Brookhaven National Laboratory ABSTRACT

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1 / , copyright The Electrochemical Society THE INFLUENCE OF NH 3 ANNEAL ON THE CRYSTALLIZATION KINETICS OF HfO 2 GATE DIELECTRIC FILMS Patrick S. Lysaght 1), J. C. Woicik 2), Brendan Foran 3), Joel Barnett 1), Gennadi Bersuker 1), Byoung-Hun Lee 4) 1) SEMATECH, 2706 Montopolis Dr. Austin, TX, ) National Institute of Standards & Technology, National Synchrotron Light Source, Brookhaven National Laboratory 3) Advanced Technology Development Facility, 2706 Montopolis Dr. Austin, TX, ) IBM Assignee to SEMATECH, 2706 Montopolis Dr. Austin, TX, ABSTRACT HfO 2 gate dielectric thin films have been exposed to anneal processing in NH 3 and N 2 ambient in order to decouple the influence of N incorporation from that of the thermal cycle alone. We report on the effectiveness of NH 3 processing to introduce N into the dielectric film system and characterize the local coordination and crystallization kinetics that give rise to the resultant high-k film microstructure as determined from a variety of high resolution spectroscopic and imaging analysis techniques. INTRODUCTION Metal-oxide-semiconductor field effect transistor (MOSFET) device scaling has driven an industry wide effort to replace the conventional transistor gate dielectric layer, SiO 2, with a high permittivity (high-k) material. Physically thicker high-k films offer lower leakage current characteristics for equivalent capacitance relative to SiO 2 (1-4) which has prompted extensive evaluation of Hf based thin film systems as potential alternative gate dielectric materials. Additionally, nitrogen incorporation into Hf based films is being evaluated for associated performance advantages since N is expected to reduce the diffusion rate of boron and all other elements in the bulk medium, thereby elevating the onset of crystallization temperature since crystallization is a diffusionlimited phase transition where the rate depends on the Gibbs free-energy difference of the phases and the diffusion rate of the species involved (5-6). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) using X-ray spectra and electron energy loss spectra (EELS) were used to produce high resolution cross-sectional images and elemental profiles of nitrogen, oxygen, silicon, and hafnium to provide interfacial chemical information and to convey changes in concentration across the bulk HfO 2 /Si film system as a function of anneal process. In conjunction with these imaging techniques and chemical profiles, we have utilized depth profiling capabilities of synchrotron X-ray photoelectron spectroscopy (XPS) to determine microstructure variations in HfO 2 gate dielectric thin films (~ 3 nm thick) deposited on Si (100) substrates. Films exposed to a post deposition anneal (PDA) process consisting of NH 3 ambient exhibit a significant amount of N in the bulk film, quantified by nuclear reaction analysis (NRA), and corroborated by Hf-N and Si-N bond peaks in the XPS binding energy spectra. Finally, samples exposed to the PDA with each ambient have also been 313

2 exposed to a high temperature rapid thermal anneal (RTA) process in N 2 which has produced full polycrystalline reference samples for direct comparison (7-8). By comparing films exposed to the NH 3 PDA process with otherwise identical films exposed to an N 2 PDA, which does not incorporate an appreciable amount of N into the bulk high-k film, for the same time and temperature, the effect of N incorporation may be decoupled from that of the thermal cycle alone. It is important to note that N incorporation resulting from NH 3 dissociation at the PDA temperature likely consists of - NH 2 and a proton, H +. Further dissociation of the NH 2 molecule may lead to the formation of NH species where NH 0 is stable while NH - is unstable, suggesting the likely final products of the NH 3 PDA process are NH - 2, NH 0 and H +, H - (9). Therefore, although evidence of N in these films may more accurately be represented as an NH 2 molecular ion, for the most part, we will consider N as it has been detected by various advanced techniques. Nitrogen may exchange for O via displacement of lattice oxygen ions in the grain structures of HfO 2 films and/or incorporate in lattice interstitials as excess N. In contrast to the N 2 molecule, the atomic N interstitial has no affinity for a second electron and is also stable as a positive ion. Additionally, grains consisting of Hf-N bonds are not consistent with metal bonds but behave much like the dielectric Hf 3 N 4, since the permittivity of these films remains high, although shorter Hf-N bond lengths will contribute local strain in tetragonal and monoclinic HfO 2 (9). EXPERIMENT Samples have been prepared to enable direct comparison of the influence of Si (100) substrate starting conditions on subsequent NH 3 nitridation of thin film systems consisting of an in situ steam generated (ISSG) 2 nm thick thermal SiO 2 reference (sample A) and H-terminated substrates (samples B and C) exposed to an NH 3 chemical anneal processing. Sample A did not receive a NH 3 chemical treatment prior to HfO 2 deposition while samples B and C were exposed to NH 3 directly on the H-terminated SI substrate. These chemical treatments were followed by atomic layer deposition (ALD) of HfO 2 (3 nm thick) and characterized as-deposited and with an NH 3 post deposition anneal (PDA) process, as per the process splits indicated in Table I. Sample B maintained the HfO2 film as-deposited, without the NH3 PDA. Performance advantages associated with nitrogen incorporation into HfO 2 are being evaluated since N is expected to reduce the diffusion rate of boron (and all other elements) and elevate the onset of crystallization temperature since crystallization is a diffusion-limited phase transition where the rate depends on the Gibbs free-energy difference of the phases and the diffusion rate of the species involved (6). The nitrogen dose indicated in Table I for the corresponding chemical substrate treatment and anneal process combination has been quantified by nuclear reaction analysis (NRA) at the University of Western Ontario utilizing a Van degraaf accelerator sensitive to 14 N from the 14 N(d,α) 12 C reaction at 1.1 MeV incident deuteron energy with a detection limit of 5 x atoms/cm 2. The total N dose for sample C that was exposed to NH 3 both pre and post HfO 2 deposition is 2.34 x atoms/cm 2 greater than the 2 nm SiO 2 sample A (which only received the NH 3 PDA process) and 3.77 x atoms/cm 2 greater than sample B (which only received the pre- DA NH 3 process). The N dose difference between sample B and C indicates the contribution associated with the NH 3 PDA process for film systems processed in this 314

