARXP angles suggests that N bonding states near the dielectric surface and at the dielectricy substrate interface are similar in nature. This observation
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corroborates, therefore, the SIMS data (Fig. 3), in which N is most likely evenly distributed across the entire thickness of the dielectric after the post-nitridation process. This is different from the nitridation mechanism of the N2O-grown oxynitrides, in which a nitridation front was found to proceed from the dielectric surface towards the bulk. Indeed, a critical N concentration within the dielectric layer and the initial oxynitride thickness seem to play an important role in the final N concentration profile. SIM spectra of NO-based oxynitrides suggest that the FWHM of the N concentration profile is approximately 65–70% of the total film thickness; this is higher than that of oxynitrides grown in N2O at similar conditions w1,12–14x. Thus, a short nitridation in NH3 appears to be sufficient to yield a rather uniform N concentration profile across the dielectric film. Increasing the nitridation time only increases the amount of nitrogen in the dielectric film. For a thicker oxynitride film in NO ambient, a SiO2-based oxide of desired thickness could be grown first, followed by a NO anneal. Since, N would incorporate near the interface in the thicker dielectric film; the N front moving from the surface during a NH3 nitridation step would not readily merge with the N peak near the
Fig. 4. N 1s ARXP Spectra for the two-step processing sequence NO (3 h)™NH3 (1 h) at 158 (a), 908 (b) and survey (c). Other conditions are the same as in Fig. 3.
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surface helps prevent dopant diffusion during subsequent high temperature processing steps. A similar trend is also observed in the ARXP N 1s ˚ oxide, grown in O2 at 1000 8C for 4 spectra of a 84 A min and post nitrided in NH3 for 1 h 25 min (Fig. 6). At 158 (Fig. 6a), the N 1s peak is observed at 398.0 eV, while at 908 (Fig. 6b) the N 1s binding energies is observed at higher BE by approximately 0.8 eV. Both have similar FWHMs of 1.5 eV. At 158, the peak at 398.0 eV can be assigned to SiN1.33 or SiN1.7, while at 908, the peak at 398.8 eV can most likely be assigned to non-stoichiometric SiOxNy or Si–N_ H2. The presence of more stoichiometric silicon nitride related compounds at 158 (i.e. near the surface) suggests that the bulk and interface have not been nitrided to the same extent as the top surface of the dielectric, when a pre-grown oxide is nitrided in NH3. Yet, a bimodal N concentration profile appears to be possible with controlled ammonia post nitridation of an oxide layer, although the N peak near the dielectricysubstrate interface may be weak. A close comparison of the N 1s spectra of all three different processing sequences show that there is an N
Fig. 5. N and O SIMS depth profiles for the two-step processing sequence O2 (10 min)™NH3 (1 h) (a), O2 (8 min)™NH3 (4 h) (b). Ts1000 8C, Ps1 atm, feed flow rates1 slpm.
interface. This could be another method to grow a bimodal N concentration profile with higher N concentrations at both the interface (due to NO oxynitridation) and surface (due to NH3 nitridation), although it would really be a three-step process. Fig. 5a includes SIM Spectra of N and O for films ˚ 184 A-thick grown in O2 (10 min)™NH3 (1 h). The resulting nitrogen concentration profile yields one peak near the dielectric surface and another, albeit very weak, near the dielectricysubstrate interface. When the duration of the nitridation step is increased to 4 h, the entire dielectric film is nitridated (although this film was oxidized for 8 min (Fig. 5b)). It is interesting to point out that NH3 nitridation suggests that some N reaches the interface relatively early (Fig. 5a). This is possibly due to the absence of N in the oxide layer, since N is known to retard diffusion. Such a N concentration profile indicating higher N incorporation near the dielectric surface and minimal at the dielectricysubstrate interface could be important in the context of mobility degradation. It is known that excessive N near the interface could lead to mobility degradation w27x. Moreover, lower N near the interface suggests lower H incorporation at the interface from the NH3 nitridation step w26x and better electrical properties (lower interface trap densities), while higher N concentration near the dielectric
Fig. 6. N 1s ARXP Spectra for the two-step processing sequence O2 (4 min)™NH3 (1 h 25 min) at 158 (a), 908 (b) and survey (c). Other conditions are the same as in Fig. 5.
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1s peak shift of approximately 0.5 eV for the processing N2O™NH3, 0.8 eV for O2™NH3, and no shift for NO™NH3, when the ARXP angle changes from 15 to 908. Unlike oxynitrides, absence of any N near the interface before the NH3 nitridation step can explain the apparent maximum peak shift for the O2™NH3 sequence. These observations suggest that N2O grown oxynitrides and O2 grown oxides are more favorably nitridated near the top surface compared to the bulk and the dielectricysubstrate interface. On the other hand, there is practically no change in NO-grown oxynitrides post-nitrided by ammonia. 4. Summary–conclusion SIMS and ARXPS studies suggest that bimodal N concentration profiles can be obtained with a controlled nitridation of an oxynitride grown in N2O or perhaps an oxide grown in O2, although the bimodal N profile appears to be much less pronounced in nitrided oxides. This process does not appear to be effective with oxynitrides grown in NO, where the nitridation process is found to take place all across the film. Shorter durations of the nitridation step enhance the separation of the two N peaks at the dielectricysubstrate interface and the top dielectric surface. Increasing the nitridation duration in such two-step processes increases the N content near the dielectric surface with an apparent N front moving towards the bulk of the dielectric film. These experimental findings are in good agreement with theoretical predictions of preliminary modeling studies. In contrast to three-step processing, two-step processes eliminate an entire step and contribute to savings of the chemical (and thermal) budgets. Unlike threestep processes, the two-step ones eliminate an oxide step in between the two oxynitridation steps and, therefore, do not contribute to undesirable nitrogen removal or to the formation of potentially thick films. Also, the choice of precursor gases for the initial oxide or oxynitride growth, the processing temperature, and variation of the post-nitridation duration lend added degrees of freedom for tailoring desirable bimodal N concentration profiles in ultra-thin dielectrics of interest. Acknowledgments We thank Mr Rick Haasch and Ms Judy Baker at the Center for Microanalysis of Materials at UIUC for helping us with the Angles Resolved X-Ray Photoelectron Spectroscopy and Secondary Ion Mass Spectroscopy data; that facility is supported by the US Department of Energy under grant DEFG02-96-
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