3 manner and illustrates the efficiency for N incorporation of H-terminated Si since the N dose values range from approximately 7.6 to 20 atomic % N. Table I Sample set indicating NH 3 exposure both pre and post HfO 2 deposition and the resultant N dose measured by nuclear reaction analysis using 14 N(d,α) 12 C. Sample Interface Pre-treatment HfO2 Post Anneal [N] 1E15/cm-1 A 2 nm SiO2 none 3 nm NH3 PDA B HF-Last NH3-Pre-DA 3 nm none C HF-Last NH3-Pre-DA 3 nm NH3 PDA In order to investigate the physical cause for the electrical performance differences associated with these process parameters and identified via extensive transistor characterization, STEM EELS elemental chemical profile scans were performed on this sample set. STEM, EELS and Energy Dispersive X-ray Spectroscopy (EDXS) data were recorded in an FEI TECNAI F30 instrument operated at 300 kv using focused electron probes of 0.3 nm FWHM. The probe forming convergence semi-angle was 10 mrad and the spectrometer collection angle for EELS was 20 mrad. EELS and EDXS data were recorded simultaneously. EDXS integration windows were chosen to provide profiles of high-z element, Hf, with maximum signal to noise ratio and scaled to overlay on EELS profiles of Si, O and N to exhibit accurate qualitative changes in the concentration of each element as a function of position. The profiles were produced by integration of the spectral intensities of background corrected spectra for each element profiled, as illustrated for samples A, B and C in Fig. 1. The intensity in the HAADF-STEM Z- contrast image in the top portion of each sample in fig. 1 is roughly proportional to the atomic number squared and is therefore dominated by Hf, Z = 72. Unlike phase contrast images of high resolution transmission electron microscopy (HRTEM) which are primarily sensitive to crystallinity, Z-contrast clearly distinguishes bright, high density HfO 2 from the dark, low density SiO 2 -like interfacial layer (IL) which enables more accurate determination of the physical thickness of the layers (10). Element profiles plotted in the (lower) spectra portion of each sample in fig. 1 were acquired along a straight line scan from the Si substrate through the IL and uncapped HfO 2 layer of the stack where the length profiled matches the width of the STEM image shown above. The physical separation and slope of the Hf and O signals at the IL region are consistent with previous reports that claim Hf is not present in measurable quantities in the IL near the Si substrate (6, 11-15). The influence of the starting Si substrate chemical treatment is clearly illustrated by the position and relative intensity of the N profile of each sample. The HF-last (H-terminated) substrate condition of sample B and C result in (substrate) Si- N bond formation which does not occur for sample A where the Si-N bonding is due to SiO x N y formation of an oxidized substrate. Sample A represents a reference film system in that the 3 nm HfO 2 was deposited on a standard ISSG SiO 2 followed by the NH 3 PDA process. Beginning in the Si substrate and following the EELS profiles outward across the film system in cross section reveals an oxidized substrate which evolves into a SiO x N y interlayer between Si and HfO 2. The detection sensitivity, which may be influenced by local composition due to scattering of incident electrons and associated signal interference, is on the order of a few atomic percent. 315

4 Figure 1. HAADF-STEM z-contrast images and corresponding EELS chemical scan element profiles illustrating the position and relative abundance of N incorporation as a function of NH 3 processing and starting substrate condition for samples A, B and C as specified in Table I. RESULTS AND DISCUSSION The high resolution synchrotron XPS Si 2p spectra of fig. 2 was measured at the National Synchrotron Light Source at Brookhaven National Laboratory on beamline X24A and provides additional information regarding the significant difference in Si coordination in the bottom interfacial layer. The kinetic energy axis of fig. 2 pertains to the incident photon energy, hν, minus the Si 2p binding energy. The data are normalized to the substrate Si (Si-Si bonding) doublet Si 0 3/2. As shown in the EELS spectra of fig.1, nitrogen is detected with a peak profile in the bottom interface of sample A due to the NH 3 PDA process. This apparent SiO x N y interlayer between Si and HfO 2 appears much like thermal SiO 2 from the perspective of Si coordination and the evidence of the strong Si 4+ peak associated with sample A in fig. 2. By comparison, the hydrogen terminated (HF-last) Si substrate sample, C, which was exposed to both pre and post HfO 2 deposition NH 3 processing, clearly does not exhibit much of a Si 4+ peak and therefore does not consist of significant amounts of stoichiometric SiO 2. Samples processed to replicate the film system of sample C have been measured by medium energy ion scattering (MEIS) and the IL was fit with a model of 0.5 nm Si 3 N 4. Although this is consistent with the EELS spectra of fig. 1, this substrate Si-N bond formation is not as strong as stoichiometric Si 3 N 4 given that the N redistributes during subsequent RTA processing. 316

5 Intensity ECS Transactions, 1 (5) (2006) Si 2p Normalized to Si 0 Si 4+ No Si 4+ Sample C SiO 2 -like Sample A Kinetic Energy Figure 2. The starting Si substrate process of HF-last results in H-terminated Si which restricts Si from achieving four coordination (Si 4+ ) with oxygen as indicated in sample C. The reference thermal oxide example of sample A exhibits a significant Si 4+ peak although N is detected in the bottom interface resulting from NH 3 PDA. There have been numerous approaches to incorporating nitrogen into thin Hf-based gate dielectric films including reoxidation or annealing of physical vapor deposition (PVD) HfN metal to form HfO x N y films. HfN has low electron affinity and resistivity (35 uohm-cm) (16). It is found that Hf-N bonds in reactive sputtered (Hf target in Ar/N 2 /O 2 mixed ambient) HfO x N y films are not stable during PDA, compared with CVD HfO x N y films, due to the substitutional characteristics of oxygen, resulting in significant loss of nitrogen from the bulk films (17-18). This is consistent with the indication that two types of Hf-N bonds of different energy may be formed (19). One concern centers on the effective transformation of conductive HfN into the desired dielectric produced via oxidation and the avoidance of electrically nanostructured heterogeneous (ENH) HfO x N y compositions where conductive nanoscale inclusions are embedded in a dielectric matrix (20-22). Furthermore, the origin of N 2 may be different due to nitridation method. Plasma nitridation has been shown to incorporate molecular nitrogen, N 2, as interstitials in SiO x N y films from reaction of Si 2 = N species with an O or NO (23-24). Under these conditions, neutral or ionized N 2 species which are rich in N 2 plasma are likely to be incorporated into the film. No atomic dissociation from N 2 is expected, which is consistent with the fact that nitrogen is known to be an inert anneal ambient for oxides at 900 C (25). The Hf 4f core level spectra of samples A, B and C illustrated in fig. 3 clearly indicate 317

6 Intensity ECS Transactions, 1 (5) (2006) the influence of the NH 3 PDA on the local coordination of the Hf atom in these thin HfO 2 films. The well resolved transition metal core level doublet spectra (Hf 4f 7/2 and Hf 4f 5/2 ) of sample B corresponds to as-deposited HfO 2 while the relatively poorly resolved spectra of samples A and C have been exposed to the NH 3 PDA process. The modeling associated with achieving a very good fit with the measured spectra for samples A and C in fig. 3 indicate the contribution of Hf-N bonding (shift to lower binding energy) to the resultant spectra. Although sample A illustrates a significant degree of crystallinity by HRTEM, Hf-N bonds broaden the line to lower binding energy and the simulated Hf 4f 5/2 peak aligns with the trough between the measured Hf 4f 7/2 and 4f 5/2 peaks associated with HfO 2, thereby compromising the resultant measured resolution. Figure 3 exhibits Hf 4f resolution that is significantly broadened following exposure to NH 3 relative to the asdeposited 3 nm HfO 2 sample (B). Similarly, sample E exhibits increased peak broadening due to increased Hf-N bond formation from an additional NH 3 process (pre-deposition of HfO 2 ). 200 Hf 4 f Hf 4f Hf 4f Sample 9 A HfO 2 component HfO 2 component 100 Intensity Intensity A Sample 09 C Sample 24 B Sample 22 Hf 4f Hf 4f Sample C Sample 24 Hf-N HfO 2 component HfO 2 component Hf-N Kinetic Energy Kinetic Energy Figure 3 illustrates the Hf 4f core level spectra of samples A, B and C. Samples A and C have been exposed to NH 3 PDA which introduces Hf-N bonding and accounts for the peak broadening. Sample A has 2 nm SiO 2 bottom interface while samples B and C where both processed on HF-last substrates. The substitution of N for O in the unit cell leads to a hole in the valence band (p-type material) and also favors the formation of anion vacancies. This process can be represented as a formal reaction between oxygen in the HfO 2 unit cell (O 0 ) and interstitial nitrogen leading to the substitution of nitrogen for oxygen at an anion site (N 0 ), vacancy formation (V 0 ), and oxygen release: 3O 0 + 2N = 2N 0 + V 0 +3O. Hafnium oxynitride may 318

7 be formed by nitridation of the oxide or oxidation of the nitride. The nitridation of HfO 2 increases the barrier for oxygen and boron diffusion (26). The nitridation reaction of HfO 2 + 1/2N 2 = HfN + O 2 is thermodynamically unfavorable. Hf 2 N 2 O has the simplest stoichiometric formula and may be prepared by 1:1 mixing of HfO 2 and Hf 3 N 4 as: 1/2HfO 2 + 1/2Hf 3 N 4 = Hf 2 N 2 O, which has the cubic Bixbyite-type crystal structure. This structure is close to the fluorite-type structure of cubic HfO 2 in which one-fourth of anion positions are unoccupied and two-thirds of the remaining oxygen atoms are replaced by nitrogen atoms (27). Given the NRA data which indicated the N dose for each sample exposed to NH 3 processing and the Hf-N bonding component in the Hf 4f spectra, the N 1s core level binding energy spectra of this sample set is of great interest. Consider the N 1s spectra measured for samples A, B and C in fig. 4. Sample D was exposed to the pre-da process only and there is evidence for only Si-N bonding in this sample. Sample A was only exposed to the PDA NH 3 process and it appears to exhibit only Hf-N bonds (no appreciable Si-N bonding evidence, although EELS data for this sample does indicate N in the bottom interface). Finally, sample E indicates the N 1s spectra comprised of both components; the Si-N corresponding to the pre-da and the Hf-N resulting from the PDA. No appreciable N-O or N-H bond formation was captured for this sample set utilizing high resolution synchrotron XPS N 1s Si-N: 398 ev BE Hf-N: ev BE hv = ev Hf-N NH 3 PDA A Intensity NH 3 Pre-DA & PDA ~1.5eV C 8000 NH 3 Pre-DA Si-N [N]:5.9E15/cm 2 B [N]:2.1E15/cm Kinetic Energy Figure 4. N 1s core level signal for samples A (PDA only), C (pre-da and PDA), and B (pre-da only). 319

8 Figure 5. Electron diffraction peaks illustrating position and intensity spectra of (a) two 3 nm thick HfO 2 samples comparing N 2 and NH 3 PDA and (b) the same N 2 PDA sample compared with N 2 RTA. Figure 5 (a) illustrates the crystalline phase differences in 3 nm thick HfO 2 resulting from N 2 and NH 3 PDA processing. Although both samples exhibit tetragonal phase composition, the relative peak intensities are different and the overall structural intensity is greater for the sample processed with the N 2 PDA. It has been reported that in HfO x N y, N bonds to Hf replacing Hf-O bonds with Hf-N bonds. Hf is tetravalent with 4 d valence electrons, a closed shell electronic structure, with these electrons transfer to the oxygen anions (28-30). HfN has the rocksalt structure and is a metal, because it has one more electron than needed for a closed shell. Hf is tetravalent like Si, so Hf 3 N 4 has a closed shell and has a smaller band gap. In cubic HfO 2 the Hf site is eightfold coordinated and the O site is fourfold coordinated. A stable Hf 8 O 10 N 4 cell structure may evolve where the nitrogen atoms each have fourfold coordination. The oxygen atoms also retain their fourfold coordination. Compared with Hf 8 O 16, there are two oxygen vacancies, therefore, the mean coordination of Hf falls from 8 to 7. This is also found for Hf 10 O 8 N 8 and Hf 8 O 4 N 8 in each case N is fourfold and the mean Hf coordination falls to 6.4 and 6, respectively (31). The addition of N to Hf 8 O 10 N 4 has reduced the band gap by ~ 1.2 ev. This is due to the VB edge. The local DOS shows that N adds states above the VB edge of HfO 2. The VB maximum of HfO 2 is due to O 2p states. These are rather nonbonding and lie at the same energy as in the free atom. Incorporating N adds N 2p states, which lie 1.1 ev above the O 2p states in free atoms. The CB minimum is due to nonbonding Hf 5d states, and also lies at fixed energy. Therefore, the gap of HfO x N y is set primarily by atomic energies, and adding N reduces the gap by a fixed amount of about 1.2 ev by raising the VB. The evidence of phase change between the PDA and RTA process illustrated in fig. 5 (b) pertains to both N 2 and NH 3 PDA. Therefore, the intermediate temperature PDA process that introduces nitrogen into HfO 2 (NH 3 ambient) does so in a manner that evolves indistinguishably from that of the N2 PDA once subjected to the RTA process. Introducing some N adds states about 1.2 ev above the VB which lowers the VB offset to about 2.2 ev in HfO 2. Nitrogen also creates gap states when incorporated in SiO 2. There, N has a planar trivalent site and O is divalent. Both states have nonbonding p π orbitals (32). In HfO x N y, coordinations are controlled by ionic radii so N tends to be fourfold 320

9 when bonded to Hf which is a driving force for lower Hf coordination. The effect of adding N is to reduce the Hf coordination, and thus the mean coordination, while maintaining a closed shell configuration. CONCLUSION Physical characterization of the influence of nitrogen incorporation into thin HfO 2 gate dielectric film systems has been evaluated for NH 3 anneal ambient relative to N 2 PDA processing. HAADF-DTEM with EELS, synchrotron XPS and electron diffraction of HRTEM samples in plan view have been utilized to compare and characterize Hf-N and Si-N bonding and the corresponding crystallographic phase differences that evolve from these anneal processes. It has been shown that NH 3 PDA introduces an appreciable amount of N into HfO 2 where N exchanges for O in the evolving unit cell structure. While electron diffraction could not identify a change in d-spacing associated with the expected shorter bond length of Hf-N relative to Hf-O bonding, a measured difference in overall peak intensity as well as the peak intensity ratio varied for the NH 3 PDA sample relative to the N 2 sample. However, both samples evolve indistinguishably following N 2 RTA processing of uncapped films. REFERENCES 1. A. I. Kingon, J-P. Maria, and S. K. Streiffer, Nature 406 (2001). 2. International Technology Roadmap for Semiconductors, (2003). 3. G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys (2001). 4. D. G. Schlom and J. H. Haeni, MRS Bulletin 27, 198 (2002). 5. C. S. Kang, H. J. Cho, K. Onishi, R. Nieh, R. Choi, S. Gopalan, S. Krishnan, J. H. Han, and J. C. Lee, Appl. Phys. Lett. 81, 2593 (2002). 6. S. Stemmer, Z. Chen, C.G. Levi, P. Lysaght, B. Foran, J. A. Gisby and J. R. Taylor, Jpn. J. Appl. Phys. 42, 3593 (2003). 7. P. S. Lysaght, P. J. Chen, R. Bergmann, T. Messina, R. W. Murto and H. R. Huff, J. of Non-Crys. Solids, 303, pp 54-63, (2002). 8. P. S. Lysaght, B. Foran, G. Bersuker, P. J. Chen, R. W. Murto and H. R. Huff, Appl. Phys. Lett., 82, (2003). 9. J. L. Gavartin, A. L. Shluger, A. S. Foster, G. I. Bersuker, J. Appl. Phys. 97, (2005). 10. A. C. Diebold, B. Foran, C. Kisielowski, D. A. Muller, S. J. Pennycook, E. Principe and S. Stemmer, Microsc. Microanal. 9, 493 (2003). 11. G. Wilk and D. Muller, Appl. Phys. Lett. 83, 3984 (2003). 12. J.-C. Lee, S.-J. Oh, M. Cho, C. S. Hwang and R. Jung, Appl. Phys. Lett. 84, 1305 (2004). 13. S. Ramanathan, D. A. Muller, G. D. Wilk, C. M. Park and P. C. McIntyre, Appl. Phys. Lett. 79, 3311 (2001). 14. G. Bersuker, J. Barnett, N. Moumen, B. Foran, C. D. Young, P. Lysaght, J. Peterson, 321

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